Effects of Oprozomib in Combination with Pomalidomide and/or Dexamethasone on Human Multiple Myeloma Tumors Growing in SCID Mice
Authors: Sanchez Eric , Li Mingjie , Gillespie Abigail , Mehta Puja , Vardanyan Suzie , Garzio Gigi , Tang George , Wang Cathy , Chen Haiming , Berenson@imbcr.org James R 
Published: October 2019

Abstract
Introduction: Oprozomib (OPZ) is an irreversible, orally administered proteasome inhibitor (PI).It decreases tumor burden and prevents tumor-related bone loss in preclinical multiple myeloma (MM) studies. In recently published clinical trials, the combination of the PI carfilzomib with the immunomodulatory agent (IMiD) lenalidomide and dexamethasone (Dex) has shown high response rates with durable responses for previously treated and untreated MM patients. Carfilzomib with the IMiD pomalidomide (Pom) and Dex for the treatment of RRMM patients shows promising results (Shah et al, 2013). We evaluated OPZ in combination with Pom and Dex using two of our human MM xenograft models in severe combined immunodeficient (SCID) mice.
Methods: Each SCID mouse (n=10/group) was surgically implanted with a 20 - 40 mm3 MM tumor piece into the hind limb. Seven days post-implantation mice were randomized into treatment groups based on human immunoglobulin (Ig) G levels. OPZ stock solution (4 mg/ml) was diluted to 40 mg/kg using 1% carboxymethylcellulose (CMC) and administered twice weekly on two consecutive days via oral gavage.Dex stock solution (10 mg/ml) was diluted to 1 mg/kg using NaCl and administered daily via intraperitoneal injection.Pom stock solution (1 mg/ml) was diluted to 10 mg/kg using 1% CMC and administered daily via oral gavage. Tumor size was measured using calipers and IgG levels by ELISA.
Results: Using our human MM model LAGk-1A, treatment with single agent OPZ or Pom produced a minimal reduction in tumor volume when compared with vehicle-treated mice, whereas Dex alone or OPZ + Pom produced more anti-MM effects, and no differences were observed between these two groups. Mice treated with OPZ + Dex or Pom + Dex also showed greater anti-MM activity than OPZ + Pom or Dex alone but the differences were not significant. All three agents together resulted in much smaller tumors when compared to OPZ + Pom on days 35, 42, 49 and 56 (P = 0.0006, P = 0.0001, P = 0.0002 and P < 0.0001, respectively). The same triplicate resulted in a smaller tumors when compared to OPZ + Dex on days 35, 42, 49, 56, and 63 (P = 0.0112, P = 0.0030, P = 0.0060, P = 0.0035 and P = 0.0021, respectively). Although Pom + Dex had some anti-MM effects when compared to the three single agents and one of the doublets (OPZ + Pom), mice receiving the triplicate demonstrated markedly smaller tumors when compared with Pom + Dex on days 35, 42, 49, 56, 63, 70, and 77 (P = 0.0250, P = 0.0018, P < 0.0001, P = 0.0014, P = 0.0018, P = 0.0017 and P = 0.0014, respectively). Mice receiving Pom + Dex had to be euthanized on day 77, whereas mice receiving all three drugs had very small tumors at study termination. We obtained similar results in a second MM xenograft model that produces IgG (LAGλ-1). Although day 21 post-tumor implantation mice receiving Pom alone had lower IgG levels compared with vehicle-treated mice (P = 0.0053), mice receiving OPZ + Pom had smaller tumors when compared to Pom alone (P = 0.0387), OPZ alone (P = 0.0004), or vehicle-treated mice (P = 0.0001). Although mice receiving Pom alone had a reduction in tumor size when compared with vehicle-treated mice (P = 0.0021), mice receiving OPZ + Pom had smaller tumors when compared with Pom alone (P = 0.0081), OPZ alone (P = 0.0007), or vehicle-treated mice (P < 0.0001). Mice receiving the triplicate showed smaller tumors on day 28 compared with mice treated with either the doublets of Pom + Dex (P = 0.0120) or OPZ + Pom (P = 0.0043).
Conclusions: These in vivo human MM xenograft studies show that the combination of OPZ + Pom + Dex shows greater anti-MM activity than doublets (Pom + Dex, OPZ + Dex or OPZ + Pom) or single agents, and provides further support for this three drug combination to treat MM.

Disclosures
Tang: Onyx: Research Funding. Berenson@imbcr.org:Onyx: Consultancy, Honoraria, Research Funding, Speakers Bureau.