Antimicrobial effects of continuous versus intermittent administration of carbapenem antibiotics in an in vitro dynamic model

Challenges for global antibiotic regimen planning and establishing antimicrobial resistance targets: implications for the WHO Essential Medicines List and AWaRe antibiotic book dosing

SUMMARYThe World Health Organisation's 2022 AWaRe Book provides guidance for the use of 39 antibiotics to treat 35 infections in primary healthcare and hospital facilities. We review the evidence underpinning suggested dosing regimens. Few (n = 18) population pharmacokinetic studies exist for key oral AWaRe antibiotics, largely conducted in homogenous and unrepresentative populations hindering robust estimates of drug exposures. Databases of minimum inhibitory concentration distributions are limited, especially for community pathogen-antibiotic combinations. Minimum inhibitory concentration data sources are not routinely reported and lack regional diversity and community representation. Of studies defining a pharmacodynamic target for ß-lactams (n = 80), 42 (52.5%) differed from traditionally accepted 30%-50% time above minimum inhibitory concentration targets. Heterogeneity in model systems and pharmacodynamic endpoints is common, and models generally use intravenous ß-lactams. One-size-fits-all pharmacodynamic targets are used for regimen planning despite complexity in drug-pathogen-disease combinations. We present solutions to enable the development of global evidence-based antibiotic dosing guidance that provides adequate treatment in the context of the increasing prevalence of antimicrobial resistance and, moreover, minimizes the emergence of resistance.

Meropenem-Tobramycin Combination Regimens Combat Carbapenem-Resistant Pseudomonas aeruginosa in the Hollow-Fiber Infection Model Simulating Augmented Renal Clearance in Critically Ill Patients

Augmented renal clearance (ARC) is common in critically ill patients and is associated with subtherapeutic concentrations of renally eliminated antibiotics. We investigated the impact of ARC on bacterial killing and resistance amplification for meropenem and tobramycin regimens in monotherapy and combination. Two carbapenem-resistant Pseudomonas aeruginosa isolates were studied in static-concentration time-kill studies. One isolate was examined comprehensively in a 7-day hollow-fiber infection model (HFIM). Pharmacokinetic profiles representing substantial ARC (creatinine clearance of 250 ml/min) were generated in the HFIM for meropenem (1 g or 2 g administered every 8 h as 30-min infusion and 3 g/day or 6 g/day as continuous infusion [CI]) and tobramycin (7 mg/kg of body weight every 24 h as 30-min infusion) regimens. The time courses of total and less-susceptible bacterial populations and MICs were determined for the monotherapies and all four combination regimens. Mechanism-based mathematical modeling (MBM) was performed. In the HFIM, maximum bacterial killing with any meropenem monotherapy was ∼3 log₁₀ CFU/ml at 7 h, followed by rapid regrowth with increases in resistant populations by 24 h (meropenem MIC of up to 128 mg/liter). Tobramycin monotherapy produced extensive initial killing (∼7 log₁₀ at 4 h) with rapid regrowth by 24 h, including substantial increases in resistant populations (tobramycin MIC of 32 mg/liter). Combination regimens containing meropenem administered intermittently or as a 3-g/day CI suppressed regrowth for ∼1 to 3 days, with rapid regrowth of resistant bacteria. Only a 6-g/day CI of meropenem combined with tobramycin suppressed regrowth and resistance over 7 days. MBM described bacterial killing and regrowth for all regimens well. The mode of meropenem administration was critical for the combination to be maximally effective against carbapenem-resistant P. aeruginosa.

An investigation of the stability of meropenem in elastomeric infusion devices

Purpose

To evaluate the stability of meropenem trihydrate in elastomeric infusion devices at a range of selected concentrations (6, 12, 20 and 25 mg/mL) at ambient, refrigeration and freezing temperatures.

Methods

Meropenem Ranbaxy^® (meropenem trihydrate equivalent to anhydrous meropenem 1 g) vials for injection were reconstituted with 0.9% sodium chloride and adjusted to pH 6.5 using 1 M hydrochloric acid. Following preparation, solutions were stored for 7 days at either 6.7°C in elastomeric infusion devices or at −19°C in glass vials; samples of each concentration were removed from the infusion devices at specific time-points and stored for 24 hrs at 22.5°C. All solutions were assayed at specific time-points using high-performance liquid chromatography. Forced degradation in hydrochloric acid, sodium hydroxide and hydrogen peroxide was carried out at 40°C.

Results

The lowest concentration of meropenem (6 mg/mL) displayed the highest stability. It maintained >90% of its initial concentration for up to 144 hrs when stored at 6.7°C and 72 hrs following 24 hrs storage at 22.5°C, having been initially refrigerated for 48 hrs. Meropenem 20 mg/mL required immediate administration following preparation under ambient temperatures, whilst meropenem 25 mg/mL did not remain stable following 24 hrs storage at ambient temperatures. Frozen meropenem solutions displayed good stability in all concentrations but were physically unstable due to the formation of a precipitate.

Conclusion

At lower concentrations, meropenem showed suitable stability for storage and administration in elastomeric infusion devices, at refrigerated temperatures. To enhance the stability of lower concentration solutions when exposed to ambient temperatures by ambulatory patients, a more adept method of maintaining lower temperatures that reflect refrigerated conditions for elastomeric infusion devices should be devised.

Continuous and Prolonged Intravenous β-Lactam Dosing: Implications for the Clinical Laboratory

Beta-lactam antibiotics serve as a cornerstone in the management of bacterial infections because of their wide spectrum of activity and low toxicity. Since resistance rates among bacteria are continuously on the rise and the pipeline for new antibiotics does not meet this trend, an optimization of current beta-lactam treatment is needed. This review provides an overview of optimization through use of prolonged- and continuous-infusion dosing strategies compared with more traditional intermittent infusions. Included is an overview of the scientific basis for using these nontraditional prolonged- and continuous-infusion-based regimens, with a focus on major areas in which the clinical laboratory can support the clinical use of these regimens.

Meropenem by Continuous Versus Intermittent Infusion in Ventilator-Associated Pneumonia due to Gram-Negative Bacilli

Background:

It is known that β-lactam antibiotics exhibit time-dependent bactericidal activity. Several studies have found continuous infusion of meropenem more effective than intermittent infusion in maintaining constant serum concentrations in excess of the minimum inhibitory concentration. However, limited data exist on the clinical efficacy of meropenem administered by continuous infusion.

Objective:

To evaluate the clinical efficacy of continuous versus intermittent infusion of meropenem for the treatment of ventilator-associated pneumonia (VAP) due to gram-negative bacilli.

Methods:

A retrospective cohort study was conducted of patients with VAP caused by gram-negative bacilli who received initial empiric antibiotic therapy with meropenem. We analyzed 2 contemporary cohorts: one group received meropenem by continuous infusion (1 g over 360 min every 6 h), the other by intermittent infusion (1 g over 30 min every 6 h). The administration method was prescribed according to the physician's discretion. Patients received meropenem plus tobramycin for 14 days.

Results:

There were no significant differences between patient groups with regard to gender, age, APACHE-II at intensive care unit admission, diagnosis, microorganism responsible for VAP, or organ dysfunction severity at the time VAP was suspected. The group receiving medication by continuous infusion showed a greater clinical cure rate than the group treated with intermittent infusion (38 of 42, 90.47%, vs 28 of 47, 59.57%, respectively, with OR 6.44 [95% Cl 1.97 to 21.05; p < 0.001]).

Conclusions:

Meropenem administered by continuous infusion may have more clinical efficacy than intermittent infusion.

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.

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.

Designing Antibacterial Compounds through a Topological Substructural Approach

A novel application of TOPological Substructural MOlecular DEsign (TOPS-MODE) was carried out in antibacterial drugs using computer-aided molecular design. Two series of compounds, one containing antibacterial and the other containing non-antibacterial compounds, were processed by a k-means cluster analysis in order to design training and predicting series. All clusters had a p-level < 0.005. Afterward, a linear classification function has been derived toward discrimination between antibacterial and non-antibacterial compounds. The model correctly classifies 94% of active and 86% of inactive compounds in the training series. More specifically, the model showed a global good classification of 91%, i.e., 263 cases out of 289. In predicting series, the model has shown overall predictabilities of 91 and 83% for active and inactive compounds, respectively. Thereby, the model has a global percentage of good classification of 89%. The TOPS-MODE approach, also, similarly compares with respect to one of the most useful models for antimicrobials selection reported to date.

Pharmacological principles of antibiotic prescription in the critically ill

The goal of antimicrobial prescription is to achieve effective drug concentrations. Standard antimicrobial dosing regimens are based on research performed often decades ago and for the most part with patients who were not critically ill. More recent insights into antibiotic activity (e.g. the importance of high peak/MIC ratios for aminoglycosides and time above MIC for beta-lactam antibiotics), drug pharmacokinetics (e.g. increased volume of distribution and altered clearances) and the pathogenesis of sepsis (e.g. third space losses and altered creatinine clearances) have made re-evaluation of dosing regimens necessary for the critically ill. The inflammatory response associated with sepsis results in a rapid decrease in serum albumin levels, large fluid shifts and third space losses, initially with a high cardiac output. In turn these changes result in increased creatinine clearance and increased renal drug clearance. Unless these effects are offset by ensuing renal and/or hepatic impairment, with subsequent drug accumulation, antibiotic levels may be too low for optimal efficacy. The institution of continuous renal replacement therapy separately affects antibiotic clearances, and therefore dosing, even further. This article reviews relevant literature and offers principles for more effective and appropriate antibiotic dosing in the critically ill, based on the pharmacokinetic and pharmacodynamic principles of the main antibiotic groups (aminoglyosides, glycopeptides, beta-lactams, carbapenems and quinolones) and knowledge of the pathophysiology of the inflammatory response syndrome. Finally it also provides some guidance on the basic principles of drug prescription for patients receiving continuous renal replacement therapy.

Pharmacokinetics and pharmacodynamics of meropenem in critically ill patients

The pharmacokinetics and pharmacodynamics of meropenem were investigated in 14 critically ill patients with sepsis. Patients with creatinine clearance (CrCl) higher than 50 ml/min received 1 g meropenem three times daily (Group I) and patients with CrCl lower than 50 ml/min received 1 g meropenem twice daily (Group II). Meropenum concentrations in plasma were determined by high performance liquid chromatography with UV detection. The pharmacokinetic parameters differed between the two groups as follows, Group I, maximal concentration 56.3 +/- 19.1 microg/ml; trough concentration 3.3 +/- 2.5 microg/ml; elimination half life 2.5 +/- 1.2 h; clearance (Cl) 155.8 +/- 40.6 ml/min; MRT 2.2 +/- 0.4 h; steady state volume of distribution (V(ss)) 21.7 +/- 5.7 l, and AUC(-8) 119.4 +/- 32.6 microg/ml h. Group II, maximal concentration 71.1 +/- 5.1 microg/ml; trough concentration 3.4 +/- 1.8 microg/ml; elimination half life 3.9 +/- 1.6 h; Cl 77.7 +/- 15.8 ml/min; MRT 3.5 +/- 0.6 h; V(ss), 17.1 +/- 2.1 l, and AUC(0-12) 230.2 +/- 43.3 microg/ml h. The most frequently isolated bacteria from blood and wound infections were Acinetobacter baumanii, Pseudomonas aeruginosa, Klebsiella pneumoniae and Escherichia coli; their meropenem minimal inhibitory concentrations (MICs) ranged from 0.064 to 3.0 mg/l. In most cases the pharmacodynamic parameters, measured as T>MIC index, were higher than 75%. In both groups, patients with susceptible pathogens (MIC<1 mg/l) had meropenem plasma levels which exceeded the MIC for the whole dosing interval. When pathogens were highly resistant (A. baumanii or P. aeruginosa) the T>MIC indices were lower.

Comparative pharmacokinetics of the carbapenems: clinical implications

During the last few decades, several carbapenems have been developed. The major characteristic of the newer drugs, such as MK-826, is a prolonged half-life. Alternatively, some carbapenems have been developed that can be given orally, such as CS-834 and L-084. Although imipenem and panipenem have to be administered with a co-drug to prevent degradation by the enzyme dehydropeptidase-1 and reduce nephrotoxicity, the newer drugs such as meropenem, biapenem and lenapenem are relatively stable towards that enzyme. Structural modifications have, besides changes in pharmacology, also led to varying antimicrobial properties. For instance, meropenem is relatively more active against Gram-negative organisms than most other carbapenems, but is slightly less active against Gram-positive organisms. Except for half-life and bioavailability, the pharmacokinetic properties of the carbapenems are relatively similar. Distribution is mainly in extracellular body-water, as observed both from the volumes of distribution and from blister studies. Some carbapenems have a better penetration in cerebrospinal fluid than others. In patients with renal dysfunction, doses have to be adjusted, and special care must be taken with imipenem/cilastatin and panipenem/betamipron to prevent accumulation of the co-drugs, as the pharmacokinetic properties of the co-drugs differ from those of the drugs themselves. However, toxicity of the co-drugs has not been shown. The carbapenems differ in proconvulsive activity. Imipenem shows relatively the highest proconvulsive activity, especially at higher concentrations. Pharmacodynamic studies have shown that the major surrogate parameter for antimicrobial efficacy is the percentage of time of the dosage interval above the minimum inhibitory concentration (MIC). The minimum percentage percentage of time above the MIC (TaM) needed for optimal effect is known in animals (30 to 50%), but not in humans. It is probably less than 100%, but may be higher than 50%. Dosage regimens currently in use result in a TaM of about 50% at 4 mg/L, which is the current 'susceptible' breakpoint determined by the National Committee for Clinical Laboratory Standards (NCCLS) for most micro-organisms. Dosage regimens in patients with reduced renal clearance should be based on the TaM. The increased half-life of the newer carbapenems will probably lead to less frequent administration, although continuous infusion may still be the optimal mode of administration for these drugs. The availability of oral carbapenems will have a profound effect on the use of carbapenems in the community.

Continuous infusion versus intermittent administration of meropenem in critically ill patients

This prospective crossover study compared the pharmacokinetics of meropenem by continuous infusion and by intermittent administration in critically ill patients. Fifteen patients were randomized to receive meropenem either as a 2 g iv loading dose, followed by a 3 g continuous infusion (CI) over 24 h, or by intermittent administration (IA) of 2 g iv every 8 h (q8h). Each regimen was followed for a period of 2 days, succeeded by crossover to the alternative regimen for the same period. Pharmacokinetic parameters (mean +/- SD) of CI included the following: concentration at steady state (Css) was 11.9+/-5.0 mg/L; area under the curve (AUC) was 117.5+/-12.9 mg/L x h. The maximum and minimum serum concentrations of meropenem (Cmax, Cmin) and total meropenem clearance (CItot) for IA were 110.1+/-6.9 mg/L, 8.5+/-1.0 mg/L and 9.4+/-1.2 L/h, respectively. The AUC during the IA regimen was larger than the AUC during CI (P < 0.001). In both treatment groups, meropenem serum concentrations remained above the MICs for the most common bacterial pathogens. We conclude that CI of meropenem is equivalent to the IA regimen and is therefore suitable for treating critically ill patients. Further studies are necessary to compare the clinical effects of CI and IA in this patient group.

Continuous infusion of beta-lactam antibiotics

There are considerable laboratory data and information from animal and continuous culture in vitro models to support continuous infusion therapy for penicillins and cephalosporins, but, as yet, the only existing clinical data relate to cephalosporins. Penicillins do not exert concentration-dependent killing in the therapeutic range but have a post-antibiotic effect (PAE) against Gram-positive cocci but not Gram-negative rods. Animal models indicate the time (T) during which the serum concentrations exceed the minimum inhibitory concentration (MIC) of the pathogen [T > MIC] determines outcomes. Pharmacokinetic studies in humans indicate that continuous infusion with penicillins is possible but there are no clinical data on efficacy. Cephalosporins have similar pharmacodynamic properties to penicillins; T > MIC determines outcome. Data related to ceftazidime indicate that the drug concentration at steady-state (Css) should exceed the pathogen MIC by > 1-fold and perhaps by 4- to 5-fold or more. Human pharmacokinetics of ceftazidime administered by continuous infusion to a wide variety of patient groups indicates that Css of > 20 mg/L can easily be achieved using conventional daily doses. Clinical data indicate increased effectiveness of a continuous regimen in neutropenic patients with Gram-negative infection. Furthermore cefuroxime administration by continuous infusion has resulted in lower doses and shorter course durations. Little is known of the pharmacodynamics of monobactams and there are few clinical data on continuous infusion therapy. Carbapenems have different pharmacodynamics to other beta-lactams as they have concentration-dependent killing and a PAE with both Gram-positive and Gram-negative bacteria. While T > MIC has a role in determining outcomes, the proportion of the dosing interval for which serum drug concentrations should exceed the pathogen MIC is less than for other beta-lactams. In vitro models have shown that continuous infusion is effective, as is less frequent dosing. There are few data on continuous infusion of carbapenems but some patients have been treated with once-daily dosing. Clinically, continuous infusion therapy with penicillins and cephalosporins should be considered in patients infected with susceptible Gram-negative rods not responding to conventional therapy. As an approximation, the same total daily dose should be given but a bolus intravenous injection should be give at the start of continuous infusion to ensure Css is reached rapidly. The Css may be difficult to predict and determination of serum drug concentrations may be indicated. Ideally, the Css should be calculated based on the MIC of the potential pathogen and may be higher or lower than the Css achieved by a conventional daily dose.

Determinants of response to antibiotic therapy

The in vivo situation is far more complex than that of the standard in vitro susceptibility test yet in vitro tests have stood the test of time and are often good predictors of clinical outcome. Nevertheless, our understanding of the pharmacodynamics of antibiotic microbe interaction are giving us new insights into how to improve our performance and interpretation of these tests. These factors include consideration of inoculum effect, antibiotic interactions, cidal effects and the area under the inhibitory time curve (AUIC). There are however other variables which it is difficult to incorporate into in vitro tests, especially the immune status of the patient, which can be crucial to outcome. While the immune system can be boosted in certain instances, e.g. by growth factors or immunoglobulin infusions, our ability to modify the immune response to infection has been frustrated. Understanding the interaction of antibiotics with the immune system and the consequences of the differing actions of the various antibiotic classes on the immune response is another door that is opening for the future.

Single-dose pharmacokinetics of meropenem during continuous venovenous hemofiltration

The pharmacokinetic properties of meropenem were investigated in nine critically ill patients treated by continuous venovenous hemofiltration (CVVH). All patients received one dose of 1 g of meropenem intravenously. High-flux polysulfone membranes were used as dialyzers. Meropenem levels were measured in plasma and ultrafiltrate by high-performance liquid chromatography. The total body clearance and elimination half-life were 143.7 +/- 18.6 ml/min and 2.46 +/- 0.41 h, respectively. The post- to prehemofiltration ratio of meropenem was 0.24 +/- 0.06. Peak plasma drug concentrations measured 60 min postinfusion were 28.1 +/- 2.7 microgram/ml, and trough levels after 6 h of CVVH were 6.6 +/- 1.5 microgram/ml. The calculated total daily meropenem requirement in these patients with acute renal failure and undergoing CVVH was 2,482 +/- 321 mg. Based on these data, we conclude that patients with severe infections who are undergoing CVVH can be treated effectively with 1 g of meropenem every 8 h.