Introduction

Pharmacokinetics sometimes abbreviated as PK, is a branch of pharmacology dedicated to determine the fate of substances administered to a living organism. The substances of interest include any chemical xenobiotic such as: pharmaceutical drugs, pesticides, food additives, cosmetics, etc.

It attempts to analyze chemical metabolism and to discover the fate of a chemical from the moment that it is administered up to the point at which it is completely eliminated from the body. Pharmacokinetics is the study of how an organism affects a drug, whereas Pharmaco Dynamics (PD) is the study of how the drug affects the organism. Both together influence dosing, benefit, and adverse effects, as seen in PK/PD models.

Pharmacokinetics describes how the body affects a specific xenobiotic/chemical after administration through the mechanisms of absorption and distribution, as well as the metabolic changes of the substance in the body, and the effects and routes of excretion of the metabolites of the drug. Pharmacokinetic properties of chemicals are affected by the route of administration and the dose of administered drug. These may affect the absorption rate.

Liberation is the process in which a pharmaceutical substance is released from the formulation it is delivered in. This must occur before the drug can be absorbed into the body.

Absorption

Absorption is the process in which a pharmaceutical substance enters the blood circulation in the body. The pharmacokinetic parameters for absorption include: Absorption rate constant: absorption rate/amount of drug remaining to be absorbed, Bioavailability: amount of drug absorbed/drug dose.

Distribution

Distribution is the process in which a pharmaceutical substance is dispersed through the fluids and tissues in the body. The pharmacokinetic parameters for distribution include: Apparent volume of distribution: amount of drug in body/drug concentration in plasma, unbound fraction: unbound drug concentration in plasma/total drug concentration in plasma.

Metabolism

Metabolism is the process in which a pharmaceutical substance is transformed into other substances, called metabolites, in the body. The pharmacokinetic parameters for metabolism include: Metabolic clearance: drug metabolism rate/drug concentration in plasma.

Excretion

Excretion is the process in which a pharmaceutical substance is removed from the body. In rare cases, some drugs may never be completely excreted from the body. They then irreversibly accumulate in the tissues. The pharmacokinetic parameters for excretion include: Renal clearance: renal excretion rate/drug concentration in plasma, Fraction excreted unchanged: renal excretion rate/drug elimination rate.

Other factors are related to individual physiology. The effects of some individual factors (eg, renal failure, obesity, hepatic failure, dehydration) can be reasonably predicted, but other factors are idiosyncratic and thus have unpredictable effects. Because of individual differences, drug administration must be based on each patient’s needs traditionally, by empirically adjusting dosage until the therapeutic objective is met.

This approach is frequently inadequate because it can delay optimal response or result in adverse effects. Knowledge of pharmacokinetic principles helps prescribers adjust dosage more accurately and rapidly. Application of pharmacokinetic principles to individualize pharmacotherapy is termed therapeutic drug monitoring.

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Regards

Mary Wilson

Editorial office

Clinical Pharmacology and Toxicology Research

E-mail: pharmatoxicol@eclinicalsci.com