Background:  Previous studies have demonstrated synergy between zidovudine (AZT), a nucleoside reverse transcriptase inhibitor (NRTI), and efavirenz (EFV), a non-NRTI (NNRTI). The aim of the current study was to define the molecular mechanisms involved in this synergistic interaction at the enzyme level. 

Methods:  Recombinant wild type, NNRTI-resistant (G190A), AZT-resistant (AZT^R, D67N/K70R/T215Y/K219Q), and RNase H-deficient (D443N) HIV-1 RT were purified to homogeneity. The ability of EFV to inhibit incorporation of AZT-triphosphate and ATP-mediated excision of AZT-monophosphate (AZT-MP) was investigated using both RNA/DNA and DNA/DNA template/primer.  The effect of EFV on RNase H activity was also investigated.

Results:  EFV exhibited a significantly greater capacity (~29-fold decrease) to inhibit ATP-mediated excision of AZT-MP by wild type HIV-1 RT on an RNA/DNA template/primer (IC₅₀ = 7.2 nM) compared with the identical DNA/DNA template/primer (IC₅₀ = 208 nM). A similar trend was also noted for AZT^R RT, however, EFV exhibited a limited capacity to inhibit AZT-MP excision reactions carried out by G190A RT (IC₅₀> 250 nM for both template/primers). Interestingly, we found that EFV accelerates wild type and AZT^R HIV-1 RT RNase H activity resulting in more secondary and tertiary RNA cleavage events.

Conclusions:  Because AZT potency may be governed by a balance between nucleotide excision and RNase H activity, we hypothesize that the observed increase in RNase H cleavage in the presence of EFV may destroy the RNA template before excision has occurred, leading to dissociation of the template/primer and inhibiting excision. These results might help to explain the observed synergy between AZT and EFV.