Abstract:

Nonclinical reproductive toxicity studies commonly utilize mammalian species such as rats, rabbits, and cynomolgus monkeys. Among these, successful animal pregnancy remains a critical factor influencing study outcomes. This article focuses on the key challenges and corresponding mitigation strategies in the animal pregnancy phase of such studies. Rats are typically employed in fertility and early embryonic development toxicity studies (Segment I), embryo-fetal development toxicity studies (Segment II), and peri-/postnatal development toxicity studies (Segment III). The estrous cycle of female rats can be determined through vaginal smears, and mating is achieved by pairing one female with one male in a cage, with successful copulation confirmed by checking for vaginal plugs the following day. Rabbits are commonly used in embryo-fetal development toxicity studies. Mating occurs when males are paired with females in estrus, and the timing of conception is determined based on observed mating behavior. Challenges frequently encountered in these studies include females not entering estrus, significant variations in estrous cycles between batches of animals, and unsuccessful mating. These issues can be mitigated by extending light exposure, increasing protein intake, selecting appropriate mating methods, and modifying housing conditions (such as keeping males and females in adjacent enclosures). Non-human primates (NHPs) are typically employed in peri- and postnatal toxicity studies (Segment III). Their use also presents several difficulties. One key challenge is determining sexual maturity in male animals, which can be assessed by evaluating age, body weight, and testicular volume. Generally, male macaques are considered sexually mature when they are over 4.5 years old, weigh more than 4.5 kg, and have a single testis volume exceeding 10 mL or a combined bilateral testis volume greater than 20 mL. Confirming pregnancy is another critical concern. Ultrasound examination is used to detect distinct gestational sacs, making it essential for experienced clinical veterinarians to establish a robust B-scan methodology. In summary, for reproductive toxicity studies, veterinarians should develop tailored observation, identification, and evaluation protocols based on the reproductive system characteristics of different animal species to ensure successful mating—a key factor in the smooth progression of reproductive toxicity testing.

Key words: Nonclinical, Reproductive toxicity study, Animal gestation, Key technology