Emergence of Dual Resistance to Zidovudine and Lamivudine in Clinical HIV-1 Isolates from Patients Receiving Zidovudine/Lamivudine Combination Therapy

Susceptibility to zidovudine and lamivudine was determined on human immunodeficiency virus type 1 (HIV-1) isolates obtained from patients who added lamivudine after 6 months of treatment with zidovudine. Lamivudine-resistant isolates that were also zidovudine-resistant were recovered from 13/16 (81%) patients after 6 months of dual therapy. In contrast to findings in anti-retroviral therapy-naive patients, these results suggest that dual resistance to zidovudine and lamivudine emerges relatively quickly when lamivudine is added to zidovudine as a single agent in the majority of patients with extensive prior zidovudine treatment.