BACKGROUND: PD 0332991 is an orally bioavailable selective inhibitor of cyclin-dependent kinase (CDK) 4/6. Inhibition of CDK4/6 phosphorylation of retinoblastoma (Rb) induces prolonged early G1 cell cycle arrest (pG1) and synchronous progression to S phase (pG1-S) upon withdrawal, which sensitizes human multiple myeloma (MM) cells to killing by bortezomib (B) or dexamethasone (D) in vitro and in animal models. Based on these observations, a phase 1/2 study in combination with B plus D in patients (pts) with relapsed and/or refractory MM was initiated. The phase 1 part of the study (completed) determined the recommended phase 2 dose and schedule to be PD 0332991 100 mg QD 12 days on followed by 9 days off treatment in a 21-day cycle with intravenous B 1.0 mg/m2 plus oral D 20 mg administered on Days 8 and 11 in pG1 and 15 and 18 in pG1-S (Niesvizky et al. ASH 2010). We present preliminary data from the phase 2 part of the study.
METHODS: Pts with Rb protein-positive, measurable (as defined by International Myeloma Working Group [IMWG]) progressive, relapsed or refractory MM after ≥1 prior treatment were eligible. Prior B was allowed only if there was a response and disease progression occurred off therapy. Pts received oral PD 0332991 once daily on Days 1-12 in a 21-day cycle in combination with intravenous B 1.0 mg/m2 plus oral D 20 mg administered on Days 8, 11, 15, and 18. The primary endpoint is overall response rate (ORR); secondary endpoints include time to progression (TTP), progression-free survival (PFS), overall survival, duration of response, and safety. PD 0332991-mediated inhibition of CDK4/6-specific phosphorylation of Rb (pSRb) and Ki67 in bone marrow MM cells were also assessed. The phase 2 part of the study is a Simon Two-Stage Minimax design; 25 response evaluable patients were to be enrolled into the first stage.
RESULTS: 39 pts have been tested for Rb and 36 pts (92%) were positive. Of the 36 pts, 30 pts have been enrolled to date including 2 pts who did not receive the study treatment, and 23 pts are considered response evaluable as of the data cut-off. 56% of pts had an Eastern Cooperative Oncology Group performance status (ECOG PS) of 1 and 8% had ECOG PS of 2. At baseline, median β2 microglobulin was 3.1 (range 1.6–26.2), median hemoglobin was 11.2 (7.2–13.6), median calcium was 9.4 (8.7–11.9). The median number of prior therapies was 2 (range 1–8); 55% had received prior B. Sixteen pts have discontinued (9 due to progressive disease, 3 due to AE, 2 consent withdrawal, and 2 not treated). The most common treatment-related AEs were thrombocytopenia (44%), nausea (20%), anemia, constipation, fatigue, and neutropenia (all 16%); 32% of pts reported grade ≥3 thrombocytopenia. IHC data showed on-treatment reduction in pSRb and Ki67 in MM cells from bone marrow of 3/3 patients with available samples. To date, 1 pt achieved a complete response (CR), 1 achieved a very good partial response (VGPR), 1 partial response (PR), 1 minor response (MR), and 5 stable disease (SD); 6 pts are too early for assessment.
CONCLUSIONS: To date, the combination of PD 0332991 and B plus D has shown response in 4 pts with relapsed/refractory MM. The most commonly reported AEs were cytopenias, consistent with the known safety profiles of PD 0332991 and B. PD 0332991 inhibited phosphorylation of Rb and cell cycle progression in MM cells. The accrual to stage 1 is ongoing. Updated efficacy and safety data will be presented.