On improving the physiological stability of curcuminoids: Curcumininoid-silver nanoparticle complex as a better and efficient therapeutic agent

Highlights

• Curcuminoid silver nanoparticles (CUR-AgNP) were prepared by green chemistry approach.

• CUR-AgNPs showed improved physiological stability.

• CUR-AgNPs showed excellent antimicrobial activity.

• CUR-AgNPs exhibited cytotoxic effect on MDA-MB231 cells.

Abstract

Curcumin, together with its analogs, namely, demethoxycurcumin and bisdemethoxycurcumin, are phenolic pigments extracted from turmeric. These curcuminoids are considered to be a potent antioxidants possessing a multitude of properties including anti-inflammatory, antitumor, anti-HIV and antimicrobial properties. One of the serious drawbacks of these curcuminoids as therapeutic agent is their fast degradation at physiological pH leading to poor bioavailability in the biological milieu. With a view to address this stability issue we synthesized curcuminoid-silver nanoparticle (CUR-AgNP) complexes, utilizing curcuminoids as reducing agents. The stability of individual curcuminoids in the CUR control and in the CUR-AgNP complex was compared after incubation in phosphate buffer of pH 7.4. Interestingly the initial degradation of curcuminoids at 4 h was only 9.6% in the CUR-AgNP complex when compared to 49.6% degradation observed in the CUR control. Among the three curcumin analogs, bisdemethoxycurcumin showed better pH stability compared to curcumin and demethoxycurcumin whereas all the three curcuminoids exhibited the same degradation pattern in the case of the CUR-AgNP complex. In vitro studies with the CUR-AgNP complex showed excellent antibacterial property against P. aeruginosa. CUR-AgNP complex also exhibited good cytotoxic effect on MDA-MB231 cells. The storage stability of CUR-AgNP complex, kept as a suspension at room temperature ($\sim$28 ${}^{\mathrm{o}}$C), was tested by measuring its hydrodynamic size and zeta potential. It was found that after 340 days of storage the particle size (Z-average) and zeta potential were 65.5 ± 1.1 nm and -22.9 ± 0.2 mV, respectively, indicating that the nanoparticle suspension was stable The results obtained in the present study are promising and could be extended to improve the therapeutic efficacy of curcuminoids in the biological pH conditions.

Introduction

Turmeric a common spice that has long been recognized for its medicinal properties [1] has received much interest being the primary source of curcumin. Historically, turmeric has been extensively used in Ayurveda and is believed to be effective against disorders of the skin, bone and digestive systems [2]. The active ingredients in turmeric are yellow-colored pigments called curcuminoids with curcumin being its major constituent [3]. Curcumin exists with demethoxycurcumin and bisdemethoxycurcumin [3], in commercial extracts in a ratio 71:17:3 [4]. The chemical name of curcumin is 1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione [5]. Curcumin is now being widely promoted as a therapeutic owing to its antioxidant [6], antimicrobial [7], antitumor [8] and anti-inflammatory properties [9]. However, its poor aqueous stability and rapid degradation in physiological and higher pH [10], leads to limited bioavailability reducing its potential for therapeutic translation. The stability of curcumin is very important to retain its physiological properties [11]. Zhu et al. [12] reported that the degradation products of curcumin such as ferulic acid, vanillin, ferulaldehyde, feruloyl methane and bicyclopentadione were biologically active but their biological activities were substantially less when comparing with curcumin. Therefore, keeping the curcumin stable in the biological pH is important to keep its biological activity.

There exists a plethora of literature on methods to improve the stability of curcumin. Nanoencapsulation in polymers and cyclodextrin [13], redox active antioxidants [14], nanoliposomes [15], conjugation with hyaluronic acid [16], chitosan-chlorogenic acid [17] and binding to metallic cations such as zinc, copper and palladium [18], [19], [20], [21] effectively enhanced curcumin stability. In addition there are reports on the application of curcumin silver nanoparticles for various therapeutic needs. Jaiswal and Mishra [22] demonstrated the effectiveness of curcumin silver nanoparticle against both Gram-positive and Gram-negative bacteria and Loo et al. [23] reported enhanced antibiofilm activity. These nanoparticles also exhibit efficient inhibition against respiratory syncytial virus infection with no host cell toxicity [24] and could be utilized for nucleic acid sensing [25] and collagen stabilization [26]. Though several research groups have focused on the synthesis, stability and applications of these curcumin-silver nanoparticles, there is inadequate information on the stability of the complexes formed by individual curcuminoids with silver at physiological pH. Our work mainly focuses on this stability aspect among the three curcumin analogs.

The main objective of our study was to develop a stable curcuminoid capped silver nanoparticle (CUR-AgNP) complex by green synthesis route without the involvement of any harsh chemicals employing curcuminoids as a reducing agent. The formation of the CUR-AgNP complex was monitored and confirmed using UV–Vis absorption and fluorescence spectroscopic techniques. The nanoparticles were characterized by UV–Vis absorption and fluorescence spectroscopy, X-ray powder diffraction, transmission electron microscopy, dynamic light scattering,fourier transform infrared spectroscopy, inductively coupledplasma optical emission spectrometry, X-ray photoelectron spectroscopy and high-performance liquid chromatography.

The physiological stability of the CUR-AgNP complex was evaluated using HPLC and UV–Vis​ absorption spectroscopy. In vitro antibacterial activities of the CUR-AgNP complex were carried out on three different bacterial strains Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus. Cytotoxicity of the CUR-Ag NP complex against MDA-MB231cells was studied and quantified using MTT assay. We envision that these relatively stable NPs at physiological pH could serve as effective and potential therapeutic agents.