Pharmacokinetics: General
Pharmacokinetics describes quantitatively the various steps of drug disposition in the body including absorbtion of drugs, distribution of the drugs to various organs, and their elimination by excretion and biotransformation. The rates of these processes are important in characterizing the fate of a medication in the body.
The actual percentage of a drug contained in a drug product that enters the circulation unchanged after its administration, combined with the rate of entry into the body, determines the bioavailability of a drug.
Once absorbed, most drugs are carried from their site of action and elimination by the circulating blood. Some drugs simply dissolve in serum water, but many others are carried bound to proteins, especially albumin. Plasma protein binding influences the fate of drugs in the body, since only the free (unbound) drug reaches the site of drug action. This interaction with binding sites is reversible.
The intensity of drug action is most frequently related to the concentration of the drug at the site of action. The duration of drug effect is related to the persistence of its presence at this site. The time to reach maximum drug concentrations (or peak effects) is usually referred to as tmax.
Whenever the fate of a drug in the body is described by pharmacokinetic parameters, a model of the body is assumed. The fundamental principles of pharmacokinetics are based on the most elementary model. The body is considered a single compartment. Distribution of the drug is considered uniform. The "volume" in which the drug is distributed is referred to as the volume of distribution (Vd). It is typically expressed in liters per kilogram (L/kg).
Elimination of the drug is assumed to be exponential. The rate of elimination of a drug is usually described by its half-life (t1/2), which is the time required for 50 percent elimination of the drug. This is typically expressed in hours (h). Another way to express drug elimination is the clearance, which represents the volume of drug cleared from the body per unit of time. This is usually expressed in milliliters per minute per kilogram (ml/min/kg) but can also be expressed in liters per hour per kilogram (L/h/kg).
An effect of a single dose of a drug may be characterized by its latency, the time needed for drug concentrations to reach maximum levels (tmax). Magnitude of peak effects and duration of action dosage and rates of absorption and elimination are influenced by these parameters. As dosage increases, latency is reduced and peak effect increased without change in the time of peak effect. Reduced elimination (long half-life, reduced clearance) results in an expected prolongation of drug effects and, in some cases, drug accumulation. Using more complex models than a single compartment model, physicians use pharmacokinetic data not only to characterize the fate of a drug in the body but also to calculate doses and frequency of drug administration for each particular patient. This is important because there are wide variations among individuals in the absorption, distribution, and elimination of drugs.
Tables 1 through 4 are a summary of the available data on the kinetic properties of alcohol and other abused drugs. Some of the drugs of abuse included in this summary are illicit drugs (e.g., COCAINE) while others are effective pharmacological agents that have the potential to be abused (e.g., OPIOIDS).
Although some of the drugs included in the tables have been used for centuries (e.g., ALCOHOL, CAFFEINE), knowledge of their kinetics and metabolism is very recent and, in some cases, still incomplete.
This is due partly to their complex metabolism and partly to the difficulties of studying drugs of abuse in humans.
The tables show the route of administration, the type of subjects used in the study, the doses used, and the most important kinetic parameters such as protein binding, half-life, volume of distribution, and clearance.
Drug Metabolism; Pharmacokinetics of Alcohol)
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