Principles of pharmacology and pharmacokinetics - mechanisms of action

Overview

Drug mechanisms of action span receptor agonism/antagonism, enzyme inhibition/induction, ion channel modulation, and transporter blockade - understanding the specific mechanism for a given drug class explains its therapeutic effect, characteristic adverse effects, and predictable drug interactions
Foundational pharmacology underlying rational understanding of every drug class discussed throughout clinical practice

Agonists activate a receptor to produce a biological response mimicking the endogenous ligand; antagonists bind without activating, blocking the endogenous ligand's effect; partial agonists produce a submaximal response even at full receptor occupancy, explaining phenomena like buprenorphine's ceiling effect and its ability to precipitate withdrawal in patients already using full opioid agonists (by displacing the full agonist while providing only partial activation)

Not directly tested via a specific investigation, but mechanism-based understanding predicts drug interactions (e.g. co-administering a full agonist and antagonist at the same receptor produces competitive antagonism) and explains characteristic adverse effect patterns
Mechanism-based rational polypharmacy - combining drugs with complementary (not overlapping/redundant) mechanisms for synergistic effect where appropriate, and avoiding combinations with directly opposing or dangerously additive mechanisms
Predicting a new drug's likely effects/interactions from its stated receptor mechanism, even before extensive clinical experience with that specific agent

Partial agonist precipitated withdrawal (buprenorphine displacing full opioid agonists), competitive receptor antagonism as an interaction mechanism, receptor downregulation/upregulation with chronic exposure (relevant to tolerance and withdrawal phenomena)
Buprenorphine, a partial opioid agonist, can precipitate acute withdrawal if given to a patient with a full opioid agonist still active in their system - it displaces the full agonist from the receptor (high receptor affinity) while providing only partial activation, causing a net reduction in opioid effect despite being an opioid itself, which is why buprenorphine initiation requires the patient to already be in mild-moderate withdrawal from their previous full agonist

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