Hydroxyurea in the treatment of HIV infection: clinical efficacy and safety concerns

Data from basic science and clinical studies suggest that hydroxyurea (hydroxycarbamide)-based regimens are effective treatment options for patients with HIV at various stages of disease. In vitro studies of HIV-infected lymphocytes have shown that hydroxyurea: (i) inhibits viral DNA synthesis; (ii) synergistically interacts with nucleoside reverse transcriptase inhibitors (NRTI); and (iii) increases the antiviral activity of didanosine. Clinical studies have confirmed that hydroxyurea in combination with didanosine produces potent and sustained viral suppression in patients with HIV infection. However, some concerns have been recently raised on the use of hydroxyurea in association with NRTI. Hydroxyurea can cause myelosuppression, skin toxicities, mild gastrointestinal toxicity, and abnormalities of renal and liver functions. In addition, hydroxyurea may accentuate the toxic effects of nucleoside analogues. In fact, some clinical data seem to indicate an increased risk of pancreatitis and neuropathy when hydroxyurea is combined with didanosine and stavudine. Since hydroxyurea-related toxicity is dose dependent, a systematic study of hydroxyurea optimal dosage and schedule was initiated to monitor patients for possible nucleoside toxicity. In the Research Institute for Genetic and Human Therapy (RIGHT) 702 study it was shown that a low, well-tolerated hydroxyurea dose (600 mg daily) achieved better antiretroviral activity than higher doses, together with better CD4+ cell count increase and fewer adverse effects. In this paper the effects of hydroxyurea as salvage therapy for heavily pretreated patients with advanced HIV disease are presented. These studies have shown that some patients with extensive pretreatment experience and advanced disease can respond substantially to the addition of hydroxyurea. The addition of hydroxyurea to didanosine does not prevent the emergence of resistance to didanosine; nonetheless, the efficacy of this therapeutic regimen may not be attenuated by the presence of didanosine-resistant HIV mutants. Since CD4 T lymphocyte activation is essential for virus replication and CD8 T lymphocyte activation may contribute to pathogenesis, the combination of hydroxyurea with other drugs may lead to the inhibition of HIV, by blocking the 'cell activation-virus production-pathogenesis' cycle. Clinical data indicate that hydroxyurea may play a role in attenuation of viral rebound and immune reconstitution by decreasing CD4 T cell proliferation, as well as preventing the exhaustion of CD8 T cell populations that may result from excessive activation during HIV infection. While the combination of hydroxyurea with didanosine has provided hope, future studies including those that evaluate optimal dosing and long-term toxicity are needed to define the role for this agent in the treatment of HIV infection.