Carbidopa And Levodopa

INDICATIONS AND USAGE Carbidopa and levodopa extended-release tablets are indicated in the treatment of Parkinson’s disease, post-encephalitic parkinsonism, and symptomatic parkinsonism that may follow carbon monoxide intoxication or manganese intoxication.

DOSAGE AND ADMINISTRATION Carbidopa and levodopa extended-release tablets contain carbidopa and levodopa in a 1:4 ratio as either the 50-200 tablet or the 25-100 tablet. The daily dosage of carbidopa and levodopa extended-release tablets must be determined by careful titration. Patients should be monitored closely during the dose adjustment period, particularly with regard to appearance or worsening of involuntary movements, dyskinesias or nausea. Carbidopa and levodopa extended-release tablets should not be chewed or crushed. Standard drugs for Parkinson’s disease, other than levodopa without a decarboxylase inhibitor, may be used concomitantly while carbidopa and levodopa extended-release tablets are being administered, although their dosage may have to be adjusted. Since carbidopa prevents the reversal of levodopa effects caused by pyridoxine, carbidopa and levodopa extended-release tablets can be given to patients receiving supplemental pyridoxine (vitamin B 6 ). Initial Dosage Patients Currently Treated with Conventional Carbidopa-Levodopa Preparations Studies show that peripheral dopa-decarboxylase is saturated by the bioavailable carbidopa at doses of 70 mg a day and greater. Because the bioavailabilities of carbidopa and levodopa in carbidopa and levodopa immediate-release tablets and carbidopa and levodopa extended-release tablets are different, appropriate adjustments should be made, as shown in Table 2. Table 2: Approximate Bioavailabilities at Steady State This table is only a guide to bioavailabilities since other factors such as food, drugs, and inter-patient variabilities may affect the bioavailability of carbidopa and levodopa. Tablet Amount of Levodopa (mg) in Each Tablet Approximate Bioavailability Approximate Amount of Bioavailable Levodopa (mg) in Each Tablet Carbidopa and Levodopa Extended-Release Tablets 50 mg/200 mg 200 0.70-0.75 The extent of availability of levodopa from carbidopa and levodopa extended-release tablets was about 70-75% relative to intravenous levodopa or standard carbidopa and levodopa immediate-release tablets in the elderly. 140-150 Carbidopa and Levodopa Immediate-Release Tablets 25 mg/100 mg 100 0.99 The extent of availability of levodopa from carbidopa and levodopa immediate-release tablets was 99% relative to intravenous levodopa in the healthy elderly. 99 Dosage with carbidopa and levodopa extended-release tablets should be substituted at an amount that provides approximately 10% more levodopa per day, although this may need to be increased to a dosage that provides up to 30% more levodopa per day depending on clinical response (see DOSAGE AND ADMINISTRATION: Titration with Carbidopa and Levodopa Extended-Release Tablets ). The interval between doses of carbidopa and levodopa extended-release tablets should be 4-8 hours during the waking day (see CLINICAL PHARMACOLOGY: Pharmacodynamics ). A guideline for initiation of carbidopa and levodopa extended-release tablets is shown in Table 3. Table 3: Guidelines for Initial Conversion from Carbidopa and Levodopa Immediate-Release Tablets to Carbidopa and Levodopa Extended-Release Tablets Carbidopa and Levodopa Immediate-Release Tablets Total Daily Dose For dosing ranges not shown in the table see DOSAGE AND ADMINISTRATION: Initial Dosage: Patients Currently Treated with Conventional Carbidopa and Levodopa Preparations. Levodopa (mg) Carbidopa and Levodopa Extended-Release Tablets Suggested Dosage Regimen 300-400 200 mg b.i.d. 500-600 300 mg b.i.d. or 200 mg t.i.d. 700-800 A total of 800 mg in 3 or more divided doses (e.g., 300 mg a.m., 300 mg early p.m., and 200 mg later p.m.) 900-1000 A total of 1000 mg in 3 or more divided doses (e.g., 400 mg a.m., 400 mg early p.m., and 200 mg later p.m.) Patients Currently Treated with Levodopa Without a Decarboxylase Inhibitor Levodopa must be discontinued at least twelve hours before therapy with carbidopa and levodopa extended-release tablets is started. Carbidopa and levodopa extended-release tablets should be substituted at a dosage that will provide approximately 25% of the previous levodopa dosage. In patients with mild to moderate disease, the initial dose is usually 1 tablet of 50 mg/200 mg carbidopa and levodopa extended-release tablets b.i.d. Patients Not Receiving Levodopa In patients with mild to moderate disease, the initial recommended dose is 1 tablet of 50 mg/200 mg carbidopa and levodopa extended-release tablets b.i.d. Initial dosage should not be given at intervals of less than 6 hours. Titration with Carbidopa and Levodopa Extended-Release Tablets Following initiation of therapy, doses and dosing intervals may be increased or decreased depending upon therapeutic response. Most patients have been adequately treated with doses of carbidopa and levodopa extended-release tablets that provide 400 to 1600 mg of levodopa per day, administered as divided doses at intervals ranging from 4 to 8 hours during the waking day. Higher doses of carbidopa and levodopa extended-release tablets (2400 mg or more of levodopa per day) and shorter intervals (less than 4 hours) have been used, but are not usually recommended. When doses of carbidopa and levodopa extended-release tablets are given at intervals of less than 4 hours, and/or if the divided doses are not equal, it is recommended that the smaller doses be given at the end of the day. An interval of at least 3 days between dosage adjustments is recommended. Maintenance Because Parkinson’s disease is progressive, periodic clinical evaluations are recommended; adjustment of the dosage regimen of carbidopa and levodopa extended-release tablets may be required. Addition of Other Antiparkinson Medications Anticholinergic agents, dopamine agonists, and amantadine can be given with carbidopa and levodopa extended-release tablets. Dosage adjustment of carbidopa and levodopa extended-release tablets may be necessary when these agents are added. A dose of carbidopa and levodopa immediate-release tablets 25/100 or 10/100 (one half or a whole tablet) can be added to the dosage regimen of carbidopa and levodopa extended-release tablets in selected patients with advanced disease who need additional immediate-release levodopa for a brief time during daytime hours. Interruption of Therapy Sporadic cases of hyperpyrexia and confusion have been associated with dose reductions and withdrawal of carbidopa and levodopa immediate-release tablets or carbidopa and levodopa extended-release tablets. Patients should be observed carefully if abrupt reduction or discontinuation of carbidopa and levodopa extended-release tablets is required, especially if the patient is receiving neuroleptics (see WARNINGS ). If general anesthesia is required, carbidopa and levodopa extended-release tablets may be continued as long as the patient is permitted to take oral medication. If therapy is interrupted temporarily, the patient should be observed for symptoms resembling NMS, and the usual dosage should be administered as soon as the patient is able to take oral medication.

ADVERSE REACTIONS In controlled clinical trials, patients predominantly with moderate to severe motor fluctuations while on carbidopa and levodopa immediate-release were randomized to therapy with either carbidopa and levodopa immediate-release or carbidopa and levodopa extended-release. The adverse experience frequency profile of carbidopa and levodopa extended-release did not differ substantially from that of carbidopa and levodopa immediate-release, as shown in Table 1. Table 1: Clinical Adverse Experiences Occurring in 1% or Greater of Patients Adverse Experience Carbidopa and Levodopa Extended-Release n = 491 % Carbidopa and Levodopa Immediate-Release n = 524 % Dyskinesia 16.5 12.2 Nausea 5.5 5.7 Hallucinations 3.9 3.2 Confusion 3.7 2.3 Dizziness 2.9 2.3 Depression 2.2 1.3 Urinary tract infection 2.2 2.3 Headache 2.0 1.9 Dream abnormalities 1.8 0.8 Dystonia 1.8 0.8 Vomiting 1.8 1.9 Upper respiratory infection 1.8 1.0 Dyspnea 1.6 0.4 ‘On-Off’ phenomena 1.6 1.1 Back pain 1.6 0.6 Dry mouth 1.4 1.1 Anorexia 1.2 1.1 Diarrhea 1.2 0.6 Insomnia 1.2 1.0 Orthostatic hypotension 1.0 1.1 Shoulder pain 1.0 0.6 Chest pain 1.0 0.8 Muscle cramps 0.8 1.0 Paresthesia 0.8 1.1 Urinary frequency 0.8 1.1 Dyspepsia 0.6 1.1 Constipation 0.2 1.5 Abnormal laboratory findings occurring at a frequency of 1% or greater in approximately 443 patients who received carbidopa and levodopa extended-release and 475 who received carbidopa and levodopa immediate-release during controlled clinical trials included: decreased hemoglobin and hematocrit; elevated serum glucose; white blood cells, bacteria and blood in the urine. The adverse experiences observed in patients in uncontrolled studies were similar to those seen in controlled clinical studies. Other adverse experiences reported overall in clinical trials in 748 patients treated with carbidopa and levodopa extended-release, listed by body system in order of decreasing frequency, include: Body as a Whole: Asthenia, fatigue, abdominal pain, orthostatic effects. Cardiovascular: Palpitation, hypertension, hypotension, myocardial infarction. Gastrointestinal: Gastrointestinal pain, dysphagia, heartburn. Metabolic: Weight loss. Musculoskeletal: Leg pain. Nervous System/Psychiatric: Chorea, somnolence, falling, anxiety, disorientation, decreased mental acuity, gait abnormalities, extrapyramidal disorder, agitation, nervousness, sleep disorders, memory impairment. Respiratory: Cough, pharyngeal pain, common cold. Skin: Rash. Special Senses: Blurred vision. Urogenital: Urinary incontinence. Laboratory Tests: Decreased white blood cell count and serum potassium; increased BUN, serum creatinine and serum LDH; protein and glucose in the urine. The following adverse experiences have been reported in postmarketing experience with carbidopa and levodopa extended-release: Cardiovascular: Cardiac irregularities, syncope. Gastrointestinal: Taste alterations, dark saliva. Hypersensitivity: Angioedema, urticaria, pruritus, bullous lesions (including pemphigus-like reactions). Nervous System/Psychiatric: Increased tremor, peripheral neuropathy, psychotic episodes including delusions and paranoid ideation, pathological gambling, increased libido including hypersexuality, impulse control symptoms. Skin: Alopecia, flushing, dark sweat. Urogenital: Dark urine. Other adverse reactions that have been reported with levodopa alone and with various carbidopa-levodopa formulations and may occur with carbidopa and levodopa extended-release are: Cardiovascular: Phlebitis. Gastrointestinal: Gastrointestinal bleeding, development of duodenal ulcer, sialorrhea, bruxism, hiccups, flatulence, burning sensation of tongue. Hematologic: Hemolytic and nonhemolytic anemia, thrombocytopenia, leukopenia, agranulocytosis. Hypersensitivity: Henoch-Schonlein purpura. Metabolic: Weight gain, edema. Nervous System/Psychiatric: Ataxia, depression with suicidal tendencies, dementia, euphoria, convulsions (however, a causal relationship has not been established); bradykinetic episodes, numbness, muscle twitching, blepharospasm (which may be taken as an early sign of excess dosage; consideration of dosage reduction may be made at this time), trismus, activation of latent Horner’s syndrome, nightmares. Skin: Malignant melanoma (see also CONTRAINDICATIONS ), increased sweating. Special Senses: Oculogyric crises, mydriasis, diplopia. Urogenital: Urinary retention, priapism. Miscellaneous: Faintness, hoarseness, malaise, hot flashes, sense of stimulation, bizarre breathing patterns. Laboratory Tests: Abnormalities in alkaline phosphatase, SGOT (AST), SGPT (ALT), bilirubin, Coombs test, uric acid.

Pharmacokinetics Carbidopa reduces the amount of levodopa required to produce a given response by about 75% and, when administered with levodopa, increases both plasma levels and the plasma half-life of levodopa, and decreases plasma and urinary dopamine and homovanillic acid. Elimination half-life of levodopa in the presence of carbidopa is about 1.5 hours. Following carbidopa and levodopa extended-release, the apparent half-life of levodopa may be prolonged because of continuous absorption. In healthy elderly subjects (56-67 years old) the mean time-to-peak concentration of levodopa after a single dose of 50 mg/200 mg carbidopa and levodopa extended-release was about 2 hours as compared to 0.5 hours after standard carbidopa and levodopa immediate-release. The maximum concentration of levodopa after a single dose of carbidopa and levodopa extended-release was about 35% of the standard carbidopa and levodopa immediate-release (1151 vs. 3256 ng/mL). The extent of availability of levodopa from carbidopa and levodopa extended-release was about 70-75% relative to intravenous levodopa or standard carbidopa and levodopa immediate-release in the elderly. The absolute bioavailability of levodopa from carbidopa and levodopa extended-release (relative to I.V.) in young subjects was shown to be only about 44%. The extent of availability and the peak concentrations of levodopa were comparable in the elderly after a single dose and at steady state after t.i.d. administration of 50 mg/200 mg carbidopa and levodopa extended-release. In elderly subjects, the average trough levels of levodopa at steady state after the extended-release tablet were about 2-fold higher than after the standard carbidopa and levodopa immediate-release (163 vs. 74 ng/mL). In these studies, using similar total daily doses of levodopa, plasma levodopa concentrations with carbidopa and levodopa extended-release fluctuated in a narrower range than with carbidopa and levodopa immediate-release. Because the bioavailability of levodopa from carbidopa and levodopa extended-release relative to carbidopa and levodopa immediate-release is approximately 70-75%, the daily dosage of levodopa necessary to produce a given clinical response with the extended-release formulation will usually be higher. The extent of availability and peak concentrations of levodopa after a single dose of 50 mg/200 mg carbidopa and levodopa extended-release increased by about 50% and 25%, respectively, when administered with food. At steady state, the bioavailability of carbidopa from carbidopa and levodopa immediate-release tablets is approximately 99% relative to the concomitant administration of carbidopa and levodopa. At steady state, carbidopa bioavailability from 50 mg/200 mg carbidopa and levodopa extended-release is approximately 58% relative to that from carbidopa and levodopa immediate-release. Pyridoxine hydrochloride (vitamin B 6 ), in oral doses of 10 mg to 25 mg, may reverse the effects of levodopa by increasing the rate of aromatic amino acid decarboxylation. Carbidopa inhibits this action of pyridoxine.