Mechanism of Action

Current evidence indicates that symptoms of Parkinson’s disease are related to depletion of dopamine in the corpus striatum. Administration of dopamine is ineffective in the treatment of Parkinson’s disease apparently because it does not cross the blood-brain barrier. However, levodopa, the metabolic precursor of dopamine, does cross the blood-brain barrier, and presumably is converted to dopamine in the brain. This is thought to be the mechanism whereby levodopa relieves symptoms of Parkinson’s disease.

Pharmacodynamics

When levodopa is administered orally it is rapidly decarboxylated to dopamine in extracerebral tissues so that only a small portion of a given dose is transported unchanged to the central nervous system. For this reason, large doses of levodopa are required for adequate therapeutic effect and these may often be accompanied by nausea and other adverse reactions, some of which are attributable to dopamine formed in extracerebral tissues.

The incidence of levodopa-induced nausea and vomiting is less when LODOSYN is used with levodopa than when levodopa is used without LODOSYN. In many patients this reduction in nausea and vomiting will permit more rapid dosage titration.

Carbidopa inhibits decarboxylation of peripheral levodopa. Carbidopa has not been demonstrated to have any overt pharmacodynamic actions in the recommended doses. It does not appear to cross the blood-brain barrier readily and does not affect the metabolism of levodopa within the central nervous system at doses of carbidopa that are recommended for maximum effective inhibition of peripheral decarboxylation of levodopa.

Since its decarboxylase-inhibiting activity is limited primarily to extracerebral tissues, administration of carbidopa with levodopa makes more levodopa available for transport to the brain. However, since levodopa and carbidopa compete with certain amino acids for transport across the gut wall, the absorption of levodopa and carbidopa may be impaired in some patients on a high protein diet.

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.

In clinical pharmacologic studies, simultaneous administration of separate tablets of carbidopa and levodopa produced greater urinary excretion of levodopa in proportion to the excretion of dopamine when compared to the two drugs administered at separate times.

Supplemental pyridoxine (vitamin B6) can be given to patients when they are receiving carbidopa and levodopa concomitantly or as SINEMET* CR (Carbidopa-Levodopa) Sustained-Release or SINEMET* (Carbidopa-Levodopa). Previous reports in the medical literature cautioned that high doses of vitamin B6 should not be taken by patients on levodopa therapy alone because exogenously administered pyridoxine would enhance the metabolism of levodopa to dopamine. The introduction of carbidopa to levodopa therapy, which inhibits the peripheral decarboxylation of levodopa to dopamine, counteracts the metabolic-enhancing effect of pyridoxine.

Carbidopa is combined with levodopa in SINEMET (Carbidopa-Levodopa) and SINEMET CR (Carbidopa-Levodopa) Sustained-Release tablets. These combination tablets are available in three strengths for SINEMET: SINEMET 10-100 (Carbidopa-Levodopa), SINEMET 25-250 (Carbidopa-Levodopa) (1:10 ratio of carbidopa to levodopa) and SINEMET 25-100 (Carbidopa-Levodopa) (1:4 ratio of carbidopa to levodopa), and in two strengths for SINEMET CR: SINEMET CR 50-200 (Carbidopa-Levodopa) Sustained-Release and SINEMET CR 25-100 (Carbidopa-Levodopa) Sustained-Release (1:4 ratio of carbidopa to levodopa). Clinical trials show that these ratios of carbidopa and levodopa provide useful therapeutic effects in most patients.

LODOSYN (Carbidopa) has no antiparkinsonian effect when given alone. It is indicated for use with SINEMET (Carbidopa-Levodopa) or levodopa. LODOSYN (Carbidopa) does not decrease adverse reactions due to central effects of levodopa.

When LODOSYN (Carbidopa) is to be given to carbidopa-naive patients who are being treated with levodopa alone, the two drugs should be given at the same time. At least twelve hours should elapse between the last dose of levodopa and initiation of therapy with LODOSYN (Carbidopa) and levodopa in combination. Start with no more than one-fifth (20%) to one-fourth (25%) of the previous daily dosage of levodopa when given without LODOSYN (Carbidopa). See the DOSAGE AND ADMINISTRATION section before initiating therapy.

As with levodopa, concomitant administration of LODOSYN and levodopa may cause involuntary movements and mental disturbances. These reactions are thought to be due to increased brain dopamine following administration of levodopa. All patients should be observed carefully for the development of depression with concomitant suicidal tendencies. Patients with past or current psychoses should be treated with caution. Because LODOSYN (Carbidopa) permits more levodopa to reach the brain and, thus, more dopamine to be formed, dyskinesias may occur at lower levodopa dosages and sooner with concomitant use of LODOSYN (Carbidopa) and levodopa or carbidopa-levodopa combination products than with levodopa alone. The occurrence of dyskinesias may require levodopa dosage reduction.

Levodopa, with or without LODOSYN, should be administered cautiously to patients with severe cardiovascular or pulmonary disease, bronchial asthma, renal, hepatic, or endocrine disease.

Care should be exercised in administering levodopa, with or without LODOSYN, to patients with a history of myocardial infarction who have residual atrial, nodal, or ventricular arrhythmias. In such patients, cardiac function should be monitored with particular care during the period of initial dosage adjustment, in a facility with provisions for intensive cardiac care.

As with levodopa alone there is a possibility of upper gastrointestinal hemorrhage in patients with a history of peptic ulcer.

Neuroleptic Malignant Syndrome (NMS): Sporadic cases of a symptom complex resembling NMS have been reported in association with dose reductions or withdrawal of certain antiparkinsonian agents such as levodopa, SINEMET (Carbidopa-Levodopa), or SINEMET CR (Carbidopa-Levodopa) Sustained-Release. Therefore, patients should be observed carefully when the dosage of levodopa is reduced abruptly or discontinued, especially if the patient is receiving neuroleptics.

NMS is an uncommon but life-threatening syndrome characterized by fever or hyperthermia. Neurological findings, including muscle rigidity, involuntary movements, altered consciousness, mental status changes; other disturbances, such as autonomic dysfunction, tachycardia, tachypnea, sweating, hyper- or hypotension; laboratory findings, such as creatine phosphokinase elevation, leukocytosis, myoglobinuria, and increased serum myoglobin, have been reported.

The early diagnosis of this condition is important for the appropriate management of these patients. Considering NMS as a possible diagnosis and ruling out other acute illnesses (e.g., pneumonia, systemic infection, etc.) is essential. This may be especially complex if the clinical presentation includes both serious medical illness and untreated or inadequately treated extrapyramidal signs and symptoms (EPS). Other important considerations in the differential diagnosis include central anticholinergic toxicity, heat stroke, drug fever, and primary central nervous system (CNS) pathology.

The management of NMS should include: 1) intensive symptomatic treatment and medical monitoring and 2) treatment of any concomitant serious medical problems for which specific treatments are available. Dopamine agonists, such as bromocriptine, and muscle relaxants, such as dantrolene, are often used in the treatment of NMS; however, their effectiveness has not been demonstrated in controlled studies.

General

As with levodopa alone, periodic evaluations of hepatic, hematopoietic, cardiovascular, and renal function are recommended during extended concomitant therapy with LODOSYN and levodopa, or with LODOSYN and SINEMET (Carbidopa-Levodopa), or any combination of these drugs.

Patients with chronic wide-angle glaucoma may be treated cautiously with LODOSYN and levodopa or SINEMET, or any combination of these drugs, just as with levodopa alone, provided the intraocular pressure is well controlled and the patient is monitored carefully for changes in intraocular pressure during therapy.

Laboratory Tests

Abnormalities in laboratory tests may include elevations of liver function tests such as alkaline phosphatase, SGOT (AST), SGPT (ALT), lactic dehydrogenase, and bilirubin. Abnormalities in blood urea nitrogen and positive Coombs test have also been reported. Commonly, levels of blood urea nitrogen, creatinine, and uric acid are lower during concomitant administration of carbidopa and levodopa than with levodopa alone.

Levodopa and carbidopa-levodopa combination products may cause a false-positive reaction for urinary ketone bodies when a test tape is used for determination of ketonuria. This reaction will not be altered by boiling the urine specimen. False-negative tests may result with the use of glucose-oxidase methods of testing for glucosuria.

Drug Interactions

Caution should be exercised when the following drugs are administered concomitantly with LODOSYN (Carbidopa) given with levodopa or carbidopa-levodopa combination products.

Symptomatic postural hypotension has occurred when LODOSYN, given with levodopa or carbidopa-levodopa combination products, was added to the treatment of a patient receiving antihypertensive drugs. Therefore, when therapy with LODOSYN, given with or without levodopa or carbidopa-levodopa combination products, is started, dosage adjustment of the antihypertensive drug may be required.

For patients receiving monoamine oxidase inhibitors, see CONTRAINDICATIONS. Concomitant therapy with selegiline and carbidopa-levodopa may be associated with severe orthostatic hypotension not attributable to carbidopa-levodopa alone (see CONTRAINDICATIONS).

There have been rare reports of adverse reactions, including hypertension and dyskinesia, resulting from the concomitant use of tricyclic antidepressants and carbidopa-levodopa preparations.

Dopamine D2 receptor antagonists (e.g., phenothiazines, butyrophenones, risperidone) and isoniazid may reduce the therapeutic effects of levodopa. In addition, the beneficial effects of levodopa in Parkinson’s disease have been reported to be reversed by phenytoin and papaverine. Patients taking these drugs with LODOSYN and levodopa or carbidopa-levodopa combination products should be carefully observed for loss of therapeutic response.

Iron salts may reduce the bioavailability of carbidopa and levodopa. The clinical relevance is unclear.

Although metoclopramide may increase the bioavailability of levodopa by increasing gastric emptying, metoclopramide may also adversely affect disease control by its dopamine receptor antagonistic properties.

Carcinogenesis, Mutagenesis, Impairment of Fertility

Pregnancy

Nursing Mothers

It is not known whether carbidopa is excreted in human milk. Because many drugs are excreted in human milk, and because of their potential for serious adverse reactions in nursing infants, a decision should be made whether to discontinue nursing or to discontinue the drug, taking into account the importance of the drug to the nursing woman.

Pediatric Use

Safety and effectiveness in pediatric patients have not been established, and use of the drug in patients below the age of 18 is not recommended.

Patients Receiving SINEMET (Carbidopa-Levodopa) Who Require Additional Carbidopa

Some patients taking SINEMET (Carbidopa-Levodopa) may not have adequate reduction in nausea and vomiting when the dosage of carbidopa is less than 70 mg a day, and the dosage of levodopa is less than 700 mg a day. When these patients are taking SINEMET 10-100** (Carbidopa-Levodopa), 25 mg of LODOSYN may be given with the first dose of SINEMET (Carbidopa-Levodopa) each day. Additional doses of 12.5 mg or 25 mg may be given during the day with each dose of SINEMET (Carbidopa-Levodopa). When patients are taking SINEMET 25-250*** (Carbidopa-Levodopa) or SINEMET 25-100† (Carbidopa-Levodopa), 25 mg of LODOSYN may be given with any dose of SINEMET (Carbidopa-Levodopa) as required for optimum therapeutic response. The maximum daily dosage of carbidopa, given as LODOSYN and as SINEMET (Carbidopa-Levodopa), should not exceed 200 mg.

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**SINEMET 10-100 (Carbidopa-Levodopa) contains 10 mg of carbidopa and 100 mg of levodopa.

***SINEMET 25-250 (Carbidopa-Levodopa) contains 25 mg of carbidopa and 250 mg of levodopa.

†SINEMET 25-100 (Carbidopa-Levodopa) contains 25 mg of carbidopa and 100 mg of levodopa.

Patients Requiring Individual Titration of Carbidopa and Levodopa Dosage

Although SINEMET (Carbidopa-Levodopa) is the preferred method of carbidopa and levodopa administration, there may be an occasional patient who requires individually titrated doses of these two drugs. In these patients, LODOSYN (Carbidopa) should be initiated at a dosage of 25 mg three or four times a day. The two drugs should be given at the same time, starting with no more than one-fifth (20%) to one-fourth (25%) of the previous or recommended daily dosage of levodopa when given without LODOSYN (Carbidopa). In patients already receiving levodopa therapy, at least twelve hours should elapse between the last dose of levodopa and initiation of therapy with LODOSYN (Carbidopa) and levodopa. A convenient way to initiate therapy in these patients is in the morning following a night when the patient has not taken levodopa for at least twelve hours. Physicians who prescribe separate doses of LODOSYN and levodopa should be thoroughly familiar with the directions for use of each drug.

Dosage Adjustment

Dosage of LODOSYN may be adjusted by adding or omitting one-half or one tablet a day. Because both therapeutic and adverse responses occur more rapidly with combined therapy than when only levodopa is given, patients should be monitored closely during the dose adjustment period. Specifically, involuntary movements will occur more rapidly when LODOSYN and levodopa are given concomitantly than when levodopa is given without LODOSYN. The occurrence of involuntary movements may require dosage reduction. Blepharospasm may be a useful early sign of excess dosage in some patients.

Current evidence indicates other standard antiparkinsonian drugs may be continued while carbidopa and levodopa are being administered. However, the dosage of such other standard antiparkinsonian drugs may require adjustment.

Interruption of Therapy

Sporadic cases of a symptom complex resembling the Neuroleptic Malignant Syndrome (NMS) have been associated with dose reductions and withdrawal of SINEMET (Carbidopa-Levodopa) or SINEMET CR (Carbidopa-Levodopa) Sustained-Release. Patients should be observed carefully if abrupt reduction or discontinuation of SINEMET (Carbidopa-Levodopa) or SINEMET CR (Carbidopa-Levodopa) Sustained-Release is required, especially if the patient is receiving neuroleptics. (See WARNINGS.)

If general anesthesia is required, therapy may be continued as long as the patient is permitted to take fluids and medication by mouth. When therapy is interrupted temporarily, the patient should be observed for symptoms resembling NMS, and the usual daily dosage may be resumed as soon as the patient is able to take medication orally.

Storage

Store at 25°C (77°F), excursions permitted to 15–30°C (59–86°F).