The first cephalosporin released for clinical use was cephalothin in 1964. Since that time, more than 20 cephalosporin antibiotics have been introduced. These are the most commonly prescribed antibiotics in the United States and account for the largest share of hospital expenditures. To determine which agent to use for an infection in a particular patient involves weighing competing claims of improved antibacterial spectrum, side effects, pharmacokinetics, and cost. This review will address these issues, as well as the basic mechanisms of action of the cephalosporins and their mechanisms of bacterial resistance. In addition, instances in which cephalosporins serve as appropriate alternatives or in which they offer distinct advantages over previously available antimicrobial agents will be described.
Chemistry and Mechanism of Action
Cephalosporins resemble penicillins (Fig.1) in that they have a β-lactam structure, but the five-member thiazolidine ring characteristic of the penicillins is replaced by a six-member dihydrothiazine ring (Fig. 2). The dihydrothiazine ring of the cephalosporins provides the molecule with the ability to resist bacterial enzymes; the antibacterial activity emanates from the β-lactam ring shared by penicillins and cephalosporins. Modifications at position 7 of the cephalosporin nucleus generally affect the antibacterial spectrum, and substitutions at position 3 of the dihydrothiazine ring alter the pharmacokinetics and metabolic parameters of the drug.