Skin of frog, venom of spider and saliva of leech.
Ingredients for the witch's brew at the beginning of "Macbeth"? No, they are potential sources of drugs.
The pharmaceutical industry is going back to nature, scouring the oceans for algae, the soil for fungi and the jungles for plants, insects and beasts that might provide chemicals to fight cancer, acquired immune deficiency syndrome and other diseases.
Such chemical prospecting could also provide an economic incentive for preserving rain forests and endangered species, conservationists and drug company officials say.
The resurgence of interest in such natural sources is surprising because biotechnology was supposed to have ushered in the age of "rational drug design," in which drugs would be formulated using scientific principles, doing away with the need to rely on Mother Nature.
Instead, biotechnology has produced techniques that let companies screen tens of thousands of substances a year in an attempt to find just one that will fight disease.
"The testing procedures have become like factories," said Dr. John H. Fried, vice chairman of Syntex Corp. and president of its research division. "You can run a lot of products through."
Syntex signed an agreement last year under which sci
ence academies in China will supply it with up to 10,000 plant extracts a year for testing. SmithKline Beecham has hired Biotics Ltd., a young British company, to supply it with hundreds of plant extracts each year from the jungles of Ghana and Costa Rica.
The National Cancer Institute, which dropped its natural-drug screening program in the early 1980s, resurrected it a few years ago. It collects nearly 4,000 plant samples each year from around the world, plus thousands more samples of marine organisms and microorganisms.
Biotechnology companies are also working with natural drug sources.
Natural Product Sciences Inc. of Salt Lake City and Cambridge Neuroscience Inc. of Cambridge, Mass., hope that spider venom, which can paralyze the spider's prey, will yield drugs for human brain and nervous system disorders.