Iid - Antihistamines And Allergy Pharmacology

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First Generation Antihistamines

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Chlorpheniramine
Clemastine
Cyproheptadine
Dimenhydrinate
Diphenhydramine
Hydroxyzine
Meclizine
Promethazine

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Chlorpheniramine

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H1 First Generation Antagonist

OTC "cold medicine"

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Clemastine

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H1 First Generation Antagonist

OTC "cold medicine"

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Cyproheptadine

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H1 First Generation Antagonist

Used in paraplegic spasticity, appetite stimulation

Also has anti-serotonin effects

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Dimenhydrinate

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H1 First Generation Antagonist

Motion sickness

Salt of diphenhydramine

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Diphenhydramine

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H1 First Generation Antagonist

Motion sickness; treatment of antipsychotic-induced extrapyramidal symptoms

Available IV

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Hydroxyzine

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H1 First Generation Antagonist

Widely used for pruritus; adjunctive pain control

Available IV

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Meclizine

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H1 First Generation Antagonist

Motion sickness; dizziness

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Promethazine

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H1 First Generation Antagonist

Widely used as an antiemetic; also a neuroleptic; sedation; anesthetic adjunct

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Second Generation H1 Antagonists

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Fexofenadine
Loratadine
Desloratadine
Cetirizine
Levocetirizine

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Fexofenadine

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Second Generation H1 Antagonist

No sedation; large therapeutic window, OTC

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Loratadine

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Mild sedation at higher doses, OTC

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Desloratadine

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Active metabolite of loratadine

Approximately 10% of population are slow metabolizers, prescription

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Cetirizine

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Causes most sedation of 2nd generation agents, OTC

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Levocetirizine

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Active metabolite of cetirizine, prescription

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Urticaria

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Intensely pruritic, circumscribed, raised, and erythematous eruption with central palor. It represents mast cell activation in the subcutaneous later. Angioedema is the manifestation of deeper mast cell degranulation.

Often caused by autoantibody IgG to the IgE or to the IgE receptor, causing activation in the absence of allergic trigger

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Mast cell stabilizers

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Cromolyn and nedocromil

Inhibits release of histamine and other autocoids from mast cells, probably by stabilizing the cellular membrane.

Drug used prophylactically; once symptoms develop drug is much less effective

Safety outstanding, efficacy modest

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Leukotriene modifiers

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Montelukast

High affinity competitive antagonist at leukotriene receptor (cys-LT1).

Leukotrienes are associated with allergic nasal congestion, histamine is associated with sneezing and nasal itching

Drug of this class have efficacy in asthma

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Ipratropium

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Topical anticholinergic spray with minimal absorption

Very effective for rhinorrhea, but not for sneezing, congestion or pruritus

Safe; also effective for vasomotor rhinitis (runny nose associated with cold weather exposure)

Adverse events of nasal dryness

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Phenylephrine

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Nasal decongestant

Sympathomimetic

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H2 receptor antagonists vs proton pump inhibitors

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H2 antagonists have inferior acid suppression compared to potent proton pump inhibitors

H2 antagonists have much quicker onset of action, so they are good for quick symptom relief

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H2 receptor antagonists

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Cimetidine
Ranitidine
Famotidine
Nizatidine

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Cimetidine

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H2 receptor antagonists

First agent available; associated with more side effects and drug interactions than the other agents

Binds to androgen receptor

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Ranitidine

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H2 receptor antagonists

Occasionally associated with thrombocytopenia

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Famotidine

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H2 receptor antagonists

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Nizatidine

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H2 receptor antagonists

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Pharmacokinetics of H2 agents

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Duration of effect is 6-12 hours

Dosage needs to be reduced in renal insufficiency

Hepatic metabolism is minor

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Adverse effects and drug interactions of H2 antagonists

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Antiandrogen - Gynecomastia, galactorrhea, and impotence have been associated with the antiandrogen effect of cimetidine after extended dosing

Cytopenias - Cimetidine and ranitidine have been associated with reversible decreases in blood cell types, most notably platelets

Drug interactions - Cimetidine inhibits multiple cytochrome P450 enzymes, so it will increase other drugs metabolized by this enzyme.