The Characterisation of FcγRIIa Antagonists
Increasing evidence for FcγRIIa as a key participant in the pathogenesis of autoimmune inflammatory disorders like rheumatoid arthritis and systemic lupus erythematosus marks this cell surface receptor as a potential target for treatment. The direct blockade of the FcγRIIa-IgG interaction in the prevention of platelet activation and inevitable inflammation with the use of novel inhibitors is a strategy that has been gaining interest. The discovery of mAbs and small molecules to inhibit the FcγRIIa in the treatment of immune complex-mediated inflammatory diseases has shown auspicious results. The FcγRIIa is the only Fc receptor expressed on platelets. Small molecule inhibitors synthesised in RCSI and clinically approved drugs have previously demonstrated inhibition in the binding of platelets to immobilised IgG and the aggregation of platelets induced by heat-agglutinated IgG. It is necessary to characterise the effect of these compounds on platelets and their specificity for the FcγRIIa in order for a lead compound to be selected. Characterisation and specificity tests were carried out in this study with FcγRIIa and the compounds. Significant binding of the compound MT-84 to FcγRIIa in a thermal shift assay was detected at a concentration of 10μM. At 50 μM, MT-84 demonstrated its potential to induce possible conformational changes to the FcγRIIa structure in both a thermal shift assay and in an ELISA. The compound MT-222 significantly inhibited the binding of FcγRIIa to IgG in an ELISA. In silico docking of the compounds into the crystal structures of the FcγRs suggests that these compounds are non-specific for the FcγRIIa. It was demonstrated that vinorelbine significantly induced partial activation of platelets and the loss of single platelets suggesting that vinorelbine induces thrombocytopenia through the FcγRIIa. This work suggests a direct interaction between MT-84 and MT-222 with the FcγRIIa. These compounds inhibit IgG mediated aggregation and adhesion, without the partial agonism seen with vinorelbine. Future work should focus on further characterisation of the interaction using more sensitive techniques such as surface plasmon resonance and isothermal titration calorimetry.