This is a Phase Ib/II open label clinical trial in patients with metastatic castration resistant prostate cancer. The objective of the phase Ib portion of the study is to establish the maximum tolerated dose (MTD) and dose limiting toxicities (DLT) of docetaxel (75 mg/m2 IV q21 days) and prednisone (5mg orally BID) in combination with ribociclib in escalating oral daily doses in patients with metastatic castrate resistant prostate cancer (mCRPC) with prior resistance to abiraterone and/or enzalutamide who have not undergone prior chemotherapy for metastatic disease. Up to three cohorts will be enrolled to determine the MTD and DLT profile of this combination during Phase 1b. Dose escalation will follow the standard 3+3 design. The dosing schedule is being chosen to allow patients to be exposed to the most efficacious dosing schedule of docetaxel (75 mg/m2 every 3 weeks). If there is excess toxicity observed with the treatment combination at the first dose level (dose level I), an alternative dosing schema may be pursued with weekly docetaxel treatment (35 mg/m2 weekly), which has demonstrated activity in mCRPC and decreased risk of cytopenias compared with every 3 week dosing schedule.

The Phase II portion (N = 29) of the study is a single arm, two stage, open-label study of ribociclib (dosed at the RP2D) in combination with docetaxel and prednisone to determine the efficacy and further define the safety of the treatment combination. Patients will be treated with the combination of ribociclib plus docetaxel + prednisone for up to 9 cycles. If there is no evidence of radiographic or clinical disease progression after 9 cycles of protocol therapy, patients may continue on single agent maintenance ribociclib until the time of disease progression. Patients will have the option of starting maintenance ribociclib after 6 cycles of docetaxel if stable disease or better on re-staging scans. The dose of ribociclib used during maintenance will be the same dose as that immediately preceding cessation of docetaxel treatment.

PRIMARY OBJECTIVES:

I. To determine the safety profile, maximally tolerated dose (MTD), and recommended phase 2 dose of ribociclib in combination with docetaxel plus prednisone in patients with metastatic castration-resistant prostate cancer (mCRPC). (Phase IB) II. To determine the 6-month radiographic progression-free survival rate with the combination of ribociclib, docetaxel, and prednisone in patients with mCRPC. (Phase 2)

SECONDARY OBJECTIVES:

I. To determine the median radiographic progression-free survival with the combination of ribociclib, docetaxel, and prednisone in patients with mCRPC.

II. To determine the objective response rate and median duration of response among patients with measurable disease at baseline.

III. To determine the prostate-specific antigen (PSA) response proportion and time to PSA progression.

IV. To characterize the safety profile of ribociclib in combination with docetaxel.

V. To determine if there is evidence of drug-drug interaction between docetaxel + prednisone with ribociclib.

EXPLORATORY OBJECTIVES:

I. To determine whether baseline or percent change from baseline in gallium citrate uptake on positron emission tomography (PET) scan is associated with clinical outcomes. (For University of California San Francisco (USCF) Patients Only) II. To determine whether genomic assessment of MYC pathway activation (MYC amplification or overexpression, Rb1 deletion, cyclin D/E and CDK 4/6 overexpression) assessed within metastatic tumor tissue, circulating tumor cells (CTCs), and/or cell-free circulating tumor deoxyribonucleic acid (ctDNA) is predictive of clinical outcomes with the combination of ribociclib plus docetaxel.

III. To determine whether MYC activation score as determined by validated expression signature can distinguish those with and without clinical benefit with ribociclib in combination with docetaxel.

IV. To use an unbiased approach with integration of clinical, genomic, and proteomic data (differential pathway signature correlation; DiPSC) to define a signature associated with response to taxane + CDK4/6 inhibition in mCRPC.

OUTLINE: This is a phase Ib, dose-escalation study of ribociclib followed by a phase II study.

Patients receive docetaxel intravenously (IV) over 1 hour on day 1 or on days 1 and 8, prednisone orally (PO) twice daily (BID) on days 1-21, and ribociclib PO once daily (QD) on days 1-4, and 8-15. Treatment repeats every 21 days for 9 cycles in the absence of disease progression or unacceptable toxicity. Patients achieving stable disease or better on re-staging scans after 6 cycles and patients without radiographic or clinical disease progression after 9 cycles of treatment may continue on single agent maintenance ribociclib PO QD on days 1-14 of every 21 day cycle in the absence of disease progression or unacceptable toxicity.

After completion of study treatment, patients are followed up every three months.

Inclusion Criteria:

• Histologically confirmed prostate cancer. Small cell/neuroendocrine differentiated allowed but not required for study participation.

• Progressive metastatic prostate cancer (as defined below in Item #5) despite castrate levels of testosterone (< 50 ng/dL).

• Patients may have either non-measurable disease OR measurable disease

• Progressive disease during (or within 4 weeks of completion) with abiraterone, enzalutamide, and/or ARN-509 based on any one of the following:

  1. For patients with measurable disease, progression by the RECIST criteria.
  2. PSA evidence for progressive prostate cancer consists of a PSA level of at least 2 ng/ml which has risen on at least 2 successive occasions, at least one week apart. If the confirmatory PSA (#3) value is less (i.e., #3b) than the screening PSA (#2) value, then an additional test for rising PSA (#4) will be required to document progression for the purposes of eligibility.
  3. Radionuclide bone scan: At least two new foci consistent with metastatic lesions

• Testosterone < 50 ng/dL. Patients must continue primary androgen deprivation with a luteinizing hormone-releasing hormone (LHRH) analogue if they have not undergone orchiectomy.

• Patients treated with first generation anti-androgen as most recent systemic therapy (bicalutamide, nilutamide) must have at least 4 weeks elapsed from treatment discontinuation to start of protocol therapy with evidence of disease progression by Prostate Cancer Clinical Trials Working Group 2 (PCWG2) criteria following discontinuation of prior anti-androgen.

• Eastern Cooperative Oncology Group (ECOG) Performance Status 0 or 1.

• Patient has adequate bone marrow and organ function as defined by the following laboratory values:

• Absolute neutrophil count ≥ 1.5 × 109/L.

• Platelets ≥ 100 × 109/L.

• Hemoglobin ≥ 9 g/dl.

• Potassium, total calcium (corrected for serum albumin) and magnesium within normal limits for the institution or corrected to within normal limits before first dose of study medication.

• International normalized ratio (INR) ≤ 1.5 unless on direct thrombin inhibitor at time of study entry.

• Serum creatinine ≤ 1.5mg/dL or estimated creatinine clearance ≥ 50 ml/min

• In the absence of liver metastases, alanine aminotransferase (ALT) and aspartate aminotransferase (AST) <2.5 x upper limit of normal (ULN). If the patient has liver metastases, ALT and AST <5 x ULN

• Total bilirubin < ULN; or total bilirubin ≤3.0 x ULN or direct bilirubin ≤1.5 x ULN in patients with well-documented Gilbert's Syndrome.

• No other systemic therapies for prostate cancer within 28 days or 5 half-lives, whichever is shorter, prior to day 1 of study therapy.

• Sexually active males must use a condom during intercourse while taking the drug and for 30 days after stopping treatment and should not father a child in this period. A condom is required to be used also by vasectomized men in order to prevent delivery of the drug via seminal fluid. Fertile males must use a condom with spermicide (double barrier method).

• Age ≥ 18 years

• Written informed consent must be obtained prior to any screening procedures and according to local guidelines.

Exclusion Criteria:

• Patient has a known hypersensitivity to ribociclib or any of its excipients, or prior treatment with cyclin-dependent kinase (CDK) 4/6 inhibitor.
• Prior chemotherapy for metastatic castration-resistant prostate cancer. Chemotherapy administered in the castration-sensitive setting is allowed provided last dose of chemotherapy was greater than 6 months prior to study entry
• Patient has a concurrent malignancy or malignancy within 3 years of randomization, with the exception of adequately treated, basal cell skin cancer, squamous cell carcinoma, non-melanomatous skin cancer or curatively resected cervical cancer.
• Patients with central nervous system (CNS) involvement unless they meet all of the following criteria:
• At least 4 weeks from prior therapy completion (including radiation and/or surgery) to starting the study treatment
• Clinically stable CNS tumor at the time of screening and not receiving steroids and/or enzyme-inducing anti-epileptic medications for brain metastases.
• Baseline screening for CNS metastases is not required unless presence of signs and/or symptoms of involvement
• Patient is not able to swallow oral medication and/or has impairment of gastrointestinal (GI) function or GI disease that may significantly alter the absorption of the study drugs (e.g., ulcerative diseases, uncontrolled nausea, vomiting, diarrhea, malabsorption syndrome, or significant small bowel resection).
• Clinically significant, uncontrolled heart disease and/or recent events including any of the following:
• History of acute coronary syndromes (including myocardial infarction, unstable angina, coronary artery bypass grafting, coronary angioplasty or stenting) or symptomatic pericarditis within 12 months prior to screening
• History of documented congestive heart failure (New York Heart Association functional classification III-IV)
• Patient has a left ventricular ejection fraction (LVEF) < 50% as determined by Multiple Gated acquisition (MUGA) scan or echocardiogram (ECHO) obtained during Screening.
• History of any cardiac arrhythmias, e.g., ventricular, supraventricular, nodal arrhythmias, or conduction abnormality within 12 months of screening. Patients with rate-controlled atrial fibrillation or flutter are permitted.
• Bradycardia (heart rate < 50 bpm at rest), by ECG or pulse, at screening
• Congenital long QT syndrome or family history of long QT syndrome
• Any of the following abnormalities on screening 12-lead ECG:
• QT interval with Fridericia's correction (QTcF) > 450 msec
• Bradycardia (heart rate < 50 bpm at rest)
• Tachycardia (heart rate > 100 bpm at rest)
• P-R interval > 220msec,
• QRS interval >109 msec
• Documented cardiomyopathy
• Systolic blood pressure >160 mmHg or <90 mmHg at screening
• AST and/or ALT > 1.5 x ULN with concomitant alkaline phosphatase > 2.5 x ULN
• Patient receiving any of the following medications within 7 days of day 1 of study treatment.
• Known strong inducers or inhibitors of CYP3A4/5, including grapefruit, grapefruit hybrids, pummelos, star-fruit, and Seville oranges
• That have a narrow therapeutic window and are predominantly metabolized through CYP3A4/5.
• That have a known risk to prolong the QT interval or induce Torsades de Pointes.
• Herbal preparations/medications
• Patient is currently receiving or has received systemic corticosteroids ≤2 weeks prior to starting study drug at a dose greater than the equivalent of 10 mg prednisone/day, or who have not fully recovered from the side effects of such treatment
• The following uses of corticosteroids are permitted: short duration (<5 days) of systemic corticosteroids; any duration of topical applications (e.g., for rash), inhaled sprays (e.g., for obstructive airways diseases), eye drops or local injections (e.g., intra-articular).
• Patient is currently receiving warfarin or other coumarin-derived anticoagulant for treatment, prophylaxis or otherwise. Therapy with heparin, direct thrombin inhibitors, low molecular weight heparin (LMWH) or fondaparinux is allowed.
• Participation in a prior investigational study within 30 days prior to enrollment or within 5 half-lives of the investigational product, whichever is longer
• Major surgery within 14 days prior to starting study drug or has not recovered from major side effects (tumor biopsy is not considered as major surgery).
• Patient who has received radiotherapy ≤4 weeks or limited field radiation for palliation ≤2 weeks prior to starting study drug, and who has not recovered to grade 1 or better from related side effects of such therapy (exceptions include alopecia), and/or in whom ≥25% of the bone marrow was irradiated.
• Patient has a known history of HIV infection (testing not required)
• Patient has not recovered from all toxicities related to prior anticancer therapies to NCI-CTCAE version 4.03 to less than or equal to Grade 1 (Exception to this criterion: patients with grade 1 taxane-induced neuropathy, any grade of alopecia, amenorrhea or other toxicities not considered a safety risk for the patient as per investigator's discretion, are allowed to enter the study).
• Patients with chronic liver disease with a Child-Pugh score B or C.
• Patients with serious intercurrent infections, or nonmalignant medical illnesses that are uncontrolled or whose control may be jeopardized by the complications of this therapy.
• Patients with severe psychiatric illness/social situations that would limit compliance with study requirements in the judgment of study investigator.
• History of bleeding diathesis. Patients receiving anti-coagulation must be able safely interrupt treatment for tumor biopsy (Phase 2 only)
• Patient has a history of non-compliance to medical regimen or inability to grant consent

Participants assigned to the recommended phase 2 dose (RP2D) during their Phase 1b enrollment, were included in the Phase 1b/2 RP2D group for data analysis

Up to 2 years

Participants assigned to the Phase 1b RP2D cohort were included in a combined Phase 1b/2 RP2D group for some of the analyses.