L-dopa induces dyskinesia in normal monkeys: behavioural and pharmacokinetic observations.

RATIONALE: L-Dopa induces dyskinesias during the treatment of Parkinson's disease and also in primates with nigrostriatal lesions produced by MPTP, but it is claimed that L-dopa does not provoke dyskinesia in humans or monkeys with an intact or mildly damaged nigrostriatal system. OBJECTIVES: This study assessed the behavioural and pharmacokinetic effects of chronic oral administration of L-dopa plus carbidopa alone, or with co-administration of the peripheral COMT inhibitor entacapone, to normal macaque monkeys. Repeated high dose L-dopa administration was shown to induce marked dyskinesias in monkeys with an intact nigrostriatal system, and the threshold for dyskinesia expression was increased by peripheral catechol-O-methyltransferase inhibition with entacapone. METHODS: Six groups of normal macaque monkeys (n=8 per group; Macaca fascicularis) were treated with L-dopa (20, 40 or 80 mg/kg) plus carbidopa (5, 10 or 20 mg/kg) with or without the catechol-O-methyltransferase inhibitor entacapone (20, 40 or 80 mg/kg), or with entacapone alone (80 mg/kg), by oral administration once daily for 13 weeks. RESULTS: Eleven of 16 animals receiving high dose L-dopa (80 mg/kg plus carbidopa 20 mg/kg PO with or without entacapone 80 mg/kg PO for 13 weeks) gradually developed reproducible and idiosyncratic combinations of chorea, athetosis and dystonia maximal at 60-100 min after L-dopa administration, which progressively intensified over the course of the study. The dyskinesias observed were similar in type and distribution to L-dopa-induced dyskinesia observed in patients with Parkinson's disease and in MPTP-treated primates. The occurrence of dyskinesia correlated with plasma concentrations of L-dopa, with animals displaying the most severe dyskinesias having significantly higher plasma concentrations of L-dopa one hour after dosing than animals with mild or moderate dyskinesia or no dyskinesia. Co-administration of entacapone with L-dopa plus carbidopa significantly lowered peak plasma concentrations of L-dopa and this was reflected by a decrease in the severity of dyskinesias, with only one animal receiving entacapone and high dose L-dopa plus carbidopa showing severe dyskinesia, while four receiving high dose L-dopa plus carbidopa alone did so. CONCLUSIONS: These results show for the first time that chronic oral L-dopa administration can provoke dyskinesias in primates independently of nigrostriatal damage, and that this effect is dose related.

Comparative toxicological study on the hepatic safety of entacapone and tolcapone in the rat.

Entacapone and tolcapone are novel COMT (catechol-O-methyltransferase) inhibitors indicated for the adjunctive treatment of Parkinson's disease (PD) in combination with levodopa. The marketing authorisation of tolcapone was suspended in the European Union (EU) in 1998 mainly due to severe abnormal hepatic reactions. This fact raised concern about the safety of COMT inhibitors in the treatment of parkinsonian patients. In order to investigate whether these COMT inhibitors exhibit different effects on the liver comparative toxicological studies were performed in the rat. Short term toxicological studies in rats at high oral doses of entacapone and tolcapone (200, 400 or 600mg/kg daily) were carried out. Tolcapone (400 mg/kg/day or 600 mg/kg/day) increased mortality after only one week treatment and induced signs of toxicity such as a rise in body temperature, stimulation of respiration and rapid onset of rigor mortis after death. Entacapone did not show any adverse effects at the tested dose levels. In the histopathological examination liver cell necrosis was observed in the tolcapone (400 and 600mg/kg/day) treated rats, but it revealed no treatment related signs of toxicity in entacapone-treated rats. We conclude that the toxicological profile of the two COMT inhibitors, entacapone and tolcapone, differ from each other, tolcapone--unlike entacapone--showed hepatotoxicity.

Conditioned place preference induced by a combination of L-dopa and a COMT inhibitor, entacapone, in rats.

The interaction of dopamine (DA) precursor L-dopa and catechol-O-methyltransferase (COMT) inhibitor, entacapone, was examined in rats using conditioned place preference (CPP) paradigm to assess reinforcement, and by measuring DA metabolism in the striatum and the limbic forebrain. Neither L-dopa (100 mg/kg i.p.) nor entacapone (30 mg/kg i.p.) alone induced CPP, but in combination they induced significant CPP. Entacapone alone had no effect on limbic or striatal DA concentrations, while it reduced the concentrations of the COMT products 3-methoxytyramine (3-MT), a metabolite reflecting DA release, and homovanillic acid (HVA) in both brain areas. L-dopa elevated limbic but not striatal 3-MT. L-dopa also slightly elevated limbic DA but had no effect on striatal DA concentration. L-Dopa-induced increase of 3-MT was attenuated by entacapone. Our results show for the first time that L-dopa is able to produce CPP in intact animals. This effect may be related to the findings that L-dopa increases synaptic DA concentrations in the limbic areas, and entacapone may enhance this elevation as it prevents the synaptic metabolism of DA.

The role of cAMP- and cGMP-dependent protein kinases in the cardiac actions of the new calcium sensitizer, levosimendan.

OBJECTIVE: The role of phosphodiesterase III inhibition and calcium sensitization in the cardiac actions of levosimendan, (R)-[[4-(1,4,5, 6-tetrahydro-4-methyl-6-oxo-3-pyridazinyl)phenyl]hydrazono]propane dinitrile, was studied. METHODS: Various heart preparations were used to investigate positive inotropy, chromotropy, coronary flow and calcium sensitivity of contractile proteins. The cAMP- and cGMP-dependent protein kinases (PKA and PKG) were inhibited by KT5720 and KT5823, respectively. Furthermore, the synthesis of cAMP was stimulated by forskolin and increased phosphorylation of troponin I was induced by isoprenaline. RESULTS: In Langendorff guinea-pig heart, levosimendan (0.01-1 microM) and milrinone (0.1-10 microM) increased the left ventricular systolic peak pressure almost to the same extent. In the presence of KT5720 (1 microM) milrinone was devoid of positive inotropic activity. In contrast, KT5720 did not antagonize the inotropic effect of levosimendan at < or = 0.03 microM (-up to the EC50 of levosimendan). The effects of levosimendan and milrinone on heart rate and coronary flow were not affected by KT5720. The PKG inhibitor, KT5823 (1 microM), on the other hand, potentiated the levosimendan-induced increase in coronary flow while it had no effect on the increase induced by milrinone. The mechanical parameters were not affected by KT5823. In the papillary muscle, the positive inotropic effect of milrinone but not that of levosimendan was potentiated by forskolin (0.1 microM). In contrast to milrinone, the positive inotropy by levosimendan was decreased by isoprenaline pretreatment (0.1 microM; 3 min). In line with this, the calcium-sensitizing effect of levosimendan was decreased in skinned fibers prepared from isoprenaline-treated hearts. CONCLUSIONS: Our results indicate that the cardiac effects of levosimendan at its therapeutically relevant concentrations were not mediated through PKA or PKG and its positive inotropy is therefore most probably due to the previously reported troponin-C-mediated calcium sensitization of contractile proteins.

Entacapone, a novel catechol-O-methyltransferase inhibitor for Parkinson's disease, does not impair mitochondrial energy production.

Entacapone, a novel mainly peripherally acting catechol-O-methyltransferase inhibitor used in the treatment of Parkinson's disease, was evaluated for its possible uncoupling activity in cell culture, in rat liver mitochondria, and in isolated guinea-pig heart. Entacapone did not stimulate respiration in the L1210 murine T cell lymphoma cell line at the concentrations studied (5-40 microM). Furthermore, entacapone neither increased mitochondrial respiration nor impaired cardiac function at pharmacologically relevant concentrations (< 10 microM). In fact, the threshold concentration for increased mitochondrial oxygen consumption was 20 microM and half-maximal stimulation of respiration was not detected until 58 microM. Surprisingly, tolcapone, another catechol-O-methyltransferase inhibitor, which acts both peripherally and centrally, stimulated respiration in L1210 cells at the lowest concentration studied (5 microM). In addition, 1 microM tolcapone increased mitochondrial respiration, indicating that it caused uncoupling at a much lower concentration than that of 2,4-dinitrophenol, a well-known uncoupler of oxidative phosphorylation. Tolcapone also impaired the mechanical function and oxygen consumption of the isolated guinea-pig heart at 1 microM. These results show that peripherally acting entacapone, unlike the brain-penetrating tolcapone, is a safe catechol-O-methyltransferase inhibitor for the treatment of Parkinson's disease, since it does not interfere with mitochondrial energy metabolism at pharmacologically effective concentrations.

Troponin C-mediated calcium sensitization by levosimendan accelerates the proportional development of isometric tension.

The effects of various calcium sensitizers on myosin-actin crossbridge kinetics were evaluated in intact, paced guinea-pig papillary muscle by analysing the velocity of the development of isometric tension (dT/dt) in detail. The effect on association (the whole sequence of events from troponin onward) and dissociation rates of crossbridges was estimated from the rising phase and from the early decay phase of the normalized dT/dt curve. Levosimendan, a calcium sensitizer acting through troponin C, accelerated the proportional association rate and decelerated the dissociation rate of crossbridges. The effect of levosimendan on crossbridge kinetics occurred before the peak twitch tension was achieved. Thus, the compound did not change the actual relaxation phase of twitch tension. Since the effect on the association was more pronounced than on the dissociation of crossbridges, levosimendan shifted the entire twitch tension curve to the left. Based on the dissociation rate analysis levosimendan seems to act preferentially as a calcium sensitizer at low concentrations. At high concentrations the phosphodiesterase III (PDE III) inhibitory properties of levosimendan modulated its effect on the early relaxation processes. In contrast, PDE III inhibition is probably the primary mechanism of action for MCI-154. Pimobendan, and EMD 53998 at low concentrations, whereas their direct effects on crossbridge kinetics contributed to the positive inotropic action at high concentrations. The calcium sensitizing mechanisms of these compounds seemed to be based almost exclusively on the decelerating effect on dissociation of crossbridges.

Cardiac troponin C as a target protein for a novel calcium sensitizing drug, levosimendan.

The role of cardiac troponin C (cTnC) as a target protein for the calcium sensitization by levosimendan, pimobendan, MCI-154 and EMD 53998 was evaluated using purified recombinant human cTnC. For determination of calcium- and magnesium-dependent binding of the compounds to cTnC a new type of cTnC-HPLAC column was used. Furthermore, dansylated cTnC was utilized to study the effect of the calcium sensitizing compounds on calcium-induced conformation of cTnC. Only levosimendan showed calcium-dependent and to a lesser extent magnesium-dependent retention in the cTnC column. The findings indicate that levosimendan binds both to the N-terminal and C-terminal domains of cTnC. In agreement with this, only levosimendan shifted the calcium-induced fluorescence curve of dansylated cTnC to the left. In the control experiments Ca50 and KCa2+ were calculated to be 2.73 microM and 4 x 10(5) M-1, respectively. Levosimendan at 3 microM decreased the value of Ca50 to 1.19 microM. In conclusion, it is suggested that the mechanism of calcium sensitizing effect of levosimendan, unlike that of the other calcium sensitizers, is based on calcium-dependent binding to the N-terminal domain of cTnC. This is proposed to amplify the trigger of contraction induced by cTnC in the cardiac muscle.

Troponin C-mediated calcium sensitization induced by levosimendan does not impair relaxation.

Levosimendan is a novel positive inotropic drug targeted to increase contraction force of the heart through its calcium-dependent binding to troponin C (cTnC). We investigated the calcium-sensitizing effect of levosimendan on contractile proteins as well as its positive inotropic and lusitropic effects in paced guinea pig papillary muscle. We also studied the effect on energy consumption of myosin-actin crossbridges in a myosin ATPase assay. The calcium sensitization induced by levosimendan in fibers skinned with saponin was dependent on the perforation velocity of cell membranes. Levosimendan was almost ineffective in slowly perforated fibers, but was the most potent calcium sensitizer in fibers with rapidly perforated cells. The perforation-dependent calcium sensitization was probably due to changes in phosphorylation state of contractile proteins during the slow dissection of fibers. It is noteworthy that the calcium-sensitizing effect of levosimendan was not affected by acidic pH. Levosimendan at therapeutically relevant (0.3-10 microM) concentrations markedly increased calcium sensitivity both at pH 6.7 and 7.0, being more potent than EMD 53998, pimobendan, and MCI-154. The lack of effect of levosimendan on maximum tension supports the hypothesis that levosimendan increases calcium sensitivity through its action on cTnC. Unlike EMD 53998, levosimendan did not increase myosin ATPase activity, indicating that it did not increase the cycling rate of myosinactin crossbridges. In paced papillary muscles, levosimendan induced positive inotropic effect without changing relaxation time. Thus, levosimendan was devoid of the main negative factors described for calcium sensitizers.

Antioxidant properties of nitecapone are potentiated by glutathione.

The effect of nitecapone (3[(3,4-dihydroxy-5-nitrophenyl) methylene]-2,4-pentanedione) on lipid peroxidation in guinea pig liver microsomes was assessed with the thiobarbituric acid assay. Nitecapone inhibited dose-dependently hydroxyl and peroxyl radical induced lipid peroxidation with IC50-values of 11.1 microM and 16.2 microM, respectively. Nitecapone was an effective antioxidant in native microsomes, where it potentiated glutathione-dependent protection against oxidative stress. Nitecapone provided dose-dependent reduction in lipid peroxidation lasting up to three hours in the presence of glutathione. The long-lasting effect was lost, when the microsomes were boiled. These data suggest that nitecapone is an effective scavenger of oxygen derived free radicals, and that its antioxidant properties are potentiated by glutathione.

Mechanisms of action of calcium-sensitizing drugs.

This review compares the mechanisms of action of the calcium-sensitizing agents levosimendan, pimobendan, MCI-154, and EMD 53998. By using purified human recombinant troponin-C (cTnC), the role of cTnC as a target protein for these compounds was investigated. Accordingly, the calcium-dependent binding to cTnC in a cTnC-high-performance liquid affinity chromatography (HPLAC) column and the stabilizing effects of the compounds on the calcium-induced conformation of dansylated cTnC were studied. Only levosimendan showed calcium-dependent action on cTnC. Of the studied compounds, levosimendan was the most potent calcium sensitizer in skinned fiber experiments. Furthermore, EMD 53998 and MCI-154, but not levosimendan and pimobendan, increased myosin ATPase activity, indicating that they may enhance the cycling rate of myosin-actin crossbridges. By analyzing the velocity (dT/dt) of isometric tension development in paced papillary muscles, it was shown that levosimendan probably enhances the association rate but decreases the dissociation rate of myosin-actin crossbridges. These effects occurred before the peak twitch tension was achieved. Therefore, levosimendan does not seem to affect the actual relaxation phase. The other calcium sensitizers, however, appear to act mainly by decreasing the dissociation rate of crossbridges. The weak calcium-sensitizing effect of pimobendan may be based on indirectly mediated increase in affinity of cTnC for calcium. MCI-154 might act in a similar way but, like EMD 53998, MCI-154 also acts on myosin-actin crossbridges. We suggest that levosimendan binds in a calcium-dependent manner to the N-terminal domain of cTnC, which magnifies the extent of the contraction produced by cTnC when it is calcium-activated.

Protection by nitecapone against sodium taurocholate-induced damage to cultured gastric cells.

The goal of this study was to observe if nitecapone protected against taurocholate-induced damage in primary cultured rat gastric mucosal cells, as well as in a well-differentiated human gastric epithelial cell line (MKN 28). Prostaglandins were measured to analyze the protection mechanism. In primary rat gastric mucosal cell culture, nitecapone 125-250 microM protected the cells significantly against damage induced by sodium taurocholate, increasing cell viability by 31-38%. In the human gastric epithelial cell line, in which mitochondrial activity was measured as an indication of cell viability, nitecapone (62.5-250 microM) protected the cells against sodium taurocholate-induced damage by 12-20%. Prostaglandin E2, thromboxane B2, and 6-keto-prostaglandin F1 alpha measurements in the primary cultured rat gastric mucosal cells showed that nitecapone (125 microM and 250 microM) significantly stimulated prostaglandin E2 production (84.7% and 61.0%, respectively), and inhibited thromboxane B2 formation (50% at 250 microM), while the 6-keto-prostaglandin F1 alpha formation was unaffected. Nitecapone had no effect on prostaglandin E2 production in the MKN 28 epithelial cell line. Indomethacin or aspirin, at concentrations that did not affect cell viability, antagonized the stimulative effect of nitecapone on prostaglandin E2 formation in the primary cultured rat gastric mucosal cells. Although the prostaglandin E2 synthesis was blocked, nitecapone still protected against cell damage induced by taurocholate. These results demonstrated the direct and efficacious protection of nitecapone on gastric cell level and suggest that the "cytoprotection" by nitecapone against taurocholate may not be mediated through the mechanism of stimulated synthesis of prostaglandin E2.

Biochemical and pharmacological properties of a peripherally acting catechol-O-methyltransferase inhibitor entacapone.

Entacapone, OR-611, was found to be a potent peripherally acting inhibitor of catechol-O-methyltransferase (COMT). IC50 values of 10 nmol/l and 160 nmol/l were obtained for rat duodenum and liver-soluble COMT, respectively. There were no effects on other catecholamine metabolizing enzymes. Entacapone showed reversible, tight-binding type of inhibition of soluble rat liver COMT with a Ki-value of 14 nmol/l and it also caused 50% inhibition of rat duodenal, erythrocyte, liver and striatal COMT activity 1 h after oral dosing with 1.1, 5.4, 6.7 and 24.2 mg/kg, respectively. However, penetration of entacapone into the brain was poor, since the formation of homovanillic acid (HVA), the O-methyl metabolite of dopamine in the striatum, was not reduced, even after the highest dose of 30 mg/kg. In rat blood serum, the concentration of 3-O-methyldopa (3OMD), the O-methylated product of L-dopa, was reduced in a dose-dependent manner, and the concentration of L-dopa was increased after the administration of entacapone (3-30 mg/kg p.o.) together with L-dopa + carbidopa. These changes were reflected, in the striatum, by a significant rise in the dopamine concentration and a reduction in the 3OMD concentration. Consequently, when entacapone was added to the treatment with L-dopa + carbidopa, the dose of L-dopa could be lowered from 50 mg/kg to 15 mg/kg in order to produce the same striatal dopamine concentrations as with 50 + 50 mg/kg of L-dopa + carbidopa alone.

Role of gastric mucosal eicosanoid production in the cytoprotection induced by nitecapone.

Nitecapone (3-100 mg/kg orally) dose-dependently (40-97%) decreased the macroscopic gastric lesions induced by ethanol, NaOH, or HCl in the rat. The duration of protection was long, being still 70% at 6 h after dosing. Nitecapone (10-100 mg/kg orally) induced at 1 h after dosing a significant and dose-dependent increase in gastric mucosal prostaglandin E2 release. After the dose of 30 mg/kg the release was sixfold at 2 h and threefold at 12 h. Colloidal bismuth subcitrate (30 mg/kg) stimulated the prostaglandin E2 release only transiently, and sucralfate (400 mg/kg) showed only a tendency to stimulate the release. Indomethacin prevented the nitecapone-induced stimulation of prostaglandin E2 but was unable to counteract the cytoprotective activity of the compound. Nitecapone (30 mg/kg) also caused transient increase (twofold) in the release of 6-keto-prostaglandin F1 alpha and thromboxane B2 and a decrease in both basal and ethanol-induced release of leukotriene C4.

Catechol-O-methyltransferase activity in red blood cells in threatened preterm labor; effect of indomethacin and nylidrin.

Catecholamines that are released in excess during human labor are inactivated mainly by catechol-O-methyltransferase (COMT). To ascertain whether uterine contractions are associated with changes in COMT activity in red blood cells (RBCs), we studied 25 women with established threat of preterm labor between 25 and 33 weeks of gestation, 25 gestational age-matched control women not experiencing uterine contractions, 25 women who were in term labor, and 25 non-pregnant healthy women. COMT activity in pregnant women without uterine contractions (median 0.3, range 0.1-0.8 pmol/mg/min) was lower (p less than 0.05) than that in non-pregnant control series (median 0.5, range 0.3-0.7 pmol/mg/min). RBCs' COMT activity in women with preterm labor (median 0.6, range 0.2-1.1 pmol/mg/min) was greater (p less than 0.05) than that in pregnant and non-pregnant control women, but similar to that during term labor (median 0.5, range 0.2-1.7 pmol/mg/min). Women with preterm labor were treated with indomethacin (12 women) or nylidrin (13 women). Nylidrin treatment was accompanied by a 35% rise in COMT activity 3 h later, whereas indomethacin caused no significant change. Apart from cessation of uterine contractions during tocolysis, 13 women went into labor before the 37th gestational week, but their pretreatment COMT activity (median 0.7, range 0.2-1.1 pmol/mg/min) did not differ from COMT activity in women whose pregnancy proceeded to term (median 0.5, range 0.3-1.0 pmol/mg/min).(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis of some novel potent and selective catechol O-methyltransferase inhibitors.

A series of disubstituted catechol derivatives was synthesized and tested as potential COMT inhibitors. The most active compounds were more than 1000 times more potent (IC50 = 3-6 nM) in vitro than the known COMT inhibitor, 3',4'-dihydroxy-2-methylpropiophenone (U 0521, IC50 = 6000 nM). The new compounds were also highly selective COMT inhibitors with no activity against other essential enzymes involved in the synthesis and metabolism of catecholamines.

Favorable effect of catechol-O-methyltransferase inhibition by OR-462 in experimental models of Parkinson's disease.

A selective catechol-O-methyltransferase inhibitor, OR-462, was studied for its ability to affect pharmacokinetic properties of L-dopa after the p.o. administration of the inhibitor to rats and mice. When OR-462 was given to rats at the dose range of 0.3 to 30 mg/kg in conjunction with L-dopa and carbidopa, a dose-related and long-lasting (greater than 5 hr) increase in striatal L-dopa and dopamine levels as well as a reduction in 3-O-methyldopa levels were shown. For a 50% reduction of the 3-O-methyldopa levels a dose of 6 mg/kg of OR-462 was needed. The increase in striatal homovanillic acid, an O-methylated metabolite of dopamine which poorly penetrates the blood brain barrier, indicates that O-methylation was not inhibited in the brain. In order to get the same dopamine levels in striatum the L-dopa dose could be lowered to one-fourth when OR-462 was added. The L-dopa-sparing effect of OR-462 given p.o. was also demonstrated in two behavioral parkinsonian models. OR-462 given at doses of 3 to 30 mg/kg in conjunction with L-dopa and carbidopa, dose-dependently potentiated the L-dopa-induced reversal of hypoactivity in reserpinized mice. Likewise, the same doses of OR-462 caused a marked potentiation of L-dopa-induced contralateral turning behavior in rats with unilateral nigrostriatal lesions produced by 6-hydroxydopamine. The data suggest a possible beneficial effect of OR-462 in the therapy of Parkinson's disease.

Inhibition of catechol-O-methyltransferase activity by two novel disubstituted catechols in the rat.

Catechol-O-methyltransferase (COMT) has an important role in the extraneuronal inactivation of catecholamine neurotransmitters and drugs with a catechol structure. Two novel COMT inhibitors, OR-462 and OR-486, were highly effective (IC50 = 18 and 12 nM, respectively) and selective in inhibiting COMT activity in vitro. Tyrosine hydroxylase was not inhibited until micromolar concentrations of these compounds were used: the IC50 values for OR-462 and OR-486 were 10 and 14 microM, respectively. The IC50 values for dopamine-beta-hydroxylase, dopa-decarboxylase and monoamine oxidase forms A and B were greater than 50 microM. In studies ex vivo oral OR-462 inhibited mainly the COMT activity in the duodenum while OR-486 inhibited COMT activity in the liver and red blood cells as well. Oral OR-462 did not penetrate into the brain in doses up to 30 mg/kg while the same dose of OR-486 had some effect on striatal COMT activity. When tested in combination with levodopa-carbidopa, orally administered OR-462 and OR-486 were more effective in reducing the formation of 3-O-methyldopa from levodopa than was the levodopa-carbidopa treatment alone. These results indicate that OR-462 and OR-486 are effective and long-lasting inhibitors of COMT activity.

Properties of novel effective and highly selective inhibitors of catechol-O-methyltransferase.

Novel bisubstituted catechols were found to be potent and highly selective COMT inhibitors in vitro. One of them, OR-462 (3-(3,4-dihydroxy-5-benzylidene)-2,4-pentanedione), was studied also in vivo. When administered to rats orally together with levodopa and carbidopa, OR-462 greatly improved the bioavailability of levodopa and effectively reduced the formation of 3OMD. The levels of levodopa and dopamine were increased also in the striatum, and the 3OMD levels were decreased. The metabolic profile of dopamine demonstrated that COMT inhibition occurred in the peripheral tissues but not in the striatum. OR-462 thus resembled the peripheral inhibitors of dopadecarboxylase. These potent, selective and orally active COMT inhibitors offer a new tool for interfering in the metabolism of various COMT substrates.

Haem arginate: a new stable haem compound.

Intravenous administration of haem in acute hepatic porphyrias inhibits the induction of delta-aminolaevulinic acid synthase, reduces the formation of potentially harmful metabolites of porphyrin synthesis and corrects the haem deficiency. Typically, haem therapy has been given in the form of haematin--haem dissolved in alkali. Such haematin solutions are, however, extremely unstable. Thus, the rapid decomposition of this therapeutic agent may have been responsible for the ineffectiveness of treatment in some clinical states and adverse reactions may have been caused by haematin degradation products. There is, therefore, a need for a stable, effective and well-tolerated haem preparation. We have prepared certain highly soluble haem compounds of which haem arginate has proved to be the most promising. Pure haemin was isolated from HIV and hepatitis B negative human blood. The haem derivatives prepared were screened as substrates for haem oxygenase. Haem arginate and haem lysinate were found to be as good substrates as methaemalbumin. Stock solutions of haem arginate were stable for 2 years at +6 degrees C. After dilution with sterile isotonic saline the haem arginate infusion was clearly more stable than haematin solutions made in the laboratory or prepared by dissolving commercial lyophilized haematin. The antiporphyrogenic effect of haem arginate (even after storage for two years) in 2-allyl-2-isopropylacetamide-induced experimental porphyria of rats was equal to that of freshly prepared haematin. The acute oral toxicity of haem arginate was low compared with the parenterally administered drug, indicating poor oral bioavailability. The acute toxic effects after high intravenous or intraperitoneal doses were directed to the liver.(ABSTRACT TRUNCATED AT 250 WORDS)