Illustrated Glossary of Organic Chemistry

Lead compound: In drug discovery, a compound that shows some interesting biological activity. The molecular structure of the lead compound is then varied -- perhaps through a chemical library -- to find compounds that have better activity and/or less side effects.

Molecule A
Lead compound

Molecule B

Molecule C

Molecule D

Final drug structure
Molecule A is a (hypothetical) lead compound, having some interesting activity. Molecule A was modified to give molecule B, which has better activity and selectivity than molecule A. Further changes and refinements of molecular structure (molecule C, molecule D, etc.) ultimately resulted in the final drug having the best potency, selectivity, and other desirable features.