Levodopa CAS NO.59-92-7

L-dopa is used to increase dopamine levels for the treatment of Parkinson's disease and dopamine-responsive dystonia, since it is able to cross the bl…

L-dopa is used to increase dopamine levels for the treatment of Parkinson's disease and dopamine-responsive dystonia, since it is able to cross the blood-brain barrier, whereas dopamine itself cannot. Once levodopa has entered the central nervous system (CNS), it is metabolized to dopamine by aromatic L-amino acid decarboxylase. Pyridoxal phosphate (vitamin B6) is a required cofactor for this decarboxylation, and may be administered along with levodopa, usually as pyridoxine. Conversion to dopamine also occurs in the peripheral tissues, i.e., outside the brain. This may be the mechanism of the adverse effects of levodopa, listed below. It is standard clinical practice to co-administer a peripheral DOPA decarboxylase inhibitor,carbidopa or benserazide,and often a catechol-O-methyl transferase (COMT) inhibitor, like entacapone, to prevent synthesis of dopamine in peripheral tissue. Co-administration of pyridoxine without a decarboxylase inhibitor accelerates the extracerebral decarboxylation to such an extent that it cancels out the effects of levodopa administration, a circumstance that historically caused great confusion.