Preclinical Evaluation of a Potent Anti-Bcma CD3 Bispecific Molecule for the Treatment of Multiple Myeloma

Multiple myeloma (MM) is a debilitating disease characterized by the abnormal accumulation of malignant plasma cells in the bone marrow. Despite recent advances in myeloma therapy, including proteasome inhibitors, immunomodulatory drugs, and targeted antibody therapies, patients relapse and the disease remains incurable and one of high unmet need. T cell redirecting therapies are a new and exciting class of therapeutics that harness the potent cytotoxic activity of T cells and redirect it to target tumor cells. T cell redirecting therapies are only as good as their targeted tumor associated antigen (TAA) and the potent nature of the therapy requires a lack of TAA expression in essential normal tissue. B-cell Maturation Antigen, BCMA, is a tumor necrosis factor superfamily member highly expressed on the surface of myeloma cells. Detectable normal BCMA tissue expression appears limited to plasmablasts and mature plasma cells, making it an ideal T cell redirecting target for the treatment of MM.

Other groups have developed T cell redirecting therapies against BCMA, including CAR T and BiTE therapy (a short half-life CD3 bispecific). Here we present preclinical studies on a fully-human IgG CD3 bispecific molecule targeting BCMA (half-life in mice of ~3 days). This molecule utilizes anti-BCMA and anti-CD3 targeting arms paired through hinge mutation technology and placed in an IgG2A backbone. The molecule binds to BCMA-expressing myeloma cell lines and to T cells with affinities of 20pM and ~40nM, respectively.

T cells co-cultured with MM cell lines were activated and de-granulated in the presence of BCMA bispecific. In vitro cytotoxicity assays revealed the high potency of the molecule, as it was able to drive lysis of MM target cells with an EC₅₀ of 6± 8 pM (mean ± SD). We also observed strong in vitro potency with the BCMA bispecific in four different MM primary patient samples, EC₅₀ =0.093±0.1 nM (mean ± SD). When the same four samples were targeted with a BCMA antibody drug conjugate (ADC), 3 of the samples gave EC₅₀ values of 1.25±0.7 nM (mean ± SD) - i.e. a 43 fold decrease in potency compared to the CD3 bispecific. The fourth patient did not respond to the ADC. Together, these results illustrate the potential advantages of a CD3 bispecific over an ADC for targeting BCMA.

In orthotopic, established, tumor mouse models utilizing three different MM cell lines, (OPM2, MM.1S and MOLP8), a single injection of BCMA bispecific effectively treated tumors in a dose-dependent manner. Re-dosing the bispecific was able to provide additional and prolonged efficacy.

The extreme potency of T cell redirecting therapies results in outstanding efficacy, but can also lead to lysis of normal cells expressing even minute levels of target. The species cross-reactivity of the BCMA bispecific allowed for exploratory toxicity studies in cynomologus monkeys. The molecule was able to effectively deplete normal plasma B cells expressing low levels of BCMA, providing evidence of activity. Activity was accompanied by a cytokine spike following initial dosing. No cytokine release was observed following a second bispecific dose. Encouragingly, animals experienced no additional adverse events (AEs), confirming the favorable safety profile of BCMA as a target for MM.

In summary, we report on a fully human IgG CD3 bispecific molecule targeting BCMA for the treatment of multiple myeloma. Our BCMA bispecific is expected to have an antibody-like half-life in humans and, taken together, our findings support that the molecule has the potential to be both a potent and safe therapeutic.