Drug release from injectable biodegradable polymeric adhesives for bone repair

Health care-associated infections: Controlled delivery of cationic antiseptics from polymeric excipients

Nowadays, the treatment of health care-associated infections represents a serious issue, due to the increasing number of bacterial strains resistant to traditional antibiotics. The use of antiseptics like quaternary ammonium salts and biguanides is a viable alternative to face these life-threatening infections. However, their inherent toxicity as well as the necessity of providing a sustained release to avoid the formation of pathogen biofilms are compelling obstacles towards their assessment in the hospitals. Within this framework, the role of polymeric drug delivery systems is fundamental to overcome the aforementioned problems. Biocompatibility, biodegradability and excipient-drug interactions are crucial properties determining the efficacy of the formulation. In this work, we provide an in-depth analysis of the polymer drug delivery systems that have been developed or are under development for the sustained release of positively charged antiseptics, highlighting the crucial characteristics that allowed to achieve the most relevant therapeutic effects. We reported and compared natural occurring polymers and synthetic carriers to show their pros and cons and applicability in the treatment of health care-associated infections. Then, the discussion is focused on a particularly relevant class of materials adopted for the scope, represented by polyesters, which gave rise, due to their biodegradability, to the field of resorbable drug delivery devices. Finally, a specific analysis on the effect of the polymer functionalization over the formulation performances for the different types of polymeric carriers is presented.

Highly adjustable biomaterial networks from three-armed biodegradable macromers

Biocompatible material platforms with adjustable properties and option for chemical modification are warranted for site-specific biomedical applications. To this end, three-armed biodegradable macromers of well-defined chemical characteristics were prepared from trivalent alcohols with different degrees of ethoxylation and different lengths of oligoester domains. A platform of 15 different macromers was established. The macromers were designed to exhibit different hydrophilicities and molecular weights and contained various types of oligoesters such as d,l-lactide, l-lactide and ε-caprolactone. Macromers chemical composition was determined and molecular weights ranged from 900 to 3000 Da. Thermally induced cross-linking of methacrylated macromers was monitored by oscillation rheology. A novel variant of the solid lipid templating technique was established to fabricate macroporous tissue engineering scaffolds from these macromers. Scaffold properties were thoroughly investigated regarding mechanical properties, compositional analysis including methacrylic double bond conversion, microstructure and porosity. Material properties could be controlled by macromer chemistry. By variation of the fabrication procedure and processing parameters scaffold porosity was increased up to 88%. Basic cytocompatibility was assessed including indirect and direct contact methods. The established macromers hold promise for various biomedical purposes.

STATEMENT OF SIGNIFICANCE

Specific biomedical applications require tailored biomaterials with defined properties. We established a macromer platform for preparation of tissue engineering scaffolds with adjustable chemical and mechanical characteristics. Macromers were composed of trivalent core alcohols with different degrees of ethoxylation to which biodegradable domains - lactide or ε-caprolactone - were oligomerized before final methacrylation. The solid lipid templating technique was adapted to fabricate macroporous scaffolds with controlled pore structure and porosity from the developed macromers, which can also be processed by solid freeform fabrication techniques. The material platform relies on clinically established chemistries of the biodegradable domains and the macromer concept enables the fabrication of networks in which cross-polymerization kinetics, mechanical properties and surface hydrophobicity is predefined by macromer chemistry. Cytocompatibility was confirmed by indirect and direct cell contact experiments.

Viscoelastic and biological performance of low-modulus, reactive calcium phosphate-filled, degradable, polymeric bone adhesives

The aim of this study was to investigate the effect of reactive mono- and tricalcium phosphate addition on the mechanical, surface free energy, degradation and cell compatibility properties of poly(lactide-co-propylene glycol-co-lactide) dimethacrylate (PPGLDMA) thin films. Dry composites containing up to 70 wt.% filler were in a flexible rubber state at body temperature. Filler addition increased the initial strength and Young’s modulus and reduced the elastic and permanent deformation under load. The polymer had high polar surface free energy, which might enable greater spread upon bone. This was significantly reduced by filler addition but not by water immersion for 7 days. The samples exhibited reduced water sorption and associated bulk degradation when compared with previous work with thicker samples. Their cell compatibility was also improved. Filler raised water sorption and degradation but improved cell proliferation. The materials are promising bone adhesive candidates for low-load-bearing areas.

Reactive calcium-phosphate-containing poly(ester-co-ether) methacrylate bone adhesives: setting, degradation and drug release considerations

This study has investigated novel bone adhesives consisting of fluid photo-polymerizable poly(lactide-co-propylene glycol-co-lactide)dimethacrylate (PGLA-DMA) mixed with systematically varying fillers of β-tricalcium phosphate (β-TCP) and monocalcium phosphate monohydrate (MCPM), for the delivery of an antibacterial drug chlorhexidine (CHX). All formulations were found to polymerize fully within 200 s after exposure to blue light. In addition, water sorption by the polymerized materials catalyzed varying filler conversion to dicalcium phosphate (DCP) (i.e. brushite and monetite). With greater DCP levels, faster degradation was observed. Moreover, increase in total filler content enhanced CHX release, associated with higher antibacterial activity. These findings thus suggest that such rapid-setting and degradable adhesives with controllable drug delivery property could have potential clinical value as bone adhesives with antibacterial activity.

Cell attachment and response to photocured, degradable bone adhesives containing tricalcium phosphate and purmorphamine

The aim of this study was to quantify and provide evidence as to how addition of tricalcium phosphate (β-TCP) and the Hedgehog agonist purmorphamine to a degradable bone adhesive affects cell attachment/proliferation and Hedgehog pathway activation. Fourier transform infrared spectroscopy demonstrated that high levels (75 wt.%) of β-TCP addition reduced the photocure rate of the chosen poly(propylene glycol-co-lactide) dimethacrylate (PPLM) bone adhesive, but this problem was overcome by increased light exposure. In phosphate-buffered saline the total surface mass loss of set 15 mm diameter PPLM films was ∼3.2 mg in 12 weeks, irrespective of thickness (200 or 400 μm) or β-TCP level (50 or 75 wt.%). With 400 μm samples there was additional bulk material loss. Proliferation of pre-osteoblast cells (MC3T3-E1) on the set adhesive surfaces was enhanced by decreased sample thickness or filler content increase. Degradation evidence suggested that both effects were due to reduced acidic polymeric degradation products. Activation of the Hedgehog pathway was quantified by measuring Gli expression in Light II reporter cells. The 0.01 and 0.1 wt.% purmorphamine in composite discs (400 μm, 75 wt.% β-TCP) enhanced Gli expression of attached cells 2- and 5-fold, respectively, without influencing their number. Pre-storage of the composite samples in culture medium had no detrimental effect on this response. Furthermore, sample storage medium gave no enhanced Gli expression in cells on tissue culture plastic. This suggests drug release levels were very low. Purmorphamine and β-TCP incorporation in PPLM adhesives might, therefore, provide prolonged enhancement of in vivo bone repair without systemic drug side-effects.

Chemical, modulus and cell attachment studies of reactive calcium phosphate filler-containing fast photo-curing, surface-degrading, polymeric bone adhesives

The initial structure, setting and degradation processes of a poly(lactide-co-propylene glycol-co-lactide) dimethacrylate adhesive filled with 50, 60 or 70 wt.% reactive calcium phosphates (monocalcium phosphate monohydrate (MCPM)/beta-tricalcium phosphate (beta-TCP)) have been assessed using nuclear magnetic resonance, Fourier transform infrared spectroscopy, Raman, X-ray powder diffraction and gravimetric studies. Filler incorporation reduced the rapid light-activated monomer polymerization rates slightly, but not the final levels. Upon immersion in water for 24h, the set composite mass and volume increased due to water sorption. This promoted initial soluble MCPM loss from the composite surfaces, but also its reaction and monetite precipitation within the specimen bulk. After 48 h, composite gravimetric and chemical studies were consistent with surface erosion of polymer with reacted/remaining filler. The filled formulations exhibited more rapid early water sorption and subsequent surface erosion than the unfilled polymer. Calcium and phosphate release profiles and solution pH measurements confirmed early loss of surface MCPM with protons from polymer degradation products. At later times, the slower release of monetite/beta-TCP buffered composite storage solutions at approximately 5 instead of 3.2 for the unfilled polymer. Incorporation of filler increased both the early and later time material modulus. At intermediate times this effect was lost, presumably as a result of enhanced water sorption. The early modulus values obtained fell within the range reported for cancellous bone. Despite surface degradation, initial human mesenchymal cell attachment to both composites and polymer could be comparable with a non-degrading positive Thermanox control. These studies indicate that the filled formulations may be good candidates for bone repair. Release of calcium and phosphate ions provides components essential for such repair.

Reactive calcium-phosphate-containing poly(ester-co-ether) methacrylate bone adhesives: chemical, mechanical and biological considerations

A poly(propylene glycol-co-lactide) dimethacrylate adhesive with monocalcium phosphate monohydrate (MCPM)/beta-tricalcium phosphate (beta-TCP) fillers in various levels has been investigated. Water sorption by the photo-polymerized materials catalyzed varying filler conversion to dicalcium phosphate (DCP). Polymer modulus was found to be enhanced upon raising total calcium phosphate content. With greater DCP levels, faster release of phosphate and calcium ions and improved buffering of polymer degradation products were observed. This could reduce the likelihood of pH-catalyzed bulk degradation and localized acid production and thereby may prevent adverse biological responses. Bone-like MG-63 cells were found to attach, spread and have normal morphology on both the polymer and composite surfaces. Moreover, composites implanted into chick embryo femurs became closely apposed to the host tissue and did not appear to induce adverse immunological reaction. The above results suggest that the new composite materials hold promise as clinical effective bone adhesives.

Development of remineralizing, antibacterial dental materials

Light curable methacrylate dental monomers containing reactive calcium phosphate filler (monocalcium phosphate monohydrate (MCPM) with particle diameter of 29 or 90microm) and beta-tricalcium phosphate (beta-TCP) at 1:1 weight ratio in a powder:liquid ratio (PLR) of 1:1 or 3:1 and chlorhexidine diacetate (0 or 5 wt.%), were investigated. Upon light exposure, approximately 90% monomer conversion was gained irrespective of the formulation. Increasing the PLR promoted water sorption by the set material, induced expansion and enhanced calcium, phosphate and chlorhexidine release. Concomitantly, a decline in compressive and biaxial flexural strengths occurred. With a reduction in MCPM particle diameter, however, calcium and phosphate release was reduced and less deterioration in strength observed. After 24h, the remaining MCPM had reacted with water and beta-TCP, forming, within the set materials, brushite of lower solubility. This provided a novel means to control water sorption, component release and strength properties. Measurable chlorhexidine release was observed for 6weeks. Both diffusion rate and total percentage of chlorhexidine release decreased with lowering PLR or by adding buffer to the storage solutions. Higher chlorhexidine release was associated with reduced bacterial growth on agar plates and in a biofilm fermenter. In cell growth media, brushite and hydroxyapatite crystals precipitated on the composite material surfaces. Cells spread on both these crystals and the exposed polymer composite surfaces, indicating their cell compatibility. These formulations could be suitable antibacterial, biocompatible and remineralizing dental adhesives/liners.

Chemical characterization of a degradable polymeric bone adhesive containing hydrolysable fillers and interpretation of anomalous mechanical properties

An experimental, light-curable, degradable polyester-based bone adhesive reinforced with phosphate glass particles ((P(2)O(5))(0.45)(CaO)(x)(Na(2)O)(0.55-)(x), x=0.3 or 0.4mol) or calcium phosphate (monocalcium phosphate/beta-tricalcium phosphate (MCPM/beta-TCP)) has been characterized. Early water sorption (8wt.% at 1week) by the unfilled set adhesive catalysed subsequent bulk degradation (4wt.% at 2weeks) and substantial decline in both elastic and storage moduli. Addition of phosphate glass fillers substantially enhanced this water sorption, catalysed greater bulk mass loss (40-50 and 52-55wt.%, respectively) but enabled generation of a microporous scaffold within 2weeks. The high levels of acidic polymer degradation products (38-50wt.% of original polymer) were advantageously buffered by the filler, which initially released primarily sodium trimetaphosphate (P(3)O93-). Calcium phosphate addition raised polymer water sorption to a lesser extent (16wt.%) and promoted intermediate early bulk mass loss (12wt.%) but simultaneous anomalous increase in modulus. This was attributed to MCPM reacting with absorbed water and beta-TCP to form more homogeneously dispersed brushite (CaHPO(4)) throughout the polymer. Between 2 and 10weeks, linear erosion of both polymer (0.5wt.%week(-1)) and composites (0.7-1.2wt.%week(-1)) occurred, with all fillers providing long-term buffer action through calcium and orthophosphate (PO43-) release. In conclusion, both fillers can raise degradation of bone adhesives whilst simultaneously providing the buffering action and ions required for new bone formation. Through control of water sorption catalysed filler reactions, porous structures for cell support or substantially stiffer materials may be generated.