Abstract 5334: Mechanism of cysteine-mcMMAF release from the stably linked antibody-drug conjugate SGN-75

Abstract The antibody-drug conjugate SGN-75 is composed of the humanized monoclonal antibody (mAb) h1F6 and the drug-linker mcMMAF conjugated through interchain disulfide cysteine residues. SGN-75 has been shown to be highly active against tumor cell lines both in vitro and in vivo. The mcMMAF drug-linker was designed to be protected from protease cleavage, and we have shown that cysteine-mcMMAF is the major released drug in vitro. We demonstrate that the cysteine residue in cysteine-mcMMAF comes from the mAb, implying antibody degradation as the mechanism for generation of cysteine-mcMMAF. Two isotope cysteine labeling approaches were utilized in which either total cysteine in tumor cells or the mAb cysteines of h1F6 were labeled with heavy isotope cysteine. Metabolic incorporation of heavy cysteine into CD70-expressing tumor cells or mAb-producing CHO cells using media depleted of cysteine and supplemented with heavy isotope cysteine gave complete labeling by heavy cysteine. Heavy cysteine-labeled cells were treated with unlabeled SGN-75, and unlabeled cells were treated with heavy-cysteine-labeled SGN-75. The results demonstrate that the mAb backbone of SGN-75 is the origin of the cysteine in cysteine-mcMMAF providing evidence that an active drug component can be released from a stably linked ADC through proteolytic degradation of the mAb inside of target cells. Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5334.