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Histamine and antihistaminics
 

Histamine and antihistaminics

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    Histamine and antihistaminics Histamine and antihistaminics Presentation Transcript

    • Histamine and Antihistaminics
    • Case study • A 24 year old male patient John , suffers from allergic rhinitis. Every winter, she develops a runny nose, itchy eyes, and sneezing. • To relieve his symptoms, he takes an over-the-counter antihistamine, diphenhydramine. • John is annoyed by the unpleasant effects that accompany his allergy medication. Every time he takes his 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 • Why does John develop seasonal rhinitis? • Why does diphenhydramine relieve John`s symptoms? • Why does diphenhydramine cause drowsiness? • Why doesn't loratadine cause drowsiness?
    • Histamine • Histos: Tissue NH2 5 H  skin, lungs, GIT Mucosa 4 3 1 N N •Present mostly in mast cells: 2 •Non mast cell histamine: Brain, Gastric Mucosa Histamine Histamine is a biogenic amine present in many animal and plant tissues .
    • Histamine • Biogenic amine present in many animal and plant tissues • Also present in venoms and stinging secretions • One of the mediators involved in inflammatory & hypersensitivity reactions.
    • Synthesis, storage & metabolism of histamine • Synthesized by decarboxylation of amino acid histidine • Histamine is present in storage granules of mast cells & also found in skin, lungs, liver, gastric mucosa etc.
    • Mechanism of Action of Histamine Histamine H1 Receptors ↑ Ca2+ Smooth muscle contraction Increased capillary permeability Vasodilation Sensory nerve endings pain & itching H2 Receptors ↑ cAMP ↑ Gastric acid secretion Blood vessels: vasodilation Increased capillary permeability H3 Receptors (presynaptic auto receptors) ↓ cAMP ↓ histamine release ↓secretion Vasodilation
    • Pharmacological actions Stimulating H1, H2, H3 Receptors • CVS: – Dilates arterioles, capillaries, venules, • IV injection- decreased BP • Intradermal- Triple response Red spot Wheal Flare(Reflex arteriolar dilatation)
    • Pharmacological actions (Contd) • Visceral smooth muscles: – Bronchospasm, abdominal cramps • Secretions: – Increased gastric secretion (H2) – Increased nasal secretions (H1) • Sensory Nerve Endings: Itching • CNS: – Wakefulness on intra Cerebroventricular Injection
    • Therapeutic Uses • Betahistine – To control vertigo in Meniere`s disease 8 mg tab ½ tablet QID Histamine releasers • stings and venom •Ag-Ab reaction •Drugs  d-tubocurarine Morphine
    • Adverse effects of histamine release • • • • • • • • Itching, Urticaria Flushing Hypotension Tachycardia Bronchospasm Angioedema Wakefullness Increased acidity (Gastric acid secretion)
    • Classification of H1 Antagonists
    • Mechanism of action Competitive antagonism Histamine General formula of H1 Blocker
    • Pharmacological actions • CNS depression: (More with first generation) – Sedation and drowsiness – Some have antiemetic and antiparkinsonian effects • Antiallergic action • Anticholinergic actions (More with first generation) – Dryness of mouth , Blurring of vision – Constipation – Urinary retention
    • Pharmacological Actions • Antimotion sickness effect: Dimenhydrinate, Promethazine • Antiemetic: Promethazine • Antiparkinsonism: Diphenhydramine, orphenadrine, promethazine(IV) • Antivertigo: cinnarizine
    • Preparations & dosage (Daily) Drug 1. 2. 3. 4. 5. 6. 7. Dose Diphenhydramine Dimenhydrinate Promethazine Chlorpheniramine Pheniramine Cinnarizine Cyprohepatidine 25-50 mg oral 25-50 mg oral 25-50 mg oral/injection 2-4 mg oral 25- 50 mg oral/im 25-150 mg oral 4 mg oral
    • Therapeutic uses 1. 2. 3. 4. 5. 6. Allergic rhinitis & common cold Allergic dermatitis, itching, urticaria Wasp stings/ bite: pain and itching decreases Mild blood transfusion reactions Allergic conjunctivitis Motion sickness: dimenhydrinate, promethazine 7. Morning sickness: promethazine
    • Therapeutic uses (Contd..) 8. Vertigo: cinnarizine 9. Chronic urticaria 10. Appetite stimulant: cyprohepatidine 11.Drug induced parkinsonism: Diphenhydramine,
    • Adverse effects • • • • Sedation Anticholinergic effects Dermatitis on local use Cyclizine, meclizine : teratogenicity
    • 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
    • Drug interactions • Increased effect of CNS depressants • MAO inhibitors increase anticholinergic effect of antihistaminics • First generation antihistaminics can decrease effectiveness of cholinesterase inhibitors used in Alzheimer`s disease like donepezil and rivastigmine
    • Answers • The IgE-mediated hypersensitivity reaction is responsible for initiation of certain inflammatory disorders, like allergic rhinitis . • John suffered from allergic rhinitis, with a runny nose, itchy eyes, and sneezing. • An environmental allergen, such as pollen, crosses the nasal epithelium and enters the underlying tissue. There, the allergen encounters previously sensitized mast cells and crosslinks IgE/Fc receptor complexes on the mast cell surface. • The mast cell degranulates and releases histamine, which binds to H1 receptors in the nasal mucosa and local tissues.
    • Answers • Stimulation of H1 receptors causes blood vessel dilation and ↑es vascular permeability, leading to edema. This swelling in the nasal mucosa is responsible for the nasal congestion experienced in allergic rhinitis. • The accompanying itching, sneezing, runny nose, etc result from the combined action of histamine and other inflammatory mediators, including kinins, prostaglandins etc. • These molecules initiate the hypersecretion and irritation characteristic of allergic rhinitis.