The addition of matrix formers within a formulation provides a means for enhancing the redispersibility of nanoparticles (NPs) enabling them to retain their advantageous properties imparted onto them by their sub-micron size. In this work, NPs were isolated in the solid state via spray drying with a range of sugars. The processed powders were characterized, establishing that itraconazole (ITR) nanostructured microparticles (NMPs) spray dried in the presence of mannitol and trehalose had favorable redispersibility confirmed by dynamic light scattering and nanoparticle tracking analysis. Solid-state analysis confirmed the crystalline nature of NMPs based on mannitol and the amorphous character of trehalose-based NMPs. The NMPs powders were compacted at a range of pressures, producing tablets with high tensile strength without compromising their disintegration time. A greater amount of ITR was solubilized from trehalose NMPs compared to the mannitol-based compacts in 0.1 M HCl, showing a promise for enhanced in vivo activity. Overall, as trehalose exhibited superior carrier properties for ITR NMPs, this type of excipient included in the formulation warrants careful consideration. The structured approach to matrix former selection and tabletting studies can reduce the amount of material and time required for testing in the initial stages of product development.
New drug candidates generally have poor water solubility and/or dissolution rates. Nanoparticles (NPs) benefit from high surface-area-to-volume ratio, thus exhibit improved dissolution rates, and often demonstrate increased solubility resulting in enhanced bioavailability (1). Dissolution rate enhancement imparted by NPs may exist regardless of fasted or fed state, while the absorption behavior of NPs was found to be unaffected by bile salt concentration (2). In addition, NPs may exhibit improved adhesiveness further contributing to their enhanced bioavailability, can be manipulated to achieve highly specific site delivery, and have the ability to transverse different biological barriers through small capillaries and into cells (3,4). Aqueous nanodispersions, especially those prepared by the bottom-up approach, are however associated with problems such as aggregation and term instability. To overcome these issues, they can be isolated in the dry state (5). A variety of drying techniques exist for the isolation of NPs from liquid to powder form including freeze drying, spray drying, and spray-freeze drying (6).
Spray drying is a process whereby a liquid feed is atomized into a fine spray and dried using a hot gas resulting in evaporation of solvent (7). This process is considered industrially preferable due to its favorable speed and cost and product characteristics can be fine-tuned via careful manipulation of process parameters. Isolation of NPs by spray drying is usually based on co-processing with carrier/matrix materials and thus converting NPs into easily redispersible powders (8). For instance, candesartan cilexetil NPs prepared by wet bead milling were converted into powders using a spray drying process. Mannitol was used as the bulking agent and its amount used was 73.33% (w/w) with respect to candesartan cilexetil to ensure adequate redispersion of the NPs (9). It has been established that during secondary processing additional stresses can destabilize the NPs (10) and solid-state form change of the matrix material and/or NPs via input of energy can be induced (11). An understanding of the impact of critical process parameters upon product characteristics such as NP redispersibility is an essential part of the formulation process. Hence, careful consideration must be given to matrix former type and concentration used in the downstream processing of NPs. Chaubal and Popescu previously investigated the impact of surfactants and sugars on the processing and redispersibility of itraconazole (ITR) NPs obtained by microprecipitation–homogenization with mannitol reported to be the most favorable carrier for spray drying of the NPs (10). Mou et al. spray dried ITR NPs made by facile acid-base neutralization using either Poloxamer 407 or hydroxypropyl methylcellulose as stabilizers also using mannitol as the bulking agent (12).
While the literature is rich in information regarding spray drying of nanodispersions, fewer reports exist regarding their processing into solid oral dosage forms. Spray drying, as a continuous, one step process, has a great advantage to become the process of choice for producing powders that might be directly compressible into a tablet. During consolidation of a powder bed, porosity of the material decreases, the compact volume is reduced, particles move closer to one another, bonds are created, particles stick together, and a compact is formed (13). Wetting and disintegration of particles should be good to maintain NP dissolution enhancement. From the compaction point of view, appropriate matrix formers might be water soluble sugars including mannitol, lactose, trehalose, and raffinose (14,15). Sugar-based powders containing NPs formed during spray drying should therefore rapidly redisperse when hydrated to release NPs with their original particle size. Thus, particle dissolution should occur in a short time frame to facilitate drug absorption in vivo (10). Nevertheless, production of a dosage form with sufficient tensile strength, capable of rapid release of NPs, presents a challenging task (16).
This work explores the feasibility of production of easily redispersible nanostructured microparticles (NMPs) via spray drying, as a one step process, with subsequent testing of the powders for their suitability in compaction studies. Itraconazole (ITR) was made into nanodispersions via a bottom up anti-solvent precipitation method as described and characterized previously (17,18,19,20). Subsequently, saccharides were added to these aqueous nanodispersions prior to spray drying. The obtained NMPs were then compacted into solid oral dosage forms. A number of sugars were investigated and assessed in terms of their ability to adequately redisperse ITR NPs. ITR was selected as a model drug as being a Biopharmaceutics Classification System (BCS) class II compound, it exhibits poor aqueous solubility as well as a strong food effect (21). In this study, five different water-soluble sugars and one sugar alcohol (collectively referred to later as sugars) exhibiting a variety of physical properties were explored for inclusion as potential matrix formers. ITR NMPs were characterized in terms of their solid state and physical properties, particle morphology, and redispersibility. A subset of these formulations was selected for further tabletting studies. Resulting tablets were examined using a number of standard or modified pharmacopeial tests. The impact of the presence of ITR NPs upon the processability and tabletability of SD sugars was assessed. This work thus encompasses the complete pharmaceutical manufacturing process, starting with the making of NPs through to final solid dosage form design. During the initial stages of formulation development, often only limited amounts of material are available. Tests performed in this work are done so on a small scale, making them beneficial in terms of cost and time during product development from an industrial perspective.
MATERIALS AND METHODS
Itraconazole (ITR) was a gift from Welding GmbH (Hamburg, Germany). Trehalose dihydrate, D-mannitol, sucrose, raffinose pentahydrate, maltose monohydrate, and lactose monohydrate were all obtained from Sigma-Aldrich (Arklow, Ireland). Acetone Chromasolv® HPLC grade was obtained from Sigma-Aldrich. Milli-Q water was used in all instances.
Details regarding preparation of ITR nanodispersions were presented in previous work (14,15). Solutions of ITR dissolved in acetone (solvent phase) and water (anti-solvent phase) were maintained at 25°C. Solvent phase was filtered using a 0.45-μm syringe polytetrafluoroethylene (PTFE) filter (VWR, Ireland), and anti-solvent phase was filtered using a hydrophilic PTFE 0.2 μm syringe filter (VWR, Ireland) prior to NP formation. ITR NPs were formed in a round bottom flask using a solvent phase (2 mg/ml ITR in acetone = 80% saturated solution) rapidly injected via a Sterican 12 mm needle into an anti-solvent phase, using a 1:10 solvent to anti-solvent (v/v) ratio (2 ml solvent phase added to 20 ml anti-solvent phase). The dispersion was subsequently transferred to a rotary evaporator and subjected to 60 mbar pressure, 160 rpm at 30°C for 30 min in order to evaporate acetone. The particle size of NPs was measured after the acetone evaporation process by dynamic light scattering (DLS) as described below, and it varied between 203 and 266 nm with a polydispersity index of 0.04–0.19 depending on the batch.
ITR nanodispersions were prepared as described above. Solid sugar was added to the liquid medium (20 ml) to achieve the desired sugar (taking an anhydrous equivalent) concentration, 1, 3, or 5% (w/v), and this was used as the liquid feed for NMP production using a Buchi B-290 mini spray dryer (Flawil, Switzerland) with a 1.5-mm cap and 0.7-mm tip. The pump speed was 30% (9–10 ml/min), the aspirator was set to 100%, and a mixture of nitrogen (with a pressure of 6 bar) and air was used as a drying gas (22). Inlet temperature was set to 160°C (23) and selection of this parameter was also informed by an in-house statistical factorial design study. The ITR/sugar ratios in the dry products were as follows, 1:50 (w/w) for 1% (w/v) sugar feed, 1:150 (w/w) for 3% (w/v) sugar feed, and 1:250 (w/w) for 5% (w/v) sugar feed. Sugar solutions (5% (w/v)) without NPs, made in deionized water, were also spray dried for comparison purposes and ITR microparticles were processed from dichloromethane as described previously (17).
Powder X-Ray Diffraction
Powder X-ray diffraction (PXRD) was carried out at room temperature using a Rigaku Miniflex II, desktop X-ray diffractometer (Tokyo, Japan) equipped with a Cu Kαradiation X-ray source and a Haskris cooler (IL, USA). The samples were mounted on a low background silicon sample holder and scanned over a 2θ range 2–40° with a step width of 0.05, scan rate 0.05° per second, and signal collection time of 1 s per step. The output voltage and current of the tube (Cu, 1 kW normal focus) were 30 kV and 15 mA, respectively. PXRD diffraction patterns for alpha, beta, and delta mannitol were obtained using Cambridge Crystallographic Data Centre (CCDC) reference codes; DMANTL 08, DMANTL 09, and DMANTL 10 (24).
Differential Scanning Calorimetry
Differential scanning calorimetry (DSC) was performed using a Mettler Toledo DSC (Schwerzenbach, Switzerland). The purge gas was nitrogen. Approximately 5–10 mg samples were analyzed in sealed 40-μl aluminium pans with pierced lids. Samples were heated from 25 to 250°C at a rate of 10°C/min. Thermograms were analyzed using the Mettler Toledo STARe software (version 6.10).
Thermal Gravimetric Analysis
Thermal gravimetric analysis (TGA) was carried using a Mettler TG50 measuring module coupled to a Mettler Toledo MT5 balance (Schwerzenbach, Switzerland). Approximately 2–5 mg samples were analyzed in an open aluminium pans, using nitrogen as the purge gas. Samples were heated from 25 to 100°C at a rate of 10°C/min. The Mettler Toledo STARe software (version 6.10) was used to analyze thermograms obtained.
Dynamic Vapor Sorption
Powdered samples were analyzed with a Dynamic Vapor Sorption (DVS) Advantage-1 automated gravimetric vapor sorption analyzer (Surface Measurement Systems Ltd., London, UK). The temperature was maintained constant at 25.0 ± 0.1°C. In all measurements, approximately 30 mg of powder was loaded into a sample net basket and placed in the system. All samples were equilibrated at 0% of RH until constant mass (dm/dt ≤ 0.002 mg/min). The reference mass was recorded as a mass equilibrated at 0% RH. Sorption-desorption analysis was then carried between 0 and 90% RH, in steps of 10% RH. At each stage, the sample mass was equilibrated (dm/dt ≤ 0.002 mg/min for at least 10 min) before moving to the next RH level. Two sequential cycles of sorption and desorption were performed. An isotherm was calculated from the complete sorption and desorption profile. PXRD was performed on all samples following DVS analysis to detect possible crystallization. DVS analysis was performed in duplicate.
Dynamic Light Scattering
Particle size measurements were performed at two stages for nanodispersions: upon removal from the rotary evaporator and after redispersion of ITR NMPs in deionized water using dynamic light scattering (DLS). Measurements were obtained using a Zetasizer Nano ZS (Malvern Instruments, UK). Measurement position and attenuator factor were automatically optimized by the software. All measurements were carried out at 25°C in a folded zeta cell (DTS1061). The analysis was performed in triplicate for each sample. Viscosity of the continuous phase was measured using a Vibro Viscometer SV-10 (A&D, Japan) at 25°C and particle size values corrected for the actual viscosity of the dispersion medium. Redispersibility index was used as an assessment tool to evaluate the redispersibility of NMPs and calculated as follows:
Nanoparticle Tracking Analysis
Particle size characterization of ITR NMPs was performed with a NanoSight NS300 and analyzed using the nanoparticle tracking analysis (NTA) 3.2 software as described previously (20). A defined mass of NMPs was redispersed in 1 ml of deionized water producing a dispersion containing the original concentration of sugar (as in the spray drying feed) or 1% (w/v) sugar. Samples were injected into the sample chamber using disposable syringes. All measurements were performed at 25°C. The software was programmed to capture five 30-s videos, with the sample being advanced in between each video. Samples were measured using manual shutter and gains adjustments.
Scanning Electron Microscopy
Scanning electron microscopy was performed using a Zeiss Ultra Scanning electron microscope (Germany) equipped with a secondary electron detector. Samples were sputter coated with gold palladium for 90 s under vacuum prior to analysis and analyzed using a 6-kV accelerating voltage. Analysis of spray-dried powders of NMPs was performed in addition to redispersed ITR NPs. NPs were redispersed by addition of a defined mass of NMP powder to deionized water, followed by gentle, manual shaking. Sample aliquots were mounted directly onto aluminium stubs. Sample surface was washed using deionized water to remove traces of sugar and left to dry under nitrogen purge overnight. Dry NMP powders were glued onto carbon tabs and mounted onto aluminium stubs prior to coating.
Flat-faced tablets (200 mg) were compressed using a Natoli NP-RD10 (Saint Charles, MO, USA) laboratory scale single punch tablet press supplied with an Enerpac (Menomonee Falls, WI, USA) P-392 manual pump with a RC-104 hydraulic cylinder and standard 8 mm diameter punch and die tooling (I Holland, Limited, UK). Compaction properties of tablets were assessed using a number of different compaction pressures in the range of 49 to 249 MPa. Pressure was released 60 s after the desired compaction pressure was achieved.
Tablet Hardness, Tensile Strength, Solid Fraction, Density
Weight (Wt in g) of freshly produced tablets was recorded immediately after compaction. Tablet thickness (t in cm) and diameter (D in cm) were measured by an electronic caliper. Crushing strength (F) is a measure of the load at which a tablet breaks, whereas tensile strength (σ) is a measurement of the resistance to fracture. Hardness (N) of the tablets was measured using a portable tablet hardness tester (Electrolab, India). Each hardness value reported is an average of five measurements. Tensile strength (σ) of the 8 mm round flat faced tablet was calculated based on breaking force values using Eq. 1:
where d is tablet diameter and h is tablet thickness. Compaction pressure was then calculated based on the applied force and cross-sectional area of the punch. Solid fraction (SF) and porosity (ε) were calculated based on true density (ρtrue) of SD powders which were measured using a gas pycnometer AccuPyc II 1340 (Micrometrics, GA, USA). SF was calculated according to true density (ρtrue, given in Table. SI.1), tablet volume (v) and weight (Wt) as per Eq. 2:
Porosity (ε) was calculated based on SF as per Eq. 3:
SEM, PXRD, and NTA Analyses
Subsequent to hardness testing, broken cross sections of tablets were monitored for changes using SEM, with samples being glued onto carbon tabs and mounted onto aluminium stubs prior to coating.
Following compaction under pressure, material was analyzed using PXRD. A defined mass of material consisting of ITR NMPs was redispersed in deionized water following compaction into tablets. This liquid medium which contained redispersed ITR NPs was analyzed using NTA. SEM, PXRD, and NTA analyses were performed as described above.
Disintegration of tablets was studied using the disintegration tester, Dist 300 (PharmaTest, Germany) as per the United States Pharmacopeia (USP) compendial test (701) Disintegration (25). A volume of 900 ml of deionized water at 37°C was used as the media and disintegration time was recorded. All samples were measured in triplicate.
Dissolution of ITR NPs Released from Tablets in 0.1 M Hydrochloric Acid
Disintegration of tablets made of ITR NMPs with subsequent evaluation of ITR NP dissolution was performed in 500 ml 0.1 M hydrochloric acid (HCl), with the intent of replicating the gastric conditions, using the disintegration tester, Dist 300 apparatus, at 37°C. Dissolution studies on an equivalent amount of spray-dried ITR were also conducted for comparison purposes. Sample aliquots (2 ml) were taken at specific time points over a 30-min period of study. Each sample was filtered through a 0.1-μm PTFE membrane filter (Sartorius Stedim, Germany) (17,26). The concentration of ITR in each sample was then measured using a Shimadzu Pharmspec UV-1700 UV-vis spectrophotometer (Japan) at 262 nm without dilution using low volume 10-mm quartz cuvettes. Analysis was performed in triplicate.
Statistical analysis was performed using the Minitab 16 software. Data was analyzed using two-sample student t tests or a one-way analysis of variance (ANOVA) with Tukey’s multiple comparison test. A p value of ≤ 0.05 was considered significant.
RESULTS AND DISCUSSION
Spray Drying of Itraconazole Nanostructured Microparticles (ITR NMPs)
In the liquid dispersion, NPs have physical and chemical stability issues (27). Transformation of nanodispersions into solids can overcome these issues. Spray drying offers a viable approach for the production of intermediate powders which can undergo further processing steps, resulting in the production of optimal dosage forms for patient use. The addition of matrix formers prior to drying prevents particle aggregation while enhancing redispersibility of NPs upon hydration (28). Tables I and II contain information regarding the physical properties of matrix formers studied in this work.
As all ITR NMPs spray dried with sugars were subjected to identical processing conditions within the spray dryer, any differences observed on the micro and nano-scale can be attributed to the presence of the sugar itself. Figure 1 confirms the presence of a MAN polymorphic mixture obtained upon spray drying, consisting of beta and alpha polymorphs.
The simultaneous presence of different mannitol polymorphs produced upon spray drying is well documented (33). All other sugars investigated were amorphous upon spray drying, as verified by PXRD and DSC analysis (Figs. SI.1 to SI.5). Glass transition temperatures (Tgs) for all amorphous SD powders were consistent with literature values provided in Table I, being below 115°C. This was followed by an exothermic event indicative of crystallization and finally a melting endotherm. The amorphous form is not at thermodynamic equilibrium, meaning these materials have a strong tendency to revert back to their stable crystalline state (29). TGA analysis of all SD sugars processed from 5% (w/v) solutions confirmed the weight loss to be between 4.7 and 7.1%, despite the use of a high inlet temperature (160°C) in this case.
Figure 2 presents findings from the morphological examination of SD samples.
Morphological examination of SD powders revealed spherical morphology generally, with LAC and TRE NMPs having a more fused appearance (Fig. 2a, c) compared with MAN and RAF equivalents, which are more discrete in nature (Fig. 2e, g, Fig. SI.6). Notably, the morphology of SD powders does not appear to affect the NP size upon redispersibility (Fig. 3 and Table III).
Surface pores and hollow particles were evident particularly for LAC (Fig. 2a), MAN (Fig. 2e), and RAF (Fig. SI.6) NMPs. The inlet temperature 160°C employed in this work would be considered in the high range of temperatures normally utilized for the spray drying of nanodispersions, with literature reporting 80 to 150°C as being typical (9,10,12); however, this allowed to minimize the residual moisture levels in the samples. Hollow particles therefore seem likely to be present in all samples as these are determined by evaporation rate which is a function of inlet temperature in the spray dryer. Figure 2g and Fig. SI.6 confirm that ITR NPs are embedded within most likely hollow sugar MPs. SEM images of redispersed powders confirm the presence of NPs and their nano-size range (Fig. 2). NPs from redispersed LAC and TRE NMPs (Fig. 2b, d) show an irregular appearance with a smooth surface texture compared with those contained in MAN and RAF NMPs, which are uniform (Fig. 2f, h).
Upon closer inspection, DLS measurements of dispersion formed by LAC NMP samples appeared to have two populations: one smaller than the original particles, alongside a dominant larger sized population. This pointed towards a possible interaction between LAC and the amine moiety of ITR, a feature noted elsewhere in the literature (37). For this reason, LAC was excluded from further investigation. With respect to redispersibility, assessed via DLS and NTA (Fig. 3 and Table III), TRE, SUC, and MAN displayed the best redispersibility and showed the smallest NTA particle size values, ranging from 127 to 139 nm, again making them potential options warranting further exploration. In terms of ranking redispersibility by DLS and NTA, good agreement was demonstrated between the techniques with RAF displaying the largest particle size upon redispersion with both techniques (Fig. 3 and Table III). DLS is an intensity-based technique, where larger particles have the potential to skew the particle size distribution towards larger sizes. NTA is a number-based technique, where all particles are given equal weighting within the distribution regardless of their size. Also, DLS sizes accurately in the range of 10 to 10 μm whereas NTA sizes between 30 nm and 1 micron (20). With this in mind, large undispersed aggregates present in samples would generate larger average particle sizes for DLS compared with NTA. These micron-sized aggregates may also not be seen with NTA. The abovementioned factors may collectively lead to NTA generating smaller particle size values. It seems likely that the poor redispersibility of ITR NPs, imparted by the presence of RAF as a matrix former (Fig. 3 and Table III) may be ascribed to its lower aqueous solubility, in comparison to other investigated sugars, as denoted in Table I. This may result in a slower dissolution rate and therefore release of ITR NPs into the surrounding aqueous medium.
Upon spray drying into ITR NMPs, SUC immediately presented processability issues, namely extremely poor flowability and a “stickiness” making it impossible to work with. For this reason, it was discarded from further investigations. Amorphous, TRE-based ITR NMP samples were placed inside a desiccator and stored at 4°C for 4 days. Subsequently, this sample was assessed using PXRD, for evidence of crystallization. Figure SI.5 confirms no crystallization occurred during that time. Despite MAN exhibiting polymorphism as shown in Fig. 1a, b, its confirmed crystallinity upon spray drying set it aside as a suitable candidate for further exploration. Moving forward, all efforts were focused on MAN and TRE as potential matrix formers for enhancing the redispersibility of ITR NPs upon rehydration.
Physical Stability of ITR NMP Powders
Upon consideration of the different sugar concentrations employed in initial spray drying tests, 5% (w/v) concentration was selected for further studies. A favorable production yield was the basis for this decision. Achieving an optimum yield should not be overlooked as a desirable process output from an industrial perspective (38). As TRE and MAN showed the most promise for use as ITR NP matrix formers, physical stability of the sugars, processed with and without NPs, were investigated by DVS. Figure 4 presents isotherms for DVS analysis. PXRD analysis of all samples post-DVS testing is shown in Fig. SI.7.
Cycle 1 of sorption/desorption showed a moisture uptake of 0.18% for SD MAN (on its own) and 0.13% for MAN ITR NMPs up to 90% RH (Fig. 4a). Between 70 and 80% RH, a decrease in mass was observed in both MAN samples. The kinetic profile presented in Fig. 4b confirms this event at 80% RH, markedly so for ITR NMPs. It is likely that these events are caused by the moisture-induced reorganization of small crystallites present within the sample. Furthermore, simultaneous crystallization of a small number of amorphous regions present within these samples introduced as a result of the spray drying process is likely. As gravimetric sorption techniques are capable of quantifying amorphous content below 1% (39), PXRD is not capable of detecting these low levels of disorder (Fig. SI.7a to f). The decrease in mass occurs earlier for the NMP powder in comparison to the SD powder with no NPs, suggesting that moisture can penetrate the sample more easily, maybe due to extra voids created by the presence of NPs. However, the greater porosity of the NMPs did not result in an increased moisture sorption, perhaps due to the high crystallinity of MAN. Furthermore, DVS did not appear to induce any polymorphic changes in MAN samples (Fig. SI.7c to f).
Cycle 1 of sorption/desorption shows a remarkable moisture uptake of amorphous SD TRE powders up to 50% RH (Fig. 4c). TRE SD alone showed a mass increase of 11.85% at 50% RH, with ITR NMPs exhibiting a similar trend with a comparable mass of 11.84% at 50% RH (Fig. 4c). Subsequently, this is followed by a decrease in mass resulting from expulsion of water caused by crystallization of amorphous regions in the samples (40). This RH threshold is unaffected by the presence of ITR NPs (Fig. 4c). The crystalline state of all TRE samples was confirmed by PXRD analysis after DVS testing (Fig. SI.7 g to l). Cycle 2 of sorption/desorption for TRE SD powders shows little moisture uptake, suggesting all of the amorphous content of the samples crystallized during cycle 1 (Fig. SI.8b). Excess water retained within the crystal lattice of both TRE samples is responsible for the desorption isotherm not returning to its original value (Fig. SI.8b). This sugar has the propensity to crystallize to its dihydrate form, supported by a 10–11% weight gain seen in Fig. 4c. Similar to the mannitol SD samples, solid-state changes in TRE-based ITR NMPs occurred quicker than in SD TRE processed without NPs, although TRE NMPs took much longer to achieve a stable mass at 60% RH compared with SD TRE.
Due to a limited availability of material, a range of compaction pressures were screened using the SD powders without NPs and the optimum selected for ongoing tabletting studies including ITR NMPs.
A continuous increase in tensile strength was achieved up to a specific limit for both MAN and TRE tablets (studies on SD powders with no NPs, Fig. 5a), beyond which any increase in compaction pressure did not result in any further improvement in tensile strength (13). Therefore, it was concluded that the relationship between tablet hardness and compression force is non-linear over a broad range. For TRE tablets, this compaction pressure threshold (200 MPa) resulted in close to zero porosity and was accompanied by a plateau in tensile strength (2.5 MPa) (Fig. 5a, b). This threshold was reached at 239 MPa for MAN tablets (Fig. 5b). It is known that compaction profiles require a careful inspection to avoid selecting a compaction pressure which imparts no further improvement upon tablet tensile strength but only a declining dissolution rate (41).
Good compactibility describes a material capable of achieving desired tablet hardness at low compaction pressure (42). With this in mind, careful selection of optimum compaction pressures to be employed for ITR NP tabletting studies was imperative. When SD TRE was compacted at 99 MPa, a tensile strength of 1.5 MPa was achieved (Fig. 5a). Furthermore, it was necessary to compact SD MAN at 149 MPa to achieve a tablet with a tensile strength of 0.9 MPa (Fig. 5a). Statistical analysis using a one-way ANOVA confirmed the significantly greater tensile strength of TRE over MAN tablets (p = 0.0005). This increased tensile strength of TRE tablets was accompanied by low porosity, ranging from 0.025 to 0.175, compared with MAN tablets of 0.1 to 0.32 in the same pressure range (Fig. 5b). An increase in compaction pressure for TRE tablets above 200 MPa produced no further volume reduction, attributed to limited space available for fragmentation and plastic deformation (13). It is also important to bear in mind that any unnecessary increase in compaction pressure can induce physical changes upon the compacted materials. For amorphous TRE, application of surplus energy during compaction could cause crystallization resulting in altered physiochemical material properties. Likewise for MAN, the polymorphic mixture present within the sample may be susceptible to transformation to the most stable polymorphic form with a different set of physical properties.
The density of SD uncompressed powders can be found within Table SI.1. It seems likely that the presence of ITR NPs in the microparticles contributed to an increase in inter-particle voids, as also conjectured from the DVS study, therefore reducing density of the NMP powders. Notably, despite the same density of powders and porosity values for compacted MAN- and TRE-based ITR NMPs, TRE tablets exhibited much greater tensile strength (Fig. 5). Therefore, it can be concluded that this increase in tensile strength is likely due to the difference in the solid-state form of the SD powders, and not density. Compaction did not induce any changes in the physical form of SD MAN, and SD TRE showed itself to be resistant to physical change upon compaction (Fig. SI.9).
The shape of the compressibility profiles can be valuable. The MAN tablets made of SD powder containing no NPs showed a linear decline in porosity suggesting a plateau had not yet been reached within this pressure range. A further decrease in porosity could have been achieved with greater compaction pressure above 239 MPa (Fig. 5). The TRE profile (for compacts produced using SD powder containing no NPs) displays a different shape, whereby a plateau in porosity is reached at 200 MPa; this would suggest no further decrease in porosity would have been achieved beyond this compaction pressure range (Fig. 5b). The presence of ITR NPs within MAN and TRE SD samples (NMPs versus SD powders with no NPs) had no impact upon the porosity of the compacts (p = 0.052).
SEM images shown in Fig. 6, combined with tabletability and compressibility profiles in Fig. 5, indicate adequate porosity without compromising tensile strength for compacts formed at the abovementioned pressures (Fig. 6d, e). Hence, ITR NMPs with MAN were compacted at 149 MPa and ITR NMPs with TRE at 99 MPa into single oral dosage forms (as shown in Fig. 5a). In addition, it is thought that particle morphology can explain differences in the compactibility of materials. Figure 6 contains SEM images depicting the surface cross sections of broken tablets comprised of SD TRE and SD MAN alone after hardness testing and shows that MAN particles are more regular and spherical compared with the irregular appearance of TRE. These properties of SD TRE particles allow for greater particle rearrangement, favoring interparticulate bond formation and resulting in compacts of greater tensile strength (Fig. 5a). These compacts have superior mechanical strength enabling them to withstand packaging and transportation conditions, while safeguarding the delivery of the ITR. Figure 5b confirms that TRE-based ITR NMPs tablets possess lower porosity, corresponding with greater tensile strength, when compared with MAN equivalents.
The mechanical properties and processability of materials can be affected by their solid-state form, including tensile strength and compactibility (33). Figure 7 and Fig. SI.9 confirm the crystalline nature of SD MAN when compacted, as well as the amorphous nature of SD TRE.
Crystalline material possesses greater long-range order in their structure, making them less brittle and elastic. Stronger inter-particle and intermolecular forces must be overcome in order to achieve adequate compaction and hence increased tensile strength of tablets. The superior compactibility of amorphous forms has been well documented (43,44). The findings of this work support existing evidence within the literature, with amorphous TRE compacts exhibiting superior mechanical properties when compared with crystalline MAN equivalents (Fig. 5).
The impact of compaction upon the redispersibility of SD ITR NMPs with MAN and TRE was assessed via NTA (Table IV).
SEM images in Fig. 8 confirm presence of a thin outer shell of MP protecting a core composed of ITR NPs. The presence of ITR NPs within MAN microparticles reduced the tensile strength of tablets (Fig. 5a). TRE equivalents exhibited a marked increase in tensile strength (Fig. 5a). Upon redispersion of ITR NMPs with TRE, NTA measurements confirmed similar average NP sizes of 112 nm, compared with 135 nm for the uncompressed sample (Table IV). Also, ITR NMPs with MAN displayed alike average particle sizes of around 200 nm, compared with 139 nm for uncompressed powder (Table IV). Notably, TRE samples were associated with a lower measurement variability than MAN.
Disintegration times for MAN SD and NMPs compacted at 149 MPa as well as TRE SD and NMPs compacted at 99 MPa can be found in Fig. 9a, while the solubility profiles of ITR NPs released from NMPs compacted with MAN and TRE are presented in Fig. 9b.
Dosage forms with fast disintegration time, such as those presented in this work and disintegrating within 4 min, accompanied by adequate tensile strength are considered optimal. The increased mechanical strength of TRE NMP tablets had no impact upon tablet disintegration time in water, with no change in disintegration time observed (Fig. 9a). However, a slightly longer disintegration time, around 4 min, was observed for MAN NMP tablets, despite the reduced mechanical strength of those tablets. Considering that porosity of the compacts made of NMPs were comparable to those made of SD materials containing no NPs, it appears that the solid-state character of these materials may be the dominant factor in determining their disintegration in water, with amorphous TRE exhibiting superior properties. Disintegration times for SD MAN and TRE (with no NPs) compacted at a range of pressures can be found in Table SI.2.
Both types of tablets exhibited slightly erratic solubility profiles of ITR NPs in 0.1 M HCl as seen in Fig. 9b. An initial burst in ITR concentration for TRE NMP tablets was observed after 2 min as shown in Fig. 9b, followed by a decrease in ITR concentration which can be explained by possible agglomeration and crystallization of NPs. Figure 9b also confirms that initial drug loading for both sugars was similar but evidently the presence of TRE within compacted tablets resulted in greater solubilization of ITR. TRE tablets released 100% ITR after 30 min, while MAN tablets around 55%, almost half that of TRE. Spray-dried ITR released around 25% of the drug after 30 min, and had a dissolution profile comparable with that of the MAN NMP tablet. The dissolution studies employed in this work used an ITR dose of 0.8 mg per tablet in 500 ml HCl, which means that the drug thermodynamic solubility, estimated at 4–5 μg/ml (45), was never reached. Sun et al. performed dissolution of crystalline ITR coarse powder with the particle size of approximately 15 μm and three suspensions with ITR particles varying in size. While those studies used ITR under supersaturation conditions (50 mg of ITR was dispersed in 900 ml of 0.1 M HCl), it was noted that after 30 min the coarse ITR released only around 5–6% of the drug and after 90 min it was still less than 10%, while the 300-nm ITR crystalline NPs released around 75% of ITR after 30 min (46). In relation to studies of a marketed solid dosage form of ITR, Sporanox® capsules, Matteucci and co-workers noticed around 30% ITR release after 30 min of dissolution; however, again, the studies were performed under supersaturation conditions in 0.1 M HCl, where the nominal ITR dose was 350 μg/ml and equilibrium solubility of crystalline ITR in those conditions was 4.4 μg/ml (26).
TRE has been noted for its ability to act as a “water-structure marker.” The interaction between trehalose and water is stronger than that of water and water; thus, trehalose has a “destructuring” effect on the tetrahedral hydrogen bonded network of water and it is known as a kosmotrope (35). This is supported by the high hydrogen bond donor and acceptor counts noted in Table II for TRE. The glucose ring structure of TRE may be more effective at disrupting the hydrogen bonded water network during solubilization compared with the short-chain structure of MAN. Furthermore, recent studies have shown that TRE exhibited a drug solubilizing character when physically mixed with bendroflumethiazide, a poorly soluble molecule, and an improvement in drug dissolution properties was achieved (47). These properties of TRE, accompanied by its amorphous character, may explain the reason for greater ITR solubilization in the presence of TRE compared with MAN at 30 min (p = 0.001) (Fig. 9b). Thus, matrix former type has a significant impact upon end product performance. This fact reiterates the importance of careful consideration of excipients within the final formulation and their impact upon end product performance.
Trehalose proved to be a better matrix former for ITR NPs boasting superior performance in numerous aspects when compared with mannitol. Namely, compressed ITR NMPs made with trehalose redispersed to smaller average particle sizes upon hydration compared with uncompressed equivalents, confirmed by both NTA and DLS. In addition, trehalose NMPs compressed into tablets exhibited increased tensile strength, compared with mannitol compressed NMPs, with adequate disintegration time maintained. Furthermore, trehalose was capable of solubilizing a greater amount of ITR in gastric conditions (0.1 M HCl) when compared with mannitol. Solid-state effects dominate in determining tablet properties with amorphous trehalose exhibiting favorable mechanical and disintegrant properties. The matrix former type has a profound impact upon end product performance. We can conclude that matrix former selection for NP isolation by spray drying warrants careful consideration. This research embodies the complete pharmaceutical development process, demonstrating a logical structured approach to the drug product development process.
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Research leading to these results was supported by the Synthesis and Solid State Pharmaceutical Centre (SSPC), financed by a research grant from Science Foundation Ireland (SFI) and co-funded under the European Regional Development Fund (Grant Number 12/RC/2275). The authors would like to thank Mark Lynch for his help in the study.
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McComiskey, K.P.M., McDonagh, A. & Tajber, L. Isolation of Itraconazole Nanostructured Microparticles via Spray Drying with Rational Selection of Optimum Base for Successful Reconstitution and Compaction. AAPS PharmSciTech 20, 217 (2019). https://doi.org/10.1208/s12249-019-1436-6
- spray drying
- solid state