Oxo substituents markedly alter the phase II metabolism of alpha-hydroxybutenylbenzenes: models probing the bioactivation mechanisms of tamoxifen.
The P450-catalyzed hydroxylation of tamoxifen to give alpha-hydroxytamoxifen [(E)-4-{4-[2-(dimethylamino)ethoxy]phenyl}-3,4-diphenyl-3-buten-2- ol] and subsequent formation of reactive sulfate esters which alkylate DNA has been proposed to be a potential carcinogenic pathway for tamoxifen. In the present study, the ability of alpha-hydroxytamoxifen analogs to form GSH and sulfate conjugates was investigated in order to understand the structural features influencing reactivity. The para oxo analogs 1 [1-(4-methoxyphenyl)-3-hydroxy-1-butene], 2 [1-(4-hydroxyphenyl)-3-hydroxy-1-butene], and 4 [1-(4-hydroxyphenyl)-1-phenyl-3-hydroxy-1-butene] reacted with GSH instantaneously under strong acidic conditions to yield GSH conjugates in greater than 90% yields. Interestingly, the meta phenolic analogs 3 [1-(3-hydroxyphenyl)-3-hydroxy-1-butene] and 5 [1-(3-hydroxyphenyl)-1-phenyl-3-hydroxy-1-butene] did not react with GSH to any significant extent under similar conditions. Characterization of the GSH conjugates with 1H-NMR, electrospray mass spectrometry, and UV showed that all of the conjugates resulted from attack of GSH at the alpha-position of the substrates with displacement of the hydroxyl group. The formation of a single pair of diastereomeric conjugates strongly supported adduct formation to proceed through a direct S(N)2 displacement mechanism and not through a quinone methide (4-alkyl-2,5-cyclohexadien-1-one) intermediate. At physiological pH and temperature only the para hydroxy analogs 2 and 4 gave GSH conjugates, a reaction which seems to be catalyzed by isoforms of glutathione S-transferase. Similar substituent effects were observed in the sulfotransferase-mediated formation of alpha-hydroxy sulfate esters in that only the para hydroxy analogs formed conjugates at the aliphatic hydroxyl group. Finally, the present investigation showed a remarkable difference in the reactivities of para and meta phenolic analogs of alpha-hydroxybutenylbenzenes toward GSH and sulfate conjugation reactions.
Ramakrishna
K V
KV
Department of Medicinal Chemistry and Pharmacognosy, College of Pharmacy, University of Illinois at Chicago, 60612-7231, USA.
Fan
P W
PW
Boyer
C S
CS
Dalvie
D
D
Bolton
J L
JL
D000818:Animals; D001711:Biotransformation; D004965:Estrogen Antagonists / Q000493:pharmacokinetics*; D005978:Glutathione / Q000378:metabolism; D008099:Liver / Q000378:metabolism; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D015238:Sulfotransferases / Q000502:physiology; D013629:Tamoxifen / Q000493:pharmacokinetics*
D004965:Estrogen Antagonists; D013629:Tamoxifen; D015238:Sulfotransferases; D005978:Glutathione