We have generated a range of potent Affimer antagonists to both human and mouse programmed death-ligand 1 (PD-L1) and have demonstrated that when formatted, possess the necessary affinity, pharmacokinetics (PK) and tissue penetration to modulate the immune system and demonstrate efficacy in mouse models. Affimer biotherapeutics are a new protein scaffold with great potential for the generation of biotherapeutics. Based on the human protease inhibitor Stefin A, the scaffold is small (12kDa), lacks any post translational modifications such as disulphide bonds and glycosylation. Large diverse phage display libraries have been created by engineering in two 9 amino acid peptide loops into the scaffold backbone. Using phage display, we have identified competitive binders to both mouse and human PD-L1 which are low single digit nM in affinity as determined by Biacore. By ELISA we have shown that the Affimer molecules identified compete against PD-1 and CD80 for the binding site on PD-L1 and bind to the target on a range of human cancer cell lines (e.g. lung adenocarcinoma cells), as demonstrated by FACs. Following formatting of the lead Affimer protein to the N-terminal Fc region of an IgG, we showed that we could further improve the affinity to give a KD of <40 pM as determined by Biacore. The PK properties of the Affimer-Fc fusion were determined in mouse dosed at 10 mg/kg and shown to have a terminal half-life of over 100 hours. When dosed in a tumor mouse model the Affimer-Fc fusion protein showed a statistically significant reduction in tumor volume when compared to a control antibody. We have demonstrated that the Affimer scaffold has all the necessary attributes to be developed as a therapeutic platform. The Affimer scaffold was well tolerated in vivo, demonstrated a potency and PK profile that ensured sufficient exposure in the animal to modulate the immune system to slow tumor progression in a relevant mouse model.

Citation Format: Amrik Basran, Emma Stanley. Generation and formatting of an Affimer® biotherapeutic for the inhibition of the PD-L1/PD-1 pathway: Proof of concept in mouse [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 3776.