How Drugs Work
- The objective of drug therapy is to deliver the right drug, in the right concentration, to the right site of action, and at the right time to produce the desired effect
- Only those drugs able to interact with the receptors in a particular site of action can produce effects in that site. This is why specific cells respond only to certain drugs
Concentration and Effect
- To produce an effect, a drug must achieve a minimum effective concentration (MEC). This is when there is enough drug at the site of action to produce a response
- The range between the minimum effective concentration and the minimum toxic concentration is called the therapeutic window. When concentrations are in this range, most patients receive the maximum benefit from their drug therapy with a minimum of risk
- Therapeutic drug monitoring can be useful when the blood concentration of the drug reflects the concentration at the site of action
ADME Processes and Diffusion
- Blood concentrations are the result of four simultaneously occurring processes: absorption, distribution, metabolism, and excretion (the ADME processes)
- Besides the four ADME processes, a critical factor in drug concentration and effect is how drugs move through biological membranes. Most drugs penetrate biological membranes by passive diffusion
Absorption
- One of the primary factors affecting oral drug absorption is the gastric emptying time
Distribution
- Many drugs bind to proteins in blood plasma to form a complex that is too large to penetrate biological membranes, essentially making the drug inactive
Metabolism
- Enzymes catalyze the transformation of drugs to metabolites. Most metabolites are inactive molecules that are excreted
Excretion
- The kidneys filter blood and remove wastes, drugs, and metabolites from the body
- Urinary excretion = glomerular filtration + renal secretion - urinary reabsorption
Bioavailability
- The amount of a drug that is delivered to the site of action and the rate of which it becomes available is called the bioavailability of the drug
Bioequivalence
- Bioequivalent drug products are pharmaceutical equivalents or alternatives that have essentially the same bioavailabilities (ie. rate and extent of absorption) when administered in the same dose of the active ingredient under similar conditions
- Pharmaceutical equivalents are drug products that contain identical amounts of the same active ingredient in the same dosage form, but may contain different inactive ingredients
- Pharmaceutical alternatives are drug products that contain the identical active ingredient, but not necessarily in the same salt form, same amount, or dosage form
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