Oral low-dose cyclophosphamide, as a second-line agent, in patients with primary docetaxel refractory metastatic castration-resistant prostate cancer (mCRPC).

e15210 Background: Docetaxel is the standard of care for treatment in mCRPC, yet docetaxel resistance is universal, presenting as primary refractoriness or developing after an initial response. While abiraterone and cabazitaxel are FDA approved after docetaxel failure, multiple other agents, including oral low dose cyclophosphamide (OLDC), have also been utilized with some clinical success. We have witnessed several remarkable responses to OLDC and sought to define clinical predictors of such responses. Methods: Twenty-one patients progressing on docetaxel were classified according to initial docetaxel response as either docetaxel non-responsive (DNR) or docetaxel responsive (DR). Patients were classified as DNR if an initial response to docetaxel by prostate specific antigen (PSA) working group-2 and RECIST criteria was absent, or if circulating tumor cells (CTC) numbers remained >5 at all assessed time points. Otherwise patients were classified as DR. All patients initiated OLDC (50-150 mg/day), and sub...