Abstract 1579: PA289, a prodrug linker-payload with a novel mechanism of action for the development of antibody drug conjugates

Abstract Background: PA289, is a novel seco-CBI-based prodrug payload which appears to exert cell-killing activity through the alkylation of one or more cytosolic proteins (Tietze et al, 2010). PA289, is the lead linker-payload from our ProAlk payload technology platform and is designed with tumor-selective glucuronide triggers that attenuate the potency of the payload until uptake into target cancer cells and selective release in the lysosome. The glucuronide linker and PermaLink (PL) conjugation chemistry ensure ADC stability. The unique mechanism of action of PA289 compared to currently used chemotherapies and ADC payloads, led us to generate proof-of-concept data for this novel ADC payload. Methods: The free payload and ADCs were tested for cell-killing activity in vitro by CellTiter-Glo assay. The prodrug linker-payload PA289, was conjugated using PermaLink technology to generate ADCs with a drug to antibody ratio (DAR) of approximately 4. Stability was determined in mouse and human plasma and in vivo efficacy was performed with subcutaneous human tumor xenografts in nude mice. Bystander effect was analyzed in vitro and compared to ADCs composed of MMAE and MMAF payloads. An exploratory toxicology study was conducted in cynomolgus monkeys. Results: The free payload showed highly potent in vitro activity against 248 cancer cell lines representing 15 solid cancer indications with IC50 values in the fM to pM range. Isumab01-PL-PA289, a model ADC targeting FRA, displayed excellent plasma stability and was highly potent against the JEG-3 cell line in vitro, with IC50 values in the pM range. A single administration of Isumab01-PL-PA289 in the FRA-positive JEG-3 xenograft model induced complete tumor regression by day 30 when dosed at 2.2 mg/kg, while the Isotype-PL-PA289 non-binding control resulted in weak tumor growth inhibition. Evaluation of bystander killing in vitro with a model ADC targeting CA242, Isumab04-PL-PA289, demonstrated stronger bystander cell-killing than benchmark ADCs containing MMAE and MMAF. A pilot toxicity study in cynomolgus monkeys treated with Isumab04-PL-PA289 showed the ADC was well-tolerated with minimal to no test-article related findings at the highest dose tested (7 mg/kg single dose). Conclusions: ADCs comprised of PA289, a novel protein alkylator prodrug linker-payload, demonstrate 1) strong in vitro potency and bystander effect, 2) durable tumor regressions in xenograft models, and 3) excellent tolerability in monkeys. The free payload is highly active against a broad array of cancer cell lines representing a variety of solid tumors suggesting that PA289 is a promising candidate for the development of ADCs with a novel MOA for the treatment of solid tumors. Citation Format: Justyna H. Mysliwy, Jutta Deckert, Adam Lodge, Jenny Thirlway, James Stephenson, Patrick Higgs, Daniel Williamson, Robert J. Lutz. PA289, a prodrug linker-payload with a novel mechanism of action for the development of antibody drug conjugates [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1579.