LAUSR

The placental alkaline phosphatases, ALPP and ALPPL2, are glycosylphosphatidylinositol (GPI)-anchored cell-surface proteins that are expressed in the placenta during fetal development but have very little expression on normal adult tissue. These proteins are over-expressed in a number of different solid tumor indications including ovarian, endometrial, gastric, pancreatic and non-small cell lung cancers. As a result of the highly restricted normal tissue expression and over-expression in cancer, ALPP and ALPPL2 are attractive targets for antibody and protein-drug conjugate (ADC and PDC) approaches. The recent initiation of a Phase I clinical trial utilizing a full-length antibody ADC has fueled further interest in these oncology targets. Small protein domain binders, which have the capacity to penetrate deeper into solid tumors and can be engineered into multiple therapeutic formats in a modular fashion, offer a number of potential benefits over full-length antibodies as the targeting vehicle in PDC therapeutics. We report the identification and characterization of a series of high affinity ALPP/ALPPL2 specific single domain VHH binders which, importantly, show no binding to the closely related ALPI or ALPL isoforms (which have high normal tissue expression). Through application of the Almac Discovery PDC technology platform, these VHH domains have been reformatted into a suite of homogenous site-specifically labelled drug conjugates, with defined drug to antibody ratios, in high yields. These conjugates, which are based on mono- and bi-paratopic Fc fusion formats, employ both clinically established and novel linker-toxin combinations. In pre-clinical studies, the lead bi-paratopic PDCs were well tolerated in vivo and showed excellent anti-tumor efficacy in ALPP/ALPPL2 positive cell-line derived xenograft models of gastric and pancreatic carcinoma, with sustained regressions still observed 10 weeks after administration of the final dose of agent. These agents have physicochemical and pharmaceutical properties suitable for further development and we anticipate that the excellent pre-clinical efficacy profile of lead PDCs will translate to a highly differentiated product for the treatment of a variety of solid tumor indications. Citation Format: Graham Cotton, Paul Trumper, Estelle McLean, Mark Wappett, Stacey Bell, Greg Papadakos, Jennifer Thom, Chiara Saladino, Aaron Cranston, Georgiana Parau, Stephanie Gatdula, Stephanie Burton, Aidan McCann, Tim Harrison. Development of novel protein drug conjugates (PDCs) for the selective targeting of ALPP/ALPPL2 expressing tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr LB011.