Sclerotium rolfsii scleroglucan: The promising behavior of a natural polysaccharide as a drug delivery vehicle, suspension stabilizer and emulsifier

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Abstract

Gel matrices of scleroglucans from Sclerotium rolfsii ATCC 201126 (EPS I and EPS II, from 48-h and 72-h fermentations, respectively) were evaluated on their release kinetics of theophylline (Th). Equivalent polymer (2%, w/w) and Th (0.2%, w/w) concentrations showed almost coincident drug release patterns, independently of polymer molecular weight or the microstructural properties of gel matrices. Dynamic rheological studies of scleroglucan hydrogel structures (storage, G′, and loss, G″, moduli) indicated a solid-like behavior. Differences on pore size dimensions (EPS I = 20 μm and EPS II = 7 μm) were in accordance to the differences in G′ (EPS I = 113 Pa and EPS II = 161 Pa), a fact likely related to variations in the cross-linking density of polymer networks. Compared to already known biopolymers, EPS I and EPS II at 0.5 g/L showed a good dispersing ability against particulate suspensions of activated charcoal, bentonite, CaCO3, celite and quartz powder. Emulsifying ability of both EPSs at 2 g/L was high (E = 56–60%) when tested with kerosene, moderate (∼30%) with hexadecane, and negligible in the presence of olive oil-in-water emulsions.

Introduction

Scleroglucan, a neutral β-1,3-β-1,6-glucan produced by Sclerotium rolfsii ATCC 201126 has, as other hydrophilic polymers, the ability to form three-dimensional network structures or gel structures even at low polymer concentrations [1], [2]. Water solubility, biocompatibility, resistance to hydrolysis and the ability to maintain viscosity even at high temperatures (100 °C/60 min), high ionic strength (up to 20%, w/v NaCl) and over a wide range of pH 0–13 [3] make this polysaccharide specially attractive for a diversity of applications.

Actual or potential uses may include enhanced oil recovery, paper and painting industries, cosmetic and pharmaceutical products and quality improvement of foods [1], [4]. From the medicinal point of view, antitumor, antimicrobial and antiviral properties of scleroglucan were also attributed to immune stimulating effects [5]. Moreover, researchers have recently evaluated commercially available scleroglucans for drug delivery purposes [1], [6], [7], [8].

As further properties of scleroglucan become revealed, novel and unexpected applications can thus be suggested [9]. While flocculating or emulsifying polysaccharides have been already described, relatively few records were up to date reported on dispersing agents from microorganisms [10], [11], [12], [13]. Dispersing and emulsifying abilities have been poorly or even not explored for scleroglucan and reasonably, advances on the study of the abovementioned features will be crucial to propose the use of scleroglucan in such fields as drilling fluids, pesticide, pharmaceutical and cosmetic formulations, cements and ceramic additives [10].

Various Sclerotium species have been reported as producers of scleroglucan with variable degrees of β-1,6-glycosidic branching [14]. From the literature it appears that beginnings of scleroglucan research, mainly with academic purposes, focused on S. glucanicum polysaccharide, whilst the one produced by S. rolfsii became more attractive from the commercial point of view [9], [15]. Differences regarding molecular weight, number and length of side chains, degree of polymerization and rheological characteristics have been already reported depending on the Sclerotium species, strain, culture conditions or even the downstream processing [9], [15], [16].

In a previous work we have investigated on the S. rolfsii ATCC 201126 scleroglucan physicochemical properties [3]. Different scleroglucans commercially available and currently produced at industrial level have been recently studied, particularly concerning their application for drug delivery systems [1], [6], [7], [8]. However, very little research has been conducted on the actual or potential applications of lab-scale or pilot-plant produced scleroglucans with recently isolated Sclerotium strains.

Since polysaccharide properties could be somewhat at variance among the different available scleroglucans, their ability for specific practical applications should be evaluated in each particular case [17]. Accordingly, the drug delivery, dispersing and emulsifying properties of scleroglucans produced by S. rolfsii ATCC 201126 (EPS I and EPS II) were comparatively assessed against other polymers currently applied for a variety of industrial purposes.

Section snippets

Materials

Scleroglucan exopolysaccharide (EPS) from S. rolfsii ATCC 201126 was produced in batch culture at two different fermentation times: 48 h for EPS I and 72 h for EPS II, recovered and subsequently purified as we previously described [3]. Commercially available scleroglucans of molecular weights (Mw) 4.5 × 105 (LSCL, from CarboMer, USA) and 4 × 106 (HSCL, from Sanofi-Synthelabo, France), EPS I and EPS II (triplex Mw = 5.2 × 106 Da for both) were used with no further purification or modification. Theophylline

Scleroglucan as drug delivery matrix

The cumulative Th concentration for each Th/scleroglucan gel sample was plotted against time (Fig. 1A–C). After subtracting an initial lag-time of 58 s, fitting curves according to Eq. (2) were also constructed (Fig. 1A–C). Parameter values for m and k′ corresponding to Eq. (2) (Table 1) were not significantly different among tested scleroglucans and all prepared hydrogels showed a similar release pattern (Fig. 2).

Dynamic rheological characteristics and microstructural features of scleroglucan polymeric matrices

For all Th-free polymeric matrices, the storage modulus value (G′) exceeded the

Discussion

There is a continuous search for new and/or versatile biopolymers for different industrial purposes [4]. Many of them are still obtained from natural sources, but seasonal fluctuations in their availability or characteristics encourage the exploration for novel and consistently produced candidates. Supply of microbial polymers may satisfy this demand, but efforts should be done aimed at improving polysaccharide yields, production and/or recovery costs, as well as on the knowledge on the polymer

Concluding remarks

Results herein presented gave a clear evidence of the potential of EPSs from S. rolfsii ATCC 201126 for modified drug release and the stabilization of suspensions and emulsions, thus opening new perspectives for the use of this biopolymer. It is the first time that scleroglucan from this fungal strain is tested for the mentioned applications. Additionally, no reports or just a few ones have dealt with the suspending or emulsifying properties of scleroglucan. Already published work currently

Acknowledgements

Financial support from Universidad de Buenos Aires, Argentina (grant I032), CIUNT, Universidad Nacional de Tucumán (grant E-324) and CONICET (PIP 6202) is gratefully acknowledged. Thanks are also due to CALCITEC SRL for generously providing bentonite samples.

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