Abstract 297: Cymirafen: A protein-drug conjugate that delivers very intense short pulses of MMAE on a weekly schedule.

Abstract INTRODUCTION: Cymirafen is a novel protein-MMAE conjugate that targets the LGR4/LGR5/LGR6 receptor whose pharmacokinetics suggest that it can address many of the limitations of existing ADCs that use MMAE as their warhead. The human doses of MMAE ADCs are very small (1.2 - 4.5 mg/kg/3 wk) because larger doses are simply not tolerated. At these doses the ADCs do not penetrate deeply into tumors, the plasma concentrations are too low to saturate all receptors on those cells the ADC can reach, and the very long duration exposure to high plasma free MMAE concentrations cause neurotoxicity. However, long exposures are not required for MMAE efficacy; intracellular MMAE kills with even short durations of exposure (days rather than weeks). METHODS: PK studies used mice and Sprague-Dawley rats, and the SC OVCAR5 CDX model. Plasma and tumor lysate cymirafen was measured by ELISA using antibodies to different epitopes. Free MMAE was determined by LC-MS/MS. RESULTS: Cymirafen is very potent. Across a panel of cell lines of mixed cancer types, IC50 was 10 nM in 70% and 5 nM in 34%. Nevertheless, very high doses are well tolerated in mice and rats. In nu/nu mice the MTD is 125 mg/kg and a dose of 85 mg/kg can be given weekly for 8 weeks without net weight loss. Single dose HNSTD of MMAE ADCs in the rat is 5 - 10 mg/kg whereas the HNSTD for cymirafen is 75 mg/kg, 10 times higher than MMAE ADCs. In both mouse and rat, the initial and terminal half-lives of cymirafen are ∼ 4 h and 0.8 - 1.0 d whereas terminal half-lives for MMAE ADCs are 8.6 -14.6 d. In rats given 60 mg/kg, the peak total cymirafen was 3001 ± 813 nM. Free plasma MMAE peaked immediately after injection at 3.8 nM and declined with a half-life of 7 h to become undetectable after 48 h. Cymirafen was found to deliver MMAE to tumors very rapidly. Following an IP injection of cymirafen 85 mg/kg in nu/nu mice, free MMAE in the OVCAR5 tumor lysates rose very rapidly reaching 100 nM by 8 h and persisted for a prolonged period. The dose density of cymirafen (moles of MMAE per tolerated dose) is estimated to be ∼2.5 times higher than that of MMAE ADCs because the MW of cymirafen is only half that of an ADC and very much larger doses are tolerated. CONCLUSIONS: In contrast to MMAE ADCs, because of the much larger doses tolerated and the very much shorter half-life, cymirafen achieves very high plasma concentrations that load MMAE into tumors at a very high rate over just a few days. This favors deep penetration and complete saturation of receptors. The much higher payload dose means that many more moles of MMAE are delivered/dose, and since large doses of cymirafen can be given weekly, the rate and total amount of MMAE delivered over 3 weeks is far higher than what can be delivered by an MMAE ADC. Peak plasma free MMAE is only 0.09% of total cymirafen, and all free MMAE is cleared by 48 h. Low free MMAE plasma levels, and its short half-life, markedly reduce exposure of normal tissues to free MMAE and permits injection of very high doses. Citation Format: Maria Carmen Mulero Roig, Sukshala A. Jadhav, Willie Pi, Stephen B. Howell. Cymirafen: A protein-drug conjugate that delivers very intense short pulses of MMAE on a weekly schedule abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 297.