Cephalosporins

Cephalosporins

Many antibacterial drugs introduced for clinical use in recent years have been cephalosporins.

Through the generations

Cephalosporins are grouped into generations according to their effectiveness against different organisms, their characteristics, and their development.

§    First-generation cephalosporins include cefadroxil, cefazolin sodium, and cephalexin monohydrate.

§    Second-generation cephalosporins include cefaclor, cefprozil, cefoxitin, cefuroxime axetil, and cefuroxime sodium.

§    Third-generation cephalosporins include cefdinir, cefixime, ce-fotaxime sodium, cefpodoxime proxetil, ceftazidime, ceftibuten, and ceftriaxone sodium.

§    Fourth-generation cephalosporins include cefepime hydrochlo-ride.

A sensitive issue

Because penicillins and cephalosporins are chemically similar (they have what’s called a beta-lactam molecular structure), cross-sensitivity occurs in 10% to 15% of patients. This means that someone who has had a reaction to penicillin is also at risk for a reaction to cephalosporins.

Pharmacokinetics

Many cephalosporins are administered parenterally because they aren’t absorbed from the GI tract. Some cephalosporins are ab-sorbed from the GI tract and can be administered orally, but food usually decreases the absorption rate of these oral cephalospo-rins, though not the amount absorbed. Two cephalosporins (oral cefuroxime and cefpodoxime) actually have increased absorption when given with food.

Distribution

After absorption, cephalosporins are distributed widely and readi-ly cross the placenta.

Generational divide

Cefuroxime (second-generation) and the third-generation drugs cefotaxime, ceftriaxone, and ceftazidime cross the blood-brain barrier after I.V. or I.M. administration. Cefepime (fourth-genera-tion) also crosses the blood-brain barrier, but to what extent isn’t known.

Metabolism

Many cephalosporins aren’t metabolized at all. Cefotaxime sodium is metabolized to the nonacetyl forms, which provide less antibac-terial activity than the parent compounds. To a small extent, cef-triaxone is metabolized in the intestines to inactive metabolites, which are excreted via the biliary system.

Excretion

All cephalosporins are excreted primarily unchanged by the kid-neys with the exception of ceftriaxone, which is excreted in stool via bile.

Pharmacodynamics

Like penicillins, cephalosporins inhibit cell-wall synthesis by bind-ing to the bacterial enzymes known as PBPs, located on the cell membrane. After the drug damages the cell wall by binding with the PBPs, the body’s natural defense mechanisms destroy the bac-teria. (See How cephalosporins attack bacteria.)

Pharmacotherapeutics

The four generations of cephalosporins have particular therapeu-tic uses.

§    First-generation cephalosporins, which act primarily against gram-positive organisms, may be used as alternative therapy in the patient who’s allergic to penicillin, depending on how sensitive to penicillin he is. They’re also used to treat staphylococcal and streptococcal infections, including pneumonia, cellulitis (skin in-fection), and osteomyelitis (bone infection).

§    Second-generation cephalosporins act against gram-negative bacteria. Cefoxitin is the only cephalosporin effective against anaerobes (organisms that live without oxygen).

§    Third-generation cephalosporins, which act primarily against gram-negative organisms, are the drugs of choice for infections caused by Enterobacter, P. aeruginosa, and anaerobic organisms.

§    Fourth-generation cephalosporins are active against many gram-positive and gram-negative bacteria.

Drug interactions

The patient receiving cephalosporins who drinks alcoholic bever-ages with or up to 72 hours after taking a dose may experience acute alcohol intolerance, with such signs and symptoms as headache, flushing, dizziness, nausea, vomiting, or abdominal cramps within 30 minutes of alcohol ingestion. This reaction can occur up to 3 days after discontinuing the antibiotic. (See Adverse reactions to cephalosporins.)

In use with uricosurics

Uricosurics (drugs to relieve gout), such as probenecid and sulfinpyrazone, can reduce kidney excretion of some cephalosporins. Probenecid is used therapeutical-ly to increase and prolong plasma cephalosporin con-centrations.

Cephalosporins may also decrease estrogen absorp-tion, leading to decreased efficacy of oral contracep-tives containing estrogen and progesterone.