Now why do we talk about hydrophilic and hydrophobic
drugs. Hydrophilic drugs or lipophobic drugs are
are drugs that love waters. So hydro means water, philic
means love. Now any drug that loves water is going to hate
oil. So lipophobic and hydrophilic are the same thing.
Hydrophilic drugs are usually polar. Hydrophobic drugs
are usually not polar. Hydrophilic drugs dissolve in water
so it's aqueous diffusion. Hydrophobic drugs dissolve in
lipid or oil so they have lipid diffusion. Hydrophilic drugs
need a pore or a transporter to get across that lipid bilayer.
Hydrophobic drugs or lipophilic drugs often can cross
barriers without the need of pores or transporters.
This is an important distinguishing feature between these
two types of drug. Hydrophilic drugs, because they can't
cross that membrane usually don't cross the blood brain
barrier or the blood uterine barrier. So generally speaking,
which is'nt always true, hydrophilic drugs don't affect
cerebral function and usually are safer in pregnancy.
Hydrophobic drugs or lipophilic drugs are almost always drugs
that have some kind of a central effect and almost always
have some kind of a fetal effect. Now when we talk about
aqueous diffusion, we are talking about molecule movement
through water, and lipid diffusion is through oil. Aqueous
diffusion is always dependant on the passages of water
between cells or the passage of water through pore. It is
a passive process when you have aqueous diffusion
and similarly with lipid diffusion you have movement
through membranes and it is also a passive process.
All of the passive processes are governed by Fick's Law.
Now before we get too bogged down on the equation,
let's just remember that it is propertional to the diffusion
constant of the drug, the surface area of the membrane,
the thickness of the membrane and the concentration
difference between the inside of the cell and the outside
of the cell. Usually the outside being C1. So, let's
apply this information that we just learned to drugs.
Choose the best answer. A drug that is hydrophilic... A, can
permeate a cell without pores or transporters. B, will cross
the blood brain barrier. C, is always non-polar. D, will
always affect the fetus in a pregnant patient. And E,
will passively permeate into a cell by Fick's Law.
Which one is correct? So the answer is E.
All of the other ones are wrong. All of the other ones
describe a lipophilic drug. So a lipophilic drug permeates
a cell without pores or transporters. A lipophilic drug
will cross the brain barrier and will often affect a fetus
because it crosses the blood uterine barrier and it's almost
always non-polar. All drugs generally will passively permeate
into a cell by Fick's Law whether it's through an open
channel or through the membrane in the case of a
lipophilic drug. It's a hard question and i'll tell you why
people get this question wrong. Because people assume that
just because you are hydrophilic, you can't permeate into
a cell. That's not true. Hydrophilic drug can passively
permeate into a cell. It does'nt always need an active
transporter. And the reason why it can passively permeate
is because it can go through that cell pore
that we had talked about earlier.