This is post number two of our six part post on Pharmacology. Our posts will focus on the following topics:
Part 1. Introduction
Part 2. Pharmacokinetics
Part 3. Pharmacodynamics
Part 4. Bioavailabilty
Part 5. “Free versus Bound Drug”
Part 6. Elucidating Pharmacodynamic Effect from an Analytical Chemistry Result

Pharmacokinetics is defined as the study of the rate of drug absorption, distribution, and elimination in the body. In brief, pharmacokinetics is what happens to the drug while in the body. Pharmacokinetic studies investigate and characterize how a drug given by different routes of administration, is absorbed, distributed, and eliminated with respect to time.

Pharmacokinetic equations describe the relationships between dosage regimen and the profile of drug concentration in the blood over time.

Pharmacokinetics can be divided into distinct, but interrelated concepts: Absorption, Distribution, Metabolism, and Excretion through use of fundamental concepts of linear kinetics referred to as  Compartmental Modeling. In this concept, blood would be the central compartment and other sites of distribution within the body (for example fat, organs, or central nervous system) would be other compartments. Most drugs can be described as two- or three-compartment body models, that is, two or three linear lines for the logarithm of blood concentration versus time plots.

An extension of pharmacokinetics is toxicokinetics which describes the relationship of drugs to their non-therapeutic effect, namely, toxic effects. Knowledge of toxicokinetics enables one to understand individual factors that enhance or reduce toxicity. Toxicokinetics helps to explain why some people survive large quantities of a particular toxin whereas others succumb to a much smaller amount.

Drugs, in general, fall into two different types of pharmacokinetic characteristics: (1) Zero order kinetic drugs, and (2) Non-zero order kinetic drugs.

• Zero order kinetic drugs are defined as those drugs that have a constant rate of elimination irrespective of plasma concentration. The most famous example of a zero order kinetic drug is ethanol.
• Non-zero order kinetic drugs are defined as those drugs that have a rate of elimination proportional to plasma concentration.  The elimination rate constant (Kel) represents the fraction of drug eliminated per unit of time. Rate of elimination = constant (CL) x Conc.

The absorption, distribution metabolizing and elimination of a chemical are qualitatively similar in all individuals but in practice occur at different rates due to factors such as race, age, and sex. Each person must be considered individually and treated accordingly.

[Update on April 25, 2011]: Alfred Staubus, PharmD, PhD and professor emeritus in pharmacology comments:

Very nice introduction to pharmacokinetics. Properly trained clinical pharmacists have a full year of courses devoted to just pharmacokinetics. The subsequent additional clinical pharmacy courses then use the principles of pharmacokinetics and pharmacology to properly evaluate and dose patients.

Only one minor comment/clarification: The two basic types of drug elimination are first-order pharmacokinetics and Michaelis-Menten pharmacokinetics. For Michaelis-Menten eliminated drugs, at very low concentrations, they behave in a pseudo-first-order fashion (rate of elimination is proportional to concentration – total body clearance is essentially constant); at high concentrations, they behave in a pseudo-zero-order fashion (rate of elimination is essentially constant, independent of drug concentration – total body clearance decreases as concentration increases). Ethanol is an example a Michaelis-Menten drug that at “therapeutic concentrations” (greater than 0.02 g/dL) behaves in an apparent zero-order fashion. Below 0.02 g/dL, ethanol no longer behaves as an apparent zero-order drug. At extremely high ethanol concentrations, secondary routes of elimination can sometimes be seen.