A chlorhexidine delivery system based on reline acrylic resins

Abstract

Drug delivery systems have been widely used in dentistry to prevent and treat several diseases. The methods of fabrication of a drug delivery system vary between coating, immersion, or incorporation. Compounds with antimicrobial activity like miconazole, silver, zinc oxide, titanium dioxide, chlorhexidine, fluconazole, nystatin, peptide mimetic compounds, zirconia, ethanol, and natural compounds have been added into acrylic resins to treat denture stomatitis. Denture stomatitis is a pathological condition among people who wear removable dentures, characterized by erythema and edema of the oral mucosal areas. Usually, it is located in the upper jaw. Despite its multifactorial etiology, the infection by Candida species is considered the main etiologic factor. Trauma (caused by unstable dentures) and other local and systemic factors are also associated with denture stomatitis’ etiopathogenesis. Since conventional therapy with topical and/or systemic antifungals seems to be unsatisfying, drug delivery systems have been investigated. This treatment simultaneously targets the removable denture, the microbial biofilm, and the oral mucosa. The use of drug delivery systems in the oral cavity inhibits microbial growth and adhesion and are based on a continued, localized controlled release of a drug over a long period, promoting high therapeutic efficiency. It does not require patient compliance and presents minimal adverse risks when compared with systemic therapy. In Candida albicans-associated denture stomatitis, the treatment with the proposed novel drug delivery system consists of incorporation chlorhexidine into three different reline acrylic resins by mixing the drug with the acrylic powder. The study reported in Chapter 4 aimed to evaluate the effect of loading chlorhexidine on the antimicrobial activity against Candida albicans and Streptococcus oralis and the reline acrylic resins’ mechanical, structural, and surface properties. The amount of drug release was also determined, as well as the cytotoxicity of this novel system. Kooliner, Ufi Gel Hard, and Probase Cold were loaded with chlorhexidine diacetate. The results of an agar diffusion assay for Candida albicans defined the groups: control (with no addition of drug) and experimental (2.5% chlorhexidine (w/w) to Kooliner and 5% to Ufi Gel Hard and Probase Cold). In each group, the reline resin powder and chlorhexidine were mixed and, after incorporating the liquid into the mixture, specimens were produced using steel molds with shapes and dimensions depending on the test to be performed. The minimum inhibitory concentration of chlorhexidine for selected strains was previously determined. Specimens (n=3) were placed on agar plates inoculated with strains for antimicrobial activity study with the agar diffusion assay. Other specimens were submitted to the biofilm inhibition assay with SEM. The Knoop microhardness and the three-point bending flexural strength tests (n=8) were performed for mechanical characterization. A micro-CT device was used for morphometric and total porosity analysis and FTIR-ATR for chemical analysis. Surface characterization included porosity by the water immersion method (n=5), free energy using the Wilhelmy plate method (n=5), surface morphology with SEM, and shear bond strength testing between reline resins and the denture base resin (n=10). Chlorhexidine release was determined by spectrophotometry (n=3), and cytotoxicity was measured by the fibroblast viability endpoint MTT test (n=2). Since normality was not verified, quantitative data were submitted to the Kruskal-Wallis and Mann-Whitney tests (α=0.05). Chlorhexidine-loaded Probase Cold showed a pronounced biofilm inhibition and decreased flexural strength (p=0.005). The other two reline resins showed biofilm formation in both control and experimental groups and no significantly (p>0.05) differences in mechanical properties, except for Ufi Gel Hard with significantly (p=0.003) higher microhardness values in the experimental group. The addition of chlorhexidine in all resins decreased total porosity, not changing their chemical structure. Chlorhexidine-loaded Ufi Gel Hard and Probase Cold showed significantly (p=0.008) higher surface free energy than the controls. SEM images showed a change in the surface morphology of chlorhexidine-loaded Probase Cold. Incorporating chlorhexidine increased (p<0.05) the Kooliner’s and Ufi Gel Hard’s bond strength but decreased (p<0.001) the Probase Cold’s bond strength to the denture base resin. Chlorhexidine release followed a slow and steady pattern after 1-2 days in all resins and was higher (p<0.001) in Ufi Gel Hard. Chlorhexidine-loaded Probase Cold was considered slightly cytotoxic for the fibroblast cell line. Loading Probase Cold with 5% chlorhexidine demonstrated effective antimicrobial activity and did not negatively affect its mechanical, structural, and surface properties. Also, with a steadier long-lasting chlorhexidine diffusion, Probase Cold was the least cytotoxicity loaded resin under evaluation. In Chapter 5, an in-vitro study was developed to investigate how these drug delivery systems were affected by biodegradation phenomena in the oral cavity. The main purpose was to evaluate the effect of chlorhexidine incorporation on the microhardness, flexural strength, surface energy, and color stability of the three reline acrylic resins after a thermal or a 28-day chemical aging process. The same experimental groups previously defined in Chapter 4 were set: Kooliner with 0% and 2.5% chlorhexidine); Ufi Gel Hard with 0% and 5% chlorhexidine; and Probase Cold with 0% and 5% chlorhexidine. After thermal aging (1000 cycles, 5oC-55oC) and a 4-week chemical aging process (cycles of 6 hours at pH=3 and 18 hours at pH=7 in artificial saliva), the Knoop microhardness and the threepoint flexural strength tests (n=8) were performed. Surface energy was estimated by determining contact angles using the Wilhelmy plaque technique (n=5) and color measurement with two spectrophotometers before and after both aging methods. The values were converted to the CIELab system, and the overall color change (ΔE) (n=5) was calculated and converted to National Bureau of Standards units. Since normality was not verified, data were submitted to the Mann-Whitney nonparametric statistical tests (α=0.05). Chlorhexidine incorporation did not significantly (p>0.05) affect the microhardness, flexural strength, and surface free energy of the Kooliner and Ufi Gel Hard resins after either thermal or chemical aging, except for chlorhexidine-loaded Kooliner, which showed higher (p=0.050) microhardness values than the control group. Loading Probase Cold with chlorhexidine led to lower microhardness (p=0.010) and flexural strength (p=0.038), and higher surface free energy (p=0.008) values than the control group after thermal aging. After chemical aging, drug loading only decreased flexural strength (p=0.021). Loading Kooliner with chlorhexidine led to an increase (p<0.05) of ΔE values after either aging method. However, in the other two resins, chlorhexidine loading only caused higher ΔE values (Ufi Gel Hard – p=0.008; Probase Cold – p=0.008) than the control group after chemical aging, with no differences (p>0.05) after thermal aging. Loading Kooliner and Ufi Gel Hard with chlorhexidine does not negatively affect their microhardness, flexural strength, and surface free energy after aging, only affecting their color stability. However, loading Probase Cold with chlorhexidine decreases its microhardness and flexural strength and increases its surface free energy while not affecting the color stability after thermal aging. After chemical aging, Probase Cold led to decreased flexural strength and affected color stability. Within the limitations of these in-vitro studies, chlorhexidine delivery systems based on reline acrylic resins of removable dentures may be used in the prevention or treatment of denture stomatitis. However, further in-vivo studies are necessary to recommend clinical use.