Abstract C61: Factors determining sensitivity of tumor cells to lorvotuzumab mertansine in vitro.
Authors: Ab Olga , Goldmacher Victor S. , Bartle Laura M. , Sun Xiuxia , Erickson Hans K. 
Published: June 2012

Abstract
Lorvotuzumab mertansine (LM), is an antibody-drug conjugate (ADC) consisting of the anti-CD56 antibody lorvotuzumab, linked to a highly cytotoxic maytansinoid DM1 via the stable reducible disulfide linker SPP. The conjugate is currently being evaluated in clinical trials for the treatment of CD56-positive solid tumors and multiple myeloma. Encouraging results of the clinical trials were reported previously (Berdeja et al., ASCO 2011, Woll et al., ASCO 2011).
During preclinical studies LM was found to be highly cytotoxic towards several CD56-positive cancer cell lines. However, a subset of CD56-positive cancer cell lines was found to be resistant to the conjugate. (Resistant cell lines were defined as those that have similar sensitivities to LM and a non-targeting antibody-SPP-DM1 conjugate.) These resistant cell lines did not express PgP, which we previously found to be a major MDR transporter of maytansinoid (MAY) catabolites.
In the current study we explored the factors thought to influence the sensitivity of the targeted cancer cells to LM such as the level of CD56 expressed on the cell surface, CD56-mediated internalization rates, intracellular accumulation of maytansinoid catabolites, and the sensitivity of the cancer cells to unconjugated maytansinoid (S-methyl-DM1). The rate of internalization of Alexa-labeled lorvotuzumab was analyzed in a FACS-based internalization assay. Conjugate processing and CD56 density on the cell surface (Antibody Bound per Cell, ABC) was assayed with 3H-labeled lorvotuzumab. Our previous data indicated that Alexa-labeled and 3H-labeled lorvotuzumab were appropriate surrogate molecules to analyze internalization and processing of LM. Sensitivity of the cells to LM and S-methyl-DM1 was determined in a cytotoxicity assay on cultured cells in vitro.
Values of ABC, percent of internalized and processed antibody, and relative amount of MAY catabolites produced from lysosomal degradation (a combined parameter of ABC and the percent of antibody processed) varied depending on a cell line, but did not distinguish the conjugate-resistant from the sensitive cell lines. One parameter that differentiated the sensitive from the resistant cell lines was sensitivity to S-methyl-DM1. Among the S-methyl-DM1-sensitive cell lines, there was strong positive correlation between the amount of MAY catabolites produced and the degree of the sensitivity of the cells to LM. Cells with low sensitivity to S-methyl-DM1 were resistant to the conjugate, irrespective of their level of produced MAY catabolites.
Overall, our data suggest that the combination of the sensitivity of cells to S-methyl-DM1 and the level of MAY catabolites produced is the determining factor for the activity of LM against CD56-positive cells.
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 C61.