Ciprofloxacin vs Moxifloxacin

## Overview

Moxifloxacin is a fourth-generation fluoroquinolone with enhanced gram-positive and anaerobic coverage compared to ciprofloxacin. Ciprofloxacin retains superiority for gram-negative organisms, especially Pseudomonas. Moxifloxacin cannot be used for UTI (minimal urinary excretion).

## Mechanism of Action

Both inhibit bacterial DNA gyrase and topoisomerase IV. Moxifloxacin's C8-methoxy substituent enhances activity against gram-positive organisms and anaerobes by improving enzyme inhibition.

## Antimicrobial Spectrum

Moxifloxacin: excellent gram-positive (S. pneumoniae, MRSA variable), exceptional anaerobic activity (Bacteroides fragilis, Prevotella), good atypical coverage, no meaningful Pseudomonas activity. Ciprofloxacin: superior gram-negative/Pseudomonas, poor anaerobes and poor S. pneumoniae.

## Pharmacokinetics

Moxifloxacin: oral bioavailability ~90%, half-life ~12 h (once daily), eliminated in feces (biliary) and urine — but inadequate urinary concentration for UTI treatment. Ciprofloxacin: renal excretion with therapeutic urinary concentrations.

## QTc

Moxifloxacin has the greatest QTc-prolonging effect of the fluoroquinolones (avoid in hypokalemia, with class I/III antiarrhythmics, baseline prolonged QTc).

## Key Takeaways

- Moxifloxacin: enhanced gram-positive + anaerobic; cannot treat UTI
- Ciprofloxacin: superior gram-negative/Pseudomonas; no anaerobic activity
- Moxifloxacin: greatest QTc prolongation among fluoroquinolones