Exceptional responses to PARP inhibitors in patients with metastatic breast cancer in oncologic crisis

Purpose Cancers deficient in homologous recombination DNA repair, such as those with BRCA1 or BRCA2 ( BRCA1/2 ) mutations rely on a pathway mediated by the enzyme poly(adenosine diphosphate-ribose) polymerase (PARP). PARP inhibitors (PARPi’s) have demonstrated efficacy in treating patients with germline ( g ) BRCA1/2 , somatic ( s ) BRCA1/2 , and gPALB2 mutations in clinical trials. However, patients with a poor performance status (PS) and those with severe organ impairment are often excluded from clinical trials and cancer-directed treatment. Methods We report the cases of two patients with metastatic breast cancer who had poor PS, significant visceral disease, and gPALB2 and sBRCA mutations, who derived significant clinical benefit from treatment with PARP inhibition. Results Patient A had germline testing demonstrating a heterozygous PALB2 pathogenic mutation (c.3323delA) and a BRCA2 variant of unknown significance (c.9353T>C), and tumor sequencing revealed PALB2 (c.228_229del and c.3323del) and ESR1 (c.1610A>C) mutations. Patient B was negative for pathologic BRCA mutations upon germline testing, but tumor sequencing demonstrated somatic BRCA2 copy number loss and a PIK3CA mutation (c.1633G>A). Treatment with PARPi’s in these two patients with an initial PS of 3–4 and significant visceral disease resulted in prolonged clinical benefit. Conclusion Patients with a poor PS, such as those described here, may still have meaningful clinical responses to cancer treatments targeting oncogenic drivers. More studies evaluating PARPi’s beyond gBRCA1/2 mutations and in sub-optimal PS would help identify patients who may benefit from these therapies.