1045 Efficiency and Transport of Taxanes in Human Cancer Cell Lines and Solid Tumors

However, the low frequency of BRCA1/2 mutations limits the applicability of this therapeutic strategy. In contrast, aberrations in phosphatidylinositol 3-kinase (PI3K) pathway leading to pathway activation are frequent in TNBC. Besides regulating cell metabolism and promoting survival, the PI3K pathway has been also implicated in the maintenance of DNA integrity. In this study we hypothesized that inhibition of PI3K signaling may result in deficient HR with consequent sensitization to PARP inhibition. This phenomenon would occur in BRCA-proficient tumors, providing a therapeutic rationale for treating a larger set of patients with TNBC. Materials and Methods: PI3K inhibition was attained through either siRNA or treatment with pan-PI3K inhibitors. Clonogenic, anchorage-independent growth and xenograft assays were used to assess efficacy of treatments. Five patient-derived xenograft (PDX) models were developed by in vivo serial transplantation of tumors from metastatic breast cancer patients after informed consent. Results: Using BRCA wildtype TNBC cell-lines and PDXs, we demonstrate that PI3K blockade leads to gHistone-2AX nuclear foci accumulation, a marker of double strand DNA breaks. Concomitantly, inhibition of PI3K suppressed the expression of both BRCA1/2 and Rad51, another component of HR. However, while Rad51 was consistently downregulated in all the animal models, BRCA1/2 levels diminished only in 3 out of 5 PDXs. First analyses revealed that BRCA1/2 expression were, at least in part, modulated by the ERK pathway. Indeed, downregulation of BRCA1/2 (both mRNA and protein levels) was prevented by pharmacological MEK blockade. PI3K blockade, achieved by either siRNA of PIK3CA or pharmacological inhibition of the pathway, sensitized BRCA wildtype cell-lines to PARP inhibition in vitro. In vivo, this phenomenon was limited only to those models that presented BRCA1/2 downregulation following PI3K suppression. In these PDXs, simultaneous inhibition of PI3K and PARP resulted in profound anti-tumor response, whereas no superior activity versus single agent was observed in the PDXs where BRCA expression was retained upon PI3K blockade. Conclusions: PI3K blockade sensitizes to PARP inhibition by promoting BRCA-deficiency that may be regulated by ERK feedback signaling. This combinatorial strategy broadens the applicability of PARP inhibition to BRCAproficient patients and deserves further investigation to identify accurate predictive biomarkers.