A NOVEL POLYMORPHISM AT CODON-333 OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 REVERSE-TRANSCRIPTASE CAN FACILITATE DUAL RESISTANCE TO ZIDOVUDINEAND L-2',3'-DIDEOXY-3'-THIACYTIDINE

Recent clinical trials examining 3'-azido-3'-deoxythymidine (AZT, zido vudine, or Retrovir) combined with L-2',3'-dideoxy-3'-thiacytidine (3T C or lamivudine) have shown that combination therapy with these nucleo side analogs affords significant virological and clinical benefits. Th e addition of 3TC to AZT delays AZT resistance in therapy-naive patien ts and can restore viral AZT susceptibility in patients who previously received AZT alone. In some AZT-experienced patients, the,virological response to AZT-3TC therapy is not sustained and virus resistant to b oth drugs can be identified. To gain insight into the possible mechani sm of dual resistance, we studied a recently described variant resista nt to both AZT and 3TC and obtained by simultaneous passage of an AZT- resistant clinical isolate in cell culture with AZT and 3TC. Genetic m apping and site-directed mutagenesis experiments demonstrated that a p olymorphism at codon 333 (Gly to Glu) of human immunodeficiency virus type 1 reverse transcriptase (RT) was critical in facilitating dual re sistance in a complex background of AZT and 3TC resistance mutations. To assess the potential clinical relevance of RT codon 333 changes, we studied dually resistant viruses from patients taking AZT and 3TC. Ge netic mapping of RT molecular clones derived from patients' plasma sam ples demonstrated that in some cases polymorphism at codon 333 was res ponsible for facilitating dual resistance.