Structure of bacterial DNA gyrase complexed with DNA and two ciprofloxacin molecules (green)<unk> s are chemotherapeutic bactericidal drugs. They interfere with DNA replication by preventing bacterial DNA from unwinding and duplicating. Specifically, they inhibit the ligase activity of the type II topoisomerases, DNA gyrase and topoisomerase IV, which cut DNA to introduce supercoiling, while leaving nuclease activity unaffected. With the ligase activity disrupted, these enzymes release DNA with single- and double-strand breaks that lead to cell death. The majority of<unk> s in clinical use are fluoroquinolones, which have a fluorine atom attached to the central ring system, typically at the 6-position or C-7 position. Most of them are named with the -oxacin suffix. First and second generation<unk> s are largely active against Gram-negative bacteria, whereas third and fourth generation<unk> s have increased activity against Gram-positive and anaerobic bacteria. Some<unk> s containing aromatic substituents at their C-7 positions are highly active against eukaryotic type II topoisomerase. It has also been proposed that<unk> s cause oxidation of guanine nucleotides in the bacterial nucleotide pool, and that this process contributes to the cytotoxicity of these agents. The incorporation of oxidized guanine nucleotides into DNA could be bactericidal. Bacterial cytotoxicity could arise from incomplete repair of closely spaced 8-oxo-2'-deoxyguanosine in the DNA resulting in double-strand breaks. Cellular uptake Fluoroquinolones can enter cells easily via porins, so are often used to treat intracellular pathogens such as Legionella pneumophila and Mycoplasma pneumoniae. For many Gram-negative bacteria, DNA gyrase is the target, whereas topoisomerase IV is the target for many Gram-positive bacteria. Eukaryotic cells are not believed to contain DNA gyrase or topoisomerase IV. However, debate exists concerning whether the<unk> s still have such an adverse effect on the DNA of healthy cells. Some compounds in this class have been shown to inhibit the synthesis of mitochondrial DNA.
Quinolone antibiotic
Polypeptide antibiotic
Beta-lactam antibiotics
Lipase inhibitors
Nonsteroidal anti-inflammatory drug
Antiviral drug
Protease inhibitor 
Antibiotic