penicillinase-producing strains), and Listeria are all susceptible. Although some strains of methicillin-resistant staphylococci are susceptible, many are not. Activity is excellent against the Enterobacteriaceae, including organisms that are cephalosporin-resistant by virtue of expression of extended-spectrum b-lactamases. Most strains of Pseudomonas and Acinetobac-ter are inhibited. Anaerobes, including B. fragilis, are highly susceptible.

Pharmacokinetics and Adverse Reactions

Imipenem is hydrolyzed by a dipeptidase found in the brush border of the proximal tubule. To prolong drug activity, imipenem is combined with cilastatin, an inhibitor of the dehydropeptidase; the combined formulation is available as primaxin. Both imipenem and cilastatin have a t/2 of ~1 hour. When administered with cilastatin, ~70%o of administered imipenem is recovered in the urine as the active drug. Dosage should be reduced for patients with renal insufficiency.

Nausea and vomiting are the most common side effects. Seizures have been noted in up to 1.5% of patients, especially when high doses are given to patients with CNS lesions or with renal insufficiency. Patients who are allergic to other b-lactam antibiotics may have hypersensitivity reactions to imipenem.

Therapeutic Uses

Imipenem-cilastatin is effective for a wide variety of infections, including urinary tract and lower respiratory infections; intra-abdominal and gynecological infections; and skin, soft tissue, bone, and joint infections. The combination appears to be especially useful for the treatment of serious infections caused by cephalosporin-resistant nosocomial bacteria, as may be seen in hospitalized patients who have recently received other b-lactam antibiotics. Imipenem should not be used as monotherapy for infections with P. aeruginosa because of the risk of developing resistance during therapy.

meropenem Meropenem (merrem iv) is a derivative of thienamycin that does not require coadministration with cilastatin because it is not sensitive to renal dipeptidase. Its toxicity and clinical efficacy are similar to imipenem, except that it may be less likely to cause seizures.

ertapenem Ertapenem (invanz) differs from imipenem and meropenem by having a longer serum t1/2 that allows once-daily dosing and by having inferior activity against P. aeruginosa and Acinetobacter spp. Its spectrum of activity against gram-positive organisms, Enterobacteriaceae, and anaerobes makes it attractive for use in intra-abdominal and pelvic infections.

aztreonam Aztreonam (azactam) is a monocyclic b-lactam. Its structural formula is:

Aztreonam is resistant to the b-lactamases that are elaborated by most gram-negative bacteria. The antimicrobial activity of aztreonam differs from those of other b-lactam antibiotics and resembles that of an aminoglycoside. Aztreonam has activity only against gram-negative bacteria; it has no activity against gram-positive bacteria and anaerobic organisms. Activity against Enterobacteriaceae is excellent, as is that against P. aeruginosa. It is also highly active against H. influenzae and gonococci.

Aztreonam is administered intramuscularly or intravenously. The elimination t122 is 1.7 hours, and most drug is excreted unaltered in the urine. The t122 is prolonged to 6 hours in anephric patients. The usual dose of aztreonam for severe infections is 2 g every 6-8 hours (reduced in patients with renal insufficiency). A notable feature is that there is little cross-reactivity with most other b-lactam antibiotics. Aztreonam is therefore used to treat gram-negative infections that normally would be treated with a b-lactam antibiotic were it not for a prior allergic reaction.

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