A. Levodopa 1. Mechanisms—

Because dopamine has low bioavailability and does not readily cross the blood-brain barrier, its precursor, l-dopa (levodopa), is used. This amino acid enters the brain via an l-amino acid transporter (LAT) and is converted to dopamine by the enzyme aromatic l-amino acid decarboxylase (dopa decarboxylase), which is present in many body tissues, including the brain. Levodopa is usually given with carbidopa, a drug that does not cross the blood-brain barrier but inhibits dopa decarboxylase in peripheral tissues. With this combination, the plasma half-life is prolonged, lower doses of levodopa are effective, and there are fewer peripheral side effects.