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8/11/2019 Cell Junctions and MDR
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Epithelial Cell Junctions
&
Efflux Transporter Systems
I n relevance to
Oral Drug Absorption and Bioavailabi l i ty
Chandra Teja U
Department of Pharmaceutics
KVSR SCOPS
By
Y13MPPC140002
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What They Are;
How They are Important;
Their Types; and
Relevance in Oral Drug Absorption
We Will be iscussing
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Epithelial Cell Junctions
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Cell Junctions are the structures where long term association
between neighbouring cells and cells with ECM are established.
They
Hold the cells togetherwith enough stability to compose tissue.
Are composed of Membrane-Associated Structures, usually
Transmembrane Proteins.
Also help in communicationbetween cells.
What Are Cell Junctions!?
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How Many Types Are There!?
Cell Junctions are of the following types
1. Tight/Occluding junctions: Seal cells together into sheets.
(forming an impermeable barrier)
2. Adhering/Anchoring junctions: Attach cells to other cells or
ECM. (providing mechanical support)3. Gap/Communicating junctions: Allow exchange of chemical or
electrical information between cells
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Tight or Occluding JunctionsThe junction forms a
band around the cell
fusing cells togetherand closing
intercellular space.
The number of fusionsites often correlate
to the leakiness of the
tissue.
In renal tubule of the kidney (filtrating area) few fusion sites are
present enabling leakability.
In urinary bladder (contents should not leak out) numerous tight
junctionsare present to form a tight seal between cells.
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These junction greatly restricts the passage of water, electrolytes and
other small moleculesacross the epithelium.
They block the movement
of integral membraneproteins (red and green)
between the apical and baso-
lateral surfaces of the cell.
Thus the special functions ofeach surface, like
Receptor-mediated
endocytosisat the apical
surface
Exocytosisat the
basolateral surface
can be preserved.
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Anchoring JunctionsCells must be anchored to one another and to components of the ECM.
Junctional complexes called anchoring proteinsextend through theplasma membrane to link cytoskeletal proteins in one cell to that in
other cells as well as to the ECM.
Three types of anchoring junctions are observed: Adherens Junctions
Desmosomes
Hemidesmosomes
Anchoring junctions are most abundant in tissues that are subjected to
severe mechanical stress, such as heart, muscle, and epidermis.
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a)Adherens Junctions or Zonula Adherens
Situated atclose proximity to the
apexbut somewhat below the tight
junctions.
They are relatively permeable as
there are no fusion sites that
completely seal off the paracellularspace.
They often form a continuous
adhesion belt/band.
Their transmembrane anchors are composed of Cadherinsto anchor with
other cells andIntegrinsto anchor with ECM.
Adherens junctions share the characteristic of anchoring cells through
their cytoplasmic contractile bundle of actin filaments.
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b)Desmosomes. a.k.a. Macula AdherensCan be visualized as rivetsthrough
the plasma membrane of adjacent cells.
Can besituated anywhere on the lateral
surface of the cell.
It is a disk-shaped structurematched with
an identical structure at the surface of theadjacent cell.
Their transmembrane anchors areCadherins.
Its adhesion function is further empowered by groups of intermediate
Keratin filamentswhich provides firm adhesion.
The membranes in this region are usually further apart (30nm).
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c)Hemidesmosomes
Like desmosomes, they tie to intermediate filaments in the cytoplasm, but
their transmembrane anchors are Integrinsrather than Cadherins.
Hemidesmosomes are like Desmosomes, but form links betweenCells
and the basal laminathat underlie epithelia.
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Gap Junctions
Aresimply gaps(~2nm)betweenthe neighbouring cell membranes.
Containsprotein units called
Connexinwith a hydrophilic pore
of 1.5nm diameter that acts as anion channelbetween cells.
Present along the lateral surfaceof the epithelial cell.
A Gap-Junction is made up of six Transmembrane Connexin Subunits.
Inorganic ions, andsmall water soluble molecules(
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Efflux Transporter Systems
(Multi Drug Resistance)
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MDR Transporters belong to the family of the ATP Binding Cassette
(ABC) proteins& are present in all living organisms.
The key physiological taskof the MDR-ABC transporter network is to
provide general xenobiotic resistance, probably evolved as Complex
Cellular Defence Systems.
Hundreds of structurally diverse therapeutic agents are substrates totheseand they impedes the absorption, permeability, and retention of the
drugs, extruding them out of the cells.
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In humans, 3 major types of MDR proteins are present, that are the
members of the subfamilies
The ABCB (P-glycoprotein/ABCB1/MDR1),
The ABCC (ABCC1/MRP1, ABCC2/MRP2), and
The ABCG (ABCG2/MXR/BCRP).
The major practical causes for studying MDR-ABC transporter
expression and function are
Predicting the fate of pharmaceutical agents in our body.
The relevance of in vitro experiments to the in vivo role of
MDR-ABC transporters.
Related to cancer & infection drug resistance.
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MDR1 (P-gp) preferentially extrudes large hydrophobic molecules,
while ABCC1 and ABCG2 can transport both hydrophobic drugs and
intracel lularly formed metaboli tes, e.g., Drug Conjugates.
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P-glycoprotein is primarily found in epithelial cells lining the Small
Intestine, Colon, pancreatic & bile ducts, kidney proximal tubules, and
also in the BBB.
The highest MRP1 levels are reported at testis, cardiomyocytes,
placenta, prostate, lung, thymus and kidney. It is found in lower levels
along the GIT.
Significant expression of ABCG2 was observed in the Placental
Barrier, and at the blood-testis barrier, the BBB, and the stem cells.
Hence, P-gp is of greatest importance when talking about Oral BA of
Drugs.
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The P-GlycoProteinThe most extensively studied and experimented MDR Protein
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P-gp is encoded by theABCB1gene.
It contributes greatly in the extrusion of many drugs from the blood into
the intestinal lumen.
It is also responsible for enhancing the excretion of drugs out of
hepatocytes and renal tubules into bile and urine.
It can recognize and transport numerous structurally diverse drugs over
a molecular weight range of 250 g/mol (cimetidine) to 1200 g/mol
(cyclosporin).
Therefore, P-gp can potentially reduce the absorption and oral
bioavailabilityand decrease the retention time of a number of drugs.
The transporter is also overexpressed on the surface of many
neoplastic cellsand makes the chemotherapy almost ineffective in many
cases.
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Location of P-gp Function
GI Epithelial CellsEfflux of Cellular waste and Drugs
into lumen
Hepatocytes
Secretion of drugs into bile and re-
entry into Intestine
(Entero-Hepatic Circulation)
Renal Epithelial CellsEfflux of Drugs into Tubular
Lumen, easily Excreted
Placenta, Brain, Testis Barrier Action for Entry of Drugs
Cancer Cells Resistance to Agents
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Interactions of P-gp with Drugs
The substrate specificity of P-gp and CYP450 enzyme system has
overlap extensively.
Clinically Significant reduction in Oral Absorption and BA is seen with
Antacids
Cimetidine
Ranitidine
Antibiotics
Erythromycin
Levofloxacine
Cardiac Drugs
Digoxin
Antitumour Agents
Placitaxel
Doxorubicin
Vinblastin
Ca2+
Channel Blockers
Diltiazem
Immunosuppressants
Cyclosporine A
Anti-retrovirals
Ritonavir
Indinavir
Steroids
DexamethasoneOpioids
Morphine
Fentanyl
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P-gp also has Inducers and Inhibitors
An I nducer fur ther reduces the absorptionof other substrates.
An I nhibitor generally increases the absorption of other drugs byreducing P-gp activity.
I nducersAvasimibe
Carbamazepine
Phenytoin
Rifampin
Morphine
Dexamithazone
InhibitorsQuinidine
Reserpine
Cyclosporine
Verapamil
Azithromycin
Atorvastatin
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P-gp Follows Saturable Kinetics
In the case offast absorbing drugs having larger doses
, efflux byP-gp poses less impact on drug absorption and therefore is not
important in terms of bioavailability or pharmacokinetic properties.
This is because the transport activity of P-gp becomes saturated by
high concentrations of drugin the intestinal lumen.
This is best explained by Michaelis Menten Plot & Equation.
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Enhancement of BA by P-gp Inhibition
P-gp mediated drug efflux greatly interferes with the delivery of Potent
Drugs or the drugs with slow dissolution and dif fusion rates.
This decrease in drug absorptionof those small amounts of drugs can be
life threatening at times as sufficient plasma levels of drug cannot be
reached.
Moreover, it can make the sustained release dosage forms of the
substrates completely ineffectiveby limiting their absorptions.
This is even more important in case of MDR exhibiting Cancer.
Hence, P-gp inhibition has gained a lot of importance in Research field
and many approaches are being postulated.
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In general, P-gp can be inhibited by three mechanisms:
(i) blocking the pump competitively or non-competitively;
(ii) interfering with ATP hydrolysis; and
(iii) altering integrity of cell membrane lipids.
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P-gp inhibitors are classified into three generations based on their specificity,
affinity, and toxicity.
1stgen.inhibitors arepharmacologically active substances which are
clinically used but have the ability to inhibit P-gp.
2ndgen.inhibitorspharmacologically inactive. But, they inhibit CYP
enzymes and ABC transporters leading to complicated pharmacokineticalterations. (EgDexverapamil, valspodar, etc)
3rdgen. inhibitors are under clinical development, aiming for higher
specificity and low toxicity(EgLaniquidar, Tariquidar, etc)
Other approaches likeLiposomes (Active Targeting), Use ofP-gp bypassing
Polymers,Drug-Macromoecule Conjugates, etc are being studied.
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References
Google Images
http://www.histology.leeds.ac.uk/
http://the-aps.org
http://en.wikipedia.org
http://www.nature.com/
PubMed NCBI Articles
SlideShare and AuthorStream Presentations
http://www.histology.leeds.ac.uk/http://the-aps.org/http://en.wikipedia.org/http://www.nature.com/http://www.nature.com/http://www.nature.com/http://www.nature.com/http://www.nature.com/http://www.nature.com/http://www.nature.com/http://www.nature.com/http://en.wikipedia.org/http://en.wikipedia.org/http://en.wikipedia.org/http://en.wikipedia.org/http://en.wikipedia.org/http://en.wikipedia.org/http://en.wikipedia.org/http://en.wikipedia.org/http://the-aps.org/http://the-aps.org/http://the-aps.org/http://the-aps.org/http://the-aps.org/http://the-aps.org/http://the-aps.org/http://www.histology.leeds.ac.uk/http://www.histology.leeds.ac.uk/http://www.histology.leeds.ac.uk/http://www.histology.leeds.ac.uk/http://www.histology.leeds.ac.uk/http://www.histology.leeds.ac.uk/http://www.histology.leeds.ac.uk/http://www.histology.leeds.ac.uk/http://www.histology.leeds.ac.uk/http://www.histology.leeds.ac.uk/http://www.histology.leeds.ac.uk/8/11/2019 Cell Junctions and MDR
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