Abstract 955: Preclinical characterization of a novel SSEA-4-targeting antibody drug conjugate, OBI-998
Authors: Chen I-Ju , Wang Chun-Chung , Shia Chi-Sheng , Su Chung-Chen , Lu Chi-Huan , Chang Hui-Wen , Chiu Ping-Tzu , Wu Yueh-Chin , Lai Ming-Tain , Tang Wei-Chien , Tung Hsin-Yi , Hsu Ren-Yu 
Published: July 2021

Abstract
Stage-specific embryonic antigen-4 (SSEA-4) ceramide, a globo-series hexasaccharide glycosphingolipid, has been reported to be a tumor-associated antigen. Here, we present a novel antibody-drug conjugate (ADC) targeting SSEA-4 to evaluate its potential as a therapeutic agent for cancer treatment.
OBI-998 is an ADC comprising the humanized anti-SSEA-4 antibody (OBI-898) that is conjugated to the highly potent microtubule-disrupting agent monomethyl auristatin E (MMAE) through maleimide and PEGylated cleavable linkers. We demonstrated the specificity of the naked antibody to SSEA-4 by screening against a panel of 21 related carbohydrate antigens using ELISA. OBI-998 displayed potent cytotoxic activity against cells (SKOV3) with a high level of SSEA-4 expression at sub-nanomolar potency but no effect on the viability of cells (SKBR3) with negligible SSEA-4 expression. The bystander killing effect of OBI-998 was shown by the transferring of conditioned medium from SKOV3 cells treated with 1 or 5 nM of OBI-998 to SKBR3 cells.
Furthermore, significant bystander effects were observed by co-culturing high (NCI-N87) and low (PANC-1/GFP) SSEA-4-expressing cell lines at different ratios in the presence of 5 or 10 nM of OBI-998. OBI-998 was found to be rapidly internalized into cancer cells within 5 minutes upon binding to its target on the cell surface by confocal microscopy. OBI-998 showed significant anti-tumor efficacy in multiple cancer cell–derived xenograft models at doses of 1, 3, and 10 mg/kg in a dose-dependent manner. More importantly, OBI-998 at a dose of 10 mg/kg showed complete tumor regression in an EGFR-triple mutation non–small cell lung cancer xenograft model, which is resistant to the current last-line tyrosine kinase inhibitor, osimertinib. Pharmacokinetic analysis of OBI-998 revealed that total antibody and the conjugated antibody exhibited similar pharmacokinetic profiles, suggesting OBI-998 is highly stable in vivo. The biodistribution study in HCC1428 tumor-bearing mice indicated that MMAE accumulated in the tumor site at a higher level compared with other blood-rich organs. In addition, the MMAE levels in tumors reached peak level at 24 hours post-treatment, which was much higher than the maximum levels in other organs, and the level was sustained for 168 hours with tumor-to-muscle ratio of 329.
OBI-998 is a novel ADC targeting SSEA-4 that possesses desired properties such as high target specificity, rapid internalization, potent cytotoxicity, and significant bystander effects. Furthermore, OBI-998 showed a high level of deposition and a persistent presence of MMAE in tumors and significant anti-tumor efficacy in a variety of animal models. Taken together, these results support the further development of OBI-998 as a therapeutic agent for SSEA-4-targeting cancer therapy.
Citation Format: I-Ju Chen, Chun-Chung Wang, Chi-Sheng Shia, Chung-Chen Su, Chi-Huan Lu, Hui-Wen Chang, Ping-Tzu Chiu, Yueh-Chin Wu, Ming-Tain Lai, Wei-Chien Tang, Hsin-Yi Tung, Ren-Yu Hsu. Preclinical characterization of a novel SSEA-4-targeting antibody drug conjugate, OBI-998 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 955.