Background:  Chemoprophylaxis with tenofovir (TDF) as a strategy for the prevention of HIV transmission continues to receive considerable attention. However, drug resistance is a concern when prophylaxis fails because these persons will be receiving TDF monotherapy. Currently, simian immunodeficiency virus (SIV)-infected macaques are suitable models for assessing drug resistance emergence resulting from TDF monotherapy. As with HIV-1, TDF primarily selects for the high fitness cost K65R reverse transcriptase mutation in SIV. We used a simple and sensitive assay to monitor the emergence of K65R in TDF-treated macaques to improve the evaluation of drug resistance risks associated with the use of TDF chemoprophylaxis.

Methods:  Using a real-time polymerase chain reaction (PCR)-based resistance assay for the SIV K65R mutation, we examined longitudinal plasma specimens from 11 SIV_mac251-infected rhesus macaques treated subcutaneously with 30 mg/kg TDF daily. K65R⁺ SIV sequences serially diluted in wild type backgrounds revealed that as little as 0.2% mutant virus could be detected. However, the mean limit of detection for the validation population, equivalent to 0.4% mutant virus, was used as the assay cutoff for diagnostic purposes. Population sequencing was performed on the plasma virus for comparison to the real-time PCR results.

Results:  In testing longitudinal plasma virus specimens from the 11 TDF-treated macaques, the assay detected resistant viruses 1 week after treatment initiation in 4 animals and by week 6 in another 4 animals. Drug resistance was detected by week 9 in the remaining 3 animals, all of which underwent a 1-week treatment interruption at week 6. No K65R mutants were identified prior to treatment. In 5 of 11 animals, K65R mutants were identified at frequencies estimated to be between 1 and 10% during resistance emergence, below the detection of population sequencing. The mutants in these animals became enriched to sequence-detectable levels 2 to 4 weeks after their initial identification by the real-time PCR assay.

Conclusions:  The real-time PCR analysis of SIV-infected macaques treated with TDF revealed that, despite the high fitness cost conferred by the K65R mutation, resistance can emerge rapidly. The data may have implications for persons who become infected while on TDF prophylaxis, and reiterate the importance of close HIV and drug-resistance monitoring during chemoprophylaxis.