Furthermore, newly invented pharmacologically active moieties may have poor druglikeness and may require chemical modification to become drug-like enough to be tested biologically or clinically.
[1] This is because the discovery and selection of lead compounds occurs prior to preclinical and clinical development of the candidate.
Before lead compounds can be discovered, a suitable target for rational drug design must be selected on the basis of biological plausibility or identified through screening potential lead compounds against multiple targets.
[1] Drug libraries are often tested by high-throughput screenings (active compounds are designated as "hits") which can screen compounds for their ability to inhibit (antagonist) or stimulate (agonist) a receptor of interest as well as determine their selectivity for them.
Lead compounds are found by characterizing natural products, employing combinatorial chemistry, or by molecular modeling as in rational drug design.