Nonclinical Toxicology Studies with Zidovudine: Genetic Toxicity Tests and Carcinogenicity Bioassays in Mice and Rats

Zidovudine (ZDV), an antiviral drug active in the treatmentof acquired immunodeficiency syndrome (recommended human dose,100 mg every 4 hr while awake), was evaluated for mutagenicand carcinogenic potential in a battery of short-term in vitroand in vivo assays and in lifetime studies in mice and rats.In L5178Y mouse lymphoma cells (tk^+/– locus), a weak positiveresult was obtained only at the highest concentrations tested(4000 to 5000 µg/ml) in the absence of metabolic activation.In the presence of metabolic activation, the drug was weaklymutagenic at concentrations of 1000 µg/ml and higher.Following 24 hr treatment in the absence of metabolic activation,ZDV was moderately mutagenic at concentrations up to 600 µg/ml;dose-related structural chromosomal alterations were seen atconcentrations of 3 µg/ml and higher in cultured humanlymphocytes. Such effects were not noted at the two lowest concentrationstested, 0.3 and 1 µg/ml, and BALB/c-3T3 cells were transformedat concentrations of 0.5 µg/ml and higher. No effectswere seen in the Ames Salmonella plate incorporation and preincubationmodification assays (possibly due to bacteriocidal activityof ZDV at low concentrations) at concentrations ranging from0.01 to 10 µg/plate or in a single-dose intravenous bonemarrow cytogenetic assay in CD rats. In multidose micronucleusstudies, increases in micronucleated erythrocytes were seenin mice at doses of 100 to 1000 mg/kg/day. Similar results wereseen in rats and mice after 4 or 7 days of dosing at 500 mg/kg/day.In carcinogenicity bioassays, adjusted doses of 20, 30, or 40mg/kg/day and 80, 220, and 300 mg/kg/day were given to CD-1mice and CD rats, respectively, for up to 22 months in miceand 24 months in rats. ZDV caused a macrocytic, normochromicanemia in both species. No evidence of carcinogenicity was seenin male mice or rats. In female mice, five malignant and twobenign vaginal epithelial neoplasms occurred in animals given40 mg/kg/day. A single benign vaginal epithelial tumor was seenin a mouse given 30 mg/kg/day. In rats, two malignant vaginalepithelial neoplasms were seen in animals given 300 mg/kg/day.In a 7-day study in mice, ZDV was shown to be devoid of estrogenicactivity. In an oral pharmacokinetics study, the AUC was 17and 144 µg/ml hr in female mice and rats given 40 or 300mg/kg of ZDV, respectively. In contrast, the average steady-stateconcentration in humans at the recommended daily dose is 0.62µg/ml. Twenty-four hour urine concentrations were 1245and 4417 µg/ml in female mice and rats given 40 or 300mg/kg of ZDV, respectively. These values were approximately26-and 136-fold higher than the human urine concentration atthe recommended daily dose. In a one- to three-day study withintravenously administered sodium fluoroscein in rats and mice,retrograde flow of urine into the vagina was demonstrated. Ina subsequent lifetime carcinogenicity bioassay in mice in whichZDV was given intravaginally at concentrations of 5 or 20 mgZDV/ml in saline, 13 vaginal squamous cell carcinomas were seenat the highest concentration tested. It was concluded that thevaginal tumors seen in the oral carcinogenicity studies werethe result of chronic local exposure of the vaginal epitheliumto high urine concentrations of ZDV.