Solubility of the new antileukotrienic drug quinlukast (4-{[4-(2-quinolylmethoxy)phenyl]sulfanyl}benzoic acid) was determined in water and in aqueous solutions of α-cyclodextrin (α-CD), β-cyclodextrin (β-CD), hydroxypropyl- β-cyclodextrin (HP-β-CD, average degree of substitution 0.8), and methyl-β-cyclodextrin (M-β-CD, average degree of substitution 1.8). The determined solubility of quinlukast in water was 0.081±0.008 mmol/l (3.12± 0.30 mg/100ml) and only an insignificant increase in quinlukast solubility was observed in aqueous solutions of β-CD. However, three well soluble cyclodextrins showed a marked solubilizing effect, in aqueous solutions with a moderate cyclodextrin concentration 5 g/100ml, a 12-fold increase in quinlukast solubility was observed in the case of M-β-CD, and a 10-fold increase in the case of both HP-β-CD and α-CD. Phase solubility diagrams of quinlukast in aqueous solutions of these cyclodextrins (up to 0.05 mol/l) were determined. In the cases of M-β-CD and HP-β-CD, the solubility diagrams were linear (AL) and they corresponded to the formation of a soluble inclusion complex quinlukast – cyclodextrin 1:1 with the evaluated stability constants K11 300±35 l/mol and 260±30 l/mol for M-β-CD and HP-β-CD, respectively. The phase solubility diagram of quinlukast in aqueous solutions of α-CD showed a marked positive deviation (AP) from linearity, the solubilization efficiency of dilute α-CD solutions was relatively low but it increased progressively with the increasing α-CD concentration. In the overall evaluation, the cyclodextrins α-CD, HP-β-CD and M-β-CD appeared to be suitable for the quinlukast solubilization into aqueous solutions.

        Key words: quinlukast – antileukotrienic drug – solubility – solubilization – cyclodextrins – inclusion com plexes