Colon azoreductase

The metabolic transformation of many drugs is catalysed by various enzyme of the intestinal microflora. The anaerobic microflora and colon are rich in reductases which may be responsible for a significant proportion of the azoreductase and nitroreductase activity. The enzymes and other factors that may produce change in the nature of intestinal microflora might also produce changes in the metabolism pattern of the drugs. 

Sulfasalazine, balsalazide, and olsalazine contain 5-ASA bound by an azo (N=N) bond to an inert compound or to another 5-ASA molecule (Figure 62-8). In sulfasalazine, 5-ASA is bound to sulfapyridine in balsalazide, 5-ASA is bound to 4-aminobenzoyl-B-alanine and in olsalazine, two 5-ASA molecules are bound together. The azo structure markedly reduces absorption of the parent drug from the small intestine. In the terminal ileum and colon, resident bacteria cleave the azo bond by means of an azoreductase enzyme, releasing the active 5-ASA. Consequently, high concentrations of active drug are made available in the terminal ileum or colon. 

Of the azo compounds, 10% of sulfasalazine and less than 1% of balsalazide are absorbed as native compounds. After azoreductase breakdown of sulfasalazine, over 85% of the carrier molecule sulfapyridine is absorbed from the colon. Sulfapyridine undergoes hepatic metabolism (including acetylation) followed by renal excretion. By contrast, after azoreductase breakdown of balsalazide, over 70% of the carrier peptide is recovered intact in the feces and only a small amount of systemic absorption occurs. 

The most widely used intestinal microbial strains used to test colon targeting and enzymatic biodegradation of the carrier, respectively, are microorganisms with azoreductases and different types of glycosidases. 

The experiments can also be performed by means of isolated enzymes that occur in the human cecum and colon [64], These are, for instance, azoreductases for azo-prodrugs, dextranase, or particularly endo-glucanases that are able to internally cleave polysaccharide chains. The latter seem to be more effective than the azoreductases in metabolizing drugs, prodrugs, and drug-carrier materials. 

Bacterial azoreductase enzymes in the colon cleave the azo bond between 5-ASA and sulfapyridine, releasing the 5-ASA component. 

Redox reactions and hydrolysis are the predominant metabolic conversions triggered by the intestinal microflora. The primary reductive enzymes produced by the intestinal microflora are nitroreductase, deaminase, urea dehydroxylase, and azoreductase. The hydrolytic enzymes are p-glucuronidase, p-xylosidase, p-galactosidase, and ot-L-arabinosidase. Studies conducted by Macfarlane and coworkers have shown that proteolysis can also occur in the colon. More recent findings by the group indicate that bacterial fermentation of proteins in humans could account for 17% of the short-chain fatty acids in the cecum and for 38 /o in the sigmoid and the rectum. ... 

Colon and rectum 13-68h 7.0-7.5 Broad spectrum of bacterial enz3rmes (glycosidases, azoreductase, polysaccharidases)... 

An oligopeptide spacer with aromatic azo groups was used for selective release of 9-aminocamptothecin in colon. A combination of azoreductase and proteolytic activity was responsible for selective drug release in colon while the conjugate was stable in stomach and small intestine. Absorption of the drug in vivo after oral administration was also tested. ... 

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