Preventing release in the acidic environment of the stomach via occlusion in ordered mesoporous silica enhances the absorption of poorly soluble weakly acidic drugs

Preventing Release in the Acidic Environment of the Stomach via Occlusion in Ordered Mesoporous Silica Enhances the Absorption of Poorly Soluble Weakly Acidic Drugs MICHIEL VAN SPEYBROECK,1 RANDY MELLAERTS,2 THAO DO THI,1 JOHAN A. MARTENS,2 JAN VAN HUMBEECK,3 PIETER ANNAERT,1 GUY VAN DEN MOOTER,1 PATRICK AUGUSTIJNS1 1

Laboratory for Pharmacotechnology and Biopharmacy, Katholieke Universiteit Leuven, Leuven, Belgium

2

Centre for Surface Chemistry and Catalysis, Katholieke Universiteit Leuven, Heverlee, Belgium

3

Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Heverlee, Belgium

Received 15 January 2011; revised 24 April 2011; accepted 21 June 2011 Published online 15 July 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/jps.22703 ABSTRACT: This study aimed to assess the pharmaceutical performance of formulations consisting of either indomethacin or glibenclamide and the ordered mesoporous silica material SBA-15. Both compounds were loaded on SBA-15 via solvent impregnation. Adsorption in the SBA-15 mesopores was confirmed using nitrogen physisorption. Differential scanning calorimetry results suggested that both compounds were dispersed monomolecularly onto the SBA-15 surface. In in vitro experiments simulating the gastric-to-intestinal transition, the release of both compounds from SBA-15 remained under 1% in simulated gastric fluid (SGF, pH 1.2), whereas both drugs were completely released within 10 min after transfer to fasted state simulated intestinal fluid (FaSSIF, pH 6.5). As both drugs exhibited very rapid precipitation from the supersaturated state in SGF, the preferential release in FaSSIF—where conditions are more favourable by virtue of either much higher solubility (indomethacin) or more stable supersaturation (glibenclamide)—was considered crucial towards achieving optimal absorption. This hypothesis was confirmed by an in vivo study, where the extent of absorption of a glibenclamide–SBA-15 formulation was found to be more than fourfold higher than that R . © 2011 Wiley-Liss, Inc. and the American of the commercial glibenclamide product Daonil Pharmacists Association J Pharm Sci 100:4864–4876, 2011 Keywords: ordered mesoporous silica; SBA-15; glibenclamide; indomethacin; solubility; dissolution; supersaturation; absorption; bioavailability

INTRODUCTION For a significant portion of contemporary drug candidates, poor solubility/dissolution limits absorption. The sharp increase in the number of poorly soluble drug candidates over the last three decades has been attributed to evolutions in drug discovery strategies, which have caused drug candidates to become more lipophilic and of higher molecular weight, and hence more poorly soluble.1,2 Another contributor to the increase in the number of poorly soluble drug candidates is associated with the nature of current drug Correspondence to: Patrick Augustijns (Telephone: +32-16330301; Fax: +32-16-330305; E-mail: patrick.augustijns@pharm .kuleuven.be) Journal of Pharmaceutical Sciences, Vol. 100, 4864–4876 (2011) © 2011 Wiley-Liss, Inc. and the American Pharmacists Association

4864

targets in that the chemical requirements for adequate binding on these targets are often incompatible with the chemical requirements for good absorption potential.3 There is a growing interest to formulate poorly soluble compounds as supersaturating drug delivery systems, that is, systems that are able to present drugs to the intestinal milieu at concentrations above their equilibrium solubility.4 The rationale behind this formulation approach is that enhanced concentrations at the site of absorption may—by virtue of Fick’s First Law—increase the flux of drug across the gastrointestinal epithelium. A variety of formulation approaches, such as amorphous solid dispersions,5,6 lipid-based systems,7 crystalline salt forms8 or cocrystals,9 rely on the generation of supersaturation.

JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 11, NOVEMBER 2011

PREVENTING RELEASE IN THE STOMACH VIA OCCLUSION IN OMS

Table 1.

Structural Formulas and Physicochemical Properties of the Model Compounds Glibenclamide and Indomethacin Glibenclamide

Indomethacin

494.0 4.3 3.5 5.5 169

357.8 3.7 1.3 4.1 161

Molecular weight (g/mol) log Pa log D6.5 a pKaa Melting point (◦ C)b a Values

4865

calculated using the ADME predictor of the Pipeline Pilot software package (Accelrys, San Diego, California). experimentally using DSC as described in the section Differential Scanning Calorimetry.

b Determined

Ordered mesoporous silicates (OMSs) have recently emerged as supersaturating drug delivery systems. OMSs are porous materials that exhibit an array of uniform mesopores (i.e. pores with diameters between 2 and 50 nm—IUPAC classification). Because of their high porosity, OMSs exhibit a very high specific surface area (up to 1500 m2 /g) and pore volume (up to 1.5 cm3 /g). Various studies have demonstrated that the deposition of drugs onto the surface of OMSs (typically performed via solvent-based impregnation techniques) improves dissolution rate.10–12 The dissolution-enhancing effect of OMSs is a result from the loss of crystallinity and an increase in surface area associated with the adsorption of the drug molecules onto the OMSs surface.10,13,14 OMSs offer a major advantage over their nonporous counterparts (e.g. colloidal silicon dioxide) in that the deposited molecules are not just adsorbed onto the silica surface but also confined to pores that are only a few molecular diameters wide, which prevents the adsorbed molecules from rearranging themselves in a crystal lattice.15,16 Furthermore, the ability to tune the mesopore size enables to gain close control over the drug release rate, which may be of great value when designing a dose form.17 Adsorption on OMSs has proven to be a very effective dissolution-enhancing formulation approach applicable to a wide variety of compounds.10 However, as mentioned above, OMSs present drugs in a supersaturated state to the intestinal milieu, which on the one hand may increase transepithelial transport, but on the other hand inherently holds the risk of the drug precipitating into an energetically more favourable but less soluble form. Arguably, the pharmaceutical performance of OMS-based formulations does not only depend on the drug release rate but also—and maybe even more so—on the stability of the supersaturated solution generated. In previous DOI 10.1002/jps

work, conducted with the poorly soluble weak base itraconazole, we have demonstrated that precipitation following release could be compensated for by coadministration of the polymeric precipitation inhibitor hydroxypropylmethylcellulose.18 In a subsequent study, using the nonionisable drug fenofibrate, we have illustrated that it can be useful to decrease the release rate from OMSs (which can be accomplished by decreasing the pore size), in order to attenuate the degree of supersaturation associated with drug release, thereby minimising precipitation and maximising absorption.17 In the present study, we aimed to assess the pharmaceutical performance of OMS-based formulations comprising poorly soluble weak acids. Glibenclamide (an antidiabetic) and indomethacin (an antiinflammatory drug) (Table 1) were used as model compounds. By their very nature, these drugs exhibit higher solubility at the pH values prevailing in the small intestine. The lower solubility in the stomach, and the consequent higher driving force for precipitation, may lead to rapid precipitation of the released drug molecules. If significant precipitation takes place in the stomach, that is, prior to reaching the small intestine—the major site of absorption—the dissolution-enhancing effect of OMSs may be totally abolished. It was therefore our hypothesis that the in vivo performance of OMS-based formulations comprising a weak acid would be enhanced if release in the stomach was minimised or avoided. During the first experiments conducted within the framework of this study, it appeared that glibenclamide and indomethacin were released only to a negligibly low extent in simulated gastric fluid (SGF, pH 1.2), whereas rapid and complete release took place in fasted state simulated intestinal fluid (FaSSIF, pH 6.5). This behaviour, whereby weakly acidic drugs are released under the conditions that JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 11, NOVEMBER 2011

4866

VAN SPEYBROECK ET AL.

typically prevail in the small intestine, proved to be beneficial in terms of oral absorption, as clearly demonstrated in an in vivo study (rat model) where an OMS formulation comprising glibenclamide was benchmarked against the commercial glibenclamide R . product Daonil

MATERIALS AND METHODS Synthesis of OMSs The OMS material SBA-15 was used throughout this study.19 SBA-15 was synthesised according to the procedure described by Galarneau et al.20 The ripening was conducted at 90◦ C for 48 h. After calcination (550◦ C for 8 h), the SBA-15 powder was sieved through a 125 :m sieve (Retsch, Haan, Germany) and stored in an airtight container. The batch of SBA-15 used throughout this study had a pore diameter of 7.2 nm, a specific surface area of 606 m2 /g and a specific pore volume of 0.90 cm3 /g.

chloride was used. When the impregnated powder appeared dry, it was dried further at 40◦ C under reduced pressure (10−3 bar) for 48 h. The SBA-15 formulations of glibenclamide and indomethacin are denoted as glibenclamide–SBA-15 and indomethacin–SBA-15 in the text. The performance of glibenclamide–SBA-15 was benchmarked against that of the commercial forR (Impexeco, Moeskroen, Belgium), mulation Daonil which is a tablet formulation containing microR nised glibenclamide (5 mg dose strength). Daonil tablets were gently crushed using mortar and pestle, and the obtained powder was used for further experiments. For the in vivo experiments, glibenclamide–SBA-15 was blended with 25% of croscarmellose sodium (FMC BioPolymer, Philadelphia, Pennsylvania). Indomethacin–SBA-15 was referenced against crystalline indomethacin. Crystalline indomethacin was sieved through a 125-:m sieve and blended with 80% (w/w) of lactose (lactose monohydrate 80 mesh; Certa) prior to experimentation.

Preparation of Release Media Simulated gastric fluid (pH 1.2) was prepared by admixing 2 g of NaCl, 70 mL of 1 M HCl and 930 mL of deionised water (18.0 M; Elga, Bucks, UK). FaSSIF and fed state simulated intestinal fluid (FeSSIF) were prepared according to the formula described in Vertzoni et al., 200421 (standard media). Sodium taurocholate (practical grade) and lecithin (phospholipon 90 G) were purchased from ICN Biomedicals (Eschwege, Germany) and Nattermann Phospholipid (K¨oln, Germany), respectively. Blank FaSSIF and blank FeSSIF refer to the buffer solutions used to make up FaSSIF and FeSSIF. These blank media have the same pH as FaSSIF and FeSSIF, respectively, but do not contain sodium taurocholate or lecithin. The formula of FaSSIF used in the pH-shift experiments was modified such that the addition of 10% of SGF resulted in similar taurocholate and lecithin concentrations as standard FaSSIF, that is, 3.33 mM sodium taurocholate and 0.83 mM lecithin. Morpholineethansulfonic (45 mM) acid sodium salt (MES) (Sigma–Aldrich, Bornem, Belgium) was added as an additional buffer component to ensure that pH 6.5 was maintained upon addition of 10% (v/v) of SGF. Preparation of Formulations Glibenclamide (Sigma–Aldrich) and indomethacin (Certa, Braine-l’Alleud, Belgium) were loaded onto SBA-15 by solvent impregnation. The target drug load was 20% (w/w) for both drugs. A concentrated drug solution was added dropwise to the SBA-15 powders while the moist mixture was continuously stirred with a spatula. For glibenclamide, a 10 mg/ mL solution in chloroform was used for impregnation. For indomethacin, a 50 mg/mL solution in methylene JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 11, NOVEMBER 2011

Drug Load Quantification The drug content of the SBA-15 formulations, the R tablets and the indomethacin tritcrushed Daonil uration in lactose was determined by suspending 10 mg of powder in 10 mL of methanol (n = 5). These suspensions were sonicated for 30 min (Branson 3510, 40 kHz; Branson Ultrasonics, Danbury, Connecticut) and subsequently put in a rotary mixer (Snijders, Tilburg, the Netherlands) for 24 h. Afterwards, SBA-15/undissolved excipients were separated from the drug solution by centrifugation (2600g, 10 min). The supernatant was diluted appropriately (i.e. to fit within the linear range of the calibration curve) with mobile phase and assayed by highperformance liquid chromatography with ultraviolet de(HPLC–UV), as described in the section Analysis in vitro samples. The drug load of glibenclamide–SBA15, indomethacin–SBA-15, the indomethacin trituraR amounted to 22.5 ± 0.1%, tion in lactose and Daonil 19.3 ± 0.8%, 20.0 ± 0.2% and 2.9 ± 0.1%, respectively. Differential Scanning Calorimetry Differential scanning calorimetry experiments were performed on a TA Instruments Q-2000 apparatus (TA Instruments, Leatherhead, UK). Scans were recorded between 0◦ C and 200◦ C at a heating rate of 40◦ C/min. Dry nitrogen was used as purge gas (flow rate of 50 mL/min). All experiments were carried out in crimped aluminium pans (TA Instruments). Sample size varied between 5 and 8 mg. All thermograms were recorded in duplicate and all data handling was performed using the Universal Analysis 2000 software package (TA Instruments). DOI 10.1002/jps

PREVENTING RELEASE IN THE STOMACH VIA OCCLUSION IN OMS

4867

The solubility of glibenclamide and indomethacin was assessed in various aqueous media by the shake-flask method; approximately 1 mg of drug was dispersed in 1.5 mL of medium and shaken for 24 h at 37◦ C. Undissolved material was separated from the solution by centrifugation (20,000g, 10 min). The supernatant was diluted appropriately with mobile phase and analysed by HPLC-UV (see section Analysis in vitro samples).

placement with fresh medium as samples were considered homogeneous); samples were filtered through a 0.1 :m membrane filter (PTFE, Macherey-Nagel, ¨ Duren, Germany), diluted appropriately with mobile phase and analysed by HPLC-UV as described in the section Analysis in vitro samples. All release experiments were conducted at room temperature. Prior to experimentation, all filters were pretreated with 5 mL of a saturated solution of the compound in the medium of interest. In addition, prior to collection of the first sample, all filters were flushed with 1 mL of sample fluid. For the sink experiments, SGF+1% sodium lauryl sulphate (SLS) was used as release medium. The dose was 0.1 mg for glibenclamide and 0.5 mg for indomethacin, such that the final concentration did not exceed 10 :g/ml or 50 :g/mL, respectively. The solubility in SGF+1% SLS amounted to 38.8 :g/mL for glibenclamide and 180.3 :g/mL for indomethacin, so sink conditions (i.e. the final concentration being at least three times higher than the equilibrium solubility) were maintained throughout the experiment. In the transfer experiments (SGF to FaSSIF), a dose of 5 mg (glibenclamide) or 10 mg (indomethacin) was weighed into a test tube. This dose was dispersed in 10 mL of SGF, three samples were taken during the SGF stage as described above (at 5, 10, and 30 min), and after 30 min, 1 mL of SGF was transferred to 9 mL of FaSSIF. This transfer was thus accompanied with a 10-fold dilution, such that the (theoretical) final concentration in FaSSIF amounted to 50 :g/mL (glibenclamide) or 100 :g/mL (indomethacin). The relative amount in solution in FaSSIF after transfer from SGF was calculated based on the expected amount of drug present in FaSSIF (i.e. 10% of the nominal dose that was dispersed in SGF). We also conducted experiments where the SGF stage was omitted (formulations directly exposed to FaSSIF); in these experiments, the dose was adjusted such that the theoretical final concentration was the same as in the transfer experiments.

In Vitro Release Experiments

Supersaturation Experiments

We performed two types of release experiments: one set of experiments was conducted under sink conditions and another set implemented a transfer of SGF to FaSSIF, in order to simulate the in vivo gastric-tointestinal transition. Essentially, the approach was similar for all experiments: a fixed amount of formulation was weighed out in a test tube; release medium was added; test tubes were agitated using a rotary mixer (Snijders) (the stirring intensity was set to “1” of the nominal scale of the device, which equals ∼10 rotations per minute); 0.8-mL samples were taken at predetermined time points (without re-

In order to gain insight into the stability of glibenclamide and indomethacin supersaturation, we conducted cosolvent-induced supersaturation experiments. In these experiments, the medium of interest was spiked with a highly concentrated solution (20 mg/mL for glibenclamide; 100 mg/mL for indomethacin) in dimethylsulfoxide (DMSO). The maximum DMSO concentration in the medium did not exceed 2.5% (glibenclamide) or 1% (indomethacin). The setup, sampling protocol and analysis were the same as for the in vitro release experiments (section In Vitro Release Experiments).

Determination of Porosity Characteristics (Nitrogen Physisorption) Nitrogen adsorption–desorption isotherms were recorded at −196◦ C using a Micromeritics Tristar 3000-apparatus (Norcross, Georgia). The unloaded SBA-15 powder was pretreated at 300◦ C for 12 h under nitrogen flushing prior to analysis. For the formulations, the pretreatment was conducted at 100◦ C for 12 h, in order to avoid chemical degradation or melting of the adsorbed drug. Nitrogen adsorption–desorption isotherms were also recorded of aliquots of the SBA-15 formulations that were soaked in 0.01 M HCl. The approach here was to slurry the formulations in 0.01 M HCl for 120 min, under the same stirring conditions as mentioned in section In Vitro Release Experiments. Subsequently, the powder was separated from the medium by centrifugation (2600g, 5 min). After removal of the supernatant, the powder was dried at 40◦ C and reduced pressure (10−3 bar) for 24 h. The pretreatment of these samples was also conducted at 100◦ C for 12 h. Note that 0.01 M HCl was used instead of SGF in order to avoid sample contamination by the NaCl present in SGF. The total surface area and total pore volume were calculated using the t-plot method.18 The pore size distribution was derived from the adsorption branches of the nitrogen isotherms using the Barrett–Joyner–Halenda method (Barrett et al.22 ).

Solubility Measurements

DOI 10.1002/jps

JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 11, NOVEMBER 2011

4868

VAN SPEYBROECK ET AL.

Analysis in vitro samples All samples coming from the in vitro experiments (drug content quantification, solubility measurements, in vitro release and supersaturation experiments) were analysed by HPLC-UV. The chromatographic system consisted of a binary pump (type 1525; Waters, Milford, Massachusetts), an autosampler (717 Plus; Waters) and a dual wavelength absorbance detector (type 2487; Waters). A R 100 RP C18 column (5 :m) (Merck, Lichrospher Darmstadt, Germany) was used for chromatographic separation. For glibenclamide, the mobile phase consisted of methanol/sodium acetate buffer (50 mM, pH 3.5) 68/32 (v/v%), pumped at a flow rate of 1 mL/min. The wavelength of detection was 300 nm. The retention time of glibenclamide under these conditions was 6.2 min. Calibration curves were linear over a concentration range of 0.1–100 :g/mL. For indomethacin, the same mobile phase was used as for glibenclamide, but the flow rate was adjusted to 1.5 mL/min and the wavelength of detection was set at 266 nm. The retention time of indomethacin under these conditions was 6.5 min. Calibration curves were linear over a concentration range of 0.1–100 :g/mL. All chromatograms R software package were analysed using the Breeze (Waters).

In Vivo Study All in vivo experiments were carried out in accordance with the European Commission directive 86/609/EEC for animal experiments (license number LA1210261). Approval for this project was granted by the Institutional Ethical Committee for Animal Experimentation of the Katholieke Universiteit Leuven.

cally using a dosing syringe plunger (PCcaps kit, Capsugel).

Blood Sampling Prior to each blood draw, the rats were acclimatised in an incubator (37◦ C) (Mini-thermacage MK3; Datesand, Manchester, UK) for 15 min to promote bleeding. Subsequently, the animals were placed in a cylindrical restrainer with adjustable headgate and removable tailgate (Harvard Apparatus, Holliston, Massachusetts). Blood samples (500 :L) were taken by individual punctures of the lateral tail vein at 1, 2, 3, 4, 6, 8, 10 and 24 h after dosing. Blood was collected into heparin-coated tubes (LH, 68 IU) (Vacutainer; Becton Dickinson, Plymouth, UK) and plasma was harvested by centrifugation (2600g, 4◦ C, 10 min). All plasma samples were stored at −20◦ C pending analysis. After every blood draw, the rats were re-released into their cages.

Analysis Plasma Samples Sample Preparation. All plasma samples were quantified for unchanged glibenclamide. Prior to analysis, samples (100 :L) were spiked with internal standard (100 :L of a 200 ng/mL glipizide [Sigma–Aldrich] solution in methanol). Subsequently, 1 N HCl (0.5 mL) was added. Afterwards, all samples were extracted using 4 mL of a 1:1 methylene chloride–hexane mixture. After centrifugation (2600g, 4◦ C, 5 min), the organic phase was transferred to a clean test tube and evaporated to dryness. Lastly, the extraction residue was resuspended in 0.5 mL of mobile phase (70/30 methanol/0.1% formic acid), transferred into autosampler vials and analysed by liquid chromatography–tandem mass spectrometry (LC–MS/MS) as described below.

Animals Male Wistar rats (300–350 g, ∼9 weeks of age; Elevage Janvier, Le Genet Saint Isle, France) were used throughout this study. All animals were allowed to acclimatise for at least 1 week prior to experimentation. Water and feed (58% carbohydrates, 33% pro¨ teins, 9% lipids) (ssniff R/M-H; sniff Spezialdiaten, Soest, Germany) were available ad libitum during the acclimatisation period. Twelve hours prior to dosing, the rats were deprived from feed, while water remained available ad libitum. The animals were allowed access to feed again 4 h after dose.

Dosing All rats were dosed orally with 0.8 ± 0.05 mg of glibenR ) (n = 4). clamide (as SBA-15 formulation or Daonil The formulations were filled into hard gelatine capsules (PCcaps size 9) (Capsugel, Bornem, Belgium) by means of a stand, funnel and tamper (PCcaps kit, Capsugel). Capsules were administered intragastriJOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 11, NOVEMBER 2011

Liquid Chromatography–Tandem Mass Spectrometry. All experiments were performed on a Thermo Scientific (San Jose, California) LC–MS/MS system comR R autosampler, an Accela pump prising an Accela R  and a TSQ Quantum Access triple quadrupole mass spectrometer equipped with an electrospray ionisation source. Data acquisition and peak integraR 2.0.7 and tion were performed with the Xcalibur R  LCquan 2.5.6 software packages, respectively. R C18 column (50 × 2.10 mm, 2.6 :m; A Kinetex Phenomenex, Utrecht,the Netherlands) protected by a Krudkatcher Ultra inline filter (Phenomenex) was used for chromatographic separation. The mobile phase consisted of 0.1% (w/w) formic acid in water (A) and methanol (B). Gradient elution at a constant flow rate of 0.5 mL/min was performed as follows: 90% A for 0.5 min, linear decrease to 30% A in 0.5 min, constant flow of 30% A for 2.0 min, linear increase back to 90% A in 0.5 min, re-equilibration for 0.5 min with 90% A before the next injection. The total run DOI 10.1002/jps

PREVENTING RELEASE IN THE STOMACH VIA OCCLUSION IN OMS

Table 2.

4869

Summary solubility model compounds Solubility (:g/ml)a

Glibenclamide Indomethacin a Values

SGF

blank FaSSIF

FaSSIF

blank FeSSIF

FeSSIF

SGF+1% SLS

1.4 (0.1) 1.0 (0.2)

3.2 (0.0) 362.0 (1.3)

3.8 (0.2) 421.9 (5.9)

1.5 (0.1) 15.1 (0.55)

2.7 (0.0) 130.5 (3.6)

38.8 (0.5) 180.3 (4.5)

represent mean and (standard deviation) (n = 3).

time was 4.0 min and the injection volume was 25 :L (full loop mode). Under these conditions, the retention times of glibenclamide and the internal standard were 2.2 and 1.7 min, respectively. The mass spectrometer was operated in the positive electrospray mode. The spray voltage was 3000 V and the capillary temperature was set at 320◦ C. The vapouriser temperature was 220◦ C. Nitrogen was used as the sheath gas (50 arbitrary units). No auxiliary gas was used. Argon was used as the collision gas at a pressure of 1.5 mTorr. The mass spectrometer was operated in a three-channel selected reaction monitoring mode. Two precursor–product ion pairs were used for detection of glibenclamide: m/z 494.3 → 169.1 (collision energy: 14 V) and m/z 494.3 → 369.2 (32 V). One ion pair was used for detection of the internal standard glipizide: m/z 446.3 → 321.0 (15 V). A scan time of 0.01 s was used per ion pair. Both Q1 and Q3 were set at 0.7 unit mass resolution. An online motorised divert valve was used to introduce the eluent to the mass spectrometer over the period of 0.25–2.75 min for data acquisition. The eluent flow outside this interval was diverted to the waste. Calibration curves were linear over the concentration ranges of 8–125 ng/mL (low-concentration range) and 125–4000 ng/mL (high-concentration range). The intraday precision (relative standard deviation; n = 6) of standards with low (30 ng/mL), medium (300 ng/ mL) and high (3000 ng/mL) concentration amounted to 2.0%, 6.6% and 1.0%, respectively. The intraday accuracy (bias) of said standards was 2.0%, −2.7% and −7.9%, respectively. The interday precision (n = 3) of the same standards amounted to 6.9%, 1.9% and 5.6%, and the interday accuracy (n = 3) to −11.3%, −9.2% and −9.0%, respectively.

Data Analysis The observed maximum plasma concentration (Cmax ) and the time of its occurrence (Tmax ) were noted directly from the individual plasma concentration–time profiles. The area under the plasma concentration— time curve (AUC) was calculated from time zero to the last sampling point via linear-trapezoidal integration. Statistical analyses were performed using a Mann–Whitney nonparametric test. p values lower than 0.05 were considered significant. All statistical analyses were performed using GraphPad Prism DOI 10.1002/jps

for Windows, version 5.00 (GraphPad Software, San Diego, California).

RESULTS Differential Scanning Calorimetry The thermograms of glibenclamide–SBA-15 and indomethacin–SBA-15 did not exhibit a melting peak, nor did they exhibit a glass transition, which suggests monomolecular adsorption of the drugs onto the surface of SBA-15 (data not shown). Evidence for the effective adsorption of the drugs into the SBA-15 mesopore system is provided by nitrogen physisorption data, discussed in section In Vitro Release. Solubility Model Compounds Before addressing the in vitro release data, it is useful to briefly discuss the solubility of the model compounds in various solvent systems (Table 2). A first important observation is that the solubility in SGF is very low (around 1 :g/mL) for both model compounds. The solubility of both compounds increases in the (higher pH) simulated intestinal media, but this increase is much more pronounced for indomethacin. This may be explained, at least partly, by the lower pKa of the latter (Table 1). Another remarkable observation is that the increase in glibenclamide solubility in the simulated intestinal media is only marginal as compared to the blank media (i.e. media having the same pH but not containing sodium taurocholate or lecithin), which suggests that glibenclamide has low affinity for the mixed micelles present in FaSSIF and FeSSIF. Indomethacin solubility enhances sharply with pH, with a clear additional effect of the solubilising components present in FaSSIF and FeSSIF. It is also noteworthy that the solubility of both compounds is higher in FaSSIF than in FeSSIF, in spite of the fivefold higher concentration of solubilising components in this latter medium. The higher solubility in FaSSIF may be attributed to the higher degree of drug ionisation (pH of FaSSIF is 6.5; pH of FeSSIF is 5).

In Vitro Release In Vitro Release under Sink Conditions (SGF + 1% SLS) Figure 1 depicts the release profiles of the SBA-15 formulations under sink conditions. For comparison, JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 11, NOVEMBER 2011

4870

VAN SPEYBROECK ET AL.

Figure 2. Release profiles of glibenclamide under biorelevant conditions (mean ± SD, n = 3): after 30 min in SGF, aliquots were transferred to FaSSIF in order to simulate the in vivo gastric-to-intestinal transition. On the graph, 100% corresponds to a concentration of 500 :g/mL in SGF and 50 :g/mL in FaSSIF. The dashed line represents the equilibrium solubility (Cs ) of glibenclamide in FaSSIF (3.8 :g/mL).

Figure 1. Release profiles of glibenclamide (upper panel) and indomethacin (lower panel) under sink conditions (SGF+1% SLS) (mean ± SD, n = 3).

R the dissolution profiles of Daonil and crystalline indomethacin have been recorded under sink conditions as well. The SBA-15 formulations attain complete release within 10 min, whereas the crystalline reference formulations do not reach 100% dissolution within 120 min. The data in Figure 1 clearly illustrate that, under conditions where wettability is high and a sink is maintained throughout the experiment, SBA-15 exerts a significant dissolution-enhancing effect. Although insightful, these data may not be very predictive of in vivo performance. The experiments discussed in the section In vitro Release under Biorelevant Conditions implement a transfer of SGF to FaSSIF in order to simulate the in vivo gastric-tointestinal transition, and the results of those experiments likely provide a better indicator for in vivo performance.

In Vitro Release under Biorelevant Conditions Figure 2 depicts the release profiles of glibenclamide. From this figure, it appears that glibenclamide release in the SGF stage is only marginal, reaching not more than 1%. It should be noted that the glibenclamide–SBA-15 powder dispersed quickly JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 11, NOVEMBER 2011

upon contact with SGF to form a homogeneous suspension, without powder floating on the surface. Upon transfer to FaSSIF, more than 90% is released within 5 min. In Figure 2, 100% release in the FaSSIF stage corresponds to a concentration of 50 :g/mL, which is about 13 times the equilibrium solubility of glibenclamide in FaSSIF (3.8 :g/mL). In spite of being released well above its equilibrium solubility, glibenclamide does not precipitate rapidly. As a matter of fact, a relatively high degree of supersaturation is maintained throughout the experiment (180 min), with the relative amount in solution not dropping below 60% or 30 :g/mL. Basically, the release not exceeding 1% in SGF could be indicative of two events. The most straightforward explanation is that indeed little or no drug is released. Another—theoretically possible, although unlikely—explanation may be that the entire drug load is released instantaneously, followed by rapid and complete precipitation, such that little or no drug is in solution anymore when the first sample is collected. In the event that these precipitates are amorphous, they might quickly redissolve upon transfer to FaSSIF. In order to exclude this latter scenario, we conducted cosolvent-induced supersaturation experiments which indicated that, when SGF was spiked with a stock solution of glibenclamide in DMSO in order to obtain a (theoretical) initial concentration of 0.5 mg/mL (i.e. the same concentration as in the pH-shift release experiments), the drug indeed precipitated very rapidly; the concentration measured after 5 min amounted to 0.6 ±0.1 :g/mL or 0.12% of the initial dose (Fig. 2). However, the precipitates formed in SGF did not rapidly redissolve upon transfer to FaSSIF; only minor supersaturation was observed during the first 30 min after transfer. These findings preclude the DOI 10.1002/jps

PREVENTING RELEASE IN THE STOMACH VIA OCCLUSION IN OMS

Figure 3. Release profiles of indomethacin under biorelevant conditions (mean ± SD, n = 3): after 30 min in SGF, aliquots were transferred to FaSSIF in order to simulate the in vivo gastric-to-intestinal transition. On the graph, 100% corresponds to a concentration of 1000 :g/mL in SGF and 100 :g/mL in FaSSIF. Note that the large standard deviations of the “DMSO stock” sample are due to the variability associated with the transfer of the precipitates (see text for details).

possibility that the sharp increase in concentration following transfer of glibenclamide–SBA-15 to FaSSIF is a result of rapid dissolution of amorphous precipitates formed in SGF. Note that the commerR remained virtually completely cial product Daonil undissolved in SGF (concentrations were below the limit of detection of our analysis). Upon transfer to R dissolved rapidly to reach the equiFaSSIF, Daonil librium solubility within 5 min. Essentially, the behaviour of indomethacin–SBA15 in the pH-shift experiments was similar to that of glibenclamide–SBA-15; the release in SGF did not exceed 1% and upon transfer to FaSSIF, complete release was attained within 10 min (Fig. 3). As opposed to glibenclamide, release of indomethacin in FaSSIF is not associated with the creation of supersaturation. The solubility of indomethacin in FaSSIF amounts to 421 :g/mL, so the release in FaSSIF is essentially taking place under sink conditions (the final concentration in the release experiment amounts to 100 :g/ mL). Analogously to glibenclamide, we conducted cosolvent-induced supersaturation experiments in order to check whether the very sharp increase in concentration upon transfer to FaSSIF was due to either release from SBA-15 or redissolution of precipitates formed in SGF. Although indomethacin indeed precipitated very rapidly in SGF (concentrations dropping down to the equilibrium solubility within 30 min), the formed precipitates redissolved to reach a plateau only 60 min after transfer (as opposed to after 5 min for indomethacin–SBA-15) (Fig. 3). The fact that transfer of the stock solution resulted in a plateau around 50% instead of 100% was due to variability associated with the transfer step; since the precipitates formed in SGF were very large and sticky, it was cumbersome to transfer a homogeneous sample to FaSSIF DOI 10.1002/jps

4871

(which is also reflected in the very large standard deviations of thecorresponding profile). However, the fact that a plateau value was reached after 60 min, whereas this was only 5 min for indomethacin–SBA15, indicates that the rapid increase in concentration after transfer of indomethacin–SBA-15 is due to instantaneous release from the SBA-15 pores, and not due to rapid redissolution of precipitates. Even though crystalline indomethacin dissolves completely within 60 min after transfer, its dissolution rate is significantly slower than the release rate from SBA-15. The nonrelease at low pH of both drugs was substantiated by nitrogen physisorption experiments. The approach here was to slurry the SBA-15 formulations in 0.01 M HCl for 120 min, then separate the powder from the medium by centrifugation and subject the dried powder again to nitrogen physisorption. As drug displacement out of the SBA-15 pores is accompanied with an increase in pore volume, a change (increase) in porosity after soaking in 0.01 M HCl may provide an indication for the extent of drug release. An overview of the results is given in Table 3. First, the data in Table 3 indicate a significant decrease in surface area, pore volume and pore diameter for the formulations as compared to the unloaded SBA15, which is in line with expectations; this decrease in porosity evidences the successful loading of the drugs in the SBA-15 internal pore system. Second, it appears that the aliquots of the SBA-15 formulations that were dispersed in 0.01 M HCl exhibit only a slightly higher specific surface area, specific pore volume and average pore diameter as compared to the corresponding starting formulations, which evidences the extremely low release in 0.01 M HCl. Note that the soaking procedure had only a minor effect on the structural integrity/porosity of the SBA15 carrier (compare SBA-15 to SBA-15 soaked). On the basis of the increase in pore volume after soaking, a rough approximation of the amount released can be made. For instance, the total specific pore volume of glibenclamide–SBA-15 amounts to 0.49 cm3 /g, whereas that of the unloaded SBA-15 starting material is 0.90 cm3 /g. The volume occupied by the adsorbed drug thus amounts to 0.41 cm3 /g. After soaking in 0.01 M HCl, an amount of drug corresponding to 0.02 cm3 /g (0.51 to 0.49 cm3 /g) is displaced, which represents 4.8% of the initial 0.41 cm3 /g. A similar calculation for indomethacin–SBA-15 points to a drug release of 2.7%. All of the above results point to a substantially different release behaviour in SGF and FaSSIF, and a variety of factors may be at the basis of this difference. First, it can reasonably be assumed that charge interactions between the drugs and the silica surface contribute, at least to some extent, to the differential release behaviour. SBA-15 is weakly acidic and its isoelectric is around 1.5.23 Hence, below pH 1.5 (e.g. in JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 11, NOVEMBER 2011

4872

VAN SPEYBROECK ET AL.

Table 3. Overview Porosity Characteristics of the SBA-15 Starting Material, SBA-15 Formulations and SBA-15 Formulations That Were Soaked in 0.01 M HCl for 120 Min Sample SBA-15 SBA-15-soaked GLIB–SBA-15 GLIB–SBA-15-soaked IND–SBA-15 IND–SBA-15-soaked

Average Pore Diameter (nm)

Specific Surface Area (m2 /g)

Specific Pore Volume (cm3 /g)

7.2 7.3 6.6 6.6 6.6 6.9

606.7 568.0 336.3 349.2 377.0 383.5

0.90 0.89 0.49 0.51 0.53 0.54

SGF of pH 1.2), the silica surface carries a net positive charge, whereas indomethacin (pKa 4.1) and glibenclamide (pKa 5.5) are completely unionised. At pH 6.5, however, the drugs and the silica surface both carry a negative charge, leading to charge repulsion which is likely to promote desorption. It should be noted that the fact that glibenclamide and indomethacin are not effectively desorbed from the SBA-15 surface at pH 1.2 is inconsistent with the commonly accepted picture that weak acids normally do not adsorb strongly on silica surfaces from aqueous solutions.23,24 The data in Figure 1 indicated that rapid and complete desorption does take place if SGF is supplemented with 1% of SLS, suggesting that the nonrelease in plain SGF is also a result of poor wettability. It should be noted, however, that the SBA-15 formulations did disperse homogeneously upon contact with SGF, so the nonrelease is not just a matter of poor contact with the aqueous medium but merely reflects the inability of water to effectively displace the adsorbed drug molecules from the silica surface. The experiments described in the remainder of this section were aimed at further elucidating why release does take place in FaSSIF. As stated in the previous paragraph, charge repulsion between drug and silica at pH 6.5 is a key underlying factor. Another likely contributor is the additional wetting effect brought about by the surface-active agents sodium taurocholate and lecithin present in FaSSIF. In order to assess the effect of the FaSSIF components, we performed release experiments wherein glibenclamide–SBA-15 was directly exposed to FaSSIF and blank FaSSIF (Fig. 4, upper panel). Interestingly, although the release profile recorded in FaSSIF reaches a plateau at 100%, that recorded in blank FaSSIF levels off at around 50%. Since this lower plateau could be indicative of either (i) a less stable supersaturation and more extensive precipitation, (ii) a lower release or (iii.) a combination of these two scenarios, we performed cosolvent-induced supersaturation experiments (Fig. 4, lower panel) in an attempt to discriminate between the above-mentioned possibilities. Although it appears from Figure 4 that glibenclamide supersaturation is indeed more stable in FaSSIF, the data do indicate that blank FaSSIF is JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 11, NOVEMBER 2011

Figure 4. Left panel: Release from glibenclamide–SBA15 in blank FaSSIF and FaSSIF (mean ± SD, n = 3). Right panel: Supersaturation profiles of glibenclamide in FaSSIF and blank FaSSIF (mean ± SD, n = 3). On both graphs, the concentration corresponding to 100% amounts to 50 :g/mL. The dashed line represents the equilibrium solubility (Cs ) of glibenclamide in FaSSIF (3.8 :g/mL). The equilibrium solubility in blank FaSSIF is only slightly lower (3.2 :g/ mL) but has not been implemented on the graphs for the sake of clarity.

able to support glibenclamide supersaturation without precipitation setting in for at least 10 min. What can be inferred from this result is that the observed plateau at 50% in blank FaSSIF is not reflective of precipitation prior to the first sampling point but rather of 50% release in the strict sense of the word. Stated otherwise, if all glibenclamide would be released instantaneously in blank FaSSIF, one would DOI 10.1002/jps

PREVENTING RELEASE IN THE STOMACH VIA OCCLUSION IN OMS

expect to measure maximum relative concentrations close to 100% because blank FaSSIF is capable of maintaining supersaturation. The fact that the relative concentrations in blank FaSSIF level off at 50% therefore indicate that only 50% glibenclamide is released from SBA-15. Note that the differential concentration–time profiles in blank FaSSIF and FaSSIF are not related to differences in solubility; the maximal concentration in the experiments (50 :g/ mL) corresponds to a degree of supersaturation of 15.6 (50 :g/mL divided by 3.2 :g/mL) for blank FaSSIF and 13.2 (50 :g/mL divided by 3.8 :g/mL) for FaSSIF. Besides illustrating the effect of taurocholate and lecithin on the release behaviour in FaSSIF, the data presented in Figure 4 also provide some other useful insights. First, the release profile of glibenclamide–SBA-15 recorded in FaSSIF (Fig. 4) is highly comparable to that recorded in FaSSIF after transfer from SGF (Fig. 2). This again indicates that the formulation is essentially inert when it is being dispersed in SGF, and that the actual release process only sets in upon transfer to FaSSIF. Second, the supersaturation data in Figure 4 indicate that the solubilising components in FaSSIF exert a stabilising effect on FaSSIF supersaturation. What is also evident from the cosolvent-induced supersaturation experiments is that glibenclamide supersaturation is more stable in the neutral media FaSSIF and blank FaSSIF as compared with the acidic medium SGF, in which essentially complete precipitation from the supersaturated state takes place in as little as 5 min. The elucidation of the physicochemical principles underlying the more stable state of supersaturation formed in neutral media was beyond the scope of this study, but it is may reasonably be assumed that intermolecular charge repulsion (glibenclamide is ionised at pH 6.5) is a contributor. It should be noted that in the above-mentioned supersaturation experiments, the (theoretical) initial glibenclamide concentration in SGF (500 :g/mL) was 10-fold higher than that in (blank) FaSSIF (50 :g/mL). However, we have also conducted experiments in which SGF was spiked to obtain a (theoretical) initial concentration of 50 :g/ mL, and the trends observed were essentially similar as when the starting concentration was 500 :g/ mL, that is, virtually complete precipitation within 5 min (data not shown). The same holds true for indomethacin supersaturation in SGF (data not shown). In summary, the immediate and complete release from SBA-15 upon transfer to FaSSIF is a result from (i) the increase in pH leading to charge repulsion between drug and silica and (ii) the additional desorptive effect of the solubilising components present in FaSSIF. Following release in FaSSIF, a supersaturated solution is formed in which lecithin and taurocholate act as stabilisers. DOI 10.1002/jps

4873

Figure 5. Mean plasma concentration versus time profiles (mean ± SD, n = 4) of glibenclamide following oral administration to overnight fasted Wistar rats. Treatments consisted of glibenclamide–SBA-15 and a commercialised R ). A dose of 0.8 mg was adreference formulation (Daonil ministered for both treatments.

In Vivo Study By virtue of its lower solubility in intestinal media, glibenclamide may be considered a more challenging compound to formulate as compared to indomethacin, and therefore glibenclamide was selected for further in vivo experiments. The mean plasma concentration versus time profiles of glibenclamide following oral administration to overnight fasted Wistar rats are depicted in Figure 5. A summary of the in vivo results is provided in Table 4. Figure 5 provides compelling evidence for the potential of SBA-15 to enhance the absorption of glibenclamide; in comparison R , the Cmax and AUC of glibenclamide–SBAto Daonil 15 were 7.7-fold and 4.4-fold higher, respectively. These differences were statistically significant (p < 0.05). There was no statistically significant difference in terms of Tmax . The much better in vivo performance of glibenclamide–SBA-15 is in agreement with the in vitro results, and it may reasonably be assumed that the trends observed in the in vitro experiments also hold true in the in vivo situation, that is, very little glibenclamide is released in the stomach, whereas the drug is rapidly released as soon as the formulation reaches the small intestine, creating a state of supersaturation that is maintained for long enough to allow for absorption to take place.

DISCUSSION This study aimed to assess the pharmaceutical performance of formulations consisting of either indomethacin or glibenclamide and the OMS material SBA-15. The inherent higher solubility of these weakly acidic compounds at the pH values typically encountered in the small intestine led us to the JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 11, NOVEMBER 2011

4874

VAN SPEYBROECK ET AL.

Table 4. Summary pharmacokinetic parameters for glibenclamide after oral administration to Wistar rats (n = 4)

R Daonil

GLIB–SBA-15

Tmax (h)

Cmax (ng/ml)

AUC (ng.h/ml)

3.5 (2-4) 2.5 (1-4)

292.7 (101.8) 2255.8 (1251.8)

1413.5 (438.9) 6236.8 (2513.0)

Treatments consisted of glibenclamide:SBA-15 and a commercialised R reference formulation (Daonil ). A dose of 0.8 mg was administered for both treatments. Mean and (standard deviation) are provided for AUC and Cmax . Median and (range) are given for Tmax . The differences in Cmax and AUC between both treatments were statistically significant (p < 0.05).

assumption that it would be beneficial to target the release of these drugs to the small intestine; the lower solubility at the pH values in the stomach, and the consequent higher driving force for precipitation, may lead to more rapid precipitation of the released drug molecules. If significant precipitation takes place in the stomach, that is, prior to reaching the small intestine—the major site of absorption—the dissolution-enhancing effect of OMS may be totally abolished. The solubility data collected in this study were in line with expectations and revealed a higher solubility of both compounds in the simulated intestinal medium FaSSIF (pH 6.5) as compared to SGF (pH 1.2). In addition, cosolvent-induced supersaturation experiments demonstrated that both compounds precipitated very rapidly in SGF. In contrast, high drug concentrations could be maintained in FaSSIF. Glibenclamide supersaturation in FaSSIF was found to be very stable, which was, at least partly, due to the effect of ionisation and an additional stabilisation brought about by taurocholate and lecithin. For indomethacin, the solubility in FaSSIF was so high (421.9 ± 5.9 :g/mL) that precipitation following release was considered highly unlikely to occur in the in vivo situation (and therefore no cosolvent-induced supersaturation experiments were conducted with this compound). Taken together, the solubility and supersaturation data indicated that the conditions for drug release were more favourable in the small intestine, from both a thermodynamical (higher equilibrium solubility) as well as a kinetic (slower precipitation—only of relevance to glibenclamide) point of view. Our in vitro release experiments demonstrated that weak acids (or at least the weak acids used in this study) were not released from SBA-15 in SGF, whereas rapid and complete release took place in FaSSIF. The preferential release in FaSSIF was found to be a result of the higher pH, leading to charge repulsion between drug and silica and the additional desorptive effect exerted by the solubilising components present in FaSSIF. In view of the above-mentioned higher solubility/more stable supersaturation in intestinal media, preventing release in JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 11, NOVEMBER 2011

the acidic environment in the stomach was in fact essential to achieving optimal pharmaceutical performance (which was clearly reflected in the more than fourfold increase in extent of absorption of glibenclamide–SBA-15 as compared with the comR ), in spite of the fact that this mercial product Daonil approach reduces the time span that the formulation is effectively interacting with the gastrointestinal environment (the formulation can be considered inert when residing in the stomach). Although drug release from mesoporous silicates has frequently been explained in terms of a competitive desorption by water molecules,11,25 our results illustrate that, at least in some cases, drug desorption may not occur under the conditions prevailing in the stomach (lower pH and lower wettability as compared with the small intestine). In that respect, it is worth noticing that the model drugs used in this study are essentially completely unionised at pH values lower than 2.0, and it can therefore reasonably be assumed that nonrelease in gastric fluids is a likely prospect for a variety of other (i.e. nonacidic) drugs. It should be stressed, however, that nonrelease in the stomach does not necessarily imply a lower pharmaceutical performance. As a matter of fact, the results of this study suggest that preventing release in the stomach is actually beneficial to the absorption of supersaturating drug delivery systems comprising acidic drugs. It would be interesting to explore whether this also holds true for neutral drugs (i.e. drugs that are not ionised over the entire physiological pH range). These latter class of compound typically exhibit higher solubility in the small intestine due to the solubilising effect of endogenous bile-derived components such as bile salts and phospholipids, and most likely targeting release to the small intestine would also be useful for these compounds. Even though we made use of SBA-15 in this study, targeting release to the small intestine could easily be accomplished for a very wide variety of dosage forms through, for example, application of an enteric coating. It should be noted that weakly basic drugs are likely to benefit from release at the level of the stomach, where solubility is higher, and, as demonstrated in our previous studies, supersaturation may be more stable.18 This study is also an advocacy for the use of biorelevant release experiments (i.e. experiments in which biorelevant media such as FaSSIF and FeSSIF are used and that take into account the phenomenon of supersaturation and the associated likelihood of precipitation) in the assessment of mesoporous silicabased drug delivery systems. The large majority of studies dealing with dissolution enhancement using mesoporous silicates solely report on release experiments in simple aqueous buffer systems that are usually supplemented with high amounts of surfactants to maintain sink conditions. Although release DOI 10.1002/jps

PREVENTING RELEASE IN THE STOMACH VIA OCCLUSION IN OMS

experiments under sink conditions are of importance in the assessment of mesoporous silica-based systems, the artificial micellar solubilisation brought about by the added surfactants may lead to gross overestimations of the extent of drug release that is likely to be encountered in the in vivo situation. The discrepancy between the release results obtained in plain SGF (release did not exceed 1%) versus SGF+1% SLS (100% release within 10 min) or between blank FaSSIF (50% glibenclamide release) and FaSSIF (100% glibenclamide release) may serve to illustrate the above statement. In conclusion, the data described in this paper illustrate a number of concepts that might prove instrumental in the development of mesoporous silica-based formulations. Furthermore, some of the results, and most notably the observation that the absorption of a poorly soluble weak acid is greatly enhanced if its release is targeted to the small intestine, may also be usefully incorporated in formulation strategies for supersaturating delivery systems in general.

ACKNOWLEDGMENTS This study was supported by a grant from the Katholieke Universiteit Leuven Industrial Research Fund (IOF) and the Katholieke Universiteit Leuven Onderzoeksfonds. Michiel van Speybroeck acknowledges the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT Vlaanderen) for a PhD grant. Thao Do Thi acknowledges the IOF for a PhD grant. Randy Mellaerts is a postdoctoral researcher of the Research FoundationFlanders (FWO-Vlaanderen). Johan A. Martens acknowledges the Flemish Government for long-term structural funding (Methusalem).

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

REFERENCES 1. Gribbon P, Sewing A. 2005. High-throughput drug discovery: What can we expect from HTS? Drug Discov Today 10:17–22. 2. Lipinski CA. 2000. Drug-like properties and the causes of poor solubility and poor permeability. J Pharmacol Toxicol Methods 44:235–249. 3. Lipinski C, Hopkins A. 2004. Navigating chemical space for biology and medicine. Nature 432:855–861. 4. Brouwers J, Brewster ME, Augustijns P. 2009. Supersaturating drug delivery systems: The answer to solubility-limited oral bioavailability? J Pharm Sci 98:2549–2572. 5. Curatolo W, Nightingale JA, Herbig SM. 2009. Utility of hydroxypropylmethylcellulose acetate succinate (HPMCAS) for initiation and maintenance of drug supersaturation in the GI milieu. Pharm Res 26:1419–1431. 6. Miller DA, DiNunzio JC, Yang W, McGinity JW, Williams RO 3rd. 2008. Enhanced in vivo absorption of itraconazole via stabilization of supersaturation following acidic-to-neutral pH transition. Drug Dev Ind Pharm 34:890–902. 7. Gao P, Akrami A, Alvarez F, Hu J, Li L, Ma C, Surapaneni S. 2009. Characterization and optimization of AMG 517 superDOI 10.1002/jps

18.

19.

20.

21.

22.

4875

saturatable self-emulsifying drug delivery system (S-SEDDS) for improved oral absorption. J Pharm Sci 98:516–528. ´ HR, Tawa M, Zhang Z, Ratanabanangkoon P, Shaw P, Guzman Gardner CR, Chen H, Moreau J-P, Almarsson O, Remenar JF. 2007. Combined use of crystalline salt forms and precipitation inhibitors to improve oral absorption of celecoxib from solid oral formulations. J Pharm Sci 96:2686–2702. Bak A, Gore A, Yanez E, Stanton M, Tufekcic S, Syed R, Akrami A, Rose M, Surapaneni S, Bostick T, King A, Neervannan S, Ostovic D, Koparkar A. 2008. The co-crystal approach to improve the exposure of a water-insoluble compound: AMG 517 sorbic acid co-crystal characterization and pharmacokinetics. J Pharm Sci 97:3942–3956. Van Speybroeck M, Barillaro V, Thi TD, Mellaerts R, Martens J, Van Humbeeck J, Vermant J, Annaert P, Van den Mooter G, Augustijns P. 2009. Ordered mesoporous silica material SBA15: A broad-spectrum formulation platform for poorly soluble drugs. J Pharm Sci 98:2648–2658. Mellaerts R, Aerts CA, Van Humbeeck J, Augustijns P, Van den Mooter G, Martens JA. 2007. Enhanced release of itraconazole from ordered mesoporous SBA-15 silica materials. Chem Commun 13:1375–1377. Heikkila¨ T, Salonen J, Tuura J, Kumar N, Salmi T, Murzin DY, Hamdy MS, Mul G, Laitinen L, Kaukonen AM, Hirvonen J, Lehto V-P. 2007. Evaluation of mesoporous TCPSi, MCM41, SBA-15, and TUD-1 materials as API carriers for oral drug delivery. Drug Deliv 14:337–347. Mellaerts R, Houthoofd K, Elen K, Chen H, Van Speybroeck M, Van Humbeeck J, Augustijns P, Mullens J, Van den Mooter G, Martens JA. 2010. Aging behavior of pharmaceutical formulations of itraconazole on SBA-15 ordered mesoporous silica carrier material. Microporous Mesoporous Mater 130:154–161. Aza¨ıs T, Tourn´e-P´eteilh C, Aussenac F, Baccile N, Coelho C, Devoiselle J-M, Babonneau F. 2006. Solid-state NMR study of ibuprofen confined in MCM-41 material. Chem Mater 18:6382–6390. Alba-Simionesco C, Coasne B, Dosseh G, Dudziak G, Gubbins K, Radhakrishnan R, Sliwinska-Bartkowiak M. 2006. Effects of confinement on freezing and melting. J Phys Condens Matter 18:15–68. Alcoutlabi M, McKenna GB. 2005. Effects of confinement on material behaviour at the nanometer size scale. J Phys Condens Matter 17:461–524. Van Speybroeck M, Mellaerts R, Mols R, Thi TD, Martens JA, Van Humbeeck J, Annaert P, Van den Mooter G, Augustijns P. 2010. Enhanced absorption of the poorly soluble drug fenofibrate by tuning its release rate from ordered mesoporous silica. Eur J Pharm Sci 41:623–630. Van Speybroeck M, Mols R, Mellaerts R, Thi TD, Martens JA, Van Humbeeck J, Annaert P, Mooter G, Augustijns P. 2010. Combined use of ordered mesoporous silica and precipitation inhibitors for improved oral absorption of the poorly soluble weak base itraconazole. Eur J Pharm Biopharm 75:354–365. Zhao D, Feng J, Huo Q, Melosh N, Fredrickson G, Chmelka B, Stucky G. 1998. Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 angstrom pores. Science 279(5350):548–552. Galarneau A, Nader M, Guenneau F, DiRenzo F, Gedeon A. 2007. Understanding the stability in water of mesoporous SBA-15 and MCM-41. J Phys Chem 111:8268–8277. Vertzoni M, Fotaki N, Kostewicz E, Stippler E, Leuner C, Nicolaides E, Dressman J, Reppas C. 2004. Dissolution media simulating the intralumenal composition of the small intestine: Physiological issues and practical aspects. J Pharm Pharmacol 56:453–62. Barrett EP, Joyner LG, Halenda PP. 1951. The determination of pore volume and area distribution in porous substances. J Am Chem Soc 73:373–380.

JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 11, NOVEMBER 2011

4876

VAN SPEYBROECK ET AL.

23. Rosenholm JM, Lind´en M. 2008. Towards establishing structure–activity relationships for mesoporous silica in drug delivery applications. J Control Release 128:157–64. 24. Spildo K, Høiland H, Olsen MK. 2000. Adsorption of benzoic and 4-heptylbenzoic acid on different silica substrates

JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 11, NOVEMBER 2011

from organic and aqueous solution. J Colloid Interface Sci 221:124–132. 25. Monkhouse DC, Lach JL. 1972. Use of adsorbents in enhancement of drug dissolution. I. J Pharm Sci 61:1430– 1435.

DOI 10.1002/jps