2.1.6. Solubility Profile of RSP/RM-β-CD Inclusion Complexes

The solubility of the drug substance in the inclusion complex has been evaluated using the shake-flask method [18,19,52]. The drug concentration in saturated solution was assessed using UV spectrophotometry. RM-β-CD in phosphate buffer 0.1 M (pH 7.4) does not present absorption in the spectral range of 210–310 nm (Figure 7a); an RSP calibration curve accomplished using the absorbance values from 277 nm at 25 ◦C (Figure 7b) was employed in order to quantify the drug in the inclusion complex.

**Figure 7.** The absorption spectra of RM-β-CD and RSP in phosphate buffer 0.1 M (pH 7.4), in the spectral range of 210–310 nm, at 25 ◦C (**a**); RSP calibration curve at 277 nm (**b**).

The solubility of the included RSP as KP obtained as an average of five experimental determinations is 1392.949 ± 0.016 µg/mL. In standard controlled experiments, clear solution was obtained when 29.31 mg of RSP/RM-β-CD KP were dissolved in 5 mL 0.1 M phosphate buffer (pH 7.4) at room temperature.

The results of the saturation solubility studies reveal RM-β-CD's ability to increase RSP solubility in phosphate buffer 0.1 M (pH 7.4) by 2.58-fold as compared with free RSP (540.007 ± 0.003 µg/mL).
