Optimization of Adcitmer, a Monomethyl-Auristatin E bearing antibody-drug conjugate for the treatment of CD56-expressing cancers

The cell adhesion protein CD56 has been identified as a potential therapeutic target in several solid tumors and hematological malignancies. Recently, we developed a CD56-directed antibody-drug conjugate (ADC), called Adcitmer and demonstrated its antitumor properties in preclinical models of the rare and aggressive skin cancer Merkel cell carcinoma (MCC). The present study aims to further optimize Adcitmer to overcome the therapeutic limitations observed with previously evaluated CD56-targeting ADCs, which were partially related to toxic effects on leukocytes. To this end, we aimed to avoid interaction of Adcitmer with immune cells via Fc gamma receptor (Fc gamma R) binding. Since glycosylation is essential for Fc gamma R binding, an aglycosylated form of Adcitmer was generated and evaluated on human leukocytes and MCC cell lines using cell death (annexin V/7-aminoactinomycine D) and proliferation (2,3-Bis-(2-methoxy-4Nitro-5-sulfophenyl)-2H-tetrazolium-5carboxanilide) assays. Finally, the therapeutic performance of Adcitmer and its aglycosylated form was assessed in an MCC xenograft mouse model. Investigating the Adcitmer interaction with immune cells demonstrated that it is mostly mediated by Fc recognition. Accordingly, Adcitmer aglycosylation led to reduced immune cell toxicity and strikingly also to improved therapeutic performance even in an MCC xenograft model using immunodeficient mice. Our study suggests that aglycosylated Adcitmer should be considered as a therapeutic option in patients with advanced MCC or other CD56-positive tumors.