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Catalog > Transporters > MDR1/P-gp

MDR1/P-gp

MDR1/P-gp Transporter (ATP Binding Cassette Protein B1/ABCB1)

MDR1 (Multi Drug Resistance Protein 1), more commonly referred to as P-gp (P-glycoprotein) plays an important role in drug disposition and distribution.  P-gp is an efflux transporter that prevents drugs from penetrating tissues such as the brain and gut and is also involved in biliary excretion and renal excretion of drugs. Current FDA and EMA recommendations for testing P-gp are based mainly on its role in intestinal absorption. Drugs that are substrates of P-gp might be victims to drug drug interactions (DDI) where this transporter is inhibited. This is especially true for drugs with a narrow therapeutic index and low oral bioavailability.

Localization

P-gp is ubiquitously expressed in the human body, with higher concentrations in the luminal membrane of barrier organs such as the intestine and blood brain barrier and in the apical membranes of excretory cells e.g. hepatocytes and renal proximal tubule cells.  P-gp is also expressed at high concentrations in tumors and in transformed cell lines such as Caco-2.

Function, physiology and clinically significant polymorphisms

P-gp comprises 1276–1280 amino acids with a molecular mass of approximately 170 kDa and a tandemly duplicated structure, with each half of the molecule containing six predicted and highly hydrophobic transmembrane domains.
The function of P-gp is to export xenobiotics from cells into extracellular spaces (e.g. at the BBB) or out of the body e.g. in the gut and for renal and hepatic clearance.  P-gp preferentially transports neutral or positively charged hydrophobic molecules and contains multiple substrate binding sites within the ligand binding domain [1, 2].

Understanding the substrate liability of P-gp can provide early knowledge of tissue distribution – such as whether there is a likelihood of brain penetration.  P-gp substrate drugs include quinidine, HIV-type 1 non-nucleoside reverse transcriptase inhibitors such as ritonavir, cardiac glycosides like digoxin, chemotherapeutic agents e.g. etoposide, glucocorticoids e.g. dexamethasone, cyclin-dependent kinase inhibitors such as seliciclib [3], endogenous substances including steroids and bilirubin. There is a broad range of overlapping substrate specificities and tissue distribution for CYP3A4 and P-gp, as these proteins (along with BCRP) act synergistically as a protective barrier in the bioavailability of orally dosed drugs.  Many P-gp inhibitors contain aromatic ring structures, a tertiary or secondary amino group and have high lipophilicity.  P-gp inhibitors are typically either very high-affinity substrates that bind non-competitively (not allowing other drugs to bind), or by being efficient inhibitors of ATP hydrolysis, either at the ATP binding site or by inhibiting PKC, which is involved with ATP coupling to P-gp [4].

More than 62 coding region SNPs are reported at the ABCB1 locus. Three high frequency SNPs at the ABCB1 gene locus, e13/C1236T, e22/G2677T/A and e27/C3435T have variable frequencies across populations and are associated with differences in drug response.  G2677T and C3435T and are reported to protect Chinese but not Caucasian men from late-onset Parkinson’s disease [5-7].  Expression of e27/3435C correlates with increased efflux of the P-gp substrate, rhodamine 123 [8], reduced efflux of nelfinavir and no effect on fexofenadine efflux [9]. Significantly increased exposure to irinotecan is associated with the 1236TT genotype and increased response to temozolomide, cyclosporine A and nelfinavir with the 1236CC genotype [10].

Nuclear receptors, including the Pregnane X Receptor (PXR), constitutive Androstane Receptor (CAR) and Farnesoid X Receptor (FXR) regulate P-gp either directly (PXR) or indirectly (FXR via PXR) in response to chronic treatment with xenobiotics such as rifampicin.

Clinical significance

Direct clinical evidence of the contribution of P-gp inhibition or induction to DDI is limited due to cross specificity of P-gp substrates with the drug metabolizing enzyme CYP3A4.  Digoxin is the preferred drug for evaluations of P-gp inhibition and induction, as it is not metabolized by P450 enzymes and is a substrate of P-gp.  Mibefradil was removed from the market as it increased the Cmax and AUC of digoxin to levels that induced severe adverse reactions. [11]. Itraconazole increases the plasma concentrations of digoxin, whilst decreasing its renal clearance [12]. P-gp is implicated in DDIs between digoxin and rifampicin, as the AUC of digoxin significantly decreases when co-administered with the PXR activator rifampicin, likely due to increased intestinal expression of P-gp preventing the absorption of digoxin in the gut [13].  Phase I clinical trials with LY335979 (P-gp inhibitor) in patients with advanced malignancies reported neurotoxicity, cerebral dysfunction, hallucinations and palinopsia [14].  P-gp transporter contributes to the multi-drug resistance induced by many chemotherapeutic agents in cancer patients. In clinical studies, P-gp was induced 1.8-fold by chemotherapy, translating to a 3- to 4-fold greater incidence of treatment failure [15] .  Evaluation of 359 resected breast cancer samples revealed a strong correlation between the extent of taxol and doxorubicin resistance and P-gp expression levels [16]. 

The G2677TT genotype is associated with lower plasma concentrations of fexofenadine, a higher risk of cyclosporine A failure in steroid resistant ulcerative colitis, a positive correlation to tacrolimus neurotoxicity, increased resistance to antiepileptic drugs and increased response to cytarabine in AML patients. However, this SNP does not appear to affect plasma trough concentrations of ritonavir in HIV patients, rhodamine 123 efflux in peripheral blood lymphocytes, tacrolimus pharmacokinetics in renal transplant patients or blood, semen and saliva concentrations of ritonavir or lopinavir in HIV patients.

Regulatory Requirements

P-gp has been identified as the transporter protein with the most relevance to drug PK and interactions by regulatory agencies such as the FDA and EMA.  All drug candidates need to be screened for in vitro P-gp substrate and inhibition liability for regulatory submissions.  Based on this data, decisions are made for clinical trials. 

Location
Endogenous substrates
Substrates used experimentally
Substrate drugs
Inhibitors
Intestinal enterocytes, kidney proximal tubule, hepatocyte canalicular membrane, brain endothelia, placenta, cornea
Steroids, lipids, bilirubin, bile acids
Calcein AM, Digoxin, NMQ
Digoxin, loperamide, berberine, irinotecan, doxorubicin, vinblastine, paclitaxel, fexofenadine, seliciclib
Cyclosporine, quinidine, tariquidar, verapamil, PSC833

References

  1. Aller, S.G., et al., Structure of P-glycoprotein reveals a molecular basis for poly-specific drug binding. Science, 2009. 323(5922): p. 1718-22.
  2. Loo, T.W., M.C. Bartlett, and D.M. Clarke, Identification of Residues in the Drug Translocation Pathway of the Human Multidrug Resistance P-glycoprotein by Arginine Mutagenesis. J Biol Chem, 2009. 284(36): p. 24074-87.
  3. Rajnai, Z., et al., ATP-binding cassette B1 transports seliciclib (R-roscovitine), a cyclin-dependent kinase inhibitor. Drug Metab Dispos, 2010. 38(11): p. 2000-6.
  4. Wang, R.B., et al., Structure-activity relationship: analyses of p-glycoprotein substrates and inhibitors. J Clin Pharm Ther, 2003. 28(3): p. 203-28.
  5. Lee, C.G., et al., MDR1, the blood-brain barrier transporter, is associated with Parkinson’s disease in ethnic Chinese. J Med Genet, 2004. 41(5): p. e60.
  6. Tan, E.K., et al., Effect of MDR1 haplotype on risk of Parkinson disease. Arch Neurol, 2005. 62(3): p. 460-4.
  7. Tan, E.K., et al., Analysis of MDR1 haplotypes in Parkinson’s disease in a white population. Neurosci Lett, 2004. 372(3): p. 240-4.
  8. Hitzl, M., et al., The C3435T mutation in the human MDR1 gene is associated with altered efflux of the P-glycoprotein substrate rhodamine 123 from CD56+ natural killer cells. Pharmacogenetics, 2001. 11(4): p. 293-8.
  9. Drescher, S., et al., MDR1 gene polymorphisms and disposition of the P-glycoprotein substrate fexofenadine. Br J Clin Pharmacol, 2002. 53(5): p. 526-34.
  10. Zhang, Y.T., et al., ABCB1 polymorphisms may have a minor effect on ciclosporin blood concentrations in myasthenia gravis patients. Br J Clin Pharmacol, 2008. 66(2): p. 240-6.
  11. Siepmann, M. and W. Kirch, Drug-drug interactions of new active substances: mibefradil example. Eur J Clin Pharmacol, 2000. 56(3): p. 273.
  12. Alderman, C.P. and P.D. Allcroft, Digoxin-itraconazole interaction: possible mechanisms. Ann Pharmacother, 1997. 31(4): p. 438-40.
  13. Greiner, B., et al., The role of intestinal P-glycoprotein in the interaction of digoxin and rifampin. J Clin Invest, 1999. 104(2): p. 147-53.
  14. Rubin, E.H., et al., A phase I trial of a potent P-glycoprotein inhibitor, Zosuquidar.3HCl trihydrochloride (LY335979), administered orally in combination with doxorubicin in patients with advanced malignancies. Clin Cancer Res, 2002. 8(12): p. 3710-7.
  15. Trock, B.J., F. Leonessa, and R. Clarke, Multidrug resistance in breast cancer: a meta-analysis of MDR1/gp170 expression and its possible functional significance. J Natl Cancer Inst, 1997. 89(13): p. 917-31.
  16. Mechetner, E., et al., Levels of multidrug resistance (MDR1) P-glycoprotein expression by human breast cancer correlate with in vitro resistance to taxol and doxorubicin. Clin Cancer Res, 1998. 4(2): p. 389-98.

 

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