a. Once the drug is absorbed, it enters the circulation and is carried throughout the body. The location in the body where the drug goes 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. Finally, some of the drug will reach the target tissue where it can cause the effect for which it was administered. An equilibrium will be established between the circulating unbound drug and each area of the body.
b. The distribution of a drug in the body happens in a very systematic manner. Figure 3-4 demonstrates the concept of distribution.
Assume that 100 micrograms (100 mcg) of a drug have been absorbed and is distributed based on the percentages noted in Figure 3-4. Of the 100 micrograms absorbed, only two micrograms of the drug will arrive at the target tissue to give the desired pharmacological effect. If two micrograms is enough drugs to produce the desired pharmacological effect, the desired effect will be obtained. However, if the amount of drug required to produce the effect is four micrograms, the desired pharmacological effect will not be obtained. The dose of the drug can be increased so that 200 milligrams of the drug can be absorbed, thus providing the amount of drug needed to give the desired pharmacological effect. However, doubling the dose may present problems. Doubling the dose would also double 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 side effects of the drug. Thus, it is important to remember that the whole body must be taken into account when a drug is administered. If the problem of the side effects cannot be resolved, the drug may not be released for use.
NOTE: Another areas of concern in the distribution of drugs are those that crosses the placental barrier. Drugs may actively or passively cross the placental barrier and enter the fetal circulation. The enzyme systems of the developing fetus may not be able to adequately metabolize the drug. Toxic effects can result. At this time, it is virtually impossible to predict whether a drug will pass the placental barrier.
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