PHARMACOLOGY OG P- GLYCOPROTEINS

1. P glycoprotein
Dr. Uma Advani
Assistant Professor Pharmacology
SMS MC Jaipur

2. Learning Objectives
• Introduction
• Cellular localization of P-gp
• Structure of P-gp
• Drug/substrate recognition
• Substrates for P-gp
• Inhibition of P-gp
• Pharmacological relevance
• Pharmacokinetic implications of P-gp
• Conclusion

3. Introduction
• P-glycoproteins are part of a larger family of
efflux transporters
• They appear to have developed as a mechanism
to protect the body from harmful substances
• They transport certain hydrophobic substances in
the following directions
– into the gut
– into urine
– into bile
– out of the brain
– out of the gonads
– out of other organs

4. Contd…
• Considered as apart of the 1st pass metabolism
• P-glycoprotein has also recently been designated
CD(Cluster Differentiation)243.
• In humans, P-glycoprotein is encoded by the
ABCB1(ATP Binding Cassette Subfamily B) gene
• It functionally protects the body against toxic
xenobiotics & drugs by excreting these compound into
the bile, urine & the intestine
• It is also called Hydrophobic vaccum cleaner/flippase:
transporting drug from inner leaflet of plasma
membrane to lipid bilayer to outer leaflet or to
external medium.

5. Cellular localization of MDR1/P-gp
1
•Cancer cells
•In colon, renal & adrenal tumors
2
•Normal tissues
•Certain cells in liver, pancreas, kidney,(entire length duodenum, jejunum,colon &
rectum)
•Biliary canalicular cells , trophoblast in placenta, endothilial cells,astrocytes in brain
3
•Human fetus

6. Structure of P-gp
• It’s a 170 kDa membrane bound protein, long half life
of 24 hrs.
• Composed of 2 symmetrical halves or cassettes, each
of which contains 6 TM( TransMembrane) domains
that are separated by flexible polypeptide loops
• 2 ATP binding domains in the cytosol side also k/a
Nuclear Binding Folds ( NBF).
• Each ATP binding domain contains 3 regions
– Walker A
– Walker B
– Signature C

8. Efflux of substrate/ drug
Drug binds to
the TM binding
domain
Causes
conformational
changes &
reorganization
into compact
domains
Opening of
the central
pore & efflux
of the drug

9. Substrates for P-gp
Categories Example
Anti cancer drugs Actinomycin, cisplatin, danuorubicin,
dactinomycin, paclitaxel, etoposide,
vincristine, vinblastin etc
CVS drugs Statins (atorvastatin & lovastatin),
celiprolol, diltiazem, digioxin, losartan,
quinidine & verapamil
Anti viral drugs Amrenavir, indinavir, saquinavir, nelfinavir
& ritonavir
Antibacterial Erythomycin, rifampicin, sparfloxicin,
levofloxacin
GIT drugs Cimetidine, risperidone, domperidone,
loperamide & odansetron
Others Chloroquine, dexamethasone,
fexofenadine, morphine, phenytoin,
tacrolimus, cyclosporine

10. Antagonists of P-gp
Competitive
inhibition. E.g.
itraconazole,
dex-Verapamil
Blockage of ATP
hydrolysis. E.g.
valsopodar
(derivative of
cyclosporine D)
Interfering with
both hydrolysis
& substrate
recognition. E.g.
cyclosporine A
Allosteric
inhibition. E.g.
trans-flupentixol

11. Pharmacological relevance
(Multi drug resistance)
Anti cancer
drug resistance
Anti epileptic
drug
resistance
Chloroquine
resistance
Opiods
resistance
Anti viral
resistance
Anti
parasitical

12. Anti cancer therapy
• Over expression of MDR 1 gene conferred
resistance to multiple drugs such as vinca
alkoloids(Vincristine,vinblastine),
anthracyclines(Daunorubicin,doxorubicin) &
taxanes(Paclitaxel &Docetaxel).
• Tumors arising from tissues where MDR1/P-gp is
highly expressed show intrinsic resistance to
chemotherapy
• Recent theory suggest that there is intracellular
transfer of P-gp
• Poor brain penetration of drug like imitanib
mesylate(TKI used in CML,GISTs) is due to P-gp

13. Anti cancer contd…
• 4-aryl-1, 4- dihydropyridines & aromatized 4-
arylpyridines have been synthesized to
increase the MDR resistance reversal of
vinblastine are Chemo sensitizers
• Lamellarin D a marine alkaloid developed for
prostate cancer is a newer cytotoxic drug
insensitive to P-gp

14. Others
• Antiepileptics: like Valproate which is substrate for
Pgp results in pharmacoresistant epilepsy,studies
using Pgp inhibitor like tariquidar increased its
effect.
• Antimalarials:
– Mechanism is controversial for Mefloquine &
Artesunate
– Chloroquine resistant plasmodium falciparum strains
– In vitro can be modulated by verapamil & calcium

15. Contd…
• Opiods
– Due to induction of P-gp
– Increase the efflux from brain
• Antiviral
– Over expression of multidrug transporters is the key
mechanism for resistance to protease inhibitors
– Also involved in development of resistance against
antibiotics
– Biricodar & timcodar ( MDR inhibitors) are effective in
when use with antibiotics like fluoroquinolones

16. Biological barrier to xenobiotics
• P-gp protects us from the harmful effects of
xenobiotics & their toxic metabolites
E.g. DDT is an organochlorine pesticides
• Its metabolite p,p-DDE is an environmental
containment accumulates in animals
• Both induce the MDR1/P-gp gene expression
leading to their clearance

17. Pharmacokinetic implications of
P-gp

18. Role of Transporters
• Transporters mediate tissue specific drug
distribution (drug targeting).
• Transporters serve as protective barriers to
organs and cell types.
• P- glycoprotein in blood brain barriers protects
CNS through efflux mechanisms also.
• Relevant transporters to drug response also
control distribution, absorption and
elimination as well.

19. Absorption
• Intestinal Pgp:reduce absorption of various drugs
like protease inhibitors and anti cancer drugs that
are substrates for P-gp,limits their bioavailability
• Both the passive diffusion & the P-gp operate in
opposite directions
• E.g. Bioavailability of digoxin & cyclosporine is
enhanced by inhibitors & reduced by inducers like
Phenytoin & Rifampicin
– Quinidine(Pgp inhibitor): increases the absorption &
plasma concentrations of morphine
– The bioavailability of Saquinavir is reduced with alcohol
due to excessive expression of P-gp

20. Drug distribution
• Blood brain barrier (BBB)
– Protects the brain from the harmful endogenous &
exogenous substances
– P-gp is the key element of BBB
– Limits the central distribution of beneficial drugs. E.g.
TCA’s, risperidone & antiepileptic drugs
– Combined inhibitory effect of P-gp inhibitors like
cyclosporin A, verapamil are useful approach to increase
the drug concentrations
– Parkinson's & Alzheimer's patients have dysfunctional P-
gp in BBB

22. contd…
• Placental barrier
– Extrusion of drugs toxins & thus helps protect the
fetal malformations
– Poor maternal- fetal transfer of Indinavir has been
shown due to P-gp
– Verapamil or quinidine do not inhibit P-gp activity
in term human placenta

23. Metabolism
• It has been postulated that both the CYP3A4
& P-gp acts in concert to limit the
bioavailability of the drug
• They can "set up" or act as "gatekeepers" for
later P450 cytochrome actions
• There is a substantial overlapping of
substrates between CY3A4 & P-gp

24. P-gp inhibition
• Verapamil an inhibitor of metabolizing enzyme
CYP3A4 & P-gp increases the effect of
paclitaxel
• Less potent P-gp inhibitors such as valspodar,
cyclosporin A and ketoconazole had modest
effect on the brain plasma ratios of nelfinavir
• Piperine (component of black pepper) has
shown to inhibit both CY3A4 & P-gp in rodents
& increase the concentration of various drugs

25. P-gp induction
• Both CYP3A4 & P-gp have overlapping inducers in vitro
• A chronic treatment with rifampicin induces expression of
transport proteins & CYP3A4
• Protease inhibitor( ritonavir) & dexamethasone
• Others include
– Phenobarbitone
– Doxorubicin
– Atazanavir
– Prazocin
– Spiranolactone
– St John’s Wort( hypericum perforatum)

26. Drug excretion
• Renal excretion:Modest role in elimination
,expression of Pgp on luminal proximal tubule cells
in kidney pumps out drug into urine.
– Various drug interactions can result
• Digoxin + Clarithromycin
• Digoxin + Verapamil increased levels of digoxin
• Digoxin + cyclosporine
• Pazufloxacin + cyclosporine
• Cimitedine + itraconazole

27. Modulation Pgp by herbal drugs
• St. Johns’ wort
– Commonly used as anti depressant
– Potent inducer of CYP3A4 & P-gp: causes OCP failure &
serotonin syndrome
– Decreases the concentrations of amitriptyline, cyclosporine,
digoxin, fexofenadine, indinavir, metahdone, midazolam etc
• Piperine
– Inhibitor of P-gp & CYP3A4
• Grapefruit
– Inhibitor of P-gp & CYP3A4

28. Polymorphism of P-gp
• Mutations at 2677 & 3435 have been associated
with alterations P g expression & function
• MDR1 polymorphism also effect the drugs
associated with it.E.g. the non linear PK’s with
large inter individual differences of phenytoin
• MDR1 is highly polymorphic ,may effect the entry
of various drugs across the BBB or the placental
barrier leading to accumulation of toxic
substances
• MDR1 polymorphism may have an influence on
the disease activity in cases of RA ,IBS &
colchicine resistance.

29. Drug interactions: Pgp
• Also involved in many drug interactions
• Loperamide sustrate for Pgp which is used over
the counter for diarrhea, if given with Pgp
inhibitor it crosses BBB increase concentration in
brain& results in respiratory depression.
• Dabidatran (Anticoagulant :Direct thrombin
inhibitor) sustrate for Pgp if given with Pgp
inhibitors like verapamil , clarithromycin&
ticagrelor like drugs there will be increase
concentration and haemorrhage.

30. Conclusion
• P-glycoproteins are part of a larger family of efflux
transporters
• They form an important component of the BBB & the
placental barrier
• Have a protective role in preventing the accumulation
of harmful substances within the body
• Have been found to be a cause for the MDR in varous
diseases especially cancers
• Alterations in P-gp can lead to increased susceptibility
for development of certain diseases like Alzheimer's &
Parkinson’s
• Awareness of potential transporter related drug-drug
interaction for safe medication is need of time.

32. References
• Tandon VR, Kapoor B, Bano G, Gupta S et.al; P-
glycoprotein: Pharmacological relevance. IJP
feb 2006; 38:13-24