Quinolones and fluoroquinolones

Quinolones are a type of antibiotic. Antibiotics kill or inhibit the growth of bacteria. Nalidixic acid was the first quinolone discovered, but it has since been discontinued.

There are five different quinolone classes. One of the classes, called fluoroquinolones, was derived from quinolones by adding a fluorine atom to nalidixic acid. This enhanced its activity against bacteria, and improved its absorption, metabolism, and excretion characteristics. Although fluoroquinolones are still available, their side effects limit their use and they are not usually used first-line.  Common fluoroquinolones include:

• ciprofloxacin
• delafloxacin
• gemifloxacin
• levofloxacin
• moxifloxacin
• norfloxacin (discontinued)
• ofloxacin.

Some people use the words quinolones and fluoroquinolones interchangeably, but fluoroquinolones are the only class of quinolones still available. For this reason, the term fluoroquinolones will be used from now on.

Fluoroquinolones detrimentally affect the function of two enzymes produced by bacteria, topoisomerase IV and DNA gyrase, so that bacteria can no longer manufacture or repair their own DNA. DNA gyrase is essential for bacterial growth and is not present in eukaryotic cells (these are cells with a nucleus enclosed in a nuclear membrane and are present in animals, fungi, plants, and protozoa. 

What are fluoroquinolones used for?

Fluoroquinolones are considered broad-spectrum antibiotics. This means that they are effective against a wide range of bacteria.

However, because of the risk of serious side effects, the FDA has advised that they are not suitable for common conditions such as sinusitis, bronchitis, and uncomplicated urinary tract infections, and should only be considered when treatment with other, less toxic antibiotics, has failed.

Fluoroquinolones may also be used to treat unusual infections such as anthrax or plague. Doctors may also decide to use them for other types of infection when other alternative treatment options have failed or cannot be used.

Common fluoroquinolones

Table: List of common fluoroquinolones

Are fluoroquinolones safe?

Fluoroquinolones should be avoided in children under the age of 18 years unless they have a serious infection that cannot be treated with any other antibiotic. This is because they can damage the weight-bearing joints in children, and children are also more susceptible to other adverse effects of fluoroquinolones, including tendinitis and tendon rupture.

The FDA has some concerns about fluoroquinolones and considers them unsuitable for most common infections because they have been associated with some serious effects, including tendinitis (inflammation of a tendon) and tendon rupture. In addition, some people have reported peripheral neuropathy (nerve pain in the fingers and toes) and central nervous system effects such as agitation, attention problems, disorientation, memory impairment, nervousness, and delirium while taking fluoroquinolones. They have also been associated with low blood sugar levels (hypoglycemia). All fluoroquinolones are required by the FDA to carry safety warnings about these potentially serious side effects.

The risk of tendinitis and tendon rupture is increased in people over the age of 60, in those taking corticosteroids, or with a history of organ transplant. Previous tendon disorders or strenuous activity may also increase risk.

Occasionally, liver damage and allergic reactions have occurred in people taking fluoroquinolones.

If any of these very severe side effects happen, discontinue the fluoroquinolone immediately, and avoid all other fluoroquinolones in the future.

What are the side effects of fluoroquinolones?

The most commonly reported side effects include diarrhea, nausea, abnormal liver function tests, vomiting, and rash.

Fluoroquinolones may also cause anxiety, insomnia, psychotic reactions, nerve pain or a loss of feeling in the extremities, electrocardiogram (ECG) abnormalities, increased sensitivity to light, and other effects.

In people with myasthenia gravis, they may exacerbate muscle weakness and may trigger seizures or increase the risk of having a seizure.

In people with diabetes, fluoroquinolones may affect blood glucose levels requiring extra monitoring.

Fluoroquinolones increase the sensitivity of the skin to the sun and may cause photo-sensitivity reactions and severe sunburn on exposed areas of the skin.

Fluoroquinolones are not suitable for people with myasthenia gravis, certain heart rhythm disturbances, or children and adolescents under the age of 18 (unless the infection cannot be treated by another antibiotic).