Absorption Distribution Metabolism and Excretion

Most parenteral cephalosporins have good bioavailabil-ity after intramuscular injection, and a few members of each cephalosporin generation have good oral bioavail-ability (Table 45.2). The ester prodrugs cefuroxime ax-etil (Ceftin) and cefpodoxime proxetil (Vantin) are oral formulations in which the ester is hydrolyzed during drug passage through the intestinal mucosa; the free cephalosporin enters the systemic circulation. Concomitant ingestion of food reduces the bioavailability of some cephalosporins, e.g., cefaclor (Ceclor), and therefore, these compounds should be administered on an empty stomach.

The cephalosporins distribute in satisfactory concentrations to most tissues except the central nervous system. Only cefepime, cefuroxime (Zinacef), cefotaxime (Claforan), ceftriaxone (Rocephin), and ceftazidime (Fortaz) achieve therapeutic concentrations in cerebrospinal fluid. Cefotaxime and ceftriaxone are antibiotics of first choice for the empirical treatment of brain abscess and meningitis.

There is considerable variation in the protein binding among the cephalosporins. Drugs like ceftriaxone that have extensive protein binding (85-95%) may displace bilirubin from serum albumin. Consequently, ceftriaxone may increase the risk of kernicterus in jaundiced neonates.

Urinary excretion is the major elimination path for most cephalosporins. When prescribing cephalosporins to patients with renal failure, practitioners must consider dose reduction or dose interval extension (Table 45.2). Renal tubular secretion contributes to the elimination of some cephalosporins, and an increase in cephalosporin plasma concentrations may occur when probenecid blocks renal tubular secretion of cephalosporins. Biliary elimination is important for some cephalosporins. Cefmetazole, cefoperazone (Cefobid), cefoxitin, and cef-triaxone achieve biliary concentrations greater than those in plasma. After parenteral administration of cefoperazone, 70% of the dose appears in the bile within 24 hours. Practitioners should decrease the dose of cefop-erazone when prescribing for patients with hepatic failure or biliary obstruction. Metabolism is not a major elimination path for most cephalosporins. Cefotaxime is one of the few cephalosporins having an active metabolite, desacetyl cefotaxime.

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