Cytosolic and microsomal activation of doxifluridine and tegafur to produce 5-fluorouracil in human liver.
OBJECTIVE
The enzymatic formation of 5-fluorouracil (5-FU) from two fluoropyrimidine prodrugs, doxifluridine (5'-DFUR) and tegafur (FT), was compared in vitro in order to determine whether there are differences between the metabolic profiles of the two prodrugs.


METHODS
Conversion of the two fluoropyrimidine prodrugs to 5-FU was measured by high-performance liquid chromatography at a concentration of 500 micro M using the microsomal and cytosolic fractions of 12 human livers. The degree of correlation between the 5-FU-forming activities was determined using various cytochrome P450-dependent reactions.


RESULTS
Liver microsomes catalyzed 5-FU formation from 5'-DFUR at rates of 10.0-160.1 pmol/min per mg protein and correlated well with CYP2A6-dependent coumarin 7-hydroxylase activity. The rates of microsomal 5-FU formation from FT ranged from 44.9 to 808.3 pmol/min per mg protein and also correlated with coumarin 7-hydroxylase activity. The cytosol fractions catalyzed 5-FU formation from 5'-DFUR at rates of 3,164.6 to 6,026.6 pmol/min per mg protein, almost two orders of magnitude higher than the rates of cytosolic 5-FU formation from FT (46.8-219.0 pmol/min per mg protein).


CONCLUSIONS
The cytosolic enzymes in livers appear to be important for 5-FU formation from 5'-DFUR. Both cytosolic and microsomal enzymes were involved almost equally in 5-FU formation from FT. The increased formation of 5-FU from 5'-DFUR might provide an answer to the question of why similar blood 5-FU levels were retained despite blood 5'-DFUR levels lower than blood FT levels.
Ozawa
Shogo
S
Project Team for Pharmacogenetics, National Institute of Health Sciences, Kamiyoga, Tokyo, Japan. sozawa@nihs.go.jp
Hamada
Masashi
M
Murayama
Norie
N
Nakajima
Yukiko
Y
Kaniwa
Nahoko
N
Matsumoto
Yoshiaki
Y
Fukuoka
Masamichi
M
Sawada
Jun-ichi
J
Ohno
Yasuo
Y
D000284:Administration, Oral; D000964:Antimetabolites, Antineoplastic / Q000378:metabolism*; D001189:Aryl Hydrocarbon Hydroxylases / Q000378:metabolism; D019388:Cytochrome P-450 CYP1A2 / Q000378:metabolism; D065637:Cytochrome P-450 CYP2A6; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism*; D005467:Floxuridine / Q000378:metabolism*; D005472:Fluorouracil / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D008862:Microsomes, Liver / Q000201:enzymology*; D006899:Mixed Function Oxygenases / Q000378:metabolism; D013250:Steroid Hydroxylases / Q000378:metabolism; D005641:Tegafur / Q000378:metabolism*; D013939:Thymidine Phosphorylase / Q000378:metabolism
D000964:Antimetabolites, Antineoplastic; D005467:Floxuridine; D005641:Tegafur; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D013250:Steroid Hydroxylases; D001189:Aryl Hydrocarbon Hydroxylases; C585710:CYP2A6 protein, human; D019388:Cytochrome P-450 CYP1A2; D065637:Cytochrome P-450 CYP2A6; C022657:steroid hormone 6-beta-hydroxylase; D013939:Thymidine Phosphorylase; D005472:Fluorouracil; C025034:doxifluridine