Paper Details

ABSTRACT

Background: Autoreactive B cells and plasma cells are key drivers of pathogenic autoantibody production and are therapeutic targets in autoimmune cytopenias. B-cell Maturation Antigen (BCMA) is expressed on B-lineage cells and may provide an opportunity for selective ablation of these populations. Velinotamig (CLN-803) is a BCMA×CD3 bispecific T-cell engager that redirects T cells to BCMA-expressing B-lineage cells. We characterized BCMA expression in human peripheral blood mononuclear cells (PBMC) and bone marrow mononuclear cells (BMMC) and evaluated the pharmacological activity of velinotamig in multiple in vitro and in vivo models. Together, these nonclinical data support clinical development of velinotamig for autoimmune cytopenias.

Methods: Human PBMC and BMMC phenotyping were performed using multiparameter flow cytometry to identify BCMA-expressing B-cell populations. Velinotamig binding was evaluated by flow cytometry using BCMA-expressing target cell lines and primary human T-cells. Functional activity was assessed in T cell-dependent cellular cytotoxicity (TDCC) assays using primary human T-cells cultured with BCMA positive or negative target cells. Additional studies were conducted using healthy donor PBMC cultures alone or co-cultured with BCMA-expressing Raji cells. T-cell activation, target-cell depletion, endogenous B-cell depletion, and cytokine production were assessed by flow cytometry and multiplex cytokine analysis. Pharmacodynamic activity was further evaluated in humanized (h) BCMA/CD3 knock-in mice.

Results: Immune profiling demonstrated BCMA expression across peripheral blood and bone marrow B-cell compartments with the highest expression detected in plasmablasts, short-lived plasma cells, and bone marrow long-lived plasma cells, while naïve and memory B cells exhibited lower BCMA expression. These findings identified clinically relevant BCMA-positive cell populations associated with autoantibody production and support BCMA-directed depletion as a therapeutic strategy for autoimmune cytopenias.

Velinotamig demonstrated selective, concentration-dependent binding to BCMA-expressing target cells and primary human T cells. Target-cell binding increased with BCMA expression and was negligible in BCMA-negative cells, confirming selective target engagement.

In TDCC assays, velinotamig induced potent activation of CD4 and CD8 T cells and dose-dependent depletion of cell lines with varying BCMA expression levels. No significant activity was observed in BCMA-negative cell co-cultures or target-cell-only controls, demonstrating that cytotoxicity required both BCMA-positive target cells and CD3-positive T cells. Cytokine induction was similarly target-dependent, with responses observed in BCMA-positive cultures and minimal responses in BCMA-negative controls.

In PBMC-only cultures, velinotamig induced dose-dependent T-cell activation with partial endogenous B-cell depletion and limited cytokine release, consistent with low target abundance. Addition of BCMA-expressing Raji cells enhanced T-cell activation and cytokine production, resulting in potent Raji cell depletion and reduced endogenous B-cell depletion, consistent with target competition and reduced effector-to-target ratios. These findings demonstrate that velinotamig activity is strongly influenced by target abundance. The PBMC-only system may be translationally relevant for autoimmune diseases.

In hBCMA/hCD3 knock-in mice, velinotamig demonstrated pharmacodynamic activity, including reduced circulating B-lineage cells, altered T-cell populations, and transient cytokine responses, confirming on-target pharmacologic activity in vivo and supporting translation of its mechanism of action across experimental systems.

Conclusions: BCMA expression was detected across antibody-producing B-cell populations in peripheral blood and bone marrow, including plasmablasts, short-lived plasma cells, and long-lived plasma cells. Velinotamig demonstrated target-selective engagement, BCMA-dependent T-cell activation, and potent TDCC of BCMA-expressing cells across multiple experimental systems. Activity was modulated by target abundance, with PBMC-based assays providing a clinically relevant translational model characterized by low-frequency endogenous BCMA-positive target cells. Collectively, these findings support development of velinotamig as a BCMA-directed T-cell engager for the treatment of autoimmune cytopenias driven by BCMA-expressing B-lineage cells.