| Indication | For the treatment of idiopathic Parkinson's disease (Paralysis Agitans), postencephalitic parkinsonism, symptomatic parkinsonism which may follow injury to the nervous system by carbon monoxide intoxication, and manganese intoxication. |
| Pharmacodynamics | Levodopa (L-dopa) is used to replace dopamine lost in Parkinson's disease because dopamine itself cannot cross the blood-brain barrier where its precursor can. However, L-DOPA is converted to dopamine in the periphery as well as in the CNS, so it is administered with a peripheral DDC (dopamine decarboxylase) inhibitor such as carbidopa, without which 90% is metabolised in the gut wall, and with a COMT inhibitor if possible; this prevents about a 5% loss. The form given therapeutically is therefore a prodrug which avoids decarboxylation in the stomach and periphery, can cross the blood-brain barrier, and once in the brain is converted to the neurotransmitter dopamine by the enzyme aromatic-L-amino-acid decarboxylase. |
| Mechanism of action | Striatal dopamine levels in symptomatic Parkinson's disease are decreased by 60 to 80%, striatal dopaminergic neurotransmission may be enhanced by exogenous supplementation of dopamine through administration of dopamine's precursor, levodopa. A small percentage of each levodopa dose crosses the blood-brain barrier and is decarboxylated to dopamine. This newly formed dopamine then is available to stimulate dopaminergic receptors, thus compensating for the depleted supply of endogenous dopamine. |
| Absorption | Levodopa is rapidly absorbed from the proximal small intestine by the large neutral amino acid (LNAA) transport carrier system. |
| Volume of distribution | Not Available |
| Protein binding | High |
| Metabolism | 95% of an administered oral dose of levodopa is pre-systemically decarboxylated to dopamine by the L-aromatic amino acid decarboxylase (AAAD) enzyme in the stomach, lumen of the intestine, kidney, and liver. Levodopa also may be methoxylated by the hepatic catechol-O-methyltransferase (COMT) enzyme system to 3-O-methyldopa (3-OMD), which cannot be converted to central dopamine. |
| Route of elimination | Not Available |
| Half life | 50 to 90 minutes |
| Clearance | Not Available |
| Toxicity | Oral, mouse: LD50 = 2363 mg/kg; Oral, rabbit: LD50 = 609 mg/kg; Oral, rat: LD50 = 1780 mg/kg. |