Inclusion Criteria:

          -  Signed informed consent form (ICF) providing agreement to adhere to the dosing
             schedule, report for all trial visits and authorization, use and release of health and
             research trial information

          -  Histologically or cytologically confirmed carcinoma of the prostate (excluding
             neuroendocrine differentiation or squamous cell histology)

          -  Ongoing gonadal androgen deprivation therapy with gonadotropin-releasing hormone
             (GnRH) analogues, antagonists or orchiectomy; patients who have not had an orchiectomy
             must be maintained on effective GnRH analogue/antagonist therapy

          -  Castration resistant prostate cancer as defined by rising PSA when serum testosterone
             < 50 ng/ml (note: current testosterone results are not required if the potential
             subject has not missed any GnRH analogue/antagonist doses since their last result was
             received) AND one of the following:

               -  PSA level of at least 2 ng/ml that has risen on at least 2 successive occasions
                  at least 1 week apart

               -  Evaluable disease progression by modified RECIST (Response Evaluation Criteria in
                  Solid Tumors)

               -  Progression of metastatic bone disease on bone scan with > 2 new lesions

          -  Prior therapy with abiraterone, enzalutamide and/or docetaxel; there is no limit to
             the number of prior treatment regimens

          -  Presence of metastatic disease on scans

          -  Eastern Cooperative Oncology Group (ECOG) performance status of =< 1

          -  Life expectancy >= 12 weeks

          -  No prior malignancy is allowed except:

               -  Adequately treated basal cell or squamous cell skin cancer or

               -  In situ carcinoma of any site or

               -  Other adequately treated malignancy for which the patient has been disease-free
                  for at least one year (any prior chemotherapy is allowed)

          -  Absolute neutrophil count >= 1.5 x 10^9 cells/L

          -  Hemoglobin (Hgb) >= 9.0 g/dL

          -  Platelets >= 100,000 x 10^9/L

          -  Aspartate aminotransferase (AST), alanine aminotransferase (ALT) =< 1.5 x upper limit
             of normal (ULN)

          -  Total bilirubin levels =< 1.5 x ULN

          -  Patients must have clear and documented evidence of biallelic inactivation BRCA1,
             BRCA2 or ATM by sequencing, for example University of Washington (UW)-Oncoplex, SU2C,
             or Foundation One testing and/or patients with clearly deleterious mutations of other
             genes involved in homologous DNA repair (e.g., partner and localizer of BRCA2 [PALB2],
             BRCA1-interacting protein 1 [BRIP1], etc.) by sequencing via UW-Oncoplex, SU2C, or
             Foundation One testing may be included at the investigator's discretion

        Exclusion Criteria:

          -  Currently receiving active therapy for other neoplastic disorders

          -  Histologic evidence of neuroendocrine or small cell carcinoma of the prostate

          -  Known parenchymal brain metastasis

          -  Active or symptomatic viral hepatitis or chronic liver disease

          -  Clinically significant heart disease as evidenced by myocardial infarction, or
             arterial thrombotic events in the past 6 months, severe or unstable angina, or New
             York Heart Association (NYHA) class II-IV heart disease or cardiac ejection fraction
             measurement of < 35 % at baseline, if done

          -  Treatment with an investigational therapeutic within 30 days of cycle 1

          -  Patients with dementia/psychiatric illness/social situations limiting compliance with
             study requirements or understanding and/or giving of informed consent are not eligible

          -  Any medical conditions, which, in the opinion of the investigators, would jeopardize
             either the patient or the integrity of the data obtained are not eligible