When chemists envision novel products, they undoubtedly have a target in mind. A vast knowledge of chemistry allow them to examine the binding sites on the intended target molecule. Back at the bench, they combines basic ingredients, and by trial and error, a novel drug is created. Of course, the careful chemist has an end game and that is a testing system for the new compound.
The new drug has to be relatively non-toxic at an effective dose. Organic synthesis can take years of work with many dead ends. High throughput computer systems may reduce the discovery tasks and even identify potential compounds and targets. Bioreactors are used in the laboratory to grow organisms that produce a desired product. Biosynthesis is a multi-step, enzyme-catalyzed process where substrates are converted into more complex products in living organisms.
Sixty five years ago an inventive group of Belgium scientists fed a newly made drug to chickens and found it active against ascarids. The drug was also active against sheep ascarids, but it was inactive in the species infecting rats or mice. They collected the chicken feces and separated out the drug metabolites that were passed in the manure. Amazingly, the manure contained a new molecule active against another chicken parasite as well as parasites found in sheep. This new molecule was active against those ascaris species infecting in rats and mice! Further testing revealed activity against gastrointestinal and lung parasites found in cattle, pigs, horses, pigeons, pheasants, ducks, cats, tiers, monkeys and man! This experiment was great for discovery but is significant food for thought.
It is worth considering that the drugs that are given to horses can build up in pastures and water. Many drugs are eliminated through the feces. Unintended contamination of the environment is a serious concern and one important reason not to chronically treat animals with unnecessary medications. Not only are you medicating pasture mates at ineffective low doses or new compounds, you are treating the wildlife. Soil organisms, insects, small mammals, and pasture mates are all exposed. What effect do these metabolized drugs have on the environment? You can check the Freedom of Information Summary for the levels of compounds that are excreted and the types of laboratory experiments that were conducted to determine environmental safety. However, if several animals are treated over long periods of time the unintended consequences can be significant.