Pharmacodynamics

Excerpt

"Did we but know the mechanical affections of the particles of rhubarb, hemlock, opium, and a man...we should be able to tell beforehand that rhubarb will purge, hemlock kill and opium make a man sleepy..." - John Locke: Essay Concerning Human Understanding

Pharmacodynamics is the study of a drug's molecular, biochemical, and physiologic effects or actions. It comes from the Greek words "pharmakon," meaning "drug," and "dynamikos," meaning "power." All drugs produce their effects by interacting with biological structures or targets at the molecular level to induce a change in how the target molecule functions in regard to subsequent intermolecular interactions. These interactions include receptor binding, post-receptor effects, and chemical interactions. Examples of these types of interactions include (1) drugs binding to an active site of an enzyme, (2) drugs that interact with cell surface signaling proteins to disrupt downstream signaling, and (3) drugs that act by binding molecules like tumor necrosis factor (TNF).

Subsequent to the drug-target interaction occurring downstream, effects are elicited, which can be measured by biochemical or clinical means. Examples of this include the (1) inhibition of platelet aggregation after administering aspirin, (2) reduction of blood pressure after ACE inhibitors, and (3) the blood-glucose-lowering effect of insulin.

While these examples seem obvious, the administration of the preceding drug examples should be kept in mind, so practitioners do not administer these drugs to inhibit platelet aggregation, lower blood pressure, or lower blood glucose but to reduce the risks of cerebrovascular accident, myocardial infarction, and renal and eye complications through the drug's pharmacodynamic effects. It is important for healthcare practitioners they are treating the patient, not the symptom or the lab value.

Pharmacodynamics and pharmacokinetics are the two branches of pharmacology, with pharmacodynamics studying the action of the drug on the organism and pharmacokinetics studying the effect the organism has on the drug.

Pharmacodynamic actions include:

1. Stimulating activity by directly inhibiting a receptor and its downstream effects

2. Depressing activity by direct receptor inhibition and its downstream effects

3. Antagonistic or blocking a receptor by binding to it but not activating it

4. Stabilizing action, where the drug apparently behaves as neither an agonist nor antagonist

5. Direct chemical reactions (beneficial in therapy and also as an adverse event)

Any of these factors can work both therapeutically as well as precipitate an adverse event.