CONFORMATIONAL CHANGES IN HIV-1 REVERSE TRANSCRIPTASE RELEVANT TO THE DESIGN OF POTENT INHIBITORS. Andrew Hopkins¹, Jingshan Ren¹, Robert Esnouf¹, Carl Ross², E.Yvonne Jones¹, David Stammers² and David Stuart¹. ¹Laboratory of Molecular Biophysics, Rex Richards Building, South Parks Road, Oxford, OX1 3QU, UK; ²Structural Biology Group, Glaxo-WellcomeResearch Laboratories, Langley Court, Beckenham, Kent, BR3 3BS, UK.

Inhibition of HIV-1 reverse transcriptase (RT) is an important target for the development of anti-AIDS drugs. Nucleoside inhibitors such as AZT, 3TC and ddI suffer from serious toxicological effects and the development of drug resistant viral mutations. Non-nucleoside inhibitors (NNIs) consist of a variety of potent compounds such as TIBO, nevirapine and BHAP which are selective for HIV-1 RT and display low toxicity. The therapeutic efficacy of NNIs, however, is limited by the rapid developments of drug resistant mutations. Several crystal structures of RT-NNI complexes determined at high resolution (Ren et al, 1995, Nature Struct. Biol. 2, 303-308, Ren et al, 1995, Structure. 3, 915- 926) show NNIs bind to a hydrophobic pocket in RT about 10Å from the polymerase catalytic site. Analyses of these crystal structures illustrate the mechanisms of NNI resistance to be mainly a loss of stabilizing van der Waals contacts between the protein and the inhibitor. To dissect out the structural requirements for the design of a potent NNI, we have determined the crystal structures of a series of HEPT analogues covering a wide range of potencies (Hopkins et al, J.Med.Chem., in press). These complex structures reveal conformational changes in the protein some of which correlate with the potencies of the HEPT analogues. The major determinant of increased potency is the improved ring stacking interactions between the 6-benzyl ring of the inhibitors and Tyr181. The conformational switching of Tyr181 into its more exploitable position is caused by steric interactions with the 5-position substituent on the pyrimidine ring. All tight binding NNIs possess groups which throw this conformational switch.