A Dose Optimization Study of Enzalutamide in Elderly Patients With Advanced Prostate Cancer

Inclusion Criteria: Main Cohort: 1. Male aged 75 years or older. 2. Signed Informed Consent Form 3. Advanced prostate cancer with a Health Canada approved indication for enzalutamide amongst the following: 1. High-risk biochemical recurrent hormone sensitive prostate cancer (high risk definition: PSA doubling time of ≤9 months and a PSA level of ≥2 ng per milliliter above nadir after radiation therapy or ≥1 ng per milliliter after radical prostatectomy with or without postoperative radiation therapy). 2. Metastatic castration-sensitive prostate cancer. 3. Non-metastatic castration-resistant prostate cancer (M0CRPC) with a PSA doubling time of ≤9 months 4. Participants must receive concomitant androgen deprivation therapy with either a Luteinizing hormone-releasing hormone (LHRH) antagonist or agonist, or have had bilateral orchiectomy. 5. Participant is capable, in the opinion of the investigator, of completing all study assessments and procedures, including blood draws for PK sampling according to the required schedule as per the study protocol as well as PROs. 1. Note, for patients who speak a language other than English, PROs may be completed with the help of an interpreter, or translated PROs may be substituted, if available in the patient's primary language. Control Cohort: 1. Age \< 70 years 2. Receiving enzalutamide for advanced prostate, for any indication as per Product Monograph 3. Receiving the same dose of enzalutamide taken daily and fully compliant for ≥ 29 days 4. Signed Informed Consent Form Exclusion Criteria: Main Cohort: 1. Metastatic castration-resistant prostate cancer 2. Pain related to bone metastasis requiring the use of opioid analgesics 3. Impending spinal cord compression related to metastasis to vertebral column 4. Painful femoral neck or any other metastasis judged to be at risk of short-term pathological fracture in the investigator's opinion 5. Co-administration of medications which are moderate to strong CYP2C8 inducers or inhibitors 6. Co-administration of medications which are moderate to strong CYP3A4 inducers or inhibitors 7. History of seizure disorder 8. History of moderate-severe dementia 9. Performance status \> ECOG 2 10. Uncontrolled hypertension (defined as sustained systolic blood pressure above 160 and/or diastolic above 100 despite the adequate use of antihypertensive medication) 11. Participants with a known history of dementia, seizure disorder, intracranial lesions, moderate to severe hepatic impairment, stage ≥ 3 chronic renal failure or stroke will also be excluded Control Cohort: 1. Unable to provide written informed consent 2. Receiving a medication which is a moderate-strong CYP2C8 or CYP3A4 inducer or inhibitor