Inclusion Criteria:

Subjects must meet ALL of the following criteria to be eligible for inclusion into the
study.

- Histological or cytological diagnosis of adenocarcinoma of the prostate

- Metastatic disease on chest, abdominal, or pelvic computed tomography (CT) scan
and/or bone scan

- Currently receiving abiraterone acetate and prednisone and meeting the following
criteria:

- Any PSA decline within 12 weeks from initiation of abiraterone acetate

- Currently tolerating abiraterone acetate (1000 mg oral daily) and prednisone
(5-20 mg oral daily)

- PSA progression, defined as an increase in PSA which is ≥25% above the nadir and
an absolute value of ≥2 ng/mL, which is confirmed by a second value ≥2 weeks
later.

- No evidence of symptomatic or radiographic progression that would require
alternative therapy (e.g., needing radiation therapy for pain or significant
progression of visceral metastases or >33% increase in daily opioid use within 2
weeks prior to randomization).

- All patients who have not had a surgical orchiectomy must continue treatment with a
luteinizing hormone-releasing hormone (LHRH) agonist or antagonist to maintain a
castrate level of testosterone.

- Patient must fulfill "Prior Therapy" criteria as follows:

- Chemotherapy: no more than 1 prior chemotherapy regimen for castrate-resistant
prostate cancer (CRPC) is permitted; a minimum of at least 28 days must have
passed since the last dose of chemotherapy.

- Hormone therapy: hormonal androgen ablation therapy prior to abiraterone is
required.

- Experimental therapy: prior non-cytotoxic experimental therapy is permitted
provided a minimum of at least 14 days has passed since completing therapy.
Prior treatment with enzalutamide (MDV3100) is allowed.

- Radiation: prior external beam radiation is permitted provided a minimum of at
least 14 days have passed since completing radiotherapy (exception for
radiotherapy: at least 7 days since completing a single fraction of ≤800 cGy to
a restricted field or limited-field radiotherapy to non-marrow bearing area such
as an extremity or orbit) at the time of randomization

- Must be willing to use effective contraception throughout study treatment and for 3
months after completion of study treatment if able to father a child.

- Must be willing not to change (add or subtract) bone protecting therapy
(bisphosphonates and/or denosumab) during the study unless changed for toxicity.

- Written informed consent must be obtained prior to any protocol-specific procedures
being performed.

Exclusion Criteria:

Subjects meeting ANY of the following exclusion criteria will NOT be eligible for
inclusion into the study:

- Currently receiving abiraterone acetate in combination with any other anti-cancer
agent (except prednisone)

- Documented brain metastases, or carcinomatous meningitis, treated or untreated (Brain
imaging for asymptomatic patients is not required.)

- Cord compression requiring surgery or radiation therapy while on abiraterone
treatment

- Active second malignancy (including lymphoid malignancies such as chronic lymphocytic
leukemia or low grade lymphoma) defined, in general, as requiring anticancer therapy
or at high risk of recurrence during the study; not including adequately treated non
melanomatous skin cancer or other solid tumors curatively treated with no evidence of
disease in > 3 years

- History of allergic reactions to therapeutic antisense oligonucleotides

- Active autoimmune disease requiring treatment

- Participated in a prior Phase 3 clinical study evaluating custirsen regardless of
study arm assignment (i.e., either control or investigational arm), or prior exposure
to OGX-427

- Uncontrolled medical conditions such as myocardial infarction, uncontrolled
hypertension, stroke or treatment of a major active infection within 3 months of
randomization, as well as any significant concurrent medical illness that in the
opinion of the Investigator would preclude protocol therapy

- Planned concomitant participation in another clinical trial of an experimental agent,
vaccine, or device. Concomitant participation in observational studies is acceptable.