Sensitive ELISA Method for the Measurement of Catabolites of Antibody–Drug Conjugates (ADCs) in Target Cancer Cells

A new, sensitive ELISA method has been developed which measures catabolites in cells and media upon processing of antibody-drug conjugates (ADCs) by target cancer cells. This ELISA method, exemplified for maytansinoid ADCs, uses competitive inhibition by a maytansinoid analyte of the binding of biotinylated antimaytansine antibody to an immobilized BSA-maytansinoid conjugate. Synthetic standards of several maytansinoid catabolites derived from ADCs with different linkers were tested and showed similar inhibition curves, with an EC50 of about 0.1 nM (0.03 pmol in an assay volume of 0.25 mL). This high sensitivity allowed quantification of catabolites from a methanolic cell extract and from the medium, generated from an ADC in 1 day using only about 1 million cells. The processing of anti-EpCAM and anti-CanAg ADCs with noncleavable linker (SMCC-DM1), disulfide linker (SPDB-DM4), and charged sulfonate-bearing disulfide linker (sulfo-SPDB-DM4), each containing an average of about four maytansinoid molecules per antibody, were compared in colon cancer cell lines (COLO 205 and HT-29). An 8-10-fold higher total level of catabolite was observed for anti-CanAg ADCs than for anti-EpCAM ADCs upon processing by COLO 205 cells, consistent with a higher cell-surface expression of CanAg. In a multidrug resistant HCT-15 colon cancer cell line, the anti-EpCAM-SPDB-DM4 linker conjugate was not cytotoxic and showed a significantly lower level of catabolite within cells compared to that in medium, presumably due to Pgp-mediated efflux of the nonpolar DM4 catabolite. In contrast, sulfo-SPDB-DM4 and SMCC-DM1 linker conjugates were cytotoxic, which correlated with higher amounts of catabolites found within the HCT-15 cells relative to amounts in medium. In a nonmultidrug resistant HT-29 cell line, the anti-EpCAM-SPDB-DM4 linker conjugate was cytotoxic, with most of the catabolite found in cells and little in the medium. In conclusion, this highly sensitive ELISA method for measurement of ADC catabolite is convenient for screening multiple ADC parameters such as linkers and antibodies in a number of cell lines, does not require concentration of sample or extraction of media, and is complementary to other reported methods such as radiolabeling of ADCs or mass spectrometry.