The dissolved concentration of S-309309 in phosphate buffer (pH6.8) after 120 min was improved by mixing a water-soluble polymer, regardless of the polymer type, compared with that of S-309309 alone. Lippold et al. suggested that improved drug wettability leads to an increase of the effective surface area, which in turn results in an accelerated dissolution rate. Therefore, it was indicated that the water-soluble polymers improved the wettability of S-309309, which raised the dissolution rate of the drug and resulted in a higher S-309309 concentration in the buffer after 120 min than was the case for S-309309 alone.