Howard R. Mellor

Cardiotoxicity Associated with Targeting Kinase Pathways in Cancer

Cardiotoxicity, also referred to as drug-induced cardiac injury, is an issue associated with the use of some small-molecule kinase inhibitors and antibody-based therapies targeting signaling pathways in cancer. Although these drugs have had a major impact on cancer patient survival, data have implicated kinase-targeting agents such as sunitinib, imatinib, trastuzumab, and sorafenib in adversely affecting cardiac function in a subset of treated individuals. In many cases, adverse cardiac events in the clinic were not anticipated based on preclinical safety evaluation of the molecule. In order to support the development of efficacious and safe kinase inhibitors for the treatment of cancer and other indications, new preclinical approaches and screens are required to predict clinical cardiotoxicity. Laboratory investigations into the underlying molecular mechanisms of heart toxicity induced by these molecules have identified potentially common themes including mitochondrial perturbation and modulation of adenosine monophosphate-activated protein kinase activity. Studies characterizing cardiac-specific kinase knockout mouse models have developed our understanding of the homeostatic role of some of these signaling mediators in the heart. Therefore, when considering kinases as potential future targets or when examining secondary pharmacological interactions of novel kinase inhibitors, these models may help to inform us of the potential adverse cardiac effects in the clinic.

Induction of Heart Valve Lesions by Small-Molecule ALK5 Inhibitors

Aberrant signaling by transforming growth factor-β (TGF-β) and its type I (ALK5) receptor has been implicated in a number of human diseases and this pathway is considered a potential target for therapeutic intervention. Transforming growth factor-β signaling via ALK5 plays a critical role during heart development, but the role of ALK5 in the adult heart is poorly understood. In the current study, the preclinical toxicology of ALK5 inhibitors from two different chemistry scaffolds was explored. Ten-week-old female Han Wistar rats received test compounds by the oral route for three to seven days. Both compounds induced histopathologic heart valve lesions characterized by hemorrhage, inflammation, degeneration, and proliferation of valvular interstitial cells. The pathology was observed in all animals, at all doses tested, and occurred in all four heart valves. Immunohistochemical analysis of ALK5 in rat hearts revealed expression in the valves, but not in the myocardium. Compared to control animals, protein levels of ALK5 were unchanged in the heart valves of treated animals. We also observed a physeal dysplasia in the femoro-tibial joint of rats treated with ALK5 inhibitors, a finding consistent with a pharmacological effect described previously with ALK5 inhibitors. Overall, these findings suggest that TGF-β signaling via ALK5 plays a critical role in maintaining heart valve integrity.

Gender differences in drug toxicity

Clinical data suggest that gender dimorphic profiles are emerging in terms of both drug efficacy and adverse drug reactions (ADRs). With an increasing emphasis on individualised therapies and the need to prevent drug attrition there is a compelling need to understand the molecular basis for gender dimorphic profiles in ADRs and the consequences. Classes of agents exhibiting gender-based variation in pharmaceutical efficacy and toxicity include anaesthetics, HIV-1 therapies and antiarrhythmic drugs. Body weight differences are often cited as a reason for differences in drug pharmacokinetics and subsequent toxicity. However, some studies accounted for these factors and still found significance suggesting that dosage versus body weight does not explain the outcome. Here, we present an overview of current understanding of gender-specific drug toxicity and present rational molecular explanations for these adverse events. There is mounting evidence in support of hormonal effects underpinning the majority of the ADR differences observed between the sexes.

Using mode of action information to improve regulatory decision-making: An ECETOC/ILSI RF/HESI workshop overview

The European Centre for Ecotoxicology and Toxicology of Chemicals (ECETOC), the International Life Sciences Institute (ILSI) Research Foundation (RF), and the ILSI Health and Environmental Sciences Institute (HESI) hosted a workshop in November 2009 to review current practice in the application of mode of action (MOA) considerations in chemical risk assessment. The aim was to provide a rationale for a more general, but flexible approach and to propose steps to facilitate broader uptake and use of the MOA concept. There was consensus amongst the workshop participants that it will require substantial effort and cooperation from the multiple disciplines involved to embrace a common, consistent, and transparent approach. Setting up a repository of accepted MOAs and associated guidance concerning appropriate data to support specific MOAs for critical effects would facilitate categorization of chemicals and allow predictions of toxicity outcomes by read-across. This should in future contribute to the reduction of toxicity testing in animals. The workshop participants also acknowledged the value and importance of human data and the importance of integrating information from biological pathway analyses into current MOA/human relevance frameworks.

Targeted inhibition of the FGF19-FGFR4 pathway in hepatocellular carcinoma; translational safety considerations.

Hepatocellular carcinoma (HCC) is a leading cause of cancer‐related death and new therapies are urgently required to treat this disease. Recent data suggest that the FGF19‐FGFR4 axis may be a key driver in certain forms of HCC, making the pathway an interesting, emerging molecular target for potential therapeutic intervention. A complication is that, outside of malignant disease, FGFR4 plays an important physiological role in the regulation of hepatic bile acid (BA) synthesis. FGF19 signalling via FGFR4 suppresses de novo BA production in the liver, tightly maintaining hepatic and systemic levels of these detergent‐like molecules at a physiological threshold and preventing pathological complications of raised BA levels, such as cholestatic liver injury and bile acid diarrhoea. In some cases of HCC, the malignant disease causes bile duct obstruction, preventing BA secretion from the liver and resulting in cholestasis. Here, the role of FGFR4 signalling in both HCC and BA homoeostasis is discussed. The potential effects of therapeutic FGF19‐FGFR4 inhibition on human hepatobiliary/gastrointestinal physiology are considered along with the potential safety implications of FGF19‐FGFR4 blockade in patients with HCC.

Epigenetics – relevance to drug safety science

Epigenetics describes the study of heritable changes in gene expression that occur in the absence of a change to the DNA sequence. Specific patterns of epigenetic signatures can be stably transmitted through mitosis and cell division and form the molecular basis for developmental stage- and cell type-specific gene expression. Associations have been observed that endogenous and exogenous stimuli can change the epigenetic control of both somatic and stem cell differentiation and thus influence phenotypic behaviours and/or disease progression. In relation to drug safety, DNA methylation changes have been identified in many stages of tumour development following exposure to non-genotoxic carcinogens. However, it is not clear whether DNA methylation changes cause cancer, or arise as a consequence of the transformed state. Toxic agents could act at different levels, by directly modifying the epigenome or indirectly by altering signalling pathways. These alterations in chromatin structure may or may not be heritable but are probably reversible. That said, there is currently insufficient data to support inclusion of epigenetic profiling into pre-clinical evaluation studies. Several international collaborations aim to generate data to determine whether epigenetic modifications are causal links in disease and/or tumour progression. It will only be when an understanding of chemical mode-of-action is required that evaluation of epigenetic changes might be considered. The current toxicological testing battery is expected to identify any potential adverse effects regardless of the mechanism, epigenetic or otherwise. It is recommended that toxicologists keep a close watch of new developments in this field, in particular identification of early epigenetic markers for non-genotoxic carcinogenicity. Scientific collaborations between academia and industry will help to understand inter-individual variations in response to drug and toxin exposure to be able to distinguish between adverse and non-adverse epigenetic changes.

019 Chemotherapy-induced cardiotoxicity: could a translational cardiac MRI model help identify patients at risk?

Background Cancer survivorship is an international priority and mortality from cardiac damage is one of the greatest concerns. Anthracycline chemotherapy is cardiotoxic but remains highly effective against cancer. Methods This translational project involved the development of a pre-clinical rat model of progressive anthracycline-induced cardiotoxicity (1.25mg/m2 doxorubicin weekly for 8 doses) and a concurrent clinical study of 30 cancer patients receiving 6 cycles of curative anthracycline therapy (doxorubicin 50mg/m2). A bespoke cardiac magnetic resonance imaging (CMR) protocol was developed which included longitudinal T1 mapping (modified look-locker) to estimate extracellular volume (ECV), T2 mapping (to quantify oedema) and strain analysis. A panel of circulating biomarkers (including inflammatory, ischaemic and fibrosis markers) was evaluated by multiplex ELISA. Longitudinal histological analysis was performed in the rats and correlated with imaging and biomarker findings. Results Left ventricular function (LVEF) declined steadily during treatment in rats and humans. Persistent LV dysfunction was seen in 23% of patients 12 months after therapy. Peak Troponin I levels correlated with fall in (LVEF) and circulating levels translated well between rat and humans. However, levels did not peak until the final cycle of chemotherapy when significant LV decline had already occurred. There was no significant change in the other circulating biomarkers. ECV did not change significantly during treatment but lower baseline ECV correlated with a greater fall in LVEF. Microscopic changes were seen in the rat myocardium after 6 doses of doxorubicin but electron-microscopy revealed mitochondrial damage after just one dose. Conclusions This translational approach enabled forward and back translation leading to the development of a clinically relevant pre-clinical cardiotoxicity model. Troponin I was the most informative circulating biomarker but peaked at the end of therapy too late to modify treatment and prevent LV decline. The model has the potential to be used to identify earlier biomarkers and evaluate cardioprotective strategies. The imaging findings showed that fibrosis cannot be detected on CMR within one year of chemotherapy but generated a new hypothesis that patients with ‘healthier hearts’ may be a greatest risk of drug-induced cardiotoxicity.