Infections in the setting of implantable biomaterials remain serious complications that must be considered whenever these materials are employed clinically. It is the aim of this thesis to take an existing set of clinical problems—orthopedic and biomaterial-related infections—and explore a novel approach to their prevention and treatment. That approach is the chemical modification of existing antibiotic molecules with polymerizable functional groups such that newly formed species can be readily attached to implantable materials with the intent of retarding bacterial colonization and killing infective organisms.
Each synthetic modification of a traditional antibiotic brings with it subtle changes in antibiotic potency that must be examined if the new species is to see clinical application. This thesis makes use of numerous chemical characterization techniques including mass spectrometry and multidimensional nuclear magnetic resonance (NMR) spectroscopy in combination with various biological assays to systematically explore the functionalization of vancomycin with polymerizable groups (i.e., acrylates and acrylamides). Since the majority of anti-infective biomaterial strategies rely on the release of active compounds to kill microbial organisms, we desired to implement a surface-based antibiotic platform utilizing various graft photopolymerization techniques, including those of the living radial variety.
The first important goal of this thesis was to establish that a polymerizable derivative of the antibiotic vancomycin could be effectively attached to biomaterials already in clinical use. Ti-6Al-4V alloy, a material commonly used in load-bearing orthopedic implants such as hip prostheses, was employed for this purpose. Bactericidal polymer films based on a poly(ethylene glycol)-acrylate (PEG-acrylate) vancomycin derivative were successfully coated on alloy coupons. To our knowledge, this represents the first time such materials have been used to modify orthopedic biomaterials.
Having shown that the concept was feasible from an engineering perspective, structural questions regarding the monomer and the resulting polymers naturally arose. This culminated in NMR experiments and molecular dynamics simulations designed to explain changes in activity within the context of vancomycin's known mechanism of action. This effort was intended to establish a framework for developing new and better monomers. The data presented in subsequent chapters show that one should carefully examine both primary mechanisms of action involving the antibiotic binding pocket as well as secondary mechanisms such as antibiotic dimerization.
Polymeric structures are also key to the design of materials intended to facilitate specific interactions with bacterial cells. Various surfaces were created with non-PEGylated and PEGylated vancomycin derivatives using free-radical polymerizations. Such reactions are especially complex due to the highly reactive nature of radical species. It was for this reason that grafting chemistries utilizing dithiocarbamate-based iniferters were used in this thesis. Such chemistries were expected to increase the homogeneity of polymer chains and reduce undesirable side reactions. However, the difficulty in quantifiably discerning various underlying architectures must be acknowledged, and there is considerable room for additional work in this area.
Another focus of this thesis was the effect of polymerizable antibiotics on the inhibition of bacterial biofilm formation. Since biofilms play a significant role in many orthopedic infections, experiments were conducted with both Ti-6Al-4V alloy and poly(methyl methacrylate) (PMMA) bone cement. Polymerizable vancomycin derivatives can potentially inhibit Staphylococcus epidermidis adherence more effectively than PEG alone. Experiments with PMMA bone cement loaded with polymerizable vancomycin derivatives also showed decreased biofilm formation, and some of the mechanisms by which this may occur are explored in this thesis. Compressive mechanical tests were also conducted with various PMMA composites since loss of mechanical integrity is a problem when bone cement is loaded with conventional antibiotics. The results reported here suggest that polymerizable antibiotics may offer a new set of tools for combating biofilm infections and can potentially offer advantages in polymer composites where mechanical properties cannot be compromised.
In this thesis, it is demonstrated that polymerizable derivatives of vancomycin can be readily synthesized and polymerized to/from various surfaces to give bactericidal properties. Some of these polymerizable vancomycin analogs are effective at reducing biofilm proliferation on orthopedic biomaterials or at improving mechanical properties of PMMA bone cement. Structure-function relationships elucidated through site-specific modification of the vancomycin molecule and through molecular dynamics simulations are in good agreement with known vancomycin mechanisms of action. A PEG spacer was beneficial when polymers were applied to biomaterial surfaces, likely due to increased antibiotic mobility (versus surface-attached, non-PEGylated derivatives) and perhaps penetration into the peptidoglycan layer of test organisms. We anticipate that the current work will lead to increased interest in polymerizable antibiotics and that these species will ultimately be useful in many clinical applications.