A Study to Assess the Benefit of Treatment Beyond Progression With Enzalutamide in Men Who Are Starting Treatment With Docetaxel After Worsening of Their Prostate Cancer When Taking Enzalutamide Alone

The purpose of the study was to understand if there was benefit in continued treatment with a medicine called enzalutamide, when starting treatment with docetaxel and prednisolone (a standard chemotherapy for prostate cancer), after the prostate cancer had gotten worse when treated with enzalutamide alone.

The study was conducted in consecutive periods of open label treatment with enzalutamide followed by randomized double-blind treatment with continued enzalutamide or placebo, in combination with docetaxel and prednisolone.

Open Label (Period 1)

Participants received open label treatment (OL) with enzalutamide. At week 13, all participants were assessed by prostate-specific antigen (PSA) and imaging. Participants with no confirmed PSA response or evidence of radiographic progression were ineligible for participation in Period 2 and typically had safety follow up; however, Period 1 treatment continued for some participants as long as the investigator considered it to be of clinical benefit (stopping on initiation of any new antineoplastic therapy). Participants with confirmed PSA response continued Period 1 until disease progression.

Enrollment to Period 2 ceased after approximately 274 participants had been enrolled or 182 primary endpoint events had been reached, whichever occurred first. Participants who were not randomized into period 2 at this time continued to receive open label treatment in an extension period.

Randomization (Double Blind \[DB\]) (Period 2)

Participants with confirmed disease progression on enzalutamide alone who continued to meet all eligibility criteria proceeded to randomization. Treatment allocation was in a 1:1 ratio, stratified by disease progression in Period 1 to the following treatments:

* Enzalutamide with docetaxel and prednisolone
* Placebo with docetaxel and prednisolone

Any ongoing participants in Period 2 at the point of unblinding in the enzalutamide+docetaxel arm that were still receiving and benefitting from enzalutamide treatment, had the option to continue treatment via an extension period.

Eligibility Criteria:
Inclusion Criteria:

* Histologically confirmed adenocarcinoma of the prostate without neuroendocrine differentiation or small cell features;
* Ongoing androgen deprivation therapy (ADT) with a luteinizing hormone-releasing hormone (LHRH) agonist or antagonist at a stable dose and schedule within 4 weeks of initiation of investigational medicinal product (IMP), or bilateral orchiectomy (i.e., surgical or medical castration);
* Metastatic disease documented by at least 2 bone lesions on bone scan, or soft tissue disease documented by computed tomography (CT)/magnetic resonance imaging (MRI);
* Progressive disease at study entry defined as the following occurring in the setting of castrate levels of testosterone: Prostate specific antigen (PSA) progression defined by a minimum of three rising PSA levels with an interval of ≥ 1 week between each determination.
* Asymptomatic or minimally symptomatic prostate cancer (Brief Pain Inventory - Short Form (BPI-SF) question 3 score of \< 4);
* Eastern Cooperative Oncology Group (ECOG) performance score of 0-1;
* Estimated life expectancy of ≥ 12 months;
* Be suitable and willing to receive chemotherapy as part of the trial;
* Able to swallow the IMP and comply with study requirements;
* Subject agreed not to participate in another interventional study while on treatment.

Exclusion Criteria:

* Prior treatment with the following agents for the treatment of prostate cancer: Aminoglutethimide; Ketoconazole; Abiraterone; Enzalutamide or participation in a clinical trial of enzalutamide; 223Ra, 89Sr, 153Sm, 186Re/188Re; Immunomodulatory therapies; Cytotoxic chemotherapy; Participation in a clinical trial of an investigational agent that inhibits the AR or androgen synthesis unless the treatment was placebo;
* Current or prior treatment within 4 weeks prior to initiation of investigational medicinal product (IMP) with the following agents for the treatment of prostate cancer: Antiandrogens; 5-α reductase inhibitors; Estrogens; Anabolic steroids; Drugs with antiandrogenic properties; Progestational agents;
* Subject had received investigational therapy within 28 days or 5 half-lives whichever was longer, prior to initiation of IMP;
* Use of opiate analgesia for pain from prostate cancer within 4 weeks prior to initiation of IMP;
* Radiation therapy to bone lesions or prostatic bed within 4 weeks prior to initiation of IMP;
* Major surgery within 4 weeks prior to initiation of IMP;
* History of seizure or any condition that may predispose to seizures at any time in the past. History of loss of consciousness or transient ischemic attack within 12 months prior to Screening;
* Known or suspected brain metastasis or active leptomeningeal disease;
* History of another malignancy within the previous 5 years other than non-melanoma skin cancer;
* Clinically significant cardiovascular disease;
* Gastrointestinal disorders affecting absorption;
* Medical contraindications to the use of prednisolone or docetaxel;
* Allergies to any of the active ingredients or excipients in the study drugs