Abstract B209: The CD37-targeting ADC IMGN529 combines the potent anticancer activity of K7153A antibody with efficient maytansinoid delivery.

IMGN529 is an antibody-drug conjugate (ADC) for treatment of B-cell malignancies and consists of a humanized CD37-targeting antibody, K7153A, conjugated to the antimitotic agent, DM1, via an uncleavable, thioether-based linker, SMCC. Strong pro-apoptotic activity, antibody-dependent cell-mediated cytotoxicity and complement-dependent cytotoxicity are seen in vitro with both the K7153A antibody and IMGN529 conjugate. Conjugation of DM1 to the K7153A antibody results in a compound with further increased in vitro and in vivo potency in several preclinical models. The B-cell antigen, CD37, had not been considered an ideal target for an ADC-based therapeutic since it was believed to be poorly internalized. Surprisingly, in preclinical testing the IMGN529 conjugate was found to have greatly enhanced efficacy compared with the K7153 antibody alone, and was found to be highly effective in killing B-cell lymphomas and leukemias, suggesting IMGN529 is efficiently internalized and activated inside cancer cells. To investigate the mechanism of IMGN529 activation inside cancer cells, we first identified the maytansinoid catabolites formed within cancer cells following exposure to IMGN529. We found rapid formation of lysine-SMCC-DM1 within the cells, consistent with the identified catabolite described for other ADCs made with SMCC-DM1. We subsequently developed a highly sensitive assay using 3 H-propionate-labeled antibody as an alternative to the established 3 H-labeled antibody-maytansinoid conjugate method to follow lysosomal processing and catabolite formation in cancer cells. Using this system, we found the degree of 3 H-propionate-labeled K7153A antibody processing to be similar to that of 3 H-IMGN529 conjugate, suggesting that this method can be used effectively to estimate the amount of maytansinoid delivered to cancer cells. Rituximab, an α-CD20-binding antibody that also targets B-cell lymphomas and leukemias, has been shown to be internalized poorly on lymphoma cells. Therefore, we compared the lysosomal processing of K7153A with that of rituximab on a panel of six B-cell lymphoma human cell lines. In five of these cell lines, the degree of lysosomal processing of K7153A was at least four-fold greater than that of rituximab, while in the remaining cell line the degree of processing was similar for K7153A and rituximab. With an ADC, the amount of metabolite formation is dependent on both the number of antibody molecules bound per cell (ABC) and the rate of antigen-mediated lysosomal processing inside the cell. The total amount of potentially deliverable metabolite was determined based on ABC values and percent processing. In 5 of 6 cell lines evaluated, potential metabolite formation with K7153A was better than or similar to that of rituximab. Our results suggest that the CD37-directed conjugate, IMGN529, is effectively internalized into B-cells and processed by lysosomal degradation and thus provides a rationale for clinical development of this ADC for the treatment of CD37-positive B-cell malignancies. The data further indicate that IMGN529 achieved significantly more maytansinoid metabolites within these cells than would be possible with a rituximab-based ADC, regardless of antigen expression levels. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr B209.