Imipenem/Cilastatin : A Reappraisal of its Antibacterial Activity, Pharmacokinetic Properties and Therapeutic Efficacy

Micaela M Buckley, Rex N Brogden, Lee B Barradell, Karen L Goa
Drugs 1992, 44 (3): 408-444

SYNOPSIS: Imipenem is an antibacterial agent of the carbapenem class of β-lactams, with a very broad spectrum of activity that includes most Gram-negative and Gram-positive pathogens, aerobes and anaerobes, and with marked activity against species producing β-lactamases. It is coadministered with cilastatin, a renal dehydropeptidase inhibitor that prevents renal metabolism of imipenem. As initial monotherapy, imipenem/cilastatin provides effective and well-tolerated treatment of moderate to severe infections in various body systems, including intr-abdominal, obstetric and gynaecological, lower respiratory tract, skin and soft tissue, and urinary tract infections, and also in bacteraemia and septicaemia, and in patients with malignancy-related febrile neutropenia. It is likely to be of particular benefit in cases where bacterial pathogens have not yet been identified, such as in the treatment of serious infections in immunocompromised patients, or in an intensive care setting. Thus, imipenem/cilastatin is effective as initial monotherapy of a variety of infections, including infections in neutropenic patients, with a clear role in empirical treatment of mixed infection.

ANTIBACTERIAL ACTIVITY: Data published since the earlier review in the Journal confirm the in vitro activity of imipenem against a wide range of Enterobacteriaceae and other Gram-negative aerobic bacteria (except Xanthomonas maltophilia and Pseudomonas cepacia), Gram-positive aerobic bacteria and anaerobic microorganisms. Imipenem is generally more active than third-generation cephalosporins against Enterobacter cloacae and Citrobacter freundii and of similar activity to these antibacterial agents against other Enterobacteriaceae, but is generally less active than ciprofloxacin against Escherichia coli, Klebsieila pneumoniae, E. cloacae and Serratia marcescens. Recent studies confirm the excellent activity of imipenem against methicillin-susceptible strains of commonly isolated Gram-positive aerobic bacteria as well as less common strains such as Listeria monocytogenes and Rhodococcus equi. The activity of imipenem against the Bacteroides fragilis group, other Bacteroides species and Clostridium species is comparable with that of metronidazole and the investigational drug meropenem. The minimum bactericidal concentration in vitro of imipenem against Enterobacteriaceae, Bacteroides species and most staphylococci and streptococci is equal to or within one dilution of the minimum inhibitory concentration (MIC). The drug has a postantibiotic effect lasting 3 hours (mean). The in vitro activity of imipenem is not significantly influenced by an increase in inoculum size to 10(6) colony forming units, but the drug is inactivated in thioglycolate broth. The activity of imipenem against coagulase negative staphylococci, like that of cefuroxime and ciprofloxacin, is reduced in used peritoneal dialysis fluid and when these bacteria adhere to silicone rubber. Recent studies confirm the synergistic effect of the imipenem/amikacin combination against imipenem-susceptible Pseudomonas aeruginosa but show variable results when imipenem is combined with fluoroquinolones. Two studies have observed synergistic activity of imipenem and cefotiam, cefazolin or cefpiramide against a large proportion of tested strains of methicillin-re-sistant Staphylococcus aureus. Although a strong inducer of class I β-lactamases, imipenem is not hydrolysed by these enzymes and has remained active against a wide range of β-lactamase- and non-β-lactamase-producing bacteria; studies conducted in many countries found that over 90% of large numbers of Gram-negative clinical isolates remained susceptible to imipenem 4 mg/L. A consistent finding was the general lack of cross-resistance between imipenem and other β-lactam antibiotics. Mutational loss of susceptibility of P. aeruginosa to imipenem is apparently due to loss of a specific outer membrane porin that facilitates entry of imipenem but not other agents. The earlier reported efficacy of imipenem in eradicating experimental infection in animal models was confirmed in more recent studies. Fractional administration of imipenem to neutropenic mice, simulating treatment with 500mg 12-hourly intramuscularly or 6-hourly intravenously in humans, showed similar bactericidal efficacy with both routes, and indicated that in this model the duration that plasma antibiotic concentrations exceeded MIC was more important in determining efficacy than the magnitude of peak concentration above MIC. Imipenem and cefoxitin were generally more effective than cefotetan, ceftizoxime and ampicillin/sulbactam in reducing numbers of E. coli and B. fragilis group organisms in mice with subcutaneous abscesses. A murine peritonitis model, designed to detect and quantify resistance after short term antimicrobial therapy, suggested that resistance to third-generation cephalosporins, pefloxacin and ciprofloxacin developed more readily than to imipenem in tested Enterobacteriaceae and P. aeruginosa.

PHARMACOKINETIC PROPERTIES: In clinical practice, imipenem is administered with an equal amount of cilastatin to reduce the postexcretory metabolism of imipenem by dehydropeptidase-I, which is localised in the brush border microvilli of the proximal tubule. An additional reason for coadministration of cilastatin is to obviate nephrotoxicity observed in some animal species when imipenem is administered alone. A 1: 1 combination of imipenem/cilastatin is required to maintain inhibition of renal metabolism for more than 8 hours. Accumulation of cilastatin has been observed in patients with severe renal impairment and in neonates, but has not been associated with adverse sequelae. Single dose intravenous infusion of imipenem/cilastatin 500 or 1000mg to healthy volunteers results in mean maximum plasma concentrations of 30 to 35 mg/L and 60 to 70 mg/L, respectively, which decline to 0.5 and 2 mg/L, respectively, 4 to 6 hours later. Following intramuscular administration of 500 or 750mg mean maximum imipenem plasma concentrations were 7.4 to 10.4 and 8.2 to 11.7 mg/L, respectively, and remained above 0.4 mg/L for 12 hours. Administration of imipenem 500mg intramuscularly provided mean plasma concentrations above 4 and 2 mg/L for a time period equivalent to that following intravenous injection of the same dose 8-hourly. Steady-state volume of distribution following intravenous infusion was 14.4L and protein binding of imipenem was reported as < 10 to 20%. One to 2 hours after intravenous administration of imipenem/cilastatin 500mg, concentrations of imipenem in sputum, tonsillar tissue, prostatic tissue, female genital organs and synovial fluid exceed MICs for most susceptible Gram-negative and Gram-positive aerobic bacteria. Such concentrations are also achieved in surgical specimens of colon, peritoneum, intestinal mucosa, and abdominal muscle and in bile obtained by T-tube drainage. In patients with inflamed meninges, concentrations in cerebrospinal fluid ranged from 1.1 to 2.3 mg/L following intravenous administration of imipenem/cilastatin 1000mg. The elimination half-life of both imipenem and cilastatin is about 1 hour after intravenous administration, but increases to 2.5 to 3.8 and 2.6 to 5.1 hours after intramuscular administration of 500 and 750mg, respectively. The proportion of a dose excreted in the urine is similar after either route; however, peak urinary concentrations are lower after intramuscular injection. In patients with severely impaired renal function, the elimination half-life of cilastatin is prolonged (to between 7 and 16 hours), to a greater degree than that of imipenem (around 3 hours); in premature infants, half-lives are prolonged to 2.5 and 9.1 hours, respectively. As might be expected, the pharmacokinetic profile of imipenem/cilastatin in elderly volunteers is similar to that in young patients with mild renal impairment.

THERAPEUTIC USE: Imipenem/cilastatin has been shown to be efficacious in controlled trials in patients with infections of various body systems. In randomised nonblind trials in intra-abdominal infections, intravenous imipenem/cilastatin was as efficacious as either triple therapy (an aminoglycoside plus a penicillin plus metronidazole or clindamycin), or a 'third-generation' cephalosporin plus metronidazole, whether clinical or bacteriological efficacy was considered. In one large comparison in which patients were stratified for severity of infection, imipenem/cilastatin was significantly superior to tobramycin plus clindamycin. Intravenous imipenem/cilastatin has produced clinical efficacy rates of between 81 and 100% in the treatment of obstetric and gynaecological infections, and a rate of clinical success of 89.7% has now been reported in one trial of intramuscular imipenem/cilastatin in such patients. In severe lower respiratory tract infection, addition of an aminoglycoside has been recommended to reduce the likelihood of development of resistance to imipenem associated with P. aeruginosa infection. Using such a strategy, clinical cure and/or improvement was apparent in 72 to 100% of patients with community-acquired infection treated intravenously and in 95% of those treated intramuscularly. In patients with hospital-acquired lower respiratory tract infections, imipenem/cilastatin was of comparable clinical efficacy to cefotaxime plus amikacin plus pefloxacin. Clinical efficacy rates for imipenem/cilastatin in skin and soft tissue infection approximate 95%, with bacteriological cure in 86%, these findings for intravenous administration being sustained in one trial of intramuscular use. Intramuscular imipenem/cilastatin also has demonstrated efficacy in urinary tract infection (UTI), overall efficacy of 76% being obtained in one study in complicated UTI. Clinical cure was obtained in 82% of patients with septicaemia treated with imipenem/cilastatin in a further trial. In recent clinical research, imipenem/cilastatin has been shown to provide effective treatment of infections in neutropenic patients with malignant disease. Controlled comparative trials indicate that imipenem/cilastatin is more effective as initial monotherapy in febrile neutropenic patients than ceftazidime, or indeed, cefoperazone or ceftazidime plus piperacillin. In treatment of severe infections in patients in intensive care, rates of clinical cure with intravenous imipenem/ cilastatin have been reported variously as 77 and 83%, the latter figure being the same as that obtained with triple therapy with amoxicillin plus clindamycin plus netilmicin or tobramycin. In a small number of patients, resistance of P. aeruginosa to imipenem developed despite use of an aminoglycoside. Intravenous imipenem/cilastatin has also been used successfully in paediatric patients with cancer, as well as in trials in paediatric patients with infections of various body systems, where clinical efficacy rates approach 100%.

TOLERABILITY: Imipenem/cilastatin is in general well tolerated in clinical trials. The most frequent adverse clinical events reported, in common with other β-lactam antibacterials, are nausea, vomiting and diarrhoea, skin rashes, phlebitis at the injection site, transient elevation of liver function test results, and eosinophilia. Seizures have occurred in patients with CNS dysfunction and renal failure, in conjunction with unadjusted dosage regimens. The risk of seizure increases sharply with increasing dosages (> 2 g/day), emphasising the necessity for careful dosage adjustment in patients with impaired renal function. The intramuscular formulation provides the advantage of avoiding some adverse events, such as phlebitis and nausea, associated with intravenous administration.

DOSAGE AND ADMINISTRATION: In adults, the usual intravenous dosage of imipenem/cilastatin is 500 to 750mg, administered 6-hourly, depending on the severity of infection; a daily dosage of 2g is most frequently employed. Intramuscular dosages of between 500 and 750mg 8- to 12-hourly have been effective in treatment of infections of various body systems, including intra-abdominal, obstetric/gynaecological, urinary tract, and skin and soft tissue infections. Intravenous dosages of 60 mg/kg/day in patients weighing 40kg or less, and adult doses in patients heavier than this, appear to be effective and well tolerated in paediatric patients. The dosage of imipenem/cilastatin should be reduced in patients with renal impairment.

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