AFM11, a CD19/CD3 bispecific tandab, to facilitate T-cell-mediated killing of CD19+ cells

3068 Background: CD19, due to broader expression on B cell subtypes, is an attractive alternative to CD20 as a target for treatment of B cell malignancies. T cells are potent tumor-killing effectors that cannot be recruited by full length antibodies, however TandAb technology harnesses their cytotoxic nature for oncology indications. The CD3 RECRUIT TandAb AFM11 enables T cells to potently and specifically kill CD19+ tumors and possesses advantageous PK properties enabling intravenous dosing. Methods: We constructed AFM11, a human bispecific tetravalent antibody with two binding sites for both CD3 and CD19. In vitro efficacy and safety were evaluated on CD19+ cell lines and primary tumors. In vivo efficacy was evaluated in a murine NOD/scid xenograft model reconstituted with human PBMC. Results: In vitro assays demonstrate higher potency and efficacy of target cell lysis by AFM11 relative to a bispecific tandem scFv. CD8+ T cells dominate early cytotoxicity (4 hrs) while after 24 hrs both CD4+ and CD8+ T cells equally contribute to tumor lysis with EC50 of 0.5 – 5 pM; cytotoxicity is independent of cell CD19 density. AFM11 exhibits similar cytotoxicity at Effector:Target ratios from 5:1 to 1:5 and facilitates T cell serial killing of its targets. AFM11 activates T cells only in the presence of CD19+ cells. In PBMC cultures AFM11 induces CD69 and CD25 expression, T cell proliferation, and production of IFN-γ, TNF-α, IL-2, IL-6, and IL-10. Depletion of CD19+ cells from PBMC abrogates these effects, and indicates strict CD19+ target-dependent T cell activation. Thus, AFM11 should not elicit the devastating cytokine release observed when full length antibodies bind CD3. Cell lysis by AFM11 is restricted to CD19+ targets asCD19- bystanders are not lysed in co-culture assays. Up to one week co-incubation with AFM11 does not inhibit T cell cytotoxicity and thus it does not induce anergy. In vivo AFM11 exhibits a dose-dependent growth inhibition of Raji tumors; a single dose of AFM11 exhibits similar efficacy as 5 daily injections. Conclusions: AFM11 is a highly efficacious novel drug candidate for the treatment of CD19+ malignancies with an advantageous safety profile and anticipated dosing regimen.