Gliclazide hydroxylation by rat liver microsomes

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 V-max=256 +/- 27 mu M and 1.85 +/- 0.10 nmol/min/mg respectively). 3. Tolbutamide competitively inhibited hydroxygliclazide formation (K-i=840 mu M) and gliclazide competitively inhibited hydroxytolbutamide formation (K-i=240 mu M) with K-i 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 (r(s) = 0.82, P < 0.01). 4. Diclofenac (K-i=64 mu M), phenytoin (K-i=38 mu M), mephenytoin (K-i=66 mu M), glibenclamide (K-i=14 mu M) and glipizide (K-i=189 mu M) were fully competitive inhibitors of gliclazide hydroxylation. The rank order of K-i constants differed for gliclazide and tolbutamide suggesting that gliclazide and tolbutamide hydroxylases are not identical enzymes. 5. Quinine (K-i=0.3 mu M) and quinidine (K-i=13 mu M) were partially competitive inhibitors of hydroxygliclazide formation. Hydroxylation of gliclazide was related to the activity of CYP2D1 as assessed by dextrorphan production from dextromethorphan (r(s)= 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.