SGN-CD48A: a Novel Humanized Anti-CD48 Antibody-Drug Conjugate for the Treatment of Multiple Myeloma

Multiple myeloma (MM) is an incurable hematologic malignancy of transformed plasma cells. New targeted biological therapeutics are needed to increase the stringency and durability of remissions. In this study we describe SGN-CD48A, a potent CD48-targeting antibody-drug conjugate (ADC) utilizing a novel glucuronide-monomethylauristatin E (MMAE) linker, under development for the treatment of MM. CD48, or SLAMF2 (Signaling Lymphocyte Activation Molecule family member 2), is a GPI-anchored membrane protein in the SLAM family of immunoreceptors. CD48 is expressed on B and T lymphocytes, natural killer (NK) cells, and other immune cell types where it functions to modulate immune cell activation, proliferation, and differentiation. CD48 is also a tumor antigen broadly expressed in MM. We observed CD48 expression on the surface of malignant plasma cells in 90% (90/100) of human multiple myeloma patient samples examined by flow cytometry. Monoclonal antibodies (mAbs) specific for human CD48 were evaluated and a lead antibody was selected based on binding characteristics and cytotoxic activity against myeloma cells as an auristatin ADC. SGN-CD48A is a humanized anti-CD48 mAb to which eight molecules of MMAE, a potent microtubule disrupting cytotoxic drug, have been conjugated via a β-glucuronidase-cleavable linker. This novel glucuronide-MMAE drug-linker incorporates a PEG side chain and self-stabilizing maleimide to achieve homogenous drug-to-antibody ratio (DAR) 8 conjugates with decreased plasma clearance and increased preclinical antitumor activity. Following binding of CD48 at the myeloma cell surface, SGN-CD48A internalizes and traffics to lysosomal vesicles. Intracellular MMAE drug released from SGN-CD48A in myeloma cells induced cell cycle arrest at G2/M phase, phospho-histone H3 (Ser-10) phosphorylation, and caspase 3/7 dependent apoptotic cell death. SGN-CD48A demonstrated potent cytotoxic activity (EC50 values 1.0 - 11 ng/mL) against a panel of human MM cell lines, with CD48 expression levels of 135,000 - 480,000 receptors per cell. In contrast, SGN-CD48A had negligible cytotoxic activity against normal resting human B, NK, and T lymphocytes. We evaluated the in vivo antitumor activity of SGN-CD48A in disseminated MM cell line mouse xenograft models. In the NCI-H929 and EJM xenograft models, a single intraperitoneal dose of 0.3 mg/kg SGN-CD48A produced durable complete remissions in 8/8 and 6/8 mice, respectively. Similarly, in the U-266 xenograft model, a single dose of 1.0 mg/kg SGN-CD48A produced durable complete remissions in 7/8 mice. Neither unconjugated mAb nor a non-binding control MMAE ADC were active in these MM xenograft models, demonstrating that targeted delivery of MMAE drug through CD48 binding is required for activity. In summary, CD48 is a highly expressed new multiple myeloma target and the novel glucuronide-MMAE ADC SGN-CD48A shows potent antitumor activity against cell line models of MM.