Levodopa: History and Therapeutic Applications.

Levodopa - the aromatic amino acid L-3,4-dihydroxy phenylalanine has held the attention of neurologists and pharmacologists alike for more than half a century. Even though extensive research has been done across the globe in treatment of Parkinson's disease, with different molecules, none could replace the gold standard treatment or provide complete relief for the debilitated. Although research brought us better tips and tricks to modulate the dopamine blood levels to balance between the desired and deleterious effects, it could never replace the basic substrate. From simple oral preparation to more advanced treatment like duodenal dopa administration for better efficacy and compliance, L-dopa has sure undergone scrutiny and stayed strong as the fundamental neurotransmitter replacement therapy to pave path for many more new therapeutic strategies. So as a token of gratitude to the revolutionary agent and pioneers behind it, a trip down the memory lane is in order.

INTRODUCTION

Introduction of levodopa therapy in Parkinson's disease almost half a century back has revolutionized the therapeutic outcome of this challenging disease. Although several other agents have been subsequently added to the therapeutic armamentarium, levodopa remains the gold standard till date. The levodopa era began in 1967, when Cotzias et al. showed that orally administered levodopa had a dramatic and sustained effect on the symptoms of severely disabled Parkinsonian patients.[12]

The basic research that lead to the introduction of this wonder drug was the discovery of dopamine – a neurotransmitter that could control movements – by the Swedish pharmacologist Arvid Carlsson, who shared the Nobel prize in physiology and medicine in the year 2000.[3] Another major landmark in the development of levodopa therapy was the introduction of carbidopa the peripheral decarboxylase inhibitor. The success of levodopa therapy for Parkinson's disease also was the key factor in creating a new subspecialty of neurology, namely, movement disorders.[4]

Although the modern history of levodopa unfolds from the 19th century, it would be interesting to note that ancient Indian ayurvedic physicians used the seeds of Mucuna pruriens, which later proved to contain 4%–6% of levodopa, to treat symptoms of Parkinson's disease, as early as 300BC.[5678]

HISTORY OF LEVODOPA

L-Dopa, the naturally occurring isomer of the amino acid 3,4-dihydroxyphenylalanine, was first isolated in 1913 from legumes (seedlings of Vicia faba) by Marcus Guggenheim.[9] Casimir Funk had synthesized D, L-dopa in the laboratory two years prior to this at the Welcome labs in London, England[10] Almost 40 years later, Dale suggested its current name, dopamine for the compound synthesized earlier.[11] Both funk and Guggenheim considered the amino acid as a possible parent compound of adrenaline.

The discovery by Peter et al.[12] of an enzyme, DOPA decarboxylase, in mammalian tissue (kidney) extracts that converted L-dopa to the corresponding – biologically active amine, in 1938, represented a turning point in catecholamine research. This has led to the postulation of the possible pathway of catecholamine synthesis in the body by Blaschko and Holtz in 1939, which still holds good, and proceeds from L-tyrosine to L-dopa which get converted to dopamine, and this in turn get converted into catecholamines.[13] The discovery of the enzyme aromatic-L-amino-acid decarboxylase (also called dopa decarboxylase), provided a mechanism for the formation of dopamine in the brain from an exogenous source since L-dopa, unlike dopamine itself, can cross the blood–brain barrier.

Guggenheim, in addition to isolating L-dopa, was also the first to perform a self-experiment, by ingesting 2.5 g of L-dopa and demonstrated its emetic action which was however interpreted by him as a an unspecific irritation of the gastric mucosa but was later proved to be due to its conversion to dopamine which acts on the emetic centre in the medulla oblongata. Guggenheim believed L-dopa was biologically inactive but, in 1927, nearly 15 years later, it was disproved by Hirai and Gondo, who found that D, L-dopa caused hyperglycemia in rabbit (200–300 mg s. c.).[14] In 1930, Hasama demonstrated that in the rabbit D, L-dopa, in contrast to the vasopressor effect of adrenaline, produced a clear fall in arterial blood pressure, the lowest effective dose, given intravenously.[15]

In 1942, Holtz demonstrated that the administration of L-dopa in laboratory animals and humans results in excretion of dopamine in the urine.[16] He noticed that in humans, 50 mg L-dopa intravenous (IV) caused tachycardia. Dopamine was isolated first in the adrenal medulla by Goodall, 1950.[17] Raab and Gigee discovered the occurrence of a catecholamine-(adrenaline-) like substance in the brain of many species, including humans in 1951.[18] From then onward, reports of isolation of dopamine from body tissues started pouring in, from tumor tissue of phaeochromocytoma,[19] in the human brain,[20] in the iris and choroidal layer of the calf eye.[21]

Indisputably, the first researchers to study the effect of systemically administered dopa on brain catecholamines were Raab and Gigee in 1951. They injected, in rats, a great number of biologically active substances and found that only dopa increased the brain concentration of a catecholamine-(adrenaline-) like substance. Following these results, early dopamine and L-dopa trials[22] were started.

In 1956, Hornykiewicz working in Blaschko's laboratory, now located in Oxford, was assigned the task of determining whether blood pressure was directly affected by dopamine or if some other product of dopamine was responsible. Hornykiewicz showed that, indeed, dopamine did lower blood pressure and that L-dopa had a similar effect.[23]

CARLSSON'S DISCOVERY OF DOPAMINE

Arvid Carlsson, a Swedish postdoctoral fellow in Bernard Brodie's cardiac pharmacology laboratory at NIH, was investigating the mechanism whereby reserpine lowers blood pressure and slows heart rate. Brodie's laboratory had shown that reserpine lowered serotonin levels[24] and had proposed that loss of serotonin was responsible for reserpine's cardiovascular effect and also for its neurologic effect of “tranquilization” (now recognized as akinesia). Carlsson, however, wondered if perhaps it was not serotonin, but rather a catecholamine. At about the same time, L-dopa was found to cause central, behavioral as well as EEG activation and to antagonize reserpine's “tranquilizing” effects[25] responsible for lowering blood pressure and for the akinetic effect of reserpine. In 1957, he observed that norepinephrine and epinephrine disappeared more or less completely from tissues, including brain, after reserpine treatment[26] the same effect that Brodie and coworkers had shown for serotonin. After returning to Sweden, Carlson established his own laboratory at the University of Lund, he showed that D, L-dopa, not 5-hydroxytryptophan (the precursor of serotonin), reversed the akinetic effect of reserpine in rabbits[3] In dramatic fashion, reserpinized rabbits, after receiving D, L-dopa, went from being totally unresponsive to alert, with their drooping ears popping up almost immediately. During its initial phases, the Carlsson experiments were viewed with skepticism by the scientific community but mooted interest in several researchers to experiment on dopamine. Although a bit delayed, the revolutionary discovery of Carlson was honored by the Nobel committee in 2000 by awarding him the Nobel Prize in physiology and medicine.

Almost during the same period, Hornykiewicz, who returned back to Vienna from being a postdoc in Blaschco's lab in Oxford, began to measure postmortem brain dopamine in people with PD, postencephalitic parkinsonism, Huntington's disease, and in other extrapyramidal disorders, as well as in control brains, and in 1960, published a landmark paper showing for the first time a marked depletion of dopamine in the caudate and putamen of patients only in the PD and postencephalitic parkinsonian brains.[27] Six years later, Hornykiewicz published a major review article in Pharmacological Reviews positing that striatal dopamine deficiency is correlated with most of the motor symptoms of PD.[28]

LEVODOPA AND HUMAN EXPERIMENTS

In view of the existing evidences for use of L-dopa in Humans, in 1961 Hornykiewicz together with the neurologist Birkmayer performed a trial with i. v. L-dopa in a group of twenty patients[29] and first time reported the dramatic improvement of all motor deficits related to the symptom of akinesia, lasting for several hours. At the same time, and independently, over the next several years, some investigators around the world showed similar benefits using higher doses of L-dopa[29303132333435] while other investigators reported no benefits with similar doses.[3536373839]

The first person to use really high doses of DOPA was Patrick McGeer in Vancouver, British Columbia. Even with 5 g of D, L-dopa per day, McGeer failed to show a benefit.

The breakthrough for L-dopa as a therapeutic agent in PD came 6 years after the initial studies, in 1967, when George Cotzias in New York introduced, with great success, the high-dose oral L-dopa regimen, still used, in principle, today (Cotzias et al., 1967). George Cotzias developed a new approach, in which he used levodopa in gradually incremental dosages, eventually giving up to 16 g/day. Patients with PD tolerated this very slow, gradual build-up of dosage and then began to achieve dramatic, revolutionary benefit as therapeutic doses were reached. This was associated with less gastrointestinal side effects.

Soon after that, in 1969, Melvin Yahr in New York published the very first double-blind L-dopa study, establishing objectively the amino acids superior effectiveness as an Antiparkinsonian agent[40] [Table 1].

Table 1

Levodopa the journey so far

There have been several papers questioning the usefulness of levodopa, but none could disprove its usefulness.[414243] Most of the criticism waned off when 1975 Ken Lloyd in Toronto published a study conclusively demonstrated that in the striatum of PD patients treated with L-dopa, showed nine to fifteen-fold higher levels of dopamine than in nondopa-treated patients.[44]

THERAPUTIC APPLICATIONS OF LEVODOPA

The major therapeutic application of levodopa is still revolves around its use in Parkinsonism, Parkinsons plus syndrome and dopa-responsive dystonias. Clinical efficacy of levodopa varies in different subsets of symptoms. The classical motor symptoms of PD, bradykinesia, and rigidity, usually respond well to levodopa.

Response of tremor is less pronounced. Uniform poor response is the rule for other motor symptoms, such as speech and swallowing disorders, postural instability, and freezing gait. The response to nonmotor symptoms is poor with levodopa therapy, for example, cognitive disorders, dementia, depression psychosis autonomic dysfunction, and sleep disorders. Nocturnal treatment with levodopa may improve sleep in some patients. Chronic levodopa treatment is associated with Dyskinesia and motor fluctuations. Dyskinesias are involuntary movements that are mainly divided into peak-dose and biphasic dyskinesia. It is estimated that ~30%–50% of patients on levodopa therapy for more than 5-year experience dyskinesia.[45] Although duodopa has been developed to achieve continuous dopaminergic stimulation, the treatment remains unaffordable to most of our patients.

NEWER FORMULATIONS AND FUTURE OF LEVODOPA FORMULATIONS

Recent research has highlighted newer pharmacological properties of levodopa other than its actions on the dopaminergic system. For example, levodopa undergoes nonenzymatic conversion to form 2, 4, 5–trihydroxyphenylalanine (TOPA), a compound that reacts to form TOPA-quinone.[46] In vitro research using both TOPA and TOPA-quinone has found that these compounds are excitotoxic at glutamate receptors. These experiments showed additional properties of levodopa administration that is independent of dopaminergic receptor signaling, including alteration in neuronal firing rates, membrane depolarization, and inward current flux.[47] These responses occurred faster than the subsequent neurotransmission by dopamine at its receptors.

Nonenzymatic reactions in a physiological environment also yield a levodopa conjugate with cysteine, 5-S-cysteinyldopa. When it is decarboxylated in vivo, the resulting compound (5-S-cysteinyldopamine) produces pharmacological effects that differ from those of dopamine.[48]

The newer long-acting preparations, which have given promising result, include the latest extended-release levodopa-carbidopa product IPX066. This levodopa-carbidopa formulation utilizes microspheres created with coatings that govern the rate of drug release. By mixing a population of slower and more rapid release microspheres, the intended goal is a pharmacokinetic profile of drug release with greater constancy than that achieved with immediate-release levodopa-carbidopa. The results of clinical investigation with IPX066 show that its therapeutic-range plasma concentration of levodopa lasts 4 h or longer.[49]

Two products are under development for improving levodopa uptake are utilizing gastric retention strategies. One is DM-1992 (DepoMed), a gastric retentive formulation of levodopa and carbidopa. This product when exposed to gastric juices swells to an expanded size incapable of passing through the pylorus until it eventually dissolves up to 8 to9 h later.[50] Another gastro retentive product under development uses a proprietary drug delivery system that has been termed the accordion pill (AP). The AP is composed of layered polymer sheets folded into a capsule, that, when released, extend to exceed the size of the pylorus aperture. Consequently, the polymer sheets have several hours for the controlled gastric leaching of imbedded carbidopa and levodopa. This biodegradable drug platform remains in the stomach for up to 12 h before it dissolution.[51] Other modes of drug delivery, such as inhaled form of levodopa and subcutaneous delivery of levodopa are also under development.[5253]

CONCLUSION

The story of levodopa probably dates back from its use in ancient India as Atmagupta, the powdered form of mucuna pruriens for the treatment of tremor disorders. The revolutionary discovery of dopamine by Carlsson was probably the turning point in using this marvelous agent in modern therapeutics. However, the contributions from other scientists and researchers who have paved the way to the development of “Levodopa-the molecule of the century” cannot be undermined.

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Conflicts of interest

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