Analysis of the phase solubility diagram of a phenacetin/competitor/beta-cyclodextrin ternary system, involving competitive inclusion complexation

The competitive inclusion complexations in the ternary phenacetin/competito rs/beta -cyclodextrin (beta -CyD) systems were investigated by the solubili ty method, where m-bromobenzoic acid (m-BBA) and o-toluic acid (o-TA) were used as competitors, The solubility changes of the drug and competitors as a function of beta -CyD concentration in the ternary systems were formulate d using their stability constants and intrinsic solubilities. The decrease in solubility of phenacetin by the addition of competitors could be quantit atively simulated by the formulation, when both drug and competitor give A, type solubility diagrams. On the other hand, when one of the guests gives a B-S type solubility diagram, its solubility change was clearly reflected in that of the another guest, i.B., phenacetin gave an A, type solubility d iagram in the binary phenacetin/beta -CyD system and o-TA gave a B-S type d iagram in the binary o-TA/beta -CyD system, but in the ternary phenacetinlo -TA/beta -CyD system, a new plateau region appeared in the original A, type diagram of phenacetin. This was explained by the solubilization theory of Higuchi and Conners, The solubility analysis of the ternary drug/competitor /CyD systems may be particularly useful for determination of the stability constant of a drug whose physicochemical and spectroscopic analyses are dif ficult, because they can be calculated by monitoring the solubility change of a competitor, without monitoring that of a drug, Furthermore, the presen t results suggest that attention should be paid to the type of the phase so lubility diagram, as well as the magnitude of the stability constant and th e solubility of the complex, for a rational formulation design of CyD compl exes.