Population pharmacokinetics and pharmacodynamics of continuous versus short-term infusion of imipenem-cilastatin in critically ill patients in a randomized, controlled trial

Beta-lactams are regularly administered in intermittent short-term infusions. The percentage of the dosing interval during which free drug concentrations exceed the MIC (fT(>MIC)) is the measure of drug exposure that best correlates with clinical outcome for beta-lactams. Therefore, administration by continuous infusion has gained increasing interest recently. We studied 20 critically ill patients with nosocomial pneumonia and investigated whether continuous infusion with a reduced total dose, compared to the standard regimen of intermittent short-term infusion, results in a superior probability of target attainment as assessed by the fT(>MIC) value of imipenem. In this prospective, randomized, controlled clinical study, patients received either a loading dose of 1 g/1 g imipenem and cilastatin (as a short-term infusion) at time zero, followed by 2 g/2 g imipenem-cilastatin per 24 h as a continuous infusion for 3 days (n = 10), or 1 g/1 g imipenem-cilastatin three times per day as a short-term infusion for 3 days (total daily dose, 3 g/3 g; n = 10). Imipenem concentrations in plasma were determined by using a validated liquid chromatography-tandem mass spectrometry assay. A two-compartment open model was employed for population pharmacokinetic modeling. We simulated 10,000 intensive-care-unit patients via Monte Carlo simulations for pharmacodynamic evaluation using the target 40% fT(>MIC). The probability of target attainment by MIC for intermittent infusion was robust (>90%) up to MICs of 1 to 2 mg/liter. The corresponding value for continuous infusion was 2 to 4 mg/liter. Although all 20 patients had an fT(>MIC) of 100%, 3 patients died. Patient survival was best described by employing a sepsis-related organ failure assessment score as a covariate in a logistic regression analysis. Larger clinical trials are warranted for evaluation of continuous infusions at a reduced dose of imipenem for critically ill patients.

Imipenem/cilastatin: an update of its antibacterial activity, pharmacokinetics and therapeutic efficacy in the treatment of serious infections

The prototype carbapenem antibacterial agent imipenem has a very broad spectrum of antibacterial activity, encompassing most Gram-negative and Gram-positive aerobes and anaerobes, including most beta-lactamase-producing species. It is coadministered with a renal dehydropeptidase inhibitor, cilastatin, in order to prevent its renal metabolism in clinical use. Extensive clinical experience gained with imipenem/cilastatin has shown it to provide effective monotherapy for septicaemia, neutropenic fever, and intra-abdominal, lower respiratory tract, genitourinary, gynaecological, skin and soft tissues, and bone and joint infections. In these indications, imipenem/cilastatin generally exhibits similar efficacy to broad-spectrum cephalosporins and other carbapenems and is at least equivalent to standard aminoglycoside-based and other combination regimens. Imipenem/cilastatin is generally well tolerated by adults and children, with local injection site events, gastrointestinal disturbances and dermatological reactions being the most common adverse events. Seizures have also been reported, occurring mostly in patients with impaired renal function or CNS pathology, or with excessive dosage. Although it is no longer a unique compound, as newer carbapenems such as meropenem are becoming available, imipenem/cilastatin nevertheless remains an important agent with established efficacy as monotherapy for moderate to severe bacterial infections. Its particular niche is in treating infections known or suspected to be caused by multiresistant pathogens.

Clinical pharmacokinetics during continuous haemofiltration

Continuous haemofiltration is an extracorporeal technique that is increasingly used to remove fluid, electrolytes, and other waste products from the blood supply of critically ill patients with acute renal failure. Continuous arteriovenous haemofiltration (CAVH), where the blood exits the body from an artery and re-enters through a vein, is widely used. Continuous venovenous haemofiltration (CVVH), where blood both exits and enters through a vein by way of a mechanical pump, avoids problems that result from the variable ultrafiltration rate found during CAVH. Continuous arteriovenous or venovenous haemodiafiltration (CAVHD or CVVHD) combine continuous haemofiltration and haemodialysis. All methods involve ultrafiltration of the patient's blood through a filter that is highly permeable to water and small molecules. Drug elimination by haemofiltration depends mainly on the rate of ultrafiltration, the drug protein binding and the sieving coefficient of the membrane. Because patients undergoing continuous haemofiltration have impaired renal function, dosage reduction is often recommended so that adverse drug reactions are avoided. In contrast, if drug removal by haemofiltration is significant, dosage supplementation may be required to ensure therapeutic efficacy of the drug. Therefore, knowledge of the impact of continuous haemofiltration on drug elimination and the pharmacokinetic profile of drugs is essential to good clinical management. The currently available information on the clinical pharmacokinetic aspects of drug therapy during continuous haemofiltration are summarised. Drugs commonly associated with haemofiltration therapy are tabulated with updated pharmacokinetics and drug-monitoring information.

Pharmacodynamic evaluation of extending the administration time of meropenem using a Monte Carlo simulation

A Monte Carlo simulation demonstrated that 1 g of meropenem (MEM) every 8 h (q8h) (3-h infusion) has a higher target attainment rate against Pseudomonas aeruginosa than either 500 mg of MEM q8h (3-h infusion) or 0.5 g of imipenem-cilastatin (I-C) q6h (1-h infusion). For other pathogens, 500 mg of MEM q8h was equivalent or superior to I-C.

Imipenem/cilastatin. A reappraisal of its antibacterial activity, pharmacokinetic properties and therapeutic efficacy

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

The in vitro elution characteristics of vancomycin combined with imipenem-cilastatin in acrylic bone-cements: a pharmacokinetic study

The aim of this in vitro study was to compare the elution characteristics of vancomycin alone and in combination with imipenem-cilastatin from 3 acrylic bone-cements (CMW1 [DePuy International, Blackpool, UK], Palacos R [Schering-Plough, Wehrheim, Germany], and Simplex P [Howmedica International, London, UK]). Six groups of 3 antibiotic-loaded cement disks were prepared, incorporating 2 g of vancomycin (3 groups) and 2 g of vancomycin plus 2 g of imipenem-cilastatin (3 groups). The disks were placed in saline baths for 5 weeks, with the baths being sampled periodically and the elution rates calculated. The total amount of vancomycin released by the cements treated with vancomycin alone was 7.98 mg for CMW1, 7.74 mg for Palacos R, and 6.76 mg for Simplex P; with the addition of imipenem-cilastatin, the total amount of vancomycin released by the 3 cements increased by 30.58%, 50.52%, and 50.15%. CMW1 had better elution characteristics than the other cements when treated with vancomycin alone; the elution of Palacos R and Simplex P was better than that of CMW1 when vancomycin was combined with imipenem-cilastatin.

Pharmacokinetics of meropenem compared to imipenem-cilastatin in young, healthy males

Imipenem combined with cilastatin and meropenem was given as intravenous infusions of 1 g to eight young, healthy males on two separate occasions. Blood and urine samples were collected for up to 12 h. The terminal half-lives in plasma were 0.98 h and 1.11 h for meropenem and imipenem, respectively. The volume of distribution was smaller for meropenem than for imipenem (12.5 l and 14.4 l, respectively). The plasma clearance for meropenem was 188 (SD 31) ml/min and for imipenem 183 (SD 25) ml/min. Renal clearance was on average 139 (SD 24) ml/min and 135 (SD 11) ml/min, respectively. About 75% of the administered dose of both compounds was eliminated unchanged in urine. Non-renal clearance accounted for approximately 25% of the total clearance for both drugs. The kinetics of meropenem are very similar to those of imipenem given with cilastatin, and meropenem is as stable against renal metabolic degradation as imipenem combined with cilastatin.

Pharmacokinetics of meropenem in animals, healthy volunteers, and patients

Meropenem is a carbapenem antibiotic that appears to be widely distributed in tissues and is eliminated by both excretion and metabolism. Approximately 70% of meropenem is excreted via the kidneys, thus dosage adjustments are required for patients with renal impairment. The pharmacokinetic parameters for meropenem are similar to those for imipenem/cilastatin, with the exception of meropenem's smaller volume of distribution. The urinary recovery of meropenem is as high as that of imipenem in combination with cilastatin, an inhibitor of renal dehydropeptidase. Therefore, unlike imipenem, meropenem can be used without dehydropeptidase inhibitors to obtain a consistently high concentration in the urine without nephrotoxic effects.

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.

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.

Pharmacokinetic–Pharmacodynamic Modeling and Simulation for in Vivo Bactericidal Effect in Murine Infection Model

A pharmacokinetic (PK)/pharmacodynamic (PD) modeling strategy to simulate in vivo bactericidal effects for three carbapenem antibiotics, doripenem (DRPM), meropenem (MEPM)/cilastatin (CS), and imipenem (IPM)/CS, against a Pseudomonas aeruginosa (P. aeruginosa) strain is proposed. The PD model we have already developed to explain in vitro time-kill profiles was modified to incorporate the growth rate, bactericidal activities, and PK profiles in murine lung infection models. Plasma concentration data and bacterial time-kill data for each antibiotic consist of six and eight time points, respectively, at one dose regimen (four or five mouse/point). In vivo time-kill curves could be well simulated for each antibiotic by the PK/PD model. Simulated bacterial counts at 24 h and PK/PD indices derived from total drug concentrations (time above the minimum inhibitory concentration (MIC) (T > MIC), C(max)/MIC, and AUC/MIC) for various dose regimens were examined for MEPM/CS and IPM/CS. Simulated bacterial counts correlated only with T > MIC (correlation coefficient: 0.951 for MEPM/CS, 0.982 for IPM/CS) and T > MIC values to achieve a bacteriostatic effect and a 2-log killing effect for both antibiotics were estimated to be approximately 15 and 20%, respectively, which are similar to previously reported results. These findings suggested that the proposed PK/PD model is a good tool for predicting in vivo bactericidal effects.

Clinical pharmacology of imipenem and cilastatin in premature infants during the first week of life

The first-dose and multidose pharmacokinetics of imipenem and cilastatin were evaluated in 41 premature infants during their first week of life. Premature infants (gestational age, less than or equal to 37 weeks) were assigned to receive 10-, 15-, 20-, or 25-mg/kg doses of imipenem-cilastatin (1:1) as a single- or multiple-dose regimen. A total of 39 infants received a single dose, whereas 18 infants received multiple doses. No differences were observed in pharmacokinetic parameter estimates for either agent relative to the dose administered or infant body weight; thus, the data were pooled. Elimination half-life, steady-state volume of distribution, and body clearance averaged 2.5 h, 0.5 liter/kg, and 2.5 ml/min per kg, respectively, for imipenem and 9.1 h, 0.4 liter/kg, and 0.5 ml/min per kg, respectively, for cilastatin. Similar values for these parameter estimates were observed after multidose administration, although substantial accumulation of cilastatin in serum was observed. A total of 21% of the imipenem and 43% of the cilastatin were excreted unchanged in the urine over a 12-h collection period. Corresponding renal clearances averaged 0.4 and 0.2 ml/min per kg for imipenem and cilastatin, respectively. Substantial differences were observed in the route by which imipenem was cleared from the body compared with data from adult volunteers. These data suggest that infants should receive an imipenem dose of 20 mg/kg administered every 12 h for the treatment of bacterial infections outside the central nervous system.

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.

Clearance of imipenem/cilastatin in acute renal failure patients treated by continuous hemodiafiltration (CAVHD)

In patients with acute renal failure, who were treated with continuous arteriovenous hemofiltration (CAVH) or continuous arteriovenous hemodiafiltration (CAVHD), we measured clearance rates of imipenem and cilastatin (Tiënam-500). Literature data on volume of distribution and on the endogenous clearance in normals and in anuric patients and the observed clearance rates by CAVH/CAVHD were used to develop guidelines for dose adaptations. Based on the desired peak levels of imipenem, normal subjects should receive the fixed imipenem/cilastatin dose combination (500 mg/500 mg) q.i.d. and patients with acute renal failure should receive the same dose b.i.d. After starting treatment with either CAVH, CAVHD or continuous venovenous hemofiltration (CVVH), no further dose adjustment is necessary. The non-renal clearance rate of cilastatin is very low compared to that of imipenem. If a patient develops anuria, the clearance rate of imipenem decreases from the normal value of 245 ml/min to 116 ml/min. Clearance rate of cilastatin, however, decreases from 230 ml/min to 3 ml/min. Therefore, in patients with renal failure accumulation of cilastatin will occur. On the other hand, if the patient is treated by CAVHD, the relative contribution of the dialyser clearance to the total drug clearance is much greater for cilastatin than for imipenem. As a result, the accumulation of cilastatin is reversed. During treatment by CAVHD, the clearance rate of imipenem raises 15%-25% and that of cilastatin 335%-600%. For this reason, we conclude that the use of the fixed dose combination (500 mg/500 mg) b.i.d. in patients with acute renal failure treated by CAVHD may be justified.

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.

Pharmacodynamic Modeling of β-lactam Antibiotics for the Empiric Treatment of Secondary Peritonitis: A Report from the OPTAMA Program

BACKGROUND

In this report of the OPTAMA (Optimizing Pharmacodynamic Target Attainment using the MYSTIC Antibiogram) program, we utilized Monte Carlo simulation to compare the probabilities of achieving bactericidal time above the minimum inhibitory concentration (MIC) (%T > MIC) exposures for imipenem-cilastatin 500 mg q6h and 1000 mg q8h, meropenem 500 mg q6h and 1000 mg q8h and piperacillin/tazobactam 3.375 g q6h and 4.5 g q8h in the empiric treatment of secondary peritonitis.

METHODS

The prevalence of pathogens causing secondary peritonitis was identified from the primary surgical and infectious diseases literature. Data for these pathogens with respect to MIC were obtained from the 2003 MYSTIC surveillance study and weighted by the prevalence of each pathogen. A sensitivity analysis varying the prevalence of P. aeruginosa was performed with two additional models to determine the robustness of the data. Pharmacokinetic parameters, obtained from previously published studies in healthy volunteers were used to simulate the %T > MIC for 10,000 patients receiving imipenem-cilastatin, meropenem, and piperacillin/tazobactam. The likelihood of obtaining bactericidal exposure is reported.

RESULTS

Empiric utilization of imipenem-cilastatin and meropenem 500 mg q6h and 1000 mg q8h regimens achieved 99.6%-99.7% likelihood of bactericidal exposure. Piperacillin/ tazobactam 3.375 g q6h and 4.5 g q8h produced bactericidal target attainments of 92.9% and 85.2%, respectively. Models simulating higher prevalence of P. aeruginosa reduced the likelihood of bactericidal exposure for piperacillin/tazobactam regimens significantly and had little effect on the carbapenems.

CONCLUSION

All of the beta-lactams used in the current analysis were predicted to achieve high target attainment consistently for the empiric treatment of secondary peritonitis. However, imipenem-cilastatin 500 mg q6h and 1000 mg q8h, meropenem 1000 mg q8h and 500 mg q6h, and piperacillin/tazobactam 3.375 g q6h achieved the highest likelihood. These, in particular, would be effective choices for the empiric treatment of secondary peritonitis.

Pharmacodynamic Modeling of Imipenem-Cilastatin, Meropenem, and Piperacillin-Tazobactam for Empiric Therapy of Skin and Soft Tissue Infections: A Report from the OPTAMA Program

BACKGROUND

The bactericidal exposures necessary for positive clinical outcomes among skin and soft tissue infections are largely dependent on interpatient pharmacokinetic variability and pathogen drug susceptibility. By simulating the probability of achieving target bactericidal exposures, the pharmacodynamics of three beta-lactam agents were compared against a range of pathogens implicated commonly in complicated skin and soft tissue infections.

METHODS

Using Monte Carlo simulation, pharmacodynamic target attainment expressed as the percentage of the time interval during which the antibiotic concentration exceeded the minimal inhibitory concentration (%T > MIC) in serum and blister fluid was calculated for 5,000 simulated patients receiving imipenem-cilastatin 0.5 g q8h, meropenem 0.5 g q8h, piperacillin-tazobactam 3.375 g q6h, and piperacillin-tazobactam 4.5 g q8h. The pharmacokinetics for each antibiotic were derived from previously published healthy volunteer studies. The MICs for Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, Enterobacter sp., Klebsiella sp., coagulase-negative staphylococci, Proteus sp., beta-hemolytic streptococci, and Serratia sp. were taken from the MYSTIC 2003 surveillance study and weighted by the prevalence of each pathogen among 1,404 isolates collected from skin and soft tissue infections during the 2000 SENTRY study. The prevalence of methicillin-resistant Staphylococcus aureus (MRSA) was added into the model at increasing resistance rates.

RESULTS

Imipenem-cilastatin, meropenem, and piperacillin-tazobactam 3.375 g q6h achieved greater than 90% likelihood of achieving bactericidal exposure in serum and blister fluid until the prevalence of MRSA increased beyond 10%. Piperacillin-tazobactam 4.5 g q8h achieved a lower probability of achieving bactericidal exposure than the other regimens (88.7%, p < 0.001).

CONCLUSIONS

When the incidence of MRSA is low, imipenem-cilastatin, meropenem and piperacillin-tazobactam 3.375 g q6h would be optimal choices for the empiric treatment of complicated skin and soft tissue infections among the regimens studied. When MRSA is suspected, a drug that retains activity against this pathogen should be considered.

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.

Effect of biliary obstruction on the hepatic excretion of imipenem-cilastatin

The biliary excretion of imipenem-cilastatin studied by endoscopic cannulation of the common bile duct in patients with complete obstruction and in a group without obstruction showed that despite a 24-h prophylaxis, the bile obtained from patients with obstruction immediately after cannulation contained neither imipenem nor cilastatin, while there were 2 and 5% of peak concentrations in serum for imipenem and cilastatin, respectively, in the bile from patients without obstruction. Biliary excretion of both compounds increased rapidly after decompression, reaching a maximum of 15% of peak levels in serum within 2 h. Twenty-four hours after drainage, the biliary excretion of the drugs further improved. We conclude that since biliary obstruction impairs excretion of antibiotics, drainage is necessary for the control of sepsis in obstructed cholangitis.

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.

Pharmacokinetics of intravenous imipenem/cilastatin during intermittent haemofiltration

The pharmacokinetics of a single iv dose of imipenem/cilastatin (500/500 mg) were studied during and after intermittent haemofiltration (IHF) treatment in six patients with chronic renal failure. The elimination half-lives of imipenem and cilastatin during the IHF treatment were almost identical, 1.4 +/- 0.3 and 1.5 +/- 0.3 h, respectively. Accordingly, approximately 75% of the given dose was eliminated during a 3-h IHF session. However, there was a great difference between the elimination half-lives of the two drugs in the post-treatment period, 3.4 +/- 1.0 and 16 +/- 10 h for imipenem and cilastatin, respectively. The haemofiltration clearance of imipenem was 134 +/- 41 ml/min and that of cilastatin 109 +/- 8 ml/min. On the basis of our results, we suggest that a supplementary dose of imipenem/cilastatin (500/500 mg) should be given directly after the IHF treatment. This dose should be the starting dose for a period of 12-h dosing intervals until the next IHF procedure.

Effects of parenterally administered ciprofloxacin in a murine model of pulmonary Pseudomonas aeruginosa infection mimicking ventilator-associated pneumonia

BACKGROUND AND METHODS

We compared the bacteriological, pharmacological and histopathological effects of parenterally administered ciprofloxacin (CPFX) to those of imipenem/cilastatin (IMP/CS) and cefozopran (CZOP) in a murine model of mucoid Pseudomonas aeruginosa pneumonia mimicking ventilator-associated pneumonia.

RESULTS

The minimum inhibitory concentrations (MICs) of CPFX, IMP and CZOP were 1.0, 1.0 and 4.0 mg/l, respectively. Treatment with CPFX resulted in a significant decrease in the number of viable bacteria [control, IMP/CS, CZOP and CPFX (mean +/- SEM): 5.02 +/- 0.20, 4.96 +/- 0.38, 5.44 +/- 0.13 and 3.27 +/- 0.02 log(10) colony-forming units lung, respectively]. Histopathological examination revealed that inflammatory changes in the CPFX-treated group were less marked than in other groups. Of the drugs analyzed, the pharmacokinetic parameters of area under the time-concentration curve (AUC)/MIC, AUC exceeding MIC and the time that lung concentrations of drug remained above the MIC were highest for CPFX.

CONCLUSION

Our results suggest that parenterally administered ciprofloxacin is effective in ventilator-associated P. aeruginosa pneumonia.

Aerosolization of imipenem/cilastatin prevents pseudomonas-induced acute lung injury

Aerosolization of imipenem/cilastatin was compared with continuous intravenous infusions of the antibiotic for pharmacokinetic/pharmacodynamic analysis. The concentrations of imipenim/cilastatin in bronchoalveolar lavage fluids (BAL) obtained from rats exposed to the aerosolized antibiotic were significantly greater than the concentrations in BAL in the rats that had received intravenous infusions of imipenem/cilastatin. The two methods of antibiotic delivery were compared for their effects on bacterial-induced lung injury in rats that had Pseudomonas aeruginosa instilled into their airspaces. The aerosolization of antibiotic was associated with significantly decreased bacterial-induced lung injury. The high concentrations of antibiotic in the airspaces secondary to aerosolization appears to kill bacteria more quickly and preserve lung epithelial and endothelial integrity better than systemic delivery of the same antibiotic.