Pharmacokinetics
By Ron Aylor, CPhT
Pharmacokinetics is the study of how a patient's body will process a substance from introduction to elimination. Basically, it's the journey the substance encounters as it makes its way through the body and what the body does to it. Please don't confuse it with Pharmacodynamics, which is the study of how a drug will affect a patient, it's the other way around.
The Basics
There are four basic processes that the body puts the drug through. A.D.M.E. is the acronym in pharmacology for those four simultaneously occurring processes; Absorption, Distribution, Metabolism, and Excretion. These four processes working together, are responsible for blood concentrations of a drug within the body. When viewed altogether, the four processes are called disposition.
Absorption
For
a drug to reach its intended site of action, it must be taken into the
bloodstream. The process of releasing a drug from its dosage formulation
and transferring it into the bloodstream is called absorption. This
process occurs to some extent regardless of formulation or route of
administration; drugs injected directly into the bloodstream must still
be absorbed.
Three factors affect the
absorption of a drug: its water solubility, its fat solubility, and the
transport mechanisms of the body. It is imperative that you understand
that all drugs must be in solution before they can be absorbed.
All body fluids are water based. Therefore, a drug must be soluble in water in order to be absorbed. Dissolution of the drug in aqueous (water) solution is dependent on the pH of the solution and the disintegration of the drug. The speed at which a dosage form disintegrates is dependent upon the type of solid dosage form and the manufacturing process used to make that dosage form. Solid dosage forms can be a tablet, suppository, capsule, powder, or a suspension. Take for example a tablet. The manufacturer may add starch to a tablet in order to make it swell when added to water. A tablet may be a sub-lingual tab made to rapidly dissolve in the mouth. The contents of a tablet could be compressed under great pressure so that it will dissolve more slowly. Further, an enteric coating may be applied to a tablet so that it will pass through the stomach and dissolve in the intestine. In the case of some capsules, "extended release capsules" systematically dissolve over a period of time, prolonging the effect of the drug.
Distribution
Once a drug is absorbed, it enters the circulation and
is carried throughout the body. The destination within in the body
varies from drug to drug. The drug may be stored in bone or fat, bound
to the proteins in the blood plasma, or circulate freely as the unbound
drug. The drug will find its way into many organs. Yet finally, some of
the drug will reach the target tissue where it can cause the effect for
which it was administered.
The distribution of a drug within the body happens in a very systematic way. Assume that 10 mg of a drug have been absorbed and
distributed based on the percentages in the illustration above. Of the
10 mg absorbed, only 0.2 mg of the drug will arrive at the target tissue
to give the desired pharmacological effect. If 0.2 mg is enough drugs
to produce the desired effect, all is well. However, if the amount of
drug required to produce the effect is 1 mg, the desired effect will not
be obtained. The dose can be increased so that 50 milligrams of the
drug can be absorbed, thus providing the amount of drug needed to give
the desired effect. However, increasing the dose may present problems.
Increasing the dose would also increase the amount of drug in the other
areas of the body. Perhaps this increased dosage may produce some
response by another body organ. For example, the patient may become
nauseous, vomit, lose his hair, or go into convulsions. These are all
side effects of the drug. It is important to remember that the whole
body must be taken into account when a drug is administered.
Another
area of concern in the distribution of drugs is the crossing of the
placental barrier. Drugs may actively or passively cross the placental
barrier and enter the fetal circulation. The enzyme systems of a
developing fetus may not be able to adequately metabolize certain drugs
and toxic effects can result.
Metabolism
The above processes of absorption and distribution are
dynamic. That is, it is continually changing. Even as the drug is being
distributed, the individual cells of the body begin to chemically change
or alter the drug. This metabolic process of changing the drug is
called metabolism or bio-transformation. The liver changes drugs to make them more
water-soluble so that they may be more easily excreted from the body.
As metabolism occurs, the initial drug is converted to new compounds called metabolites. When metabolites are pharmacologically inert, metabolism deactivates the administered dose of drug and this usually reduces the effects on the body. Drugs can undergo one of four potential bio-transformations: Active Drug to Inactive Metabolite, Active Drug to Active Metabolite, Inactive Drug to Active Metabolite, Active Drug to Toxic Metabolite (bio-toxification). Metabolites may also be pharmacologically active, sometimes more so than the parent drug.
Excretion
Excretion is the process of eliminating a drug or its metabolites from the body. The major organ of excretion is the kidney. The kidneys filter blood and remove waste materials. The resulting fluid is excreted from the body as urine.
Secondary routes of excretion are hepatic (liver),
through the bile into the feces, lungs, saliva, sweat, and breast milk.
The inability of a patient to excrete drugs and other waste can be life
threatening. Patients who have limited liver and kidney function usually
require lower doses of medication, due to the fact that more of the
drug tends to stay in the body.
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