Abstract 6580: A Novel EGFR x cMET bispecific ADC PRO1286 demonstrated broad antitumor activity and promising tolerability in preclinical models

Abstract EGFR and cMET are frequently co-expressed on tumor cells, often upregulated as the escape mechanism for each other, and both well-validated targets in oncology. Small molecule TKIs and amivantamab have been approved for non-small cell lung cancer with relevant actionable genomic alterations (AGA), and monoclonal antibodies offer treatment options for EGFR-expressing head and neck and colorectal cancers without AGA. Yet high unmet need in these tumors still exists. We sought to develop an EGFR and cMET dual-targeting ADC using our proprietary technology platform with carefully chosen design features for the many tumors expressing EGFR and cMET with or without AGA. PRO1286 is comprised of a monovalent bispecific human IgG1 (dubbed as “Bsab”) with fine-tuned affinity on EGFR and cMET, and the proprietary topoisomerase 1 inhibitor-based linker-drug, sesutecan. Sesutecan previously demonstrated promising characteristics in preclinical studies with multiple targets and conferred an encouraging benefit:risk profile in the clinic with a FRα-directed ADC (rinatabart sesutecan). PRO1286 at DAR8 displays high hydrophilicity and excellent stability and developability in vitro. PRO1286 binds selectively and specifically to each target with a low nanomolar EC50 in the CHO-K1 cells overexpressing each target. PRO1286 exhibited efficient internalization (~50-80% internalized in 4 hrs) and strong cytotoxicity (sub-nanomolar IC50) against a broad panel of tumor cell lines with higher potency than the parent mAbs. PRO1286 produced marked and dose dependent anti-tumor activity in mouse xenograft models encompassing a wide range of target expression levels, tumor types (head and neck, lung, esophageal, gastric, colorectal), and genomic landscape (with or without AGA in EGFR or cMET; with diverse additional co-mutations). Sustained tumor growth inhibition or complete remission was observed in several xenograft models after treatment with a single dose (2 mg/kg) of PRO1286. PRO1286 was more efficacious than benchmarking molecules in the mouse studies. In an exploratory toxicity study in cynomolgus monkeys, PRO1286 was well tolerated at 30 mg/kg with the main toxicity being payload-driven and residing in bone marrow. PRO1286 exhibited a stable PK profile that was similar to that of the Bsab in rats and favorable PK characteristics in monkeys. In summary, PRO1286 demonstrated promising physiochemical properties, PK/PD, antitumor activity, and tolerability in preclinical studies. It may carry an expanded therapeutic index compared to other EGFR- and/or cMET-directed ADCs. It may also differentiate from existing EGFR- and/or cMET-targeting agents on broad coverage of EGFR and cMET-expressing tumors with or without the respective AGAs. Effort is under way to advance a DAR optimized molecule to the clinic for the treatment of many EGFR- or cMET-expressing solid tumors. Citation Format: Yang Xiao, Haiyu Huang, Jianhe Yin, Xuan Qiu, Lei Wang, Haidong Liu, Baiteng Zhao, Zhu Chen. A Novel EGFR x cMET bispecific ADC PRO1286 demonstrated broad antitumor activity and promising tolerability in preclinical models [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(6_Suppl):Abstract nr 6580.