ABSTRACT: Purpose: Recently, hydroxyurea (HU) has shown good clinical activity in the treatment of HIV infection when given in combination with nucleoside therapy. We treated Didox (3.4-dihydroxybenzohydroxamic acid), amore potent ribonucleotide reductase inhibitor, to determine whether it might be more active in vivo than HU.
Methods: Preliminary dose-ranging experiments were performed in the Rauscher musrine leukemia model and were used to estimate optimum HU and Didox concentrations for activity and toxicity. These optimum concentration of 600 mg/kg HU and 450 mg/kg Didox were used to treat HIV-infected SCID mice reconstituted with human PBMC. Infectious HIV recovery was measured in blood cells, splenocytes, lymph nodes and peritoneal cells by quantitative co-culture. Intracellular viral RNA copy number (viral load) was quantified by the NASBA assay. The ability of treatment to protect human CD4 lymphocytes from virus-induced cytolysis was measured by FACS analysis. HU and Didox were tested alone and in combination with didanosine, with i.p. drug therapy beginning 24 h before HIV infection.
Results: Didox showed significantly more activity than HU when given as monotherapy. For example, in peritoneal cells Didox reduced viral titers from 4.8x103 TCID50/106 cells to 2.2x101 TCID/106 cells whereas HU treatment reduced the titers to 2.7x103 TCID50/106 cells. Combination regimens were highly effective at inhibiting HIV replication and could not be differentiated in this experiment owing to the higher than expected activity of 60 mg/kg didanosine. However, the combination of didanosine plus Didox was twice as effective at protecting CD4 cells HIV-induced cytolysis as was didanosine plus HU. Additional combination experiments with more appropriate didanosine concentrations are in progress to differentiate between the two regimens.
Conclusions: Didox showed promising antiviral activity in the SCID model and warrants clinical evaluation.
