Determination of Microquantity of Ethylphenylephrine in Plasma by Radioisotope Derivative Method using p-Toluenesulfonyl [³⁵S] Chloride

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A determination of microquantity of ethylphenylephrine in plasma of dogs and human volunteers after oral administration of a therapeutic dose was investigated by the radioisotope derivative method using p-toluenesulfonyl [³⁵S] chloride (tosyl [³5S] chloride). Tosyl chloride, in a weak alkaline medium containing 33% acetone, reacted with the phenolic hydroxyl and amino group of ethylphenylephrine molecule, producing N-tosylethylphenylephrine tosylate, and reacted with the phenolic hydroxyl group of the drug in a neutral medium, producing ethylphenylephrine tosylate. A linear correlation was found between the amount of ethylphenylephrine (0.0125-0.1μg) added to human plasma and the radioactivity of N-tosylethylphenylephrine tosylate-³⁵S derivative produced from ethylphenylephrine which was extracted from the plasma samples. After the administration of ethylphenylephrine in dogs, about 56-80% of total plasma level was found as unchanged drug in plasma. On the contrary, unchanged ethylphenylephrine was not detected in human plasma after oral administration of the drug. After oral administration of the sustained-release capsule, plasma level was compared with that of the uncoated tablet in man and it was found that increase of plasma level was slower and more durable after administration of the sustained-release capsule than that of the uncoated tablet. Thus, the radioisotope derivative method using tosyl [³⁵S] chloride was useful for determination of a microquantity of ethylphenylephrine or its metabolite in plasma.