Paper I
2024 March (Supplementary) (2010 Scheme) · 40 marks · 120 min

Question

Levodopa and Carbidopa in the treatment of Parkinsonism

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Answer

Carbidopa (a peripheral dopa-decarboxylase inhibitor that does not cross the blood-brain barrier) is combined with levodopa to prevent peripheral conversion of levodopa to dopamine before it reaches the CNS. Since dopamine itself cannot cross the blood-brain barrier, levodopa (its precursor) must be given — but without carbidopa, most of an oral dose would be decarboxylated peripherally, producing dopamine-mediated peripheral adverse effects (nausea, vomiting, hypotension) while little reached the brain. Co-administration increases the CNS-available fraction of levodopa and substantially reduces the peripheral dose and adverse-effect burden needed for the same central antiparkinsonian effect.

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