It is generally appreciated that under different conditions a drug may produce diverse effects, ranging from none to a desirable effect or, in other cases, an undesirable, toxic effect. Physicians must learn how to individualize the drug dosage under different conditions to ensure effective and safe therapy.
This requires knowing pharmacokinetics—examining the movement of a drug over time through the body—and pharmacodynamics—relating drug concentration to drug effect.
[...] CLEARANCE The efficiency of elimination can be described by assessing how fast the drug clears from the circulation. Drug clearance is a measure of the volume of plasma cleared of drug per unit of time. It is similar to the measurement used clinically to assess renal function—the creatinine clearance, which is the volume of plasma from which creatinine is removed per minute. Total drug clearance (Cltot ) is the rate of elimination by all processes (Eltot ) divided by the plasma concentration of the drug (Cp Cltot = {{Eltot Cp Drugs may be cleared by several organs, with renal and hepatic clearance being the two major mechanisms. [...]
[...] DISTRIBUTION After delivery of a drug into the systemic circulation either directly by intravenous injection or after absorption, the drug is transported throughout the body, initially to the well-perfused tissues and later to areas that are less perfused. The distribution phase can be assessed best by plotting the drug's plasma concentration on a log scale versus time on a linear scale. The initial phase, from immediately after administration through the rapid fall in concentration, represents the distribution phase, during which a drug rapidly disappears from the circulation and enters the tissues. [...]
[...] Factors that influence the availability of free drug affect drug absorption from the administration site; this effect can be exploited to design medications that provide a slow release of drug into the circulation by prolonging drug absorption. With certain sustained-released oral preparations, the rate of dissolution of the drug in the gastrointestinal tract determines the rate at which the drug is absorbed (e.g., timed-release antihistamines). Similarly a prolonged drug effect can be obtained by using transdermal medications (e.g., nitroglycerin) or intramuscular depot preparations (e.g., benzathine penicillin G). [...]
[...] The time needed to eliminate the drug is described best by the drug half-life (t1/2 which is the time required during the elimination phase to decrease the plasma concentration of the drug by half. Mathematically the half-life is equal to the natural logarithm of 2 (representing a reduction of drug concentration to half) divided by ke . Substituting for ke from Equation 4 and calculating the natural logarithm of the half-life can be represented by the following equation: t1/2 = 0.693 VD From this equation, one can predict that at a given clearance, as the VD increases, the half-life increases. [...]
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