INHIBITOR-COMPLEXED STRUCTURES OF AN HIV-1 PROTEASE MUTANT DISPLAYING CROSS RESISTANCE TO ALL CLINICAL TRIAL CANDIDATES Youwei Yan, Ying Li, Hilary B. Schock, and Zhongguo Chen, Merck Research Laboratories, West Point, PA 19486

Crystal structures of a mutant HIV-1 protease (M46I/L63P/V82T/I84V) complexed with either indinavir (MK-639, Merck) [Chen et al., (1995) J. Biol. Chem. 269, 26344-26348], retonavir (ABT-538, Abbott), or VX-478 (Vertex). are resolved at 2.0, 2.2 , and 1.8 resolution, respectively, with R factors of 17 ~ 18%. The comparison is of interest because this mutant protease has been shown to display cross resistance to a panel of protease inhibitors that are either in or being considered for clinical trials [Condra et al., (1995) Nature 374, 569-571]. When superpositioned against the structures of the wild-type enzyme complexed with indinavir and VX-478 [Chen et al. (1994) J. Biol. Chem. 270, 21433-21436; Kim et al., (1995) J. Am. Chem. Soc. 117,1181-1182], no gross difference is found in the tertiary structure of the enzyme. The C₍ tracings of the native structures are essentially identical, and the four substituted sidechains are clearly revealed in the electron density maps. In the inhibitor-bound form, the V82T substitution introduces an unfavorable hydrophilic moiety for binding at the S1, but not the S1', site of the indinavir and VX-478 complexes (and at neither sites of the ABT-538 complex). The I84V substitution generates a cavity that should lead to a decrease in van der Waals contacts with the inhibitor in all three complexes. These changes are consistent with the observed increase in the K_i value for the inhibitors as a result of the mutations. Residurs 46 and 63 are not located in the binding site and have no interactions with these inhibitors. Thus, the role of the M46I and L63P substitutions in drug resistance is not obvious from our crystallographic data; the conformational perturbations induced by them are minor.