Pharmacodynamic effects of meropenem on gram-negative bacteria

Optimizing the Use of Beta-Lactam Antibiotics in Clinical Practice: A Test of Time

Despite their limitations, the pharmacokinetics (PK) and pharmacodynamics (PD) indices form the basis for our current understanding regarding antibiotic development, selection, and dose optimization. Application of PK-PD in medicine has been associated with better clinical outcome, suppression of resistance, and optimization of antibiotic consumption. Beta-lactam antibiotics remain the cornerstone for empirical and directed therapy in many patients. The percentage of time of the dosing interval that the free (unbound) drug concentration remains above the minimal inhibitory concentration (MIC) (%fT > MIC) has been considered the PK-PD index that best predicts the relationship between antibiotic exposure and killing for the beta-lactam antibiotics. Time dependence of beta-lactam antibiotics has its origin in the acylation process of the serine active site of penicillin-binding proteins, which subsequently results in bacteriostatic and bactericidal effects during the dosing interval. To enhance the likelihood of target attainment, higher doses, and prolonged infusion strategies, with/or without loading doses, have been applied to compensate for subtherapeutic levels of antibiotics related to PK-PD changes, especially in the early phase of severe sepsis. To minimize resistance and maximize clinical outcome, empirical therapy with a meropenem loading dose followed by high-dose-prolonged infusion should be considered in patients with high inoculum infections presenting as severe (Gram negative) sepsis. Subsequent de-escalation and dosing of beta-lactam antibiotics should be considered as an individualized dynamic process that requires dose adjustments throughout the time course of the disease process mediated by clinical parameters that indirectly assess PK-PD alterations.

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

OBJECTIVE

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

DATA SELECTION

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

DATA EXTRACTION

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

DATA SYNTHESIS

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

CONCLUSIONS

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

In vitro pharmacodynamics of colistin against Acinetobacter baumannii clinical isolates

BACKGROUND

Colistin is being increasingly used for treatment of infections caused by multidrug-resistant Gram-negative bacteria, including Acinetobacter baumannii.

METHODS

The in vitro pharmacodynamic properties of colistin (sulphate) were investigated by studying the time-kill kinetics and the post-antibiotic effect (PAE) against multidrug-resistant, including colistin heteroresistant, A. baumannii. Time-kill was studied with four multidrug-resistant clinical isolates at concentrations ranging from 0.5 to 64 x MIC. The PAE was examined after 20 min exposure with five clinical isolates, including the four in the time-kill study, plus ATCC 19606.

RESULTS

Colistin showed extremely rapid killing in a concentration-dependent manner; but re-growth was observed as early as 3 h and substantial re-growth at 24 h even at concentrations up to 32 x MIC or 64 x MIC for some isolates. Colistin exhibited modest PAE of 1.0, 2.3 and 3.5 h at 16, 32 and 64 x MIC, respectively, against ATCC 19606. Surprisingly, negative PAE (range: -0.8 to -8.15 h) was observed for all of the five clinical isolates.

CONCLUSIONS

These findings suggest that monotherapy with colistin methanesulphonate, the parenteral form of colistin, and long dosage intervals (e.g. 24 h) may be problematic for treatment of infections caused by colistin heteroresistant A. baumannii.

Antibiotic pharmacokinetic and pharmacodynamic considerations in critical illness

BACKGROUND

Many factors over which there may be little control may influence the response of a patient to therapy. However, therapy with antibiotics can be readily optimised.

DISCUSSION

Concentration-dependent agents such as aminoglycosides appear effective and to entail fewer side effects when given in large, infrequent doses. There is also evidence that time-dependent antibiotics often fail to reach adequate concentrations throughout the treatment period. To date no randomised controlled prospective trial has demonstrated improvement in clinical outcome following infusion rather than intermittent boluses of time-dependent antibiotics. Critical illness alters antibiotic pharmacokinetics principally through increases in volume of distribution. Other than glycopeptides and aminoglycosides, antibiotic blood concentrations are rarely monitored and therefore adequate concentrations can only be inferred from clinical response.

CONCLUSIONS

Failure to respond within the first few days of empirical treatment may be due to antibiotic resistance or inadequate doses. Therefore the same rigor should be applied to achieving adequate antibiotic concentrations as is applied to inotropes, which are titrated to achieve predetermined physiological targets.

Meropenem: an updated review of its use in the management of intra-abdominal infections

Meropenem is a carbapenem antibacterial agent with a broad spectrum of activity which encompasses gram-negative, gram-positive and anaerobic bacteria. Like other carbapenems, meropenem is stable against chromosomal and extended-spectrum beta-lactamases. In patients with moderate to severe intra-abdominal infections, empirical monotherapy with meropenem achieved clinical response rates ranging from 91 to 100% in 7 randomised comparative trials. Efficacy rates were similar to those of imipenem/cilastatin (94 to 97%), clindamycin plus tobramycin (93%) and, overall, to cefotaxime plus metronidazole (75 to 100%), although there were differences between trials versus this combination regimen. According to limited data, meropenem also achieved clinical response rates of over 80% in patients with severe intra-abdominal infections. Meropenem is well tolerated, the most common adverse events being diarrhoea, rash, nausea/vomiting and inflammation at the injection site which are reported in <2.5% of patients each. Meropenem also has an improved CNS tolerability profile compared with imipenem/cilastatin.

CONCLUSIONS

Extensive comparative clinical data demonstrate that meropenem can be used effectively as empirical monotherapy in moderate to severe intra-abdominal infections. It also shows potential in the most severe forms of infection, although experience in this infection type remains limited. Compared with standard combination regimens, meropenem offers the benefits of ease of administration without the need for monitoring. It also offers improved CNS tolerability compared with imipenem/cilastatin with the option of a higher maximum dosage, which may be a particular advantage in patients with severe intra-abdominal infections.

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.

Pharmacokinetics of meropenem in critically ill patients with acute renal failure undergoing continuous venovenous hemofiltration

OBJECTIVE

Meropenem is a broad-spectrum antibiotic used for severe infections. In patients with chronic end-stage renal failure, meropenem clearance is reduced and doses must be adjusted according to the creatinine clearance. The aim of this study was to assess pharmacokinetic data of meropenem in patients with acute renal failure and to determine the amount of drug removed by continuous venovenous hemofiltration, an often-used renal replacement therapy in patients with acute renal failure.

METHODS

Nine critically ill anuric patients with acute renal failure undergoing continuous venovenous hemofiltration received 500 mg meropenem 2 or 3 times daily. Plasma and hemofiltrate concentrations were determined during 1 dosing interval at steady state. Pharmacokinetic parameters were calculated for a 2-compartment open model and dose requirements were calculated.

RESULTS

The total meropenem clearance was 52.0 +/- 8.4 mL/min, with a hemofiltration clearance of 22.0 +/- 4.7 mL/min and a nonrenal-nonhemofiltration clearance of 29.9 +/- 5.4 mL/min; 235.9 +/- 88.6 mg, or 47.2% +/- 17.7%, of the dose were removed through continuous venovenous hemofiltration. The terminal elimination half-life was 8.7 +/- 3.5 hours and the volume of distribution at steady state was 12.4 +/- 1.8 L. Peak and trough concentrations for a dosing interval of 12 hours were 38.9 +/- 9.7 mg/L and 7.3 +/- 1.3 mg/L, respectively. The corresponding concentrations for a dosing interval of 8 hours were 44.7 +/- 10.4 mg/L and 11.9 +/- 0.7 mg/L, respectively.

CONCLUSION

Pharmacokinetic data of anuric patients with acute renal failure were similar to those of patients with end-stage renal failure. Because hemofiltration contributes significantly to meropenem elimination, the recommended dose for critically ill anuric patients receiving continuous venovenous hemofiltration should be increased by 100% to avoid potential underdosing.

The effect of subinhibitory concentrations of antibiotics on adherence of Pseudomonas aeruginosa to cystic fibrosis (CF) and non-CF-affected tracheal epithelial cells

OBJECTIVES

this project investigated the proposition that Pseudomonas aeruginosa binds preferentially to cystic fibrosis (CF) respiratory cells. In addition, disadherence of P. aeruginosa by subinhibitory concentrations of antibiotics was examined as a possible explanation for clinical improvement seen in chronically colonized patients when treated with anti-pseudomonal agents.

METHODS

we used a distinctive HPV-transformed respiratory cell line to compare adherence of two strains of P. aeruginosa to CF and non-CF respiratory cells. The effect of subinhibitory concentrations of antipseudomonal antibiotics on adherence of P. aeruginosa to cell monolayers was measured.

RESULTS

piliated P. aeruginosa bound significantly better to CF than non-CF-affected cells (P=0.003). Adherence was significantly reduced when organisms were preincubated in subinhibitory concentrations of antibiotics overnight (P<0.001).

CONCLUSIONS

these results support the presence of an altered receptor present on CF-affected cells that binds piliated strains of P. aeruginosa. Bacterial disadherence due to subinhibitory concentrations of antibiotics may partially explain why clinical improvement is observed in CF patients with acute respiratory exacerbations.

Pharmacokinetics of meropenem in febrile neutropenic patients. Swedish study group

To determine the impact of neutropenia on the pharmacokinetics of meropenem, 14 patients with fever and neutropenia were given 1 g of meropenem i.v. every 8 h as an infusion over 30 min. The volume of distribution (16.2 l/1.73 m2) and the nonrenal clearance [75 ml/(min x 1.73 m2)] in this group were significantly increased compared to healthy subjects studied previously with identical techniques. The kinetic study was repeated when the patients had a normal temperature and a raised neutrophil count; most kinetic variables did not differ from the findings on the first day of treatment. The pharmacokinetic profile of meropenem in febrile neutropenic patients differs from earlier findings in healthy subjects. Considering these data and known minimum inhibitory concentration values for common pathogens, meropenem administered every 6 to 8 h seems an appropriate regimen in patients with febrile neutropenia. The shorter time interval may be used for treatment of Pseudomonas infection.