17.9: Drug toxicity: Idiosyncratic Reactions

Idiosyncratic drug reactions represent abnormal chemical responses that vary significantly among individuals, ranging from extreme sensitivity to low doses to insensitivity to high doses. These reactions often occur due to the drug's covalent binding with serum proteins, forming a foreign hapten that triggers an immunotoxicological response. The variability in drug reactions has a strong pharmacogenetic foundation, with genetic differences crucial in how individuals metabolize drugs. For instance, about 10% of black males experience severe hemolytic anemia from primaquine due to a genetic deficiency in glucose-6-phosphate dehydrogenase. Similarly, polymorphisms in the NAT2 gene affect the metabolism of isoniazid, leading to different acetylation and clearance rates among individuals. Warfarin's effectiveness as an anticoagulant also varies due to genetic variations in CYP2C9 and VKORC1, influencing its pharmacokinetics and pharmacodynamics. Furthermore, enzymes like CYP3A4 and CYP2D6 metabolize a wide range of drugs in the liver, and variations in these genes can significantly impact drug half-life. The risk of drug overdose toxicity increases when a drug that is a substrate for a particular cytochrome P450 (CYP) enzyme is taken alongside an inhibitor of the same enzyme. Due to this, many drugs include warnings about potential drug-drug interactions in their prescribing information, underscoring the importance of considering genetic factors and possible interactions in drug therapy management.

Idiosyncratic reactions are unpredictable individual responses to chemicals, unrelated to dose and varying widely among individuals.

These typically happen when drugs bind serum proteins to form haptens, triggering a toxic immune response.

Additionally, interindividual differences in idiosyncratic reactions can be influenced by pharmacogenetic factors.

For example, 10% of black males on primaquine treatment develop severe hemolytic anemia due to a genetic deficiency of erythrocyte glucose-6-phosphate dehydrogenase.

Genetic variations in CYP2C9 affect warfarin's metabolism, while variations in VKORC1 cause the formation of clots.

Polymorphisms in CYP3A4 and CYP2D6, key hepatic enzymes, can markedly alter drug activity and half-life.

Lastly, the combined administration of a drug metabolized by cytochrome P450 with another drug that inhibits the enzyme heightens the risk of drug overdose toxicity.