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METABOLISM, TRANSPORT, AND PHARMACOGENOMICS

Substrate-Dependent Effects of Human ABCB1 Coding Polymorphisms

Department of Biopharmaceutical Sciences, University of California, San Francisco, California

One of the many obstacles to effective drug treatment is the efflux transporter P-glycoprotein (P-gp), which can restrict the plasma and intracellular concentrations of numerous xenobiotics. Variable drug response to P-gp substrates suggests that genetic differences in ABCB1 may affect P-gp transport. The current study examined how ABCB1 variants alter the P-gp-mediated transport of probe substrates in vitro. Nonsynonymous ABCB1 variants and haplotypes with an allele frequency 2% were transiently expressed in HEK293T cells, and the transport of calcein acetoxymethyl ester and 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY-FL)-paclitaxel was measured in the absence or presence of the P-gp inhibitor cyclosporin A. The A893S, A893T, and V1251I variants and the N21D/1236C>T/A893S/3435C>T haplotype altered intracellular accumulation compared with reference P-gp in a substrate-dependent manner. It is interesting that certain variants showed altered sensitivity to cyclosporin A inhibition that was also substrate-specific. These functional data demonstrate that nonsynonymous polymorphisms in ABCB1 may selectively alter P-gp transport and drug-drug interactions in a substrate- and inhibitor-dependent manner.

Address correspondence to: Dr. Deanna L. Kroetz, UCSF Box 2911, 1550 4th St., Rm. RH584E, San Francisco, CA 94158-2911. E-mail: deanna.kroetz{at}ucsf.edu