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Physicochemical characterization and solubility analysis of thalidomide and its N-alkyl analogs
Colleen Goosen1, Timothy J Laing, Plessis Jeanetta du
1Department of Pharmaceutics, School of Pharmacy, Potchefstroom, University for Christian Higher Education, Republic of South Africa. fmscg@puknet.puk.ac.za
Purpose:
The present study was primarily aimed at exploring the feasibility of improving percutaneous delivery via chemical manipulation of the thalidomide molecule to form analogs with improved physicochemical properties. N-Alkyl analogs were synthesized with the belief that these would be suitably hydrophobic and far less crystalline than the reference compound. This article presents their physicochemical properties.
Methods:
Thalidomide and three of its N-alkyl analogs were synthesized. Identification and levels of purity (>96%) were assured through element analysis, fast atom-bombardment mass spectrometry, nuclear magnetic resonance spectroscopy, and high-performance liquid chromatography. N-Octanol/water partition coefficients were determined at pH 6.4. Solubilities in water and a series of n-alkanols were obtained. Best-fit solubility parameters were determined from the solubilities of the respective compounds in London solvents and were also calculated from respective hexane solubilities. melting points and heats of fusion.
Results:
Methylation of the thalidomide molecule at its acidic nitrogen led to an aqueous solubility about 6-fold higher than thalidomide but, because the alkyl chain length was further extended from methyl to pentyl. aqueous solubilities decreased essentially exponentially. The destabilization of the crystalline structure with increasing alkyl chain length led to an increased solubility in nonpolar media. The log partition coefficient increased linearly with increasing alkyl chain length and the solubility parameters declined systematically through this series. By adding a methyl group to the thalidomide structure, the melting point dropped by more than 100 degrees C. Adding to the alkyl chain length led to further, more modest decreases. Heats of fusion decreased dramatically upon thalidomide's alkylation as well.
Conclusion:
Alkylation of the thalidomide molecule resulted in compounds with physicochemical properties that appear to be markedly better suited for percutaneous delivery.