Background:  HIV-1 prevention using daily oral use of generally well-tolerated anti-retroviral agents is being evaluated in clinical trials. Drug resistance could contribute to chemoprophylaxis failure due to transmission of drug resistant variants, or selection of drug resistance after transmission. We have searched, using methods that sensitively detect minor sequence variants, for the resistance mutations K65R and K70E using specimens from chemoprophylaxis trial participants.

Methods:  Population genotyping was performed by the Trugene assay (Seimens). A quantitative allele-specific polymerase chain reaction (PCR) was developed using a DNA polymerase with improved discrimination against mis-extensions. Customized primers were used that matched the genetic background of the virus detected in each specimen. The allele specific PCR assay was used to interrogate the pol DNA amplicons from the commercial genotypic resistance assay. The pol DNA fragments were also cloned, and individual clones were scored for the presence of K65R and K70E.

Results:  A randomized blinded placebo controlled clinical trial of daily oral tenofovir (TDF) versus placebo for HIV-1 prevention in women was completed in 2006. Specimens became available from 2 seroconverters, 1 of whom had been exposed to TDF chemoprophylaxis for 1 month prior to seroconversion. No doses had been missed according to pill count or report. The other was exposed to placebo. There was no evidence of drug resistance at codons RT 65 or 70 by population sequencing, allele-specific PCR (<0.3%), and extensive clonal analysis of 765 and 774 clones at each site, respectively. Plasma RNA levels in seroconverter previously exposed to TDF reached a peak of 115,000 copies/mL and then decreased spontaneously to 1780 copies/mL 4 weeks later with no additional exposure to antiviral drugs.

Conclusions:  There is no evidence of TDF resistance mutations K65R or K70E in 1 adherent participant who seroconverted after 1 month of exposure to TDF chemoprophylaxis. Similarly, TDF resistance has not been observed in non-human primates who became infected despite TDF chemoprophylaxis. In contrast with treatment use, prevention use of antivirals may engender drug resistance rarely, or not at all, because the viral populations being inhibited are too small to readily generate resistant mutants.