Fluoroquinolone Antibiotics
Fluoroquinolones inhibit bacterial DNA gyrase and topoisomerase IV, providing broad-spectrum coverage. FDA black box warnings for tendon rupture, neuropathy, and aortic dissection have restricted their use to situations where alternatives are unavailable.
## Mechanism of Action
Fluoroquinolones inhibit two essential bacterial enzymes involved in DNA replication and repair:
- **DNA gyrase (topoisomerase II)** -- Relieves positive supercoiling ahead of the replication fork. Primary target in gram-negative bacteria.
- **Topoisomerase IV** -- Separates interlinked daughter DNA strands after replication. Primary target in gram-positive bacteria.
By trapping enzyme-DNA complexes, fluoroquinolones create double-strand DNA breaks that are lethal to the bacterium. This produces concentration-dependent bactericidal activity.
## Classification and Spectrum
**Second generation:** Ciprofloxacin -- Strongest gram-negative and anti-pseudomonal activity. Poor gram-positive coverage. Drug of choice for anthrax and certain Salmonella infections.
**Third generation:** Levofloxacin -- Enhanced gram-positive coverage (including Streptococcus pneumoniae), termed "respiratory fluoroquinolone." Used for community-acquired pneumonia and complicated UTIs.
**Fourth generation:** Moxifloxacin -- Broadest gram-positive coverage, excellent anaerobic activity. No renal excretion (cannot treat UTIs). Used for respiratory infections and as part of TB regimens (drug-resistant TB).
## Pharmacokinetic Advantages
Fluoroquinolones have excellent oral bioavailability (70-99%), achieving serum levels comparable to IV administration. They penetrate well into lung tissue, bone, prostate, and intracellular compartments (effective against intracellular pathogens like Legionella and Chlamydia). Long half-lives permit once or twice daily dosing.
**Key interaction:** Divalent and trivalent cations (calcium, magnesium, iron, aluminum, zinc) chelate fluoroquinolones in the GI tract, dramatically reducing absorption. Separate from antacids, supplements, and dairy by at least 2 hours.
## FDA Black Box Warnings
The FDA has issued multiple black box warnings restricting fluoroquinolone use to situations where no alternative antibiotics are available:
- **Tendon rupture and tendinitis** -- Risk is 2-4x higher, particularly the Achilles tendon. Risk factors include age >60, concurrent corticosteroids, renal transplant recipients. Mechanism involves direct collagen toxicity and MMP activation.
- **Peripheral neuropathy** -- Potentially irreversible paresthesias, pain, and weakness. Can occur after just a few doses.
- **CNS effects** -- Seizures, psychosis, increased intracranial pressure, tremors. Fluoroquinolones antagonize GABA-A receptors and chelate magnesium, lowering the seizure threshold.
- **Aortic aneurysm/dissection** -- Epidemiological data suggest increased risk, possibly related to collagen degradation. Avoid in patients with Marfan syndrome or known aortic disease.
- **Hypoglycemia** -- Severe, sometimes fatal blood sugar drops, particularly with concurrent sulfonylureas.
## Resistance Concerns
Fluoroquinolone resistance develops readily through chromosomal mutations in DNA gyrase and topoisomerase IV genes, as well as efflux pump upregulation. Resistance rates in E. coli UTIs have risen dramatically in many regions, limiting empiric use for uncomplicated infections.
## Key Takeaways
- Reserve fluoroquinolones for infections where safer alternatives are inadequate.
- The FDA restricts their use for uncomplicated UTIs, sinusitis, and bronchitis due to serious adverse effects.
- Warn patients about tendon pain -- discontinue immediately if symptoms occur.
- Chelation with cations is a common cause of treatment failure if not properly counseled.