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The term pharmacokinetics refers to the principles underpinning the absorption, distribution, metabolism and elimination of a drug following its administration into the body. The drug can be administered via a range of routes; many require absorption of the drug from the site of administration to get into the blood circulation, whereas drug can also be administered directly into the blood circulation (intravenously) bypassing the absorption process.

The clinical efficacy of a drug relies on the delivery of a drug to the receptor site at the required concentration to produce a therapeutic effect. A drug, therefore, must bypass the barriers at the absorptive site and the metabolic challenges for successful delivery to the target site. The life cycle of a drug typically means that it goes ‘into’ the body, goes ‘around’ the body, exerts its pharmacological effects and comes ‘out’ of the body (Figure 2.1).

Figure 2.1 Illustration showing a typical life cycle of a drug in the body (RBCs = red blood cells).

When a drug is administered into the body, it is absorbed from the tissues where it was administered (e.g., muscles, gastrointestinal tract under the skin, etc.) to get into blood circulation. It is then distributed across the body once it is in the systemic circulation where a fraction of the drug arrives at the target sites, binds the receptors and produces a therapeutic effect. The drug is also metabolised by various tissues (mainly liver and gut) into other forms (mainly inactive) during its voyage into the body and then it is excreted out of the body, mainly via urine. The ‘pharmacokinetics’ is the study of the processes governing the ‘absorption’, ‘distribution’, ‘metabolism’ and ‘excretion’ of a drug, often referred to as ADME.