Date Published: December 1, 2017
Author(s): Panagiotis Sofokleous, Shanom Ali, Peter Wilson, Asma Buanz, Simon Gaisford, Dharmit Mistry, Adrian Fellows, Richard M. Day.
The spread of antibiotic-resistant pathogens requires new treatments. Small molecule precursor compounds that produce oxidative biocides with well-established antimicrobial properties could provide a range of new therapeutic products to combat resistant infections. The aim of this study was to investigate a novel biomaterials-based approach for the manufacture, targeted delivery and controlled release of a peroxygen donor (sodium percarbonate) combined with an acetyl donor (tetraacetylethylenediamine) to deliver local antimicrobial activity via a dynamic equilibrium mixture of hydrogen peroxide and peracetic acid. Entrapment of the pre-cursor compounds into hierarchically structured degradable microparticles was achieved using an innovative dry manufacturing process involving thermally induced phase separation (TIPS) that circumvented compound decomposition associated with conventional microparticle manufacture. The microparticles provided controlled release of hydrogen peroxide and peracetic acid that led to rapid and sustained killing of multiple drug-resistant organisms (methicillin-resistant Staphylococcus aureus and carbapenem-resistant Escherichia coli) without associated cytotoxicity in vitro nor intracutaneous reactivity in vivo. The results from this study demonstrate for the first time that microparticles loaded with acetyl and peroxygen donors retain their antimicrobial activity whilst eliciting no host toxicity. In doing so, it overcomes the detrimental effects that have prevented oxidative biocides from being used as alternatives to conventional antibiotics.
The manuscript explores a novel approach to utilize the antimicrobial activity of oxidative species for sustained killing of multiple drug-resistant organisms without causing collateral tissue damage. The results demonstrate, for the first time, the ability to load pre-cursor compounds into porous polymeric structures that results in their release and conversion into oxidative species in a controlled manner. Until now, the use of oxidative species has not been considered as a candidate therapeutic replacement for conventional antibiotics due to difficulties associated with handling during manufacture and controlling sustained release without causing undesirable tissue damage. The ultimate impact of the research could be the creation of new materials-based anti-infective chemotherapeutic agents that have minimal potential for giving rise to antimicrobial resistance.
Many antibiotics, antiseptics and disinfectants developed to date are susceptible to bacterial resistance , , . There is, therefore, an urgent need to find alternative antimicrobial compounds that provide broad spectrum activity to which resistance is less likely to be present. Re-evaluation of existing high energy oxidative species, such as hydrogen peroxide (H2O2) and peracetic acid (PAA), with well-established biocidal properties could provide a range of new therapeutic products . PAA does not give rise to bacterial resistance, whereas some resistance can occur to H2O2 for organisms producing catalase and dismutase enzymes , . However, when used alone, these low-molecular weight oxidising agents offer limited clinical therapeutic potential since they pass easily through cell membranes and at high local concentrations lead to cell death, causing unwanted off-target tissue damage , . To date, safe and efficacious delivery of oxidative species into sites of infection without causing unacceptable tissue damage has proven to be challenging , , .
The porous structure observed in unloaded TIPS microparticles was created by dimethyl carbonate (DMC) solvent crystals formed when the droplet of polymer solution froze as it entered the liquid nitrogen quenching bath. The frozen solvent was removed by lyophilisation resulting in the highly porous polymer structure. The different surface morphology observed with microparticles loaded with TAED was due to the introduction of acetonitrile (ACN) to the polymer solution as a solvent for TAED. The melting temperature of ACN (−45 °C) and the interaction with the polymer solution is likely to have changed the formation of solvent crystal shapes in the frozen droplet structure . The increased porosity observed with microparticles loaded with SP is likely to have resulted from the inclusion of water introduced to the polymer mix to dissolve the SP. This may have caused changes to the thermal energy of the system, altering the shape of the solvent crystals formed in the droplet during the freezing process . Mixing aqueous solutions into a hydrocarbon-rich polymer is challenging . It is likely only limited quantities of H2O2 produced from SP dissolved in water during manufacture would become entrapped in microparticles due to its low molecular weight, the hydrophobicity of the polymer, and the expected high diffusion coefficient from the polymers matrices . Further decomposition of H2O2 into water and oxygen whilst in the aqueous solvent and polymer solvent (DMC) is likely to have occurred , . As SP is an adduct with hydrogen peroxide it would be expected to reform during the lyophilization process if the hydrogen peroxide has not degraded. Some of the H2O2 formed may also have been removed along with water from the microparticles during lyophilisation, similar to that observed in previous studies . However, the microbiology data indicate the quantity of SP loaded is sufficient to achieve biocidal activity. Further studies are necessary to characterize the behaviour of SP during the manufacturing process.
Antibiotic resistance poses a significant risk to human health; therefore, novel approaches to combat infection are urgently needed. Re-evaluation of existing small molecules with well-established biocidal properties and a low propensity to give rise to resistance, especially in terms of optimising their mode of delivery, could offer an accelerated pathway to clinical application for a range of new therapeutic products.