Mepyramine, also known as pyrilamine, is a first-generation antihistamine that targets H1 receptors as an inverse agonist. [1] Mepyramine rapidly penetrates the brain, often causing drowsiness. [2] It is commonly sold as the maleate salt, pyrilamine maleate.

The drug has negligible anticholinergic activity and is 130,000-fold more selective for histamine H1 receptors than for muscarinic acetylcholine receptors (by comparison, diphenhydramine is 20 times). [3]

It was patented in 1943 and put into medical use in 1949. [4] It is marketed under the names Histadyl, Histalon, Neo-Antergan, Neo-Pyramine and Nisaval. [5] It was a very common ingredient in over-the-counter sleeping pills such as "Alva-Tranquil," "Dormin," "Sedacaps," "Sominex," "Nytol," and others in the 1960s and 70s. [5]

It is used in over-the-counter combination products, such as Midol Complete, to treat the common cold and menstrual symptoms. [6] It is also the active ingredient in the topical antihistamine creams Anthisan[7] and Neoantergan[1] for insect bites, stings and hives.
A simple, specific, and accurate stability-indicating RP-HPLC method was developed for the simultaneous determination of acetaminophen, pamabromide, and pyrilamine maleate in pharmaceutical dosage forms. All components were successfully separated within 10 min using a C18 column with mobile phases of methanol and acidified water (pH 1.8) in the ratio (27:73 v/v), respectively. The flow rate of the mobile phase was 1.5 mL/min, and the detection wavelength was 300 nm. The method was validated according to ICH guidelines. The response was a linear function of concentration over the range of 50-150 μg/mL for acetaminophen, 2.5-7.5 μg/mL for pamabromide, and 1.5-4.5 μg/mL for pyrilamine maleate. The method provides good separation of all analytes and their stress-induced degradation products with acceptable peak tailing and good resolution. Therefore, it can be successfully used for the simultaneous determination of acetaminophen, pamabromide and pyrilamine maleate in pharmaceutical preparations and their stability studies.
The main goal of this research work was to achieve optimal conditions for the simultaneous separation of acetaminophen, bamabromide, and pyriramine maleate in their fixed-dose combinations.

Various mobile and stationary phases were initially attempted to achieve optimal separation conditions and resolution between acetaminophen, permabromide, and pyrilamine maleate. For the initial experiments, a Hypersil BDS C8 column was chosen, and the mobile phase consisted of 1 M potassium dihydrogen phosphate and acetonitrile in different ratios (50:50, 40:60, 30:70, 20:80). Under all these conditions, only elution of acetaminophen occurred and there was no separation between pamabromide and pyrilamine maleate.