ABSTRACT
Title
In vitro inhibition of cytochrome P450 (CYP) 2C8-mediated drug metabolism by the flavonoid diosmetin
Authors
L. Quintieri, P. Palatini, S. De Martin and M. Floreani
Dept. of Pharmacology and Anesthesiology, Universityof Padova, Padova, Italy
Dept. of Pharmacology and Anesthesiology, Universityof Padova, Padova, Italy
Abstract
Background: The use of dietary supplements and/or herbal medications containing flavonoids has constantly increased in recent years, in spite of their potential for interaction with co-administered drugs. Recently, Rajnarayana et al reported that oral treatment of healthy volunteers with diosmin, the 7-rhamnosyl-D-glucoside of diosmetin (3’,5,7-trihydroxy-4-methoxyflavone), a flavone present in widely used pharmaceutical preparations, significantly increases the bioavailability of diclofenac [1]. We have recently demonstrated that diosmetin, the absorbable aglycone form of diosmin [2], to which oral diosmin is converted by rhamnosidases of Enterobacteriacae [3], is a potent in vitro inhibitor of human cytochrome P450 (CYP) 2C9 [4], the CYP form mainly responsible for diclofenac biotransformation, thereby providing an explanation for the observation of Rajnarayana et al [1].
Objectives: Since CYP2C9 shares more than 80% amino acid sequence identity with CYP2C8, a CYP form responsible for the biotransformation of various important drugs [5], e.g. the conversion of the antitumor agent paclitaxel to its inactive metabolite 6α-hydroxy-paclitaxel [6], the aim of the present study was to ascertain whether diosmetin also inhibits in vitro CYP2C8 activity, using paclitaxel conversion to 6α-hydroxy-paclitaxel as a marker reaction. As in many countries, diosmin is marketed in association (9:1, w/w) with hesperidin, the 7-rhamnosyl-D-glucoside of the flavanone hesperetin (3’,5,7-trihydroxy-4-methoxyflavanone), we also assessed the effect of hesperetin on CYP2C8.
Results: Diosmetin and hesperetin inhibited 6α-hydroxy-paclitaxel production by human liver microsomes in a concentration-dependent manner, diosmetin being about 16-fold more potent than hesperetin (mean IC50 values 4.25±0.02 and 68.47±3.27 µM, respectively). Therefore, we only characterized the mechanism of diosmetin inhibition of CYP2C8 activity, since hesperetin concentrations close to its IC50 value are unlikely to be reached in vivo. Diosmetin proved to be a reversible, dead-end, full inhibitor of CYP2C8 activity. Kinetic analysis indicated that this flavonoid caused linear mixed-type inhibition of CYP2C8-mediated paclitaxel metabolism, since it decreased Vmax and increased the Km value of the enzymatic reaction. Consistent with this observation, the apparent catalytic efficiency (Vmax/Km) of human liver microsomes for this reaction was markedly decreased. Non linear fitting of untransformed initial velocity data to the equation describing mixed inhibition gave a mean Ki (inhibitor dissociation constant) value of 3.13±0.11 µM.
Conclusions: This study provides the first demonstration that diosmetin, the absorbable aglycone form of diosmin, is a potent inhibitor of CYP2C8 at concentrations similar to those reached in vivo after administration of diosmin-containing pharmaceuticals [2]. This finding may have important clinical implications, since CYP2C8 metabolizes various important drugs. For example, more that 60% paclitaxel is converted by CYP2C8 to its inactive metabolite 6α-hydroxy-paclitaxel [7].
References:
[1] Rajnarayana et al. Drug Metabol. Drug Interact. 2007; 22: 165-174.
[2] Cova et al. Int. J. Clin. Pharmacol. Ther. Toxicol.1992; 30: 29-33.
[3] Serra et al. Bioorg. Med. Chem. 2008; 16: 4009-4018.
[4] Quintieri et al. Drug Metab. Pharmacokinet. 2010; 25: 466-476.
[5] Totah and Rettie Clin. Pharmacol. Ther. 2005; 77: 341-352
[6] Rahman et al. Cancer Res.1994; 54: 5543-5546.
[7] Cresteil et al. Drug. Metab. Dispos. 2002; 30: 438-445
Objectives: Since CYP2C9 shares more than 80% amino acid sequence identity with CYP2C8, a CYP form responsible for the biotransformation of various important drugs [5], e.g. the conversion of the antitumor agent paclitaxel to its inactive metabolite 6α-hydroxy-paclitaxel [6], the aim of the present study was to ascertain whether diosmetin also inhibits in vitro CYP2C8 activity, using paclitaxel conversion to 6α-hydroxy-paclitaxel as a marker reaction. As in many countries, diosmin is marketed in association (9:1, w/w) with hesperidin, the 7-rhamnosyl-D-glucoside of the flavanone hesperetin (3’,5,7-trihydroxy-4-methoxyflavanone), we also assessed the effect of hesperetin on CYP2C8.
Results: Diosmetin and hesperetin inhibited 6α-hydroxy-paclitaxel production by human liver microsomes in a concentration-dependent manner, diosmetin being about 16-fold more potent than hesperetin (mean IC50 values 4.25±0.02 and 68.47±3.27 µM, respectively). Therefore, we only characterized the mechanism of diosmetin inhibition of CYP2C8 activity, since hesperetin concentrations close to its IC50 value are unlikely to be reached in vivo. Diosmetin proved to be a reversible, dead-end, full inhibitor of CYP2C8 activity. Kinetic analysis indicated that this flavonoid caused linear mixed-type inhibition of CYP2C8-mediated paclitaxel metabolism, since it decreased Vmax and increased the Km value of the enzymatic reaction. Consistent with this observation, the apparent catalytic efficiency (Vmax/Km) of human liver microsomes for this reaction was markedly decreased. Non linear fitting of untransformed initial velocity data to the equation describing mixed inhibition gave a mean Ki (inhibitor dissociation constant) value of 3.13±0.11 µM.
Conclusions: This study provides the first demonstration that diosmetin, the absorbable aglycone form of diosmin, is a potent inhibitor of CYP2C8 at concentrations similar to those reached in vivo after administration of diosmin-containing pharmaceuticals [2]. This finding may have important clinical implications, since CYP2C8 metabolizes various important drugs. For example, more that 60% paclitaxel is converted by CYP2C8 to its inactive metabolite 6α-hydroxy-paclitaxel [7].
References:
[1] Rajnarayana et al. Drug Metabol. Drug Interact. 2007; 22: 165-174.
[2] Cova et al. Int. J. Clin. Pharmacol. Ther. Toxicol.1992; 30: 29-33.
[3] Serra et al. Bioorg. Med. Chem. 2008; 16: 4009-4018.
[4] Quintieri et al. Drug Metab. Pharmacokinet. 2010; 25: 466-476.
[5] Totah and Rettie Clin. Pharmacol. Ther. 2005; 77: 341-352
[6] Rahman et al. Cancer Res.1994; 54: 5543-5546.
[7] Cresteil et al. Drug. Metab. Dispos. 2002; 30: 438-445