Development of small
molecules targeting the PD-L1/PD-1 interface is advancing both in
industry and academia, but only a few have reached early-stage clinical
trials. Here, we take a closer look at the general druggability of PD-L1
using in silico hot spot mapping and nuclear magnetic resonance
(NMR)-based characterization. We found that the conformational
elasticity of the PD-L1 surface strongly influences the formation of hot spots. We deconstructed several generations of known inhibitors into fragments
and examined their binding properties using differential scanning
fluorimetry (DSF) and protein-based nuclear magnetic resonance (NMR).
These biophysical analyses showed that not all fragments bind to the PD-L1 ectodomain despite having the biphenyl scaffold. Although most of the binding fragments
induced PD-L1 oligomerization, two compounds, TAH35 and TAH36, retain
the monomeric state of proteins upon binding. Additionally, the presence
of the entire ectodomain did not affect the binding of the
hit compounds and dimerization of PD-L1. The data demonstrated here
provide important information on the PD-L1 druggability and the
structure-activity relationship of the biphenyl core moiety and
therefore may aid in the design of novel inhibitors and focused fragment libraries for PD-L1.