Ternary naproxen: beta-cyclodextrin:polyethylene glycol complex formation.
The aim of this study was to investigate the effect of the presence of the water-soluble polymer polyethylene glycol (PEG)-MW=35000 g/mol-on the complexation of the phototoxic anti-inflammatory drug naproxen, in its sodium salt form, with beta-cyclodextrin (beta-CD). The data revealed that the polymer does not interact with the uncomplexed naproxen whereas it does with the beta-CD. The presence of different proportions of PEG, in the 0-1% (w/w) range, systematically lowers K(app) of the formation of the naproxen:beta-CD inclusion complex. The reason for the decrease in the complexed drug is the presence of other competing equilibria, the first one is an interaction of the polymer with the beta-CD, which in turn reduces the amount of free CD available for including the naproxen, and the second is the formation of a naproxen:beta-CD:PEG ternary complex with lower affinity than the binary complex. The binding constant of these processes are K(2)=(4.5+/-1.0) x 10(5) M(-1) and K(3)=870+/-19 M(-1), respectively. In addition the presence of the PEG produces an important change in the driving force of the complex formation. In this case the process is enthalpically unfavoured and entropically favoured; these are typical characteristics of processes governed by hydrophobic interactions.