Structural mechanisms of drug resistance for mutations at codons 181 and 188 in HIV-1 reverse transcriptase and the improved resilience of second generation non-nucleoside inhibitors

Mutations at either Tyr181 or Tyr188 within HIV-1 reverse transcriptase (RT ) give high level resistance to many first generation non-nucleoside inhibi tors (NNRTIs) such as the anti-AIDS drug nevirapine. By comparison second g eneration inhibitors, for instance the drug efavirenz, show much greater re silience to these mutations. In order to understand the structural basis fo r these differences we have determined a series of, seven crystal structure s of mutant RTs in complexes with first and second generation NNRTIs as wel l as one example of an unliganded mutant RT., These are Tyr181Cys RT (TNK-6 51) to 2.4 Angstrom, Tyr181Cys RT (efavirenz) to 2.6 Angstrom, Tyr181Cys RT (nevirapine) to 3.0 Angstrom, Tyr181Cys RT (PETT-2) to 3.0 Angstrom, Tyr18 8Cys RT (nevirapine) to 2.6 Angstrom, Tyr188Cys RT (UC-781) to 2.6 A and Ty r188Cys RT (unliganded) to 2.8 Angstrom resolution. In the two previously p ublished structures of HIV-1 reverse transcriptase with mutations at 181 or 188 no side-chain electron density was observed within the p66 subunit (wh ich contains the inhibitor binding pocket) for the mutated residues. In con trast the mutated side-chains can be seen in the NNRTI pocket for all seven structures reported here, eliminating the possibility that disordering con tributes to the mechanism of resistance. In the case of the second generati on compounds efavirenz with Tyr181Cys RT and UC-781 with Tyr188Cys RT there are only small rearrangements of either inhibitor within the binding site compared to wild-type RT and also for the first generation compounds TNK-65 1, PETT-2 ana nevirapine with Tyr181Cys RT. For nevirapine with the Tyr188C ys RT there is however a more substantial movement of the drug molecule. We conclude that protein conformational changes and rearrangements of drug mo lecules within the mutated sites are not general features of these particul ar inhibitor/mutant combinations. The main contribution to drug resistance for Tyr181Cys and Tyr188Cys RT mutations is the loss of aromatic ring stack ing interactions for first generation compounds, providing a simple explana tion for the resilience of second generation NNRTIs, as such interactions m ake much less significant contribution to their binding. (C) 2001 Academic Press.