## Levodopa Transport Across the Blood-Brain Barrier **Key Point:** Levodopa crosses the blood-brain barrier via the large neutral amino acid transporter type 1 (LAT1), a saturable, active transport mechanism. This is why levodopa—not dopamine—is used therapeutically; dopamine cannot cross the BBB. **Mnemonic:** **LAT1** = **L**arge **A**mino acid **T**ransporter type **1**. Remember: LAT1 is saturable, so high protein meals can compete and reduce levodopa absorption. ## Why Peripheral Decarboxylase Inhibitors Are Used While levodopa IS converted to dopamine in the periphery, the statement "must always be given" is too absolute: - Peripheral conversion causes nausea, vomiting, and cardiac arrhythmias - Decarboxylase inhibitors (carbidopa, benserazide) **cannot cross the BBB** and thus block only peripheral conversion - However, in rare cases (e.g., late-stage disease with severe dyskinesias), levodopa monotherapy without a decarboxylase inhibitor may be used **High-Yield:** The combination levodopa + carbidopa allows ~75% dose reduction while improving efficacy and tolerability. ## Drug Selection by Age | Age Group | First-Line Agent | Rationale | |-----------|------------------|----------| | <60 years | Dopamine agonist | Delay levodopa to reduce motor complications | | 60–70 years | Levodopa or DA agonist | Individualize; DA agonist if dyskinesia risk high | | >70 years | **Levodopa** | Better efficacy, fewer non-motor side effects; dyskinesia risk lower due to shorter life expectancy | **Clinical Pearl:** Levodopa undergoes decarboxylation (not hepatic metabolism) as its primary elimination pathway. Bioavailability is relatively predictable when given with a decarboxylase inhibitor [cite:Harrison 21e Ch 297].
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