Meropenem: evidence of lack of proconvulsive tendency in mice

Meropenem and Vaborbactam: Stepping up the Battle against Carbapenem-resistant Enterobacteriaceae

Vaborbactam (VAB; formerly RPX7009) is a novel beta-lactamase inhibitor based on a cyclic boronic acid pharmacophore with potent inhibitory activity against Ambler class A and C beta-lactamases. It has been co-formulated with meropenem to restore its activity against Klebsiella pneumoniae carbapenemases (KPC). VAB does not inhibit class B or D carbapenemases, nor does it improve the activity of meropenem against multidrug-resistant nonfermenting gram-negative bacilli, notably Acinetobacter spp. and Pseudomonas aeruginosa. The purpose of this article is to review existing data pertaining to the biochemistry, mechanism of action, pharmacokinetics/pharmacodynamics, in vitro activity, and current progress in clinical trials of meropenem and VAB (MV). Phase 1 studies have demonstrated single and multiple doses of VAB up to 2000 mg, alone or in combination with meropenem 2000 mg administered as a prolonged infusion over 3 hours, are well tolerated with an adverse effect profile similar to that of meropenem monotherapy. The available data suggest preexisting resistance among KPC-producing isolates is rare. Strains with elevated MICs have been characterized by multiple resistance determinants including porin defects, increased drug efflux, and increased blaKPC expression. It remains uncertain whether multifactorial resistance will emerge during MV treatment and with more widespread use. Early data are positive for complicated urinary tract infections and MV compared with best available therapy in patients with serious carbapenem-resistant Enterobacteriaciae (CRE) infections. As clinicians contemplate how to incorporate MV into CRE treatment strategies, it will be important to track and understand resistance, discern the role, if any, of combination therapy in enhancing efficacy and/or preserving activity, and define the specific therapeutic niche of MV among the expanding anti-CRE armamentarium.

Imipenem and meropenem: Comparison of in vitro activity, pharmacokinetics, clinical trials and adverse effects

OBJECTIVE

To compare and contrast imipenem and meropenem in terms of in vitro activity, pharmacokinetics, clinical efficacy and adverse effects.

DATA SELECTION

MEDLINE search from 1975 to 1997 and follow-up of references.

DATA EXTRACTION

Clinical trials comparing imipenem with meropenem, or either imipenem or meropenem with standard therapy in the treatment of serious infections were selected.

DATA SYNTHESIS

Imipenem, the first carbapenem, was first marketed in 1987; meropenem was introduced to the market in 1996. In general, imipenem is more active against Gram-positive cocci while meropenem is more active against Gram-negative bacilli. The agents display similar pharmacokinetics. Clinical studies in patients with serious infections (intra-abdominal infection, respiratory infection, septicemia, febrile neutropenia) report similar bacteriological and clinical cure rates with imipenem and meropenem. Meropenem is approved for the treatment of bacterial meningitis, whereas imipenem is not. Adverse effects are similar.

CONCLUSIONS

Current literature supports the use of imipenem at a dose of 500 mg every 6 h and meropenem at 1 g every 8 h for the treatment of severe infections. For the treatment of serious infections, imipenem (500 mg every 6 h or 2 g/day [$98/day]) is more economical than meropenem (1 g every 8 h or 3 g/day [$142/day]) based on acquisition cost.

The role of carbapenems in the treatment of severe nosocomial respiratory tract infections

The prevalence of antibiotic-resistant bacteria continues to increase, particularly in patients in the intensive care unit with nosocomial pneumonia. The intention of this review is to provide an overview of severe nosocomial pneumonia, carbapenems and the problem of bacterial resistance to antimicrobial agents. Attention was focused on the efficacy, safety and pharmacodynamics of imipenem, meropenem, ertapenem and doripenem. Issues on the impact of appropriate empiric antibiotic therapy for nosocomial pneumonia patients considered at risk for resistant pathogens are discussed. Critical decision making regarding the use of carbapenems for treating severe nosocomial pneumonia requires careful consideration of the four Ds of optimal antimicrobial therapy: right Drug, right Dose, De-escalated to pathogen-directed therapy and right Duration of therapy.

Absence of convulsive liability of doripenem, a new carbapenem antibiotic, in comparison with beta-lactam antibiotics

beta-Lactam antibiotics have been suggested to have some degree of convulsive activity and neurotoxicity in experimental animals as well as in clinical situations. We examined the convulsive activities of a new carbapenem antibiotic, (+)-(4R,5S,6S)-6-[(1R)-1-hydroxyethyl]-4-methyl-7-oxo-3-[[(3S,5S)-5-[(sulfamoylamino)methyl]-3-pyrrolidinyl]thio]-1-azabicyclo[3.2.0]hept-2-ene-2-carboxic acid monohydrate (doripenem) using several animals and compared them with beta-lactam antibiotics. In intravenous (IV) injection studies, imipenem/cilastatin, at 400/400mg/kg produced seizure discharges on electroencephalogram (EEG) accompanied with clonic convulsions in rats. Meropenem showed only wet dog shaking behavior at 200 and 400mg/kg. Doripenem caused no changes in the EEG and behavior in rats at 400mg/kg. Imipenem/cilastatin IV potentiated the pentylenetetrazol (PTZ)-induced convulsions in mice at 250/250 mg/kg, while meropenem, panipenem/betamipron, cefazolin or doripenem did not cause any marked effects at up to 500 mg/kg. In mouse intracerebroventricular (ICV) injection studies, imipenem, panipenem and cefazolin induced clonic convulsions in a dose-dependent manner in mice. Doripenem and meropenem did not induce convulsions at up to 100 microg/mouse. In dog ICV injection studies, imipenem produced generalized seizure discharge with clonic convulsions at 100 microg/dog. Meropenem also produced spikes or seizure discharges at 100, 300 and 1,000 microg/dog. However, doripenem had no effects on the EEG and behavior in dogs at any doses. In in vitro binding studies, imipenem, panipenem, cefazolin and meropenem inhibited [(3)H]muscimol binding to the GABA(A) receptor in mouse brain homogenates while doripenem did not cause any inhibition at up to 10mM. In addition, doripenem had no influence on the anti-convulsant actions of valproic acid in the PTZ- or bicuculine-induced convulsive model. These results clearly indicate that doripenem has no convulsive activity, suggesting that its neurotoxicity may be negligible in clinical use.

Redefining penems

The antimicrobial class of penems has the potential to address most of the relevant resistance issues associated with beta-lactam antibiotics because of their exceptionally broad spectrum of antibacterial activity and their intrinsic stability against hydrolytic attack by many beta-lactamases including ESBL and AmpC enzymes. The subclass of carbapenems covers the spectrum of hospital pathogens whereas the subclass of penems covers community pathogens. The only currently available penem, faropenem, has a low propensity for resistance development, beta-lactamase induction and selection of carbapenem-resistant Pseudomonas aeruginosa. This makes it attractive for the treatment of community-acquired infections and for step-down or sequential therapy following carbapenem treatment without jeopardizing the activity of carbapenems or the entire beta-lactam class in the hospital environment.

Plasma pharmacokinetics and tissue fluid concentrations of meropenem after intravenous and subcutaneous administration in dogs

OBJECTIVE

To estimate pharmacokinetic variables and measure tissue fluid concentrations of meropenem after IV and SC administration in dogs.

ANIMALS

6 healthy adult dogs.

PROCEDURE

Dogs were administered a single dose of meropenem (20 mg/kg) IV and SC in a crossover design. To characterize the distribution of meropenem in dogs and to evaluate a unique tissue fluid collection method, an in vivo ultrafiltration device was used to collect interstitial fluid. Plasma, tissue fluid, and urine samples were analyzed by use of high-performance liquid chromatography. Protein binding was determined by use of an ultrafiltration device.

RESULTS

Plasma data were analyzed by compartmental and noncompartmental pharmacokinetic methods. Mean +/- SD values for half-life, volume of distribution, and clearance after IV administration for plasma samples were 0.67 +/- 0.07 hours, 0.372 +/- 0.053 L/kg, and 6.53 +/- 1.51 mL/min/kg, respectively, and half-life for tissue fluid samples was 1.15 +/- 0.57 hours. Half-life after SC administration was 0.98 +/- 0.21 and 1.31 +/- 0.54 hours for plasma and tissue fluid, respectively. Protein binding was 11.87%, and bioavailability after SC administration was 84%.

CONCLUSIONS AND CLINICAL RELEVANCE

Analysis of our data revealed that tissue fluid and plasma (unbound fraction) concentrations were similar. Because of the kinetic similarity of meropenem in the extravascular and vascular spaces, tissue fluid concentrations can be predicted from plasma concentrations. We concluded that a dosage of 8 mg/kg, SC, every 12 hours would achieve adequate tissue fluid and urine concentrations for susceptible bacteria with a minimum inhibitory concentration of 0.12 microg/mL.

Pharmacokinetic-pharmacodynamic modeling of the electroencephalogram effect of imipenem in healthy rats

A pharmacokinetic-pharmacodynamic (PK-PD) modeling approach was developed to investigate the epileptogenic activity of imipenem in rats. Initially, animals received an intravenous infusion of imipenem at a rate of 2.65 mg min(-1) for 30 min. Blood samples were collected for drug assay, and an electroencephalogram (EEG) was recorded during infusion and postinfusion. A dramatic delay was observed between concentrations of imipenem in serum and the EEG effect; this effect was accompanied by tremors and partial seizures. Indirect-effect models failed to describe these data, which were successfully fitted using an effect compartment model. The relationship between effect and concentration at the effect site was best described by a spline function. The elimination rate constant from the effect compartment was severalfold lower than that from the central compartment. The robustness of the model was then confirmed after administering the imipenem dose over 60 and 90 min. In conclusion, the successful PK-PD modeling of the imipenem EEG effect in rats constitutes a major improvement for better prediction of the epileptogenic risk associated with this antibiotic.

Meropenem versus imipenem/cilastatin as empirical monotherapy for serious bacterial infections in the intensive care unit

OBJECTIVE

To compare the efficacy and tolerability of meropenem and imipenem/cilastatin as empirical monotherapy in intensive care unit (ICU) patients with serious bacterial infections.

METHODS

A multicenter, open-label, randomized, parallel-group trial was conducted in Belgium, evaluating empirical monotherapy with meropenem or imipenem/cilastatin (both 1 g/8 h intravenously) in ICU patients with one or more of the following infections caused by sensitive pathogens: lower respiratory tract infection (LRTI) in ventilated patients, intra-abdominal infection or sepsis.

RESULTS

The overall satisfactory clinical response rate at the end of randomized treatment was 77.0% (67/87) with meropenem and 68.1% (62/91) with imipenem/cilastatin (difference 8.9%; 95% confidence interval -4.2% to 21.9%; P = 0.185). The two drugs produced similar satisfactory clinical response rates against LRTIs: 68.3% (41/60) with meropenem versus 68.6% (35/51) with imipenem/cilastatin. Meropenem appeared to be slightly more effective against intra-abdominal infections: 95.5% (21/22) versus 76.7% (23/30), respectively. All five meropenem recipients with sepsis had a satisfactory clinical response, compared to 40.0% (4/10) of those who received imipenem/cilastatin. The overall satisfactory bacteriologic response rate was 67.1% (49/73) with meropenem and 60.3% (44/73) with imipenem/cilastatin (difference 6.9%; 95% confidence interval -8.7% to 22.4%; P = 0.389). The predominant pathogens were Escherichia coli, Enterobacter spp. and Pseudomonas aeruginosa. No incidences of drug-related nausea and vomiting were reported, but one probable drug-related seizure occurred in the imipenem/cilastatin group.

CONCLUSIONS

Meropenem is at least as efficacious (clinically and bacteriologically) as imipenem/cilastatin for the empirical monotherapy of serious bacterial infections in ICU patients, and it can therefore be considered a useful option in this setting. Moreover, meropenem is well tolerated and offers several potential advantages, including greater in vitro activity against Gram-negative pathogens and the option of bolus administration.

Low convulsive activity of a new carbapenem antibiotic, DK-35C, as compared with existing congeners

Since carbapenems and cephalosporins have been suggested to induce convulsive side effects through an inhibitory action on the central gamma-aminobutyric acid (GABA)-mediated inhibitory transmission, the present study evaluated the convulsive activity of a new carbapenem antibiotic (1R,5S,6S)-6[(R)-1-hydroxyethyl]-2-[(3S,5S)-5(S-methyl-4thiomorpholin ylcarbonyl)pyrrolidin-3-thio]-l-methylcarbapen-2-em-3- carboxylic acid (DK-35C) in in vitro and in vivo experiments, in comparison with cefazolin, imipenem and meropenem. In in vitro experiments, their abilities to inhibit [3H]muscimol (5 nM) binding to GABA(A) receptors were measured using crude synaptic membranes prepared from the rat cerebral cortex. The concentrations (mM) of the antibiotics which inhibit 50% of the specific binding, were 0.6 for imipenem, 1.8 for cefazolin, 15.4 for DK-35C and 27.6 for meropenem. In in vivo experiments, intracerebroventricular (i.c.v.) injections of cefazolin, imipenem and DK-35C induced convulsions in a dose-dependent manner in rats. The doses (nmol/rat) of the antibiotics which induce convulsions in 50% of rats, were 57 for imipenem, 96 for cefazolin, 377 for DK-35C and >3000 for meropenem. In the mouse pentylenetetrazole (PTZ) convulsive model, intravenous pretreatment with cefazolin (800 mg/kg) or imipenem (200 mg/kg) shifted the dose-response curve of PTZ (i.p.) to the left, indicating enhancement of the convulsive activity of PTZ. However, pretreatment with cefazolin, meropenem or DK-35C at a dose of 400 mg/kg did not produce any marked effects on the convulsive activity of PTZ compared with the saline vehicle-pretreated control. The results clearly demonstrate a good correlation between in vitro GABA(A) receptor binding assay and in vivo i.c.v. convulsive model using rats, and suggest that DK-35C may possess a relatively weak convulsive activity mediated through an interaction with GABA(A) receptors.

Carbapenems and monobactams: imipenem, meropenem, and aztreonam

Imipenem and meropenem, members of the carbapenem class of beta-lactam antibiotics, are among the most broadly active antibiotics available for systemic use in humans. They are active against streptococci, methicillin-sensitive staphylococci, Neisseria, Haemophilus, anaerobes, and the common aerobic gram-negative nosocomial pathogens including Pseudomonas. Resistance to imipenem and meropenem may emerge during treatment of P. aeruginosa infections, as has occurred with other beta-lactam agents; Stenotrophomonas maltophilia is typically resistant to both imipenem and meropenem. Like the penicillins, the carbapenems have inhibitory activity against enterococci. In general, the in vitro activity of imipenem against aerobic gram-positive cocci is somewhat greater than that of meropenem, whereas the in vitro activity of meropenem against aerobic gram-negative bacilli is somewhat greater than that of imipenem. Daily dosages may range from 0.5 to 1 g every 6 to 8 hours in patients with normal renal function; the daily dose of meropenem, however, can be safely increased to 6 g. Infusion-related nausea and vomiting, as well as seizures, which have been the main toxic effects of imipenem, occur no more frequently during treatment with meropenem than during treatment with other beta-lactam antibiotics. The carbapenems should be considered for treatment of mixed bacterial infections and aerobic gram-negative bacteria that are not susceptible to other beta-lactam agents. Indiscriminate use of these drugs will promote resistance to them. Aztreonam, the first marketed monobactam, has activity against most aerobic gram-negative bacilli including P. aeruginosa. The drug is not nephrotoxic, is weakly immunogenic, and has not been associated with disorders of coagulation. Aztreonam may be administered intramuscularly or intravenously; the primary route of elimination is urinary excretion. In patients with normal renal function, the recommended dosing interval is every 8 hours. Patients with renal impairment require dosage adjustment. Aztreonam is used primarily as an alternative to aminoglycosides and for the treatment of aerobic gram-negative infections. It is often used in combination therapy for mixed aerobic and anaerobic infections. Approved indications for its use include infections of the urinary tract or lower respiratory tract, intra-abdominal and gynecologic infections, septicemia, and cutaneous infections caused by susceptible organisms. Concurrent initial therapy with other antimicrobial agents is recommended before the causative organism has been determined in patients who are seriously ill or at risk for gram-positive or anaerobic infection.

Synthesis and biological properties of new 1 beta-methylcarbapenems

The synthesis and biological activity of the novel series of 1 beta-methylcarbapenems, 1 and 2 were described. Most compounds displayed high potent antibacterial activity. The best compound in this series, 2a (IH201; R2 = NH2) showed an excellent and a broad spectrum as well as high renal DHP-I stability. It also possessed good in vivo efficacy and high safety.

Prospective, randomised, multicentre study of meropenem versus imipenem/cilastatin as empiric monotherapy in severe nosocomial infections

The clinical and bacteriological efficacy and the tolerability of meropenem versus imipenem/cilastatin (both 1 g t.i.d.) in severe nosocomial infections were compared in a multicentre, randomised, nonblinded study. A total of 151 patients were recruited; 133 (66 meropenem, 67 imipenem/cilastatin) were clinically evaluable and 84 (42 meropenem, 42 imipenem/cilastatin) bacteriologically evaluable. Most clinically evaluable patients (90%) were in intensive care units, required mechanical ventilation (72%), and had received previous antibiotic therapy (62%). The mean (+/- SD) APACHE II score was 15.2 (+/- 6.6) in the meropenem group and 17.8 (+/- 6.8) in the imipenem/cilastatin group. The primary infections were nosocomial lower respiratory tract infections (56% of patients), intra-abdominal infections (15%), septicaemia (21%), skin/skin structure infections (5%), and complicated urinary tract infections (3%); 35% of the patients had two or more infections. There was no significant difference between the meropenem and imipenem/cilastatin groups in the rates of satisfactory clinical (weighted percentage 87% vs. 74%) or bacteriological (weighted percentage 79% vs. 71%) response. There was a slightly higher rate of clinical success with meropenem against primary or secondary lower respiratory tract infection (89% vs. 76%). Drug-related adverse events occurred in 17% and 15% of meropenem and imipenem/cilastatin patients, respectively. Meropenem (1 g t.i.d.) was as efficacious as the same dose of imipenem/cilastatin in this setting, and both drugs were well tolerated.

Empiric treatment of hospital-acquired lower respiratory tract infections with meropenem or ceftazidime with tobramycin: a randomized study. Meropenem Lower Respiratory Infection Group

OBJECTIVE

To evaluate the efficacy and tolerability of intravenous empiric treatment with meropenem compared with ceftazidime-tobramycin in patients with hospital-acquired lower respiratory tract infections.

DESIGN

Prospective, nonblind, randomized trial.

SETTING

Multicenter trial conducted at 22 centers.

PATIENTS

Two hundred eleven patients were enrolled and 121 were evaluable for the analysis of both clinical and bacteriologic efficacy.

INTERVENTIONS

One hundred four patients were randomized to receive intravenous meropenem (1000 mg) every 8 hrs and 107 patients were randomized to receive intravenous ceftazidime (2000 mg) plus tobramycin (1 mg/kg) every 8 hrs. Sixty-three meropenem-treated patients and 58 ceftazidime-tobramycin-treated patients were eligible for the analysis of clinical and bacteriologic efficacy. In the ceftazidime-tobramycin group, 32 (55%) evaluable patients received more than six doses of tobramycin, 24 (41%) received six doses or fewer, and two (3%) did not receive any tobramycin.

MEASUREMENTS AND MAIN RESULTS

The analysis of efficacy was based on the clinical and bacteriologic responses at the end of treatment. Satisfactory clinical responses occurred in 56 (89%) of 63 of the meropenem-treated patients and in 42 (72%) of 58 of the ceftazidime-tobramycin-treated patients (p = .04). Corresponding bacteriologic response rates were 89% and 67%, respectively (p = .006). The frequency and profile of drug-related adverse events was similar across treatment groups. Seizures were reported in three meropenem-treated patients, but these seizures were considered by the investigator to be unrelated to treatment.

CONCLUSIONS

Meropenem is well tolerated and more efficacious than the combination of ceftazidime and tobramycin for the initial empiric treatment of hospital-acquired bacterial pneumonia.

Carbapenems

Carbapenems are broad-spectrum beta-lactam antibiotics that cover an absolute majority of all bacterial pathogens that possess a cell wall. The only clinically important exceptions are X. maltophilia, E. faecium, and some strains of methicillin-resistant staphylococci and penicillin-resistant pneumococci. So far, after several years of clinical use of imipenemcilastatin, emergence of resistance has been a problem mainly restricted to P. aeruginosa. The pharmacokinetics of carbapenems, especially imipenem, are complicated by the renal metabolism, necessitating the imipenem-cilastatin combination. This is not required for meropenem. The safety profile of carbapenems is favorable. With imipenem-cilastatin, nausea constitutes a practical problem in that administration times may have to be prolonged. The risk for neurologic reactions with imipenem-cilastatin has become a factor reducing the possibilities to use high doses. For all indications except bacterial meningitis, the now approved maximal dose of 4 g per day should suffice. In this respect, meropenem has an advantage over imipenem-cilastatin in that it can be used in the treatment of bacterial meningitis without apparent increased risk of seizures. Carbapenems are indicated mainly as empiric monotherapy in serious infections, such as intra-abdominal infections and infections in neutropenic patients. Combinations of carbapenems and other antibiotics should not be used routinely.

Randomized comparison of meropenem with cefotaxime for treatment of bacterial meningitis. Meropenem Meningitis Study Group

Broad-spectrum cephalosporins are drugs of choice for the treatment of meningitis in communities which can afford them. The emergence of cephalosporin-resistant pneumococci demands the clinical trial of alternate agents. Carbapenems are active against the bacteria causing meningitis, but the use of imipenem-cilastatin was frustrated by drug-associated seizures. The safety and efficacy of meropenem, a new carbapenem, were compared to those of cefotaxime in a prospective randomized trial of 190 children with bacterial meningitis. Seizures occurred within 24 h before antibiotic therapy in 16 of 98 patients (16%) randomized to receive meropenem and in 6 of 92 patients (7%) randomized to receive cefotaxime. In patients without seizures before therapy, seizures occurred during therapy in 5 of 82 patients (6%) receiving meropenem and in 1 of 86 patients (1%) receiving cefotaxime (95% confidence interval: -0.7%, 10.6%). None were thought to be drug related. Twenty-four meropenem-treated patients (24%) and 11 cefotaxime-treated patients (12%) had neurological abnormalities before therapy. In patients without pretherapy neurological abnormalities, these abnormalities were present after treatment in 4 of 74 meropenem-treated patients (5%) and in 2 of 81 cefotaxime-treated patients (2%) (95% confidence interval: -3.2%, 9.1%). Of 75 meropenem-treated and 64 cefotaxime-treated patients with pretherapy positive cerebrospinal-fluid cultures, 68 and 59, respectively, had repeat lumbar punctures. Bacterial eradication was found to be 100% in both groups. Our data suggest that meropenem may be a carbapenem agent that is well tolerated and effective in the treatment of bacterial meningitis.

Correlation between in vitro and in vivo models of proconvulsive activity with the carbapenem antibiotics, biapenem, imipenem/cilastatin and meropenem

The present study evaluated the proconvulsant liability of biapenem, a novel carbapenem antibiotic, in in vitro and in vivo experiments, in comparison with the carbapenems, imipenem/cilastatin and meropenem. Imipenem/cilastatin is a carbapenem antibiotic with known proconvulsive liability in man and in animal experiments. In in vivo studies imipenem/cilastatin, at doses of 400/400 mg/kg i.v., significantly lowered the convulsive threshold of pentylenetetrazol (PTZ) in mice and shifted the dose-response curve of PTZ. The effects of biapenem (400 mg/kg i.v.) and another reference carbapenem, meropenem (400 mg/kg i.v.), in the mouse PTZ model were not significantly different from control. In in vitro experiments the carbapenems were tested for their ability to inhibit [3H]muscimol (1.3 mM) binding to rat brain homogenates at concentrations of 1-10 mM. Similar to in vivo results, when compared to imipenem/cilastatin, biapenem and meropenem did not inhibit [3H]muscimol binding to the GABAA receptor complex in brain homogenates while imipenem/cilastatin exhibited significant inhibition (IC50 = 4.6 mM). These results further confirm the correlation between in vitro GABAA binding and in vivo PTZ convulsive testing with carbapenem antibiotics, and suggest that biapenem possesses a low proconvulsive liability.

Bacterial resistance to carbapenems

The carbapenems have the broadest spectra of all beta-lactams but resistance still occurs, caused by target modification, impermeability or beta-lactamase production. Target modification or replacement is important in methicillin-resistant staphylococci, E. faecium and some pneumococci. These organisms present the greatest current threat to carbapenem efficacy. Impermeability to carbapenems arises in P. aeruginosa mutants, where it is contingent on loss of D2 outer membrane protein, a minor porin. This resistance functions only if the Pseudomonas retains its chromosomal group 1 beta-lactamase, and so reflects the interplay of impermeability and hydrolysis rather than impermeability alone; nevertheless porin loss is the critical change that engenders resistance. Resistance based on impermeability coupled to a group 1 beta-lactamase has also been described in E. cloacae, but demands loss of a major porin and is much rarer and less stable than in P. aeruginosa. Although group 1 beta-lactamases contribute to resistance in these organisms, they have only feeble carbapenemase activity. Chromosomal beta-lactamases with potent carbapenemase activity occur in most or all X. maltophilia, A. hydrophila and F. odoratum isolates. These enzymes, which cause carbapenem resistance when expressed copiously, are all zinc-dependent. Zinc carbapenemases also are a concern in B. fragilis, where they are encoded by the chromosomal DNA of c. 3% of isolates, though expressed by only 1%. Carbapenemases are extremely rare outside these species. Nevertheless, a plasmidic zinc carbapenemase was reported from one P. aeruginosa isolate and from several S. marcescens. Further carbapenemases, some not zinc-dependent, are known from a tiny numbers of Serratia, Enterobacter, and Acinetobacter isolates. Despite these various modes of resistance, carbapenems have retained their efficacy far better than have expanded-spectrum cephalosporins. Whether this advantage will be retained indefinitely is uncertain. If resistance does become more prevalent it may be possible to derivatize the carbapenems so as to extend their activity. There is already interest in the design of carbapenems that bind beta-lactam-resistant PBPs and, to an extent, in the development of carbapenemase inhibitors.

Relationship between structure and convulsant properties of some beta-lactam antibiotics following intracerebroventricular microinjection in rats

The epileptogenic activities of several beta-lactam antibiotics were compared following their intracerebroventricular administration in rats. Different convulsant potencies were observed among the various beta-lactam antibiotics tested, but the epileptogenic patterns were similar. The patterns consisted of an initial phase characterized by wet-dog shakes followed by head tremor, nodding, and clonic convulsions. After the largest doses of beta-lactam antibiotics injected, clonus of all four limbs and/or the trunk, rearing, jumping, falling down, escape response, transient tonic-clonic seizures, and sometimes generalized seizures were observed, followed by a postictal period with a fatal outcome. At a dose of 0.033 mumol per rat, cefazolin was the most powerful epileptogenic compound among the drugs tested. It was approximately three times more potent than benzylpenicillin in generating a response and much more potent than other cephalosporins, such as ceftriaxone, cefoperazone, and cefamandole. No epileptogenic signs were observed with equimolar doses of cefotaxime, cefonicid, cefixime, and ceftizoxime in this model. The more convulsant compounds (i.e., cefazolin and ceftezole) are both characterized by the presence of a tetrazole nucleus at position 7 and show a marked chemical similarity to pentylenetetrazole. Imipenem and meropenem, the two carbapenems tested, also showed epileptogenic properties, but imipenem was more potent than meropenem, with a convulsant potency similar to those of ceftezole and benzylpenicillin. In addition, the monobactam aztreonam possessed convulsant properties more potent than those of cefoperazone and cefamandole. This suggest that the beta-lactam ring is a possible determinant of production of epileptogenic activity, with likely contributory factors in the substitutions at the 7-aminocephalosporanic or 6-aminopenicillanic acid that may increase or reduce the epileptogenic properties of the beta-lactam antibiotics. While the structure-activity relationship was also investigated, there seem to be no convincing correlations among the rank order of lipophilicities and the convulsant potencies of the compounds studied. The lack of marked convulsant properties of cefixime, cefonicid, cefuroxime, and cephradine suggests that these antibiotics may interact with a binding site which is different from that by which the beta-lactam antibiotics exert their convulsant effects or may demonstrate a reduced affinity for the relevant site(s).

Meropenem: a new carbapenem antimicrobial

OBJECTIVE

To describe and then compare an investigational carbapenem antibiotic, meropenem, with the only currently available antibiotic in this class, imipenem/cilastatin.

DATA IDENTIFICATION

An English language search using MEDLINE (1988-1993); Abstracts of the 31st Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC), 1991; and Abstracts of the 32nd ICAAC, 1992.

STUDY SELECTION

All current scientific publications were reviewed for study design and quality. Emphasis was placed on susceptibility and pharmacokinetic analysis. Phase 3 clinical trials are now being completed and have only been published in abstract form. Hence, conclusions derived regarding efficacy were tempered.

RESULTS

Meropenem is active against a broad spectrum of gram-positive and -negative pathogens including beta-lactamase producers. Meropenem appears to be two- to fourfold less active than imipenem against gram-positive organisms. Meropenem is two- to fivefold more active against enterobacteriaceae. The two compounds appear to be equally active against Pseudomonas aeruginosa. Pharmacokinetic disposition is also similar for imipenem and meropenem. Meropenem may exhibit greater tissue penetration. Meropenem is not labile to renal hydrolysis and can be administered without a competitive antagonist of dihydropeptidase, such as cilastatin. In clinical trials, meropenem appears to be as safe and effective as imipenem/cilastatin or ceftazidime in the treatment of infections involving soft tissue, urinary tract, upper respiratory tract, abdominal processes, and febrile neutropenic episodes.

CONCLUSIONS

Meropenem is comparable to imipenem in terms of in vitro susceptibility pattern and pharmacokinetic disposition. Overall, meropenem seems to offer promise as the second of the carbapenem class of antibiotics. Clinical data are preliminary, and further data are needed.

Meropenem versus imipenem/cilastatin in the treatment of intraabdominal infections requiring surgery

In a prospective randomized study meropenem was compared with imipenem/cilastatin in the treatment of 62 patients with intraabdominal infections requiring surgery. The patients were suffering from diffuse or local peritonitis of moderate severity complicating in most cases gangrenous appendicitis, stomach perforation or gallbladder disease. There were 30 patients in the meropenem group and 32 patients in the imipenem/cilastatin group. Both antibiotic regimens were given intravenously at a dosage of 1 g every 8 h for a mean duration of 7.7 days in the meropenem group versus 8.6 days in the imipenem/cilastatin group. Fifty-nine aerobic strains and 15 anaerobic strains were isolated from cultures of pus taken intraoperatively, the meropenem MICs ranging from < or = 0.25 to 2 micrograms/ml. At follow-up at least one month after treatment the outcome was considered successful in all of 27 evaluable patients given meropenem and in all of 29 evaluable patients given imipenem/cilastatin. Both antibiotic regimens were well tolerated.

Pharmacokinetics of meropenem in subjects with various degrees of renal impairment

Five healthy volunteers and 18 patients with various degrees of renal impairment received 500 mg of meropenem intravenously as a 30-min infusion. Five dialysis patients were dosed 2 h prior to hemodialysis, and four of them were also dosed between hemodialysis treatments. Plasma and urine samples were collected for up to 48 h and 12 h, respectively. Concentrations of meropenem and its open ring metabolite ICI 213,689 were determined by high-performance liquid chromatography and radioimmunoassay, respectively. The subjects were divided into four groups with glomerular filtration rates (GFR) of greater than 80, 30 to 80, 5 to 29, or less than 5 ml/min. There were linear correlations between the GFR and the rates for total plasma clearance as well as renal clearance of meropenem (group mean values for total clearance of 186, 74, 53, and 19 ml/min/1.73 m2, respectively). In subjects with normal renal function, nonrenal clearance accounted for approximately 20% of total elimination, increasing to about 50% in patients with GFR between 5 and 29 ml/min/1.73 m2. The terminal half-life of meropenem increased from 0.9 h in the healthy volunteers to 6.8 h in patients with end-stage renal disease. The half-life of ICI 213,689 was 2.31 h in the healthy volunteers and increased to 23.6 h in patients with GFR of 5 to 29 ml/min. In patients with end-stage renal disease, half-lives could not be measured, as concentrations were hardly declining during the 48-h observation period. The area under the concentration-time curve for meropenem increased more than 10-fold. Both meropenem and its open ring metabolite were readily dialyzable, with dialysis clearances of 79 and 81 ml/min/1.73 m2, respectively.