d with EKB 569 exposure. It is important to note that Barrick et al. Page 7 Toxicol Appl Pharmacol. Author manuscript, available in PMC 2009 May 18. NIH PA Author Manuscript NIH CH5132799 PI3K inhibitor PA Author Manuscript NIH PA Author Manuscript interstitial lung disease has been reported in a subset of patients receiving gefinitib in nonsmall cell lung cancer clinical trials. Although we did not observe increased pulmonary fibrosis, indirect evidence of pulmonary damage was supported by increased pulmonary proteinosis and thrombi with proteinaceous material in the RV of EGFR inhibitor treated mice. Differences between mode of inhibition, potency and selectivity between the two TKIs used in our experimental regimen may account for the discrepancy in toxicity.
EKB 569 is an irreversible inhibitor, forming a covalent bond with the Cys 773 residue within the EGFR catalytic domain, while AG 1478 is a competitive inhibitor of ATP binding. With irreversible inhibition, CP-466722 1080622-86-1 normal levels of EGFR activity are only recovered after gene transcription and translation. Recent findings suggest irreversible inhibitors may prevent the acquired resistance seen in non small cell lung cancer patients treated with competitive inhibitors such as gefitinib and erlontinib. While these properties are promising for cancer therapy, irreversible TKIs may adversely affect cardiomyocyte function and survival, since EGFR transcript levels are normally very low in the adult mouse and human heart. The AG 1478 diet resulted in an approximately 45% reduction in polyp number, while at approximately the same concentration in identical base chow, EKB 569 caused about 87% reduction in polyp number in the ApcMin mouse model.
A single oral dose of EKB 569 was previously reported to rapidly inhibit EGFR kinase activity by 90% while multiple intraperitoneal doses of AG 1478 decreased phosphorylation of EGFR and ERK1/2 by nearly 60% and over 70%, respectively, in xenograft studies. This data suggests that EKB 569 is more potent than AG 1478, and the greater toxicity observed with EKB 569 may reflect more potent EGFR TKI activity. Although the current data suggests that the observed cardiotoxities are not off target effects, but rather caused by perturbed cardiac homeostasis in the absence of normal EGFR activity, collateral inhibition of ERBB2 may contribute to the cardiotoxicity of EGFR TKIs.
Since EGFR and ERBB2 have a high sequence homology in their catalytic domains, it is not surprising that many TKIs suppress activity of both receptors. In cell free systems, AG 1478 showed higher selectivity for EGFR over ERBB2 than EKB 569 . In cell based assays using human carcinoma cell lines which overexpress EGFR or ERBB2, the IC50 for EKB 569 was 0.03 g/mL and 0.007 g/mL, respectively, consistent with effective inhibition of both receptors. Mice with myocardium specific deletion of Erbb2 resulted in a 70% decrease in myocardial Erbb2 expression and a significant increase in cardiomyocyte apoptosis with anthracycline exposure. Moreover, gene therapy with over expression of Bcl2l1 partially rescued the dilated cardiomyopathy in these mice. Recent data also demonstrated similarly depressed Bcl2l1 expression, cardiomyocyte apoptosis, and mitochondrial dysfunction in isolated cardiomyocytes with exposure to the anti ERBB2 drug Herceptin . Given the well documented roles of ERBB2 and ERBB4 signaling in cardiomyocyte