Pyrilamine Maleate
**Pyrilamine maleate** (also known as mepyramine) is a first-generation, ethylenediamine-class antihistamine used in veterinary medicine primarily to reduce or prevent histamine-mediated adverse effects. Key clinical points: * **Primary Use**: Predominantly utilized in **equine practice** for the management of allergic conditions such as insect bite hypersensitivity, hives (urticaria), and as an adjunct in respiratory conditions like equine asthma (heaves). * **Advantages**: Compared to other first-generation antihistamines (like diphenhydramine), pyrilamine is considered to be **less sedating** and possesses **fewer anticholinergic side effects**. * **Formulations**: Often found in combination products (e.g., with pseudoephedrine or guaifenesin) as oral granules for horses, or as an injectable solution. * **Regulatory Note**: In equine sports, it is classified as an ARCI Class 3 drug and requires appropriate withdrawal times before competition.
Mecanismo: Pyrilamine acts as a **competitive antagonist (inverse agonist)** at **H1-receptors**. * **Mechanism**: Allergic trigger → Mast cell degranulation → Histamine release. Pyrilamine competes with free histamine for binding sites on **H1-receptors** located on smooth muscle, endothelium, and in the CNS. * **Effects**: By blocking these receptors, it prevents histamine-induced bronchoconstriction, vasodilation, and increased capillary permeability (which leads to edema/hives). * **Limitations**: It does **not** inhibit the release of histamine from mast cells, nor does it inactivate histamine that is already bound. Therefore, it is more effective as a preventative rather than a rescue treatment during acute anaphylaxis. * **Secondary Actions**: Like other first-generation antihistamines, it can cross the blood-brain barrier, leading to mild CNS depression (sedation) or paradoxical CNS stimulation, and possesses mild anticholinergic properties.
Dosificación por especie
- Antihistamine · 0.25-0.5 gram IM · IM · Unknown
- Antihistamine · 0.5-1.5 grams IM · IM · Unknown
- Adjunctive treatment of aseptic laminitis · 55-110 mg/100 kg IV or IM · IV, IM · Unknown
- Antihistamine · 0.88-1.32 mg/kg (2-3 mL of 20 mg/mL solution per 100 lbs body weight) IV (slowly), IM or SC · IV, IM, SC · q6-12h · May repeat in 6-12 hours if necessary. NOTE: ARCI UCGFS CLASS 3 DRUG
- Antihistamine (Foals) · 0.44 mg/kg (1 mL of 20 mg/mL solution per 100 lbs. body weight) IV (slowly), IM or SC · IV, IM, SC · q6-12h · May repeat in 6-12 hours if necessary.
- Antihistamine · 1 mg/kg IV , IM or SC · IV, IM, SC · Unknown
- Antihistamine · 0.5-1.5 grams IM · IM · Unknown
- Antihistamine · 0.25-0.5 gram IM · IM · Unknown
- Antihistamine · 12.5-25 mg PO four times a day · PO · q6h
Vías de administración
Contraindicaciones
- Should not supersede the use of other emergency drugs and procedures (e.g., epinephrine for acute anaphylaxis)
Efectos adversos
- CNS stimulation (nervousness, insomnia, convulsions, tremors, ataxia)
- Palpitation
- GI disturbances
- CNS depression (sedation)
- Muscular weakness
- Anorexia
- Lassitude
- Incoordination
Interacciones farmacológicas
- Anticoagulants (heparin, warfarin) · Antihistamines may partially counteract the anticoagulation effects of heparin or warfarin.
- CNS Depressant Drugs · Increased sedation can occur if pyrilamine is combined with other CNS depressant drugs.
- Epinephrine · Pyrilamine may enhance the effects of epinephrine.
Monitorización
- Clinical efficacy (reduction in allergic signs, hives, or respiratory effort)
- Adverse effects (monitor for signs of CNS stimulation or excessive sedation)
Sobredosis
Treatment of overdosage is **supportive and symptomatic**. * **Seizure Management**: Phenytoin (IV) is recommended in the treatment of seizures caused by antihistamine overdose in humans. Most toxicologists recommend **avoiding** barbiturates and diazepam, though one veterinary manufacturer historically suggested 'careful titration' of barbiturates. * **CNS Depression**: Avoid the use of CNS stimulants (analeptics like caffeine, ephedrine, or amphetamines) to treat CNS depression, as this can exacerbate toxicity.
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