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 non-alcoholic 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.

Maternal immunization with Group B Streptococcus six-valent polysaccharide conjugate vaccine supported by lack of toxicity in rat and rabbit fertility and developmental toxicity studies

A maternal Group B Streptococcus (GBS) six-valent polysaccharide conjugate vaccine (GBS6) is being developed to protect neonates and infants up to 3 months of age through passive transfer of antibodies from the mother to the infant. Fertility and developmental toxicity studies were conducted in female Sprague Dawley rats and New Zealand White rabbits with GBS6 (20 μg capsular polysaccharide/serotype formulated with or without AlPO₄ , the highest clinical dose). Females were administered the full human dose of the GBS6 formulation intramuscularly twice prior to mating and twice during gestation, to ensure that high antibody levels were maintained throughout gestation and lactation. Approximately, half of the rats and rabbits were evaluated at the end of gestation, and the remainder were evaluated at the end of lactation. Maternal blood for GBS6 serology, to measure antibody titers to the GBS6 antigens, was collected prior to the first dose, prior to mating, and at each necropsy. Blood for serology was also collected from offspring at the end of gestation and lactation. In both species, there was no evidence of vaccine-related effects on fertility, embryo-fetal development, or postnatal development of the offspring, supporting regulatory guidance that single-species evaluation would have been sufficient. Functional serum antibodies to all six serotypes in the vaccine were confirmed in maternal animals and functional serum antibodies to one or more of the six serotypes was also confirmed in some rat offspring and most of the rabbit offspring. The results of these studies supported the safety of GBS6 vaccine administration to pregnant women.

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.

Inhibition of ACC causes malformations in rats and rabbits: comparison of mammalian findings and alternative assays

Acetyl-CoA carboxylase (ACC) is an enzyme within the de novo lipogenesis (DNL) pathway and plays a role in regulating lipid metabolism. Pharmacologic ACC inhibition has been an area of interest for multiple potential indications including oncology, acne vulgaris, metabolic diseases such as type 2 diabetes mellitus, and non-alcoholic fatty liver disease/non-alcoholic steatohepatitis. A critical role for ACC in de novo synthesis of long-chain fatty acids during fetal development has been demonstrated in studies in mice lacking Acc1, where the absence of Acc1 results in early embryonic lethality. Following positive predictions of developmental toxicity in alternative in vitro assays (positive in murine embryonic stem cell [mESC] assay and rat whole embryo culture, but negative in zebrafish), developmental toxicity (growth retardation and dysmorphogenesis associated with disrupted midline fusion) was observed with the oral administration of the dual ACC1 and 2 inhibitor, PF-05175157, in Sprague Dawley rats and New Zealand White rabbits. The results of these studies are presented here to make comparisons across the assays, as well as mechanistic insights from the mESC assay demonstrating high ACC expression in the mESC and that ACC induced developmental toxicity can be rescued with palmitic acid providing supportive evidence for DNL pathway inhibition as the underlying mechanism. Ultimately, while the battery of alternative approaches and weight-of-evidence case were useful for hazard identification, the embryo-fetal development studies were necessary to inform the risk assessment on the adverse fetal response, as malformations and/or embryo fetal lethality were limited to doses that caused near complete inhibition of DNL.

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.

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.

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.

Refinement of a morphological scoring system for postimplantation rabbit conceptuses

BACKGROUND

The rabbit is used extensively in developmental toxicity testing, yet basic information on rabbit embryo development is lacking. The goals of this study were to refine a rabbit embryo morphology scoring system, and use it to evaluate rabbit whole embryo cultures (WEC).

METHODS

A total of 265 conceptuses were harvested between GD 8.0 and 12.0 (coitus = GD 0) at 6-hr intervals and examined in detail. Discreet developmental landmarks were then established for 18 morphological features and assigned scores ranging from 0 up to 6. The scoring system was then validated on a subset of randomly selected in vivo conceptuses, and was used to evaluate conceptuses grown for 12, 24, 36, or 48 hr in WEC beginning from GD 9.0 or 10.0. A few embryos also were examined using microscopic computed tomography (microCT)-based virtual histologytrade mark to assess the utility of this technology.

RESULTS

Morphology scores of in vivo developed conceptuses increased linearly (r2 = 0.98) with advancing gestational age, from means of 0.0 on GD 8.0 to 67.9 on GD 12.0. Application of the scoring system, supplemented with evidence from Virtual histologytrade mark, indicated that the WEC system supported normal morphological development of rabbit conceptuses. However, when explanted at GD 9, the rate of development was about 20% slower than in vivo, whereas the rate of development in WEC from GD 10 was indistinguishable from in vivo.

CONCLUSIONS

This work enhances the evaluation tools available to study mechanisms of normal and abnormal development in this widely used animal testing species.