ABSTRACT
1. The herb-drug interaction potential of Hwang-Ryun-Hae-Dok-Tang (HR) extracts mediated by cytochrome P450 (CYP) inhibition was determined using human liver microsomes. 2. HR strongly inhibited CYP1A2 and moderately inhibited CYP2C19, CYP2D6, and CYP3A4 (testosterone) but not CYP2A6, CYP2B6, CYP2C8, CYP2C9, and CYP3A4 (midazolam). 3. The enzyme kinetic results suggest that CYP1A2 inhibition is competitively reversible (Ki, 13.4±1.8 µg/ml), and CYP2D6 inhibition is quasi-irreversible (KI, 0.234±0.138 µg/ml; kinact, 0.067±0.006 min(-1)). 4. Fermentation using Lactobacillus acidophilus attenuated the HR-induced inhibition of CYP2D6, but not the other isoforms. 5. Neither CYP1A2 nor CYP3A4 was markedly inhibited by berberine, palmatine, and geniposide-major components in HR-and CYP2D6 was inhibited by berberine (IC50, 13.8 µg/ml) in a metabolism-dependent manner. 6. The results suggest the possibility of HR-drug interaction through inhibition of CYP-particularly CYP2D6-which may be attenuated by fermentation using L. acidophilus.
Subject(s)
Cytochrome P-450 Enzyme Inhibitors/pharmacology , Cytochrome P-450 Enzyme System/chemistry , Drugs, Chinese Herbal/pharmacology , Microsomes, Liver/enzymology , Cytochrome P-450 Enzyme Inhibitors/chemistry , Drugs, Chinese Herbal/chemistry , Fermentation , Humans , Isoenzymes/antagonists & inhibitors , Isoenzymes/chemistry , Kinetics , Lactobacillus acidophilus/metabolismABSTRACT
Although cytochrome P450 inhibition is the major drug-drug interaction (DDI) mechanism in clinical pharmacotherapy, DDI of a number of well-established drugs have not been investigated. Rifampicin, isoniazid, pyrazinamide and ethambutol combination therapy inhibits clearance of theophylline in patients with tuberculosis. We determined the inhibitory effects of ethambutol on the activities of nine CYP isoforms including CYP1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1 and 3A4 in pooled human liver microsomes (HLM). As measured by liquid chromatography-electrospray ionization tandem mass spectrometry, ethambutol exhibited strong inhibitory potential against CYP1A2 and CYP2E1, moderate against CYP2C19 and CYP2D6 and weak against CYP2A6, CYP2C9 and CYP3A4, based on the IC50 values. The K(i) value of ethambutol for CYP1A2 was 1.4 µM and for CYP2E1 was 2.9 µM. Inhibition of CYP1A2 and CYP2E1 was not increased by preincubation with ethambutol and ß-nicotinamideadenine dinucleotide phosphate (NADPH), suggesting that the ethambutol-induced CYP inhibition may not be metabolism-dependent. Kinetic analysis showed that the inhibition of CYP1A2 and CYP2E1 by ethambutol was best fit to a competitive inhibition model. Formation of 1-methylxanthene and 1,3-dimethyluric acid from theophylline in HLM was decreased to 47% and 36%, respectively, by 3.0 µM ethambutol, which is comparable to its IC50 value against CYP1A2. Considering its maximal plasma concentrations of ~10 µM and long half-life of ~22 h, our findings raise the possibility that ethambutol causes significant DDIs in clinical situations with drugs with narrow therapeutic index, such as theophylline, in clinical situations.
