DRUG DISTRIBUTION

Introduction: Distribution involves the transport of drugs throughout the body. The simplest factor determining distribution is the amount of blood flow to body tissues. The heart, liver, kidneys, and brain receive the most blood supply. Skin, bone, and adipose tissue receive a lower blood supply; therefore, it is more difficult to deliver high concentrations of drugs to these areas. The physical properties of the drug greatly influence how it moves throughout the body after administration. Drug administration techniques

Drug solubility in lipids:  Lipid solubility is an important characteristic, because it determines how quickly a drug is absorbed, mixes within the bloodstream, crosses membranes, and becomes localized in body tissues. Lipid-soluble agents are not limited by the barriers that normally stop water-soluble drugs; thus, they are more completely distributed to body tissues. Some tissues have the ability to accumulate and store drugs after absorption. The bone marrow, teeth, eyes, and adipose tissue have an especially high affinity, or attraction, for certain medications. Examples of agents that are attracted to adipose tissue are thiopental, diazepam, and lipid-soluble vitamins. 

Tetracycline binds to calcium salts and accumulates in the bones and teeth. Once stored in tissues, drugs may remain in the body for many months and are released very slowly back to the circulation. 

Extent of drug distribution:  Not all drug molecules in the plasma will reach their target cells, because many drugs bind reversibly to plasma proteins, particularly albumin, to form drug–protein complexes. Drug–protein complexes are too large to cross capillary membranes; thus, the drug is not available for distribution to body tissues. Drugs bound to proteins circulate in the plasma until they are released or displaced from the drug–protein complex. 

Only unbound (free) drugs can reach their target cells or be excreted by the kidneys. Some drugs, such as the anticoagulant warfarin, are highly bound; 99% of the drug in the plasma is bound in drug–protein complexes and is unavailable to reach target cells. Drugs and other chemicals compete with one another for plasma protein–binding sites, and some agents have a greater affinity for these binding sites than other agents. 

Effect of drugs and food:  Drug–drug and drug–food interactions may occur when one drug displaces another from plasma proteins. The displaced medication can immediately reach high levels in the bloodstream and produce adverse effects. Drugs such as aspirin or valproates, for example, displace Coumadin from the drug–protein complex, thus raising blood levels of free Coumadin and dramatically enhancing the risk of hemorrhage. Most drug guides give the percentage of medication bound to plasma proteins; when giving multiple drugs that are highly bound, the nurse should monitor the patient closely for adverse effects.

RELATED;

1.  SOLUBILITY OF MEDICAL COMPOUNDS

2.  DYNAMICS OF DRUGS AND THE HUMAN BODY

3.  PHARMACODYNAMICS

4.  DIAZEPAM

5.  PLASMA PROTEINS