Pharmacodynamics of a fluoroquinolone antimicrobial agent in a neutropenic rat model of Pseudomonas sepsis.

We examined the impact of dose fractionation and altered MICs on survivorship in a neutropenic rat model of Pseudomonas aeruginosa sepsis employing the new fluoroquinolone antibiotic lomefloxacin. Once-daily administration of a drug dose which produced a high peak concentration/MIC (peak/MIC) ratio (ca. 20/1) produced significantly better survivorship compared with regimens employing the same daily dose but on a more fractionated schedule. The use of a smaller dose, producing lower (< 10/1) peak/MIC ratios, did not show this effect, as once-daily and twice-daily regimens produced equivalent results (the area under the concentration-time curve/MIC ratio was linked to survivorship). Challenge with resistant mutants selected for altered MICs of fluoroquinolones (two and four times the MIC for the parent strain, respectively) resulted in markedly diminished survivorship. Challenge with the parent strain and use of a drug dose which produced a peak/MIC ratio identical to that for animals challenged with the mutant for which the MIC was four times that for the parent strain and treated with the larger drug dose produced survivorship curves which were not different. For this animal model, peak/MIC ratio was linked to survivorship, particularly when high ratios (10/1 to 20/1) were obtained. At lower doses, producing peak/MIC ratios < 10/1, the area under the concentration-time curve relative to the MIC appeared to be most closely linked to outcome. The time that levels in plasma exceeded the MIC did not influence survivorship. The hypothesis most likely to explain these findings is that higher peak/MIC ratios can suppress the parent strain and mutant organisms (gyrA and transport mutants) for which the MIC is higher but limited (no more than eight times that for the parent strain).