Showing papers on "Catechol-O-methyl transferase published in 1995"

Kinetics of human soluble and membrane-bound catechol O-methyltransferase: a revised mechanism and description of the thermolabile variant of the enzyme.

Abstract: Human soluble (S) and membrane-bound (MB) catechol O-methyltransferase (COMT, EC 2.1.1.6) enzymes have been expressed at sufficiently high levels in Escherichia coli and in baculovirus-infected insect cells to allow kinetic characterization of the enzyme forms. The use of tight-binding inhibitors such as entacapone enabled the estimation of actual enzyme concentrations and, thereby, comparison of velocity parameters, substrate selectivity, and regioselectivity of the methylation of both enzyme forms. Kinetics of the methylation reaction of dopamine, (-)-noradrenaline, L-dopa, and 3,4-dihydroxybenzoic acid was studied in detail. Here, the catalytic number (Vmax) of S-COMT was somewhat higher than that of MB-COMT for all four substrates. The Km values varied considerably, depending on both substrate and enzyme form. S-COMT showed about 15 times higher Km values for catecholamines than MB-COMT. The distinctive difference between the enzyme forms was also the higher affinity of MB-COMT for the coenzyme S-adenosyl-L-methionine (AdoMet). The average dissociation constants Ks were 3.4 and 20.2 microM for MB-COMT and S-COMT, respectively. Comparison between the kinetic results and the atomic structure of S-COMT is presented, and a revised mechanism for the reaction cycle is discussed. Two recently published human COMT cDNA sequences differed in the position of S-COMT amino acid 108, the residue being either Val-108 [Lundstrom et al. (1991) DNA Cell. Biol. 10, 181-189] or Met-108 [Bertocci et al. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 1416-1420].(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetic-pharmacodynamic interaction between the COMT inhibitor tolcapone and single-dose levodopa.

Abstract: 1. Single oral doses of the catechol-O-methyltransferase (COMT) inhibitor tolcapone (10-800 mg) or placebo were administered simultaneously with a dose of levodopa/benserazide 100/25 mg to seven sequential groups of six healthy male subjects in a two-way crossover study. 2. Plasma concentrations of tolcapone, its metabolite 3-O-methyltolcapone, levodopa and 3-O-methyldopa (3-OMD) were determined in conjunction with COMT activity in erythrocytes. 3. The drug combination was well tolerated at all dose levels and there were no signs indicative of an increase in dopaminergic stimulation. 4. Tolcapone caused a rapid and reversible inhibition of COMT activity in erythrocytes in parallel with a dose-dependent decrease in the formation of 3-OMD. Tolcapone increased the area under the concentration-time curve and elimination half-life of levodopa. The maximum effects were obtained at a dose of about 200 mg when both parameters increased approximately twofold. The drug had no influence on the maximum concentration of levodopa. 5. Tolcapone was rapidly absorbed and eliminated with, on average, a tmax of 1.5 h and a t1/2 of 2.3 h. The drug showed dose-proportional pharmacokinetics, in contrast to 3-O-methyltolcapone whose formation was relatively decreased at higher doses. 6. Plasma concentrations of tolcapone correlated with inhibition of COMT activity in erythrocytes and suppression of 3-OMD levels, but not with changes in levodopa pharmacokinetics.

Integrated pharmacokinetics and pharmacodynamics of the novel catechol-O-methyltransferase inhibitor tolcapone during first administration to humans

Abstract: Objectives To assess the tolerability, pharmacokinetics and pharmacodynamics of single oral doses of the novel catechol-O-methyltransferase (COMT) inhibitor tolcapone in healthy volunteers. Methods In this double-blind, placebo-controlled, ascending-single-dose study, doses of 5 to 800 mg tolcapone were administered orally to eight sequential groups of six young healthy male volunteers. Adverse events, vital signs, and clinical laboratory variables were recorded. Pharmacokinetic parameters of tolcapone and its 3-O-methylmetabolite were determined. Pharmacodynamics were assessed by determination of COMT activity in erythrocytes. Results Tolcapone was well tolerated at all dose levels and did not exert a detectable influence on vital sign measurements. The drug was rapidly absorbed and showed dose-proportional pharmacokinetics. Its mean elimination half-life was 2.0 ± 0.8 hours (n = 42). Plasma levels of the 3-O-methylmetabolite of tolcapone were not proportional to dose, and its formation was delayed at higher doses. Its elimination half-life was 32 ± 7 hours (n = 29). Tolcapone caused a rapid and reversible inhibition of COMT activity in erythrocytes. At doses of 200 mg and higher, COMT activity was inhibited by more than 80%. The Pharmacokinetic-pharmacodynamic relationship could be described by an inhibitory Emax model and suggested that metabolites of tolcapone did not substantially contribute to its inhibitory activity. Conclusions The novel COMT inhibitor tolcapone was well tolerated at oral doses of 5 to 800 mg. Tolcapone concentration-dependently inhibited COMT activity in erythrocytes and exhibited dose-proportional kinetics. Further investigations into its applicability in the treatment of Parkinson's disease are warranted. Clinical Pharmacology & Therapeutics (1995) 57, 508–517; doi:

The role of monoamine oxidase and catechol O-methyltransferase in dopaminergic neurotransmission.

Abstract: The action of dopamine (DA) released in the synaptic cleft is mainly terminated by its reuptake and catabolism by the enzymes monoamine oxidase (MAO) and catechol O-methyltransferase (COMT) Preclinical data show that the reduction of the catabolism of DA elicited by MAO and COMT inhibitors leads to an enhancement of DA neurotransmission Moreover, there is evidence suggesting that MAO-B inhibition might protect DA neurons from oxidative stress Nevertheless, due to differences in enzyme localization and activity between man and rodents, results obtained in experimental animals might not reflect the actual situation in humans Today the availability of potent and selective MAO and COMT inhibitors makes it feasible for the clinician to test whether the blockade of catabolic enzymes would result in a symptomatic improvement in Parkinsonian patients, and whether MAO-B inhibition might additionally exert a neuroprotective effect

Effect of entacapone, a COMT inhibitor, on the pharmacokinetics and metabolism of levodopa after administration of controlled-release levodopa-carbidopa in volunteers.

Abstract: We studied the effect of entacapone, a catechol-O-methyltransferase (COMT) inhibitor, on the pharmacokinetics and metabolism of levodopa after administration of a controlled-release (CR) levodopa-carbidopa preparation (Sinemet CR) in an open, randomized trial in 12 healthy male volunteers. The inhibition of soluble COMT (S-COMT) in red blood cells (RBCs) was also measured. Single graded doses of entacapone (100-800 mg) were administered concomitant with a single oral dose of CR levodopa, or CR levodopa was given without entacapone (control treatment), at least 1 week apart. Plasma concentrations of levodopa, 3-O-methyldopa (3-OMD), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), carbidopa, and entacapone were determined for pharmacokinetic calculations. Entacapone decreased dose-dependently the activity of S-COMT in RBCs with a maximal inhibition of 66% after the highest dose (800 mg). Entacapone increased the area under the plasma concentration-time curve (AUC) of levodopa; the increase was highest (33%) after the 400-mg dose. Entacapone did not influence time to maximal concentration (Tmax) of levodopa. Entacapone was absorbed faster than levodopa from the CR preparation. The AUCs of 3-OMD and HVA decreased and that of DOPAC increased dose-dependently after entacapone, maximally by 69, 38, and 74%, respectively. Higher doses of entacapone (400 mg and 800 mg) decreased the AUC, but not Tmax of carbidopa. Over the dose range studied, entacapone was well tolerated. Entacapone is an effective COMT inhibitor. It improves the pharmacokinetic profile of levodopa when used in combination with a CR levodopa preparation, as it does with a standard levodopa preparation. The results justify further clinical studies with entacapone in combination with CR preparations of levodopa.

Acute administration of levodopa-benserazide and tolcapone, a COMT inhibitor, in Parkinson's disease

Abstract: Tolcapone, a catechol-O-methyltransferase inhibitor, can interfere with the metabolism of levodopa and dopamine and could prolong the motor effect induced by levodopa in parkinsonian patients. To test this hypothesis, we studied the motor effect induced by three acute administrations of a dose of le

Striatal [18F]fluorodopa utilization after COMT inhibition with entacapone studied with PET in advanced Parkinson's disease.

Abstract: The effect of peripheral catechol-O-methyltransferase (COMT) inhibition with entacapone on striatal uptake of 6-[18F]fluoro-L-dopa (FDOPA) was studied with PET both without and with entacapone in fifteen advanced parkinsonian patients and six healthy controls. Entacapone significantly enhanced the fraction of unmetabolized FDOPA in plasma from 16% to about 50% at 80 minutes after FDOPA injection in all subjects. The striatal to occipital ratios and the striatal FDOPA uptake, expressed as a modified decarboxylation coefficient (k3R0), was significantly increased in healthy controls, whereas in parkinsonian patients the increase was significant only in the caudate. On the other hand, the influx constant (Ki) decreased significantly in the caudate and putamen in parkinsonian patients; in healthy controls the Ki remained virtually unchanged. Effective peripheral COMT inhibition markedly increased the fraction of FDOPA in plasma and thus its availability in the brain for decarboxylation both in patients and control subjects. However, the change in striatal FDOPA uptake was modest in the advanced parkinsonian patients as compared to that in control subjects, because of the advanced disease, decreased storage capacity, or both.

Effects of tolcapone in Parkinson's patients taking L-dihydroxyphenylalanine/carbidopa and selegiline

Abstract: A double-blind, placebo-controlled, crossover trial of tolcapone (RO 40-7592), a potent reversible inhibitor of catechol-O-methyltransferase (COMT), was performed in 10 Parkinson's disease (PD) patients to determine single-dose safety and efficacy All subjects were chronically treated with stable doses of selegiline and L-dihydroxyphenylalanine (L-DOPA)/carbidopa Tolcapone was administered in four single ascending doses (50-800 mg) randomly paired with placebo Motor ratings were performed every 30 min for 6 h At higher doses (400 mg and 800 mg), tolcapone prolonged the antiparkinson response of L-DOPA Nausea was the most common adverse effect of the tolcapone-L-DOPA/carbidopa-selegiline combination Adverse cardiovascular effects were not seen The acute inhibition of amino acid decarboxylase, monoamine oxidase-B, and COMT is well tolerated and prolongs the L-DOPA response in PD patients Tolcapone may be a safe and useful adjunct to L-DOPA/carbidopa in PD patients taking selegiline

Acute effects of COMT inhibition on L-DOPA pharmacokinetics in patients treated with carbidopa and selegiline.

Abstract: The effects of acute catechol-O-methyltransferase (COMT) inhibition on L-DOPA pharmacokinetics were studied in 10 parkinsonian subjects on stable doses of L-DOPA/carbidopa and selegiline. Tolcapone, a reversible COMT inhibitor, was administered in four single ascending doses (50-800 mg) randomly paired with placebo. Serial plasma concentrations of L-DOPA and its metabolites were measured, and patient diaries and clinical ratings of dyskinesia were completed every 30 min for 6 h. Tolcapone increased the area under the curve of the plasma L-DOPA concentration versus time curve and decreased the accumulation of homovanillic acid. COMT inhibition increased "on" time and the duration of dyskinesia without affecting the maximal amplitude of dyskinesia. Tolcapone may be a useful adjunct to L-DOPA/carbidopa.

Metoprine, an inhibitor of histamine N-methyltransferase but not catechol-O-methyltransferase, suppresses feeding in sated and in food deprived rats.

Abstract: Metoprine is a histamine N-methyltransferase (HMT) inhibitor often used to elevate endogenous histamine (HA) levels when studying the role of brain HA. Since central histaminergic systems may be involved in the regulation of feeding, the effect of metoprine on food intake was studied in sated and in food deprived rats. The treatment caused a dose-dependent decrease in food intake in sated rats. It also suppressed deprivation-induced feeding. To clarify the specificity of the treatment, the effect of metoprine on another methylating enzyme, catechol-O-methyltransferase (COMT), was examined indirectly by examining the ratio of the non-methylated dopamine metabolite, dihydroxyphenylacetic acid (DOPAC) to that of its methylated product homovanillic acid (HVA). The dopamine metabolites did not change in a manner consistent with COMT inhibition, but instead a transient decrease in DOPAC levels was observed. However, the suppression of feeding is considered to be related to the metoprine-induced inhibition of brain HA catabolism and not with the changes in dopaminergic systems. Metoprine had no effect on brain concentration of serotonin (5-HT) or its metabolite 5-hydroxyindoleacetic acid (5-HIAA). The results provide further support for the role of brain HA in the control of feeding behavior.

Synergistic interactions between COMT-/MAO-inhibitors and L-Dopa in MPTP-treated mice

Abstract: Four experiments were performed to investigate the anti-akinesia effects of combining a sub-threshold dose (5mg/kg, s.c.) of L-Dopa with different doses and combinations of COMT and MAO inhibitors upon the hypokinesia observed in MPTP-treated mice. Ro 40-7592 (1 and 3 mg/kg, s.c.), a novel COMT inhibitor, 60 min before L-Dopa reinstated both locomotion and rearing during a 2-hr interval after L-Dopa in MPTP mice; control mice were unaffected. The combination of Ro 40-7592 (3 mg/kg, s.c.) and pargyline (5 mg/kg, s.c.), a MAO inhibitor, with L-Dopa produced increases in both the peak effect and duration of action indicating a distinct potentiation of the effects of Ro 40-7592 by pargyline. L-Deprenyl, a MAOB inhibitor, together with L-Dopa, restored locomotion and rearing behaviour at all three doses applied (1, 3 and 10 mg/kg, s.c.); in control mice, motor activity was stimulated at the higher doses (3 and 10 mg/kg, s.c.), independent of L-Dopa administration. Combining L-Deprenyl (3 mg/kg, s.c.) with Ro 40-7592 (3 mg/kg, s.c.) one hr before L-Dopa to MPTP mice potentiated the restorative effects of each compound by itself, although no increase in peak effect was obtained. In the control mice, L-Deprenyl plus Ro 40-7592 or L-Deprenyl, by itself, stimulated motor activity following injection of L-Dopa. Marked dopamine (DA) depletions in the striatum of MPTP-treated mice were evident. The present results demonstrate that the effects of the COMT/MAO inhibitors in combination, and in conjunction with L-Dopa (at a dose that was without effect by itself), were well in excess of a summation of their individual effects. It was concluded therefore that a synergism of the restorative, anti-akinesic action of these compounds in MPTP-treated mice could offer a broader therapeutic spectrum in the treatment of Parkinson's disorder.

The effect of entacapone on the disposition and hemodynamic effects of intravenous isoproterenol and epinephrine

Abstract: Background Entacapone is a potent, selective catechol-O-methyltransferase (COMT) inhibitor. Entacapone could potentiate the hemodynamic effects of exogenously administered catecholamines, which are substrates of the COMT enzyme. Design and methods Originally, the study was to follow a placebo-controlled, randomized crossover design. Because of two cases of ventricular arrhythmia, a decision was made to terminate the study before its completion. Six subjects went through the isoproterenol and epinephrine infusions while taking placebo and five other subjects while taking entacapone. The actual design was thus one with two parallel groups with random allocation and double-blind drug administration. The subjects were given either a single dose of 400 mg entacapone or placebo 30 minutes before the start of isoproterenol or epinephrine infusions. Four dosages of epinephrine (1.5, 3, 6, or 12 μg/min) and isoproterenol (0.5, 1, 1.5, or 2 μg/min) were infused (5 minutes for each level). Heart rate and blood pressure were measured and ECG was monitored. The concentrations of isoproterenol and epinephrine in plasma were determined by HPLC. Results The maximal increase in heart rate during isoproterenol infusion after entacapone administration (40 ± 11 beats/min, mean ± SD) was statistically greater (p = 0.0496) than after placebo administration (27 ± 7 beats/min). The increase in heart rate during epinephrine infusion was 25 ± 13 beats/min after entacapone administration and 14 ± 9 beats/min after placebo administration (p = 0.127). There were no statistically significant differences between entacapone and placebo in blood pressure or in plasma concentrations of isoproterenol and epinephrine. Conclusion We conclude that entacapone may potentiate the chronotropic and arrhythmogenic effects of exogenously administered isoproterenol and epinephrine. Clinical Pharmacology & Therapeutics (1995) 58, 221–227; doi:

Epinephrine is metabolized by the spinal meninges of monkeys and pigs.

Abstract: BackgroundEpinephrine commonly is added to epidural opioids and local anesthetics, however, little is known about the fate of epidurally administered epinephrine. Studies have identified the epinephrine metabolizing enzyme, catechol-O-methyl transferase (COMT), in the cranial meninges of several spe

Evidence for saturation of catechol-0-methyltransferase by low concentrations of noradrenaline in perfused lungs of rats

Abstract: Previous studies on the pulmonary removal and metabolism of catecholamines in rat lungs have shown that, when the lungs are perfused with a low concentration (1 nmol/1) of noradrenaline, the amine is metabolized by catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO), but is predominantly O-methylated, and the activities of COMT and MAO are 0.357 min-1 and 0.186 min-1, respectively. The aim of the present study was to examine the changes in the metabolic profile of noradrenaline in rat lungs over a range of concentrations, and to examine the kinetics of the pulmonary O-methylation of noradrenaline and adrenaline. In isolated lungs perfused with 3H-noradrenaline, there was a progressive decrease in the proportion of O-methylated metabolites and a corresponding increase in the proportion of deaminated metabolites, as the noradrenaline concentration in the perfusion solution was increased from 1 to 10 to 100 to 1000 nmol/l. Experiments designed to determine the rate of uptake of noradrenaline in lungs perfused with 1 nmol/l 3H-noradrenaline, under conditions of MAO inhibited, COMT inhibited and COMT and MAO inhibited, showed that the results were compatible with co-existence of COMT and MAO in the pulmonary endothelial cells. Hence, it appeared that the changing metabolic profile with amine concentration in the previous series of experiments was not due to saturation of noradrenaline uptake into cells that contained COMT but not MAO. Further experiments to examine the kinetics of O-methylation of noradrenaline and adrenaline (MAO inhibited) showed that the O-methylation of these amines in the lungs was predominantly saturable, with half-saturation occurring at concentrations (9.8 nmol/I and 19.4 nmol/l, respectively) that were two orders of magnitude lower than those required to half-saturate uptake1 of the amines. Saturation of O-methylation by these low concentrations of noradrenaline (1) provides the explanation for the change in the metabolic profile of noradrenaline described above and (ii) appears to occur because Vmax uptake ≫ Vmax COMT for the metabolizing system consisting of non-neuronal uptake1 + COMT in the lungs, as has been described previously for the system consisting of uptake2 + COMT in extraneuronal sites in rat heart. The results show that the metabolic profile of catecholamines in the pulmonary circulation will reflect that occurring at physiological levels only if studies are carried out with very low amine concentrations.