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Table 1 Inhibitor Constants of Selected COMT Inhibitors<br />

K i (nM) Human b K i (nM)Pig c<br />

Nitecapone 1.0 700<br />

Entacapone 0.3 N.D.<br />

Tolcapone 0.3 N.D.<br />

Vinylphenylketone 4.0 a 200<br />

a T. Lotta, unpublished results<br />

b Reference 19.<br />

c Reference 41.<br />

N.D.: not determined<br />

Page 353<br />

used as in vitro tools to investigate COMT inhibition, but because of the lack of potency and selectivity<br />

and because they were toxic, they were not clinically useful [2]. These inhibitors have inhibition<br />

constants (K i values) in the micromolar range. Many of them contain the catechol structure and are also<br />

substrates of COMT.<br />

B. Second-Generation Inhibitors<br />

The invention of a new structural family of COMT inhibitors in the late 1980s lead the COMT research<br />

into a new active epoch [25,26]. The most potent second-generation inhibitors are nitrocatechol<br />

derivatives; some examples are shown in Figure 11. Entacapone (OR-611) and tolcapone (RO-40-7592)<br />

are now in clinical trials for the treatment of Parkinson's disease, and have been extensively studied<br />

[27–40]. Both compounds are very potent and selective tightbinding inhibitors of human COMT with K i<br />

values of 0.3 nM [19]. They differ mainly in their pharmacokinetic properties. Entacapone acts<br />

peripherally while tolcapone inhibits COMT both peripherally and centrally.<br />

Recently it was reported that 2-((3,4-dihydroxy-2-nitrophenyl)vinyl) phenylketone is a tight-binding<br />

inhibitor of pig COMT [41], and it is also a potent inhibitor of human COMT (Table 1). This inhibitor<br />

has a vinylphenylketone group at position 4 of the catechol ring, i.e., in the position ortho to the nitro<br />

group.<br />

V. Enzyme Inhibitor Interactions<br />

A. Background<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_353.html (1 of 2) [4/5/2004 5:28:31 PM]

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