Obesity and Associated Comorbidities in People and Companion Animals: A One Health Perspective.

This article reviews the biology, prevalence and risks for obesity in people and companion dogs and cats, and explores the links between obesity and diabetes mellitus and cancer across these species. Obesity is a major healthcare problem in both human and veterinary medicine and there is an increasing prevalence of obesity in people and pets. In people and animals, obesity is a complex disorder involving diet, level of physical activity, behavioural factors, socioeconomic factors, environment exposures, genetics, metabolism and the microbiome. Pets and people share a number of obesity-related comorbidities. Obesity is a major risk factor for type 2 diabetes mellitus in people and in cats, but this association is not recognized in dogs. Obesity is a recognized risk factor for a number of human cancers, but there are fewer data available describing this association with canine neoplastic disease. One approach to addressing the problem of obesity is by taking a 'One Health' perspective. Comparative clinical research examining shared lifestyle and environmental risk factors and the reasons underlying species differences should provide new perspectives on the fundamental biology of obesity. One Health programmes involving human healthcare professionals and veterinarians could help address obesity with simple interventions at the community level.

A Review of Paclitaxel and Novel Formulations Including Those Suitable for Use in Dogs.

Paclitaxel is a commonly used chemotherapeutic agent with a broad spectrum of activity against cancers in humans. In 1992, paclitaxel was approved by the U.S. Food and Drug Administration (FDA) as Taxol(®) for use in advanced ovarian cancer. Two years later, it was approved for the treatment of metastatic breast cancer. Paclitaxel was originally isolated from the bark of the Pacific yew tree, Taxus brevifolia in 1971. Taxanes are a family of microtubule inhibitors. As a member of this family, paclitaxel suppresses spindle microtubule dynamics. This activity results in the blockage of the metaphase-anaphase transitions, and ultimately in the inhibition of mitosis, and induction of apoptosis in a wide spectrum of cancer cells. Additional anticancer activities of paclitaxel have been defined that are independent of these effects on the microtubules and may include the suppression of cell proliferation as well as antiangiogenic effects. Based on its targeting of a fundamental feature of the cancer phenotype, the mitotic complex, it is not surprising that paclitaxel has been found to be active in a wide variety of cancers in humans. This review summarizes the evidence in support of paclitaxel's broad anticancer activity and introduces the rationale for, and the progress in development of novel formulations of paclitaxel that may preferentially target cancers and that are not associated with the risks for hypersensitivity in dogs. Of note, a novel nanoparticle formulation of paclitaxel that substantially limits hypersensitivity was recently given conditional approval by the FDA Center for Veterinary Medicine for use in dogs with resectable and nonresectable squamous cell carcinoma and nonresectable stage III, IV and V mammary carcinoma.

Mitochondrial retrograde signaling induces epithelial-mesenchymal transition and generates breast cancer stem cells.

Metastatic breast tumors undergo epithelial-to-mesenchymal transition (EMT), which renders them resistant to therapies targeted to the primary cancers. The mechanistic link between mtDNA (mitochondrial DNA) reduction, often seen in breast cancer patients, and EMT is unknown. We demonstrate that reducing mtDNA content in human mammary epithelial cells (hMECs) activates Calcineurin (Cn)-dependent mitochondrial retrograde signaling pathway, which induces EMT-like reprogramming to fibroblastic morphology, loss of cell polarity, contact inhibition and acquired migratory and invasive phenotype. Notably, mtDNA reduction generates breast cancer stem cells. In addition to retrograde signaling markers, there is an induction of mesenchymal genes but loss of epithelial markers in these cells. The changes are reversed by either restoring the mtDNA content or knockdown of CnAα mRNA, indicating the causal role of retrograde signaling in EMT. Our results point to a new therapeutic strategy for metastatic breast cancers targeted to the mitochondrial retrograde signaling pathway for abrogating EMT and attenuating cancer stem cells, which evade conventional therapies. We report a novel regulatory mechanism by which low mtDNA content generates EMT and cancer stem cells in hMECs.

Loss-of-function screen in rhabdomyosarcoma identifies CRKL-YES as a critical signal for tumor growth.

To identify novel signaling pathways necessary for rhabdomyosarcoma (RMS) survival, we performed a loss-of-function screen using an inducible small hairpin RNA (shRNA) library in an alveolar and an embryonal RMS cell line. This screen identified CRKL expression as necessary for growth of alveolar RMS and embryonal RMS both in vitro and in vivo. We also found that CRKL was uniformly highly expressed in both RMS cell lines and tumor tissue. As CRKL is a member of the CRK adapter protein family that contains an SH2 and two SH3 domains and is involved in signal transduction from multiple tyrosine kinase receptors, we evaluated CRKL interaction with multiple tyrosine kinase receptor signaling pathways in RMS cells. While we saw no interaction of CRKL with IGFIR, MET or PI3KAKT/mTOR pathways, we determined that CRKL signaling was associated with SRC family kinase (SFK) signaling, specifically with YES kinase. Inhibition of SFK signaling with dasatinib or another SFK inhibitor, sarcatinib, suppressed RMS cell growth in vitro and in vivo. These data identify CRKL as a novel critical component of RMS growth. This study also demonstrates the use of functional screening to identify a potentially novel therapeutic target and treatment approach for these highly aggressive pediatric cancers.

Met interacts with EGFR and Ron in canine osteosarcoma.

The receptor tyrosine kinase (RTK) Met is known to be over-expressed in canine osteosarcoma (OSA). In human cancers, the RTKs Met, epidermal growth factor receptor (EGFR) and Ron are frequently co-expressed and engage in heterodimerization, altering signal transduction and promoting resistance to targeted therapeutics. We found that EGFR and Ron are expressed in canine OSA cell lines and primary tissues, EGFR and Ron are frequently phosphorylated in OSA tumour samples, and Met is co-associated with EGFR and Ron in canine OSA cell lines. Transforming growth factor alpha (TGFα) and hepatocyte growth factor (HGF) stimulation induced amplification of ERK1/2 and STAT3 phosphorylation in OSA cells and Met was phosphorylated following TGFα stimulation providing evidence for receptor cross-talk. Lastly, treatment of OSA cells with combined gefitinib and crizotinib inhibited cell proliferation in an additive manner. Together, these data support the notion that Met, EGFR and Ron interact in OSA cells and as such, may represent viable targets for therapeutic intervention.

Prospective study of thrombospondin-1 mimetic peptides, ABT-510 and ABT-898, in dogs with soft tissue sarcoma.

BACKGROUND: Exposure to anti-angiogenic thrombospondin-1 (TSP-1) mimetic peptides (MPs) has resulted in sporadic anti-tumor activity in humans and dogs. HYPOTHESIS: Novel TSP-1 MPs formulations will be safe, tolerated, and clinically active in soft tissue sarcoma (STS) in dogs. ANIMALS: Sixty-two client-owned dogs with measurable STS were enrolled, excluding hemangiosarcoma. METHODS: A prospective, single agent, multicenter, open-label study assessing ABT-510 bolus, ABT-898 bolus, or ABT-898 depot formulations of TSP-1 in dogs. Endpoints included tolerability, antitumor activity, and the assessment of ability of clinical covariates and circulating endothelial cells (CEC) concentration to predict tumor response. RESULTS: Two non-dose-limiting toxicoses possibly attributed to treatment were observed (keratitis and osteoarthritis). Antitumor activity (10/44 = 23% responses) was observed in study subjects who received treatment for >28 days (n = 44) including both partial (7) and minimal responses (3). Responses were disproportionately seen in dogs receiving ABT-898 formulations (9/28 = 32%) versus those receiving ABT-510 (1/16 = 6%; P < .045). Disease stabilization for >84 days was also documented (8/44 = 18%). Slow rates of tumor progression before study entry correlated with anti-tumor activity in treated dogs, whereas no significant association was found between changes in total CEC concentration and tumor response (P = .28) or time to progression (P = .42). CONCLUSIONS AND CLINICAL IMPORTANCE: Safely achieved antitumor activity was documented with TSP-1 MPs in dogs with STS. The most notable activity was achieved with the ABT-898 formulations.

Global methylation analysis identifies PITX2 as an upstream regulator of the androgen receptor and IGF-I receptor genes in prostate cancer.

The insulin-like growth factor-I (IGF-IR) and androgen (AR) receptors are important players in prostate cancer. Functional interactions between the IGF-I and androgen signaling pathways have crucial roles in the progression of prostate cancer from early to advanced stages. DNA methylation is a major epigenetic alteration affecting gene expression. Hypermethylation of tumor suppressor promoters is a frequent event in human cancer, leading to inactivation and repression of specific genes. The aim of the present study was to identify the entire set of methylated genes ("methylome") in a cellular model that replicates prostate cancer progression. The methylation profiles of the P69 (early stage, benign) and M12 (advanced stage, metastatic) prostate cancer cell lines were established by treating cells with the demethylating agent 5-aza-2'-deoxycytidine (5-Aza) followed by DNA microarray analysis. Comparative genome-wide methylation analyses of 5-Aza-treated versus untreated cells identified 297 genes overexpressed in P69 and 191 genes overexpressed in M12 cells. 102 genes were upregulated in both benign and metastatic cell lines. In addition, our analyses identified the PITX2 gene as a master regulator upstream of the AR and IGF-IR genes. The PITX2 promoter was semi-methylated in P69 cells but fully methylated (i. e., silenced) in M12 cells. Epigenetic regulation of PITX2 during the course of the disease may lead to orchestrated control of the AR and IGF signaling pathways. In summary, our results provide new insights into the epigenetic changes associated with progression of prostate cancer from an organ confined, androgen-sensitive disorder to an aggressive, androgen-insensitive disease.

A randomized trial investigating the efficacy and safety of water soluble micellar paclitaxel (Paccal Vet) for treatment of nonresectable grade 2 or 3 mast cell tumors in dogs.

BACKGROUND: Effective treatments for dogs with advanced stage mast cell tumors (MCT) remain a pressing need. A micellar formulation of paclitaxel (paclitaxel [micellar]) has shown promise in early-phase studies. HYPOTHESIS/OBJECTIVES: The objective was to demonstrate greater activity for paclitaxel (micellar) compared with lomustine. The null hypothesis was µ(p) = µ(L) (ie, proportion of responders for the paclitaxel [micellar] and lomustine groups, respectively). ANIMALS: Two hundred and fifty-two dogs with advanced stage nonresectable grade 2 or 3 MCT. METHODS: Prospective multicenter randomized double-blind positive-controlled clinical trial. The primary endpoint was confirmed overall response rate (CORR) at 14 weeks. A secondary endpoint, biologic observed response rate (BORR), also was calculated. Safety was assessed by the characterization and grading of adverse events (AE). RESULTS: Overall CORR (7% versus 1%; P = .048) and BORR (23% versus 10%; P = .012) were greater for paclitaxel (micellar) compared with lomustine. Paclitaxel (micellar)-treated dogs were 6.5 times more likely to have a confirmed response and 3.1 times more likely to experience a biologic observed response. The majority of AE with paclitaxel (micellar) were transient and clinically manageable. Twenty-seven dogs (33%) receiving lomustine were discontinued because of hepatopathy compared with 3 dogs (2%) receiving paclitaxel (micellar) (P < .0001; odds ratio 26.7). CONCLUSIONS AND CLINICAL IMPORTANCE: Paclitaxel (micellar)'s activity and safety profile are superior to lomustine. The addition of an active and novel taxane to the veterinary armamentarium could fill a substantial need and, as its mechanism of action and AE profile do not overlap with currently available TKI, its availability could lead to effective combination protocols.

A review of the association between osteosarcoma metastasis and protein translation.

The malignant transformation of mesenchymal cells within the bone leads to the development of osteosarcoma (OS), but the genetic underpinnings of these events are not understood. From a clinical perspective, primary tumour management can be achieved successfully in most patients. However, the development of metastasis to the lungs represents the most common cause of death in OS patients. A clearer understanding of metastasis biology is required to improve cancer mortality and improve outcomes. Modelling the genetics, biology and therapy of OS can be accomplished through research involving a number of species. Most notable is the naturally occurring form of OS that develops in dogs. Through a cross-species and comparative approach important questions can be asked within specific and suitable models to advance our understanding of this disease and its common metastatic outcome. A comparative perspective on the problem of OS metastasis that utilizes a cross-species approach may offer unique opportunities to assist in this prioritization and generate new hypotheses related to this important clinical problem.

Evaluation of the oral antimitotic agent (ABT-751) in dogs with lymphoma.

BACKGROUND: ABT-751 is a novel orally available antimitotic agent that targets microtubule polymerization. This mechanism may suggest potential activity in canine lymphoma. OBJECTIVE: Determine a maximum tolerated dose for ABT-751, and assess long-term tolerability and activity in canine lymphoma. ANIMALS: Thirty dogs with newly diagnosed (n = 19) or relapsed (n = 11) non-Hodgkin's lymphoma. METHODS: Dogs (n = 11) were enrolled in a rapid dose escalation study to define the maximum tolerated dose. Upon definition of a maximally tolerated dose, a cohort expansion of 19 dogs allowed verification of long-term tolerability and assessment of activity. Study endpoints in the cohort expansion included chronic tolerability, response rate, response duration, and time to progression. Additional endpoints included serum pharmacokinetics, lymph node drug concentrations, and changes in circulating endothelial cells. RESULTS: The maximum tolerated dose of ABT-751 was 350 mg/m(2) PO q24h. Dose-limiting toxicities included vomiting and diarrhea, which resolved with a schedule adjustment to 350 mg/m(2) PO q48h. ABT-751 was consistently detected in lymphoma tissue samples from dogs treated at or above the maximum tolerated dose. In the cohort expansion, objective responses were seen in 3/15 (20%) dogs with a response duration ranging from 21 to 111 days. Decreases in circulating endothelial cells were seen in 10 dogs at day 7 (2 responding dogs and 8 nonresponding dogs). CONCLUSION: ABT-751 was well tolerated at 350 mg/m(2) PO q24h for 7 days and then q48h thereafter. Activity of ABT-751 suggested a rationale for additional studies of ABT-751 as part of a combination chemotherapy protocol for lymphoma or other canine cancers.

Small molecule inhibitors of ezrin inhibit the invasive phenotype of osteosarcoma cells.

Ezrin is a multifunctional protein that connects the actin cytoskeleton to the extracellular matrix through transmembrane proteins. High ezrin expression is associated with lung metastasis and poor survival in cancer. We screened small molecule libraries for compounds that directly interact with ezrin protein using surface plasmon resonance to identify lead compounds. The secondary functional assays used for lead compound selection included ezrin phosphorylation as measured by immunoprecipitation and in vitro kinase assays, actin binding, chemotaxis, invasion into an endothelial cell monolayer, zebrafish and Xenopus embryonic development, mouse lung organ culture and an in vivo lung metastasis model. Two molecules, NSC305787 and NSC668394, that directly bind to ezrin with low micromolar affinity were selected based on inhibition of ezrin function in multiple assays. They inhibited ezrin phosphorylation, ezrin-actin interaction and ezrin-mediated motility of osteosarcoma (OS) cells in culture. NSC305787 mimicked the ezrin morpholino phenotype, and NSC668394 caused a unique developmental defect consistent with reduced cell motility in zebrafish. Following tail vein injection of OS cells into mice, both molecules inhibited lung metastasis of ezrin-sensitive cells, but not ezrin-resistant cells. The small molecule inhibitors NSC305787 and NSC668394 demonstrate a novel targeted therapy that directly inhibits ezrin protein as an approach to prevent tumor metastasis.

Protein kinase C regulates ezrin-radixin-moesin phosphorylation in canine osteosarcoma cells.

The development of metastasis is the most significant cause of death for both canine and human patients with osteosarcoma (OS). Ezrin has been associated with tumour progression and metastasis in human, canine and murine OS. Ezrin activation is dynamically regulated by protein kinase C (PKC) during metastatic progression in human and murine OS. To include the dog in the development of therapeutics that target ezrin biology, we characterized four new canine OS cell lines and confirmed the relationship between PKC and ezrin in these cells. Three of four cell lines formed tumours in mice that were histologically consistent with OS. All cell lines were markedly aneuploid and expressed ezrin and PKC. Finally, both ezrin phosphorylation and cell migration were inhibited using a PKC inhibitor. These data suggest that an association between PKC-mediated activation of ezrin and the metastatic phenotype in canine OS cells.

Concentration of lipocalin region of collagen XXVII alpha 1 in the serum of dogs with hemangiosarcoma.

BACKGROUND: Hemangiosarcoma (HSA) is a common malignancy of dogs with characteristic early, aggressive metastasis. Diagnosis of HSA is challenging because of lack of sensitive and specific diagnostic tests. HYPOTHESIS: Specific proteins that are increased in serum of dogs with HSA might represent useful biomarkers of the disease. ANIMALS: Thirty-four dogs with HSA and 42 healthy dogs from the Ontario Veterinary College Teaching Hospital. METHODS: This case-control study compared serum proteins in dogs with HSA and healthy dogs. Proteins were separated by 2-dimensional difference gel electrophoresis and identified by liquid chromatography and tandem mass spectrometry. RESULTS: Western blot analysis showed that serum collagen XXVII peptide concentration in serum of dogs with large metastatic HSA burdens (1,488, 231-3,754 DU; median, minimum-maximum); was, on average, 9.5-fold higher than in healthy dogs (156; 46-2,101 DU). While concentrations for dogs with osteosarcomas (678; 124-3,251 DU), lymphomas (423; 92-2,777 DU), carcinomas (1,022; 177-3,448 DU), and inflammatory disease were also increased, values were consistently lower than those for HSA. Receiver operating characteristic curves revealed an estimated area under the curve of 83% for HSA cases whereas areas for other neoplastic and nonneoplastic diseases were nondiscriminatory. Serum collagen XXVII peptide concentration before splenectomy (1,350; 1,156-1,929 DU) was reduced after tumor removal (529; 452-562 DU) and chemotherapy but increased in 2 dogs with tumor recurrence (511-945 DU; 493-650 DU). CONCLUSIONS AND CLINICAL IMPORTANCE: Collagen XXVII peptide might be useful for diagnosis and monitoring of advanced HSA.

Response evaluation criteria for peripheral nodal lymphoma in dogs (v1.0)--a Veterinary Cooperative Oncology Group (VCOG) consensus document.

Standardized assessment of response to therapy for lymphoma in dogs is lacking, making critical comparisons of treatment protocols difficult. This Veterinary Cooperative Oncology Group (VCOG) consensus document, based on the recommendations of a subcommittee of ACVIM board-certified veterinary oncologists, was unanimously adopted at the 29th Annual Conference of the Veterinary Cancer Society (VCS) by the VCOG membership. It has integrated guidance from the response assessment criteria established for lymphoma in human patients using standards available in routine veterinary oncology practices that are simple, repeatable and consistently applicable. These guidelines are intended only for use in dogs, where peripheral lymphadenopathy represents the principal component of their disease and as such do not critically assess extranodal disease (e.g., primary cutaneous, central nervous system, gastrointestinal). It is hoped these guidelines will be widely adopted and serve to facilitate the comparison of current and future treatment protocols used in the therapy of dogs.

A survey of evidence in the Journal of Veterinary Internal Medicine oncology manuscripts from 1999 to 2007.

OBJECTIVES: To survey and monitor trends in evidence for oncology manuscripts published in the Journal of Veterinary Internal Medicine (JVIM) between 1999 and 2007 based on an evidence-based medicine (EBM) standard. METHODS: All veterinary oncology-related articles published in JVIM and 7 other high-impact journals from 1999 to 2007 were collected by database searches. Relevant manuscripts then were characterized including investigator affiliation, subject matter investigated, retrospective or prospective study design, manuscript type, and classifications of manuscripts using an EBM standard. RESULTS: A total of 172 relevant veterinary oncology manuscripts were identified in JVIM between 1999 and 2007. The proportion of oncology manuscripts published each year rose with the total number of manuscripts published in JVIM (mean, 13%; range, 8-15%). The author affiliations and subject matter were similar during this evaluation period. Case series represented the most common manuscript type (40%). With the exception of a progressive increase in prospective manuscripts and a reduction in case reports, no significant changes in the classification of manuscripts using EBM standards were seen. During this same period, veterinary oncology manuscripts published in 7 high-impact journals were associated with higher standards of evidence including prospective studies and randomized trials. CONCLUSIONS: The standards of evidence for veterinary oncology manuscripts published in JVIM have remained static between 1999 and 2007. This survey provides an informative benchmark for the state of evidence in previous JVIM oncology manuscripts and may be useful in identifying specific opportunities that may raise the standards of evidence in future publications in JVIM.

Beta4 integrin promotes osteosarcoma metastasis and interacts with ezrin.

The development of pulmonary metastasis is the major cause of death in osteosarcoma, and its molecular basis is poorly understood. In this study, we show that beta4 integrin is highly expressed in human osteosarcoma cell lines and tumor samples. Furthermore, highly metastatic MNNG-HOS cells have increased levels of beta4 integrin. Suppression of beta4 integrin expression by shRNA and disruption of beta4 integrin function by transfection of dominant-negative beta4 integrin was sufficient to revert this highly metastatic phenotype in the MNNG-HOS model without significantly affecting primary tumor growth. These findings suggest a role for beta4 integrin expression in the metastatic phenotype in human osteosarcoma cells. In addition, we identified a previously uncharacterized interaction between beta4 integrin and ezrin, a membrane-cytoskeletal linker protein that is implicated in the metastatic behavior of osteosarcoma. The beta4 integrin-ezrin interaction appears to be critical for maintenance of beta4 integrin expression. These data begin to integrate ezrin and beta4 integrin expression into a model of action for the mechanism of osteosarcoma metastases.

The actin-cytoskeleton linker protein ezrin is regulated during osteosarcoma metastasis by PKC.

Ezrin is a member of the ERM (ezrin, radixin, moesin) protein family and links F-actin to the cell membrane following phosphorylation. Ezrin has been associated with tumor progression and metastasis in several cancers including the pediatric solid tumors, osteosarcoma and rhabdomyosarcoma. In this study, we were surprised to find that ezrin was not constitutively phosphorylated but rather was dynamically regulated during metastatic progression in osteosarcoma. Metastatic osteosarcoma cells expressed phosphorylated ERM early after their arrival in the lung, and then late in progression, only at the invasive front of larger metastatic lesions. To pursue mechanisms for this regulation, we found that inhibitors of PKC (protein kinase C) blocked phosphorylation of ezrin, and that ezrin coimmunoprecipitated in cells with PKCalpha, PKCiota and PKCgamma. Furthermore, phosphorylated forms of ezrin and PKC had identical expression patterns at the invasive front of pulmonary metastatic lesions in murine and human patient samples. Finally, we showed that the promigratory effects of PKC were linked to ezrin phosphorylation. These data are the first to suggest a dynamic regulation of ezrin phosphorylation during metastasis and to connect the PKC family members with this regulation.

Phase I clinical evaluation of carboplatin in tumor-bearing cats: a Veterinary Cooperative Oncology Group study.

BACKGROUND: The dosage of carboplatin in cats has been reported anecdotally and experimentally in non-tumor-bearing cats, but the dosage for carboplatin treatment in tumor-bearing cats has yet to be defined in a prospective clinical trial. PURPOSE: To determine the maximally tolerated dose (MTD) and dose-limiting toxicosis (DLT) of carboplatin in tumor-bearing cats. CATS: Fifty-nine cats with measurable solid tumors. METHODS: The starting dose of carboplatin was 160 mg/m(2) of body surface area IV. Doses were increased by 20 mg/m(2) in cohorts of 3-14 cats until the MTD was reached. RESULTS: The 59 cats entered into this multi-institutional phase I study received 1 or more doses of carboplatin at various dosages and were evaluated for toxicity, response to treatment, or both. The MTD was 240 mg/m(2) and neutropenia was the DLT. For the 1st cycle of treatment in 44 cats evaluated for neutropenia, 6 episodes of grade 3 or greater neutropenia occurred on days 7 (n=1), 14 (n=4), and 21 (n=1). There was no evidence of drug-induced nephrotoxicosis or pulmonary edema. Preliminary evidence of antitumor activity was observed in 7 of 59 (11.9%; 95% CI, 5.6-22.8%) cats evaluated for response to treatment. There was 1 complete response (cutaneous hemangiosarcoma) and 6 partial responses (4 injection site sarcomas, 1 oral squamous cell carcinoma, 1 lymphoma). Responses were of short duration (median, 42 days; range, 7-168 days). CONCLUSIONS AND CLINICAL IMPORTANCE: The dose of carboplatin recommended to treat tumor-bearing cats is 240 mg/m(2) IV every 3-4 weeks.

Spontaneous and genetically engineered animal models; use in preclinical cancer drug development.

The preclinical development of anticancer drugs has been based primarily on the transplantation of murine or human cancers into mice. Alternatives to these transplantation models are animals that naturally develop cancers with features relevant to the human disease. The first group of these models arises in mice that are genetically engineered to develop cancer. The second group includes pet dogs and cats that naturally develop cancer. This review will discuss the use and integration of these spontaneous cancer models into a comprehensive and comparative approach to preclinical drug development. Examples of their successful use and an outline of their relative strengths and weaknesses will be provided.