Pooled population pharmacokinetic model of imipenem in plasma and the lung epithelial lining fluid

AIMS

Several clinical trials have confirmed the therapeutic benefit of imipenem for treatment of lung infections. There is however no knowledge of the penetration of imipenem into the lung epithelial lining fluid (ELF), the site of action relevant for lung infections. Furthermore, although the plasma pharmacokinetics (PK) of imipenem has been widely studied, most studies have been based on selected patient groups. The aim of this analysis was to characterize imipenem plasma PK across populations and to quantify imipenem ELF penetration.

METHODS

A population model for imipenem plasma PK was developed using data obtained from healthy volunteers, elderly subjects and subjects with renal impairment, in order to identify predictors for inter-individual variability (IIV) of imipenem PK. Subsequently, a clinical study which measured plasma and ELF concentrations of imipenem was included in order to quantify lung penetration.

RESULTS

A two compartmental model best described the plasma PK of imipenem. Creatinine clearance and body weight were included as subject characteristics predictive for IIV on clearance. Typical estimates for clearance, central and peripheral volume, and inter-compartmental clearance were 11.5 l h(-1) , 9.37 l, 6.41 l, 13.7 l h(-1) , respectively (relative standard error (RSE) <8%). The distribution of imipenem into ELF was described using a time-independent penetration coefficient of 0.44 (RSE 14%).

CONCLUSION

The identified lung penetration coefficient confirms the clinical relevance of imipenem for treatment of lung infections, while the population PK model provided insights into predictors of IIV for imipenem PK and may be of relevance to support dose optimization in various subject groups.

Nonvolatile salt-free stabilizer for the quantification of polar imipenem and cilastatin in human plasma using hydrophilic interaction chromatography/quadrupole mass spectrometry with contamination sensitive off-axis electrospray

A hydrophilic interaction chromatography/mass spectrometry (HILIC-MS)-based assay for imipenem (IMP) and cilastatin (CIL) was recently reported. This orthogonal electrospray ion source-based (ORS) assay utilized nonvolatile salt (unremovable) to stabilize IMI in plasma. Unfortunately, this method was not applicable to conventional MS with off-axis spray (OAS-MS) because MS sensitivity was rapidly deteriorated by the nonvolatile salt. Therefore, we aimed to find a nonvolatile salt- and ion suppression-free approach to stabilize and measure the analytes in plasma using OAS-MS. Acetonitrile and methanol were tested to stabilize the analytes in the plasma samples. The recoveries, matrix effects and stabilities of the analytes in the stabilizer-treated samples were studied. The variations in MS signal intensities were used as the indicator of the assay ruggedness. The results show that a mixture of methanol and acetonitrile (1:1) is best for the storage and measurement of IMP and CIL in human plasma. Utilization of this precipitant not only blocked the hydrolysis of the analytes in plasma but also resulted in an ion suppression-free, fast (120 s per sample) and sensitive detection. The sensitivity obtained using the less sensitive OAS-MS (API3000, 4 pg on column) is much greater than that of the published ORS-MS-based assay (API4000, 77 pg on column). The ruggedness of the assay was demonstrated by its constant MS signal intensity. In conclusion, an improved HILIC/MS-based assay for IMP and CIL was established. The approach presented here provides a simple solution to the challenge of analyzing hydrolytically unstable β-lactam antibiotics in biological samples.

Comparison of the pharmacokinetics of imipenem after intravenous and intrathecal administration in rabbits

BACKGROUND

Intrathecal administration of antibiotics has potentially high effectiveness for the treatment for severe intracranial infections, particularly nosocomial meningitis. The use of intrathecal injection of antibiotics has been reported mostly in case reports. However, there is sparse data regarding the pharmacokinetics of antibiotics after intrathecal administration.

AIM

This study investigated whether intrathecal injection is an effective method for the administration of imipenem.

MATERIALS AND METHODS

The pharmacokinetics of imipenem after intrathecal and intravenous administration of 1:1 imipenem: cilastatin (IMI/CIL) to rabbits were compared.

RESULTS

The AUC0-t in the cerebrospinal fluid for intrathecal administration was approximately twice that of an equal dose of intravenous administration at doses of 0.35, 0.7, and 1.4 mg/kg. Brain concentrations of imipenem after intrathecal injection were three times greater than observed after intravenous injection and remained high for at least 8 hours post-injection. Elimination of imipenem after administration by either route was primarily via urine, but a transient surge of imipenem in bile and intestinal tissue was observed.

CONCLUSIONS

Results indicate that there is a clinical potential for intrathecally administered IMI/CIL. Further studies are warranted to investigate the potential for seizure and to assess the translatability of the rabbit model to human treatment.

[Assessment of efficacies of imipenem, cefoperazone-sulbactam and cefepime in rats with experimental thigh abscess model with multidrug resistant and susceptible Acinetobacter baumannii strains]

Imipenem, cefaperazon-sulbactam and cefepime are the antibiotics of choice for the treatment of soft tissue infections due to Acinetobacter baumannii. In this study, it was aimed to determine the invivo and invitro efficacy of, these antibiotics against drug susceptible and multidrug resistant A.baumannii in an experimental abscess model. Abscess models were established in Wistar-Albino type female rats. Susceptibility tests were performed by E-test. Rats were divided randomly into four groups with eight rats in one group. Standard absorbent paper discs containing 6 log10 CFU microorganisms were used to form an abscess model. The first group was regarded as the control group and the other three groups were the study group each treated with one of the test antibiotics. Cardiac blood samples for serum antibiotic efficacy test, were obtained on the fourth day of treatment and 30 minutes after the last dose. The number of live bacteria at the area of infection was determined by colony count method. All of the three antibiotics reached sufficient concentration in sera of rats and there were no statistically important difference between the efficacies of these antibiotics (p= 0.778). In all of the antibiotic-treated groups, the weight of the abscess material were less, macroscopic views were smaller and the colony counts per gram of abscess tissue were less than the control group (p< 0.001). All antibiotics were effective against susceptible and resistant strains in vitro. No resistance was detected against imipenem, cefaperazon-sulbactam and cefepime in the course of therapy. Cefaperazone-sulbactam and cefepime were as effective as imipenem against susceptible and multi-drug resistant A.baumannii both in vivo and in vitro. Since irrational use of extended spectrum cephalosporins are frequently associated with the emergence of carbapenem resistant strains, the use of relatively narrow spectrum antibiotics should better be considered in the empirical treatment of A.baumannii infections.

Hydrophilic interaction chromatography/tandem mass spectrometry for the simultaneous determination of three polar non-structurally related compounds, imipenem, cilastatin and an investigational beta-lactamase inhibitor, MK-4698, in biological matrices

A method coupling hydrophilic interaction chromatography (HILIC) with tandem mass spectrometry (MS/MS) has been developed for the simultaneous determination of three polar non-structurally related compounds--a carbapenem antibiotic, imipenem (IMP), a renal dehydropeptidase inhibitor, cilastatin (CIL), and an investigational beta-lactamase inhibitor, MK-4698 (BLI), in rat plasma, monkey plasma and mouse blood. The analytes were extracted through protein precipitation, chromatographed on a Waters Atlantis HILIC column, and detected on a Sciex API4000 mass spectrometer using a Turbo-Ion Spray ion source in positive ionization mode following multiple-reaction monitoring (MRM). The assay dynamic range was 0.1-100 microg/mL for IMP, CIL and BLI, respectively, using a total of 20-25 microL biologic samples, and the total HPLC/MS/MS run time was 4 min/injection. The assay was found to be sensitive, selective and reproducible. The challenges, namely, sample stability, blood sample processing, matrix effect in monkey study samples, and dilution re-assays for the limited mouse blood samples, are resolved and discussed. This technique allowed rapid analysis of polar compounds in biologic matrixes with satisfactory chromatographic retention and increased throughput.

Pharmacokinetics of Imipenem in Sheep with Special Reference to Its Hepato-Renal Effects

BACKGROUND

Imipenem is a carbapenem antibiotic with a broad spectrum of activity. The aim of this study was to determine the pharmacokinetics of imipenem after single intravenous and intramuscular injections and the effect of repeated intramuscular injections on hepato-renal functions in sheep.

METHODS

Imipenem concentrations in plasma and urine were determined by a microbiological agar plate assay.

RESULTS

Following intramuscular injection, imipenem was rapidly absorbed and the peak plasma concentration was 9.99 microg ml(-1) and the systemic bioavailability was 65.97%. Urine concentrations of imipenem were much higher than in plasma. Light and electron microscopy evidenced little changes in the kidney and liver.

CONCLUSION

Imipenem is likely to be efficacious in most urinary tract infections and has no adverse effects on hepato-renal structures and functions.

Activity of imipenem against VIM-1 metallo-β-lactamase-producing Klebsiella pneumoniae in the murine thigh infection model

The in-vivo activity of imipenem against VIM-1-producing Klebsiella pneumoniae (VPKP) was assessed in a thigh infection model in neutropenic mice. Animals were infected with three VPKP isolates (imipenem MICs 2, 4 and 32 mg/L, respectively) and a susceptible clinical isolate (MIC 0.125 mg/L) that did not produce any beta-lactamase with broad-spectrum activity. Bacterial density at the site of infection was determined after imipenem treatment (30 and 60 mg/kg every 2 h for 24 h). The log(10) reduction in CFU/thigh was greatest for the wild-type isolate, intermediate for the two imipenem-susceptible VPKP isolates, and lowest for the imipenem-resistant VPKP isolate. Whilst in-vivo imipenem activity appeared reduced against in-vitro susceptible VIM-1 producers compared with a VIM-1-negative control, an increased drug dosage could moderate this reduction.

Liquid Chromatographic Method for the Simultaneous Determination of Imipenem and Sulbactam in Mouse Plasma

The first analytical method is developed and validated for the simultaneous determination of imipenem and sulbactam in mouse plasma. Sample treatment is based on plasma stabilization with 4-(2-hydroxyethyl)piperazine-ethanesulfonic acid (HEPES) (0.5 mol/L; pH 7.0)-water-ethylene glycol (2:1:1, v/v/v), precipitation of plasma proteins with acetonitrile, centrifugation, evaporation, and reconstitution with borate buffer. Analytical determination is carried out by high-performance liquid chromatography with diode-array detection. Chromatographic separation is achieved within 11 min on a C(18) column by gradient elution with borate buffer (0.1 mol/L, pH 7.2) and methanol. Imipenem and sulbactam are monitored at 295 and 230 nm, respectively. The overall interday accuracy is in the range of 95% to 100% and from 98% and 101% for imipenem and sulbactam, respectively. Interday precision is below 8% and 4% for imipenem and sulbactam, respectively. Limits of quantitation of imipenem and sulbactam are 0.05 and 1.0 microg/mL, respectively. The mean extraction recoveries are 94.5% and 94.2% for imipenem and sulbactam, respectively. The described method allows an accurate, simple, and rapid identification and quantitation of imipenem and sulbactam in mouse plasma. This method is applied to the analysis of imipenem and sulbactam in mouse plasma after drug administration.

Trough serum concentrations of β-lactam antibiotics in cancer patients: inappropriateness of conventional schedules to pharmacokinetic/pharmacodynamic properties of β-lactams

Serum concentrations of beta-lactams that continuously exceed the minimum inhibitory concentration may improve therapeutic outcomes for immunosuppressed patients. The trough serum levels of ceftazidime (CAZ), cefepime (FEP) or imipenem (IMP) were prospectively determined on days 1 and 3 of treatment in cancer patients. Seventy-eight episodes of suspected infection were analysed. Trough serum levels were higher than 4 mg/L in 62%, 24% and 0% of cases in the CAZ, FEP and IMP groups, respectively, and were higher than 20 mg/L in 24% of cases in the CAZ group compared with 0% both in the FEP and IMP groups. For suspected infectious episodes in cancer patients, the traditional intermittent regimen of beta-lactams does not appear to be appropriate for the pharmacokinetic/pharmacodynamic properties of these antibiotics.

Pharmacokinetics of imipenem in healthy volunteers following administration by 2 h or 0.5 h infusion

OBJECTIVES

The aim of this study was to demonstrate the t > MIC of 0.5 and 1 g of imipenem when administered by 2 h infusion every 6 h compared with 0.5 g of imipenem when administered by 0.5 h infusion every 6 h.

METHODS

The study was a randomized three-way crossover study with a 30 h wash-out period in eight healthy volunteers. Each subject received imipenem in three regimens: (i) a 0.5 h infusion of 0.5 g every 6 h for three doses; (ii) a 2 h infusion of 0.5 g every 6 h for three doses; and (iii) a 2 h infusion of 1 g every 6 h for three doses.

RESULTS

Following 0.5 h infusion of 0.5 g, the percentages of time above four times an MIC of 4, 2 and 1 mg/L were 21.5 +/- 2.2%, 38.6 +/- 3.5% and 57.5 +/- 4% of a 6 h interval, respectively. For the 2 h infusion of 0.5 g, the percentages of time above four times an MIC of 4, 2 and 1 mg/L were 26.9 +/- 8.5%, 48.0 +/- 3.5% and 65.4 +/- 3.2% of a 6 h interval, respectively. For the 2 h infusion of 1 g, the percentages of time above four times an MIC of 4, 2 and 1 mg/L were 51.6 +/- 5.4%, 67.8 +/- 4.5% and 87.8 +/- 5.6% of a 6 h interval, respectively.

CONCLUSIONS

A 2 h infusion resulted in a greater t > MIC than those after a 0.5 h infusion and intermittent infusion may be a useful mode of administration of imipenem in tropical countries.

Pharmacokinetics of imipenem-cilastatin following intravenous administration in healthy adult horses

In two studies, six healthy adult horses were given imipenem-cilastatin by slow intravenous (i.v.) infusion at an imipenem dosage of 10 mg/kg (study 1) and 20 mg/kg (study 2). The same horses were used in each dosage schedule, with a 2-week washout period between studies. In each dosage group, serial blood and synovial fluid samples were collected for 6 h after completion of the infusion. HPLC was used to determine the imipenem concentration in all samples. Imipenem was well tolerated by all horses at both dosages; no adverse effects were noted during the study period or during the 24-hour postinfusion observation period. The pharmacokinetic profiles of imipenem in the plasma and synovial fluid indicate that an imipenem dosage of 10-20 mg/kg by slow i.v. infusion q6h (every 6 h) is appropriate for most susceptible pathogens.

Comparison of in vivo intrinsic activity of cefepime and imipenem in a Pseudomonas aeruginosa rabbit endocarditis model: effect of combination with tobramycin simulating human serum pharmacokinetics

OBJECTIVES

The purpose of this experimental study was first to compare the in vivo intrinsic activity of imipenem and cefepime administered as a continuous infusion and to determine their lowest effective serum steady-state concentration (LESSC). Secondly, we studied the effect of combining therapy with tobramycin.

METHODS

In a Pseudomonas aeruginosa (ATCC 27853) rabbit endocarditis model, beta-lactam antibiotics were administered by continuous infusion over a 24 h treatment period at different doses until the LESSC was reached, i.e. able to achieve a 2-log drop of cfu/g of vegetations versus untreated animals. The effect of adding tobramycin (3 mg/kg once daily) was then studied.

RESULTS

The LESSC was between 3 x and 4 x MIC of cefepime for P. aeruginosa and about 0.2 5x MIC of imipenem. Combination of tobramycin with each of the two beta-lactams did not result in any further significant killing.

CONCLUSION

The optimal Css/MIC ratio might differ from one molecule to another. The LESSC of imipenem is lower than that of cefepime, giving a better intrinsic activity in vivo, despite a higher MIC in vitro.

Imipenem levels are not predictable in the critically ill patient

BACKGROUND

Critically ill patients often demonstrate extremely unusual volumes of distribution (Vd) and half-lives (t1/2) of drugs. Imipenem is a widely used antibiotic in critically ill patients.

METHODS

We performed high-performance liquid chromatography analysis of imipenem in samples from 50 critically ill patients treated with either 500 or 1,000 mg.

RESULTS

Peak imipenem levels varied from 1.56 microg/mL to 58.8 microg/mL. Trough levels varied between 0.0 microg/mL and 15.62 microg/mL. Only 54% of patients maintained a trough level greater than 4 microg/mL. Both the Vd and the t1/2 of imipenem were much greater than observed in other patient populations.

CONCLUSION

The pharmacokinetic activity of imipenem in critically ill patients is different from that in other patient populations. There is a very weak correlation between dosage and serum concentrations. Therapeutic failures of imipenem may be because of unpredictable pharmacodynamics (Vd and t1/2) in critically ill surgical patients.

Pharmacokinetic interactions of ceftazidime, imipenem and aztreonam with amikacin in healthy volunteers

The common usage of extended spectrum beta-lactams co-administered with amikacin in everyday clinical practice for infections by multidrug-resistant isolates has created the need to search for pharmacokinetic interaction. Eighteen healthy volunteers were enrolled in the study; six were administered 1g of ceftazidime singly intravenously or combined with 0.5 g of amikacin; six received 0.5 g of imipenem singly or combined with 0.5 g of amikacin and six 1g of aztreonam singly or combined with 0.5 g of amikacin. Blood and urine samples were collected at regular time intervals and apparent serum levels were determined by a microbiological assay. Co-administration of ceftazidime and amikacin resulted in higher C(max) and AUC for amikacin than when administered alone. Co-administration of imipenem and amikacin resulted in higher C(max) for imipenem than when administered alone. The tested interactions did not affect plasma half-life (t(1/2)) and clearance rate of any antimicrobial compared with its single administration. All tested drugs were mainly eliminated by glomerular filtration. It is concluded that co-administration of ceftazidime, imipenem or aztreonam with amikacin in healthy volunteers might affect C(max) and AUC without influencing any other pharmacokinetic parameter. The probable clinical endpoint is that giving ceftazidime, imipenem or aztreonam with amikacin might result in a transient elevation of beta-lactam serum levels without further affecting the complete pharmacokinetic profile of each drug as obtained after administration of the drug alone.

Pharmacokinetics of imipenem in dogs

OBJECTIVE

To determine the plasma pharmacokinetics of imipenem (5 mg/kg) after single-dose IV, IM, and SC administrations in dogs and assess the ability of plasma samples to inhibit the growth of Escherichia coli in vitro.

ANIMALS

6 adult dogs.

PROCEDURE

A 3-way crossover design was used. Plasma concentrations of imipenem were measured after IV, IM, and SC administration by use of high-performance liquid chromatography. An agar well antimicrobial assay was performed with 3 E coli isolates that included a reference strain and 2 multidrug-resistant clinical isolates.

RESULTS

Plasma concentrations of imipenem remained above the reported minimum inhibitory concentration for E coli (0.06 to 0.25 microg/mL) for a minimum of 4 hours after IV, IM, and SC injections. Harmonic mean and pseudo-standard deviation half-life of imipenem was 0.80 +/- 0.23, 0.92 +/- 0.33, and 1.54 +/- 1.02 hours after IV, IM, and SC administration, respectively. Maximum plasma concentrations (Cmax) of imipenem after IM and SC administration were 13.2 +/- 4.06 and 8.8 +/- 1.7 mg/L, respectively. Time elapsed from drug administration until Cmax was 0.50 +/- 0.16 hours after IM and 0.83 +/- 0.13 hours after SC injection. Growth of all 3 E coli isolates was inhibited in the agar well antimicrobial assay for 2 hours after imipenem administration by all routes.

CONCLUSIONS AND CLINICAL RELEVANCE

Imipenem is rapidly and completely absorbed from intramuscular and subcutaneous tissues and effectively inhibits in vitro growth of certain multidrug-resistant clinical isolates of E coli.

Tissue distribution of imipenem in critically ill patients

BACKGROUND AND METHODS

Imipenem is a broad-spectrum antibiotic used mainly for serious infections in critically ill patients. Because the infection originates mostly from a certain tissue, we assessed tissue concentrations of imipenem using microdialysis in patients in intensive care with serious infections compared with healthy volunteers. Most patients were >60 years old and had renal failure; most patients also had impaired liver, heart, or lung function.

RESULTS

Muscle and subcutaneous tissue concentrations in patients (maximum of 2.3 +/- 1.5 microg/mL for both muscle and subcutaneous tissue) were significantly lower than those in healthy subjects (maximum of 12.8 +/- 1.6 and 10.7 +/- 1.0 microg/mL for muscle and subcutaneous tissue). The tissue distribution rate constants for muscle and subcutaneous tissue were also significantly lower in patients (1.95 +/- 0.6 and 1.1 +/- 0.2 h(-l), respectively) than in healthy subjects (5.2 +/- 1.0 and 6.6 +/- 1.7 h(-1), respectively), meaning that tissue distribution in patients was reduced and retarded. Values for area under the plasma concentration-time curve did not significantly differ between patients and healthy subjects (37.4 +/- 5.9 microg. h/mL and 46.0 +/- 4.4 microg. h/mL, respectively,) although the elimination of imipenem in patients was prolonged (clearance, 6.3 +/- 0.8 L/h and 13.2 +/- 1.4 L/h in patients and healthy subjects, respectively).

CONCLUSIONS

Our data suggest that the amount and velocity of imipenem tissue distribution in seriously ill patients is reduced compared with those values in healthy volunteers. Dose adjustments that are exclusively based on plasma concentration data may therefore be misleading and may result in potential underdosing.

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.

Serum bactericidal and inhibitory titres in the management of melioidosis

A retrospective evaluation of the relationship between serum bactericidal and inhibitory titres and treatment outcome in 195 adult Thai patients with severe melioidosis was conducted. Drug regimens included ceftazidime (52% of patients), co-amoxiclav (24%), imipenem (11%) or the conventional four-drug combination (11%). Pre- and 1 h post-dose serum samples were collected after 48-72 h of therapy, and serum inhibitory and bactericidal titrations determined. Median post-dose titres were: bactericidal 1:8 (range 0-1:128) and inhibitory 1:16 (range 0-1:128). Overall mortality was 26% and outcome was not influenced by either inhibitory or bactericidal titres. Pre-dose titres correlated with renal function; renal function was the most important predictor of mortality. Determination of serum inhibitory or bactericidal titres is unhelpful in the management of severe melioidosis.

Comparison of five beta-lactam antibiotics against common nosocomial pathogens using the time above MIC at different creatinine clearances

STUDY OBJECTIVE

To compare the time above the minimum inhibitory concentration (T>MIC) for five parenteral beta-lactam antibiotics against common nosocomial bacterial pathogens at different creatinine clearances (Clcr).

INTERVENTIONS

Serum concentration-time profiles were simulated for cefepime, ceftazidime, piperacillin, piperacillin-tazobactam, and imipenem at Clcr ranging from 120-30 ml/minute. The MIC data for 90% of organisms (MIC90) were collected for Escherichia coli, Klebsiella pneumoniae, Serratia marcescens, Citrobacter freundii, Enterobacter aerogenes, Enterobacter cloacae, Pseudomonas aeruginosa, and oxacillin-susceptible Staphylococcus aureus, and a weighted geometric mean MIC90 was calculated. The T>MIC was calculated as percentage of the dosing interval in which free concentrations exceeded the weighted geometric mean MIC90. A T>MIC of 70% or greater was considered desirable for all organisms except S. aureus (> or = 50%).

MEASUREMENTS AND MAIN RESULTS

Cefepime 2 g every 12 hours (Clcr > or = 70 ml/min) and every 24 hours (Clcr < or = 60 ml/min) achieved desirable T>MIC for all Enterobacteriaceae and S. aureus at every Clcr. Imipenem 0.5 g achieved desirable T>MIC for E. coli, K. pneumoniae, C. freundii, and S. aureus at every Clcr. However, imipenem T>MIC was less than 70% for the following regimens and organisms: S. marcescens 0.5 g every 6 hours (Clcr > or = 90 ml/min), E. aerogenes 0.5 g every 6 hours (Clcr > or = 80 ml/min), E. cloacae 0.5 g every 6 hours (Clcr > or = 100 ml/min), S. marcescens 0.5 g every 8 hours (Clcr 60-70 ml/min), E. cloacae 0.5 g every 8 hours (Clcr 60-70 ml/min), and E. aerogenes 0.5 g every 8 hours (Clcr 50-70 ml/min). Ceftazidime 2 g every 8 hours (Clcr 60-100 ml/min) and every 12 hours (Clcr 40-50 ml/min) achieved desirable T>MIC for E. coli, K. pneumoniae, S. marcescens, and S. aureus only. At every dose and Clcr, piperacillintazobactam achieved desirable T>MIC for S. aureus but not for any Enterobacteriaceae at Clcr > 50 ml/minute. Piperacillin did not achieve desirable T>MIC for any organism, and none of the beta-lactams attained a T>MIC of 70% or above for P. aeruginosa at any Clcr.

CONCLUSION

At every Clcr, cefepime achieved a desirable T>MIC for more nosocomial pathogens than any other beta-lactam evaluated. Based on pharmacodynamic data, cefepime is an appropriate empiric choice for treatment of nosocomial infections. However, when P. aeruginosa is a potential pathogen, empiric combination therapy should be considered.

Antibiotic dosing issues in lower respiratory tract infection: population-derived area under inhibitory curve is predictive of efficacy

Several lower respiratory tract infection (LRTI) trials have documented a correlation between clinical response and area under the inhibitory curve (24 h AUC/MIC; AUIC). The AUIC values in these studies were based on measured MICs and measured serum concentrations. This study evaluates AUIC estimates made using population pharmacokinetic parameters, and MICs from an automated microbiological susceptibility testing system. A computer database review over 2 years yielded 81 patients at Millard Fillmore Hospital with a culture-documented gram-negative LRTI who had been treated with piperacillin and an aminoglycoside, ceftazidime, ciprofloxacin or imipenem. Their AUIC values were estimated using renal function, drug dosages and MIC values. Outcome groups (clinical and microbiological cures and failures) were related to the AUIC values using Kruskal-Wallis ANOVA, linear regression and classification and regression tree (CART) analysis. A significant breakpoint for clinical cures was an AUIC value at least 72 SIT(-1) x 24 h (inverse serum inhibitory titre integrated over time). All antibiotics performed significantly better above this value than below it. Clinical cure was well described by a Hill-type equation. Within the piperacillin/aminoglycoside regimen, most of the activity came from the piperacillin, which had a higher overall AUIC value than the aminoglycoside. AUIC estimations based upon MIC values derived from the automated susceptibility testing method differed from NCCLS breakpoint data and from tube dilution derived values in this hospital by as much as three tube dilutions. These automated methods probably overestimated the MIC values of extremely susceptible organisms. The lack of precise MIC estimates in automated clinical microbiology methods impairs the use of AUIC to prospectively optimize microbiological outcome. Even ignoring this limitation and using the values as they are reported, the results of this analysis suggest that AUIC targets between 72 and 275 SIT(-1) x 24 h are useful in predicting clinical outcome.

Comparative therapeutic efficacy of clinafloxacin in a Pseudomonas aeruginosa mouse renal abscess model

A deep-seated Pseudomonas aeruginosa mouse kidney abscess model was used to compare the therapeutic efficacy of clinafloxacin, a fluoroquinolone in clinical trials, with that of clinically relevant standard drugs. Following 50 mg/kg oral doses, twice daily for five consecutive days, clinafloxacin produced a 4 log decrease in mean bacterial count, the greatest decrease of all drugs tested. The same dosage regimen resulted in complete bacterial eradication in 88% of the kidneys. No other compound produced total bacterial clearance in 50% of the kidneys at the highest dose tested.

Pharmacokinetics of imipenem-cilastatin in critically ill patients undergoing continuous venovenous hemofiltration

The pharmacokinetics of imipenem-cilastatin were investigated in 12 critically ill patients with acute renal failure (ARF) managed by continuous veno-venous hemofiltration (CVVH) while receiving a fixed combination of 500 mg of imipenem-cilastatin intravenously three or four times daily. No adverse drug reactions were observed. Plasma and hemofiltrate samples were taken at specified times during one dosing interval, and the concentrations of imipenem and cilastatin were determined by high-performance liquid chromatography. Pharmacokinetic variables were calculated by a first-order, two-compartment pharmacokinetic model for both substances. Total clearances of imipenem and cilastatin (mean +/- standard deviations) were 122.2 +/- 28.6 and 29.2 +/- 13.7 ml/min, respectively, with hemofiltration clearances of 22.9 +/- 2.5 and 16.1 +/- 3.1 ml/min, respectively, and nonrenal, nonhemofiltration clearances of 90.8 +/- 26.3 and 13.2 +/- 13.9 ml/min, respectively. Mean imipenem dosage requirements were approximately 2,000 mg/24 h (2,111.8 +/- 493.4 mg/24 h). They were calculated in order to achieve an average steady-state concentration of 12 mg/liter to ensure that concentrations in plasma exceeded the MICs at which 90% of intermediately resistent bacteria are inhibited (8 mg/liter) during the majority of the dosing interval. By contrast, the recommended dosage for patients with end-stage renal failure (ESRF) and infections caused by intermediately resistant bacteria is 1,000 mg/24 h. This remarkable difference may be due (i) to differences in the nonrenal clearance of imipenem between patients with ARF and ESRF and (ii) to the additional clearance by the hemofilter. Since the total clearance of cilastatin was low, marked accumulation occurred, and this was particularly pronounced in patients with additional liver dysfunction. Thus, in patients with ARF managed by CVVH, rather high imipenem doses are required, and these inevitably result in a marked accumulation of cilastatin. The doses of imipenem recommended for patients with ESRF, however, will lead to underdosing and inadequate antibiotic therapy.

Determination of imipenem in plasma by high-performance liquid chromatography for pharmacokinetic studies in patients

A rapid and simple HPLC method is described for the determination of imipenem in human plasma. After blood collection, plasma was separated by centrifugation and immediately stabilized with 3-morpholinopropanesulfonic acid (MOPS) and ethylene glycol solution (1:1). The sample preparation, before injection into HPLC, was ultrafiltration. The mobile phase was boric acid buffer. The imipenem was detected at 300 nm and cilastatin sodium, coadministered, did not interfere. Calibration curves in human plasma were linear from 0.1 to 100 microg/ml. The limit of detection was 0.030 microg/ml. Inter-day precision at 0.1 microg/ml, determined as the coefficient of variation, was 6.26%. Only 250 microl of plasma was required in our assay. Due to the limited stability of imipenem [G.B. Smith et al., J. Pharm. Sci., 79 (1990) 732], stability studies in plasma were done to establish appropriate storage conditions. The assay was applied to pharmacokinetic studies in patients.

Influence of changes in pancreatic tissue morphology and capillary blood flow on antibiotic tissue concentrations in the pancreas during the progression of acute pancreatitis

BACKGROUND

The ability of an antibiotic to reach bactericidal concentrations in tissue depends on numerous factors including tissue composition and regional perfusion. Although necrotising pancreatitis is characterised by progression of pancreatic necrosis over at least 96 hours and microcirculatory alterations, the impact of these changes on the concentration of antibiotics in the pancreas has not yet been investigated.

AIM

To determine and compare pancreatic tissue concentrations of imipenem and cefotaxime at different stages of acute necrotising pancreatitis in an animal model that has been shown to mimic closely the pathomorphological and bacteriological features of severe human pancreatitis.

METHOD

Acute necrotising pancreatitis was induced in rats by a standardised intraductal infusion of glycodeoxycholic acid and intravenous cerulein. Six hours (n = 16) and 48 hours (n = 16) after induction of pancreatitis, the animals were randomised for intravenous therapy with either imipenem or cefotaxime. Fifteen minutes after injection of the antibiotic, the animals were killed. Blood and the head of the pancreas were collected for determining imipenem or cefotaxime in serum and tissue; the splenic portion of the pancreas was prepared for histological examination. In an additional set of identically treated animals, pancreatic capillary blood flow (PCBF) was assessed by intravital microscopy before induction of acute necrotising pancreatitis and at the time of antibiotic therapy.

RESULTS

Imipenem accumulates in the pancreas in the initial phase of acute necrotising pancreatitis characterised by pronounced oedema and decreased PCBF, and tends to decrease with resolution of the oedema and the progression of acinar cell necrosis in the later course of the disease. Concentrations of cefotaxime are low in oedematous pancreatic tissue early after induction of acute necrotising pancreatitis and increase with the resolution of oedema and normalisation of PCBF.

CONCLUSIONS

Concentrations of antibiotics in the pancreas vary in acute necrotising pancreatitis, depending on changes in pancreatic tissue morphology and capillary blood flow. This suggests that antibiotic tissue concentrations may not be consistent from one agent to another and that efficacy of antibiotics in acute pancreatitis cannot be estimated solely on the basis of their pharmacological and microbiological properties.

Serum bactericidal activities and comparative pharmacokinetics of meropenem and imipenem-cilastatin

The pharmacokinetics and serum bactericidal activities (SBAs) of imipenem and meropenem were investigated in a randomized crossover study. Twelve healthy male volunteers received a constant 30-min infusion of either 1 g of imipenem plus 1 g of cilastatin or 1 g of meropenem. The concentrations of the drugs in serum and urine were determined by bioassay and high-pressure liquid chromatography. Pharmacokinetic parameters were based on an open two-compartment model and a noncompartmental technique. At the end of infusion, the mean concentrations of imipenem and meropenem measured in serum were 61.2 +/- 9.8 and 51.6 +/- 6.5 mg/liter, respectively; urinary recoveries were 48.6% +/- 8.2% and 60.0% +/- 6.5% of the dose in 12 h, respectively; and the areas under the concentration-time curve from time zero to infinity were 96.1 +/- 14.4 and 70.5 +/- 10.3 mg.h/liter, respectively (P < or = 0.02). Imipenem had a mean half-life of 66.7 +/- 10.4 min; that of meropenem was 64.4 +/- 6.9 min. The volumes of distribution at steady state of imipenem and meropenem were 15.3 +/- 3.3 and 18.6 +/- 3.0 liters/70 kg, respectively, and the mean renal clearances per 1.73 m2 were 85.6 +/- 17.6 and 144.6 +/- 26.0 ml/min, respectively. Both antibiotics were well tolerated in this single-dose administration study. The SBAs were measured by the microdilution method of Reller and Stratton (L. B. Reller and C. W. Stratton, J. Infect. Dis. 136:196-204, 1977) against 40 clinically isolated strains. Mean reciprocal bactericidal titers were measured 1 and 6 h after administration. After 1 and 6 h the median SBAs for imipenem and meropenem, were 409 and 34.9 and 97.9 and 5.8, respectively, against Staphylococcus aureus, 19.9 and 4.4 and 19.4 and 4.8, respectively, against Pseudomonas aeruginosa, 34.3 and 2.2 and 232 and 15.5, respectively, against Enterobacter cloacae, and 13.4 and 2.25 and 90.7 and 7.9, respectively, against Proteus mirabilis. Both drugs had rather short biological elimination half-lives and a predominantly renal route of elimination. Both carbapenems revealed high SBAs against clinically important pathogens at 1 h; meropenem had a higher SBA against E. cloacae and P. mirabilis, and the SBA of imipenem against S. aureus was greater than the SBA of meropenem.

Comparative serum bactericidal activity against Pseudomonas aeruginosa of six antipseudomonal agents

Serum levels and serum bactericidal activities of six antipseudomonal agents were studied comparatively in 60 patients. Single intravenous doses of gentamicin (1.5 mg/kg), piperacillin (4 g), ceftazidime (1 g), imipenem (0.5 g), aztreonam (1 g), and ciprofloxacin (200 mg) were given over 30 min to 10 patients each, and serum samples were obtained 30 min, 1, 2, 3, 4, 6, 8 and 12 h after beginning the infusion. Serum bactericidal activity was determined by the broth microdilution method against 10 recent isolates of Pseudomonas aeruginosa. Mean peak serum levels were as follows: gentamicin 10.4 micrograms/ml, piperacillin 227.5 micrograms/ml, ceftazidime 43.5 micrograms/ml, imipenem 17.3 micrograms/ml, aztreonam 42.3 micrograms/ml, and ciprofloxacin 3.9 micrograms/ml. All agents demonstrated effective serum bactericidal activity (geometric mean titer > 1:2) at peak serum levels. Ceftazidime was by far the most potent compound with a mean titer of 1:46.5, followed by ciprofloxacin (1:17), imipenem (1:13.7), and aztreonam (1:13.4). Ceftazidime also showed the longest duration of activity with a mean titer of 1:5.1 at 4 h. Based on our results, ceftazidime appeared to be the most potent antipseudomonal agent, while gentamicin and piperacillin were the least effective.

Determination of meropenem in plasma by high-performance liquid chromatography and a microbiological method

A rapid and selective high-performance liquid chromatographic (HPLC) method for the quantitative determination of meropenem in plasma is described. The drug was separated from plasma after plasma protein precipitation with 15% of trichloroacetic acid. The mobile phase consisted of acetonitrile-water-glacial acetic acid (21.2, 78 and 0.8% v/v, respectively) delivered at a flow rate of 1.2 ml/min. Meropenem was quantified using ultraviolet detection at 296 nm. Meropenem and the internal standard (pheniramine) were well separated from plasma components. The drug could be assayed by the HPLC method in the presence of its analogue, imipenem. The detection limit in plasma was 25 ng/ml of meropenem. The results were compared with those of agar for a microbiological diffusion method using Escherichia coli ATCC 25922 as the test organism. The sensitivity of the microbiological assay was less than 5 ng/ml, but this decreased at higher concentrations. Both methods were applied to the determination of the drug in aqueous solutions and in plasma.

Pharmacokinetics and transplacental passage of imipenem during pregnancy

Imipenem pharmacokinetics were studied in early pregnancy (n = 7; length of gestation, 8.6 +/- 1.5 weeks, mean +/- standard deviation), in late pregnancy (n = 7; length of gestation, 38.7 +/- 1.4 weeks), and in the nonpregnant state (n = 6). A single dose of 500 mg of imipenem-cilastatin (1:1) was administered as a 20-min infusion. Multiple plasma and urine samples, as well as specimens of umbilical plasma and amniotic fluid from the pregnant subjects, were collected at frequent intervals for 8 h. Imipenem concentrations were assayed by specific microbiologic assay. The mean peak concentrations in plasma were 14.7 +/- 4.9, 14.9 +/- 5.2, and 43 +/- 28.3 micrograms/ml in early pregnancy, late pregnancy, and the nonpregnant state, respectively. The volumes of distribution were significantly larger during early pregnancy (0.98 +/- 0.45 liter/kg of body weight, P < 0.005) and late pregnancy (0.59 +/- 0.19 liter/kg, P < 0.05) than in the nonpregnant state (0.33 +/- 0.10 liter/kg), and total clearances from plasma were faster in early pregnancy (12.7 +/- 7.8 ml min-1 kg-1, P < 0.05) and late pregnancy (10.7 +/- 4.6 ml min-1 kg-1, P < 0.05) than in the nonpregnant state (5.77 +/- 1.19 ml min-1 kg-1). The mean concentrations in amniotic fluid were 0.07 +/- 0.01 and 0.72 +/- 0.85 micrograms/ml in early and late pregnancy. The mean umbilical venous and arterial drug concentrations were 1.72 +/- 1.22 and 1.64 +/- 1.22 micrograms/ml. The feto-maternal ratio at the time of cesarean section was 0.33 +/- 0.12. These results indicate that an adjustment of doses of imipenem may be required when treating pregnant women because of considerable changes in imipenem pharmacokinetics during pregnancy.

Bactericidal activity of cefclidin (E1040) against Pseudomonas aeruginosa under conditions simulating plasma pharmacokinetics: lack of development of chromosomally-mediated resistance to beta-lactams

The bactericidal activity of cefclidin (E1040), a new cephalosporin, against Pseudomonas aeruginosa was compared with that of ceftazidime and imipenem in an in-vitro model in which antibiotic concentration was varied continuously. A two-compartment open in-vitro model was used to simulate the plasma pharmacokinetics of each antibiotic in man for 12 h after a 1 h infusion of 1 g iv. The bactericidal activity of each antibiotic was observed for 6 h; however, it was diminished or absent after 6 h when the antibiotic concentration fell near to the MIC. With ceftazidime and imipenem, marked regrowth was observed after 6 h. Moreover, selection of resistant variants was observed with ceftazidime, and these variants produced 200 to 500 times more beta-lactamase than the corresponding wild-type strains. With cefclidin neither marked regrowth nor emergence of resistant variants was observed. The affinity of cefclidin for the chromosomal beta-lactamase produced by P. aeruginosa was much lower than the affinities of other new beta-lactams, and cefclidin was hydrolyzed more slowly than ceftazidime at a low concentration (2 microM). The high activity of cefclidin against P. aeruginosa, which results mainly from the low affinity of cefclidin for the pseudomonal beta-lactamase, and may play a major role in the absence of regrowth and lack of selection of resistant variants.

Comparative study of pharmacokinetics and serum bactericidal activities of cefpirome, ceftazidime, ceftriaxone, imipenem, and ciprofloxacin

We compared the pharmacokinetics and the serum bactericidal activities of cefpirome, ceftazidime, ceftriaxone, imipenem, and ciprofloxacin. Fifteen healthy volunteers received 1 g of cefpirome, ceftazidime, and ceftriaxone intravenously, 500 mg of imipenem-cilastatin intravenously, and 500 mg of ciprofloxacin orally. High-performance liquid chromatographic assays were used to quantitate unchanged antibiotic in plasma and urine. Serum bactericidal activities were determined against six clinical isolates each of Staphylococcus aureus, Enterobacter cloacae, and Pseudomonas aeruginosa by using a modified microdilution method of Reller and Stratton (L. B. Reller and C. W. Stratton, J. Infect. Dis. 136:196-204, 1977). Overall, cefpirome exhibited pharmacokinetics similar to those of ceftazidime: half-life (t1/2), 1.95 h; concentration at 1 h (C1h), 47 to 49 micrograms/ml for both antibiotics. Ceftriaxone displayed the longest t1/2 (7.65 h) and the highest C1h (137.8 micrograms/ml), while we observed the shortest t1/2 (1.05 h) and the lowest C1h (19.85 micrograms/ml) with imipenem. At 1 h, cefpirome and, even more so, imipenem showed significantly better serum bactericidal activities against S. aureus (1:273 and 1:80) than did the other antibiotics (P less than 0.0005; analysis of variance with randomized block design and Bonferroni correction). Against E. cloacae, we observed the highest serum bactericidal titers at 1 h with cefpirome, and this superiority vis-à-vis the other antibiotics tested was maintained for up to 8 h after dosing. Ceftazidime remained the most active agent tested against P. aeruginosa (serum bactericidal activity titers, 1:43 at 1 h) up to 8 h. In summary, the study showed that cefpirome and imipenem provide more potent serum bactericidal activities than do broad-spectrum cephalosporins against S. aureus; thus, both of these antibiotics should be adequate against serious S. aureus infections. In addition, cefpirome appears to be a promising alternative for treatment of infections caused by E. cloacae and P. aeruginosa.

Pharmacokinetics of imipenem in serum and skin window fluid in healthy adults after intramuscular or intravenous administration

The pharmacokinetic profiles of imipenem after intramuscular (i.m.) and intravenous injections were examined in adult volunteers. Levels of imipenem in serum after i.m. injection of a microcrystalline suspension of imipenem-cilastatin (500 mg each) reached a peak (8.0 micrograms/ml) at 1.5 h after administration, and concentrations were maintained in excess of 1.5 micrograms/ml for 6 h. Serum elimination half-life (1.3 h), volume of distribution (14.5 liters), and area under the curve (AUC; 27.8 micrograms.h/ml) after i.m. injection did not significantly differ from those of a comparable dose given by intravenous infusion. Bioavailability after i.m. injection was 89%. Imipenem levels in skin window fluid after i.m. administration were maximal (4.3 micrograms/ml) at 4 h after injection, at which time imipenem concentrations exceeded those produced by intravenous infusion. The AUCskin window/AUCserum ratio for skin window fluid after i.m. injection was 68%, indicating good penetration of the drug into skin fluid. This study shows that i.m. injection of 500 mg of imipenem-cilastatin results in concentrations of imipenem in serum and skin fluid that are, for at least 6 h, consistent with antimicrobial activity against susceptible organisms.

Removal of imipenem and cilastatin by hemodialysis in patients with end-stage renal failure

The removal of imipenem and cilastatin by hemodialysis was studied in 14 (for imipenem) and 6 (for cilastatin) subjects. Following intravenous infusion of imipenem and cilastatin at a combined concentration of 10 mg/kg of body weight, drug levels in plasma were determined serially during off- and on-hemodialysis periods, which were 2 and 4 h, respectively. The biexponential decay of the drug levels in plasma was evident in each subject for both imipenem and cilastatin. Hemodialysis accelerated the elimination of both imipenem and cilastatin: the mean elimination-phase half-life of imipenem was shortened from 200 to 78 min, and that of cilastatin was shortened from 445 to 115 min. Hemodialysis clearance of imipenem and cilastatin was calculated by five different methods, each with intrinsic assumptions. The mean hemodialysis clearance of imipenem was estimated to be 74.08 +/- 13.29 ml/min, and that of cilastatin was estimated to be 65.0 +/- 8.6 ml/min, after consideration of various methodological limitations. It was estimated that in a hypothetical anephric patient weighing 60 kg, a 4-h hemodialysis treatment would remove 54.8% of the imipenem and 62.9% of the cilastatin present in the body at the start of dialysis.

[Diffusion of imipenem in synovial fluid]

The diffusion of imipenem (IMP) in the knee joint was studied after a 1 g i.v. administration of Tienam over one hour. The synovial fluid was collected under anesthesia during arthroscopy carried out for mechanical lesions of the knee (meniscal lesions after ligamental injuries or sequelae after meniscectomy), in 3 groups of six patients at one, two, or three hours after the end of injection of IMP. The concentrations of IMP determined by high performance liquid chromatography (HPLC) were: 42.5, 20.1, 9.3 and 5.7 mg/l in the blood at T0, T1, T2 and T3 hr, respectively; 20.4, 13.0 and 7.9 mg/l in the synovial fluid at T1, T2 and T3, respectively. The decrease of IMP concentrations in the synovial fluid was 1,5 times as low as in serum. On account of its broad-spectrum antibacterial activity and our data, IMP could be used in perioperative prophylaxis of the knee joint surgery.

Pharmacokinetics of parenteral imipenem/cilastatin in patients on continuous ambulatory peritoneal dialysis

We investigated the pharmacokinetics of two intravenous (iv) dose regimens of imipenem/cilastatin in Chinese patients on chronic ambulatory peritoneal dialysis (CAPD), who had an average creatinine clearance of 3.2 ml/min/1.73 m2. Doses of 0.5 and 1.0 g produced mean peak serum imipenem concentrations of 30 and 70 mg/l respectively, about 60% of cilastatin. Peritoneal dialysis fluid (PDF) imipenem concentrations reached 20-30% of the serum peak 4-5 h after iv injection, and the lowest maximum PDF concentrations were 2 mg/l after the 0.5 g dose and 14 mg/l after 1.0 g. Thus both regimes produced PDF imipenem concentrations above the MICs of susceptible pathogens. The half-life of imipenem was 6.4 h and the plasma clearance 66 ml/min; serum and PDF imipenem were in equilibration after about 5 h. Cilastatin had a prolonged half-life of 19 h and a plasma clearance of 10 ml/min, and accumulated in both serum and PDF. With a 0.5 g dose, the pharmacokinetics of imipenem/cilastatin suggest that the combination may prove an effective treatment for peritonitis associated with CAPD.

Quantification of imipenem's primary metabolite in plasma by postcolumn chemical rearrangement and UV detection

Imipenem (thienamycin formamidine) is an antibiotic active against a broad spectrum of bacteria. Its primary metabolite arises from cleavage of the lactam ring. The metabolite can be formed in-vitro by acid-catalyzed or enzymatic hydrolysis. In animals and man, this metabolite can be generated systemically as well as in the kidneys following the excretion of imipenem into the urine. In man, this dehydropeptidase-catalyzed renal metabolism is minimized by the coadministration of cilastatin, a competitive inhibitor. A specific HPLC assay has been developed to evaluate the disposition of this metabolite in humans having normal or end-stage renal function. The assay employs ion-pair, reversed-phase chromatography, and post-column acid treatment of the analyte for ultraviolet detection.

[Study of the intraperitoneal penetration of imipenem/cilastatin in acute peritonitis with perforation]

A study was performed to investigate the intraperitoneal penetration of Imipenem/cilastatin into inflammatory peritoneal fluid. Six patients undergoing abdominal surgery (acute peritonitis), were treated with Imipenem/cilastatin (4 perfusions of 0.5 g/day) during 5 days after the intervention. Plasma samples were obtained on day 1 and 4 at the pic and at the valley; peritoneal samples were obtained every days for 4 days, 1, 3 and 6 hours after the end of a perfusion. The samples were immediately stabilised following the manufacturer instructions and quick freezed at -80 degrees C. Dosages were performed using a microbiological assay. The mean peritoneal levels are above the MIC 90 of the more frequent bacteria which cause infection in abdominal surgery. Moreover none of the patients showed relapse of infection or complication during this treatment.

[Clinical pharmacokinetics of an imipenem-cilastatin combination]

In the combination of imipenem (antibiotic of the thienamycin class, in the beta-lactam family) with cilastatin (renal dehydropeptidase I inhibitor) the ratio is 1:1. The urinary excretion of imipenem shows considerable interindividual variations due to the activity of renal dehydropeptidase I which degrades the compound. This makes it difficult to determine a mean dosage for therapeutic uses. However, when the dehydropeptidase I inhibitor cilastatin is given concomitantly with imipenem the urinary excretion and clearance of imipenem reach similar values in all subjects; in addition, the antibiotic is better tolerated by the kidney. Moreover, cilastatin has been shown to increase by 15 to 20 per cent the area under the imipenem plasma concentration curve. Following an intravenous infusion of imipenem 500 mg, the main pharmacokinetic values for the antibiotic are: area under the plasma concentration curve 43.2 +/- 4.7 h.mg/l; plasma clearance 195 +/- 25 ml/min; half-life 1.0 +/- 0.1 h; apparent volume of distribution 10.4 +/- 1.7 l. With repeated infusions of 500 ml every 6 to 8 hours, imipenem and cilastatin do not accumulate. In patients with renal impairment (creatinine clearance below 30 ml/min), cilastatin is excreted more slowly than imipenem and dosage must be adjusted accordingly.

Determination of imipenem in human plasma, urine and tissue by high-performance liquid chromatography

A simple and reliable HPLC method is described for the new beta-lactam antibiotic imipenem; suitable extraction procedures for the drug in human plasma, urine and prostatic tissue are described. The figures of merit for the assays are reported and examples given of their application.

[Clinical and pharmacokinetic study of imipenem/cilastatin in children and newborn infants]

Imipenem, a new carbapenem (thienamycin) beta lactam antibiotic which is clinically used in a 1:1 combination with cilastatin, an inhibitor or renal metabolism of imipenem, was evaluated in 25 patients; 11 children and 14 neonates. A mean daily dose of 60 mg/kg was given to children and the dose in neonates was 50 mg/kg. Clinically, 21 patients were cured, two failed to respond to treatment and two were not evaluable. Pharmacokinetic studies were performed in the 11 children and in 10 of the neonates. The mean elimination half-life of imipenem was 0.87 h in children and 2.1 h in neonates. The mean cilastatin elimination half-life was 0.73 h in children and 5.1 h in neonates. This difference in half-life between children and neonates is similar to the one noted between healthy adults and adults with renal insufficiency. No accumulation of imipenem was seen in neonates studied on the first and fifth days of treatment.

[Intraocular passage of imipenem in man]

Twenty two patients (mean age: 69 years) undergoing cataract surgery were perfused with imipenem (1g for 1 hour). The samples of serum and aqueous humor were taken simultaneously 0.5-1-2-5-7 hours after the end of perfusion and immediately stabilized by MES or MOPS buffers and ethylene glycol. The antibiotic concentrations were determined with Bacillus subtilis ATCC 6633. Aqueous humor levels increased from the end of perfusion to 2nd hour (peak 5.2 mg/l) and decreased after (mean level 2.8 mg/l). The concentrations were well above MIC-90 of the species commonly involved in bacterial endophthalmitis. Imipenem appears to have an exceptional penetration into ocular fluids.

Evaluation of FCE 22101 in experimental meningitis caused by Escherichia coli and Streptococcus pneumoniae

FCE 22101 is a new penem antibiotic with a spectrum of activity suggesting a possible role in the empirical treatment of meningitis. It appears to achieve a mean reduction in bacterial titre in CSF comparable with currently accepted agents for both pneumococcal and Escherichia coli meningitis. Its efficacy may, however, be variable. It does not achieve CSF level/MIC ratios as favourable as imipenem for the pathogens studied. Further studies are necessary to determine its role, if any, in this disease.

Imipenem concentrations in colorectal surgery and impact on the colonic microflora

Twenty patients undergoing colorectal surgery were given, as prophylaxis, imipenem-cilastatin intravenously. Ten of them received a dose of 0.5/0.5 g of imipenem-cilastatin at induction of anesthesia, followed by subsequent doses of 0.5/0.5 g every 6 h for 48 h. The other 10 patients were given 1.0/1.0 g imipenem-cilastatin in the same way for 48 h. Samples from serum, intestinal mucosa, and feces were taken for analysis of imipenem concentrations during the day of surgery. The mean concentrations in serum at 1 h after the first imipenem dose were 15.9 +/- 1.7 micrograms/ml for the 0.5-g dose and 68.2 +/- 8.2 micrograms/ml for the 1.0-g dose. The mean half-lives were 1.5 and 1.4 h, respectively, and the mean areas under the serum concentration-time curve were 41.2 +/- 6.0 and 128.3 +/- 13.5 mg.h/liter, respectively. The imipenem concentrations in the intestinal mucosa varied between less than 0.1 and 3.6 mg/kg for the 0.5-g dose and 3.2 and 13.4 mg/kg for the 1.0-g dose. The concentrations in the fecal samples varied between less than 0.1 and 5.0 mg/kg for the 0.5-g dose and 0.7 and 11.3 mg/kg for the 1.0-g dose. Fecal samples were also collected during the investigation period for cultivation of aerobic and anaerobic bacteria. The aerobic bacteria--staphylococci, streptococci, enterococci, and enteroaerobic enterococci, and enterobacteria--were and anaerobic bacteria. The aerobic bacteria--staphylococci, streptococci, enterococci, and enterobacteria--were suppressed significantly during the imipenem prophylaxis period. Among the anaerobic bacteria, cocci, bifidobacteria, eubacteria, lactobacilli, clostridia, fusobacteria, and bacteroides decreased markedly during the same period. The microfloras were normalized after 2 weeks. There were no differences between the patients receiving 0.5-g doses of imipenem and those receiving 1.0-g of imipenem. No postoperative infections occurred.

Amikacin, ciprofloxacin, and imipenem treatment for disseminated Mycobacterium avium complex infection of beige mice

The Mycobacterium avium complex (MAC) is a common cause of disseminated infection in patients with acquired immunodeficiency syndrome and is increasingly seen as a cause of infection in other immunocompromised patients. Traditional antimycobacterial therapy often is ineffective, and there is a clear need for antibiotics with proven activity against the MAC. Three agents, amikacin, ciprofloxacin, and imipenem, were tested in vitro for activity against MAC strain 101. Amikacin was bacteriostatic, with an MIC of 4.8 micrograms/ml, which is significantly lower than the concentration in serum obtained with standard dosing. Imipenem and ciprofloxacin had little or no activity alone (MICs, greater than 16 and 4.7 micrograms/ml, respectively), but when they were combined with amikacin there was bactericidal activity. Each agent was tested individually and in combination by using the beige mouse model of disseminated MAC infection. There was no mortality in a group of animals infected with MAC 101 and treated with amikacin alone; also, there was a significant decrease in the infection of the blood, liver, and spleen. There was no apparent improvement in therapeutic effectiveness when amikacin was combined with the other agents. Neither ciprofloxacin nor imipenem was active as a single agent, which was consistent with the in vitro activities of these agents. Amikacin in combination with traditional antimycobacterial agents warrants further study as potential therapy for disseminated MAC infections.