Entacapone is an important adjuvant drug for the treatment of Parkinson’s disease, usually used in combination with levodopa/carbidopa.

I. Drug Overview of Entacapone

1. Drug Class

Entacapone is a selective and reversible catechol‑O‑methyltransferase (COMT) inhibitor, belonging to the nitrocatechol structural compounds.

2. Clinical Position

(1) As an adjunct to levodopa/carbidopa, entacapone is used to improve the end‑of‑dose “on‑off” phenomena in patients with Parkinson’s disease.

(2) It has no anti‑Parkinsonian effect on its own and must be used in combination with levodopa/carbidopa; it must not be used alone.

II. Mechanism of Action and Drug Interactions

1. Core Mechanism of Action

Entacapone exerts its effect by inhibiting the activity of the COMT enzyme.

2. Pharmacokinetic Properties

(1) Absorption and distribution:

Oral absorption of entacapone is rapid, with peak plasma concentrations achieved at approximately 1 hour, and absolute bioavailability of 35%. Food does not affect its pharmacokinetics. The volume of distribution is small (20 L), and 98% of the drug is bound to plasma proteins (mainly albumin).

(2) Metabolism and excretion:

The drug is almost completely metabolized before excretion, with only 0.2% excreted unchanged in urine. The major metabolic pathway is direct glucuronidation after isomerization, and the metabolites are inactive. Following oral administration, 10% is excreted in urine and 90% in feces.

(3) Dose‑related characteristics:

Entacapone exhibits linear pharmacokinetics over the dose range of 5 mg to 800 mg. A single 200‑mg dose inhibits erythrocyte COMT activity by an average of 65%, with activity returning to baseline within 8 hours.

3. Important Drug Interactions

(1) With non‑selective MAO inhibitors:

Concomitant use with non‑selective monoamine oxidase (MAO) inhibitors (e.g., phenelzine, tranylcypromine) may theoretically cause excessive blockade of catecholamine metabolic pathways, leading to arrhythmias and severe blood pressure fluctuations; such combinations should be avoided. Selective MAO‑B inhibitors (e.g., selegiline) can be safely combined with entacapone.

(2) With drugs metabolized by COMT:

Drugs metabolized by COMT, including isoprenaline, adrenaline, noradrenaline, dopamine, and dobutamine, regardless of the route of administration, may cause tachycardia, arrhythmias, and excessive blood pressure fluctuations when combined with entacapone.

(3) With warfarin:

Entacapone increases the mean AUC of R‑warfarin by 18% and the mean INR by 13%. Markedly increased INR has been reported post‑marketing; INR monitoring is recommended during co‑administration.

(4) With drugs affecting biliary excretion:

Since entacapone is primarily excreted via bile, caution is advised when co‑administered with drugs that may interfere with biliary excretion, glucuronidation, or intestinal β‑glucuronidase activity, such as probenecid, cholestyramine, erythromycin, rifampicin, ampicillin, and chloramphenicol.

(5) Other interactions:

In vitro studies show that entacapone inhibits CYP enzymes only at very high concentrations (IC50 200 μM to >1000 μM), and no clinically relevant CYP‑mediated interactions are expected. No interactions have been observed with imipramine or selegiline.

III. Precautions and Monitoring Points

1. Administration and Dosage Adjustment

(1) Recommended dose:

The recommended dose of entacapone is 200 mg administered with each levodopa/carbidopa dose, up to 8 times daily (total dose not exceeding 1600 mg). Clinical experience with daily doses above 1600 mg is limited.

(2) Levodopa dosage adjustment:

In clinical studies, most patients with a daily levodopa dose ≥800 mg or moderate‑to‑severe dyskinesia before treatment required levodopa reduction. The mean daily levodopa dose was reduced by approximately 25% in those requiring dose reduction, and more than 58% of patients with a daily dose >800 mg needed reduction.

(3) Administration:

May be taken with or without food, and may be combined with immediate‑release or extended‑release levodopa/carbidopa formulations.

2. Other Precautions

(1) Urine discoloration:

Approximately 10% of patients develop brownish‑orange urine, which is a normal finding due to drug metabolites and has no clinical significance.

(2) Iron chelation:

Entacapone can chelate iron. A trend toward decreased serum iron has been observed in clinical studies, but no significant changes in serum ferritin or anemia incidence were seen after one year of treatment.

(3) Melanoma risk:

Epidemiological studies indicate that patients with Parkinson’s disease have a 2–6‑fold higher risk of melanoma than the general population. Regular skin examinations are recommended during treatment.

(4) Laboratory monitoring:

INR monitoring is required during co‑administration with warfarin. Caution is advised in patients with hepatic impairment, although no specific monitoring parameters are required.

For further information, please refer to the prescribing information for Entacapone.