Capecitabine efficacy is correlated with TYMP and RB expression in PDX established from triple-negative breast cancers

Purpose: triple-negative breast cancer (TNBC) patients with residual disease after neoadjuvant chemotherapy have a poor outcome. We developed patient-derived xenografts (PDX) from residual tumors to identify efficient chemotherapies and predictive biomarkers in a context of resistance to anthracyclines and taxanes-based treatments. Experimental Design: PDX were established from residual tumors of primary breast cancer patients treated in neoadjuvant setting. TNBC PDX were treated by anthracyclines, taxanes, platins and capecitabine. Predictive biomarkers were identified by transcriptomic and immunohistological analysis. Downregulation of RB1 was performed by siRNA in a cell line established from a PDX. Results: residual TNBC PDX were characterized by a high tumor take, a short latency and a poor prognosis of the corresponding patients. With the exception of BRCA1/2 mutated models, residual PDX were resistant to anthracyclines, taxanes, and platins. Capecitabine, the oral prodrug of 5-FU, was highly efficient in 60% of PDX with two models showing complete responses. Prior treatment of a responder PDX with 5-FU increased expression of thymidylate synthase and decreased efficacy of capecitabine. Transcriptomic and IHC analyses of 32 TNBC PDX, including both residual tumors and treatment-naive derived tumors, identified RB and TYMP proteins as predictive biomarkers for capecitabine response. Finally, RB1 knockdown in a cell line established from a capecitabine-responder PDX decreased sensitivity to 5-FU treatment. Conclusions: we identified capecitabine as efficient chemotherapy in TNBC PDX models established from residual disease and resistant to anthracyclines, taxanes and platins. RB positivity and high expression of TYMP were significantly associated with capecitabine response.