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l-DOPA, also known as l-3,4-dihydroxyphenylalanine and used medically as levodopa, is made and used as part of the normal biology of some plants [2] and animals, including humans. Humans, as well as a portion of the other animals that utilize l -DOPA, make it via biosynthesis from the amino acid l -tyrosine .
In addition, levodopa, co-administered with a peripheral DDCI, is efficacious for the short-term treatment of restless leg syndrome. [11] The two types of response seen with administration of levodopa are: The short-duration response is related to the half-life of the drug.
Carbidopa reduces the amount of levodopa required to produce a given response by about 75% and, when administered with levodopa, increases both plasma levels and the plasma half-life of levodopa, and decreases plasma and urinary dopamine and homovanillic acid. Elimination half-life of levodopa in the presence of carbidopa is about 1.5 hours.
Carbidopa/levodopa, also known as levocarb and co-careldopa, is the combination of the two medications carbidopa and levodopa. [6] It is primarily used to manage the symptoms of Parkinson's disease , but it does not slow down the disease or stop it from getting worse. [ 6 ]
Droxidopa, also known as (–)-threo-3-(3,4-dihydroxyphenyl)-L-serine (L-DOPS), is a substituted phenethylamine and is chemically analogous to levodopa (L-3,4-dihydroxyphenylalanine; L-DOPA). Whereas levodopa functions as a precursor and prodrug to dopamine, droxidopa is a precursor and prodrug of norepinephrine. [citation needed]
Absorption half-life 1 h, elimination half-life 12 h. Biological half-life (elimination half-life, pharmacological half-life) is the time taken for concentration of a biological substance (such as a medication) to decrease from its maximum concentration (C max) to half of C max in the blood plasma.
All of these metabolites are inactive except the reduced derivative. Opicapone is eliminated with a terminal half-life of 0.7 to 3.2 hours. It is mainly excreted via the faeces (67%), and in form of the glucuronide also via the kidney (13%). The sulfate has a much longer half-life of 94 to 122 hours. [11] [13] [15]
Tolcapone is used in the treatment of Parkinson's disease as an adjunct to levodopa/carbidopa or levodopa/benserazide medications. Levodopa is a prodrug for dopamine, which reduces Parkinson symptoms; carbidopa and benserazide are aromatic L-amino acid decarboxylase (AADC) inhibitors.