Pharmacology of Antihistamines, Antineoplastics, and Diuretics

Posted on Jun 16, 2026 in Medicine

Antihistaminic Agents

Antihistamines are drugs that inhibit the action of histamine in the human body. Histamine is a chemical substance classified as an autacoid (a local hormone released from tissue that acts at the site of synthesis).

Histamine Overview

  • Chemical Name: 2-(1H-Imidazol-4-yl)ethanamine
  • Discovery: Sir Henry Dale, 1910.
  • Synthesis: Decarboxylation of Histidine, catalyzed by Histidine Decarboxylase (HDC).
  • Storage: Primarily in mast cells and basophils, also in enterochromaffin-like (ECL) cells and histaminergic neurons.

Release Mechanism

Histamine is stored as an inactive complex with Heparin and Chondroitin. Release occurs via mast cell degranulation triggered by the interaction of an antigen with IgE antibodies, leading to a calcium-dependent signaling pathway.

Physiological Effects

  • Vasodilation: Causes hypotension.
  • Gastric Secretion: Increases HCl secretion.
  • Oedema: Plasma protein leakage into extracellular spaces.
  • Smooth Muscle: Bronchoconstriction.
  • Hypersensitivity: Itching, sneezing, and inflammation.

Histamine Receptors (GPCRs)

  • H1: Smooth muscle, CNS, heart; mediates allergic responses.
  • H2: Gastric parietal cells; increases cAMP and gastric acid.
  • H3: CNS; modulates neurotransmitter release.
  • H4: Hematopoietic cells; modulates immune function.

Classification of Antihistamines

  • First Generation (Sedative): Highly lipophilic (e.g., Diphenhydramine, Promethazine).
  • Second Generation (Non-Sedative): Hydrophilic (e.g., Cetirizine, Loratadine).
  • H2-Antagonists: Cimetidine, Famotidine, Ranitidine.
  • Proton Pump Inhibitors (PPI): Omeprazole, Lansoprazole.

Antineoplastic Agents

Antineoplastic agents are used to treat cancer by opposing abnormal tissue growth (neoplasms). Cancer is characterized by uncontrolled cell division and failure of apoptosis.

Classification

  1. Alkylating Agents: Form covalent bonds with DNA (e.g., Cyclophosphamide, Mechlorethamine).
  2. Antimetabolites: Compete with natural metabolites (e.g., 6-Mercaptopurine, Methotrexate).
  3. Plant Products: Vinca alkaloids (Vincristine) and Epipodophyllotoxins (Etoposide).
  4. Antibiotics: Doxorubicin, Bleomycin.

Anti-Anginal Drugs

Used to treat angina pectoris (ischemic chest pain) caused by an imbalance between oxygen supply and demand.

Types of Angina

  • Stable: Predictable; caused by exertion.
  • Variant (Prinzmetal): Occurs at rest due to vasospasm.
  • Unstable: High risk of myocardial infarction; occurs at rest.

Drug Classes

  • Nitrates: Convert to Nitric Oxide (NO), increasing cGMP to cause vasodilation (e.g., Nitroglycerin).
  • Calcium Channel Blockers (CCBs): Inhibit L-type calcium channels to reduce cardiac workload (e.g., Nifedipine, Verapamil).

Diuretics and Hypertension

Diuretics increase urine excretion to manage hypertension and edema.

Classification

  • Carbonic Anhydrase Inhibitors: Acetazolamide.
  • Thiazides: Inhibit Na+/Cl- cotransporter in the DCT (e.g., Chlorothiazide).
  • Loop Diuretics: Inhibit Na+/K+/2Cl- cotransporter in the Loop of Henle (e.g., Furosemide).
  • Potassium-Sparing: Spironolactone, Amiloride.

Hypertension Management

Hypertension is treated using ACE inhibitors (Captopril), Beta-blockers (Timolol), and centrally acting agents (Methyldopate) to reduce peripheral vascular resistance and blood volume”