Potent B-Cell Depletion by IMGN529, a CD37-Targeting Antibody-Maytansinoid Conjugate for the Treatment of B-Cell Malignancies,

Abstract 3726

CD37 is a B-cell surface antigen which is widely expressed on malignant B cells in non-Hodgkin's lymphoma (NHL) and chronic lymphocytic leukemia (CLL). In normal tissues CD37 expression is limited to blood cells and lymphoid tissues. This restricted expression profile makes CD37 an attractive therapeutic target for antibodies and antibody-drug conjugates.

We developed a novel anti-CD37 antibody, K7153A, which provides a unique combination of functional properties: it demonstrated strong pro-apoptotic and direct cell killing activity against NHL cell lines and could mediate effector activity such as CDC and ADCC. The antibody-maytansinoid conjugate, IMGN529, was produced by conjugation of K7153A with the potent maytansinoid, DM1, via the non-cleavable linker, SMCC. The direct cytotoxic potency of the K7153A antibody was superior to that of the CD20-directed rituximab and was further enhanced with maytansinoid conjugation in IMGN529. In vivo, IMGN529 demonstrated better anti-tumor activity than the K7153A antibody in established subcutaneous follicular lymphoma (FL), diffuse large B-cell lymphoma (DLBCL), and CLL xenograft models in SCID mice. A single administration of IMGN529 showed similar or improved efficacy compared to anti-CD20 antibodies or standard chemotherapy where tested.

Immunohistochemical (IHC) staining of formalin fixed paraffin-embedded (FFPE) NHL tissue sections was performed to evaluate CD37 expression. CD37 exhibited a similar prevalence to CD20 in subtypes of NHL such as FL, DLBCL, Burkitt's lymphoma (BL) and mantle cell lymphoma (MCL).

B-cell depletion is an important measure of efficacy for targeted therapies, such as CD20-directed antibodies, in B-cell malignancies. CD37 expression in blood cells from healthy human donors was measured by quantitative flow cytometry in comparison to CD20. The greatest CD37 expression was found in B cells at approximately 77,000 antibodies bound per cell (ABC), which was similar to CD20 expression in B cells at 95,000 ABC. In other blood cell types CD37 staining was seen at low levels, about 2,000 – 5,000 ABC, in monocytes, NK cells and T cells. In vitro depletion experiments were performed with purified peripheral blood mononuclear cells (PBMCs) and with whole blood, both derived from several healthy donors. Cells were incubated for 1 hr with 10 μg/mL of either K7153A, IMGN529, CD37-targeting TRU-016, rituximab or the anti-CD52 antibody alemtuzumab, with cell depletion determined relative to counting beads by flow cytometry. The K7153A antibody and the IMGN529 conjugate efficiently and specifically depleted B-cells in a dose-dependent manner in the context of purified PBMCs and whole blood. With purified PBMCs, both K7153A and IMGN529 caused 50–60% depletion of B cells, with little to no depletion of T cells or monocytes. IMGN529 was more potent than rituximab, which led to 30–40% B-cell depletion, or TRU-016, which caused 20–30% B-cell depletion. IMGN529 also was more specific than alemtuzumab, which depleted T-cells and monocytes as well as B cells. With whole blood samples, both K7153A and IMGN529 resulted in 30–40% B-cell depletion with no effect on T cells, NK cells or monocytes. IMGN529 was again more potent than rituximab or TRU-016, which caused approximately 10% B-cell depletion, and was more specific than alemtuzumab, which depleted the majority of T cells in addition to 40% of B cells.

IMGN529 embodies a unique B-cell targeted agent as it combines the intrinsic pro-apoptotic, CDC and ADCC activities of its anti-CD37 antibody component with the potent cytotoxic mechanism provided by the targeted delivery of its maytansinoid payload. It is highly active in vitro and in vivo against B-cell lymphoma and CLL cell lines. In addition, it mediates specific B-cell depletion in vitro that is greater than B-cell depletion by CD20-directed rituximab. Together, these findings indicate that IMGN529 is a promising therapeutic candidate for the treatment of B-cell malignancies.