Quinolones

The quinolones are a newer class of antibiotic. One of the earlier quinolones was nalidixic acid, often used to teat urinary tract infections.

More recently, modified quinolones (the fluoroquinolones) have been designed. The original fluoroquinolones were ofloxacin and ciprofloxacin. However, even newer ones are coming onto the market, such as moxifloxacin, gatifloxacin and grepafloxacin.

Mechanism of action

Quinolones kill bacteria by stopping replication of bacterial DNA. They accomplish this by binding to and inhibiting the action of one of the enzymes involved in DNA replication.

Route of administration

Nalidixic acid is only available in oral formulations. However, the fluoroquinolones are available both intravenously and orally (as well as topically).

One of the great advantages of the quinolones is that the absorption of these drugs is so good that giving it orally is essentially the same as giving it intravenously.

The only situations where one would give an intravenous quinolone would be when someone was unable to absorb orally, as may occur after surgery to the stomach, for example.

Spectrum of action

The older fluoroquinolones (ciprofloxacin and ofloxacin) had excellent activity against Gram negative bacteria, but little activity against some Gram positive bacteria (such as Streptococcus pneumoniae, Streptococcus pyogenes).

They are also active against some of the more unusual or uncommon bacteria, such as Legionella (which causes Legionnaire’s disease) and Rickettsia (which causes tick bite fever).

Infections that can be treated with these agents include:

• Urinary tract infections
• Typhoid
• Dysentery
• Tick bite fever
• Tuberculosis (quinolones are often used to treat drug resistant TB)
• Gonorrhoea (quinolones are in fact preferable to ceftriaxone, since a quinolone can be taken orally)

The newer fluoroquinolones have been designed to have better action against some of the Gram positive bacteria mentioned above, and they are thus being increasingly used for infections of the respiratory tract.

There is some debate around this issue, as some people feel that excessive use of the quinolones in this setting (when something like amoxicillin would also be appropriate) could lead to an increasing level of bacterial resistance to the quinolones.

Side effects

Very few side effects have been attributed to the quinolones. There have been some reports of liver toxicity, although this is very, very unusual. Initial animal studies of these drugs showed some effects on the cartilage and bones of animal foetuses, and the drugs have thus not been licensed for use in pregnancy or childhood.

There is a growing opinion, however, that they are safe for use in these settings. This information comes as a result of increasing numbers of pregnant women and children who have been treated with these drugs (when there was no alternative), with no reported side effects.

However, this is still an issue that is being debated, and some doctors would still not use quinolones in these settings.