Identification of the major human hepatic and placental enzymes responsible for the biotransformation of glyburide

Abstract

One of the factors affecting the pharmacokinetics (PK) of a drug during pregnancy is the activity of hepatic and placental metabolizing enzymes. Recently, we reported on the biotransformation of glyburide by human hepatic and placental microsomes to six metabolites that are structurally identical between the two tissues. Two of the metabolites, 4-trans- (M1) and 3-cis-hydroxycyclohexyl glyburide (M2b), were previously identified in plasma and urine of patients treated with glyburide and are pharmacologically active. The aim of this investigation was to identify the major human hepatic and placental CYP450 isozymes responsible for the formation of each metabolite of glyburide. This was achieved by the use of chemical inhibitors selective for individual CYP isozymes and antibodies raised against them. The identification was confirmed by the kinetic constants for the biotransformation of glyburide by cDNA-expressed enzymes. The data revealed that the major hepatic isozymes responsible for the formation of each metabolite are as follows: CYP3A4 (ethylene-hydroxylated glyburide (M5), 3-trans-(M3) and 2-trans-(M4) cyclohexyl glyburide); CYP2C9 (M1, M2a( 4-cis-) and M2b); CYP2C8 (M1 and M2b); and CYP2C19 (M2a). Human placental microsomal CYP19/aromatase was the major isozyme responsible for the biotransformation of glyburide to predominantly M5. The formation of significant amounts of M5 by CYP19 in the placenta could render this metabolite more accessible to the fetal circulation. The multiplicity of enzymes biotransforming glyburide and the metabolites formed underscores the potential for its drug interactions in vivo.

Glyburide Disposition During Pregnancy

“…These clinical studies appear to suggest that CYP2C9 contributes significantly to glyburide metabolism in vivo. On the other hand, in vitro studies using human liver microsomes have shown that CYP3A4 contributes greater than 50% of glyburide metabolism, while CYP2C9 contributes a much smaller percentage (Naritomi et al, 2004;Zharikova et al, 2009;Zhou et al, 2010a). Additionally, Lilja et al showed that oral administration of clarithromycin, an inhibitor of CYP3A but not CYP2C9, significantly increased C max and the plasma AUC of glyburide (Lilja et al, 2007).…”

“…It is possible that glyburide is metabolized in vivo through the joint actions of hepatic CYP3A and CYP2C9. In vitro metabolism studies using human liver microsomes or recombinant systems revealed that, besides CYP3A4 and CYP2C9, glyburide was also metabolized by other cytochrome P450 enzymes such as CYP3A5, CYP2C8 and CYP2C19, but to a much lesser extent (Naritomi et al, 2004;Zharikova et al, 2009;Zhou et al, 2010a). Zharikova et al determined five metabolites of glyburide formed in human liver microsomes: M1 (4-transhydrocyclohexyl glyburide), M2a (4-cis-hydrocyclohexyl glyburide), M2b (3-cishydrocyclohexyl glyburide), M3 (3-trans-hydrocyclohexyl glyburide), M4 (2-transhydrocyclohexyl glyburide) and M5 (ethylene-hydroxylated glyburide) (Zharikova et al, 2009;Zharikova et al, 2007).…”

“…CYP2C8 catalyzes the formation of M1, M2b, M3 and M4. CYP2C19 catalyzes the formation of M2a, M2b and M3 (Zharikova et al, 2009;Zharikova et al, 2007). The two major metabolites of glyburide, M1 and M2b, which account for approximately half of all the metabolites formed in vitro by human liver microsomes (Zharikova et al, 2007), are excreted into the bile and urine (~50% each) (Feldman, 1985).…”

“…Several groups have shown that CYP3A is a major enzyme responsible for the in vitro metabolism of glyburide (Naritomi et al, 2004;Zharikova et al, 2009;Zhou et al, 2010a). It has also been well established that hepatic CYP3A activity is significantly induced by pregnancy Tracy et al, 2005).…”

“…Human term placentas have been used in several studies to characterize placental metabolism of glyburide (Jain et al, 2008;Zharikova et al, 2009;Zharikova et al, 2007). Zharikova et al reported that placental microsomes converted ~87% of glyburide to the M5 metabolite (ethylene-hydroxylated glyburide), and the rest to other metabolites (Zharikova et al, 2007).…”