ABCB1 single nucleotide polymorphisms (1236C > T, 2677G > T, and 3435C > T) do not affect transport activity of human P-glycoprotein David Dickens, Andrew Owen, Ana Alfirevic and Munir Pirmohamed Background P-glycoprotein (P-gp) is a multidrug efflux transporter that has a defined role in the absorption and disposition of drugs. Many studies have investigated the potential influence of ABCB1 polymorphisms on the disposition of its substrates. However, there remains significant controversy regarding the role of these polymorphisms. Our aim was to generate a P-gp expression system for single nucleotide polymorphisms (SNPs) and the reference sequence to assess the functional significance of these variants on transport. Materials and methods P-gp reference, a P-gp ATPase deficient mutant (G534D) and a triple SNP variant of P-gp (1236C > T, 2677G > T, and 3435C > T) were expressed in Xenopus laevis oocytes and used to assess the influence of these SNPs on transport of digoxin and imatinib. The inhibition of P-gp-mediated transport in Caco-2 cells and oocytes was also assessed. Results No effect of the triple SNP variant of P-gp on molecular transport of digoxin or imatinib was observed. The rank order of inhibition of P-gp in Caco-2 cells and ABCB1-injected oocytes was tariquidar > elacridar > PSC- 833 > laniquidar > cyclosporine > verapamil > dipyridamole. Conclusion These data suggest there is no functional consequence of these SNPs for molecular transport of model substrates or inhibition by model inhibitors for P-gp. Transporter-injected oocytes may be a useful tool for probing the mechanism for unexplained drug–drug interactions or to characterize therapeutic transport inhibitors. Pharmacogenetics and Genomics 23:314–323  c 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins. Pharmacogenetics and Genomics 2013, 23:314–323 Keywords: ABCB1, 1236C > T, 2677G > T, and 3435C > T, IC 50 , inhibition, MDR1, P-glycoprotein Department of Molecular and Clinical Pharmacology, The Wolfson Centre for Personalised Medicine, Institute of Translational Medicine, University of Liverpool, Liverpool, UK Correspondence to Munir Pirmohamed, PhD, FRCP, FRCP(E), Department of Molecular and Clinical Pharmacology, The Wolfson Centre for Personalised Medicine, Institute of Translational Medicine, University of Liverpool, Block A Waterhouse Building, 1-5 Brownlow Street, Liverpool L69 3GL, UK Tel/fax: + 44 151 794 5549; e-mail: munirp@liv.ac.uk Received 26 September 2012 Accepted 3 March 2013 Introduction P-glycoprotein (P-gp, ABCB1, MDR1) plays an important role in the whole body bioavailability and disposition of many drugs and xenobiotics [1]. P-gp can limit drug entry into the body after oral administration by enhancing excretion into the gut lumen [2]. Once the drug has reached the systemic circulation, ABCB1 can also modulate cellular distribution, for example at the canalicular membrane of hepatocytes, at the luminal membrane of proximal tubule kidney cells, and at tissues such as testis and brain [3]. P-gp shows significant interindividual variability in expression [4]. Part of this variability may be genetically determined, although many different single nucleotide polymorphisms (SNPs) and haplotypes have been identi- fied in the ABCB1 gene [5]. Three SNPs, either as a haplotype or individually (1236C > T, rs1128503; 2677G > T, rs2032582; 3435C >T, rs1045642) have attracted the most attention [4], and have been linked to several different phenotypes of drug response. There are, however, conflicting data on the effect of these SNP variants in both in-vivo and in-vitro studies [6]. The ABCB1 haplotype has also been suggested to affect substrate specificity of P-gp [7]. Despite the controversy regarding the functional importance of these ABCB1 SNPs, numerous studies are still investigating their influence on the pharmacokinetics and response to P-gp substrates. For example, recent manuscripts have shown ABCB1 SNPs to be important for response to imati- nib [8,9]. P-gp inhibition has been suggested as an adjuvant therapy to boost accumulation in cancer cells or to alter bioavailability and distribution of substrate drugs into tissues such as the brain and peripheral tissues. Inhibitors can be categorized into three groups: first-generation transport inhibitors are licensed drugs such as cyclospor- ine, shown to be high-affinity substrates; second-genera- tion inhibitors such as PSC-833, which are more specific for P-gp; and third-generation inhibitors such as tariqui- dar, which show the highest specificity for P-gp. The first- generation and second-generation transport inhibitors have failed in clinical trials as adjuvants for cancer therapy, due to either a lack of clinical benefit or high incidence of toxicity [10]. As a result, no P-gp inhibitor 314 Original article 1744-6872  c 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins DOI: 10.1097/FPC.0b013e328360d10c Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.