Background:  Currently approved NRTI have significant limitations, including the selection of drug-resistant HIV-1 variants that are cross-resistant to other NRTI. Based on knowledge of both resistance mechanisms and structural components of NRTI that affect sensitivity to these resistance mechanisms, we have identified a lead class of 3’-azido-2’,3’-dideoxypurines (ADP) that retain potent activity against NRTI-resistant HIV-1.

Methods:  3’-Azido-dideoxyguanosine (3’-azido-ddG) and 3’-azido-2’,3’-dideoxyadenosine (3’-azido-ddA) were subjected to extensive in vitro evaluations of antiviral potency, activity against purified HIV-1 reverse transcriptase (RT), cytotoxicity, and cellular pharmacology.

Results:  3’-Azido-ddA and 3’-azido-ddG showed excellent antiviral activity in primary human mononuclear cells, HeLa, and T cell lines (EC₅₀ ranged from 0.19 to 2.1 μM for 3’-azido-ddG and from 0.36 to 10 μM for 3’-azido-ddA). Both 3’-azido-ddGTP and 3’-azido-ddATP were incorporated by HIV-1 RT as efficiently as the natural dGTP or dATP substrates. Furthermore, both 3’-azido-ddA and 3’-azido-ddG retained activity against viruses containing K65R, L74V, or M184V (EC₅₀ fold-change <2.0) and against those containing 3 or more thymidine analog mutations (EC₅₀ fold-change <3.5). Even at high concentrations of drug (100 μM), 3’-azido-ddG did not exhibit cytotoxicity in primary lymphocytes, epithelial or T cell lines, and did not decrease the mitochondrial DNA content of HepG2 cells. 3’-Azido-ddG was efficiently phospshorylated to 3’-azido-ddGTP in primary human mononuclear cells with an intracellular half-life of the nucleoside triphosphate of 9 hr.

Conclusions:  The ADP exhibit a favorable activity profile against both wild type and NRTI-resistant viruses. 3’-Azido-ddG also exhibited minimal cytotoxicity and favorable cellular pharmacologic profile. Based on these findings, additional preclinical studies are warranted to assess the potential of ADP for treatment of HIV-1 infection.