Inhibition of human erythrocyte and gastroduodenal catechol-O-methyltransferase activity by nitecapone

Summary The effect of increasing single oral doses of the novel catechol-O-methyltransferase (COMT) inhibitor, nitecapone, on enzyme activity in red cells (RBC) and gastroduodenal COMT activity has been studied in healthy male volunteers.A dose-dependent decrease in RBC COMT activity was seen in all cases after 1 to 150 mg of the drug. The highest dose of 300 mg did not produce much more inhibition of COMT than 150 mg. The inhibition was not complete; at the highest doses the COMT activity was reduced by 50–60%. The effect and the duration of the inhibition in RBC COMT was strongly correlated with plasma nitecapone concentrations in the dose range up to 150 mg. RBC COMT activity recovered fully in 4 h after medication.Gastric mucosal COMT activity was several-fold higher than that in RBCs. It was also dose-dependently inhibited at the two doses (25 and 100 mg) studied. The inhibition of gastric and duodenal COMT was greater than that in RBCs. This also indicates that nitecapone is locally active in the gastroduodenal tract.The results confirm nitecapone as a potent COMT inhibitor in human tissues. New COMT inhibitors may provide a valuable approach to the treatment of Parkinson's disease in combination with L-dopa and dopa decarboxylase inhibitor therapy.     

Effect of moclobemide, a new reversible monoamine oxidase inhibitor, on absorption and pressor effect of tyramine

We determined in healthy subjects the pressor effect and the plasma level of free tyramine in response to intravenous and oral tyramine doses before and after therapeutic doses (3 X 100 mg/day) of moclobemide, a new reversible, preferential type A monoamine oxidase (MAO) inhibitor. In fasting subjects moclobemide increased the pressor effect of intravenously and orally administered tyramine; the tyramine dose-pressor curve was shifted to the left by factors of 2.4 and 4.1, respectively. No increase in systolic blood pressure occurred at free plasma tyramine concentrations lower than 70 ng/ml before, and 20 ng/ml after, moclobemide. Peak plasma tyramine concentrations increased dose-dependently after oral tyramine; after moclobemide similar peak plasma concentrations of tyramine were obtained with 2.6 times smaller doses of tyramine. Thus, the potentiation by moclobemide of the pressor effect of oral tyramine appears to be due to inhibition of tyramine first-pass metabolism, as well as to inhibition of tyramine catabolism by MAO within adrenergic nerve terminals. The peak concentrations of free tyramine in plasma and the concomitant increase of systolic blood pressure were significantly (p less than 0.01) smaller when tyramine was administered with a meal (before or after moclobemide) than when given with tap water. We conclude that at doses of 3 X 100 mg/day moclobemide induces only a mild potentiation of the pressor effect of tyramine. This potentiation is virtually absent under natural conditions when tyramine is given with a meal.     

The effects of the COMT inhibitor entacapone on haemodynamics and peripheral catecholamine metabolism during exercise

1. Catechol-O-methyltransferase (COMT) inhibition might be assumed to potentiate the effects of circulating catecholamines, particularly under conditions of enhanced catecholamine release. 2. The purpose of the present study was to establish whether the novel COMT inhibitor, entacapone, changes haemodynamic responses and catecholamine metabolism during exercise. 3. Entacapone was given orally to 12 healthy male subjects (age 23-30 years) in increasing single doses from 0 mg (control day) to 200 mg. A submaximal exercise test was performed on a bicycle ergometer, and blood pressure, heart rate and ECG were recorded. The concentrations of noradrenaline, adrenaline, DHPG (3,4-dihydroxyphenylglycol), MHPG (3-methoxy-4-hydroxyphenyl-glycol) and, DOPAC (3,4-dihydroxyphenylacetic acid) in plasma were determined. 4. Entacapone did not influence haemodynamics or ECG at rest or during exercise. 5. Entacapone did not influence plasma catecholamine levels, either at rest or during exercise. However, it altered the metabolic profile of catecholamines, which was shown by increases in the plasma concentrations of the monoamine oxidase-dependent metabolites DHPG (by up to 100%) and DOPAC (by up to 53%), and by a decrease of the COMT-dependent metabolite MHPG (by up to 29%).     

Lithium does not alter the renal response to a pressor dose of tyramine in man.

Renal clearance of lithium has been used as a marker of proximal tubular function in man. Recently, lithium pre-treatment has been shown to interfere with the natriuretic actions of some natriuretic agents in man. We have therefore investigated the effects of oral lithium carbonate (500 mg) on the natriuretic response to a pressor dose of tyramine (15 micrograms kg-1 min-1) in six normal volunteers. Lithium had no effect on baseline sodium excretion, nor did it affect the tyramine-induced increase in blood pressure and subsequent natriuresis. These results show that oral lithium carbonate (500 mg) does not appear to interfere with the pressure natriuretic response to tyramine in man.     

Potentiation of the pressor effect of oral and intravenous tyramine during administration of the selective MAO-A inhibitor moclobemide in healthy volunteers

The interaction between tyramine and the new short-acting and reversible mono amine oxidase inhibitor moclobemide was investigated in a double-blind placebo-controlled study in six healthy volunteers. There were two consecutive study periods of 8 days during which the subjects received moclobemide three x 200 mg daily or placebo. On day 5 of each study period changes in systolic blood pressure (SBP) were determined after incremental intravenous bolus doses of tyramine and on days 6, 7 and 8 changes in SBP were determined after oral tyramine (100, 200 and 300 mg, respectively). Oral tyramine was administered together with a standard breakfast, before which moclobemide had been given. On days 5- 8 blood was taken for determination of blood drug levels. On days 6-8 blood samples were taken before and at 15, 30 and 45 min after tyramine administration for determination of plasma tyramine and plasma norepinephrine concentrations. When SBP had increased by approximately 30 mmHg no further doses of either intravenous or oral tyramine were given. Moclobemide was well tolerated by all subjects. Plasma trough levels of moclobemide were within the therapeutic range. The tyramine induced increases in SBP were greater during moclobemide than during placebo. After intravenous tyramine the dose-response curve for SBP was shifted to the right by a factor of approximately 3. When compared to placebo the pressor response to 100 mg tyramine orally was not significantly different, but the pressor response to the other two doses was enhanced during moclobemide.     

(-)-Deprenyl inhibits tyramine-induced noradrenaline release, but not tyramine-induced dopamine release or potassium-induced noradrenaline release, from rat brain synaptosomes.

The effect of (-)-deprenyl (selegiline), a therapeutic agent for Parkinson's disease, on the tyramine-induced release of catecholamine from rat brain synaptosomes was studied using a superfusion system. Tyramine (10(-7) to 10(-5)M) enhanced the release of [3H]noradrenaline (NA) and [3H]dopamine (DA) from forebrain and striatal synaptosomes in a dose-dependent manner. (-)-Deprenyl (5x10(-5)M) had no effect on spontaneous catecholamine release, suggesting that it has no tyramine-like catecholamine releasing effect. Pretreatment with (-)- or (+)-deprenyl (5x10(-5)M) significantly prevented the tyramine (10(-6)M)-induced NA release, but not DA release. The inhibitory action of (-)-deprenyl was not observed on potassium (15mM)-induced NA release. (-)-Desmethyldeprenyl (5x10(-5)M), a metabolite of (-)-deprenyl, and a monoamine oxidase-A (MAO-A) inhibitor, clorgyline (5x10(-5)M), failed to block the tyramine-induced NA and DA release. Although (+)-deprenyl, a potent DA uptake inhibitor, did not inhibit tyramine-induced DA release, a catecholamine uptake inhibitor nomifensine (5x10(-5)M) did. In summary, (-)-deprenyl at a dose inhibiting tyramine-induced NA release did not have any effect on tyramine-induced DA release or potassium-induced NA release.     

Assessment of the serotonin and norepinephrine reuptake blocking properties of duloxetine in healthy subjects.

Duloxetine is a dual inhibitor of norepinephrine (NE) and serotonin (5-HT) uptake. Initial trials conducted in depressed patients using regimens of 20 mg/day or less did not convincingly demonstrate its efficacy as an antidepressant. The aim of this study was to assess the effects of duloxetine on the 5-HT and NE reuptake processes in healthy human volunteers. Twenty-seven healthy young males without a history of psychiatric disorder were randomly assigned to four groups, each group receiving one of the following daily drug regimens: placebo, clomipramine (a potent 5-HT/NE reuptake blocker) 100 mg/day, duloxetine 20 mg/day, or duloxetine 60 mg/day. In order to assess the NE reuptake process, the pressor response to intravenous tyramine (4 and 6 mg) was measured. Determination of the whole blood 5-HT content was used to evaluate the 5-HT reuptake blockade. These measurements were performed at baseline and repeated after 7 and 14 days of drug intake. Both duloxetine, at doses of 20 to 60 mg/day, and clomipramine significantly interfered with the 5-HT reuptake process, as demonstrated by marked decreases in blood 5-HT concentrations. However, the same doses of duloxetine, unlike clomipramine, failed to impede the usual increase in blood pressure that follows a tyramine intravenous infusion, indicating that clomipramine but not duloxetine blocked NE reuptake. At doses tested in a population of healthy volunteers, duloxetine acted as a selective 5-HT reuptake inhibitor, having no clear effect on the NE reuptake process. Nevertheless, given that the highest dose of duloxetine increased supine systolic blood pressure, it is possible that it represents the threshold regimen for NE reuptake inhibition.     

Influence of food on the tyramine pressor effect during chronic moclobemide treatment of healthy volunteers

Summary An open study was carried out to examine the effect of moclobemide, a new antidepressant reversible inhibitor of MAO-A, on the pressor response induced by oral tyramine added to meals of different lipid and protein composition, and to correlate the blood pressure increase in the tyramine test with that obtained during an exercise test.Eight healthy volunteers of both sexes participated in the study. A tyramine sensitivity and an exercise test were performed beforehand. Subjects were included if, under fasting condition, their systolic blood pressure (SBP) increased by more than 30 mmHg after administration of 400 or 600 mg tyramine. Exercise tests were performed to determine the grade of effort that corresponded to a rise in SBP of 30 mmHg.Subjects received moclobemide 600 mg/d. Starting on Day 7, each subject consumed a standardized meal (52 g lipids, 43 g proteins, 86 g carbohydrates) just before taking moclobemide. Tyramine was added to these meals in daily increasing doses of 50, 100, 150 ... mg until an increase in SBP > 30 mmHg was obtained. On moclobemide treatment, an average dose of 250 mg tyramine (range 150-400 mg) increased SBP by 36.6 mmHg. The time to reach peak SBP was longer (175 min) than in the fasting condition before the trial (40.6 min).The administration of the same dose of tyramine both during a protein-rich (75 g proteins, 85 g lipids, 90 g carbohydrates) and lipid-rich (110 g lipids, 55 g proteins, 100 g carbohydrates) meal significantly reduced the average increase in SBP to 21 mmHg, but did not significantly modify the time of appearance of the peak SBPIn the exercise test, an increase in SBP of 30 mmHg was produced by the low load of about 100 W. During moclobemide treatment, oral doses of tyramine considerably larger than the amounts present in normal meals did not increase SBP by more than the effort exerted during every day life. Concomitant administration of a large quantity of lipids significantly reduced the pressor response.     

The effects of the COMT inhibitor nitecapone for one week on exercise haemodynamics and catecholamine disposition

Summary We have studied resting and exercise haemodynamics and catecholamine disposition after catechol-O-methyl-transferase (COMT) inhibition with nitecapone 100 mg t. d. s. for 7 days in 15 healthy men (aged 21 to 28 y) in a placebo-controlled, cross-over study.Nitecapone did not alter resting or exercise heart rates, blood pressure, or plasma catecholamine concentrations, but it altered the metabolic profile of endogenous catecholamines, as shown by: (1) a fall in the concentrations of the COMT-dependent metabolite 3-methoxy-4-hydroxy-phenylglycol (MHPG) by 22% (P>0.01), (2) increases in plasma concentrations of the monoamine oxidase-dependent metabolites 3,4-dihydroxyphenylglycol (DHPG) by up to 90% (P<0.001) and 3,4-dihydroxyphenylacetic acid (DOPAC) by 67% (P<0.05), (3) a three-fold increase in the concentration of circulating conjugated adrenaline (P<0.001).     

Beta-blockade antagonism of tyramine-induced rise in blood pressure

Summary The effect -adrenoceptor blockade on the pressor response to tyramine has been investigated in 6 healthy volunteers, each submitted to an i.v. tyramine pressor test before and after 7 days of propranolol 40 mg b.d. or indenolol 60 mg o.d. Tyramine was given as i.v. boluses of 1–6 mg, alternating with saline, in a randomized, single blind fashion.Prior to treatment tyramine caused a temporary, dose-dependent increase in systolic and diastolic blood pressure, whilst the heart rate remained unaffected. Both propranolol and indenolol reduced the pressor response to tyramine, as shown by a significant increase in ED₁₅, i.e. the dose of tyramine required to increase systolic blood pressure by 15%.     

The effect of angiotensin II on haemodynamic and plasma noradrenaline responses to tyramine infusion in man

Summary Six normal volunteers were studied on four separate occasions. On each occasion they received two concomitant infusions which were either placebo/placebo, placebo/tyramine, angiotensin II/placebo or angiotensin II/tyramine. Angiotensin II infusion was given at a constant rate of 2ng/kg/min whereas the tyramine infusion consisted of 10 min increments at 1.25, 2.5, 3.75, 5, 7.5 and 10 g·kg^–1·min^–1.Tyramine infusion caused a dose dependent increase in systolic blood pressure with increases in diastolic blood pressure and plasma noradrenaline only at the highest doses. These changes were not affected by concomitant angiotensin infusion.We have therefore found no evidence to support the enhancement of haemodynamic or plasma noradrenaline responses to tyramine infusion by low dose infusion of angiotensin II in man.     

Adrenoceptors in intracerebral resistance vessels.

1. The effects of tyramine and isoprenaline on hypothalamic blood flow (HBF) were measured in conscious rabbits. 2. Injections of small doses of tyramine caused an increase in HBF while larger doses caused a decrease in HBF. 3. Isoprenaline injections also produced an increase in HBF. 4. The vasodilatation induced by isoprenaline and the small dose of tyramine was blocked by propranolol. 5. The vasoconstriction induced by the larger doses of tyramine was abolished by phenoxybenzamine. 6. Chemical sympathectomy of the hypothalamus with 6-hydroxydopamine and depletion of biogenic amines by reserpine also abolished tyramine-induced vasocoonstriction. 7. These results suggest the presence of alpha- and beta-adrenoceptors in cerebral resistance vessels, and that these receptors may be activated by released (endogenous) noradrenaline.     

Effects of LM 5008, a selective inhibitor of 5-hydroxytryptamine uptake, on blood pressure and responses to sympathomimetic amines

LM 5008 (4-[2-(3-indolyl)ethyl]piperidine) (10, 20 and 50 mg kg-1) had no significant effect on pressor responses to noradrenaline or tyramine in rats anaesthetized with urethane. Desmethylimipramine (1 mg kg-1) blocked the response to tyramine but chlorimipramine (5 mg kg-1) had no significant effect on responses to noradrenaline or tyramine. In the rabbit, anaesthetized with chloralose, LM 5008 (5 mg kg-1) had no effect on pressor responses to noradrenaline, tyramine or angiotensin II, while desmethylimipramine (0.25 mg kg-1) inhibited responses to tyramine and potentiated those to noradrenaline. LM 5008 (10 mg kg-1) had no effect on resting blood pressure of conscious normotensive or DOCA-saline hypertensive rats. Tranylcypromine (5 mg kg-1) produced a fall in blood pressure in conscious normotensive and in DOCA hypertensive rats. Treatment with a combination of LM 5008 (10 mg kg-1) and tranylcypromine (5 mg kg-1) resulted in the appearance of a behavioural hyperactivity syndrome, but blood pressure was not different from that of animals treated with tranylcypromine alone. These results further demonstrate the selectivity of LM 5008 for 5-hydroxytryptamine as opposed to catecholamine uptake.     

Differential effects of D 600 on release of catecholamines by acetylcholine, histamine, tyramine and by cyclic AMP from canine adrenal medulla.

The isolated canine adrenal glands were perfused retrogradely with Locke's solution, and the catecholamine contents of the effluents were measured by the trihydroxyindole-fluorimetric method. Stimulation of the glands by acetylcholine, histamine, tyramine and cyclic AMP caused an increase in release of catecholamines from the glands. Introduction of D 600 of the perfusion medium reduced release of catecholamines in response to acetylcholine, and this reduction was overcome by raising calcium ion concentrations of the perfusion medium. Similarly, D 600 reduced release of catecholamines in response to histamine. The release of catecholamines evoked by tyramine was also inhibited by D 600, although to a lesser degree than the release by acetylcholine. In contrast, D 600 was entirely ineffective on the catecholamine release in response to cyclic AMP. D 600 had no effect on the spontaneous catecholamine output. From these results it was concluded that release of catecholamines from adrenal chromaffin cells by acetylcholine and histamine, and by tyramine in part requires the entry of calcium ions across the cell membane, whereas that by cyclic AMP does not.     

Pressor response to oral tyramine during co-administration with safinamide in healthy volunteers

The aim of this study was to evaluate the pressor response to oral tyramine during repeated administration of oral safinamide in healthy volunteers. Twelve females and eight males aged 52.7 ± 4.9 years entered the study. An oral tyramine screening test was conducted to select subjects sensitive to the tyramine pressor effect on systolic blood pressure (SBP) in the dose range of 200–400 mg. Safinamide 300 mg was then administered once daily under fasting conditions. Starting on day 5 (safinamide pharmacokinetic steady state), single ascending doses of tyramine were co-administered daily: 50, 100 and 200 mg were administered on days 5, 6 and 7, respectively. Vital parameters were monitored by telemetry. No SBP increase ≥30 mmHg over baseline was observed when tyramine was co-administered with safinamide. Less than one third of the 400 mg responders reported SBP increases between 22 and 27 mmHg, which were below the threshold of 30 mmHg over baseline. SBP increases, as well as time interval to pressor response measured after co-treatment with safinamide and tyramine 200 mg, were not significantly different from those measured after administration of oral tyramine 200 mg alone. Safinamide 300 mg, administered o.d. under fasting conditions, does not change the tyramine pressor response as evaluated at steady state after 6–7 days of treatment as compared with the effect of tyramine administered alone. Safinamide, which inhibits monoamine oxidase (MAO)-B, does not affect oral tyramine metabolism mediated mostly by the intestinal MAO-A.     

The antihypertensive effect of a selective central muscarinic cholinergic antagonist: N-(4-diethylamino-2-butynyl)-succinimide

N-(4-Diethylamino-2-butynyl)-succinimide (DKJ-21), a cholinergic antagonist selective for muscarinic receptors in the central nervous system, has an antihypertensive effect in conscious spontaneously hypertensive rats (SHR) but not in normotensive Wistar-Kyoto (WKY) control rats. Intravenous doses over a range of 3.1 to 25.0 mg/kg produced a dose-dependent decrease in systolic blood pressure. This effect was still apparent 24 hr after drug administration, but blood pressure returned to predrug levels by 48 hr after injection of DKJ-21. In a dose–response study, the maximum antihypertensive response in a group receiving 25.0 mg/kg was 43 ± 8 mm Hg. Marked variability of the maximum response was observed in all age groups and with all doses. Decreases in blood pressure up to 75 mm Hg were obtained in individual animals. The magnitude of the antihypertensive effect is not age related since 10-, 15-, 22-, and 36-week-old SHR responded to the same degree after injection of 25 mg/kg DKJ-21. Smaller doses (50 μg) of DKJ-21 decreased blood pressure when administered into the lateral cerebral ventricle of SHR. Vascular responses to norepinephrine, acetylcholine, dimethylphenylpiperazinium, angiotensin I, and tyramine were not inhibited by i.v. injection of DKJ-21; however, the centrally mediated pressor response to physostigmine was reduced by 60%.     

Effect of a novel monoamine-oxidase inhibitor, moclobemide on the sensitivity to intravenous tyramine and norepinephrine in humans

This study compared the effects of moclobemide (Ro11-1163), a selective and reversible inhibitor of monoamine-oxidase (MAOI) type A and phenelzine, an irreversible non-selective MAOI, on the pressor responses to IV tyramine and norepinephrine. Because of the reversibility of this inhibition, the pressor effect of tyramine was expected to be minimal. Twelve healthy men participated in this randomized double-blind, placebo-controlled, crossover study. Volunteers began with oral treatment of moclobemide (100 mg TID) or phenelzine (15 mg TID) for 1 week followed by placebo treatment for 2 weeks and then moclobemide or phenelzine treatment for another week. The tyramine and norepinephrine challenge tests were conducted at baseline and then at weekly intervals, for a total of five challenges. The average tyramine dose that was required to increase systolic blood pressure by 25 mm Hg (PD25) was 1.6 +/- 0.2 mg after moclobemide treatment, which was lower (P less than .01) than the baseline value of 3.6 +/- 0.7 mg and that after phenelzine (3.0 +/- 0.5 mg) treatment. Moclobemide did not influence norepinephrine sensitivity. In conclusion, moclobemide mildly decreased the sensitivity to IV tyramine as compared with placebo and phenelzine.     

Blood pressure changes and sympathetic function in rats given cyclohexylamine by intravenous infusion.

Intravenous infusion of cyclohexylamine (30-120 mg/kg) caused a dose-dependent increase in blood pressure in urethane-anesthetized rats. The increase in blood pressure was inversely related to the duration of the infusion. The blood pressure returned to baseline values (+/- 5 mm Hg) within 60 min of the end of the infusion after doses of 30 and 60 mg/kg were administered over 20 and 40 min. The plasma concentrations of cyclohexylamine were related linearly to the administered dose and decreased only 27% between the end of the infusion and 60 min later. The concentration-effect relationship showed clockwise hysteresis, indicative of tachyphylaxis, as has been reported in humans (Eichelbaum et al., 1974). Administration of bolus doses of tyramine at the end of the infusion or 60 min later demonstrated the presence of an indirect sympathomimetic response, although this was attenuated to a greater extent by high doses and more rapid infusions of cyclohexylamine. An almost complete loss of response to tyramine was found only in animals given 120 mg/kg over 40 min. The presence of a tyramine response 60 min after the infusion of 60 mg/kg occurred when there was an essentially complete reversal of the hypertensive effect of cyclohexylamine. These data indicate that the hypertensive effect of the indirectly acting sympathomimetic amine cyclohexylamine occurs primarily during rapid increases in plasma concentrations. Tachyphylaxis develops rapidly after the cessation of the infusion which is probably due largely to reuptake of released noradrenaline at low doses and depletion of releasable noradrenaline at high doses.     

Nitecapone and selegiline as effective adjuncts to L-DOPA in reserpine-induced catatonia in mice

Reserpine-induced catatonia is a widely accepted animal model of Parkinson's disease. In the present study, reserpine (5 mg/kg i.p.) and alpha-methylpara-tyrosine (AMPT) (200 mg/kg i.p.) induced catatonia in mice 20 h and 1 h before the experiment, respectively, as assessed using the rota-rod and bar tests after reserpine treatment. There was a significant decrease in fall-off time in the rota-rod test and a significant increase in time spent on the bar in the bar test as compared to the untreated control mice. Combination therapy with L-DOPA (100 mg/kg i.p.) and carbidopa (10 mg/kg i.p.) was less effective in reversing catatonia as compared to higher doses of L-DOPA (200 mg/kg i.p.) and carbidopa (20 mg/kg i.p.), which showed intense hyperactivity in reserpinized mice. Pretreatment with nitecapone (30 mg/kg i.p.), a COMT inhibitor, or selegiline (10 mg/kg i.p.), a MAO-B inhibitor potentiated the motor stimulant actions of subthreshold doses of the L-DOPA (100 mg/kg i.p.) and carbidopa (10 mg/kg i.p.) combination. Amantadine (40 mg/kg i.p.), but not bromocriptine, potentiated the effects of L-DOPA treatment. The NMDA antagonistic action of amantadine may have beneficial effects. It is concluded that COMT and MAO-B enzymes play an important role in the metabolism of dopamine and administration of a COMT or MAO-B inhibitor may prove to be a better adjunct to L-DOPA therapy than a dopamine receptor agonist in Parkinson's disease.