Calcitonin gene-related peptide (CGRP) is a neuropeptide that plays a key role 
in the pathophysiology of migraine headache. MK-0974 (telcagepant) is a potent 
and selective antagonist of the human and rhesus CGRP receptors and is currently 
in Phase III clinical studies for the acute treatment of migraine. The 
pharmacology of MK-0974 has been studied extensively, but there has not been a 
thorough characterization of its binding properties. Here, we characterize the 
binding of a tritiated analog of MK-0974 on human neuroblastoma (SK-N-MC) 
membranes and rhesus cerebellum. [(3)H]MK-0974 displayed reversible and 
saturable binding to both SK-N-MC membranes and rhesus cerebellum with a K(D) of 
1.9 nM and 1.3 nM, respectively. Agonists and antagonists of the CGRP receptor 
displaced [(3)H]MK-0974 in a concentration-dependent manner in competition 
binding experiments. Both CGRP and adrenomedullin demonstrated biphasic 
competition while MK-0974 and the peptide antagonist CGRP(8-37) displaced 
[(3)H]MK-0974 in a monophasic fashion. In competitive binding studies with 
[(3)H]MK-0974 and CGRP, the fraction of high-affinity binding was reduced 
significantly by incubating the membranes with GTPgammaS. In kinetic binding 
experiments, the off-rate of [(3)H]MK-0974 was determined to be 0.51 min(-1) 
with a half-life of 1.3 min. In conclusion, the radioligand [(3)H]MK-0974 has 
proven to be a useful tool for studying the binding characteristics of MK-0974 
and has broadened our understanding of this promising molecule.