2. Slide index
• Introduction
• Membrane transporters
• Structure
• Physiological sites
• Cellular localisation
• Mechanism of action
• Physiological relevance
• Pharmacokinetic
importance
• P gp substrates
• P gp inhibition
• Pgp inhibitors
• Pgp stimulators
• Clinical implications
• polymorphism
3. Introduction
• P-glycoprotein (P-gp) belongs to superfamily of ATP-binding
cassette (ABC) transporters.
• These proteins bind ATP and use the energy to drive the
transport of various molecules across all the cell membranes.
• The P-gp efflux transporter can functionally protect the body
against toxic xenobiotics and drugs by excreting these
compounds into bile, urine, and the intestinal lumen and by
preventing their accumulation in brain, gonads, and placenta.
4. Membrane transporters
They are membrane proteins that control influx of
essential nutrients and ions, and the efflux of cellular
waste, environmental toxins, drugs and other
xenobiotics.
They exhibit selectivity, saturability and competitive
inhibition by co-transported substances.
7. Need to study p glycoprotein
Although MDR is a multifactorial process, the
main obstacle is the expression of multidrug-
efflux pumps that lowers the intracellular drug
levels.
P-glycoprotein (P-gp) is the longest identified
efflux pump involved in this.
8. structure
• P-gp is a 170 kDa membrane-bound protein, an energy-
dependent efflux transporter driven by ATP hydrolysis.
• It is composed of 2 homologous and symmetrical halves
(cassettes), each of which contains six TM domains that
are separated by an intracellular flexible linker
polypeptide loop, with an ATP-binding motif.
• There are two ATP-binding domains of P-gp, located in
the cytosol side.
9.
10. Structure continued
• ATP binding site:
• Walker A-ATP binding
• Walker B- Magnesium ion
• Signature C-?ATP hydrolysis
• Trans membrane domain:
• TM1, TM4, TM6, TM10, TM11,TM12
• TM1: pocket deciding drug size
• TM6, TM12: drug binding sites
12. Cellular localisation
• In liver, P-gp is found exclusively on the biliary
canalicular front of hepatocytes and on the apical
surface of epithelial cells in small biliary ductules.
• In pancreas, P-gp is found on the apical surface of
the epithelial cells of small ductules but not larger
pancreatic ducts.
• In kidney, P gp is found concentrated on the apical
surface of epithelial cells of the proximal tubules.
13. • Both colon and jejunum show high levels of P-gp on the
apical surfaces of superficial columnar epithelial cells.
• Adrenal gland shows high levels of P-gp, diffusely
distributed on the surface of cells in both the cortex and
medulla.
• Its expression is also detected in specialized epithelial cells
with secretory or excretory functions, trophoblasts in the
placenta, and on endothelial cells of capillary blood
vessels at blood–tissue barrier site
14. Mechanism of p gp
• Drug / substrate recognition:
Amino acids in TM1 are involved in the formation of a
binding pocket that plays a role in determining the suitable
substrate/drug size for P-gp, whereas Gly residues in TM2
and TM3 are important in determining substrate specificity.
15. ATP-binding and subsequent hydrolysis:
• Around 2molecules of ATP are hydrolyzed for every molecule
of the drug transported outside the cell.
• one in the transport of substrate and the other in effecting
conformational changes to reset the pump for the next
catalytic cycle.
• Alternate catalytic cycle of ATP hydrolysis and ADP release is
the rate-limiting step in the catalytic cycle and the substrates
exert their effect by modulating ADP release.
16.
17. Efflux of substrate/drug through central pore:
• P gp intercepts lipophilic drugs as they move
through the lipid membrane and flips the drugs
from inner leaflets to the outer leaflet and to
extra cellular medium.
• A major reorganization of the TM domains
occurs throughout the entire depth of the
membrane resulting in central pore formation of
2-3 nm diameter and 5-6 nm in depth on binding
18.
19. Physiological significance
• Transport mechanisms for the extrusion of toxic
xenobiotics and their metabolites from cellular
environment
Example:
1,1-Bis(4-chlorophenyl)-2,2,2-trichloroethane (DDT)
plastic derived xenobiotics
21. P gp substrates
• Neutral or cationic compounds form the substrates
• Anticancer drugs: Actinomycin, cyclosporine-A, cisplatin,
• Cardiovascular drugs: Atorvastatin, lovastatin, digitoxin,
losartan,
• Antiviral drugs: amprenavir, indinavir, saquinavir,
nelfinavir, and ritonavir
• Antibacterial agents: erythromycin, rifampin,
sparfloxacin, levofloxacin
• GIT drugs: Cimetidine,domperidone, loperamide and
ondansetron
• Other drugs: Chloroquine, dexamethasone, morphine,
phenytoin
22. Mechanisms of P gp inhibition
• Competitive inhibition: Itroconazole, verapimil
• ATP hydrolysis
• ATP hydrolysis blockage& Competitive inhibition:
cyclosporine A
• Allosteric inhibition: flupenthixol
24. Pharmacokinetic importance
• ABSORPTION:
Enterocytes of the GI tract.
• DISTRIBUTION:
• Trophoblasts in the placenta
• Endothelial cells of capillary blood vessels
at blood–tissue barrier siteslike blood
brain barrier
25.
26. • METABOLISM:
Intestinal CYP3A4-mediated bio
transformation and active efflux of absorbed drug
by Pgp are major determinants of bioavailability of
orally administered drugs.
• EXCRETION:
Renal P-gp forms a transepithelial
tubular drug transport pathway that is responsible
for the net urinary excretion of various xenobiotics
and drugs
27.
28. Drug interactions
• Digoxin:
Absorbtion is reduced when given along with
rifampicin(inducer)
Excretion is retarded when given with verapimil
(inhibitor)
• Protease inhibitors
• Azithromycin can modify the hepatobiliary excretion of
doxorubicin-a.
30. ANTICANCER DRUGS:
Intrinsic resistance: Tumors arising from
tissues where MDR1/Pgp are expressed like
pancreatic carcinoma
Acquired resistance: Increased expression of
MDR1/P-gp is seen in tumours where cisplatin,
doxorubicin are used
31. PREVENTION OF RESISTANCE:
• Agents that are insensitive to P-gp-mediated drug
efflux:
Lamellarin D (LAM-D), a marine alkaloid a
potent proapoptotic agent used in prostrate ca and
leukaemia is an example of drug which is insensitive for
p gp
• P gp inhibitor:
verapimil, flupenthixol
32. a) ANTI-EPILEPTIC DRUG (AED) THERAPY:It has been
hypothesized that overexpression of P-gp and other efflux
transporters in the cerebrovascular endothelium in the
region of the epileptic focus may lead to drug resistance in
epilepsy
b) CHLOROQUINE-RESISTANT PLASMODIUM FALCIPARUM
STRAINS: recently a study presented evidence for a
saturable and energy-dependent chloroquine efflux system
to be present in chloroquine-resistant strains.
c) PROTEASE INHIBITORS:Overexpression of multidrug
transporters significantly reduces the accumulation of
protease inhibitors (PIs) at major sites of virus replication
33. P gp polymorphism
• These variations could contribute to the
interpatient variabilities in plasma protease
inhibitor concentrations and might be of clinical
use.
• Intracellular concentration of HIV PIs and
antiretroviral efficacy is affected by variable P-gp
expression, as a result of the polymorphism of
MDR1
