Natural product drug discovery limitations when compared to
synthetic chemical drug discovery:
1- Building up and maintaining a high-quality natural product library
requires a skill set that is not generally available in industry.
2- Natural products are often synthesized in small quantities and
present as mixtures in extracts, which require labor-intensive and
time consuming purification procedures.
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3- Rediscovery of known compounds is a major problem when screening natural
product libraries. This is caused by a lack of efficient dereplication methodologies
for both natural product sourcing and compounds in the natural product libraries.
4-The time-consuming processes of dereplication and purification are not compatible
with the present regime of screening campaigns in which assay support is only
available for a limited duration (three months).
5- Natural products are often structurally complex. Modification of complex natural
products using organic chemistry is frequently challenging.
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Advances in natural product research to address these
limitations.
* Recent technological advances and the development of
new methods have revolutionized the screening of natural
products and offer a unique opportunity to re-establish natural
products as major source of drug leads.
* The new methods and technologies can address the
aforementioned limitations of screening of natural products.
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Examples of recent advances in the application of these
technologies that have immediate impact on the discovery of
novel drugs are:
•Development of a streamlined screening process for natural
products.
•Improved natural product sourcing; and advances, Organic
synthetic methodologies; combinatorial biosynthesis; microbial
genomics.
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Pharmacodynamics:
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The study of the effect of a drug and its mechanism of
action.
In cerebro:
Defined as: using ones own mind (knowledge) to think
through a problem, interpret data, or make conclusions.
In silico:
The use of software to analyze data or make prediction
(estimations) through some form of logic or knowledge
(learning rules or database).
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•Biological screening methods.
Using biological assays to identify the active samples.
Used Assays must be:
* Rapid
* Simple
* Consume small amount of samples
* Not very expensive
* Specific
•All steps are done using the biological assay starting from
fractionation till the isolation of pure active compounds.
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Biological Screening Methods in the Search for
Pharmacologically Active Natural Products
Natural products have served as an important source of drugs since ancient
times and about half of the useful drugs today are derived from natural sources.
Chemodiversity in nature, e.g. in plants, microorganisms and marine
organisms, still offers a valuable source for novel lead discovery, but rapid
identification of the bioactive compounds of natural product mixtures remains a
critical factor to ensure that this tool of drug discovery can compete with recent
developed technologies such as chemical compound libraries and high-throughput
screening of combinatorial synthetic efforts.
Corrado T. (2004). Bioactive Compounds from Natural Sources, isolation, characterization
and biological properties.2nd ed. Taylor & Francis.
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* Rapid screening of natural product mixtures requires the availability
of a library of reference of natural compounds and methods for simple
identification of putative lead structural classes avoiding to a large extent,
the potential for false-positive results.
* The coupling of chromatographic methods such as high pressure
liquid chromatography (HPLC), mass spectrometry (MS) or nuclear
magnetic resonance spectroscopy (NMR) or, and with, on-line bioactivity
assays, is an important tool for high throughput screening of natural product
mixtures.
The effective use of automated procedures and databases in the
isolation, identification and biological profiling of bioactive compounds
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from natural sources will be the best guarantee to the continued discovery
of novel chemotypes from nature.
ROLES OF BIOASSAYS
Four major roles of bioassays can be distinguished, i.e. prescreens,
screens, monitors and secondary testing.
In a prescreen a bioassay is applied to large numbers of initial samples to
determine whether or not they have any bioactivity of the desired type. Such
bioassays must have high capacity, low cost and must give rapid answers.
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• A bioassay in a screen is used to select materials for secondary testing.
• In a monitor, a bioassay is used to guide fractionation of a crude material towards
isolation of the pure bioactive substances. It must, therefore, be fast and cheap,
have high capacity, and be readily available to the phytochemist.
• In the secondary testing, lead compounds are evaluated in multiple models and
test conditions to select candidates for development towards clinical trials.
Secondary testing is consequently characterized by a low capacity and expensive
and slow bioassays.
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CLASSIFICATION OF BIOASSAYS
The methods for the detection of biological activity of natural
product mixtures can best be divided into two groups for screening
purposes: General screening bioassays and Specialized screening
bioassays.
Depending on the aims of the screening program, either a
general screening which can pick up many different effects, or a
specific assay which is directed at finding some effect against a
specific disease.
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A broad screening bioassay is probably most useful if one is randomly
screening chosen organisms for any kind of pharmacological activity.
The alternative to using broad screening would be the setting up of a battery
of specific test methods, which is cumbersome and expensive.
Another drawback of using a general test for screening and monitoring is
that one does not know, until the active compound has been isolated, if the work was
worth doing.
Since, in most phytochemical laboratories engaged in the bioassay-guided
isolation of actives from natural product mixtures no complex bioassays can be used,
efforts have been made to introduce single, inexpensive ‘front line’ or ‘bench-top’
bioassays for the rapid screening of extracts and fractions.
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Specialized screening bioassays can be subdivided
according to the target organisms which are used in the model.
These can be * lower organisms, e.g. microorganisms, insects,
molluscs, protozoa, helminths, * isolated subcellular systems, e.g.
enzymes, receptors, organelles, * isolated intact cells of human or
animal origin, * isolated organs or vertebrates, * or whole
animals.
In any case, specialized bioassays have to be relevant
which means they should predict the intended therapeutic
indications.
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Firstly, the bioassay must be sensitive in a dose dependent
fashion to standard compounds that are known to possess the desired
therapeutic property.
Secondly, the relative potency of known active agents in the
bioassay should be comparable to their relative potency in clinical
use.
Thirdly, the bioassay should be selective.
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General screening bioassays:
Broad screening bioassays:
* Broad assaying procedures can be performed by
1- Hippocratic screening, Hippocratic method is performed on intact rats and
involves observation of about 30 parameters at 3- to 5-dose levels.
* It is a time-consuming and expensive method, which requires much
experience in observation and comparatively large amounts of test substances, but the
screening of a wide variety of biological activities is possible.
* The large amount of extract and the time factor makes the method
unsuitable for monitoring the isolation of an active component from plant extracts.
* Substituting mice for rats and observation of only the most pronounced
effect in a limited number of animals could potentially overcome the drawback of
using the Hippocratic screening for monitoring purposes.
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2- Isolated guinea-pig ileum, The isolated organ method, using, e.g. guinea pig
ileum for the observation of contraction or inhibition of contraction, has been used
as a non-specific screen for pharmacological effects of crude plant extracts by
different research groups. Two tests are performed for each extract.
* First, the extract’s ability to contract the ileum is tested.
* In a second test, the ileum is induced to contract by electrical stimulation and the
extract is tested for ability to inhibit these contractions . As the ileum is a piece of
smooth muscle, innervated by many different nerve systems, all of which can be
electrically stimulated at the same time, a positive response can indicate many
different mechanisms for the pharmacological activity of the extract.
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