Objective
The Affimer biotherapeutic protein scaffold is based on human Stefin A and is a small ~14 kDa protein about 1/10th the size of an antibody. It has no post translational modifications making it easier to manufacture. Two surface loops in Stefin A have been engineered into the scaffold backbone to create large Affimer phage libraries (1x10
11). This library has been used to screen for Affimer binders to human Programmed death-ligand 1 (PDL1) that compete for the PD-1 epitope. One of the single digit nM affinity PD-1 antagonists (AVA04-261) is presented here, co-crystallised with a domain of PDL1. This is the first non-antibody protein scaffold structure bound to PDL1.
PDL1 plays an important role in the modulation of the immune system and has been clinically validated as a target for a number of human cancers with monoclonal antibodies (mAbs). Crystal structures elucidating how antibodies, antibody fragments - and small molecules bind and inhibit PD-1 are available. The Affimer scaffold offers an alternative to mAbs as therapeutics due to their flexibility in formatting options and easy of manufacture. Here we describe an Affimer protein that specifically binds to human PDL1 and competes with PD-1/CD80 binding. The crystal structure of the Affimer protein bound to human PDL1, solved here to ~ 2.5 A resolution shows how the two engineered loops interact directly with the flat PDL1 binding surface.We present the protein production and complex formation.
