The physicochemical properties and solubility of pharmaceuticals – Methyl xanthines

• Solubility of methyl xanthines in water and alcohols was measured.
• Solubility in water, or alcohols was of the order of 10−4 in mole fraction.
• Experimental aqueous pKa’s values are reported for the selected drugs.
• The basic thermodynamic functions were determined.

The aim of this study was to evaluate the physio-chemical properties and solubility of three pharmaceuticals (Phs): theophylline, 7-(β-hydroxyethyl) theophylline, and theobromine in binary systems in different solvents. The solvents used were water, ethanol, and 1-octanol. Score of the solubility of these substances is being important for their dissolution effect inside the cell, the transportation by body fluids and the penetration possibility of lipid membranes.The Phs were classified to the group of methyl xanthines, which contain purine in their structure. Although they are mainly obtained via chemical synthesis, they can be also found in natural ingredients such as cocoa beans and tea leaves. These drugs are mainly acting on the central nervous system but are also used in the treatment of asthma or blood vessels.Solubility of 7 (β-hydroxyethyl) theophylline and theophylline were tested using synthetic method. In case of theobromine, which solubility is very small in the solvents noted, the spectrophotometric method has been used to measure its solubility. After designating phase diagrams of each of the solubility in the bipolar system, the experimental points have been correlated with the equations: Wilson, NRTL, UNIQUAC. Results show that theophylline and its derivatives show the best solubility from all tested Phs.Another method also used during this study was the differential scanning calorimetry (DSC), which allowed designation of the thermal properties of Phs. The fusion temperature and the enthalpy of melting were measured. Unfortunately, it was not possible to determine the fusion temperature and enthalpy of melting of theobromine, because of the decomposition of Ph at high temperature.The important property tested was the constant acidity, to this end, the spectrophotometric method of Bates–Schwarzenbach was used. Unfortunately, with this method it was not possible to determine the value of pKa 7-(β-hydroxyethyl) theophylline. For other Phs, these values do not differ significantly from those proposed in the literature.Both awareness and knowledge of values of the drug pKa and solubility are important in Phs production. This allows the selection of a suitable solvent and allows estimation of the correct dose and its capacity to absorb in human body.

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