Levodopa is
the “gold standard” for the medical treatment of the motor symptoms of Parkinson’s
disease (PD) [1].
How levodopa was
discovered and the process by which it was found to have anti-parkinsonian
effects is an interesting scientific story.
Levodopa is
the precursor to the neurotransmitter dopamine, i.e., it is converted to
dopamine in the body. A neurotransmitter
is a chemical that “transmits” information from one neuron (nerve cell) to
another [2]. Dopamine is involved in the ability to move, form
memories and learn, experience pleasure, sleep, and keep stable emotional
states. It was first synthesized in 1911, by Casimir
Funk in London [3]. At the same time, an
Italian pharmacologist, Torquato Torquati, isolated and crystallized levodopa from
the seeds of a broad bean plant (Vicia
faba), while researching naturally occurring compounds in plants [4].
In 1913,
Swiss biochemist Marcus Guggenheim discovered that the compound that Torquati
had produced was levodopa [3, 4]. Guggenheim
used himself as a test subject and ingested 2.5 g of levodopa (the equivalent
of taking 25 Sinemet 25/100 pills at once) and recorded his body’s
reaction. He became nauseous and began
to vomit, which he assumed incorrectly was due to an irritation of the mucus
membrane layer of his stomach [5].
Four years
after Guggenheim’s work with levodopa, Swiss dermatologist Bruno Bloch correctly
postulated that levodopa was the parent compound—a chemical composite from
which derivatives can be created—of melanin [3]. Melanin is responsible for determining skin
and hair color, i.e., their pigmentation [6].
This discovery showed that levodopa
occurred naturally in human skin cells.
For many
years it was not realized that levodopa had any useful biological functions. In the early 1940s, however, it was found that
the body converted levodopa into dopamine and in the 1950s dopamine was also
found to occur naturally in the body [7].
Swedish scientist Arvid Carlsson discovered that dopamine was a
neurotransmitter in the brain [8] and established a scientific method for measuring
dopamine levels in brain tissue [9]. Subsequent
analysis showed that the basal ganglia, an area of the brain essential for movement,
normally contained high levels of dopamine [10]. By giving animals a drug
called reserpine—which reduces the levels of dopamine—Carlsson induced a loss
of spontaneous movements [11]. In 2000, he was awarded the Nobel Prize in
Physiology or Medicine for his work.
In 1959 two
Austrian doctors, Ehringer and Hornykiewicz, measured dopamine levels in the
brains of deceased patients. Using 17 human control brains, they compared dopamine
levels to those in the brains of 2 patients with Huntington’s disease, 6
patients with movement disorders of unknown origin, and 6 brains of patients
with PD. Compared to the controls and the patients with other neurodegenerative
diseases, only the brains of the 6 PD patients showed severe dopamine loss [12].
A treatment hypothesis
was soon formed: PD patients suffer from a loss of dopamine; therefore, the
introduction of dopamine to the body should have a beneficial effect. Certain substances, including dopamine, are
unable to penetrate from the blood into the brain. Accordingly, levodopa —which
does penetrate this blood-brain barrier —was used instead as the therapeutic agent. In 1961, two independent medical studies found
that levodopa led to significantly improved motor function of PD patients [13,
14]. Seven years later, neurologist
George Cotzias outlined a high-dose levodopa treatment for PD patients that is
still in use today [15, 16].
While
levodopa can cross the blood-brain barrier, researchers discovered that the majority
of levodopa administered was being metabolized by the body before it reached
the brain. The inclusion of the compound
carbidopa to the levodopa inhibited this extra-cerebral breakdown of levodopa,
so that more levodopa was able to get into the brain. An added benefit of slowing the extra-cerebral
breakdown of levodopa was a significant reduction in the nausea experienced by patients. Consequently, patients were able to take much
smaller doses of levodopa with the same (or more) therapeutic benefit.
In 1972 a combination
carbidopa/levodopa treatment was approved in the United States under the trade
name Sinemet© (a Latin combination of Sin-“without”
and Emet-“vomiting”) in four different
dose strengths, 10/100, 25/100, 25/250 and 50/250. Madopar© was created for distribution in
Europe and other countries, using a combination of levodopa and benserazide (a
compound with the same inhibitory capability as carbidopa). Over forty years have passed since
symptomatic levodopa treatment was introduced for PD, and to this day it
remains the most widely used course of treatment for combatting the motor
symptoms of PD.
References
- Aminoff MJ. Parkinson's disease.
Neurol Clin. 2001;19(1):119-28, vi.
- National Institute of Health. Brain
Basics. Available at http://www.nimh.nih.gov/health/educational-resources/brain-basics/brain-basics.shtml.
Accessed on March 23, 2016
- Hornykiewicz O. L-Dopa: A historical
perspective. In: Yoshimi Misu, Yoshio Goshima, eds. Neurobiology of DOPA as a Neurotransmitter. Boca Raton, FL:
CRC Press; 2005: 3-16
- Hauser RA. Levodopa: past, present,
and future. Eur Neurol. 2009;62(1):1-8.
- Hornykiewicz O. A brief history of
levodopa. J Neurol. 2010;257(Suppl 2):S249-52.
- Mandal, A. What is Melanin? http://www.news-medical.net/health/What-is-Melanin.aspx
Accessed 12/17/2015
- Carlsson A, Lindqvist M, Magnusson T,
Waldeck B. On the presence of 3-hydroxytyramine in brain. Science.
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- Barbeau A, Sourkes TL, Murphy GF. Les
catécholamines dans la maladie de Parkinson. In: de Ajuriaguerra J (ed)
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