Comtan (Entacapone)
Indications
| Indication | Approved Population | Therapy Type | Status |
|---|---|---|---|
| Parkinson’s disease — adjunct to levodopa/carbidopa for end-of-dose “wearing-off” | Adults | Always adjunctive to LD/CD (or LD/benserazide) | FDA Approved |
Entacapone is a selective, reversible, peripherally acting COMT inhibitor indicated exclusively as an adjunct to levodopa/carbidopa (or levodopa/benserazide) therapy. By blocking peripheral COMT-mediated methylation of levodopa, entacapone increases the AUC of levodopa by approximately 35% and extends its elimination half-life from about 1.3 hours to 2.4 hours, leading to more sustained dopaminergic stimulation. It is specifically targeted at patients experiencing end-of-dose motor fluctuations (“wearing-off”). Entacapone has no anti-parkinsonian effect when used alone and must always be administered alongside levodopa.
Unlike tolcapone, entacapone acts only peripherally (does not cross the blood-brain barrier significantly) and does not require mandatory liver function monitoring. Tolcapone carries an FDA boxed warning for fatal hepatotoxicity; entacapone does not.
Dosing
Standard Dosing — Adults With Motor Fluctuations
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| End-of-dose wearing-off — adjunct to LD/CD | 200 mg with each LD/CD dose | 200 mg with each LD/CD dose | 200 mg × 8/day = 1,600 mg/day (FDA) | No titration required; flat dose with every LD intake EMA allows up to 200 mg × 10/day (2,000 mg/day) |
| Patients on LD ≥800 mg/day or with moderate-severe dyskinesia | 200 mg with each LD/CD dose | 200 mg with each LD/CD dose + LD reduction | 1,600 mg/day | Reduce LD dose by ~25% on average; >58% of patients on LD >800 mg/day required LD reduction in trials Extending dosing interval may also be needed |
Special Populations
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| Renal impairment (any severity) | 200 mg (standard) | Standard | 1,600 mg/day | No important PK effects of renal impairment (FDA PI) EMA suggests dialysis patients may need increased dosing interval |
| Hepatic impairment | Use with caution; AUC and Cmax approximately doubled in cirrhosis patients. EMA contraindicates use in hepatic impairment. | |||
| Elderly (≥65 years) | 200 mg (standard) | Standard | 1,600 mg/day | PK independent of age; no dose adjustment required |
Unlike dopamine agonists, entacapone requires no dose titration. The fixed 200 mg dose is taken with each levodopa dose from day one. The key dosing decision is whether to proactively reduce the levodopa dose by approximately 25% at initiation — strongly recommended when the patient is already on levodopa ≥800 mg/day or has significant dyskinesia.
Although no cases of NMS following abrupt entacapone withdrawal were reported in clinical trials, postmarketing cases of a symptom complex resembling NMS have been reported in association with rapid dose reduction or withdrawal of dopaminergic drugs. Withdraw gradually and monitor closely, adjusting other dopaminergic therapies as needed.
Pharmacology
Mechanism of Action
Entacapone is a selective and reversible inhibitor of catechol-O-methyltransferase (COMT), a ubiquitous enzyme that catalyses the O-methylation of catechol substrates including levodopa, dopamine, and catecholamine neurotransmitters. In the presence of an aromatic amino acid decarboxylase inhibitor such as carbidopa, COMT becomes the major peripheral metabolising pathway for levodopa. By blocking this pathway, entacapone prevents the conversion of levodopa to 3-O-methyldopa (3-OMD) in peripheral tissues, increasing the proportion of each levodopa dose reaching the brain. This manifests clinically as an approximately 35% increase in levodopa AUC and prolongation of levodopa half-life from about 1.3 to 2.4 hours, reducing motor fluctuations.
ADME Profile
| Parameter | Value | Clinical Implication |
|---|---|---|
| Absorption | Rapidly absorbed; Tmax ~1 h; bioavailability 35%; Cmax ~1.2 mcg/mL after 200 mg; food has no effect on PK | Can be taken with or without food; taking with food may help if nausea occurs |
| Distribution | Vd 20 L (IV); 98% bound to serum albumin; does not cross BBB significantly; low lipophilicity | Small Vd and high protein binding confine entacapone to the peripheral compartment, explaining its peripheral-only COMT inhibition |
| Metabolism | Almost completely metabolised; isomerisation to cis-isomer → direct glucuronidation; glucuronide conjugate is inactive; 0.2% excreted unchanged in urine; non-CYP mediated | No CYP-based drug interactions; hepatic impairment doubles AUC (biliary excretion is major route) |
| Elimination | Biphasic: t½β 0.4–0.7 h; t½γ 2.4 h (γ-phase ~10% of AUC); total clearance 850 mL/min (IV); 10% urine / 90% faeces | Short half-life necessitates dosing with each levodopa dose; renal impairment does not significantly affect PK |
Side Effects
Most adverse effects of entacapone result from enhanced dopaminergic activity (due to increased levodopa exposure) and can be managed by reducing the levodopa dose. Diarrhea and urine discolouration are unique to entacapone itself. Data below are from FDA PI double-blind, placebo-controlled trials (N=1,003 total; 603 entacapone, 400 placebo).
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Dyskinesia | 25% (vs 15% placebo) | Most common adverse effect; due to increased LD exposure; reduce LD dose by ~25%; DC for dyskinesia 1.5% vs 0.8% placebo |
| Urine discolouration (brownish-orange) | 10% (vs 0% placebo) | Benign; due to entacapone metabolites; reassure patient that it is harmless |
| Diarrhea | 10% | May have delayed onset (4–12 weeks after initiation); can be severe/watery; may indicate colitis; most common reason for serious discontinuation |
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Nausea | ≥3% > placebo | Dopaminergic effect; usually manageable with LD dose reduction; take with food |
| Hyperkinesia | ≥3% > placebo | Related to enhanced LD exposure; reduce LD dose |
| Abdominal pain | ≥3% > placebo | Evaluate for diarrhea-related causes or colitis if persistent |
| Vomiting | ≥3% > placebo | Dopaminergic; monitor hydration and electrolytes |
| Dry mouth | ≥3% > placebo | May overlap with anticholinergic medications in PD regimen |
| Adverse Effect | Estimated Frequency | Typical Onset | Required Action |
|---|---|---|---|
| Diarrhea with colitis | Uncommon (subset of 10% diarrhea) | 4–12 weeks after initiation; can be delayed months | Discontinue entacapone; consider colonoscopy/biopsy if diarrhea persists after stopping; monitor hydration, potassium, weight |
| Rhabdomyolysis | Rare (postmarketing) | Variable; may relate to severe prolonged dyskinesia | Check CPK; discontinue if confirmed; may be linked to NMS-like syndrome |
| NMS-like syndrome (hyperpyrexia, rigidity, altered consciousness) | Rare (postmarketing) | During rapid withdrawal or dose reduction of dopaminergic therapy | Withdraw gradually; supportive care; monitor CPK, temperature |
| Hallucinations / psychosis | Uncommon; led to DC and hospitalisation in trials | Variable | Reduce LD dose or discontinue entacapone; evaluate for pimavanserin or low-dose quetiapine |
| Impulse control disorders | Uncommon (class effect of dopaminergic therapy) | Weeks to months | Screen at every visit; dose reduction of dopaminergic therapy; involve caregivers |
| Reason for Discontinuation | Entacapone | Placebo |
|---|---|---|
| Psychiatric disorders | 2% | 1% |
| Diarrhea | 2% | 0% |
| Dyskinesia / hyperkinesia | 2% | 1% |
| Nausea | 2% | 1% |
| Abdominal pain | 1% | 0% |
| Aggravation of PD symptoms | 1% | 1% |
Diarrhea affects approximately 10% of patients and may have a uniquely delayed onset (4–12 weeks). In some patients, biopsy-confirmed colitis has been identified. If prolonged diarrhea is suspected to be drug-related, entacapone should be discontinued. Diarrhea has recurred upon rechallenge in some patients with confirmed colitis. Monitor hydration, weight, and potassium levels in affected patients.
Drug Interactions
Entacapone does not undergo CYP-mediated metabolism and has a distinct interaction profile from dopamine agonists. Its primary interactions stem from COMT inhibition affecting catechol-structured drugs and its effects on levodopa pharmacokinetics.
Monitoring
- DyskinesiaEach visit; closely at initiation
RoutineMost common adverse effect (25%); typically requires LD dose reduction. Evaluate within days to weeks of starting entacapone. - Diarrhea / GIEach visit for first 3 months; then ongoing
RoutineDelayed onset (4–12 weeks); monitor hydration, weight, potassium. If prolonged, consider colonoscopy; discontinue entacapone. - Blood PressureBaseline, each visit
RoutineOrthostatic hypotension may occur, especially during initial therapy. Lying and standing measurements. - NeuropsychiatricEach visit
RoutineHallucinations, psychosis, confusion; enhanced dopaminergic tone is the mechanism. Screen for ICDs (gambling, hypersexuality). - Hepatic FunctionBaseline (if clinical suspicion)
Trigger-BasedUnlike tolcapone, mandatory LFT monitoring is NOT required. However, check baseline LFTs if hepatic disease suspected, as AUC doubles in cirrhosis. - Skin ExaminationAnnually
RoutinePD patients have an elevated melanoma risk (disease-related, not drug-specific). - CPKIf symptoms arise
Trigger-BasedCheck if fever, rigidity, myalgia, or altered consciousness develop (rhabdomyolysis/NMS concern).
Contraindications & Cautions
Absolute Contraindications
- Hypersensitivity to entacapone or any excipient (FDA PI).
- Hepatic impairment (EMA SmPC; FDA PI advises caution rather than absolute contraindication).
- Pheochromocytoma (EMA SmPC; risk of hypertensive crisis via COMT inhibition of catecholamines).
Relative Contraindications (Specialist Input Recommended)
- Concomitant non-selective MAO inhibitors: Risk of hypertensive crisis. Selective MAO-B inhibitors (selegiline, rasagiline) are safe.
- History of rhabdomyolysis or NMS: Postmarketing cases reported; exercise caution.
Use with Caution
- Biliary obstruction: Major excretion route is biliary; impaired biliary flow may increase exposure.
- Patients with history of diarrhea-related colitis: Entacapone-associated diarrhea/colitis has recurred on rechallenge.
- Pregnancy/Lactation: Always co-administered with LD/CD (known teratogen in animals); excreted in rat milk.
Patients treated with dopaminergic medications, including entacapone in combination with levodopa, have reported suddenly falling asleep without warning during daily activities. Patients should be advised not to drive or engage in hazardous activities until they have gained sufficient experience with the medication.
Patient Counselling
Purpose of Therapy
Entacapone is an “add-on” medication that makes your levodopa work longer. It does not replace levodopa and has no effect on its own. By taking entacapone with each levodopa dose, the levodopa stays active in your body for a longer period, which helps reduce the return of symptoms (“wearing off”) between doses.
How to Take
Take one 200 mg tablet at the same time as each levodopa/carbidopa dose, up to 8 times per day. The tablet can be taken with or without food. Do not break or crush the tablet. Do not take extra doses — the dose is always 200 mg.
Sources
- COMTAN (entacapone) Tablets — FDA Prescribing Information. Revised 2014. accessdata.fda.govPrimary regulatory source for dosing, PK data, adverse reaction tables, drug interactions, and discontinuation rates.
- COMTAN (entacapone) Tablets — FDA Prescribing Information. Revised 2010. accessdata.fda.govEarlier label version with identical PK parameters and clinical trial results; confirms consistency of regulatory data.
- Comtan (entacapone) — EMA Summary of Product Characteristics. ema.europa.euEuropean regulatory source; contraindicates hepatic impairment and pheochromocytoma; allows max 10 doses/day (2000 mg).
- Rinne UK, Larsen JP, Siden A, Worm-Petersen J. Entacapone enhances the response to levodopa in parkinsonian patients with motor fluctuations. Neurology. 1998;51(5):1309–1314. doi:10.1212/WNL.51.5.1309Nordic pivotal trial (N=171) demonstrating +1.5 h “on” time and −87 mg/day LD dose reduction with entacapone.
- Parkinson Study Group. Entacapone improves motor fluctuations in levodopa-treated Parkinson’s disease patients. Ann Neurol. 1997;42(5):747–755. doi:10.1002/ana.410420511North American pivotal trial (N=205) supporting FDA approval; primary endpoint was proportion of awake time “on.”
- Stocchi F, Rascol O, Kieburtz K, et al. Initiating levodopa/carbidopa therapy with and without entacapone in early Parkinson disease: the STRIDE-PD study. Ann Neurol. 2010;68(1):18–27. doi:10.1002/ana.22060Large RCT (N=747) that found earlier onset of dyskinesia with initial LD/CD/entacapone vs LD/CD alone; shaped current use for fluctuators only.
- Fox SH, Katzenschlager R, Lim SY, et al. International Parkinson and Movement Disorder Society Evidence-Based Medicine Review: Update on Treatments for the Motor Symptoms of Parkinson’s Disease. Mov Disord. 2018;33(8):1248–1266. doi:10.1002/mds.27372MDS evidence review classifying COMT inhibitors as “efficacious” for reducing off-time in motor fluctuations.
- National Institute for Health and Care Excellence (NICE). Parkinson’s disease in adults. NICE guideline [NG71]. Updated 2024. nice.org.ukUK guideline recommending COMT inhibitors as adjunctive therapy for motor fluctuations in PD.
- Kaakkola S. Clinical pharmacology, therapeutic use and potential of COMT inhibitors in Parkinson’s disease. Drugs. 2000;59(6):1233–1250. doi:10.2165/00003495-200059060-00004Comprehensive review of COMT pharmacology; source for mechanism details and peripheral vs central COMT inhibition.
- Nissinen E, Lindén IB, Schultz E, Pohto P. Biochemical and pharmacological properties of a peripherally acting catechol-O-methyltransferase inhibitor entacapone. Naunyn Schmiedebergs Arch Pharmacol. 1992;346(3):262–266. doi:10.1007/BF00173538Original preclinical characterisation of entacapone as a peripheral COMT inhibitor.
- Entacapone. In: DailyMed [Internet]. Bethesda (MD): U.S. National Library of Medicine. dailymed.nlm.nih.govGeneric label confirming PK parameters (t½, Vd, bioavailability, clearance) and adverse reaction data.
- Entacapone. In: Drugs.com Professional Drug Information. drugs.com/proConsolidated prescribing information with detailed pharmacokinetic tables and drug interaction data.
- Brooks DJ, Sagar H, and the UK-Irish Entacapone Study Group. Entacapone is beneficial in both fluctuating and non-fluctuating patients with Parkinson’s disease: a randomised, placebo controlled, double blind, six month study. J Neurol Neurosurg Psychiatry. 2003;74(8):1071–1079. doi:10.1136/jnnp.74.8.1071UK-Irish trial demonstrating benefit of entacapone in both fluctuating and non-fluctuating PD patients; an additional phase III study beyond the three pivotal trials.