Discovery of Potent Antiallodynic Agents for Neuropathic Pain Targeting P2X3 Receptors.

Antagonism of the P2X3 receptor is one of the potential therapeutic strategies for the management of neuropathic pain because P2X3 receptors are predominantly localized on small to medium diameter C- and Aδ-fiber primary afferent neurons, which are related to the pain-sensing system. In this study, 5-hydroxy pyridine derivatives were designed, synthesized, and evaluated for their in vitro biological activities by two-electrode voltage clamp assay at hP2X3 receptors. Among the novel hP2X3 receptor antagonists, intrathecal treatment of compound 29 showed parallel efficacy with pregabalin (calcium channel modulator) and higher efficacy than AF353 (P2X3 receptor antagonist) in the evaluation of its antiallodynic effects in spinal nerve ligation rats. However, because compound 29 was inactive by intraperitoneal administration in neuropathic pain animal models due to low cell permeability, the corresponding methyl ester analogue, 28, which could be converted to compound 29 in vivo, was investigated as a prodrug concept. Intravenous injection of compound 28 resulted in potent antiallodynic effects, with ED50 values of 2.62 and 2.93 mg/kg in spinal nerve ligation and chemotherapy-induced peripheral neuropathy rats, respectively, indicating that new drug development targeting the P2X3 receptor could be promising for neuropathic pain, a disease with high unmet medical needs.

Synthesis and Biological Evaluation of Novel σ1 Receptor Ligands for Treating Neuropathic Pain: 6-Hydroxypyridazinones.

By use of the 6-hydroxypyridazinone framework, a new series of potent σ1 receptor ligands associated with pharmacological antineuropathic pain activity was synthesized and is described in this article. In vitro receptor binding studies revealed high σ1 receptor affinity (Ki σ1 = 1.4 nM) and excellent selectivity over not only σ2 receptor (1366-fold) but also other CNS targets (adrenergic, µ-opioid, sertonerigic receptors, etc.) for 2-(3,4-dichlorophenyl)-6-(3-(piperidin-1-yl)propoxy)pyridazin-3(2H)-one (compound 54). Compound 54 exhibited dose-dependent antiallodynic properties in mouse formalin model and rats chronic constriction injury (CCI) model of neuropathic pain. In addition, functional activity of compound 54 was evaluated using phenytoin and indicated that the compound was a σ1 receptor antagonist. Moreover, no motor impairments were found in rotarod tests at antiallodynic doses and no sedative side effect was evident in locomotor activity tests. Last but not least, good safety and favorable pharmacokinetic properties were also noted. These profiles suggest that compound 54 may be a member of a novel class of candidate drugs for treatment of neuropathic pain.

Inhibition of N-type calcium channels by fluorophenoxyanilide derivatives.

A set of fluorophenoxyanilides, designed to be simplified analogues of previously reported ω-conotoxin GVIA mimetics, were prepared and tested for N-type calcium channel inhibition in a SH-SY5Y neuroblastoma FLIPR assay. N-type or Cav2.2 channel is a validated target for the treatment of refractory chronic pain. Despite being significantly less complex than the originally designed mimetics, up to a seven-fold improvement in activity was observed.

P2X7 receptor-mediated analgesia in cancer-induced bone pain.

Pain is a common and debilitating complication for cancer patients significantly compromising their quality of life. Cancer-induced bone pain involves a complex interplay of molecular events, including mechanisms observed in inflammatory and neuropathic pain states, but also changes unique for cancer-induced bone pain. The P2X7 receptor (P2X7R) is involved in a variety of cellular functions and has been linked to both inflammatory and neuropathic pain. Here we study the analgesic potential of P2X7R antagonism in a rat model of cancer-induced bone pain. In cancer-bearing animals, the P2X7R antagonist A839977 attenuated dorsal horn neuronal responses in a modality and intensity-specific way. Spinal application of 0.4-mg/kg and 1.2-mg/kg A839977 significantly reduced the evoked responses to high-intensity mechanical and thermal stimulation, whereas no effect was seen in response to low-intensity or electrical stimulation. In contrast, A839977 had no effect on the tested parameters in naïve or sham animals. In awake animals, 40-mg/kg A839977 (i.p.) significantly reduced both early- and late-stage pain behavior. In contrast, no effect was observed in sham or vehicle-treated animals. The results suggest that the P2X7R is involved in the mechanisms of cancer-induced bone pain, and that P2X7R antagonism might be a useful analgesic target. No effect was observed in sham or naïve animals, indicating that the P2X7R-mediated effect is state-dependent, and might therefore be an advantageous target compared to traditional analgesics.

HYP-1, a novel diamide compound, relieves inflammatory and neuropathic pain in rats.

In the present study, we investigated whether a novel compound, 2-(2-(4-((4-chlorophenyl)(phenyl)methyl) piperazin-1-yl)-2-oxoethylamino)-N-(3,4,5-trimethoxybenzyl)acetamide (HYP-1), is capable of binding to voltage-gated sodium channels (VGSCs) and evaluated both its inhibitory effect on Na+ currents of the rat dorsal root ganglia (DRG) sensory neuron and its in vivo analgesic activity using rat models of inflammatory and neuropathic pain. HYP-1 showed not only high affinity for rat sodium channel (site 2), but also potent inhibitory activity against the TTX-R Na+ currents of the rat DRG sensory neuron. HYP-1 co-injected with formalin (5%, 50 µl) under the plantar surface of rat hind paw dose-dependently reduced spontaneous pain behaviors during both the early and late phases. This result was confirmed by c-Fos immunofluorescence in the L4-5 spinal segments. A large number of c-Fos-positive neurons were observed in rat injected with a mixture of formalin and vehicle, but not in rat treated with a mixture of formalin and HYP-1. In addition, the effectiveness of HYP-1 (6 and 60 mg/kg, i.p.) in suppression of neuropathic pain, such as mechanical, cold and warm allodynia, induced by rat tail nerve injury was investigated. HYP-1 showed limited selectivity over hERG, N-type and T-type channels.Our present results indicate that HYP-1, as a VGSC blocker, has potential analgesic activities against nociceptive, inflammatory and neuropathic pain.

Glutathione reductase is inhibited by acetaminophen-glutathione conjugate in vitro.

The aim of the present work was to investigate a new mechanism likely contributing to the toxic action of acetaminophen, especially to explore the possible inhibition of glutathione reductase through an acetaminophen-glutathione conjugate (APAP-SG). APAP-SG conjugate was synthesized by organic synthesis and purified by column chromatography. The inhibitory effect of the conjugate on two types of glutathione reductase (from yeasts and rat hepatocytes) was tested spectro-photometrically. We found that the enzyme activity was reduced similarly after the treatment with 2.96 mM acetaminophen-glutathione conjugate in both yeast and hepatocyte glutathione reductases (GR); the enzyme activity was inhibited to 52.7+/-1.5 % (2.4+/-0.3 mU/ml) in yeast GR (control activity was 5.6+/-0.3 mU/ml) and to 48.1+/-8.8 % (2.2+/-0.2 mU/ml) in rat hepatocytes lysate GR (control activity was 5.2+/-0.2 mU/ml). In addition, the enzyme activity (from hepatocytes lysate) was decreased to 79+/-7 %, 67+/-2 % and 39+/-7 %, in 0.37, 1.48 and 3.7 mM concentration of the conjugate, respectively. We found that glutathione reductase, the essential enzyme of the antioxidant system, was dose-dependently inhibited by the product of acetaminophen metabolism - the conjugate of acetaminophen and glutathione.

Analgesic, antipyretic, anti-inflammatory and toxic effects of andrographolide derivatives in experimental animals.

Andrographolide (1) and 14-deoxy-11,12-didehydroandrographolide (2) are active constituents of Andrographis paniculata (Burm. f.), family Acanthaceae. A. paniculata extracts are reported to have antiviral, antipyretic, immunostimulant and anticancer activities. In this study, 1 and its 14-acetyl- (4) and 3,19-isopropylidenyl- (3) derivatives, as well as 2 and its 3,19-dipalmitoyl-derivative (5), were intraperitoneally tested for their analgesic, antipyretic, anti-inflammatory and acute toxicity effects in animal models. Analgesic effects were tested in mice using hot plate and writhing tests to distinguish the central and peripheral effects, respectively. The results showed that, at 4 mg/kg, all tested substances have significant analgesic effects, and the highest potency was seen with 3, 4 and 5. Increasing the dose of 3 and 5 to 8 mg/kg did not increase the analgesic effect. In the writhing test, 3 and 5, but not 1, showed significant results. In a baker's yeast-induced fever model, 3 and 5 significantly reduced rats' rectal temperature (p < 0.05). In a carrageenan-induced inflammation model, 1, 3 and 5 significantly reduced rats' paw volume. Doses of 3 and 5 up to 100 mg/kg did not show any serious toxic effects. From this study, 3 and 5 are the most interesting derivatives, showing much greater potency than their parent compounds. These could be further developed as analgesic, antipyretic and anti-inflammatory agents, without any serious toxicity.

Synthesis, biological evaluation and docking studies of novel benzopyranone congeners for their expected activity as anti-inflammatory, analgesic and antipyretic agents.

8-Acetyl-7-hydroxy-4-phenyl-2H-benzopyran-2-one as starting material a number of 8-substituted derivatives (i.e., hydrazones 2a,b, imine 2c, chalcones 3, pyrazoles 4, 3-cyano-2-oxo-dihydropyridines 5, and/or 3-cyano-2-imino-dihydropyridines 6) were synthesized and assayed for their anti-inflammatory, analgesic and antipyretic activities. Compounds 3c, 4b and 4i showed significant anti-inflammatory, analgesic and antipyretic activities. In addition, 1, 3b, 4d, 4e, 5b, 6a, 6c, 6d, 6e showed anti-inflammatory activity, 2b, 4h, 5e exhibit analgesic activity, and 2b, 4h, 5e showed antipyretic effect. In addition, molecular modeling and docking of the tested compounds into cyclooxygenase II complexed with its bound inhibitor indomethacin (4COX) using molsoft icm 3.4-8C program was performed in order to predict the affinity and orientation of the synthesized compounds at the active site. Also, it was found that the active compounds 1, 4i, 6a-e interact with both Serine 530, and Tyrosine 385 amino acids which are the main amino acids involved in the mechanism of cyclooxygenase II inhibition. The synthesis of the pyrazole-containing new compounds 4 proved a successful hit; also, the 2-imino derivatives of 3-cyano-dihydropyridines were more successful than the 2-oxo derivatives. According to these results, we can conclude that compounds 1, 3c, 4b, 4i, and 6c appear to be the most interesting and seem potentially attractive as anti-inflammatory, analgesic, and antipyretic agents.

Discovery of potent furan piperazine sodium channel blockers for treatment of neuropathic pain.

The synthesis and pharmacological characterization of a novel furan-based class of voltage-gated sodium channel blockers is reported. Compounds were evaluated for their ability to block the tetrodotoxin-resistant sodium channel Na(v)1.8 (PN3) as well as the Na(v)1.2 and Na(v)1.5 subtypes. Benchmark compounds from this series possessed enhanced potency, oral bioavailability, and robust efficacy in a rodent model of neuropathic pain, together with improved CNS and cardiovascular safety profiles compared to the clinically used sodium channel blockers mexiletine and lamotrigine.

Conotoxins containing nonnatural backbone spacers: cladistic-based design, chemical synthesis, and improved analgesic activity.

Disulfide-rich neurotoxins from venomous animals continue to provide compounds with therapeutic potential. Minimizing neurotoxins often results in removal of disulfide bridges or critical amino acids. To address this drug-design challenge, we explored the concept of disulfide-rich scaffolds consisting of isostere polymers and peptidic pharmacophores. Flexible spacers, such as amino-3-oxapentanoic or 6-aminohexanoic acids, were used to replace conformationally constrained parts of a three-disulfide-bridged conotoxin, SIIIA. The peptide-polymer hybrids, polytides, were designed based on cladistic identification of nonconserved loci in related peptides. After oxidative folding, the polytides appeared to be better inhibitors of sodium currents in dorsal root ganglia and sciatic nerves in mice. Moreover, the polytides appeared to be significantly more potent and longer-lasting analgesics in the inflammatory pain model in mice, when compared to SIIIA. The resulting polytides provide a promising strategy for transforming disulfide-rich peptides into therapeutics.

Synthesis of some bioactive 2-bromo-5-methoxy-N'-[4-(aryl)-1,3-thiazol-2-yl]benzohydrazide derivatives.

Eight novel 2-bromo-5-methoxy-N'-[4-(aryl)-1,3-thiazol-2-yl]benzohydrazide derivatives were prepared and characterized by analytical and spectral analyses. All the compounds were screened for their analgesic, antifungal and antibacterial activities and three of the compounds were screened for antiproliferative activity. Two of the newly synthesized compounds exhibited promising analgesic activity and one compound exhibited in vitro antiproliferative activity.

Progress in 5H[1]benzopyrano[4,3-d]pyrimidin-5-amine series: 2-methoxy derivatives effective as antiplatelet agents with analgesic activity.

A series of 2-methoxy-5H[1]benzopyrano[4,3-d]pyrimidin-5-amines were prepared and screened for their in vitro antiplatelet activity inducing the aggregation by ADP, arachidonic acid (AA) and collagen. In vivo experiments were performed in order to evaluate their antiphlogistic, analgesic and antipyretic activities. Title compounds showed antiplatelet activity in aggregation AA or collagen-induced, and a good analgesic activity without any gastric toxicity. Comparison with a number of analogue benzopyrano[4,3-d]pyrimidine derivatives and some SAR consideration were reported.

Metal-ion speciation in blood plasma incorporating the tetraphosphonate, N,N-dimethylenephosphonate-1-hydroxy-4-aminopropilydenediphosphonate (APDDMP), in therapeutic radiopharmaceuticals.

In a quest for more effective radiopharmaceuticals for pain palliation of metastatic bone cancer, this paper relates results obtained with 166Ho and 153Sm complexed to the bone seeking phosphonate, N,N-dimethylenephosphonate-1-hydroxy-4-aminopropylidenediphosphonate (APDDMP). APDDMP is synthesised from the known bone cancer pain palliation agent 1-hydroxy-3-aminopropylidenediphosphonate (APD) and was complexed to lanthanide trivalent metal ions. This work is performed to utilise the idea that the energetic beta-particle emitter, 166 Ho, coupled with phosphonate ligands such as APD and APDDMP could afford a highly effective radiopharmaceutical in the treatment of bone cancer. Complex-formation constants of APDDMP with the important blood plasma metal-ions, Ca2+, Mg2+, and Zn2+ and the trivalent lanthanides Ho3+ and Sm3+ were measured by glass electrode potentiometry at 37 degrees C and I = 150 mM. Blood plasma models were constructed using the computer code ECCLES and the results compared with those gathered from animal tests. The 166Ho-APDDMP complex was found to have little liver or bone uptake while 153Sm-APDDMP had a moderate bone uptake. This was primarily due to the high affinity of APDDMP for Ca(II). Clinical observations could be explained by the blood plasma modelling.

Synthesis and pharmacological screening of novel non-acidic gastroprotective antipyretic anti-inflammatory agents with anti-platelet properties. 5-Alkyl/cycloalkylamino substituted 2-amino-5H-[1]benzopyrano[4,3-d] pyrimidines.

The preparation and screening of antipyretic, anti-inflammatory, analgesic, gastroprotective and antiplatelet activities of original non-acidic aminobenzo-pyranopyrimidine derivatives are described. Major and dose-dependent analgesic and antipyretic properties were detected in all the compounds after oral administration (6.25-100 mg kg-1) in the mouse writhing test and in the E. coli lipopolysaccharide-induced fever in the rat, respectively. Unlike the reference drug indometacin (ED50 ulcer = 9.1 mg kg-1), no gastrolesivity was displayed by all the new compounds up to 150 mg kg-1 so that therapeutical indices equal or higher than that of indometacin were calculated. Furthermore, at 100 mg kg-1 they were able to prevent ethanol-induced gastric mucosal injury in rats. At a 1 mmol/l concentration the compounds under study were as effective as acetylsalicylic acid in inhibiting in vitro platelet aggregation induced by adenosine diphosphate.

Analgesic activity of acylated 2-benzoxazolinone derivatives.

Ten new benzoxazolinone derivatives having a disubstituted benzoyl group at the six position of the ring were synthesized by reacting 2-benzoxazolinone with aromatic carboxylic acids in the presence of polyphosphoric acid. The structure of the compounds were elucidated by IR, 1H NMR, MS and elemental analysis. Analgesic activity was evaluated by a modified Koster test. Seven compounds showed analgesic activities higher than that of acetylsalicylic acid.

Aminophosphinic inhibitors as transition state analogues of enkephalin-degrading enzymes: a class of central analgesics.

Inhibition of aminopeptidase N and neutral endopeptidase-24.11, two zinc metallopeptidases involved in the inactivation of the opioid peptides enkephalins, produces potent physiological analgesic responses, without major side-effects, in all animal models of pain in which morphine is active. Dual inhibitors of both enzymes could fill the gap between opioid analgesics and antalgics. Until now, attempts to find a compound with high affinity both for neutral endopeptidase and aminopeptidase N have failed. We report here the design of dual competitive inhibitors of both enzymes with KI values in the nanomolar range. These have been obtained by selecting R1, R2, and R3 determinants in aminophosphinic-containing inhibitors: NH2---CH(R1)P(O)---(OH)CH2---CH(R2)CONH---CH(R3)COOH, for optimal recognition of the two enkephalin inactivating enzymes, whose active site peculiarities, determined by site-directed mutagenesis, have been taken into account. The best inhibitors were 10x more potent than described dual inhibitors in alleviating acute and inflammatory nociceptive stimuli in mice, thus providing a basis for the development of a family of analgesics devoid of opioid side effects.

New orally active enkephalinase inhibitors: their synthesis, biological activity, and analgesic properties.

A series of (4S)-4-[(2S)-benzyl-3-mercaptopropionylamino]-4-(N-phenylcarbamoyl )-butyric acids has been identified as potent systemically active enkephalinase inhibitors. Structure-activity relationships (SAR) are discussed. Further chemical modification of the inhibitors was carried out in order to identify the inhibitors which are orally active in an animal model. Compounds of particular interest are the prodrug-like analogues, including 5b (ONO-9902). Their analgesic effects after oral administration were evaluated.

Broad-spectrum, non-opioid analgesic activity by selective modulation of neuronal nicotinic acetylcholine receptors.

Development of analgesic agents for the treatment of severe pain requires the identification of compounds that are devoid of opioid receptor liabilities. A potent (inhibition constant = 37 picomolar) neuronal nicotinic acetylcholine receptor (nAChR) ligand called ABT-594 was developed that has antinociceptive properties equal in efficacy to those of morphine across a series of diverse animal models of acute thermal, persistent chemical, and neuropathic pain states. These effects were blocked by the nAChR antagonist mecamylamine. In contrast to morphine, repeated treatment with ABT-594 did not appear to elicit opioid-like withdrawal or physical dependence. Thus, ABT-594 may be an analgesic that lacks the problems associated with opioid analgesia.

N-substituted-2-oxo-(2H)1-benzopyran-3-carboxamide derivatives with analgesic and/or diuretic activities.

The synthesis of N-substituted-2-oxo-(2H)1-Benzopyran-3-Carboxamide derivatives starting from semicarbazones or thiosemicarbazones and Carbon Suboxide (ratio 1:2) is described. Some compounds showed an interesting analgesic and/or diuretic activity in mice.