Nonclinical Development of Next-generation Site-specific HER2-targeting Antibody-drug Conjugate (ARX788) for Breast Cancer Treatment

Conventional antibody-drug conjugates (ADC) utilize native surface-exposed lysines or cysteines on the antibody of interest to conjugate cytotoxic payload. The nonspecific conjugation results in a mixture with variable drug-to-antibody ratios (DAR), conjugation sites, and ADCs that are often unstable in systemic circulation. ARX788 is an ADC consisting of a HER2-targeting antibody site-specifically conjugated with a potent antitubulin cytotoxic drug-linker, AS269. The site-specific conjugation is achieved by first incorporating the nonnatural amino acid, Para-acetyl phenylalanine (pAF), into the antibody, followed by covalent conjugation of AS269 to the pAF to form a highly stable oxime bond resulting in a DAR 2 ADC. ARX788 exhibits significant, dose-dependent antitumor activity against HER2- expressing breast and gastric xenograft tumors. Pharmacokinetic (PK) studies in multiple species showed the highly stable nature of ARX788 with overlapping PK profiles for the intact ADC and total antibody. Metabolism studies demonstrated that pAF-AS269 was the sole major metabolite of ARX788, with no evidence for the release of free drug often observed in conventional ADCs and responsible for adverse side effects. Furthermore, ARX788 demonstrated a favorable safety profile in monkeys with a highest nonseverely toxic dose of 10 mg/kg, which was well above the efficacious dose level observed in preclinical tumor models, thus supporting clinical development of ARX788.