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.

The inhibition of soluble catechol-O-methyl-transferase (S-COMT) in red blood cells (RBCs) by entacapone, and the pharmacokinetics of entacapone after single oral (5-800 mg) and i.v. (25 mg) doses have been examined in an open study in 12 healthy young male volunteers. Oral entacapone dose-dependently decreased the activity of S-COMT in RBCs with a maximum inhibition of 82% after the highest dose (800 mg). The inhibition of S-COMT in RBCs was reversible and the activity recovered within 4-8 h. Entacapone showed linear pharmacokinetics over the dose range studied: Cmax and AUC were correlated with the dose of the drug. Oral absorption of entacapone was fast, with a tmax ranging from 0.4 to 0.9 h, depending on the dose. Systemic availability of entacapone varied between 30 and 46%. Entacapone was rapidly eliminated by metabolism with a half-life of 0.27-0.30 h after oral doses of 5 to 50 mg. After doses from 100 to 800 mg the disposition was best described by two phases with a t1/2 alpha of 0.27-0.37 h and t1/2 beta of 1.59-3.44 h. Over the dose range studied, the single oral and i.v. doses of entacapone were well tolerated. No haematological, biochemical or haemodynamic adverse effects were seen. The results show that entacapone is an orally effective and reversible COMT inhibitor in man and has simple, linear pharmacokinetics.

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