title: Gliclazide hydroxylation by rat liver microsomes.
abstract: 1. The metabolism of gliclazide to hydroxygliclazide has been investigated in Sprague-Dawley rat liver microsomes. 2. The kinetics of hydroxygliclazide formation are consistent with Michaelis-Menten kinetics (mean (+/- SD, n = 3) apparent K(m) and Vmax = 256 +/- 27 microM and 1.85 +/- 0.10 nmol/ min/mg respectively). 3. Tolbutamide competitively inhibited hydroxygliclazide formation (Ki = 840 microM) and gliclazide competitively inhibited hydroxytolbutamide formation (Ki = 240 microM) with Ki similar to K(m). Therefore gliclazide and tolbutamide may be metabolized by the same enzyme in the rat. In nine livers the formation of hydroxygliclazide correlated with the formation of hydroxytolbutamide (rs = 0.82, p < 0.01). 4. Diclofenac (Ki = 64 microM), phenytoin (Ki = 38 microM), mephenytoin (Ki = 66 microM), glibenclamide (Ki = 14 microM) and glipizide (Ki = 189 microM) were fully competitive inhibitors of gliclazide hydroxylation. The rank order of Ki constants differed for gliclazide and tolbutamide suggesting that gliclazide and tolbutamide hydroxylases are not identical enzymes. 5. Quinine (Ki = 0.3 microM) and quinidine (Ki = 4.3 microM) were partially competitive inhibitors of hydroxygliclazide formation. Hydroxylation of gliclazide was related to the activity of CYP2D1 as assessed by dextrorphan production from dextromethorphan (rs = 0.83, p = 0.01). 6. In the rat gliclazide is metabolized to hydroxygliclazide by at least two cytochrome P450 isoforms, including tolbutamide hydroxylase and 2D1, which have similar affinities for gliclazide.
authors.affiliation: Department of Clinical Pharmacology, Royal North Shore Hospital, St Leonards, NSW, Australia.
authors.forename: A
authors.initials: A
authors.lastname: Rieutord
authors.forename: I
authors.initials: I
authors.lastname: Stupans
authors.forename: G M
authors.initials: GM
authors.lastname: Shenfield
authors.forename: A S
authors.initials: AS
authors.lastname: Gross
mesh_terms: D000818:Animals; D002851:Chromatography, High Pressure Liquid; D019389:Cytochrome P-450 CYP2D6; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005907:Gliclazide / Q000031:analogs & derivatives / Q000378:metabolism*; D005913:Glipizide / Q000494:pharmacology; D005905:Glyburide / Q000494:pharmacology; D006900:Hydroxylation; D007004:Hypoglycemic Agents / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D008862:Microsomes, Liver / Q000378:metabolism*; D006899:Mixed Function Oxygenases / Q000037:antagonists & inhibitors / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D014044:Tolbutamide / Q000031:analogs & derivatives / Q000378:metabolism; D000096926:Benzenesulfonamides
chemical_list: D019389:Cytochrome P-450 CYP2D6; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System; D004791:Enzyme Inhibitors; D005907:Gliclazide; D005913:Glipizide; D005905:Glyburide; D007004:Hypoglycemic Agents; D006899:Mixed Function Oxygenases; D014044:Tolbutamide; C031014:hydroxymethyltolbutamide; D000096926:Benzenesulfonamides; C099074:hydroxygliclazide