Smithers Avanza White Paper Reproductive and Developmental Toxicity Testing

Toxicokinetics (TK) is generation of kinetic data for systemic exposure and toxicity assessment of the drug. These studies help us to estimate the observed toxicity to that dose. TK evaluation is very important in drug development phase in both regulatory and scientific perspective. There are several guidelines to conduct TK study in animals recommended by regulatory bodies (OECD). TK evaluation is useful in selection of dose, dosing form, alternative dosing route, evaluation of toxicological mechanism, and also used for the setting safe dose level in clinical phases. This TK studies also used to reduces the animal number (replacement, reduction and refinement). On the other hand, TK data are practically used for the purpose of drug discovery such as lead-optimization and candidate-selection. Email:marketing@medicilon.com.cn web:www.medicilon.com

Prior to registration of chemicals, a series of guideline-required toxicological studies are conducted in a number of mammalian species to demonstrate their safety to humans. A number of guidelines have outlined studies to assess toxicity of chemicals to reproductive and developmental stages, from fertilization through adulthood, and in some cases to second generation. Traditionally, a series of animal studies are conducted to assess the developmental and reproductive risk of test chemicals to adults and developing progeny on specific life stage. For example, ICH guideline S5(R2) requires three separate studies to determine effects of test chemicals on fertility and early embryonic development, peri- and postnatal development including maternal function, and embryo fetal development. A similar approach is taken by other regulatory guidelines with the exception of OECD 443 (Extended One-Generation Reproductive Toxicity Study, EOGRTS). The EOGRTS design makes the toxicity testing more progressive, comprehensive, and efficient by assessing the key toxicity profiles across life stages at relevant doses in minimum number of animals, combining studies/evaluations across multiple life stages into one study, and proposing a tiered testing approach for flexibility based on the needs and available data. EOGRTS determines toxicity during preconception, development of embryo/fetus and newborn, adolescence, and adults with specific emphasis on the nervous, immunological, and endocrine systems, EOGRTS also assesses maternal and paternal toxicity. However, the EOGRTS guideline is difficult to follow, the criteria for selecting the highest dose is somewhat unclear, and monitoring of the systemic dose during the course of the study for better interpretation and human relevance of the animal data is not included. This whitepaper discusses ways to make the EOGRTS guideline easier to follow; suggesting procedures to select relevant doses, and monitor the systemic exposure at different life stages for better interpretation of the data. An important aspect of safety assessment of test chemicals is determining their potential reproductive and developmental toxicity. There are several guideline studies routinely conducted, primarily in rats, to determine immediate and latent effects of test chemical exposure. Assessment of toxicity to reproduction includes possible effects of test chemicals on fertility, embryonic and fetal development, pre- and postnatal development, and maternal function. Traditionally, separate reproductive/developmental toxicity studies are conducted to evaluate these effects. Determining Effects on Fertility. Effects of test chemicals on fertility is determined by administration to cycling females and sexually mature males starting at least two weeks prior to mating. In some cases, males are exposed for four or even ten weeks, covering a full spermatogenic cycle, prior to mating to evaluate functional effects on male fertility. Extended exposure of males prior to mating may be necessary when results of other repeat-dose general toxicity studies of at least 4-week duration remain equivocal for effects on male reproductive organs. Administration continues in males during mating and in females through the completion of implantation for the assessment of the impact of test chemicals on maturation of gametes, fertility, mating behavior, and implantation. Determining Effects on Embryo-Fetal Development. Effects of test chemicals on embryo-fetal development including embryo/fetal death, altered growth and structural changes are determined by administering chemicals to females starting from implantation to the day prior to parturition. This study is conducted in one rodent (rat) and one nonrodent (rabbit) species to identify susceptibility across species. Animals are exposed to test chemicals starting from implantation, gestational day 6-7 through the day prior to parturition. Females are sacrificed one day prior to parturition and fetuses are examined for viability and abnormalities. Determining Effects on Pre- and Postnatal Development. Evaluation of the effects of test chemicals on perinatal and postnatal development is conducted in females by dosing them from implantation through the end of lactation (ICH, 2005). This study is designed to evaluate impact of test chemicals on birth and development including length of gestation, dystocia, postnatal mortality and impaired maternal behavior. Randomly selected F1 pups are monitored up to the age of sexual maturity, paired within dose groups to determine fertility. Determining Effects on Reproduction. The OECD 421 and OPPTS 870.3550 guidelines (OECD, 1995; USEPA, 2000a) are incorporated in the three ICH evaluations (ICH, 2005) described above. In these studies cycling females and sexually mature males are dosed with a chemical of interest starting at least two weeks prior to mating through postnatal day (PND) three and sacrificed on PND 4. In these studies, effects of test chemicals on maternal behavior, length of gestation, dystocia, number and sex of pups, stillbirths, live births, runts, presence of gross abnormalities, and abnormal behavior in pups on PND 4 are determined. A modified study design is outlined in OECD 422 and OPPTS 870.3650 combining repeat dose general toxicity with the reproduction/ developmental toxicity endpoints (OECD, 1996; USEPA, 2000b). Males and females in OECD 422 study are dosed and cared for as described for OECD 421; however, additional parameters related to general toxicity endpoints are added. Two additional guidelines determine effects of chemicals on reproduction in one- and two-generation studies. In one-generation study, sexually mature males are exposed to the chemicals for ten weeks prior to and through mating. Cycling females (5-9 weeks of age) are exposed to the test chemical for two weeks prior to mating through weaning of the offspring. In the one-generation study, effects of chemicals on fertility, maternal behavior, length of gestation, dystocia, number and sex of pups, stillbirths, live births, runts, presence of gross abnormalities, and abnormal behavior in pups are assessed.The two-generation reproduction toxicity study is considered the most comprehensive design to assess reproductive toxicity and the effects of chemicals on the reproductive performance of the F1 parents. Parental males and females (5-9 weeks of age) are exposed to a test chemical for at least ten weeks prior to mating and exposure continues in males during mating and in females through weaning of F1 offspring. Actual dosing of offspring starts at weaning; however, they may get exposure prior to weaning through milk during nursing and/or when chemicals are provided in diet or drinking water. The offspring from each dose group are mated after reaching sexual maturity and exposure to test chemical for at least 10 weeks to produce the second generation of animals. Dosing of F1 males continues through mating and of females through weaning of F2 offspring. The two-generation reproduction toxicity study assesses the effects of chemicals on fertility, maternal behavior, length of gestation, dystocia, number and sex of pups, stillbirths, live births, runts in parental and F1 generation and the presence of gross abnormalities and abnormal behavior in F1 and F2 animals. The NTP’s Modified One-Generation Study design determines effects of test chemicals on animals from gestation through weaning of F2 animals (Foster, 2014); however, no formal guideline document exists. The difference between NTP design and other approved guidelines include retention of multiple pups per litter rather than 1 pup/sex/litter/dose group and premating treatment of males for full 10 weeks.