Presence of 2 ',5 '-bis-O-(tert-butyldimethylsilyl)-3 '-spiro-5 ''-(4 ''-amino-1 '',2 ''-oxathiole-2 '',2 ''-dioxide) (TSAO)-resistant virus strains in TSAO-inexperienced HIV patients

HIV-1 samples from six patients undergoing diverse anti-HIV therapies posse ssed the E138A mutation in their reverse transcriptase (RT) genome. Patient s were receiving the following therapies: TIBO monotherapy tone patient); z idovudine plus didanosine combination therapy tone); zidovudine monotherapy tone); sequential therapy with zidovudine, then stavudine and finally zalc itabine plus didanosine tone); and two were drug naive. E138K, not E138A, i s a known TSAO-specific resistance mutation, emerging under selective press ure in vitro, Our phenotypic data on the patient isolates, confirmed by dat a on an E138A mutant acquired through in vitro mutagenesis, indicated that an alanine substitution for glutamate at codon 138 of the HIV-1 RT renders the virus TSAO resistant, confirming the importance of this amino acid resi due in the activity of TSAO derivatives. In addition, we have demonstrated through phenotypic analysis of the E138A and A98S mutants (after in vitro m utagenesis) that the mutation A98S, found in one of these patients, could b e partially responsible for the phenotypic reversal of TSAO resistance. Thi s reversal could be explained by the restoration of a hydrogen bond between 98S and the main-chain residue L349, which compensates for the loss of the E138-G99 main-chain hydrogen bond, As TSAO derivatives have not been used in the clinical setting, the presence of the E138A mutation at a frequency of 6.7% in our study of 90 TSAO-inexperienced HIV-seropositive individuals implies that 138A of the RT must be a natural variant and that the mutant v irus is replication competent. Our observations suggest that the E138A muta tion may likely arise in patients under the selective pressure of TSAO or r elated compounds that show a decreased antiviral potency toward the E138A v ariant.