2. Introduction
P-glycoprotein also known as multidrug resistance protein 1 (MDR1) or ATP-
binding cassette sub-family B member 1 (ABCB1) is an important protein of
the cell membrane that pumps many foreign substances out of cells. More
formally, it is an ATP-dependent efflux pump with broad substrate
specificit.
3. Mechanism of action
o They are membrane proteins that control influx of essential nutrients and
ion ,the efflux of cellular waste ,environmental toxin ,drug and other
xenobiotics.
In the intestine and in the liver, P-gp increases exposure of drug to CYP
enzymes. Because of this, there is increase in drug metabolism and
decrease in bioavailability.
o In tumor cells, P-gp transports drugs out of the cells and contributes to the
multidrug resistance phenomenon (MDR).
4. Struture
P-gp is a single 170 kDa polypeptide ) consisting of 1280 amino acids
. As previously mentioned, the protein appears to have arisen by a gene
duplication event, by fusing two homologous halves, each consisting of six
highly hydrophobic transmembrane α-helices (TMHs) and one nucleotide
binding domain (NBD) located on the cytoplasmic side of the membrane,
which binds and hydrolyses ATP .The two half molecules are separated by
a highly charged cytoplasmic “linker region”, which is phosphorylated at
several sites by protein kinase C (PKC)
5.
6. Transe member domain
o TM1,TM4,TM6,TM10,TM11,TM12
o TM1:pocket deciding drug site
o TM6,TM12:drug binding sites
7. o ATP binding and hydrolysis at the two cytoplasmic NBDs was found to
be essential for P-gp-mediated drug transport . Site- directed
mutagenesis approaches have demonstrated that the three NBDs highly
conserved motifs
o Walker A
o Walker B
o signature c
8. it is now generally accepted that the NBDs must dimerize in order
to hydrolyse ATP ) and, according to this so-called “ATP sandwich dimer”,
the NBDs are arranged in a head-to-tail arrangement, with two ATP
molecules bound along the interface.
9. Encod
In humans, P-gp is encoded by two multidrug resistance (MDR) genes,
MDR1/ABCB1 and MDR3/ABCB4 (also designated MDR2), The MDR
phenotype is associated with the MDR1 isoform.
The human MDR3 isoform functions as a phosphatidylcholine (PC)
translocase, or “flippase”, exporting this phospholipid into the bile .
10. Physiological site
Three different P-gp isoforms were identified (P-gp class I, II and III); only P-
gp class I and III were characterized in various normal human tissues with
potential role in the normal physiology of these tissues .P-gp class III is
expressed in liver hepatocytes; and mice lacking its expression fail biliary
phopholipid secretion. P-gp is expressed as well in a wide range of
epithelia with potential transport function, such as colon,
small intestine, liver, pancreas, kidney, uterus and placenta.
In addition, P-gp was found expressed in highly specialized
capillary transport endothelia such as brain and testis .
12. Substrates
substrates are weakly amphipathic and relatively hydrophobic, often (but
not always) containing aromatic rings and a positively charged nitrogen
atom . P-gp substrates vary greatly in size, structure, and function, ranging
from small molecules, such as organic cations, carbohydrates, amino
acids, and some antibiotics, to macromolecules such as polysaccharides
and proteins .
some physicochemical characteristic features, such as lipophilicity,
hydrogen-bonding ability, molecular weight, and surface area, may
contribute to the drug binding ability of the substrate to P-gp .
13. Chemistry of Substrates
Class I elements which is having two electron releasing groups-[2.5 °A],
whilst Class II elements have three electron releasing group-[4.6 °A].
Electron releasing group are those who have lone pair of electrons on
electron withdrawing atoms or those have unsaturated system containing
Π electron cloud.
Substrates for P-gp depend on number and strength of proton acceptor group.
16. Mechanism Of P-gp Inhibitor
o Competitive inhibition: itraconazole, verapamil
o ATP hydrolysis
o ATP hydrolysis blockage & Competitive inhibition: cyclosporine A
o Allosteric inhibition: flupenthixol
17. P-gp Inhibitor
• First generation P-gp blockers
Verapamil (VRP) is the prototype P-gp blocker and was
found to enhance intracellular accumulation of many anticancer
drugs, including DOX in numerous cancer cell lines
Further studies found that the P-gp inhibiting activity
is shared by many other calcium channel blockers such as,
diltiazem ,bepridil ,nicardipine, nifedipine ,felodipine,
18. Second generation
These compounds may significantly inhibit the metabolism and excretion of
cytotoxic agents leading to unacceptable toxicity, thus requiring
chemotherapy dose reductions in clinical trials.
The search for non-toxic second generation P-gp blockers resulted in newer
analogs of the first generation agents with more potent P-gp inhibition and
considerably less toxicity.
second generation agents were also substrate of the CYP 3A4. So
these may lead to unpredictable absorption and metabolism and these
ultimately resulted into modified bioavaibility.
19. Third generation
These compounds demonstrated a potency approximately 10-fold higher
than the first- and second-generation inhibitors.
These agents were not the substrates of CYP 3A4 so used to overcome
drawback of second generation agents and these agents
selectively and potentially inhibit P-gp.
21. First generation Second generation Third generation
verapamil ® verapamil Triquidar(XR9576)
Cyclosporine A dexniguldipine Zosuquidar(LY335979)
Vincristine biricodar Laniquidar
Quinidine dofequidar mitotane
tamoxifen valsprodar annamycin
trifluoperazine
22. Novel Approaches To Bypass P-gp Efflux
Pump
o Reversal agents
o Natural and Synthetic Polymers
o Nanocarrier drug delivery system
o Liposomes
o Stealth liposomal carriers
o Niosomes
o Polymeric nanoparticles
o Dendrimers
o Polymeric drug conjugate
