Ertapenem

Overview
Ertapenem is a carbapenem antibiotic marketed by Merck as Invanz®. It is structurally very similar to meropenem in that it possess a 1-β-methyl group.

Indications
Ertapenem has been designed to be effective against Gram negative bacteria. It is not active against MRSA, ampicillin-resistant enterococci, Pseudomonas aeruginosa or Acinetobacter species. Ertapenem also has clinically useful activity against anaerobic bacteria.

Ertapenem is marketed by Merck as a first-line treatment for community-acquired infections. It should not be used as empirical treatment for hospital-acquired infections because of its lack of activity against Pseudomonas aeruginosa. In practice, it is reserved primarily for use against ESBL-producing and high level AmpC-producing Gram-negative bacteria.

Dosage
Ertapenem is dosed as 1g given by intravenous injection over 30 minutes, or 1g diluted with 3.2ml of 1% lidocaine given intramuscularly. There is no oral preparation of ertapenem available. Ertapenem cannot be mixed with glucose.

The marketing slogan for ertapenem is "The Power of One", because the dose is one gram, once a day.

Pharmacology
Unlike imipenem and meropenem, ertapenem is highly protein bound, which explains its long half life (4 hours).

Ertapenem is excreted primarily (80%) by the kidneys. Metabolism by the liver is not clinically important and does not affect dosing.

Patients on haemodialysis should be given ertapenem at least 6 hours before dialysis. If it is given less than six hours before dialysis, then the patient should be given an additional dose of 150mg IV after dialysis. Ideally, patients on haemodialysis should be given ertapenem immediately following dialysis.

Resistance
Acquired resistance to ertapenem is usually mediated by up-regulation of efflux mechanisms and by the selection of porin-deficient mutants. Organisms that produce a metallo-&beta;-lactamase are innately immune to ertapenem (as well as all carbapenems) (Reference needed).

Side effects
There are few adverse effects of ertapenem. The only absolute contra-indication is a previous anaphylactic reaction to ertapenem or other β-lactam antibiotic. There are no studies done in pregnant women, so the manufacturers cannot comment on its safety in pregnancy. In 2006, Ertapenem is now approved for pediatric use in certain infections. Ertapenem is not recommended for children under 3 months of age and children with meningitis.

Use of all antibiotics is associated with increased rates of resistance (although carbapenem resistance is currently rare). There is particular worry that although ertapenem has no clinically useful activity against Pseudomonas aeruginosa, widespread use of ertapenem could still lead to increased carbapenem resistance in Pseudomonas (Livermore et al. 2005).

Like many antibiotics, Clostridium difficile colitis has been associated with its use.