Breakpoint determination when multiple organisms are tested for effect targets.

Determination of the susceptibility breakpoint for antibiotics is important, as it guides the use of agents in the clinical setting. Currently, breakpoints are often evaluated using a Probability of Target Attainment Analysis in which the targets are set through pre-clinical experiments, often by examining a strain of a target pathogen in a murine model such as a neutropenic thigh infection model. However, regulatory authorities are often rightly concerned about the setting of breakpoints when a number of isolates of target pathogens are evaluated and there is a sizeable spread of the drug exposures necessary to achieve the target with a sufficiently high (usually 90%) probability. Here, we propose a method for supporting a breakpoint determination for this circumstance. We examined 8 isolates of resistant Enterobacteriaceae in a neutropenic murine thigh infection model. The stasis exposure was determined and ranged from 5.70 to 43.5 AUC/MIC Ratio. The mean ± standard deviation was 20.05 ± 13.05. A 5000-iterate Monte Carlo simulation was performed to generate a range of stasis targets and Probability of Target Attainment Analyses were calculated at the 1st, 5th, 10th, 25th, 50th, 75th, 90th, 95th, and 99th percentiles of the distribution. Breakpoints were determined at each percentile. Breakpoints ranged from 2 mg/L to 32 mg/L. A weighted (by the percentages of the distribution) breakpoint was calculated and determined to be 4 mg/L. This method is a rational approach to identifying breakpoints when there is substantial between-isolate variability in exposure targets.