Sean Brown

Role of peripheral hyperpolarization-activated cyclic nucleotide-modulated channel pacemaker channels in acute and chronic pain models in the rat

Hyperpolarization-activated, cyclic nucleotide-modulated (HCN) channels contribute to rhythmic spontaneous activity in the heart and CNS. Ectopic spontaneous neuronal activity has been implicated in the development and maintenance of acute and chronic hyperalgesia, allodynia and spontaneous pain. Previously, we documented that systemic administration of ZD7288, a specific blocker of pacemaker current (I(h)), decreased ectopic activity in dorsal root ganglion (DRG) and reversed tactile allodynia in spinal nerve ligated (SNL) rats [Chaplan SR, Guo HQ, Lee DH, Luo L, Liu C, Kuei C, Velumian AA, Butler MP, Brown SM, Dubin AE (2003) Neuronal hyperpolarization-activated pacemaker channels drive neuropathic pain. J Neurosci 23:1169-1178]. Spontaneous pain is the chief clinical manifestation of peripheral nerve injury; however, a role for I(h) in spontaneous pain has not been described. Here, in further rat studies, we report that systemic administration of ZD7288 reversed spontaneous pain induced by mild thermal injury (MTI) and tactile allodynia induced by SNL and MTI. In contrast, ZD7288 did not reduce thermal hyperalgesia. An important locus of action appears to be in the skin since intraplantar (local) administration of ZD7288 completely suppressed tactile allodynia arising from MTI and SNL and reduced spontaneous pain due to MTI. Immunohistochemical staining of plantar skin sections detected HCN1-HCN4 expression in mechanosensory structures (e.g., Meissners corpuscles and Merkel cells). Collectively, these data suggest that expression and modulation of I(h) in the peripheral nervous system, including specialized sensory structures, may play a significant role in sensory processing and contribute to spontaneous pain and tactile allodynia.

Thiadiazolopiperazinyl ureas as inhibitors of fatty acid amide hydrolase

A series of thiadiazolopiperazinyl aryl urea fatty acid amide hydrolase (FAAH) inhibitors is described. The molecules were found to inhibit the enzyme by acting as mechanism-based substrates, forming a covalent bond with Ser241. SAR and PK properties are presented.

Biochemical and Biological Properties of 4-(3-phenyl-[1,2,4] thiadiazol-5-yl)-piperazine-1-carboxylic acid phenylamide, a Mechanism-Based Inhibitor of Fatty Acid Amide Hydrolase

Fatty acid amide hydrolase (FAAH) is an integral membrane enzyme within the amidase-signature family. It catalyzes the hydrolysis of several endogenous biologically active lipids, including anandamide (arachidonoyl ethanolamide), oleoyl ethanolamide, and palmitoyl ethanolamide. These endogenous FAAH substrates have been shown to be involved in a variety of physiological and pathological processes, including synaptic regulation, regulation of sleep and feeding, locomotor activity, pain and inflammation. Here we describe the biochemical and biological properties of a potent and selective FAAH inhibitor, 4-(3-phenyl-[1,2,4]thiadiazol-5-yl)-piperazine-1-carboxylic acid phenylamide (JNJ-1661010). The time-dependence of apparent IC50 values at rat and human recombinant FAAH, dialysis and mass spectrometry data indicate that the acyl piperazinyl fragment of JNJ-1661010 forms a covalent bond with the enzyme. This bond is slowly hydrolyzed, with release of the piperazinyl fragment and recovery of enzyme activity. The lack of inhibition observed in a rat liver esterase assay suggests that JNJ-1661010 is not a general esterase inhibitor. JNJ-1661010 is >100-fold preferentially selective for FAAH-1 when compared to FAAH-2. JNJ-1661010 dose-dependently increases arachidonoyl ethanolamide, oleoyl ethanolamide, and palmitoyl ethanolamide in the rat brain. The compound attenuates tactile allodynia in the rat mild thermal injury model of acute tissue damage and in the rat spinal nerve ligation (Chung) model of neuropathic pain. JNJ-1661010 also diminishes thermal hyperalgesia in the inflammatory rat carrageenan paw model. These data suggest that FAAH inhibitors with modes of action similar to JNJ-1661010 may be useful clinically as broad-spectrum analgesics.

Novel ketooxazole based inhibitors of fatty acid amide hydrolase (FAAH).

Efforts to improve the properties of the well studied ketooxazole FAAH inhibitor OL-135 resulted in the discovery of a novel propylpiperidine series of FAAH inhibitors that has a modular design and superior properties to OL-135. The efficacy of one of these compounds was demonstrated in a rat spinal nerve ligation model of neuropathic pain in rats.

Aryl Piperazinyl Ureas as Inhibitors of Fatty Acid Amide Hydrolase (FAAH) in Rat, Dog, and Primate

A series of aryl piperazinyl ureas that act as covalent inhibitors of fatty acid amide hydrolase (FAAH) is described. A potent and selective (does not inhibit FAAH-2) member of this class, JNJ-40355003, was found to elevate the plasma levels of three fatty acid amides: anandamide, oleoyl ethanolamide, and palmitoyl ethanolamide, in the rat, dog, and cynomolgous monkey. The elevation of the levels of these lipids in the plasma of monkeys suggests that FAAH-2 may not play a significant role in regulating plasma levels of fatty acid ethanolamides in primates.

Heteroarylureas with spirocyclic diamine cores as inhibitors of fatty acid amide hydrolase

A series of mechanism based heteroaryl urea fatty acid amide hydrolase (FAAH) inhibitors with spirocyclic diamine cores is described. A potent member of this class, (37), was found to inhibit FAAH centrally, elevate the brain levels of three fatty acid ethanolamides [FAAs: anandamide (AEA), oleoyl ethanolamide (OEA) and palmitoyl ethanolamide (PEA)], and was moderately efficacious in a rat model of neuropathic pain.

Heteroaryl urea inhibitors of fatty acid amide hydrolase: Structure-mutagenicity relationships for arylamine metabolites

The structure-activity relationships for a series of heteroaryl urea inhibitors of fatty acid amide hydrolase (FAAH) are described. Members of this class of inhibitors have been shown to inactivate FAAH by covalent modification of an active site serine with subsequent release of an aromatic amine from the urea electrophile. Systematic Ames II testing guided the optimization of urea substituents by defining the structure-mutagenicity relationships for the released aromatic amine metabolites. Potent FAAH inhibitors were identified having heteroaryl amine leaving groups that were non-mutagenic in the Ames II assay.

The Role of Pacemaker Currents in Neuropathic Pain

Abstract:  Hyperpolarization‐activated cation nonselective cyclic nucleotide‐gated (HCN) channels mediate pacemaker currents that control basic rhythmic processes including heartbeat. Alterations in HCN channel expression or function have been described in both epilepsy and cardiac arrhythmias. Recent evidence suggests that pacemaker currents may also play an important role in ectopic neuronal activity that manifests as neuropathic pain. Pacemaker currents are subject to endogenous regulation by cyclic nucleotides, pH and perhaps phosphorylation. In addition, a number of neuromodulators with known roles in pain affect current density and kinetics. The pharmacology of a number of drugs that are commonly used to treat neuropathic pain includes effects on pacemaker currents. Altered pacemaker currents in injured tissues may be an important mechanism underlying neuropathic pain, and drugs that modulate these currents may offer new therapeutic options.