*3.2. Characterization and Selection of Mesoporous Carriers as Adsorbent Carrier*

The powder materials used as a carrier for preparation of liquisolid systems should have high liquid adsorbent power, and, at the same time, good flow and compaction properties, to allow uniform feed and reproducible filling of tablet dies and good tableting. Then, the flow and compaction properties of the mesoporous silicas and clay materials considered as potential adsorbent carriers for preparation of liquisolid systems, namely Aeroperl®300, Zeopharm®5170 and Neusilin®US2, were firstly investigated. The results of these studies, in terms of apparent and tapped density, Carr's Index and Hausner ratio, flowability (Copley test) and compactability (Wells test) are presented in Table 1.


**Table 1.** Apparent (DA) and tapped (DT) Density, Carr Index % (CI), Hausner Ratio (HR), flowability (as flow through an orifice), and compactability (determined by Wells test A, B and C) of Aeroperl®300, Neusilin®US2, Zeoparm®5170.

Based on Carr's Index and Hausner Ratio values, Zeopharm®5170 showed the best fluidity level, followed by Neusilin®US2 and then by Aeroperl®300. However, according to the flow Copley test, all powders presented excellent flowability, freely falling through the smallest apparatus hole (4 mm diameter). As for the compaction properties, the results of the Wells test showed that all powders samples exhibited a fragmenting behavior since similar crushing strength values were obtained in the different conditions of the test (A≈B≈C). This is considered a desirable characteristic of powders to be compressed, since the crushing strength of tablets made with fragmenting materials should be less negatively affected by the presence of hydrophobic lubricant, such as Mg stearate, with respect to plastic-deforming materials [55,56]. Neusilin®US2 gave rise to the tablets with the highest hardness, but acceptable breaking strength values (around 30 N) were obtained also with Aeroperl®300. On the contrary, the crushing strength values of Zeopharm®5170 tablets were very low, probably due to its poor binding properties, and then this excipient was discarded in subsequent studies.

The selected mesoporous clay and silica were then further characterized by BET analysis. Both compounds showed a very extended specific surface area, which confirmed their highly porous nature. However, the chosen clay Neusilin®US2 showed a clearly greater surface area (355.5 vs. 268.4 m2/g) and also a significantly higher volume (1.0 vs. 0.5 cm3/g) of mesopores (i.e., pores in the 2.0–50 nm range) compared to the chosen silica Aeroperl®300.

#### *3.3. Preparation and Characterization of Liquisolid Systems*

Liquisolid systems were then prepared by dissolving 5 mg GLY (drug therapeutic single dose) in 0.05 mL of the selected solvents (DMA or 2-PYR), and then gradually adding, under continuous mixing, Neusilin®US2 or Aeroperl®300, selected as carrier-coating materials. The strong adsorptive power of the selected mesoporous clay and silica allowed to use a liquid load factor (i.e., the w/w ratio of liquid medication to the carrier powder) of 1.1, clearly higher than the values commonly used in conventional liquisolid systems [22,27,29,47], and obtain dry-looking powders, with practically unchanged flow properties (measured according to the Copley flow test) compared to the respective pure carriers.

The obtained liquisolid systems were then characterized by LPS for granulometric distribution and compared with the corresponding pure carriers. As can be observed in Figure 2, both Aeroperl®300 and Neusilin®US2 exhibited a satisfactorily homogeneous distribution curve, with a mean volumetric diameter of 31 μm and 76 μm, respectively. The formation of liquisolid systems did not substantially change the original granulometric distribution of the corresponding carriers, irrespective of the type of solvent used, indicating in all cases the absence of appreciable aggregation phenomena and the obtainment of homogeneous systems looking as dry powders.

**Figure 2.** Granulometric analysis, by Laser Particle Size (LPS), of pure Aeroperl®300 and Neusilin®US2, and of the corresponding liquisolid systems with glyburide (GLY) containing 2-pyrrolidone (2-PYR) or dimetylacetamide (DMA) as non-volatile water miscible solvent.

DSC analyses were performed on pure drug and mesoporous clay and silica and on the corresponding liquisolid systems, in order to evaluate possible solid-state modifications or interactions between the components (Figure 3). The DSC curve of GLY was typical of a crystalline, pure, anhydrous compound, showing a flat profile before the sharp endothermic peak at 175 ◦C (ΔH 180 J/g) due to the drug melting. On the contrary, the thermal curves of both carriers indicated their amorphous nature, being characterized by a broad endothermal band in the range 70–140 ◦C, due to evaporation of associated water molecules, followed, in the case of Neusilin®US2, by another endothermic effect at a higher temperature (240 ◦C), due to decomposition phenomena.

**Figure 3.** Differential Scanning Calorimetry (DSC) curves of pure glyburide (GLY), Aeroperl®300 and Neusilin®US2 and of the corresponding liquisolid systems with 2-pyrrolidone (2-PYR) or dimetylacetamide (DMA) as non-volatile water miscible solvent.

DSC curves of liquisolid systems containing 2-PYR as a solvent showed two broad endothermic effects: the first, which peaked around 100 ◦C, was due to the carrier (Neusilin®US2 or Aeroperl®300) dehydration, while the second one, which peaked around 240 ◦C, was due to the solvent evaporation (boiling point 245 ◦C), partially superimposed, in the case of Neusilin®US2, to the carrier decomposition phenomena. An analogous thermal behavior was displayed by liquisolid systems prepared with DMA as a solvent; however, in this case, due to the lower boiling point of this solvent (165 ◦C), a partial superimposition of the carrier dehydration band to that of solvent evaporation happened. Interestingly, in all cases the complete disappearance of the drug melting peak was observed, which can be considered indicative of drug amorphization and/or solubilization in the liquisolid system, i.e., its almost molecular dispersion within the liquisolid matrix [26,27].

The results of PXRD studies were substantially in agreement with those of DSC analysis. In fact, as can be observed in Figure 4, the diffraction pattern of pure GLY exhibited the presence of numerous sharp peaks, in particular at 18.8, 19.4, 20.8 and 22.9◦ 2Θ, indicative of its crystalline nature, while a diffuse halo pattern, characteristic of amorphous powders, was shown by both mesoporous clay and silica.

The typical diffraction peaks of the drug were no more detectable in the X-ray spectra of all the liquisolid formulations, thus confirming the conversion of the drug in an amorphous or solubilized form within the liquisolid matrix, as suggested by DSC analysis, and allowing to exclude any possible artifact of this last technique, due to the sample heating during the scan.

**Figure 4.** X-Ray powder diffraction patterns of pure glyburide (GLY), Neusilin®US2 and Aeroperl®300 and of the corresponding liquisolid systems with 2-pyrrolidone (2-PYR) or dimetylacetamide (DMA) as non-volatile water miscible solvent.

The ESEM outcomes (Figure 5) further supported the results of DSC and XRPD analyses. The ESEM image of pure micronized GLY showed its crystalline nature and its very homogeneous particle size. Neusilin®US2 appeared instead as particles of almost spherical form, with a highly porous surface,

and Aeroperl®300 as spherical particles, many of which characterized by the presence of an internal cavity. The morphology of liquisolid systems obtained with both Neusilin®US2 or Aeroperl®300 as carrier-coating material was very similar to that of the corresponding pure samples, thus confirming their excellent adsorbent power and the absence of agglomeration phenomena.

**Figure 5.** ESEM images of pure glyburide (**A**), Aeroperl®300 (**B**) and Neusilin®US2 (**C**) and of the corresponding liquisolid systems (**B'** and **C'**).

Dissolution studies were then performed to evaluate the ability of the various liquisolid systems to improve the GLY dissolution rate and select the more effective ones for the preparation of liquisolid tablets. As can be seen in Figure 6, all the developed liquisolid systems showed a very marked improvement of GLY dissolution rate compared to the plain GLY, with an about 27 times increase of percent drug dissolved after only 2 min, and an about 4 times increase at the end of the test (60 min). No statistically significant (*p* > 0.05) differences were found among the different kinds of liquisolid systems and then they were all employed for tablet preparation.

**Figure 6.** Dissolution profiles of glyburide (GLY) as such or from the different liquisolid systems.
