Pharmacodynamics

Pharmacodynamics

Pharmacodynamics is the study of the drug mechanisms that produce biochemical or physiologic changes in the body. The interaction at the cellular level between a drug and cellular components, such as the complex proteins that make up the cell membrane, enzymes, or target receptors, represents drug action. The response resulting from this drug action is the drug effect

It’s the cell that matters

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 accomplishing.

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).

Agonist drugs

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 binding with the receptor is referred to as intrinsic activity.

Antagonist drugs

If a drug has an affinity for a receptor but displays little or no intrinsic activity, it’s called an antagonist. An antagonist prevents a response from occurring.

Reversible or irreversible

Antagonists can be competitive or noncompetitive.

• A competitive antagonist competes with the agonist for receptor sites. Because this type of antagonist binds reversibly to the receptor 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 agonist can’t reverse the antagonist’s action.

Regarding receptors

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 example, 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 receptors (which act primarily on the heart) and beta₂ receptors (which act primarily on smooth muscles and gland cells).

Potent power

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 graphically represent the relationship between the dose of a drug and the response it produces.

Maximum effect

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.

Margin of safety

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 difference between:

• an effective dose for 50% of the patients treated
• the minimal dose at which adverse reactions occur

Narrow index = potential danger

Drugs with a narrow, or low, therapeutic index have a narrow margin of safety. This means that there’s a narrow range of safety between an effective dose and a lethal one. On the other hand, a drug with a high therapeutic index has a wide margin of safety and poses less risk of toxic effects.