Carbidopa¿¡ ´ëÇÑ Mechanism_Of_Action Á¤º¸ When mixed with levodopa, carbidopa inhibits the peripheral conversion of levodopa to dopamine and the decarboxylation of oxitriptan to serotonin by aromatic L-amino acid decarboxylase. This results in increased amount of levodopa and oxitriptan available for transport to the CNS. Carbidopa also inhibits the metabolism of levodopa in the GI tract, thus, increasing the bioavailability of levodopa. Entacapone¿¡ ´ëÇÑ Mechanism_Of_Action Á¤º¸ The mechanism of action of entacapone is believed to be through its ability to inhibit COMT and alter the plasma pharmacokinetics of levodopa. When entacapone is given in conjunction with levodopa and an aromatic amino acid decarboxylase inhibitor, such as carbidopa, plasma levels of levodopa are greater and more sustained than after administration of levodopa and an aromatic amino acid decarboxylase inhibitor alone. It is believed that at a given frequency of levodopa administration, these more sustained plasma levels of levodopa result in more constant dopaminergic stimulation in the brain, leading to greater effects on the signs and symptoms of Parkinson's disease. Levodopa¿¡ ´ëÇÑ Mechanism_Of_Action Á¤º¸ 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.
Pharmacology
Carbidopa¿¡ ´ëÇÑ Pharmacology Á¤º¸ Carbidopa, a noncompetitive decarboxylase inhibitor, is used in combination with levodopa for the treatment of Parkinson's disease. Entacapone¿¡ ´ëÇÑ Pharmacology Á¤º¸ Entacapone is used in the treatment of Parkinson¡¯s disease as an adjunct to levodopa/carbidopa therapy. Entacapone is a selective and reversible inhibitor of catechol-O-methyltransferase (COMT). In mammals, COMT is distributed throughout various organs with the highest activities in the liver and kidney. COMT also occurs in the heart, lung, smooth and skeletal muscles, intestinal tract, reproductive organs, various glands, adipose tissue, skin, blood cells and neuronal tissues, especially in glial cells. COMT catalyzes the transfer of the methyl group of S-adenosyl-L-methionine to the phenolic group of substrates that contain a catechol structure. Physiological substrates of COMT include dopa, catecholamines (dopamine, norepinephrine, and epinephrine) and their hydroxylated metabolites. The function of COMT is the elimination of biologically active catechols and some other hydroxylated metabolites. In the presence of a decarboxylase inhibitor, COMT becomes the major metabolizing enzyme for levodopa, catalyzing the metabolism to 3-methoxy-4-hydroxy-L-phenylalanine (3-OMD) in the brain and periphery. Levodopa¿¡ ´ëÇÑ Pharmacology Á¤º¸ 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.
Absorption
Carbidopa¿¡ ´ëÇÑ Absorption Á¤º¸ Rapidly decarboxylated to dopamine in extracerebral tissues so that only a small portion of a given dose is transported unchanged to the central nervous system. Entacapone¿¡ ´ëÇÑ Absorption Á¤º¸ Entacapone is rapidly absorbed (approximately 1 hour). The absolute bioavailability following oral administration is 35%. Levodopa¿¡ ´ëÇÑ Absorption Á¤º¸ Levodopa is rapidly absorbed from the proximal small intestine by the large neutral amino acid (LNAA) transport carrier system.
Carbidopa¿¡ ´ëÇÑ Toxicity Á¤º¸ Symptoms of a carbidopa toxicity include muscle spasms or weakness, spasms of the eyelid, nausea, vomiting, diarrhea, an irregular heartbeat, confusion, agitation, hallucinations, and unconsciousness. Entacapone¿¡ ´ëÇÑ Toxicity Á¤º¸ Side effect include increase the occurrence of orthostatic hypotension, severe rhabdomyolysis, dyskinesia, hallucinations, hyperkinesia, hypokinesia, dizziness, fatigu,e gastrointestinal effects including abdominal pain constipation diarrhea nausea Levodopa¿¡ ´ëÇÑ Toxicity Á¤º¸ Oral, mouse: LD50 = 2363 mg/kg; Oral, rabbit: LD50 = 609 mg/kg; Oral, rat: LD50 = 1780 mg/kg.
Drug Interactions
Carbidopa¿¡ ´ëÇÑ Drug_Interactions Á¤º¸ Not Available Entacapone¿¡ ´ëÇÑ Drug_Interactions Á¤º¸ Apomorphine Entacapone increases the effect and toxicity of sympathomimeticsBitolterol Entacapone increases the effect and toxicity of sympathomimeticsDobutamine Entacapone increases the effect and toxicity of sympathomimeticsDopamine Entacapone increases the effect and toxicity of sympathomimeticsEpinephrine Entacapone increases the effect and toxicity of sympathomimeticsIsocarboxazid Possible hypertensive crisis with this combinationIsoetharine Entacapone increases the effect and toxicity of sympathomimeticsIsoproterenol Entacapone increases the effect and toxicity of sympathomimeticsMethyldopa Entacapone increases the effect and toxicity of sympathomimeticsNorepinephrine Entacapone increases the effect and toxicity of sympathomimeticsPhenelzine Possible hypertensive crisis with this combinationTranylcypromine Possible hypertensive crisis with this combinationBitolterol Entacapone increases the effect and toxicity of sympathomimetics Levodopa¿¡ ´ëÇÑ Drug_Interactions Á¤º¸ Not Available
Carbidopa¿¡ ´ëÇÑ Description Á¤º¸ An inhibitor of DOPA decarboxylase, preventing conversion of levodopa to dopamine. It is used in parkinson disease to reduce peripheral adverse effects of levodopa. It has no antiparkinson actions by itself. [PubChem] Entacapone¿¡ ´ëÇÑ Description Á¤º¸ Entacapone is a catechol-O-methyl transferase inhibitor for the treatment of Parkinson's disease. When administered in conjunction with dopaminergic agents such as L-DOPA, entacapone increases the bioavailability of these compounds by facilitating their passage across the blood-brain barrier.It is a member of the class of nitrocatechols.The most frequent undesirable effects caused by entacapone relate to the increased effects of L-DOPA, such as involuntary movements (dyskinesias).These occur most frequently at the beginning of entacapone treatment. Others common side effects are gastrointestinal problems, including diarrhoea, nausea and abdominal pains. The substance may cause urine to turn reddish-brown. This is a harmless side effect and is not a cause for concern. In studies with entacapone, some people have reported experiencing a dry mouth. Levodopa¿¡ ´ëÇÑ Description Á¤º¸ The naturally occurring form of dihydroxyphenylalanine and the immediate precursor of dopamine. Unlike dopamine itself, it can be taken orally and crosses the blood-brain barrier. It is rapidly taken up by dopaminergic neurons and converted to dopamine. It is used for the treatment of parkinsonian disorders and is usually given with agents that inhibit its conversion to dopamine outside of the central nervous system. [PubChem]