Abstract 5854: BT5528, a Bicycle Toxin Conjugate targeting EphA2 has potent anti-tumor activity without bleeding or coagulation abnormalities in preclinical models
Authors: Bennett Gavin , Huxley Philip , Brown Amy , Mudd Gemma , Park Peter U. , Keen Nicholas 
Published: August 2018

Ephrin receptor A2 (EphA2) is a member of the Ephrin receptor family of cell-cell junction proteins and is both highly overexpressed in several solid tumors and associated with poor prognosis in patients. EphA2 is considered a high-value target for cancer therapeutics, but development of an antibody drug conjugate targeting EphA2 (MEDI-547) was stopped in early clinical development after severe adverse events, including bleeding toxicity were seen (Annunziata et al, 2013). We have developed a series of Bicycle Toxin Conjugates comprising a constrained bicyclic peptide (Bicycle®) that binds with high affinity and specificity to EphA2, which is covalently linked to a toxin payload via a cleavable linker. The small size of BTCs offers a significant advantage over other targeted cytotoxic approaches such as antibody-drug conjugates due to rapid extravasation, renal clearance and improved tumor penetration. BT5528 was selected from a panel of &gt;75 Bicycle Toxin Conjugates (BTCs) with variations in Bicycle binder, molecular spacer, cleavable linker and toxin payload, based on in vivo efficacy and tolerability. BT5528 is effective in EphA2-expressing xenograft models, with efficacy seen from 1mg/kg qw and complete tumor regression at 3mg/kg qw. Rapid, complete regression of tumors is seen across a range of EphA2-expressing cell lines, including HT1080 fibrosarcoma line, MDA-MB-231 triple negative breast cancer line and NCI-H1975 lung cancer line and NSCLC PDX models. In order to test the benefit of the enhanced tumor penetration of BTCs, NSCLC PDX tumors were grown to &gt;1000mm3 before commencing treatment. Complete regression of tumor was seen following 3 weeks of dosing. The bleeding events reported in early clinical trials of MEDI-547 (EphA2 ADC) were consistent with effects on the coagulation system in preclinical models. Dose-limiting toxicology in non-human primate (NHP) was reportedly Disseminated Intravascular Coagulation (DIC) and changes were seen in APTT and D-Dimer, with associated elevations in liver enzymes. Toxicology studies of BT5528 in rat and NHP showed no laboratory or clinical evidence of DIC, with normal levels of platelets, D-Dimer, APTT, and other coagulation-related parameters. Bicycle Toxin Conjugates (BTC) targeting EphA2 show potent antitumor activity in a range of solid tumor xenograft models. The BTCs show profound efficacy, without the limiting toxicity observed with previous Antibody Drug Conjugate approaches. IND-enabling studies for BT5528 are currently underway.

Citation Format: Gavin Bennett, Philip Huxley, Amy Brown, Gemma Mudd, Peter U. Park, Nicholas Keen. BT5528, a Bicycle Toxin Conjugate targeting EphA2 has potent anti-tumor activity without bleeding or coagulation abnormalities in preclinical models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5854.