6 Reproductive and Developmental

: Apical tests for reproductive or developmental toxicity assess the potential for a compound to affect any of the thousands of steps involved in making gametes and in the successful development of a fully functional offspring. Conventionally, this is thought to require at least 21 days for rodent female reproduction and for development, and close to 70 days for spermatogenesis. Short-term tests can evaluate some subset of these processes, so multiple tests must be used. The best use of in vitro tests currently is for evaluating a series of structurally-related molecules with an endpoint which reports a specific type of toxicity known to affect at least some members of that class. Because no in vitro tests have been found to correlate well with the breadth of reproductive and developmental toxicity observed in vivo, test-tube or culture-based tests should not be used as a first-pass, general screen for these effects. Even though short-term (21 or 28 day) in vivo studies will miss a variety of transgenerational effects, they remain the best means of identifying the more potent developmental and reproductive toxicants. We will review the rationale for the current versions of definitive tests for reproductive and developmental toxicity, the approaches taken in reducing the duration of these tests and documenting what is gained and lost by such alternatives. Finally, we will address in vitro and genotoxicity tests, and review briefly their advantages and shortcomings, and their relationship to in vivo developmental/reproductive toxicity results. To be explicit, this consideration moves from the best to the worst, in terms of confidence in the information generated. It is the feeling of this group that good screens for toxicity evaluate as much of a process at once as possible. This is the standard against which we will judge the value of a potential screen. ❚ DEFINITIVE TESTS FOR REPRODUCTIVE TOXICITY Definitive tests for reproduction and development are, essentially, set up to maximize confidence in a negative result. The definitive in vivo tests are apical, that is, they evaluate the integrated function of the entire system in one test. The benefit: if the results are negative, then one has reasonable assurance that there has been no effect anywhere in the process. The down-side is that identifying the location of a lesion or adverse effect can be slightly more time-consuming when starting from apical data. As apical tests, these designs expose the entire process to the toxicant in question. Although this also depends on the pharmacokinetics of the compound in question, some default durations have been evolved, based on biology: For female rodent reproduction, the adult females should be exposed for 3-4 weeks prior to conception, as this exposes 4 or 5 estrous cycles of 5 or 4 days, respectively. In practice, the females may be exposed for 2-3 times this length of time, but 21-28 days is generally considered the minimum. For spermatogenesis, the concept is to expose the gamete from a spermatogonium until it is ejaculated (again, the concept of exposing all stages of the process to the toxicant). In the rat, this is approximately 60-70 days. In practice, this often winds up being a 90 day exposure. The field of developmental toxicology is in transition. In the past, the concept held that most terata resulted from exposures during organogenesis. In the rat, this begins about 6 days after mating, and continues until gdl5 (gestation day 15, out of a 22 day gestation period). Recent evidence shows that significant effects on the fetus can occur shortly after fertilization, and well before implantation (which occurs approximately on gd6), so many newer studies begin exposure of the pregnant female the day after mating, and continue until the day before delivery, when she