Characterization and optimization of experimental variables within a reproducible bladder encrustation model and in vitro evaluation of the efficacy of urease inhibitors for the prevention of medical device‐related encrustation

Abstract

This study presents a reproducible, cost-effective in vitro encrustation model and, furthermore, describes the effects of components of the artificial urine and the presence of agents that modify the action of urease on encrustation on commercially available ureteral stents. The encrustation model involved the use of small-volume reactors (700 mL) containing artificial urine and employing an orbital incubator (at 37 degrees C) to ensure controlled stirring. The artificial urine contained sources of calcium and magnesium (both as chlorides), albumin and urease. Alteration of the ratio (% w/w) of calcium salt to magnesium salt affected the mass of encrustation, with the greatest encrustation noted whenever magnesium was excluded from the artificial urine. Increasing the concentration of albumin, designed to mimic the presence of protein in urine, significantly decreased the mass of both calcium and magnesium encrustation until a plateau was observed. Finally, exclusion of urease from the artificial urine significantly reduced encrustation due to the indirect effects of this enzyme on pH. Inclusion of the urease inhibitor, acetohydroxamic acid, or urease substrates (methylurea or ethylurea) into the artificial medium markedly reduced encrustation on ureteral stents. In conclusion, this study has described the design of a reproducible, cost-effective in vitro encrustation model. Encrustation was markedly reduced on biomaterials by the inclusion of agents that modify the action of urease. These agents may, therefore, offer a novel clinical approach to the control of encrustation on urological medical devices.

Validation of the CDC biofilm reactor as a dynamic model for assessment of encrustation formation on urological device materials

“…The authors note that in the 7–8 pH range struvite crystallized first and in effect may act as a nidus for the subsequent crystallization of hydroxyapatite, and as elevating levels of magnesium result in higher levels of struvite deposition, levels of deposition of calcium phosphate as hydroxyapatite will be observed. A similar observation was made by Jones et al13 who noted that in a model system inoculated with Jack Bean urease, a similar “u‐shaped” response in the deposition of calcium salts is observed with alterations in the concentration of magnesium within artificial urine. At physiological pH values, Robertson and Nordin29 note that the formation products of calcium phosphates are exceeded at approximately 0.02% calcium in solution; however, the solubility product is exceeded at 0.012%.…”

“…Within any model intended to investigate the formation of encrustation on urological devices, the composition of the urine used will have a significant effect on the type and quantity of encrustation formed 13. Figures 17 and 18 demonstrate the effect of altering the concentration of calcium and magnesium, both present as chlorides in terms of the available concentration of calcium and magnesium ions, respectively.…”

“…Within any model intended to investigate the formation of encrustation on urological devices, the composition of the urine used will have a significant effect on the type and quantity of encrustation formed. 13 Figures 17 and 18 demonstrate the effect of altering the concentration of calcium and magnesium, both present as chlorides in terms of the available concentration of calcium and magnesium ions, respectively. Results shown in Figure 17 demonstrate that altering the concentration of Ca 21 within the artificial urine used in the experimental procedure does not have any statistically significant effect on the deposition of magnesiumcontaining encrustation on analysis with a one-way ANOVA.…”

“…Static systems have also been used to investigate the precipitation of urinary salts8 and the formation of encrustation on catheters 9. Encrustation models have also used constant stirring of urine, either by utilization of Teflon stirring bars or by placing reactor vessels within orbital incubators 10–13. Although there is a degree of turbulence within the systems, and precipitation of salts is minimized, levels of turbulence within the system may not be constant, with some materials being subject to more turbulent flow and pockets of higher solution concentration within the vessels.…”

Optimization of NMR analysis of biological fluids for quantitative accuracy

“…A synthetic urine buffer was used which provides a solution with very similar chemical characteristics to that of real urine, allows for a known and reproducible buffering system, and has a defined metabolic profile (408 mM urea, 77.9 mM NaCl, 20.5 mM KH 2 PO 4 , 16.0 mM Na 2 SO 4 , 18.6 mM NH 4 Cl, 21.2 mM KCl, 0.02%w/v NaN 3 , 3.26 mM CaCl 2 , 3.17 mM MgCl 2 AE 6H 2 O, 5% D 2 O, and 0.49 mM DSS (disodium-2, 2-dimethyl (2-silapentane-5-sulphonate)) (Jones et al, 2005). Two samples containing specific chemical compounds were prepared in the laboratory using the synthetic urine buffer.…”