RAB5A expression is a predictive biomarker for trastuzumab emtansine in breast cancer

PURPOSETargeted therapeutics strongly depends on validated biomarkers in order to select patients most likely to benefit from the treatment. HER2 serves as a predictive biomarker for HER2-targeted tyrosine kinase inhibitors and monoclonal antibodies. HER2 may, however, also be utilized as a transport gate for delivery of cytotoxic agents into the cell, such as for HER2-targeted antibody drug conjugates (ADCs; e.g. trastuzumab emtansine (T-DM1)). The predictive biomarkers for such ADCs may be more complex, also reflecting the intracellular transport.METHODSFive HER2-positive breast and ovarian cancer cell lines were evaluated with respect to T-DM1 sensitivity and correlated to the expression levels of proteins involved in endocytic trafficking including RAB4A, RAB5A and RAB11A, with possible impact on ADC pharmacology. The results were confirmed in a clinical cohort consisting of patients from the adaptive breast cancer clinical trial I-SPY2 where pathological complete response (pCR) was correlated to the RNA expression level of RAB4A, RAB5A and RAB11A. A subset of the clinical KAMILLA trial including 19 patients was used as a verification cohort where semi-quantitative IHC of RAB5A was correlated to progression free survival (PFS).RESULTSThe early endosome marker RAB5A, was found to correlate positively to T-DM1 sensitivity in the cell line panel. Correlation between RAB5A expression and T-DM1 sensitivity (pCR) was confirmed in patients treated with trastuzumab emtansine/pertuzumab in the I-SPY2 trial, but not in the trastuzumab/paclitaxel control arm. The clinical correlation was verified in the patients from the KAMILLA trial where semi-quantitative RAB5A IHC staining correlated significantly positive to PFS.CONCLUSIONThe present results indicate that RAB5A is a predictive biomarker for T-DM1 and outline, for the first time, proteins involved in endocytic trafficking as predictive biomarkers for ADCs.