 This is a first in human, multinational, phase 1, open-label study of TAS3681 in patients with metastatic castration resistant prostate cancer mCRPCevaluating safety, tolerability, pharmacokinetics and preliminary antitumor activity in patients with mCRPC for which there is no standard therapy. Eligible patients will either be enrolled in the dose escalation phase to evaluate safety and determine the maximum tolerated dose for TAS3681 or the expansion phase in which additional cohorts of patients may be enrolled to further evaluate safety and preliminary efficacy of TAS3681 at the MTD or at a lower dose. 