levodopa-carbidopa

**Mechanism** - Levodopa is a precursor of dopamine. It is absorbed in the proximal small intestine and crosses the BBB via the large neutral amino acid transporter. In the CNS, it is decarboxylated by DOPA decarboxylase (aromatic L-amino acid decarboxylase) in surviving nigrostriatal neurons to dopamine. This replenishes striatal dopamine lost from degeneration of the substantia nigra pars compacta. - Dopamine itself cannot cross the BBB, so levodopa is given instead. - Carbidopa inhibits peripheral DOPA decarboxylase and does NOT cross the BBB. Benefits: (1) about 4-5 fold lower levodopa dose needed, (2) less peripheral dopamine, which reduces nausea/vomiting (via the area postrema chemoreceptor trigger zone, which lies outside the BBB), tachycardia/arrhythmias, and orthostatic hypotension, and (3) more predictable CNS levels.

**Pathophysiology context (Parkinson disease)** - Loss of dopaminergic neurons in the substantia nigra pars compacta, with Lewy bodies (alpha-synuclein) on pathology. - Decreased dopamine leads to decreased direct pathway (D1) activation and increased indirect pathway (D2) inhibition. The result is reduced motor cortex activation and bradykinesia. - Classic features: TRAP (Tremor at rest, Rigidity [cogwheel], Akinesia/bradykinesia, Postural instability), plus a shuffling gait, masked facies, and micrographia.

**Adverse effects** - Early: nausea, vomiting, orthostatic hypotension (reduced by carbidopa), vivid dreams, and somnolence. - Long-term (usually after 5 or more years): - Dyskinesias (choreoathetoid movements, dystonia) from peak-dose effects. - "Wearing-off" phenomenon (symptoms return before the next dose). - "On-off" fluctuations (sudden, unpredictable changes in motor function). - Neuropsychiatric: hallucinations, psychosis, confusion, impulse control problems (these are more common with dopamine agonists). - Treat levodopa-induced psychosis with pimavanserin (a 5-HT2A inverse agonist) or quetiapine/clozapine. Avoid typical antipsychotics, which worsen parkinsonism.

**Drug interactions / pearls** - Dietary protein (large neutral amino acids) competes for absorption and BBB transport, so give the drug 30-60 minutes before meals. - Nonselective MAO inhibitors are contraindicated because they risk hypertensive crisis. Selective MAO-B inhibitors (selegiline, rasagiline) are OK and are used as adjuncts. - Pyridoxine (B6) in high doses increases peripheral decarboxylation of levodopa when it is given without carbidopa. Carbidopa largely eliminates this issue. - Carbidopa does not prevent central effects such as dyskinesias or psychosis. - Antipsychotics and metoclopramide (D2 blockers) antagonize its effect. - The most common side effect when carbidopa is inadequate is nausea. Domperidone (not available in the US) is a peripheral D2 antagonist used for this. - Abrupt withdrawal can precipitate neuroleptic malignant-like syndrome (parkinsonism-hyperpyrexia syndrome).

**Adjuncts to levodopa-carbidopa (to reduce "off" time)** - COMT inhibitors (entacapone, tolcapone [hepatotoxic], opicapone) block peripheral breakdown of levodopa to 3-O-methyldopa, which prolongs its half-life. - MAO-B inhibitors (selegiline, rasagiline, safinamide) reduce central dopamine breakdown. - Dopamine agonists (pramipexole, ropinirole [non-ergot], bromocriptine [ergot]). - Amantadine (helps dyskinesias). - Anticholinergics (benztropine, trihexyphenidyl) mainly for tremor in younger patients. - Deep brain stimulation (subthalamic nucleus or globus pallidus interna) for refractory motor fluctuations.

**Exam tips** - Levodopa is the most effective drug but is often delayed in younger patients because of the risk of motor complications. Dopamine agonists are sometimes started first in that group. - In an elderly patient with significant functional impairment, levodopa-carbidopa is first-line. - Know which side effects carbidopa prevents (peripheral) and which it does not (central). - Formulations include immediate-release, controlled-release, orally disintegrating, enteral suspension (Duopa, via jejunal infusion), and subcutaneous infusion formulations for advanced disease.