The purpose of this phase Ib/II study is to (a) in Phase Ib evaluate the safety, tolerability, and pharmacokinetics (PK) of AMO959 when given in combination with 177Lu-PSMA-617with an androgen receptor pathway inhibitor (ARPI) in participants with metastatic castration resistant prostate cancer (mCRPC) who have failed one prior ARPI and with or without prior taxane exposure, and (b) in Phase II evaluate the preliminary efficacy of AMO959 in combination with AAA617 and ARPI in participants with mCRPC who have failed one prior ARPI, but who have not yet been exposed to taxane treatment.