A phase II clinical trial of 6-mercaptopurine (6MP) and methotrexate in patients with BRCA defective tumours: a study protocol

Background: BRCA1 and BRCA2 genes are critical in homologous recombination DNA repair and have been implicated in familial breast and ovarian cancer tumorigenesis. Tumour cells with these mutations demonstrate increased sensitivity to cisplatin and poly (ADP-ribose) polymerase (PARP) inhibitors. 6MP was identified in a screen for novel drugs and found to selectively kill BRCA-defective cells in a xenograft model as effectively as the PARP inhibitor AGO14699, even after these cells had acquired resistance to a PARP inhibitor or cisplatin. Exploiting the genetic basis of these tumours enables us to develop a more tailored approach to therapy for patients with BRCA mutated cancers. Methods: This multi-centre phase II single arm trial was designed to investigate the activity and safety of 6-mercaptopurine (6MP) 55 mg/m(2) per day, and methotrexate 15 mg/m(2) per week in patients with advanced breast or ovarian cancer, ECOG PS 0-2, progressing after >= one prior regimen and known to bear a BRCA1/2 germ line mutation. Accrual was planned in two stages, with treatment continuing until progression or unacceptable toxicity; in the first, if less than 3/30 evaluable patients respond at 8 weeks after commencing treatment, the trial will be stopped for futility; if not, then accrual would proceed to a second stage, in which if more than 9/65 evaluable patients are found to respond at 8 weeks, the treatment will be regarded as potentially effective and a phase III trial considered subject to satisfactory safety and tolerability. The primary outcome is objective response at 8 weeks, defined by RECISTS v1.1 as complete response, partial response or stable disease. Secondary outcomes include safety, progression-free and overall survival, and quality of life. Discussion: This study aims to investigate whether 6MP might be an effective treatment for BRCA deficient tumours even after the development of resistance to PARP inhibitors or platinum drugs. The study has surpassed the first stage analysis criteria of more than 3 out of 30 evaluable patients responding at 8 weeks, and is currently in the second stage of recruitment.