Enkephalinase inhibitors

Neurokinin effects are terrninated by proteolysis. In vitro acetylcholinesterase (ACE) and enkephalinase can hydrolyze substance P. However, there appears to be no clear evidence that either acetylcholinesterase or ACE limit the actions of released substance P. Enkephalinase inhibitors, eg, thiorphan, can augment substance P release or action in some systems but the distribution of enkephalinase in the brain does not precisely mirror that of substance P. There appears to be a substance P-selective enzyme in brain and spinal cord. 

R = R = H), the first reported enkephalinase inhibitor, produced analgesia in post-myelography headache pain, but not in shock-induced pain (38). There was no tolerance or dependence observed. 

Roge J, Baumer P, Berard H, Schwartz JC, Lecomte JM The enkephalinase inhibitor, acetorphan, in acute diarrhoea. A doubleblind, controlled clinical trial versus loperamide. Scand J Gastroenterol 1993 28 352-354. 

J. C. Schwartz, Enkephalinase Inhibitors as Drugs , in Design of Enzyme Inhibitors as Drugs , Eds. M. Sandler, H. J. Smith, Oxford University Press, Oxford, 1989, p. 206-226. 

D. M. Lambert, F. Mergen, C. F. Berens, J. H. Poupaert, P. Dumont, Synthesis and Pharmacological Properties of 2-[S-Acetylthiophan]-l,3-diacylaminopropan-2-ol Derivatives as Chimeric Lipid Drug Carriers Containing an Enkephalinase Inhibitor , Pharm. Res. 1995, 12, 187-191. 

Independently, simple peptide hydroxamic acids (Z-Gly-L-Leu-NHOH and others) were first observed to inhibit the metalloprotease thermolysin in 1977 9,101 The structure was then further improved to the hydroxamidoalkylmalonyl-peptide moiety by considering the substrate specificity of thermolysin and other metalloproteases 10-121 A summary of hydroxamic acids reported to be inhibitors of various metalloenzymes up to 1983 has been published 131 In 1985 hydroxamido-benzylsuccinyl-L-alanine (kelatorphan) was synthesized and found to be one of the best enkephalinase inhibitors 141... 

Scheme 6 A Synthetic Route to Kelatorphan, an Enkephalinase Inhibitor 14 ...

Enkephalins are pentapeptides that bind to opiate receptors. In the gut, enkephalins promote the absorption of sodium, chloride and water (Dobbins et al 1980). Racecadotril is an oral enkephalinase inhibitor used in France and the Philippines for the treatment of acute diarrhea. It prevents the degradation of endogenous opioids (enkephalins) and thus promotes absorption of water and electrolytes from the intestinal lumen (Matheson Noble 2000). Studies have demonstrated the efficacy of racecadotril in two models of hypersecretory diarrhea infusion of cholera toxin and castor oil induced diarrhea. Moreover, unlike loperamide, racecadotril did not prolong transit time in the small intestine or colon. Further experiments have shown that racecadotril does not promote bacterial overgrowth in the small intestine (Duval-Iflah et al 1999). There are no reports on the use of racecadotril in horses. 

True antisecretory agents, those that block a biochemical process that promotes intestinal fluid secretion, are not available for clinical use in horses. Loperamide can reduce the volume of diarrhea in foals with a primarily small intestinal secretory disorder. However, treated foals may become colicky as a result of fluid distention in the intestines because the mechanism of action is primarily retention of fluid within the intestine. Also, retention of intestinal content may promote the proliferation of enteropathogens. The enkephalinase inhibitor racecadotril appears to have true antisecretory effect in animal models and in humans with diarrhea (Izzo et al 1998). Its safety or effectiveness in foals and horses has not been reported. 

Borson, D.B., Corrales, R., Varsano, S. et al. (1987). Enkephalinase inhibitors potentiate substance P-induced secretion of 35S04-macromolecules fix>m ferret trachea. Exp. Lung Res. 12, 21-36. 

Sekizawa, K., Tamaoki, J., Nadel, J.A. et al. (1987). Enkephalinase inhibitor potentiates substance P- and electrically induced contraction in ferret trachea. J. Appl. Physiol. 63, 1401-1405. 

Kefzol cephazolin. kelatorphan is a substituted alanine derivative, a NEUTRAL ENDOPEPTIDASEINHIBHOR ( enkephalinase inhibitor), with ANALGESIC activity. 

Salyrgan mercuderamide. sampatrilat [ban, inn] is a pseudopeptide, an ace INHIBITOR and neutral ENDOPEPTIDASE INHIBITOR ( enkephalinase inhibitor). It is a VASODILATOR and ANTIHYPERTENSIVE. 

Eournie-Zaluski, M. C., Lucas-Soroca, E., Devin, J., Roques, B. P. H NMR configural correlation for retro-inverso dipeptides application to the determination of the absolute configuration of enkephalinase inhibitors. J. Med. Chem. 1986, 29, 751-757. 

In washed brain slices, an in vitro model for the evaluation of effects of membrane-bound enzymes on enkephalin hydrolysis, the main hydrolysis products have been identified as Tyr and Tyr-Gly-Gly. The formation of Tyr-Gly-Gly is dependent on the action of enkephalinase and is reduced in the presence of thiorphan (an enkephalinase inhibitor). In the presence of bestatin (a general aminopeptidase inhibitor), the formation of Tyr is reduced Tyr is also reduced, to a lesser extent, in the presence of puromycin (an aminopeptidase Mil inhibitor). In the presence of thiorphan the formation of Tyr increases whereas in the presence of bestatin there is an increase in formation of Tyr-Gly-Gly. The level of Tyr-Gly in this model is low and is unaffected by either thiorphan or bestatin indicating that the action of DAP is unimportant. Recovery of endogenous enkephalins released by depolarisation of brain slices is enhanced in the presence of thiorphan or bestatin and is complete when both inhibitors are present. This does not occur in the case of puromycin or captopril (an ACE inhibitor) [31]. 

The biological activities of enkephalinase and aminopeptidase inhibitors have been found to be very weak when they are administered alone. However, enkephalinase inhibitors potentiate the effects of enkephalin analogues which are resistant to degradation by the aminopeptidases such... 

The presence of a C-2 substituent is essential for good enkephalinase inhibition and is more important than the absolute configuration of the compound. However, the (5)-isomer exhibits greater potency as an enkephalinase inhibitor [79,81] although the (/ )- and (5)-isomers interact with the enzyme with the same order of potency. This indicates flexibility within the region of the active site containing the Sj subsite and zinc. 

C-Terminal carboxylic acid. The presence of a free terminal carboxylate in the P2 amino acid residue in a substrate increases the interaction with enkephalinase. Esterification or amidification of the C-terminal group of [Met] -enkephalin leads to a 6- and 30-fold decrease in binding [78], respectively. Replacement by a hydroxymethyl also leads to a drastic loss of activity [73,79]. This feature is also apparent in enkephalinase inhibitors [79,84] although the differences are smaller (Table 6.3). 

Retro-inversion of hydroxamate enkephalinase inhibitors [89] leads to a 5-fold difference in potency between the (/ ) and (5) isomer compared with the 100-fold difference obtained with retrothiorphan. This difference between the thiol and hydroxamate inhibitors may be explained in terms of the geometrical parameters existing in their inhibitor-zinc complexes since in the thermolysin-thiol inhibitor complex, the zinc is tetra-coordinated [87], whereas in the thermolysin-hydroxamic acid inhibitor complex it is penta-coordinated [88]. The conformational space accessible to the thiol function is less than that accessible to the hydroxamate function, as there is only one degree of freedom in retrothiorphan (a rotation around the alpha... 

Enkephalinase inhibitors have been developed from early studies which utilized simple dipeptides as probes for identifying the structural requirements of the active site of the enzyme [78] together with information evolved from the development of ACE inhibitors. The different enkephalinase inhibitors have been classified according to the zinc binding ligand in the inhibitor molecule namely sulphydryl, carboxyalkyl and phosphoryl [33,71,91] (see Figure 6.5). 

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