Cardiotoxicity of Doxorubicin. A Study of Methods and Protective Interventions in Rat Models

Introduction: The clinical use of anthracyclines, like doxorubicin, is a double-edged sword. On one side, anthracyclines play an undisputed key role in the treatment of many neoplastic diseases. On the other side; administration of anthracyclines induces cardiomyopathy and congestive heart failure usually refractory to common medications. Therefore, interventions to reduce the cardiotoxicity of doxorubicin are important and clinically relevant, and research should be performed to achieve a better understanding of the toxic mechanisms of doxorubicin. Generation of reactive oxygen species (ROS), cellular damage mediated by ROS, mitochondrial dysfunction, and impaired calcium handling have been proposed as toxic mechanisms to explain both acute and delayed cardiotoxicity of anthracyclines. The mechanisms behind the cardiotoxicity of doxorubicin are numerous, and unfortunately not fully understood. However, the anticancer mechanisms and the cardiotoxic mechanisms seem to be quite distinct, leading us to hope that interventions targeted towards the cardiotoxic effects will not interfere or diminish the anticancer effect of this wildly used drug. In order to intervene, and hopefully prevent cardiotoxicity and heart failure, we need tools and biomarkers for early detection of heart damage. A common clinical tool for evaluation of cardiac function is monitoring left ventricular ejection fraction (EF). A weakness in this method is that cardiac damage is usually detected only when an irreversible functional impairment has already occurred, which leaves little room for early, preventive strategies. Measurement of biomarkers, on the other hand, can be a useful diagnostic tool for early identification, assessment, and monitoring of cardiotoxicity. Release of biomarkers like cardiac troponin T (TnT) and indices of ROS generation like hydrogen peroxide (H2O2) are of relevance to study myocardial damage, and can be supplemented by measurement of myocardial accumulation of doxorubicin and its metabolite doxorubicinol in experimental studies, in order to get a better understanding of the pharmacokinetics and pharmacodynamics of this drug. Thus, animal models where functional, biochemical and pharmacological indices can be studied within an acceptable time-frame, are of interest. Aims: To establish a rat model to assess interventions to reduce cardiotoxicity of doxorubicin based on physiological, biochemical and pharmacological indices. To study and assess whether pharmacological pretreatment or pharmacological preconditioning with morphine and diazoxide, could reduce cardiotoxicity of doxorubicin. Results: We developed a short-time model (STM) (Paper I) that reduced animal stress as well as time- and resource consumption in experimental protocols, compared to a long-time model (LTM). Furthermore, we found that morphine enhanced doxorubicin cardiotoxicity (Paper II), and we found diazoxide to be protective against doxorubicin cardiotoxicity (Paper III). Conclusion: The principles of pharmacological preconditioning (mimicking ischemic preconditioning) represent promising protective interventions in doxorubicin cardiotoxicity, and can be studied in a STM. In our rat model, diazoxide, but not morphine, showed protective effects that could be related to preconditioning. However, the results could be related to other effects of the two drugs