Abstract 950: Src tyrosine kinase contributes to lapatinib resistance in human breast cancer models.

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Abstract Background. The dual EGFR/HER2 tyrosine kinase inhibitor (TKI) lapatinib is approved in metastatic breast cancer patients after trastuzumab failure. Unfortunately, several patients do not respond to treatment due to constitutive resistance, and even in responders the occurrence of disease progression is often observed. Recently, the activation of Src TK has been associated with resistance to anti-HER2 drugs, both trastuzumab and lapatinib, in HER2 overexpressing breast cancer cells. However, the molecular mechanisms by which Src activation may affect response to HER2 antagonists are still partially unclear. Methods. We used a panel of HER2 expressing human breast cancer cell lines with different degree of response to lapatinib: SKBR3, BT474 and MDA-MB-361 (high sensitivity), KPL-4 (moderate sensitivity), JIMT-1 (low sensitivity). Moreover, we generated a cell line with acquired resistance to lapatinib, MDA-MB-361-LR (Lapatinib Resistant). On these models, we investigated the effect of Src silencing via small interference RNA (siRNA) or pharmacological inhibition by saracatinib on lapatinib response, both in vitro and in vivo. Results. The combination of Src inhibition with lapatinib synergistically inhibited survival of human breast cancer cells with both intrinsic or acquired resistance to lapatinib, by interfering with signal transducers involved in cell proliferation and survival. Moreover, both saracatinib and Src specific siRNAs were able to reduce migration and invasion capabilities of breast cancer cells, by affecting Focal Adhesion Kinase (FAK) and paxillin activation. In nude mice xenografted with JIMT-1 cells, the combined treatment with saracatinib and lapatinib produced a cooperative antitumor effect with inhibition of tumor growth, interference with signal transduction and prolongation of mice survival. The combination strongly reduced the incidence of lung metastasis, as revealed through experimental metastasis assays. Interestingly, a significant reduction of EGFR/HER2 co-immunoprecipitation was found in resistant cells, thus suggesting that growth signals could be transduced by other type of TKRs dimers in these cells. Moreover, in breast cancer cell lines with both intrinsic or acquired resistance to lapatinib, Src interacted with EGFR rather than with HER2, in opposition to what was observed in sensitive cells. Consistently, EGFR inhibition through cetuximab or EGFR specific siRNAs was able to partially restore lapatinib sensitivity in resistant cells, by interfering with the reciprocal activation of EGFR and Src. Conclusions. Src plays an important role in the onset of resistance to lapatinib in breast cancer cell lines, and its interaction with EGFR may be involved in this event. Therefore, the combined inhibition of Src and EGFR/HER2 axis may represent a potentially effective therapeutic strategy in order to prevent or overcome resistance to lapatinib. Citation Format: Roberta Rosa, Lucia Nappi, Luigi Formisano, Claudia D'Amato, Valentina D'Amato, Vincenzo Damiano, Roberta Marciano, Ana Paula De Maio, Lucia Raimondo, Umberto Malapelle, Giancarlo Troncone, Sabino De Placido, Roberto Bianco. Src tyrosine kinase contributes to lapatinib resistance in human breast cancer models. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 950. doi:10.1158/1538-7445.AM2013-950 Note: This abstract was not presented at the AACR Annual Meeting 2013 because the presenter was unable to attend.