The following abstract presents results of a study conducted by a contract laboratory for the National Toxicology Program. The findings were not evaluated in accordance with the levels of evidence for reproductive or developmental criteria established by NTP in March 2009. The findings and conclusions for this study should not be construed to represent the views of NTP or the U.S. Government.
Azidothymidine (AZT) was evaluated for potential reproductive toxicity using the multigenerational Continuous Breeding paradigm. With the increasing prophylactic use of AZT comes concerns about the function of the offspring of treated parents. A dose-range finding study (Task 1) was conducted by administering AZT to adult male and female CD 1 mice twice daily by gavage using split doses of equal amounts approximately six hours apart to achieve a total daily dose of 0, 50, 100, or 200 mg/kg (N=8 adult mice/sex). Since there was no offspring lethality, and since previous studies had shown that this high dose was effective at reducing sperm motility in F0 mice, these dose levels were selected for the F0 Cohabitation (Task 2).
Task 2 was conducted to test for effects of prenatal/lactational exposure on fertility and the development of the subsequent generation. AZT was administered to adult F0 male and female mice (N=20) twice daily by gavage using split doses of equal amounts approximately six hours apart to achieve a total daily dose of 0, 50, 100, or 200 mg/kg (N=20 adult mice/sex). Dosing of F0 animals began one week prior to cohabitation and continued through lactation of F1 pups to F0 necropsy. Thus, F0 males were dosed from Study Day 1-92 and F0 females were dosed from Study Day 1-129. F0 mating pairs were allowed to produce three litters (F1a, F1b, and F1c). None of the F1 animals were dosed. F1a and F1b litters were evaluated and killed on post-natal day 1. F1c litter was reared to adulthood and on post-natal day 74 ± 10, F1c adults were assigned to mating pairs and were allowed to produce three litters (F2a, F2b, and F2c). Endpoints evaluated for F0 and F1 adult animals included body weight, food consumption, water consumption, clinical signs, sperm parameters, vaginal cytology, number and weight of pups per litter, organ weights, and gross and microscopic pathology. Endpoints evaluated for pups in the F1 and F2 litters included number and weight, sex, and anogenital distance (AGD).
For the F0 generation, the pregnancy index in the high-dose group was 65% and 60% for treated F1b and F1c litters, respectively, compared to 100% in controls; it was unaffected in other litters and dose groups. There was an increase (3-11 %) in days to litter in all treated groups. There was a dose-related decrease (16-61%) in the number of pups/litter in all litters of treated F0 mice. In addition, there was a trend toward decreased percent males in the litters of all treated groups, particularly in the F1c litter. F0 sperm analysis revealed decreases in sperm motility, velocity, linearity, density, and spermatid head counts at all dose levels. There were no differences in estrous cycle measures of the F0 generation females.
Following F1 Cohabitation, there were no effects on pregnancy index or number of pups/litter, except for a 23% decrease in the number of pups/litter during F2c litter at 200 mg/kg/day. Sperm parameters and vaginal cytology for the F1 generation were unchanged from controls.Following F1 Cohabitation, there were no effects on pregnancy index or number of pups/litter, except for a 23% decrease in the number of pups/litter during F2c litter at 200 mg/kg/day. Sperm parameters and vaginal cytology for the F1 generation were unchanged from controls.
Under the conditions of this study, AZT had a significant reproductive effect, consisting of a dose-related decrease in the number of live pups/litter at 50, 100, and 200 mg/kg/day in the F0 generation. However, the F1 offspring of dosed F0 mice, who were exposed only in utero and during lactation, were capable of successfully producing healthy litters. The reduction in the size of the F2c litter suggests a treatment effect on total reproductive capacity or on early reproductive senescence, which can only be confirmed by further study.