Background-Tyrosine kinase inhibitors (TKIs) have advanced cancer treatment. Sunitinib, a recently-approved, multi-targeted TKI, prolongs survival for patients with metastatic renal cell carcinoma (RCC) and gastrointestinal stromal tumor (GIST), but concerns about cardiac safety have arisen with this agent.

Based on extensive preclinical data, glycogen synthase kinase-3 (GSK-3) has been proposed to be a viable drug target for a wide variety of disease states, ranging from diabetes to bipolar disorder. Since these new drugs, which will be more powerful GSK-3 inhibitors than lithium, may potentially be given to women of childbearing potential, and since it has controversially been suggested that lithium therapy might be linked to congenital cardiac defects, we asked whether GSK-3 family members are required for normal heart development in mice. We report that terminal cardiomyocyte differentiation was substantially blunted in Gsk3b -/-embryoid bodies. While GSK-3α-deficient mice were born without a cardiac phenotype, no live-born Gsk3b -/-pups were recovered. The Gsk3b -/-embryos had a double outlet RV, ventricular septal defects, and hypertrophic myopathy, with near obliteration of the ventricular cavities. The hypertrophic myopathy was caused by cardiomyocyte hyperproliferation without hypertrophy and was associated with increased expression and nuclear localization of three regulators of proliferation -GATA4, cyclin D1, and c-Myc. These studies, which we believe are the first in mammals to examine the role of GSK-3α and GSK-3β in the heart using loss-of-function approaches, implicate GSK-3β as a central regulator of embryonic cardiomyocyte proliferation and differentiation, as well as of outflow tract development. Although controversy over the teratogenic effects of lithium remains, our studies suggest that caution should be exercised in the use of newer, more potent drugs targeting GSK-3 in women of childbearing age.

Tyrosine kinase inhibitors (TKIs) are transforming the treatment of patients with malignancies. One such agent, sunitinib (Sutent, Pfizer, New York, NY, USA), has demonstrated activity against a variety of solid tumors. Sunitinib is "multitargeted," inhibiting growth factor receptors that regulate both tumor angiogenesis and tumor cell survival. However, cardiac dysfunction has been associated with its use. Identification of the target of sunitinib-associated cardiac dysfunction could guide future drug design to reduce toxicity while preserving anticancer activity. Herein we identify severe mitochondrial structural abnormalities in the heart of a patient with sunitinib-induced heart failure. In cultured cardiomyocytes, sunitinib induces loss of mitochondrial membrane potential and energy rundown. Despite the latter, 5' adenosine monophosphate-activated protein kinase (AMPK) activity, which should be increased in the setting of energy compromise, is reduced in hearts of sunitinib-treated mice and cardiomyocytes in culture, and this is due to direct inhibition of AMPK by sunitinib. Critically, we find that adenovirus-mediated gene transfer of an activated mutant of AMPK reduces sunitinib-induced cell death. Our findings suggest AMPK inhibition plays a central role in sunitinib cardiomyocyte toxicity, highlighting the potential of off-target effects of TKIs contributing to cardiotoxicity. While multitargeting can enhance tumor cell killing, this must be balanced against the potential increased risk of cardiac dysfunction.

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