Background:  Entecavir, an analogue of 2’-deoxyguanosine, is approved by the FDA for the treatment of chronic hepatitis B virus (HBV). The package insert states that entecavir has no clinically relevant activity against HIV. We became aware of 2 HIV/HBV co-infected patients on entecavir monotherapy who had a 1-log decline in their HIV RNA. Thus, we tested the hypothesis that entecavir has clinically relevant activity against HIV.

Methods:  Dose/response curves for inhibition of HIV infection were generated in vitro using a precise infectivity assay. HIV pseudoviruses carrying pol gene sequences from either a reference or a patient-derived HIV isolate were used to infect CD4⁺ lymphoblasts in the presence of increasing concentrations of antiviral drugs. To assess development of resistance mutations, HIV RNA was extracted from an HIV-HBV co-infected patient at several time points while on entecavir monotherapy, amplified via RT-PCR, cloned and sequenced. Finally, a real-time polymerase chain reaction assay was used to determine reverse transcriptase activity in the presence of entecavir.

Results:  We found that entecavir potently inhibited HIV replication with an IC₅₀ between 0.1 and 1 nM. This IC₅₀ is below the plasma concentrations achieved in vivo at doses given for hepatitis B and is 100- to 1000-fold lower than the IC₅₀ of zidovudine in the same system. The prospective analysis of our patient samples demonstrated accumulation of the M184V mutation with 0%, 61%, and 96% of the clones harboring this variant at the start of entecavir, and at 4 and 6 months after entecavir, respectively. Phylogenetic analysis of these M184V clones was consistent with selection for the mutation on entecavir. Both the laboratory and patient-derived strains with the M184V were resistant to entecavir. Using real-time PCR, we found that entecavir decreased the level of early, intermediate, and late reverse transcripts suggesting that it inhibits HIV replication at or before the reverse transcription step. 

Conclusions:  This study demonstrates that entecavir is a potent (but partial) inhibitor of HIV replication in vivo and in vitro and that it can select for viruses bearing the M184V mutation, which confers high-level resistance to entecavir. These data have important implications for treatment of hepatitis B in HIV-infected patients since they indicate that current guidelines recommending entecavir as the first line treatment in co-infected persons who do not require anti-HIV therapy should be reconsidered.