Would you use a PARP inhibitor in metastatic ER/PR positive breast cancer with somatic BRCA1 mutation but negative for germline mutation?

I certainly would. This is based on data from TBCRC 048 that showed efficacy in sBRCA1/2 as well as gPALB2.

Tung et al., PMID 33119476

The Tung et al. presentation of TBCRC 048 at ASCO 2020 was a phase II study of olaparib in metastatic breast cancer patients with germline, non-BRCA1/2 mutations OR somatic mutations in BRCA1/2 and other HR-pathway genes. The primary endpoint of this study was ORR. Fifteen patients had a somatic BRCA 1 or 2 mutation and one additional patient had a germline CHEK2 mutation and somatic BRCA1 mutation. Of those 16 patients, 8 (50%) experienced a partial response, and clinical benefit rate at 18 weeks was 67%. This is very encouraging data, but it is a very small dataset, and thus, if clinical trials can be identified/offered to these patients, that would remain my first choice. If not, I do think it is reasonable to offer patients a PARP inhib...