Mechanism of Action and Pharmacokinetics
Dopamine itself is unable to cross the blood-brain barrier (BBB). Therefore, its precursor, levodopa, is used to replenish central nervous system (CNS) dopamine levels. The drug is absorbed in the intestine via an active transport system shared by large neutral amino acids. A protein-rich meal can compete for this transport carrier and decrease absorption efficiency.
Once it enters intact CNS neurons, levodopa is converted into dopamine by the enzyme DOPA decarboxylase. Released from nerve terminals, dopamine stimulates $D_2$ receptors on cholinergic interneurons in the neostriatum. This suppresses their pathological hyperactivity and, consequently, relieves the motor symptoms of parkinsonism.
Bioavailability Issues and Combination Drugs
The primary pharmacokinetic challenge is that over 90% of a levodopa dose undergoes extensive peripheral metabolism by DOPA decarboxylase (in the intestinal wall and liver) into dopamine, which cannot cross the BBB. As a result, only 1–3% of the administered dose reaches the brain. Additional degradation occurs via monoamine oxidase (MAO) and catechol-$O$-methyltransferase (COMT).
To optimize therapy, levodopa is combined with peripheral DOPA decarboxylase inhibitors (carbidopa or benserazide), which do not cross the BBB. This prevents premature peripheral conversion, increasing the fraction reaching the brain to about 10%. Such combinations lower the required therapeutic dose of levodopa and reduce peripheral adverse effects.
Examples of combination drugs:
- Levodopa + Carbidopa: Sinemet, Duopa.
- Levodopa + Benserazide: Madopar.
Side Effects and Management
Adverse reactions to levodopa are categorized into peripheral and central effects:
Peripheral Effects (mediated by dopamine outside the CNS):
- Cardiovascular: Tachycardia, arrhythmias, orthostatic hypotension.
- Gastrointestinal: Nausea and vomiting due to stimulation of $D_2$ receptors in the area postrema (chemoreceptor trigger zone) of the vomit center. This is managed using domperidone, a peripheral dopamine receptor antagonist that does not cross the BBB and thus does not diminish the antiparkinsonian effect.
Central Effects (caused by excessive dopamine in the CNS):
- Neuropsychiatric: Psychosis, hallucinations, anxiety, insomnia, somnolence.
- Motor: Choreiform dyskinesias (rapid, involuntary, purposeless movements).
To manage psychosis, clozapine is used as an atypical antipsychotic with preferential affinity for $D_4$ receptors, avoiding exacerbation of parkinsonian symptoms.
Long-Term Complications and Contraindications
After more than 5 years of treatment, the efficacy of levodopa often declines due to the progressive degeneration of dopaminergic neurons. Dopamine metabolites are also hypothesized to exert neurotoxic effects.
A classic long-term complication is the "on-off" syndrome—sudden fluctuations in clinical state ranging from severe parkinsonian symptoms («off») to dyskinesias caused by excessive drug levels («on»), despite stable dosing. This is managed using controlled-release formulations or adjunctive medications.
Levodopa is contraindicated in narrow-angle glaucoma, diabetes mellitus, severe cardiovascular disease (arrhythmias), active psychoses, severe hepatic or renal impairment, individuals under 25 years of age, and during pregnancy or breastfeeding.