Anti-AIDS drug candidate and non-nucleoside reverse transcriptase inhibitor (NNRTI) TMC125-R165335 (etravirine) caused an initial drop in viral load similar to that observed with a five-drug combination in naïve patients and retains potency in patients infected with NNRTI-resistant HIV-1 variants. TMC125-R165335 and related anti-AIDS drug candidates can bind the enzyme RT in multiple conformations and thereby escape the effects of drug-resistance mutations. Structural studies showed that this inhibitor and other diarylpyrimidine (DAPY) analogues can adapt to changes in the NNRTI-binding pocket in several ways: (1). DAPY analogues can bind in at least two conformationally distinct modes; (2). within a given binding mode, torsional flexibility ("wiggling") of DAPY analogues permits access to numerous conformational variants; and (3). the compact design of the DAPY analogues permits significant repositioning and reorientation (translation and rotation) within the pocket ("jiggling"). Such adaptations appear to be critical for potency against wild-type and a wide range of drug-resistant mutant HIV-1 RTs. Exploitation of favorable components of inhibitor conformational flexibility (such as torsional flexibility about strategically located chemical bonds) can be a powerful drug design concept, especially for designing drugs that will be effective against rapidly mutating targets.

TMC278 is a diarylpyrimidine (DAPY) nonnucleoside reverse transcriptase inhibitor (NNRTI) that is highly effective in treating wild-type and drug-resistant HIV-1 infections in clinical trials at relatively low doses (ϳ25-75 mg/day). We have determined the structure of wild-type HIV-1 RT complexed with TMC278 at 1.8 Å resolution, using an RT crystal form engineered by systematic RT mutagenesis. This highresolution structure reveals that the cyanovinyl group of TMC278 is positioned in a hydrophobic tunnel connecting the NNRTI-binding pocket to the nucleic acid-binding cleft. The crystal structures of TMC278 in complexes with the double mutant K103N/Y181C (2.1 Å) and L100I/K103N HIV-1 RTs (2.9 Å) demonstrated that TMC278 adapts to bind mutant RTs. In the K103N/Y181C RT/TMC278 structure, loss of the aromatic ring interaction caused by the Y181C mutation is counterbalanced by interactions between the cyanovinyl group of TMC278 and the aromatic side chain of Y183, which is facilitated by an ϳ1.5 Å shift of the conserved Y183MDD motif. In the L100I/K103N RT/ TMC278 structure, the binding mode of TMC278 is significantly altered so that the drug conforms to changes in the binding pocket primarily caused by the L100I mutation. The flexible binding pocket acts as a molecular ''shrink wrap'' that makes a shape complementary to the optimized TMC278 in wild-type and drug-resistant forms of HIV-1 RT. The crystal structures provide a better understanding of how the flexibility of an inhibitor can compensate for drug-resistance mutations.drug design ͉ drug resistance ͉ nonnucleoside reverse transcriptase inhibitor ͉ polymerase ͉ x-ray crystallography C ombinations of drugs have been used to successfully treat HIV-1 infections, permitting dramatic reduction in viral loads and restoration of the immune system. However, drug treatments do not eliminate the infection and treatment must be life-long. There are problems with drug toxicity, and prolonged drug exposure can lead to the emergence of drug-resistant mutant viruses. Because toxicity and drug resistance complicate treatment strategies, the development of new anti-AIDS drugs remains essential., also referred to as rilpivirine and R278474, is a diarylpyrimidine (DAPY) nonnucleoside reverse transcriptase inhibitor (NNRTI) that was developed in a multidisciplinary structure-based design effort (3, 4) intended to discover drugs that can inhibit a wide range of the known drug-resistant HIV-1 strains. In cell-based assays, TMC278 inhibits a broad spectrum of HIV-1 variants (Table 1), including strains that are partially or completely resistant to the existing NNRTI drugs (3). Phase I and II clinical trials have shown that TMC278 effectively reduces viral load and maintains low viral load levels with no virus rebound in patients even after 48 weeks of treatment (5, 6).Crystal structures of HIV-1 RT/NNRTI complexes were solved in the course of developing TMC278 (7, 8) along a path that originated with the first-generation TIBO compounds (9). Most of those structures were obtained at ϳ...

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