DRI-C21045 (compound 10) is a potent and selective inhibitor of the CD40-CD40L costimulatory protein-protein interaction (PPI) with an IC50 of 0.17 μM. DRI-C21045 shows concentration-dependent inhibition of the activation of NF-κB and B cell proliferation all induced by CD40L with IC50s of 17.1 μM and 4.5 μM, respectively.