The 3′-azido group is not the primary determinant of 3′-azido-3′-deoxythymidine (AZT) responsible for the excision phenotype of AZT-resistant HIV-1

The mechanism of human immunodeficiency virus (HIV) 1 resistance to 3'-azido-3'-deoxythymidine (AZT) involves reverse transcriptase (RT)-catalyzed phosphorolytic excision of the chain-terminating AZT-5'-monophosphate ( AZTMP). Primers terminated with AZTMP are generally better substrates for this reaction than those terminated with 2',3'-dideoxynucleoside-5'-monophosphate (2', 3'-ddNMP) analogs that lack a 3'-azido moiety. This led to the hypothesis that the 3'-azido group is a major structural determinant for maintaining the primer terminus in the appropriate site for phosphorolytic excision of AZTMP by AZT-resistant (AZTR) RT. To test this hypothesis, we evaluated the incorporation, phosphorolytic excision, and antiviral activity of a panel of 3'-azido-2', 3'-ddN including 3'-azido-2', 3'-ddA (AZddA), 3'-azido-2',3'-ddC (AZddC), 3'-azido-2',3'-ddG(AZddG), AZT, and 3'-azido-2', 3'-ddU ( AZddU). The results indicate that mutations correlated with resistance to AZT (D67N/K70R/T215F/K219Q) confer resistance to the 3'-azidopyrimidine nucleosides (AZddC, AZT, and AZddU) but not to the 3'-azidopurine nucleosides ( AZddA and AZddG). The data suggest that the presence of a 3'-azido group on the 3'-terminal nucleotide of the primer does not confer increased phosphorolytic excision by AZTR RT for all 3'-azido-ddNMP analogs. Thus, the 3'-azido group cannot be the only structural determinant important for the enhanced phosphorolytic excision of AZTMP associated with HIV resistance to AZT. Other structural components, such as the base, must play a role in defining the specificity of the excision phenotype arising from AZT resistance mutations.