Hematologic toxicity of AZT and ddC administered as single agents and in combination to rats and mice

Toxicity studies of 3'-azido-2',3'-dideoxythymidine (AZT) and 2',3'-dideoxycytidine (ddC) were conducted in F344/N rats and B6C3F1 mice. The drugs were administered as single agents and in combination. In all studies, animals were treated by oral gavage twice a day, 7 days a week. In studies of the individual compounds, each was administered for 13 weeks at the following concentrations: AZT in rats, 0, 125, 250, 500, 1000 mg/kg and in mice, 0, 25, 50, 100, 400, 1000 mg/kg; ddC in rats and mice, 0, 250, 1000, 2000 mg/kg. Additional male rats and female mice that were treated with 0, 250, 1000, or 2000 mg/kg ddC and male and female mice treated with 0, 50, 400, 1000 mg/kg AZT were maintained for 30 days after treatment was stopped (at 94 days) to evaluate the reversibility of toxic effects. Hematologic variables were measured on Days 5, 23, and 94 (last day of dosing), and on Day 123 (after a 30-day period without treatment). AZT and ddC produced dose-related, poorly regenerative, macrocytic anemias as evidenced by decreases in erythrocyte counts, hematocrits, and hemoglobin concentrations and increases in mean corpuscular hemoglobin and mean corpuscular volume. Bone marrow samples in rats treated with AZT were hyperplastic whereas those in mice treated with AZT and rats and mice treated with ddC were hypoplastic. The hematologic toxicity of AZT was more severe than that of ddC. Generally, toxic effects of either chemical were greater in mice than in rats and more pronounced in female than in male animals. After 30 days without dosing, hematologic effects either resolved or dramatically improved. In studies in which ddC and AZT were administered in combination for 4 weeks at concentrations of 0/0, 0/500, 500/0, 10/500, 100/500, 500/500, and 500/1000 mg/kg ddC/AZT, there was macrocytic anemia in animals in the lower doses and marked microcytic anemia in surviving male mice in higher dose groups. Most female mice died in the 500/500 and 500/1000 mg/kg ddC/AZT dose groups. At lower concentrations (100/500, 500/1000 mg/kg ddC/AZT), effects of the two drugs were similar to those in the single drug studies. At higher concentrations (500/500 and 500/1000 mg/kg ddC/AZT), the combination treatment produced enhanced hematopoietic toxicity. These studies demonstrated the early and progressive time course of toxicity of AZT and ddC, species differences in sensitivities and responses, and reversibility of effects after termination of treatment. Based on these findings, a chronic toxicity study is being conducted with AZT in mice.