Taste-masking properties of solid lipid based micropellets obtained by cold extrusion-spheronization

International Journal of Pharmaceutics 506 (2016) 361–370

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International Journal of Pharmaceutics journal homepage: www.elsevier.com/locate/ijpharm

Taste-masking properties of solid lipid based micropellets obtained by cold extrusion-spheronization Gustavo Freire Petrovicka,* , Jörg Breitkreutza , Miriam Pein-Hackelbuscha,b a b

Institute of Pharmaceutics and Biopharmaceutics, Heinrich-Heine-University, Universitaetstrasse 1, 40225, Duesseldorf, Germany Life Science Technologies, University of Applied Sciences Ostwestfalen-Lippe, Georg-Weerth-Strasse 20, 32756 Detmold, Germany

A R T I C L E I N F O

Article history: Received 21 March 2016 Received in revised form 20 April 2016 Accepted 21 April 2016 Available online 28 April 2016 Chemical compounds studied in this article: Metformin hydrochloride (PubChem CID: 14219) glyceryl trimyristate (PubChem CID: 11148) glyceryl distearate (PubChem CID: 114690) Keywords: Metformin hydrochloride Taste masking Lipid pellets Cold extrusion Electronic tongue Drug release Spheronization

A B S T R A C T

Taste-masked properties of micropellets based on hard fat and/or solid lipid mixtures (prepared by solvent-free cold extrusion/spheronization), containing metformin hydrochloride were investigated. An in-line and an on-line drug release profile evaluation of the pellets was performed and further correlated with an electronic tongue investigation. The pellets based on more than 30% of lipid binders showed metformin HCl releases below 10% after 30 s of dissolution. Micropellets based on 20 and 30% of lipids showed immediate drug release profiles. Likewise, the electronic tongue assay showed a decrease in the sensor responses related to the increase of lipid amount in the formulations denoting a significant improvement in the taste masking properties of pellets based on more than 30% of lipid binders. A slight difference between pellets based on hard fat only and pellets based on ternary lipid mixtures was evidenced. Solvent-free cold extrusion/spheronization using solid lipids showed to be a robust method to obtain high drug loaded metformin HCl micropellets with adequate taste-masked properties and immediate drug release profile. ã 2016 Elsevier B.V. All rights reserved.

1. Introduction To overcome API bitter taste issues, several taste masking methodologies can be employed, such as use of flavors or sweeteners, coating of drug particles using polymers, using ion exchange resins, hot melt extrusion, spray drying, or by inclusion complex formation (Bandgar et al., 2009; Bhise et al., 2008; Bora et al., 2008; Mizumoto et al., 2008; Ono et al., 2010; Pimparade et al., 2015; Xu et al., 2008). During the last decade, the use of solid lipid excipients to taste mask bitter taste of drugs received more attention, in particular due to their lacking toxicity and diverse application possibilities (Suzuki et al., 2004; Witzleb et al., 2011). The use of such excipients was for example investigated based on the production of multiparticulate taste-masked systems, such as extrudates (Witzleb et al., 2011) or granules (Eckert et al., 2014; Kharb et al., 2014). These small multiple-unit oral systems are beneficial compared to monolithic dosage forms, when it comes to

* Corresponding author. E-mail address: [email protected] (G.F. Petrovick). http://dx.doi.org/10.1016/j.ijpharm.2016.04.058 0378-5173/ ã 2016 Elsevier B.V. All rights reserved.

appropriate and individual dosing. Due to their small size, solid lipid based granules can also easily be swallowed, especially when mixed with food. This facilitates the administration to pediatric and geriatric patients (Kluk and Sznitowska, 2014). To obtain solid lipid based pellets, a particular extrusion/ spheronization method employing powdered solid lipid binders, known as solid lipid cold extrusion followed by spheronization (SLCE), can be applied (Breitkreutz et al., 2003; Vaassen et al., 2012). In this gentle SLCE process lipids are not molten (at least not the bulk pat of the lipid) but “softened” due to a thermomechanical treatment that takes place some degrees Celsius below the melting point or melting range of the lipids (Kleinebudde, 2013). The extrusion is followed by a spheronization step, which similarly to the extrusion is based on a specific thermomechanical process. Lipid based pellets obtained by this method can present immediate release (Krause et al., 2009), modified release (Güres and Kleinebudde, 2011; Windbergs et al., 2010), and/or taste-masked properties (Vaassen et al., 2012). The according taste-masked characteristics are attributed to the formation of a thin melted layer during the extrusion step or due to a further lipid coating

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originated during the spheronization step. These modifications could result in lag times of a few seconds up to minutes (Krause et al., 2009; Vaassen et al., 2012) or modify the drug release profile (Reitz and Kleinebudde, 2007; Reitz et al., 2008). Although this methodology has already been used for quite a while, taste masking properties of lipid based pellets were not investigated in detail. The most common method evaluate the taste is by human taste panels. However, in vivo testing is expensive and subject to ethical and toxicological considerations and inter-subject variability. Thus, in vitro taste assessments becomes progressively popular (Gittings et al., 2014; Pein et al., 2014). Several in vitro analytical methods to assess taste-masking efficacy of dosage forms are available. Most frequently used is the spectroscopic drug dissolution analysis. As taste-masked efficiency is related to a reduction or inhibition of the interaction between the API and the oral taste buds (Sohi et al., 2004), the hindered release of API in the oral cavity is critical for taste masking. Therefore, the use of dissolution assays is an essential tool to confirm this (Gittings et al., 2014). Dissolution assays require off-line, at-line, on-line or in-line monitoring of the drug dissolution, which is usually done by UV, resp. UV/Vis spectroscopy. However, electronic tongue analysis gained importance during the last years, in particular for tastemasking assessment (Pein et al., 2014). Electronic tongues are sensor array systems capable to detect single substances or complex mixtures by means of particular sensor membranes and electrochemical techniques. From an analytical point of view, these systems are based on a different composition of sensors with variable properties and characteristics of partial or cross-selectivity which can detect a range of substances of different tastes and intensities (Gittings et al., 2014; Woertz et al., 2011). In the present investigation metformin HCl was used as model drug. Metformin HCl is an oral antihyperglycemic biguanide widely used for treatment of type II diabetes mellitus (also known as non-insulin dependent diabetes mellitus—NIDDM) due to its activity reducing fasting blood glucose levels (Campbell et al., 1996). It presents high water solubility (BCS class III) and shows an intense bitter taste (Bhoyar and Biyani, 2010). Aim of this study is to investigate the taste-masked properties of metformin HCl lipid based micropellets obtained by SLCE followed by spheronization method.

2. Material and methods 2.1. Materials Metformin hydrochloride (Wanbury, Maharashtra, India), a semi-synthetic powdered hard fat (Witocan1 42/44, Cremer Oleo, Witten, Germany), glyceryl distearate (Precirol1 ATO 5, Gattefossé, Weil am Rhein, Germany) and glyceryl trimyristate (Dynasan1 114, Cremer Oleo, Witten, Germany) were sieved through a 300 mm sieve prior to usage.

2.2. Lipid extrusion The powdered solid lipids were blended for 15 min at 25 rpm (LM40, Bohle, Ennigerloh, Germany) with metformin hydrochloride in ratios as listed in Table 1. The extrusion was performed on a co-rotating twin-screw extruder (Mikro 27GL-28D, Leistritz, Nuremberg, Germany). The blends were transferred into a dosing device (KT20K-Tron Soder, Lenzhard, Swizerland) which fed the powder gravimetrically into the barrel of the twin-screw extruder (feeding rate: 40 g min
1, screw speed: 50 rpm). The extruder was equipped with an axial screen plate with 91 dies of 0.5 mm in diameter and 1.35 mm length. Temperature of the equipment barrels was kept at 25  C except the last barrel, which was set to 33  C. 2.3. Spheronization Batches of 300 g extrudate were spheronized (15 min, 1500 rpm) using a spheronizer (RM 300, Schlüter, Neustadt, Germany) equipped with a cross-hatchet rotor plate of 300 mm in diameter. The spheronizer jacket temperature was adjusted to 30  C. An infrared light source was employed according to our prior work (Petrovick et al., 2015) aiming to heat the material up to 33  C. The produced pellets were sieved and the fraction between 250 mm and 1000 mm was used for further evaluation. 2.4. Pellets characterization 2.4.1. Particle size and size distribution Particle size and size distribution were determined using a Camsizer1 XT (Retsch Technology, Haan, Germany) by using a freefall module. Aspect ratio (AR) of the micropellets was determined as the ratio between the particle Feret diameter which is the longest chord of the measurement particle projection (particle width, B) and the particle diameter which is the longest Feret diameter of the measured particle projection (particle length, L) according to Eq. (1). Each batch was evaluated in triplicate. AR ¼ L=B

ð1Þ

2.4.2. Scanning electron microscopy (SEM) The surface morphology of the particles was evaluated using a Phenom1 G2 PRO Desktop scanning electron microscope (Phenom-World, Eindhoven, Netherlands). 2.4.3. Drug release profile evaluation A dissolution tester (Sotax1 AT7 Smart, Sotax, Loerrach, Germany) equipped with baskets (stirring speed: 150 rpm) as described in European Pharmacopoeia (2005) was used. The dissolution media consisted of 900 mL of demineralized water containing 0.001% (w/w) polysorbate 20, at 37  0.5  C. Drug release was monitored by UV/Vis spectrometry (Lambda 40, Perkin Elmer, Rodgau, Germany) in a continuous flow-through cuvette, at a wavelength of 232 nm.

Table 1 Extrudate formulations. Components (%; w/w)

F1

F2

F3

F4

F5

F6

Placebo A

Placebo B

Metformin HCl Hard fat (Witocan1 42/44) Glyceryl distearate (Precirol1 ATO 5) Glyceryl trimyristate (Dynasan1 114)

80 20 – –

70 30 – –

50 50 – –

80 15 2.5 2.5

70 22.5 3.75 3.75

50 37.5 6.25 6.25

– 100 – –

– 66.6 16.7 16.7

Extruder was equipped with an axial screen plate with 91 dies of 0.5 mm in diameter and 1.35 mm length. Temperature of the first six equipment barrels were kept at 25  C. The barrel near to the die plate was set to 33  C.

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2.5. Taste masking evaluation 2.5.1. UV/VIS probe assay For in-line measurements, an UV transmission probe (T300-RTUV-VIS, Ocean Optics, Ostfildern, Germany) with a path length of 10 mm, connected via an optical fiber with a deuterium light source (Mikropak1 DH-2000-BAL, Ocean Optics, Ostfildern, Germany), was used.

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analysis, Euclidean distances were calculated for all the used variables (n), according to Eq. (2). The values were further normalized in percentage considering the distance of the placebo to metformin HCl (bitter taste reference) as 100%. rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2 Xn p
q ð2Þ dðp;qÞ ¼ j i i¼1 j¼1

3. Results and discussion 2.5.1.1. Calibration procedure. Solutions of metformin HCl in demineralized water containing 0.001% (w/w) of polysorbate 20, at concentrations from 0.001 to 0.015 mg mL
1, were prepared and evaluated in triplicate (l = 232 nm).

3.1. Lipid pellets formulations

2.5.1.2. In-line measurements. Pellet samples (corresponding to 10 mg of metformin HCl) were placed into a cylindrical sinker. Dissolution studies were performed in 900 mL of demineralized water containing 0.001% (w/w) polysorbate 20, at 37  0.5  C applying a paddle apparatus (stirring speed: 100 rpm). Drug release was monitored using the UV probe (l=232 nm). The data was collected using SpectraSuite v.2.0.162 Software (Ocean Optics, Ostfildern, Germany).

Due to the high solubility of metformin HCl, applied lipid binders are expected to have major impact on the release profile and thus on the taste masking properties of the micropellets. The choice of employed lipid binders (Table 1) was based on previous works, which reported that hard fat, glyceryl trimyristate and glyceryl distearate exhibit adequate properties to the extrusion/ spheronization process (Krause et al., 2009; Reitz and Kleinebudde, 2007; Vaassen et al., 2012; Witzleb et al., 2011). The extrusion and spheronization parameters were based on the methodology developed in our previous work (Petrovick et al., 2015).

2.5.2. Electronic tongue assay

3.2. Characterization of the pellets

2.5.2.1. Electronic tongue measurements. All measurements were performed using a taste sensing system TS-5000Z (Insent Inc., Atsugi-Chi, Japan). The electronic tongue is equipped with seven lipid membrane sensors, which are dedicated to different taste qualities (umami, saltiness, sourness, astringency, bitterness, Table 2), and reference electrodes. The equipment parameters, such as washing steps and data collect times, were performed according to Woertz et al. (2010).

3.2.1. Morphological characteristics of the pellets The quality of micropellets produced by spheronization is commonly related to their particle size distribution and particle shape. There are several methods to evaluate this last characteristics, such as sphericity, shape factor, and circularity (Bouwman et al., 2004). In the present work, aspect ratio (AR) was chosen as width-length factor to characterize the pellet shape. The morphological characteristics of the produced lipid micropellets are shown in Table 3. Concerning shape and particle size distribution, micropellets of differing content presented comparably good values (e.g. AR < 1.3). Results were in good agreement to Kleinebudde (1995), who considered these AR values as sufficient for pharmaceutical pellets regarding adequate material flow and our previous work (Petrovick et al., 2015). SEM images (Fig. 1a) show smooth and slightly porous surfaces for all lipid pellet formulations. Crystals of metformin HCl are fully embedded in the lipid matrix, more prominently in micropellets based on higher amounts of lipids (F5 and F6).

2.5.2.2. Electronic tongue calibration procedure. Metformin HCl solutions of 0.01, 0.1, 0.5, 1.0, 2.5, 5.0, 8.5, 10.0, and 15.0 mg mL
1 were prepared and evaluated. 2.5.2.3. Sample preparation. Micropellets corresponding to 1.0 g of metformin HCl were stirred (100 rpm) in 100 mL demineralized water at 37  C. After 30, resp. 60 s, samples were immediately filtered through a 20 mm paper filter using a vacuum pump. 2.5.2.4. Evaluation of taste-masking properties. E-tongue results were expressed as raw data in mV of the relative measurement of the samples to the reference according to Woertz et al. (2011). Sensor signals were evaluated univariate and multivariate. Data processing and graphical illustration of the results were carried out using Excel 2013 (Microsoft, Redmond, USA) and SIMCA1 v.13.0.3 (Umetrics AB, Umea, Sweden). To determine the relative distance between the sample and the placebo (p, q) after multivariate data

3.2.2. Drug release profile Due to the high solubility of the API and applied sink conditions during the dissolution studies, drug diffusion through the lipid matrix is assumed to be the main mechanism controlling the drug release. According to literature (Güres and Kleinebudde, 2011; Guse et al., 2006; Kreye et al., 2011; Rosiaux et al., 2014), investigated micropellets remain intact during dissolution, which

Table 2 Sensors for the taste sensing system and corresponding taste sensations. Sensor type

Sensor name

Corresponding taste sensation

SB2AAE SB2CT0 SB2CA0 SB2AE1 SB2AC0 SB2ANO SB2C00 Reference electrode

Umami sensor Saltiness sensor Sourness sensor Astringency sensor Bitterness sensor 1 Bitterness sensor 2 Bitterness sensor 3

Umami Saltiness Sourness Astringency Bitterness of cationic substances Bitterness of cationic and neutral substances Bitterness of anionic substances

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Table 3 Pellet characteristics. Property (x  s)

F1

F2

F3

F4

F5

F6

d10 (mm) d50 (mm) d90 (mm) AR

427.97  38.78 719.00  5.29 927.90  126.90 1.21

630.53  4.46 763.67  1.33 918.43  52.59 1.15

660.57  0.95 759.00  1.31 913.93  40.67 1.29

400.67  21.04 734.53  8.32 841.70  30.17 1.25

646.60  19.67 760.90  2.56 851.40  5.97 1.22

682.23  2.53 785.47  3.63 957.47  82.34 1.37

Fig. 1. (A) SEM micrographs from the lipid pellets surface (taken at 880 magnification) and (B) SEM micrographs of the pellets after dissolution (taken at 450x magnification).

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gradient, which is higher at the matrix extrudate surface and lower within the matrix due to hindered exchange with the release medium because of fine pore capillaries (Güres et al., 2011; Reitz et al., 2008; Rosiaux et al., 2014). 3.3. Taste masking investigation

Fig. 2. (A) Drug release from the lipid pellets trough online measurements using dissolution apparatus and (B) drug release from the lipid pellets trough in-line measurements using a UV/VIS probe.

could be proven by SEM images of the pellet surfaces after dissolution (Fig. 1b). A highly porous structure is formed due to the dissolution of metformin HCl particles. Regarding the drug release profiles (Fig. 2a), micropellets based on 20 and 30% (w/w) of lipids can be classified as immediately releasing (USP 34, 2011), since they release more than 85% (w/w) of metformin HCl after 10 (F1) an 22 min (F4), resp. 36 (F2) and 54 min (F5). A complete drug release was achieved after 20–25 min for the micropellets based on 20% (w/w) of lipids, and 58–70 min to the micropellets based on 30% (w/w) of lipids. Micropellets F3 and F6 showed differing drug release kinetics. A faster initial drug release followed by slower and sustained release of metformin HCl was observed. The fast dissolution of the particles located on the extrudate surfaces produces the burst effect. Followed drug release is controlled by the concentration

3.3.1. In-line measurements using an UV probe According to literature, micropellets obtained by solid lipid extrusion provide a lag time or slow drug release rate within the first minutes of dissolution (Kharb et al., 2014; Vaassen et al., 2012; Witzleb et al., 2011). Therefore, drug dissolution was carefully monitored in-line over 5 min (Fig. 2b). However, results were similar compared to those in Fig. 2a, with no lag time visible. To judge about taste masking properties, several authors refer to the FIP/AAPS guideline, suggesting a successful taste-masking with a drug release of <10% within the first 5 min (Siewert et al., 2003). Based on this suggestion, only pellet F6 can be characterized as taste-masked, since all the other formulation released more than >10% (w/w) of metformin HCl after 5 min. Though, micropellets containing glyceryl distearate and trimyristin show different behavior in this time window. Small differences in the drug release kinetic are observed due to the increase of the lipid amount comparatively to the micropellets based on hard fat only. Similar observations were described by Krause et al. (2009), who reported the influence of these lipid binders on the drug release kinetic of a highly soluble API. The micropellets F4 show not more than 40% (w/w) of metformin HCl released after 5 min, while F1 releases around 90% (w/w) of the API. On the other hand, micropellets F5 and F6 show similar drug release profiles within these first 5 min, releasing 19.6 and 15.9% (w/w) of metformin, respectively. These previous results could indicate that the spheronization of the formulations based on the ternary lipid mixture lead to micropellets presenting taste-masked properties even at high drug loads comparatively to formulations based on only hard fat. However, 5 min can be considered as “unrealistic” residence time for a solid dosage form in the oral cavity. Within a study on children with attention deficit hyperactive disorder and autistic disorder Beck et al. (2005) defined “successful and secure swallowing time”, when the residence time did not exceed 30 s. Thus, Pein et al. (2014) suggested 30 s of dissolution as more reliable to investigate taste-masking properties of solid oral dosage forms. Released metformin HCl concentrations after 30 and 60 s are accordingly summarized in Table 4. After 60 s, micropellets F1 and F4 presented drug release values > 20% (w/V), while the micropellets based on 30 and 50% (w/w) of lipids presented values around 10 and 6% (w/V), respectively. Taking into consideration that drugs usually do not remain that long in the oral cavity, values for 30 seconds become more significant. At this time, all micropellets presented drug release rates between 4 and 13% (w/V). These results indicate a stronger modification in the taste perception of these formulations and thus, all formulations, with exception of the micropellet F1, are assumed to present tastemasked properties.

Table 4 Drug release from lipid pellets after 30 and 60 s. Time (s)

30 60

Drug release (%; w/V; n = 3, mean  SD) F1

F2

F3

F4

F5

F6

37.06  1.72 52.51  0.92

9.19  0.50 12.26  0.28

4.12  0.11 5.78  0.08

11.70  1.08 22.99  0.77

7.59  0.41 11.87  0.25

4.46  0.25 6.94  0.09

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Fig. 3. Individual sensor responses to metformin HCl.

3.3.2. Electronic tongue 3.3.2.1. Electronic tongue calibration. In order to investigate the sensor responses of the electronic tongue to metformin HCl, a calibration procedure was conducted, assessing each individual response for metformin HCl solutions of 0.01–15 mg mL
1. Each sensor presented a concentration dependent response to metformin HCl (Fig. 3,Table 5). A log-linear range was observed for all sensors beginning at concentrations of 0.50 mg mL
1. Proving the literature known unpleasant taste of metformin HCl, sensor SB2AC0, which is dedicated to bitterness of cationic substances, showed sensitivity at even lower concentrations. 3.3.2.2. Taste masking investigation of sample pellets. The sample preparation (dissolution/dispersion) to this assay is considered a critical point, as for example pre-treatment times could offer safety margins for evaluating taste-masking effects (Pein et al., 2014). The micropellets based on hard fat lonely (F1, F2, and F3) and the micropellets based on the ternary lipid mixture (F4, F5, and F6) were evaluated separately by an electronic tongue. To investigate the taste-masking properties of these micropellets, samples were stirred in 100 mL of purified water for 30 s and 60 s resp., prior to filtering. The principal component analysis (PCA) (Fig. 4a) combines sensor responses of the micropellets based on hard fat only, metformin HCl, and drug-free micropellets. The first principal component (PC1) contains 90.5% of the information. Therefore, horizontal distances between the samples count the most. Pure metformin HCl is positioned in the left corner in the Scores Scatter Plot, while the drug-free formulation is located on the right. The loading scatter plot (Fig. 4b) proves the results from the calibration study—those samples, which contain metformin, are directly correlated to the sensor responses of SB2AC0.

The lipid micropellets F1 are located nearest to the pure drug and a small difference between the samples of 30 and 60 s dissolution time is observed. Scattering of the data points of this and the other pellet samples are due to the fast drug release and are in agreement to the higher standard deviations displayed in Fig. 2. However, second principal component (PC2) contains only 5.7% of the information is displayed. The micropellets F2 are positioned in the central part of the PCA map, shifting from the pure drug towards the placebo formulation. Although a better taste-masking compared to micropellets F1 can be assumed, no great difference between the 30 and 60 s is perceived. Formulation F3 showed the highest proximity to the drug-free micropellets indicating the most taste-masked properties. This result is in good agreement with the drug release investigations, since a relation between the lipid amount in the micropellets and their drug release kinetic was found. F2 (after 30 s of stirring) and F3 are positioned in the same quadrant on the PCA map (Fig. 4a). This indicates that these samples are positively correlated based on similar sensor responses and, therefore, similar taste-masked properties might be assumed. The PCA representing sensor responses formulation F4-F6 is depicted in Fig. 5a. PC1 contains 80% of the MVDA information, PC2 9.9%. A similar loading scatter plot is generated (Fig. 5b) compared to the observed in the MVDA of micropellets based on hard fat lonely (Fig. 4b), showing similar correlation between the bitter tasting metformin and the sensors SB2AC0 and SB2AN0. Concerning the PCA results, and initially regarding the PC1 (xaxis) information, a similar pattern can be observed. In comparison to Fig. 4a, sample location is related to the amount of lipids and the proximity to the drug-free formulation. The higher the amount of lipids, the closer the sample lies next to the drug-free formulation. These results also can be correlated with the drug release results (Fig. 2), since slower metformin HCl releases were observed for formulations based on higher amounts of lipids. The less metformin HCl is released, the lower the responses of the sensors SB2AN0 and SB2AC0 to the metformin HCl concentration are and, thus, the better the taste is assumed. In the case of F4, this could also clearly be seen for the samples taken at 30, resp. 60 s. 3.3.2.3. Euclidean distance. The Euclidean distance is an ordinary distance between two Cartesian points and the use of this equation allows the conception of a metric space using the PCA map results. These calculated distances can be used to compare formulations in different PCA plots allowing a better visual comparison. Employing this approach, the distance between metformin HCl and a drugfree formulation can be considered as 100% of “pleasant-tounpleasant taste variation” or the achievement of an adequate taste masking effect. However, this premise is only valid considering metformin HCl as the bitter and unpleasant taste sample while the drug-free formulation is considered as good tasting. Euclidean distances for the samples of 30 and 60 s for F1–F6 are depicted in Fig. 6. Displayed results indicate that the lower the

Table 5 Sensor responses. Sensor

Log-linear range [mg mL
1]

Sensitivity [mV]

Slope

Coefficient of determination (R2)

SB2AAE SB2CT0 SB2CA0 SB2C00 SB2AE1 SB2AC0 SB2AN0

0.5–1.5 0.5–1.5 0.5–1.5 0.5–1.5 0.1–1.5 0.01–1.5 0.5–1.5

28.74 67.48 38.18 57.10 106.68 143.76 42.96


8.94
20.09
11.73
15.03
21.57 20.47 12.95

0.9875 0.9960 0.9965 0.9371 0.9994 0.9903 0.9938

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Fig. 4. (A) PCA map for the lipid pellets based on hard fat lonely (F1 to F3) and (B) respective loading plot for the set of sensors.

residence time of the dosage form in the oral cavity, the lower the bitter taste perception by the patient is assumed. It is interesting to notice, that the micropellets based on 50% (w/w) of lipids (F3 and F6) the Euclidean distances after 30 or 60 s are quite small, indicating that these formulations present better taste-masked properties, even at longer contact times. Micropellets containing only hard fat presented generally lower Euclidean distances values to the drug-free formulation compared to the ternary lipid micropellets formulations. This result could indicate that the presence of only hard fat as lipid binder in the pellet formulation proportionate better taste-masking properties to dosage form. Interestingly, the presence of glyceryl trimyristate and glyceryl distearate in the formulation did not alter the drug release of metformin HCl compared to the formulations containing only hard fat. However, different responses of the electronic tongue to the presence of different lipids were observed. This variation of the responses of these formulations can be explained due to an intrinsic taste response of the lipids or even by the presence of lipid particles in contact with the lipid membranes of the sensors. This observation shows the importance of evaluating taste masking effects electronic taste sensing systems.

3.4. Electronic tongue and UV probe results interrelation UV spectroscopy can be applied for taste-masking evaluation based on two reasons: the unpleasant taste is related to the API and this API needs to be dissolved to cause an unpleasant taste. However, this only counts, if the taste is not altered by complex formulations or added excipients (Pein et al., 2014), since human psychophysical studies suggested that the taste of chemical compounds can be different in a mixture than when presented alone (Bartoshuk, 1975; Moskowitz, 1972). The presence of the lipids presented an influence on the electronic taste sensing responses, which is not detected by UV analysis. Mixtures of mono-, di- and triglycerides, where the small molecules could partially dissolve in water, can interact with the electronic tongue sensors, and therefore causing a response. Furthermore, lipids are also know for presenting themselves a specific taste and certain bitter taste intensity (Suzuki et al., 2004) and can therefore not be neglected in taste-masking studies. Likewise, electronic tongue results need to be carefully interpreted. Applied sensors are based on a potentiometric measurement principle and therefore interact differently to charged and

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Fig. 5. (A) PCA map for the lipid pellets based on lipid mixture as binders (F4 to F6) and (B) respective loading plot for the set of sensors.

uncharged molecules. Furthermore, molecular interactions between different formulation components could induce to misinterpretation of the results (Pein et al., 2014). Therefore, combining these both different methodologies results to evaluate taste masking properties could be beneficial

to a better interpretation of the taste-masked properties of formulations. In the present study, UV and electronic tongue analysis have been applied and both methods respond to the variation of concentrations of the API. As the both methods present an increase in their responses related to the concentration of metformin HCl, the sensor responses of bitter sensors SB2AN0 and SB2AC0 against the according UV absorption units were correlated, and the results are plotted in Fig. 7. Although the linear relationship between concentration and responses for the both methods is in different ranges (0.001–0.01 mg mL
1 in the UV Probe and 0.01–15 mg mL
1 in the electronic tongue) a good logarithmic linear correlation is observed for the both electronic tongue sensors. This correlation could indicate that the both methodologies provide relatively complementary information to the investigation of taste-masking properties, since they provide a set of information at different concentration ranges.

4. Conclusion

Fig. 6. Euclidean distances to placebo.

Taste-masking properties of metformin HCl pellets, containing diverse amounts of hard fat or ternary solid lipid mixtures and different concentrations of API, were investigated by a taste sensing system and an in-line applied UV/VIS probe.

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method to obtain high drug loaded metformin HCl pellets with adequate taste-masked with fast drug release rates. Conflict of interest The authors declare no conflict of interest. Acknowledgement This work was supported by the German Academic Exchange Service (DAAD). References

Fig. 7. Relation between the sensors (A) SB2AC0 and (B) SB2AN0 and the UV probe absorption results for different metformin HCl concentrations.

Both methods showed results dependent on the concentration of released metformin HCl per time, which were related to the amount and type of applied solid lipid: higher amounts of lipids led to a slower drug release rate within the first minutes of dissolution. Both investigation methods moreover provided adequate sensibility to the taste-masking properties of lipid pellets at different concentration ranges. Results of the electronic tongue were more embracing, since it takes sensor responses to other components of the formulation into account. Besides this, complementary results but a more complete drug release profile were shown by the UV/ VIS probe. Each of the investigated formulations presented improved taste-masked properties compared to pure metformin HCl. Higher amount of lipids resulted in micropellets with higher taste-masked characteristics due to the retardation of the drug release within the first minutes. Pellets based on only hard fat as lipid binder showed similar Euclidean distances to the placebo in the PCA map compared to pellets based on the ternary lipid mixture. However, formulations containing the lipid mixture presented lower drug release rates within the first 5 min of dissolution. The experiments showed an improvement on the results regarding the drug release within the first minutes and a major proximity of the electronic tongue sensor responses comparing the pellets and a free-drug formulation. Though, more information concerning the human threshold to bitter taste of metformin HCl is still required to define acceptable taste levels of this API in oral solid dosage forms. Regarding the observed results, solvent-free cold extrusion/ spheronization, using powdered solid lipids, showed to be a robust

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