Abstract 4481: BT5528, an EphA2-targeting Bicycle Toxin Conjugate (BTC): Profound efficacy without bleeding and coagulation abnormalities in animal models (2019)

Ephrin receptor A2 (EphA2) is part of the Ephrin receptor family of cell-cell junction proteins highly overexpressed in several solid tumours, and is associated with poor prognosis. EphA2 has long been 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 were seen (Annunziata et al, 2013). We have developed a series of Bicycle Toxin Conjugates (BTC®) 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 tumour penetration. Bicycle binders for EphA2 were identified using a proprietary phage display peptide technology consisting of highly diverse phage libraries of Bicycles, conjugated to cleavable linkers & toxins to form Bicycle Toxin Conjugates (BTCs). We selected the candidate BTC BT5528 from a panel of >75 BTCs, based on in vivo efficacy, tolerability and drug-like properties. BT5528 is effective in EphA2-expressing xenograft models, with complete tumour regression seen from 0.5mg/kg weekly. Expression of EphA2 is high across a range of cancers of high unmet medical need. A good correlation with efficacy was seen across cell- and patient- derived xenograft models including lung, breast, oesophageal, ovarian, prostate, gastric and sarcoma tumours. No significant efficacy is seen in tumours without EphA2 expression. Rapid tumour penetration, renal elimination and significant bystander effect produce efficacy through a “hit & run” approach, with low systemic exposure to conjugate or free toxin. Efficacy is seen with dosing intervals of 1 or 2 weeks, despite a terminal half-life of ~30min for BTCs across species. In simple xenografts (CDX, ~200mm3), efficacy of BT5528 & anti-EphA2 ADC are comparable. In large & more complex models (heterogeneous PDX, >1000mm3), the ADC shows reduced efficacy, while the efficacy of BT5528 is maintained. Clinical development of MEDI-547 was terminated when patients showed profound bleeding & coagulation toxicity, consistent with the nature of toxicology seen in preclinical studies. In contrast, BT5528 shows no sign of bleeding in toxicology studies in rat and NHP and no changes in coagulation parameters or liver enzymes. The EphA2 targeting BTC BT5528 shows potent anti-tumour activity in a range of solid tumour xenograft models without the limiting toxicity observed with previous Antibody Drug Conjugate approaches. IND-enabling studies for BT5528 are currently underway.Citation Format: Gavin S. Bennett, Amy Brown, Gemma Mudd, Johanna Lahdenranta, Nicholas Keen. BT5528, an EphA2-targeting Bicycle Toxin Conjugate (BTC): Profound efficacy without bleeding and coagulation abnormalities in animal models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4481.