Solubility characteristics of weak bases and their hydrochloride salts in hydrochloric acid solutions.

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The pH-solubility profiles of four basic drugs were studied at 37°. The solubility of papaverine hydrochloride in acetate buffer showed a typical pH-profile, which increased with decrease in pH. On the other hand, the pH-solubility profile of the hydrochloride in HCl-sodium acetate buffer was not as simple as that in acetate buffer, showing as solubility maximum at approximately 3.7. Similar pH-profiles of solubility were observed for trihexyphenidyl hydrochloride, isoxsuprine hydrochloride, and oxyphencyclimine hydrochloride. The decrease in the solubility of these drugs at more acidic pH values could be rationalized on the basis of the common ion effect. The dissolution behavior of the free bases and that of the hydrochloride salts of papaverine, trihexyphenidyl, and isoxsuprine were compared in dilute hydrochloric acid solutions, in the pH range from 1.0 to 1.8. It was confirmed that a much higher solution concentration and dissolution rate could be obtained using the free bases than from the hydrochloride salts in the pH range of the stomach (pH 1.0-1.2 for papaverine, and pH 1.2-1.4 for trihexyphenidyl and isoxsuprine) due to the common ion effect. It is suggested that salt formation does not always result in an enhancement of solubility characteristics.

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