PKPD modeling of the inoculum effect of combined ceftazidime/avibactam and colistin against KPC-3 <i>Klebsiella pneumoniae</i> isolate

The inoculum effect (IE) characterizes a decrease in the antimicrobial effect of antibiotics with increasing inoculum. To face antimicrobial resistance, antibiotic combinations are progressively used. In this context, the effect of combination may be affected by IE, especially drugs for which an IE has been described. The objective was to characterize the IE of a carbapenemase (KPC-3) <i>Klebsiella pneumoniae</i> isolate on the combination of ceftazidime/avibactam (CZA) and colistin (CST). <i>In vitro</i> time-kill curves with single and combined drugs were performed at four different inocula. The IE of each drug was described using pharmacokinetic/pharmacodynamic modeling, and interactions on IE were investigated with the general pharmacodynamic interaction model when drugs were combined. The IE was assessed by evaluating the significance of the parameters associated with the IE model compared to the no IE model and by comparing the CFU counts over time predicted with the IE model vs the no IE model. Rapid bacterial killing was observed at 10⁴ CFU/mL. For both 5·10⁵ and 10⁷ CFU/mL inocula, initial decays followed by re-growth were observed with drugs alone, while the combination prevented the emergence of resistance. Eradication was never achieved at 10⁸ CFU/mL. The IE was best modeled as a reduction of CZA maximum bactericidal effect and as an increase in CST EC₅₀ with increasing inoculum. However, no interaction between IEs was significant, meaning that CST did not modify the IE of CZA and inversely. IE may be important at least as demonstrated by <i>in vitro</i> antibiotic combination studies.