Preparation of Ultra-fine Salbutamol Sulfate Particles by Reactive Precipitation and Characterization of Dry Powder Inhalant *

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1 Chinese Journal of Chemical Engineering, 16(5) (2008) Preparation of Ultra-fine Salbutamol Sulfate Particles by Reactive Precipitation and Characterization of Dry Powder Inhalant * XU Jing ( 续京 ) 1, LIU Xiaolin ( 刘晓林 ) 2,** and CHEN Jianfeng ( 陈建峰 ) 2 1 Ningxia University, Yinchuan , China 2 Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing , China Abstract The preparation of ultra-fine particles of salbutamol sulphate (SS) was accomplished with a reactive precipitation pathway, in which salbutamol and sulphuric acid were used as reactants with the solvents of ethanol. The effects of sulphuric acid concentration, reaction temperature, stirring rate, and reaction time on the size of the particle were investigated. A binary mixture composed of lactose and SS was prepared to evaluate SS. The results showed that ultra-fine SS particles with controlled diameters ranging between 3 μm and 0.8 μm and with a narrow distribution could be achieved. The morphology consisting of clubbed particles was successfully obtained. The purity of the particles reached above 98% with UV detection. The dose of dry powder inhalation was obtained by blending the particles with recrystallized lactose, which acted as a carrier. The deposition quantity of the drug in breathing tract was estimated using a twin impinger apparatus. Compared with the Shapuer powder (purchased in the market), the results showed that SS particles had more quantities subsided in simulative lung. Keywords salbutamol sulphate, ultra-fine particle, reactive precipitation, preparation, dry powder inhalant, deposition 1 INTRODUCTION Salbutamol sulphate [(C 13 H 21 NO 3 ) 2, H 2 SO 4 ] is the best choice in several asthma drugs [1, 2]. Aerosol requires ultra-fine drug particles with controlled particle size and particle size distribution (PSD) [3, 4]. Aerosols with diameters ranging between 1 and 5 μm are deposited primarily in the tracheobronchial and pulmonary regions, whereas, aerosols with diameters less than 1 μm are deposited predominantly in the pulmonary region [5, 6]. Thus, ultra-fine particles of less than 5 μm diameter are better for improving drugs bioavailability. There are several different methods to prepare salbutamol sulphate. For example, salbutamol sulphate was prepared using salicylaldehyde and p-hydroxy acetophenone as reactants [7], and salicylaldehyde was used to synthesize salbutamol sulphate in the experiment [8, 9]. Unfortunately, not all of these processes are very successful. For example, the reaction conditions are restricted severely and the environment pollution cannot be avoided. Besides, the drug particles of salbutamol sulphate obtained by these methods are larger. Some particles are more than 10 µm, which is not thought to be the optical particles size for inhalation and therefore it needs to be re-micronized. Recently, ultra-fine materials with very interesting properties have attracted more attention. However, those studies are based on the preparation of larger particles. Recrystallization of vapor phase or liquid phase has been achieved by the characteristics of supercritical fluid to obtain ultra-fine particles [5]. However, the operation conditions of this method are complex and the fees of production are high. Generally, from larger particles, spray drying provides an alternative method of forming micron-sized drug particles, which are usually spherical with or without dimples. Spherical particles are considered to have good flow properties. However, use of elongated particles has attracted interest in the field of aerosol science [10, 11]. Fibers and needle-like crystals with aerodynamic diameters are almost independent of their length, and the diameter is approximately equal to the shortest dimension of the particles in question [12]. Such particles may travel further in the air stream allowing a greater probability of drug detachment from the surface of the carrier, and thereby, may be equal to spherical particles for the pulmonary delivery [13, 14]. This article describes a new method for preparing ultra-fine particles of salbutamol sulphate by reactive precipitation. In the experiment, salbutamol and sulphuric acid were used as reactants with the solvents of ethanol. The effects of various factors on the preparation process were investigated. The characteristics of the ultra-fine particles obtained by reactive precipitation were also studied. Using recrystallized lactose as carrier, the dose of dry powder inhalation of salbutamol sulphate was made [15]. Making more deposition of the drug in the pulmonary region was the chief purpose. Twin impinger apparatus was used in estimated deposition of the drug in breathing tract. Compared with the Shapuer dry powder inhalant (commercial), the results showed that salbutamol sulphate particles had uniform or more quantities subsided in simulative lung. 2 MATERIALS AND METHODS 2.1 Materials Salbutamol was purchased from Dianjiang Chongqing Ltd. (China, purity 99.0%) and hard gelation Received , accepted * Supported by the National High Technology Research and Development Program of China (2001AA218061) and the National Natural Science Foundation of China ( ). ** To whom correspondence should be addressed. Liuxl@mail.buct.edu.cn

2 792 Chin. J. Chem. Eng., Vol. 16, No. 5, October 2008 capsules (size 3) were supplied by Shaihai Diyi medicine store. Sulfuric acid (purity 98.0%) and ethanol (purity 99.7%) were bought from Chemical Factory of Beijing (China) and lactose was obtained from USA (Proliant5030). 2.2 Preparation and deposition tests of ultra-fine particles of salbutamol sulfates Crystallization procedures Salbutamol and sulphuric acid were used as reactants with the solvents of ethanol. Salbutamol (18 g) was dissolved in 500 ml ethanol at 50 C with the aid of stirring using a magnetic stirrer. When the salbutamol was completely dissolved, the solution was allowed to cool to room temperature. According to 1:1 (molar ratio), the salbutamol solution in ethanol and sulphuric acid solution were introduced into the flask by two peristaltic pumps (BT01-100, China) separately and simultaneously. The ultra-fine salbutamol sulfate particles can be precipitated after adding within a minute. When the precipitation was complete, the crystals were collected by filtration under vacuum, through a filter paper (0.2µm) and fitted into the filtration unit. The resultant crystals were washed with absolute ethanol several times to remove the trace mother solution. The crystals growing should be prevented by washing. The collected crystals were spread on a glass petri dish, and allowed to dry overnight in a vacuum oven at 50 C. The crystals were then transferred to a vial, sealed, and placed in a desiccator over silica gel until required for further investigation. In this study, the flux of salbutamol solution was 50 ml min -1. The other process parameters, i.e., sulphuric acid concentration, reaction temperature, stirring rate, and reaction time were varied, one at a time Preparation of powder formulation Salbutamol sulphate ultra-fine particle of 4 different particle sizes and lactose were mixed in a mass ratio of and the blends were homogenized by sieving. All blends were filled into hard gelatin capsules (size 3) manually so that each capsule contained ( ± 14.60) µg of salbutamol sulphate as a nominal dose Deposition test of salbutamol sulfate The deposition of salbutamol sulphate was determined using a twin-stage impinger (TSI, Peking University, China) after perforating the capsules after aerosolisation at 60 L min -1, via a Rotahaler ; 7 and 30 ml, respectively, of the normal saline (mass concentration 0.9%) was introduced into the throat, the upper, and lower stage of the impinger [16]. The capsule tested was placed in the inhaler device (Rotahaler, TianPing Ltd., ShangHai, China), which was fitted into a moulded rubber mouthpiece attached to the throat of the impinger. Once the assembly had been checked and found to be airtight and vertical, the dose was released and the switches on the pump were allowed to run for 7 s at 60 L min -1 and then switched off. The capsule shell was then removed from the inhaler device and the deposition test was repeated with two more capsules being actuated in the same manner. The inhaler device, the throat, and the upper and lower stages were all washed with the normal saline and made up to separate 50 ml fractions. All the drug particles obtained were analyzed for the concentration of salbutamol sulphate using the UV method outlined below. Deposition of salbutamol sulphate from each formulation was determined five times and a variety of particles sizes were employed to characterize the deposition profiles of the drug. Three capsules were prepared for each drug tested; and each capsule was analyzed five times (results mean of n=15). The recovered dose (RD) was the sum of the drug mass (μg) recovered from the inhaler device, the throat, and the upper and lower stages of the twin impinger, whilst the emitted dose (ED) was from the inhaler device and was deposited in the upper and lower stages of the twin impinger. Fine particle dose (FPD) was the amount of drug recovered from the lower stage. The fine particle fraction (FPF) was calculated as the ratio of FPD to RD and the dispersibility as the ratio of FPD to ED (both expressed as percentages). The percent recovery was calculated as the ratio of RD to the expected dose and the percent emission was defined as the ratio of ED to RD (expressed as a percentage). 2.3 Measurement of salbutamol sulphate content The purity of each particle was analyzed using UV spectrophotometer according to pharmacopoeia absorbance (A) of the solution containing salbutamol sulphate was detected at 276 nm. The coefficient of absorbance 1% 1cm E was X-ray diffractometry (XRD) X-ray diffraction pattern analysis was performed using an X-ray diffractometer (XRD-6000, Japan) to ascertain the structural characteristics of precipitated ultra-fine particles and compare with the salbutamol sulphate purchased. 2.5 FT-IR spectrophotometry FT-IR spectra were recorded with a Bruker IFS66 spectrophotometer in the wave range of cm -1, using a resolution of 2 cm -1 and 32 scans. All ultra-fine particles were diluted with KBr mixing powder at 1%, dried at 120 C for 30 min, and pressed to obtain self-supporting disks. 2.6 Transmission electron microscopy (TEM) The morphology of salbutamol sulphate ultra-fine particle was examined using a transmission electron microscope (H-800, Japan). Small ultra-fine particles were dispersed by a sonication (KQ-400DB) for 10 min before measurements, and isopropyl alcohol was used as the dispersion agent. The ultra-fine particles were dripped on copper gauze and covered

3 Chin. J. Chem. Eng., Vol. 16, No. 5, October with carbon. The median particle size and particle size distribution were analyzed from more than 200 particles of the TEM image. 3 RESULTS AND DISCUSSION 3.1 Preparation of ultra-fine particles The ultra-fine particles were affected by reaction temperature, stirring rate, reaction time, and sulphuric acid concentration. The results are shown in Table Effect of sulphuric acid concentration Table 1 shows that the particle size (brachyaxis) decreased from 3 to 1.5 μm with only increase of H 2 SO 4 concentration from 0.8 to 2.5 mol L -1. The increase of reactant concentration is beneficial to prepare high yield product ranging from 84% to 94%. The product obtained did not dissolve in ethanol; therefore, increasing the H 2 SO 4 concentration occurred as a consequence of the reduced solubility of salbutamol sulphate, which led to an increase in the degree of supersaturation. Supersaturation is the impetus on the crystal process, and affects nucleation and crystal growth directly [17, 18]. When the supersaturation reaches a certain extent, the rate of crystal growth is less than the rate of nucleation and smaller particles are precipitated easily. In the meantime, increasing the concentration of H 2 SO 4 may also increase the yield of salbutamol sulphate. If the concentration of H 2 SO 4 increases further, the acidity and causticity of H 2 SO 4 become stronger, so that the requirement of equipment is more rigorous and the reaction of elimination and dehydration between sulphuric acid and ethanol occur easily, which may affect the purity of the product Effect of reaction temperature Elevating reaction temperature can increase the reaction rate and shorten the reaction time to equilibrium. In the experiment, the reactions proceed at different temperatures of 5 C, 15 C, 30 C, and 50 C. Table 1 shows that lower temperature is liable to prepare smaller particles. Reducing reaction temperature occurred as a consequence of solubility decrease and supersaturation increase of salbutamol sulphate in ethanol, resulting in smaller particles being produced. If the reaction temperature lowers further, the reaction process will become more complicated and the operation will be more difficult. Therefore, the suitable reaction temperature was selected at 15 C Effect of stirring rate Stirring rate was another important factor for the particle size. In the experiment, Table 1 shows that the size of particles minished gradually and the morphology of particles tended to uniformity with the stirring rate increase from 100 to 900 r min -1. When the stirring rate exceeded 500 r min -1, the granularity and particle size distribution went into stabilization. The size and morphology of the particles at higher stirring rate were better than those at lower rate. Since salbutamol sulphate hardly dissolved in ethanol, its solubility reached saturation rapidly as soon as salbutamol reacted with H 2 SO 4. Otherwise, enhancing the stirring rate can intensify the mass transfer process, which can improve the uniformity of micromixing, narrow the metastable region, increase the relative supersaturation, and make the rate of crystal growth lesser than that of nucleation and obtain smaller particles easily [19, 20]. Thus, the stirring rate must be over 500 r min Effect of reaction time Table 1 shows that the percent of the particles in brachyaxis within (800±200)nm was 85% after 5 min, 81% for 10 min, and 70% for 20 min. When the supersaturation of the solute reaches a certain extent, shorter reaction time is prone to forming smaller particles. Here, 20 min or less was selected as the reaction time. The yield of salbutaml sulphate was also affected by different reaction times. It showed that the yield of salbutaml sulphate obtained could increase with the reaction time increasing. However, the difference of the yield was not evident when the reaction exceeded 10 min. On the other hand, longer reaction No. Temperature / C Table 1 Particle size and yield of salbutamol sulphate particle varying with effect factor Stirring rate /r min -1 Reaction time /min Concentration of H 2 SO 4 /mol L -1 Size of particles (brachyaxis) /μm ± ± ± ± ± ± ± ± ± ± ±0.2 (81%) ±0.2 (85%) ±0.2 (70%) ±0.2 (62%) 92.6 Yield /%

4 794 Chin. J. Chem. Eng., Vol. 16, No. 5, October 2008 time prolonged the residence time of salbutaml sulphate in ethanol. In this process, the equilibrium of dissolution-crystallization turned into a dominant step, and smaller particles were dissolved and recrystallized producing larger particles, which influenced the size and uniformity of the particles obtained. Here, 10 min was selected as the feasible reaction time. 3.2 Characterization of synthesized ultra-fine particles Four types of clubbed ultra-fine particles of salbutamol sulphate with different sizes were obtained using the reactive precipitation process according to the conditions of Table 2. The size of all particles was less than 5μm, which was acceptable for pulmonary drug delivery, and deposited in the deep lung and not exhaled. Fig. 1 depicts the TEM photo of particles. Table 2 Size in brachyaxis of salbutamol sulphate obtained under different reactive conditions H 2 SO 4 concentration /mol L -1 Reaction temperature / C Stirring rate /r min -1 Reaction time/min Particle Brachyaxis/μm A ±0.5 B ±0.5 C ±0.2 D ±0.2 (a) (b) Figures 2 and 3 depict the IR and XRD of salbutamol sulphate obtained and that purchased as contrast. It was seen that all salbutamol sulphate obtained had the same structure as that purchased. The purity of each salbutamol sulphate was measured using UV spectrophotometer and the contents were more than 98.0%. (c) 3.3 Evaluation of the dry powder inhaler Table 3 depicts the deposition of different formulations of the drug in breathing tract. It can be seen that all formulations and Shapuer had similar RD, ranging from 405 μg to 448 μg. These RDs correspond to percentage recoveries ranging between 84% and 93%. Compared with Shapuer, the formulations produced slightly small RD and ED, which may be explained by poorer flow ability of the formulation originating from the electrostatic attraction among particles of the drug owing to smaller particle size. However, the formulations compared with Shapuer Figure 1 (d) TEM of salbutamol sulphate provided higher FPD dispersibility and FPF. It can be thought that less than 1 μm particles enter into the pulmonary region more easily. Table 3 Different formulations of deposition of the drug in breathing tract Formulation RD/μg ED/μg FPD/μg FPF/% Dispersibility/% Percent recovery Percent emission synthesized (A) SS 421.6± ± ± ± ± ± ±0.70 synthesized (B) SS 405.2± ± ± ± ± ± ±1.15 synthesized (C) SS 417.5± ± ± ± ± ± ±1.46 synthesized (D) SS 419.5± ± ± ± ± ± ±1.68 purchased (<5 μm) SS 448.5± ± ± ± ± ± ±1.08

5 Chin. J. Chem. Eng., Vol. 16, No. 5, October Using the reactive precipitation pathway, the most interesting morphology consisted of clubbed ultra-fine particles of salbutamol sulphate obtained successfully. The higher concentration of sulphuric acid and stirring rate, and the lower reaction temperature and reaction time were favorable for forming smaller particles of salbutamol sulphate. Using recrystallized lactose as carrier, the formulation of dry powder inhalation of salbutamol sulphate was prepared. Twin impinger apparatus was used to estimate the deposition of the drug in the breathing tract. The experimental results showed that the size of salbutamol sulphate particles reached between 3 μm and 0.8 μm in brachyaxis with the narrow distribution obtained respectively. Compared with the Shapuer powder (available from the market), salbutamol sulphate superparticles had more quantities subsided in simulative lung. The size range of the particle obtained was suitable to allow pulmonary delivery. REFERENCES Figure 2 X-ray patterns of salbutamol sulphate 1 salbutamol sulphate purchased; 2 salbutamol sulphate Figure 3 IR of salbutamol sulphate 1 salbutamol sulphate purchased; 2 salbutamol sulphate 4 CONCLUSIONS 1 State Pharmacopoeia Committee of the People s Republic of China, Pharmacopoeia of the People s Republic of China (II), Chemical Industry Press, Beijing (2000). 2 The British Pharmacopoeia Commission, British Pharmacopoeia, HMSO, London (1993). 3 Weda, M., Zanen, P., de Boer, A.H., Equivalence testing of salbutamol dry powder inhalers: In vitro impaction results versus in vivo efficacy, International Journal of Pharmaceutics, 249 (1), (2002). 4 Ashurst, I.C., Metered dose inhaler for albuterol, U.S.Pat., (2000). 5 Reverchon, E., Della Porta, G., Pallado, P., Supercritical antisolvent precipitation of salbutamol microparticles, Powder Technology, 114, (2001). 6 Ganderton, D., The generation of respirable cloud from coarse powder aggregates, Biopharm. Sci., 3, (1992). 7 Esther, B., Nicholas, I.C., Robert, S.T., A short synthesis of albuterol, Synthesis, 2 (13), (1988). 8 Chen, F.E., Yu, H.X., Wan, J.L., Studies on synthesis of salbutamol sulfate I, Chinese Journal of Medicinal Chemistry, 5 (3), (1993). 9 He, W., Qin, H., Luo, X.D., Improved method of synthesis albuterol, Huaxi Journal of Medicinal, 13 (4), (1998). 10 Zeng, X.M., Martin, G.P., Marriott, C., Particle Interactions in Dry Powder Formulations for Inhalation, Taylor & Francis, London, (2001). 11 Zeng, X.M., Martin, G.P., Marriott, C., Pritchard, J., The influence of carrier morphology on drug delivery by dry powder inhalers, Int. J. Pharm., 200 (1), (2000). 12 Hinds, W.C., Aerosol Technology, Wiley, New York (1982). 13 Zhang, X.G., Effect of lactose on physical-mixed powder subside in breathing tract, Chinese J. Pharm., 32 (8), (2001). 14 Hino, T., Serigano, T., Yamamoto, H., Particle design of wogon extract dry powder for inhalation aerosols with granulation method, International Journal of Pharmaceutics, 168 (1), (1998). 15 Zeng, X.M., Martin, G.P., Tee, S.K., Effects of particle size and adding sequence of fine lactose on the deposition of salbutamol sulphate from a dry powder inhalers, Adv. Drug Delivery Rev., 182 (2), (1999). 16 Hassan, L., Martin, G.P., Marriott, C., Prime, D., The influence of carrier and drug morphology on drug delivery from dry powder formulations, Int. J. Pharm., 257, (2003). 17 Violanto, M.R., Fishcher, H.W., Method for making uniformly sized particles form water-insoluble organic compounds, U.S.Pat., (1989). 18 Zhang, K.C., Zhang, L.H., Crystal Growth, Science Press, Beijing (1981). 19 Chen, J.F., High Gravity Technology and Its Application, Chemical Industry Press, Beijing (2003). 20 Zhu, W.C., Wang, Y.H., Chen, J.F., Synthesis of ultra-fine needle-like calcium carbonate particles by high-gravity reactive precipitation, J. Chem. Eng. Chinese Univ., 16 (5), (2002).