Fabrication of composite microparticles of artemisinin for dissolution enhancement

Powder Technology 203 (2010) 277–287

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Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c

Fabrication of composite microparticles of artemisinin for dissolution enhancement Nanda Gopal Sahoo a, Mitali Kakran a, Lin Li a,⁎, Zaher Judeh b a b

School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, 637459 Singapore

a r t i c l e

i n f o

Article history: Received 2 February 2010 Received in revised form 17 April 2010 Accepted 14 May 2010 Available online 21 May 2010 Keywords: Artemisinin Crystallinity Dissolution Microparticles Spray drying

a b s t r a c t The main aim of this study is to enhance the dissolution of a poorly water soluble antimalarial drug, artemisinin (ART) by fabricating its microparticles and composites with selected hydrophilic polymers using a spray drier with a modified multi-fluid nozzle. We investigated the spray drying of ART with polyvinylpyrrolidone (PVP) considering the effect of feed ratio (ART:PVP) on the physical properties and dissolution of spray dried ART. Other hydrophilic carriers such as polyethylene glycol (PEG) were selected for comparing the dissolution with that of spray dried ART with PVP. The drug and polymer solutions were supplied through different liquid passages of the modified four-fluid nozzle to fabricate ART and composite microparticles. Characterization of the original ART powder, spray dried ART microparticles and ARTpolymer composite microparticles was carried out by scanning electron microscopy (SEM), Fourier transform infrared (FTIR), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and dissolution tester. The DSC and XRD studies suggested that the crystallinity of ART decreased after spray drying and depended on the weight ratio of drug to polymer. Percent dissolution efficiency (%DE); relative dissolution (RD); mean dissolution time (MDT); difference factor (f1) and similarity factor (f2) were calculated for the statistical analysis. The dissolution of ART from the spray dried ART–PVP composite microparticles was more rapid than that from their respective physical mixture, spray dried ART–PEG composite microparticles and original ART powder. In the mathematical modeling, the Weibull and Korsmeyer-Peppas model were found to best fit to the in vitro dissolution data and the drug release kinetics could be recognized as Fickian diffusion. This study demonstrated that the modified multi-fluid spray drier can be used for the preparation of drug microparticles to improve the dissolution ability of poorly water soluble drugs and overcome the problem of finding a common solvent for drugs and carriers. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Recent drug discovery has led to an increasing number of new drug molecules with low water solubility and hence poor bioavailability, especially those administered orally [1]. Poorly water soluble drugs tend to be eliminated from the gastrointestinal tract before they get an opportunity to fully dissolve and be absorbed into the blood circulation. Since about 65% of the human body is made up of water, a drug must have a certain hydrophilicity or polarity to be water soluble and thus possess an acceptable bioavailability level. Therefore, it is a great challenge to develop reliable and efficient processing methods to increase the oral bioavailability of poorly water soluble drugs. Many approaches have been developed to enhance the solubility as well as bioavailability of poorly water soluble drugs, including both modifications to the drug substance itself and the creation of specific formulations. Physical modifications to increase the surface area, solubility and wettability of the drug particles, therefore focus on

⁎ Corresponding author. Tel.: + 65 6790 6285; fax: + 65 6791 1859. E-mail address: [email protected] (L. Li). 0032-5910/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2010.05.019

particle size reduction [2] or development of amorphous states [3,4]. The most common method is to increase the surface area of a drug by micronization. There are several methods for the production of drug micro/nano particles such as pulverization of large particles using a ball or jet mill, spray freezing, spray drying, supercritical antisolvent technique (SAS) [5–10] etc. to improve drug solubility. Spray drying is the most commonly used industrial process because the spray dried powders meet the highest quality standards with respect to the particle size distribution of products, homogeneity and shape. Different types of particles have been produced by this method, such as conventional particles, encapsulated particles and porous particles [8,9]. By modifying the spray drying operation, it is possible to control the properties of spray dried particles towards enhancement of drug bioavailability and delivery. The present study is concerned with the improvement of the dissolution of a poor water soluble drug by using this method. Artemisinin is a potent antimalarial drug that remains effective against multidrug resistant strains of Plasmodium falciparum malaria. It has good intestinal permeability and can readily cross the intestinal monolayers via passive diffusion [11]. Major problem with artemisinin (ART) is its poor aqueous solubility [12], resulting in poor absorption

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upon oral administration. This poor solubility, its short half life and high first-pass metabolism, might lead to incomplete clearance of the parasites resulting in recrudescence [13]. Few studies investigated the enhancement of the solubility and/or dissolution of ART using a carrier like cyclodextrin [14,15]. They prepared β-cyclodextrin-ART complexes by using a slurry method at a molar ratio of 1:1 and showed that the β-cyclodextrin-ART complex has a faster rate and higher extent of dissolution and possessed the enhanced bioavailability in vivo compared with a commercial preparation containing the normal form of the drug [14,15]. In our present study, attempts are made to improve the dissolution of ART by using an in-house modified multi-fluid nozzle spray drier that allows co-spray drying of ART with polyvinylpyrrolidone (PVP) that acts as a hydrophilic carrier. We have also selected other hydrophilic carriers such as polyethylene glycol (PEG) for comparing the dissolution with that of spray dried ART with PVP. Characterization of the prepared drug particles has been carried out by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and dissolution study. We have also investigated the mechanism of drug release through mathematical modeling of dissolution data for all samples. 2. Materials and methods 2.1. Materials Artemisinin was obtained from Kunming Pharmaceutical Corporation (Kunming, China). The samples of PVP-K30 and PEG (average molecular weights of 40,000 and 4000, respectively) were bought from Sigma-Aldrich. All reagents used were of technical grade. 2.2. Fabrication method A modified multi-fluid nozzle pilot spray drier (PSD 52, Anhydro A/S, Denmark) was used to prepare ART microparticles in this work. The pilot spray drier is shown in Fig. 1. The drier consists of two main units, the drying chamber and the filter. The maximum evaporation capacity at inlet/outlet temperatures of 350/90 °C is 9.3 kg/h, the air heater is electrical with a capacity of 12 KW, the controlled volumetric air flowrate is 125 m3/h. With the original nozzle setup, shown in Fig. 2i, two fluids, one compressed air and the other product to be dried, can be fed simultaneously through the nozzle atomizer into the

drying chamber. In this setup, the product to be dried is kept in the feeding tank. From here the liquid is drip fed through the siphon hose into the nozzle atomizer. For regulation of the feed rate to the atomizer, a manually adjustable clamp is used around the feed hose. The drying air is drawn through an air filter mat and through the electrical air heater and into the chamber. The temperature of the drying air is controlled variably and monitored from the instrument panel. The dried powder exiting the chamber enters the cyclone separator where it is collected into a bucket. The moist drying air continues and passes through a filter trapping any remaining particles in a second bucket. For preparation of composite drug particles, it is necessary to find a common solvent for the drug and the carrier. For a poorly water soluble drug, the drug is more hydrophobic while the drug carrier is hydrophilic so it is very difficult to find common solvent for the drug and carrier. That's why this manufacturer's setup was modified as shown in Fig. 2ii, allowing for four fluids to be fed simultaneously which is more advantageous to produce composite drug particles compared to the existing spray drying process. Three of the fluids are for liquid while one remains as the compressed air stream. Three steel pipes with very small diameters (diameter 1/16″ or 1.6 mm) were connected to the calibrated digital dosing pumps (Grundfos, Denmark) and the feed solutions. The original process stream feed port was modified to accommodate the three steel pipes for liquids, which were inserted alongside each other inside the nozzle feed pipe and joined at a point such that fluids come into contact with another 5 mm before the nozzle tip. The atomizer head was also slightly modified at the nozzle tip. In this setup, we are able to perform experiments under controlled feeding of solutions entering separately into the nozzle, thereby avoiding solvent interaction effects associated with mixing upstream of the nozzle. A multi-fluid nozzle has previously been used by Beppu et al. [16] who recognized that with such an alternative multi-fluid nozzle design, it is possible to use different liquid feed lines for the separate addition of solvents into the nozzle, thereby overcoming the problem of finding and using a common solvent. The original ART powder was dissolved in ethyl alcohol at a feed concentration of 10 g/L and PVP was dissolved in water at different concentrations to maintain the composite feed ratios of 1:1, 1:2, 1:4 and 1:6 (w/w). The drug and PVP solutions were supplied through different liquid passages of the modified four-fluid nozzle. Spray drying was conducted at an inlet temperature of 140 °C. Only two of the three liquid feed lines were used. The flow rate for the ART and PVP solutions was 250 ml/h. The outlet temperature was maintained

Fig. 1. Engineering drawing of the pilot spray drier used in this study.

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Fig. 2. i. Original two-fluid nozzle atomizer for co-current atomization (left). Original setup for siphon or gravity feeding (right). ii. Modified 4-fluid nozzle pump-fed setup.

within the range 40–50 °C while air pressure was controlled at 1.4 bar. For comparing with the spray dried ART–PVP composite particles, we have prepared ART–PEG composite particles with 1:6 ratio using the same conditions. Single-component spray dried ART particles were also prepared under the same conditions.

2.3. Particle morphology The morphology of samples was observed using a scanning electron microscope (JSM-6390LA-SEM, Jeol Co., Japan). The powder samples were spread on a SEM stub and sputtered with gold before the SEM observations. For observation of ART particles in composites, PVP was removed by dissolving the ART–PVP composite particles in

water for only 5 min under sonication. The ART suspensions were filtered to remove PVP and dried. 2.4. FTIR analysis Fourier transform infrared (FTIR) spectroscopic measurements were performed using a DIGILAB FTS 3100 system using the KBr disk method. 2.5. DSC analysis Differential scanning calorimetric (DSC) measurements were carried out using a PerkinElmer DSC 7 thermal analyzer in a

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temperature range of 30–250 °C at a heating rate of 10 °C/min in nitrogen gas. The melting point and heat of fusion were calculated using the DSC software.

Furthermore, the enthalpy change (ΔH) for the ART–PVP and ART– PEG binary systems were investigated using the temperature dependent characteristic of the stability constant based on the van't Hoff equation [15,20] as shown below:

2.6. X-Ray diffraction analysis X-ray diffraction was studied using the Bruker AXS D8 Advance X-ray diffractometer with Cu Kα — targets at a scanning rate of 0.010 2θ/s, applying 40 kV, 40 mA, to observe the crystallinity of samples. 2.7. Dissolution studies The in vitro dissolution of the spray dried ART samples as well as the original ART was determined using the paddle method (USP apparatus II) (Verkin Dissolution Tester DIS 8000) in 900 mL of distilled water, under nonsink conditions. The paddle rotation was set at 200 rpm. The temperature was maintained at 37 ± 0.5 °C. The original ART, spray dried ART, ART–PEG and ART–PVP samples containing an equivalent 360 mg of ART were tested for their dissolution in water. The dissolved solution samples of 1 ml were collected at 0.5, 1, 2, 3 and 4 h of dissolution time. For each sample the dissolution test was done 3 times. 2.8. Analysis of ART concentration The ART concentrations for the dissolution studies were determined using a high performance liquid chromatography (HPLC) method with ultraviolet detection [17,18]. The HPLC used was Agilent 1100 series. The column used was Kromasil C18 (150 mm × 4.6 mm id × 3.5 μm) (Eka Chemicals AB, Sweden). The mobile phase consisted of 75% of 0.01 M disodium hydrogen phosphate and 25% acetonitrile (HPLC grade) and the mobile phase was adjusted to pH 6.5 with glacial acetic acid. The flow rate was set at 0.8 mL/min. The detector was operated at a wavelength of 254 nm. The samples were filtered through 0.45 μm polypropylenereinforced Teflon membrane with polypropylene housing (Ministart-SRP 15, Saritorius, Germany). The samples were subjected to pretreatment prior to injection into the HPLC system. 1 mL of sample was added into 200 μL of 10 M sodium hydroxide and the mixture was heated at 45 °C for 25 min, which was then cooled to room temperature. Finally, 150 μL of glacial acetic acid was added into the above mixture before injection into the HPLC system. 2.9. Phase solubility study The phase solubility study was performed using the method reported by Higuchi and Connors [19]. Excess amount of ART was added into 6 solutions of PVP (or PEG) in screw capped vials. The concentrations of PVP and PEG used were 0, 2, 4, 6, 8 and 10 mM. The vials were shaken continuously in a thermostatically controlled water bath at 24, 37 and 52 °C for 72 h until equilibrium was achieved. After equilibrium, a 1 mL sample of each solution was filtered through a 0.45-μm polypropylene-reinforced Teflon membrane and diluted with water before HPLC analysis. The stability constants at the three different temperatures were calculated from the linear section of the phase solubility diagrams. The stability constant Ks was calculated using the following relationship:

Ks =

slope S0 ð1−slopeÞ

ð1Þ

Where So is the intrinsic solubility of ART in the absence of polymers, and the slope refers to the gradient of the plot of ART solubility (mM) vs. polymers concentration (mM).

ln Ks = −

ΔH RT

ð2Þ

where Ks is as defined above, T is the absolute temperature (Kelvin), and R is the gas constant (8.314 J/mol/K). The ΔH (enthalpy change) was calculated from the slope of the plot of ln Ks vs. 1/T after least square linear regression analysis. The Gibbs free energy of transfer (ΔG) of ART from pure water to aqueous solution of a polymer was calculated using the following equation [21]:
ΔG = −2:303RT log

SS SO

 ð3Þ

where SS/S0 is the ratio of molar solubility of ART in aqueous solution of a polymer to that of the pure water. 2.10. In vitro dissolution/statistical analysis Percent dissolution efficiency (%DE) was calculated to compare the relative performance of the spray dried ART–PVP, ART–PEG composites with the spray dried ART and original ART powder. The %DE at 30 min (%DE30 min) and 1 h (%DE1 h) for each formulation was computed as the percent ratio of area under the dissolution curve up to the time t, to that of the area of the rectangle described by 100% dissolution at the same time [22]. 0 t 1 ∫ y⋅dt B C B C %DE = B 0 C100 @ y100 ⋅t A

ð4Þ

The mean dissolution time (MDT) was calculated by the following expression [22]: MDT =

∑ni = 1 tmid ΔM ∑ni = 1 ΔM

ð5Þ

where i is the dissolution sample number, n is the number of dissolution times, tmid is the time at the midpoint between times ti and ti − 1, and ΔM is the amount of ART dissolved (μg) between times ti and ti − 1. The difference factor (f1), evaluating the percent error between two curves over all time points [22]: f1 =

∑ni = 1 jRi −Ti j × 100 ∑ni = 1 Ri

ð6Þ

where i is the dissolution sample number, n is the number of dissolution times, Ri and Ti are the amounts dissolved of the reference drug and the test drug at each time point i. The percent error is zero when the test drug and reference profiles are identical and increase proportionally with the dissimilarity between the two dissolution profiles. The similarity factor (f2) is a logarithmic transformation of the sum-squared error of differences between the test Ti and reference Ri over all time points [22]:

f2 = 50 × log

h

1 + ð1= nÞ∑i = 1 jRi −Ti j n

i 2 −0:5

 × 100

ð7Þ

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It is 100 when the test and reference profiles are identical and tends to 0 as the dissimilarity increases. 2.11. Mathematical modeling of release kinetics The in vitro drug release data were fitted to various release kinetic models viz. Higuchi, Korsmeyer-Peppas and Weibull model employing the following set of equations [22]: Higuchi model pffiffi Mt = K t

281

hygroscopic polymer and absorbs moisture from the environment. The spectrum of PEG (Fig. 5ii) showed a broad band at 3450 cm− 1, which was attributed to the presence of –OH stretching. Other important band was observed at 1105 cm− 1 due to the presence of C– O stretching. Comparison of the spectra of spray dried composites of ART with both polymers showed no differences in the position of the absorption peak. The absences of shifts in the wavenumbers of the FTIR peaks of the spray dried composites and the physical mixture indicated the lack of significant interaction between the ART and both polymers in the spray dried composites.

ð8Þ 3.3. X-ray diffraction

Korsmeyer-Peppas model Mt n = kt M∞

ð9Þ

Weibull model  Mt n = 1− exp −ks ⋅t M∞

ð10Þ

where Mt and M∞ correspond to the drug amount dissolved at a particular time t and at infinite time, respectively. Various other terms viz. K, k, and ks refer to the release kinetic constants obtained from the linear curves of Higuchi, Korsmeyer-Peppas, and Weibull model, respectively. Model fitting using Eqs. (8)–(10) was accomplished using the Sigma plot software. 3. Results and discussion 3.1. Particle morphology SEM microphotographs of the original ART powder, spray dried ART particles, spray dried ART–PVP and ART–PEG composite particles are shown in Fig. 3. It was observed that the particle size of spray dried ART was in the range 1–5 µm with rods and cubic morphology. The original ART powder exhibited particles lacking uniformity in size and being relatively larger than the spray dried ones and had different morphology. The spray dried particles are more uniform. Fig. 3(c)–(f) shows that the ART and PVP or ART and PEG were not mixed homogeneously but both could clearly be observed in all particles for all of the ART–PVP and ART–PEG co-spray dried products. Ozeki et al. reported the same observation in flurbipron (FP) and sodium salicylate (SS) spray dried composite particles [23]. The feed ratio was influential on the particle size of the spray dried particles. Fig. 4 (a) and (b) shows the SEM photographs of ART micro particles dispersed in composites after removal of PVP by complete dissolution in water. The ART particles in the composites were similar in shape but much lower in size than the spray dried ART particles. The ART particle size decreased as the ART concentration decreased in the composites as shown by the smaller particle size of ART particles in ART–PVP composites at 1:4 ratio in Fig. 4(b) as compared to the same at 1:2 ratio in Fig. 4(a). The outlet spray drier temperature was not found to influence the morphology of produced particles. 3.2. FTIR study The FTIR spectra of original ART powder, PVP, PEG, spray dried composite particles and the corresponding physical mixture are shown Fig. 5i and ii. The FTIR spectrum of ART powder (Fig. 5i) shows the absorption peaks at 1736 cm− 1, the stretching vibrations of C=O due to the lactone and at 832, 883, 1117 cm− 1 due to the peroxide. The spectrum of PVP showed an important band at 1652 cm− 1 due to the presence of C=O stretching. A very broad band was also visible at 3430 cm− 1 which was attributed to the presence of water as PVP is a

The X-ray diffraction patterns of the original ART powder, spray dried ART particles, PVP, PEG, spray dried composite particles and the corresponding physical mixture are shown in Fig. 6. The figures reflect the changes in the drug crystal structure. The X-ray patterns of the original ART powder in Fig. 6i (a) displayed the presence of numerous distinct peaks at 2θ of 7.29°, 11.78°, 14.65°, 15.63°, 16.64°, 18.23°, 20.0°, and 22.1°, which suggests that the drug was of a crystalline form. On the other hand, the diffraction spectrum of PVP in Fig. 6i (h) showed no prominent peak, indicating the amorphous nature of the PVP. The diffraction spectrum of PEG in Fig. 6ii (a) showed two prominent peaks with the highest intensity at 2θ of 19.2° and 23.3°. The spray dried ART particles in Fig. 6i (a) showed the similar diffraction pattern with a lower peak intensity, suggesting that the crystallinity of the spray dried ART particles decreased during the spray drying process. For the ART–PVP physical mixture, all the peaks from ART were present with slightly lower intensity and no new peaks were observed, suggesting the absence of interaction between the drug and the PVP in their physical mixture. However, in the spray dried composite samples in Fig. 6i (c–g), all the diffraction peaks of ART were observed with remarkably decreased intensity compared to original ART powder and their corresponding physical mixture in Fig. 6i (b). It is very interesting to note in Fig. 6ii (b) that the new diffraction peak appeared at 2θ = 9.24° for the spray dried ART–PEG composite sample at 1:6 ratio as compared to other composite samples. These results suggest a change in the crystal structure of ART in ART–PEG composite. According to the XRD data, the decrease in XRD peak intensity in the case of ART–PVP composite (1:6) (Fig. 6ii) was greater than that of ART–PEG composite (1:6), suggesting that the lower crystallinity of the spray dried ART–PVP compared to the spray dried ART–PEG composite. The new peaks observed in case of spray dried ART–PEG composite suggested some physical interaction between the drug and the carrier, which led to the change in the crystal structure. The FTIR spectra showed no chemical interaction between ART and the carrier, but XRD data suggested that the addition of the carrier introduced some changes in the crystal structure of ART. From XRD observations, we can conclude that the crystalline nature of the drug was still maintained, but the relative reduction of diffraction intensity of ART in the spray dried composite suggests that the quality of the crystals was reduced. 3.4. Thermal properties In order to understand the effect of spray drying on the thermal properties of ART, DSC was conducted. The DSC thermograms of the original ART powder, PVP, spray dried composite particles and the corresponding physical mixtures are presented in Fig. 7. The melting temperature (Tm) and heat of fusion (ΔHf) obtained from the DSC study are summarized in Table 1. The original ART powder used in this study had a sharp melting endothermic peak at 156 °C. As an amorphous and hygroscopic polymer, PVP did not show any melting peak or phase transition, apart from a broad endotherm due to dehydration, which lies between 80 and 120 °C (not shown in the

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Fig. 3. SEM photographs of (a) original ART powder, (b) spray dried ART particles, (c)–(e) spray dried ART–PVP and (f) ART–PEG composite particles.

figure). The endothermic melting peak of the spray dried ART particles slightly shifted to the lower temperature side. For the spray dried ART–PVP composite particles, the ART endothermic peak was observed at lower temperature compared to the original ART powder, spray dried ART particles and their corresponding physical mixtures, which was indicative of a certain loss of crystallinity. The heat of fusion of the original ART powder was higher than that of the spray dried ART particles. The heat of fusion of the spray dried composite ART–PVP particles depended on feed ratio. From Table 1, it is clearly

seen that the heat of fusion of the spray dried composite ART–PVP particles decreased with increasing polymer concentration. From Fig. 7 and Table 1, it is observed that the heat of fusion of the spray dried ART–PVP composites was lower than their corresponding physical mixtures. The heat of fusion of spray dried ART–PVP composite (1:6) was lower than that of the ART–PEG composite (1:6). Since heat of fusion is proportional to the amount of crystallinity in the samples, these results suggest that the crystallinity of ART was decreased when they were dispersed in the spray dried

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Fig. 4. SEM photographs of ART micro particles dispersed in composites (a) ART–PVP = 1:2 (b) ART–PVP = 1:4, after removal of PVP.

composite particles. Therefore, we conclude that the spray dried ART– PVP composites were less crystalline than the ART–PEG composites, which was supported by the XRD analysis.

3.5. Phase solubility studies

Fig. 5. i. FTIR spectra of (a) PVP, (b) physical mixing of ART-PVP1:1, (c) spray dried ART–PVP composite particles 1:1, (d) spray dried ART–PVP composite particles1:2 and (e) original ART powder; ii. FTIR spectra of (a) PEG, (b) spray dried ART–PEG composite particles1:6 and (c) original ART powder.

Fig. 6. i. X-ray diffractograms of (a) Original ART, (b) spray dried ART, (c) physical mixing of ART–PVP 1:1, spray dried ART–PVP composite particles of (d) 1:1, (e) 1:2, (f) 1:4, (g) 1:6, and (h) PVP; ii. X-ray diffractograms of (a) PEG, (b) spray dried ART–PEG composite particles 1:6 (b), and (c) spray dried ART–PVP composite particles 1:6.

The phase solubility profiles of ART in the presence of PVP and PEG are shown in Fig. 8. The phase solubility of ART increased linearly with

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Fig. 7. i. DSC thermograms of original ART powder (a), physical mixing of ART–PVP 1:1 (b), and spray dried ART particles (c); ii. DSC thermograms of spray dried ART–PVP composite particles of (a) 1:1, (b) 1:2, (c) 1:4, (d) 1:6, and (e) spray dried ART–PEG composite particles (1:6).

increasing PVP and PEG concentrations and temperature. The linear solubility curve could be classified as type AL, suggesting that the system followed the first order in nature [19]. The linear relationship also suggested that dilution of a solution of an ART–PVP and ART–PEG during administration into the body would not cause precipitation of an ART regardless of the extent of dilution. The solubility increased approximately 10 fold for PVP and 9-fold for PEG at the highest polymer concentration at 24 °C. The same tendency was observed at 37 °C and 52 °C. Similar results have been reported with several other drugs using the water soluble carriers, due to the formation of weekly soluble complexes and/or cosolvent effect of the carrier [21,24,25].

Table 1 DSC parameters of original ART, spray dried ART, ART–PVP and ART–PEG composite particles. Samples

Tm (°C)

ΔHf (J/g)

Original ART powder Spray dried ART Phy. mix. ART:PEG = 1:1 Spray dried ART–PVP composites

156.0 152.6 155.6 126.5 128.2 130.1 118.2 116.0

76.19 50.44 65.83 9.32 7.81 6.10 4.61 7.19

Spray dried ART–PEG composites

1:1 1:2 1:4 1:6 1:6

Fig. 8. Solubility of ART in aqueous solutions of PVP (i) and PEG (ii) in water at different temperatures.

The thermodynamic parameters (ΔG, ΔH, and Ks) for the system are shown in Table 2. The correlation coefficient (r2) of van't Hoff plot was found to be 0.9846 and 0.9963 for PVP and PEG solutions, respectively, which indicated a good fit. An indication for the transfer of ART from pure water to aqueous solutions of PVP and PEG was obtained from the Gibbs free energy change. The Gibbs free energy values provide the information whether the reaction condition is favorable or not for drug solubilization in the aqueous carrier solution. The negative Gibbs free energy indicates the favorable conditions. ΔG values were negative for the polymers at different temperatures, demonstrating the spontaneity of ART solubilization. The ΔG values became more negative with increasing the polymer concentration, showing that the process of ART transfer from pure water to a polymer solution was more favorable at higher polymer concentrations. The negative enthalpy indicates that the reaction is exothermic, i.e. there was release of energy which favored the formation of the ART–PVP and ART–PEG composite. However, ΔG and ΔH in the case of PVP were more negative than PEG, demonstrating that the formation of ART–PVP composite was more favorable than the ART–PEG composite. The stability constant (Ks) value was higher in the case of spray dried ART–PVP composite particles. Thus, it can be concluded that the solubilization was favored more by PVP, than PEG. However, the stability constant (Ks) decreased with increasing temperature. This may be due to the decrease in the interaction forces, i.e. Van der Waals and hydrophobic forces [26].

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Table 2 Thermodynamic parameters for solubilization process of ART in aqueous solutions of PEG and PVP at 24 °C, 37 °C and 52 °C. a. Concentration (mM)

2 4 6 8 10

ΔG (J/mol) PEG

PVP

24 °C

37 °C

52 °C

24 °C

37 °C

52 °C

− 2213.07 − 3630.90 − 4506.09 − 5025.12 − 5480.77

− 1545.71 − 3281.46 − 3997.14 − 4587.93 − 5194.03

− 1434.04 − 2830.42 − 3712.52 − 4349.57 − 4919.99

− 2262.97 − 3848.15 − 4752.32 − 5299.81 − 5807.20

− 1693.05 − 3527.15 − 4326.67 − 5029.29 − 5580.85

− 1550.72 − 3013.71 − 3829.67 − 4545.01 − 5006.14

b. Temperature (°C) 24 37 52

Ks (M− 1)

ΔH (kJ/mol)

r2

PEG

PVP

PEG

PVP

PEG

PVP

− 11.648

− 15.069

1002.4010 804.7858 667.3635

1200.3970 1014.97 710.8913

0.9963

0.9846

3.6. Dissolution study

Fig. 9. i. In vitro dissolution profiles of (a) original ART powder, (b) physical mixture of ART–PVP 1:1, (c) physical mixture of ART–PVP 1:6, (d) spray dried ART, (e) spray dried ART–PVP 1:6, and (f) ART–PEG composite particles at 1:6 ratio; ii. Weibull release model fit to the dissolution profiles of the original ART powder, Spray dried ART and ART/PVP composites.

Fig. 9i shows the dissolution profiles of original ART powder, spray dried ART particles, ART–PEG, and spray dried ART–PVP composite particles, and the corresponding physical mixture of ART–PVP. The dissolution parameters such as DC (concentration of drug dissolved at particular time), %DE (percent dissolution efficiency at particular time), RD (relative dissolution rate at particular time), and MDT (mean dissolution time) are presented in Table 3. It is evident from Fig. 9i and Table 3 that the dissolution of original ART was very low (DC30 min = 20 μg/ml). However, the spray dried ART particles enhanced the dissolution (DC30 min = 55.45 μg/ml) within 30 min as compared to the original ART powder. The dissolution of ART from all spray dried ART–PVP composite particles was markedly increased compared to the original ART powder and spray dried ART particles. The dissolution profile of ART from the physical mixture of ART and PVP at ART–PVP ratio of 1:1 was measured. The dissolution of ART from the physical mixture was slightly higher (DC30 min = 30.3 μg/ml) as that from original ART. The drug dissolution was increased with increasing the amount of drug carrier in the spray dried ART–PVP composite particles. The highest improvement (235.97 μg/ml) was observed at the highest carrier level i.e. ART–PVP = 1:6. Comparing with enhancement in the dissolution of ART using PEG as a carrier, the maximum amount dissolved for the spray dried ART–PEG composite particles at the ratio of 1:6 was 216.83 μg/ml after 4 h, which was much higher than the original ART powder and spray dried ART, but lower than that of the spray dried ART–PVP composite particles at the same ratio. The value of %DE30 min was enhanced from 2.5 for original ART to 6.9 for the spray dried ART, then to 19.4 for the spray dried ART–PVP composite at 1:6 ratio. Similar to %DE30 min values, the value of RD30 min was the lowest for the spray dried ART (2.67) and the highest for the spray dried ART–PVP composite (7.76) at 1:6 ratio. Both %DE30 min and RD30 min values for the physical mixture of ART and PVP at an ART-PVP ratio of 1:1 were lower (3.78 and 1.51 respectively) than the spray dried ART and the spray dried ART–PVP 494 composite at 1:1 ratio (11.95 and 4.78 respectively). The highest dissolution improvement with the spray dried ART–PVP composite was observed at the highest PVP concentration. The spray dried ART– PEG composite particles at ratio 1:6 also showed improvement in drug dissolution, but lower than the corresponding spray dried ART–PVP composite particles. It is also verified that the dissolution of the ART can be influenced by PVP and PEG. The calculated MDT values for all the samples

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Table 3 Dissolution parameters of ART and spray dried composites. Dissolution parameters

Original ART

Physical mixing ART–PVP = 1:1

Spray dried ART

Spray dried ART–PVP composites 1:1

1:2

1:4

1:6

1:6

DC30mina DC1 h DC2 h DC3 h DC4 h %DE30 minb RD30 minc MDTd f1 e f2 f

20.00 30.10 41.10 48.84 52.40 2.50 – 0.99 – –

30.30 41.10 59.20 65.50 68.60 3.78 1.51 0.89 42.51 44.14

55.45 80.95 110.23 118.60 128.56 6.93 2.68 0.84 172.26 20.85

95.65 148.76 166.31 176.78 182.43 11.95 4.78 0.65 387.84 11.66

115.65 168.23 182.43 190.34 198.57 14.45 5.78 0.60 466.63 10.14

130.45 180.34 198.12 210.72 218.56 16.31 6.52 0.58 520.34 9.32

155.31 190.67 210.87 225.57 235.97 19.41 7.76 0.57 590.58 8.46

130.45 175.97 198.93 208.78 216.83 16.31 6.30 0.58 481.99 9.54

a b c d e f

Spray dried ART–PEG composite

DC: concentration of drug dissolved at particular time (μg/ml). %DE: percent dissolution efficiency at particular time. RD: relative dissolution rate at particular time. MDT: mean dissolution time (hour) at 3 h. f1: difference factor at 1 h. f2: similarity factor at 1 h.

investigated (Table 3) support this finding. MDT reflects the time for the drug to dissolve and is the first statistical moment for the cumulative dissolution process that provides an accurate drug release rate [27]. A higher MDT value indicates a greater drug retarding ability [28]. The MDT values of original ART at 3 h were higher than that of the spray dried ART, ART–PEG and ART–PVP composite particles, which suggested a lower dissolution rate. The lowest MDT value was observed in the case of the spray dried ART–PVP composite particles at 1:6 ratios, which reflected a higher dissolution rate compared to other ART–PVP samples and ART–PEG composite particles at the same ratio. Furthermore, comparison between the dissolution profiles of ART from different formulations was made by the difference factor (f1) and similarity factor (f2). According to the FDA's guidelines, f1 values lower than 15 (0–15) and f2 values greater than 50 (50–100) show the similarity of the dissolution profiles [29]. The calculated f1 and f2 values are reported in Table 3. It was observed that the dissolution profiles of ART from all the samples (i.e. spray dried ART, spray dried ART–PVP composite particles at all ratios and spray dried ART–PEG composite particles at 1:6) and from original ART were not similar as f1 values for all these formulations were higher than 15, where as their f2 values were lower than 50. But in the case of the physical mixing of ART–PVP at 1:1 ratio, the f1 value was slightly higher than 15 and f2 value was close to 50. The dissolution profiles of ART from the spray dried ART–PVP at different ratios were not similar. From these studies, we can conclude that the spray dried ART–PVP at 1:6 ratio showed the better in vitro dissolution profile and lower MDT, so the dissolution of ART from this sample was the highest as compared to the rest of the samples. According to the Noyes–Whitney equation, the saturation solubility and dissolution rate of a drug can be increased by reducing the particle size to increase the particle surface area [30,31]. The particle size of original ART powder was reduced to a size in a range of

approximately 1–5 µm in the composites prepared by the spray drier, which resulted in the increased dissolution of ART. The DSC and XRD studies also revealed that the crystallinity of ART in the composites was lower, especially in the case of the spray dried ART–PVP composite at ratio of 1:6. The extent of crystallinity also influences the dissolution of the drug. An amorphous or metastable form will dissolve at the faster rate because of its higher internal energy and greater molecular motion, as compared to crystalline materials [32,33]. From the above results, it was also observed that the dissolution of ART from the spray dried ART–PVP composite particles was higher than from the spray dried ART–PEG composite particles. This may be due to the higher amorphizing properties of PVP than the PEG, as reported in an earlier study [21]. Our spray dried ART with the increased dissolution rate could translate into an enhanced bioavailability upon oral administration. 3.7. Kinetic mechanism To understand the mechanism of drug release, various models were used to fit the dissolution kinetics of the ART. The regression parameters obtained after fitting various release kinetic models to the in vitro dissolution data are shown in Table 4. In our studies for the spray dried ART–PVP and ART–PEG composites, the best fit to various models followed the order of Korsmeyer-Peppas = Weibull N Higuchi. The analysis of experimental data in the light of the KorsmeyerPeppas equation [33], as well as the interpretation of the corresponding release exponent values (n) leads to a better understanding of the balance between the purely diffusional and purely erosioncontrolled mechanisms. The values of diffusional exponent ‘n’, obtained from the slopes of the fitted Korsmeyer-Peppas model, ranged from 0.217 to 0.438. From these values, we can conclude that all of our samples tended to exhibit the Fickian diffusional characteristic which was more prominent for the spray dried ART–

Table 4 Statistical parameters of various samples obtained after fitting the drug release data to various release kinetic models. Samples

Original ART Spray dried ART Phy. mix. ART:PVP = 1:1 Spray dried ART–PVP composites

Spray dried ART–PEG composite

Korsmeyer-Peppas

1:1 1:2 1:4 1:6 1:6

Higuchi 2

k

n

r

0.0736 0.1979 0.1052 0.3330 0.3795 0.4161 0.4592 0.4126

0.4383 0.3716 0.3838 0.2568 0.2153 0.2159 0.1882 0.2172

0.9954 0.9904 0.9900 0.9755 0.9799 0.9878 0.9976 0.9899

Weibull 2

K

r

ks

n

r2

27.79 70.34 37.8 106.9 117.6 129.0 139.2 128.1

0.9901 0.9644 0.9692 0.8607 0.8120 0.8208 0.7899 0.8244

0.0765 0.2209 0.1112 0.4058 0.4782 0.5398 0.6175 0.5338

0.4614 0.4306 0.4128 0.3333 0.2903 0.3027 0.2733 0.3033

0.9959 0.9920 0.9908 0.9781 0.9819 0.9897 0.9983 0.9917

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PVP composite at 1:6, as the corresponding values of ‘n’ were always lower than the standard value of 0.45 for the Fickian release behavior [34]. The Weibull model also showed the excellent level of fitting with the experimental data as shown in Fig. 9ii and hence can be used to analyze the drug release kinetics. The n values obtained for all the samples were smaller than 1, which implied the parabolic shape of the dissolution curve with a higher initial slope and after that becoming consistent with the exponential [22]. From Table 4, the release constant ks values for the spray dried ART and the ART/PVP composites were smaller than 1 and in both cases greater than that for the original ART and the physical mixture, indicating a faster dissolution rate for the spray dried ART and the ART/PVP composites. Our spray dried formulations were also observed to yield statistically valid correlations with the Higuchi model. The results reflected the prevalence of diffusional mechanistic phenomena, in consonance with the results obtained while fitting to the KorsmeyerPeppas model. As can be seen from Table 4, the values of k, K and ks release kinetic constants increased from those for the spray dried ART to the significantly higher values for the ART composites. The increases in those values were greater in the case of ART–PVP composites than in the ART–PEG composites, suggesting a higher dissolution rate for the ART–PVP composites than the ART–PEG composites. 4. Conclusions This study demonstrated that our modified multi-fluid nozzle spray drier is able to prepare ART–PVP and ART–PEG composite microparticles with significantly higher dissolution without the problem of finding and using a common solvent for a drug and a carrier. The dissolution of ART in the composite particles depended on the type of drug carrier, carrier concentration and crystallinity. The highest improvement in dissolution was observed in the case of the spray dried ART–PVP composite microparticles with the higher polymer concentration. The Weibull and Korsmeyer-Peppas model most fitted the in vitro dissolution data and showed the drug release kinetics as the Fickian diffusion. ART microparticles fabricated using our systematic methods could have a high potential for delivery in much smaller doses compared with commercial preparation containing the normal form of the drug. Acknowledgement Authors acknowledge the financial support from Lee Kuan Yew Postdoctoral Fellowship and SUG grant M58050023, NTU, Singapore. References [1] C.A. Lipinsk, F. Lombardo, B.W. Dominy, P.J. Feeney, Experimental and computational approaches to estimate solubility and permeability in drug discovery and development settings, Adv. Drug Deliv. Rev. 46 (2001) 3–26. [2] B. Subramaniam, R.A. Rajewski, K. Snavely, Pharmaceutical processing with supercritical carbon dioxide, J. Pharm. Sci. 86 (1997) 885–890. [3] B.C. Hancock, G. Zografi, Characteristics and significance of the amorphous state in pharmaceutical systems, J. Pharm. Sci. 86 (1997) 1–12. [4] M.J. Grau, O. Kayser, R.H. Muller, Nanosuspensions of poorly soluble drugsreproducibility of small scale production, Int. J. Pharm. 196 (2000) 155–157. [5] E. Merisko-Liverside, G.G. Liversidge, E.R. Cooper, Nonosizing: a formulation approach for poorly-water soluble compounds, Eur. J. Pharm. Sci. 18 (2003) 113–120.

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