Identification of second-generation P2X3 antagonists for treatment of pain.

A second-generation small molecule P2X3 receptor antagonist has been developed. The lead optimization strategy to address shortcomings of the first-generation preclinical lead compound is described herein. These studies were directed towards the identification and amelioration of preclinical hepatobiliary findings, reducing potential for drug-drug interactions, and decreasing the projected human dose of the first-generation lead.

Localization of CGRP, CGRP receptor, PACAP and glutamate in trigeminal ganglion. Relation to the blood-brain barrier.

Calcitonin gene-related peptide (CGRP) receptor antagonists have demonstrated anti-migraine efficacy. One remaining question is where do these blockers act? We hypothesized that the trigeminal ganglion could be one possible site. We examined the binding sites of a CGRP receptor antagonist (MK-3207) and related this to the expression of CGRP and its receptor in rhesus trigeminal ganglion. Pituitary adenylate cyclase-activating polypeptide (PACAP) and glutamate were examined and related to the CGRP system. Furthermore, we examined if the trigeminal ganglion is protected by the blood-brain barrier (BBB). Autoradiography was performed with [(3)H]MK-3207 to demonstrate receptor binding sites in rhesus trigeminal ganglion (TG). Immunofluorescence was used to correlate binding and the presence of CGRP and its receptor components, calcitonin receptor-like receptor (CLR) and receptor activity-modifying protein 1 (RAMP1), and the distribution of PACAP and glutamate in rhesus and rat TG. Evans blue was used to examine large molecule penetration into the rat TG. High receptor binding densities were found in rhesus TG. Immunofluorescence revealed expression of CGRP, CLR and RAMP1 in trigeminal cells. CGRP positive neurons expressed PACAP but not glutamate. Some neurons expressing CLR and RAMP1 co-localized with glutamate. Evans blue revealed that the TG is not protected by BBB. This study demonstrates CGRP receptor binding sites and expression of the CGRP receptor in rhesus and rat TG. The expression pattern of PACAP and glutamate suggests a possible interaction between the glutamatergic and CGRP system. In rat the TG is outside the BBB, suggesting that molecules do not need to be CNS-penetrant to block these receptors.

Identification of a novel RAMP-independent CGRP receptor antagonist.

Identification of an HIV integrase inhibitor with micromolar affinity for the CGRP receptor led to the discovery of a series of structurally novel CGRP receptor antagonists. Optimization of this series produced compound 16, a low-molecular weight CGRP receptor antagonist with good pharmacokinetic properties in both rat and dog. In contrast to other nonpeptide antagonists, the activity of 16 was affected by the presence of divalent cations and showed evidence of an alternative, RAMP-independent CGRP receptor binding site.

Pharmacological properties of MK-3207, a potent and orally active calcitonin gene-related peptide receptor antagonist.

Calcitonin gene-related peptide (CGRP) has long been hypothesized to play a key role in migraine pathophysiology, and the advent of small-molecule antagonists has clearly demonstrated a clinical link between blocking the CGRP receptor and migraine efficacy. 2-[(8R)-8-(3,5-Difluorophenyl)-10-oxo-6,9-diazaspiro[4.5]dec-9-yl]-N-[(2R)-2'-oxo-1,1',2',3-tetrahydrospiro[indene-2,3'-pyrrolo[2,3-b]pyridin]-5-yl]acetamide (MK-3207) represents the third CGRP receptor antagonist to display clinical efficacy in migraine trials. Here, we report the pharmacological characterization of MK-3207, a potent and orally bioavailable CGRP receptor antagonist. In vitro, MK-3207 is a potent antagonist of the human and rhesus monkey CGRP receptors (K(i) = 0.024 nM). In common with other CGRP receptor antagonists, MK-3207 displays lower affinity for CGRP receptors from other species, including canine and rodent. As a consequence of species selectivity, the in vivo potency was assessed in a rhesus monkey pharmacodynamic assay measuring capsaicin-induced changes in forearm dermal blood flow via laser Doppler imaging. MK-3207 produced a concentration-dependent inhibition of dermal vasodilation, with plasma concentrations of 0.8 and 7 nM required to block 50 and 90% of the blood flow increase, respectively. The tritiated analog [3H]MK-3207 was used to study the binding characteristics on the human CGRP receptor. [3H]MK-3207 displayed reversible and saturable binding (K(D) = 0.06 nM), and the off-rate was determined to be 0.012 min(-1), with a t(1/2) value of 59 min. In vitro autoradiography studies on rhesus monkey brain slices identified the highest level of binding in the cerebellum, brainstem, and meninges. Finally, as an index of central nervous system penetrability, the in vivo cerebrospinal fluid/plasma ratio was determined to be 2 to 3% in cisterna magna-ported rhesus monkeys.

Examining the binding properties of MK-0974: a CGRP receptor antagonist for the acute treatment of migraine.

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.

Benzodiazepine calcitonin gene-related peptide (CGRP) receptor antagonists: optimization of the 4-substituted piperidine.

In our continuing effort to identify CGRP receptor antagonists for the acute treatment of migraine, we have undertaken a study to evaluate alternative 4-substituted piperidines to the lead dihydroquinazolinone 1. In this regard, we have identified the piperidinyl-azabenzimidazolone and phenylimidazolinone structures which, when incorporated into the benzodiazepine core, afford potent CGRP receptor antagonists (e.g., 18 and 29). These studies produced a potent analog (18) which overcomes the instability issues associated with the lead structure 1. A general pharmacophore for the 4-substituted piperidine component of these CGRP receptor antagonists is also presented.