MLN9708 Elicits Pharmacodynamic Response in the Bone Marrow Compartment and Has Strong Antitumor Activity in a Preclinical Intraosseous Model of Plasma Cell Malignancy.
Authors: Lee Edmund , Bannerman Bret , Fitzgerald Michael , Terkelsen Jennifer , Bradley Daniel , Li Zhi , Li Ping , Janz Siegfried , Van Ness Brian G , Manfredi Mark , Kupperman Erik 
Published: October 2019

Abstract
Abstract 1834
Poster Board I-860

Introduction
The clinical success of VELCADE® (bortezomib) for Injection has validated the proteasome as a therapeutic target for the treatment of human cancer. The novel proteasome inhibitor MLN9708 is a potent, reversible, and specific inhibitor of the b5 site of the 20S proteasome identified in preclinical studies. MLN9708 is currently in human clinical development for both hematological and non-hematological malignancies. Here we describe the pharmacodynamic (PD) response of MLN9708 in the murine bone marrow compartment and its strong antitumor activity in an intraosseous xenograft model of plasma cell malignancy.

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

Methods
It has been previously shown that double transgenic iMycCa/Bcl-XL mice develop de novo plasma cell malignancies (J. Clin. Invest. 113:1763-1773, 2004) in which neoplastic plasma cell development is driven by the targeted expression of the transgene Myc (c-myc; myelocytomatosis oncogene) and Bcl-x (Bcl2l1; encodes the oncoprotein Bcl-XL). DP54 is a plasma cell tumor cell line derived from the bone marrow of a syngeneic mouse previously inoculated with an iMycCa/Bcl-XL tumor (Cancer Res. 67:4069-4078, 2007). In vitro, DP54 cells express both the Myc and Bcl-XL transgenes, various plasma cell and B-cell markers including CD38, CD138 and B220, and has gene expression profile very similar to human multiple myeloma. To establish a preclinical intraosseous model of plasma cell malignancy for efficacy studies, freshly dissociated DP54-Luc cells (constitutively expressing firefly luciferase under a mouse Ig-k promoter) were aseptically injected into the bone marrow space of the upper shaft of the right tibia of NOD-SCID mice. Once tumor growth has been established, mice were randomized into treatment groups and then treated intravenously (IV) with vehicle, bortezomib (at 0.8 mg/kg twice weekly [BIW]) or MLN2238 (at 11 mg/kg BIW) for 3 consecutive weeks. Tumor burden was measured by bioluminescent imaging.

Results
MLN2238 strongly inhibited proteasome activity in the blood and bone marrow compartments of mice (maximum b5 inhibition of 84% and 83%, respectively). In vivo, when DP54 cells were aseptically injected into the bone marrow space of the mouse tibia, signs of bone erosion in the tibia, femur and cranial sagittal sultures (as determined by ex-vivo mCT imaging) were observed which resembled osteolytic lesions frequently seen in human multiple myeloma. Dissemination of DP54-Luc cells after intratibia inoculations were detected by in vivo bioluminescent and confirmed by ex vivo imaging where luminescent tumor nodules were detected in the spleen, kidneys, intestine, lymph nodes and bones including right tibia, spine and cranium. To assess the antitumor activity of MLN2238 in the bone marrow compartment, an efficacy study was performed using the DP54-Luc intraosseous xenograft model of plasma cell malignancy. Tumor burden (bioluminescence), osteolytic lesions (mCT) and overall survival after treatment with bortezomib and MLN2238 will be presented.

Conclusion
The novel proteasome inhibitor MLN9708 demonstrates strong activity in the bone marrow compartment in vivo. MLN9708 is currently in human clinical development for both hematological and solid tumor indications.

Disclosures
Lee: Milllennium: Employment, Equity Ownership. Bannerman:Milllennium: Employment. Terkelsen:Milllennium: Employment. Bradley:Milllennium: Employment, Equity Ownership, Research Funding. Li:Milllennium: Employment. Li:Milllennium: Employment. Janz:Milllennium: Research Funding. Van Ness:Milllennium: Research Funding. Manfredi:Milllennium: Employment. Kupperman:Milllennium: Employment.