Pmd-107: Marizomib, Pomalidomide and Low Dose-Dexamethasone Combination Study in Relapsed/Refractory Multiple Myeloma (NCT02103335): Full Enrollment Results from a Phase-1 Multicenter, Open Label Study

Marizomib (MRZ) is a novel, irreversible, pan subunit proteasome inhibitor (PI) with preclinical evidence demonstrating in vitro and in vivo activity in multiple myeloma (MM). This study was designed to evaluate the safety and antimyeloma activity of pomalidomide (POM), MRZ and low dose dexamethasone (Lo-DEX) (PMD) in patients with relapsed and refractory multiple myeloma (RRMM). Thirty-eight heavily pretreated patients with RRMM were enrolled [dose-escalation cohort (n=14); recommended Phase 2 dose (RP2D) cohort (n=24)]. IV MRZ (0.3 to 0.5 mg/m2) was administered on Days (D) 1, 4, 8, 11; POM (3 or 4 mg) on D1 through 21; and Lo-DEX (5 or 10 mg) on D1, 2, 4, 5, 8, 9, 11, 12, 15, 16, 22, 23 of every 28-D cycle. Patients received a median of 4 (range 1-9) prior lines of therapy; 100% received prior lenalidomide (LEN) and bortezomib (BTZ), 34% carfilzomib (CFZ), and 50% thalidomide. 53% of patients were refractory to both LEN and BTZ and 21% were refractory to LEN, BTZ, and CFZ. There were no dose limiting toxicities during the study. The most common study treatment related ≥Grade 3 adverse events (AEs) were neutropenia (11/38 pts: 29%), pneumonia (4/38 pts 11%), anemia (4/38 pts; 11%), thrombocytopenia (4/38 pts; 11%), and febrile neutropenia (2/38 pts; 5%), with two grade 4 AEs (neutropenia related to POM and viral infection related to DEX), and one grade 5 AE (cardio-respiratory arrest from a suspected PE related to POM). Overall, MRZ was well tolerated, did not add to the incidence or severity of POM/Lo-DEX AEs and the regimen may have fewer hematological and infectious AEs compared to that observed with POM/Lo-DEX. MRZ pharmacokinetic analysis revealed that it was rapidly cleared with a short T1/2 (6.2-11mins) and a large volume of distribution (41-86L) suggesting extensive tissue distribution. Pharmacodynamic analysis demonstrated rapid and robust inhibition of chymotrypsin-like activity in both packed whole blood (PWB) and peripheral blood mononuclear cells (PMBCs), reflecting the irreversible binding nature of MRZ. Evolving inhibition of trypsin-like and caspase-like proteasome activity was also observed in PWB and PBMC with continued dosing. The overall response rate (ORR) and clinical benefit rate (CBR) for the 36 response evaluable patients was 53% (19/36) and 64% (23/36), respectively (Table 1). Subpopulation analysis demonstrated an ORR of 50% (5/10) in high risk cytogenetic patients, 56% (10/18) in LEN/BTZ refractory patients, 71% (5/7) in LEN/BTZ/CFZ refractory patients and 80% (8/10) in CFZ refractory patients. These data compare favorably against POM/Lo-Dex with a near doubling of ORR in both the total patient population and the double refractory patients. Substantial activity in high-risk patients that are triple refractory and in patients that are refractory to CFZ in prior last regimen was observed. MRZ activity in RRMM patients exposed and/or refractory to multiple PIs is likely a consequence of its unique pan proteasome subunit inhibitory actions. In conclusion, MRZ in combination with POM and Lo-DEX was well tolerated and demonstrated promising activity in heavily pretreated, high-risk RRMM patients. Disclosures Harrison:Janssen-Cilag: Research Funding, Speakers Bureau; Celgene: Honoraria. Zonder:Prothena: Consultancy, Honoraria; Seattle Genetics: Consultancy, Honoraria; Bristol Myers Squibb: Consultancy, Honoraria; Janssen: Consultancy, Honoraria; Celgene: Consultancy, Honoraria, Research Funding; Takeda: Consultancy, Honoraria; Pharmacyclics: Other: DSMC membership. Khot:Amgen: Honoraria; Janssen: Consultancy; Pfizer: Speakers Bureau. Anderson:C4 Therapeutics: Equity Ownership; Millennuim: Membership on an entity9s Board of Directors or advisory committees; Oncoprep: Equity Ownership; C4 Therapeutics: Equity Ownership; Gilead: Membership on an entity9s Board of Directors or advisory committees; Gilead: Membership on an entity9s Board of Directors or advisory committees; Celgene: Membership on an entity9s Board of Directors or advisory committees; Celgene: Membership on an entity9s Board of Directors or advisory committees; Acetylon: Equity Ownership; Acetylon: Equity Ownership; Oncoprep: Equity Ownership; Millennuim: Membership on an entity9s Board of Directors or advisory committees; Bristol Myers Squibb: Membership on an entity9s Board of Directors or advisory committees; Bristol Myers Squibb: Membership on an entity9s Board of Directors or advisory committees. MacLaren:Triphase Accelerator: Employment, Equity Ownership. Reich:Triphase Accelerator: Consultancy. Trikha:Encycle Therapeutics: Consultancy, Equity Ownership; Triphase Accelerator: Employment, Equity Ownership. Richardson:Jazz Pharmaceuticals: Consultancy, Membership on an entity9s Board of Directors or advisory committees.