Interaction of naproxen with ionic cyclodextrins in aqueous solution and in the solid state.

The possible role of the cyclodextrin charge in the interaction with an acidic drug such as naproxen (pK(a) 4.8) has been evaluated. Sulfobutylether-B-cyclodextrin (SBE-β Cyd) and trimethylammonium-β-cyclodextrin (TMA-β Cyd) were selected as, respectively, anionically and cationically charged carriers and their performance was compared with that of the parent β-cyclodextrin (β Cyd) and of its methyl-derivative (Meβ Cyd) previously found as the best partner for the drug. Interactions in solution were investigated by phase-solubility, fluorescence and circular dichroism analyses. Equimolar drug-carrier products prepared by different techniques (blending, cogrinding, sealed-heating, colyophilization) were characterized by differential scanning calorimetry and X-ray powder diffractometry and tested for drug dissolution properties. Anionic charges of SBE-β Cyd did not negatively influence interactions in unbuffered aqueous solutions (pH ≈ 5) with the acidic drug. In fact, it was a very effective carrier, exhibiting solubilizing and complexing properties considerably better than the parent β Cyd and comparable to those of Meβ Cyd. On the contrary, the positive charges of TMA-β Cyd did not favour interactions with the counter-ionic drug (despite the presence of about 60% ionised drug) and it was less efficacious also than native β Cyd. Therefore, the role of the Cyd charge on the complexing and solubilizing properties towards naproxen was not important whereas other factors, such as steric hindrance effects and favourable hydrophobic interactions were significant in determining the drug affinity for the Cyd inclusion. Solid state studies evidenced similar amorphizing properties of both charged Cyds towards naproxen. On the other hand, dissolution tests, in agreement with solution studies, showed that all products with SBE-β Cyd exhibited significantly better dissolution properties than the corresponding ones with TMA-β Cyd. A clear influence of the preparation method of drug-Cyd solid systems on the performance of the end product was also observed. Colyophilization was the most effective technique, followed by the cogrinding one. Colyophilized product with SBE-β Cyd allowed a 10-times increase in drug dissolution efficiency (D.E.) (with respect to the five-times increase obtained with the corresponding coground product) and a reduction of t(50%) from about 60 min (for the coground product) to less than 2 min.