Extracts from natural product sources have served as a valuable source of moleculardiversity in many drug discovery programs, and several important drugs have been isolated from natural products. In any natural product isolation program in which the endproduct is to be a drug or a lead compound, some type of bioassay screening or pharmacological evaluation must necessarily be used to guide the isolation process towards the pure bioactive component. The pharmacological evaluation of extracts of organisms and pure isolates is an essential aspect of the drug discovery process and developments in the area of in vitro techniques have substantially transformed this facet of natural product chemistry. While it previously took weeks or even months to test a sample for some assays, it can now take only a few hours. One should also distinguish between the “primary bioassay screens” from “secondary screens”. Primary bioassays are assays which can be rapidly applied to a large number of samples to determine if any bioactivity of the desired type is present. They should therefore have high capacity, low cost and provide the results quickly. They need not be quantitative. “Secondary testing” procedures involve more detailed testing of lead compounds on a number of model systems in order to select compounds for clinical trials. They are usually low capacity, slow and costly assays. The main requirements which a primary bioassay screen should meet are the following:
(1) The bioassay results should predict some type of therapeutic potential, either directly or by analogy with clinically effective drugs which have also been screened by the same procedure.
(2) Potentially useful pharmacological activity should not go undetected even though the activity may be either unexpected or unique.
(3) The probable nature of the activity should be indicated so that subsequent research can be organized intelligently.
(4) The primary bioassay screening test should be tolerant of the many impurities present in a crude extract and yet it should be sensitive enough to reveal presence the potentially interesting substances present in low concentrations (levels of about 0.0001% of an active compound in an extract, based on the dried weight of the extracted organism, should be detectable).
(5) The bioassay procedure should be unbiased and it should allow for the coding of all samples, including both “known” reference materials (standards) and “unknown” test samples.
(6) The results obtained should be reproducible.
(7) The screen should allow the use of both crude materials and pure isolates so that the procedure can be used to direct the extraction, isolation and purification work of the natural product chemist.
(8) Completion of a single bioassay screen should not require more than 1.0–2.0 g of the crude dry natural material (plant or animal extract).
(9) The primary bioassay screen should have a high throughput, even if the information content is low, with the results becoming available quickly.
(10) The procedure should not require expensive equipment or a sophisticated laboratory environment so that the primary level screening experiment can be conducted synchronously with the fractionation process.
(11) The procedure should be compatible with the use of dimethyl sulfoxide (DMSO) since DMSO is commonly employed to solubilize extracts or pure polar compounds for screening.
(12) The procedure should be simple enough to be taught easily to laboratory technicians so that highly trained and qualified researchers are not required for the routine operation of the bioassay program.
(13) The test animals (if required for the bioassay) should be easily obtainable, easily handled, easily bred and resistant to infections.
(14) Finally the bioassay should be economical to conduct over extended time periods. A hit rate of 1% or less is generally considered a reasonable and one then proceeds from primary screening to secondary screening for profile selectivity and in order to establish biological activity.