Levodopa is used in Parkinson's disease. Why is it combined with carbidopa?
- A Carbidopa inhibits peripheral DOPA decarboxylase, reducing systemic conversion of levodopa to dopamine; this increases levodopa availability to cross the BBB and reduces peripheral side effects ✓
- B Carbidopa crosses the blood-brain barrier and inhibits central MAO-B, enhancing dopamine availability
- C Carbidopa inhibits COMT in the periphery, slowing levodopa metabolism to 3-O-methyldopa
- D Carbidopa is an α2 adrenoceptor agonist that reduces levodopa-induced dyskinesias
Correct answer: A. Carbidopa inhibits peripheral DOPA decarboxylase, reducing systemic conversion of levodopa to dopamine; this increases levodopa availability to cross the BBB and reduces peripheral side effects
Explanation
Carbidopa is a peripheral DOPA decarboxylase inhibitor that does not cross the blood-brain barrier. When combined with levodopa, it prevents ~80% of peripheral conversion of levodopa to dopamine, reducing nausea, vomiting, and cardiovascular effects caused by peripheral dopamine. This also allows a 75% reduction in the levodopa dose required, since more levodopa reaches the CNS unchanged. COMT inhibitors (entacapone, tolcapone) perform the complementary role of blocking 3-O-methylation.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.