Pyrido[2,3-b]pyrazines, discovery of TRPV1 antagonists with reduced potential for the formation of reactive metabolites.

The transient receptor potential cation channel, subfamily V, member 1 (TRPV1) is a non-selective cation channel that can be activated by a wide range of noxious stimuli, including capsaicin, acid, and heat. Blockade of TRPV1 activation by selective antagonists is under investigation in an attempt to identify novel agents for pain treatment. During pre-clinical development, the 1,8-naphthyridine 2 demonstrated unacceptably high levels of irreversible covalent binding. Replacement of the 1,8-naphthyridine core by a pyrido[2,3-b]pyrazine led to the discovery of compound 26 which was shown to have significantly lower potential for the formation of reactive metabolites. Compound 26 was characterized as an orally bioavailable TRPV1 antagonist with moderate brain penetration. In vivo, 26 significantly attenuated carrageenan-induced thermal hyperalgesia (CITH) and dose-dependently reduced complete Freund's adjuvant (CFA)-induced chronic inflammatory pain after oral administration.

Discovery of novel 6,6-heterocycles as transient receptor potential vanilloid (TRPV1) antagonists.

The transient receptor potential cation channel, subfamily V, member 1 (TRPV1) is a nonselective cation channel that can be activated by a wide range of noxious stimuli, including capsaicin, acid, and heat. Blockade of TRPV1 activation by selective antagonists is under investigation in an attempt to identify novel agents for pain treatment. The design and synthesis of a series of novel TRPV1 antagonists with a variety of different 6,6-heterocyclic cores is described, and an extensive evaluation of the pharmacological and pharmacokinetic properties of a number of these compounds is reported. For example, the 1,8-naphthyridine 52 was characterized as an orally bioavailable and brain penetrant TRPV1 antagonist. In vivo, 52 fully reversed carrageenan-induced thermal hyperalgesia (CITH) in rats and dose-dependently potently reduced complete Freund's adjuvant (CFA) induced chronic inflammatory pain after oral administration.

Aminoquinazolines as TRPV1 antagonists: modulation of drug-like properties through the exploration of 2-position substitution.

A focused SAR exploration of the lead 4-aminoquinazoline TRPV1 antagonist 2 led to the discovery of compound 18. In rats, compound 18 is readily absorbed following oral dosing and demonstrates excellent in vivo potency and efficacy in an acute inflammatory pain model.

From arylureas to biarylamides to aminoquinazolines: discovery of a novel, potent TRPV1 antagonist.

Bioisosteric replacement of piperazine with an aryl ring in lead VR1 antagonist 1 led to the biarylamide series. The development of B-ring SAR led to the conformationally constrained analog 70. The resulting aminoquinazoline 70 represents a novel VR1 antagonist with improved in vitro potency and oral bioavailability vs the analogous compounds from the lead series.

Activation of protein kinase C sensitizes human VR1 to capsaicin and to moderate decreases in pH at physiological temperatures in Xenopus oocytes.

The two-electrode voltage-clamp technique was used to evaluate the effect of protein kinase C (PKC) activation on ion current flow in Xenopus laevis oocytes injected with cRNA coding for the human vanilloid receptor (VR1). In the presence of 30 nM phorbol-12,13-dibutyrate (PDBu), current evoked by an effective concentration (EC(30)) of capsaicin (CAP) was potentiated by 638+/-117% (n=8). PDBu exhibited an EC(50) of about 17+/-3 nM for this effect (n=8). Potentiation was not observed when VR1 expressing oocytes were exposed to both 30 nM PDBu and 1 microM staurosporine. In the presence of 300 nM PDBu, the EC(50) for CAP shifted from 899+/-78 to 139+/-2 1 nM (n=11 and 5, respectively). In the presence of 30 nM PDBu, the maximal current amplitude evoked by application of CAP increased by 86+/-21% (n=10), in a staurosporine sensitive manner. Application of 1 microM PDBu alone elicited a capsazepine sensitive current within 3 min of exposure. This effect was observed in the absence of previous exposure of the oocyte to CAP and was abolished in the presence of 1 microM staurosporine. No current was elicited during a 10 min application of 300 nM PDBu, the longest interval assessed. Prior to 30 nM PDBu exposure, no current was evoked at temperature ramps from room temperature (22-23 degrees C) up to 37 degrees C at pH 6.8, 7.0, or 7.4. Following PDBu treatment, VR1 mediated current was evoked at 26 degrees C at pH 7.0. Likewise, following 30 nM PDBu treatment, current was evoked by application of pH 6.8 alone and a further increase in current amplitude was evoked by heat at 24 degrees C in a staurosporine sensitive manner. These data provide direct evidence that PKC activation can increase VR1 current evoked by candidate physiological activators, pH and heat. This observation provides an empirical foundation for explaining some types of inflammatory pain in terms of PKC activation, small decreases in tissue pH levels, and small increases in skin temperature, all of which can accompany inflammatory conditions.