BACKGROUND: Preveon (adefovir dipivoxil; bis-POM PMEA) and PMPA Prodrug are both nucleotide analogs currently in clinical trials for the treatment of HIV. In vitro experiments showed that a K65R or K70E mutation developed in the presence of adefovir, while the K65R mutation arose under the selective pressure of PMPA. To determine whether viruses carrying mutations in RT selected by Preveon in patients would remain susceptible to PMPA, viruses were constructed expressing RT genes from patients who had received extensive Preveon therapy. Additionally, samples from SIV-infected monkeys and HIV-infected patients who had received PMPA therapy were characterized to determine whether resistance to PMPA arose in either setting.

METHODS: Genotypic analyses were conducted on RT-PCR products containing nucleotides 1-900 of HIV-RT generated from patient plasma samples. Phenotypic analyses were conducted on recombinant viruses constructed by site directed mutagenesis of HXB2D or generated from patient plasma samples.

RESULTS: Recombinant HIV strains were constructed from all 8 of the 29 patients completing an extended Preveon dosing phase who developed sequence changes from baseline which were possibly associated with Preveon therapy. Viruses constructed from 2 patients each who developed either a K70E or T69D mutation in RT showed no decreased susceptibility to PMPA. The remaining 4 patients had developed AZT-associated resistance mutations in HIV RT; these viruses showed a < or = 3-fold decrease in PMPA susceptibility compared to matched pre-therapy viruses. Experiments using site directed HXB2D recombinants expressing the corresponding mutations yielded similar results. We also investigated the emergence of resistance to PMPA as a result of PMPA treatment in vitro and in vivo. In vitro the K65R mutation in RT was selected and conferred a 3-fold decrease in PMPA susceptibility to HIV and SIV. Importantly, in a study investigating PMPA for the treatment of SIV, the K65R mutation developed in 4/4 treated monkeys soon after therapy began, yet at up to 3 years post infection, animals who are still receiving PMPA therapy continue to show a durable suppression of viral replication. Recently, 28 patients received 1 month of PMPA Prodrug monotherapy and none on the 16 patients analyzed to date developed any genotypic changes in RT from baseline.

CONCLUSIONS: Based on in vitro data, mutations which arose during Preveon therapy would not be expected to affect subsequent response to PMPA treatment. Additionally, no clinically significant PMPA resistance has developed in SIV-infected monkeys and no genotypic changes have been observed in RT of HIV-infected patients as a result of PMPA therapy.