TOLCAPONE
Clinical safety rating: safe
Animal studies have demonstrated safety
Tolcapone is a selective and reversible inhibitor of catechol-O-methyltransferase (COMT). It inhibits both peripheral and central COMT activity, thereby reducing the metabolism of levodopa to 3-O-methyldopa and increasing the bioavailability and duration of action of levodopa in the brain.
| Metabolism | Tolcapone is extensively metabolized in the liver, primarily via glucuronidation by UGT1A6, UGT1A9, and UGT2B7. Minor metabolism via catechol-O-methyltransferase (COMT) and cytochrome P450 (CYP3A4 and CYP2A6) also occurs. |
| Excretion | Primarily hepatic metabolism (glucuronidation and methylation), with minimal renal excretion of unchanged drug. Fecal excretion accounts for approximately 40% of the dose, with 14% excreted in urine as glucuronide conjugates. Less than 1% of the dose is recovered as unchanged tolcapone in urine. |
| Half-life | Terminal elimination half-life is 2–3 hours in healthy subjects; in patients with hepatic impairment, it may be prolonged up to 8–10 hours, necessitating dose reduction. |
| Protein binding | Extensively bound (>99.9%) to plasma proteins, primarily albumin, with a minor fraction bound to alpha-1-acid glycoprotein. |
| Volume of Distribution | Volume of distribution is approximately 0.3–0.5 L/kg, indicating distribution mainly into extracellular water and tissues. |
| Bioavailability | Oral bioavailability is approximately 65% under fasting conditions; food may reduce absorption by about 20%. |
| Onset of Action | Oral administration: onset of clinical effect (reduction in motor fluctuations) occurs within 1–2 hours, corresponding to peak plasma concentrations. |
| Duration of Action | Clinical effect duration is approximately 6–8 hours, due to reversible binding to catechol-O-methyltransferase (COMT); dosing three times daily is recommended. |
100 mg orally three times daily. The first dose of the day should be taken together with the first daily dose of levodopa/carbidopa.
| Dosage form | TABLET |
| Renal impairment | No dosage adjustment required for GFR ≥25 mL/min. Safety and efficacy not established for GFR <25 mL/min; use is not recommended. |
| Liver impairment | Contraindicated in patients with any degree of hepatic impairment. Use is not recommended in patients with Child-Pugh Class A, B, or C. |
| Pediatric use | Safety and efficacy in pediatric patients have not been established; no dosing recommendations available. |
| Geriatric use | No specific dose adjustment recommended, but monitor for increased risk of adverse effects such as diarrhea, dyskinesia, and orthostatic hypotension due to age-related physiological changes. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Nonselective MAOIs are contraindicated Can cause hepatotoxicity requiring frequent liver monitoring.
| Breastfeeding | Unknown if excreted in human milk. M/P ratio not available. Due to potential for hepatotoxicity, breastfeeding not recommended during therapy. |
| Teratogenic Risk | Pregnancy Category C. Animal studies show embryo/fetal toxicity at maternal toxic doses. No adequate human studies. First trimester: potential for developmental toxicity; use only if benefit outweighs risk. Second/third trimester: limited data, monitor fetal growth and hepatic function. |
| Fetal Monitoring |
■ FDA Black Box Warning
Tolcapone has been associated with cases of fatal fulminant hepatotoxicity. Tolcapone should not be used in patients with liver disease or elevated liver enzymes. Liver function monitoring is required before and during treatment.
| Common Effects | Diarrhea |
| Serious Effects |
["Patients with liver disease or elevated liver enzymes (e.g., baseline ALT/AST > upper limit of normal).","History of tolcapone-induced hepatotoxicity.","Known hypersensitivity to tolcapone or any components of the formulation.","Concurrent use with nonselective monoamine oxidase inhibitors (MAOIs) (e.g., phenelzine, tranylcypromine)."]
| Precautions | ["Hepatotoxicity: potentially fatal liver injury; monitor liver enzymes before and every 2-4 weeks for first 6 months, then as clinically indicated.","Hypotension/Syncope: may cause orthostatic hypotension, especially during dose escalation.","Dyskinesia: may exacerbate levodopa-induced dyskinesia.","Hallucinations and psychosis (including hallucinations, delusions, and agitation).","Diarrhea: may be severe and persistent; monitor for dehydration and electrolyte imbalance."] |
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| Monitor maternal liver function tests (ALT, AST) every 2-4 weeks; fetal ultrasound for growth and development; assess for signs of hepatic failure in mother. |
| Fertility Effects | No formal fertility studies in humans. Animal studies show no impairment of fertility at clinically relevant doses. Possible reversible effects on hormonal balance due to COMT inhibition. |