This is a phase II, single-centre, pragmatic, non-randomized, dose escalation study to evaluate optimal dose levels of enzalutamide in elderly patients using pharmacokinetic blood sampling concentrations.

Enzalutamide is an androgen receptor pathway inhibitor (ARPI) drug that is commonly used in prostate cancer. It is safe, effective, and is generally well tolerated by patients at the standard dose of 160mg, however researchers have seen that it may cause unfavorable side effects in elderly patients 75 and older such as fatigue and cognitive impairment that impact their quality of life more significantly than those under 75.

To date there is limited research comparing differences in side effects and quality of life experienced by elderly patients on standard versus a lower dose of enzalutamide. Dose reductions are common, particularly in elderly patients, although they are not explicitly recommended on the drug label, and as a result this study seeks to build on the existing research and practice experience to evaluate how enzalutamide is metabolized in elderly patients and whether this difference warrants different dosing standards in these patients.

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