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netLibrary - eBook Summary Structure-based Drug Design by ...

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Page 360<br />

therapy of Parkinson's disease L-dopa (precursor of dopamine) is administered together with a<br />

peripheral dopadecarboxylase (DDC) inhibitor (such as carbidopa or benserazide). The major peripheral<br />

L-dopa metabolizing enzymes are DDC and COMT. After inhibition of DDC, COMT is responsible for<br />

the main catabolism of L-dopa. In the central nervous system COMT together with mono- amino<br />

oxidase (MAO) participitates in the metabolism of L-dopa and dopamine. Large amounts of orally<br />

administered L-dopa are converted <strong>by</strong> COMT to 3-O-methyldopa (3-OMD). Having a long plasma halflife<br />

(approximately 15 h compared with the dopamine 1-h half-life), 3-OMD accumulates in the plasma<br />

during the L-dopa treatment. L-Dopa and 3-OMD also compete for the same active transport system into<br />

the brain. It has been proposed that 3-OMD could cause some side effects of the L-dopa treatment<br />

(dyskenisia, on-off phenomenon). Inhibition of COMT enzyme decreases the 3-OMD formation and<br />

improves the brain entry and bioavailability of L-dopa. The use of COMT inhibition should prolong the<br />

L-dopa effects and permit a decreased does [27– 29].<br />

Preclinical and clinical results indicate that both entacapone and tolcapone are orally active, nontoxic<br />

and well-tolerated drugs. The adjuvant L- dopa therapy with DDC inhibitor + COMT-inhibitor (+<br />

possible MAO inhibitor) may substitute for the present double therapy in the treatment of Parkinson's<br />

disease [27–40]. Together with the development of dopamine agonists and MAO inhibitors, the<br />

inhibition of COMT will constitute major progress in the treatment of Parkinson's disease in the near<br />

future.<br />

B. Other Possible Indications of the COMT Inhibition<br />

It has been proposed that COMT inhibitors co-administered with L-dopa could have beneficial effects in<br />

the treatment of depressive illness [40]. This can be caused <strong>by</strong> either the better availability of dopamine<br />

or <strong>by</strong> the elevated noradrenaline levels in the brain. Another hypothesis suggests that the increasing level<br />

of AdoMet caused <strong>by</strong> COMT inhibition may cause an antidepressive effect [33].<br />

Dopamine has also natriuretic and diuretic effects in kidney. There has been evidence that abnormalities<br />

of the renal dopamine system can lead to salt- sensitive hypertension [48]. In rat kidney, deamination<br />

represents the major pathway in the metabolism of dopamine, but when MAO is inhibited, methylation<br />

appears to offer an alternative metabolic pathway [49]. Thus COMT inhibition may be important in the<br />

regulation of renal sodium excretion.<br />

References<br />

1. Axelrod J, Tomchick R. Enzymatic O-methylation of epinephrine and other catechols. Journal of<br />

Biological Chemistry 1958; 233:702–705.<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_360.html [4/5/2004 5:30:11 PM]

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