Pharmacodynamics

Excerpt

f"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 studies 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 regarding subsequent intermolecular interactions. These interactions include receptor binding, post-receptor effects, and chemical interactions. Examples of these interactions include drugs binding to an active site of an enzyme, drugs that interact with cell surface signaling proteins to disrupt downstream signaling, and drugs that act by binding molecules like tumor necrosis factor.

After the drug-target interaction occurs downstream, effects are elicited, which can be measured by biochemical or clinical means. Examples include the inhibition of platelet aggregation after administering aspirin, the reduction of blood pressure after ACE inhibitors, and 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. Healthcare practitioners must treat the patient, not the symptom or the lab value. Pharmacodynamics and pharmacokinetics are the 2 branches of pharmacology, with pharmacodynamics studying the action of the drug on the organism and pharmacokinetics studying the effect the organism has on processing the drug.

Pharmacodynamic actions include:

1. Stimulating activity by directly acting on 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 behaves as neither an agonist nor an antagonist

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

Any of these factors can be therapeutically effective and can also precipitate an adverse event.