PO-492 Ex vivo functional homologous recombination (HR) test detects BRCA reversal and resistance to PARPi in metastatic breast cancer patients

Introduction Better predictive biomarkers for response to Poly ADP-Ribose inhibitors (PARPi) are required, since on the one hand evidence is emerging that PARPi are also effective beyond germline BRCA mutated (gBRCAm) cancers and on the other hand gBRCAm cancers can become resistant to PARPi. Therefore, we previously developed a functional homologous recombination (HR) assay exploiting the formation of RAD51 foci in proliferating cells after ex vivo irradiation of fresh primary breast cancer (BrC) tissue (n=148): the REpair CAPacity (RECAP) test. The aim of the current study is to validate feasibility of this test on histological biopsies from metastatic BrC and to explore the utility of the RECAP test as a predictive biomarker for PARPi treatment of metastatic BrCs. Material and methods Fresh tissue biopsies from metastatic BrC lesions were collected in customised DMEM/F12 medium, irradiated with 5 Gy and cultured for 2 hours. Molecular characterisation of functional HR deficient (HRD) biopsies as well as platinum/PARP resistant biopsies was performed. Results and discussions 41 biopsies were derived from 38 patients with recurrent or metastatic BrC. The RECAP test had a high success rate (93%) when performed on core needle or punch biopsies. Final test outcomes were available within 1 week after the biopsy procedure. HRD was detected in 13 out of 41 biopsies (32%). Among these 13 HRD tumours, 5 were gBRCAm, indicating that the RECAP test identifies more patients who may benefit from PARPi treatment than germline BRCA analysis only. In three gBRCAm patients BRCA reversion was detected, as the HRD tumours became HR proficient (HRP) after showing in vivo progressive disease (PD) on cisplatin/PARPi treatment. One of these patients obtained a secondary BRCA1 mutation that restored the open reading frame and led to production of full-length BRCA1 protein, while the causative molecular event in the other patients is still elusive. Conclusion The RECAP test is a functional HRD test that reflects the real-time HR status regardless of BRCA mutational status. Thus, RECAP shows great potential as a predictive biomarker for PARPi treatment of metastatic BrC.