Fluoroquinolones

Discovery

During the commercial preparation of chloroquine (an anti-malarial agent), George Lesher and his team discovered a compound with antibacterial properties (Lesher et al., 1962). This first quinolone molecule, nalidixic acid (Figure 1), was clinically tested in the 1960s and used for treating urinary tract infections (Appelbaum and Hunter, 2000).

Figure 1. 2D structure of Nalidixic acid.

The original quinolone molecule has undergone various structural modifications to yield several generations of this class of antibiotic (Oliphant and Green 2002). Today fluoroquinolones are broad-spectrum bactericidal drugs used to treat infections caused by gram-negative and gram-positive bacteria, including genitourinary infections, pneumonia, and infections from urinary catheters. They target type IIA topoisomerase enzymes (DNA gyrase and topoisomerase IV), which modify DNA topology to prevent bacterial DNA replication. When fluoroquinolones bind to these enzymes, their ability to reseal DNA is corrupted, generating single- and double-strand breaks that lead to cell death (Aldred et al., 2014).

Overview of Chemistry

All fluoroquinolone molecules have a bicyclic core structure with various substituents (Figure 2). The substituent on the nitrogen in the quinolone ring (marked as R1) affects the overall potency of the molecule - bulkier groups improved the antibiotic activity against both gram-negative and gram-positive species. The substituent R2 is often a nitrogen-based heterocyclic group and may vary in different members of the family. The fluoro group on the benzene ring improved activity against gram-positive organisms. The benzene ring plays a key role in binding to the target via stacking interactions with the DNA bases. The exo-cyclic oxygen and the carboxylic acid side chain (marked R3) play key roles in target binding - they interact with a Mg²⁺ ion coordinated by target protein amino acids in all fluoroquinolones (Appelbaum and Hunter 2000).

Figure 2. A 2D diagram of moxifloxacin showing key chemical features of the fluoroquinolone class of antibiotics. The substituents R1, R2, and R3 are described in the text. The figure was generated using ChemAxon.

Types

Quinolones are classified into generations based on their antibacterial spectra (see Table 1, Sharma et al., 2009). Members of the first generation of quinolones were related to the naphthyridine family (nalidixic acid). They were used to treat urinary tract infections in the 1960s and 70s but are rarely used clinically. Beginning in the second generation, a fluorine atom was included in the C6 (or C7) position (hence the name fluoroquinolone. This class had expanded gram-negative activity and atypical pathogen coverage, but limited gram-positive activity. They were most active against aerobic gram-negative bacilli. Ciprofloxacin remains the quinolone most active against Pseudomonas aeruginosa. Third-generation quinolones retain expanded gram-negative and atypical intracellular activity but have improved gram-positive coverage. Finally, fourth-generation agents improve gram-positive coverage, maintain gram-negative coverage, and gain anaerobic coverage.

Most clinically-relevant quinolones are fluoroquinolones, which have a fluorine atom in the all-carbon-containing ring at the C-6 position (see Figure 2). The two-ringed nitrogen-containing system with a ketone is called a quinolone. Frequently used fluoroquinolones include Avelox (moxifloxacin), Cipro (ciprofloxacin), and Levaquin (levofloxacin).

Table 1. Classification of quinolones, based on information presented in Sharma et al., 2009.

Resistance

Resistance to quinolones can evolve rapidly, even during a course of treatment. Numerous pathogens, including Escherichia coli, commonly exhibit resistance. Common mechanisms of resistance include chromosomal mutations in the drug target that prevent drug binding (Oliphant and Green, 2002). Efflux pumps also play a role in resistance by moving the drug out of the cell rapidly. Learn more about Moxifloxacin resistance.

Safety Information

Eukaryotic cells do not contain DNA gyrase or topoisomerase IV, so the drug should not affect human cells. However, some compounds in this class have been shown to impact mitochondrial function in pancreatic beta cells (Ghaly et al., 2014) while some inhibit the synthesis of mitochondrial DNA (Hangas et al., 2018). Increased risk of tendinitis and tendon rupture when taking this class of antibiotics has led the US FDA to add a black-box warning label on fluoroquinolones (Tanne, 2008). Some and

Across the fluoroquinolone antibiotic class, a range of health side effects are described in the Warnings and Precautions section of the US FDA drug labeling and vary by individual drug.
* In 2016, the US FDA issued enhanced warnings about the association of fluoroquinolones with disabling and potentially permanent side effects involving tendons, muscles, joints, nerves, and the central nervous system.
* In 2018, the U.S. Food and Drug Administration made safety labeling changes for fluoroquinolones to strengthen the warnings about the risks of various health side effects and serious blood sugar disturbances (even instances of hypoglycemic coma), when using fluoroquinolones, including levofloxacin (Levaquin), ciprofloxacin (Cipro), ciprofloxacin extended-release tablets, moxifloxacin (Avelox), ofloxacin, gemifloxacin (Factive) and delafloxacin (Baxdela). (FDA, 2018).

Fluoroquinolones are still effective in treating serious bacterial infections, and the benefits of these drugs outweigh the risks. The US FDA recommends that patients should only be given fluoroquinolones when there are no other treatment options.

References

Appelbaum, P. C., Hunter, P. A. (2000) The fluoroquinolone antibacterials: past, present and future perspectives. Int J Antimicrob Agents. 16(1):5-15. https://doi.org/10.1016/s0924-8579(00)00192-8

Aldred, K. J., Kerns, R. J., Osheroff, N. (2014). Mechanism of quinolone action and resistance. Biochemistry. 53 (10): 1565–74. https://doi.org/10.1021/bi5000564

Bush, N. G., Diez-Santos, I., Abbott, L. R., Maxwell, A. (2020). Quinolones: Mechanism, lethality and their contributions to antibiotic resistance. Molecules, 25(23), 5662. https://doi.org/10.3390/molecules25235662

FDA updates warnings for fluoroquinolone antibiotics on risks of mental health and low blood sugar adverse reactions. U.S. Food and Drug Administration. https://www.fda.gov/news-events/press-announcements/fda-updates-warnings-fluoroquinolone-antibiotics-risks-mental-health-and-low-blood-sugar-adverse

Ghaly, H., Jörns, A., Rustenbeck, I. (2014) Effect of fluoroquinolones on mitochondrial function in pancreatic beta cells. Eur J Pharm Sci. 52:206-14. https://doi.org/10.1016/j.ejps.2013.11.011

Hangas, A., Aasumets, K., Kekäläinen, N. J., Paloheinä, M., Pohjoismäki, J. L., Gerhold, J. M., Goffart, S. (2018) Ciprofloxacin impairs mitochondrial DNA replication initiation through inhibition of Topoisomerase 2. Nucleic Acids Res. 46(18):9625-9636. https://doi.org/10.1093/nar/gky793

Lesher, G. Y., Froelich, E. J., Gruett, M. D., Bailey, J. H. (1962) 1,8-NAPHTHYRIDINE DERIVATIVES. A NEW CLASS OF CHEMOTHERAPEUTIC AGENTS. J Med Pharm Chem. 1063-5. https://doi.org/10.1021/jm01240a021

Oliphant, C. M., Green, G. M. (2002) Quinolones: a comprehensive review. Am Fam Physician. 65(3):455-64. https://www.aafp.org/pubs/afp/issues/2002/0201/p455.html

Sharma, P. C., Jain, A., Jain, S. (2009). Fluoroquinolone antibacterials: a review on chemistry, microbiology and therapeutic prospects. Acta Pol Pharm, 66(6), 587-604. https://www.ptfarm.pl/pub/File/Acta_Poloniae/2009/6/587.pdf

Tanne, J. H. (2008) FDA adds "black box" warning label to fluoroquinolone antibiotics. BMJ. 337(7662):a816. https://doi.org/10.1136/bmj.a816

March 2025, Helen Gao, Shuchismita Dutta; Reviewed by Dr. James Berger
https://doi.org/10.2210/rcsb_pdb/GH/AMR/drugs/antibiotics/dna-synth/topo2/flqs