Delivery of the antibiotic gentamicin sulphate from precipitation cast matrices of polycaprolactone
Introduction
Bacterial infection of bone (osteomyelitis), soft tissue, the eye and foot ulcers in patients with diabetes constitutes a serious healthcare problem particularly when drug-resistant bacteria such as methicillin-resistant Staphylococcus aureus are involved [1], [2], [3]. Infection following musculoskeletal injuries and surgical implantation is generally treated by debridement and irrigation and systemic antibiotic therapy. However, it is often difficult to treat topical infection using the latter approach since only a fraction of the dose reaches the target site, a problem which is accentuated when the local blood supply is compromised. A wide variety of polymeric carriers has therefore been investigated for local antibiotic delivery [3], [4], [5], [6], [7], [8], [9] in attempts to maintain drug levels above the minimum inhibitory concentration (MIC) without achieving systemic toxicity. Beads of non-biodegradable polymethylmethacrylate (PMMA) containing aminoglycoside antibiotics such as gentamicin sulphate (GS) are commonly used for treating bone and soft tissue infection [5], [6]. Almost 40% of the drug is released in 14 days resulting in local concentrations of 30 mg/l within the first day postoperatively and 5 mg/l for several weeks which are sufficient to inhibit non-resistant microorganisms. However incomplete drug release, colonisation of PMMA devices by bacteria, decreased function of immune cells and the generally accepted requirement for surgical removal of the implants have prompted investigations of biodegradable polymers as alternatives. Porous collagen matrices result in total and rapid release of gentamicin within 1–4 days in vivo in well perfused sites, maintaining levels above the MIC for gentamicin-sensitive organisms (4 μg/ml) for relatively short time periods of around 7 days [6].
The synthetic biodegradable polymers such as poly(l-lactide) (L.PLA) and poly(dl lactide co-glycolide) (PLGA) feature prominently in research into controlled antibiotic delivery. GS-loaded microspheres (250–550 μm) produced by emulsification-solvent evaporation techniques, can provide gradual and complete drug release in 7–10 days in vitro (following an initial burst release of 10–25%) by blending 50 : 50 PLGA copolymers of different molecular weight [8]. GS-loaded beads, disks and cylinders are often formulated by compression of polymer/drug powders but this approach can result in major burst release of the antibiotic (around 40%) and limited duration of release in vitro (70% in 15 days) [4], [9], [10], [11]. Incorporation of 10% lactide or glycolide monomer in compression moulded PLGA disks, however, has been shown to extend GS delivery over 1 month in PBS at 37 °C and gave rise to zero order release profiles (following 30% burst release of antibiotic) [10]. In a related though complex approach, GS–calcium phosphate–DL.PLA disks were prepared by compression of granules [11]. However, coating of the disks with DL.PLA was necessary to avoid major burst release (50%) of the drug load and to provide gradual delivery of approximately 50% of the GS content over 5 weeks. Melt processing has been applied widely as an alternative to compression moulding of antibiotic delivery devices. Injection moulded GS-loaded polyanhydride beads (12 × 3 mm) have been developed for implantation at sites where joint prostheses are replaced due to infection [12]. Zhang et al. [13] produced a series of cylindrical, GS-loaded devices from poly dl.lactide (DL.PLA) and poly(dl lactide co-caprolactone) copolymers by melt extrusion. Control over the release profile was achieved by incorporation of dl.lactic acid oligomers in the formulations, by coating the devices with DL.PLA and by adjustment of the drug loading and length of the devices. Small burst effects, coupled with gradual and sustained GS release in vitro were achieved by limiting the antibiotic loading to 20–30% in coated DL.PLA cylinders.
Poly(ε-caprolactone) (PCL) is characterised by a resorption time in excess of two years but ease of processing, biocompatibility and successful clinical usage as a delivery device for contraceptive hormones have stimulated investigation of the polymer in a number of formats for local antibiotic delivery. Vancomycin was incorporated in 200–250 μm PCL microspheres by using powder suspensions in the solvent evaporation method or by using an aqueous vancomycin solution to form the primary emulsion in the double emulsion technique [14]. PCL rods (10 × 3 mm dia.) loaded with the aminoglycoside, tobramycin, were produced from solvent cast films containing dispersed drug. The devices were more effective at eliminating infection in its early stage than irrigation alone and delivered higher levels of antibiotic than PMMA rods when inserted in rabbit femurs [3], [15]. Microporous PCL matrices (MPM), formulated by precipitation casting, have shown advantages for drug delivery [16], [17]. Incorporation of particulates of inulin polysaccharide (as a model of a hydrophilic, macromolecular drug) in the matrices resulted in limited release of 18% of the content in 12 months in PBS at 37 °C due to effective encapsulation by the PCL phase. In contrast, a low molecular weight lipophilic drug (progesterone), incorporated by co-solution techniques, was released with high efficiency over 10 days [17]. The aim of the present study was to extend the range of applications of microporous PCL matrices to encompass delivery devices for low molecular weight hydrophilic drugs. Gentamicin sulphate provided the candidate molecule and the influence of formulation conditions on GS loading, subsequent release kinetics in vitro and the antibacterial effect of released drug on cultures of S. epidermidis are described.
Section snippets
Materials and methods
Gentamicin sulphate powder and o-phthaldialdehyde reagent were purchased from Sigma. PCL (CAPA 650, Mw 115,000) was obtained from Solvay Interox, Warrington, UK. Nutrient broth was prepared from 25 g nutrient stand (AM132, Amyl Meida Pty Ltd., Australia) in 1 L de-ionised water. Nutrient agar (1.5%) was made from 15 g agar (Ajax Finechem, Australia) in 1 L nutrient broth. S. epidermidis were supplied by Ms Helen Agus, School of Molecular and Microbial Biosciences, University of Sydney.
Results and discussion
Since gentamicin sulphate is insoluble in most organic liquids (including acetone and methanol) but freely soluble in water (> 1 g/ml) [20] the antibiotic was incorporated in microporous matrices by precipitation casting at room temperature using dispersions of GS powder in PCL solution. GS-loaded MPM prepared using 12.5% w/v PCL solutions with a theoretical 10% w/w drug loading showed very low (19%) loading efficiency (Table 1). Moreover, the distribution of GS was not uniform (Fig. 1) due to
Conclusions
Microporous PCL matrices can be loaded with the hydrophilic antibiotic gentamicin sulphate by dispersing powder in a PCL solution prior to precipitation casting. Cooling of the suspension to 4 °C improved antibiotic loading by a factor of 4 to approximately 7% w/w, doubled the delivery efficiency to 80% and extended the time of GS release by a factor of 10 in PBS at 37 °C to 20 weeks. GS-loaded PCL matrices retained anti-bacterial activity after immersion in PBS at 37 °C over 14 days as
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Present address: School of Pharmacy, Steele Building, The University of Queensland, Brisbane, QLD 4072, Australia.