The L -dopa story: Translational neuroscience ante verbum

Abstract

Since almost 50 years L-Dopa is the gold standard for the treatment of patients with Parkinson’s disease (PD). For the first time, a specific chemical abnormality was found in a specific brain disorder. It has been shown that the striatal dopamine (DA) content is greatly reduced in PD patients. The substitution of DA by its precursor L-dopa greatly enhanced the quality of life of PD patients.

In the late 60s and the early 70s of the 20th century, the introduction of levodopa (L-dopa) in treatments markedly changed the fate of patients suffering from Parkinson’s disease (PD) in Western countries. It was preceded by a series of antecedents during several decades. Nowadays, L-dopa combined with a decarboxylase inhibitor is the gold standard treatment for PD. This conclusion has been reached along a long and winding path.

L-dopa and dopamine

Dihydroxyphenylalanine (D,L-dopa) was first synthesized by Funk. ¹ Casimir Funk (1884–1967) was a Polish biochemist. He worked in Berlin, Berne (where he wrote his doctoral thesis), London and the United States. He coined the term ‘vitamine’. The young Swiss chemist Markus Guggenheim (1884–1967; Figure 1) at Hoffmann-La Roche isolated L-dopa from broad bean seeds (Vicia faba), characterized it chemically and developed a simplified method for its synthesis. ² The substance was licensed by Hoffmann-La Roche. In animal experiments, he could not detect any obvious effect. On self-ingesting up to 2.5 g of L-dopa, Guggenheim developed minimal nausea and vomiting, and he observed no clinical use for it and the substance disappeared in his drawer for many years. Later, in his comprehensive textbook Die biogenen Amine, Guggenheim ³ dedicated not more than two pages to L-dopa.

Figure 1.

Markus Guggenheim (1884–1967).

In 1939, just at the start of World War II, Peter Holtz (1902–1970) showed in Germany that L-dopa is metabolized to dopamine (DA) by dopa decarboxylase. ⁴ It is not surprising that his contribution, published in German, received little attention at the time. Holtz and Credner ⁵ have injected 50 mg of L-dopa intravenously to a volunteer, resulting merely in an increase in the pulse rate.

For a long time, DA was considered as a mere metabolic intermediate in the formation of the catecholamines adrenaline and noradrenaline. Only Carlsson et al. ^6,7 demonstrated that it is a neurotransmitter occurring on its own]. In animal experiments, ‘catalepsy’ induced by reserpine was completely abolished by L-dopa. It replenished brain DA depleted by reserpine. Sano et al. ⁸ in Japan discovered that DA is normally highly concentrated in human brains. Bertler and Rosengren, ⁹ working at Carlsson’s lab in Lund (Sweden), found the same in dog brains. They showed that about 80% of the total brain DA was concentrated in the striatum, especially in the caudate nucleus and the putamen.

Carlsson (Figure 2) was born in 1926 at Uppsala (Sweden), and he became a professor of pharmacology at the University of Gothenburg in 1959. In 2000, the Nobel Prize was awarded jointly to Arvid Carlsson, Paul Greengard and Eric R. Kandel for their discoveries concerning signal transduction in the nervous system. Many neurologists and neuroscientists regretted that Oleh Hornykiewicz was not awarded at the same time for his crucial contributions to the L-dopa treatment of patients with PD.

Figure 2.

Arvid Carlsson (*1926).

Oleh Hornykiewicz (Figure 3) was born 1926 in Galicia, at the time part of Poland, now in Ukraine. His father, an orthodox priest, moved with his family to Vienna to escape the Stalinist terror. After having learned German, he studied medicine in Vienna after World War II. He graduated in 1953 and started to work at the Department of Pharmacology, at the time chaired by Franz von Brücke. He obtained a scholarship from the British Council with the intention to work with the German-born physician, biochemist and Nobel Laureate Sir Hans Krebs in Oxford. As Krebs had no appropriate project for him, he was recommended to Hugh (Hermann) Blaschko, another German émigré. Under his auspices, Hornykiewicz worked on the role of DA in peripheral tissues. Blaschko ¹⁰ had argued that DA in addition to being a metabolic intermediate may have ‘some regulatory functions of its own which are not yet known’.

Figure 3.

Oleh Hornykiewicz (*1926).

Back to Vienna, encouraged by Kathleen Montagu’s ¹¹ finding of DA in the brains of different animals and the above-mentioned papers by Sano et al. ⁸ and by Bertler and Rosengren, ⁹ Hornykiewicz decided to investigate DA in human brains. Together with his assistant Herbert Ehringer, he examined post-mortem brain material, 3–20 h after death, from 20 controls, 2 patients with Huntington’s chorea, 6 with other extrapyramidal diseases and 6 with PD (4 postencephalitic and 2 PD) with a rather insensitive colorimetric method. In the basal ganglia of controls as well as in the Huntington cases, they found high concentrations of DA, whereas it was diminished by a factor of approximately 10 only in the Parkinsonian patients. ¹² In 2007, Hornykiewcz stated, ‘For the first time, a specific chemical abnormality was found in a specific brain region in specific brain disorder…’.

A few months later, Barbeau et al. ¹³ published that DA excretion in the urine of PD patients is reduced.

Later, with a more sensitive spectrofluorometric method, Hornykiewcz ¹⁴ demonstrated DA depletion in the pars compacta of substantia nigra. He concluded that the striatal DA deficiency is a consequence of the nigral cell loss and that there must exist an anatomical connection originating in the melanin-containing substantia nigra and terminating in the striatum.

Treating PD patients with L-dopa

Hornykiewicz was determined to replace the missing DA in PD patients by L-dopa. He chose the intravenous route, which had been used successfully and without severe side effects by Degkwitz et al. ¹⁵ in their attempt to overcome the side effects of reserpine, and because he disposed only a very limited amount of L-dopa delivered by Hoffmann-La Roche in Basle.

He contacted Walther Birkmayer (1910–1996; Figure 4), at the time head of the neurological ward of the Municipal Home for the aged in Lainz-Vienna that permanently housed PD patients, for collaboration. In the beginning, Birkmayer was not very enthusiastic and he delayed the start of the project by 6 months. Later, he confessed that he was taking revenge because 2 years earlier Hornykiewicz had refused to collaborate in a project which was not appealing to him. ¹⁶

Figure 4.

Walther Birkmayer (1910–1996).

They injected intravenously 50–150 mg of L-dopa in saline to 21 patients with PD or with postencephalitic Parkinsonism. The effect of a single injection was spectacular: Patients who had been almost completely immobilized were able to move almost freely. The effect on bradykinesia lasted from 3 to 24 h after a single injection. Intravenous injections with chemically related substances (i.e. D-dopa, DA, 3-O-methyldopa, dihydroxyphenylalanine [DOPS], 5-hydroxytryptophane [5-HTP]) resulted in no similar effect. ^17,18

Subsequently, several authors reported similar effect with intravenous L-dopa (cited by Calne ¹⁹ ). Two controlled double-blind studies did not provide any therapeutic benefit from 1.5 mg/kg of intravenous L-dopa. ^20,21 Oral L-dopa in PD patients gave rather controversial results (Greer and Williams, 1963). ^18,22,23,24

The therapeutic value of L-dopa remained very controversial. Even Hornykiewicz ²⁵ felt that treatment with L-dopa is impractical, and Fahn (2015) ²⁶ speculated that neurologists might have given up on further trials with this substance. Eventually, the reports of Cotzias et al. in 1967 ²⁷ and 1969 ²⁸ mark a turning point, and they had an enormous impact in the acceptance of L-dopa treatment. They have shown that chronic oral treatment with L-dopa is practical and that it provides a sustained improvement.

George Cotzias (1918–1977; Figure 5) was born in the Isle of Crete and started his medical studies at the University of Athens. Because he had served in the Greek army, he had to leave Greece in 1941 when the German army invaded Greece. He immigrated to the United States, and despite initially speaking hardly a word of English, he was, eventually, able to continue his medical education at Harvard Medical School, where he graduated summa cum laude in 1943. He came in contact with Parkinsonism when he was studying toxicity of manganese. Miners from Chile excavating manganese suffered from Parkinsonism. ²⁹

Figure 5.

George Cotzias (1918–1977).

Cotzias was aware of the publications from Lund and Vienna; nevertheless, he started dopa treatment with another goal than increasing striatal DA level. He suspected that melanin depletion was the major problem in PD. By giving precursors of melanin, such as melanocyte-stimulating hormone, phenylalanine and dopa, he attempted to increase nigral melanin concentration. Only dopa shows clinical improvement. Nowadays, it is generally accepted that this concept was wrong. Cotzias et al. ³⁰ later spoke of an ‘erroneous but fruitful hypothesis’.

Cotzias et al. ^27,28 first used D,L-dopa and later L-dopa in daily doses up to 16 and 8 g, respectively. They started at low doses and gradually increased the daily dosage divided in several small amounts. The highest level that could be achieved in an individual patient was maintained over several weeks. They were able to achieve high doses with minimal nausea and vomiting. In earlier studies, these gastrointestinal problems frequently have been dose limiting. In about three quarters of the 28 patients, a marked and sustained improvement was achieved.

In a prospective double-blind placebo-controlled study, Yahr et al. ³¹ confirmed the therapeutic effect of L-dopa. In this trial, 49 of 60 patients significantly improved during the double-blind phase. By 4 months, all but four patients had improved by at least 20%. Improvement included rigidity, bradykinesia and tremor.

Oliver Sacks ³² has very vividly described the ‘awakening’ of patients suffering from postencephalitic Parkinsonism by L-dopa.

The advent of decarboxylase inhibitors

It has been mentioned that in the mid-60s of the 20th century, L-dopa treatment for PD was not generally not accepted. Astonishingly, the obvious beneficial effect of L-dopa was met with opposition and disbelief. Several alternative theories have been proposed to explain its therapeutic action. Bertler and Rosengreen ³³ have written that ‘the effect of levodopa was too complex to permit conclusions about disturbances of the dopamine system in Parkinson’s disease’.

It goes without saying that the promoters of the DA theory have tried to find more evidence in its favour. One study aimed to prevent L-dopa to be metabolized to DA by adding a decarboxylase inhibitor to L-dopa. It was expected that the L-dopa effect would be abolished by preventing its transformation to DA.

Hoffmann-La Roche in Basle disposed of such an inhibitor (benserazide) which had been tested before without effect in patients with arterial hypertension. Based on previous animal experiments, it was assumed to interfere with the formation of DA from L-dopa in vivo and to attenuate the therapeutic effect of L-dopa. ³⁴ However, such attenuation could not be observed. On the contrary, benserazide seemed to enhance the L-dopa effect in PD patients. ³⁵

This seemingly paradoxical clinical observation could best be explained by poor penetration of the inhibitor from the blood to the brain. ³⁶ Subsequently, the combination of L-dopa with a decarboxylase inhibitor (benserazide or carbidopa) became the gold standard treatment for PD. Since the decarboxylase inhibitors protect L-dopa from decarboxylation in extracerebral organs, larger amounts of L-dopa are available for penetration into the brain, where it is transformed to DA. With the combination of L-dopa and a decarboxylase inhibitor, the amount of L-dopa to achieve a therapeutic effect can markedly be reduced, and at the same time, peripheral sides are significantly attenuated.

The L-dopa story is an example of how important and fruitful a close collaboration between basic scientists and clinicians can be. It also demonstrates that tortuous paths may finally lead to a successful end.