Preclinical Evaluation of MEDI2228, a BCMA-Targeting Pyrrolobenzodiazepine-Linked Antibody Drug Conjugate for the Treatment of Multiple Myeloma

Despite remarkable progress in treatment over the past two decades, multiple myeloma (MM) remains an incurable disease. B-cell maturation antigen (BCMA, TNFRSF17) is a suitable therapeutic target for the treatment of MM due to its restricted expression on normal plasma cells and universal expression in myeloma cells. Targeting BCMA with an antibody-drug conjugate (ADC) is an attractive therapeutic option, combining the specificity of a monoclonal antibody with a potent cytotoxic drug to preferentially eliminate antigen-positive cells for the treatment of cancer.

Here we describe MEDI2228, a novel ADC that targets BCMA. MEDI2228 is composed of a fully human antibody site-specifically conjugated to a DNA cross-linking pyrrolobenzodiazepine (PBD) dimer via a protease-cleavable linker. Flow cytometry studies show that MEDI2228 is rapidly internalized and trafficked to lysosomes. Upon release, the warhead binds to the minor-groove and cross-links DNA, leading to DNA damage and apoptotic cell death.

The anti-tumor activity of MEDI2228 was assessed in vitro in 10 MM and plasma cell leukemia (PCL) cell lines encompassing several cytogenetic subtypes. MEDI2228 was highly active in 8 of 10 MM cell lines tested (IC50 range 6 to 210 ng/mL) including cell lines with high (~19,000 receptors/cell) or low (~930 receptors/cell) expression of BCMA. In addition, MEDI2228 is active in the presence of bone marrow stromal cells, which have been shown to play a role in chemotherapy resistance, and in cell models resistant to Lenalidomide. In vivo antitumor activity was evaluated in 4 separate MM and PCL subcutaneous xenograft models and 2 PCL disseminated xenograft models, where a single injection of MEDI2228 induced tumor regression at doses as low as 0.1 mg/kg.

A soluble form of BCMA (sBCMA) has been detected in the serum of MM patients (Sanchez et. al. Br J Haematol 2012) and is comprised of the entire extracellular domain of the molecule (Laurent et. al. Nat Commun 2015). Therefore, sBCMA could diminish the effects of antibody-based therapies. We found that levels of sBCMA are elevated in the sera of MM patients compared with healthy donor controls, with levels ranging from 23 ng/mL to 728 ng/mL. The functional features of sBCMA and recombinant monomeric human BCMA are similar (Laurent et. al. Nat Commun 2015). Therefore, to mitigate the potential effects of sBCMA on the efficacy of our drug, the antibody component of MEDI2228 was selected because it possessed weak binding to recombinant monomeric human BCMA and strong binding to membrane-bound BCMA. The in vitro cytotoxicity of MEDI2228 in the presence of clinically-relevant levels of sBCMA was evaluated using NCI-H929 and MM.1S cells. MEDI2228 was cytotoxic to both MM.1S and NCI-H929 in vitro, killing an average of 95% of tumor cells in the presence of sBCMA at levels up to 720 ng/mL with little impact on IC50. ADCs developed from antibodies that possessed a similar affinity between monomeric BCMA and membrane-bound BCMA exhibited a sBCMA-dose dependent drop in potency, with a 20-fold shift in IC50 in the presence of 720 ng/mL sBCMA.

MM is a clonally heterogeneous disease, and previously it has been reported that a small population of CD19+CD138- clonogenic cells can be identified in the bone marrow (BM) of MM patients (Matsui et. al. Blood 2004). We used flow cytometric sorting of CD19 and CD138 populations and outgrowth in methocult to confirm that CD19+CD138- cells are the clonogenic cells in MM BM samples. Furthermore, we found that this population contains the same light chain restriction, genetic translocation, and aberrant phenotype as the MM plasma cells. These cells express BCMA and can be killed by MEDI2228 in methocult outgrowth assays.

These data demonstrate that MEDI2228 exhibits potent antitumor activity in preclinical models of MM. Importantly, in vitro experiments suggest that this activity is maintained in the presence of sBCMA. MEDI2228, bearing a potent PBD payload, may effectively target both the bulk myeloma plasma cells as well as the more quiescent, CD19+CD138- clonogenic cells, which may offer an opportunity for more durable clinical response in this genetically heterogeneous disease.