Prophylactic window therapy with the clinical poly(ADP-ribose) polymerase inhibitor olaparib delays BRCA1-deficient mammary tumour formation in mice.

Women with heterozygous germline mutations in the BRCA1 tumour suppressor gene are strongly predisposed to developing early-onset breast cancer through loss of the remaining wild-type BRCA1 allele and inactivation of TP53. Although tumour prevention strategies in BRCA1-mutation carriers are still limited to prophylactic surgery, several therapeutic strategies have been developed to target the DNA repair defects (also known as 'BRCAness') of BRCA1-deficient tumours. In particular, DNA-damaging agents such as platinum drugs and poly(ADP-ribose) polymerase (PARP) inhibitors show strong activity against BRCA1-mutated tumours. However, it is unclear whether drugs that target BRCAness can also be used to prevent tumour formation in BRCA1-mutation carriers, especially as loss of wild-type BRCA1 may not be the first event in BRCA1-associated tumourigenesis. We performed prophylactic treatments in a genetically engineered mouse model in which de novo development of BRCA1-deficient mammary tumours is induced by stochastic loss of BRCA1 and p53. We found that prophylactic window therapy with nimustine, cisplatin or olaparib reduced the amount and size of mammary gland lesions, and significantly increased the median tumour latency. Similar results were obtained with intermittent prophylactic treatment with olaparib. Importantly, prophylactic window therapy with nimustine and cisplatin resulted in an increased fraction of BRCA1-proficient mammary tumours, suggesting selective survival and malignant transformation of BRCA1-proficient lesions upon prophylactic treatment with DNA-damaging agents. Prophylactic therapy with olaparib significantly prolonged mammary tumour-free survival without any significant increase in the fraction of BRCA1-proficient tumours, warranting the evaluation of this PARP inhibitor in prophylactic trials in BRCA1-mutation carriers. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.