Regional brain distribution of risperidone and its active metabolite 9-hydroxy-risperidone in the rat

Risperidone is a new benzisoxazole antipsychotic. 9-Hydroxy-risperidone is the major plasma metabolite of risperidone. The pharmacological properties of 9-hydroxy-risperidone were studied and appeared to be comparable to those of risperidone itself, both in respect of the profile of interactions with various neurotransmitters and its potency, activity, and onset and duration of action. The absorption, plasma levels and regional brain distribution of risperidone, metabolically formed 9-hydroxy-risperidone and total radioactivity were studied in the male Wistar rat after single subcutaneous administration of radiolabelled risperidone at 0.02 mg/kg. Concentrations were determined by HPLC separation, and off-line determination of the radioactivity with liquid scintillation counting. Risperidone was well absorbed. Maximum plasma concentrations were reached at 0.5-1 h after subcutaneous administration. Plasma concentrations of 9-hydroxy-risperidone were higher than those of risperidone from 2h after dosing. In plasma, the apparent elimination half-life of risperidone was 1.0 h, and mean residence times were 1.5 h for risperidone and 2.5 h for its 9-hydroxy metabolite. Plasma levels of the radioactivity increased dose proportionally between 0.02 and 1.3 mg/kg. Risperidone was rapidly distributed to brain tissues. The elimination of the radioactivity from the frontal cortex and striatum--brain regions with high concentrations of 5-HT2 or dopamine-D2 receptors--became more gradual with decreasing dose levels. After a subcutaneous dose of 0.02 mg/kg, the ED50 for central 5-HT2 antagonism in male rats, half-lives in frontal cortex and striatum were 3-4 h for risperidone, whereas mean residence times were 4-6 h for risperidone and about 12 h for 9-hydroxy-risperidone. These half-lives and mean residence times were 3-5 times longer than in plasma and in cerebellum, a region with very low concentrations of 5-HT2 and D2 receptors. Frontal cortex and striatum to plasma concentration ratios increased during the experiment. The distribution of 9-hydroxy-risperidone to the different brain regions, including frontal cortex and striatum, was more limited than that of risperidone itself. This indicated that 9-hydroxy-risperidone contributes to the in vivo activity of risperidone, but to a smaller extent than would be predicted from plasma levels. AUCs of both active compounds in frontal cortex and striatum were 10-18 times higher than those in cerebellum. No retention of metabolites other than 9-hydroxy-risperidone was observed in any of the brain regions investigated.

Survey on the pharmacodynamics of the new antipsychotic risperidone

This review reports on the pharmacodynamics of the new antipsychotic risperidone. The primary action of risperidone is serotonin 5-HT2 receptor blockade as shown by displacement of radioligand binding (Ki: 0.16 nM), activity on isolated tissues (EC50: 0.5 nM), and antagonism of peripherally (ED50: 0.0011 mg/kg) and centrally (ED50: 0.014 mg/kg) acting 5-HT2 receptor agonists in rats. Risperidone is at least as potent as the specific 5-HT2 receptor antagonist ritanserin in these tests. Risperidone is also a potent dopamine D2 receptor antagonist as indicated by displacement of radioligand binding (Ki: 1.4 nM), activity in isolated striatal slices (IC50: 0.89 nM), and antagonism of peripherally (ED50: 0.0057 mg/kg in dogs) and centrally acting D2 receptor agonists (ED50: 0.056-0.15 mg/kg in rats). Risperidone shows all effects common to D2 antagonists, including enhancement of prolactin release. However, some central effects such as catalepsy and blockade of motor activity occur at high doses only. Risperidone is 4-10 times less potent than haloperidol as a central D2 antagonist in rats and it differs from haloperidol by the following characteristics: predominant 5-HT2 antagonism; LSD antagonism; effects on sleep; smooth dose-response curves for D2 antagonism; synergism of combined 5-HT2/D2 antagonism; pronounced effects on amphetamine-induced oxygen consumption; increased social interaction; and pronounced effects on dopamine (DA) turnover. Risperidone displays similar activity at pre- and postsynaptic D2 receptors and at D2 receptors from various rat brain regions. The binding affinity for D4 and D3 receptors is 5 and 9 times weaker, respectively, than for D2 receptors; interaction with D1 receptors occurs only at very high concentrations. The pharmacological profile of risperidone includes interaction with histamine H1 and alpha-adrenergic receptors but the compound is devoid of significant interaction with cholinergic and a variety of other types of receptors. Risperidone has excellent oral activity, a rapid onset, and a 24-h duration of action. Its major metabolite, 9-hydroxyrisperidone, closely mimics risperidone in pharmacodynamics. Risperidone can be characterized as a potent D2 antagonist with predominant 5HT2 antagonistic activity and optimal pharmacokinetic properties.

Genomic cloning, heterologous expression and pharmacological characterization of a human histamine H1 receptor

A human histamine H1 receptor gene lacking introns was isolated by screening a human genomic library with a bovine histamine H1 receptor probe. The deduced protein of 487 amino acids showed characteristic properties of G-protein-coupled receptors. The coding region was subcloned into the expression vector pSVL (Pharmacia), and the resulting construct transfected into COS-7 cells. Binding studies with [3H]pyrilamine on membranes from transfected cells revealed saturable specific binding with a KD of 1.2 nM and a Bmax of 3400 fmol/mg protein. Binding affinities of histamine and known histamine antagonists were similar to those for histamine H1 receptors in guinea-pig cerebellum. In transfected COS-7 cells, histamine induced inositol phosphate formation, that was inhibitable by pyrilamine.

Occupancy of central neurotransmitter receptors by risperidone, clozapine and haloperidol, measured ex vivo by quantitative autoradiography

Risperidone (Risperdal) is a novel antipsychotic drug, with beneficial effects on both positive and negative symptoms of schizophrenia, and with a low incidence of extrapyramidal side effects (EPS). These particular properties have been attributed to the predominant and very potent serotonin 5-HT2 receptor antagonism of the drug combined with less potent dopamine D2 antagonism. In order to provide data on the degree to which various central neurotransmitter receptors are occupied in vivo, we performed ex vivo receptor occupancy studies with risperidone in comparison with clozapine and haloperidol in rats and guinea pigs. Various types of receptors, to which the compounds were known to bind to in vitro, were investigated precisely using receptor autoradiography in sections of the same rat brain except for histamine H1 receptors that were measured in the guinea-pig cerebellum. Risperidone (2 h after s.c. treatment) occupied 5-HT2 receptors at very low doses (ED50 = 0.067 mg/kg). Nearly full occupancy (> 80%) was achieved before H1, D2, alpha 1 and alpha 2 receptors became occupied (ED50 = 0.45, 0.66, 0.75 and 3.7 mg/kg, respectively). Clozapine displayed occupancy of H1 and alpha 1 receptors at low doses (ED50 = 0.15 and 0.58 mg/kg, respectively) and of 5-HT2, 5-HT1C, D2, alpha 2, cholinergic muscarinic and 5-HT1A receptors at higher doses (ED50 = 1.3, 1.8, 9.0, 9.5, 11 and 15 mg/kg, respectively). Haloperidol occupied D2 and alpha 1 receptors at low doses (ED50 = 0.13 and 0.42 mg/kg, respectively) and 5-HT2 receptors at a higher dose (ED50 = 2.6 mg/kg). Occupancy of receptor types occurred with similar ED50-values in various brain areas, e.g. D2 receptors in striatum and mesolimbic areas. The ED50-values for the ex vivo measured occupancy of 5-HT2 and D2 receptors were in good agreement with ED50-values for functional effects putatively mediated by these central receptors. The dose-dependent occupancy of D2 receptors proceeded more gradually with risperidone (slope in the caudate-putamen: 0.85) than with clozapine (slope: 1.44) or haloperidol (slope: 1.51). It has previously been suggested that partial D2 receptor occupancy may suffice to control the positive symptoms of schizophrenia, whereas higher D2 receptor occupancy would induce extrapyramidal symptoms (EPS). The dose ratio for high (75%) vs. low (25%) D2 receptor occupancy in the caudate-putamen, was 37.3 for risperidone, 8.4 for clozapine, and 7.9 for haloperidol.(ABSTRACT TRUNCATED AT 400 WORDS)

Receptor cloning and heterologous expression--towards a new tool for drug discovery

The explosion in the number of cloned receptors presents the pharmaceutical industry with challenges to discover new drugs targeting those receptors; to find more-selective drugs for all novel receptor subtypes; and to learn more about the function of the receptors in order to discern the conditions where such drugs may be applied usefully as therapeutics. At the same time, receptor cloning affords an unprecedented opportunity to address these challenges: heterologously expressed recombinant human receptors can be used for drug screening and - through an improved understanding of structure-function relationship - possibly for drug design, while the receptor clones permit mobilization of the full power of molecular biology to elucidate the function of the receptors in health and disease.

Towards an improved survival of rat brain neurons in culture by cerebrospinal fluid of patients with senile dementia of Alzheimer's type

Neuronal cell survival was investigated in rat brain cortical cultures in the presence of increasing concentrations of human brain extracts or cerebrospinal fluid (CSF) from control and Senile Dementia of Alzheimer's type (SDAT) patients. Using hippocampal brain extracts, converted 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) was compared to the content of the neuronal marker MAP2 in foetal rat brain neuronal cultures in order to test converted MTT as a quantitative parameter for neuronal cell survival. A significant correlation was found between both parameters. SDAT frontal cortex brain extracts induced a two four-fold increase in neuronal cell survival at 25 to 125 micrograms protein extract, whereas control brain extracts induced at similar protein concentrations a decline in neuronal cell survival. The enhanced survival yielded by SDAT brain extracts was fully abolished in the presence of control brain extract. Control CSF concentration-dependently increased neuronal cell survival in postnatal rat brain neuronal cultures independent of the difference in the protein content of CSF samples and age of the patients. SDAT CSF also concentration-dependently enhanced neuronal cell survival, however, the effect was more pronounced compared to control CSF. These observations are in favour of the hypothesis that there might be a higher neurotrophic activity in SDAT brain tissue.

Synthesis and biological activities of psi (CH2NH) pseudopeptide analogues of the C-terminal hexapeptide of neurotensin

Each peptide CO-NH function in the biologically important C-terminal 8-13 sequence of neurotensin was replaced by the reduced CH2-NH isostere using the rapid in situ solid phase procedure developed by Sasaki & Coy. In general this modification resulted in a drop in receptor affinity except for the [Arg psi(CH2NH) Arg]-NT8-13 analogue (PIC50 9.23 vs. NT8-13 pIC50 8.03). This analogue also showed enhanced enzymatic stability, but acted as a full agonist as shown by the observation of relaxations of guinea-pig colon ascendens.

Activity of serotonin (5-HT) receptor agonists, partial agonists and antagonists at cloned human 5-HT1A receptors that are negatively coupled to adenylate cyclase in permanently transfected HeLa cells

The activity of serotonin (5-HT) receptor agonists, partial agonists and antagonists, and various other neurotransmitter receptor antagonists at human 5-HT1A receptors that are negatively coupled to adenylate cyclase in permanently transfected HeLa cells was investigated. 5-HT1A receptor-mediated inhibition of adenylate cyclase was studied by measuring inhibition of cAMP accumulation, induced by forskolin. At 100 microM forskolin produced a 100-fold increase in cAMP formation: 5-HT concentration dependently inhibited the cAMP formation; maximal inhibition was attained at 1 microM 5-HT and represented 90% of the stimulated cAMP formation. Full inhibition was observed with 5-HT1A receptor agonists: N,N-dipropyl-8-hydroxy-2-aminotetralin (8-OH-DPAT) and flesinoxan, and non-selective 5-HT receptor agonists: d-lysergic acid diethylamide (d-LSD), RU 24,969, bufotenine, methysergide and tryptamine. The rank order of potency of the compounds for inhibiting the cAMP formation corresponded to the rank order of the binding affinities of the drugs for the 5-HT1A receptor. Partial inhibition was obtained with submicromolar concentrations of buspirone, spiroxatrine and ipsapirone. A slight inhibition was observed with 1 microM 5-HT receptor agonist CP 93129 and 1 microM 5-HT receptor antagonists mesulergine and BW-501. No inhibition was found with: the 5-HT receptor agonists quipazine, sumatriptan and 1-(2,5-dimethoxy-4-methylphenyl)-2- aminopropane (DOM); the 5-HT receptor antagonist ICS-205,930; and other neurotransmitter receptor antagonists such as pindolol, CGP 20712-A, prazosin, sulpiride and pyrilamine. Spiperone and pindolol fully antagonized the agonist-mediated inhibition of forskolin-stimulated cAMP formation. Partial inhibition of the agonist-mediated inhibition of forskolin-stimulated cAMP formation was apparent with 1 microM ocaperidone and 1 microM ipsapirone. It can be concluded that HeLa cells, permanently expressing human 5-HT1A receptors, are a valid cellular system for studying the negative coupling of 5-HT1A receptors to adenylate cyclase and the action of compounds thereupon.

Interaction of antipsychotic drugs with neurotransmitter receptor sites in vitro and in vivo in relation to pharmacological and clinical effects: role of 5HT2 receptors

In the introductory section an overview is given of the strategies which have been proposed in the search for side-effect free antipsychotics. Special attention is paid to the role of predominant 5HT2 receptor blockade over D2 blockade. Whereas D2 receptor blockade seems to be essential for the treatment of positive symptoms of schizophrenia, it also underlies the induction of extrapyramidal side effects (EPS). Predominant 5HT2 receptor blockade may reduce the EPS liability and can ameliorate negative symptoms of schizophrenia. We further report a nearly complete list of neuroleptics that are on the European market and eight new antipsychotics that recently entered clinical trial, 5HT2 and D2 receptor binding affinity (Ki values) and the rank order in affinity for various neurotransmitter receptor subtypes are also discussed. For the eight new antipsychotics and for six reference compounds the complete receptor binding profile (including 33 radioligand receptor binding and neurotransmitter uptake models) is reported. Furthermore, for a series of 120 compounds the relative affinity for D2 receptors and D3 receptors (a recently cloned new dopamine receptor subtype) is compared. Finally, original findings are reported for the new antipsychotic risperidone and for haloperidol and clozapine on the in vivo occupation of neurotransmitter receptors in various brain areas after systemic treatment of rats or guinea pigs. The receptor occupation by the drugs was measured ex vivo by quantitative receptor autoradiography. The receptor occupancy was related to the motor activity effects of the test compounds (measurements were done in the same animals) and to the ability of the drugs to antagonize various 5HT2 and D2 receptor mediated effects. With risperidone a high degree of central 5HT2 receptor occupation was achieved before other neurotransmitter receptors became occupied. This probably co-underlies the beneficial clinical properties of the drug. Antagonism of the various D2 receptor-mediated effects was achieved at widely varying degrees of D2 receptor occupancy, from just about 10% to more than 70%. For therapeutic application it may be of prime importance to carefully titrate drug dosages. Antipsychotic effects may be achieved at a relatively low degree of D2 receptor occupancy at which motor disturbances are still minimal. With drugs such as risperidone that produce shallow log dose-effect curves, differentiation between the various D2 receptor mediated effects may be made more easily, allowing EPS-free maintenance therapy of schizophrenic patients.

Comparison of in vitro binding properties of a series of dopamine antagonists and agonists for cloned human dopamine D2S and D2L receptors and for D2 receptors in rat striatal and mesolimbic tissues, using [125I] 2'-iodospiperone

We investigated the ligand binding properties in vitro of two splice variants of the cloned human dopamine D2 receptor (the 443 and 414 amino acids long forms called D2L and D2S, respectively), expressed in 293 human kidney cells, in comparison with those of the dopamine D2 receptors in rat striatum, nucleus accumbens and tuberculum olfactorium. The new radioligand, [125I]2'-iodospiperone, showed a similar high binding affinity (KD:0.056-0.122 nM) for cloned human D2S and D2L receptors and for the D2 receptors in the three rat brain areas. Binding affinities of 25 dopamine antagonists and of 10 dopamine agonists belonging to different chemical classes were measured. The IC50 values of the antagonists were virtually identical in the five preparations: spiperone was the most potent compound (pIC50 approximately 9.9), remoxipride the least potent one (pIC50 approximately 5.7). The agonists showed similar IC50 values for the cloned human D2S and D2L receptors but their affinity for rat brain D2 receptors was 2- to 5-fold higher. Dopamine showed shallow inhibition curves, the high affinity binding was 10-fold lower for the cloned human D2 receptors than for the rat brain D2 receptors. Addition of stable guanosine-5'-triphosphate (GTP) analogues shifted the D2 receptors in the rat brain tissues to the "low" affinity state, the low affinity binding of dopamine was equal to the affinity for the cloned human receptor. None of the dopamine antagonists or agonists could differentiate between the two splice forms of the cloned human D2 receptors or between the D2 receptors in rat striatal and mesolimbic tissues. The lower apparent affinity of some agonists and of dopamine in the absence of stable GTP analogues suggests a less appropriate receptor G-protein coupling for the cloned human D2 receptors expressed in the 293 human kidney cells. Unexpectedly, guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma-S) reduced the [125I]2'-iodospiperone binding to the D2 receptors by 20-35% in the rat brain tissues and the cloned human D2L receptor, and by 75% to the cloned human D2S receptor. The inhibition in the last case could be prevented partly by submicromolar concentrations of dopamine. The GTP-gamma-S effect is suggested to be due to reduction of disulphide bonds in the receptor. Recent molecular modelling studies indicated an important role of the disulphide bridge between Cys107 at the start of transmembrane domain three and Cys182 in the third extracellular loop, for the binding of dopamine to the D2 receptor.

Chronic treatment with sabeluzole protects cultured rat brain neurons from the neurotoxic effects of excitatory amino acids

The neuroprotective properties of the cognitive enhancer sabeluzole were investigated in rat brain neuronal cultures derived from the hippocampal formation of 17-day-old rat embryos. Measurement of the neuronal cytoskeletal microtubule-associated protein, MAP2, was used to assess survival of neurons after exposure of neuronal cultures to glutamate. MAP2 was quantified in neuronal cell homogenates by means of an enzyme-linked immunosorbent assay (ELISA) using a mouse monoclonal MAP2 antibody, peroxidase-labeled goat anti-mouse Ig antiserum, and 2,2'-azido-di-[3-ethylbenz-thiazoline] sulphonate (ABTS) as substrate. Exposure of 7-day-old neuronal cultures to 1 mM glutamate for 16 hours led to a three-fold increase in released lactate dehydrogenase (LDH) and a 40% decrease in cellular MAP2 content. Acute treatment of neuronal cultures with 10 microM sabeluzole yielded a 40% drop in released LDH induced by glutamate. Cultures treated chronically with 0.1 microM sabeluzole on days 1 and 4 in culture showed, after 1 week in culture, a MAP2 content and total LDH activity that was not different from control cultures. A 16-hour exposure to 1 mM glutamate did not induce LDH release or changes in MAP2 levels in sabeluzole-treated cultures. A single treatment with 0.1 microM sabeluzole between day 1 to 5 induced a 70-80% drop in glutamate-induced released LDH in 7-day-old neuronal cultures. Full and partial neuronal protection after chronic sabeluzole treatment at 0.1 microM was also observed for neurotoxicity induced by 5 mM N-methyl-D-aspartate (NMDA) and 1 mM kainic acid or 30 microM veratridine, respectively. Within a series of compounds such as Ca++ and Na+ channel antagonists, glutamate receptor antagonists and various neurotransmitter receptor antagonists, sabeluzole, chronically given, were the most potent for inhibition of released LDH induced by 1 mM glutamate (IC50-value: 34 +/- 13 nM). In conclusion, chronic sabeluzole treatment protects cultured rat brain neurons from excitotoxic aggression.

Blockade of nitric oxide formation does not prevent glutamate-induced neurotoxicity in neuronal cultures from rat hippocampus

This study examined the role of nitric oxide (NO) in glutamate-induced, N-methyl-D-aspartate (NMDA) receptor-mediated neurotoxicity in rat hippocampal neuronal cultures grown under serum-free conditions. Formation of cGMP was used as an indirect measure of NO formation. Neuronal cell degeneration was monitored by measuring the release of lactate dehydrogenase (LDH). Neuronal cells showed a 4-fold increase in cGMP formation and release of LDH upon exposure to 30 microM glutamate. cGMP formation was fully inhibited by 1 microM nitro-arginine (N-Arg), 100 microM hemoglobin or 1 microM MK-801. In the presence of 1 microM MK-801, glutamate induced neither cGMP formation nor neuronal cell degeneration. However, when NO formation was inhibited by means of 100 microM N-Arg, glutamate still induced neurotoxicity. Therefore, in serum-free hippocampal cultures glutamate neurotoxicity occurs notwithstanding complete inhibition of the NO-synthase enzyme by N-Arg. Our data provide evidence that NO, synthesized upon glutamate exposure, has not a primary toxic action in pure hippocampal neuronal cultures.

Autoradiographic evidence for the occlusion of rat brain dopamine D3 receptors in vivo

[125I]Iodosulpride binding was studied in frontal rat brain sections by quantitative autoradiography. Using preincubated (= washed) sections, selective labelling and identification of dopamine D3 receptors was obtained using 0.2 nM [125I]iodosulpride in the presence of 100 nM domperidone for the occlusion of the D2 receptors. A high density of D3 receptors was noticed in the islands of Calleja. When preincubation of the sections was omitted, no D3 receptor labelling could be achieved, indicating tight binding to the receptor of an endogenous inhibitor. Such a tight receptor occupancy was not observed for the D2 receptor and various other neurotransmitter receptors. The occlusion of the D3 receptor could be prevented by tetrabenazine-induced monoamine depletion of the rats. It can be concluded, therefore, that D3 receptors are massively occupied by a monoamine, likely to be dopamine. This observation prompts the question to what extent dopamine D3 receptors can become occupied in vivo by systematically applied exogenous compounds.

In vitro and in vivo receptor binding and effects on monoamine turnover in rat brain regions of the novel antipsychotics risperidone and ocaperidone

Risperidone and ocaperidone are new benzisoxazol antipsychotics with particularly beneficial effects in schizophrenia. We report a comprehensive study on the in vitro and in vivo receptor binding profile of the new compounds, compared with haloperidol, and on the drug effects on monoamine and metabolite levels in various brain areas. The in vitro receptor binding and monoamine uptake inhibition profiles, comprising 29 receptors and four monoamine uptake systems, revealed that ocaperidone and risperidone bound primarily, and with the highest affinity thus far reported, to serotonin 5HT2 receptors (Ki values of 0.14 and 0.12 nM, respectively). Further, the drugs bound at nanomolar concentrations to the following receptors (Ki values, in nM, for ocaperidone and risperidone, respectively): alpha 1-adrenergic (0.46 and 0.81), dopamine D2 (0.75 and 3.0), histamine H1 (1.6 and 2.1), and alpha 2-adrenergic (5.4 and 7.3). In contrast, haloperidol showed nanomolar affinity for D2 receptors (1.55) and haloperidol-sensitive sigma sites (0.84) only. The in vitro binding affinity of ocaperidone, risperidone, and haloperidol for D2 receptors was exactly the same when measured in membranes from rat striatum, nucleus accumbens, tuberculum olfactorium, and human kidney cells expressing the cloned human D2 receptor (long form). In vivo binding in rats, using intravenous administration of [3H]spiperone, revealed very potent occupation by ocaperidone and risperidone of 5HT2 receptors in the frontal cortex (ED50 of 0.04-0.03 mg/kg); in this respect, they were 6, 30, and 100 times more potent than ritanserin, haloperidol, and clozapine, respectively. Ocaperidone occupied D2 receptors in the striatum and the nucleus accumbens with similar potency as did haloperidol (ED50 of 0.14-0.16 mg/kg). Risperidone revealed biphasic inhibition curves in the latter brain areas, indicating that [3H] spiperone labeled both 5HT2 receptors (occupied by risperidone at less than 0.04 mg/kg) and D2 receptors (risperidone ED50 of approximately 1 mg/kg). In the tuberculum olfactorium, 5HT2 and D2 receptors were also distinguished with risperidone. The ED50 values for occupation of the latter were for ocaperidone and risperidone 2 times lower and for haloperidol 2 times higher than in the striatum. Ocaperidone, risperidone, and haloperidol readily increased the levels of the dopamine metabolites 3,4-dihydroxybenzene acetic acid and homovanillic acid in the striatum, the nucleus accumbens, the tuberculum olfactorium, and, to some extent, the frontal cortex. Dose-response curve shapes were markedly different; with ocaperidone maximal levels were reached at 0.16 mg/kg and maintained to 10 mg/kg; with risperidone the levels tended to increase continuously up to 10 mg/kg. Haloperidol produced dome-shaped curves (maximum at 0.16-0.63 mg/kg).(ABSTRACT TRUNCATED AT 400 WORDS)

Synthesis and biological activities of linear and cyclic enkephalin analogues containing a psi (E,CH = CH) or psi (CH2CH2) isosteric replacement

The peptide CO-NH function was replaced by a trans carbon-carbon double bond or by a CH2-CH2 isostere in enkephalin analogues of DADLE, DCDCE-NH2 or DPDPE. In DADLE the 2-3 and the 3-4 peptide bond was modified, whereas in the cyclic analogues the Gly3-Phe4 bond was replaced by the isosteres Gly psi (E,CH = CH)Phe [5-amino-2-(phenylmethyl)-3(E)-pentenoic acid] or Gly psi (CH2CH2)Phe [5-amino-2-(phenylmethyl)pentanoic acid]. In general, the modification results in a drop in potency which is the largest for the flexible CH2-CH2 replacement. The Gly3 psi (E,CH = CH)Phe4 DCDCE-NH2 analogue retains considerable potency. These results confirm the importance of the peptide function at the 2-3 and 3-4 position in enkephalin analogues for biological potency.

Human beta 1- and beta 2-adrenergic receptor binding and mediated accumulation of cAMP in transfected Chinese hamster ovary cells. Profile of nebivolol and known beta-adrenergic blockers

The interaction of nebivolol and its SRRR and RSSS enantiomers, and of known beta-adrenergic blockers, with human beta 1- and beta 2-adrenergic receptors expressed separately in Chinese hamster ovary cells in culture (CHO-Hu beta 1 and CHO-Hu beta 2), was investigated. We studied [3H]CGP-12177 binding to the intact cells and the accumulation of cAMP induced by isoproterenol. Each of the receptor subtypes displayed saturable [3H]CGP-12177 binding on intact cells with sub-nanomolar affinity. The density of beta 1- and beta 2-adrenergic receptor sites was 1.1 x 10(6) receptor binding sites per CHO-Hu beta 1 cell and 0.2 x 10(6) receptor binding sites per CHO-Hu beta 2 cell, respectively. The beta-adrenergic antagonists CGP 20712-A, ICI 118-551 and propranolol showed the same binding properties as beta-adrenergic receptors in previously described tissues or cells. The potencies of these compounds in inhibiting beta-adrenergic receptor mediated accumulation of cAMP corresponded well with their binding affinities. d-Nebivolol (SRRR) and nebivolol showed combined high affinity and selectivity for inhibition of beta 1-adrenergic receptor coupled accumulation of cAMP in CHO-Hu beta 1 cells (0.41 and 0.42 nM for d-nebivolol and nebivolol, respectively). l-Nebivolol (RSSS) was 1460 times less potent than d-nebivolol in CHO-Hu beta 1 cells. The binding affinities of d-nebivolol and nebivolol for human beta 1-adrenergic binding sites correlated well with their potencies in inhibiting beta 1-adrenergic receptor coupled accumulation of cAMP. CHO cells transfected with human beta 1- and beta 2-adrenergic receptors are a valid model system for studying the interaction of compounds with human beta-adrenergic receptors.

Thromboxane A2/prostaglandin endoperoxide (TXA2/PG-END) receptor binding properties in human platelets of ridogrel, a combined TXA2 synthase inhibitor--TXA2/PG-END receptor antagonist

Ridogrel, a potent thromboxane A2 (TXA2) synthase inhibitor, also has thromboxane A2 prostaglandin endoperoxide (TXA2/PG-END) receptor antagonistic properties as documented in functional studies of human platelets. In the present study, the binding affinities of the TXA2 synthase inhibitors, ridogrel, dazoxiben, dazmegrel and pirmagrel, and the TXA2/PG-END receptor antagonists, GR32191, L670596, SQ29548, ICI159995, AH69212 and sulotroban, for the TXA2/PG-END receptor labelled with [3H]SQ29548 on intact human platelets were assessed. The potencies of the TXA2/PG-END receptor antagonists to inhibit specific [3H]SQ29548 binding to intact human platelets ranged between 1.2 nM and 6,200 nM and corresponded to the ability of the drugs to suppress human platelet aggregation induced by TXA2/PG-END receptor stimulation with U46619 and collagen. The TXA2 synthase inhibitors dazoxiben, dazmegrel and pirmagrel could not inhibit specific [3H]SQ29548 binding to intact human platelets, tested up to 10(-5) M, nor suppress human platelet aggregation, indicating lack of any receptor antagonistic properties. Ridogrel, however, directly bound to the TXA2/PG-END receptor with micromolar affinity (IC50 = 5.2 microM) and inhibited U46619-27, or collagen-induced platelet aggregation, with ED50-values of 27 microM and 4.7 microM respectively. The present study thus demonstrates that antagonism by ridogrel of TXA2/PG-END receptor activation on platelets as defined in functional tests, coincides with inhibition of specific ligand binding to the receptors.

Receptor-based assays

Receptor-based assays have benefitted from the newest advances in biotechnology and electronics in three main ways: genetically engineered cells expressing single receptor subtypes have been developed for many natural and synthetic ligands; assays have been designed which take advantage of a variety of signals triggered in cells by binding, or inhibition of binding, of ligands to surface-bound receptors; and radiolabelled ligand assays have been considerably improved and simplified by novel electronic devices.

[Non-sedative antihistaminics and binding to central and peripheral H1 histamine receptors]

Four non-sedating antihistamines (astemizole, cetirizine, loratadine and terfenadine) were investigated for in vitro and ex vivo binding to histamine-H1 receptors in guinea-pig cerebellum and lung. In vitro, all the drugs dissociated slowly from H1 receptors (half-times greater than or equal to 100 min), Ki,app-values decreased with longer incubation times for potent lipophylic agents (astemizole and terfenadine) Ki,app-values were lower with more dilute tissue suspensions. In optimized assay conditions astemizole showed a Ki,app-value of 0.2 microM. Terfenadine, cetirizine and loratadine bound with 30-, 80- and 100-times lower affinity to H1 receptors. The occupancy of lung and cerebellar H1 receptors was investigated after oral administration of various dosages of the drugs and at several times after drug administration, using ex vivo binding techniques. Astemizole was a very potent compound showing complete differentiation between lung and cerebellar receptor occupation (with 0.63 mg/kg: 70% of lung H1 receptors were occupied, with less than 10% of cerebellar H1 receptor occupancy). A 7-times higher dose of terfenadine was required to induce the same effect. Astemizole produced a rapid and complete occupancy of lung receptors, which was maintained up to 72 h after administration. In contrast, terfenadine produced a peak effect at 1 h and was completely eliminated from lung receptors in 24 h. Loratadine and cetirizine only poorly differentiated between lung and cerebellar receptor occupancy (at 2.5 mg/kg: 70 and 60% of lung receptor occupancy, 50 and 70% of cerebellar receptor occupancy).(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of non-serotonergic [3H]ketanserin binding sites on human platelets and their role in serotonin release

[3H]Ketanserin was found to label (besides a low amount of 5-HT2 receptors) non-serotonergic binding sites on human platelet membranes. The latter binding was detected in the presence of excess of the 5-HT2 antagonist BW501, and was potently inhibited by tetrabenazine. [3H]Ketanserin revealed a KD value = 19 +/- 4 nM and a Bmax = 425 +/- 82 fmol/10(9) platelets for these binding sites. [3H]Ketanserin binding in the presence of BW501 was inhibited by the tetrabenazine derivative RO-4-1284 (IC50 = 1.4 nM), tetrabenazine (IC50 = 8.6 nM) and the ketanserin derivatives R 71,278 (IC50 = 6.3 nM), R 47,288 (IC50 = 17 nM) and R 71,428 (IC50 = 100 nM). Ketanserin revealed an IC50 = 32 nM. The drugs were found to trigger the release of 3H from [3H]5-HT-loaded human platelets in superfusion experiments in vitro. The amount of 3H released by the drugs correlated with their binding affinities for the non-serotonergic sites. The non-serotonergic [3H]ketanserin binding sites on human platelets and their possible role in triggering monoamine release corresponded to the properties of non-serotonergic ketanserin binding sites previously characterized in rat striatum. The possible role of the action of ketanserin on the non-serotonergic sites in the reported partial reduction by ketanserin of the monoamine content in cardiovascular tissues is discussed.

Ca++ and Na+ channels involved in neuronal cell death. Protection by flunarizine

Flunarizine, a class IV Ca++ antagonist non-selective for slow Ca++ channels, has been shown to be beneficial in the prophylactic treatment of migraine, the treatment of vertigo, and as add-on treatment in therapy-resistant forms of epilepsy. Flunarizine protects the brain against functional and/or structural neuronal damage in various animal models of cerebral ischemia. In addition to its cerebrovascular effect, flunarizine has also direct neuroprotective actions. New data have emerged on flunarizine with regard to Ca++ and Na+ channels in neuronal cells. There are several possible mechanisms involved in the mode of action of flunarizine. Flunarizine may block Ca++ and Na+ channels, both of which may flux Ca++ as well as Na+. A decrease in Ca++ influx may prevent further release of glutamate, and activation of NMDA receptor gated Ca++ channels at physiological pH. A decrease in Na+ influx may prevent cytotoxicity secondary to a large gain in intracellular Ca++, by reverse operation of the Na+/Ca++ exchanger. This mechanism may be important when the glycolytic rate is increased with concomitant acidosis, and phospholipids are broken down as occurs typically during ischemia. Given the complexity of biochemical events leading to cell death, blocking exclusively one channel subtype is not likely to yield sufficient protection. Hence, it may be useful to develop anti-ischemic compounds which act on a series of pathways involved in Ca++ overload, rather than selectively block one such channel.

Neurotoxic action of veratridine in rat brain neuronal cultures: mechanism of neuroprotection by Ca++ antagonists nonselective for slow Ca++ channels

The effect of various Ca++ antagonists and local anesthetics on neuronal cell degeneration induced by veratridine was studied in primary rat brain neuronal cultures. Cell death was quantified by measuring lactate dehydrogenase (LDH) released in the culture medium. The neuronal cell degeneration was Ca+(+)-dependent because, in the absence of extracellular Ca++, 16 hr of exposure to 30 microM veratridine failed to produce release of LDH. Ca++ antagonists, nonselective for slow Ca++ channels (flunarizine, cinnarizine, lidoflazine, prenylamine and bepridil) inhibited veratridine-induced release of LDH with IC50 values between 0.11 and 0.47 microM. Ca++ antagonists selective for slow Ca++ channels were less potent and inhibited veratridine-induced release of LDH at concentrations in the following order of potency: nicardipine greater than gallopamil and verapamil greater than niludipine greater than nitrendipine greater than nifedipine greater than nimodipine greater than diltiazem. Tested local anesthetics were incomplete inhibitors of veratridine-induced release of LDH. A good correlation was found between the potency of the drugs to inhibit released LDH induced by 30 microM veratridine in neuronal cultures and their binding affinity for the batrachotoxin binding site of Na+ channels in rat cortex synaptosomal preparation. It is concluded that protection against veratridine-induced neurotoxicity can be mediated by blocking a veratridine-sensitive Na+ channel. It is a property of certain nonselective Ca++ antagonists. There is apparently no direct relationship with Ca++ antagonistic activity. The effect is unrelated to local anesthetic activity.

Gaps and peculiarities in 5-HT2 receptor studies

After an introductory review of the properties of 5-HT2 receptors, three issues in 5-HT2 receptor binding research are discussed. 1. Is radioligand binding a reliable tool for distinguishing receptor subtypes? It is argued that apparent differences in the occurrence and density of 5-HT2 receptors labeled with radioactive agonists and antagonists may be due to tissue preparation and assay conditions. There are no substantial indications for the existence of 5-HT2 receptor subtypes. 2. 5-HT2 receptor roles and activity in physiological conditions. Properties of 5-HT2 receptors (behavioral effects of agonists and antagonists, receptor regulation) were compared to dopamine D2 receptor properties. The hypothesis that 5-HT2 receptors probably receive little stimulation under normal physiologic conditions, but that they may only function under emergency or in pathologic conditions, could provide an explanation for various anomalous findings. 3. There are a plethora of 5-HT2 antagonists but few selective agents. Using receptor binding data of 48 compounds with nanomolar affinity for 5-HT2 receptors, but showing diverse receptor binding profiles and belonging to 20 different chemical classes, structure activity relationships for 5-HT2 and 5-HT1C receptor binding were analyzed. Structural rules appeared to be difficult to define; it was suggested that different binding possibilities on the receptor molecule may exist.

Chronic 5-HT2 receptor antagonist treatment alters 5-HT1A autoregulatory control of 5-HT release in rat brain in vivo

Rats treated chronically (14 days) with the 5-HT2 receptor antagonist ritanserin, show decreased 5-HT2 receptor numbers in the frontal cortex. The present experiments were designed to investigate the effects of acute and chronic ritanserin treatment on the autoregulatory control of the release of 5-HT and its metabolite 5-HIAA in vivo in rats using intracerebral dialysis. Neither acute nor chronic ritanserin treatment altered basal extracellular levels of 5-HT or 5-HIAA, suggesting that 5-HT2 receptors do not directly influence 5-HT release. In the control animals, systemic stimulation of somatodendritic 5-HT1A receptors with the 5-HT1A receptor agonist 8-OH-DPAT, inhibited the release of 5-HT presumably via inhibitory feedback autoregulation; an effect also seen in animals treated acutely with ritanserin. However, in the animals treated chronically with ritanserin, administration of 8-OH-DPAT produced an initial increase in extracellular 5-HT which declined gradually to the end of the experiment. These results suggest that chronic, but not acute, 5-HT2 receptor antagonist treatment attenuates the 5-HT1A receptor-mediated autoregulation of 5-HT release. The underlying mechanisms have yet to be ascertained.