Antibody-drug conjugates (ADC) are designed to selectively bind to tumor antigens via the antibody and release their cytotoxic payload upon internalization. Controllable payload release through judicious design of the linker has been an early technological milestone. Here, we examine the effect of the protease-cleavable valine-citrulline [VC(S)] linker on ADC efficacy. The VC(S) linker was designed to be cleaved by cathepsin B, a lysosomal cysteine protease. Surprisingly, suppression of cathepsin B expression via CRISPR-Cas9 gene deletion or shRNA knockdown had no effect on the efficacy of ADCs with VC(S) linkers armed with a monomethyl auristatin E (MMAE) payload. Mass spectrometry studies of payload release suggested that other cysteine cathepsins can cleave the VC(S) linker. Also, ADCs with a nonprotease-cleavable enantiomer, the VC(R) isomer, mediated effective cell killing with a cysteine-VC(R)-MMAE catabolite generated by lysosomal catabolism. Based on these observations, we altered the payload to a pyrrolo[2,1-c][1,4]benzodiazepine dimer (PBD) conjugate that requires linker cleavage in order to bind its DNA target. Unlike the VC-MMAE ADCs, the VC(S)-PBD ADC is at least 20-fold more cytotoxic than the VC(R)-PBD ADC. Our findings reveal that the VC(S) linker has multiple paths to produce active catabolites and that antibody and intracellular targets are more critical to ADC efficacy. These results suggest that protease-cleavable linkers are unlikely to increase the therapeutic index of ADCs and that resistance based on linker processing is improbable.

This study was conducted to investigate the clinicopathological features of synchronous cancers and treatment options according to their locations.Records of 8368 patients with colorectal cancer treated at our center between July 2003 and December 2010 were analyzed retrospectively. All synchronous colorectal cancer patients who underwent surgical treatment were included.Synchronous cancers were identified in 217 patients (2.6%). Seventy-nine patients underwent either total colectomy, subtotal colectomy, or total proctocolectomy; 116 underwent 1 regional resection, including local excision; and 22 underwent 2 regional resections. The mean age was 62 years, slightly higher than that for the single-cancer patients. Synchronous cancers were more common in male patients, more frequently located in the left colon, had more microsatellite instability-high status, and showed more advanced stage than single cancer. Extensive resection was mainly performed for synchronous cancers located in both the right and left colon. Two regional resections were performed for cancers in the right colon and rectum. There were no differences in complication rates or the occurrence of metachronous cancer between the 2-region resection and extensive resection groups. Eight years postoperatively, the mean number of daily bowel movements for these 2 groups were 1.9 and 4.3, respectively.We found that synchronous cancer was different from single cancer in terms of age, gender, location, and pathologic features. Synchronous colorectal cancer requires different treatment strategy according to the distribution of lesions. Comparison between the 2 regional resections and extensive resection approaches suggests that 2 regional resections are preferable.