Abstract 3138: Discovery of AT65474, a highly selective anti-CLDN6 ADC with a proprietary payload

Abstract CLDN6 is a membrane-bound oncofetal protein and a promising target for cancers such as ovarian and testicular cancer. CLDN6 shares high sequence homology with other CLDN family members which are expressed in normal tissues, making a high degree of selectivity required for CLDN6-targeting therapies. In particular, CLDN6 is closest to CLDN9 in sequence with a difference of only three residues in the extracellular loops, making it challenging to generate a highly selective antibody. We report the development of a humanized anti-CLDN6 antibody with high affinity and high selectivity for targeted cells which displays 800-fold higher binding affinity for CLDN6 over CLDN9. The antibody was refined for use as an ADC and incorporates AxcynCYSTM technology for site-specific conjugation which can reproducibly achieve high DAR4 ratios of greater than 97% by HIC analysis. AT65474 is an ADC candidate that incorporates a proprietary payload derived from an FDA approved drug that was optimized in-house for increased potency and improved PK properties to limit systemic exposure. In vitro, the payload displays broad sub-nanomolar activity across a wide range of cancer cell lines including those resistant to paclitaxel, DM1, MMAE and DXd. The payload is conjugated via an enzyme-cleavable linker optimized for solubility, stability, and high drug to antibody ratios. We provide evidence characterizing AT65474 as a highly potent ADC with high DAR4 homogeneity and strong in vivo activity at low doses. Compared with a benchmark ADC in clinical development, AT65474 shows stronger binding, higher internalization, superior in vitro cytotoxicity, and stronger bystander killing effect. When evaluated in PA-1 and OV90 xenograft models of ovarian cancer, AT65474 displays significantly higher efficacy over the benchmark ADC. Furthermore, we show that AT65474 displays an ADCC effect that contributes to tumor suppression in the OV90 model. AT65474 was shown to be well tolerated in all xenograft mouse models and a pre-tox study in non-human primates. The combined data support further development of AT65474. ​ Citation Format: Chen Zhong, You Xue Wu, Xi Yun Zhang, Yin Wen Cheng, Qian Wang, Ping Du, Shao Jun Liu, Bin Zou. Discovery of AT65474, a highly selective anti-CLDN6 ADC with a proprietary payload abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 3138.