Pharmacokinetic properties of the newer cephalosporins. A valid basis for drug selection?

The total process cost of parenteral antibiotic therapy: beyond drug acquisition cost

Intravenous antibiotic therapy represents a considerable expense to hospital pharmacy budgets; however, when evaluating the cost of these therapies one needs to look beyond acquisition cost and consider the total "process" cost of treatment. These additional costs include the personnel time and the materials required for drug preparation and administration, maintenance of intravenous access, waste disposal, and therapeutic drug monitoring. This paper provides an examination of the daily process costs of intravenous therapy with cefazolin, cefotaxime, ceftazidime, once-daily ceftriaxone, cefuroxime, or aminoglycoside (tobramycin or gentamicin) combination therapy, where the aminoglycoside is given once daily or in divided doses. This analysis demonstrates that the costs associated with drug preparation and administration can equal or exceed drug acquisition costs and are highly dependent on dosing frequency. On this basis, ceftriaxone, at $52.21, is the least expensive of these antibiotic regimens in terms of total daily process cost, followed by the remaining cephalosporins at $53.29 to $94.57, aminoglycoside once-daily combinations at $93.44 to $99.65, and aminoglycoside multidose combinations at $103.26 to $111.42, respectively (values are given in constant 1995 Canadian dollars). Once-daily ceftriaxone offers the potential for cost savings compared with other antibiotic regimens whose pharmacokinetics require multiple daily doses, due largely to the reduced resources required for ceftriaxone preparation and administration.

Choosing cephalosporins for the formulary

Proper selection of antibiotics for any formulary requires cooperation and communication with members of the pharmacy and therapeutics (P&T) committee and departments of infectious diseases, microbiology, and epidemiology. Ideally, P&T members need to know their institution's incidence and sources of infection (hospital- or community-acquired), antibiotic cure rates, and the adverse effects profiles of antimicrobial agents used. In addition, members need to be briefed on the findings of antibiotic utilization reviews performed. In vitro activity should be compared among similar classes of antibiotics and, when possible, local resistance rates should be considered. The pharmacokinetic properties of an antibiotic, such as peak concentration, protein binding, distribution characteristics, and elimination half-life, should be evaluated during the selection process, as should total cost of the drug, including costs associated with monitoring therapy. The three generations of cephalosporins are used as examples here to delineate differences in potency, pharmacokinetics, efficacy, and potential for causing adverse reactions during the ideal formulary evaluation process.

Temocillin efficacy in experimental Klebsiella pneumoniae meningitis after infusion into rabbit plasma to simulate antibiotic concentrations in human serum

An infusion system was developed to simulate in the plasma of rabbits the concentrations of temocillin in human serum measured after administration of a 2-g intravenous bolus dose. The efficacy of therapy with this infusion against experimental Klebsiella pneumoniae meningitis was compared with that of a conventional bolus dose to the animals. The marked difference between the elimination half-life (t1/2) of temocillin in rabbit plasma and human serum (0.3 and 5 h, respectively) was reflected in concentrations in cerebrospinal fluid (CSF). The mean peak concentration after infusion occurred 3.5 h later than after bolus dosing, and levels were more prolonged (t1/2 in CSF was 6.3 h compared with 0.83 h following the bolus dose). After infusion, the mean viable count in CSF decreased by 4 log10 CFU/ml, whereas the bolus dose was ineffective because of the rapid fall to subinhibitory concentrations. These results suggest that the infusion system used is valuable for experimental studies with antibacterial agents whose elimination kinetics differ markedly between animals and humans.

Cephalosporin, carbapenem, and monobactam antibiotics

Cephalosporin and related antibiotics are highly effective bactericidal agents of relatively low toxicity. The spectrum of activity varies with the drug but is usually broad. The first-generation cephalosporins, and especially cefazolin, are most active against sensitive staphylococci and streptococci. Most second-generation (except cefoxitin) and third-generation cephalosporins show substantial activity against Haemophilus influenzae. All cephalosporins (except cefsulodin) are active against Klebsiella, Escherichia coli, and Proteus mirabilis, whereas only the third-generation agents have pronounced activity against the other Enterobacteriaceae. Imipenem (a carbapenem) is active against essentially all pathogenic organisms, but aztreonam (a monobactam) is active against only aerobic gram-negative bacilli. Advantages associated with some of the new cephalosporins are once-daily administration and high cerebrospinal fluid levels. With the development of new cephalosporins, however, new toxicities have become apparent, and superinfections and induction of resistance have become greater problems. The cephalosporins are among the most expensive antibiotics in use today; thus, use of these expensive agents must be justified by lower toxicity, greater efficacy, or both in comparison with drugs of more reasonable cost.

Pharmacokinetics of cefotaxime and its desacetyl metabolite in plasma and in cerebrospinal fluid

The aim of the study was to investigate the pharmacokinetic modelling of Cefotaxime (CTX) and its main metabolite Desacetyl Cefotaxime (DCTX) which has a less antibacterial activity than the CTX. After intravenous administration of 1g of CTX to 26 patients, the plasma concentrations determined by HPLC showed that the pharmacokinetics of CTX and transformation to DCTX can be described with an open five-compartment model. The implications of this are discussed from the clinical point of view.

Effect of broad-spectrum parenteral antibiotics on composition of intestinal microflora of humans

We compared the effects of four beta-lactam drugs with widely differing antibacterial and pharmacological properties on the composition of the intestinal flora. Cefoxitin, piperacillin, cefoperazone, and aztreonam were given intravenously for 9 days to healthy volunteers. Cefoperazone reduced the numbers of aerobic and anaerobic bacteria to undetectable levels. At the other extreme, cefoxitin had little effect on the normal flora. Aztreonam markedly reduced the numbers of aerobes, whereas piperacillin had a variable effect on both aerobic and anaerobic bacteria. There was extensive overgrowth of enterococci in subjects given cefoxitin or aztreonam, which have little activity against this species, and of yeasts in subjects given cefoperazone or piperacillin. Cefoperazone reached concentrations of 2,727 to 8,840 micrograms/g in the feces, whereas the other agents were generally undetectable. These results show that the new beta-lactam antibiotics produce widely varying effects on the fecal microflora after parenteral administration and that these effects are consistent with the antibacterial and pharmacological properties of the drugs.

Dosage adjustment for ceftazidime in patients with impaired renal function

Ceftazidime has good antibacterial activity against many Gram-negative micro-organisms including Ps. aeruginosa. The aim of the present study was to calculate a dosage adjustment regimen for renal failure patients and to test it in a second group of patients. A study was made of the pharmacokinetics of ceftazidime 1 g given as a single bolus i.v. injection in 20 patients in an intensive care unit with varying degrees of renal function, including patients on regular haemodialysis. The serum half-life of elimination (t1/2 beta) varied from 1.6 to 45 h depending on renal function. During haemodialysis the mean t1/2 was 4.7 h. A good correlation between the renal clearance of creatinine and ceftazidime was observed. In most patients protein binding was lower than previously observed. From the pharmacokinetic data, a dosage adjustment regimen for patients with renal insufficiency was calculated, which studies in 7 further patients showed to be effective.