Abstract P6-06-02: Functional homologous recombination REpair CAPacity (RECAP) test in metastatic breast cancer biopsies

Abstract

Abstract Introduction Tumors of germline BRCA1/2 mutated carriers cannot repair DNA double strand breaks (DSB) due to homologous recombination deficiency (HRD), rendering them highly sensitive to Poly ADP Ribose Polymerase (PARP) inhibitors and DSB inducing chemotherapy. Although evidence is emerging that the use of these therapies could be extended beyond germline BRCA1/2 mutated carriers, a robust validated test to detect HRD tumors is lacking. We have developed a functional HR assay exploiting the formation of RAD51 foci in proliferating cells after ex vivo irradiation of fresh primary breast cancer (BrC) tissue: the RECAP test. Here, we investigated feasibility of this test on small biopsies from metastatic BrC lesions. Methods Patients with advanced or recurrent BrC with easily accessible metastases were eligible. Patients with pulmonary or bone metastases were excluded due to risk of pneumothorax and technical issues with calcifications. Biopsies were collected in customized DMEM/F12 medium, irradiated with 5Gy and cultured for 2 hours. Final test outcomes were available within 1 week after the biopsy procedure. The primary endpoint was the proportion of patients with a successful test result defined as either less than 20% (i.e. HRD), 20-50% (i.e. HR intermediate), or more than 50% (i.e. HR proficient) of S/G2 phase cells showing RAD51 foci. Using a Simon 2-stage design (p0=60%, p1=80%, α=0.1, β=0.1), 38 patients were required. When at least 27 had a successful test result, we decided to further develop this test. Results 51 patients enrolled in the RECAP trial. Core needle biopsies were collected from liver (n=16), breast (n=4), lymph nodes (n=13) and other sites (n=6), as well as punch biopsies from skin metastases (n=5). In 6 cases a biopsy could not be obtained; 1 patient was excluded as the biopsy appeared to be a metastasis of a neuroendocrine tumor. Of 44 obtained biopsies, 42 biopsies contained tumor cells. Successful test results were obtained in 40 of the 42 metastatic biopsies (95%, CI 89-100%). The reason for the two unsuccessful tests, both derived from ER+/PR+/HER2-liver metastases, was absence of tumor cells in S-phase. Success of the test in biopsies was independent of BrC subtype (p=0.12), histological grade (p=0.65), receptor status (p=0.74), biopsy type (p=0.82) and metastatic site (p=0.78). We found that 25 metastatic lesions were HR proficient, 13 were HR deficient and 2 were HR intermediate. Germline BRCA testing showed mutations in 6/13 HRD lesions. Somatic and epigenetic analyses are ongoing. Conclusion In this study, we enhance the diagnostic potential and clinical applicability of the functional RECAP test by demonstrating novel feasibility on metastatic biopsies from BrC patients. Robustness of the test is shown, as its applicability extends from core needle biopsies from different metastatic sites to punch biopsies from skin metastases. Comparison of HRD status of primary versus metastatic tumors and among BrC subtypes will be presented at the conference. Citation Format: Meijer TG, Verkaik NS, Naipal KAT, van Deurzen CHM, Kanaar R, van Gent DC, Jager A. Functional homologous recombination REpair CAPacity (RECAP) test in metastatic breast cancer biopsies [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P6-06-02.