Self-assembly of cyclodextrin complexes: effect of temperature, agitation and media composition on aggregation.

Recently it has been shown that aggregation of drug/cyclodextrin inclusion complexes is strongly influenced by the drug molecule in addition to self-assembling tendencies of the cyclodextrin itself in aqueous media. Whereas the mechanistic basis of cyclodextrin self-assembly is known, the driving forces for complex aggregation are still unknown. In the present study, the influence of temperature on hydrocortisone/2-hydroxypropyl-β-cyclodextrin complex aggregation is investigated as are influences associated with the addition of ethanol or water soluble polymers to the aqueous systems. Furthermore the effect of stirring on the aggregation is assessed. Size exclusion permeability studies were conducted to estimate complex aggregation tendencies. The results indicate that self-assembled complex aggregates are metastable and notably become smaller with increasing temperature and the addition of ethanol. Water soluble polymers also reduce the size of the complex aggregates. Specifically, hexadimethrine bromide had the greatest impact, since addition of this compound eliminated aggregates from the systems or reduced their size below the molecular weight cut-off of the sizing membrane (8 kDa). Similar observations are made when aqueous solutions of hydrocortisone and 2-hydroxypropyl-β-cyclodextrin are equilibrated by stirred.