Cloning and pharmacological characterization of the dog P2X7 receptor

BACKGROUND AND PURPOSE

Human and rodent P2X7 receptors exhibit differences in their sensitivity to antagonists. In this study we have cloned and characterized the dog P2X7 receptor to determine if its antagonist sensitivity more closely resembles the human or rodent orthologues.

EXPERIMENTAL APPROACH

A cDNA encoding the dog P2X7 receptor was isolated from a dog heart cDNA library, expressed in U-2 OS cells using the BacMam viral expression system and characterized in electrophysiological, ethidium accumulation and radioligand binding studies. Native P2X7 receptors were examined by measuring ATP-stimulated interleukin-1beta release in dog and human whole blood.

KEY RESULTS

The dog P2X7 receptor was 595 amino acids long and exhibited high homology (>70%) to the human and rodent orthologues although it contained an additional threonine at position 284 and an amino acid deletion at position 538. ATP possessed low millimolar potency at dog P2X7 receptors. 2'-&3'-O-(4benzoylbenzoyl) ATP had slightly higher potency but was a partial agonist. Dog P2X7 receptors possessed relatively high affinity for a number of selective antagonists of the human P2X7 receptor although there were some differences in potency between the species. Compound affinities in human and dog blood exhibited a similar rank order of potency as observed in studies on the recombinant receptor although absolute potency was considerably lower.

CONCLUSIONS AND IMPLICATIONS

Dog recombinant and native P2X7 receptors display a number of pharmacological similarities to the human P2X7 receptor. Thus, dog may be a suitable species for assessing target-related toxicity of antagonists intended for evaluation in the clinic.

Negative and positive allosteric modulators of the P2X(7) receptor

BACKGROUND AND PURPOSE

Antagonist effects at the P2X(7) receptor are complex with many behaving in a non-competitive manner. In this study, the effects of N-[2-({2-[(2-hydroxyethyl)amino]ethyl}amino)-5-quinolinyl]-2-tricyclo[3.3.1.1(3,7)]dec-1-ylacetamide (compound-17) and N (2)-(3,4-difluorophenyl)-N (1)-[2-methyl-5-(1-piperazinylmethyl)phenyl]glycinamide dihydrochloride (GW791343) on P2X(7) receptors were examined and their mechanism of action explored.

EXPERIMENTAL APPROACH

Antagonist effects were studied by measuring agonist-stimulated ethidium accumulation in cells expressing human or rat recombinant P2X(7) receptors and in radioligand binding studies.

KEY RESULTS

Compound-17 and GW791343 were non-competitive inhibitors of human P2X(7) receptors. Receptor protection studies using decavanadate and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) showed that neither compound-17 nor GW791343 competitively interacted at the ATP binding site and so were probably negative allosteric modulators of the P2X(7) receptor. GW791343 prevented the slowly reversible blockade of the human P2X(7) receptor produced by compound-17 and inhibited [(3)H]-compound-17 binding to the P2X(7) receptor suggesting they may bind to similar or interacting sites. At rat P2X(7) receptors, compound-17 was a negative allosteric modulator but the predominant effect of GW791343 was to increase agonist responses. Antagonist interaction and radioligand binding studies revealed that GW791343 did not interact at the ATP binding site but did interact with the compound-17 binding site suggesting that GW791343 is a positive allosteric modulator of the rat P2X(7) receptor.

CONCLUSIONS

Compound-17 was a negative allosteric modulator of human and rat P2X(7) receptors. GW791343 was a negative allosteric modulator of the human P2X(7) receptor but at the rat P2X(7) receptor its predominant effect was positive allosteric modulation. These compounds should provide valuable tools for mechanistic studies on P2X(7) receptors.

Direct labelling of the human P2X7 receptor and identification of positive and negative cooperativity of binding

BACKGROUND AND PURPOSE

The P2X(7) receptor exhibits complex pharmacological properties. In this study, binding of a [(3)H]-labelled P2X(7) receptor antagonist to human P2X(7) receptors has been examined to further understand ligand interactions with this receptor.

EXPERIMENTAL APPROACH

The P2X(7) receptor antagonist, N-[2-({2-[(2-hydroxyethyl)amino]ethyl}amino)-5-quinolinyl]-2-tricyclo[3.3.1.1(3,7)]dec-1-ylacetamide (compound-17), was radiolabelled with tritium and binding studies were performed using membranes prepared from U-2 OS or HEK293 cells expressing human recombinant P2X(7) receptors.

KEY RESULTS

Binding of [(3)H]-compound-17 was higher in membranes prepared from cells expressing P2X(7) receptors than from control cells and was inhibited by ATP suggesting labelled sites represented human P2X(7) receptors. Binding was reversible, saturable and modulated by P2X(7) receptor ligands (Brilliant Blue G, KN62, ATP, decavanadate). Furthermore, ATP potency was reduced in the presence of divalent cations or NaCl. Radioligand binding exhibited both positive and negative cooperativity. Positive cooperativity was evident from bell shaped Scatchard plots, reduction in radioligand dissociation rate by unlabelled compound-17 and enhancement of radioligand binding by KN62 and unlabelled compound-17. ATP and decavanadate inhibited binding in a negative cooperative manner as they enhanced radioligand dissociation.

CONCLUSIONS

These data demonstrate that human P2X(7) receptors can be directly labelled and provide novel insights into receptor function. The positive cooperativity observed suggests that binding of compound-17 to one subunit in the P2X(7) receptor complex enhances subsequent binding to other P2X(7) subunits in the same complex. The negative cooperative effects of ATP suggest that ATP and compound-17 bind at separate, interacting, sites on the P2X(7) receptor.

(E)-2(R)-[1(S)-(Hydroxycarbamoyl)-4-phenyl-3-butenyl]-2'-isobutyl-2'-(methanesulfonyl)-4-methylvalerohydrazide (Ro 32-7315), a selective and orally active inhibitor of tumor necrosis factor-alpha convertase

Tumor necrosis factor-alpha (TNF-alpha), a cytokine secreted by inflammatory cells, has been implicated in several inflammatory disease states. (E)-2(R)-[1(S)-(Hydroxycarbamoyl)-4-phenyl-3-butenyl]-2'-isobutyl-2'-(methanesulfonyl)-4-methylvalerohydrazide (Ro 32-7315), is a potent, orally active inhibitor of the TNF-alpha convertase (TACE), an enzyme responsible for proteolytic cleavage of the membrane bound precursor, pro-TNF-alpha. Ro 32-7315 inhibited a recombinant form of TACE (IC(50) = 5.2 nM) with selectivity over related matrix metalloproteinases. In a cellular assay system, THP-1 cell line, and in human and rat whole blood, Ro 32-7315 significantly reduced lipopolysaccharide (LPS)-induced TNF-alpha release with IC(50) values of 350 +/- 14 nM (n = 5), 2.4 +/- 0.5 microM (n = 5), and 110 +/- 18 nM (n = 5), respectively. Oral administration of Ro 32-7315 to Wistar rats caused a dose-dependent inhibition of LPS-induced release of systemic TNF-alpha with an ED(50) of 25 mg/kg. Treatment (days 0-14) of Allen and Hamburys hooded rats with Ro 32-7315 (2.5, 5, 10, and 20 mg/kg, i.p., twice daily) significantly reduced adjuvant-induced secondary paw swelling (42, 71, 83, and 93%, respectively) as compared with the vehicle group. In the Ro 32-7315-treated group, the reduced paw swelling was associated with improved lesion score and joint mobility. Furthermore, in a placebo-controlled, single-dose study, Ro 32-7315 given orally (450 mg) significantly suppressed ex vivo, LPS-induced TNF-alpha release in the whole-blood samples taken from healthy male and female volunteers (mean inhibition of 42% over a 4-h duration, n = 6). These data collectively support the potential use of such a compound for the oral treatment of inflammatory disorders.

Radical-mediated synthesis of alpha-C-glycosides based on N-acyl galactosamine

[structure] C-Glycosides of N-acyl 2-amino-2-deoxygalactose (acyl = MeCO, CF(3)CO, t-BuOCO) are available in a stereoselective manner by trapping of an anomeric radical with an activated alkene. Using anomeric selenides, radical generation and trapping is carried out under conditions that avoid competitive reduction, and this chemistry has been applied to the synthesis of the novel C-glycoside analogue of O-benzyl alpha-D-GalNAc.

Selective alpha 2-adrenoceptor blockade does not enhance glucose-evoked insulin release

An investigation has been made of the effects of the selective alpha 2-adrenoceptor antagonist, idazoxan, on the plasma immunoreactive insulin and glucose responses following a glucose stimulus in conscious euglycaemic rats. UK 14304 (100 micrograms/kg), a selective alpha 2-adrenoceptor agonist, reduced the insulin response and potentiated the hyperglycaemia elicited by an intra-arterial glucose load (0.25 g/kg), thereby confirming previous findings that alpha 2-adrenoceptors can influence pancreatic insulin secretion and glycaemia. The effects of UK 14304 were totally abolished by idazoxan (1.0 mg/kg), indicating that idazoxan, at the dose studied, effectively antagonized alpha 2-adrenoceptor-mediated responses. However, idazoxan (1.0 mg/kg) by itself did not significantly affect the plasma glucose and insulin responses to glucose challenge. The data indicate that selective alpha 2-adrenoceptor blockade per se does not potentiate glucose-evoked insulin secretion.

The role of alpha- and beta-adrenoceptor subtypes in mediating the effects of catecholamines on fasting glucose and insulin concentrations in the rat

1. The role of alpha- and beta-adrenoceptor subtypes in the regulation of plasma glucose and immunoreactive insulin (IRI) levels has been investigated in normal conscious fasted rats by employing selective agonists and antagonists. 2. Adrenaline (0.2 mg kg-1)-induced hyperglycaemia was abolished by the selective alpha 2-adrenoceptor antagonist idazoxan (1.0 mg kg-1), unaltered by non-selective beta-adrenoceptor blockade (propranolol, 1.0 mg kg-1) and potentiated by the selective alpha 1-adrenoceptor antagonist prazosin (0.3 mg kg-1). Adrenaline increased plasma IRI levels in the presence of idazoxan but not in the presence of either prazosin or propranolol. 3. The selective alpha 2-adrenoceptor agonists UK 14304 (0.1 and 0.3 mg kg-1) and BHT-920 (0.2 and 0.5 mg kg-1) elicited dose-dependent hyperglycaemic responses, but did not alter plasma IRI levels. UK 14304 (0.1 mg kg-1)-evoked hyperglycaemia was blocked by idazoxan but not by prazosin. 4. The selective alpha 1-adrenoceptor agonists methoxamine (0.3 mg kg-1) and phenylephrine (0.3 mg kg-1) failed to modify either plasma glucose or IRI levels. 5. Isoprenaline (0.2 mg kg-1) elicited hyperglycaemic and insulinotropic responses which were attenuated by propranolol (1.0 mg kg-1) and the selective beta 2-adrenoceptor antagonist ICI 118551 (1.0 mg kg-1), but not by the beta 1-selective antagonists atenolol (1.0 mg kg-1) and betaxolol (1.0 mg kg-1). 6. None of the antagonists per se affected basal plasma glucose or IRI concentrations, except prazosin (1.0 mg kg-1). 7. The results indicate that adrenoceptors do not appear to be involved in regulating basal plasma glucose and IRI concentrations in the fasted rat. However, the effects of catecholamines on these parameters are mediated by alpha 2- and beta 2-adrenoceptors, whereas alpha,- or beta l-adrenoceptors do not appear to be involved.

Substituted 1,3,4-thiadiazoles with anticonvulsant activity. 4. Amidines

Two different structural types of 2-aryl-1,3,4-thiadiazole amidines were synthesized and evaluated for anticonvulsant activity. Enhancement of the inherent anticonvulsant activity therein and separation of this activity from the accompanying sedative action of these compounds were attempted. The most potent compounds occurred in the 2-(trifluoromethyl)phenyl series of type 3 amidines, but they also possessed a relatively high level of neurotoxicity and sedation as demonstrated in the rotorod test.

Determination of idazoxan in plasma by radioreceptor assay

A radioreceptor assay to determine the plasma concentration of idazoxan, a potent, highly selective antagonist for the alpha(2)-adrenoreceptor, is described. The assay is based upon a technique in which plasma extracts containing idazoxan compete with radiolabelled ligand for binding sites on receptor-rich tissue prepared from beef brain cortex. Using a logistic data-fit the limit of detection is of the order of 1 ng ml(-1) and represents a 10-fold increase in sensitivity over that from an established HPLC procedure. Comparison of human plasma data from the two assays indicates a correlation coefficient of 0.92 (N = 27) although the chromatographic method gave consistently higher values than the binding assay. The binding assay requires no sample extraction or pretreatment of plasma and its accuracy, precision and inherent specificity are such that the method represents a useful alternative to HPLC for therapeutic drug monitoring.

Substituted 1,3,4-thiadiazoles with anticonvulsant activity. 3. Guanidines

The synthesis and anticonvulsant activity of a number of 2-aryl-5-guanidino-1,3,4 thiadiazoles are described. The unsubstituted guanidine 2a was found to possess potent anticonvulsant properties; considerable reduction or loss of activity however was observed with the majority of the substituted guanidines. Incorporation of the guanidine group into an imidazoline ring also resulted in a loss of activity. Secondary pharmacological evaluation confirmed the anticonvulsant properties of 2a but also revealed that the compound exhibited a considerable degree of sedative activity.

Substituted 1,3,4-thiadiazoles with anticonvulsant activity. 1. Hydrazines

The synthesis and anticonvulsant activity of a series of 2-aryl-5-hydrazino-1,3,4-thiadiazoles are described. The combination of preferred aromatic substituents in the 2-position coupled with alkyl substitution on the hydrazine moiety led to a number of potent compounds lacking sedation, ataxia, or lethality. 5-(2-Biphenylyl)-2-(1-methylhydrazino)-1,3,4-thiadiazole (4m) represents a new class of anticonvulsant agent and compares favorably with the standard drugs phenytoin, phenobarbital, and carbamazepine.

Substituted 1,3,4-thiadiazoles with anticonvulsant activity. 2. Aminoalkyl derivatives

This paper describes the synthesis and pharmacological evaluation of a number evaluation of a number of substituted 1,3,4-thiadiazoles. The first member of the series, 2-(aminomethyl)-5-(2-biphenylyl)-1,3,4-thiadiazole (7) was found to possess potent anticonvulsant properties in rats and mice and compared favorably with the standard anticonvulsant drugs phenytoin, phenobarbital, and carbamazepine in a number of test situations. The potency of compound 7 was maintained on alkylation of the side-chain nitrogen atom; however, aryl substitution or chain lengthening caused a drop in potency. Replacement of the 2-biphenylyl group by phenyl or benzyl also lead to inactive compounds.

2-Alkyl analogue of idazoxan (RX 781094) with enhanced antagonist potency and selectivity at central alpha 2-adrenoceptors in the rat

Four 2-alkyl (methyl, ethyl, n-propyl and isopropenyl) analogues of idazoxan (RX 781094) have been synthesized and assessed in terms of their central alpha 2/alpha 1-adrenoceptor selectivity and alpha 2-adrenoceptor antagonist potency using both in vitro and in vivo tests in the rat. In cortical binding assays using [3H]-idazoxan and [3H]-prazosin, idazoxan had a 5 times greater alpha 2/alpha 1-selectivity than yohimbine. The 2-alkyl substituted analogues all showed improved selectivity, being between 17 and 29 times more selective than yohimbine for [3H]-idazoxan binding sites. In terms of central antagonist potency in vivo, the most favourable substitutions were 2-ethyl (RX 811033) and 2-n-propyl (RX 811054). Compared with yohimbine, these analogues were, respectively, 36 and 18 times more potent intravenously and 5 and 7.5 times more potent orally in their antagonism of guanoxabenz-induced mydriasis in the pentobarbitone-anaesthetized rat. All the analogues had a duration of action similar to that of idazoxan, which was significantly shorter than that of yohimbine. The results indicate that introduction of alkyl groups in the 2-position of idazoxan greatly increases the alpha 2/alpha 1-adrenoceptor selectivity as measured in binding studies. Improved alpha 2-adrenoceptor affinity and antagonist potency were particularly associated with the 2-ethyl and 2-n-propyl analogues.

Effects of idazoxan on catecholamine systems in rat brain

Experiments have been performed to assess the potency of idazoxan (RX 781094) at alpha and beta-adrenoceptors and dopamine receptors and on catecholamine uptake processes in rat brain. The effects of idazoxan on the turnover rates of noradrenaline and dopamine have been determined. Radioligand binding studies with cerebral cortex membranes have demonstrated that idazoxan exhibits 46-fold selectivity for alpha 2-adrenoceptors labelled by (3H)-idazoxan (Mean Ki +/- S.E.M. = 3.1 +/- 0.4 nM) compared with alpha 1-adrenoceptors labelled by (3H)-prazosin (Mean Ki +/- S.E.M. = 142 +/- 27 nM). Under the same conditions, yohimbine showed 6-fold selectivity for alpha 2-adrenoceptors. Idazoxan had low affinity for beta-adrenoceptors labelled by (3H)-dihydroalprenolol (IC50 value greater than 10 microM), for dopamine receptors labelled by (3H)-domperidone (IC50 value greater than 20 microM), for the (3H)-noradrenaline uptake site in rat hypothalamus (IC50 = 31 microM) and for the (3H)-dopamine uptake site in rat striatum (IC50 value approximately 800 microM). In rats treated with alpha-methyl-p-tyrosine, idazoxan (10-80 mg/kg, po) produced a marked increase (63% at 10, 217% at 20 mg/kg, po) in the apparent rate of turnover of noradrenaline in rat cortex/striatum, without affecting the rate of turnover of dopamine. This was in contrast to yohimbine (5-20 mg/kg, po) which increased the turnover rates of both catecholamines. In the absence of alpha-methyl-p-tyrosine, idazoxan (5-40 mg/kg, po) produced a dose related increase in the MHPG concentration and a small (20-30%) reduction in the steady state concentration of NA; the duration of the reduction was dose-related. DA steady state concentrations were unaffected. Idazoxan is a new selective alpha 2-adrenoceptor antagonist which should prove a valuable investigative tool in neurochemical studies and which may be a useful clinical agent in the management of the affective disorders.

The relationship between plasma concentration of valproic acid and its anticonvulsant and behavioural effects in the rat

The relationship between the plasma concentration of valproic acid (VPA) and anticonvulsant or neurotoxic effects was studied in the rat. Anticonvulsant activity was assessed against; (1) maximal seizures induced either by electroshock or by intravenous injection of pentylenetetrazol; and (2) kindled amygdaloid epilepsy. Drug-induced neurotoxicity was determined by the rotarod test and by observation of behaviour. In the maximal seizure tests, tonic hindlimb extension was always abolished at plasma valproic acid concentrations of 225 microgram ml-1 and above. Tonic forelimb extension was not consistently blocked until the plasma drug concentration exceeded 530 microgram ml-1. In fully-kindled rats, plasma valproic acid concentrations of 300 microgram ml-1 and above markedly reduced the duration of amygdala afterdischarge activity and the intensity of behavioural seizures produced by amygdala stimulation. Analysis of the data from individual kindled rats revealed that there was a significant correlation between the estimated plasma concentration of valproic acid and the degree of seizure protection. Impairment of rotarod performance and marked ataxia occurred at plasma valproic acid concentrations above 510 microgram ml-1 and loss of righting reflex became evident at 970 microgram ml-1. From these results, it is concluded that the plasma concentration of valproic acid is closely correlated with the anticonvulsant and neurotoxic effects observed in individual rats after acute administration of sodium valproate.