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.

กลไกการออกฤทธิ์: 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.

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