HIV-1 reverse transcriptase (RT) genotype and susceptibility to RT inhibitors during abacavir monotherapy and combination therapy

AIDS. 2000 Jan 28;14(2):163-71. doi: 10.1097/00002030-200001280-00012.


Objective: To examine changes in HIV-1 susceptibility (genotype and phenotype) during an initial abacavir monotherapy phase followed by the addition of zidovudine and lamivudine.

Design: Sixty HIV-1 infected, antiretroviral therapy-naive subjects were randomized to receive 100, 300 or 600 mg abacavir twice daily. Subjects completing 24 weeks of randomized therapy or meeting a protocol defined switch criterion could switch to open label abacavir/zidovudine/lamivudine.

Methods: Plasma HIV-1 reverse transcriptase was genotyped at baseline, week 12, and at the last time point on ABC monotherapy. Drug susceptibility was analysed at baseline and on subsequent samples with sufficient HIV-1 RNA levels using the recombinant virus assay. Virological responses (week 24) were correlated to week 24 genotypes.

Results: Mutant viruses were not detected before week 12 with the exception of one subject. At the latest time point on abacavir monotherapy (range, weeks 6-48), 21 out of 43 subjects harboured virus with resistance conferring mutations including single, double and triple combinations of K65R, L74V, Y115F and M184V. The most common mutational pattern was L74V + M184V (11/21 cases). Twenty of the 21 subjects with isolates containing abacavir-associated mutations reached week 48, and upon addition of lamivudine/zidovudiine, 16 out of 20 (80%) had week 48 plasma HIV-1 -RNA below 400 copies/ml. At week 48, 16 out of 46 genotypes were obtained; one of these was wild-type; 15 contained M184V either alone, in combination with K65R and/or L74V and/or Y115F or with thymidine analogue-associated mutations. Week 48 viral load levels for these 15 subjects was low (median 3.43 log10 copies/ml or -1.99 log10 copies reduction from baseline). Genotype correlated well with phenotypic resistance to ABC; four samples with three abacavir-associated mutations had high level abacavir resistance (> 8-fold) and six samples with two or three mutations showed intermediate (4-8-fold) resistance. All samples with single mutations retained full ABC susceptibility.

Conclusions: Resistance conferring mutations to abacavir were relatively slow to develop during the monotherapy phase, and did not preclude durable efficacy of abacavir/lamivudine/zidovudine up to 48 weeks.

Publication types

  • Clinical Trial
  • Comparative Study
  • Randomized Controlled Trial
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Cohort Studies
  • Dideoxynucleosides / therapeutic use*
  • Drug Resistance, Microbial
  • Drug Therapy, Combination
  • Genotype
  • HIV Infections / drug therapy*
  • HIV Infections / virology
  • HIV Reverse Transcriptase / genetics*
  • HIV-1 / drug effects*
  • HIV-1 / enzymology
  • HIV-1 / isolation & purification
  • Humans
  • Lamivudine / therapeutic use
  • Mutation
  • Phenotype
  • RNA, Viral / blood
  • Reverse Transcriptase Inhibitors / therapeutic use*
  • Viral Load
  • Zidovudine / therapeutic use


  • Dideoxynucleosides
  • RNA, Viral
  • Reverse Transcriptase Inhibitors
  • Lamivudine
  • Zidovudine
  • HIV Reverse Transcriptase
  • abacavir