A unique strategy for the enhancement of secondary binding of an inhibitor to an enzyme has been demonstrated in the design of new human immunodeficiency virus (HIV) protease inhibitors. When the planar benzene ring of a 4-hydroxycoumarin lead compound (1a, Ki = 0.800 microM) was replaced with medium-sized (i.e., 7-9), conformationally-flexible, alkyl rings, the enzyme inhibitory activity of the resulting compounds was dramatically improved, and inhibitors with more than 50-fold better binding (e.g., 5d, Ki = 0.015 microM) were obtained. X-ray crystal structures of these inhibitors complexed with HIV protease indicated the cycloalkyl rings were able to fold into the S1' pocket of the enzyme and fill it much more effectively than the rigid benzene ring of the 4-hydroxycoumarin compound. This work has resulted in the identification of a promising lead structure for the design of potent, deliverable HIV protease inhibitors. Compound 5d, a small (MW = 324), nonpeptidic structure, has already shown several advantages over peptidic inhibitors, including high oral bioavailability (91-99%), a relatively long half-life (4.9 h), and ease of synthesis (three steps).