J Therm Anal Calorim (2017) 127:871–880 DOI 10.1007/s10973-016-5918-4

Magnesium aluminium silicate–gentamicin complex for drug delivery systems Preparation, physicochemical characterisation and release profiles of the drug A. Rapacz-Kmita1 • E. Stodolak-Zych1 • M. Dudek2 • M. Gajek1 • M. Zia˛bka1

Received: 9 December 2015 / Accepted: 18 October 2016 / Published online: 1 November 2016 Ó Akade´miai Kiado´, Budapest, Hungary 2016

Abstract This paper presents the characteristics of magnesium aluminium silicate–gentamicin complexes for drug delivery systems. The work describes the results of studies on the successful introduction of gentamicin (an aminoglycoside antibiotic) into the interlayers of smectite clay and examines the possible use of intercalated smectite as a carrier for sustained drug release. Characterisation of magnesium aluminium silicate–gentamicin complexes was carried out by means of X-ray diffraction, Fourier transform infrared spectroscopy, thermal analysis and scanning electron microscopy with EDX analysis. The possibility of using the gentamicin intercalated smectite as a carrier for sustained release of the drug was investigated during in vitro study, in which the release rate of gentamicin from the smectite clay matrix was monitored based on absorption at 330 nm using a UV–Vis spectrometer and the kinetic of drug release was evaluated based on the zeroorder, first-order, Higuchi and Korsmeyer–Peppas models. The results confirmed the efficiency of intercalation and indicate the potential for introducing gentamicin into the interlayer space of montmorillonite. Accordingly, the obtained material may thus be used as a drug carrier in modulated drug delivery systems. Keywords Clays  Gentamicin  Drug release  Intercalation  Biomedical applications

& A. Rapacz-Kmita [email protected] 1

Faculty of Materials Science and Ceramics, AGH University of Science and Technology, Mickiewicza 30, 30-059 Krako´w, Poland

2

Faculty of Energy and Fuels, AGH University of Science and Technology, Mickiewicza 30, 30-059 Krako´w, Poland

Introduction Clays are commonly used in the pharmaceutical industry and play an important role in the field of health products following evaluation and/or modification to meet regulatory requirements. They may be accepted for use in the medicinal applications as both active substances and carriers for active substances, but the critical issue is then to understand the interactions between clays and medicines. These interactions may be beneficial from the biopharmaceutical perspective, improving solubility, reducing oral absorption and/or modifying the drug release profile [1, 2]. Numerous studies have been published related to beneficial interactions of clays with various medicines, but co-administration of medicines with clays and resulting interactions may also be used to obtain technological and biopharmaceutical advantages. This paradigm is considered as the starting point for the use of clays in drug delivery systems (DDSs) with modified drug release, which is motivated by the fact that the use of drugs in conventional forms may lead to unwanted fluctuations of active substances in a living organism. These concentrations may reach levels lower than the minimum effective (MEC) or higher than the minimum toxic (MTC) concentration, resulting in undesirable side effects or the absence of therapeutic benefits intended for the patient. The use of materials which may modify the release of drugs and help to reduce undesirable levels, thus reducing side effects and/ or enhancing the drug’s therapeutic effect, eventually improve the patient’s compliance with the treatment, and add commercial value to medicines [3, 4]. In the pharmaceutical field, the use of clays as effective materials for drug delivery systems has received much attention in view of the minerals’ structural availability, capacity for inclusion and spontaneous dispersion in

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aqueous solutions [5]. The intercalation of medicines, which are usually organic species, into layered inorganic solids provides a useful and convenient route for the preparation of organic–inorganic hybrids containing properties of both the inorganic host and organic guest in a single material. Montmorillonite (MMT) has been investigated as a hybrid carrier for the controlled delivery of numerous drugs, for example ibuprofen, vitamin B1, nicotine and others [6–9]. The properties of clays used for pharmaceutical applications which determine antibacterial activity and the ability to immobilise cell toxins are directly related to their colloidal size and crystalline structure in layers, i.e. a high specific surface area, optimum rheological characteristics and/or excellent sorptive ability [10–14]. This, however, depends on their capacity for interaction with the molecules of the drug through either surface adsorption or ion exchange reaction [15–17]. In the present work, a more detailed characterisation of magnesium aluminium silicate–gentamicin complexes for drug delivery systems is described. Preliminary attempt to introduce gentamicin into the interlayer space of smectite clay (magnesium aluminium silicate) was performed, however, in a previous work [18] to evaluate the clay’s capacity to be used as a hybrid carrier for gentamicin. This paper characterises smectite–gentamicin complexes by using XRD, FTIR, DTA/TG and SEM with EDX analysis, and the possibility of using the gentamicin intercalated smectite as a carrier for sustained release of the drug is evaluated based on the release profile of the antibiotic from the smectite–gentamicin hybrid during in in vitro tests performed according to The United States Pharmacopeia: Content of gentamicin sulfate.

Experimental Materials Gentamicin (structure shown in Fig. 1) is one of the few thermally stable antibiotics, belonging to the aminoglycoside family of antibiotics used in medical practice as sulphate to treat bone infection problems caused by poor circulation of blood in osseous tissue [19] and in the form of eye ointments or drops and intramuscular injections. The gentamicin antibiotic used in the study was purchased from OH H3 C

HN H3C

O HO

O

HO O H2N

Fig. 1 Structure of gentamicin (G)

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CH3

H2N O

NH2 NH2

Sigma-Aldrich (USA) in the form of gentamicin sulphate (C21H43N5O7H2SO4). The clay mineral was pharmaceutical magnesium aluminium silicate (MAS), VeegumÒF, manufactured by the R. T. Vanderbilt Company, Inc. (USA). The intercalation of gentamicin into the structure of smectite was done according to the procedure described in the following section; the abbreviations given for the materials and used in the study are: G—gentamicin, MAS—magnesium aluminium silicate, MAS-G—hybrid intercalated with gentamicin. Preparation of smectite–gentamicin complex Smectite powder was weighed in the amount of 2 g, then mixed with 50 mL of distilled water using a magnetic stirrer while being heated at 60 °C for 24 h. Then, 0.5 g of gentamicin sulphate was added to the prepared suspensions of the powder to obtain a 4:1 MAS/G ratio (initial concentration of gentamicin 0.434 mmol g-1). Following the addition of gentamicin, the solution was mixed again with a magnetic stirrer at 60 °C for an additional 24 h. The mixed solution was then dried in a standard process at 60 °C and grated in an agate mortar. A part of the mixed solution was lyophilised at minus 51 °C to maintain the proper microstructure in the MAS-G intercalated smectite for SEM observation purpose. Methods The powder of clay intercalated with gentamicin (MAS-G), the starting smectite powder (MAS) and the pure gentamicin (G) were examined using XRD, FTIR, DTA, TG/DTG and SEM (?EDX analysis). The study aimed at observing the behaviour of gentamicin in the presence of smectite and identifying possible ways for gentamicin to be incorporated into the interlayer spaces of smectite. X-ray diffraction (XRD) analysis was performed in the range 3 \ 2h \ 70 with Cu Ka radiation using a PANalytical Empyrean diffractometer with a step of 0.008 degrees (total time about 4 h). The analysis focussed on the range 5 \ 2h \ 40, where the most characteristic diffraction peaks were expected. The FTIR spectra of the MAS, MAS-G and G powders were recorded with a Bio-Rad FTS 60 V spectrometer in the range of 400–4000 cm-1 using KBr pellets. The thermal properties of the powders (20 mg samples) were studied via DTA and TG/DTG analysis (in air flow 20 mL min-1, temperature range 35–760 °C, heating rate of 10 °C min-1) using a STA 449 F3 Jupiter (Netzsch) simultaneous thermal analyser. Microstructural observations of the studied smectite and gentamicin powders were performed with the use of a Nova NanoSEM 200 scanning electron microscope (FEI Europe). During the follow-up, EDX analysis was also used in microareas. All types of powders were covered with a carbon layer prior to observation.

Magnesium aluminium silicate–gentamicin complex for drug delivery systems

The in vitro gentamicin release tests from the smectite clay matrix were performed according to The United States Pharmacopeia: Content of gentamicin sulfate [20]. The powder of clay intercalated with gentamicin (MAS-G) in an amount of 20 mg was incubated in a phosphate buffer (PBS), pH 7.4, at 37 °C for 12 days, maintaining the ratio of the buffer volume of the powder mass ratio at 1:5 9 10-6. Measurement of drug release was carried out over 12 days. In each case, a suspension of 1 mL of supernatant was collected and drug content was determined spectrophotometrically. The collected release medium was measured at k = 330 nm using a Lambda Bio UV–Vis spectrometer (PerkinElmer), and each test was carried out in triplicate with the reported results representing the average of the three tests. To determine the amount of gentamicin released during incubation in vitro, two specific chemicals were used, namely: o-phthalaldehyde and b-mercaptoethanol (both Sigma-Aldrich). The concentration of the drug released into the PBS solution was determined using the Lambert– Beer law. The calibration curve was constructed by preparing solutions of known concentrations of gentamicin (0–500 lg mL-1). According to this procedure, the amount of drug released (%) is shown as a function of the release time (days). Based on the data available in the drug database [21], which describes the maximum daily dose of the drug (10–12 lg mL-1), the referenced gentamicin concentration obtained in the PBS solution was re-calculated to the maximum value of the drug (10 lg mL-1). The kinetic of drug release was evaluated based on the zeroorder, first-order, Higuchi and Korsmeyer–Peppas models, according to Eqs. (1)–(4). Zero-order model [22, 23]: f t ¼ f0  K0  t

ð1Þ

First-order model [24, 25]: f t ¼ 1  expðK1  tÞ

ð2Þ

Higuchi model [26, 27]: f t ¼ KH  t0:5

ð3Þ

Modified Korsmeyer–Peppas model [28, 29]: f t ¼ a  tn þ b

ð4Þ

where ft is the amount of drug dissolved in time t, f0 is the initial amount of drug in the solution, K0, K1 and KH are the zero-order, first-order and Higuchi release constants, respectively, expressed in units of concentration/time, t is the time of release, and a is the constant incorporating the structural and geometric characteristics of the drug dosage form, n is the release exponent, characteristic of the release mechanism, and b represents the burst effect in the release.

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Results and discussion XRD analysis The XRD analysis was used to assess the variation in the basal spacing in the smectite-based carrier, which is a primary indicator of the effectiveness of the drug intercalation. Figure 2 shows the XRD patterns of pure magnesium aluminium silicate (MAS), gentamicin (G) and MAS following intercalation with gentamicin (MAS-G). The diffractogram of pristine MAS material shows the primary reflection at about 7.22° 2h, which corresponds to a ˚ . The obtained montmorillonite basal spacing d001 of 12.3 A X-ray pattern is in agreement with other reports on magnesium aluminium silicate powders with a water monolayer in the interlayer space [30–33]. Following interaction with gentamicin, the basal spacing of the MAS hybrid ˚ , which corresponds to material reached a value of 14.5 A the shifted principle reflection at 6.10° 2h. Reflection peaks for hybrid clay materials increased in intensity (MAS-G vs. MAS), and additional peaks also appeared in XRD patterns at 2h = 11.64°, 18.91° and 20.90°, as a consequence of structural changes occurring in the presence of gentamicin. Similar findings had been reported previously by other authors studying e.g. the intercalation of nicotine into magnesium aluminium silicate [8]. The location of the principal peak of MAS powder shifted significantly after introducing the gentamicin into the interlayer space of the clay, which was a consequence of the increase in d-spacing, indicating that gentamicin had been effectively intercalated into the MAS and was positioned on the surface of clay as a monolayer [34]. Since the thickness of a single montmorillonite layer, which can be calculated using molecular simulations or known crystallographic data [35, 36], is only slightly lower ˚ [37, 38], and d001 is the sum of layer thickness than 7.0 A and gallery height (hg), the value of hg can be easily found. Based on this, the estimated gallery heights in the MAS-G hybrid after intercalation with gentamicin equalled ˚ . According to the size of the G approximately 7.6 A molecule predicted by molecular modelling (length 15.3, ˚ ) [39], the values of interlayer spacing conheight 5.2 A firmed that gentamicin was located as a monolayer between two 2:1 montmorillonite layers, most probably together with a monolayer of water. FTIR analysis The FTIR spectra of the studied gentamicin sulphate, MAS and MAS-G powders, registered in the range of 4000–400 cm-1, are shown in Fig. 3.

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874 Fig. 2 XRD patterns of magnesium aluminium silicate (MAS), MAS with adsorbed gentamicin (MAS-G) and gentamicin (G)

MAS-G

G

MAS

12.3 Å

Intensity/a.u.

Intensity/a.u.

MAS-G

MAS 5

10

15

20 2θ /°

25

The spectrum of gentamicin sulphate shows characteristic absorption bands between 3500 and 3100 cm-1, which correspond to the occurrence of stretching vibration bands for N–H amino groups. A large number of bands from alkyl (C–H) stretching vibrations of the drug are visible in the range between 3000 and 2800 cm-1. Numerous molecules which are components of gentamicin sulphate contribute to the FTIR spectrum and in consequence produce a broad diffuse band in the range of 3600–2500 cm-1 [40]. Bands at 1650–1400 cm-1 correspond to bending vibrations of N–H, stretching vibrations

30

35

40

5

6

7

8

9

10 11 2θ /°

12

13

14

15

of C–H and stretching vibrations of C–N. The strong absorption band between 1300 and 900 cm-1 corresponds to stretching vibrations of C–N as well as numerous stretching vibrations of group C–O [40, 41]. The spectrum of the MAS powder shows characteristic bands for magnesium aluminium silicate, that is: hydroxyl stretching of Si–OH at 3630 cm-1, hydroxyl stretching of hydrogen bonded water at 3433 cm-1, hydroxyl bending at 1637 cm-1 and stretching of Si–O–Si at 1022 cm-1. Absorption bands at 925 and 467 cm-1 are also visible in the spectrum [9, 38, 42–48]. Detailed FTIR band 1013

Fig. 3 FTIR patterns of the studied powders (MAS, MAS-G, G)

623 623

795

1452

G

4000

3600

3200

2800

2400

2000

Wavenumber/cm–1

123

1600

612 1466

1532

1626

1044

MAS

1637

3433

3630

Absorbance/%

523

797

467

1022 1421

1531

1462

MAS-G

1626

2934

3406

3551

3626

3688

523

467

0

1200

800

400

Magnesium aluminium silicate–gentamicin complex for drug delivery systems

of N–H amino groups of gentamicin. The 1532 cm-1 absorption band also comes from gentamicin and may be related to stretching vibrations of C–H. The bending vibration of the H–O–H band disappears at the same time, which may be caused by displacement of water from the interlayer space of the smectite. In the FTIR spectra of the MAS-G hybrids, a shift of the hydroxyl bending band of water at 1637 cm-1 to a lower wavenumber (1626 cm-1) and its overlap with bending vibrations of N–H from gentamicin were also observed, which may confirm the formation of intermolecular hydrogen bonding water and gentamicin [51–54]. In the FTIR spectrum for the MAS-G material, an increase in the intensity of the bands in the range 3300–2500 cm-1 is also evident. This is attributed to the presence of the stretching vibration band of N–H amino groups and alkyl (C–H) stretching vibrations of gentamicin [51–54]. Characteristic changes in the FTIR curves of the MAS-G materials in the range 3500–2500 cm-1 clearly show that gentamicin molecules have been introduced into the interlayer space of magnesium aluminium silicate [55], which indicates that the studied clay can potentially be used as a drug carrier.

assignment for MAS was previously investigated and the results were discussed in our prior work [18]. In the FTIR spectra of the gentamicin-MAS hybrids (MASG), a shift of the hydroxyl stretching band of water at 3433 cm-1 to a lower wavenumber (3406–3414 cm-1) compared to non-intercalated MAS can be observed. This indicates the possible formation of intermolecular hydrogen bonding between water and the introduced gentamicin [49]. The hydroxyl stretching band of Si–OH at 3630 cm-1 is weaker in intensity and is shifted to a lower wavenumber (3626 cm-1), accompanied by the hydrogen bonding formation of Si–OH with an amine group of gentamicin and water bound with gentamicin acting as a water bridging mechanism [1]. Furthermore, the occurrence, following the MAS-G hybrid formation, of a new peak at 3688–3690 cm-1 indicates the presence of free OH groups on the inner surface of the silicate layer of MAS [50]. The stronger vibration of free hydroxyl groups on the inner surface of the silicate layers is enabled when the basal spacing of the MAS increases due to the intercalation of the gentamicin molecules [8]. The band at 1652 cm-1 registered for the MAS material, corresponding to bending vibrations of OH, shifts its position to 1621 cm-1 for the MAS-G material. The decrease in intensity and shift of the maximum of the peak originating from the vibrations of water molecules indicates a reduction in interlayer water content. This is again caused by the displacement of water molecules from the interlayer space by organic molecules derived from gentamicin. In the MAS-G hybrid, the absorption band at 3551 cm-1 and characteristic sequence of bands at 1626, 1530 and 1466 cm-1 are attributed to the presence of gentamicin. The presence of new bands confirms that gentamicin interacts with the smectite layers. An additional band in the MAS-G spectrum at 3551 cm-1 is attributed to a stretching vibration band Fig. 4 DTA curves of MAS, MAS-G and G

DTA/μV mg–1 Exo 4.0

875

Thermal analysis DTA and TG/DTG profiles for MAS, MAS-G and pure gentamicin (G) are shown in Figs. 4–6. It is seen that the total mass loss for the non-intercalated MAS powder in the range up to 760 °C reaches 9.2% where two major mass loss steps, in the temperature ranges 35–185 °C and 500–760 °C, can be observed. The first mass loss (6.3%) corresponds to free water desorption from the particle surfaces at around 100 °C and to dehydration of the hydrated cations in the interlayers (hydration water is lost at 185 °C, but the last step of dehydration ends up at around 500 °C) [56]. The second mass loss

[1] G DTA [2] MAS

[3]

DTA

3.5

[3] MAS-G DTA

3.0 2.5 2.0 1.5 1.0 [2]

0.5

[1]

0.0 –0.5 100

200

300

400

500

600

700

Temperature/°C

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A. Rapacz-Kmita et al.

TG/%

TG/%

100

[2]

95 [1] G

[2] Mass Change: –9.19%

TG

40

[2] Mass Change: –9.19%

[3]

80 60

100

[2] MAS

[3] Mass Change: –21.34%

TG [3] MAS-G TG

90

[1] Mass Change: –97.35%

[2] MAS TG [3] MAS-G TG

[2] [3] Mass Change: –21.34%

85 20 [1]

80

[3]

0

100

200

300

400

500

600

100

700

200

Temperature/°C

300

400

500

600

700

Temperature/°C

(a)

(b)

Fig. 5 TG investigation of MAS, MAS-G and G materials: a TG curves for MAS, MAS-G and G, b a close-up comparison between MAS and MAS-G

[2] MAS

TG/% [3] MAS-G

100

TG DTG

DTG/% min–1

TG DTG

[2]

0.1

TG/% [2] Mass Change: –6.30%

100

[3] Mass Change: –3.28%

0.0

95 [2]

–0.1

[2] Mass Change: –2.88%

95

–0.2

90

–0.3 [3]

85

[3]

[2] MAS [3] MAS-G

–0.4 –0.5

80

90

–0.6

[2] [3] Mass Change: –8.57%

TG TG

[3] Mass Change: –9.49%

85 80

[3]

–0.7

100

200

300

400

500

600

700

Temperature/°C

(a)

100

200

300

400

500

600

700

Temperature/°C

(b)

Fig. 6 TG/DTG investigation of MAS and MAS-G materials: a TG and DTG curves for MAS and MAS-G with arrows indicating the mass change analysis points, b detailed mass change analysis for MAS and MAS-G

(2.88%) corresponds to the loss of the structural hydroxyl group in montmorillonite [57] and thermal decomposition of MAS starts with the release of structural hydroxyl OH groups, but the structure is still maintained above 750 °C [58, 59]. The total mass loss of pure gentamicin sulphate (G) shows an initial sharp mass loss at around 238 °C, corresponding to the decomposition of gentamicin sulphate (melting point around 218–235 °C), followed by total decomposition of the drug which reached almost 100% at 760 °C. This initial decomposition peak is not seen, however, in thermal spectra of the MAS-G hybrid material, but a stable decomposition of the drug is rather visible. The total mass loss of the MAS-G hybrid reached 21.3% up to 760 °C and can be represented in a three-step process in the temperature regions of 35–185 (3.28%), 185–450 (8.57%) and 450–760 °C (9.49%), respectively. The first and the third step is characteristic for the water loss in the clay, but an additional effect in the

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temperature range 185–450 °C is explicable in terms of the decomposition of intercalated gentamicin, which confirms the introduction of the drug in MAS-G hybrid. A clear difference is visible in the kinetics of water loss between non-intercalated smectite and gentamicin intercalated clay. As can be seen, in the initial stage, the water loss is significantly decelerated in MAS-G materials, which is due to the fact that the higher amount of structural water was introduced into the clay as a result of intercalation. The opposite behaviour can be observed above 250 °C, when the total mass loss in MAS-G material is higher than in non-intercalated MAS and is accompanied with decomposition of gentamicin followed by the release of structural water out of the interlayer space which, as it has been concluded before, was filled with a monolayer of gentamicin together with a monolayer of water. Moreover, in the MAS-G hybrid, a sharp mass loss at around 238 °C, corresponding to the decomposition of gentamicin sulphate cannot be clearly distinguished, which may denote

Magnesium aluminium silicate–gentamicin complex for drug delivery systems

877

2.5

S Ka

2

C Ka

N Ka Na Ka 1.00

Cl Kb Cl Ka 2.00 3.00 S Ka

C Ka

Absorption/nm

O Ka

1.5 y = 0.0044x + 0.0984 R 2 = 0.9916

1

0.5 O Ka Cl Kb Cl Ka

N Ka 1.00

2.00 3.00

0

0

100

200

300

400

500

600

Concentration/μg mL–1

Fig. 7 SEM micrograph and EDX analysis of gentamicin sulphate powder

Fig. 9 Calibration curve of correlation between absorption intensity and concentration of gentamicin in PBS 40 35

Drug release/%

the decelerated decomposition kinetics of gentamicin in the MAS-G intercalates. Based on those decompositions, the total amount of gentamicin in the hybrids can be found to be about 12 mass% (drug-loaded amount of gentamicin 0.208 mmol g-1). Taking into consideration, that the initial concentration of gentamicin was 0.434 mmol g-1 (0.25 g of gentamicin sulphate was added to the prepared suspension per each 1.0 g of the clay), the efficiency of intercalation process at 60 °C is at the level of 48%.

30 25 20 15 10 5 0

0

2

4

6

8

10

12

Time/day

SEM analysis

Fig. 10 Release profile of gentamicin from MAS-G at 37 °C

The SEM micrographs of gentamicin sulphate and EDX (point) analysis are shown in Fig. 7. The EDX analysis, performed at two different points of the powder, shows that the major characteristic peak of gentamicin is a sulphur peak.

Figure 8 presents SEM micrographs before and after the intercalation process and EDX analysis for the MAS-G powder. A change in the agglomerated microstructure of the starting powder MAS is clearly visible after contact

C Ka Si Ka O Ka

Mg Ka Na Ka 1.00

Al Ka S Ka 2.00

Fig. 8 SEM micrograph of MAS and MAS-G (smectite before and after intercalation with gentamicin)

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Table 1 Parameters of the kinetic release models obtained for the MAS-G complex Zero order

First order

ft = f0 - K0t

Higuchi

ft = 1 - exp(-K1t)

ft = KHt

Modified Korsmeyer–Peppas 0.5

ft = atn ? b

K0

R2

K1

R2

KH

R2

a

n

b

R2

0.526

0.9736

0.030

0.8998

5.602

0.9948

0.011

0.828

0.057

0.9827

intercalation with gentamicin. There is a clear ‘house of cards’ microstructure typical of montmorillonite, proving the separation of layers of smectite clay after intercalation. The EDX analysis was made at two different points in the powder where major peaks coming from sulphur are clearly visible. The presence of sulphur, noted as characteristic for pure gentamicin, confirms the presence of the antibiotic in the form of gentamicin sulphate in the structure of the smectite. Drug release: in vitro study The release of gentamicin was evaluated based on the intensity of the maximum absorbance of the PBS fluid measured at 330 nm and its correlation with the prepared calibration curve for the drug (Fig. 9). The release profile of gentamicin from MAS-G powder at 37 °C (pH = 7.4) over 12 days is shown in Fig. 10, and it is clearly visible that the drug intercalated into the smectite clay powder exhibits obvious three-step release behaviour. It starts with an initial stage of fast release up to the 12th hour, followed by a subsequent stage of relatively stable release between the 12th and the 48th hour and a plateau region with a slow release rate starting from the 48th hour of measurement. The initial burst release in PBS in the first step started with 6.8% after 2 h and reached around 9.0 mass% of the total amount of loaded gentamicin after 12 h. Then, the release of gentamicin ran at almost constant speed and stabilised at about 30 mass% over the next 2 days. The results of the in vitro release experiment at pH = 7.4 showed that the release rate of gentamicin from the smectite is stable up to the 48th hour after the initial burst and, thus is sustained in character, which may be decisive for the successful use of smectite clay as a potential drug carrier for gentamicin. The kinetics of this release can be described by the zero-order and first-order models, but the best fit was obtained for the Higuchi model with the KH constant of 5.6. However, only the modified Korsmeyer–Peppas release model provided a description of release including the interpretation of the initial burst effect. The n value, which described the kinetics for the MAS-G material was 0.828, indicating that the release mechanism of gentamicin from MAS was non-Fickian diffusion (the parameters of all the models are given in Table 1).

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Conclusions A preliminary study on the use of smectite clay as a potential drug carrier for gentamicin was performed. XRD, FTIR and TG data showed that gentamicin, in the form of gentamicin sulphate, was introduced into smectite, which led to the intercalation of the drug molecules in the interlayer spaces of the clay. The intercalation of gentamicin was confirmed in XRD study by an increase in montmorillonite interlayer basal spacing d001, the appearance of absorption bands in the FTIR spectrum of the smectite–gentamicin hybrid, and by the improved thermal stability of gentamicin in the hybrid material, which is visible in the TG/DTG study. The in vitro release experiment showed that the release of gentamicin from the smectite is sustained in character and the amount of the drug released within 48 h is stable following the initial burst release, which may be a critical feature deciding the successful use of smectite clay as a drug carrier for gentamicin. The parameters, which describe the kinetics of drug release for the MAS-G material indicated a non-Fickian diffusion as the release mechanism of gentamicin from MAS-G complex material. The overall results of this study indicate that MAS smectite clay can be potentially used as a sustained-release carrier of gentamicin in drug delivery applications. Acknowledgements This study was performed within the framework of funding for statutory activities of AGH University of Science and Technology in Cracow, Faculty of Materials Science and Ceramics (11.11.160.617).

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