3. Histamine
• Histamine: Tissue amine
N N
NH2
H
1
2
3
4
5
Histamine
1. Present mostly in mast cells:
Skin, lungs, GIT Mucosa, liver,
placenta
2. Non mast cell histamine:
Brain, Gastric Mucosa, Epidermis.
β imidazolylethylamine
4. Histamine
Histamine is a biogenic amine present in many animal
and plant tissues
Also present in blood, body secretions, venom &
stinging nettle, pathological fluids
One of the mediators involved in inflammatory &
hypersensitivity reactions
Present within storage granules of mast cells.
5. Synthesis, storage & metabolism of histamine
• Synthesized by decarboxylation of amino acid
histidine
• Degrades rapidly by oxidation & methylation
8. 1. Blood vessels:
Marked dilatation of smaller BV including arterioles, capillaries &
venules
S.C inj- flushing, ↑HR & CO, No fall in BP
Rapid IV inj – Fall in BP
Dilatation of cranial vessels – Pulsatile HA
Vasodilatation by H1→mediated by EDRF (NO)
Vasodilatation by H2 → located directly on the vascular SM
Pharmacological actions
9. Pharmacological actions
2. CVS:
– Dilates arterioles, capillaries, venules,
• IV injection- decreased BP
• Intradermal- Triple response Red spot
(Capillary dilatation)
Wheal (exudation of
fluid from capillaries
& venules)
Flare (Reflex
arteriolar dilatation)
15. Pharmacological actions
1. CNS depression: (More with first generation)
– Sedation and drowsiness
– Some have antiemetic and antiparkinsonian effects
2. Antiallergic action – type 1 HSV reaction
3. Anticholinergic actions (More with first generation)
• Antagonize muscarinic actions of ACh
– Dryness of mouth , Blurring of vision
– Constipation
– Urinary retention
16. Pharmacological Actions
4. Antagonism of histamine:
Block the histamine induced bronchoconstriction, triple
response, fall in BP, release of adrenaline.
5. Local anaesthetic: Membrane stabilizing property
Pheniramine, promethazine, diphenhydramine
6. BP: fall in BP on IV injection (direct smooth muscle
relaxation)
17. PHARMACOKINETICS
• Well absorbed oral and parenteral routes
• Metabolized- Liver
• Excretion – Urine
• Widely distributed in the body and enter brain.
• DOA → 4 – 6hours
• Except Meclozine, CPM, loratadine, cetrizine &
fexofenadine → 12 – 24hours
• Newer drugs doesn’t cross BBB – less sedative
25. Advantages of second generation
antihistaminics
1. Absence of CNS depressant property.
2. No impairment of psychomotor performance , produce no
subjective effects, no sedation.
3. Do not potentiate alcohol or benzodiazepines.
4. Higher H1 selectivitiy
5. No anticholinergic side effects.
6. Additional antiallergic mechanisms apart from histamine blockade
7. Inhibit late phase allergic reaction by acting on leukotrienes
8. or by antiplatelet activating factor effect
26. Banned antihistaminics
1. Terfenadine
First non-sedating SGA that was withdrawn
Due to polymorphic VT (torsades de pointes) due to higher
doses
When it was co-administered with CYP3A4
inhibitors(erythromycin, clarithromycin, ketoconazole,
itraconazole)
Blockade of delayed rectifier k+ channels in the heart at
higher concentrations.
2. Astemizole is another SGA banned for the same reason
27. 1. Fexofenadine
1. Active metabolite of terfenadine
2. Does not prolong QTc interval.
3. No interaction with CYP3A4 inhibitors
4. Free of arrhythmogenic potential
2. Loratadine
1. Long & fast acting
2. Lacks CNS depressant
3. Partly metabolized by CYP3A4 & t½ of 17 hour
4. No cardiac arrhythmia
5. Seizures are reported
6. Uses: Urticaria and atopic dermatitis
3. Desloratadine
1. Active metabolite of loratadine
2. Cardiac safety are documented
4. Cetrizine
1. Metabolite of hydroxyzine
2. Upper respiratory allergies, pollinosis
3. Urticaria and atopic dermatitis;
4. Used as adjuvant in seasonal asthma
28. 5. Levocetirizine
1. Active R(–) enantiomer of cetirizine.
2. Effective at half the dose
3. Less sedation
6. Azelastine
1. Good topical activity
2. Nasal spray - seasonal and perennial allergic rhinitis
3. Side effects- Stinging in the nose, altered taste
perception, weight gain
7. Mizolastine 1. Non-sedating antihistaminic,
2. Effective in allergic rhinitis and urticaria
8. Ebastine 1. Converted to the active metabolite carbastine.
2. Non-sedating
3. Nasal and skin allergies.
4. Prolong Q-Tc interval and CYP3A4 interaction
31. VERTIGO
Diuretics
• They decrease labyrinthine fluid pressure—acetazolamide, thiazides,
furosemide.
Anxiolytics, antidepressants - diazepam, amitriptyline
• These drugs appear to modify the sensation of vertigo
Corticosteroids
• They suppress intra labyrinthine edema due to viral infection or other
causes.
Sir Henry Dale,
discovered histamine Histamine was first discovered in 1910 by Sir Henry Hallett Dale as a contaminant of ergot generated by bacterial action. It was first synthesized before its significance was known, and due to its wide range of biological activity, has become one of the most important biologically produced amines in medicine and biology
Meniere`s disease: betahistine acts by improving blood flow in inner ear. The side efects are nausea, vomiting, headache and pruritis. It should be avoided in patients with bronchial asthma and peptic ulcer.
Azelastine
H1 blocker, also decrease histamine release, PAF & Leukotriene release
Good topical activity
T ½ = 24 hrs, also have active metabolite
Nasal spray: 0.14 mg/puff/nasal spray, 2 puffs per day for seasonal and perennial allergic rhinitis