By the name of Allah
Biopharma L4
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Note : read this sheet with a smile on your face  
Yesterday we started to talk about the first mechanism of drug absorption which is the simple
passive diffusion and it is the most common mechanism for drug transport across membranes
in general , this mechanism is controlled by fick's first law of diffusion and in this law there is
different parameters that control the rate of diffusion , we can divide these parameters to :
* parameters related to the drug which is D ( diffusion coefficient in a certain environment )
and K ( oil- water partition coefficient )
* 2 constants that are related to the site of action which is A (surface area available for
permeation ) and H ( the membrane thickness )
So if we talk about drug which is transported across a defined membrane then D, A, K, H are
going to be constant , and if we talk about perfect conditions where the concentration of the
drug at the absorption site is much higher than in the plasma then the rate of diffusion =
permeability constant * concentration in the absorption site ( C1 )
1st order kinetics
so by increasing the concentration in the absorption site we will achieve very high peak within
a short time and thus increase the rate of absorption
note : C1 represent the amount of drug that available for absorption not the dose itself 
The second mechanism is " carrier mediated transport " which is a specialized carrier needed
to transport the substance from the apical side to the basolateral side
the first subtype of this mechanism is the ACTIVE TRANSPORT which is energy requiring
mechanism and it takes place against con gradient ( uphill )
another subtype is FACILLITAED TRANSPORT in this mechanism the substance move with con
gradient ( downhill ) so can you ask yourself why facilitated diffusion not simple diffusion ?
well , the substance use facilitated transport when they don’t have a sufficient lipid solubility
to allow their permeation by simple passive diffusion so they would need a carrier .
definitely these molecules share structural similarity with indigenous substance which allow
them to pass , this mechanism require no energy . ( refer to the slides for some examples )
note : some drugs can be transported by more than one mechanism
all mechanism that involve carriers have certain characteristics :
1. There is a limited number of carriers ( upregulated in some tissues whereas downregulated
in another tissues ) and the mechanism that describes the rate of transport is MichaelisMenten kinetics.
km reflects the affinity , so as we decrease "km" we increase the affinity
In most cases we are working on a concentration far below "km" that means the rate of
absorption represents 1st order kinetics , but if the concentration is much higher than "km "
we will reach the saturation and the absorption will be a zero order kinetics and this is very
rare .
2. Selectivity and specificity
3. Competition
4. Inhibition
‫ ) واعطانا مركب وحكالنا شوفولنا ياه هل بنتقل عن طريق‬:p ‫هأل اجى علينا دكتور يوسف ( من الطابق التالت‬
simple diffusion or carrier mediated ?
well , we can make a diffusion experiment in this experiment we will need a donor chamber
and acceptor chamber and we will put a layer of cells ( one type of cell ) between these two
chamber and allow them to grow , then I put the drug in the donor chamber , then I start to
take a sample from the acceptor at different time and calculate the rate of absorption and
draw a curve if the curve is linear then it would be simple passive diffusion , and if the curve is
sigmoidal then it would be carrier mediated diffusion, then I further study the other features
of carrier mediated transport ( competition and inhibition ) .
another mechanism of transport is the
PARACELLULAR which takes place between the
cells and it is responsible for transfer water
and small molecules carrying by water so it is
molecular size limitation . sometime we need
to weakens the tight junctions to enhance the
transport of larger molecules . the last
mechanism of transport is the VESICULAR TRANSPORT where we have a small spherical
membranous sac formed by budding off from an existing membrane and it is subdivided into
Endocytosis and Exocytosis . in endocytosis we brought the materials into the cell by
1. Phagocytosis ( for a large particles ) and it is not used for drugs absorption
2. Pinocytosis which is nonselective uptake of droplets .
Thanks for your patience
Done By : Asma Qatameen
spread peace and love , be kind for one another, stop judging and hated , embrace the
rainbow of diversity , respect and accept , it is the least we can do as spirits towards our
surroundings