Abordaje fisiopatologico de la infección de biomaterialesrecubrimiento polimérico con liberación controlada de antibiótico

Resumo

The use of biomaterials in healthcare has become routine practice and is growing exponentially. The main impediments for the use of a biomaterial are infection and a lack of integration of the material within host tissue. On many occasions, both these problems are interrelated. Despite ongoing research in the area, the infection of a medical device continues to pose a problem and given the expanding use of prosthetic materials, the number of affected patients is likely to rise. If we consider that hernioplasty using a prosthetic mesh has an infection rate around 2% and that some 20 million of such procedures are undertaken each year, the many patients that could suffer infection will determine a high social and healthcare burden. Given that systemic antibiotic prophylaxis does not reduce the infection rate in hernia surgery, other approaches will be needed to avoid this complication. The bacterial count needed to provoke an infection during surgery is of the order of one million times lower than if there were no prosthetic material. This is partly because the implant itself induces a reduction in the efficiency of the immune system, making the biomaterial surface more susceptible to bacterial adhesion. Besides being prone to infection, once the biomaterial becomes infected it is very difficult to eradicate and often the implant has to be removed. One of the main reasons why the infection of a biomaterial is difficult to resolve is that some microorganisms are able to form a coating or biofilm over the surface of the prosthetic material. This biofilm protects the microorganisms from the host immune response and from the action of antimicrobials. Host tissue cells and bacteria will compete in the race to colonize the foreign surface. Bacterias colonize most biomaterial surfaces easier than tissue cells and this initial adhesion is the first step in the formation of a biofilm. Nevertheless if host tissue integration exists, this will protect the implant from bacterial adhesion. The microorganisms that most often cause the infection of a biomedical device are Staphylococcus aureus and Staphylococcus epidermidis. In most cases, implant infection takes place at the time of surgery. Thus, it is at this precise time point that any measure designed to avoid infection should be taken, before the biofilm has had time to form, as one of the main causes of treatment failure. The local release of antibiotic or other measure that might avoid bacterial adhesion at the initial stages of infection could be a good therapeutic or prophylactic option. Thus, it is important to act early on the biomaterial surface to avoid biofilm formation and prevent bacterial adhesion. Systemic antibiotherapy has the drawback of poor penetration in ischaemic tissues and specifically in the area around the prosthetic implant, a site of surgical insult with oedema, haematoma and compromised blood flow. Because of this reduced blood flow, low levels of antibiotic reach the place where they are needed most and there is also a risk of systemic toxicity. This study was designed to develop a polymer coating containing vancomycin that releases this antibiotic at the prosthetic mesh surface immediately after its placement in the recipient, in a local and sustained fashion for a given time period to avoid the mesh’s infection. As the experimental model, we used a lightweight polypropylene monofilament mesh to establish three experimental groups: untreated mesh (PP group), mesh coated with the new polymer (POL group) and mesh coated with the new polymer loaded with vancomycin (VC group). The loaded polymer coating was subjected to biocompatibility and vancomycin release tests. To induce infection, inocula of S. aureus or S. epidermidis were independently used. In the in vitro stage of the study, we performed an antibiotic plate diffusion test to confirm the inhibition of bacterial growth and a time-kill assay to check the biocidal efficiency of the vancomycin loaded polymer in tubes containing 10 ml of Mueller Hinton broth with a bacterial concentration of 1.5 x 106 cfu/mL, according to the protocol described by the Clinical and Laboratory Standards Institute (CLSI). At the end of the experiment, the prosthetic meshes were processed for scanning electron microscopy For the in vivo stage, the New Zealand White rabbit was used as the experimental animal. Partial thickness defects of 3 x 5 cm were created in the abdominal wall to be repaired using the different meshes. The defect site was then infected using a 0.5 mL suspension of 108 cfu/mL of S. aureus or S. epidermidis before mesh placement. The concentration of microorganisms was determined by nephelometry (0.5 McFarland units). In total, 102 rabbits were used as follows: 24 for the control group (12 rabbits sacrificed at 14 days and 12 at 30 days); 36 rabbits inoculated with S. aureus (18 rabbits sacrificed at 14 days and 18 at 30 days); 36 inoculated with S. epidermidis (18 rabbits sacrificed at 14 days and 18 at 30 days); and 6 rabbits used to determine vancomycin levels in peripheral blood by high performance liquid chromatography (HPLC). None of the animals received systemic antibiotherapy. Once sacrificed, the abdominal wall fragments were obtained for tensile strength testing and for microscopy and immunohistochemistry (to detect S. aureus or S. epidermidis). The developed polymeric coating is able to slowly release vancomycin; it is biocompatible, biodegradable (i.e. does not need to be subsequently removed) and shows an antibiotic release curve that fulfils clinical requirements. Accordingly, it shows an initial antibiotic peak, just the time when the biomaterial is most vulnerable, achieving high local antibiotic levels above the MIC (minimum inhibitory concentration) without the risk of systemic toxicity, since peripheral blood levels are undetectable. Antibiotic release lasts 4 weeks to avoid developing antibiotic resistance. In vitro, the polymer coating was able to avoid colonization of the mesh surface and an inhibition halo was observed on the agar plate. Its bactericidal capacity was also demonstrated by the kill curve obtained in liquid medium. In the in vivo experiments in the New Zealand White rabbit, the coating was efficient at avoiding S. aureus infection, which was confirmed both macroscopically and microscopically. Our macroscopic observations also suggest the method’s efficacy against S. epidermidis, but we were unable to immunohistochemically identify S. epidermidis in the VC, PP or POL groups. The findings of this study suggest that staphylococcal infection could be reduced through the use of the prosthetic mesh polymer coating proposed. Although its use in human patients needs to be investigated, the loaded polymer was able to achieve high local antibiotic concentrations and thus seems as a reasonable strategy to prevent or treat biomaterial centered infections. The use of a polypropylene mesh coated with the polymer could be justified as complementary treatment to systemic antibiotherapy following debridement and removal of an infected abdominal wall mesh in elderly, cancer or immunocompromised patients.