Pharmacology for Dentistry

Pharmacokinetics 27

nasal mucosa, as these drugs diffuse more
rapidly.

Topical administration: It is employed
for local action in the form of ointments,
creams, jellies etc. for its antiseptic and local
anaesthetic action. The lipid soluble drugs
penetrate the skin easily and rapidly.

Rectal administration: The drug is also
administered rectally in the form of
suppositories and enema preparations
which are absorbed from the colon.

PHYSICO-CHEMICAL FACTORS

The physicochemical factors affecting
the absorption are lipid solubility,
dissolution rate, salt from complexation,
viscosity and drug stability in the GIT.

Lipid Solubility Dissociation Constant
and pH
The drug solution of weak acid in the
stomach (pH = 1.0; acidic) will be in more
unionized form, which is more lipid soluble
and gets more easily absorbed in the stomach.

A solution of weak base in stomach, less
unionized is most unabsorbable e.g. quinine.

The high pH in intestine favours the
absorption of weak bases.

Dissolution Rate
All the drugs in any solid dosage form
or suspension when administered will first
change into drug solution in body fluids. So,
dissolution rate is important factor affecting
the rate of absorption.

When a drug is more rapidly or
completely absorbed from solution, it is very
likely that its absorption will be dissolution
rate limited.

Salt Form

The sodium or potassium salts of weak

acids have higher absorption rate than those

of acids themselves. For example, the

biological activity of sodium salt of

novobiocin (weak acid) is twice as compared

to its calcium salt, and about 50 times larger

in comparison to its free acid.

The dissolution rate of tolbutamide

sodium is much greater than the rate of its

free acid.

Crystal Form

The metastable forms are preferred in

pharmaceutical preparations due to their

higher solubility and dissolution rate e.g. the

amorphous form of novobiocin is absorbed

readily as compared to its crystalline form.

Complexation

The complexation of calcium of the

mucosal cells reduces the absorption of

certain drugs e.g. the barbiturates and

sulfonamides. Presence of EDTA increases

the absorption of mannitol.

Viscosity

Viscosity limits the dissolution rate and

thereby affect the rapid absorption e.g.

aqueous solution of sodium salicylate showed

its rapid appearance in plasma while the same

drug in suspension form failed to reach the

target as quickly as with aqueous solution.

PHARMACEUTICAL FACTORS

These factors are mainly related to the

various dosage form of pharmaceuticals.

Their absorption depends upon their

physical nature like aqueous solution,

suspension, powder, tablets, capsules etc.