RESUMEN

Seizures induced by organophosphorus nerve agent exposure become refractory to treatment with benzodiazepines because these drugs engage synaptic γ-aminobutyric acid-A receptors (GABAARs) that rapidly internalize during status epilepticus (SE). Extrasynaptic GABAARs, such as those containing α4ß3δ subunits, are a putative pharmacological target to comprehensively manage nerve agent-induced seizures since they do not internalize during SE and are continuously available for activation. Neurosteroids related to allopregnanolone have been tested as a possible replacement for benzodiazepines because they target both synaptic and extrasynaptic GABAARs receptors. A longer effective treatment window, extended treatment efficacy, and enhanced neuroprotection represent significant advantages of neurosteroids over benzodiazepines. However, neurosteroid use is limited by poor physicochemical properties arising from the intrinsic requirement of the pregnane steroid core structure for efficacy rendering drug formulation problematic. We tested a non-steroidal enaminone GABAAR modulator that interacts with both synaptic and extrasynaptic GABAARs on a binding site distinct from neurosteroids or benzodiazepines for efficacy to control electrographic SE induced by diisopropyl fluorophosphate or soman intoxication in rats. Animals were treated with standard antidotes, and experimental therapeutic treatment was given following 1 h (diisopropyl fluorophosphate model) or 20 min (soman model) after SE onset. We found that the enaminone 2-261 had an extended duration of seizure termination (>10 h) in the diisopropyl fluorophosphate intoxication model in the presence or absence of midazolam (MDZ). 2-261 also moderately potentiated MDZ in the soman-induced seizure model but had limited efficacy as a stand-alone anticonvulsant treatment due to slow onset of action. 2-261 significantly reduced neuronal death in brain areas associated with either diisopropyl fluorophosphate- or soman-induced SE. 2-261 represents an alternate chemical template from neurosteroids for enhancing extrasynaptic α4ß3δ GABAAR activity to reverse SE from organophosphorous intoxication.

RESUMEN

Chronic neuropathic pain may be caused, in part, by loss of inhibition in spinal pain processing pathways due to attenuation of local GABAergic tone. Nociception and nocifensive behaviors are reduced after enhancement of tonically activated extrasynaptic GABAAR-mediated currents by agonist ligands for δ subunit-containing GABAARs. However, typical ligands that target δ subunit-containing GABAARs are limited due to sedative effects at higher doses. We used the spinal nerve ligation (SNL) and gp120 models of experimental neuropathic pain to evaluate compound 2-261, a nonbenzodiazepine site positive allosteric modulator of α4ß3δ GABAARs optimized to be nonsedative by selective activation of ß2/3-subunit-containing GABAARs over receptor subtypes incorporating ß1 subunits. Similar levels of 2-261 were detected in the brain and plasma after intraperitoneal administration. Although systemic 2-261 did not alter sensory thresholds in sham-operated animals, it significantly reversed SNL-induced thermal and tactile hypersensitivity in a GABAAR-dependent fashion. Intrathecal 2-261 produced conditioned place preference and elevated dopamine levels in the nucleus accumbens of nerve-injured, but not sham-operated, rats. In addition, systemic pretreatment with 2-261 blocked conditioned place preference from spinal clonidine in SNL rats. Moreover, 2-261 reversed thermal hyperalgesia and partially reversed tactile allodynia in the gp120 model of HIV-related neuropathic pain. The effects of 2-261 likely required interaction with the α4ß3δ GABAAR because 2-301, a close structural analog of 2-261 with limited extrasynaptic receptor efficacy, was not active. Thus, 2-261 may produce pain relief with diminished side effects through selective modulation of ß2/3-subunit-containing extrasynaptic GABAARs.

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A series of novel arylpyrid-3-ylmethanones (7a-aa) were designed as modulators of α7 nicotinic acetylcholine receptors (nAChRs). The methanones were found to be type I positive allosteric modulators (PAMs) of human α7 nAChRs expressed in Xenopus ooctyes. Structure-activity relationship (SAR) studies resulted in the identification of compound 7v as a potent and efficacious type I PAM with maximum modulation of a nicotine EC5 response of 1200% and EC50 = 0.18 µM. Compound 7z was active in reversing the effect of scopolamine in the novel object recognition (NOR) paradigm with a minimum effective ip dose of 1.0 mg/kg (2.7 µmol/kg). This effect was blocked by the selective α7 nAChR antagonist methyllycaconitine (MLA). These compounds are potent type I positive allosteric modulators of α7 nAChRs that may have therapeutic value in restoring impaired sensory gating and cognitive deficits in schizophrenia and Alzheimer's disease.

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A series of enaminone esters and amides have been developed as potent allosteric modulators of gamma-aminobutyric acidA (GABAA) receptors. The compounds bind to a novel modulatory site that is independent of the benzodiazepine (BZ), isosteric GABA, and neuroactive steroid binding sites. Structure-activity relationship (SAR) studies resulted in the synthesis of the c-Bu amide 16h with an in vitro potency of 7 nM based on inhibition of [35S]TBPS binding. The activity of the enaminones as positive allosteric modulators was confirmed with electrophysiological measurements in oocytes expressing alpha1beta2gamma2L GABAA receptors. The i-Pr, s-Bu, c-Pr, and c-Bu amides (16e-h) were orally active in mice with profound central nervous system depressant effects. The i-Pr amide 16e was an orally active anxiolytic in the mouse light-dark paradigm.

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The endogenous opioid peptide leu-enkephalin (ENK) was chemically modified by a method known as reversible aqueous lipidization (REAL) with a novel amine-reacting lipophilic dimethylmaleic anhydride analog, 3,4-bis(decylthiomethyl)-2,5-furandione. The binding affinity of the product, REAL-ENK, to opioid receptors was greatly reduced. This prodrug was stable in neutral and basic phosphate buffers but underwent rapid hydrolysis under acidic conditions in the presence of 50% acetonitrile. It also showed increased stability toward enzymatic degradations in various tissue preparations. The half-lives of REAL-ENK in mouse small intestinal mucosal homogenate and liver homogenate were 12 and 80 min, representing a 12- and 32-fold increase over those of ENK itself. In contrast to ENK (t(1/2) 6.7 min), REAL-ENK was stable in mouse plasma. More importantly, REAL-ENK produced significant and sustained antinociception mediated by peripheral opioid receptors in a rodent inflammatory pain model. Pharmacokinetic studies employing a radioimmunoassay (RIA) demonstrated that significantly higher and sustained plasma peptide levels were detected up to 24 h following the oral administration of REAL-ENK in normal mice. The peak concentration and area under the curve of oral REAL-ENK were 4.4 and 21 times higher than that of oral ENK. Our results indicate that like its disulfide-based counterpart, amine-based REAL may be an enabling technology which can be applied to enhance metabolic stability, increase oral absorption, and preserve and possibly prolong the pharmacological activity of peptide drugs.

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Patients taking fluoroquinolone antibiotics such as norfloxacin exhibit a low incidence of convulsions and anxiety. These side effects probably result from antagonism of the neurotransmitter gamma-aminobutyric acid (GABA) at the brain GABA(A) receptor complex (GRC). Modification of norfloxacin yields molecules such as compound 4 that potentiate GABA action with alpha(2) subunit selectivity. Compound 4 is anxiolytic but does not cause sedation, and may represent a new class of ligands that have anxiolytic activity without sedative liability.

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