Comparative bactericidal activity of ceftazidime against isolates of Pseudomonas aeruginosa as assessed in an in vitro pharmacodynamic model versus the traditional time-kill method

Abstract

Bactericidal activity, historically assessed by in vitro tests which employ fixed drug concentrations, may also be evaluated in in vitro pharmacodynamic models in which in vivo pharmacokinetics and bacterial growth conditions can be simulated. However, systematic comparisons between the two methods are lacking. We evaluated the bactericidal activities of ceftazidime, at two different concentration/MIC ratios (C/MICs), against 10 clinical isolates of Pseudomonas aeruginosa in a two-compartment model with continuous-infusion conditions and a 2-h half-life. These values were compared to those determined by traditional 24-h time-kill (TTK) methods at the same C/MICs. Bactericidal activities were compared by using area under the colony count-time curves. Antibiotic exposure (area under the drug concentration-time curve) was also evaluated. Although bactericidal activity appeared greater by the TTK method (P = 0.05), when it was normalized for drug exposure, these differences disappeared (P = 0.2). This disparity was likely due to differences in drug exposure in the TTK method and in the peripheral compartment of the model (site of bacteria) over the first 8 h of the experiment, during which the antibiotic accumulated to target concentrations. This suggests that the bactericidal effects with constant antibiotic concentrations are similar in the two methods; however, this may not hold true with fluctuating drug concentrations. Further, results from the pharmacodynamic model may theoretically be more relevant, as in vivo pharmacokinetics and bacterial growth conditions call be more faithfully simulated.

Cefepime free minimum concentration to minimum inhibitory concentration (fCmin/MIC) ratio predicts clinical failure in patients with Gram-negative bacterial pneumonia

“…4-6 times the MIC [6]. Li et al showed that whilst free %T > MIC of 54% was associated with microbiological clearance in patients receiving meropenem for the treatment of lower respiratory tract infections, only a meropenem fC min /MIC > 5 was associated with clinical efficacy [7].…”

A Pilot Study of the Efficacy of Constant‐Infusion Ceftazidime in the Treatment of Endobronchial Infections in Adults with Cystic Fibrosis

“…4,15,16 In previous studies in our laboratory we found that for at least some P. aeruginosa isolates from patients with cystic fibrosis, concentrations as high as 6.6 times the MIC are necessary to maximize effect. 12,13 Using traditional time-kill techniques 12 and later in an in vitro pharmacodynamic model, 13 we exposed 10 clinical isolates of P. aeruginosa to various multiples of their respective ceftazidime MICs. Using a sigmoid E max model, we showed that antimicrobial effect continued to increase with concentration for some isolates up to approximately 6.6 times the MIC.…”

“…Dosages were calculated to achieve a steady-state concentration of 6.6 times the MIC of the least susceptible P. aeruginosa isolate, which is the multiple shown to maximize antibacterial effect. 12,13 Constant infusion was administered by programmable infusion pumps. The adequacy of the dosage was confirmed based on actual MIC determination from current patient isolates obtained on admission.…”

“…Ceftazidime concentrations were determined using a validated high-performance liquid chromatographic assay. 13 Briefly, blood samples were allowed to clot on ice, and serum was harvested and deproteinized with a 25% acetonitrile solution. Samples were injected onto a reverse-phase C18 column (Versapak; Alltech Associates, Deerfield, IL) and eluted with a mobile phase consisting of 9% acetonitrile in a 0.15 molar monobasic potassium phosphate buffer.…”

What In Vitro Models of Infection Can and Cannot Do

“…One study made a direct comparison of the time-rate of killing of P. aeruginosa in a two-compartment versus a static setting after treatment with ceftazidime. 72 In this study, ceftazidime was administered by continuous infusion to achieve the same fixed concentration used in the time-kill study. The medium, inoculum, and length of the study were the same.…”