A fluorimetric, potentiometric and conductimetric study of the aqueous solutions of naproxen and its association with hydroxypropyl-beta-cyclodextrin

The dissociation constant, K-a, of naproxen, a nonsteroidal antiinflammator y drug of the family of arylpropionic acids, has been determined in aqueous solutions at 25 degrees C by using a potentiometric and a conductimetric t echniques. The solubility limit of the drug in water, a controversial point in the literature, has been found to be less than 3 x 10(-5) M. The intera ction of naproxen with hydroxypropyl-beta-cyclodextrin (HPBCD), in terms of the binding constants of the complexes formed by the CD and the nonionic ( HNAP) and ionic (NAP(-)) species of the drug, has been evaluated at 25 degr ees C as well by means of steady-state fluorescence enhancement studies. A discussion of the results, K-HPBCD:HNAP = 6500 +/- 400 M-1 and KHPBCD:NAP- = 1400 +/- 80 M-1, emphasizing the crucial importance of the choice of the pH at a value that pH greater than or equal to pK(a) + 2 or pH less than or equal to pK(a) - 2, is also included. (C) 1999 Elsevier Science B.V. All r ights reserved.