Evaluation of a cationic calix[4]arene: Solubilization and self-aggregation ability

Water-soluble calixarenes are promising macrocyclic compounds which have found numerous applications in chemistry and biology. However, these compounds have been less studied in regard to their behavior in aqueous solutions and mechanisms of drug solubilization. The present work is devoted to the evaluation of the solubilizing properties and estimation of self-aggregation ability of positively charged 5,11,17,23-tetrakis(trimethylammoniomethyl)-25,26,27,28-tetrapropoxy-calix[4]arene tetrachloride (aminocalix), including comparisons with a series of pharmaceutically relevant cyclodextrins. Phase-solubility measurements of the drugs with aminocalix and various cyclodextrins were carried out. Aminocalix showed a solubilizing ability comparable to the cyclodextrins. The drug solubility enhancement caused by the aminocalix was studied and was found to be maximal for steroid drugs. An attempt to understand the solubilizing mechanism of aminocalix was undertaken based on correlation analysis between physical and physico-chemical properties of the drugs from one side and the solubilizing ability of aminocalix from the other. Correlation analysis supports the supposition that the solubilizing effect of aminocalix is based on interaction of the drug with aminocalix aggregates rather than on inclusion complexation. UV-absorbance, osmolality and surface tension concentration dependences of aminocalix showed an inflection at 1% (w/v) which was initially related to the transition from monomers to micelles. However, dynamic light scattering and transmission electron microscopy measurements revealed that likely vesicles of diverse size exist at 0.1% (w/v) concentration. Thus the 1% (w/v) inflection point was interpreted to be spontaneous reordering of the vesicles between two different size populations.