What Is Carbidopa/Levodopa Therapy?
When Parkinson’s disease (PD) damages the neurons in the brain that produce dopamine, the motor symptoms of PD appear, including tremor, rigidity, impaired balance, and loss of spontaneous movement. The motor symptoms of PD are related to depletion of the neurotransmitter (chemical messenger) dopamine in the brain.
Dopamine is the neurotransmitter responsible for producing smooth, purposeful movement. Research has shown that by the time motor symptoms of PD appear, approximately 60-80% of the dopamine-producing neurons in the brain have been damaged or destroyed.
The combination of levodopa and carbidopa is the most effective treatment available for the management of motor symptoms of PD. Levodopa, also called L-dopa, was first discovered as a treatment for the symptoms of Parkinson’s disease over fifty years ago.
How carbidopa/levodopa therapy works
Giving dopamine as a therapy by itself is ineffective, because it cannot cross the blood-brain barrier. However, levodopa can cross the blood-brain barrier and can relieve the motor symptoms of PD.1,3,4 Levodopa is the precursor to dopamine – it is the substance that is used to make dopamine.1,2
Carbidopa is added to levodopa to prevent levodopa from being converted into dopamine in the bloodstream, which allows more of the medication to get to the brain. This also means that lower doses of levodopa can be given. The addition of carbidopa also reduces the risk of the side effects caused by dopamine in the rest of the body, such as nausea and vomiting.1,2
Formulations of carbidopa/levodopa therapy
There are several different formulations of carbidopa/levodopa therapy, including:
- Sinemet® (carbidopa levodopa)
- Sinemet® CR (carbidopa levodopa ER)
- Parcopa® (carbidopa levodopa) orally disintegrating tablet
- Stalevo® (carbidopa/levodopa/entacapone)
- Rytary® (carbidopa levodopa ER)
- Inbrija® (levodopa inhalation powder)
- Duopa® (carbidopa levodopa) enteral suspension1
There are different dosages for each of these formulations, and patients may be prescribed different dosages at different points in their disease to manage their symptoms. More information about each formulation is provided below:
- Sinemet®: Sinemet is a tablet that comes in three different strengths.
- Sinemet® CR: Sinemet CR is a sustained-release tablet that releases the ingredients over a 4-6 hour time frame. It comes in two different strengths.
- Parcopa®: Parcopa is taken orally and rapidly disintegrates on the tongue. No water is required to take Parcopa.
- Stalevo®: Stalevo is tablet that includes a combination of carbidopa, levodopa and entacapone, a COMT inhibitor. Entacapone is believed to allow more of levodopa to reach the brain by inhibiting the enzyme (COMT) that breaks down levodopa and dopamine.
- Rytary®: Rytary is an extended-release tablet that comes in four different strengths.
- Inbrija®: Inbrija is levodopa can be inhaled when patients begin to have an "off period".
- Duopa®: Duopa comes in a suspension form (gel) that is delivered directly into the intestines by a pump for up to 16 hours. It requires a procedure to make a small hole (called a stoma) in your stomach wall, and it provides another option for patients who still experience PD symptoms with oral treatments.1
Side effects of carbidopa/levodopa therapy
Some of the most common side effects of carbidopa/levodopa therapy include nausea, vomiting, orthostatic hypotension (falling blood pressure that occurs upon standing), worsening of glaucoma, dyskinesia, hallucination, psychosis, low blood pressure, confusion, dry mouth, and dizziness.1 Note that these are not all the possible side effects of carbidopa/levodopa therapy. Patients should talk to their doctor about what to expect with treatment with carbidopa/levodopa therapy.
It is important to talk with your healthcare provider before starting or stopping any new treatments, including over-the-counter medications and alternative treatments.
Dyskinesias are spontaneous, involuntary movements. They are a common side effect of long-term levodopa therapy. Dyskinesia can greatly affect the patient’s quality of life, and some patients find them very disturbing. The severity of dyskinesia due to levodopa therapy (also called levodopa-induced dyskinesia) varies among patients, and the risk of dyskinesia is higher in early-onset PD. For those people who experience dyskinesia, medications may be added or adjusted. Deep brain stimulation may also be an option.5,7
In people whose symptoms of PD are not controlled adequately by carbidopa/levodopa therapy, additional treatments may be added, or the person may be switched to other medications. Some additional treatments for PD include dopamine agonists, monoamine oxidase-B (MAO-B) inhibitors, and surgery for deep brain stimulation.6