Pharmacodynamics is the study of the drug mechanisms that pro-duce biochemical or physiologic changes in the body. The inter-action at the cellular level between a drug and cellular compo-nents, such as the complex proteins that make up the cell mem-brane, enzymes, or target receptors, represents drug action. The response resulting from this drug action is the drug effect.
A drug can modify cell function or rate of function, but it can’t impart a new function to a cell or to target tissue. Therefore, the drug effect depends on what the cell is capable of accomplish-ing.
A drug can alter the target cell’s function by:
· modifying the cell’s physical or chemical environment
· interacting with a receptor (a specialized location on a cell membrane or inside a cell).
Many drugs work by stimulating or blocking drug receptors. A drug attracted to a receptor displays an affinity for that receptor. When a drug displays an affinity for a receptor and stimulates it, the drug acts as an agonist. An agonist binds to the receptor and produces a response. This ability to initiate a response after bind-ing with the receptor is referred to as intrinsic activity.
If a drug has an affinity for a receptor but displays little or no in-trinsic activity, it’s called an antagonist. An antagonist prevents a response from occurring.
Antagonists can be competitive or noncompetitive.
· A competitive antagonist competes with the agonist for recep-tor sites. Because this type of antagonist binds reversibly to the re-ceptor site, administering larger doses of an agonist can overcome the antagonist’s effects.
· A noncompetitive antagonist binds to receptor sites and blocks the effects of the agonist. Administering larger doses of the ago-nist can’t reverse the antagonist’s action.
If a drug acts on a variety of receptors, it’s said to be nonselective and can cause multiple and widespread effects. In addition, some receptors are classified further by their specific effects. For exam-ple, beta receptors typically produce increased heart rate and bronchial relaxation as well as other systemic effects.
Beta receptors, however, can be further divided into beta1 re-ceptors (which act primarily on the heart) and beta2 receptors (which act primarily on smooth muscles and gland cells).
Drug potency refers to the relative amount of a drug required to produce a desired response. Drug potency is also used to compare two drugs. If drug X produces the same response as drug Y but at a lower dose, then drug X is more potent than drug Y.
As its name implies, a dose-response curve is used to graphi-cally represent the relationship between the dose of a drug and the response it produces. (See Dose-response curve)
On the dose-response curve, a low dose usually corresponds to a low response. At a low dose, a dosage increase produces only a slight increase in response. With further dosage increases, the drug response rises markedly. After a certain point, however, an increase in dose yields little or no increase in response. At this point, the drug is said to have reached maximum effectiveness.
Most drugs produce multiple effects. The relationship between a drug’s desired therapeutic effects and its adverse effects is called the drug’s therapeutic index. It’s also referred to as its margin of safety.
The therapeutic index usually measures the differ-ence between:
· an effective dose for 50% of the patients treated
· the minimal dose at which adverse reactions occur.
Drugs with a narrow, or low, therapeutic index have a narrow margin of safety. This means that there’s a nar-row range of safety between an effective dose and a lethal one. On the other hand, a drug with a high thera-peutic index has a wide margin of safety and poses less risk of toxic effects.