Drug distribution is the process of delivering a drug from the bloodstream to the tissues of the body – especially the tissue(s) where its actions are needed. The process of transferring a drug from the bloodstream to tissues is referred to as distribution. The same principles that govern drug absorption (e.g. ionization of a drug, lipophillicity of a drug, size of a drug, pH of the environment, etc.) also govern the rate and extent that a drug will distribute to various tissues in the body. In addition, there are additional factors at play, particularly non-specific binding to proteins. Commonly, drugs
bind non-specifically to albumin in the plasma. Additionally, one drug, digoxin, tends to bind non-specifically to skeletal muscle, when, in fact, its desired actions occur in the heart. When drugs
bind non-specifically to proteins, their movement is limited. That is because the large proteins to which they are bound will not be able to readily distribute to other parts of the body. The protein acts as a “reservoir” of sorts. As long as a drug is bound non-specifically to a protein, it cannot have a therapeutic action, nor can it be eliminated (metabolized hepatically by the liver or excreted by the kidneys). Non-specific binding to drugs
can also play a role in drug-drug interactions; if two or more drugs
are competing for the same binding site, one drug will displace the other, thereby, leading to potential toxicity caused by the drug that was displaced.
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