A CD19/CD3 Bispecific Tandab, AFM11, Recruits T Cells To Potently and Safely Kill CD19⁺ Tumor Cells

To harness the potent tumor-killing capacity of T cells for the treatment of CD19⁺ malignancies, we developed a humanized bispecific tetravalent antibody, with two binding sites for CD3 and CD19, the CD19/CD3 RECRUIT-TandAb AFM11. CD19 is expressed from early B cell development through differentiation into plasma cells, and is an attractive alternative to CD20 as a target for the development of therapeutic antibodies to treat B cell malignancies such as Non Hodgkin Lymphoma. Since native antibodies cannot recruit T cells, we engineered a bispecific anti-CD19/anti-CD3 TandAb. The tumor-specific CD19 antigen module targets the TandAb to cancer cells, while simultaneously, the CD3 effector module recruits and activates T cells, leading to cancer cell lysis. The advantages of the TandAb technology, relative to other bi-functional fragment antibody scaffolds, include: improved pharmacokinetics (PK) enabling intravenous dosing, more drug-like properties, and avidity-enhanced efficacy for the targeting and killing of tumor cells. We evaluated in vitro efficacy and safety using CD19⁺ cell lines, and in vivo efficacy in a murine NOD/scid xenograft model reconstituted with human PBMC. Further, we used standard preclinical IND enabling assays to evaluate tissue cross reactivity, PK, and toxicological profile (local tolerance, hematocompatibility, effects on hematopoesis, etc).

In vitro assays demonstrated the higher potency and efficacy of target cell lysis by AFM11 relative to a bispecific tandem scFv (that is currently in clinical evaluation). CD8⁺ T cells dominate early AFM11-mediated cytotoxicity (4 hrs) while after 24 hrs both CD4⁺ and CD8⁺ T cells equally contribute to tumor lysis with EC₅₀ between 0.5 – 5 pM; cytotoxicity was independent of CD19 cell-surface density. AFM11 exhibited similar cytotoxicity over effector:target ratios ranging from 5:1 to 1:5, and facilitated serial T cell-killing of its targets. The advantage of AFM11 over the bispecific tandem scFv was most pronounced at lower effector:target ratios. AFM11 activated T cells only in the presence of CD19⁺ cells. In PBMC cultures, AFM11 induced 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 abrogated these effects, demonstrating that the T cell activation is strictly CD19⁺ target-dependent. Thus, AFM11 should not elicit the devastating cytokine release observed when full-length antibodies bind CD3. Up to one week co-incubation with AFM11 did not inhibit T cell cytotoxicity, suggesting that the TandAb does not induce anergy. In vivo, AFM11 induced dose-dependent growth inhibition of Raji tumors; a single 0.5 mg/kg dose exhibited efficacy similar to 5 daily injections. In the tissue cross reactivity study, only tissues containing CD19⁺ and CD3⁺ cells were stained by AFM11; all other tissues, including vital organs, displayed no cross reactivity. Similarly, no local intolerance was observed in rabbits, and no effect on myeloid and erythroid progenitors was observed in a colony-forming assay. Strong accumulation of ¹²⁵I-labeled AFM11 was observed in the tumors of mice engrafted with CD19⁺ cancer cells, and no unspecific organ accumulation was observed. Finally, evaluated on the basis of C_max and the area under the curve (AUC), AFM11 exhibited dose linearity (20 – 500 mg AFM11 dose range) upon single i.v. bolus administration in mice; half-life (T_1/2) ranged from 18.4 to 22.9 hr.

In summary, AFM11 is a highly efficacious novel drug candidate for the treatment of CD19⁺ malignancies with an advantageous safety profile and anticipated dosing regimen.