Fig. 2.

Drug discovery: Stage-by-stage quality assessment to reduce costly late-stage attrition. (Hits and leads refer to candidate molecules/structures for drugs. Hits are compounds that show specificity and activity. Leads are hits, or types of hits, that show specificity and activity in a “biochemically relevant” screen.) Typical important milestones are: validated hist series (VHS), lead series identified (LSI), and clinical candidate selection (CCS), which ensure that only drug candidates with an appropriately high-potential profile are advanced to the next phase [19]. (Figure and caption reprinted with permission from Macmillan Publishers, Nature Reviews Drug Discovery, 2003.) As depicted, the earlier a poor lead can be discarded, the more money is saved. The testing methods and set of decisions to cull leads along the way defines this particular heuristic method of drug discovery.