ACR246, a first in next-generation 5T4-ADC, as a potential choice of treating 5T4-positive solid tumors.

e15110 Background: 5T4 oncofetal antigen shows limited expression in normal adult tissues but highly expressed by many different solid tumors. Besides, 5T4 has a highly efficient endocytosis ability, indicating it to be an ideal target for ADC development. However, there are currently no approved drugs for targeting 5T4-expressing tumors. ACR246, a first in next-generation 5T4-targeted ADC consisting of specific fully human mAb site-specifically conjugated with a potent topoisomerase I inhibitor payload D2102 through an innovative stable and cleavable linker, was designed to improve the safety and efficacy in treating tumor patients. Methods: A panel of 5T4 positive tumor cell lines (lung, esophageal, gastric, etc) and esophageal cancer PDOs were used to analyse the in vitro effect on cell viability (CCK8 and CTG assays). Affinity (FACS), internalization (pHrodo assay) and mechanism of action of ACR246 was also researched in 5T4 positive tumor cell lines. In vivo anti-tumor activity of ACR246 was evaluated in 5T4-positive pancreatic, breast, esophageal and lung cancer CDX models and esophageal and gastric cancerr PDX models. The synergistic effect of ACR246 and ICIs was explored in murine esophageal cancer model expressing human 5T4. The in vivo safety and PK study of ACR246 was evaluated in cynomolgus monkeys. Results: Our results showed that ACR246 significant inhibited the proliferation of 5T4-positive cells, with IC 50 values ranging from 40.0 to 372.4 nM. It bound specifically to 5T4 positive tumors cell with sub-nanomolar EC 50 , and was internalized into tumor cells rapidly, but these were not observed in 5T4 negative cells. In the 5T4-positive esophageal cancer PDO models, ACR246 also exhibited stronger anti-tumor activity than other clinical drugs, such as irinotecan and anlotinib. Mechanically, ACR246 treatment induced cell cycle arrest at the G2/M or S phase, DNA damage and cell apoptosis in 5T4 positive tumor cells, mediated by payload D2102. In vivo anti-tumor studies confirmed the higher efficacy of ACR246 compared to chemotherapy or Dxd-ADC in inhibiting tumor growth, in both 5T4-positive CDX and PDX models ( P < 0.05). In murine esophageal cancer model expressing human 5T4 protein, ACR246 could sensitize the anti-PD1 treatment through creating a more immunologically active tumor microenvironment. PK study demonstrated low systemic exposure of payload D2102 after intravenous administration of ACR246 in cynomolgus monkeys, confirming that ACR246 was stable with low off-target toxicity. GLP toxicity studies showed that ACR246 was well tolerated in cynomolgus monkeys, with an estimated therapeutic index of 24. Conclusions: ACR246 exhibits a promising anti-tumor activity and potentiate the anti-PD1 efficacy in multiple 5T4-positive tumors. We have now translated this preclinical result into a phase 1/2a clinical trial (NCT06238401) to assess the safety and efficacy of ACR246 treatment.