Abstract 1226: Lapatinib could not prevent heregulin-induced HER2-HER3 signalling, and addition of pertuzumab to lapatinib increases drug sensitivity and inhibition of HER signalling

Abstract

Lapatinib is an EGFR/HER2 tyrosine kinase inhibitor (TKI) that has been approved for clinical use in HER2 positive breast cancer patients who have progressed on previous Trastuzumab containing regimens. However, lapatinib monotherapy only delays tumour growth in HER2 positive xenograft models and has only modest clinical activity in breast cancer patients. There is a further need to understand its mechanisms of action and resistance. We aimed to assess the acute effect of lapatinib on HER2 phosphorylation in HER2 positive breast cancer cells since previous work from our lab showed that inadequate suppression of HER2 phosphorylation by gefitinib and trastuzumab may lead to acquired drug resistance. Traditionally, the phosphorylation status of HER proteins is determined by phosphor-tyrosine western blotting followed by immunoprecipitation. The limitation of this technique includes false positive due to signals from co-precipitated proteins of the same size as target, which is even more prominent with the HER family of protein. With the use of phosphorylated protein-specific antibody, we found that, contrary to general belief, lapatinib could not abolish HER2 phosphorylation in HER2 over-expressed breast cancer cell lines under serum-supplemented condition. It was thought that the growth factors in the serum activate other HER receptors to maintain HER2 phosphorylation. Indeed, Lapatinib could reduce phospho-HER2 level in serum-starved breast cancer cell lines; however, it failed to reverse heregulin-induced HER signalling activation and downstream MAPK and AKT signalling. The combination of lapatinib and pertuzumab drastically inhibited ligand-induced HER family phosphorylation and downstream signalling. HER3 was reported as an important biomarker for resistance in HER family targeted therapy. This novel combination could inhibit HER3 phosphorylation and its downstream AKT pathway more than lapatinib or pertuzumab alone; this finding also correlated with cancer cell viability, as the combination of lapatinib and pertuzumab reduced cell viability more than either drug alone. These results might indicate a new strategy of drug combination to tackle resistance in HER2 targeted therapy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1226. doi:1538-7445.AM2012-1226