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Introduction

Indomethacin is an acidic non-steroidal anti-inflam-matory drug (NSAID). It is a non-selective inhibitor of cyclooxygenase (COX) 1 and 2 enzyme that participates in prostaglandin synthesis from arachidonic acid. It is used to reduce pain/swelling involved in osteoarthritis, rheumatoid arthritis, bursitis, tendinitis, gout, ankylos-ing spondylitis, headaches, eye disorders, and pediat-ric disorders.[1,2] However, indomethacin is practically insoluble in water, which precludes its use in aqueous solution formulations. Thus, indomethacin for injection or eye drops had been on shortage due to manufacturing difficulties. The solubility enhancement of indomethacin by the hydrotropes such as urea, nicotinamide, resorci-nol, sodium benzoate and sodium p-hydroxy benzoate, water–cosolvent mixtures, Gelucire 44/14, and cyclodex-trin was demonstrated.[3–10]

The eye drops and injections in Chinese market are all through adding the polyethylene glycol (PEG) 400 or surface active agent to prepare solution. They are very irritant to both eyes and blood vessels. The stability is relatively poor, and the shelf life is short, hence affect-ing the use of indomethacin. Many manufacturers and research departments have been looking for a method to increase the water solubility of indomethacin, such

as using the inclusion complex with cyclodextrin. However, the process is complex and the inclusion rate is lower, along with a higher production cost of industry.

The hydrotropic effect by arginine was observed in increasing the solubility of proteins and organic compounds.[11–13] The present study investigates the effect of arginine (l-arginine base) on the solubility of indomethacin, and attempts to develop aqueous for-mulations and freeze-dried power. Formulations were also studied for physical and chemical stability.[14,15] The chemical structures of drug and arginine used in this study are shown in Figure 1.

Materials and methods

MaterialsIndomethacin was obtained from Wuhan Pharmaceutical Co., Ltd., Wuhan, Hubei, China. Arginine was obtained from Shijiazhuang Jing-jing Pharmaceutical Co., Ltd., Shijiazhuang, Hebei, China.

Estimation of indomethacinIn the present study, UV spectrophotometric method was used for the estimation of indomethacin. The calibration

RESEARCH ARTICLE

Solubility and stability of indomethacin in arginine-assisted solubilization system

Xiaodan Qi, Jing Zhang, Wei Wang, and Deying Cao

Department of Pharmaceutics, School of Pharmacy, Hebei Medical University, ShiJiaZhuang, P.R. China

AbstractIndomethacin is a non-steroidal anti-inflammatory drug (NSAID).It is practically insoluble in water, which precludes its use in aqueous solution formulations. The effect of arginine on the solubility of indomethacin was investigated in this study. The solubility enhancement of indomethacin by the arginine was observed. Aqueous solution formulation and freeze-drying formulation using arginine as hydrotropes were developed and studied for physical and chemical stability. Freeze-drying is considered as a suitable formulation to enhance shelf life of indomethacin.Keywords: Indomethacin, arginine, solubility, stability, hydrotrope

Address for Correspondence: Deying Cao, Department of Pharmaceutics, School of Pharmacy, Hebei Medical University, ShiJiaZhuang 050017, P.R. China. Tel: +86-311-86265633. Fax: +86-311-86266050. E-mail: caody001@sina.com

(Received 21 January 2011; revised 25 May 2011; accepted 30 May 2011)

Pharmaceutical Development and Technology, 2013; 18(4): 852–855© 2013 Informa Healthcare USA, Inc.ISSN 1083-7450 print/ISSN 1097-9867 onlineDOI: 10.3109/10837450.2011.595797

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curve of indomethacin was prepared in distilled water at 320 nm using TU-1901 double-beam UV–vis spectropho-tometer (Beijing Purkinje General Instrument Co., Ltd., Haidian, Beijing, China).

Solubility studyAn excess quantity of indomethacin was added to screw-capped 10-mL glass vials containing the distilled water, and arginine solutions of different concentra-tions (0.25, 0.2, 0.1, 0.05, 0.02, 0.01, 0.006, and 0.002 M) in water. The vials were shaken on a horizontal rotary shaker (KYC-100B; FUMA Laboratory Equipment Co., Ltd., China) at a speed of 150 rpm at 25 ± 1°C for 72 h and shielded from light. The resultant suspension was filtered through a 0.45 μm membrane filter to obtain a clear solution, filtrate diluted with distilled water, and analyzed spectrophotometrically at 320 nm against respective arginine solution diluted accordingly as blank. The solubility of indomethacin was determined in triplicate.

Formulation of aqueous solution and freeze-dried powderOn the basis of solubility data obtained, aqueous solution of indomethacin was prepared, which contained 10 mg/mL of indomethacin in 0.02 M arginine solution. This aqueous solution was freeze-dried by a vacuum freeze-dryer (Alpha 2-4, Christ, Germany) to obtain lyophilizate: the condenser was refrigerated to −60°C, a vacuum of 133 × 10−2 mbar within 10 min, and to achieve an ultimate vacuum of 33 × 10−2 mbar within 18 h, and ampoules were then sealed.

Stability studiesPhysical stability studiesThe sealed vials of the formulations were visually inspected at regular intervals against black and white backgrounds to see the changes occurring, if any, in phys-ical appearance of aqueous solution and solids, includ-ing color, turbidity, precipitation, sub-visual particulate matter (Chinese Pharmacopoeia 2010), reconstitution, and so on, on storage at 4 ± 2°C in a refrigerator, 25 ± 2°C (RT), 37 ± 2°C, 45 ± 2°C, and 60 ± 2°C in thermostatically controlled ovens.

Chemical stability studiesThe formulations were subjected to exhaustive chemical stability at 25 ± 2°C (RT) at regular intervals. The formu-lations were analyzed spectrophotometrically initially and at intervals to calculate the drug content. The per-centage drug remaining for each formulation at different time intervals as well as at different temperatures was calculated.

Results and discussion

The results of solubility study indicated that the solubil-ity of indomethacin sharply increased in the presence of arginine. This may be due to the acidic nature of indo-methacin and alkaline amino acid nature of arginine, or maybe because of arginine interacts with the aromatic moiety of the drug.[12,16–20] The aqueous solubility of indo-methacin was increased to 165 mg/mL in 0.25 M arginine solution. The solubility of indomethacin as a function of arginine concentration is shown in Figure 2. Phase solubility diagram (Figure 2) was AN type; this shows that increase in total solubility of drug was not linear function of hydrotropic concentration. This negative deviation from linearity may be associated with formation of self-polymer or dissociation of drug at high concentration. This is because indomethacin is stable in pH 2–8, and can be hydrolyzed in strong alkaline conditions. Under the relatively high concentration of arginine, the pH was higher (0.2 M, pH 9.18), and the hydrolysis of indometha-cin can be accelerated. Thus, the measured content decreased, and it was showed in the decrease of solubil-ity at the rear solubility curve.

The physical stability study showed that except for a slight to moderate color change in solution formulation, which are stored at 45°C or 60°C, all other formulations remain unchanged with respect to color change and no turbidity or precipitate formation was observed under different storage conditions. The lyophilizate could be completely dissolved in water forming a stable, clear solution even after 12 months storage. The change in color may be due to accelerated degradation of indo-methacin in solution at higher temperature in presence of oxygen that may get entrapped during fabrication despite precautions.

Figure 1. Chemical structures of indomethacin and arginine.

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The data on chemical stability at different time inter-vals are shown in Table 1, which show fast degradation rate in aqueous solution. The solution formulation remained stable for not more than 4 months, whereas freeze-dried powder formulation was stable for at least 12 months when stored at room temperature. The results show that long-term stability can be achieved by freeze-drying formulation to inhibit degradation of indomethacin.

Conclusion

The use of arginine is an effective means of solubiliz-ing indomethacin. As arginine is less toxic, it should be a much safer hydrotropic for biological systems. This enabled to avoid using organic solvent or surface active agents that are highly toxic and irritating to eyes and

vascular. The goal of this study is to prepare a stable indo-methacin formulation, resulting in lyophilized formula-tion with arginine as suitable product for long shelf life and efficient reconstitution.

Declaration of interest

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the article.

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Figure 2. Solubility of indomethacin in arginine solution.

Table 1. Chemical stability data of formulations.

Time interval Percent indomethacin remaininga

Aqueous solution Freeze-dried powder1 day 100 1003 days 100 1005 days 100 1007 days 100 10014 days 99.8 10021 days 99.5 10028 days 99.4 1002 months 95.5 99.93 months 93.1 99.74 months 89.6 99.55 months 89.0 99.46 months 87.4 99.37 months 86.5 99.18 months 84.1 99.19 months 82.8 98.910 months 81.9 98.911 months 81.2 98.812 months 80.5 98.7aAverage of three observations.

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