Quinolones

Classification (generation = spectrum expansion)

1st gen(historical): nalidixic acid — narrow Gram-, UTI only. 2nd gen: ciprofloxacin, ofloxacin, norfloxacin — good Gram- incl. Pseudomonas, limited Gram+/atypical. 3rd gen: levofloxacin — adds Gram+(pneumococcus)+atypicals(“respiratory fluoroquinolone”). 4th gen: moxifloxacin — further Gram+/anaerobic, also respiratory.

Mechanism

Inhibit 2 type II topoisomerases: DNA gyrase(introduces negative supercoils ahead of replication fork) + topoisomerase IV(decatenates daughter DNA circles post-replication). Gram-negative: DNA gyrase = primary target. Gram-positive: topoisomerase IV = primary target (differential pattern, not same enzyme equally in all organisms — explains why single-enzyme mutation ≠ automatic complete cross-organism resistance). Blocks DNA unwinding/religation → double-strand breaks → BACTERICIDAL. CONCENTRATION-dependent killing(General Considerations classification) → once/twice-daily high-dose regimens.

Adverse Effects

Tendinopathy/rupture(class-defining, esp Achilles) — >60yo, concurrent corticosteroids, transplant recipients at ↑risk; Mg2+ chelation in tendon tissue theory. Cartilage damage in developing joints(juvenile animal studies) → traditional CI in children/pregnancy(increasingly reconsidered in severe/resistant infections, but classical teaching point stands). QT prolongation(esp moxifloxacin). CNS: headache, dizziness, ↓seizure threshold(worsened by NSAIDs — additive GABA-A antagonism, specific mechanism not vague caution). Peripheral neuropathy(can be IRREVERSIBLE, black-box warning). Photosensitivity.

Drug Interactions

Chelation with divalent/trivalent cations(antacids Al3+/Mg2+, iron, zinc, dairy Ca2+) → insoluble complex, ↓↓oral absorption → manage by TIMING SEPARATION, not avoidance. CYP1A2 inhibition(esp ciprofloxacin) → ↑levels of substrates, classic example = theophylline(→toward its own narrow-TI toxic range).

Resistance

Chromosomal mutations: gyrA/gyrB(DNA gyrase) or parC/parE(topo IV) → ↓drug binding affinity. STEPWISE accumulation(single mutation = modest resistance; high-level needs mutations in BOTH enzymes) → once established, stable/high-level, not easily reversed. + efflux pump upregulation, ↓porin expression(general Gram- mechanisms, General Considerations).

Uses

UTI(historically core, rising resistance→reserved for complicated cases in many guidelines) · GI infections(traveller’s diarrhoea, typhoid) · respiratory(esp levofloxacin/moxifloxacin — pneumococcal coverage) · anthrax(ciprofloxacin — Bacillus anthracis, bioterrorism-preparedness). Ciprofloxacin’s excellent Pseudomonas coverage(unusual among oral antibiotics) = specific distinguishing feature(cystic fibrosis, malignant otitis externa).

Clinical Relevance

Generational spectrum-expansion + DNA gyrase(Gram-) vs topo IV(Gram+) primary-target distinction explains: why later-gen agents chosen for pneumococcal/atypical respiratory infections; why ciprofloxacin(earlier-gen) retains a specific still-relevant niche(Pseudomonas) some later agents don’t match — spectrum here = genuine mechanism-linked evolution, NOT “later always better across every organism.”

Classification (by Generation and Spectrum)

• 1st generation (non-fluorinated, now largely historical): nalidixic acid — narrow Gram-negative, urinary-tract-only
• 2nd generation (fluoroquinolones): ciprofloxacin, ofloxacin, norfloxacin — good Gram-negative including Pseudomonas, limited Gram-positive/atypical
• 3rd generation: levofloxacin — extends coverage to Gram-positive (particularly pneumococcus) and atypicals, the “respiratory fluoroquinolones”
• 4th generation: moxifloxacin — further improved Gram-positive/anaerobic coverage, also a respiratory fluoroquinolone

Each successive generation broadens Gram-positive/atypical coverage while generally retaining Gram-negative activity — the generational classification tracks spectrum expansion, not simply chronological release.

Mechanism of Action

Fluoroquinolones inhibit two related bacterial type II topoisomerase enzymes essential for DNA replication: DNA gyrase (topoisomerase II), which introduces negative supercoils ahead of the replication fork to relieve the topological strain of unwinding double-stranded DNA, and topoisomerase IV, which separates (decatenates) interlinked daughter DNA circles after replication so they can segregate into daughter cells. In Gram-negative organisms, DNA gyrase is generally the primary target; in Gram-positive organisms, topoisomerase IV is generally the primary target — this differential primary-target pattern (rather than both drugs hitting the same enzyme equally in every organism) is a specific, examined point explaining why a single mutation in one enzyme doesn’t automatically confer complete cross-organism resistance. By blocking these enzymes, fluoroquinolones prevent DNA unwinding/religation, leading to accumulated double-strand breaks and bactericidal cell death — concentration-dependent bactericidal killing (per the pharmacodynamic classification under Antimicrobials — General Considerations), consistent with once/twice-daily high-dose regimens.

Adverse Effects

Tendinopathy and tendon rupture — a specific, class-defining risk (particularly Achilles tendon), more common in patients over 60, those on concurrent corticosteroids, and organ transplant recipients; thought related to fluoroquinolones chelating Mg²⁺ within tendon tissue, impairing collagen/matrix integrity. Cartilage damage in developing joints (demonstrated in juvenile animal studies) is the mechanistic basis for the traditional contraindication in children and pregnancy — though this restriction has been increasingly reconsidered in specific severe/resistant infections where benefit may outweigh theoretical risk, the classical teaching point remains a standard exam answer. QT prolongation (particularly moxifloxacin) — caution with other QT-prolonging drugs. CNS effects: headache, dizziness, and — a specific, examined point — lowered seizure threshold, worsened by concurrent NSAID use (an interaction thought to involve additive antagonism at GABA-A receptors, a mechanistically specific rather than vague “avoid NSAIDs” caution). Peripheral neuropathy (can be irreversible, a black-box-level warning). Photosensitivity.

Drug Interactions

Chelation with divalent/trivalent cations — antacids (Al³⁺/Mg²⁺), iron, zinc, and dairy products (Ca²⁺) all form insoluble complexes with fluoroquinolones in the gut, substantially reducing oral absorption — the practical, frequently-tested management point is timing separation (fluoroquinolone taken well before or after the interacting agent), not avoidance of the antibiotic itself. CYP1A2 inhibition (particularly ciprofloxacin) raises levels of co-administered CYP1A2 substrates, theophylline being the classic, specifically examined example (raising theophylline toward its own narrow-therapeutic-index toxic range).

Resistance

Chromosomal mutations in the gyrA/gyrB (DNA gyrase) or parC/parE (topoisomerase IV) genes reduce drug binding affinity — resistance typically develops through stepwise accumulation of multiple mutations (a single mutation often producing only modest resistance, with high-level resistance requiring mutations in both target enzymes), which is part of why fluoroquinolone resistance, once established, tends to be stable and high-level rather than easily reversed. Efflux pump upregulation and reduced porin expression (the general Gram-negative resistance mechanisms already introduced under Antimicrobials — General Considerations) also contribute.

Uses

Broad, spanning urinary tract infections (historically a core indication, though rising resistance has shifted many guidelines toward reserving fluoroquinolones for more complicated cases), gastrointestinal infections (traveller’s diarrhoea, typhoid), respiratory infections (particularly the newer-generation “respiratory fluoroquinolones,” levofloxacin/moxifloxacin, given their improved pneumococcal coverage), and anthrax (ciprofloxacin, a specific, well-known indication given activity against Bacillus anthracis and utility in bioterrorism-preparedness contexts) — ciprofloxacin’s genuinely excellent Pseudomonas coverage, unusual among oral antibiotics, is a specific, examined distinguishing feature relevant to Pseudomonas-prone infections (e.g. in cystic fibrosis, malignant otitis externa).

Clinical Relevance

The generational spectrum-expansion pattern and the DNA gyrase (Gram-negative) versus topoisomerase IV (Gram-positive) primary-target distinction together explain why later-generation agents are specifically chosen for pneumococcal/atypical respiratory infections rather than earlier ones, and why ciprofloxacin — despite being an earlier-generation agent — retains a specific, still-relevant niche for Pseudomonas coverage that some later agents don’t match as well; spectrum in this drug class tracks a genuine, mechanism-linked evolution rather than later always meaning strictly “better” across every organism.

• DNA gyrase vs topoisomerase IV as primary targets in Gram-negative vs Gram-positive organisms respectively.
• Chelation interaction: fluoroquinolone plus divalent/trivalent cation forming an insoluble, unabsorbed complex in the gut.

Long Essays (3)

Short Essays (2)

Essays (9)

Short Answers (10)

Short Notes (7)

Multiple Choice Questions (2)