L-DOPA: From a biologically inactive amino acid to a successful therapeutic agent

The article traces the development of research on the naturally occurring amino acid L-3,4-dihydroxyphenylalanine (L-dopa), from the first synthesis of its D,L racemate in 1911, and the isolation of its L-isomer from seedling of Vicia faba beans to the amino acid's successful application, from 1961 onward, as the most efficacious drug treatment of Parkinson's disease (PD). Upon its isolation from legumes in 1913, L-dopa was declared to be biologically inactive. However, two early pharmacological studies, published in 1927 and 1930 respectively, proved (in the rabbit) that D,L-dopa exerted significant effects on glucose metabolism (causing marked hyperglycemia) and on arterial blood pressure. Interest in L-dopa's biological activity increased considerably following the discovery, in 1938, of the enzyme L-dopa decarboxylase and the demonstration that in the animal and human body L-dopa was enzymatically converted to dopamine (DA), the first biologically active amine in the biosynthetic chain of tissue catecholamines. This prompted, in the 1940s, many studies, both in animals and in humans, especially concerned with the vasopressor potential of L-dopa/DA. In the 1950s, the focus of L-dopa research shifted to its potential for replenishing the experimentally depleted (by insulin or reserpine) peripheral and brain catecholamine stores and the concomitant restoration of normal function. During that period, of special interest were the observations that L-dopa reversed the reserpine-induced state of "tranquilisation" and that its decarboxylation product DA occurred in high amounts in animal and human brain, with a preferential localization in the basal ganglia. These observations set the stage for the beginning of DA studies in PD brain. In 1960, the severe brain DA deficit, confined to patients with PD was discovered, and a year later L-dopa's strong therapeutic effect in patients with PD was demonstrated. In 1967, the chronic high-dose oral L-dopa regimen was successfully introduced into clinical practice. Despite some initial doubts about L-dopa's mechanism of action in PD, it is now generally recognized that L-dopa use in PD is a classic example of a brain neurotransmitter replacement therapy. However, the DA replacement potential of L-dopa may not be its sole action of interest, as suggested by recent evidence that L-dopa may also have its own biological activity in the CNS, independent of DA.