Cyclodextrins as solubilizers: formation of complex aggregates.

Three different techniques were applied to investigate aggregation of drug/cyclodextrin complexes, that is, drug permeation through semi-permeable membranes, determination of changes in the value of activity coefficients of drug/cyclodextrin complex solutions and transmission electron microscopy (TEM). The aqueous solutions studied contained gamma-cyclodextrin, 2-hydroxypropyl-gamma-cyclodextrin or mixtures thereof, and hydrocortisone, amphotericin B, diclofenac sodium or indomethacin. The permeation studies indicated that drug/cyclodextrin complex monomers (i.e., unaggregated complexes) were dominating at cyclodextrin concentrations below 5% (w/v). Then formation of aggregates gradually increased with increasing cyclodextrin concentration until all increase in dissolved drug/cyclodextrin complexes was due to formation of cyclodextrin aggregates. This happened even though the observed phase-solubility diagrams were linear, that is, were of A(L)-type. The activity coefficients showed positive deviation from ideal state. This positive deviation is due to concurrent of several processes, that is, hydration, aggregation and complex formation. The observed deviations from ideality indicated that complex aggregates were formed in the aqueous complexation media. TEM pictures showed formation of aggregates in both pure aqueous cyclodextrin solutions as well as in cyclodextrin solutions that had been saturated with hydrocortisone. The aggregate diameter was between 10 and 100 nm but larger aggregates with diameter of about 200 nm were formed through assemble of smaller aggregates.