604 Resistant-Repellant HIV-1 Protease Inhibitors: How They Work J. Erickson*, S. Gulnik, A. Silva, M. Eissenstat Sequoia Pharm, Inc, Gaithersburg, MD
Background: Multi-drug resistant strains of HIV-1 (mdrHIV) pose a clear and present danger to the public health and require novel approaches to antiretroviral therapy. Drug resistance to HIV-1 protease inhibitors (PIs) has been attributed to mutations in more than half of the amino acids in the enzyme. The design of drug candidates to combat mdrHIV, on a strain-by-strain basis, poses a combinatorial problem of seemingly staggering proportions. We have developed a new approach.
Methods: X-ray crystallography and molecular modeling were used to compare and contrast the effects of drug resistance-conferring mutations on the binding of 1) a resistant-repellant PI, UIC-94003 (TMC-126); 2) a structurally-related but resistant-susceptible analog, amprenavir; and 3) a structurally-unrelated, resistant-susceptible PI, ritonavir. Mutant enzymes containing the I84V and V82F mutations were chosen because they exhibit strong biochemical cross-resistance to many first-line PIs, including amprenavir and ritonavir, but remain susceptible to UIC-94003. Crystal structures of UIC94003 and ritonavir complexed with wild type and drug resistant enzymes were solved and refined to below 2.0 Å resolution.
Results: Comparative analysis indicates that ritonavir makes structural adjustments in binding to mutant enzymes, whereas UIC-94003 binds in a nearly identical manner to both wild type and mutant enzymes. The latter makes a unique set of hydrogen bonding interactions with the wild type enzyme that was strictly conserved in the complex with the double mutant. None of the drug resistant-susceptible PIs, including amprenavir, could achieve this hydrogen bonding contact profile, either in the wild-type or mutant enzymes.
Conclusions: The mode of binding of the resistant-repellant PI is rigidly conserved in both the wild-type and mutant enzymes. Thus, structural flexibility is not necessarily a key factor in the ability of a PI to maintain potent biological activity in the face of mutation-induced changes in the active site structure. Our analysis has led to the identification of a specific constellation of interactions that is a necessary condition for endowing PIs with the property of being resistant-repellant. We have incorporated these findings into a predictive algorithm for the efficient design of resistant-repellant PIs and believe that our algorithm is applicable to other drug targets.