Pharmacology · Antibacterial Agents

Fluoroquinolones

Mechanism, generations, adverse effects, and resistance

Fluoroquinolones are concentration-dependent bactericidal antibiotics that inhibit DNA gyrase (gram-negatives) and topoisomerase IV (gram-positives) — dual-target engagement slows the emergence of resistance compared to single-target agents.
Moxifloxacin has the best gram-positive and anaerobic coverage but has no reliable Pseudomonas activity — ciprofloxacin remains the fluoroquinolone of choice when Pseudomonas coverage is required.
Oral bioavailability is high for all fluoroquinolones (70–99%), but absorption is substantially reduced by polyvalent cations — take at least 2 hours before or 4–6 hours after antacids, calcium, iron, or zinc.
Five FDA black box warnings cover tendon rupture, peripheral neuropathy, CNS effects, myasthenia gravis exacerbation, and aortic aneurysm/dissection — reserve fluoroquinolones for infections where no safer alternative exists.
Plasmid-mediated resistance (qnr genes) provides low-level resistance that enables stepwise selection of high-level target mutations; qnr genes are frequently co-located with ESBL genes, making fluoroquinolone resistance near-universal in ESBL-producing organisms.

Abbreviations: DNA = deoxyribonucleic acid · AUC/MIC = area under the curve / minimum inhibitory concentration · QTc = corrected QT interval · CAP = community-acquired pneumonia · UTI = urinary tract infection · QRDR = quinolone resistance-determining region · ESBL = extended-spectrum beta-lactamase · FDA = US Food and Drug Administration · CYP = cytochrome P450 · MSSA = methicillin-susceptible Staphylococcus aureus

Fluoroquinolone Generations — Spectrum and Clinical Use

Gen	Key Agents	Gram-Positive	Gram-Negative	Atypicals	Primary Clinical Use
1st	Nalidixic acid	Minimal	Enterobacteriaceae only	No	Historical only; uncomplicated UTI
2nd	Ciprofloxacin, Ofloxacin	MSSA (not S. pneumoniae)	Broad Enterobacteriaceae; Pseudomonas	Yes	Pseudomonas; UTI; pyelonephritis; anthrax; typhoid
3rd	Levofloxacin	+ S. pneumoniae (including penicillin-resistant)	Broad; less Pseudomonas potency than ciprofloxacin	Yes	CAP; respiratory infections; TB regimens; pyelonephritis
4th	Moxifloxacin	Best gram-positive of class	Good Enterobacteriaceae; NO reliable Pseudomonas	Yes + anaerobes	CAP; aspiration pneumonia; skin infections — never for Pseudomonas

Pharmacokinetics and Key Drug Interactions

Distribution and Elimination

High Oral Bioavailability, Tissue Penetration

Oral bioavailability 70–99% — IV-to-oral switch appropriate when patient tolerating oral intake
Excellent tissue penetration — lung, prostate, bone, intracellular
Most renally eliminated — dose adjustment required in renal impairment (except moxifloxacin: hepatic/biliary)
Concentration-dependent killing; AUC/MIC >30–40 drives efficacy
Post-antibiotic effect supports once- or twice-daily dosing

Drug Interactions

Cation Chelation and CYP Inhibition

Polyvalent cations (antacids, Ca²⁺, Fe²⁺, Zn²⁺, sucralfate) chelate fluoroquinolones — take oral dose ≥2 hr before or 4–6 hr after
Ciprofloxacin inhibits CYP1A2 — reduce theophylline dose; tizanidine combination contraindicated (severe hypotension)
QTc-prolonging drugs: additive risk with moxifloxacin and levofloxacin — avoid combination with antiarrhythmics, antipsychotics
NSAIDs: increased seizure risk when combined with fluoroquinolones
Warfarin: ciprofloxacin inhibits CYP1A2 — INR monitoring required

Black Box Warnings and Key Adverse Effects

Musculoskeletal — Black Box

Tendinopathy and Rupture

Achilles tendon most commonly affected; can occur bilaterally
Risk factors: age >60, corticosteroid use, renal transplant
Stop drug immediately if tendon pain, swelling, or inflammation develops
Rupture can occur during therapy or months after completion

Neurological — Black Box

Neuropathy, CNS, and Myasthenia

Peripheral neuropathy — may be irreversible; stop immediately if symptoms develop
CNS: seizures, psychosis, confusion, anxiety, depression (2016 warning)
Contraindicated in myasthenia gravis — may cause acute respiratory failure
NSAIDs increase CNS seizure risk

Cardiac / Vascular / Metabolic — Black Box

QTc, Aorta, and Dysglycemia

QTc prolongation: moxifloxacin > levofloxacin > ciprofloxacin — avoid in baseline QTc >450 ms
Aortic aneurysm / dissection risk (2018 warning) — avoid in known aortic aneurysm or high-risk patients
Dysglycemia: hypoglycemia with sulfonylureas/insulin; hyperglycemia also reported
Gatifloxacin withdrawn from US market due to severe dysglycemia

Resistance Mechanisms

Mechanism	How It Works	Clinical Implication
QRDR mutations (gyrA, parC)	Sequential mutations in primary then secondary topoisomerase target — first mutation lowers susceptibility, second confers high-level resistance	Stepwise; sub-therapeutic dosing selects resistance; dual-target engagement (higher-generation agents) suppresses emergence
Efflux pump overexpression	AcrAB-TolC (Enterobacteriaceae), MexAB-OprM (Pseudomonas) actively extrude drug from periplasm	Often part of multidrug-resistant phenotype; synergizes with porin loss to produce high-level resistance
Plasmid-mediated (qnr genes)	Qnr proteins protect topoisomerases from drug binding; AAC(6')-Ib-cr enzyme acetylates ciprofloxacin — low-level resistance	Stepping stone to high-level resistance via QRDR mutations; frequently co-located with ESBL genes — fluoroquinolone resistance nearly universal in ESBL producers

Clinical Rules: Stewardship, Moxifloxacin, and Interactions

FDA 2016 guidance: reserve fluoroquinolones for serious infections where no safer alternative exists. Do not use for uncomplicated UTI, acute bronchitis, or acute sinusitis — the risks of tendinopathy, neuropathy, and CNS effects outweigh the benefit when first-line alternatives are available.

Moxifloxacin has no reliable Pseudomonas activity — never select it when Pseudomonas coverage is required. It also lacks adequate urinary concentrations and should not be used for UTI or pyelonephritis. Ciprofloxacin remains the fluoroquinolone of choice for Pseudomonas and urinary tract infections.

Ciprofloxacin is the most potent CYP1A2 inhibitor in the class — always reduce theophylline dose when starting ciprofloxacin, and never combine with tizanidine.

Suggested References

Author / Source	Title	Publication
Katzung BG, ed.	Basic and Clinical Pharmacology, 15th ed. — Chapter 44: Fluoroquinolones	McGraw-Hill, 2021
Brunton LL, Knollmann BC, eds.	Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed. — Chapter 51: Protein Synthesis Inhibitors and Miscellaneous Antibacterial Agents	McGraw-Hill, 2023
Hooper DC, Jacoby GA	Mechanisms of drug resistance: quinolone resistance	Ann N Y Acad Sci. 2015;1354:12–31
Aldred KJ, Kerns RJ, Osheroff N	Mechanism of quinolone action and resistance	Biochemistry. 2014;53(10):1565–1574
Hooper DC, Jacoby GA	Topoisomerase inhibitors: fluoroquinolone mechanisms of action and resistance	Cold Spring Harb Perspect Med. 2016;6(9):a025320
Craig WA	Pharmacokinetic/pharmacodynamic parameters: rationale for antibacterial dosing of mice and men	Clin Infect Dis. 1998;26(1):1–12
Mandell LA, Wunderink RG, Anzueto A, et al.	Infectious Diseases Society of America/American Thoracic Society consensus guidelines on the management of community-acquired pneumonia in adults	Clin Infect Dis. 2007;44(Suppl 2):S27–S72
Gupta K, Hooton TM, Naber KG, et al.	International clinical practice guidelines for the treatment of acute uncomplicated cystitis and pyelonephritis in women	Clin Infect Dis. 2011;52(5):e103–e120
Turnidge J	Pharmacokinetics and pharmacodynamics of fluoroquinolones	Drugs. 1999;58(Suppl 2):29–36
Croom KF, Goa KL	Levofloxacin: a review of its use in the treatment of bacterial infections in the United States	Drugs. 2003;63(24):2769–2802
Neuhofel AL, Wilton JH, Victory JM, Hejmanowski LG, Amsden GW	Lack of bioequivalence of ciprofloxacin when administered with calcium-fortified orange juice: a new twist on an old interaction	J Clin Pharmacol. 2002;42(4):461–466
Owens RC Jr, Ambrose PG	Antimicrobial safety: focus on fluoroquinolones	Clin Infect Dis. 2005;41(Suppl 2):S144–S157
Noel GJ, Natarajan J, Chien S, Hunt TL, Goodman DB, Abels R	Effects of three fluoroquinolones on QT interval in healthy adults after single doses	Clin Pharmacol Ther. 2003;73(4):292–303
FDA Drug Safety Communication	FDA updates warnings for oral and injectable fluoroquinolone antibiotics due to disabling side effects	US Food and Drug Administration; July 26, 2016
van der Linden PD, Sturkenboom MC, Herings RM, Leufkens HG, Stricker BH	Fluoroquinolones and risk of Achilles tendon disorders: case-control study	BMJ. 2002;324(7349):1306–1307
Lee CC, Lee MT, Chen YS, Lee SH, Chen SC, Chang SC	Risk of aortic dissection and aortic aneurysm in patients taking oral fluoroquinolone	JAMA Intern Med. 2015;175(11):1839–1847
Mohr JF, McKinnon PS, Peymann PJ, et al.	A retrospective, comparative evaluation of dysglycemias in hospitalized patients receiving gatifloxacin, levofloxacin, ciprofloxacin, or ceftriaxone	Pharmacotherapy. 2005;25(10):1303–1309
Jacoby GA	Mechanisms of resistance to quinolones	Clin Infect Dis. 2005;41(Suppl 2):S120–S126