18.3: Inhibitors of Bacterial DNA Synthesis

Bacterial pathogens depend on precise and efficient DNA replication to sustain infection. Two type II topoisomerases—DNA gyrase and topoisomerase IV—are critical to this process, as they resolve DNA supercoiling and unlink chromosomes during replication. Fluoroquinolones, synthetic derivatives of quinolones, exploit this mechanism by stabilizing the transient DNA–enzyme cleavage complex, preventing strand religation, and causing lethal double-strand breaks. These antibiotics are selectively toxic to bacteria due to structural differences between bacterial and eukaryotic topoisomerases.

Fluoroquinolones offer broad-spectrum coverage against both Gram-negative and Gram-positive pathogens, including intracellular organisms such as Legionella and Mycoplasma, due to efficient cellular uptake via porins.

Specific agents like ciprofloxacin, levofloxacin, and moxifloxacin are employed in urinary tract infections, respiratory infections, skin and soft tissue infections, osteomyelitis (e.g., Salmonella in sickle cell patients), and hospital-acquired pneumonia.

Side Effects
Although fluoroquinolones are generally well tolerated, they are associated with several rare but serious adverse effects.

• Tendinitis and tendon rupture, particularly involving the Achilles tendon, are notable complications, with a higher incidence in elderly patients and those receiving corticosteroids.
• Central nervous system side effects are diverse, ranging from mild symptoms such as headache and insomnia to severe manifestations including seizures, psychosis, and, in rare instances, suicidal ideation.
• Peripheral neuropathy is another concern, with some cases resulting in irreversible nerve damage.
• Fluoroquinolones have been linked to an increased risk of aortic aneurysm and dissection, particularly among older adults with preexisting vascular disease.
• Gastrointestinal complications, such as Clostridioides difficile–associated colitis, are comparable in frequency to those seen with other broad-spectrum antibiotics.

Pharmacovigilance
Due to these serious adverse effects and rapidly emerging resistance, fluoroquinolones are now generally reserved for conditions where benefits outweigh risks, or when first-line agents are ineffective. Regulatory bodies such as the FDA and EMA recommend limited use for uncomplicated infections and emphasize antimicrobial stewardship to minimize resistance development and adverse outcomes.