Sequencing Abiraterone and Enzalutamide in mCRPC

Inclusion Criteria:

1. Willing and able to provide informed consent
2. Adult males ≥ 18 years age
3. History of adenocarcinoma of the prostate diagnosed histologically without evidence of neuroendocrine or small cell differentiation
4. Prior surgical orchiectomy or if on luteinizing hormone-releasing hormone (LHRH) agonist/antagonist then testosterone \< 1.7 nmol/L at screening visit (patients must maintain LHRH agonist/antagonist therapy for duration of study treatment if not surgically castrated)
5. Evidence of metastatic disease on bone scan or CT scan
6. Evidence of biochemical or imaging progression in the setting of surgical or medical castration. Progressive disease for study entry is defined by one of the following three criteria:

   1. PSA progression: minimum of two rising PSA values from a baseline measurement with an interval of ≥ 1 week between each measurement. Minimum PSA at screening visit is \> 2.0 ug/L
   2. Soft tissue or visceral disease progression (see Appendix B for definition of measurable disease as per Response Evaluation Criteria in Solid Tumours (RECIST) 1.1 criteria)
   3. Bone progression: ≥ 2 new lesions on bone scan
7. ECOG performance status 0-2 (see Appendix C)
8. Eligible for treatment with either abiraterone acetate or enzalutamide as per standard of care guidelines
9. Adequate organ function defined as:

   1. Absolute neutrophil count ≥ 1.5 x 109/L, platelet count ≥ 100 x 109/L and hemoglobin ≥ 80 g/L
   2. Creatinine clearance ≥ 30 ml/min (calculated by Cockcroft-Gault formula, see Appendix D)
   3. Serum potassium within normal limits
   4. Total bilirubin ≤ 1.5 x upper limit of normal (ULN) except for patients with known Gilbert's syndrome (direct bilirubin ≤ 1.5 x ULN)
   5. Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 5 x ULN
10. Able to swallow study drug and comply with study requirements including provision of peripheral blood samples at specified time points for correlative studies
11. Recovery from all prior treatment-related toxicity to grade ≤ 2 (as per Common Terminology Criteria for Adverse Events 4.0)

Exclusion Criteria:

1. Severe concurrent illness or co-morbid disease that would make the subject unsuitable for enrolment
2. Prior therapy with CYP17 inhibitors (including abiraterone acetate, TAK-700, TOK-001 and ketoconazole), enzalutamide or other experimental anti-androgens (e.g. ARN-509, TOK-001)
3. Prior systemic chemotherapy for mCRPC
4. Life expectancy \< 6 months
5. Active concurrent malignancy (with the exception of non-melanomatous skin cancer)
6. Wide-field radiotherapy or radioisotopes such as Strontium-89 or Radium-223 ≤ 28 days prior to starting study drug (limited-field palliative radiotherapy for 1-5 fractions is permitted at anytime prior to commencement protocol therapy)
7. Brain metastases or active epidural disease (treated epidural disease is permitted)
8. Use of herbal products that may lower PSA level (e.g. saw palmetto)
9. Contraindication to prednisone therapy including poorly controlled diabetes mellitus
10. History of seizure or seizure disorder, or history of any cerebrovascular event within 6 months of study entry.
11. Gastrointestinal disorder affecting absorption
12. Major surgery within 4 weeks of starting study treatment