*3.3. Fourier Transform Infrared Spectroscopy*

FTIR spectra of PZQ, SEP and VHS were previously described in detail [33,43,46,47]. In Figure 4, the two most intense bands of the PZQ appear at 1665–1621 cm−1, which correspond to the ν(C=O) stretching vibration mode. The spectra of sepiolite and montmorillonite showed intense bands at 1200–800 cm<sup>−</sup>1, corresponding to the ν(Si–O) and δ(MOHM') (M, M' = Al, Mg) vibrations [47].

**Figure 4.** Fourier Transform Infrared (FTIR) spectra of sepiolite products (**a**) and of montmorillonite products (**b**).

In the interaction products prepared with sepiolite (Figure 4a) and montmorillonite (Figure 4b), the bands of the pristine PZQ were found, such as the bands that appeared at 1500–1400 cm<sup>−</sup>1, which are characteristic of the δ(CH) bands of bending vibrations. This corroborated that the drug is present in the drug–clay interaction products regardless of the solvent used in the elaboration process, but the bands of the PZQ are masked by the clay in all cases, since the clay is in a much higher proportion.

#### *3.4. In Vitro Release Studies*

Drug release profiles of the interaction products with sepiolite (PZQ–SEPet, PZQ–SEPac and PZQ–SEPdic) and montmorillonite (PZQ–VHSet, PZQ–VHSac and PZQ–VHSdic) compared with the pristine PZQ were studied in sink conditions in acid aqueous medium with HCl 0.001 M (pH = 3) and in simulated intestinal fluid (pH = 6.8) (Figure 5).

The PZQ–SEP interaction products in acid medium revealed that all the products increased the dissolution rate of the PZQ drug. This enhance in the drug release profile is higher in the following order PZQ–SEPdic > PZQ–SEPet > PZQ–SEPac (Figure 5a). The same results were obtained in the SIF medium (Figure 5b). Therefore, the interaction products with sepiolite increase the dissolution rate of the PZQ greatly using any of the three solvents in the preparation method, with PZQ–SEPdic being the one that showed the best results in both media. This higher dissolution rate is due to the loss of crystallinity of PZQ that occurred during the intercalation in sepiolite. The amorphization overcomes the high cohesion energy in the packing of the PZQ crystal lattice [43] needed for its dissolution. In particular, this amorphization is higher in PZQ–SEPdic (Figure 2a) and, hence, its dissolution rate is the highest one.

The PZQ–VHS interaction products showed differences between them in acid medium (Figure 5c). In this medium, a strong increase in the dissolution rate of the drug was observed in the PZQ–VHSac product, whereas PZQ–VHSet showed only a slightly higher dissolution rate than the pristine PZQ. On the contrary, a surprising result of PZQ–VHSdic was found in acidic medium. In this case, the drug is released at a constant rate. This, in principle, leads to better control of plasma concentration and offers

several advantages. Therefore, the PZQ–VHSdic in acidic medium showed a release that is interesting for new PZQ controlled release systems (Figure 5c). In simulated intestinal fluid, all the PZQ–VHS interaction products showed an increase in the dissolution rate with respect to the pristine PZQ drug. The material prepared with acetonitrile (PZQ–VHSac) presented the fastest release profile, followed by PZQ–VHSet and PZQ–VHSdic, which also presented a similar profile. In general, the increase in the dissolution rate of the PZQ–VHS interaction products also owes to the amorphization of PZQ. The peculiar behavior of PZQ–VHSdic at low pH can be due to a possible effect of the pH on the opening of the pores and interlayers of VHS that does not occur at higher pH, where the swelling is faster. Among the solvents used in this work, the dichloromethane is the only solvent non-miscible with water. It is likely that this behavior changed the macroscopic structure of the solid.

**Figure 5.** In vitro drug release profiles of sepiolite interaction products in HCl 0.001 M (**a**) and in simulated intestinal fluid (**b**), and of montmorillonite interaction products in HCl 0.001 M (**c**) and in simulated intestinal fluid (**d**); (mean values ± 6 SD; *n* = 7).

Moreover, in general, the PZQ–clay dissolution rate was higher in acidic medium (pH = 3) than in simulated intestinal fluid (pH = 6.8), in concordance with the results in no-sink conditions previously obtained in PZQ–clay interaction products prepared with ethanol [33]. Therefore, the interaction of the drug with the clay minerals induced an increase in dissolution rates with the independence of pH. This improvement was demonstrated in all interaction products with sepiolite in both media, and with montmorillonite in SIF medium. The interaction products with montmorillonite in acid medium showed a different behavior between them, with the PZQ–VHSac being the one with the highest dissolution rate.

Therefore, according to the results obtained in vitro, PZQ–VHS and PZQ–SEP interaction products might improve the oral bioavailability of the drug by increasing both dissolution rate and amount of drug dissolved in the medium that simulates the stomach, obtaining new accelerated oral release systems of the PZQ. Moreover, the PZQ–VHSdic in the acidic medium could be a new controlled release system of the drug.

Subsequently, the experimental dissolution data of PZQ, PZQ–SEP and PZQ–VHS interaction products were fitted to various kinetic models in order to analyze the drug release. The correlation coefficient (R2) and Akaike Information Criterion (AIC) obtained from the dissolution model's fitting are summarized in Tables S1 and S2. In general, the results showed that zero-order and first-order models were not appropriate to study these dissolution kinetics. In the same way, Square Root (Higuchi) and Power Law (Peppas) are not appropriate for the adjustments of the experimental dissolution values since there are no sufficient values lower than 63.2% of the drug release to adjust a kinetic [37]. In Tables S1 and S2, correlation coefficient and AIC values of PZQ, PZQ–SEPac, PZQ–SEPdic, PZQ–SEPet, PZQ–VHSac, PZQ–VHSet in acid and SIF media suggested that the Weibull model could be considered as an adequate model to describe the release kinetic, because the R2 obtained was the highest value and AIC value was the lowest once compared with the rest of the proposed models. The Weibull model presented an initial burst release [48,49], which is increased in the interaction products with clays. However, PZQ–VHSdic in an acid medium (pH = 3) showed a different dissolution profile, as can be seen in Table S2 and Figure 5c. In this case, the results suggested that the Cube Root (Hixson–Crowell) is the more accurate model to describe the release kinetics. Therefore, the PZQ–VHSdic IP in an acid medium (pH = 3) presented a release that is controlled by the dissolution rate of the drug particles and it is assumed that the particles of the interaction product are isometric and monodisperse. This geometric shape of the particles remains constant and there is a decrease in the surface area associated with the dissolution of the pharmaceutical form [50,51].

#### *3.5. Solubility Studies*

The PZQ–SEPet and PZQ–VHSet showed an increase in the solubility compared to the pristine drug in the studied media. The results in the acid aqueous medium with HCl 0.001 M (pH = 3) indicated that the solubility of the PZQ–SEPet and PZQ–VHSet interaction products were similar in both cases (0.71 and 0.74 mg/mL, respectively). Similar results are obtained in simulated intestinal fluid (pH = 6.8), where the solubility of PZQ–SEPet and PZQ–VHSet was 0.61 and 0.65 mg/mL, respectively. Therefore, the interaction products enhanced the solubility of the drug, and this increase was in the range of 36–48% with respect to pristine PZQ (Table 1).

**Table 1.** Solubility values (in mg/mL) of praziquantel (PZQ), PZQ–SEPet and PZQ–VHSet and increase solubility (in %) with respect to that of PZQ in acid and SIF media (solubility mean values ± 0.07 SD; *n* = 3).

