Mechanisms of Toxicity and Toxicodynamics
This domain tests whether you can move from a molecular event to a clinical syndrome and back again. Questions present a target organ or a metabolite and ask you to identify the initiating event, explain the latency, and predict which patients are susceptible. The exam expects mechanism-level vocabulary, not a list of poisons.
Three organizing mechanisms
- Receptor- and channel-mediated effects: agonism or antagonism at neurotransmitter receptors, or blockade of ion channels (sodium-channel blockade by tricyclics widening QRS).
- Enzyme inhibition: acetylcholinesterase by organophosphates, cytochrome c oxidase by cyanide, alcohol dehydrogenase by fomepizole. Inhibition explains both the toxidrome and the antidote.
- Physicochemical and metabolic injury: caustic corrosion, surfactant disruption, reactive metabolite formation, oxidative stress, mitochondrial permeability transition.
Bioactivation determines target organ
A stable parent compound is converted by phase I enzymes to a reactive electrophile. Organ selectivity often follows enzyme distribution - centrilobular hepatocytes are richest in CYP2E1, which is why acetaminophen injury is zone 3. Covalent binding depletes glutathione; once the pool falls by roughly 70%, oxidative injury and mitochondrial failure proceed.
Amplification and mode of cell death
One reactive molecule can propagate a lipid peroxidation chain, and a single DNA adduct can become a fixed mutation. Apoptosis is ATP-dependent and immunologically quiet; necrosis releases damage-associated molecular patterns that drive secondary inflammation and organ failure.
Worked example. Acetaminophen at therapeutic doses is mostly glucuronidated and sulfated, with about 5–10% shunted through CYP2E1 to NAPQI, which glutathione conjugates. In overdose those pathways saturate, NAPQI formation continues, glutathione is depleted, and NAPQI binds mitochondrial proteins. The result is centrilobular necrosis that is histologically and biochemically silent for 24 hours before transaminases rise. This explains the latency and the treatment window: N-acetylcysteine replenishes glutathione and is most effective before injury is established, but is still given late.
Traps that cost marks
- Assuming a metabolite is always more toxic - many are detoxification products.
- Extrapolating rodent mechanisms to humans without checking relevance (peroxisome proliferation is the classic mismatch).
- Confusing genotoxicity with carcinogenicity, or intrinsic reactivity with delivered dose to the target site.
- Forgetting that susceptibility varies with enzyme polymorphisms, nutritional glutathione status, and co-ingested inducers.