A Brief History of Use of Animals in Biomedical Research and Perspective on Non-Animal Alternatives.

Animals have been closely observed by humans for at least 17 000 years to gain critical knowledge for human and later animal survival. Routine scientific observations of animals as human surrogates began in the late 19th century driven by increases in new compounds resulting from synthetic chemistry and requiring characterization for potential therapeutic utility and safety. Statistics collected by the United States Department of Agriculture's Animal and Plant Health Inspection Service and United Kingdom Home Office show that animal usage in biomedical research and teaching activities peaked after the mid-20th century and thereafter fell precipitously until the early 21st century, when annual increases (in the UK) were again observed, this time driven by expansion of genetically modified animal technologies. The statistics also show a dramatic transfer of research burden in the 20th and 21st centuries away from traditional larger and more publicly sensitive species (dogs, cats, non-human primates, etc) towards smaller, less publicly sensitive mice, rats, and fish. These data show that new technology can produce multi-faceted outcomes to reduce and/or to increase annual animal usage and to redistribute species burden in biomedical research. From these data, it is estimated that annual total vertebrate animal usage in biomedical research and teaching in the United States was 15 to 25 million per year during 2001-2018. Finally, whereas identification and incorporation of non-animal alternatives are products of, but not an integral component of, the animal research cycle, they replace further use of animals for specific research and product development purposes and create their own scientific research cycles, but are not necessarily a substitute for animals or humans for discovery, acquisition, and application of new (eg, previously unknown and/or unsuspected) knowledge critical to further advance human and veterinary medicine and global species survival.

Fit for Purpose Assessment: A New Direction for IACUCs.

The organization and function of the institutional animal care and use committee (IACUC) is the key component of government regulation and oversight of necessary scientific research using live animals and of AAALAC - International accreditation of animal care and use programs in the United States. The regulations, roles, and responsibilities of IACUCs have evolved since their inception 35 years ago from a limited focus on animal welfare and specific animal procedures to embracing scientific quality, data reproducibility and translation, and animal welfare as inextricably interdependent and critical components of generation of new scientific knowledge and medical treatments. A current challenge for IACUCs is in evaluating whether benefits to be derived (eg, new knowledge or treatments) justify any unavoidable pain, stress, or injury associated with proposed research protocols, because the former are long-term and at best speculative outcomes, whereas the latter are immediate and tangible for the study animals. Scientific consensus is that research most likely to generate significant new knowledge and medical treatments is that conducted to high scientific, technical, and quality standards and reported with full transparency to facilitate reproducibility. As an alternative to current benefits evaluations included in risk benefit and harm benefit constructs, the authors propose that IACUCs assess the proposed research for scientific quality and alignment of study elements with the study purpose (e.g., Fit for Purpose [FfP]), including justifications for study design components, selection of primary endpoints and technologies, rationale for data and statistical analyses, and research communication plans. Fit for Purpose endpoints are objective, immediate, and impactful as are the potential risks for study animals, and at the same time they are the best predictors for achievement of longer-term benefits. We propose that IACUCs and any revision of The ILAR Guide consider FfP concepts in place of traditional benefits assessment to accelerate the generation of new knowledge and treatments benefiting medical and veterinary patients and the environment through better science and animal welfare rather than to continue to rely on speculative future outcomes.

Exposure assessments in reproductive and developmental toxicity testing: An IQ-DruSafe industry survey on current practices and experiences in support of exposure-based high dose selection.

The draft ICH S5(R3) guideline includes an exposure-based endpoint as an option for selecting the high dose in developmental and reproductive toxicity (DART) studies. In 2016, IQ DruSafe conducted an anonymous survey to identify industry practices and experiences related to pharmacokinetic assessments in DART studies in order to facilitate a pragmatic data-driven approach to development of an acceptable multiple of the clinical exposure to be proposed for dose selection in the guideline. Questions in the survey were designed to explore pharmacokinetic differences in pregnant versus non-pregnant animals, and to assess exposure levels attained in the absence of maternal toxicity as well as DART outcomes in animal studies associated with those exposures. Small molecule and therapeutic proteins were analyzed separately. The key findings for small molecules were: a) differences in exposures between pregnant and non-pregnant animals were generally ≤3-fold, b) Cmax or AUC exposures ≥25-fold the clinical exposure were achieved in the absence of maternal toxicity for 31% and 23% of rat and rabbit developmental toxicity studies, respectively, and c) only 3.3% (5/153) and 1.6% (2/128) of the developmental toxicity studies were positive for malformations or embryofetal lethality in rats and rabbits, respectively, that were not observed until exposure margins were ≥25-fold.

Analysis of exposure margins in developmental toxicity studies for detection of human teratogens.

The draft Step 2 ICH S5(R3) guideline includes an exposure-based endpoint as an option for selecting the high-dose in reproductive and developmental toxicity studies. To help determine an appropriate exposure margin for embryofetal developmental toxicity testing, a retrospective analysis was undertaken to determine what threshold would have been sufficient to detect hazards to embryofetal development in rats and rabbits for 18 known and 4 presumed human teratogens. The analysis showed that using a high dose that provided at least a 6-fold exposure margin in the developmental toxicity studies would have been sufficient to detect the teratogenic hazard with relevance for humans for all these therapeutics. With regards to human risk assessment practices for developmental toxicity, the analysis showed that, after excluding lenalidomide and pomalidomide data in rats, all available AUC margins at the NOAEL for the induction of malformations or embryofetal lethality were <4-fold of the exposure at the MRHD for all 22 therapeutics. These data support the proposed general approach of increased level of concern for human risk when exposure margins of the NOAEL to the MRHD are <10-fold, reduced concern when the exposure margins are 10- to 25-fold, and minimal concern when the exposure margin is > 25-fold.

An Industry Perspective on the 2017 EMA Guideline on First-in-Human and Early Clinical Trials.

The European Medicines Agency (EMA) in 2017 issued a revised guideline on nonclinical and clinical aspects of first-in-human (FIH) and early clinical trials (CTs). External input was solicited during a draft comment phase, and although some industry suggestions were adopted, others were not. We agree that subject safety is of utmost priority, and believe that minimizing risk must be balanced with efficient and informative study designs to bring new medicines to patients.

Current nonclinical testing paradigms in support of safe clinical trials: An IQ Consortium DruSafe perspective.

The transition from nonclinical to First-in-Human (FIH) testing is one of the most challenging steps in drug development. In response to serious outcomes in a recent Phase 1 trial (sponsored by Bial), IQ Consortium/DruSafe member companies reviewed their nonclinical approach to progress small molecules safely to FIH trials. As a common practice, safety evaluation begins with target selection and continues through iterative in silico and in vitro screening to identify molecules with increased probability of acceptable in vivo safety profiles. High attrition routinely occurs during this phase. In vivo exploratory and pivotal FIH-enabling toxicity studies are then conducted to identify molecules with a favorable benefit-risk profile for humans. The recent serious incident has reemphasized the importance of nonclinical testing plans that are customized to the target, the molecule, and the intended clinical plan. Despite the challenges and inherent risks of transitioning from nonclinical to clinical testing, Phase 1 studies have a remarkably good safety record. Given the rapid scientific evolution of safety evaluation, testing paradigms and regulatory guidance must evolve with emerging science. The authors posit that the practices described herein, together with science-based risk assessment and management, support safe FIH trials while advancing development of important new medicines.

Cardiac drug-drug interaction between HCV-NS5B pronucleotide inhibitors and amiodarone is determined by their specific diastereochemistry.

Severe bradycardia/bradyarrhythmia following coadministration of the HCV-NS5B prodrug sofosbuvir with amiodarone was recently reported. Our previous preclinical in vivo experiments demonstrated that only certain HCV-NS5B prodrugs elicit bradycardia when combined with amiodarone. In this study, we evaluate the impact of HCV-NS5B prodrug phosphoramidate diastereochemistry (D-/L-alanine, R-/S-phosphoryl) in vitro and in vivo. Co-applied with amiodarone, L-ala,SP prodrugs increased beating rate and decreased beat amplitude in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), but D-ala,RP produgs, including MK-3682, did not. Stereochemical selectivity on emerging bradycardia was confirmed in vivo. Diastereomer pairs entered cells equally well, and there was no difference in intracellular accumulation of L-ala,SP metabolites ± amiodarone, but no D-ala,RP metabolites were detected. Cathepsin A (CatA) inhibitors attenuated L-ala,SP prodrug metabolite formation, yet exacerbated L-ala,SP + amiodarone effects, implicating the prodrugs in these effects. Experiments indicate that pharmacological effects and metabolic conversion to UTP analog are L-ala,SP prodrug-dependent in cardiomyocytes.

Role of chronic toxicology studies in revealing new toxicities.

Chronic (>3 months) preclinical toxicology studies are conducted to support the safe conduct of clinical trials exceeding 3 months in duration. We have conducted a review of 32 chronic toxicology studies in non-rodents (22 studies in dogs and 10 in non-human primates) and 27 chronic toxicology studies in rats dosed with Merck compounds to determine the frequency at which additional target organ toxicities are observed in chronic toxicology studies as compared to subchronic studies of 3 months in duration. Our review shows that majority of the findings are observed in the subchronic studies since additional target organs were not observed in 24 chronic non rodent studies and in 21 chronic rodent studies. However, 6 studies in non rodents and 6 studies in rodents yielded new findings that were not seen in studies of 3-month or shorter duration. For 3 compounds the new safety findings did contribute to termination of clinical development plans. Although the incidence of compound termination associated with chronic toxicology study observations is low (∼10%), the observations made in these studies can be important for evaluating human safety risk.

Interaction between amiodarone and hepatitis-C virus nucleotide inhibitors in human induced pluripotent stem cell-derived cardiomyocytes and HEK-293 Cav1.2 over-expressing cells.

Several clinical cases of severe bradyarrhythmias have been reported upon co-administration of the Hepatitis-C NS5B Nucleotide Polymerase Inhibitor (HCV-NI) direct-acting antiviral agent, sofosbuvir (SOF), and the Class-III anti-arrhythmic amiodarone (AMIO). We model the cardiac drug-drug interaction (DDI) between AMIO and SOF, and between AMIO and a closely-related SOF analog, MNI-1 (Merck Nucleotide Inhibitor #1), in functional assays of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), to provide mechanistic insights into recently reported clinical cases. AMIO co-applied with SOF or MNI-1 increased beating rate or field potential (FP) rate and decreased impedance (IMP) and Ca(2+) transient amplitudes in hiPSC-CM syncytia. This action resembled that of Ca(2+) channel blockers (CCBs) in the model, but CCBs did not substitute for AMIO in the DDI. AMIO analog dronedarone (DRON) did not substitute for, but competed with AMIO in the DDI. Ryanodine and thapsigargin, decreasing intracellular Ca(2+) stores, and SEA-0400, a Na(+)/Ca(2+) exchanger-1 (NCX1) inhibitor, partially antagonized or suppressed DDI effects. Other agents affecting FP rate only exerted additive or subtractive effects, commensurate with their individual effects. We also describe an interaction between AMIO and MNI-1 on Cav1.2 ion channels in an over-expressing HEK-293 cell line. MNI-1 enhanced Cav1.2 channel inhibition by AMIO, but did not affect inhibition of Cav1.2 by DRON, verapamil, nifedipine, or diltiazem. Our data in hiPSC-CMs indicate that HCV-NI agents such as SOF and MNI-1 interact with key intracellular Ca(2+)-handling mechanisms. Additional study in a Cav1.2 HEK-293 cell-line suggests that HCV-NIs potentiate the inhibitory action of AMIO on L-type Ca(2+) channels.

Assessment of the clinical cardiac drug-drug interaction associated with the combination of hepatitis C virus nucleotide inhibitors and amiodarone in guinea pigs and rhesus monkeys.

In 2015, European and U.S. health agencies issued warning letters in response to 9 reported clinical cases of severe bradycardia/bradyarrhythmia in hepatitis C virus (HCV)-infected patients treated with sofosbuvir (SOF) in combination with other direct acting antivirals (DAAs) and the antiarrhythmic drug, amiodarone (AMIO). We utilized preclinical in vivo models to better understand this cardiac effect, the potential pharmacological mechanism(s), and to identify a clinically translatable model to assess the drug-drug interaction (DDI) cardiac risk of current and future HCV inhibitors. An anesthetized guinea pig model was used to elicit a SOF+AMIO-dependent bradycardia. Detailed cardiac electrophysiological studies in this species revealed SOF+AMIO-dependent selective nodal dysfunction, with initial, larger effects on the sinoatrial node. Further studies in conscious, rhesus monkeys revealed an emergent bradycardia and bradyarrhythmia in 3 of 4 monkeys administered SOF+AMIO, effects not observed with either agent alone. Morever, bradycardia and bradyarrhythmia were not observed in rhesus monkeys when intravenous infusion of MK-3682 was completed after AMIO pretreatment. CONCLUSIONS: These are the first preclinical in vivo experiments reported to replicate the severe clinical SOF+AMIO cardiac DDI and provide potential in vivo mechanism of action. As such, these data provide a preclinical risk assessment paradigm, including a clinically relevant nonhuman primate model, with which to better understand cardiovascular DDI risk for this therapeutic class. Furthermore, these studies suggest that not all HCV DAAs and, in particular, not all HCV nonstructural protein 5B inhibitors may exhibit this cardiac DDI with amiodarone. Given the selective in vivo cardiac electrophysiological effect, these data enable targeted cellular/molecular mechanistic studies to more precisely identify cell types, receptors, and/or ion channels responsible for the clinical DDI. (Hepatology 2016;64:1430-1441).

Characterization of an investigative safety pharmacology model to assess comprehensive cardiac function and structure in chronically instrumented conscious beagle dogs.

INTRODUCTION: There has been an increasing need to conduct investigative safety pharmacology studies to complement regulatory-required studies, particularly as it applies to a comprehensive assessment of cardiovascular (CV) risk. METHODS: We describe refined methodology using a combination of telemetry and direct signal acquisition to record concomitant peripheral hemodynamics, ECG, and left ventricular (LV) structure (LV chamber size and LV wall thickness) and function, including LV pressure-volume (PV) loops to determine load independent measures of contractility (end systolic elastance, Ees, and preload recruitable stroke work, PRSW) in conscious beagle dogs. Following baseline characterization, 28days of chronic rapid ventricular pacing (RVP) was performed and cardiac function monitored: both as a way to compare measures during development of dysfunction and to characterize feasibility of a model to assess CV safety in animals with underlying cardiac dysfunction. RESULTS: While ±dP/dT decreased within a few days of RVP and remained stable, more comprehensive cardiac function measurements, including Ees and PRSW, provided a more sensitive assessment confirming the value of such endpoints for a more clear functional assessment. After 28days of RVP, the inodilator pimobendan was administered to further demonstrate the ability to detect changes in cardiac function. Expectedly pimobendan caused a leftward shift in the PV loop, improved ejection fraction (EF) and significantly improved Ees and PRSW. DISCUSSION: In summary, the data show the feasibility and importance in measuring enhanced cardiac functional parameters in conscious normal beagle dogs and further describe a relatively stable cardiac dysfunction model that could be used as an investigative safety pharmacology risk assessment tool.

An Evaluation of the Impact of PD-1 Pathway Blockade on Reproductive Safety of Therapeutic PD-1 Inhibitors.

This report discusses the principles of reproductive toxicity risk assessment for biopharmaceuticals blocking the PD-1/programmed cell death ligand 1 (PD-L1) pathway, which have been developed for the treatment of patients with advanced malignancies. The PD-1/PD-L1 pathway is a T-cell co-inhibitory pathway that normally maintains immune tolerance to self. Its role in pregnancy is to maintain immune tolerance to the fetal allograft. In cancer patients, this signaling pathway is hijacked by some neoplasms to avoid immune destruction. PD-1/PD-L1-blocking agents enhance functional activity of the target lymphocytes to eventually cause immune rejection of the tumor. A therapeutic blockade of PD-1/PD-L1 pathway that occurs at full target engagement provides a unique challenge to address the risk to pregnancy because disruption of the same pathway may also reduce or abrogate maternal immune tolerance to the fetal alloantigens inherited through the father. Typically, nonclinical reproductive and developmental toxicity (DART) studies in animals (rats and rabbits) with clinical drug candidates are conducted to identify potential risk in humans and to determine exposure margin for the effects on reproduction as part of the risk assessment. However, for biopharmaceuticals for which the desired mechanism of action cannot be separated from potential deleterious effects to the fetus and when the only relevant toxicology species is nonhuman primate (NHP), the risk to reproduction can be predicted by a mechanism-based assessment using data generated from murine surrogate models as supportive information without conducting DART in NHPs. Such an approach has been used in the evaluation of pregnancy risk of anti-PD-1 agent, pembrolizumab, and has been demonstrated as an important alternative to performing DART studies in NHPs.

Scientific Knowledge and Technology, Animal Experimentation, and Pharmaceutical Development.

Human discovery of pharmacologically active substances is arguably the oldest of the biomedical sciences with origins >3500 years ago. Since ancient times, four major transformations have dramatically impacted pharmaceutical development, each driven by advances in scientific knowledge, technology, and/or regulation: (1) anesthesia, analgesia, and antisepsis; (2) medicinal chemistry; (3) regulatory toxicology; and (4) targeted drug discovery. Animal experimentation in pharmaceutical development is a modern phenomenon dating from the 20th century and enabling several of the four transformations. While each transformation resulted in more effective and/or safer pharmaceuticals, overall attrition, cycle time, cost, numbers of animals used, and low probability of success for new products remain concerns, and pharmaceutical development remains a very high risk business proposition. In this manuscript we review pharmaceutical development since ancient times, describe its coevolution with animal experimentation, and attempt to predict the characteristics of future transformations.

Regulatory Forum Commentary* Counterpoint: Dose Selection for Tg.rasH2 Mouse Carcinogenicity Studies.

High-dose selection for 6-month carcinogenicity studies of pharmaceutical candidates in Tg.rasH2-transgenic mice currently primarily relies on (1) estimation of a maximum tolerated dose (MTD) from the results of a 1-month range-finding study, (2) determination of the maximum dose administrable to the animals (maximum feasible dose [MFD]), (3) demonstration of a plateau in systemic exposure, and (4) use of a limit dose of 1,500 mg/kg/day for products with human daily doses not exceeding 500 mg. Eleven 6-month Tg.rasH2 carcinogenicity studies and their corresponding 1-month range-finding studies conducted at Merck were reviewed. High doses were set by estimation of the MTD in 6, by plateau of exposure in 3, and by MFD in 2 cases. For 4 of 6 studies where MTD was used for high-dose selection, the 1-month study accurately predicted the 6-month study tolerability whereas in the remaining 2 studies the high doses showed poorer tolerability than expected. The use of 3 or more drug-treated dose levels proved useful to ensure that a study would successfully and unambiguously demonstrate that a drug candidate was adequately evaluated for carcinogenicity at a minimally toxic high dose level, especially when the high dose may be found to exceed the MTD.

Recommendations from a global cross-company data sharing initiative on the incorporation of recovery phase animals in safety assessment studies to support first-in-human clinical trials.

An international expert group which includes 30 organisations (pharmaceutical companies, contract research organisations, academic institutions and regulatory bodies) has shared data on the use of recovery animals in the assessment of pharmaceutical safety for early development. These data have been used as an evidence-base to make recommendations on the inclusion of recovery animals in toxicology studies to achieve scientific objectives, while reducing animal use. Recovery animals are used in pharmaceutical development to provide information on the potential for a toxic effect to translate into long-term human risk. They are included on toxicology studies to assess whether effects observed during dosing persist or reverse once treatment ends. The group devised a questionnaire to collect information on the use of recovery animals in general regulatory toxicology studies to support first-in-human studies. Questions focused on study design, the rationale behind inclusion or exclusion and the impact this had on internal and regulatory decisions. Data on 137 compounds (including 53 biologicals and 78 small molecules) from 259 studies showed wide variation in where, when and why recovery animals were included. An analysis of individual study and programme design shows that there are opportunities to reduce the use of recovery animals without impacting drug development.

Phenazin-5-ium hydrogen sulfate monohydrate.

The crystal structure of the title salt, C12H9N2 (+)·HSO4 (-)·H2O, comprises inversion-related pairs of phenazinium ions linked by C-H⋯N hydrogen bonds. The phenazinium N-H atoms are hydrogen bonded to the bis-ulfate anions. The bis-ulfate anions and water mol-ecules are linked by O-H⋯O hydrogen-bonding inter-actions into a structural ladder motif parallel to the a axis.

Importance of halogen···halogen contacts for the structural and magnetic properties of CuX2(pyrazine-N,N'-dioxide)(H2O)2 (X = Cl and Br).

The structural and magnetic properties of the newly crystallized CuX(2)(pyzO)(H(2)O)(2) (X = Cl, Br; pyzO = pyrazine-N,N'-dioxide) coordination polymers are reported. These isostructural compounds crystallize in the monoclinic space group C2/c with, at 150 K, a = 17.0515(7) Å, b = 5.5560(2) Å, c = 10.4254(5) Å, ß = 115.400(2)°, and V = 892.21(7) Å(3) for X = Cl and a = 17.3457(8) Å, b = 5.6766(3) Å, c = 10.6979(5) Å, ß = 115.593(2)°, and V = 950.01(8) Å(3) for X = Br. Their crystal structure is characterized by one-dimensional chains of Cu(2+) ions linked through bidentate pyzO ligands. These chains are joined together through OH···O hydrogen bonds between the water ligands and pyzO oxygen atoms and Cu-X···X-Cu contacts. Bulk magnetic susceptibility measurements at ambient pressure show a broad maximum at 7 (Cl) and 28 K (Br) that is indicative of short-range magnetic correlations. The dominant spin exchange is the Cu-X···X-Cu supersuperexchange because the magnetic orbital of the Cu(2+) ion is contained in the CuX(2)(H(2)O)(2) plane and the X···X contact distances are short. The magnetic data were fitted to a Heisenberg 1D uniform antiferromagnetic chain model with J(1D)/k(B) = -11.1(1) (Cl) and -45.9(1) K (Br). Magnetization saturates at fields of 16.1(3) (Cl) and 66.7(5) T (Br), from which J(1D) is determined to be -11.5(2) (Cl) and -46.4(5) K (Br). For the Br analog the pressure dependence of the magnetic susceptibility indicates a gradual increase in the magnitude of J(1D)/k(B) up to -51.2 K at 0.84 GPa, suggesting a shortening of the Br···Br contact distance under pressure. At higher pressure X-ray powder diffraction data indicates a structural phase transition at ∼3.5 GPa. Muon-spin relaxation measurements indicate that CuCl(2)(pyzO)(H(2)O)(2) is magnetically ordered with T(N) = 1.06(1) K, while the signature for long-range magnetic order in CuBr(2)(pyzO)(H(2)O)(2) was much less definitive down to 0.26 K. The results for the CuX(2)(pyzO)(H(2)O)(2) complexes are compared to the related CuX(2)(pyrazine) materials.

Promise of new translational safety biomarkers in drug development and challenges to regulatory qualification.

One promise of new translational safety biomarkers (TSBs) is their ability to demonstrate that toxicities in animal studies are monitorable at an early stage, such that human relevance of potential adverse effects of drugs can be safely and definitively evaluated in clinical trials. Another is that they would provide an earlier, more definitive and deeper insight to patient prognosis compared with conventional biomarkers. Recent experience with regulatory authorities indicates that resource demands for new TSB qualifications under the current framework are daunting and the rate of their expansion will be slow, particularly in light of mounting financial pressures on the pharmaceutical industry. Sponsors face a dilemma over engaging in safety biomarker qualification consortia. While it is clear new TSBs could be considered catalysts to drug development and that patient health, business and scientific benefits, described here using examples, should outweigh qualification costs, concerns exist that early ambiguities in biomarker interpretations at the introduction of such new TSBs might hinder drug development.

An analysis of pharmaceutical experience with decades of rat carcinogenicity testing: support for a proposal to modify current regulatory guidelines.

Data collected from 182 marketed and nonmarketed pharmaceuticals demonstrate that there is little value gained in conducting a rat two-year carcinogenicity study for compounds that lack: (1) histopathologic risk factors for rat neoplasia in chronic toxicology studies, (2) evidence of hormonal perturbation, and (3) positive genetic toxicology results. Using a single positive result among these three criteria as a test for outcome in the two-year study, fifty-two of sixty-six rat tumorigens were correctly identified, yielding 79% test sensitivity. When all three criteria were negative, sixty-two of seventy-six pharmaceuticals (82%) were correctly predicted to be rat noncarcinogens. The fourteen rat false negatives had two-year study findings of questionable human relevance. Applying these criteria to eighty-six additional chemicals identified by the International Agency for Research on Cancer as likely human carcinogens and to drugs withdrawn from the market for carcinogenicity concerns confirmed their sensitivity for predicting rat carcinogenicity outcome. These analyses support a proposal to refine regulatory criteria for conducting a two-year rat study to be based on assessment of histopathologic findings from a rat six-month study, evidence of hormonal perturbation, genetic toxicology results, and the findings of a six-month transgenic mouse carcinogenicity study. This proposed decision paradigm has the potential to eliminate over 40% of rat two-year testing on new pharmaceuticals without compromise to patient safety.