Zidovudine: five years later.

Zidovudine, a nucleoside analog, was the first agent proved to be effective in the management of human immunodeficiency virus type 1 (HIV-1) infection. After demonstration of zidovudine's in-vitro activity against HIV-1 in 1985, the drug was rapidly evaluated in phase I and phase II clinical trials and was found to be effective in decreasing both mortality and the incidence of opportunistic infections in patients with the acquired immunodeficiency syndrome (AIDS) and advanced AIDS-related complex; the drug was also found to have a substantial but tolerable toxicity profile. Since the licensure of zidovudine in 1987, an intensive clinical research effort has established the drug's efficacy in the prevention of disease progression in asymptomatic and mildly symptomatic HIV-infected persons and has established the success of lower-dose therapy in patients at all stages of disease. The current recommendation is to use zidovudine at a dose of 500 to 600 mg/d in both symptomatic and asymptomatic persons with CD4 counts of less than 500/mm3. The major toxicities of anemia and neutropenia are less frequent at the lower doses presently used and can be managed by dose reduction or by use of hematopoietic growth factors. The inexorable disease progression seen despite zidovudine therapy and the isolation of clinical strains of HIV-1 resistant to zidovudine in vitro highlight the limitations of prolonged monotherapy with this agent. Although alternative dideoxynucleoside agents (for example, didanosine [dideoxyinosine and zalcitabine dideoxycytidine]) are available for the management of HIV-infected persons, zidovudine remains the cornerstone of antiretroviral therapy. Current research efforts are directed at elucidating the clinical relevance of zidovudine resistance and studying regimens in which zidovudine is used in combination with other agents. This latter approach holds great promise for improving efficacy, limiting toxicity, and perhaps preventing the emergence of viral resistance. For the forseeable future, zidovudine will continue to play a role in the development and in our understanding of antiretroviral therapy.