Long-Acting Tumor-Activated Prodrug of a TGFßR Inhibitor.

Inhibition of TGFß signaling in concert with a checkpoint blockade has been shown to provide improved and durable antitumor immune response in mouse models. However, on-target adverse cardiovascular effects have limited the clinical use of TGFß receptor (TGFßR) inhibitors in cancer therapy. To restrict the activity of TGFßR inhibitors to tumor tissues and thereby widen the therapeutic index, a series of tumor-activated prodrugs of a selective small molecule TGFßR1 inhibitor 1 were prepared by appending 1 to a serine protease substrate and a half-life extension fatty acid carbon chain. The prodrugs were shown to be selectively metabolized in tumor tissues relative to the heart and blood and demonstrated a prolonged favorable increase in the tumor-to-heart ratio of the active drug in tissue distribution studies. Once-weekly administration of the most tissue-selective compound 10 provided anti-tumor efficacy comparable to the parent compound and reduced systemic exposure of the active drug.

TGFß2 and TGFß3 mediate appropriate context-dependent phenotype of rat valvular interstitial cells.

This study focused on characterizing the potential mechanism of valvular toxicity caused by TGFß receptor inhibitors (TGFßRis) using rat valvular interstitial cells (VICs) to evaluate early biological responses to TGFßR inhibition. Three TGFßRis that achieved similar exposures in the rat were assessed. Two dual TGFßRI/-RII inhibitors caused valvulopathy, whereas a selective TGFßRI inhibitor did not, leading to a hypothesis that TGFß receptor selectivity may influence the potency of valvular toxicity. The dual valvular toxic inhibitors had the most profound effect on altering VIC phenotype including altered morphology, migration, and extracellular matrix production. Reduction of TGFß expression demonstrated that combined TGFß2/ß3 inhibition by small interfering RNA or neutralizing antibodies caused similar alterations as TGFßRis. Inhibition of TGFß3 transcription was only associated with the dual TGFßRis, suggesting that TGFßRII inhibition impacts TGFß3 transcriptional regulation, and that the potency of valvular toxicity may relate to alteration of TGFß2/ß3-mediated processes involved in maintaining proper balance of VIC phenotypes in the heart valve.

T4-mediated rescue of aortic malformations in hypothyroid rats indicates maternal thyroid status can affect great vessel development.

Pexacerfont is a corticotrophin-releasing factor subtype 1 receptor (CRF-1) antagonist developed for potential treatment of anxiety and stress-related disorders. In male rats, pexacerfont caused hepatic enzyme induction leading to increased thyroxine (T4) clearance. When administered to pregnant rats on gestation day 6 to 15, pexacerfont at 300 mg/kg/day (30× mean AUC in humans at 100 mg/day) produced similar effects on thyroid homeostasis with serum T4 and thyroid-stimulating hormone levels that were 0.3-0.5× and 3.3-3.7× of controls, respectively. At this dose, fetuses of pexacerfont-treated dams presented findings associated with maternal hypothyroidism including growth retardation and increased skeletal alterations. Additionally, there were unexpected great vessel malformations that were mostly derived from the 4th pharyngeal arch artery in 5 (4.3%) fetuses from 3 (15.8%) litters. The etiology was unclear whether the vascular malformations were related to insufficient thyroid hormones or another mechanism. To better understand this relationship, pregnant rats were implanted with a subcutaneous L-thyroxine pellet designed to provide a sustained release of T4 throughout organogenesis in rat embryos (GD 6 to 15; the dosing period of pexacerfont). T4 supplementation produced a near euthyroid state in pexacerfont-treated dams and completely prevented the fetal vascular malformations. These results suggest maternal T4 levels during organogenesis may have a role in great vessel morphogenesis associated with patterning and/or regression of pharyngeal arch arteries. Although previous clinical reports have speculated a potential relationship between thyroid hormone homeostasis and early cardiovascular development, this is the first report to experimentally demonstrate this relationship in great vessel morphogenesis.

Discovery of BMS-986260, a Potent, Selective, and Orally Bioavailable TGFßR1 Inhibitor as an Immuno-oncology Agent.

Novel imidazole-based TGFßR1 inhibitors were identified and optimized for potency, selectivity, and pharmacokinetic and physicochemical characteristics. Herein, we report the discovery, optimization, and evaluation of a potent, selective, and orally bioavailable TGFßR1 inhibitor, 10 (BMS-986260). This compound demonstrated functional activity in multiple TGFß-dependent cellular assays, excellent kinome selectivity, favorable pharmacokinetic properties, and curative in vivo efficacy in combination with anti-PD-1 antibody in murine colorectal cancer (CRC) models. Since daily dosing of TGFßR1 inhibitors is known to cause class-based cardiovascular (CV) toxicities in preclinical species, a dosing holiday schedule in the anti-PD-1 combination efficacy studies was explored. An intermittent dosing regimen of 3 days on and 4 days off allowed mitigation of CV toxicities in one month dog and rat toxicology studies and also provided similar efficacy as once daily dosing.

Intermittent dosing of the transforming growth factor beta receptor 1 inhibitor, BMS-986260, mitigates class-based cardiovascular toxicity in dogs but not rats.

Small-molecule inhibitors of transforming growth factor beta receptor 1 (TGFßRI) have a history of significant class-based toxicities (eg, cardiac valvulopathy) in preclinical species that have limited their development as new medicines. Nevertheless, some TGFßRI inhibitors have entered into clinical trials using intermittent-dosing schedules and exposure limits in an attempt to avoid these toxicities. This report describes the toxicity profile of the small-molecule TGFßRI inhibitor, BMS-986260, in rats and dogs. Daily oral dosing for 10 days resulted in valvulopathy and/or aortic pathology at systemic exposures that would have been targeted clinically, preventing further development with this dosing schedule. These toxicities were not observed in either species in 1-month studies using the same doses on an intermittent-dosing schedule of 3 days on and 4 days off (QDx3 once weekly). Subsequently, 3-month studies were conducted (QDx3 once weekly), and while there were no cardiovascular findings in dogs, valvulopathy and mortality occurred early in rats. The only difference compared to the 1-month study was that the rats in the 3-month study were 2 weeks younger at the start of dosing. Therefore, a follow-up 1-month study was conducted to evaluate whether the age of rats influences sensitivity to target-mediated toxicity. Using the same dosing schedule and similar doses as in the 3-month study, there was no difference in the toxicity of BMS-986260 in young (8 weeks) or adult (8 months) rats. In summary, an intermittent-dosing schedule mitigated target-based cardiovascular toxicity in dogs but did not prevent valvulopathy in rats, and thus the development of BMS-986260 was terminated.

Discovery of 4-Azaindole Inhibitors of TGFßRI as Immuno-oncology Agents.

The multifunctional cytokine TGFß plays a central role in regulating antitumor immunity. It has been postulated that inhibition of TGFß signaling in concert with checkpoint blockade will provide improved and durable immune response against tumors. Herein, we describe a novel series of 4-azaindole TGFß receptor kinase inhibitors with excellent selectivity for TGFß receptor 1 kinase. The combination of compound 3f and an antimouse-PD-1 antibody demonstrated significantly improved antitumor efficacy compared to either treatment alone in a murine tumor model.

A Developmental Toxicology Assay Platform for Screening Teratogenic Liability of Pharmaceutical Compounds.

Increasing need for proactive safety optimization of pharmaceutical compounds has led to generation and/or refinement of in vitro developmental toxicology assays. Our laboratory has developed three in vitro developmental toxicology assays to assess teratogenic liability of pharmaceutical compounds. These assays included a mouse molecular embryonic stem cell assay (MESCA), a dechorionated zebrafish embryo culture (ZEC) assay, and a streamlined rat whole embryo culture (rWEC) assay. Individually, the assays presented good (73-82%) predictivity. However, it remains to be determined whether combining or tiering the assays could enhance performance. Seventy-three compounds representing a broad spectrum of pharmaceutical targets and chemistry were evaluated across the assays to generate testing strategies that optimized performance. The MESCA and ZEC assays were found to have two limitations: compound solubility and frequent misclassification of compounds with H1 receptor or GABAnergic activity. The streamlined rWEC assay was found to be a cost-effective stand-alone assay for supporting poorly soluble compounds and/or ones with H1 or GABAnergic activity. For all other compounds, a tiering strategy using the MESCA and ZEC assays additionally optimized throughput, cost, and minimized animal use. The tiered strategy resulted in improved performance achieving 88% overall predictivity and was comparable with 89% overall predictivity achieved with frequency analysis (final teratogenic classification made from most frequent teratogenic classification from each individual assay). Furthermore there were 21 compounds in the test set characterized as definitive or suspect human teratogens and the multiassay approach achieved 95 and 91% correct classification using the tiered or frequency screening approach, respectively.

In Vitro Developmental Toxicology Screens: A Report on the Progress of the Methodology and Future Applications.

There has been increasing focus on generation and assessment of in vitro developmental toxicology models for assessing teratogenic liability of chemicals. The driver for this focus has been to find reliable in vitro assays that will reduce or replace the use of in vivo tests for assessing teratogenicity. Such efforts may be eventually applied in testing pharmaceutical agents where a developmental toxicology assay or battery of assays may be incorporated into regulatory testing to replace one of the two species currently used in teratogenic assessment. Such assays may be eventually applied in testing a broader spectrum of chemicals, supporting efforts aligned with Tox21 strategies and responding to REACH legislation. This review describes the developmental toxicology assays that are of focus in these assessments: rodent whole embryo culture, zebrafish embryo assays, and embryonic stem cell assays. Progress on assay development as well as future directions of how these assays are envisioned to be applied for broader safety testing of chemicals are discussed. Altogether, the developmental model systems described in this review provide rich biological systems that can be utilized in better understanding teratogenic mechanisms of action of chemotypes and are promising in providing proactive safety assessment related to developmental toxicity. Continual advancements in refining/optimizing these in vitro assays are anticipated to provide a robust data set to provide thoughtful assessment of how whole animal teratogenicity evaluations can be reduced/refined in the future.

Optimization and Performance Assessment of the Chorion-Off [Dechorinated] Zebrafish Developmental Toxicity Assay.

The Dechorinated Zebrafish Embryo Developmental toxicity assay was originally developed from a training set of 31 compounds and reported to be 87% concordant with in vivo teratogenicity data (Brannen, K. C., Panzica-Kelly, J. M., Danberry, T. L., and Augustine-Rauch, K. A. (2010). Development of a zebrafish embryo teratogenicity assay and quantitative prediction model. Birth Defects Res. 89, 66-77.). The assay includes scoring larva treated in a concentration range for malformations of specific morphological structures/organ systems. The model includes identifying a no-adverse-effect-level (NOAEL) and the concentration resulting in 25% lethality (LC25) at 5 days postfertilization. An LC25/NOAEL ratio ≥10 classifies a compound positive for teratogenic potential. A consortium effort evaluated a modified version of this assay which involved enzymatic chorion treatment instead of manual dissection and used experimental replicates for final classification. The modified assay achieved an 85% overall predictivity (Gustafson, A. L., Stedman, D. B., Ball, J., Hillegass, J. M., Flood, A., Zhang, C. X., Panzica-Kelly, J., Cao, J., Coburn, A., Enright, B. P., et al. (2012). Inter-laboratory assessment of a harmonized zebrafish developmental toxicology assay - progress report on phase I. Reprod. Toxicol. 33, 155-164.). The objective of this study was to perform a thorough performance evaluation of the dechorinated assay by repeating the original training set and testing additional compounds in experimental replicates. When the initial training set was repeated with inclusion of experimental replicates, the overall predictivity was 83%. Model performance was tested with an additional 34 compounds and achieved overall predictivity of 74%. When the training and test sets were combined (63 compounds) the assay's final sensitivity was 83% and the specificity was 71%. Total predictivity was 78% with relatively balanced predictivity for nonteratogens (77%) and teratogens (78%). The chorion-off assay achieved superior sensitivity (83%) compared with sensitivity (63-74%) reported by consortium efforts testing a similar assay with chorion-intact embryos (Ball, J. S., Stedman, D. B., Hillegass, J. M., Zhang, C. X., Panzica-Kelly, J., Coburn, A., Enright, B. P., Tornesi, B., Amouzadeh, H. R., Hetheridge, M., et al. (2014). Fishing for teratogens: a consortium effort for a harmonized zebrafish developmental toxicology assay. Toxicol. Sci. 139, 210-219.). Additional protocol modifications were made to increase assay throughput.

Fishing for teratogens: a consortium effort for a harmonized zebrafish developmental toxicology assay.

A consortium of biopharmaceutical companies previously developed an optimized Zebrafish developmental toxicity assay (ZEDTA) where chorionated embryos were exposed to non-proprietary test compounds from 5 to 6 h post fertilization and assessed for morphological integrity at 5 days post fertilization. With the original 20 test compounds, this achieved an overall predictive value for teratogenicity of 88% of mammalian in vivo outcome [Gustafson, A. L., Stedman, D. B., Ball, J., Hillegass, J. M., Flood, A., Zhang, C. X., Panzica-Kelly, J., Cao, J., Coburn, A., Enright, B. P., et al. (2012). Interlaboratory assessment of a harmonized Zebrafish developmental toxicology assay-Progress report on phase I. Reprod. Toxicol. 33, 155-164]. In the second phase of this project, 38 proprietary pharmaceutical compounds from four consortium members were evaluated in two laboratories using the optimized method using either pond-derived or cultivated-strain wild-type Zebrafish embryos at concentrations up to 100µM. Embryo uptake of all compounds was assessed using liquid chromatography-tandem mass spectrometry. Twenty eight of 38 compounds had a confirmed embryo uptake of >5%, and with these compounds the ZEDTA achieved an overall predictive value of 82% and 65% at the two respective laboratories. When low-uptake compounds (≤ 5%) were retested with logarithmic concentrations up to 1000µM, the overall predictivity across all 38 compounds was 79% and 62% respectively, with the first laboratory achieving 74% sensitivity (teratogen detection) and 82% specificity (non-teratogen detection) and the second laboratory achieving 63% sensitivity (teratogen detection) and 62% specificity (non-teratogen detection). Subsequent data analyses showed that technical differences rather than strain differences were the primary contributor to interlaboratory differences in predictivity. Based on these results, the ZEDTA harmonized methodology is currently being used for compound assessment at lead optimization stage of development by 4/5 of the consortium companies.

Establishment of a molecular embryonic stem cell developmental toxicity assay.

The mouse embryonic stem cell test (EST) is a 10-day screen for teratogenic potential developed to reduce animal use for embryotoxicity testing of chemicals (Spielmann, 2005; Spielmann et al., 1997). In this study, we used the cytotoxicity IC(50) values and transcriptional expression changes as primary endpoints in a shorter 4-day version of the EST, the molecular embryonic stem cell assay. Mouse D3 embryonic stem cells were used for cytotoxicity assessment (monolayers) or grown as embryoid bodies in low attachment plates for transcriptional profiling. Sixty-five compounds with known in vivo teratogenicity (33 teratogens and 32 nonteratogens) were evaluated to develop a model for classifying compounds with teratogenic potential. The expression of 12 developmentally regulated gene targets (nanog, fgf5, gsc, cd34, axin2, apln, chst7, lhx1, fgf8, sox17, foxa2, and cxcr4) was measured following exposure of embryoid bodies to a single compound concentration (0.1 × the cytotoxicity IC(20)) for 4 days. In the decision-tree model, compounds with IC(50) values < 22 µM were categorized as teratogens, whereas compounds in the two groups with IC(50) values between 22-200 µM and > 200 µM were categorized as teratogens if ≥ 8 and 12 genes, respectively, were deregulated by at least 10%. Forty-seven of 65 compounds of the training set were correctly identified (72% total concordance). In a test set of 12 additional compounds (5 teratogens, 7 nonteratogens), 10 were correctly classified by this approach (83% concordance). The false positive rate in the training and test sets was 24 and 0%, respectively, indicating that this assay has potential to identify teratogens.

The rat whole embryo culture assay using the Dysmorphology Score system.

The rat whole embryo culture (WEC) system has been used extensively for characterizing teratogenic properties of test chemicals. In this chapter, we describe the methodology for culturing rat embryos as well as a new morphological score system, the Dysmorphology Score (DMS) system for assessing morphology of mid gestation (gestational day 11) rat embryos. In contrast to the developmental stage focused scoring associated with the Brown and Fabro score system, this new score system assesses the respective degree of severity of dysmorphology, which delineates normal from abnormal morphology of specific embryonic structures and organ systems. This score system generates an approach that allows rapid identification and quantification of adverse developmental findings, making it conducive for characterization of compounds for teratogenic properties and screening activities.

Zebrafish embryo developmental toxicology assay.

A promising in vitro zebrafish developmental toxicology assay was generated to test compounds for their teratogenic potential. The assay's predictivity is approximately 87% in AB strain fish (Brannen KC et al., Birth Defects Res B Dev Reprod Toxicol 89:66-77, 2010). The procedure entails exposing dechorionated gastrulation-stage embryos to a range of compound concentrations for 5 days throughout embryonic and larva development. The larvae are evaluated for viability in order to identify an LC25 (the compound concentration in which 25% lethality is observed) and morphological anomalies using a numerical score system to identify the NOAEL (no observed adverse effect level). These values are used to calculate the teratogenic index (LC25/NOAEL ratio) of each compound. If the teratogenic index is equal to or greater than 10 then the compound is classified as a teratogen, and if the ratio is less than 10 then the compound is classified as a nonteratogen (Brannen KC et al., Birth Defects Res B Dev Reprod Toxicol 89:66-77, 2010).

Development of a streamlined rat whole embryo culture assay for classifying teratogenic potential of pharmaceutical compounds.

This study describes a novel rat whole embryo culture (rWEC) teratogenicity assay that applies a simplified experimental design and statistical prediction model, resulting in reduced animal requirements and increased throughput with low prediction error rate for classifying teratogenic potential of compounds. A total of 70 compounds (38 teratogens and 32 nonteratogens) were evaluated, and the prediction model was generated from a dataset of 59 compounds. The rWEC assay requires only one test concentration (1µM) and three structural endpoints (group average morphological scores of spinal cord, heart, and number of somite pairs), which are used in a recursive partition model for classifying teratogenic liability. The model fitting concordance between the WEC assay and in vivo outcome was 83% with a standard deviation (SD) of 4.9%. The predictivity for future compounds was evaluated by using two statistical methods. Fivefold cross-validation estimated the predictivity of this model at 73% (SD 5.8%). A second estimation of predictivity was obtained from an independent test set of 11 compounds that were not used to build the prediction model and reached 82% (SD 11.6%). The overall estimate for prediction concordance is 74% (SD 5.2%). There is no statistically significant difference (p value > 0.50) in the predictivity between this model and the model supporting European Center for the Validation of Alternative Methods WEC assay with predictivity of 80% (SD 10.6%). Overall, the streamlined WEC assay is estimated to reduce animal use and operational costs by more than 50%. It substantially improves results turnaround with no loss of predictivity.

A dysmorphology score system for assessing embryo abnormalities in rat whole embryo culture.

BACKGROUND: The rodent whole embryo culture (WEC) system is a well-established model for characterizing developmental toxicity of test compounds and conducting mechanistic studies. Laboratories have taken various approaches in describing type and severity of developmental findings of organogenesis-stage rodent embryos, but the Brown and Fabro morphological score system is commonly used as a quantitative approach. The associated score criteria is based upon developmental stage and growth parameters, where a series of embryonic structures are assessed and assigned respective scores relative to their gestational stage, with a Total Morphological Score (TMS) assigned to the embryo. This score system is beneficial because it assesses a series of stage-specific anatomical landmarks, facilitating harmonized evaluation across laboratories. Although the TMS provides a quantitative approach to assess growth and determine developmental delay, it is limited to its ability to identify and/or delineate subtle or structure-specific abnormalities. Because of this, the TMS may not be sufficiently sensitive for identifying compounds that induce structure or organ-selective effects. METHOD: This study describes a distinct morphological score system called the "Dysmorphology Score System (DMS system)" that has been developed for assessing gestation day 11 (approximately 20-26 somite stage) rat embryos using numerical scores to differentiate normal from abnormal morphology and define the respective severity of dysmorphology of specific embryonic structures and organ systems. This method can also be used in scoring mouse embryos of the equivalent developmental stage. RESULT AND CONCLUSION: The DMS system enhances capabilities to rank-order compounds based upon teratogenic potency, conduct structure- relationships of chemicals, and develop statistical prediction models to support abbreviated developmental toxicity screens.

Morphological score assignment guidelines for the dechorionated zebrafish teratogenicity assay.

BACKGROUND: Recently we reported the development and optimization of a zebrafish teratogenicity assay using dechorionated AB strain embryos, a promising assay that was 87% concordant in correctly identifying in vivo teratogens and non-teratogens from a set of 31 compounds (Brannen et al., 2010: Birth Defects Res 89:66-77). METHODS: This assay utilizes a zebrafish morphological score system to characterize adverse effects and identify the no-observed-adverse-effect level (NOAEL). RESULTS: This report describes in detail the morphological score system used in the dechorionated zebrafish embryo culture teratogenicity assay. The morphological assessment includes evaluation of most structures and organ systems and grades relative severity of abnormalities. CONCLUSIONS: To this end, the morphological score system provides information of tissue-specific teratogenicity that has been found to have good concordance with structures found affected in vivo and can also be used to rank compounds based on the severity of malformations.

In vitro developmental toxicology assays: A review of the state of the science of rodent and zebrafish whole embryo culture and embryonic stem cell assays.

In vitro developmental model systems have been an important tool for advancing basic research in the embryology and teratology fields. The rat and zebrafish embryo models have had broad utility in both fields for many decades. Furthermore embryonic stem cells, applied as a basic research tool, have broad applications across the development fields and many other fields including cancer, regeneration and epigenetic research. These models have historically been applied in mechanistic studies but were also considered promising for evaluating teratogenic potential of test substances. In recent years, in vitro teratogenicity assays have become an area of interest for supporting the 3 Rs (reduction, refinement, and replacement of animal use). Generation of such assays also provides a means to facilitate early assessment of test agents at a higher throughput without excessive use of animals. In this review, the three models are described with an emphasis of how they are being developed and/or refined to support teratogenicity assessment as screening tools. An overview of the state of the science and future directions are described.

Development of a zebrafish embryo teratogenicity assay and quantitative prediction model.

BACKGROUND: A zebrafish (Danio rerio) teratogenicity assay has been developed and evaluated for its ability to predict the teratogenic potential of chemicals. METHODS: Zebrafish embryos were dechorionated and then exposed to a test solution from 4-6 hours post-fertilization, and embryos or larvae were assessed up to 5 days post-fertilization (dpf) for viability and morphology. In preliminary experiments, the potential time points for assessment of compound-induced dysmorphology and general toxicity parameters were evaluated, and 5 dpf was found to be the optimum developmental stage for evaluation. Additionally, a morphological scoring system was devised to identify the developmental no-observed-adverse-effect level (NOAEL). For assay evaluation, 34 compounds with adequate in vivo developmental toxicity data were chosen. The compound set represented diversity in teratogenic potencies, structural classes, and pharmacologic targets. For 31 test compounds, each was evaluated over a concentration range, while 3 others were insufficiently aqueous-soluble to be fully tested. For each of the 31 tested compounds, the 5 dpf NOAEL was determined, and the concentration resulting in 25% lethality (LC25) was calculated by curve-fitting. Teratogenic potential of each compound was predicted based on the ratio of the LC25 to the NOAEL. LC25/NOAEL ratios of 10 or greater were considered predictive of teratogenicity. RESULTS: The model successfully categorized 87% of the compounds as teratogens or non-teratogens, with only 2 false-positives (dimethyl phthalate and a Bristol-Myers Squibb (BMS) investigative compound) and 2 false-negatives (valproic acid and a BMS compound). CONCLUSIONS: The results indicate that this assay is promising for screening compounds for teratogenic potential. Birth Defects Res (Part B) 89:66-77, 2010. (c) 2010 Wiley-Liss, Inc.

State of the art in developmental toxicity screening methods and a way forward: a meeting report addressing embryonic stem cells, whole embryo culture, and zebrafish.

A meeting was convened so that users of three models for in vitro developmental toxicity (embryonic stem cells, whole embryo culture, and zebrafish) could share their experiences with each model, and explore the areas for improvement. We present a summary of this meeting and the recommendations of the group.

Alternative experimental approaches for interpreting skeletal findings in safety studies.

Standard evaluations for characterizing selective developmental toxicity are traditionally undertaken in vivo. These studies incur significant cost in animal use, labor and compound, ultimately limiting the selection of compounds that can be evaluated in vivo. Such limitations hinder the ability to address questions regarding whether teratogenic outcome was caused by intended pharmacology or attributed to off-target effects associated with the structure of the small molecule. Ascertaining a better understanding of the published literature can enhance interpretation of existing in vivo datasets and hypotheses regarding critical windows of sensitivity and underlying mechanisms of teratogenicity. Thoughtful execution of investigative in vivo and in vitro studies can test and further define the underlying mechanism of teratogenicity. Skeletal variations and malformations are frequently encountered in in vivo studies and can be difficult to interpret in context of defining hazard assessment and mechanisms of abnormal development. This commentary reviews how investigative approaches can be integrated to better understand teratogenic mechanism as it pertains compounds that produce skeletal abnormalities. Approaches are discussed in context of how they could be used to study a compound that has been found to produce fused and wavy ribs in rat fetuses. An investigative approach is described that utilizes three strategies: 1) maximizing the data available from in vivo studies; 2) performing critical window studies in vivo; and 3) performing mechanism of action evaluations using gene expression studies and developmental model systems.