Abstract 639: Site specific conjugation of ARX-788, an antibody drug conjugate (ADC) targeting HER2, generates a potent and stable targeted therapeutic for multiple cancers

Abstract Creation of ADCs by conjugating drug linkers and toxin payloads to available native lysines or cysteines generates ADCs with highly variable drug to antibody loading and linker stability profiles. Variable drug loading, amino acid position, conjugation chemistry and linker design affect the pharmacokinetic characteristics, potency and toxicity of the ADC. We describe here the synthesis and pharmacology of an ADC comprising a site-specifically conjugated dolastatin analog to an antibody targeting the EGF family receptor, HER2. To specify the site of conjugation and drug loading, a non-natural amino acid, para acetyl phenylalanine (pAcF), was incorporated at defined sites within the antibody primary sequence. This non-natural amino acid provides a linkable platform for covalent conjugation of a diverse array of payloads through a stable oxime bond. The dolastatin analog, MMAF, was coupled via a non-cleavable linker to the HER2 antibody at two specific sites to generate the ADC; ARX-788. In vitro cytotoxicity assays of ARX-788 generated picomolar IC50's across a spectrum of HER2 + cell lines from breast, ovarian, lung and gastric cancer. Multiple in vivo murine xenograft studies demonstrated a potent increase in anti-tumor activity when compared to T-DM1. A single injection of ARX-788 in established xenograft models of HER2 positive ovarian, gastric and breast cancer cell lines (SKOV3, BT474, N-87, and HCC1954) induced rapid regression in all models. ARX-788 induced regression in a Herceptin-resistant derived breast xenograft (JIMT-1) and was significantly more effective than T-DM1 at equivalent doses. Pharmacokinetic evaluation of ARX-788 in rodent and non-human primates (NHP) revealed long term stability of 12 and 8 days respectively. Importantly, the conjugated form of ARX-788 remained intact over the course of the 3 week study in NHP. Precise payload stability and effective half-life of ARX-788 due to site-specific conjugation provides an improved therapeutic window for this HER2-targeting ADC. ARX-788 is currently completing non-human safety and PK studies in anticipation of clinical trial evaluation in 2015. Citation Format: Robin C. Humphreys, Jessica Kirtely, Amha Hewit, Sandra Biroc, Nick Knudsen, Lillian Skidmore, Alan Wahl. Site specific conjugation of ARX-788, an antibody drug conjugate (ADC) targeting HER2, generates a potent and stable targeted therapeutic for multiple cancers. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 639. doi:10.1158/1538-7445.AM2015-639