DEVELOPMENT OF HIV-1 RESISTANCE TO (-)2'-DEOXY-3'-THIACYTIDINE IN PATIENTS WITH AIDS OR ADVANCED AIDS-RELATED COMPLEX

Objective: To determine the rate of development of in vitro HIV resist ance to (-)2'-deoxy-3'-thiacytidine (3TC) and relate the effect of dos e to emergence of resistance. Methods: HIV-infected men and non-pregna nt women, aged greater than or equal to 18 years, with a CD4 count les s than or equal to 300x10(6)/I cells were followed in a Phase I/II stu dy, in which they were evaluated for tolerance to 3TC and effect of th is agent with regard to viral susceptibility. Peripheral blood and pla sma samples were collected at regular intervals for analysis. HIV was isolated using umbilical cord blood mononuclear cells as targets. Thes e cells were also used in determinations of median inhibitory drug con centration. Specific amplification of the 184 mutation site, associate d with HIV resistance to 3TC, was performed by polymerase chain reacti on, using specific primer pairs, on DNA harvested from infected periph eral blood mononuclear cells (PBMC) of donors or, alternatively, on DN A that had been reverse transcribed from plasma-associated HIV RNA. Re sults: Phenotypic resistance was detected in approximately one-third o f individuals studied, who were followed between 8 and 56 weeks. Devel opment of 3TC resistance occurred independently of dose, although time of first appearance of resistant HIV-1 variants appeared reduced at h igh 3TC doses. Amino-acid changes at codon 184 in HIV-1 reverse transc riptase were associated with, and preceded, the development of phenoty pic 3TC resistance. Most commonly, a Met to Ile substitution appeared transiently before being superceded by a Val substitution at codon 184 . Conclusions: In vitro resistance to 3TC developed in a high proporti on of subjects who received prolonged monotherapy with this drug. The development of resistance to 3TC was associated with appearance of mut ated viral forms and the disappearance of wild-type virus, with regard to codon 184, in both patient plasma and PBMC.