Anti-tumor and cardiotoxic effects of microtubule polymerization inhibitors: The mechanisms and management strategies

Microtubule polymerization inhibitors (MPIs) have long been used as anticancer agents because they inhibit mitosis. Microtubules are thought to play an important role in the migration of tumor cells and the formation of tumor blood vessels, and new MPIs are being developed. Many clinical trials of novel MPIs have been conducted in humans, while some clinical studies in dogs have also been reported. More attempts to apply MPIs not only in humans but also in the veterinary field are expected to be made in the future. Meanwhile, MPIs have a risk of cardiotoxicity. In this paper, we review findings on the pharmacological effects and cardiotoxicity of MPIs, as well as the mechanisms of their cardiotoxicity. Cardiotoxicity of MPIs involves not only the direct effects of MPIs on cardiomyocytes but also their effects on vascular function. For example, hypertension induced by impaired vascular function also contributes to the exacerbation of myocardial damage, and blood pressure control may be useful in reducing cardiotoxicity. By combined administration of MPIs and other anticancer agents, MPI efficacy may be enhanced, thereby potentially allowing to keep MPI dosage low. Measurement of myocardial injury markers in blood and echocardiography may be useful for monitoring cardiotoxicity. In particular, two-dimensional speckle tracking may have high sensitivity for the early detection of MPI-induced cardiac dysfunction. The exploration of the potential of new MPIs while understanding their toxicity and how to deal with them will lead to the further development of cancer chemotherapy.

A phase 2 trial of verubulin for recurrent glioblastoma: a prospective study by the Brain Tumor Investigational Consortium (BTIC)

Treatment options are limited for recurrent glioblastoma (GBM). Verubulin is a microtubule destabilizer and vascular disrupting agent that achieve high brain concentration relative to plasma in animals. Adults with recurrent GBM who failed prior standard therapy were eligible. The primary endpoint was 1-month progression-free survival (PFS-1) for bevacizumab refractory (Group 2) and 6-month progression-free survival (PFS-6) for bevacizumab naïve patients (Group 1). Verubulin was administered at 3.3 mg/m(2) as a 2-h intravenous infusion once weekly for 3 consecutive weeks in a 4-week cycle. The planned sample size was 34 subjects per cohort. 56 patients (37 men, 19 women) were enrolled, 31 in Group 1 and 25 in Group 2. The PFS-6 for Group 1 was 14% and the PFS-1 for Group 2 was 20%. Median survival from onset of treatment was 9.5 months in Group 1 and 3.4 months in Group 2. Best overall response was partial response (n = 3; 10% in Group 1; n = 1; 4.2% in Group 2) and stable disease (n = 7; 23% in Group 1; n = 5; 21% in Group 2). In Group 1, 38.7% of patients experienced a serious adverse event; however only 3.2% were potentially attributable to study drug. In Group 2, 44% of patients experienced a serious adverse event although none were attributable to study drug. Accrual was terminated early for futility. Single agent verubulin, in this dose and schedule, is well tolerated, associated with moderate but tolerable toxicity but has limited activity in either bevacizumab naïve or refractory recurrent GBM.

Cardiovascular toxicity profiles of vascular-disrupting agents

BACKGROUND

Vascular-disrupting agents (VDAs) represent a new class of chemotherapeutic agent that targets the existing vasculature in solid tumors. Preclinical and early-phase trials have demonstrated the promising therapeutic benefits of VDAs but have also uncovered a distinctive toxicity profile highlighted by cardiovascular events.

METHODS

We reviewed all preclinical and prospective phase I-III clinical trials published up to August 2010 in MEDLINE and the American Association of Cancer Research and American Society of Clinical Oncology meeting abstracts of small-molecule VDAs, including combretastatin A4 phosphate (CA4P), combretastatin A1 phosphate (CA1P), MPC-6827, ZD6126, AVE8062, and ASA404.

RESULTS

Phase I and II studies of CA1P, ASA404, MPC-6827, and CA4P all reported cardiovascular toxicities, with the most common cardiac events being National Cancer Institute Common Toxicity Criteria (version 3) grade 1-3 hypertension, tachyarrhythmias and bradyarrhythmias, atrial fibrillation, and myocardial infarction. Cardiac events were dose-limiting toxicities in phase I trials with VDA monotherapy and combination therapy.

CONCLUSIONS

Early-phase trials of VDAs have revealed a cardiovascular toxicity profile similar to that of their vascular-targeting counterparts, the angiogenesis inhibitors. As these agents are added to the mainstream chemotherapeutic arsenal, careful identification of baseline cardiovascular risk factors would seem to be a prudent strategy. Close collaboration with cardiology colleagues for early indicators of serious cardiac adverse events will likely minimize toxicity while optimizing the therapeutic potential of VDAs and ultimately enhancing patient outcomes.