Preclinical Development of a Bcma Targeting Antibody-Drug Conjugate with Novel Payload for Multiple Myeloma Therapy

BCMA (B-cell maturation antigen) is an integral membrane protein that belongs to the TNF receptor family with expression restricted to B cell lineage cells. The RNA is near universally detected in multiple myeloma (MM) cells and the protein is expressed on the surface of malignant plasma cells from patients with MM. In contrast, BCMA expression in normal tissues is very limited, making BCMA a promising target for antibody-drug conjugate (ADC) therapy.

We have developed a BCMA-targeting ADC, employing a fully human anti-BCMA monoclonal antibody (mAb) identified from Sorrento's G-MAB antibody library, which was conjugated using proprietary Concortis linker-Duo 5.2 toxin technology resulting in BCMA-077 ADC. The mAb has a unique binding profile for BCMA and demonstrated strong preferential binding for BCMA-overexpressing cells but showed much less binding to lower BCMA-expressing cells. This property allows for more selective binding of the ADC on high BCMA-expressing cells, which are usually tumor cells while sparing low BCMA-expressing normal cells. In addition, we modified the Duo 5.2 payload decreasing the potency of the unconjugated toxin while retaining activity when conjugated to the mAb. The resulting ADC, BCMA-024, was compared to BCMA-077 using in vitro assays, including binding, internalization and cytotoxicity against tumor cell lines. The two ADCs exhibited strong activity and no difference in cytotoxic potency evident. The toxicity of the payload derivative was evaluated in a rodent model and it was found to be well tolerated not showing toxicity at a dose 10 times higher than the lethal dose of the parental toxin.

Both ADCs carrying either the parental Duo 5.2 toxin or the derivative toxin payload were evaluated in vivo for anti-tumor activity in three different multiple myeloma xenograft models using different dose regimens. The data showed that both ADCs demonstrated potent BCMA-dependent in vivo anti-tumor activity in all xenograft BCMA-positive tumor models. The PK of the parental anti-BCMA mAb was investigated in non-human primates (NHP) and the parameters indicated a T_1/2 of about 10 days. The GLP toxicity studies are ongoing.

Our BCMA-ADCs have shown favorable anti-tumor activities combined with good safety profiles resulting in an expanded therapeutic window. The data make BCMA-077 and BCMA-024 promising candidates for continued preclinical development. Based on the totality of our preclinical data, we anticipate selecting a BCMA ADC clinical candidate for the treatment of multiple myeloma.