Mechanism of Action of a Nanomolar Potent, Allosteric Antagonist of the Thyroid-Stimulating Hormone Receptor

Background and purpose  Graves' disease (GD) is an autoimmune disease in which the thyroid is overactive, producing excessive amounts of thyroid hormones, caused by TSHR-stimulating immunoglobulins (TSIs). A large proportion of GD patients also suffer from thyroid eye disease (Graves' ophthalmopathy or GO), with the TSIs considered to activate TSHRs in orbital tissue also. We recently developed LMW TSHR antagonists as a novel therapeutic strategy for the treatment of GD and GO. In the present study, we determined the molecular pharmacology of a prototypic, nanomolar potent LMW TSHR antagonist, Org 274179-0. Experimental approach  First, we determined the potency and efficacy of Org 274179-0 in antagonizing TSH- and TSI-induced TSHR signaling and its cross-reactivity at the related FSHR and LHR. Second, we explored in depth the allosteric mode of interaction of Org 274179-0. Third, we determined whether Org 274179-0 is an inverse agonist at five naturally occurring, constitutively active TSHR mutants. Key results  Org 274179-0 fully inhibited TSH (and TSI)-mediated TSHR activation with nanomolar potency without hardly affecting the potency of TSH, in accordance with an allosteric mechanism of action. On the reverse, increasing levels of TSHR stimulation only marginally reduced the antagonistic potency of Org 274179-0. Finally, Org 274179-0 fully blocked the increased basal activity of all tested constitutively active TSHR mutants with nanomolar potencies. Conclusions and implications  We conclude that nanomolar potent TSHR antagonists like Org 274179-0 have the potential of being developed to treat GD and GO