I[ntroduction]{.smallcaps} {#sec1-1}
==========================

Severe sepsis and septic shock are common problems in the Intensive Care Units (ICUs) and produce high morbidity and mortality.\[[@ref1][@ref2]\] Early initiation of effective antimicrobial treatment is an important component of therapy against severe sepsis and septic shock.\[[@ref3][@ref4]\] Increasing antibiotic resistance is a major health-care problem in the ICU, but the discovery and development of suitable antibacterial drugs are slow paced and have become more costly.\[[@ref5][@ref6]\] It is critical to use antibiotics at optimal doses and appropriate routes to improve their efficacy and decrease the chances of developing drug resistance in bacteria.\[[@ref7]\]

Meropenem is a common choice for the treatment of severe sepsis and septic shock. Similar to other beta-lactam antibiotics, it is time dependent and its antibacterial activity is related to the duration of the maintenance of its free concentration above the minimal inhibitory concentration (MIC) during each dosing interval (referred to as % of T\>MIC). The T\>MIC required for carbapenems has been reported to be at least 40%,\[[@ref8]\] and a T\>MIC of 100% displayed significantly greater clinical and bacteriological outcomes in patients with serious bacterial infections.\[[@ref9]\] Extended infusions, especially continuous infusion of beta-lactam antibiotics, can prolong the T\>MIC and improve antibacterial activity. Furthermore, the pathophysiological changes associated with severe sepsis and septic shock often affect the volume of distribution (Vd), drug clearance, and pharmacokinetic parameters.\[[@ref10][@ref11]\] The use of continuous administration of meropenem was studied in some trials and indicated greater pharmacokinetic efficacy,\[[@ref10]\] bacteriological eradication,\[[@ref12]\] and clinical cure rates.\[[@ref13]\] However, these trials have not been conducted among patients with severe sepsis and septic shock, and combination pharmacokinetic/pharmacodynamic studies are scarce.

The aim of this study was to compare the pharmacokinetic, bacteriological, and clinical efficacy of continuous infusion of meropenem versus traditional intermittent administration (as recommended by the manufacturer\[[@ref14]\]) in patients with severe sepsis and septic shock.

M[ethods]{.smallcaps} {#sec1-2}
=====================

Ethical approval {#sec2-1}
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This was a single-center, prospective, randomized, comparative study; the trial was approved by the local research Ethics Committee of our institution (2012\[19\]) and conformed to the tenets of the *Declaration of Helsinki*. Written informed consent was obtained from patients' next of kin or legal representatives.

Inclusion criteria {#sec2-2}
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All the patients who were diagnosed with severe sepsis or septic shock and were admitted to the ICU, received meropenem therapy, and provided informed consent were included in the study. Meropenem administration was indicated as empirical therapy for severe infection without a proven pathogen, or as a second-line antibiotic based on microbiological findings. Concomitant antimicrobial therapy was permitted. The diagnosis of severe sepsis and septic shock was made in accordance with the *International Guidelines for Management of Severe Sepsis and Septic Shock*.\[[@ref15]\]

Exclusion criteria {#sec2-3}
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Exclusion criteria included age \<18 years, pregnancy, acute or chronic renal failure with a glomerular filtration rate (GFR; calculated with the Cockcroft formula) \<50 ml/min, immunodeficiency or taking immunosuppressant medication, allergy to meropenem, and previous application of meropenem in the past 2 weeks. According to the manufacturer\'s product information, meropenem dose reduction is required when the GFR is \<50 ml/min.\[[@ref14]\]

Patient randomization and treatment protocol {#sec2-4}
--------------------------------------------

Patients meeting the inclusion criteria with no presence of exclusion criteria were randomized into equally numbered groups using sealed opaque envelopes without stratification. They were assigned to receive either continuous infusion (continuous group, *n* = 25) or intermittent intravenous (i.v.) application (intermittent group, *n* = 25). The patients in the continuous group received a loading dose of 0.5 g of meropenem in 100 ml of normal saline i.v. infused over 30 min followed immediately by continuous infusion of 3 g of meropenem over 24 h. Regarding meropenem stability, 0.5 g of meropenem was continuously infused over 4 h in 50 ml of normal saline.\[[@ref16][@ref17]\] The patients in the intermittent group received the first dose of 1.5 g of meropenem in 100 ml of normal saline infused over 30 min, and then 1 g in 100 ml of normal saline infused over 30 min for every 8 h. The dose for both groups on day 1 was 3.5 g and 3 g/day thereafter. Patients in both groups were treated during their ICU stays by the regular team of ICU physicians and received standard intensive care (the researchers were not involved in the clinical strategy). Meropenem administration was stopped under the following conditions: further bacterial cultures and MIC testing indicated resistance to meropenem, bacterial cultures and MIC testing indicated increased sensitivity to other narrow-spectrum antibiotics, which could better permeate the infection region (de-escalation of antimicrobial therapy), and significant resolution of clinical symptoms and negative bacterial cultures.

Clinical end points {#sec2-5}
-------------------

The primary end points were clinical and microbiological results of meropenem therapy. Clinical success was defined as complete or partial resolution of temperature, clinical signs and symptoms of infection, and leukocytosis. Clinical failure was defined as the appearance of any of the following: persistent or progressing signs and symptoms of infection, or death because of infection. Microbiological outcomes included microbiological eradication and superinfection (which was defined as requiring other antibiotics to target a new Gram-negative bacterial infection). Appropriate routine bacterial cultures (including two sets of blood cultures) were obtained before commencing antimicrobial therapy and were repeated daily if clinical manifestations did not resolve or were exacerbated. Secondary end points included ICU mortality, length of ICU stay (LOS), and duration of meropenem treatment.

The following clinical data were collected: sex, age, weight, diagnosis, site and etiology of infection treated by meropenem, pathogens, MICs of identified pathogens, the Acute Physiology and Chronic Health Evaluation II (APACHE II) and Sequential Organ Failure Assessment (SOFA) scores at the start of meropenem therapy, white blood cell (WBC) counts at 1 and 5 days of meropenem therapy, procalcitonin (PCT) at 1 and 5 days of meropenem therapy, daily body temperature, serum creatinine at therapy commencement, and the total fluid infusion in the first 24 h.

Microbiologic methods {#sec2-6}
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Identification of antimicrobial susceptibility and MIC testing were performed in the clinical microbiology laboratory using the VITEK 2 automated system (bioMérieux, Marcy l'Etoile, France).

Blood sampling {#sec2-7}
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Two milliliters of blood were collected using an indwelling arterial catheter for each blood sample to determine plasma meropenem concentrations. In the first dosing period (the first 8 h), samples were collected at 0, 30, 60, 150, 200, 360, and 480 min. In the third dosing period (the first 8 h; steady state), blood samples were acquired in line with an intermittent infusion dose or change of continuous infusion bag at 0, 30, 60, 150, 200, and 480 min. The 200-min time point corresponded to nearly 40% of the dosing interval and was regarded as T40%. Specimens were centrifuged at 3000 rpm for 10 min and then frozen at −20°C for subsequent analysis. All samples were assayed individually within 7 days of collection.

Drug assay {#sec2-8}
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Plasma meropenem concentrations were measured using an ultra-high-performance liquid chromatography (UPLC)-diode array detector-column switching method with the Shimadzu LC-20A Prominence System. The chromatographic column was the ACQUITY UPLC^®^ BEH C18 column using the gradient elution method (mobile Phase A and extracting mobile Phase C: methanol-0.05 mol/L K~2~HPO~4~ 5:95, adjusted to a pH of 7; mobile Phase B: methanol). The absorbance wavelength was 299 nm.

Pharmacokinetic analysis {#sec2-9}
------------------------

The pharmacokinetic profile of meropenem in the intermittent group was individually assessed using the WinNonlin Professional version 5.0.1 software (Pharsight Corporation, Mountain View, CA, USA). A one-compartment model with the first-order elimination was selected to fit the data. Investigated pharmacokinetic parameters included the Vd and total clearance (CL).

Statistical analysis {#sec2-10}
--------------------

Kolmogorov--Smirnov test was used to check for the normal distribution of data. Continuous normally distributed data were tested with unpaired *t*-tests; nonnormally distributed data were analyzed using Mann--Whitney *U*-test. Categorical data were evaluated using Chi-square test. Unless stated otherwise, normally distributed data are presented as mean ± standard deviation, and as median (interquartile ranges) where nonnormally distributed. A two-sided *P* \< 0.05 was considered statistically significant for all tests. Statistical analyses were performed using the SPSS statistical package version 17.0 (SPSS Inc., Chicago, IL, USA).

R[esults]{.smallcaps} {#sec1-3}
=====================

Patient demographics {#sec2-11}
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Fifty patients who fulfilled the participation criteria were enrolled between June 2012 and December 2014; 25 patients were randomized to each of the intermittent group and the continuous group. The main infection site was intra-abdominal (54%); other sites included the lung (38%), bloodstream (16%), urinary tract (6%), wound (2%), central nervous system (2%), and multiple sites (18%). Bacterial cultures of 43 patients (86%) were positive for *Escherichia coli* (26%), *Pseudomonas aeruginosa* (24%), *Klebsiella pneumonia* (16%), *Acinetobacter baumannii* (12%), *Enterobacter cloacae* (2%), *Enterobacter aerogenes* (2%), *Providencia stuartii* (2%), and *Burkholderia cepacia* (2%).

Comparison of clinical characteristics between the two groups {#sec2-12}
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There were no significant differences in age, weight, severity of illness (APACHE II and SOFA scores), site of infection, or rate of isolated bacteria and other evaluated parameters \[[Table 1](#T1){ref-type="table"}\]. In addition, the rates of clinical success (the primary endpoint) were similar at 16 (64%) in the continuous group and 14 (56%) in the intermittent group (*P* = 0.564). The duration of meropenem treatment was significantly shorter in the continuous group (*P* = 0.035); however, there were no significant differences in other secondary end points including ICU mortality and LOS. Moreover, the rate of superinfection among participants in the continuous group (4%) was less than that in the intermittent group (16%), although the difference was not statistically significant (*P* = 0.157).

###### 

Comparison of clinical characteristics between the continuous and intermittent groups

  Clinical characteristics                 Continuous group (*n* = 25)   Intermittent group (*n* = 25)   Statistical value   *P*
  ---------------------------------------- ----------------------------- ------------------------------- ------------------- -------
  Male sex, *n* (%)                        10 (40.0)                     11 (44.0)                       0.082\*             0.774
  Age (years)                              68.0 ± 15.4                   67.0 ± 12.2                     0.275^‡^            0.309
  Weight (kg)                              60.5 ± 10.2                   63.8 ± 11.8                     −1.067^‡^           0.445
  APACHE II score                          19.4 ± 5.0                    19.7 ± 5.9                      −0.181^‡^           0.523
  SOFA score                               8.0 ± 2.8                     8.5 ± 2.4                       −0.657^‡^           0.577
  GFR (ml/min)                             97.5 ± 43.4                   91.1 ± 34.0                     0.578^‡^            0.295
  WBC1 (×10^9^/L)                          11.5 ± 4.0                    11.9 ± 5.0                      −0.268^‡^           0.410
  WBC5 (×10^9^/L)                          9.2 ± 3.9                     10.2 ± 4.3                      −0.822^‡^           0.325
  PCT1 (µg/L), value (range)               1.3 (0.3--4.0)                1.2 (0.3--23.8)                 292.500^†^          0.696
  PCT5 (µg/L), value (range)               0.2 (0.1--0.6)                0.3 (0.1--1.3)                  80.000^†^           0.610
  Fluid infusion (ml)                      4267.8 ± 1074.7               4225.7 ± 858.3                  0.153^‡^            0.477
  Site of infection, *n* (%)                                                                                                 
   Lung                                    9 (36.0)                      10 (40.0)                       0.085\*             0.771
   Intra-abdominal                         14 (56.0)                     13 (52.0)                       0.081\*             0.777
   Bloodstream                             5 (20.0)                      3 (12.0)                        0.595\*             0.440
   Urinary tract                           1 (4.0)                       2 (8.0)                         0.355\*             0.552
   Wound                                   1 (4.0)                       0                               1.020\*             0.312
   Central nervous system                  0                             1 (4.0)                         1.020\*             0.312
   Multiple sites                          5 (20.0)                      4 (16.0)                        0.136\*             0.713
   Isolated bacteria, *n* (%)              22 (88.0)                     21 (84.0)                       1.296\*             0.684
   Length of ICU stay (days)               10.0 (7.0--26.5)              10.0 (8.0--27.0)                263.000^†^          0.336
  Duration of meropenem treatment (days)   7.6 ± 2.3                     9.4 ± 5.5                       −2.258^‡^           0.035
  ICU mortality, *n* (%)                   7 (28.0)                      8 (32.0)                        0.095\*             0.758
  Clinical success, *n* (%)                16 (64.0)                     14 (56.0)                       0.333\*             0.564
  Superinfection, *n* (%)                  1 (4.0)                       4 (16.0)                        2.000\*             0.157

\**χ*^2^ value; ^†^*U* value; ^‡^*t* value. APACHE II: Acute Physiology and Chronic Health Evaluation II; SOFA: Sequential Organ Failure Assessment; GFR: Glomerular filtration rate (calculated with Cockcroft formula); WBC1 and WBC5: White blood cell count at the 1^st^ and the 5^th^ days of meropenem therapy; PCT1 and PCT5: Procalcitonin at the 1^st^ and the 5^th^ days of meropenem therapy; ICU: Intensive Care Unit.

Comparison of microbiological characteristics between the two groups {#sec2-13}
--------------------------------------------------------------------

The main bacterial MIC in both groups was ≤0.25 (68.2% of the continuous group and 61.9% of the intermittent group); the difference was not significant. The rate of microbiological eradication between participants was slightly, but not significantly, higher in the continuous group (81.8%) than in the intermittent group (66.7%; *P* = 0.255) \[[Table 2](#T2){ref-type="table"}\].

###### 

Comparison of microbiological characteristics between the continuous and intermittent groups

  Microbiological characteristics        Continuous group (*n* = 22)   Intermittent group (*n* = 21)   *χ*^2^   *P*
  -------------------------------------- ----------------------------- ------------------------------- -------- -------
  Bacterial MIC, *n* (%)                                                                                        
   ≤0.25                                 15 (68.2)                     13 (61.9)                       4.322    0.364
   1                                     0                             2 (9.5)                                  
   2                                     1 (4.5)                       3 (14.2)                                 
   4                                     3 (13.6)                      1 (4.8)                                  
   ≥16                                   3 (13.6)                      2 (9.5)                                  
  Microbiological eradication, *n* (%)   18 (81.8)                     14 (66.7)                       1.296    0.255

MIC: Minimal inhibitory concentration.

Pharmacokinetic data {#sec2-14}
--------------------

The observed concentration-time profiles of meropenem administered to the intermittent and continuous groups are shown in [Figure 1a](#F1){ref-type="fig"} (the first dosing period) and [Figure 1b](#F1){ref-type="fig"} (the third dosing period). The comparative Cmax, Cmin, and concentrations of T40% (C~T40%~) in both dosing periods are shown in [Table 3](#T3){ref-type="table"}. The Vd was 27.3 ± 4.8 L while the CL was 0.23 ± 0.1 L/min.

![Plasma concentrations of meropenem administered to patients with severe sepsis or septic shock by intermittent infusion and continuous infusion for (a) the first dosing period and (b) third dosing period.](CMJ-130-1139-g001){#F1}

###### 

Comparison of pharmacokinetic data between the continuous and intermittent groups

  Pharmacokinetic data (mg/L)       Continuous group (*n* = 22)   Intermittent group (*n* = 21)   *U*      *P*
  --------------------------------- ----------------------------- ------------------------------- -------- -------
  The first dosing period Cmax      19.8 (16.0--21.4)             51.8 (44.3--54.25)              0.000    0.000
  The first dosing period Cmin      11.2 (7.6--11.7)              0.5 (0--1.0)                    0.000    0.000
  The first dosing period C~T40%~   10.98 (8.18--12.38)           5.62 (4.22--6.83)               65.000   0.000
  The third dosing period Cmax      12.5 (8.7--12.9)              46.4 (40.8--49.0)               0.000    0.000
  The third dosing period Cmin      11.4 (7.9--12.5)              0.6 (0--1.1)                    0.000    0.000
  The third dosing period C~T40%~   11.7 (8.4--12.6)              5.4 (4.1--6.7)                  41.000   0.000

Cmax: Peak concentrations; Cmin: Trough concentrations; C~T40%~: Concentrations of T40% (the time point of 200 min was nearly 40% of a dosing interval and was marked as T40%).

D[iscussion]{.smallcaps} {#sec1-4}
========================

In this prospective randomized pilot study, we found that continuous infusion of 3 g of meropenem per day preserved a similar rate of clinical cure as intermittent infusion, but showed a tendency for higher microbiological efficacy and required a significantly shorter duration of treatment. We also found that continuous infusion of meropenem achieved a better steady concentration. Both intermittent and continuous infusion regimens for meropenem were effective against most Gram-negative pathogens that remain highly susceptible (MIC ≤0.25). Against intermediate-susceptibility bacteria (MIC = 4 for nonfermentative bacteria and MIC = 2 for enterobacteriaceae), better pharmacokinetic results are likely to be achieved by continuous infusion than by intermittent infusion of the same daily dose. However, administering 3 g of meropenem using either infusion regimen was pharmacokinetically ineffective against resistant bacteria (i.e., MIC ≥8 and particularly MIC ≥16) in our study.

There were no significant differences in the primary clinical outcomes (cure rates) between the continuous and intermittent groups. Our results are most comparable to those of Chytra *et al*. who found a nonsignificant difference in cure rates in a trial of 240 critically ill patients randomized to receive meropenem by continuous infusion versus intermittent (bolus) administration (83% vs. 75%, respectively).\[[@ref12]\] Recently, Dulhunty *et al*.\[[@ref18]\] conducted a randomized controlled trial in 25 ICUs to evaluate the efficacy of continuous versus intermittent infusion of beta-lactam antibiotics in patients with severe sepsis and found no difference between treatment groups in clinical cure rates (52.4% vs. 49.5%). In contrast, a retrospective study of meropenem\[[@ref13]\] and that of other beta-lactam agents found that continuous infusion resulted in better results than intermittent bolus administration did.\[[@ref19][@ref20][@ref21]\]

Several factors could explain our results. First, ours was a pilot study, and our sample size was small. Second, the theoretical advantage of continuous infusion is crucially dependent on the MIC. In our study, 86% of participants had an identified pathogen, and the MICs in a majority of our isolated pathogens (65.1%) were ≤0.25 (MICs were measured by semi-quantitative determination). However, according to our pharmacokinetic data, both the continuous and intermittent groups achieved T\>MIC values close to 100% for the highly susceptible pathogens \[[Table 3](#T3){ref-type="table"} and [Figure 1](#F1){ref-type="fig"}\]. For intermediate-susceptibility bacteria (MIC = 4 for nonfermentative bacteria and MIC = 2 for enterobacteriaceae), continuous infusion may achieve a better pharmacokinetic profile. However, only 18.6% of the pathogens were in this category; 11.6% of the isolated pathogens were resistant bacteria (MIC ≥16) against which adequate pharmacokinetic efficacy was not achieved in either group with 3 g of meropenem per day (the C~T40%~ of the intermittent group was 5.4 mg/L, while the Cmin of the continuous group was 11.7 mg/L). It is possible that increasing the dosage to 6 g/day in renal function normal patients can achieve better efficacy against resistant bacteria in the continuous group than in the intermittent group. Jamal *et al*.\[[@ref22]\] observed the pharmacokinetics of meropenem in critically ill patients receiving continuous venovenous hemofiltration and found that administration of 3 g/day with continuous infusion resulted in a higher steady-state concentration of 21.91 mg/L. Third, physiological changes associated with severe sepsis and septic shock can increase Vd and drug clearance\[[@ref10][@ref23]\] leading to lower plasma\[[@ref11][@ref24]\] and tissue concentrations\[[@ref25]\] than is commonly observed in noncritically ill patients. In our study of patients with severe sepsis and septic shock, Vd (27.3 L) and CL (0.23 L/min) were higher, and plasma concentrations of meropenem were lower than that in other critically ill patients.\[[@ref11][@ref26][@ref27]\] Taccone *et al*.\[[@ref28]\] conducted a prospective, multicenter study to determine whether the first dose of beta-lactam antibiotics would result in inadequate serum drug concentrations in patients with severe sepsis and septic shock and found that the target pharmacokinetic profiles were achieved in only 75% of patients for meropenem.

Although there was no significant difference in the rate of superinfection and microbiological eradication between participants in the continuous and intermittent groups, the former tended to show more favorable results. Other studies had similar outcomes that showed an improved bacteriological efficacy associated with the continuous application of meropenem\[[@ref12]\] and beta-lactams.\[[@ref20]\] A possible explanation for the better microbial clearance in the continuous group may be the higher tissue concentrations of meropenem in patients of this group than in those of the intermittent group;\[[@ref10][@ref25]\] patients with pneumonia in particular experience poor penetration of most beta-lactams into lung tissue.\[[@ref29]\]

We observed a significantly shorter duration of meropenem treatment in the continuous group. Similar results were observed in the study of Chytra *et al*.\[[@ref12]\] However, we did not find differences in ICU mortality rates, LOS, and values of WBC and PCT. Our results are concurrent with those of Dulhunty *et al*.,\[[@ref18]\] although previous studies had conflicting results.\[[@ref30]\] Meta-analyses by Roberts *et al*.\[[@ref31]\] and Shiu *et al*.\[[@ref32]\] found no significant differences in cumulative mortality between the two groups. In contrast, Falagas *et al*. found a significant mortality difference between continuous and intermittent infusion in their meta-analysis of observational and randomized controlled trials comparing infusion methods of carbapenems and piperacillin-tazobactam.\[[@ref33]\] Such studies have not been conducted in patients with severe sepsis and septic shock; their mortality rates were much lower than that in our patients.

Our study has the following limitations. This was a pilot investigation, and our results may have been influenced by our small sample size (only fifty patients). Furthermore, the study was not blinded and was conducted at a single center; therefore, bias in clinical evaluations may exist. We did not record the time between the onset of sepsis and administration of antibiotic therapy, but all the patients were treated according to the Surviving Sepsis Campaign guideline. We set the third dosing period as the steady state despite it being longer than 5 half-lives; the sixth dosing period may be more stable.

In conclusion, we found that continuous infusion of meropenem *in lieu* of intermittent dosing provides equal clinical efficacy and mortality, but also produces a significantly shorter duration of meropenem treatment plus a tendency toward superior bacteriological efficacy in patients with severe sepsis and septic shock. Optimal administration modalities and dosing should be chosen carefully according to the MICs of the pathogens. Both continuous and intermittent infusion of meropenem can achieve good pharmacokinetics against high-susceptibility pathogens; however, continuous infusion is better for intermediate susceptibility. Neither method is effective against drug-resistant bacteria in regular doses, although a continuous regimen may be better with incrementally increasing doses. Therapeutic drug monitoring based on pathogen MICs and doses must be the strategic focus of the future studies.

Financial support and sponsorship {#sec2-15}
---------------------------------

This study was supported by grants from the Clinical Research Special Foundation of Chinese Medical Association (No. 11030140258).

Conflicts of interest {#sec2-16}
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There are no conflicts of interest.

**Edited by:** Li-Min Chen
