Quaternary ammonium-linked glucuronidation of tamoxifen by human liver microsomes and UDP-glucuronosyltransferase 1A4.
Tamoxifen (TAM), a nonsteroidal antiestrogen, is the most widely used drug for chemotherapy of hormone-dependent breast cancer in women. In the present study, we found a new potential metabolic pathway of TAM via N-linked glucuronic acid conjugation for excretion in humans. TAM N(+)-glucuronide was isolated from a reaction mixture consisting of TAM and human liver microsomes fortified with UDP-glucuronic acid (UDPGA) and identified with a synthetic specimen by high-performance liquid chromatography-electrospray ionization-mass spectrometry. However, no TAM-glucuronidating activity was detected in microsomes from rat, mouse, monkey, dog, and guinea pig livers. A strong correlation (r(2) =0.92 ) was observed between N-glucuronidating activities toward TAM and trifluoperazine, a probe substrate for human UDP-glucuronosyltransferase (UGT) 1A4, in human liver microsomes from eight donors (five females, three males). However, no correlation ( (r(2) =0.02 )) was observed in the activities between 7-hydroxy-4-(trifluoromethyl)coumarin and TAM. Only UGT1A4 catalyzed the N-linked glucuronidation of TAM among recombinant UGTs (UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A9, UGT2B4, UGT2B7, UGT2B15, and UGT2B17) expressed in insect cells. Apparent K(m) values for TAM N-glucuronidation by human liver microsomes and recombinant UGT1A4 were 35.8 and 32.4 microM, respectively. These results strongly suggested that UGT1A4 could play a role in metabolism and excretion of TAM without Phase I metabolism in human liver. TAM N(+)-glucuronide still had binding affinity similar to TAM itself for human estrogen receptors, ERalpha and ERbeta, suggesting that TAM N(+)-glucuronide might contribute to the biological activity of TAM in vivo.
Kaku
Teppei
T
Department of Drug Metabolism and Molecular Toxicology, School of Pharmacy, Tokyo University of Pharmacy and Life Science, 1432-1 Horinouchi, Hachioji-shi, Tokyo 192-0392, Japan.
Ogura
Kenichiro
K
Nishiyama
Takahito
T
Ohnuma
Tomokazu
T
Muro
Kei
K
Hiratsuka
Akira
A
D000328:Adult; D000368:Aged; D000818:Animals; D018931:Antineoplastic Agents, Hormonal / Q000378:metabolism; D004285:Dogs; D047628:Estrogen Receptor alpha; D005260:Female; D020719:Glucuronides / Q000378:metabolism*; D014453:Glucuronosyltransferase / Q000378:metabolism*; D006168:Guinea Pigs; D006801:Humans; D007313:Insecta / Q000166:cytology; D008252:Macaca fascicularis; D008297:Male; D051379:Mice; D008862:Microsomes, Liver / Q000378:metabolism*; D008875:Middle Aged; D051381:Rats; D017207:Rats, Sprague-Dawley; D011960:Receptors, Estrogen / Q000378:metabolism; D013629:Tamoxifen / Q000378:metabolism*; D014535:Uridine Diphosphate Glucuronic Acid / Q000494:pharmacology
D018931:Antineoplastic Agents, Hormonal; D047628:Estrogen Receptor alpha; D020719:Glucuronides; D011960:Receptors, Estrogen; C019787:bilirubin glucuronoside glucuronosyltransferase; D013629:Tamoxifen; D014535:Uridine Diphosphate Glucuronic Acid; D014453:Glucuronosyltransferase