Regulatory Experience Assessing the Carcinogenic Potential of a Monoclonal Antibody Inhibiting PCSK9, Bococizumab, Including a 2-Year Carcinogenicity Study in Rats.

Bococizumab is an anti-PCSK9 monoclonal antibody that was intended for the treatment of hypercholesterolemia. After reviewing the 6-month rat toxicity study data, in which there was a low spontaneous tumor incidence, unrelated to bococizumab administration, the U.S. FDA granted a carcinogenicity waiver request based on a weight-of-evidence assessment of low carcinogenic risk. Subsequently, after reviewing 6-month rat toxicity study data from another anti-PCSK9 antibody, RN317, with a similar low tumor incidence (unrelated to RN317), the U.S. FDA rescinded the bococizumab carcinogenicity study waiver and requested a full 2-year rat carcinogenicity study be conducted. The resulting 2-year carcinogenicity study demonstrated no bococizumab-related increase in tumors, confirming the weight-of-evidence evaluation and alleviating concerns regarding the carcinogenic potential. Here we report the scientific and regulatory background that led to the request for a rat carcinogenicity study, the feedback on the design of the carcinogenicity study, and the results from this study which affirmed the original weight-of-evidence assessment of low carcinogenic risk.

Comparing rat and rabbit embryo-fetal developmental toxicity data for 379 pharmaceuticals: on systemic dose and developmental effects.

A database of embryo-fetal developmental toxicity (EFDT) studies of 379 pharmaceutical compounds in rat and rabbit was analyzed for species differences based on toxicokinetic parameters of area under the curve (AUC) and maximum concentration (Cmax) at the developmental lowest adverse effect level (dLOAEL). For the vast majority of cases (83% based on AUC of n = 283), dLOAELs in rats and rabbits were within the same order of magnitude (less than 10-fold different) when compared based on available data on AUC and Cmax exposures. For 13.5% of the compounds the rabbit was more sensitive and for 3.5% of compounds the rat was more sensitive when compared based on AUC exposures. For 12% of the compounds the rabbit was more sensitive and for 1.3% of compounds the rat was more sensitive based on Cmax exposures. When evaluated based on human equivalent dose (HED) conversion using standard factors, the rat and rabbit were equally sensitive. The relative extent of embryo-fetal toxicity in the presence of maternal toxicity was not different between species. Overall effect severity incidences were distributed similarly in rat and rabbit studies. Individual rat and rabbit strains did not show a different general distribution of systemic exposure LOAELs as compared to all strains combined for each species. There were no apparent species differences in the occurrence of embryo-fetal variations. Based on power of detection and given differences in the nature of developmental effects between rat and rabbit study outcomes for individual compounds, EFDT studies in two species have added value over single studies.

Comparison of rat and rabbit embryo-fetal developmental toxicity data for 379 pharmaceuticals: on the nature and severity of developmental effects.

Regulatory non-clinical safety testing of human pharmaceuticals typically requires embryo-fetal developmental toxicity (EFDT) testing in two species (one rodent and one non-rodent). The question has been raised whether under some conditions EFDT testing could be limited to one species, or whether the testing in a second species could be decided on a case-by-case basis. As part of a consortium initiative, we built and queried a database of 379 compounds with EFDT studies (in both rat and rabbit animal models) conducted for marketed and non-marketed pharmaceuticals for their potential for adverse developmental and maternal outcomes, including EFDT incidence and the nature and severity of adverse findings. Manifestation of EFDT in either one or both species was demonstrated for 282 compounds (74%). EFDT was detected in only one species (rat or rabbit) in almost a third (31%, 118 compounds), with 58% (68 compounds) of rat studies and 42% (50 compounds) of rabbit studies identifying an EFDT signal. For 24 compounds (6%), fetal malformations were observed in one species (rat or rabbit) in the absence of any EFDT in the second species. In general, growth retardation, fetal variations, and malformations were more prominent in the rat, whereas embryo-fetal death was observed more often in the rabbit. Discordance across species may be attributed to factors such as maternal toxicity, study design differences, pharmacokinetic differences, and pharmacologic relevance of species. The current analysis suggests that in general both species are equally sensitive on the basis of an overall EFDT LOAEL comparison, but selective EFDT toxicity in one species is not uncommon. Also, there appear to be species differences in the prevalence of various EFDT manifestations (i.e. embryo-fetal death, growth retardation, and dysmorphogenesis) between rat and rabbit, suggesting that the use of both species has a higher probability of detecting developmental toxicants than either one alone.

Drug-induced Skin Lesions in Cynomolgus Macaques Treated with Metabotropic Glutamate Receptor 5 (mGluR5) Negative Allosteric Modulators.

Three orally administered metabotropic glutamate receptor 5 (mGluR5) negative allosteric modulators caused skin lesions consistent with delayed type-IV hypersensitivity in cynomolgus macaques in 2- and 12-week toxicity studies. Several monkeys developed macroscopic skin lesions in multiple locations after 8 to 9 days of dosing; the most prominent effects involved the genital region of males and generalized erythema occurred in both sexes. Microscopic lesions occurred in both clinically affected and unaffected areas and were characterized by lymphocytic interface inflammation, subepidermal bullae, and individual keratinocyte vacuolation/necrosis. In the 12-week study, clinical effects in 2 animals resolved with continued dosing, whereas in others the inflammatory process progressed with 1 female exhibiting systemic lymphocytic inflammation in multiple tissues. The inflammatory infiltrate consisted of CD3 and CD4 positive T lymphocytes with minimal CD68 positive macrophages and only rare CD8 positive T lymphocytes. A subset of animals given a dosing holiday was subsequently rechallenged with similar lesions developing but with a more rapid clinical onset. These skin lesions were consistent with type-IV delayed hypersensitivity with some features comparable to bullous drug eruptions in humans. A relationship between these findings and the intended mode of action for these compounds could not be ruled out, given the occurrence across different chemotypes.

Decreased maternal and fetal cholesterol following maternal bococizumab (anti-PCSK9 monoclonal antibody) administration does not affect rat embryo-fetal development.

Bococizumab is a humanized monoclonal IgG2Δa antibody against proprotein convertase subtilisin/kexin type 9 (PCSK9) for the treatment of hyperlipidemia. The evaluation of potential effects on embryo-fetal development was conducted in the rat. In a pharmacokinetic/pharmacodynamic study bococizumab was administered intravenously to pregnant Sprague-Dawley (SD) rats (n = 8/group) at 0, 10, 30, and 100 mg/kg during organogenesis. Maternal and fetal bococizumab, total cholesterol and HDL concentrations were determined. Bococizumab was well tolerated and there were no effects on ovarian or uterine parameters. Maternal and fetal bococizumab exposure increased with increasing dose, with a corresponding dose-dependent decrease in fetal cholesterol levels. Maternal cholesterol levels were decreased significantly, with reductions that were of a similar magnitude regardless of dose. In the definitive embryo-fetal development study bococizumab was administered to pregnant SD rats (n = 20/group) at 0, 10, 30, and 100 mg/kg and no adverse maternal or developmental effects were observed up to 100 mg/kg. These studies have provided an appropriate and relevant safety assessment of bococizumab in pregnant rats to inform human risk assessment, demonstrating no adverse effects on embryo-fetal development at magnitudes greater than anticipated clinical exposure and in the presence of maximal reductions in maternal cholesterol and dose-dependent reductions in fetal cholesterol.

Reversible effects on female rat fertility with abrocitinib, a Janus kinase 1 inhibitor.

BACKGROUND: Abrocitinib is a Janus kinase (JAK) 1 selective inhibitor approved for the treatment of atopic dermatitis. Female reproductive tissues were unaffected in general toxicity studies, but an initial female rat fertility study resulted in adverse effects at all doses evaluated. A second rat fertility study was conducted to evaluate lower doses and potential for recovery. METHODS: This second study had 4 groups of 20 females each administered abrocitinib (0, 3, 10, or 70 mg/kg/day) 2 weeks prior to cohabitation through gestation day (GD) 7. In addition, 2 groups of 20 rats (0 or 70 mg/kg/day) were dosed for 3 weeks followed by a 4-week recovery period before mating. All mated females were evaluated on GD 14. RESULTS: No effects were observed at ≤10 mg/kg/day. At 70 mg/kg/day (29x human exposure), decreased pregnancy rate, implantation sites, and viable embryos were observed. All these effects reversed 4 weeks after the last dose. CONCLUSIONS: Based on these data and literature on the potential role of JAK signaling in implantation, we hypothesize that these effects may be related to JAK1 inhibition and, generally, that peri-implantation effects such as these, in the absence of cycling or microscopic changes in nonpregnant female reproductive tissues, are anticipated to be reversible.

Juvenile toxicity study of PF-07256472/recifercept, a recombinant human soluble fibroblast growth factor receptor 3, in 2-3-month-old cynomolgus monkeys.

Achondroplasia is an autosomal disorder caused by point mutation in the gene encoding fibroblast growth factor receptor 3 (FGFR3) and resulting in gain of function. Recifercept is a potential disease modifying treatment for achondroplasia and functions as a decoy protein that competes for ligands of the mutated FGFR3. Recifercept is intended to restore normal bone growth by preventing the mutated FGFR3 from negative inhibitory signaling in pediatric patients with achondroplasia. Here we evaluated the potential effects of twice weekly administration of recifercept to juvenile cynomolgus monkeys (approximately 3-months of age at the initiation of dosing) for 6-months. No adverse effects were noted in this study, identifying the high dose as the no-observed-adverse-effect-level and supporting the use of recifercept in pediatric patients from birth. Considering that juvenile toxicity studies in nonhuman primates are not frequently conducted, and when they are conducted they typically utilize animals ≥9 months of age, this study demonstrates the feasibility of executing a juvenile toxicity study in very young monkeys prior to weaning.

Object discrimination reversal as a method to assess cognitive impairment in nonhuman primate enhanced pre- and postnatal developmental (ePPND) studies: statistical power analysis.

An important aspect of the enhanced pre- and postnatal developmental (ePPND) toxicity study in nonhuman primates (NHP) is that it combines in utero and postnatal assessments in a single study. However, it is unclear if NHP ePPND studies are suitable to perform all of the evaluations incorporated into rodent PPND studies. To understand the value of including cognitive assessment in a NHP ePPND toxicity study, we performed a power analysis of object discrimination reversal task data using a modified Wisconsin General Testing Apparatus (ODR-WGTA) from two NHP ePPND studies. ODR-WGTA endpoints evaluated were days to learning and to first reversal, and number of reversals. With α = 0.05 and a one-sided t-test, a sample of seven provided 80% power to predict a 100% increase in all three of the ODR-WGTA endpoints; a sample of 25 provided 80% power to predict a 50% increase. Similar power analyses were performed with data from the Cincinnati Water Maze (CWM) and passive avoidance tests from three rat PPND toxicity studies. Groups of 5 and 15 in the CWM and passive avoidance test, respectively, provided 80% power to detect a 100% change. While the power of the CWM is not far superior to the NHP ODR-WGTA, a clear advantage is the routine use of larger sample size, with a group of 20 rats the CWM provides ~90% power to detect a 50% change. Due to the limitations on the number of animals, the ODR-WGTA may not be suitable for assessing cognitive impairment in NHP ePPND studies.

Reproductive toxicity assessment of sunitinib, a multitargeted receptor tyrosine kinase inhibitor, in male and female rats.

BACKGROUND: Sunitinib (SUTENT, Pfizer Inc., New York, NY) is a multitargeted inhibitor of selected receptor tyrosine kinases, which produces an antiproliferative and antiangiogenic effect by blocking pathways fundamental to tumor growth and survival. We investigated the effects of sunitinib on male and female fertility and early embryonic development in the rat. METHODS: In the female fertility and early embryonic development phase, untreated males were paired with treated females dosed at 0 (control), 0.5, 1.5, and 5 mg/kg/day from 14 days premating, through mating, to gestation day 7. In the male fertility phase, the same males were then treated 58 days at doses of 0 (control), 1, 3, and 10 mg/kg/day, mated with untreated females, with continued daily dosing for a total of 74 days. RESULTS: There was no systemic toxicity- or treatment-related effects on fertility in female rats. Females exposed at 5 mg/kg/day had an increase in the number of early resorptions with associated decrease in viable embryos. In the males, body weight and food consumption were decreased at 10 mg/kg/day compared to the controls. Male reproductive capacity, as assessed by copulation, fertility, and conception indices, was not impacted at any dose level. Sperm morphology, concentration, and motility were also unaffected by treatment. CONCLUSIONS: There were no effects on male reproduction. An increase in corpora lutea and an increase in early resorptions with associated reduction in viable embryos was noted in the females dosed 5 mg/kg/day. Sunitinib at doses up to 1.5 and 10 mg/kg/day had no effects on female and male reproduction, respectively.

Developmental toxicity study of lersivirine in mice.

Lersivirine is a second-generation nonnucleoside reverse transcriptase inhibitor undergoing clinical development for the treatment of HIV-1. An embryo-fetal developmental toxicity study was performed to evaluate the maternal and developmental toxicity of lersivirine in pregnant mice. Mated Crl:CD1(ICR) mice were administered 0, 150, 350, and 500 mg/kg lersivirine once daily by oral gavage on gestation days 6 to 17, followed by cesarean section on gestation day 18. The first 2 days of dosing for the high-dose group were done at 250 mg/kg to allow induction of hepatic metabolizing enzymes, after which the dose was increased to 500 mg/kg/day. This dosing paradigm allowed for maintenance of exposure in the high-dose group despite the considerable autoinduction that occurs in rodents following lersivirine treatment. Lersivirine did not cause an increase in external, visceral, or skeletal malformations. Intrauterine growth retardation, demonstrated by reduced fetal body weights and increased variations associated with delayed skeletal ossification, was noted at 350 and 500 mg/kg/day. The results of these studies indicate that lersivirine is not teratogenic in mice.

Sensitive windows of skeletal development in rabbits determined by hydroxyurea exposure at different times throughout gestation.

The critical periods of axial skeletal development in rats and mice have been well characterized, however the timing of skeletal development in rabbits is not as well known. It is important to have a more precise understanding of this timing of axial skeletal development in rabbits due to the common use of this species in standard nonclinical studies to assess embryo-fetal developmental toxicity. Hydroxyurea, a teratogen known to induce a variety of fetal skeletal malformations, was administered to New Zealand White rabbits as a single dose (500 mg/kg) on individual days during gestation (gestation day, GD 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, or 19) and fetal external, visceral, and skeletal morphology was examined following cesarean sections on GD 29. A wide range of fetal skeletal effects was observed following hydroxyurea treatment, with a progression of malformations from anterior to posterior structures over time, as well as from proximal to distal structures over time. The sensitive window of axial skeletal development was determined to be GD 8 to 13, while disruption of appendicular and cranio-facial skeletal development occurred primarily from GD 11 to 16 and GD 11 to 12, respectively. The results of this study provide a better understanding of the critical developmental window for different segments of the rabbit skeleton, which will aid in the design of window studies to investigate teratogenicity in rabbits.

Developmental toxicity of lersivirine in rabbits when administered throughout organogenesis and when limited to sensitive windows of axial skeletal development.

BACKGROUND: Lersivirine is a second-generation nonnucleoside reverse transcriptase inhibitor undergoing clinical development for the treatment of human immunodeficiency virus-1. An embryo-fetal development study was performed to evaluate the potential for maternal and developmental toxicity of lersivirine. METHODS: Pregnant New Zealand White rabbits were administered 0, 100, 250, and 500 mg/kg lersivirine by oral gavage once daily on gestation days (GDs) 7 to 19, followed by cesarean section on GD 29 and fetal evaluation. RESULTS: Maternal toxicity was noted at all dose levels (decreased food consumption and body weight gain), with fetal toxicity at 500 mg/kg (decreased fetal weights, increased postimplantation loss). Equivocal findings for axial skeletal malformations were observed in three fetuses at 500 mg/kg. To better understand if these malformations were related to treatment with lersivirine, a follow-up rabbit embryo-fetal development study was performed with 1000 mg/kg/day lersivirine (500 mg/kg BID, 12-hr interdose interval) for two different 3-day windows, GDs 8 to 10 or GDs 11 to 13, which represent the sensitive windows of axial skeletal development in rabbits. Control rabbits were administered vehicle following the same dosing regimen from GDs 8 to 13. Cesarean sections were performed on GD 29, and fetal skeletons were examined for the potential of lersivirine to cause skeletal malformations in rabbits. At maternal exposure levels higher than the initial study, lersivirine did not induce fetal skeletal malformations when administered in the sensitive windows of axial skeletal development. CONCLUSION: The results of these studies indicate that lersivirine did not exhibit any evidence of teratogenicity in rabbits.

Isoflurane reduces motile sperm counts in the Sprague-Dawley rat.

Animal and care use practices are constantly evolving. These can have unexpected consequences on the data collected from such procedures. One example is the recent change in our animal facility, based on recommendations from the Newcastle Consensus Meeting on Carbon Dioxide Euthanasia of Laboratory Animals, from CO(2) to isoflurane for anesthesia. The current study was conducted to determine the effects of isoflurane on sperm motility, as compared to two different CO(2) euthanasia procedures. Sperm motility was evaluated after euthanasia by a standard 5-minute CO(2) euthanasia procedure, an extended 10-minute CO(2) euthanasia procedure, or by isoflurane anesthesia followed by exsanguination (iso/exsanguination). The 5-minute CO(2) procedure produced sperm motility of 94.3 ± 1.7% motile sperm with 65.6 ± 16.8 sperm/field. By comparison, iso/exsanguination reduced that count to 3.3 ± 2.3 sperm/field and only 60.7 ± 32.0% motile sperm. The reduction in sperm motility after iso/exsanguination appeared to have been due primarily to the reduction in the number of sperm expelled from the vas deferens (3.3), compared to that after 5-minute CO(2) (65.6). This reduction in number of sperm available for evaluation, in the presence of a constant level of background debris, which was counted by the computer optics system as nonmotile sperm, resulted in an apparent reduction in motility. Using the extended 10-minute CO(2) procedure produced sperm data in between the other two extremes: 77.6 ± 36.1% motile sperm with 34.6 ± 28.3 sperm/field. The results of this study support the hypothesis that isoflurane inhibits contraction of the smooth muscle of the vas deferens, resulting in a decreased number of expelled sperm. Given these findings, it is important that careful consideration be taken to select an appropriate anesthesia/euthanasia method.

The Postnatal Resolution of Developmental Toxicity Induced by Pharmacological Diacylglycerol Acyltransferase 2 (DGAT2) Inhibition During Gestation in Rats.

Ervogastat (PF-06865571) is a small molecule diacylglycerol acyltransferase 2 (DGAT2) inhibitor being developed for the oral treatment of nonalcoholic steatohepatitis (NASH) with liver fibrosis. DGAT2 is a key enzyme in triglyceride synthesis in tissues and in regulating energy metabolism. Fertility and developmental toxicity studies with ervogastat were conducted in female rats and rabbits. There were no effects on female rat fertility or rabbit embryo-fetal development. Administration of ervogastat to pregnant rats during organogenesis reduced fetal weight and caused higher incidences of bent bones in fetuses that were shown to resolve by postnatal day 28 and were therefore considered to be transient variations secondary to developmental delay. Extended dosing in rats through the end of gestation and lactation (pre- and post-natal development study) caused impaired skin development, reduced offspring viability, and growth retardation. The spectrum of developmental effects in rats is consistent with the intended pharmacology (altered triglyceride metabolism) and the transient nature of the skeletal findings, along with the late gestational window of sensitivity for the effects on skin barrier development, reduce the concern for potential adverse developmental effects following unintended early gestational exposure to ervogastat in humans where treatment can be discontinued once pregnancy is determined.

Histologic and cytologic detection of endocrine and reproductive tract effects of exemestane in female rats treated for up to twenty-eight days.

The objective of this study was to determine the shortest period of time necessary to detect histologic evidence of estrous cycle disruption in Sprague-Dawley rats treated for up to 28 days with the aromatase inhibitor exemestane at 1,000 mg/kg. Rats were evaluated on day 5, 8, 15, or 29. Vaginal mucification, uterine and cervical epithelial atrophy, uterine luminal epithelial vacuolation, decreased uterine granulocytes, and hypertrophy/hyperplasia of mammary ducts and alveoli were noted by day 5 and persisted throughout the study. From day 8 to day 29, absence of recent basophilic corpora lutea, increased atresia of antral follicles, interstitial cell hyperplasia, and increased luteinized follicles were present in the ovaries of treated rats. Vaginal smears detected persistent diestrus, confirming estrous cycle disruption between days 5 and 8. Ovary and uterine weights were largely unaffected. Serum hormone levels were not useful due to the study design employed. Other effects of exemestane included decreased adrenal weights and decreased cell size in both the adrenal zona fasciculata and the pituitary pars distalis. While early histologic changes were evident on day 5, only after 8 days of treatment were findings considered sufficient to clearly identify exemestane-induced estrous cycle disruption using microscopy alone.

Nonclinical support of pediatric drug development in a global context: an industry perspective.

The earlier inclusion of children into clinical trials has challenged toxicologists to develop nonclinical strategies to support these trials early in the drug development process, and the routine practise of global development strategies (i.e., concomitant development and filing in multiple geographical regions) adds another complication. Ideally, one would like to develop a stagey that would meet regulatory requirements from all regions. This presentation illustrated the challenges faced in developing a strategy regarding the need to perform a toxicity study in juvenile animals and the design of any necessary study that will receive global regulatory agreement.

Toxicity study in juvenile rats with the α4ß2 nicotinic acetylcholine receptor partial agonist CP-601,927.

BACKGROUND: CP-601927 is a selective α(4) ß(2) nicotinic acetylcholine receptor (nAChR) partial agonist. The objective of this study was to assess the potential effects persisting into adulthood when CP-601,927 was administered to neonatal/juvenile rats. Since the juvenile toxicity study was being performed early in the development program and this study would represent the longest dosing period yet evaluated, the study design incorporated standard endpoints typically evaluated in a general toxicity screening study. METHODS: CP-601,927 was administered to Sprague-Dawley rats from postnatal day (PND) 7-70 by oral gavage at doses of 0.3, 1, or 3 mg/kg. During treatment animals were evaluated for growth, development, and sexual maturation. At the end of the treatment period general toxicity screening endpoints were collected (e.g., organ weights, histology, clinical chemistry). Following a 2-week latency period, animals were evaluated for CNS function in a comprehensive behavioral training battery consisting of a functional observational battery, motor activity, acoustic startle response, and learning and memory evaluations. Reproductive competency was evaluated by mating treated rats and allowing pregnant dams to deliver and rear their litters until PND 10. RESULTS AND CONCLUSIONS: Treatment-related findings included the death of 2 males receiving 3 mg/kg CP-601,927, and transient reductions in body weight for both males and females during the third week of dosing which quickly recovered to control levels. The only treatment-related alteration in behavior was decreased motor activity, which occurred only in females at the highest dose tested. CP-601,927 had no effect on acoustic startle response, learning and memory, sexual maturation, reproductive capacity, or general toxicity endpoints.

Toxicology and toxicokinetics of oral pantoprazole in neonatal and juvenile dogs.

BACKGROUND: Pantoprazole is an irreversible inhibitor of H(+) /K(+) adenosine triphosphatase proton pump. This study encompassed the period of postnatal stomach development to determine whether immature animals are uniquely sensitive to progression of PPI-induced enterochromaffin-like cell hyperplasia. METHODS: Pantoprazole was administered to beagle dogs at 3, 10, or 30 mg/kg/day (10/sex/group) from PND 1 for 13 weeks, subsets of animals had a 13-week recovery period. Clinical signs, body weights, growth, clinical chemistry, and neurobehavioral endpoints were assessed. Selected organs were weighed and histologically examined. RESULTS: There were no effects on body weights, growth, landmarks of physical and reproductive development, or sensory and neurobehavioral function. Cholesterol and triglyceride levels were increased at 10 and 30 mg/kg/day, but resolved during the recovery period. Stomach weight was increased at all doses, but after recovery the differences in stomach weights resolved for females although male stomach weights remained slightly increased. Pantoprazole-related microscopic findings in the stomach consisted of increased mucosal height, glandular necrosis, and glandular dilation at all doses; and ECL cell hyperplasia, parietal cell vacuolation, and atrophy of chief cells are noted at 10 and/or 30 mg/kg/day. There was a partial recovery of these microscopic changes indicated by a decreased incidence and/or severity of increased mucosal height, glandular necrosis, ECL cell hyperplasia, and chief cell atrophy, and complete resolution of other microscopic observations. CONCLUSION: Pantoprazole administered to beagles from PND 1 for 13 weeks resulted in findings similar to those in adult dogs and juvenile dogs, which showed no increase in severity or progression of ECL hyperplasia.

Impaired reproduction in adult male, but not female, rats following juvenile treatment with the aromatase inhibitor, exemestane.

BACKGROUND: Exemestane is an irreversible steroidal inhibitor of cytochrome-P450 aromatase required for estrogen synthesis. The safety of the drug in the pediatric population, particularly in males, has not previously been evaluated. Given the increased interest in treating children with aromatase inhibitors, we undertook a study in rats to assess the potential for exemestane to alter reproductive development and function when administered to juveniles. METHODS: Male and female rats were treated with exemestane at doses anticipated to produce exposures approximately 2- and 35-fold the expected clinical plasma exposure in young adult males during the period of reproductive maturation. After maturation, treated rats were mated to evaluate the potential impact on reproductive function. RESULTS AND CONCLUSION: There were no effects on sexual maturation in either sex or on female reproductive function. Treatment of juvenile male rats caused increased cohabitation time and decreased copulation rates; pregnancy rates and litter size were not affected in rats that mated. Decreased testis (10-15%) and epididymis (20-30%) weights, and decreased Sertoli cell numbers were noted at all doses. This indicates that exemestane can reduce Sertoli cell proliferation during maturation. The sensitive window for this effect is expected to be limited to the period of Sertoli cell proliferation, which is completed by around postnatal day 15 in rats and before puberty in humans. Treatment beginning at a later time relative to the window for Sertoli cell proliferation or for a longer duration is not expected to have additional adverse effect as the effect was not shown to be degenerative.

Knockout mouse models are predictive of malformations or embryo-fetal death in drug safety evaluations.

Traditionally, understanding potential developmental toxicity from pharmaceutical exposures has been based on the results of ICH guideline studies in two species. However, support is growing for the use of weight of evidence approaches when communicating the risk of developmental toxicity, where the intended pharmacologic mode of action affects fundamental pathways in developmental biology or phenotypic data from genetically modified animals may increasingly be included in the overall assessment. Since some concern surrounds the use of data from knockout (KO) mice to accurately predict the risk for pharmaceutical modulation of a target, a deeper understanding of the relevance and predictivity of adverse developmental effects in KO mice for pharmacological target modulation is needed. To this end, we compared the results of embryo-fetal development (EFD) studies for 86 drugs approved by the FDA from 2017 to 2019 that also had KO mouse data available in the public domain. These comparisons demonstrate that data from KO mouse models are overall highly predictive of malformations or embryo-fetal lethality (MEFL) from EFD studies, but less so of a negative outcome in EFD studies. This information supports the use of embryo-fetal toxicity data in KO models as part of weight of evidence approaches in the communication of developmental toxicity risk of pharmaceutical compounds.