H1 Antihistamines
H₁-Receptor Antagonists (Inverse Agonists) — First- vs Second-Generation Classification, Mechanism, Pharmacokinetics, Clinical Uses & Adverse Effects
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H1 Antihistamines (H1-Receptor Antagonists)
1. Definition, scope & historical context
- H1 antihistamines are drugs that competitively antagonise the actions of histamine at the H1 receptor; conventionally called "antihistaminics," they are the classical antiallergic agents (G&G 14e Ch.43, p.861; KDT 8e Ch.11, p.178).
- Histamine is a biogenic amine autacoid (β-imidazolylethylamine) — a "local hormone" produced by diverse cells, intensely bioactive, acting locally at the site of synthesis/release; it is the prototype amine autacoid alongside 5-HT (KDT 8e Ch.11, pp.173–4).
- Histamine acts through four GPCR subtypes (H1–H4); H1 antihistamines are selective for H1 and have negligible potency at H2 and little at H3 (Katzung 16e Ch.16, p.292; G&G 14e Ch.43, p.859).
- Therapeutic rationale: H1 antagonists suppress the histamine-attributable component of immediate (type I) hypersensitivity — itch, wheal, flare, urticaria, rhinorrhoea, conjunctival symptoms — but do not suppress the antigen–antibody reaction itself (palliative, not curative) (KDT 8e Ch.11, p.182; G&G 14e Ch.43, p.863).
- Historical milestones:
- Antihistamine activity first demonstrated by Bovet and Staub in 1937; the first compound protected guinea pigs against lethal histamine but was too toxic for clinical use (G&G 14e Ch.43, p.861).
- By 1944 Bovet's group described pyrilamine (mepyramine) maleate, the first clinically effective H1 antagonist; diphenhydramine and tripelennamine followed (G&G 14e Ch.43, p.861).
- First-generation agents introduced from the late 1930s; second-generation (less/non-sedating) agents added after 1980 (KDT 8e Ch.11, pp.178, 181).
- Histaminergic receptors classified into H1 and H2 by Ash and Schild (1966); Sir James Black (1972) produced the first H2 blocker (burimamide), confirming the classification, and shared the 1988 Nobel Prize (with Elion and Hitchings) (KDT 8e Ch.11, p.174; G&G 14e Ch.43, p.861).
- H3 receptor identified 1983 (autoreceptor); H4 receptor revealed by molecular cloning in 2001 (KDT 8e Ch.11, p.174).
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Antihistamines H1 Antagonists
PharmaNotes Pro · Comprehensive
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