RT Journal Article
SR Electronic
T1 Identification of the Human Liver Cytochrome P450 Isoenzyme Responsible for the 6-Methylhydroxylation of the Novel Anticancer Drug 5,6-Dimethylxanthenone-4-acetic Acid
JF Drug Metabolism and Disposition
JO Drug Metab Dispos
FD American Society for Pharmacology and Experimental Therapeutics
SP 1449
OP 1456
VO 28
IS 12
A1 Shufeng Zhou
A1 James W. Paxton
A1 Malcolm D. Tingle
A1 Philip Kestell
YR 2000
UL http://dmd.aspetjournals.org/content/28/12/1449.abstract
AB In vitro studies were conducted to identify the hepatic cytochrome P450 (CYP) isoenzyme involved in the 6-methylhydroxylation of 5,6-dimethylxanthenone-4-acetic acid (DMXAA) by using a human liver library (n = 14). The metabolite 6-hydroxymethyl-5-methylxanthenone-4-acetic acid (6-OH-MXAA) was determined by HPLC with fluorescence detection. The metabolite formed in human liver microsomes and by cDNA-expressed CYP isoform was identified by liquid chromatography mass spectrometry as 6-OH-MXAA. In human liver microsomes (n = 14), 6-methylhydroxylation of DMXAA followed monophasic Michaelis-Menten kinetics, with a mean apparent Km of 21 ± 5 μM and Vmax of 0.043 ± 0.019 nmol/min/mg. An approximate 10-fold interindividual variation in the intrinsic clearance (Vmax/Km) of DMXAA 6-methylhydroxylation in human liver microsomes was observed. The involvement of CYP1A2 in DMXAA metabolism by human livers was demonstrated by the following: 1) the potent inhibition of DMXAA metabolism by furafylline (kinact = 0.23 ± 0.04 min−1, K′app= 15.6 ± 6.7 μM) and α-naphthoflavone (Ki = 0.036 μM), but not by cimetidine, ketoconazole, tolbutamide, quinidine, chlorzoxazone, diethyldithiocarbamate, troleandomycin, and sulfaphenazole; 2) when incubated with human lymphoblastoid cell microsomes containing cDNA-expressed CYP isoenzymes, DMXAA was metabolized only by CYP1A2, with an apparent Km of 6.2 ± 1.5 μM and Vmax of 0.014 ± 0.001 nmol/min/mg, but not by CYP2A6, CYP2B6, CYP2C9 (Arg144), CYP2C19, CYP2D6 (Val374), CYP2E1, and CYP3A4; 3) a significant correlation (r = 0.90; P < .001) between 6-methylhydroxylation of DMXAA and 7-ethoxyresorufinO-deethylation; and 4) a significant correlation (r = 0.75; P < .01) between the CYP1A protein level determined by Western blots and DMXAA 6-methylhydroxylation. The American Society for Pharmacology and Experimental Therapeutics