Personalized use of metronomic cyclophosphamide for DNA repair deficient castration-resistant prostate cancer: A phase II trial.

Abstract

TPS346 Background: There is a continued need to identify novel targets for the treatment of metastatic, castration-resistant prostate cancer (mCRPC). DNA damage repair (DDR) aberrations are emerging as such a target: 20%-30% of mCRPCs harbor DDR gene aberrations, rendering tumors particularly sensitive to DNA damaging agents and poly ADP-ribose polymerase inhibitor (PARPi) therapy. 88% of men with DDR deficient mCRPC responded to the PARPi olaparib in a phase II trial, whereas in unselected mCRPC patients the metronomic use of the DNA damaging agent cyclophosphamide (CPA) resulted in response rates of 25-60%. Intriguingly, in randomized phase II trials of unselected ovarian and triple-negative breast cancer (ie tumor types enriched for DDR defects), metronomic CPA alone was as active as metronomic CPA plus the PARPi veliparib. Based on this we hypothesize that DDR deficient mCRPC is particularly sensitive to metronomic CPA. To the best of our knowledge this is the first attempt to utilize metronomic CPA in a personalized manner. Our study has the potential to define metronomic CPA as an affordable and well-tolerated alternative to PARPi therapy in men with DDR deficient mCRPC. Methods: To study if metronomic CPA achieves a similar response rate (ie ≥85%) in DDR deficient mCRPC as seen with olaparib, men with mCRPC progressing after 1-2 lines of systemic therapy will undergo circulating tumor DNA based testing for BRCA1/2 or ATM aberrations. Patients with such aberrations will proceed with metronomic CPA (50 mg po daily). Primary endpoint: RECIST 1.1 and/or ≥50% PSA response rate at 12 weeks. Secondary endpoints include biochemical, radiological and clinical progression-free survival. Applying the Optimal Simon's Two-Stage design, and using a type I error rate of 0.05 and a power of 0.8, in the first stage we plan to enroll 14 patients. If there are ≤10 or fewer responses, the study will be stopped. Otherwise, another 19 patients will be accrued as part of the second stage.