The Novel Proteasome Inhibitor MLN9708 Demonstrates Efficacy in a Genetically-Engineered Mouse Model of DeNovo Plasma Cell Malignancy.
Authors: Janz Siegfried , Van Ness Brian G , Neppalli Vishala , Liu Ray , Pickard Michael D , Terkelsen Jennifer , Bradley Daniel , Hu Ling , Kupperman Erik , Manfredi Mark , Lee Edmund 
Published: October 2019

Abstract
Abstract 3849
Poster Board III-785

Introduction
The first-in-class proteasome inhibitor VELCADË (bortezomib) is a critical component of chemotherapeutic strategy in treating multiple myeloma (MM), a plasma cell malignancy characterized by monoclonal gammopathy and osteolytic lesions. However, patients eventually develop bortezomib resistance, highlighting the continuous challenge to further improve myeloma therapy. We have recently identified a novel proteasome inhibitor MLN9708 that shows greater antitumor activity than bortezomib in a number of preclinical xenograft models. Here we describe antitumor activity of MLN9708 in a genetically engineered mouse model of human MM. In this model, neoplastic plasma cell development is driven by deregulated expression of Myc (c-myc, myelocytomatosis oncogene) and Bcl-x (official gene name: Bcl2l1; encodes the anti-apoptotic Bcl-XL oncoprotein). This is the first preclinical in vivo study of proteasome inhibitors employing a transgenic mouse model of human MM in which plasma cell neoplasms develop de novo.

Materials
Upon exposure to aqueous solutions or plasma, MLN9708 immediately hydrolyzes to MLN2238, the biologically active form. MLN2238 was used for all preclinical studies described below.

Methods
We have previously demonstrated that intercrossing C57BL/6 mice carrying the iMycCa transgene (insertion of Myc in the immunoglobulin heavy-chain locus, Igh) with FVB/N mice carrying the 3'KE-Bcl-XL transgene (enforced expression of Bcl-x driven by the immunoglobulin light-chain 3' k enhancer and Vk21 promoter) produces double transgenic F1 hybrid iMycCa/Bcl-XL mice that develop plasma cell malignances with short onset (135 days on average) and full penetrance (100% tumor incidence; J. Clin. Invest. 113:1763-1773, 2004). These double transgenic iMycCa/Bcl-XL mice develop hypergammaglobulinemia and tissue plasmacytosis by 6-8 weeks of age and rapidly succumb to malignant plasma cell tumors by less than 200 days of age. Preliminary studies based on 11 tumor-bearing iMycCa/Bcl-XL mice showed this form of plasma cell malignancy recapitulates important features of human MM, including serum paraproteins, infiltration of bone marrow with malignant plasma cells, osteolytic lesions and myeloma-like global gene expression profiles (Cancer Res. 67:4069-4078, 2007). Here we used the iMycCa/Bcl-XL mouse model of human MM to assess the antitumor activity of bortezomib and MLN9708 in a preclinical setting.

Results
Untreated double transgenic iMycCa/Bcl-XL mice (n=30) invariably developed plasma cell tumors with short onset (median tumor-free survival = 112 days) and showed marked elevations in plasma immunoglobulin IgM, IgG1, IgG2a and IgG2b levels. To assess antitumor activity of bortezomib and MLN2238, 9-week-old iMycCa/Bcl-XL mice were treated intravenously (IV) twice per week (BIW) with bortezomib (1.2 mg/kg) or MLN2238 (18 mg/kg) for 6 consecutive weeks. These doses represent the maximum tolerated dose (MTD) for each drug as previously determined in normal C57BL/6 x FVB/N F1 hybrid mice. Treatment of iMycCa/Bcl-XL mice with bortezomib (n=30) significantly prolonged tumor-free survival compared to untreated controls (median tumor-fee survival = 139 days; hazard ratio = 0.116; 95% CI = 0.057 to 0.235; p<0.0001). Importantly, treatment of iMycCa/Bcl-XL mice with MLN2238 (n=30) also significantly prolonged tumor-free survival relative to untreated controls (median tumor-free survival = 148 days; hazard ratio = 0.144; 95% CI = 0.071 to 0.289; p<0.0001). Immunoglobulin levels, overall tumor burden, osteolytic lesions (mCT) and biochemical parameters of myeloma bone disease after treatment with bortezomib and MLN2238 will be presented.

Conclusion
The novel proteasome inhibitor MLN9708 demonstrates striking antitumor activity in a genetically engineered mouse model of human MM, significantly prolonging tumor-free survival of double transgenic iMycCa/Bcl-XL mice. MLN9708 is currently in human clinical development for both hematological and solid tumor indications.

Disclosures:
Janz: Millennium – The Takeda Oncology Company: Research Funding. Van Ness:Millennium: Research Funding; Scientific Advisory Board, International Myeloma Foundation: Membership on an entity's Board of Directors or advisory committees. Liu:Milllennium: Employment, Equity Ownership. Pickard:Milllennium: Employment. Terkelsen:Milllennium: Employment. Bradley:Milllennium: Employment, Equity Ownership, Research Funding. Kupperman:Milllennium: Employment. Manfredi:Milllennium: Employment. Lee:Milllennium: Employment, Equity Ownership.