Quaternary ammonium-linked glucuronidation of tamoxifen by human liver microsomes and UDP-glucuronosyltransferase 1A4

• Published in: Biochemical pharmacology 2004-06, Vol.67 (11), p.2093-2102
• Main Authors: Kaku, Teppei, Ogura, Kenichiro, Nishiyama, Takahito, Ohnuma, Tomokazu, Muro, Kei, Hiratsuka, Akira
• Format: Article
• Language: English
• Subjects: Adult; Aged; Animals; Antiestrogen; Antineoplastic Agents, Hormonal - metabolism; Biological and medical sciences; Dogs; Estrogen receptor; Estrogen Receptor alpha; Female; Glucuronides - metabolism; Glucuronosyltransferase - metabolism; Guinea Pigs; Human liver microsomes; Humans; Insecta - cytology; Macaca fascicularis; Male; Medical sciences; Mice; Microsomes, Liver - metabolism; Middle Aged; N-Glucuronidation; Pharmacology. Drug treatments; Rats; Rats, Sprague-Dawley; Receptors, Estrogen - metabolism; Tamoxifen; Tamoxifen - metabolism; UDP-glucuronosyltransferase; Uridine Diphosphate Glucuronic Acid - pharmacology
• ISSN: 0006-2952; 1873-2968
• DOI: 10.1016/j.bcp.2004.02.014

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 μM, 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, ERα and ERβ, suggesting that TAM N +-glucuronide might contribute to the biological activity of TAM in vivo.