Learning Objectives
By the end of this session, the learner will be able to explain the pharmacology of Levodopa/Carbidopa, identify the roles of COMT and MAO-B inhibitors in prolonging dopamine activity, and recognize high-yield side effects like the “on-off” phenomenon.
1. Levodopa & Carbidopa
Levodopa is the metabolic precursor to Dopamine. Because Dopamine itself cannot cross the blood-brain barrier (BBB), we must administer it in the form of a prodrug.
[Image showing Levodopa crossing the BBB while Carbidopa remains in the periphery to inhibit AAAD]
| Drug | Mechanism | Clinical Purpose |
|---|---|---|
| Levodopa | Converted to DA by AAAD (decarboxylase). | Directly increases CNS dopamine levels. |
| Carbidopa | Inhibits peripheral AAAD. | Prevents peripheral conversion of L-DOPA to DA, reducing nausea and increasing CNS availability. |
2. COMT Inhibitors
COMT (Catechol-O-methyltransferase) is an alternative pathway for Levodopa metabolism. Blocking it increases the “half-life” of each dose.
| Drug | Clinical Use | Toxicity / Note |
|---|---|---|
| Entacapone | Peripheral COMT inhibition. | Enhances Levodopa efficacy. |
| Tolcapone | Central and peripheral COMT inhibition. | Hepatotoxic; monitor liver function tests (LFTs). |
3. MAO-B Inhibitors
These drugs selectively block the breakdown of Dopamine within the brain without the “Cheese Effect” risks associated with non-selective MAOIs.
| Drug | Unique Side Effect | Benefit |
|---|---|---|
| Selegiline | Insomnia (metabolized to amphetamine). | Used as initial monotherapy or as an adjunct. |
| Rasagiline | Less insomnia than selegiline. | Potent MAO-B selective inhibitor. |
Critical Toxicity: Levodopa Side Effects
As Dopamine levels rise in the brain and body, several high-yield side effects occur:
- Psychosis: Excessive DA in the mesolimbic pathway causes hallucinations.
- Dyskinesias: Chorea-like movements when levels are at their peak.
- “On-Off” Phenomenon: Sudden, unpredictable return of rigidity as drug levels fluctuate.
- Hypotension & Vomiting: Due to residual peripheral conversion to DA.
Clinical Pearls:
- Why Carbidopa? Without it, 99% of Levodopa is converted to DA in the blood, causing massive nausea/vomiting (via CTZ) and leaving only 1% to reach the brain.
- 3-O-methyldopa: COMT converts L-dopa into this metabolite, which is a partial agonist that competes with L-dopa for transport. Inhibiting COMT prevents this competition.
- Selegiline Mnemonic: It keeps the Dopamine “Selectively” in the “B”-rain.