A broad-spectrum antibiotic under close watch
Ciprofloxacin is a synthetic antibiotic belonging to the fluoroquinolone family. With a broad spectrum of activity, it has proven particularly effective against many Gram-negative bacteria.
When it first appeared, it marked a major advance in the treatment of infections. Still widely used today, it is nevertheless subject to increasingly restricted use, owing to its side-effect profile and to concerns around bacterial resistance.
From nalidixic acid to a modern fluoroquinolone
Ciprofloxacin descends directly from nalidixic acid, the very first antibacterial quinolone. It was discovered in 1962 by George Lesher and his team at Sterling-Winthrop, quite by chance, as a byproduct of the synthesis of chloroquine (an antimalarial drug). Although narrow in spectrum, nalidixic acid paved the way for an entire family of molecules.
The decisive breakthrough came from adding a fluorine atom and a piperazine ring to the quinolone core, giving rise to the fluoroquinolones — far more potent and with a much broader spectrum. Ciprofloxacin was developed by the German company Bayer in the early 1980s, with synthesis work carried out notably by Klaus Grohe. It was patented in 1983 and launched on the market from 1987, quickly establishing itself as one of the most widely prescribed fluoroquinolones in the world.
Its public profile rose further during the 2001 anthrax crisis in the United States, when it was recommended for prophylaxis.
Mechanism of action and antibacterial spectrum
Ciprofloxacin is bactericidal: it kills bacteria rather than simply slowing their growth. It works by inhibiting two enzymes essential to bacterial DNA replication, DNA gyrase (topoisomerase II) and topoisomerase IV. By blocking these enzymes, it prevents proper DNA unwinding and duplication, leading to bacterial death.
Its spectrum is broad, with particularly strong activity against Gram-negative bacteria, including Pseudomonas aeruginosa, as well as against certain Gram-positive and intracellular bacteria. It is used in urinary, digestive, respiratory, and bone infections, as well as in certain severe or complicated infections.
Safety and precautions for use
Despite its effectiveness, ciprofloxacin has a safety profile that calls for caution. Fluoroquinolones are associated with a risk of tendinopathy and tendon rupture (notably of the Achilles tendon), neurological effects, QT interval prolongation, photosensitivity, and digestive disorders, including Clostridioides difficile colitis.
It is generally not recommended for children or for pregnant or breastfeeding women, except in specific indications, owing to possible effects on growing cartilage. Health authorities (the FDA and EMA) have in fact restricted the use of fluoroquinolones to situations where no alternative is available for mild infections. Finally, rational use is essential to limit the development of bacterial resistance.
Dosage forms and pharmaceutical formulation
Ciprofloxacin is available in several dosage forms: tablets (most often as the more soluble ciprofloxacin hydrochloride), oral suspensions, injectable solutions for infusion, as well as eye drops and ear drops for ocular and ear infections.
From a formulation standpoint, ciprofloxacin base is poorly soluble in water and has a very bitter taste, which explains the use of the hydrochloride salt and the film-coating applied to mask the taste and ease swallowing. Like most tablets, its solid forms rely on standard excipients — diluents, binders, disintegrants (such as microcrystalline cellulose or croscarmellose sodium) and lubricants — to ensure correct manufacturing and release of the active ingredient.