HIV-1 reverse transcriptase can discriminate between two conformationally locked carbocyclic AZT triphosphate analogues

It has been proposed that the preference of 3'-azido-3'-deoxythymidine (AZT) for the extreme E-3 (south) conformation, as observed in its X-ray structure, is responsible for its potent anti-HIV activity. However, it has also been suggested that the antipodal north conformation may be required for the strong interaction of AZT 5'-triphosphate with its target enzyme, HIV reverse transcriptase (RT). To resolve this issue, we have constructed two conformationally rigid carbocyclic analogues of AZT which are locked permanently into opposite E-2 (north) and E-3 (south) conformations in order to test the ability of the corresponding 5'-triphosphates to inhibit RT. The two isomeric carbocyclic analogues of AZT, (N)-methano-carba-AZT (1) and (S)-methano-carba-AZT (2), were constructed on a bicyclo[3.1.0]hexane template that exhibits a rigid pseudoboat conformation, capable of mimicking the furanose pucker in the classical north and south conformations that are characteristic of standard nucleosides. The unique conformational properties of 1 and 2 observed by both X-ray and solution NMR studies showed the existence of the same invariant conformations in solution and in the solid state. In addition, differences observed in the outcome of the Mitsunobu inversion of a secondary hydroxyl function attempted with both bicyclo[3.1.0]hexane nucleoside analogues could be explained by the rigid pseudoboat nature of this system. In one case, the bicyclic system facilitated formation of an anhydronucleoside intermediate, whereas in the other it completely prevented its formation. The chemically synthesized 5'-triphosphates of 1 and 2 were evaluated directly as RT inhibitors using both a recombinant enzyme and enzyme obtained and purified directly from wild-type viruses. The results showed that inhibition of RT occurred only with the conformationally locked E-2 (N)-methano-carba-AZT 5'-triphosphate. This inhibition was equipotent to and kinetically indistinguishable from that produced by AZT 5'-triphosphate. The antipodal E-3 (S)-methano-carba-AZT 5'-triphosphate, on the other hand, did not inhibit RT.