What fluoroquinolone (FQ) is most similar to Marbofloxacin in terms of sensitivity if an organism is sensitive to Marbofloxacin at a minimum inhibitory concentration (MIC) of <= 0.125?

Fluoroquinolone Most Similar to Marbofloxacin in Sensitivity

For an organism sensitive to marbofloxacin at MIC ≤0.125 μg/mL, enrofloxacin is the most similar fluoroquinolone in terms of sensitivity profile, followed by ciprofloxacin.

Direct Comparative Evidence

The most relevant veterinary pharmacology research directly comparing fluoroquinolones demonstrates that marbofloxacin and enrofloxacin have nearly identical pharmacodynamic profiles and achieve similar target indices against susceptible organisms 1. When comparing five fluoroquinolones in dogs and cats, the pattern for achieving therapeutic AUC/MIC ratios (target ≥125) was: enrofloxacin+ciprofloxacin ≥ marbofloxacin ≥ ciprofloxacin ≥ enrofloxacin 1.

Key Pharmacodynamic Similarities

• Marbofloxacin and enrofloxacin demonstrate concentration-dependent killing with bactericidal activity observed within 1 hour for most strains, making them functionally equivalent in their mechanism of action 2
• At high doses, marbofloxacin, enrofloxacin, and ciprofloxacin all reached target PK/PD indices in 73-77% of susceptible isolates, significantly outperforming other fluoroquinolones like orbifloxacin (51%) and difloxacin (40%) 1
• Direct in vitro comparison shows marbofloxacin was slightly more effective than enrofloxacin, especially against P. aeruginosa and β-Streptococci, though both remained highly effective against most isolates 3

MIC Correlation and Cross-Susceptibility

The mutant prevention concentration (MPC) data reveals critical insights about cross-susceptibility:

• Marbofloxacin MPC values are only 2- to 4-fold higher than MIC values for Pasteurellaceae strains, indicating a narrow resistance selection window 2
• For organisms with marbofloxacin MIC of 0.12-0.25 μg/mL (close to your 0.125 threshold), both enrofloxacin and marbofloxacin maintain excellent activity 2
• Resistance development to either enrofloxacin or marbofloxacin is transferable to other fluoroquinolones, confirming this is essentially a class effect 4

Clinical Implications for MIC ≤0.125

For an organism at this specific MIC threshold:

• Enrofloxacin would be expected to have an MIC in the same range (0.03-0.25 μg/mL) based on comparative susceptibility patterns 1, 3
• Ciprofloxacin represents the next closest alternative, though it may require higher dosing to achieve equivalent PK/PD targets 1
• Marbofloxacin actually demonstrates superior mutant prevention characteristics compared to enrofloxacin at conventional dosing regimens, with AUC/MPC ratios of 32 vs 39 required to prevent resistance selection 4

Important Caveats

The primary pitfall is assuming all fluoroquinolones are interchangeable—while enrofloxacin is most similar to marbofloxacin, drugs like orbifloxacin and difloxacin consistently underperform and should not be considered equivalent alternatives 1. Additionally, fluoroquinolone resistance is a class effect, so if an organism develops resistance to marbofloxacin, cross-resistance to enrofloxacin and ciprofloxacin should be anticipated 4.