Direct ex vivo observation of homologous recombination defect reversal after DNA-damaging chemotherapy in patients with metastatic breast cancer

PURPOSE Biomarkers that predict response to poly (ADP-ribose) polymerase inhibitors (PARPis) are required to detect PARPi sensitivity beyond germline BRCA-mutated (gBRCAm) cancers and PARPi resistance among reverted gBRCAm cancers. Therefore, we previously developed the Repair Capacity (RECAP) test, a functional homologous recombination (HR) assay that exploits the formation of RAD51 foci in proliferating cells after ex vivo irradiation of fresh primary breast cancer tissue. The aim of the current study was to validate the feasibility of this test on histologic biopsy specimens from metastatic breast cancer and to explore the utility of the RECAP test as a predictive tool for treatment with DNA-damaging agents, such as PARPis. METHODS Fresh tissue biopsies from easily accessible metastatic lesions from patients with locally advanced or metastatic breast cancer were irradiated with 5 Gy and cultured for 2 hours followed by detection of RAD51 foci presence (HR proficient) or absence (HR deficient [HRD]). HRD biopsy specimens as well as platinum/PARPresistant specimens were subjected to BRCA1/2 sequencing. RESULTS RECAP had a success rate of 93% on biopsy specimens from metastatic breast cancer lesions (n = 44). Although HRD was detected in 13 (32%) of 41 specimens, only five showed a gBRCAm. In three patients with gBRCAm, post-treatment RECAP tests showed HR phenotype reversion after in vivo progressive disease on platinum and PARPi treatment, which was explained in one patient by a secondary BRCA1 mutation. CONCLUSION The RECAP test, which reflects real-time HR status regardless of BRCA mutations, is feasible in metastatic breast cancer biopsy specimens. Compared with gBRCA analysis, it may identify twice as many candidates for PARPi treatment.

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