SMALL MOLECULES WHICH CAN KEEP THE ECTODOMAINS OF HUMAN FcγRIIIa “CLOSED”: THERAPY FOR AUTOIMMUNE THROMBOCYTOPENIA PURPURA?

Comparative analysis of crystal structures of the ectodomains of the unliganded Fcγ receptor A (FcγRIIIa), its complex to IgG Fc and two designer proteins named “affimers” revealed that if the two ectodomains of FcγRIIIa remain “closed”, then they are conformationally incompetent to bind Fc region of antigen or infected cell bound IgG and protect them from degradation in the endosomes. By tightly binding to its ectodomains, these affimers derailed binding of FcγRIIIa to Fc portion of IgG leading to latter’s degradation. This ability to decrease antibody titer suggests use of these affimers as therapeutic interventions in autoimmune conditions like immune thrombocytopenia purpura (ITP) which is caused by autoantibodies against platelets. I queried: are there any small molecules which can dock and lock the ectodomains of FcγRIIIa analogous to affimers? Applying MTiOpenScreen on few libraries, I shortlisted 30 drug-like molecules which preferentially docked in the cleft between ectodomains of human FcγRIIIa. Their complexes were subsequently analyzed by MD simulations at theoretical pH 7.4 (and 5.7) at 310 K. For all trajectories, we tracked two characteristic molecular descriptors of FcγRIIIa: θ and δ, the angle and distance between the center masses of the two ectodomains, respectively. The values of these descriptors, in conjunction with estimated binding energies from MD simulations allow me to report three new compounds which may act as and/or lead to small molecule interventions in autoimmune conditions like ITP.