Drug Resistance Mutation L76V Alters Nonpolar Interactions at the Flap–Core Interface of HIV-1 Protease

[Image: see text] Four HIV-1 protease (PR) inhibitors, clinical inhibitors lopinavir and tipranavir, and two investigational compounds 4 and 5, were studied for their effect on the structure and activity of PR with drug-resistant mutation L76V (PR(L76V)). Compound 5 exhibited the best K(i) value of 1.9 nM for PR(L76V), whereas the other three inhibitors had K(i) values of 4.5–7.6 nM, 2–3 orders of magnitude worse than for wild-type enzymes. Crystal structures showed only minor differences in interactions of inhibitors with PR(L76V) compared to wild-type complexes. The shorter side chain of Val76 in the mutant lost hydrophobic interactions with Lys45 and Ile47 in the flap, and with Asp30 and Thr74 in the protein core, consistent with decreased stability. Inhibitors forming additional polar interactions with the flaps or dimer interface of PR(L76V) were unable to compensate for the decrease in internal hydrophobic contacts. These structures provide insights for inhibitor design.