Volume 2, Issue 3, September 2012, Pages 76–96
Monograph
Skullcap Scutellaria lateriflora L.: An American nervine
Abstract
The
efficacy and safety of herbal medicines are dependent upon the
standards by which they are made and our knowledge base when prescribing
them. Skullcap in particular is commonly associated with adulterations
with other Scutellaria species as well as the potential hepatotoxic Teucrium
spp. Additionally, while a staple among American and British
herbalists, formal clinical studies regarding its use are lacking. The
following botanical profile is excerpted from the American Herbal Pharmacopoeia® and Therapeutic Compendium.
Keywords
- Skullcap;
- Scutellaria lateriflora L.;
- Standards;
- Adulteration
Nomenclature
- Latin name
- Scutellaria lateriflora L.
- Botanical name
- Lamiaceae
Common names
- English
- Skullcap
- French
- Scutellaire, toque
- German
- Helmkraut, Fiberkraut, Schildkraut
- Spanish
- Escutelaria
- Part used
- aerial parts
1. Introduction
Skullcap (Scutellaria lateriflora)
is one of the most widely used nervines in North America and Western
herbal medicine. The herb has enjoyed a long history of use among
Eclectic and Thomsonian herbal practitioners and lacks robust clinical
and pharmacological investigation, therefore limiting our understanding
of its actions and full clinical potential. Additionally, skullcap has
historically been adulterated with the potentially hepatotoxic germander
(Teucrium canadense also known as “pink skullcap”). Whether
serving as a practitioner or manufacturer, this necessitates that
appropriate testing methodologies be applied to ensure procurement of
the authentic botanical. The American Herbal Pharmacopoeia (AHP)
developed a monograph detailing the various methods of botanical
morphologic, and chemical differentiation of the species. The monograph
also provides a detailed review of the historical and modern scientific
investigation of pharmacology and safety. This review was modified from
the original AHP monograph.
2. Morphological description
Skullcap
aerial parts are harvested when in flower and traded fresh or dried.
Dried material is available whole, cut and sifted, or powdered. Skullcap
may be adulterated with other species of Scutellaria (see Section 4.6) and the potentially hepatotoxic Teucrium
spp., making detailed attention to proper identification crucial.
Commercial material typically consists of stem, leaf, and calyces, with a
fairly small but variable amount of corolla fragments, depending on
time of harvest, and few seeds.
2.1. Leaves
Simple;
petioles slender, 0.5–2.5 cm, glabrate; blade thin, ovate to
ovate-lanceolate, 3–11 cm long, 1.5–5.5 cm wide; base rounded,
subcordate, to acute; apex acute; margin coarsely serrate to
sub-crenate; upper surface medium green, dull, hairs occasional; lower
surface lighter, with scattered hairs concentrated over the veins,
glandular scales may be visible as dark dots; venation pinnate with
reticulate secondary veins, major veins raised on the lower surface.
2.2. Flowers
Racemes
mostly axillary; calyx 1.5–2.5 mm, bell-shaped, 2-lipped, lips entire,
with a prominent dorsal crest (scutellum) and short, stiff hairs; calyx
persistent and enlarged in fruit, to 3 mm; corolla blue when fresh,
rarely white or pink, 5–8 mm, 2-lipped, upper lip hood-like (galea),
lower lip flat, 3-lobed, exterior surface pubescent, ring of hairs on
the inside of the floral tube absent; stamens 4, included, anthers
bearded; ovary deeply 4-lobed.
2.3. Fruit
Nutlets
4, attached to the base of the persistent calyx; ovoid, 1–1.8 mm long,
with 2 flattened faces and one rounded face; surface reddish-brown and
warty-papillate.
2.4. Stems
Quadrangular,
sides sunken between corners, up to 4 mm across at base; light
yellowish-green with some purple, glabrate to sparsely pubescent, leaf
insertion decussate; in transverse section, cortex and vascular cylinder
thin, pith large, white, and hollow.
Fracture: Leaf: Brittle. Stem: Fibrous.
3. Organoleptic profile
Color: Deep green to olive-green. Aroma: Inodorous to weakly fragrant, aroma fading quickly with age. Taste: Slightly bitter.
Powder: Medium green with lighter fragments of stem; somewhat gritty, tends to clump when rubbed between the fingers (Table 1).
- Table 1. Botanical differentiation of Scutellaria lateriflora and other common species of Scutellaria.
- Images: S. lateriflora by Jeanne R. Janish, Vascular Plants of the Pacific Northwest, Leo C. Hitchcock; Arthur Cronquist, and Marion Ownbey (Eds.), with permission University of Washington Press (1969). S. incana Britton and Brown 1913. Illustrated Flora of the Northern States and Canada. Vol. 3. S. galericulata Schenk 1884. Flora von Deutschland; Hegi 1906–1931. Illustrierte Flora von Mittel Europa 5th volume, 4th part.
- Full-size table
4. Microscopic identification
4.1. Leaf
Surface view:
Upper epidermal cells have sinuous anticlinal walls; stomata absent;
cuticular striations occur over veins and around bases of covering
trichomes; small glandular scales found predominately along veins, each
head consisting of 4–8 cells and ∼30 μm diameter; covering trichomes
rare, occurring mostly along veins, 1–3 cells long, up to 100 μm long,
with walls slightly thickened, cuticular striations conspicuous, and
apex acute; epidermal cells around the base of covering trichomes are
arranged in a rosette pattern; lower epidermis has diacytic and
anomocytic stomata, 25–30 μm long; glandular scales and covering
trichomes more frequent than on upper surface; small glandular scales
typically have a 4-celled head ∼30 μm diameter; large glandular scales
have a 6–8-celled head ∼45–50 μm diameter; covering trichomes up to
300 μm long, bent, with conspicuous cuticular striations; leaf margin
has small covering trichomes of 1–2 cells.
Transverse section: Bifacial; palisade cells in 1–2 rows; spongy parenchyma as broad as palisades.
4.2. Flower
Calyx:
2-lipped, with a dorsal crest (scutellum); outer epidermis with dense
cuticular striations; stomata diacytic and anomocytic; densely covered
with short, 1–2-celled covering trichomes up to 50 μm in length and
small glandular scales; longer uniseriate covering trichomes rare; few
glandular trichomes with a uniseriate stalk up to 100 μm long and
unicellular spheroidal head ∼20 μm diameter.
Corolla:
Epidermal cells on the margins are papillose, those in the central
portion have sinuous anticlinal walls; glandular scales small; glandular
trichomes with a uniseriate stalk up to 100 μm long and unicellular
spheroidal head ∼20 μm diameter; covering trichomes up to 200 μm long
have 1 short basal cell and 1 long, acute, terminal cell; few covering
trichomes along the margin may reach 400 μm in length.
Stamens:
Filament glabrous except in the lower half where unicellular and
uniseriate thin-walled covering trichomes up to 400 μm occur; glandular
trichomes absent; apical region of the anthers has straight unicellular
trichomes and papillae ranging from short to 100 μm in length.
4.3. Stem
Surface view: Rare covering or glandular scales similar to those found on the leaf.
Transverse section:
Quadrangular, alate; collenchyma occurs in the wings; endodermis
apparent; in each corner is a crescent-shaped vascular bundle containing
occasional fibers; vessels up to 25 mm diameter; pith present ( Table 2).
- Table 2. Microscopic differentiation of Scutellaria lateriflora and Teucrium spp.
Scutellaria lateriflora Teucrium canadense Teucrium chamaedrys Leaf Glandular scales 4–8 celled, 30–50 μm Simple 4-Celled, up to 50 μm diameter Glandular trichomes with bicellular heads Absent Non-glandular, trichome ca 100–200 mm long, along leaf margins and veins; sessile capitate glands ca 30–40 mm, more on the lower surface Unicellular stalk, bicellular head Corolla Color Blue, rarely white or pink Pink turning yellow when dry Rose-red, seldom white Shape 2-Lipped 2-Lipped 1 lip, 5-lobed Length 6 mm 15 mm long 10–15 mm Covering trichomes Mostly 2-celled, up to 200 (–400) μm Simple, clavate, glandular trichome with 2–4-celled stalk terminating into micro-papillae Mostly more than 2-celled, up to 1000 mm Calyx Color Green Green with purple teeth and purple along veins Greenish-reddish Shape 2-Lipped, lips entire Radially symmetric, 5 toothed, Radially symmetric, 5-toothed Length 2 mm 4 mm 5 mm Dorsal scale Erect dorsal scale (scutellum) Absent Absent Covering trichomes 1–2-Celled, to 50 μm Simple, 2–4-celled, non glandular trichome; sessile, capitate gland, clavate, glandular Up to 7-celled, to 1500 μm Stamens Anther covering trichomes Apical region with straight unicellular trichomes Covering trichome absent Covering trichomes absent Filament Glandular trichomes absent Capitate glandular trichome with micro-papillae Glandular trichomes present Stem Pith cavity Present Present Usually absent Vascular bundles In each corner In a ring located at each corner Ring-shaped Surface Glabrous (trichomes rare) Densely hairy Densely hairy
Powder:
Fragments of leaf epidermis with diacytic and anomocytic stomata;
glandular scales and bases of broken covering trichomes; fragments of
covering trichomes with striated cuticle; parenchyma and fibers of the
stem, with some vessels; few fragments of petals.
5. Commercial sources and handling
Skullcap
is widely traded and used in products in the North American herbal
products market. Most supplies on the domestic market appear to be
obtained from cultivated sources both domestically (e.g. Minnesota,
Missouri, North Carolina, Oregon, Washington) and abroad (e.g. Chile,
Costa Rica, Mexico).
Skullcap has historically been adulterated with various species of the potentially hepatotoxic germander (Teucrium canadense, T. chamaedrys) due to a close morphological similarity between S. lateriflora and T. canadense. It has also been reported that some seed stock sold as S. lateriflora may be incorrect to species (likely S. incana). This suggests that those cultivating what they believe to be S. lateriflora
based on the purchase of commercial seeds should verify the identity of
their population when flowering, including ensuring that multiples
species are not mixed. This makes it crucial for suppliers of dried
materials to conduct the appropriate tests to ensure identity to
species.
Skullcap may be susceptible to various pathogens (e.g. Botrytis cinerea, Erysiphe rots, etc.; Greenfield and Davis, 2004) and so supply shortages may occur, raising the potential for species adulteration.
5.1. Collection
Skullcap
can be obtained from either wild or cultivated sources. In the US, wild
sources are harvested sporadically throughout its range of
distribution. According to one report, approximately 70% of cultivated
material that supplies the domestic market comes from small growers
outside of North America. Numerous, relatively small producers are
geographically widespread throughout the US (e.g. Pacific Northwest,
Midwest, Minnesota, North Carolina) and in Central and South America
(e.g. Chile, Costa Rica, Mexico). Many companies who manufacture
products containing skullcap grow their own materials. In 2001, it was
reported that 85% of the entire harvest was obtained from cultivated
material (Greenfield and Davis, 2004).
Both the historical literature (Kraemer, 1920 and Spalding, 1819)
and modern analysis suggest skullcap should be harvested in the
flowering stage, between July and September. When collecting, cut the
aerial part leaving approximately 4 in. of stem, leaving at least 2 leaf
nodes on each stem. The new growth will emerge from these nodes, and
the stand will re-establish itself. Take care not to disturb the root so
as to allow the aerial portion to grow back the following year. Leave a
sufficient quantity of plants throughout the population to reseed.
In
material cultivated in Australia, researchers found flavonoid content
to be highest in flowering and prior to the fruit development and
differed according to plant part as follows: leaf 5.29%; roots 3.24%;
stems 2.29%, calculated on dry weight (Wills and Stuart, 2004).
The major compound in leaf, stem, and root sections was baicalin at
40–50% of the total flavonoids. The young leaves had higher
concentrations of flavonoids than older leaves but the increased biomass
of mature plants more than made up for the relative decrease (Wills and Stuart, 2004).
Once
flowering begins the plant can be cut by hand or with a mower. A light
cutting is possible the first year with two annual cuttings obtainable
thereafter. When harvested, material should not be allowed to heat up
but should be transported to a shaded area as soon as possible (Greenfield and Davis, 2004).
Historically, skullcap has been adulterated with the potentially hepatotoxic Teucrium
species (spp.). Because of this, it is critical that material be
obtained from sources in which someone with the requisite botanical
skills has confirmed the identity of the plant population.
5.2. Cultivation
Skullcap naturally grows in shady moist ecosystems but can be successfully grown as a field crop and over winters (Janke and DeArmond, 2004).
Skullcap can be grown commercially from seed or propagated by root
division. Skullcap prefers part to full shade and moist loamy soils but
can do well in sandy and clay soils as well as long as the soil does not
dry out in the summertime. It is hardy to zone four (Greenfield and Davis, 2004). According to Grieve plants grown in rich soils are not as long-lived (Grieve, 1931).
It can grow in full sun but the biomass will not develop as fully and
if grown in hot areas, a certain amount of shade is required. Skullcap
seed remains viable in cool, dry storage for at least 3 years. According
to one report from Australia, optimal growing conditions were reported
to be in dry summer weather in moist soil with some shade (Wills and Stuart, 2004).
Seed
sown in the late fall will germinate reliably in the spring. Seed sown
in the spring (in flats or cells in the greenhouse, or directly in the
garden or in the field) will germinate within 1–2 weeks. For
germination, barely cover the seed with soil, tamp in securely, and keep
evenly moist until germination occurs. Seedlings are ready to
transplant after the formation of the second set of true leaves.
Alternate guidance suggests that most Scutellaria species do
not like rich organic soils, so they should not be over fertilized. When
starting the seeds use a soil mix with generous amounts of coarse grit
and do not over water. Some sources suggest that seeds should be
stratified prior to sowing.
Plants
can also be propagated through root divisions. The mature root system
of skullcap is a matrix of rhizomes with fine hair roots below and
multiple upright stems. A good division consists of a piece of the
rhizome with root hairs and at least one upright stem. If the stem is
longer than an inch or two, it should be cut back to the second node, in
order to balance below ground and aerial portions at transplant.
Seedlings or divisions are planted out to the field in the spring.
Plants should be placed quite close together (on 6 in. centers) in order
to approximate the natural growth conditions, where the plant will grow
in monotypic patches. Close spacing also serves to reduce weed pressure
in the first year.
Young
plants are best cultivated, shallowly and often, to prohibit weed
emergence. Regular watering is essential. The plants will respond
favorably to foliar feeding with compost tea or other organic,
nitrogen-rich fertilizers. Another method of fertilization is to
broadcast organic compost over the beds and then water thoroughly.
Harvest when the plants reach the early flowering stage. Cut back to
about 4 in. above the ground level, leaving at least 2 leaf nodes on
each stem. The new growth will emerge from these nodes, and the stand
will re-establish itself. In this manner, it is relatively easy to make 2
or 3 cuttings in a growing season.
Skullcap is generally not damaged by insects but is susceptible to tomato spotted wilt virus and impatiens necrotic spot virus (Joshee et al., 2002).
The main challenge in cultivation of skullcap is weed control,
especially after the first year of growth. Grasses and forbs tend to
enter the beds, and weeding can be quite difficult, because the skullcap
is easily damaged or overrun. Weeding should be accomplished often and
early, followed by fertilization and watering, to encourage the skullcap
to produce a monotypic stand. Skullcap acts much like a pioneer plant,
quickly covering the disturbed soil in the first year and increasingly
challenged by weeds in subsequent years. The highest yields are usually
obtained in the second year, and many growers cultivate skullcap on a
2-year cycle, tilling under the crop after the second harvest of the
second year. A yield of approximately 2000 pounds per acre is typical (Sturdivant and Blakley, 1999).
For
production of seed, the plants should not be harvested twice in 1 year.
Instead, they must be allowed to mature completely within the growing
season. When the seedpods are ready for harvest, they will rattle
audibly on the plant (when the plant is shaken by the stem). Once this
occurs, the plants may be harvested for seed, by cutting the seeded tops
and laying them out on sheets in the sun to dry. Turn often, then when
the seeded tops are completely dry, rub them through a screen and winnow
out the seed.
The market
for skullcap appears to be on the increase with approximately 35,000
pounds sold in 2001, which was 2.5 times higher than 1997 levels. In
2001, 85% of the harvest was obtained from material cultivated outside
the US. Approximately 50% of herbal companies in the US market use
skullcap either as a stand-alone product (22%) or as an ingredient in
herbal formulas (33%) (Greenfield and Davis, 2004). For information regarding farming costs incurred in skullcap cultivation see BCMAFF (2002).
5.3. Handling and processing
No
significant loss of flavonoid content in the dried material has been
observed with the cutting of the fresh plant, however mechanical
stressing (e.g. compression of plant as to be visibly crushed) of the
plant does seem to incur substantial flavonoid loss. For example, plants
that were compressed after being cut exhibited a much higher loss of
flavonoids than plants that were cut without undergoing compression (Wills and Stuart, 2004).
In the investigations of Wills and Stuart (2004),
mature skullcap plants from the Central Coast of New South Wales
(Australia) were harvested and subjected to various manual treatments
designed to inflict varying degrees of physical damage to the plant
cells and structure. After stress treatment, plants were dried at 40 °C
in a hot air drier for 24 h. The effects of these various degrees of
handling stress on flavonoid concentration show there was no significant
effect on flavonoid concentration in dried material cut into sections.
However, there was a significant loss of flavonoids when plants were
mechanically stressed. Thus, care must be taken in physical handling of
fresh skullcap plants to maintain the flavonoid content.
5.4. Drying
Investigations
regarding optimal drying conditions for skullcap are lacking. However,
drying at 45 °C is appropriate for many leaf products and should be
appropriate for skullcap, though it has been established that drying at
temperatures up to 70 °C is acceptable. When drying, the herb should be
turned frequently to allow for full aeration, even drying, and to
prevent mold growth. If properly dried, the herb will retain its full
color (Greenfield and Davis, 2004). Wills and Stuart (2004)
recommend finished moisture content of 10%. Using flavonoid content as a
marker, high temperature drying of skullcap (40–70 °C) does not seem to
affect the quality of the herb (Wills and Stuart, 2004).
Historically, and in modern times, some herbal practitioners have
believed that skullcap loses much of its efficacy upon drying and should
only be used fresh or as a hydro-alcohol extract prepared from fresh
material (e.g. Felter and Lloyd, 1898 and Moore, 1993).
Wills and Stuart (2004)
reported that freshly harvested skullcap stored for 30 days at 20 °C
did not exhibit a significant decrease in flavonoid content. This seems
to indicate that drying the plants immediately after harvest is not a
necessity as long as the plants are maintained in relatively cool
conditions.
5.5. Storage
Follow
general storage guidelines by protecting from light, air, moisture,
insect infestation, and heat, with specific attention to protect from
humidity. The flavonoid profile of skullcap is sensitive to both
humidity and storage time. Storage of dried ground skullcap at
temperatures up to 30 °C results in a loss of about 0.1% flavonoid
content per day and up to 10% loss in material stored for 100 days (Wills and Stuart, 2004).
Dried
ground material stored in ‘moist air’ exhibited a greater loss of total
flavonoids as compared to material stored in ‘dry air’. It is therefore
suggested that the ground material is specifically susceptible to
degradation by humidity (Wills and Stuart, 2004).
After
storage, hydro-ethanol extracts (40–60%) stored at 20 °C exhibited
almost a 50% greater loss of flavonoid content than the loss observed in
dried skullcap powder (0.17% vs. 0.1% per day). After 10 weeks of
storage of the extract, approximately 12% loss of flavonoids was
observed. This may indicate a necessity to include flavone stabilizers
(not specified) to extend product shelf life. Specific data regarding
the mechanism of degradation for the ethanolic extracts is yet to be
determined (Wills and Stuart, 2004).
5.6. Adulterants
Skullcap
has historically been adulterated with various species of the
potentially hepatotoxic germander, which can be commonly called pink
skullcap (e.g. Teucrium canadense, T. chamaedrys). More recently, other Scutellaria (e.g. S. alpina and S. galericulata)
have also been reported as adulterants. Adulteration with germander has
been evident in the American market as recent as 2005 (Kelly, personal
communication, unreferenced). It is critical that a positive test for
identity be conducted by all those trading skullcap or that a critical
review of identification documentation and chain of custody be
performed. For assuring that adulterations are avoided, it is optimal to
apply both physical and chemical tests to avoid mixtures of the two
species and for chemically assuring the absence of verbascoside and
teucrioside, two key markers of Teucrium species.
Other skullcap species may be either, intermingled with or substituted for S. lateriflora.
Whether or not these species can be used interchangeably has not been
adequately ascertained. Some herbalists have reported using S. galericulata
for the same indications (Hedley, 2006, personal communication,
unreferenced). However, other references to the use of alternate species
are lacking. Other species of Scutellaria that may be mixed or substituted for S. lateriflora include: S. incana, S. galericulata, and the Chinese species S. baicalensis, of which the root is typically used. Numerous methods are available for differentiating between closely related Scutellaria and Teucrium species including genetic sequencing of Scutellaria species ( Hosokawa et al., 2005) and physical and chemical analyses ( Avula et al., 2003, Bedir et al., 2003, Gafner et al., 2003 and Peck et al., 1993).
5.7. Qualitative differentiation
The
leaf exhibits higher flavonoid content than other parts of the plant.
Therefore material that maximizes the leaf to stem ratio will provide a
higher level of total flavonoids. Additionally, younger leaves are
higher in flavonoids than older leaves. However, the increased biomass
of mature plants result in greater total yield of flavonoids (Wills and Stuart, 2004).
5.8. Preparations
Skullcap
is commercially available in the form of tea, tablets, capsules, and
liquid extracts. Efficacy studies on the optimal skullcap preparation
are lacking. Historically infusions and hydro-alcohol extracts were used
(see Section 6.10). In one in vitro investigation of the
anti-inflammatory effects of various preparations of skullcap a hot
water extract was more active than an alcohol extract. While both
preparations exhibited activity, activity did not appear to be
correlated with flavonoids concentration as the water extract, though
similar to the tincture, contained less total flavonoids than the
tincture (Gafner et al., 2004). Some herbalists recommend use of a fresh plant tincture, as drying is believed to reduce the potency of the herb (Kuhn and Winston, 2001 and Moore, 1993), while others feel both alcohol extracts and dried herb preparations are effective (Bergner, 2007;
Bone, 2006, personal communications). Lacking specific comparative data
between fresh, freshly dried, and aged material, it is likely that the
age of the material can play a significant role in efficacy.
For
the manufacture of tinctures, skullcap is typically extracted using a
mixed ethanol:water solvent. Maximum flavonoid content (about 70%) is
generally achieved within the 40–60% ethanol range with a flavonoid loss
of approximately 20–30% due to degradation during extraction. Flavonoid
extraction has been shown to continually decrease as ethanol levels are
raised above 60% (Wills and Stuart, 2004).
As noted above, there is the potential for relatively rapid flavonoid
degradation in stored hydro-alcohol extracts. In an analysis of
commercial ethanol extract products in Australia, it was found that
those tested lacked the characteristic chromatographic fingerprint of
skullcap, suggesting degradation is substantial in stored extracts. When
extracted, a purple film is often left on the sides of extracting jars.
Accelerated
solvent extraction using water, water extraction, and aqueous ethanolic
extraction were the best ways to obtain the flavonoid glycosides,
whereas the Supercritical Fluid Extraction (SFE-CO2) and to a lesser extent 70% ethanolic extraction gave the best results for aglycones (Awad et al., 2003, Bergeron et al., 2005 and Gafner et al., 2000) (Table 2).
6. Constituents
There are few studies on the chemistry of American skullcap (Scutellaria lateriflora). However, the data that are available suggest that the chemistry is similar to that of Chinese skullcap (aka Baikal skullcap; Scutellaria baicalensis),
which has been extensively investigated. Specifically, the flavonoids
found in both species are similar, but are present in different
proportions. In S. lateriflora flavonoid glycosides predominate with baicalin as the predominant constituent. S. baicalensis contains more than sixty flavonoids ( Malikov and Yuldaskev, 2002),
of which the aglycones baicalein (∼5.9%) and wogonin and the flavonoid
glycosides baicalin (∼5.8%) and wogonoside are most prominent.
6.1. Flavonoids
The aerial parts of S. lateriflora
contain principally flavonoid glycosides with baicalin as the
predominating constituent (∼5%), followed by dihydrobaicalin,
lateriflorin, ikonnikoside I, scutellarin (scutellarein-7-O-glucuronide), and oroxylin A-7-O-glucuronide, and 2′-methoxy-chrysin-7-O-glucuronide as well as the aglycones baicalein, baicalin, oroxylin A, wogonin, and lateriflorein ( Bergeron et al., 2005, Gafner et al., 2004 and Lehmann et al., 2000).
It is to be noted that lateriflorin, dihydrobaicalin, and scutellarein
have similar eluting times and detailed analysis is needed to
distinguish them from one another. The presence of wogonoside in S. lateriflora has also been reported but has yet to be published (Gafner 2007, personal communication).
As
noted, the leaf has been found to contain a significantly higher
concentration of flavonoids (5.29%) than the roots (3.24%) or stems
(2.29%). The flowering plant contains the highest amount of flavonoids.
The major compound in leaf, stem, and root sections was baicalin at
40–50% of the total flavonoids (Wills and Stuart, 2004).
Historical
data collected over a few years with material from different suppliers
showed that the content of flavonoids, expressed as a percentage of
dried material, depended on the year of harvest and the supplier. The
percentage of total flavonoids in the dried material varied from 1.78%
to 13.76% with a mean of 7.56%. S. lateriflora contained the
flavonoid glycosides baicalin (0.83–5.95%), dihydrobaicalin
(0.58–4.43%), ikonnikoside I (0.02–1.78%), lateriflorin (0.07–1.35%),
and oroxylin A 7-O-glucuronide (0.07–0.27%) and the aglycones
baicalein (0.02–0.51%), oroxylin A, wogonin, and
5,6,7-trihydroxy-2′-methoxyflavone (lateriflorein) (Gafner, unpublished
results). The content of oroxylin A, wogonin and
5,6,7-trihydroxy-2′-methoxyflavone was not high enough to be quantified.
Accelerated solvent extraction using water, water extraction, and
aqueous ethanolic extraction were the best ways to obtain the flavonoid
glycosides, whereas the Supercritical Fluid Extraction (SFE-CO2) and to a lesser extent 70% ethanolic extraction gave the best results for aglycones ( Awad et al., 2003, Bergeron et al., 2005 and Gafner et al., 2000).
6.2. Phenols
Caffeic acid, cinnamic acid, p-coumaric
acid, and ferulic acid were found in both aqueous-glycerin and
ethanolic extracts. The total phenylpropanes, expressed as a sum of the
caffeic acid, cinnamic acid, p-coumaric acid, and ferulic acid, varied between 0.005% (50% ethanol) and 0.01% (65% glycerin) in the dried plant material ( Gafner et al., 2000).
6.3. Essential oils
The
concentration and composition of the essential oil of skullcap is
uncertain because of limited published and conflicting unpublished data
that are contradictory. In a study of S. lateriflora harvested form Northern Iran ( Yaghmai, 1988),
distillation of dried aerial parts yielded 0.06% of a yellow oil. The
identity of the species used in this study may be questioned, as the
geographic source is not consistent with its endemic range. The oil in
this study was composed of at least 73 compounds, of which 56 have been
identified (93.8%). The hydrocarbons and the oxygenated compounds
account for 81.3% and 18.6% of the composition of the oil respectively.
Terpenoids constitute the major part of the oil. Sesquiterpenes form the
main components of the essential oil (78.3%). The predominant
structures include δ-cadinene (27%), calamenene (15.2%), β-elemene
(9.2%), α-cubenene (4.2%), α-humulene (4.2%), and α-bergamotene (2.8%).
Ten oxygenated sesquiterpenoids (6.5%) were also present with guaiol
(1.5%) and caryophyllene oxide (1.1%) being the major compounds in this
group. Among hydrocarbons, 9 monoterpenes (2.8%) were detected, of which
β-pinene (0.9%) and camphene (0.4%) had the highest concentrations.
Twenty oxygenated monoterpenoids (7%) were also present in the oil of S. lateriflora. Although detected in the oil of S. lateriflora
in low concentrations, six non-terpenoid aromatic compounds including
cinnamaldehyde (0.4%) and safrole (0.4%) have been identified. Several
components having aliphatic structure have also been detected in the oil
( Yaghmai, 1988).
6.4. Diterpenoids
Five neo-cleordane diterpenoids were reported in an acetone extract of the dried aerial parts of Scutellaria lateriflora. They were identified as scutelaterin A, scutelaterin B, scutelaterin C, ajugapitin, and scutecyprol A ( Bruno et al., 1998).
In the validation of AHP's high pressure liquid chromatography (HPLC)
method for this monograph, ajugapitin was absent in 7 samples of
authenticated S. lateriflora tested.
6.5. Amino acids
In different extracts of the herb of S. lateriflora
(aqueous, ethanolic, and accelerated solvent extraction), the major
amino acids were glutamine and GABA. The concentration varied between
0.34 and 3.12% for glutamine and 0.15–1.68% for GABA. Tryptophane,
phenylalanine, proline, glutamic acid, arginine, asparagine, aspartic
acid, tyrosine, isoleucine, leucine, and valine are all less than 0.1%
of the dry aerial plant extract ( Awad et al., 2003 and Bergeron et al., 2005). Very small amounts of amino acids were found in the supercritical fluid extraction-CO2 ( Bergeron et al., 2005).
6.6. Other constituents
The aerial parts of Scutellaria lateriflora
gave a wax yield of 1.2% calculated on the dry weight of the plant
material. The hydrocarbon fraction constitutes 20% of the wax. The four
major components in the n-alkane fraction were reported as n-nonacosane (7%), n-hentriacontane (14%), n-tritricontane (38%), and n-pentatriacontane
(24%) (Yaghmai and Benson 1979). The concentration of branched alkanes
was low (1.2%). Three series of branched alkanes were identified as
3,9-dimethyl alkanes (72%), 2-methyl alkanes (18%) and 3-methyl alkanes
(10%) ( Yaghmai and Benson, 1979 and Yaghmai and Khayat, 1987). The alkanes were reported as chemotaxonomic markers of the Lamiaceae. Catalpol and other bitter substances, lignin, resin, and tannin, have been reported in many popular books (e.g. Grieve, 1931 and Newall et al., 1996). However, recent primary chemical investigations supporting the presence of these compounds are lacking.
Melatonin was found in Scutellaria lateriflora (unidentified parts) at a low concentration of 0.09 μg/g dry weight ( Murch et al., 1997).
6.7. Chemical differentiation between Scutellaria lateriflora and Teucrium
Various species of germander (e.g. T. canadense, Teucrium chamaedrys) have been cited as historical or potential adulterants of S. lateriflora. S. lateriflora
is characterized by the presence of baicalin and scutellarin as
prominent flavonoids, along with baicalein, wogonin, and chrysin and the
absence of teucrioside and verbascoside. Teucrium species are characterized by the presence of phenylpropanoid glycosides, specifically, teucrioside in T. chamaedrys and verbascoside and teucrioside in T. canadense,
and the absence of the major flavonoids present in skullcap (e.g.
baicalin, baicalein, chrysin, dihydrobaicalin, ikonnicoside I,
lateriflorin, scutellarin, wogonin). Teucrium canadense also contains as primary diterpenoids teuflin and teucvidin (and possibly acteoside), while these were not detected in S. lateriflora ( Lin et al., 2009).
6.8. Chemical differentiation between Scutellaria lateriflora and Scutellaria baicalensis
There is potential for confusion in the supply chain between S. lateriflora and S. baicalensis root and potentially, S. baicalensis leaf. In S. lateriflora
leaf baicalin is the predominant flavonoid at approximately 5% whereas
baicalein occurs at much lower concentrations at approximately 0.06% and
wogonin-7-O-glucuronide at ∼0.01%. In roots of S. baicalensis baicalin occurs at approximately the same concentration as in S. lateriflora (both ∼5%) whereas in S. baicalensis root baicalein occurs in much higher concentrations at ∼5.9% and wogonin-7-O-glucuronide in higher concentrations than S. lateriflora at ∼0.9%. S. baicalensis
leaf has very low concentrations of both baicalin and wogonin. There
are other differences in the flavonoid profile between the species that
are readily observable with routine chromatography.
7. Therapeutics
The
aerial portions of skullcap are among the most widely used botanicals
by western medical herbalists for supporting a healthy nervous system
and as a popular ingredient in herbal sedative products. There have been
a wide variety of indications for Scutellaria lateriflora in past centuries, but nowadays, the plant is mostly reputed for its nerve tonic, sedative, and anti-spasmodic effects ( Hoffmann, 1983).
Despite its widespread use there has been very little scientific
investigation of this botanical, though modern herbal practitioners
remain convinced of its clinical efficacy. The single clinical trial
that is available reported on its efficacy in relieving anxiety
according to a non-validated subjective assessment scale and there is
pre-clinical data suggesting pharmacological mechanisms (e.g. GABA
receptor modulation) that may at least be partly associated with
skullcap's putative effects. In a monograph published by Health Canada (2004),
skullcap was approved for use as a mild sedative and in the management
of headaches. There is a tremendous amount of research on Chinese S. baicalensis and some of the flavonoids and putative activity between the two species are similar. Thus there is a potential that S. lateriflora may share some of the same pharmacological activity with S. baicalensis, an assumption that requires confirmation with future study.
7.1. Pharmacokinetics
To date, no studies on the pharmacokinetics of skullcap (Scutellaria lateriflora) extracts have been found. A fair amount of research, however, has been done on extracts and isolates of Chinese skullcap (Scutellaria baicalensis), mostly looking at the fate of baicalin, baicalein, scutellarin, wogonin-7-O-glucuronide, and oroxylin A-7-O-glucuronide. As these are some of the predominant flavonoids in S. lateriflora, the metabolic pathways are expected to be very similar.
Zuo et al. (2003) found that in general, constituents in a compound prescription that contains S. baicalensis had delayed absorption and elimination, a longer residence time in the body, and higher Cmax and AUC0-lim than those in single herb decoctions in a rat model. This was further confirmed by Lu et al. (2007) in a recent study. Baicalin (AUC0-lim:
1875 ng/mL h) had a particularly low plasma concentration in mice after
administration of a decoction (corresponding to 10 g/kg) of S. baicalensis roots, which may be partly due to the low amount of baicalin in the preparation initially. Wogonin-7-O-glucuronide (AUC0-lim: 10162 ng/mL h), oroxylin A-7-O-glucuronide (AUC0-lim: 11443 ng/mL h), wogonin (AUC0-lim: 8715 ng/mL h), and oroxylin A (AUC0-lim:
1950 ng/mL h) showed higher values. The plasma concentration of
baicalein was too low to be measured in this study, possibly due to
extensive liver and intestinal first-pass glucuronidation ( Zhang et al., 2006). Elimination halftimes (T1/2[Ke]) ranged from 3.55 h for baicalin to 27.37 h for oroxylin A-7-O-glucuronide.
Concerning their metabolism, researchers have pointed out that a
cleavage of the sugar moiety of baicalin, wogonin-7-O-glucuronide, and oroxylin A-7-O-glucuronide by the intestinal flora in rats and humans occurs ( Akao et al., 2000, Zuo et al., 2002 and Zuo et al., 2003).
While baicalein is readily absorbed in the intestine, it is extensively
metabolized to baicalin in the intestinal mucosa cells, and there is
evidence that baicalin undergoes enterohepatic circulation after oral
administration in rats ( Xing et al., 2005).
Other baicalein metabolites, which have been found in the bile and
urine of rats, include mainly baicalein-glucuronides,
baicalein-sulfates, and their methylated derivatives ( Feng et al., 2005).
Findings of a low oral bioavailability in rats due to an extensive
gastrointestinal first-pass effect have been reported for scutellarin ( Hao et al., 2005).
The traditional use of Scutellaria lateriflora as a sedative raises the question of whether any of the flavonoids can pass the blood-brain barrier (BBB). Tsai and Tsai (2004)
reported that baicalin was not detected in the striatum after
intravenous administration (30 mg/kg) alone or combined with
cyclosporine or quinidine (each at 10 mg/kg) at a detection limit of
5 ng/mL. However, recently Zhang et al. (2006) demonstrated that baicalin (90 mg/kg) could pass through the BBB and distribute to the cerebral nuclei in normal rats. The Tmax
varied from 110 to 160 min with clearance rate in different nuclei
ranging from 0.18 to 0.75 mg/kg/min. Baicalin tended to concentrate in
the striatum, thalamus, and hippocampus with 4.7% of the dosage
concentrated in the brain and a long mean residence time. This provides
evidence that baicalin may affect the CNS function directly.
Shen and Feng (2006)
reported a human pharmacokinetic study in 20 volunteers administered
with a single dose of scutellarin (60 mg p.o.). The reported half-life
was 2.27 h; Cmax of 12.02 ng/mL, Tmax of 5.9 h, and AUC0–∞ of 63.54 ng h/mL.
7.2. Clinical efficacy and pharmacodynamics
7.2.1. Sedative and anxiolytic effects
7.2.1.1. Human clinical studies
The only human clinical trial on the efficacy of skullcap (S. lateriflora) is a study by Wolfson and Hoffmann (2003).
This small single-dose study was performed according to a double-blind,
placebo-controlled crossover design. The endpoints were the subjective
assessment of the various preparations on the participants overall
energy, cognition, and occasional anxiety. A total of 19 healthy
subjects were treated with either of the 4 preparations: two capsules of
placebo; one capsule containing 350 mg of freeze-dried aboveground
parts of skullcap (Eclectic Institute, Sandy, OR); one capsule
containing 100 mg of freeze-dried skullcap extract; two capsules
containing 100 mg of freeze-dried skullcap extract.
The
authors did not give details about the characterization of their
extracts, but did specify that the material was subjected to HPLC
characterization. The effects of the treatments were evaluated over time
(baseline, and at 30, 60, 90, and 120 min after oral administration).
There was little impact on energy and cognition, but a noticeable
decrease in anxiety based on a non-standardized subjective assessment
scale with treatments of 350 mg freeze-dried plant material and 2
capsules of 100 mg extract. The effect was most pronounced 60 min after
the administration of the 2 capsules at 200 mg. Patients were assessed
before and after the study for a variety of hematological markers
including CBC, standard blood panels, and liver function tests. No
evidence of toxicity or side effects was reported by any of the study's
participants. The findings of this study are greatly limited. The test
subjects did not suffer from the condition purported to be the primary
endpoint, anxiety. And, a non-validated subjective assessment scale was
used as a measure, one of being either relaxed or tense. Thus, the only
conclusion that can be suggested is that the skullcap preparation
improved subjective perception of mental well being; or possibly, that
subjects taking skullcap perceived having a greater level of relaxation
as compared to controls.
7.2.2. Animal and in vitro studies
There have been two published animal studies involving S. lateriflora extracts, although one of the studies ( Peredery and Persinger, 2004) used a combination of S. lateriflora, Gelsemium sempervirens, and Datura stramonium.
The investigators concluded that a mixture of the three herbal fluid
extracts (provided by GAIA Herbs) prevented the development of induced
seizures in rats. As the study did not include an evaluation of single
ingredients, conclusions on the efficacy of a treatment with S. lateriflora herb alone cannot be made.
In the second study, Awad et al. (2003)
evaluated a water extract of skullcap alone in behavioral studies in
rats. Each rat was fed milk containing 100 mg/mL extract; control rats
received milk without the herbal extract. In the “open-field” test, the
treated rats entered the center (or anxiogenic) part of the box more
often and spent more time in the center part than control rats. In the
“elevated plus maze” paradigm, the treated rats entered the open arms
more frequently and spent more time in the open compared to control
rats, suggesting that treated rats were less apprehensive or anxious in
the unfamiliar environment of the open arms. In addition, rats treated
with skullcap displayed a greater number of unprotected head dips at the
exit of the maze, indicating the lack of fear in exploring an
unfamiliar territory. Interestingly, analysis of the aqueous extract
revealed only a small amount of baicalin and no measurable baicalein,
but it contained a large amount of the amino acid glutamine (31.2 mg/g).
The same approach (elevated plus-maze) was used by Xu et al. (2006), who evaluated baicalin alone and in combination with dl-tetrahydropalmatine,
an anxiolytic-hypnotic alkaloid, or diazepam in mice. Baicalin
treatment (7.5–30 mg/kg) significantly increased the number of entries
into and time spent in open arms, suggesting an anxiolytic effect. This
effect was blocked by the benzodiazepine-antagonist flumazenil,
suggesting that the anxiolysis was likely due to an activation of the
benzodiazepine-binding site. The anxiolysis was not accompanied by
motor-depressive or muscle relaxant side effects in the mice, and
baicalin treatment did not alter the number of head-dips in the
hole-board test nor increase the number of mice dropping from the wire
in the horizontal wire test. Co-administration of baicalin (3.75 mg/kg)
with dl-tetrahydropalmatine (0.25 mg/kg) or diazepam (0.5 mg/kg) induced an additive effect.
In
vitro tests have mainly focused on interactions with brain receptors
and enzymes involved in anxiolysis. At least six papers (Huen et al., 2003a, Huen et al., 2003b, Hui et al., 2000, Liao et al., 1998, Wang et al., 2002a and Wang et al., 2002b) report on the interaction of flavones isolated from S. baicalensis (some of which occur in S. lateriflora) with the benzodiazepine site of the GABAA receptor complex. Flavone-glucuronides (e.g. baicalin, wogonin-7-O-glucuronide) had a low affinity for the benzodiazepine binding site ( Wang et al., 2002a).
The binding affinity of flavones was much higher, especially for
flavones with a 2′-hydroxyl group. Of the flavones known to occur in S. lateriflora, oroxylin A showed the highest affinity to the receptor, followed by wogonin, while baicalein was not very active ( Huen et al., 2003a and Wang et al., 2002a). Hui et al. (2002)
investigated the anxiolytic activities of wogonin in mice and found a
similar response between wogonin (7.5–30 mg/kg oral) and diazepam
(1 mg/kg) in the elevated plus-maze test. Using electrophysiological
techniques, the authors showed that wogonin enhanced the GABA-activated
current in rat dorsal root. The effects of wogonin were partially
reversed by the co-application of the benzodiazepine site antagonist
flumazenil. However, the authors concluded that wogonin exerts its
anxiolytic effects through positive allosteric modulation of the GABAA receptor complex via interaction at the benzodiazepine site.
A paper by Gafner et al. (2003b) reported on the interaction of extracts and isolated flavones from S. lateriflora with the 5-HT7
receptor. This receptor is involved in the mechanisms of antidepressant
action, and circadian rhythms. The authors observed a weak interaction
with this receptor. The most active isolates were the flavone
glucuronides scutellarin and ikonnikoside I, neither of which are likely
to be available at high enough levels in the brain to exert any
physiological effect on the 5-HT7 receptor. Awad et al. (2003) have tested aqueous and alcoholic S. lateriflora
extracts on GABA-transaminase, which is the enzyme that metabolizes
GABA and terminates its action. Although both skullcap extracts
exhibited some inhibitory activities (an IC50 of 5.25 mg/mL
and 2.11 mg/mL for the aqueous and ethanolic extracts, respectively),
the results were not considered to be promising enough to warrant
further investigation.
7.2.3. Summary
Though
skullcap is widely used by traditional herbalists and is a common
ingredient in popular herbal sedative products, there has been very
little scientific investigation into its use. The one clinical study
found reported positive effects for reducing anxiety but the study was
methodologically weak, partly due to it being substantially undersized, a
short-term study, not based on objective evaluation scales, and only
testing for subjective feelings of reduced anxiety. Some pharmacological
data supports anti-anxiety mechanisms for a number of the flavones
contained in skullcap and points to an involvement of the GABA receptor
system as a potential mechanism of action. These flavones have been
shown to interact with the GABAA receptor at very low
concentrations, but the fact that flavones undergo an extensive
first-pass metabolism suggests they may not be the actives. Much more
work, including pharmacokinetic studies of some of the minor flavones in
skullcap, is needed to support its efficacy and to relate the activity
to its chemical composition coupled with concentrations that would be
physiologically relevant. With the exception of the flavones and amino
acids, little is known about skullcap pharmacology. Therefore, other
phytochemicals may play a role in the putative biological activity of
skullcap.
7.3. Anti-inflammatory effects
7.3.1. Animal and in vitro studies
In
past centuries, skullcap was used for the treatment of infections due
to bites, such as rabies. Little investigation regarding this putative
effect has been done. The anti-inflammatory effects of flavonoid-rich
extracts from different Scutellaria species have been described in a US patent ( Jia et al., 2007),
though the available evidence suggests any anti-inflammatory effect to
be very weak. To obtain this extract, the whole plant of S. lateriflora
was dried and ground. Sixty grams of plant material were extracted
twice with 600 mL methanol-dichloromethane (1:1) for 1 h. The two
extracts were combined and evaporated to dryness to provide the material
for the studies. This S. lateriflora extract inhibited human and ovine cyclooxygenase (COX)-1 and COX-2. The IC50
values were 20 μg/mL (human COX-2), 30 μg/mL (ovine COX-2), and
80 μg/mL (ovine COX-1). Despite the promising activity in vitro, the S. lateriflora
extract (50–200 mg/kg orally or intraperitoneally) had no effect in the
ear-swelling model in mice. The COX inhibitory activities in vitro were
confirmed in the study by Gafner et al. (2004),
which used hot water and 70% aqueous alcohol extracts, these extracts
also failed to show a significant selectivity towards either COX-1 or
COX-2. The same authors found that the two extracts decreased
prostaglandin E2 production in human keratinocyte cell
cultures. This may be due in part to an inhibition of COX-2 expression,
which has been proposed as a mechanism of anti-inflammatory action of
wogonin ( Chen et al., 2001, Chi et al., 2003, Chi and Kim, 2005, Lim et al., 2004 and Park et al., 2001). Baicalin, oroxylin A-7-O-glucuronide, scutellarin, and baicalein ( Chen et al., 2001;
Kondratyuk and Pezzuto, unpublished results) did not affect COX-2
protein expression in the mouse macrophage cell line (RAW 264.7).
Wogonin and, to a lesser extent, baicalein have been shown to reduce the
activity of the transcription factor NF-kappaB, which may explain many
of the anti-inflammatory and anti-tumor activities described for the two
flavones. The inhibitory effect of baicalein on the DNA binding
activity of transcription factors was significantly greater for nuclear
factor IL-6 (NF-IL6) than in NF-kappaB or activated protein 1 (AP-1) ( Chen et al., 2004).
Neither
the hot water nor 70% aqueous alcohol extract inhibited 5-lipoxygenase,
neither did they influence the amount of cytokine (IL-1α, IL-1β, IL-8,
TNFα) production in a cell based assay, but the 70% aqueous alcohol
extract was able to inhibit the neutrophil elastase (IC50 = 40 μg/mL). The hot water extract did not show any elastase-inhibitory activity (Gafner et al., 2004).
Many papers have reported on the in vitro inflammatory effects of pure flavones. Some of the S. lateriflora flavones (baicalin, baicalein, and wogonin) have been reported as 5- and 12-lipoxygenase (LOX) inhibitors ( Kimura et al., 1985 and You et al., 1999). Baicalein had greater inhibition of 5-LOX (IC50 7.1 μM) than COX (IC50 55.3 μM), while baicalin was less potent and only inhibited 5-LOX (IC50 180 μM), and wogonin only inhibited COX with an IC50 of 14.6 μM ( Kimura et al., 1985).
Studies looking at the reduction of TNFα by wogonin in cell-based
bioassays and animal studies have shown surprising results: in the
cerebral ischemia rat model, in cultured brain microglia and in the
mouse macrophage cell line RAW 264.7, LPS-induced TNFα production was
decreased by wogonin ( Chiu et al., 2002, Lee et al., 2003 and Piao et al., 2004),
while the same compound, without LPS induction, at a concentration of
1–10 μM, actually induced TNFα gene expression in the mouse macrophages (
Chiu et al., 2002).
Other mechanisms (inhibition of NO production and iNOS protein
expression in RAW 264.7 cells, inhibition of the binding of chemokines
to human leukocytes) for the anti-inflammatory actions of baicalin,
baicalein, oroxylin A, and wogonin have been described in a
comprehensive overview by Calixto et al., 2003 and Calixto et al., 2004.
A study by Kubo et al. (1984a)
showed that baicalin, baicalein, and wogonin, at oral doses of
50–100 mg/kg in mice, all inhibited increases in vascular permeability
induced by acetic acid. These compounds likewise reduced chemically
induced acute paw edema in rats. In developing adjuvant-induced
arthritis in rats, these compounds also suppressed the secondary lesion
and so can be considered effective for both acute and chronic
inflammation. Baicalein and wogonin were shown to inhibit histamine
release from rat mast cell in vitro with IC50 of 52.1 μM and 40.0 μM, respectively (Kubo et al., 1984b).
Another activity of baicalein, oroxylin A, and baicalin (in diminishing
order of potency) associated with inflammation is in vitro inhibition
of eotaxin production at 10 μg/mL (Nakajima et al., 2001).
Eotaxin acts to increase inflammation by recruiting eosinophils to
allergic inflammatory sites such as the bronchi in asthma. In another
asthma-associated, anti-inflammatory, leukocyte-migration effect,
baicalein inhibited the thrombin-induced endothelial leukocyte adhesion
molecule-1 and intercellular adhesion molecule-1 by 50% at
concentrations of 5.5 μM and 2.4 μM, respectively (Kimura et al., 2001). Baicalein also inhibited their expression when induced by a protein kinase C activator (phorbol myristate acetate).
7.3.2. Summary
There is only one animal study available with very limited data on the anti-inflammatory activity of S. lateriflora extracts. There is a considerable amount of data supporting the anti-inflammatory effects of flavonoids derived from S. baicalensis root, some of which occur in S. lateriflora.
However, it cannot be assumed that the same therapeutic effects will be
achieved using the North American species. Therefore, more work is
needed to determine if skullcap and its isolates can be recommended for
use as an anti-inflammatory agent.
7.4. Anti-tumor effects
7.4.1. Animal and in vitro studies
Many researchers have investigated the anti-tumor effects of S. baicalensis
due to the fact that an extract of this plant was part of the promising
prostate cancer drug PC-SPES, an adulterated herbal compound that
included estradiole, among other conventional pharmaceutical agents
(e.g. warfarin). No studies have been carried out with S. lateriflora,
but there are a large number of publications on some of its flavonoids.
Oral treatment of mice with baicalein (10, 20, and 40 mg/kg per day for
28 days) demonstrated a statistically significant (P < 0.01) prostate tumor volume reduction ( Miocinovic et al., 2005).
A similar dosage of baicalein (20 mg/kg per day p.o.) reduced the
growth of prostate cancer xenografts in nude mice by 55% after 2 weeks
compared to placebo, and delayed the average time for tumors to achieve a
volume of 1 cm3 from 16 to 47 days. These anti-tumor activities were reportedly related to the inhibition of the androgen signaling pathway ( Bonham et al., 2005).
Furthermore, intragastric administration of baicalein (130 and
260 mg/kg) significantly inhibited testosterone-induced prostatic
hyperplasia in rats, although the dosage levels used in this study were
extremely high ( Guo et al., 2004).
Baicalin was found to induce apoptosis in several prostate cancer cell lines, including DU145, PC-3, LNCaP and CA-HPV-10 (Chan et al., 2000).
Baicalin and baicalein were reported to exhibit growth inhibitory
activity towards the human hepatoma cell lines Hep 3B, SK-Hep1 and
Hep-G2 (Chang et al., 2002).
The induction of apoptosis by baicalein in cell lines of hepatocellular
carcinoma may be partly due to an inhibition of topoisomerase II,
although other mechanisms seem to be involved as well (Matsuzaki et al., 1996).
Cytotoxic effects of baicalin, baicalein, and wogonin were also
observed against the human bladder cell lines KU-1 and EJ-1, and on the
murine bladder cancer cell line MBT-2 (Ikemoto et al., 2000). Finally, baicalein potently inhibited the growth of the human breast carcinoma cell line, MDA-MB-435, with an IC50 of ∼6 μg/mL (So et al., 1996).
7.4.2. Summary
To date, there is no scientific evidence to support the use of S. lateriflora
extract as an anti-tumor agent. Based on the positive data obtained for
baicalin, baicalein, and wogonin, however, it may be worthwhile to
investigate the potential anticancer activity of other Scutellaria species, including S. lateriflora.
7.5. Other effects
7.5.1. Spasmolytic effects
Many authors in the traditional medicine literature refer to the use of S. lateriflora as a spasmolytic agent. Both the hot water and the fluidextract of the aerial parts of S. lateriflora had a weak uterine relaxation effect in female guinea pigs in a study published by Pilcher et al. (1916). To date, there is no other published evaluation of the spasmolytic properties of this herb and the model of Pilcher et al. (1916) is irrelevant to human oral use.
Some
skullcap flavonoids have demonstrated smooth muscle spasmolytic
effects. One means of inhibiting smooth muscle contractions, e.g. in the
bronchi or coronary arteries, is through inhibition of cAMP
phosphodiesterase. Baicalein and other Scutellaria flavones have shown potent cAMP phosphodiesterase inhibition in vitro ( Nikaido et al., 1988).
7.5.2. Antimicrobial effects
In an antimicrobial screen using TLC bioautography, the dichloromethane and methanol extracts of S. lateriflora were tested for inhibition of the yeast, Candida albicans, the plant-pathogenic fungus, Cladosporium cucumerinum, and the two bacteria Bacillus subtilis and Escherichia coli. Both extracts were active against C. cucmerinum at 100 μg/mL and against E. coli at 10 μg/mL. The methanol extract was also active against C. albicans at 100 μg/mL. No other activities were reported in this study ( Bergeron et al., 1996).
In regards to the isolated flavones, an early study found that wogonin inhibits Vibrio cholerae and Staphylococcus aureus and did not show antibacterial activity against Salmonella typhi or Shigella dysenteriae ( Hsu, 1954 as cited in Chang and But, 1987). More recently, baicalin demonstrated a synergism against S. aureus when used with the β-lactam antibiotic, benzylpenicillin ( Liu et al., 2000). When combined with 16 μg/mL baicalin, the minimum inhibitory concentration of benzylpenicillin against methicillin-resistant S. aureus and penicillin-resistant S. aureus
was reduced from 125 and 250 μg/mL to 4 and 16 μg/mL, respectively.
With 25 μg/mL of baicalin, the killing of methicillin-resistant S. aureus
was potentiated for ampicillin, amoxycillin, benzylpenicillin,
methicillin, and cefotaxime (concentration of each 10–15 μg/mL). This
study suggests the potential use of baicalin as an adjunct to β-lactam
antibiotic-resistant S. aureus infections.
7.5.3. Antioxidant effects
The radical scavenging activities of a hot water extract of S. lateriflora herb were evaluated in two different assays. The extract was able to scavenge the radical DPPH with an IC50
value of 19.7 μg/mL. In an electrochemical antioxidant activity test,
which measures the amount of superoxide radical scavengers in a sample,
the S. lateriflora extract gave a value of 72.5 μg/g extract, which was slightly less compared to the results obtained for an aqueous extract of Melissa officinalis (Bergeron and Gafner, unpublished results). Silva et al. (2005) evaluated the antioxidant potential of an ethanolic skullcap extract. The extract was able to scavenge DPPH free radicals (EC50: 83 μg/mL) and to reduce lipid peroxidation in rabbit synaptosomes (EC50: 10 μg/mL). The same authors also reported that the ethanolic skullcap extract reduced the ascorbate/Fe2+-induced lipid peroxidation in PC12 cells, without affecting cell viability.
A
large number of publications exist, which describe the antioxidant
activities of baicalein and baicalin. Both compounds have been reported
to scavenge free radicals, to inhibit xanthine oxidase, and to protect
lecithin liposome membranes from photo-induced or hydrogen
peroxide-induced lipid peroxidation (Gabrielska et al., 1997, Gao et al., 1999, Gao et al., 2001, Hwang et al., 2005a, Hwang et al., 2005b, Shieh et al., 2000 and Wozniak et al., 2004). Wogonin and wogonin-7-O-glucuronide
showed little effect on the scavenging of DPPH radicals, but wogonin
had some xanthine oxidase inhibitory activities. Antioxidant effects
have also been reported for scutellarin, which was shown to scavenge
hydrogen peroxide radicals ( Liu et al., 2002),
and was able to attenuate hydrogen peroxide or glutamate induced
cytotoxicity, intracellular accumulation of reactive oxygen species, and
lipid peroxidation in rat pheochromocytoma cells ( Hong and Liu, 2004a and Hong and Liu, 2004b).
Baicalin,
baicalein, and wogonin reduced lipid peroxidation in the liver of rats
when given in doses of 100 mg/kg for 3 days 1 h prior to intraperitoneal
injection of a peroxidizing reagent mixture of ferrous chloride,
ascorbic acid, and ADP (Kimura et al., 1981b).
Likewise, liver transaminase levels in rats induced by oral doses of an
oxidized vegetable oil mixture were also reduced by baicalin. In rats
fed a mixture of corn oil, cholesterol, and sodium cholate, 100 mg/kg
oral baicalin, baicalein, and wogonin inhibited liver triglyceride
deposition, while baicalin and baicalein reduced serum free fatty acid
and triglyceride levels and wogonin increased serum HDL-cholesterol (Kimura et al., 1981a).
In rats given ethanol, the same oral flavonoid dosage led to a
reduction of serum triglycerides by wogonin, an increase in
HDL-cholesterol by baicalein, and a decrease of cholesterol and
triglyceride content of the liver by baicalein and baicalin (Kimura et al., 1982).
7.6. Conclusion
The
primary use for which skullcap is employed today is as a nervous system
restorative (nervine), relaxant, and antispasmodic. For these uses,
herbal clinicians today believe strongly in skullcap's efficacy, based
partly on the extensive clinical use of the herbal practitioners (e.g.
Eclectics and Physiomedicalists) of the late 1800s and the empirical
experience of modern herbal practitioners. Despite this, little formal
clinical or pharmacological investigation has been undertaken. What
little clinical data there are does provide some supporting evidence for
an anxiolytic effect and the scant pharmacological data suggests this
may be due to modulation of the GABA receptor system (benzodiazepine
site). Clearly more investigation is needed for this popular and widely
used botanical. It should also be noted that the mechanisms of action
studied (i.e. GABA binding) are greatly influenced by the currently
understood mechanisms associated with conventional pharmaceuticals. The
true pharmacology of skullcap may be different than currently
articulated mechanisms.
7.7. Medical indications supported by clinical trials
Only
one clinical trial was found which reported positive effects of
skullcap extract as an anxiolytic. While this trial suffers from too
small of a patient population and other methodological flaws to stand
alone as evidence of skullcap's efficacy, it supports the use of
skullcap of historical and modern herbal practitioners.
7.8. Medical indications supported by traditional or modern experience
The
primary use of skullcap is for its nervine and antispasmodic
properties, which are considered by modern herbalists to be very well
established. Skullcap is therefore recommended for a myriad of
conditions where nervines and antispasmodics are indicated. Since
Constantine Rafinesque wrote about skullcap in 1830, the majority of its
medicinal uses and indications have changed very little. Rafinesque
reported that skullcap possessed tonic and antispasmodic properties and
was primarily used to treat nervous diseases, convulsions, tetanus, St.
Vitis’ Dance, tremors, and possibly rabies. This herb is no longer used
to treat rabies or tetanus, but based on typical use by modern
herbalists, the other indications appear to still be valid.
King,
in his American Dispensatory (1866) regarded skullcap much as it is
regarded today, as a tonic, nervine, and antispasmodic and recommended
its use in all cases of “nervous excitability, restlessness, or
wakefulness”, including its use in teething children. King made a
notable observation in noting that it does not leave the system
excitable or irritable when its calming effects wear off, “as with the
case of some other nervines.” alluding to its tonic properties.
Physiomedical and Eclectic authors give us the clearest indications for
the precise clinical uses of this herb. T.J. Lyle (1897), the noted Physiomedicalist physician, stated Scutellaria
“is an excellent agent in nervous prostration” and wrote that its
influence “extends to the brain, spinal cord, and sympathetic system”.
Lyle believed the hot infusion was the most effective preparation and he
used it with lobelia (Lobelia inflata), cayenne pepper (Capsicum spp.), and lady's slipper (Cypripedium spp.; a species today cited as environmentally threatened), or simply with blue cohosh (Caulophyllum thalictroides),
as an active antispasmodic for chorea, mild or petit mal epilepsy,
hysteria, and other spasmodic conditions in either children or adults.
Lyle also mentions these formulas are effective for delirium tremens,
morphine withdrawal, cranial neuralgia, uterine neuralgia, insomnia,
hysteria, convalescence from fevers, and general nervousness.
Felter
and Lloyd, in the 18th edition of King's American Dispensatory (1898),
reported on the attitude of orthodox physicians regarding the use of
skullcap saying skullcap is “one of those valuable agents which a
certain class [allopaths] consider inert yet it has proved especially
useful in chorea, convulsions, tremor, intermittent fever, neuralgia,
and many nervous afflictions”. They note that the infusion taken freely
will allow a patient with delirium tremens to fall into a calm sleep.
Combined with bugleweed (Lycopus virginicus) these Eclectics
claimed skullcap is effective for intermittent fevers (malaria, yellow
fever, or dengue fever). The tea is also recommended for teething
children, nervous excitability, or restlessness from excessive physical
or mental work and for stress-induced palpitations. For antispasmodic
activity, Cook (1869) recommended combining skullcap with blue cohosh (Caulophyllum thalictroides) and lobelia (Lobelia inflata), a half-ounce of the mixture given every 30 min to an hour. Felter and Lloyd's specific indications for Scutellaria are divided into two categories similar to what Finley Ellingwood also mentioned in his classic work, American Materia Medica, Therapeutics, and Pharmacognosy
(1919). Both works state that it is beneficial for nervousness
(anxiety) caused by acute or chronic illness, or physical and/or mental
exhaustion and is effective for treating nervousness manifesting as
muscular spasms, with the inability to control voluntary and involuntary
muscular activity. These authorities recommended using an infusion made
with 1/2 oz. of the recently dried skullcap leaves to 1/2 pint of
boiling water. Three to four cups of this tea were recommended to be
taken throughout the day. This very strong decoction is very different
from what many modern herbalists recommend. In fact, some modern
herbalists see dried skullcap as mostly inert ( Kuhn and Winston, 2001), while others consider both fresh and dry material to be effective ( Bergner, 2007;
Bone, 2006, personal communications), and Bergner cautions against
boiling. Bergner further describes exhaustion from chronic stress, sleep
deprivation, overwork, or caffeine withdrawal as primary indications
for skullcap.
The
difference of opinion regarding fresh versus dry preparations is
probably based on several different issues. The Eclectics clearly noted
the herb must be “recently dried”. This usually means that the material
is carefully dried and stored and is no more than 6 months old. Cook (1869)
counseled that boiling rendered skullcap inert and recommended an
infusion taken cold for tonic properties, and warm for acute
nervousness.
Cook also recommended a hydro-alcohol (1:5) and fluid extract (1:1). Ellingwood (1919)
similarly recommended infusion and fluid extract dosage forms. Neither
commented on whether the material should be extracted fresh or dry. When
some American and British herbalists were learning to use this herb in
the late 1960s and early 1970s, the skullcap in the marketplace was
often poorly dried, old, or adulterated with the potentially hepatotoxic
germander (Teucrium species). Therefore it is no surprise that
questions regarding skullcap's efficacy have been posed historically,
and, as with many botanicals, more investigation is needed to determine
what preparations are most efficacious.
Modern works continue to see this herb as a valuable nervine and have added to our knowledge of its appropriate utilization. Priest and Priest (1982),
the modern British Physiomedicalists, classify skullcap as a
“diffusive, stimulating, and relaxing nervine-cerebral vasodilator and
trophorestorative, indicated for nervous irritation of the
cerebro-spinal nervous system.” They suggest either skullcap alone or
combining it with windflower (Pulsatilla) or black cohosh (Actaea racemosa) for epileptiform convulsions, chorea, and agitation and when combined with passion flower (Passiflora incarnata) for nightmares, insomnia, and restless sleep.
Hoffmann, in his text, Medical Herbalism (2003)
further states that “skullcap may be used to treat any condition
associated with exhaustion or depressed states and can be used with
complete safety to ease premenstrual tension”.
In Kuhn and Winston's (2001)Herbal Therapy & Supplements,
there is further elucidation on the uses of skullcap as a nervine and
antispasmodic. The authors suggest combining tinctures of skullcap with
chamomile (Matricaria recutita), lemon balm (Melissa officinalis), fresh oat (Avena sativa), and/or St. John's wort (Hypericum perforatum)
for anxiety with muscle tension, mild forms of obsessive-compulsive
disorder, and neurasthenia. It is also stated that its antispasmodic
effects can be of benefit for reducing Parkinson's disease tremors,
restless leg syndrome, temporo-mandibular joint (TMJ) pain, bruxism, and
the physical symptoms of Tourette's syndrome. The recommended dose
forms in this source are a fresh plant tincture (3–6 ml TID) and
freeze-dried herb capsules (2 capsules TID).
Mills and Bone (2000)
note that skullcap can be of benefit for stress-induced dyspepsia,
gastro-esophageal reflux disease (GERD), irritable bowel syndrome (IBS),
and palpitations. They list skullcap as a nervine tonic or
trophorestorative and mention that nerve trophorestoratives are
indicated for neuralgias, herpetic infections, depressive states, late
night wakefulness, convalescence, and neurosis.
Naturopathic physicians continue to utilize this herb and according to one recent text (Mitchell, 2003)
it is utilized in a fairly large dose (approximately 4–5 mL of the
tincture) before bed to promote sleep. In this text it is also
prescribed for mania, non-specific drug withdrawal, teething pain, and
stress-induced cardiac symptomology.
There are some reports of herbalists using other species of Scutellaria (e.g. S. galericulata) for many of the same indications as S. lateriflora) (Hedley, 2006, personal communication, unreferenced).
7.9. Actions
Anxiolytic, antioxidant, antispasmodic, nervine, sedative.
7.10. Dosages
Powder: 1–2 g three times daily Infusion: 1–2 g three times daily Tincture (1:5, 45% alcohol): 5–10 mL three times daily Fluid extract (1:1): 1–2 mL three times daily
Due
to the lack of modern clinical trials, these dosages were derived from
the traditional literature and reflect use by modern practitioners.
Eclectic references suggested up to 14 g (dry weight) used in infusion,
recommended a dose of 1 fluid ounce three times daily of the infusion
thus made for tonic purposes, and further recommended that does could be
given every other hour for acute nervousness.
8. Safety profile
8.1. Adverse reactions
Skullcap is considered by modern herbalists to be a very safe botanical and is given a Class 1 safety rating in the Botanical Safety Handbook of the American Herbal Products Association (AHPA). In the historical literature of the Eclectic physicians ( Ellingwood, 1919, King, 1866 and Lloyd, 1921; and Physiomedicalists ( Cook, 1869)
of the late 1800s–early 1900s, skullcap was considered a safe and
valuable nervine that did not exert a strong effect on the body and
these practitioners cited no cautions regarding its use. The recent
skullcap monograph produced by Health Canada for their regulation of
skullcap as a Natural Health Product similarly reports no health
concerns. In the clinical trial of Wolfson and Hoffmann (2003)
patients were assessed before and after the study for a variety of
laboratory values including CBC, standard blood panels, and liver
function tests. No evidence of toxicity or side effects was reported by
any of the study's participants. The primary investigator of this
clinical trial reported conducting further toxicology studies utilizing
dosages that were “many orders of magnitude higher quantities than would
be used as a maximal human dosage” and reported no abnormalities
regarding hematological parameters and liver functions (Wolfson, 2007,
personal communication).
8.1.1. Reports of hepatotoxicity
In
recent years, reports have been made regarding the hepatotoxicity of
products presumably containing skullcap, including some that have
resulted in fatalities (Buajordet and Bodd, 1992, Harvey and Colin-Jones, 1981, Hullar et al., 1999, MacGregor et al., 1989, Moum et al., 1992 and Weeks and Proper, 1989).
In none of these cases could causality between skullcap intake and the
adverse effect be established. The hepatotoxicity appears to be clearly
linked with adulteration of skullcap with the known hepatotoxin
germander, which frequently occurs.
A
report of hepatotoxicity of “skullcap” by Harvey and Colin-Jones
involved a product reportedly containing skullcap and mistletoe, which
led to inflammatory infiltration of the portal tracts of the liver in a
female subject. The hepatotoxicity was initially attributed to mistletoe
(species not disclosed), based on the belief by the authors that
mistletoe contains hepatotoxic compounds. The same was the case for a
Norwegian case report (Buajordet and Bodd, 1992),
where the authors initially considered mistletoe (species not
disclosed) as the cause for the development of hepatitis by a
49-year-old female. They later reconsidered their comments and included
skullcap as a potential culprit, after more data on the safety of
mistletoe had been published (Bruseth and Enge, 1992 and Buajordet and Bodd, 1992). A second case report from Norway (Moum et al., 1992)
involved a patient, who had used a alleged skullcap product (a total of
200 tablets) for 1 month in addition to a tricyclic antidepressant and
birth control pills, which she had taken for 1 year prior to the
“skullcap” product. Though she suffered from depression, her physical
condition was good. However, liver biopsy revealed fibrosis and iron
precipitation. An analysis of immunological parameters revealed the
possibility of an allergic or immunologic mechanism involved in the
liver damage. Her use of the preparation was discontinued, and the use
of the birth control pills and the antidepressant continued. A control
after 3 months showed a reduction of fibrosis but no change in the liver
iron-level. After 6 months all clinical tests were normal. The authors
concluded that the use of the herbal medicine was the cause of the
hepatitis.
MacGregor et al. (1989)
described four cases of jaundice in women after the use of either Kalms
or Neurelax tablets, both of which are herbal combination products. In
each case, liver function returned to normal after the patients stopped
taking the tablets. It is not clear if the Kalms tablets actually
contained skullcap, as the formulation underwent changes and no
investigation of whether it contained germander were conducted. Another
case report was described in Australia (Weeks and Proper, 1989),
where a female presented with jaundice and hepatomegaly. After a liver
biopsy, the woman was diagnosed with a chronic active hepatitis. The
woman had reportedly taken several herbal medicines (including valerian,
passion flower, mistletoe, and skullcap) along with conventional
medicines (verapamil, chlordiazepoxide, and thyroxine). The case report
by Hullar et al. (1999)
involved a man with jaundice, who had a mild multiple sclerosis, for
which he treated with zinc, skullcap (6 tablets per day for 6 months),
and pau d’arco (Tabebuia impetiginosa, no dosage specified).
Examination of cultured liver cells revealed evidence of veno-occlusive
disease, suggestive of pyrrolizidine alkaloid (PA) exposure. Again, the
authors point out the possibility of an alkaloid-containing adulteration
of one of the herbal products, which may have caused the liver failure.
Analyses of skullcap have not revealed the presence of PAs.
8.1.2. Potential hepatotoxicity of neoclerodane diterpenes
Hepatotoxicity of the adulterating species Teucrium
is due to neoclerodane diterpenes, most specifically, teucrin A.
Specifically, skullcap does not contain teucrin A. It has been reported
that the hepatotoxicity of Teucrium diterpenes is associated with the furan ring moieties contained therein, and their oxidative capabilities ( Kouzi et al., 1994).
The potential of skullcap hepatotoxicity has been suggested by the fact
that skullcap contains low levels of neoclerodane diterpenes. However,
the structures of the diterpenes between the species are different.
Unlike the neoclerodane diterpenes isolated from Teucrium, in
which the hydroxy groups are esterified with acetic acid, the
neoclerodanes from skullcap possess a large range of acyl groups, the
most common being acetyl, 2-methylbutanoyl, tigloyl, and benzoyl ( Bruno et al., 2002).
In high concentrations, skullcap diterpenes have been shown to cause
apoptosis (programmed cell death) to rat hepatocytes in vitro
(100 μg/mL) ( Haouzi et al., 2000).
However, the clinical relevance of this is doubtful as the
concentrations used are magnitudes higher (200–2000 times, depending on
diterpene concentration in skullcap) than what would normally be
consumed in oral administrations of whole plant or crude skullcap
extract. Additionally, the compounds were solubilized with dimethyl
sulfoxide (DMSO) and other agents, which have a profound effect on the
uptake of these compounds.
In
contrast to concerns regarding potential hepatotoxicity of skullcap
consumption, there is increasing evidence of a hepatoprotective effect
of certain skullcap flavones. Baicalin and baicalein have shown
protective effects towards liver toxicity induced by tert-butyl hydroperoxide, acetaminophen, benzo[a]pyrene and aflatoxin B(1) in mice or rats ( Hwang et al., 2005a, Hwang et al., 2005b, Jang et al., 2003 and Ueng et al., 2001). In this regard, it would have been of value to determine if the hepatotoxic effects observed by Haouzi et al. (2000), would have been attenuated by administering a complete S. lateriflora
extract in contrast to administration of the purified diterpene
fraction. It would also be of interest to determine if any liver
toxicity occurred after in vivo administration of the extract.
Finally, a report that the hepatotoxicity of skullcap is potentially due to pyrrolizidine alkaloids (Ernst and Pittler, 2002),
which are known liver toxins, is devoid of any scientific basis. No
pyrrolizidine alkaloids have been reported in skullcap to date.
8.2. Interactions
No relevant interactions have been reported between skullcap and conventional medications. However, because S. lateriflora
may promote sleepiness, it theoretically may intensify the effects of
other sedative or hypnotic drugs (e.g. benzodiazepines, barbiturates,
alcohol, tricyclic antidepressive drugs, or certain antihistamines). A
study in mice ( Xu et al., 2006) showed that co-administration of baicalin (3.75 mg/kg) with the anxiolytic drugs dl-tetrahydropalmatine
(0.25 mg/kg) or diazepam (0.5 mg/kg) induced an additive effect,
resulting in increased anxiolysis. The clinical significance of this
report is somewhat questionable. Never-the-less, caution is warranted
regarding the co-administration of skullcap preparations with other
sedatives.
There is one report for a potential herb-drug interaction between S. lateriflora herb and the cytochrome P450 family of drug metabolizing enzymes. Awad et al. (2003)
reported a strong inhibition of CYP3A4 (>84%) by enormous
concentrations of commercial skullcap tinctures, glycerin extracts, and
crude aqueous and 95% ethanol extracts in vitro using a final dilution
of 1:40 to perform the assay. Interestingly, a hot water extract of
skullcap, tested in the same laboratory, did inhibit CYP3A4 and CYP19
(aromatase) by less than 20% at 50 μg/mL (McCollom et al., 2003,
unpublished results). The difference in the results is most likely due
to a higher concentration of actives tested by Awad et al., 2003.
Due to the high concentrations of isolated compounds administered and
uncertainty regarding the bioavailability of skullcap compounds, no
extrapolation between these in vitro findings and clinical relevance can
be made.
Several authors
have investigated the potential of skullcap isolates for interaction
with liver enzymes. A group of researchers from Taiwan (Tsai et al., 2002)
found that the inhibition of the multidrug transporter p-glycoprotein
by cyclosporine led to an increase of unbound baicalein in the brain of
Sprague-Dawley rats. In this study, baicalein (10, 30, and 60 mg/kg) was
administered through the femoral vein. The authors found that
concomitant administration of baicalein and cyclosporine did not result
in alterations of baicalein concentrations in blood, but to a reduction
of baicalein in the bile, another place where p-glycoprotein is
expressed. These results suggest that baicalein is a substrate of
p-glycoprotein and that baicalein may compete with medications, which
are transported by the same enzyme. However, this would seem unlikely
since plasma concentrations of baicalein are small. In a later
communication, the same group (Tsai and Tsai, 2004)
was looking at the metabolism of baicalin. Both cyclosporine and
quinidine, which are known inhibitors of p-glycoprotein, promoted the
transport of baicalin into the bile, but this was also the case after
co-administration of SKF-525A, a cytochrome P450 (CYP450) inhibitor.
Based on the fact that cyclosporine and quinidine are inhibitors of
CYP450 as well, the association of p-glycoprotein in the active efflux
of baicalin into the bile was excluded, suggesting that baicalin was
rather metabolized in rats by enzymes of the CYP450 family.
In a separate study, Lai et al. (2004),
found a decrease in cyclosporine bioavailability in Sprague-Dawley rats
after co-administration of a decoction of Chinese skullcap (Scutellaria baicalensis)
roots (corresponding to 90.2 mg/kg baicalin, 13.2 mg/kg baicalein, and
7.2 mg/kg wogonin), which they attributed to an activation of intestinal
CYP3A4. However, the co-administration with baicalein (30 mg/kg) and
especially baicalin (50 mg/kg) markedly increased the plasma levels of
cyclosporine. The authors were unable to explain the discrepancy in
their findings, but speculated that other more important constituents in
the S. baicalensis extract might overpower the effects of
baicalein and baicalin and thus reduce cyclosporine bioavailability.
These data suggest that both baicalein and baicalin elicit an inhibitory
effect on p-glycoprotein, which would result in an increase in
cyclosporine, which is transported by this enzyme ( Fricker et al., 1996).
In addition, baicalein has been reported to inhibit CYP3A4 (IC50: 17.4 μM), CYP1A2 (IC50: 12.2 μM), and CYP2C9 (IC50: 27.1 μM) in human liver microsomes in vitro (Kim et al., 2002). Wogonin strongly inhibited CYP1A2 (IC50: 0.74 μM). Baicalein and wogonin decreased the levels of hepatic CYP2E1 and CYP3A proteins (Ueng et al., 2000).
Treatment of mice with baicalin (300 mg/kg p.o., 30 min after
acetaminophen) decreased acetaminophen-induced liver toxicity by
inhibiting CYP2E1 enzyme activity (Jang et al., 2003).
While baicalein increased lung CYP1A and hepatic CYP1A2 protein levels,
wogonin decreased the CYP1A2 protein levels in the same assay. Finally,
both baicalein and wogonin suppressed renal UDP-glucuronosyl
transferase (UGT) activities in mice (Ueng et al., 2000). Oroxylin A inhibited CYP2C9 activity (IC50: 6.1 μM) in human liver microsomes (Kim et al., 2002).
Again, caution is warranted in relating in vitro data to human use
especially when high concentrations of pure compounds are used.
There
is some in vitro evidence showing that huge concentrations of certain
skullcap flavonoids (0.5 and 1.0 mM) have antithrombotic activity that
is comparable to the same concentration of aspirin (Kubo et al., 1985).
Baicalein, wogonin, and oroxylin A inhibit collagen-induced platelet
aggregation, while baicalein and wogonin also inhibit aggregation
induced by arachidonic acid. In addition, baicalein and baicalin inhibit
the thrombin-induced conversion of fibrinogen to fibrin. Both of these
compounds at 50 mg/kg (but not at 20 mg/kg) significantly reduced the
decrease of fibrinogen in rats following endotoxin-induced disseminated
intravascular coagulation, but not as much as 50 mg/kg aspirin. If
sufficient doses of skullcap extract to produce these blood
concentrations of these specific flavonoids in humans could be ingested,
there would be a risk from regular use of such doses in combination
with anticoagulants like warfarin or antiplatelet drugs like aspirin.
Further, the inhibition of NADPH quinone reductase (a vitamin K
reductase) by baicalein and oroxylin A and their glucuronides (e.g.
baicalin) in nanomolar concentrations indicated that these flavones can
theoretically act as anticoagulants (Chen et al., 1993 and Liu et al., 1990). However, according to an unpublished report, Merkel and Wnorowski (2002)
reported no such findings in a single-dose and 5-day repeat-dose
toxicity study using high doses of a total skullcap extract administered
to rats.
Based on the
available data and the extremely high concentrations of pure compounds
needed to elicit these reported interactions, it is highly unlikely that
there is a risk of such interactions from oral consumption of standard
skullcap doses.
8.3. Reproductive and developmental effects
There
is no published data on the use of skullcap in pregnancy. The skullcap
monograph of Health Canada cautions against the use of skullcap in
pregnancy and gives no explanation as to why. In a survey of
professional American medical herbalists, several respondents
specifically reported on the extensive use of skullcap in pregnancy with
no side effects noted. There does not seem to be any contraindications
of skullcap in pregnancy in the historical literature.
8.4. Carcinogenicity
No data available.
8.5. Toxicology
No
data on toxicity studies of skullcap have been published, although a
private manufacturer sponsored a single-dose acute and 5-day repeat-dose
acute toxicity of a freeze-dried alcohol extract of S. lateriflora
in rats (unpublished). In the acute oral toxicity study, 10 rats were
treated with one dose of 5000 mg/kg extract p.o. There were no signs of
toxicity or adverse pharmacologic effects after 14 days in any of the
animals ( Merkel and Wnorowski, 2002).
As noted previously, there are in vitro and animal studies regarding
the potential for anticoagulation activity with high concentrations of
isolated skullcap constituents. This finding should have been evident in
this model at these concentrations, suggesting that concerns raised by
these previous reports may not be clinically relevant in terms of
consumption of a total extract.
In
the 5-day repeat-acute toxicity test, five male and five female rats
were treated with 1000 mg/kg skullcap extract for five consecutive days.
The animals were observed for mortality, signs of gross toxicity, and
behavioral changes at least once daily. After 8 days, the rats were
euthanized and tissues and organs were examined. During the test period,
all animals survived, gained weight, and appeared healthy and active.
Histopathological evaluation of the animals in the 5-day repeat dose
study revealed a small number of minimal changes in the liver, which
were considered incidental by the study investigators (Merkel and Wnorowski, 2002).
8.6. Contraindications
None noted.
8.7. Precautions
It
is important that consumers and health professionals be acutely aware
of the source of the skullcap purchased to avoid potential adulterants
with germander. In its monograph, Health Canada suggests that the
duration of use should not exceed 10 days though no explanation for this
restriction was provided or accessible.
8.8. Lactation
There
is no published data available regarding the use of skullcap during
lactation. In a survey of professional American medical herbalists,
several respondents specifically reported on the extensive use of
skullcap in lactating women with no side effects noted. There does not
seem to be any contraindications of skullcap in lactation in the
historical literature.
8.9. Influence on driving
When used within its normal dosage range, no impairment of driving is to be expected.
8.10. Overdose
Grieve in her Modern Herbal
wrote that tincture of skullcap can cause giddiness, stupor, confusion
of the mind, twitching of the limbs, intermission of the pulse, and
other symptoms of epilepsy, for which in diluted strength and small
doses it has been successfully given. These cautions are reminiscent of
homeopathic cautions with material herb doses and it is difficult to
know how much of Grieve's opinion may have been influenced by
homeopathic practices of the day. Additionally, the doses used for the
treatment of hydrophobia were substantial, unlike the “small doses”
reported by Grieve.
8.11. Treatment of overdose
No data available.
8.12. Classification of the American Herbal Products Association
Class 1: Herbs that can be used safely when used appropriately (McGuffin et al., 1997).
8.13. Conclusion
There is very little scientific data available on the safety of Scutellaria lateriflora.
Most historical literature and textbooks written by modern herbal
practitioners consider skullcap to be a very safe botanical. There are a
few case reports, which link preparations allegedly containing skullcap
to liver toxicity, for which the neo-clerodane diterpenes have been
considered to be potentially responsible ( Haouzi et al., 2000).
In none of these cases, however, could the causality between the liver
damage and skullcap intake be established. Neither were the products in
question analyzed to confirm the presence of S. lateriflora.
This is particularly important due to the long history of adulteration
of skullcap with germander, a known potential hepatotoxin.
Based
on in vitro and animal studies of isolated flavonoids, there is a
theoretical potential for interaction between skullcap extracts and
conventional medications due to modulation of the CYP450 enzyme family
and p-glycoprotein. In vitro and animal studies do not necessarily
correlate to human results, nor can findings with the high
concentrations of isolates typically used in these studies be correlated
with oral consumption of crude preparations at normal dosages.
The
American Herbal Pharmacopoeia is a California-based non profit research
organization. For more information about AHP and its monographs, visit
www.herbal-ahp.org or contact Roy Upton at herbal@got.net.
9. Original reference
Upton R, editor. Scutellaria lateriflora:
American Herbal Pharmacopoeia. Winston D, Graff A, Mermell L, Laenger
R, Joshi V, Upton R, Bergeron C, Reich E, Schaneberg B, Gafner S, Harnly
J, Lin LZ, Brinckmann. Scotts Valley, CA; 2009.
Comment
from the Author and the Editor-in-Chief: “The goal of the American
Herbal Pharmacopoeia is to present a comprehensive review of the
totality of the evidence that is available on any given botanical. This
is primarily to establish both the safety and efficacy of the botanical
in light of antagonistic regulatory and social pressures against herbal
medicines. This is especially important due to the prevalence of
adulteration of skullcap with the potentially hepatotoxic germander.
Whilst neither AHP nor JHM support or endorse the use of animal studies
it is hoped that by reporting on all evidence on studies that have
already been done will prevent the same experiments being repeated”.