A Phase I Trial of PD 0332991, a Novel, Orally-Bioavailable CDK4/6-Specific Inhibitor Administered in Combination with Bortezomib and Dexamethasone to Patients with Relapsed and Refractory Multiple Myeloma.
Authors: Niesvizky Ruben , Ely Scott , Jayabalan David S , Manco Megan C. , Singhal Seema , Crann Mary , Courtney Rachel , DuFresne Cynthia , Wilner Keith D. , Chen Isan , Mark Tomer , Leonard John P. , Coleman Morton , DiLiberto Maurizio , Huang Xiangao , Chen-Kiang Selina 
Published: October 2019

Abstract
Abstract 1877
Poster Board I-902

Background:
PD 0332991 (PD) a potent orally bioavailable small molecule, is the only known CDK4/CDK6-specific inhibitor. Through selective inhibition of CDK4/6, PD induces G1 cell cycle arrest thereby preventing DNA replication. As PD acts reversibly, it can induce synchronous G1-S progression upon its discontinuation. It has been shown that PD potently induces G1 arrest in primary human bone marrow (BM) myeloma cells/lines in the presence of BM stroma and prevents tumor growth in animal models (IC50, ∼60 nM) (Baughn et al 2006). Moreover, it has been shown that induction of prolonged G1 arrest and synchronous S phase entry by PD profoundly enhances bortezomib (B) and dexamethasone (D) killing of primary BM myeloma cells in vitro and in animal models (Huang et al, unpublished). The synergistic anti-myeloma effect provides a compelling rationale for evaluating two methods of targeting CDK4/6 using PD in combination with B and D: a “concurrent regimen” aims at enhancing B killing during prolonged G1 arrest and a “sequential regimen” to enhance B killing during both G1 arrest and synchronous progression to S phase.

Methods:
Patients (pts) who had relapsed and refractory myeloma after at least 2 previous treatments and who were Rb positive were eligible. This phase 1 dose escalation study is being conducted to assess the safety, tolerability and pharmacokinetics of the combination including dose limiting toxicities (DLTs). PD was given orally once a day for 3 weeks (21 days) followed by one week (7 days) off (Schedule A), starting on Day 1 of each cycle. B was given during G1 arrest by IV bolus together with 20 mg PO of D on Days 8, 11, 15 and 18 of each cycle (up to a maximum of 10 cycles). The study design followed a 3+3 dose-escalation scheme, with planned doses of PD/B starting from 100 mg/1.0 mg/m2 and escalating up to a maximum dose of 125 mg/1.3 mg/m2, respectively. Alternatively, in case of toxicity, de-escalation was planned to a minimum dose of 50 mg/0.7 mg/m2 for PD/B, respectively.

Results:
Nine patients were enrolled in Schedule A. Pt characteristics included 90% &gt; SD stage II, 50% &gt; ISS stage II with median β2 M 4.0 (range 1.6–10.5), median serum albumin 3.8 (2.2–4.6), median Hgb 10.5 (8.1–14.4) and median calcium 9.9 (8.7–10.7) values at screening. The median number of prior therapies was 6 (2–10) with 7/9 pts showing progression to their latest prior therapy. One patient achieved VGPR (12.5%) while 1 patient each achieved MR and SD respectively for an ORR 25%. The VGPR and MR were achieved with the lowest dose combination (75 mg/0.7 mg/m2 PD/B). Six pts had progression of disease while on therapy. The most commonly reported adverse events were &gt;Grade 3 reversible uncomplicated cytopenias. PD was absorbed relatively slowly with a median Tmax of 4 hours (range 2–8 hours). PD plasma exposures (normalized to the 100 mg dose level) ranged from 345–1128 ng.hr/mL for AUC 0–12 and from 36–111 ng/mL for Cmax and were consistent with those observed in prior solid tumor studies. Immunohistochemistry of BM on Day 8 (prior to initiation of BD) in 7/8 pts demonstrated preferential and complete inhibition of CDK4/6-specific phosphorylation of Rb and Ki67 in tumor cells. Follow up BMs after 21 days, showed G1-S cell cycle progression upon PD withdrawal, confirming PDs synchronization effect.

Conclusions:
Targeting CDK4/6 with PD to induce prolonged G1 arrest in combination with B/D represents the first mechanism-based targeting of the cell cycle in cancer, and it appears to be effective in MM. Pts are being actively accrued to Schedule B consisting of sequential PD-B/D (12 days of PD followed by B/D as in Schedule A) to assess the safety of this novel schedule and the efficacy following cell cycle synchronization.

Disclosures:
Niesvizky: Celgene: Consultancy, Research Funding, Speakers Bureau; Millennium Pharmaceuticals, Inc.: Consultancy, Research Funding, Speakers Bureau; Proteolix: Consultancy, Research Funding. Courtney:Pfizer: Employment. DuFresne:Pfizer: Employment. Wilner:Pfizer: Employment. Chen:Pfizer: Employment. Mark:Celgene: Speakers Bureau; Millenium: Speakers Bureau. Coleman:Bristol-Myers Squibb Research & Development: Consultancy.