Screening of Herbal Drugs
Once the botanical identity of a herb is established, the next step is phytochemical
screening, which involves bioassays, extraction, purification, and characterization
of the active constituents of pharmaceutical importance . The herb
or herbal drug preparation in its entirety is regarded as the active substance. These
constituents are either of known therapeutic activity or are chemically defined sub-
stances or a group of substances generally accepted to contribute substantially to
the therapeutic activity of a herbal drug. In any program in which the end product
is to be a drug, some type of pharmacological screening, or evaluation, must obvi-
ously be done.
Pharmacological screening programs are not without problems. Ideally the active
principles should be isolated, preferably using bioassay guided isolation processes,
which can be problematic. The ideal pharmacological screen would be to identify
those extracts or pure compounds that are highly active and nontoxic. Such a screen
is rare to find. Failure to duplicate pharmacological results is another problem.
There are many pharmacological screening tests available [87]. In the random se-
lection program of the National Cancer Institute (NCI) in the US, plants are ran-
domly selected, extracted, and the extracts are evaluated against one or more in vitro
tumor systems and in vitro cytotoxicity tests. An extension of this procedure is
to isolate metabolites or “active compounds” from the plant that had shown most
promising activity and subject them to pharmacological tests. In another approach,
plants containing specific types or classes of chemical compounds, for example al-
kaloids, are tested. Simple tests such as color reactions are carried out on various
parts of the plant in the field, and assays are carried out in the laboratories [87]. In
terms of cost–benefit ratio, these “shotgun” approaches are considered to be very
unsatisfactory.
Another method involves random collection of plants and subjection of their ex-
tracts to several broad screening methods and pharmacological tests. The success
of this method depends on the number of samples assayed, adequate funding, and
appropriate predictable bioassay protocols. Broad-based empirical screening,
which is time consuming and expensive, can detect novel activities but is not suit-
ed for screening large numbers of samples [29, 81, 82, 122, 123].
Diagnosis by observation, a method introduced by the “father” of medicine, Hip-
pocrates, is still one of the most powerful tools of today’s physicians. In vitro
screening methods, though restricted to the detection of defined activities, are sim-
pler and more useful [124]. Recently, biochemical and receptor–ligand binding as-
says have gathered momentum. This has been made possible by the increasing
availability of human receptors from molecular cloning, and extracts and com-
pounds can be tested for binding directly to the presumed therapeutic target pro-
2.8 Screening of Herbal Drugs 45
tein. Clone receptors can be expressed in a functional state linked to receptor pro-
teins in cells such as yeast, and this has been made possible by applications of mo-
lecular biology. Combined with automated instrumentation and computer data-
bases, hundreds of such assays can be completed in relatively short periods of time
[83, 88, 125–129]. These screening processes are successfully used by internation-
al agencies such as the National Cancer Institute (NCI) in the United States and
the Central Drug Research Institute in India [29, 124, 130].
The technology of plant medicinal screening processes has even advanced to en-
zyme isolation. The enzymes that cause the disease are first isolated and the plant
extracts are tested to determine if they block enzyme action [131]. An enzyme im-
munoassay for the quantification of femtomole quantities of therapeutically im-
portant alkaloids has been established [132]. Ethanolic extracts, tinctures, and pure
plant compounds from commercially available herbs have been analyzed for their
in vitro cytochrome P450 3A4 (CYP3A4) inhibitory capability via a fluorometric mi-
crotiter plate assay. These studies indicate that high-throughput screening meth-
ods for assessing CYP3A4 inhibition by natural products have important implica-
tions for predicting the likelihood of potential herb–drug interactions [133].
Higher plants contain both mutagens and antimutagens and are susceptible to
mutagenesis, but screening programs for the detection of antimutagenesis rarely
employ higher plant systems. However, using modified screening tests to detect
antimutagenic agents, higher plants have been shown to contain a variety of struc-
turally novel antimutagenic agents [134–136]. Short-term bacterial and mammal-
ian tissue culture systems are the standard methods employed.
Once the botanical identity of a herb is established, the next step is phytochemical
screening, which involves bioassays, extraction, purification, and characterization
of the active constituents of pharmaceutical importance . The herb
or herbal drug preparation in its entirety is regarded as the active substance. These
constituents are either of known therapeutic activity or are chemically defined sub-
stances or a group of substances generally accepted to contribute substantially to
the therapeutic activity of a herbal drug. In any program in which the end product
is to be a drug, some type of pharmacological screening, or evaluation, must obvi-
ously be done.
Pharmacological screening programs are not without problems. Ideally the active
principles should be isolated, preferably using bioassay guided isolation processes,
which can be problematic. The ideal pharmacological screen would be to identify
those extracts or pure compounds that are highly active and nontoxic. Such a screen
is rare to find. Failure to duplicate pharmacological results is another problem.
There are many pharmacological screening tests available [87]. In the random se-
lection program of the National Cancer Institute (NCI) in the US, plants are ran-
domly selected, extracted, and the extracts are evaluated against one or more in vitro
tumor systems and in vitro cytotoxicity tests. An extension of this procedure is
to isolate metabolites or “active compounds” from the plant that had shown most
promising activity and subject them to pharmacological tests. In another approach,
plants containing specific types or classes of chemical compounds, for example al-
kaloids, are tested. Simple tests such as color reactions are carried out on various
parts of the plant in the field, and assays are carried out in the laboratories [87]. In
terms of cost–benefit ratio, these “shotgun” approaches are considered to be very
unsatisfactory.
Another method involves random collection of plants and subjection of their ex-
tracts to several broad screening methods and pharmacological tests. The success
of this method depends on the number of samples assayed, adequate funding, and
appropriate predictable bioassay protocols. Broad-based empirical screening,
which is time consuming and expensive, can detect novel activities but is not suit-
ed for screening large numbers of samples [29, 81, 82, 122, 123].
Diagnosis by observation, a method introduced by the “father” of medicine, Hip-
pocrates, is still one of the most powerful tools of today’s physicians. In vitro
screening methods, though restricted to the detection of defined activities, are sim-
pler and more useful [124]. Recently, biochemical and receptor–ligand binding as-
says have gathered momentum. This has been made possible by the increasing
availability of human receptors from molecular cloning, and extracts and com-
pounds can be tested for binding directly to the presumed therapeutic target pro-
2.8 Screening of Herbal Drugs 45
tein. Clone receptors can be expressed in a functional state linked to receptor pro-
teins in cells such as yeast, and this has been made possible by applications of mo-
lecular biology. Combined with automated instrumentation and computer data-
bases, hundreds of such assays can be completed in relatively short periods of time
[83, 88, 125–129]. These screening processes are successfully used by internation-
al agencies such as the National Cancer Institute (NCI) in the United States and
the Central Drug Research Institute in India [29, 124, 130].
The technology of plant medicinal screening processes has even advanced to en-
zyme isolation. The enzymes that cause the disease are first isolated and the plant
extracts are tested to determine if they block enzyme action [131]. An enzyme im-
munoassay for the quantification of femtomole quantities of therapeutically im-
portant alkaloids has been established [132]. Ethanolic extracts, tinctures, and pure
plant compounds from commercially available herbs have been analyzed for their
in vitro cytochrome P450 3A4 (CYP3A4) inhibitory capability via a fluorometric mi-
crotiter plate assay. These studies indicate that high-throughput screening meth-
ods for assessing CYP3A4 inhibition by natural products have important implica-
tions for predicting the likelihood of potential herb–drug interactions [133].
Higher plants contain both mutagens and antimutagens and are susceptible to
mutagenesis, but screening programs for the detection of antimutagenesis rarely
employ higher plant systems. However, using modified screening tests to detect
antimutagenic agents, higher plants have been shown to contain a variety of struc-
turally novel antimutagenic agents [134–136]. Short-term bacterial and mammal-
ian tissue culture systems are the standard methods employed.
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