Telithromycin

Telithromycin is the first ketolide antibiotic to enter clinical use. It is used to treat mild to moderate respiratory infections. Telithromycin is sold under the brand name of Ketek.

Telithromycin is a semi-synthetic erythromycin derivative. It is created by substituting a ketogroup for the cladinose sugar and adding a carbamate ring in the lactone ring. An alkyl-aryl moiety is attached to this carbamate ring. Furthermore, the carbon at position 6 has been methylated, as is the case in clarithromycin, to achieve better acid-stability.

History
French pharmaceutical company Hoechst Marion Roussel (later Sanofi-Aventis) began phase II/III clinical trials of telithromycin (HMR-3647) in 1998. Telithromycin was approved by the European Commission in July 2001 and subsequently went on sale in October 2001. In the US, telithromycin received FDA approval on April 1, 2004.

Available forms
Telithromycin is administered as tablets. The standard dosing is two 400 mg tablets to be taken together daily, with or without food.

Mechanism of action
Telithromycin prevents bacteria from growing, by interfering with their protein synthesis. Telithromycin binds to the subunit 50S of the bacterial ribosome, and blocks the progression of the growing polypeptide chain. Telithromycin has over 10 times higher affinity to the subunit 50S than erythromycin. In addition, telithromycin strongly bind simultaneously to two domains of 23S RNA of the 50 S ribosomal subunit, where older macrolides bind strongly only to one domain and weakly to the second domain. Telithromycin can also inhibit the formation of ribosomal subunits 50S and 30S.

Pharmacokinetics
Unlike erythromycin, telithromycin is acid-stable and can therefore be taken orally while being protected from gastric acids. It is fairly rapidly absorbed, and diffused into most tissues and phagocytes. Due to the high concentration in phagocytes, telithromycin is actively transported to the site of infection. During active phagocytosis, large concentrations of telithromycin is released. The concentration of telithromycin in the tissues is much higher than in plasma.Telithromycin fulfills a role that has arisen due to the rise of microbial resistance to existing macrolides and appears to be effective against macrolide-resistant Streptococcus pneumoniae. The defining differentiating characteristic of the ketolides as opposed to other macrolides is the removal of the neutral sugar, L-cladinose from the 3 position of the macrolide ring and the subsequent oxidation of the 3-hydroxyl to a 3-keto functional group.

Metabolism
Telithromycin is metabolized mainly in the liver, the main elimination route being the bile, a small portion is also excreted into the urine. About one third is excreted unchanged in bile and urine, the biliary route being favoured. Telithromycin's half-life is approximately ten hours.

Adverse effects
Most common side-effects are gastrointestinal, including diarrhea, nausea, abdominal pain and vomiting. Headache and disturbances in taste also occur. Less common side-effects include palpitations, blurred vision, and rashes. Prolonged QTc intervals may also be caused by telithromycin.

Rare but severe side effects reported in January 2006 involve damage to the liver. Three different incidents have been reported: one ending in death, one in a liver transplant and one case of drug-induced hepatitis.

In the United States the FDA's Office of Epidemiology and Surveillance identified 12 cases of acute liver failure, resulting in four deaths, and an additional 23 cases of acute, serious liver injury in patients taking telithromycin up to April 2006.

Telithromycin has been known to cause false positive readings in drug screenings for cocaine and amphetamines.

By April 2008, independent analysis using FDA data has linked Ketek (Telithromycin) to 18 deaths and at least 134 cases of liver damage. Some researchers say the total may be far greater.

In 2010, a published report described the likely mechanism of action underlying not only the cases of liver failure but also cases of visual disturbances and exacerbations of myasthenia gravis. The study showed that a pyridine moiety that is part of the telithromycin molecule acts as an antagonist on cholinergic receptors located in the neuromuscular junction, the ciliary ganglion of the eye and the vagus nerve innervating the liver. Other macrolides, such as azithromycin and clarithromycin and the fluoroketolide, solithromycin, do not contain the pyridine moiety and do not antagonize these cholinergic receptors significantly.

FDA Warning
On February 12, 2007, the Food and Drug Administration announced a revision to the labeling of Ketek to improve patient safety. The changes included the removal of two of the three previously approved indications: acute bacterial sinusitis and acute bacterial exacerbations of chronic bronchitis. The agency determined that the balance of benefits and risks no longer supported approval of the drug for these indications. Ketek will remain on the market for the treatment of community acquired pneumonia of mild to moderate severity (acquired outside of hospitals or long-term care facilities). In addition, the FDA worked with the manufacturer to update the product labeling with a "black box warning," their strongest form of warning. Ketek's warning states that it should not be used in patients with myasthenia gravis, a disease that causes muscle weakness.