Endeavours made by trade associations, pharmaceutical companies and regulators in the replacement, reduction and refinement of animal experimentation in safety testing of pharmaceuticals.

Following the European Commission decision to develop a roadmap to phase out animal testing and the signing of the US Modernisation Act, there is additional pressure on regulators and the pharmaceutical industry to abandon animal experimentation in safety testing. Often, endeavours already made by governments, regulators, trade associations, and industry to replace, reduce and refine animal experimentation (3Rs) are unnoticed. Herein, we review such endeavours to promote wider application and acceptance of 3Rs. ICH guidelines have stated 3Rs objectives and have enjoyed many successes driven by global consensus. Initiatives driven by US and European regulators such as the removal of the Abnormal Toxicity Test are neutralised by reticent regional regulators. Stream-lined testing requirements have been proposed for new modalities, oncology, impurity management and animal pharmacokinetics/metabolism. Use of virtual controls, value of the second toxicity species, information sharing and expectations for life-threatening diseases, human specific or well-characterised targets are currently being scrutinised. Despite much effort, progress falls short of the ambitious intent of decisionmakers. From a clinical safety and litigation perspective pharmaceutical companies and regulators are reluctant to step away from current paradigms unless replacement approaches are validated and globally accepted. Such consensus has historically been best achieved through ICH initiatives.

ICH S1 prospective evaluation study and weight of evidence assessments: commentary from industry representatives.

Industry representatives on the ICH S1B(R1) Expert Working Group (EWG) worked closely with colleagues from the Drug Regulatory Authorities to develop an addendum to the ICH S1B guideline on carcinogenicity studies that allows for a weight-of-evidence (WoE) carcinogenicity assessment in some cases, rather than conducting a 2-year rat carcinogenicity study. A subgroup of the EWG composed of regulators have published in this issue a detailed analysis of the Prospective Evaluation Study (PES) conducted under the auspices of the ICH S1B(R1) EWG. Based on the experience gained through the Prospective Evaluation Study (PES) process, industry members of the EWG have prepared the following commentary to aid sponsors in assessing the standard WoE factors, considering how novel investigative approaches may be used to support a WoE assessment, and preparing appropriate documentation of the WoE assessment for presentation to regulatory authorities. The commentary also reviews some of the implementation challenges sponsors must consider in developing a carcinogenicity assessment strategy. Finally, case examples drawn from previously marketed products are provided as a supplement to this commentary to provide additional examples of how WoE criteria may be applied. The information and opinions expressed in this commentary are aimed at increasing the quality of WoE assessments to ensure the successful implementation of this approach.

Scientific and Regulatory Policy Committee Points to Consider: Review of the United States Food and Drug Administration (FDA) Guidance on Pathology Peer Review in Nonclinical Toxicology Studies.

In December 2021, the United States Food and Drug Administration (FDA) issued the final guidance for industry titled Pathology Peer Review in Nonclinical Toxicology Studies: Questions and Answers. The stated purpose of the FDA guidance is to provide information to sponsors, applicants, and nonclinical laboratory personnel regarding the management and conduct of histopathology peer review as part of nonclinical toxicology studies conducted in compliance with good laboratory practice (GLP) regulations. On behalf of and in collaboration with global societies of toxicologic pathology and the Society of Quality Assurance, the Scientific and Regulatory Policy Committee (SRPC) of the Society of Toxicologic Pathology (STP) initiated a review of this FDA guidance. The STP has previously published multiple papers related to the scientific conduct of a pathology peer review of nonclinical toxicology studies and appropriate documentation practices. The objectives of this review are to provide an in-depth analysis and summary interpretation of the FDA recommendations and share considerations for the conduct of pathology peer review in nonclinical toxicology studies that claim compliance to GLP regulations. In general, this working group is in agreement with the recommendations from the FDA guidance that has added clear expectations for pathology peer review preparation, conduct, and documentation.

The Dog as a Second Species for Toxicology Testing Provides Value to Drug Development.

The objective of the pharmaceutical industry is to develop new drugs that are safe for human use. In many cases, the accepted approach codified in guidance from regulatory authorities to assess the nonclinical safety profile of potential pharmaceuticals is to perform toxicity testing in two species. However, the use of a second species to establish the safety of new pharmaceuticals has been the subject of much scrutiny in recent years and the industry has been repeatedly challenged to reduce, refine, or replace some or all of the animals used to establish the safety of these pharmaceutical candidates. Specifically, the value of the dog in this testing paradigm has been questioned. Publications reviewing available data for marketed drugs suggest that for many drugs, the dog does not identify unique toxicities critical to human safety. The weakness of this approach, however, is that many of the cases where the dog (or any other species) has the greatest impact on drug development are cases for which development decisions based on safety concerns are not shared publicly. The European Federation of Pharmaceutical Industries and Associations (EFPIA) Preclinical Development Expert Group (PDEG) decided to share case studies collected from its membership and the literature to illustrate the value of the dog in drug development decision-making and clinical monitoring practices to protect the safety of trial subjects.

International Harmonization of Nomenclature and Diagnostic Criteria (INHAND): Non-proliferative and Proliferative Lesions of the Non-human Primate (M. fascicularis).

The INHAND (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions Project (www.toxpath.org/inhand.asp) is a joint initiative of the Societies of Toxicologic Pathology from Europe (ESTP), Great Britain (BSTP), Japan (JSTP) and North America (STP) to develop an internationally accepted nomenclature for proliferative and nonproliferative lesions in laboratory animals. The purpose of this publication is to provide a standardized nomenclature for classifying microscopic lesions observed in most tissues and organs from the nonhuman primate used in nonclinical safety studies. Some of the lesions are illustrated by color photomicrographs. The standardized nomenclature presented in this document is also available electronically on the internet (http://www.goreni.org/). Sources of material included histopathology databases from government, academia, and industrial laboratories throughout the world. Content includes spontaneous lesions as well as lesions induced by exposure to test materials. Relevant infectious and parasitic lesions are included as well. A widely accepted and utilized international harmonization of nomenclature for lesions in laboratory animals will provide a common language among regulatory and scientific research organizations in different countries and increase and enrich international exchanges of information among toxicologists and pathologists.

Inspiration and Exasperation: The Challenges of Inhaled Biologics.

The delivery of biotherapeutic molecules (antibodies, proteins, peptides) and nucleic acids via the respiratory route has presented challenges for regulatory approval, due in part to a lack of understanding of the expected pathology, mechanisms of toxicity, and immunogenicity induced by the inhalation route. Although the first inhaled biotherapeutic was approved some time ago (Dornase Alfa, Pulmozyme; Genetech, 1993), no other inhaled biotherapeutics have been marketed for the treatment of human disease other than the inhaled insulins (Exubera; Pfizer, 2006 and Afrezza; Mannkind Corporation, 2014). As a result, scientific knowledge within the toxicologic pathology community is fragmented with precious little publicly available data. Therefore, one of the aims of this special edition was to generate a collection of manuscripts that pathologists and toxicologists could refer in order to understand the pathology, mechanisms of toxicity, immunogenicity, and challenges associated with the development of inhaled biotherapeutics.

Immunogenicity and Immune Complex Disease in Preclinical Safety Studies.

This article summarizes a continuing education presentation on immunogenicity that was part of a continuing education course entitled, "Clinical Pathology of Biotherapeutics." Immunogenicity of a biotherapeutic can have diverse impacts including altered systemic exposure and pharmacologic responses and, in a fraction of the cases, safety concerns including cross-reactive neutralization of endogenous proteins or sequela related to immune complex disease (ICD). In most cases, immune complexes are readily cleared from circulation; however, based on physiochemical properties, insoluble complexes form, activate complement, and deposit in tissues. Using published information and personal experience, a set of repeat-dose monkey toxicity studies with manifestations suggestive of ICD was reviewed to summarize the spectrum of clinical and pathology findings. The most common live-phase observation linked to ICD was an acute postdosing reaction following multiple dose administrations characterized by generalized collapse and attributed to acute complement activation. Less common live-phase observations were related to syndromes such as a consumptive coagulopathy or a protein losing nephropathy. The most common histologic change attributed to ICD was multi-organ vascular/perivascular inflammation followed by glomerulonephritis. The presentation concluded with a description of the challenges in assessing the relevance of immunogenicity-related reaction in monkey to human clinical use.

Pancreatic Effects of a Bruton's Tyrosine Kinase Small-molecule Inhibitor in Rats Are Strain-dependent.

Inhibitors of Bruton's tyrosine kinase (BTK) are under development as potential therapies for various autoimmune diseases. In repeat-dose toxicity studies, small-molecule BTK inhibitors (BTKi) have been reported to cause a constellation of histologic effects at the pancreatic endocrine-exocrine interface in male rats; however, similar findings were not reported in other species. Since the BTKi-induced pancreatic effect is morphologically similar to well-documented spontaneous changes (predominantly characterized by insular/peri-insular hemorrhage, pigment deposition, chronic inflammation, and fibrosis) that are known to vary by rat strain, we investigated potential strain-dependent differences in the pancreatic effects of a small-molecule BTKi, LY3337641. Following 13 weeks of LY3337641 treatment, Crl:CD(SD) rats were most sensitive, Crl:WI(Han) rats were of intermediate sensitivity, and Hsd:SD rats were least sensitive. These strain differences appear to be related to differences in rate of weight gain across strains and sexes; however, a definitive mechanism was not determined. This study demonstrated that BTKi-induced pancreatic effects were highly dependent on rat strain and correlated with differences in the incidence and severity of the spontaneous background change. When considered with the lack of pancreas effects in nonrat species, these changes in rats are unlikely predictive of similar changes in humans administered a BTK inhibitor.

Use of toxicogenomics in drug safety evaluation: Current status and an industry perspective.

Toxicogenomics held great promise as an approach to enable early detection of toxicities induced by xenobiotics; however, there remain questions regarding the impact of the discipline on pharmaceutical nonclinical safety assessment. To understand the current state of toxicogenomics in the sector, an industry group surveyed companies to determine the frequency of toxicogenomics use in in vivo studies at various stages of drug discovery and development and to assess how toxicogenomics use has evolved over time. Survey data were compiled during 2016 from thirteen pharmaceutical companies. Toxicogenomic analyses were infrequently conducted in the development phase and when performed were done to address specific mechanistic questions. Prior to development, toxicogenomics use was more frequent; however, there were significant differences in approaches among companies. Across all phases, gaining mechanistic insight was the most frequent reason cited for pursing toxicogenomics with few companies using toxicogenomics to predict toxicities. These data were consistent with the commentary submitted in response to survey questions asking companies to describe the evolution of their toxicogenomics strategy. Overall, these survey data indicate that toxicogenomics is not widely used as a predictive tool in the pharmaceutical industry but is used regularly by some companies and serves a broader role in mechanistic investigations and as a complement to other technologies.

Safety testing of monoclonal antibodies in non-human primates: Case studies highlighting their impact on human risk assessment.

Monoclonal antibodies (mAbs) are improving the quality of life for patients suffering from serious diseases due to their high specificity for their target and low potential for off-target toxicity. The toxicity of mAbs is primarily driven by their pharmacological activity, and therefore safety testing of these drugs prior to clinical testing is performed in species in which the mAb binds and engages the target to a similar extent to that anticipated in humans. For highly human-specific mAbs, this testing often requires the use of non-human primates (NHPs) as relevant species. It has been argued that the value of these NHP studies is limited because most of the adverse events can be predicted from the knowledge of the target, data from transgenic rodents or target-deficient humans, and other sources. However, many of the mAbs currently in development target novel pathways and may comprise novel scaffolds with multi-functional domains; hence, the pharmacological effects and potential safety risks are less predictable. Here, we present a total of 18 case studies, including some of these novel mAbs, with the aim of interrogating the value of NHP safety studies in human risk assessment. These studies have identified mAb candidate molecules and pharmacological pathways with severe safety risks, leading to candidate or target program termination, as well as highlighting that some pathways with theoretical safety concerns are amenable to safe modulation by mAbs. NHP studies have also informed the rational design of safer drug candidates suitable for human testing and informed human clinical trial design (route, dose and regimen, patient inclusion and exclusion criteria and safety monitoring), further protecting the safety of clinical trial participants.

Role of Anatomic Pathology in Skeletal Evaluations: Applying INHAND Diagnostic Criteria.

In animal studies, light microscopic examination remains an important tool in the detection and characterization of effects on the skeleton. In the case of both anticipated and unanticipated effects on bone histology, pathologists must carefully select terminology that accurately conveys skeletal morphology without overstating what information can be derived from a standard decalcified paraffin-embedded section. The International Harmonization of Nomenclature and Diagnostic Criteria (INHAND) project issued standardized nomenclature for toxicologic pathologists to use with respect to the skeleton. Consistent with general INHAND principles, the nomenclature emphasizes a descriptive approach to classifying lesions, rather than using terms that imply a specific pathogenesis or disease process. This article, which is based on a presentation at the Society of Toxicologic Pathology's 36th Annual Symposium, will discuss the role of anatomic pathology evaluation in the context of skeletal evaluation in toxicity studies. The integration of anatomic pathology data with quantitative bone end points is reviewed. An overview of the INHAND diagnostic scheme is provided and accompanied by case examples in which the INHAND terminology has been applied. In addition, this review summarizes key considerations for toxicologists and pathologists assigning adversity designations when considering bone end points.

Nonclinical pharmacology and toxicology of the first biosimilar insulin glargine drug product (BASAGLAR®/ABASAGLAR®) approved in the European Union.

Basaglar®/Abasaglar® (Lilly insulin glargine [LY IGlar]) is a long-acting human insulin analogue drug product granted marketing authorisation as a biosimilar to Lantus® (Sanofi insulin glargine [SA IGlar]) by the European Medicines Agency. We assessed the similarity of LY IGlar to the reference drug product, European Union-sourced SA IGlar (EU-SA IGlar), using nonclinical in vitro and in vivo studies. No biologically relevant differences were observed for receptor binding affinity at either the insulin or insulin-like growth factor-1 (IGF-1) receptors, or in assays of functional or de novo lipogenic activity. The mitogenic potential of LY IGlar and EU-SA IGlar was similar when tested in both insulin- and IGF-1 receptor dominant cell systems. Repeated subcutaneous daily dosing of rats for 4 weeks with 0, 0.3, 1.0, or 2.0 mg/kg LY IGlar and EU-SA IGlar produced mortalities and clinical signs consistent with severe hypoglycaemia. Glucodynamic profiles of LY IGlar and EU-SA IGlar in satellite animals showed comparable dose-related hypoglycaemia. Severe hypoglycaemia was associated with axonal degeneration of the sciatic nerve; the incidence and severity were low and did not differ between LY IGlar and EU-SA IGlar. These results demonstrated no biologically relevant differences in toxicity between LY IGlar and EU-SA IGlar.

Chronic Toxicology Studies of Basal Insulin Peglispro in Rats and Dogs: A Novel, PEGylated Insulin Lispro Analog with a Prolonged Duration of Action.

Basal insulin peglispro (BIL) consists of insulin lispro with a 20-kDa polyethylene glycol (PEG) moiety covalently attached to lysine B28. Because chronic parenteral administration of PEGylated proteins to animals has sometimes resulted in PEG vacuolation of tissue macrophages, renal tubular cells, and choroid plexus ependymal cells, we investigated whether chronic subcutaneous (sc) injection of BIL in rats (52 weeks) and dogs (39 weeks) was associated with systemic toxicities or other changes, including vacuolation of tissue macrophages, renal tubular cells, and ependymal cells. Rats and dogs received daily sc injections of BIL (rats: 0.17, 0.45, or 1.15 mg/kg/d and dogs: 0.025, 0.10, or 0.20 mg/kg/d) and the reference compound, HUMULIN N® (neutral protamine Hagedorn [NPH] human insulin; rats: 0.15 mg/kg/d and dogs: 0.02-0.03 mg/kg/d). Animals were evaluated for standard end points including mortality, clinical signs, body weights, toxicokinetics, glucodynamics, clinical pathology, and morphological pathology. Nonadverse injection site lipohypertrophy occurred for all BIL and NPH doses but more frequently with BIL. No BIL-related hyperplasia or neoplasia was observed. There was no vacuolation of tissue macrophages, renal tubular cells, or ependymal cells attributable to PEG. These studies demonstrate BIL is not associated with tissue vacuolation attributable to PEG at 4- to 6-fold multiple of the median clinical exposure in patients with diabetes.

Nonproliferative and Proliferative Lesions of the Rat and Mouse Skeletal Tissues (Bones, Joints, and Teeth).

The INHAND (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions in Rats and Mice) Project (www.toxpath.org/inhand.asp) is an initiative of the Societies of Toxicological Pathology from Europe (ESTP), Great Britain (BSTP), Japan (JSTP) and North America (STP) to develop an internationally accepted nomenclature for proliferative and nonproliferative lesions in laboratory animals. The purpose of this publication is to provide a standardized nomenclature for classifying microscopic lesions observed in the skeletal tissues and teeth of laboratory rats and mice, with color photomicrographs illustrating examples of many common lesions. The standardized nomenclature presented in this document is also available on the internet (http://www.goreni.org/). Sources of material were databases from government, academic and industrial laboratories throughout the world.

An Investigative Study of Pancreatic Exocrine Biomarkers, Histology, and Histomorphometry in Male Zucker Diabetic Fatty (ZDF) Rats Given Dulaglutide by Subcutaneous Injection Twice Weekly for 13 Weeks.

Glucagon-like peptide-1 (GLP-1) receptor agonist therapy has been implicated as a possible risk factor for acute pancreatitis in patients with type 2 diabetes. Dulaglutide is a long-acting GLP-1 receptor agonist in development for treatment of type 2 diabetes. The effects of dulaglutide were evaluated in male Zucker diabetic fatty (ZDF) rats to examine whether dulaglutide may induce or modulate pancreatitis. Rats were randomized to dose groups receiving twice-weekly subcutaneously administered dulaglutide 0.5, 1.5, and 5.0 mg/kg/dose (corresponding human plasma exposures following twice-weekly dosing are 3-, 8-, and 30-fold, respectively) for 13 weeks or to vehicle control. Following termination, serially trimmed sections of pancreases were stained with hematoxylin and eosin or co-stained with an epithelial marker and a marker of either proliferation or apoptosis. Efficacious reductions in glucose and hemoglobin A1c occurred at all dulaglutide doses. Lipase activity was unaffected, and there were modest increases in total and pancreatic amylase activities at all doses without individual microscopic inflammatory correlates. Microscopic dulaglutide-related pancreatic changes included increased interlobular ductal epithelium without ductal cell proliferation (≥0.5 mg/kg), increased acinar atrophy with/without inflammation (≥1.5 mg/kg), and increased incidence/severity of neutrophilic acinar pancreatic inflammation (5.0 mg/kg). In summary, dulaglutide treatment was associated with mild alterations in ductal epithelium and modest exacerbation of spontaneous lesions of the exocrine pancreas typically found in the ZDF rat model.

Effects of the GLP-1 Receptor Agonist Dulaglutide on the Structure of the Exocrine Pancreas of Cynomolgus Monkeys.

Clinical and nonclinical studies have implicated glucagon-like peptide-1 (GLP-1) receptor agonist therapy as a risk factor for acute pancreatitis in patients with type 2 diabetes. Therefore, it is critical to understand the effect that dulaglutide, an approved GLP-1 receptor agonist, has on the exocrine pancreas. Dulaglutide 8.15 mg/kg (approximately 500 times the maximum recommended human dose based on plasma exposure) was administered twice weekly for 12 months to cynomolgus monkeys. Serum amylase and lipase activities were measured and 6 sections of each pancreas were examined microscopically. Ductal epithelial cell proliferation was estimated using Ki67 labeling. Dulaglutide administration did not alter serum amylase or lipase activities measured at the end of treatment compared to control values. An extensive histologic evaluation of the pancreas revealed no changes in the acinar or endocrine portions and no evidence of pancreatitis, necrosis, or pancreatic intraepithelial neoplasia. An increase in goblet cells noted in 4 of the 19 treated monkeys was considered an effect of dulaglutide but was not associated with dilation, blockage, or accumulation of mucin in the pancreatic duct. There was no difference in cell proliferation in ductal epithelium between control and dulaglutide-treated monkeys. These data reveal that chronic dosing of nondiabetic primates with dulaglutide does not induce inflammatory or preneoplastic changes in exocrine pancreas.

International Harmonization of Nomenclature and Diagnostic Criteria (INHAND) progress to date and future plans.

The INHAND Proposal (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions in Rats and Mice) has been operational since 2005. A Global Editorial Steering Committee (GESC) manages the overall objectives of the project and the development of harmonized terminology for each organ system is the responsibility of the Organ Working Groups (OWG), drawing upon experts from North America, Europe and Japan.Great progress has been made with 9 systems published to date - Respiratory, Hepatobiliary, Urinary, Central/Peripheral Nervous Systems, Male Reproductive and Mammary, Zymbals, Clitoral and Preputial Glands in Toxicologic Pathology and the Integument and Soft Tissue and Female Reproductive System in the Journal of Toxicologic Pathology as supplements and on a web site - www.goreni.org. INHAND nomenclature guides offer diagnostic criteria and guidelines for recording lesions observed in rodent toxicity and carcinogenicity studies. The guides provide representative photo-micrographs of morphologic changes, information regarding pathogenesis, and key references. During 2012, INHAND GESC representatives attended meetings with representatives of the FDA Center for Drug Evaluation and Research (CDER), Clinical Data Interchange Standards Consortium (CDISC), and the National Cancer Institute (NCI) Enterprise Vocabulary Services (EVS) to begin incorporation of INHAND terminology as preferred terminology for SEND (Standard for Exchange of Nonclinical Data) submissions to the FDA. The interest in utilizing the INHAND nomenclature, based on input from industry and government toxicologists as well as information technology specialists, suggests that there will be wide acceptance of this nomenclature. The purpose of this publication is to provide an update on the progress of INHAND.

Effects of Dulaglutide on Thyroid C Cells and Serum Calcitonin in Male Monkeys.

Glucagon-like peptide-1 (GLP-1) receptor agonists, used for the treatment of type 2 diabetes, have caused hyperplasia/neoplasia of thyroid C cells in rodent carcinogenicity studies. Studies in monkeys have not identified an effect of GLP-1 receptor agonists on thyroid C cells; however, group sizes were small. Dulaglutide is a once-weekly, long-acting human GLP-1 receptor agonist recently approved in the United States and the European Union. The objective of this study was to determine whether dulaglutide altered C-cell mass in monkeys. Male cynomolgus monkeys (20 per group) were sc injected with dulaglutide 8.15 mg/kg (∼500-fold maximum human plasma exposure) or a vehicle control twice weekly for 52 weeks. Basal and calcium gluconate-stimulated serum calcitonin concentrations were obtained at 3, 6, 9, and 12 months. Thyroid glands were weighed, fixed, and sectioned at 500-µm intervals. C-cell volumes were measured using an automated image analysis. C-cell proliferation was estimated using Ki67/calcitonin colabeling and cell counting. Administration of dulaglutide 8.15 mg/kg twice weekly for 52 weeks did not increase serum calcitonin in monkeys or affect thyroid weight, histology, C-cell proliferation, or absolute/relative C-cell volume. This study represents a comprehensive evaluation of the monkey thyroid C cells after dosing with a GLP-1 receptor agonist, with a large group size, and measurement of multiple relevant parameters. The lack of effect of dulaglutide on C cells is consistent with other studies in monkeys using GLP-1 receptor agonists and suggests that nonhuman primates are less sensitive than rodents to the induction of proliferative changes in thyroid C cells by GLP-1 receptor agonists.

Chronic Toxicity and Carcinogenicity Studies of the Long-Acting GLP-1 Receptor Agonist Dulaglutide in Rodents.

The tumorigenic potential of dulaglutide was evaluated in rats and transgenic mice. Rats were injected sc twice weekly for 93 weeks with dulaglutide 0, 0.05, 0.5, 1.5, or 5 mg/kg corresponding to 0, 0.5, 7, 20, and 58 times, respectively, the maximum recommended human dose based on plasma area under the curve. Transgenic mice were dosed sc twice weekly with dulaglutide 0, 0.3, 1, or 3 mg/kg for 26 weeks. Dulaglutide effects were limited to the thyroid C-cells. In rats, diffuse C-cell hyperplasia and adenomas were statistically increased at 0.5 mg/kg or greater (P ≤ .01 at 5 mg/kg), and C-cell carcinomas were numerically increased at 5 mg/kg. Focal C-cell hyperplasia was higher compared with controls in females given 0.5, 1.5, and 5 mg/kg. In transgenic mice, no dulaglutide-related C-cell hyperplasia or neoplasia was observed at any dose; however, minimal cytoplasmic hypertrophy of C cells was observed in all dulaglutide groups. Systemic exposures decreased over time in mice, possibly due to an antidrug antibody response. In a 52-week study designed to quantitate C-cell mass and plasma calcitonin responses, rats received twice-weekly sc injections of dulaglutide 0 or 5 mg/kg. Dulaglutide increased focal C-cell hyperplasia; however, quantitative increases in C-cell mass did not occur. Consistent with the lack of morphometric changes in C-cell mass, dulaglutide did not affect the incidence of diffuse C-cell hyperplasia or basal or calcium-stimulated plasma calcitonin, suggesting that diffuse increases in C-cell mass did not occur during the initial 52 weeks of the rat carcinogenicity study.