P842: NOVEL MULTIFUNCTIONAL TETRAVALENT CD38 NKP46 FLEX-NK ENGAGERS ACTIVELY TARGET AND KILL MULTIPLE MYELOMA CELLS

Background: Given that CD38 is a clinically validated target for natural killer (NK) cell mediated cytotoxicity in multiple myeloma (MM), we sought to leverage our FLEX-NKTM platform to create a NK engager antibody targeting CD38. FLEX-NKTM is a proprietary platform for production of tetravalent IgG1-like multifunctional NK engager antibodies with a novel FLEX-linker to allow for simultaneous binding of both the targeted cancer cells and NK cells via the activation receptor NKp46. Aims: Derive preclinical proof of concept with CD38 targeted NK-cell engager bispecific antibody in Mulitple Myeloma Methods: CYT-338 was expressed in CHOZEN cells and purified by step column chromatography. PB-NK cells were purified from healthy donor blood. PB-NK cytotoxicity against MM tumors was evaluated in the presence of CYT-338 or isotype control antibodies at a fixed E/T ratio. CYT-338 pharmacokinetics in mice was evaluated following a single intravenous injection. Anti-tumor efficacy of CYT-338 was evaluated in NSG-mice injected with PB-NK cells and MM cell line MM1S expressing luciferase. CYT-338 mediated fratricide was evaluated in purified PB-NK's. CYT-338 cytokine release assessments and hemato-cytotoxicity was evaluated in-vitro in human PBMC assays. Results: CYT-338 showed dose dependent binding to CD38 expressing MM cell lines MM1S and KMS11 and no binding to a CD38 knock out MM.1S cell line. CYT-338 bound MM cell lines with ~ 2-fold higher mean fluorescence intensity than anti-CD38 monoclonal antibody (mAb) or daratumumab alone. Epitope mapping studies for our CD38 mAb binder using alanine scanning mutagenesis showed that amino acid S274 on CD38, critical for binding to daratumumab, is not important for CD38 binding, suggesting a distinct epitope detected by our CD38 binder. CYT-338 showed greater dose dependent NK cell redirected cytolysis, degranulation and cytokine production against MM1S cells compared to daratumumab. The CYT-338 pharmacokinetics study in mice showed a terminal half-life of 41 hrs. Preliminary in-vivo tumor model studies in NSG mice using PB-NK and CYT-338 injections showed tumor growth inhibition of MM1S tumor cells. Low NK cell fratricide was observed with CYT-338 while daratumumab showed significantly higher NK cell fratricide. In peripheral blood mononuclear cell hemato-toxicity studies, depletion of monocytes and NK cells were observed with daratumumab but no significant depletion was observed with CYT-338. Cytokine release assessment of CYT-338 in the human PBMC assay showed no evidence of cytokine release while high levels of cytokine release was observed with daratumumab, anti-CD28 (TGN1412) and CD3 antibody controls. Summary/Conclusion: These results suggest that the CYT-338 engager has a favorable NK cell engager profile for targeting CD38 expressing multiple myeloma distinct from daratumumab.