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Histamine and antihistaminic

John experiences seasonal allergies each winter, taking an over-the-counter antihistamine that makes him drowsy and causes dry mouth. His doctor diagnoses him with allergies and prescribes loratadine, a second-generation antihistamine that relieves his symptoms without adverse effects. Antihistamines work by blocking histamine H1 receptors to reduce allergic responses. First-generation antihistamines are more sedating due to additional anticholinergic effects, while second-generation antihistamines like loratadine selectively block H1 receptors without crossing the blood-brain barrier.

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Autacoids (Histamine & Antihistamine) Dr Naser Ashraf Tadvi 1
Case study • A 24-year-old male, John develops a runny nose, itchy eyes, and sneezing every winter. • To relieve his symptoms, he takes an OTC antihistamine • John is annoyed by the unpleasant effects that accompany his allergy medication. Every time he takes this antihistamine, he feels drowsy and his mouth feels dry. • He makes an appointment with his doctor who, advises him to take loratadine. Upon taking new allergy medication, his symptoms are relieved, and he experiences no drowsiness or other adverse effects.
Questions • What Is Johns Problem • Which OTC is probably John taking? • What is the reason of drowsiness and dryness of mouth in John ? • What is loratadine?Why did Physician prescribe it for John?
Objectives • Describe the pharmacology of histamine & enumerate its related drugs. • Classify Histamine-1(H1) antagonists. • Discuss the pharmacology of H1 Antihistaminics with emphasis on clinical uses, adverse drug reactions & interactions 4
What are Autacoids • Autos = self; Akos = remedy (Greek). • Secreted locally to increase or decrease the activity of nearby cells. • Includes – Amine autocoids: Histamine, Serotonin – Lipid derived: Prostaglandins and Leukotrines – Peptides: Bradykinin, angiotensin – Others: Cytokines 5
Histamine • Histos: Tissue N N NH2 H 1 2 3 45 Histamine •Present mostly in mast cells:  skin, lungs, GIT Mucosa •Non mast cell histamine: Brain, Gastric Mucosa Histamine is a biogenic amine present in many animal and plant tissues .
Synthesis of histamine • Synthesized by decarboxylation of amino acid histidine
Release of histamine • Immunologic (Type 1 reaction) • Chemical/ Mechanical release – Morphine – Succinylcholine – D tubocurarine – Dextran
9 Release of histamine
Histamine Receptors Histamine H1 Receptors H2 Receptors H3 Receptors (presynaptic auto receptors) ↑ Ca2+ Smooth muscle contraction ↑ capillary permeability Vasodilation Sensory nerve endings pain & itching ↑ cAMP ↑ Gastric acid secretion Blood vessels: vasodilation Increased capillary permeability ↓ histamine release ↓secretion Vasodilation ↓ cAMP Smooth muscle, vascular endothelium, brain Gastric parietal cells, cardiac muscle, Brain Heart CNS and some peripheral nerves. H4 Receptors ↓ cAMP Hematopoietic cells, gastric mucosa.
Actions of histamine (H1) Tissues Effects Peripheral nerves Sensitization of afferent nerve terminals Smooth muscles Bronchoconstriction Intestine: contraction Exocrine glands Increased secretions Hypothalamus, mid brain Wakefulness 11
Actions of Histamine (H1 & H2) • Arterioles and post capillary venules – Dilatation and decreased BP • Increased capillary permeability • Heart: increased HR and Force of contraction • Triple response • Severe hypersensitivity reaction
Actions of histamine (contd.) Tissue Effects Clinical manifestations Receptor subtype Stomach Increased gastric acid secretion Peptic ulcer disease, heartburn H2 CNS Decreased release of Histamine/ NE - H3 (Auto- receptor) 13
Adverse effects of histamine release • Itching, Urticaria • Flushing • Hypotension • Tachycardia • Bronchospasm • Angioedema • Wakefullness • Increased acidity (Gastric acid secretion)
Betahistine • H1 Selective histamine analog • Use: – Vertigo of Meniere`s Disease • Acts by Vasodilation in inner ear • Contraindication – Asthmatics – Peptic Ulcer
Classification of antihistaminics First Generation • Diphenhydramine • Dimenhydrinate • Doxylamine • Promethazine • Hydroxyzine • Pheniramine • Cyroheptadine • Meclizine • Chlorpheniramine • Dexchlorpheniramine Second Generation • Cetirizine • Levocetirizine • Fexofenadine • Loratidine • Desloratidine • Ebastine • Astemizole • Acrivastine
H1-Antihistaminics /First generation
H1-Antinistaminics /second generation
Therapeutic uses • Allergic diseases – Rhinitis, urticaria, conjunctivitis. – Atopic dermatitis – Bronchial asthma – Anaphylaxis • Antiemetics, motion sickness, morning sickness • Insomnia • Preanaesthetic medication • Cough • Parkinsonism • Vertigo • Acute Muscle dystonia 19
• As Antiemetic: Hydroxyzine, promethazine • Motion sickness and Vestibular disturbances – first generation antihistamines like • Diphenhydramine and Promethazine are useful • Cyclizine and Meclizine- less sedative • Morning sickness: Doxylamine 20 THERAPEUTIC USES
Uses: second Generation Antihistamines • Allergies Reduce symptoms of itching, sneezing, rhinorrhea and allergic conjunctivitis 21 Topical preparations- Azelastine Olopatadine
Common Adverse Effects & Contraindications • Adverse effects: – Anticholinergic adverse effects like blurred vision, dry mouth, tachycardia, urinary retention, dizziness, drowsiness – Paradoxical excitement in infants and children – Overdose of Astemizole and Terfenadine may cause arrhythmias • Contraindications : – Acute angle-closure glaucoma, Hypersensitivity, Urinary obstruction
Drug interactions • Terfenadine and astemizole – Ketoconazole – Macrolide antibiotics Lethal ventricular arrhythmias. Withdrawn from market • Grapefruit juice also inhibits CYP3A4. – Increased levels of antihistamines. 23
Mechanism of action Competitive antagonism Histamine General formula of H1 Blocker
Second generation H1 Blockers (Non Sedative:Less anticholinergic property) • Fexofenadine • Astemizole • Loratidine • Cetrizine • Levocetrizine • Azelastine • Terfenadine Uses: • Allergic rhinitis • Allergic Dermatitis • Allergic conjunctivitis • Urticaria • Common cold
Advantages of second generation antihistaminics • They have no anticholinergic side effects • Do not cross blood brain barrier (BBB), hence cause minimal or no drowsiness and sedation • Do not impair Psychomotor performance
Summary • Histamine • Antihistaminics – First generation – Second generation