Chemistry - An ethylenediamine antihistamine, pyrilamine maleate occurs as a white, crystalline powder with a melting range of 99-103°. One gram is soluble in approximately 0.5 ml of water or 3 ml alcohol.
Storage/Stability/Compatibility - Avoid freezing the injectable product.
Pharmacology - Antihistamines (H1-receptor antagonists) competitively inhibit histamine at H1 receptor sites. They do not inactivate, nor prevent the release of histamine, but can prevent histamine’s action on the cell. Besides their antihistaminic activity, these agents also have varying degrees of anticholinergic and CNS activity (sedation). Pyrilamine is considered to be less sedating and have much less anticholinergic effects when compared to most other antihistamines.
Uses/Indications - Antihistamines are used in veterinary medicine to reduce or help prevent histamine mediated adverse effects.
Pharmacokinetics - The pharmacokinetics of this agent have apparently not been extensively studied.
Contraindications/Precautions - The manufacturer indicates that the use of this product “should not supercede the use of other emergency drugs and procedures.”
Adverse Effects/Warnings - Adverse effects in horses can include CNS stimulation (nervousness, insomnia, convulsions, tremors, ataxia), palpitation, GI disturbances, CNS depression (sedation), muscular weakness, anorexia, lassitude and incoordination.
Overdosage - Treatment of overdosage is basically supportive and symptomatic. The manufacturer (Schering - Histavet-P®) suggests using “careful titration” of barbiturates to treat convulsions, and analeptics (caffeine, ephedrine, or amphetamines) to treat CNS depression. Most toxicologists however, recommend avoiding the use of CNS stimulants in the treatment of CNS depressant overdoses. Phenytoin (IV) is recommended in the treatment of seizures caused by antihistamine overdose in humans; barbiturates and diazepam are to be avoided.
Drug Interactions - Increased sedation can occur if diphenhydramine is combined with other CNS depressant drugs. Antihistamines may partially counteract the anticoagulation effects of heparin or warfarin. Pyrilamine may enhance the effects of epinephrine.
Laboratory Interactions - Antihistamines can decrease the wheal and flare response to antigen skin testing. In humans, it is suggested that antihistamines be discontinued at least 4 days before testing.
a) 0.88 - 1.32 mg/kg (2-3 mls of 20 mg/ml solution per 100 lbs body weight) IV (slowly), IM or SQ; may repeat in 6-12 hours if necessary. Foals: 0.44 mg/kg (1 ml of 20 mg/ml solution per 100 lbs. body weight) IV (slowly), IM or SQ; may repeat in 6-12 hours if necessary. (Package Insert; Histavet-P® - Schering)
b) 1 mg/kg IV, IM or SQ (Robinson 1987)
c) 0.5 -1.5 grams IM (Swinyard 1975)
Monitoring Parameters -
1) Clinical efficacy and adverse effects
Dosage Forms/Preparations/FDA Approval Status/Withholding Times -
Pyrilamine Maleate Injection 20 mg/ml; 100 ml vial; Histavet-P® (Schering); (Rx) Approved for use in horses not intended for food only.
Pyrilamine Maleate Tablets 25 mg; Generic; (Rx/OTC)
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