Elisidepsin (elisidepsin trifluoroacetate, Irvalec®, PM02734) is a new synthetic depsipeptide, a result of the PharmaMar Development Program that seeks synthetic products of marine origin-derived compounds. Elisidepsin is a drug with antiproliferative activity in a wide range of tumors. In the present work we studied and characterized the mechanisms associated with sensitivity and resistance to elisidepsin treatment in a broad panel of tumor cell lines from breast and pancreas carcinomas, focusing on different factors involved in epithelial-mesenchymal transition (EMT) and the use of HER family receptors in predicting the in vitro drug response. Interestingly, we observed that the basal protein expression levels of EMT markers show a significant correlation with cell viability in response to elisidepsin treatment in a panel of 12 different breast and pancreatic cancer cell lines. In addition, we generated three elisidepsin treatment-resistant cell lines (MCF-7, HPAC and AsPC-1) and analyzed the pattern of expression of different EMT markers in these cells, confirming that acquired resistance to elisidepsin is associated with a switch to the EMT state. Furthermore, a direct correlation between basal HER3 expression and sensitivity to elisidepsin was observed; moreover, modulation of HER3 expression levels in different cancer cell lines alter their sensitivities to the drug, making them more resistant when HER3 expression is downregulated by a HER3-specific short hairpin RNA and more sensitive when the receptor is overexpressed. These results show that HER3 expression is an important marker of sensitivity to elisidepsin treatment. © 2013 Teixidó et al.