Novel antibody-drug conjugates targeting EphA3 in tumours and the tumour microenvironment

The Eph receptor tyrosine kinase EphA3 is over-expressed in a range of tumours, most commonly in the tumour microenvironment (TME), including in immunosuppressive cancer-associated fibroblasts (CAFs). In contrast, normal homeostatic tissues show little or no expression of EphA3, making it an attractive therapeutic target. We previously developed a humanized EphA3 antibody (Ifabotuzumab) which was tested in preclinical models and in phase I clinical trials, showing tumour specificity and biological activity. To improve its efficacy, we have tested novel antibody-drug conjugates (ADCs) of Ifabotuzumab with the microtubule disruptor monomethyl auristatin E (MMAE), the Topoisomerase I inhibitor deruxtecan (DXd), and the DNA binding cytotoxin pyrrolobenzodiazepine dimer (PBD), using cleavable linkers and site-specific 'AJICAP' peptide-mediated conjugation. In vitro analysis showed retention of specific binding of Ifabotuzumab-ADCs to EphA3 and specific, dose-dependent killing of EphA3-expressing cells. Ifabotuzumab-PBD showed the highest in-vitro potency and was alone effective against EphA3-expressing cancer-associated fibroblasts. Treatment of mice bearing EphA3-expressing tumour xenografts with all Ifabotuzumab-ADCs decreased tumour growth and increased survival, again with Ifabotuzumab-PBD the most efficacious. Ifabotuzumab-PBD was also able to inhibit growth of tumours that express EphA3 only in stromal cells in the TME, using syngeneic tumour models in immunocompetent mice. Notably, combination with the immune-checkpoint inhibitor anti-PD1 increased CD8+ T cells in tumours, and improved tumour inhibition, compared to anti-PD1 alone. This data indicates that Ifabotuzumab-ADCs have potential as novel specific anti-tumour agents capable of targeting the TME in solid tumours.