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The intensity of pharmacological effect (or toxic effect) is often related to drug concentration at the receptor site (in tissue). Measuring plasma drug concentration may help to adjust the dose to optimise the drug response or prevent serious toxic effects in an individual patient.

Measuring drug concentration in blood during bioequivalence studies also helps to establish therapeutic equivalences between two different products of a drug (such as brand vs. generic) or different formulations of a drug (such as tablet vs. capsule).

The fraction bound can also be subject to a potential drug interaction; if a co‐administered drug had a higher affinity for the binding site at the plasma protein or the RBC, it could displace the other drug. This may result in a significant increase in the free fraction of a drug which can result in increased therapeutic effect (or toxicity) and increased elimination.

The potency of drug response (the pharmacological or toxic effect) depends on the drug concentration at the target site in the tissues; measuring drug concentration in the blood (or in serum or plasma) helps to estimate the intensity of the drug response at a given dose.