Effects of uric acid, creatinine clearance, and infusion duration on the population pharmacokinetics of meropenem in critically ill patients

Background: Meropenem is a carbapenem antibiotic that has demonstrated excellent in vitro activity against gram-negative clinical isolates and is commonly used in critically ill patients. This study aimed to nd the pharmacokinetic/ pharmacodynamic of meropenem in critically ill patients and whether prolonged injection duration is really benecial to meropenem therapy. Method: We included 209 samples in 64 patients in this prospective study. PPK analysis and Monte Carlo dosing simulations were developed using Phoenix. Results: A two-compartment model described the data adequately. Clearance (CL), volume (V), clearance of peripheral compartment (CL 2 ), volume of peripheral compartment (V 2 ) were 6.15 L/h, 2.83 L/h, 17.40L, and 17.48L, respectively. Creatinine clearance and uric acid were signicant covariates. Patients with creatinine clearance of 60 ml/min or less and uric acid greater than 400 μmol/l could achieve the target > 90% under the minimum inhibitory concentration (MIC) of 8 mg/L, even with the administration dose of 500 mg/8 h with a 2-h infusion. Prolonging the infusion time signicantly improved the therapeutic effect when MIC(cid:0)4. However, for the pharmacodynamic (PD) effects of 100% fT > MIC and 100% fT > 4MIC, no signicant statistical difference was observed in critically ill patients. Conclusions: Critically ill patients with lower creatinine clearance and higher uric acid levels were likely to need a lower dosage of meropenem. Prolonged infusion time were not always benecial for those who need a higher therapeutic target (100% fT > MIC,100% fT > 4 MIC) or with MIC 4mg/L. Increasing dose or alternative therapeutic strategies may be required for critically ill patients with drug-resistant or severe infections. The study is of great signicance to guide the rational use of meropenem in critically ill patients. safety of meropenem in critically ill patients. It is the rst time to nd that uric acid level signicantly impacts meropenem use and to assess the achievement of various PK/PD targets (40% time free concentration above MIC (fT>MIC), 100% fT>MIC and 100% fT>4MIC) with MIC values ranging from 1–8 mg/L in critically ill patients with severe pneumonia. During clinical empirical therapy, dose adjustment based on creatinine clearance and uric acid appears to be reasonable. Patients with a lower level of creatinine clearance and a high uric acid level tend to require lower dosages. There was signicant clinical benet from prolonged infusion time when MIC ≥ 4 mg/L. Increasing dose or alternative therapeutic strategies may be required for critically ill patients with drug-resistant or severe infections who require higher therapeutic targets. least-squares method was used to estimate model parameters. The goodness of t and visual predictive check (VPC) were used to evaluate the model. Objective function values (OFV) were used to compare the model t. Covariates were retained in the model if the additional covariates were signicant at a P-value of 0.01 ( △ OFV > 6.635). VPC was used to evaluate the goodness of t 12, 31-34 . attainment.


Introduction
Severe pneumonia is a risk factor for in-hospital mortality [1][2][3][4] . In recent years, carbapenems have been widely used in patients and are considered the last line of defense in treating gram-negative bacterial infections [5][6][7] . Meropenem is a second-generation carbapenem antibiotic. Unlike the rst generation, the 1β methyl modi cation of the chemical structure enhances the stability of the drug to renal dehydropeptidase I 8,9 . It is also a broad-spectrum antimicrobial used as empirical or directed therapy in critically ill patients. Meropenem shows time-dependent antibacterial activity and is characterized by linear pharmacokinetics in vivo; higher doses correspond to higher peak and trough concentrations 10,11 .
In healthy volunteers, the elimination half-life of meropenem in plasma was about 1 hour 12,13 .
Severe pathophysiological changes in critical illness can lead to dramatically altered antimicrobial pharmacokinetics (PK). In populations such as children, elderly, and obese, those with severe burns, those treated with continuous renal replacement therapy, and patients on extracorporeal membrane oxygenation, meropenem shows signi cant individual differences in plasma concentrations and pharmacokinetic parameters [14][15][16][17][18] . These effects are related to the time that the free concentration is maintained above the minimum inhibitory concentration (MIC) (fT > MIC), at least 40% 19 . Several clinical studies suggest that 100% fT > MIC in plasma is associated with better therapeutic effects [20][21][22][23][24] .
Additionally, it has been widely reported that prolonged injection duration can improve the therapeutic effect of meropenem 25,26 . Nevertheless, it remains unclear whether standard meropenem dosing regimens achieve this target in critically ill patients. Therefore, this study aimed to measure the meropenem pharmacokinetic in critically ill patients and improve pharmacokinetic/pharmacodynamic and patient outcomes.

Study design and patients
This prospective study was conducted at the Department of Respiratory and Intensive Care Unit, the Second Xiang-ya Hospital of Central South University, between January and December 2019. The Ethics Committee of our hospital approved the study (number 2019-005) that was registered as a China Clinical Trial (ChiCTR1900020672).
Patients treated with meropenem and admitted to the Department of Respiratory and Critical Care Medicine were eligible. Written informed consent was obtained from all participants. Inclusion criteria were as follows: (i) severe lung infection; (ii) clear indications for the use of meropenem; (iii) the time of continuous medication exceeded two days; (iv) at least one steady-state plasma concentration could be obtained; (v) age > 18 years, and (vi) gram-negative bacilli were isolated from specimen culture. Exclusion criteria were as follows: (i) pregnancy and lactation; (ii) allergy to carbapenems; (iii) concomitant uses of sodium valproate; (iv) isolation of gram-positive cocci, viruses, or fungi before enrollment; and (v) incomplete dosing information or clinical data.
Based on the standard recommendations for meropenem use, the conventional dosage regimen was 500 mg/8 h or 1000 mg/8 h, two times/8 h, and continuous infusion for 30 min, 1 h or 2 h. From the electronic medical record information system, we recorded demographic information, clinical data, and laboratory test results using a standardized data collection form on the day of serum sampling. The endogenous creatinine clearance rate was calculated using the Cockcroft-Gault formula 27,28 .

Pharmacokinetic study
The PK model of meropenem in critically ill patients was developed using Phoenix NLME software (Version 8.1, Pharsight, A Certara Company, USA). Serum meropenem concentrations were tted to a twocompartment model using the logarithmic additive residual. The rst-order conditional estimationextended least-squares method was used to estimate model parameters. The goodness of t and visual predictive check (VPC) were used to evaluate the model. Objective function values (OFV) were used to compare the model t. Covariates were retained in the model if the additional covariates were signi cant at a P-value of 0.01 (△OFV > 6.635). VPC was used to evaluate the goodness of t 12, 31-34 .
Probability of target attainment.
We use Monte Carlo simulations (n = 3,000) to determine the probability of target attainment (PTA) with different signi cant covariates. Meropenem doses of 500 mg, 1,000 mg, and 2,000 mg given intravenously every 8 h (q8h) with a duration of 0.5 h, 2 h, and 4h were simulated at different levels of selected covariates. The PTA was calculated after three days of therapy. The MIC at which PTA was equal to 90% was derived to enable a numeric comparison among the regimens 16,18,35 . MIC values were selected for the most common value of pathogenic bacterias such as Enterobacter cloacae, Pseudomonas aeruginosa, Klebsiella pneumoniae, and Acinetobacter baumannii obtained from our hospital. PTA was calculated for single doses of 500 mg,1000 mg, and 2000 mg. The therapeutic target adopted the effect of 40% fT > MIC, 100% fT > MIC, and 100% fT > 4MIC 19,23,24 .

Statistical analysis.
Continuous variables are expressed as means (standard deviations [SD]) or medians (interquartile ranges) depending on the normality of distribution. Enumeration data were expressed as absolute numbers and relative frequencies. The Kolmogorov-Smirnov, and Shapiro-Wilk tests were used to test for normality. A two-sided P-value of < 0.05 was considered statistically signi cant. One-way analysis of variance was used to test the differences in selected signi cant covariate groups. All analyses were performed using IBM SPSS Statistics version 25 (IBM, New York, NY). Figures were generated using Phoenix NLME and Graphpad Prism version 8 (San Diego, CA, USA).

Demographic and clinical data of study patients
Sixty-four patients were enrolled in this prospective study. A total of 210 meropenem plasma samples were obtained; 73.43% of the patients were male, with an average weight of 62.5 kg. The average age was 63.5 years, and the mean Acute Physiology and Chronic Health Evaluation (APACHE) score was 17.2. More details about the demographic and clinical characteristics are shown in Table 1.

Pharmacokinetic model
A total of 210 meropenem plasma concentrations were included in the population analysis. The meropenem PPK was best described by a two-compartment linear model with rst-order elimination. A stepwise method was used to determine all the covariates that may affect the pharmacokinetic parameters. For covariates, we selected gender, age, body weight, APACHE score, Cockcroft-Gault CLCR (CG-CLCR), white blood cell, red blood cell, platelets, hemoglobin, alanine aminotransferase, aminotransferase, albumin, urea nitrogen, and uric acid. In ammatory indicators were also included in the covariate selection. Despite various covariates having relationships with the estimated clearance, they were not included in the nal model. Uric acid was nally found to be closely connected to meropenem V2 and CL. CG-CLCR was closely connected to CL. These two factors for meropenem V2 and CL improved the model t best. When they were added to the model, the log-likelihood value from the previous model was signi cantly improved (P < 0.01). The covariate model was as follows: The parameters for the basic and nal covariate model are shown in Table 3. Individual and population predicted serum meropenem concentrations vs. observed concentrations are shown in Fig. 1. The distribution of conditional weighted residuals is presented in Fig. 2. The values of conditional weighted residuals were between -2 to 2. Both plots indicated the tting advantages of the nal model. The nal covariate model was used for Monte Carlo dosing simulations.

Simulations
Monte Carlo simulations and meropenem probabilities of target attainment for various CG-CLCR values, uric acid values, dosage regimens, and MICs with a duration of 0.5 h, 2 h, and 4 h are presented in an additional le. Details are shown in Additional le 1-3. MIC values listed in the tables were chosen according to the sensitivity of pathogenic bacteria to meropenem at our hospital. We found that at the lowest dosage (500 mg/q8h), patients with uric acid levels of > 400 mol/L can achieve an optimal PTA ( of 40% fT > MIC for isolates with MICs of 8 mg/L with a duration of 4 h. However, those with uric acid levels > 40 mol/L and CG-CLCR of 120 ml/min could not achieve optimal PTA ( of 40% fT>MIC for isolates with MICs of 1 mg/L, even with the highest dosage of 2000 mg/8h with a duration of 0.5 h. Moreover, the numbers of targeted PTA for the three infusion time groups were 55.56%, 62.96%, and 77.78%, respectively. These ndings suggest that to achieve an optimal PTA, a prolonged infusion time, higher dose, or alternative administration protocol is needed for this cohort. Notably, patients with uric acid levels of 800 μmol/L and CG-CLCR of 30 ml/min can achieve optimal PTA ( for all targeted therapeutic effects, including 100% fT > 4MIC for isolates with MICs of 8 mg/L using a dosage of 2000 mg/8h with infusion durations of 2 h and 4 h. The low uric acid group of 40 mol/L failed to achieve the PK/PD target of 100% fT>MIC and 100% fT>4MIC for all the simulated dosing regimens. In general, high levels of CG-CLCR and low levels of uric acid were associated with lower PTA. Detailed in uences of creatinine and uric acid on PTA are shown in Fig. 3. Patients with high creatinine clearance rates were likely to have lower PTA (P = 0.003), while those with high levels of uric acid were likely to have higher PTA (P < (Fig. 3).
We analyzed the PTA obtained from the simulation result and found that the infusion times of 2 h and 4 h appeared to have a higher PTA value than 0.5 h on average (P = 0.047, Fig. 4). However, the number of targeted PTA ( ) showed no signi cant difference among the three groups (P = 0.6847). Given this surprising result, various MICs and therapeutic targets were analyzed. The results are presented in Fig. 5.
We found that the duration of infusion affected the improvement of PTA (Fig. 5). For the target of 40% fT MIC, PTA was signi cantly different among the three groups of simulated data when MIC < mg/L (P < 0.05). Under these circumstances, PTA could be improved by prolonged infusion time. The difference in PTA was close to signi cant when MIC was 4 mg/L (P = 0.0568). Notably, the P-value of the three groups was 0.234 for the target of 40% fT MIC when MIC was 8 mg/L This nding suggests that, for drugresistant bacteria with high MICs, prolonged infusion time does not improve PTA level. There was no signi cant difference in PTA among the three groups for the targets 100% fT MIC and 100% fT 4MIC, even though MIC was 1 mg/L.

Discussion
I We developed a PPK model of meropenem in patients with severe pulmonary infection. In the previous literature, correlations of antibiotic CL with creatinine clearance were often reported. To the best of our knowledge, our study is the rst to determine that uric acid is a signi cant covariate describing the pharmacokinetic parameters of meropenem. See Table S1-S3 in the electronic supplementary material for details. These tables display the PTAs for all simulated dosage regimens using 40% fT > MIC, 100% fT > analysis, we observed that higher levels of CG-CLCR and lower levels of uric acid were associated with the lower achievement of PK/PD targets for critically ill patients. Many studies found that the characteristics of meropenem pharmacokinetics could be described in different populations using a two-compartment model, which is consistent with the results of our study 16,17,[36][37][38] . Adela et al. found that the administration of 2000 mg/8 h of meropenem as a continuous infusion allowed higher serum meropenem concentrations 37 . Similar results were found in other studies of meropenem 14,16,36,37,39 .
We found that the duration of infusion had a complex effect on the improvement of PTA. It was signi cant only when using the traditional target of 40% fT > MIC with MIC < 4 (Fig. 4). For the PD effects of 100% fT > MIC and 100% fT > 4MIC, no signi cant statistical differences were discovered. This nding suggests that, for patients with sensitivity to meropenem and mild infection, prolonging the infusion time can improve the therapeutic effect (MIC < 4). By contrast, those with meropenem-resistance or severe infections (who require a higher therapeutic target) had no signi cant clinical bene t from prolonged infusion time. A. baumannii and K. pneumoniae generally have high MICs. Therefore, higher dosages are needed to achieve the targeted therapeutic effect. However, Mohd et al. conducted an observational study of 211 patients receiving piperacillin/tazobactam and meropenem and found that administration of meropenem by prolonged infusion in critically ill patients was bene cial. Several studies showed similar results and encouraged extended infusions because this maximizes the likelihood of achieving target blood concentrations 37, 39-42 .
The reason for this distinction is most likely that few studies have compared the differences in therapeutic responses of 100% fT > MIC,100% fT > 4MIC, and 40% fT > MIC caused by infusion time; we did so and identi ed the distinction. It is worth noting that De Waele et al. mentioned that, in a signi cant subpopulation of critically ill patients with normal renal function, a 100% fT>MIC target is not reached, even with 3-hour extended infusions. This nding agrees with our results.
We also assessed the achievement of different PK/PD target (40% fT > MIC,100% fT > MIC and 100% fT > 4MIC) under MIC values ranging from 1 mg/L to 8 mg/L. The effect of meropenem dosage and infusion duration was also assessed. In particular, patients with creatinine clearance of 60 ml/min or less and uric acid greater than 400 μmol/L can achieve the target of PTA 90% under the MIC of 8 mg/L, even with the administration dose of 500 mg/8 h with a 2-h infusion (Additional le 2). This nding suggests that 500 mg/8 h is su cient for critically ill renal failure patients with high uric acid levels.
Although the correlation of antibiotic CL with creatinine clearance has been widely reported 11,12,16 , this study represents the rst nding of uric acid having a signi cant impact on meropenem use. We also found that patients with lower creatinine clearance and high uric acid levels tend to require lower dosages. Our ndings suggest that dose adjustment based on these two factors appears to be reasonable.
However, our study also has some limitations. First, the sample size is small, and it is only a single-center study. Second, adverse effects and the in uence of plasma concentration were not assessed. Therefore, actual tissue concentrations are unknown 26,43 . Measurements of concentrations in the epithelial lining uid of the lung are needed in further studies 37, 44 . In addition, most of the samples were collected at the trough concentration time; this may affect the tting of the model and intraindividual variability during the treatment period that could not be measured 26,45 .
Nevertheless, our study still provides essential information about the optimized dosage regimen of meropenem in critically ill patients. During empirical therapy of severe pneumonia caused by gramnegative bacteria, clinicians should consider both the achievement of clinical cure and the prevention of drug resistance. Therapeutic drug monitoring is one of the best means to achieve precision therapy.

Conclusions
Lower CG-CLCR and higher uric acid levels were likely to achieve higher exposure in serum and associate with lower PTA. The dose of 500 mg/8 h may be necessary to achieve an optimal coverage in critically ill patients for all susceptible isolates (MIC≤8mg/L) in patients with high uric acid levels associated with severe renal injury. Moreover, for those with drug-resistant or severe infections (MIC>4mg/L and critically ill patients who need a higher therapeutic target (100% fT > MIC,100% fT > 4 MIC), prolonged infusion time does not appear to be bene cial. Increasing dose or alternative therapeutic strategies may be required for critically ill patients with drug-resistant or severe infections who need a higher therapeutic target.     The effect of injection time to PTA Distinctions of PTA (a) and the number of achieved targeted PTA≥90% (b) in different injection time (0.5h, 2h and 4h) group. MIC=8mg/L(a3,b3 and c3) groups, respectively. Distinctions of injection time (0.5h, 2h and 4h) on