Juxtaposing CD20 and CD74 with Novel Bispecific Antibodies Evokes Potent Cytotoxicity in Mantle Cell Lymphoma (MCL)

Abstract 599 Introduction: Initial response rates with first-line conventional or high-dose chemotherapy, are high in MCL, but most patients relapse. With 3,500 new cases detected every year in the US, there is an unmet medical need for new therapeutic interventions in MCL. Combination of two different targeting mAbs to achieve improved efficacy without increased toxicity was demonstrated first in NHL patients given both rituximab (anti-CD20 chimeric antibody) and epratuzumab (humanized anti-CD22 IgG). Recently, the potential advantage of targeting both CD20 and CD74 was reported in a preclinical study showing that combining rituximab and milatuzumab (humanized anti-CD74 IgG) together with a crosslinking antibody resulted in anti-tumor activity in MCL lines and patient samples in vitro (Alinari L, et al. Blood 2011; 117:4530-41). Here we describe the generation of two novel bispecific hexavalent antibodies [HexAbs; IgG-(Fab)4] constructed from veltuzumab (humanized anti-CD20 IgG) and milatuzumab (anti-CD74 IgG), and show both are capable of inducing potent cytotoxicity in mantle cell lymphoma (MCL) and other lymphoma/leukemia cell lines, as well as primary MCL and CLL patient samples, without requiring a crosslinking antibody. To identify these HexAbs, we assign each of them a code of X-(Y)-(Y), where × and Y are specific numbers given to differentiate the antibodies, and a designated number enclosed in a parenthesis representing the antibody as a stabilized Fab dimmer (e.g., 20-(74)-(74) designates the bispecific HexAb comprising a divalent anti-CD20 IgG of veltuzumab and a pair of stabilized dimers of Fab derived from milatuzumab). Methods: The two bispecific HexAbs, 74-(20)-(20) and 20-(74)-(74), as well as the two monospecific HexAbs, 74-(74)-(74) and 20-(20)-(20), were generated by the Dock-and-Lock method by reacting cognate CH3-AD2-IgG and CH1-DDD2-Fab modules under mild redox conditions and purified by Protein A. The in vitro activities were determined by cell viability and Annexin V binding assays. In addition, the role of signal transduction, homotypic adhesion, actin reorganization, and lysosomal volume changes were evaluated. Results: Each HexAb was purified to >95 % homogeneity and were stable in vitro and in serum. Both 20-(74)-(74) and 74-(20)-(20) potently inhibited the growth of MCL lines, JeKo-1, Granta-519 and Mino, at 10 nM. In contrast, neither parental antibody, alone or in combination, nor the two monospecific counterparts, 74-(74)-(74) and 20-(20)-(20), inhibited the growth of JeKo-1 under the same conditions, suggesting the requirement of apposing CD74 and CD20 for the observed cytotoxicity. Treatment of primary MCL and CLL patient cells with 74-(20)-(20) and 20-(74)-(74) induced 25–30% apoptosis, compared to 10–15% apoptosis with parental IgGs, alone or in combination. The two bispecific anti-CD20/CD74 HexAbs, but not the parental mAbs, induced strong homotypic adhesion, actin reorganization to cell-cell junctions, loss of mitochondrial membrane potential, generation of ROS, deactivation of the PI3K/Akt signaling pathway, as well as rapid and sustained activation of ERK and JNK MAPKs, and enlargement of lysosomes followed by release of cathepsin B in target cells. In SCID mice bearing JeKo-1 xenografts, both 20-(74)-(74) and 74-(20)-(20) were effective at the 370-μg dose level, resulting in 60% and 30% increases in median survival time, respectively, compared to saline controls (P = 0.001). Conclusion: Juxtaposing CD20 and CD74 in close proximity by HexAbs induces potent in vitro cytotoxicity in MCL lines and primary samples. These novel bispecific mAbs warrant further evaluation in B-cell lymphomas that are refractory or poorly responsive to anti-CD20 antibodies. Disclosures: Gupta: Immunomedics, Inc.: Employment. Goldenberg:Immunomedics, Inc.: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties. Rossi:Immunomedics, Inc.: Employment. Cardillo:Immunomedics, Inc.: Employment. Furman:Centocor Ortho Biotech Research & Development: Research Funding. Chang:Immunomedics, Inc.: Employment, Equity Ownership, Patents & Royalties.