P-glycoproteins

1. P-glycoproteins Jessica R Oesterheld, MD

2. MDR1->P-gps • 27 years ago, Juliano and Ling noted after initial efficacy, many anti-CA drugs stopped being effective at the same time= Multiple Drug Resistance (MDR) • The gene MDR1 on chromosome 7 encodes a glycoprotein named P (permeability) glycoprotein that actively pumps out compounds from cell • Multiple drugs are effected at same time since ALL are P-gp substrates

3. Pgp is on the apical surface of endothelial cells lining the brain capillaries that forms BBB (From David Flockhart)

4. P-Glycoprotein

5. P-gps • A member of a large family of “transporters” that use ATP (ATP-Binding Cassette transporters= ABC transporters)to flip compounds across a cellular gradient= “flippases” or “bouncers”or “hydrophobic vaccuum cleaners” • Classification:http://nutrigene.4t.com/humanabc.htm since October 1999 • More than 45 transporters; divided into 7 subfamilies • Different transporters effect glucuronides, organic anions etc • Pgp-1=PGY1 (ABCB1) in MDR subfamily

6. Limits cell uptake and extrudes drugs • P-gps exist in membranes of tumor cells and in liver, bile duct, kidneys, placenta, BBB, luminal cells of jejunum, heart • BBB, testes, placenta, P-gps blocks entry of compounds into these “sanctuaries” • In kidney, liver and small intestine, P-gps efflux compounds into the gut lumen, bile, urine

7. Some drugs are P-gp substrates • Drugs can be P-gp substrates or not (see list for P-gp substrates) • Older antihistamines are not P-gp substrates therefore they enter the BBB, but non-sedating antihistamines are P-gp substrates and are excluded from the CNS and have less CNS side effects

8. Some drugs are P-gp inhibitors/inducers • New kind of drug-drug interaction • Usually anti-diarrheal drug loperamide is excluded by BBB from CNS because it is a P-gp substrate. If add quinidine which is a P-gp inhibitor at sufficient dosage, patient will get CNS side effect or respiratory depression (Sadeque and Wandel 2000)

9. Some drugs are P-gp inhibitors/inducers-2 • Long known that quinidine given with digoxin-> increased digoxin concentration • Only a small portion of digoxin is handled by CYPs, but digoxin is a P-gp substrate • Quinidine blocks P-gp in kidney, more digoxin is retained

10. Variabilities in amount of P-gps 8-fold variation in gut in renal transplant patients (Lown et al 1997); 4-fold in nonmedicated adults (Johnson 2002) • Controversial whether men have 2 fold higher levels of P-gps than women • C3435T in exon 26 TT (mutant) associated with lower intestinal P-gp activity and 2-fold higher dig concentrations since not a CYP3A substrate, less kicked into lumen (Hoffmeyer et al 2000,) and increased frequency of nortriptyline- induced postural hypotension, (Roberts et al 2002) **in a non-coding, non-promoting position and is probably linked to another region=surrogate

11. Variabilities-amount of P-gps-2 • C3435T in exon 26 CC (associated with higher P-gp activity)- and anti-seizure medication resistance (Siddiqui et al 2003)

12. Variabilities in P-gps • In gut, more P-gp protects against gastroenteritis in tropics---natural selection breeds those with higher amounts • wild type (cc) 83% Ghanians 61% African-Americans 26% Whites (Schaeffeler et al 2001) -to get similar blood levels of tacrolimus or cyclosporin in African Americans, need higher dosing as compared to causasians

13. Intestinal Cells drug pgp 3A4 drug LUMEN pgp 3A4 drug pgp 3A4 P-gps in “front”- with repeated shuttles of absorption/efflux

14. Intestinal P-gps (Lin and Yamazaki 2003) • Misconception that if a drug is a P-gp substrate, then it has low bioavailability • Amount of drug absorbed = influx (passive diffusion + active uptake) - efflux (P-gp +amount metabolized by CYPs +Phase 2) • P-gp functional activity is saturable, therefore when drug at low dose or poor permeability, P-gp effect more important

15. CYP3A and P-gps work together in the gut • Many substrates of P-gps are also CYP3A4 substrates • In small intestine, P-gps efflux compound to lumen, then compound is reabsorbed; this “shuttle” leads to increased “exposure” of compounds to CYP3A4 and maximizes their activity

16. Double Inhibitors/Double Inducers in the gut • Many drugs are both P-gp and CYP3A4 inhibitors (e.g., gfj, erythromycin, cyclosporin) • Many drugs are both P-gp and CYP3A4 inducers (e.g., St Johns wort, rifampin) • DDIs related to CYPs or P-gps or both • Coordination - PXR and perhaps other mechanisms

17. Pgp knockout mice Pgp knockout mice show increased sensitivity to ivermectin since it can now cross BBB (Smit et al 1999) • Also digoxin, cyclosporin, vinblastine colchicine • But if give cyclosporin (Pgp inh) before you give ivermectin->brain conc 2.5 times higher (Margues-Santos 1999) • Induce first with dexamethasone or morphine--> decreased brain concentration of Pgp substrates

18. Clinical use of P-gp Inhibitors • Cli nical use of P-gp inhibitors to block development of MDR in pts with Ca have no absolute success despite 15 yrs of trying • 3 generations 1-cyclosporin -intrinsic toxicity 2- valsopodar -analog devoid of intrinsic toxicity 3- tariquidar • 3rd gen drugs are not CYP3A4 inhibitors or other transporter inhibitors, very potent, and perhaps effect ATP

19. Very difficult to predict P-gp DDIs • Not a simple docking station • At least 2 binding sites and 2 ATP-binding sites on 2 symmetric non-identical halves of P-gp • Sites work cooperatively • Competitive inhibition, non-competitive inhibition and collective stimulation • Substrate specific • Must also tease out CYP3A4 effects and other transporters • Important with digoxin, cyclosporin