Received for publication July 24, 2006.
Revised September 1, 2006.
Accepted for publication September 6, 2006.

The Role of P-glycoprotein in the Bioactivation of Raloxifene

Abstract

Drug transporters have been shown to alter drug metabolism. Similarly, bioactivation of drugs may also be altered by drug transporters. The aim of this work was to examine the role of P-glycoprotein (Pgp) in the bioactivation of a Pgp substrate, raloxifene, and a non-Pgp substrate, naphthalene. To evaluate the extent of bioactivation, covalent binding was measured. In both freshly isolated and cryopreserved hepatocytes, the extent of raloxifene covalent binding significantly increased (p<0.001) in the presence of verapamil whereas no change was observed with the covalent binding of naphthalene. To ascertain that the change was a Pgp effect, covalent binding was examined in microsomes where raloxifene and naphthalene covalent binding was not altered in the presence of verapamil. In addition, the measure of raloxifene-GSH adducts in the cryopreserved hepatocytes showed that the formation of the adducts increased in the presence of verapamil which supports the idea that blocking Pgp in the liver increases metabolism, and therefore, the bioactivation of raloxifene. Because raloxifene and naphthalene are known to undergo bioactivation mediated by CYP3A4, covalent binding in the presence of ketoconazole was examined. In both hepatocytes and microsomes, raloxifene covalent binding significantly decreased (p<0.001). Interestingly, naphthalene covalent binding was not affected. In the presence of CYP2E inhibitor 4-methylpyrazole, a decrease in naphthalene covalent binding was observed, suggesting that the formation of the 1,2-epoxide may be the main culprit contributing to naphthalene covalent binding. In conclusion, these data suggest that in addition to other "protective" mechanisms, Pgp may attenuate bioactivation of drugs.

Key words: ABC transporters, active transport, bioactivation, drug transport, glutathione conjugates, p-glycoprotein, transporters

This article has been cited by other articles:

				C. E. Garner, E. Solon, C.-M. Lai, J. Lin, G. Luo, K. Jones, J. Duan, C. P. Decicco, T. Maduskuie, S. E. Mercer, et al. Role of P-Glycoprotein and the Intestine in the Excretion of DPC 333 [(2R)-2-{(3R)-3-Amino-3-[4-(2-methylquinolin-4-ylmethoxy)phenyl]-2-oxopyrrolidin-1-yl}-N-hydroxy-4-methylpentanamide] in Rodents Drug Metab. Dispos., June 1, 2008; 36(6): 1102 - 1110. [Abstract] [Full Text] [PDF]