Which pharmacokinetic/pharmacodynamic index best predicts the bactericidal efficacy of fluoroquinolones, and what implication does this have for dosing frequency?

• A AUC/MIC ratio — once-daily dosing maximizes the 24-hour area under curve relative to MIC ✓
• B Time above MIC (T>MIC) — multiple daily doses maintain continuous inhibitory concentrations
• C Cmax/MIC ratio — split dosing maintains peak concentration above a threshold throughout the day
• D Trough/MIC ratio — pre-dose concentrations must remain above MIC to prevent regrowth

Explanation

Fluoroquinolones display concentration-dependent killing, and their bactericidal activity is best predicted by the AUC/MIC ratio (also written as AUIC). Target AUC/MIC values for clinical efficacy are typically ≥125 for gram-negative pathogens. This concentration-dependent profile means that once-daily dosing maximizes the peak-related AUC contribution while also taking advantage of a post-antibiotic effect, making once-daily high-dose regimens preferable. This is in contrast to time-dependent antibiotics like penicillins (T>MIC drives efficacy) or aminoglycosides (Cmax/MIC drives efficacy). The AUC/MIC target also reduces emergence of resistance compared to subinhibitory trough-driven dosing.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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