Impact of initial vancomycin pharmacokinetic/pharmacodynamic parameters on the clinical and microbiological outcomes of methicillin-resistant Staphylococcus aureus bacteremia in children.

Optimal vancomycin exposure is important to minimize treatment failure of methicillin-resistant Staphylococcus aureus (MRSA) bacteremia. We aimed to analyze the impact of initial vancomycin pharmacokinetic/pharmacodynamic (PK/PD) parameters, including the initial vancomycin C trough and the area under the curve (AUC)/minimal inhibitory concentration (MIC) on the outcomes of pediatric MRSA bacteremia. The study population consisted of hospitalized children aged between 2 months and 18 years with MRSA bacteremia, in whom C trough was measured at least one time within the time period of January 2010 to March 2018. Demographic profiles, underlying diseases, and clinical/microbiological outcomes were abstracted retrospectively. During the study period, 73 cases of MRSA bacteremia occurred in children with a median age of 12.4 months. Severe clinical outcomes leading to intensive care unit stay and/or use of mechanical ventilation occurred in 47.5% (35/73); all-cause 30-day mortality was 9.7% (7/72). The median dosage of vancomycin was 40.0 mg/kg/day. There was a weak linear relationship between C trough and the corresponding AUC/MIC (r = 0.235). ROC curves for achieving an AUC/MIC of 300 suggested that the initial C trough at 10 μg/mL could be used as a cut-off value with a sensitivity of 90.5% and a specificity of 44%. Although persistent bacteremia at 48-72 hours after vancomycin administration was observed more frequently when the initial C trough was < 10 μg/mL and initial AUC/MIC was < 300, initial AUC/MIC < 300 was the only risk factor associated with persistent bacteremia at 48-72 hours (adjusted OR 3.05; 95% CI, 1.07-8.68). Initial C trough and AUC/MIC were not associated with 30-day mortality. Although there was a weak relationship between C trough and AUC/MIC, initial AUC/MIC < 300 could be used as a predictor of persistent MRSA bacteremia at 48-72 hours. Further prospective data on optimal vancomycin dosing are necessary to improve clinical and microbiological outcomes in pediatric MRSA bacteremia.

Introduction

Methicillin-resistant Staphylococcus aureus (MRSA) is one of the major health concerns in both healthcare-associated and community-acquired infections in children, and clinical outcomes of MRSA bacteremia are considered to be worse than cases caused by methicillin-susceptible S. aureus (MSSA) [1, 2]. Vancomycin is an important treatment option for invasive MRSA infections [1]. To achieve favorable clinical outcomes while avoiding vancomycin toxicity associated with an overdose, the methods of monitoring of vancomycin administration have been studied. The results of these studies suggested that a ratio of the area under the curve to the minimum inhibitory concentration (AUC/MIC) has been correlated with efficacy in experiments conducted in vitro [3]. Although there is an assumption that a vancomycin C trough of > 15 μg/mL when the MIC is ≤ 1 μg/mL can be used as a good surrogate marker for AUC/MIC ratio of ≥ 400 with the MIC determined by broth microdilution (BMD) for achieving clinical effectiveness in adults [4], vancomycin dosing information to ensure optimal drug exposure in the pediatric population remains limited [5]. However, recent studies have suggested that high concentrations of vancomycin may not be necessary for pediatric populations; vancomycin C trough 7–10 μg/mL could predict the achievement of AUC/MIC > 400 at a dose of 15 mg/kg/dose every 6 hours in the case of patients with normal renal function, if the MIC ≤ 1 μg/mL [6], and the risk of vancomycin nephrotoxicity may increase if C trough exceeds the predetermined range [7-9]. In addition, C trough is not highly correlated with AUC/MIC and is insufficient as a surrogate marker for clinical outcomes, not only in pediatric populations but also in adults [10, 11].

According to our previous study, initial vancomycin C trough of < 10 μg/mL was associated with microbiologic failure at 48 hours after vancomycin administration for pediatric MRSA bacteremia [12]. Another study on the impact of vancomycin C trough on the duration of MRSA bacteremia in children also suggested a C trough of > 10 μg/mL within the first 72 hours was the most predictive factor for persistent MRSA bacteremia lasting for > 3 days [13]. However, both studies focused only on the parameter of C trough without any evaluation of the impact of AUC/MIC on the clinical and microbiological outcomes.

Herein, we conducted a study elucidating the pharmacokinetics/pharmacodynamics (PK/PD) parameters, including both initial C trough and AUC/MIC, for optimal vancomycin dosing to achieve favorable clinical and microbiological outcomes in pediatric MRSA bacteremia. In addition, we aimed to determine whether C trough could be used as an optimal surrogate marker replacing AUC/MIC in clinical practice.

Materials and methods

Study design

This retrospective study was performed at a single center, the Asan Medical Center Children’s Hospital, which is a 253-bed academic-tertiary medical center located in Seoul, South Korea. Pediatric patients aged between 2 months and 18 years who were admitted during the study period from January 2010 to March 2018 with culture-confirmed MRSA bacteremia were eligible for inclusion. Patients were included if they received vancomycin for at least 48 hours, and their initial C trough had been sampled before the 4th or 5th dose within the 30 minutes before the next dose of vancomycin. The following MRSA bacteremia cases were excluded from this study: patients with chronic renal diseases requiring renal replacement therapy such as hemodialysis or peritoneal dialysis, and cases treated in the neonatal intensive care unit.

The analysis only included the first MRSA isolates detected during a single clinical episode occurring within a 4-week period, and duplicates from the same patient were excluded. Demographic profile and clinical data including primary sites of infection, underlying diseases, duration of hospital stay, and blood laboratory results, including serum creatinine, and initial vancomycin dose were extracted from the electronic medical records. The antimicrobial susceptibilities of the S. aureus isolates were decided using a MicroScan Walk-Away 96-Combo Pos 28 panel (Siemens, West Sacramento, CA, USA).

The primary outcome of this study is to clarify the relationship between the initial C trough and AUC/MIC, and the potential impact of the variables on clinical and microbiological outcomes. This study was approved by the Institutional Review Board of Asan Medical Center with a waiver of informed consent due to the study being retrospective and using de-identified data collection and analysis (IRB No. 2019–1638).

Definitions

Most of the definitions used in this study, including fever, the primary focus of infection, and persistent bacteremia, were the same as those described in a previous study [12]. Recurrent MRSA bacteremia was defined as another MRSA bacteremia event 1 month after MRSA bacteremia resolution. Co-infection was defined as the case in which clinically significant bacteria were identified from the blood culture at the same time as the diagnosis of MRSA bacteremia. Clinically severe cases included those who required an intensive care unit stay, mechanical ventilation, and/or fatal cases. Acute kidney injury (AKI) was defined as an increase in serum creatinine levels by 0.5 mg/dL [14]. The clinically significant renal toxicity of vancomycin was defined as an additional need for renal replacement therapy during vancomycin treatment or the need to replace vancomycin with an alternate antibiotic because of serum creatinine elevation.

Clinical and microbiological outcomes

The clinical and microbiological outcomes were analyzed according to the initial vancomycin C trough, vancomycin MIC of MRSA isolates, and initial AUC/MIC, respectively. As measures of clinical outcomes, recurrence of MRSA bacteremia and 30-day all-cause mortality were evaluated. Microbiological outcomes were determined using the time-to-negative conversion of blood culture and failure of clearance of bacteremia at 48–72 hours after the initiation of vancomycin administration.

Pharmacokinetic data

PK parameters including AUC and vancomycin clearance were calculated as follows:

AUC = vancomycin daily dose (mg)/vancomycin clearance (mL/hr/kg) [15].

Vancomycin clearance (mL/hr/kg) = 0.248×Wt (Kg)0.75×[0.48/serum Cr (mg/dL)]0.361×[ln(age in days)/7.8]0.995 [5].

Statistical analysis

Categorical data were analyzed with X tests and Fisher’s exact test, and continuous variables with independent t-tests or one-way ANOVA. Spearman`s correlation coefficient was used to evaluate relationships between C trough and AUC/MIC. A receiver operating characteristic (ROC) curve was used to determine the sensitivity and specificity of each PK/PD parameter. A multivariate logistic regression analysis was used to assess the potential impact of the variables on clinical and microbiological outcomes. A P value of < 0.05 was considered statistically significant for the comparisons. Statistical Program for Social Science release 21 (SPSS Inc., Chicago, IL, USA) was used for all statistical calculations.

Results

Patient characteristics

During the study period from January 2010 to March 2018, a total of 98 patients aged between 2 months and 18 years experienced MRSA bacteremia in our institute. Excluding 25 patients because of hemodialysis (n = 17), unavailable data of C trough (n = 3), and use of an alternative antibiotic instead of vancomycin (n = 5), a total of 73 patients who fulfilled the inclusion criteria were analyzed in this study. The patients’ median age was 12.4 months (range, 2 months– 17.3 years), and 97.3% (71 out of 73) had underlying medical comorbidities, of which congenital heart disease was the most common underlying disease (28.8%; 21/73) (Table 1). With the exception of one case, all other cases of MRSA bacteremia were healthcare-associated infections. Bacterial co-infection occurred in a total of 5 patients, all of which were bacteremia due to Enterococcus faecalis (n = 2), Klebsiella aerogenes (n = 1), K. pneumoniae (n = 1), and Pseudomonas aeruginosa (n = 1).

Table 1

Demographic and clinical characteristics of 73 pediatric patients with MRSA bacteremia.

Characteristics	Number of cases (%) or median values (interquartile range)
Median age, months	12.4 (5.3–36.1)
Sex (number; % of male)	43 (58.9%)
Hospital stay before the onset of the MRSA bacteremia, days	13.0 (4.0–33.0)
Presence of bacterial co-infectiona	5 (6.8%)
Presence of invasive device b	58 (79.5%)
Presence of fever ≥ 38°C at the onset of bacteremia	55 (75.3%)
Presence of underlying diseases	71 (97.3%)
Congenital heart disease	21 (28.8%)
Malignancy	12 (16.4%)
Chronic lung disease	4 (5.5%)
Neurologic diseases	5 (6.8%)
Others c	15 (20.5%)
Primary focus of MRSA bacteremia
None	31 (42.5%)
Central vascular catheter	25 (34.2%)
Lung	8 (11.0%)
Others d	9 (12.3%)
Initial laboratory findings
Serum WBC (/uL)	13,500 (8,600–19,300)
CRP (mg/dL)	3.1 (0.8–6.7)
Serum Creatinine (mg/dL)	0.3 (0.2–0.4)
Parameters related to vancomycin regimen
Initial vancomycin dose (mg/kg/day)	40.0 (40.0–55.0)
Initial vancomycin C trough concentration (mg/L)	6.4 (4.3–11.6)
Initial AUC/MIC	336.4 (271.4–422.6)
Simultaneous use of other antibiotics (%)	63 (86.3%)
Recurrent bacteremia e	14 (19.4%)
Persistent bacteremia at 48–72 hours	29 (39.7%)
Acute kidney injury	3 (4.1%)
All-cause 30 day-mortality e	7 (9.7%)

a Bacterial co-infection occurred in a total of 5 patients, all of which were bacteremia due to Enterococcus faecalis (n = 2), Klebsiella aerogenes (n = 1), K. pneumoniae (n = 1), and Pseudomonas aeruginosa (n = 1).

b Invasive devices included central vascular catheter (n = 46), gastrostomy (n = 5), tracheostomy (n = 5), ventriculo-peritoneal shunt (n = 2).

c Others included congenital diaphragmatic hernia (n = 2), congenital megacolon (n = 2), short bowel syndrome (n = 3), acute respiratory distress syndrome (n = 3), omphalocele (n = 1), trachea-esophagus fistula (n = 1), jejunal atresia (n = 1), pseudohypoaldosteronism (n = 1), VACTERL (vertebral defects, anal atresia, cardiac defects, tracheoesophageal fistula, renal anomalies, and limb abnormalities association) (n = 1).

d Others included skin and soft tissue infection (n = 7), ventriculo-peritoneal shunt infection (n = 2).

e One out of 73 patients with MRSA bacteremia was transferred to another hospital during treatment, allowing analysis of mortality and recurrence rates in a total of 72 patients excluding it.

A primary focus of MRSA bacteremia was found in 57.5% (42 out of 73), which consisted of central vascular catheters (59.5%, 25 out of 42), pneumonia (19.0%, 8 out of 42), skin and soft tissue infections (16.7%, 7 out of 42), and ventriculoperitoneal shunt (4.8%, 2 out of 42); the remaining 42.5% (31 out of 73) were primary bacteremia. Among the 25 cases of central line-associated bloodstream infection, 12 (48%) retained the central vascular catheter as salvage therapy. The simultaneous use of other antibiotics with vancomycin was observed in 86.3% (63 out of 73).

Fever was the most common initial symptom (75.3%, 55 out of 73). One out of 73 patients with MRSA bacteremia was transferred to another hospital during treatment, allowing analysis of mortality and recurrence rates in a total of 72 patients: all-cause 30-day mortality was 9.7% (7 out of 72). Recurrent MRSA bacteremia and persistent bacteremia at 48–72 hours occurred in 19.4% (14/72) and 39.7% (29/73) patients, respectively. Although AKI occurred in 3 out of 73 in this study, there was no clinically significant renal toxicity associated with vancomycin use.

Vancomycin MIC of MRSA isolates

All of the 73 MRSA isolates were susceptible to vancomycin with MIC ≤ 2.0 μg/mL. Most of the MRSA isolates belonged to the group with vancomycin MIC 1.0 μg/mL (n = 53; 72.6%), followed by the group with vancomycin MIC ≤ 0.5 μg/mL (n = 10; 13.7%), and 2.0 μg/mL (n = 10; 13.7%), respectively (Table 2). During the study period, the proportion of MRSA isolates with vancomycin MIC ≤ 0.5 μg/mL showed a decreasing trend; 30.8% (2010–2012), 3.3% (2013–2015), and 5.9% (2016–2018) (P for trend = 0.246).

Table 2

Annual trend of vancomycin MIC of MRSA isolates.

Year	Vancomycin MIC range determined by Microscan
	< 0.5 μg/mL	1.0 μg/mL	2.0 μg/mL
2010–2012	30.8% (8/26)	53.8% (14/26)	15.4% (4/26)
2013–2015	3.3% (1/30)	86.7% (26/30)	10.0% (3/30)
2016–2018	5.9% (1/17)	76.5% (13/17)	17.6% (3/17)

PK/PD of vancomycin use

The initial median dose of vancomycin was 40.0 mg/kg/day [interquartile range (IQR), 40.0–55.0 mg/kg/day], and an initial vancomycin dose of ≥ 60 mg/kg/day was used in 28.3% of cases (17 out of 73). Initial median C trough was 6.4 μg/mL (IQR, 4.3–11.6 μg/mL); 71.2% (52 out of 73) had an initial C trough ≤ 10 μg/mL and an initial C trough > 15 μg/mL was achieved in only 8 patients. The median initial AUC/MIC was 336.4 (IQR, 271.4–422.6), and an initial AUC/MIC ≥ 300 and ≥ 400 was achieved in 64.4% and 32.9%, respectively. There was a weak positive linear relationship between C trough and AUC/MIC (r = 0.235) (Fig 1).

Fig 1

Correlation between initial C trough and AUC/MIC.

ROC curves for predicting the achievement of AUC/MIC ≥ 300 suggested that the initial C trough of 10 μg/mL could be used as a cut-off value with a sensitivity of 90.5% and a specificity of 44.0%; an AUC of 0.721 [95% confidence interval (CI), 0.593–0.849]. An achievement of AUC/MIC ≥ 400 suggested sensitivity of 57.1% and specificity of 78.8% at the cut-off value of initial C trough of 10 μg/mL.

Impact on clinical and microbiological outcomes

Compared to the group with initial C trough ≥ 10 μg/mL, persistent bacteremia at 48–72 hours was observed more frequently in those with an initial C trough < 10 μg/mL (44.2% vs 19.0%; P = 0.044) (Table 3). The mean time for negative conversion of MRSA bacteremia in those with an initial C trough < 10 μg/mL was 4.1 ± 4.0 days, but it was 2.5 ± 1.5 days in those with a C trough ≥ 10 μg/mL (P = 0.018). However, there was no difference in the percentage of persistent bacteremia or duration of MRSA bacteremia with a cut off value of C trough of 15 μg/mL. In addition, there was no statistical significance between the initial vancomycin C trough and the 30-day mortality rate or recurrence of MRSA bacteremia.

Table 3

Clinical and microbiological outcomes of children with MRSA bacteremia according to the pharmacokinetic parameters of vancomycin and the MIC of MRSA isolates.

	Microbiological outcome		Clinical outcome a
	Time to negative conversion (days)	Persistent bacteremia at 48–72 hours	Recurrent MRSA bacteremia b (n = 14)	All-cause 30-day mortality (n = 7)
Initial vancomycin Ctrough (μg/mL)
< 10 (n = 52)	4.1 ± 4.0	23 (44.2%)	9/52 (17.3%)	6/52 (11.5%)
≥ 10 (n = 21)	2.5 ± 1.5	4 (19.0%)	5/20 (25.0%)	1/20 (5.0%)
P-value c	0.018	0.044	0.460	0.402
< 15 (n = 65)	3.7 ± 3.7	24 (36.9%)	12/64 (18.8%)	7/64 (10.9%)
≥ 15 (n = 8)	2.9 ± 1.9	3 (37.5%)	2/8 (25.0%)	0/8 (0%)
P-value c	0.546	0.975	0.674	Not applicable
Initial vancomycin AUC/MIC
< 300 (n = 25)	3.9 ± 3.0	14 (53.8%)	6/25 (24.0%)	3/25 (12.0%)
≥ 300 (n = 48)	3.4 ± 3.8	13 (27.7%)	8/47 (17.0%)	4/47 (8.5%)
P-valuec	0.632	0.026	0.476	0.634
< 400 (n = 49)	3.2 ± 2.6	19 (38.8%)	10/48 (20.8%)	4/48 (8.3%)
≥ 400 (n = 24)	4.4 ± 4.9	8 (33.3%)	4/24 (16.7%)	3/24 (12.5%)
P-valuec	0.184	0.651	0.674	0.574
Vancomycin MIC (μg/mL)
≤ 1.0 (n = 63)	6.5 ± 17.4	21 (33.3%)	10/62 (16.1%)	7/62 (11.3%)
2.0 (n = 10)	4.0 ± 3.5	6 (60.0%)	4/10 (40.0%)	0/10 (0%)
P-valuec	0.659	0.105	0.077	Not applicable
Initial vancomycin dose
≥ 60 mg/kg/day (n = 17)	8.2 ± 23.5	3 (17.6%)	5/17 (29.4%)	1/17 (5.9%)
< 60 mg/kg/day (n = 56)	5.6 ± 13.3	24 (42.9%)	9/55 (16.4%)	6/55 (10.9%)
P-valuec	0.154	0.059	0.235	0.541

a One out of 73 patients with MRSA bacteremia was transferred to another hospital during treatment, allowing analysis of mortality and recurrence rates in a total of 72 patients excluding it.

b Recurrent MRSA bacteremia defined as another MRSA bacteremia event 1 month after MRSA bacteremia resolution.

c Categorical data were analyzed with X tests.

Persistent bacteremia at 48–72 hours was observed more frequently in the group with initial AUC/MIC < 300 than those with AUC/MIC ≥ 300 (53.8% vs 27.7%; P = 0.026). However, in terms of initial microbiological outcomes, there was no statistically significant difference between the two groups with an initial AUC/MIC < 400 and ≥ 400. Furthermore, the time to negative conversion of MRSA bacteremia, 30-days mortality, or recurrent MRSA infection were indifferent to the initial AUC/MIC.

Compared to the group with vancomycin MIC ≤ 1.0 μg/mL, persistent bacteremia at 48–72 hours and recurrent MRSA bacteremia were more frequently observed in those with vancomycin MIC 2.0 μg/mL although there was no statistical significance (33.3% vs 60.0%; P = 0.105, and 16.1% vs 40.0%; P = 0.077, respectively).

Compared to the initial vancomycin dose ≥ 60 mg/kg/day group, the <60 mg/kg/day group tended to have more cases of persistent bacteremia at 48–72 hours, but there was no statistical significance (17.6% vs 42.9%; P = 0.059). Clinical outcomes, including recurrent MRSA infection and 30-day all-cause mortality, were not influenced by the initial vancomycin dose.

Risk factors for persistence of MRSA bacteremia and 30-day all-cause mortality

Unadjusted logistic regression analysis revealed initial C trough < 10 μg/mL and initial AUC/MIC < 300 were risk factors associated with persistent bacteremia at 48–72 hours [odds ratio (OR), 3.37; 95% confidence interval (CI), 1.00–11.41, and 3.43; 95% CI, 1.24–9.45, respectively] (Table 4).

Table 4

Risk factors for persistence of MRSA bacteremia at 48–72 hours and 30-day all-cause mortality.

Variable	Persistent bacteremia at 48–72 hours		30-day all-cause mortality
	OR (95% CI)		OR (95% CI)
	Unadjusted	Adjusted	Unadjusted	Adjusted
Age	1.00 (0.99–1.01)	1.00 (0.99–1.01)	1.01 (0.99–1.02)	1.00 (0.99–1.02)
Bacterial co-infection a	1.15 (0.18–7.34)	1.37 (0.20–9.49)	0.00	0.00
Presence of invasive device b	1.81 (0.51–6.37)	1.60 (0.42–6.13)	2.57 (0.30–22.07)	2.42 (0.27–21.76)
Presence of primary focus	2.38 (0.87–6.51)	1.92 (0.66–5.56)	1.87 (0.44–7.89)	1.85 (0.41–8.28)
Initial Ctrough < 10 μg /mL	3.37 (1.00–11.41)	3.14 (0.86–11.41)	1.73 (0.34–8.91)	1.42 (0.26–7.78)
Initial AUC/MIC < 300	3.43 (1.24–9.45)	3.05 (1.07–8.68)	0.80 (0.19–3.40)	0.60 (0.13–2.73)

a Bacterial co-infection occurred in a total of 5 patients, all of which were bacteremia due to E. faecalis (n = 2), K. aerogenes (n = 1), K. pneumoniae (n = 1), and P. aeruginosa (n = 1).

b Invasive devices included central vascular catheter (n = 46), gastrostomy (n = 5), tracheostomy (n = 5), ventriculo-peritoneal shunt (n = 2).

Although initial C trough < 10 μg/mL tended to increase the risk of persistent bacteremia at 48–72 hours (adjusted OR, 3.14; 95% CI, 0.86–11.41), in multivariate analysis adjusting for age, co-infection, indwelling medical device, and presence of primary focus, initial AUC/MIC < 300 was the only statistically significant risk factor associated with persistent bacteremia at 48–72 hours (adjusted OR, 3.05; 95% CI, 1.07–8.68). Co-infection, indwelling medical device, ICU stay, concurrent antibiotics use, and vancomycin MIC were not statistically significant risk factors for persistent bacteremia in unadjusted and adjusted logistic regression analysis.

Furthermore, 30-day all-cause mortality was not influenced by initial C trough nor initial AUC/MIC by adjusted or unadjusted logistic regression analysis. Age, co-infection, indwelling medical device, and presence of primary focus of bacteremia were not risk factors associated with 30-day all-cause mortality under adjusted nor unadjusted logistic regression analysis.

Discussion

In this study, we analyzed the relationship between initial vancomycin C trough and AUC/MIC, and the impact of these PK/PD parameters on clinical/microbiological outcomes of MRSA bacteremia in a pediatric population. Initial C trough < 10 μg/mL and AUC/MIC < 300 were associated with microbiological failure at 48–72 hours of vancomycin use, and initial AUC/MIC < 300 nearly tripled the risk for persistent MRSA bacteremia in children. However, these initial PK/PD parameters were not correlated with clinical outcomes in terms of recurrent MRSA infection and all-cause mortality within 30 days.

Our previous study analyzed the impact of initial C trough on the clinical and microbiological outcomes in pediatric MRSA bacteremia without data regarding AUC/MIC [12]. Herein, we extended the previous study, including more cases with MRSA bacteremia and estimated the AUC/MIC using vancomycin dose, vancomycin clearance, patient`s weight, and serum creatinine in a calculation proposed by Le et al. [5]. This study revealed a weak linear relationship between C trough and estimated AUC/MIC, but it is still inconclusive as to whether there is a positive correlation between C trough and AUC [16, 17]. A recent pediatric study suggested a positive correlation between C trough and AUC; a C trough of 10 μg/mL, 15 μg/mL, and 20 μg/mL can predict AUC of 413, 548, and 714, respectively [18]. However, Chhim et al. reported that the correlation between C trough and estimated AUC/MIC was poor (R2 = 0.14–0.20) [19]. Among the various methods for calculating AUC, Le’s equation tended to present the highest AUC value [17]. Extrapolation of AUC from serum C trough might underestimate AUC by up to 25%, and the AUC value varied between patients with similar results by up to 30-fold [8]. Accordingly, simultaneous monitoring of C trough and C peak has been suggested for accurate calculation of AUC/MIC. In a comparison of methods for calculating AUC, the trapezoidal method using both C trough and C peak could estimate AUC/MIC more accurately than the other methods [5, 16, 17]. Considering the trough-only concentration monitoring method might not reflect the actual AUC value in clinical settings and it is not always feasible to draw blood from young children at least two times per day, the Bayesian computer software programs could be used to estimate the appropriate vancomycin AUC value with minimal sampling [14, 20].

Although a recent study in adult patients showed that AUC/MIC of > 300 could be used as a surrogate marker for favorable clinical outcomes [10], a value of AUC/MIC 300–400 might be subtherapeutic, because AUC/MIC > 400 has been used as a standard surrogate marker for predicting the effectiveness of vancomycin use [21]. In this study, initial vancomycin AUC/MIC < 300 could be used as a predictor of persistent MRSA bacteremia at 48–72 hours, and AUC/MIC was a more reliable pharmacokinetic parameter predicting favorable microbiological outcomes compared to C trough. However, given that the initial AUC/MIC > 400 was only achieved in 33% in this study, the role of MIC should be addressed in correlating AUC with MIC, dose, C trough, and bacteremia clearance.

There is a controversy about a limited activity of vancomycin against MRSA isolates with a MIC at the high end of the susceptibility range [22, 23]; vancomycin MIC ≥ 1.5 μg/mL is associated with treatment failure in MRSA bacteremia [24]. In other study, however, the clinical outcomes of MRSA bacteremia were not different between vancomycin MIC < 2.0 μg/mL or 2.0 μg/mL [25]. Our study suggested that persistent or recurrent bacteremia tended to occur more often in the group with vancomycin MIC 2.0 μg/mL compared to those with MIC ≤ 1.0 μg/mL although there was no statistical significance. Even though it is difficult to say clearly that vancomycin “MIC creep” was observed during this study, which was defined as a gradual increase in the central tendency of the vancomycin MIC above 1 or 1.5 μg/mL for the dominant wild-type population [26], there was no increased trend of the proportion of MRSA isolates with vancomycin MIC > 1.0 μg/mL. However, it is important to clearly distinguish MIC creep from an increased occurrence of specified epidemic clones for which vancomycin MIC are elevated. So, it is necessary to monitor long-term changes in vancomycin MIC and the increase of specific clones with higher MIC. Additionally, there is considerable variability in MIC results between the susceptibility testing methods [27, 28], and the current critical PK/PD index was accepted with the MIC determined by BMD [14]. Because our institution perform MIC testing using automated systems, MicroScan WalkAway, it may be helpful to consider performing BMD in future studies to minimize method-dependent differences in MIC results.

Recent studies of pediatric MRSA bacteremia suggested that a higher C trough does not correlate with clinical outcomes [11], and the therapeutic discordance between AUC and C trough may lead to suboptimal outcomes among patients with infections due to pathogens with higher MIC values [18, 20]. In addition, targeting vancomycin trough levels > 15 μg/mL in pediatrics could expose them to adverse effects such as nephrotoxicity or ototoxicity [18]. It is difficult to give clear meaning due to the small number, but in a total of 3 patients who experienced transient AKI in this study, the initial C trough were all < 10 μg/mL.

The recurrence rate was relatively high in this study, although it was not possible to clearly distinguish between recurrence or reinfection of MRSA bacteremia because genotyping analysis using pulsed field gel electrophoresis or multilocus sequence typing was not performed. This might be explained by a high proportion of patients with severe underlying disease and long-term hospitalization, and clinician`s preference for salvage therapy for catheter associated infection.

This study had some limitations. Because of the small study population from a single center, its generalizability may be poor, and the low event rate of clinical outcome including mortality makes multivariable analysis challenging. We conducted this study retrospectively, and checked only C trough during vancomycin treatment as the standard of care. So, the estimated AUC without C peak might not reflect the actual AUC in pediatric patients. However, there have been few clinical studies to determine the impact of the PK/PD parameters of vancomycin on clinical and microbiological outcomes in MRSA bacteremia especially among children population. We adopted methods for calculating AUCs that could be applicable for children under 2 years old, and determined the impact of the vancomycin PK/PD parameters of C trough, AUC, and MIC on clinical and microbiological outcomes in MRSA bacteremia which occurred in young children. Finally, this is a study of initial C trough or AUC/MIC, rather than a study of PK/PD parameters over time during a treatment course. Given the clinical significance of subsequent C trough or AUC/MIC, a delicate analysis of the relationship between PK/PD parameters and clinical/microbiological outcomes throughout the treatment period will be essential.

In conclusion, initial vancomycin AUC/MIC < 300 increased the risk for early microbiologic failure during treatment for pediatric MRSA bacteremia, although it did not impact 30-day fatalities or recurrence. In addition, C trough and AUC/MIC had only weak relationships in children. Future studies using more accurate PK/PD parameters are needed to determine the optimal vancomycin exposure and to improve the clinical and microbiological outcomes in children.

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8 Oct 2020

PONE-D-20-22107

Impact of initial vancomycin pharmacokinetic/pharmacodynamic parameters on the clinical and microbiological outcomes of Methicillin-resistant Staphylococcus aureusbacteremia in children

PLOS ONE

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Reviewer #1: Yes

Reviewer #2: Yes

**********

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

**********

3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

**********

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Reviewer #1: Yes

Reviewer #2: Yes

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5. Review Comments to the Author

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Reviewer #1: Thank you for the opportunity to your manuscript.

Line 48-49 - Reword to "Vancomycin is an important treatment option...", as saying it's "an important treatment of choice" implies there are multiple choices.

Line 52 - Vancomycin is concentration dependent, not independent.

Line 55 - This is true when MIC is 1 or less than 1

Line 61 - Ctrough is correlated if MIC is 1 or less but otherwise it is correlated. Trough of 15-20 will achieve AUC/MIC > 400 for S. aureus > 90% of the time if MIC is 1 or less. The problem is the "MIC creep" in which AUC/MIC > 400 is more difficult to achieve. I suggest reviewing Pai et al. Advanced Drug Delivery Reviews. 2014;77:55-57 and Alsutan et al. Peditr Infect Dis J. 2018;37:880-885.

Line 106 - Why did you not define renal toxicity as also a need to adjust the vancomycin dose based on SCr? Or even a clinical definition of AKI? Would have been more useful to capture a more inclusive definition/data.

Line 125 - Did you calculate power? It is unclear to me which outcome is your primary outcome.

Line 176 - The phenomenon of "MIC creep" could have skewed your data. There is now a higher proportion of "susceptible" S. aureus isolates that have an MIC of 2 versus 0.5 or 1 and increased clinical failure.

Line 179 - You mention the creep here; how are you defining it?

213 - Make sure you have %s in the parentheses where appropriate throughout the entire chart.

Line 222 - Why did those whose vancomycin MIC was 1 or less have more persistent bacteremia? What confounders were at play? Dose differences?

Line 226 - This is an important finding. I would recommend also stratifying by > 80 mg/kg, as doses this high would warrant a switch to a different antibiotic. I would also stratify AUC > 400 attainment by < 60, 60-80, and > 80 mg/kg/day.

Line 273 - You reference a study showing AUC/MIC > 300 can be used as a marker of clinical efficacy, but the standard is > 400. A value of 300-400 would be subtherapeutic and even though you found an initial AUC/MIC > 300 is superior to < 300, you did not (as far as I can tell) address the role of the MIC. What is important is correlating AUC with MIC, dose, trough, and bacteremia clearance.

Line 293 - Why would you have to draw and peak and trough 2 hours apart for this method?

Line 297 - You mention there was no difference between MC Line 300 - You state there is no MIC creep; how are you defining this? 60% of patients with MIC 2 had persistent bacteremia. I would say that the fact that initial AUC > 400 was only achieved 33% of time could be due to MIC creep.

It is a privilege to review this manuscript. Thank you.

Reviewer #2: Yoo and colleagues have evaluated vancomycin PK/PD parameters in the treatment of pediatric MRSA bacteremia with regards to clinical and microbiological outcomes. In a retrospective single center cohort study over 8 years, they found that there was a weak relationship between trough concentration and AUC/MIC, and that initial AUC/MIC <300 was associated with persistent MRSA bacteremia at 48-72 hours. Despite the long study period, there were only 73 children in the final analysis and even fewer children who experienced the outcomes of interest. Given the dearth in PK/PD and clinical outcome data in pediatric S. aureus bacteremia, the authors are to be commended for their research, and show that the published AUC/MIC target of 400 for adults is also reasonable for children.

Comments:

Lines 72-73:

The authors proposed to study Ctrough and AUC/MIC for optimal dosing to achieve favorable clinical and microbiological outcomes. However this really is a study of initial Ctrough or AUC/MIC, rather than a study of PK/PD parameters over time during a treatment course. It maybe that subsequent Ctrough or AUC/MIC are also important in outcomes, but this has not been evaluated. This should be highlighted in the introduction and in the discussion.

Line 96:

Vancomycin MIC was performed using MicroScan. The establishment of AUC/MIC targets were established using BMD, and there are known method-based differences that lead to differences in MIC results. Was there any consideration for performing BMD?

Lines 108-109:

The definition of renal toxicity included … “the need to replace vancomycin with an alternate antibiotic because of serum creatinine elevation”. Although no patients experienced renal toxicity, the latter definition is very vague. How much serum creatinine elevation was required to meet this definition? Did it differ between treating clinicians? This may underestimate the true incidence of renal toxicity.

Line 114:

One of the clinical outcomes was “recurrence of MRSA bacteremia”. The definition for this outcome does not appear until the footnote of Table 2. Suggest insert into the Methods section here.

Lines 142-148, 164-170, 171-172:

These paragraphs are simply repeating information that is found in Table 1. There is no need to repeat data in the text that are also found in the Table. Suggest re-work text/table so that there is no repetition of material.

Lines 187-191:

The authors have reported mean Ctrough and AUC/MIC values. Were these normally distributed? Looking at the mean and range distributions provided, I suspect the data are non-parametric, so median values might be better.

Also, it is not clear that mean +/- SD are reported because SD has not been indicated in the text.

Lines 271-280:

The authors have referenced predominantly adult literature for PK/PD targets. The updated vancomycin consensus guideline has been released (Rybak Am J Health Syst Pharm 2020) and now includes pediatric sections. Suggest that this is included in discussion and references, rather than the superceded 2009 version. This updated guideline also recommends similar AUC/MIC targets to the adult population.

Table 1:

Please also include number of patients with 30-day mortality, not just percentage (to be consistent with formatting of the rest of the table). What was the rate of recurrent bacteremia?

Table 2:

Although it is not summarized elsewhere, the number of children with recurrent bacteremia was 14. From the denominator population of 62 children (presumably there were incomplete/missing data for the remaining 11 children), this is 23%. Additionally, there were 27 children with persistent bacteremia (37%). These are extraordinarily high rates of recurrent and persistent bacteremia. Do the authors have any explanation for these high rates?

Table 3:

Presentation of the adjusted multivariable model for persistent MRSAB and mortality should only include the final parsimonious model (ie. only variables with p<0.05).

Note that the small numbers of children with these outcomes (27 for persistent SAB and 9 for mortality) makes multivariable analysis challenging due to the low event rate. This should be added to the discussion.

Minor errors:

Line 126: Fischer should be spelt Fisher

Table 2:

The numbers in each group of AUC/MIC category should also be presented (as has been done for Ctrough)

The numbers in each group (persistent MRSAB, recurrent MRSAB, mortality) should be presented in the top columns.

Figure 1:

Both axes should have a unit of measurement presented.

**********

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Reviewer #2: No

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10 Jan 2021

Dear Taeok Bae, Academic Editor

Thank you for giving us a positive review of our study. We did our best to make corrections based on the opinions of the reviewers.

Journal Requirements:

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2. Thank you for providing the following ethics information in the methods section of your manuscript “This study was approved by the Institutional Review Board of Asan Medical Center with a waiver of informed consent due to the study being retrospective and using de-identified data collection and analysis (IRB No. 2019-1638)”. Please also provide this information in the ethics statement on the online submission form.

=> I wrote down the information in the ethics statement on the online submission form. Thanks.

3.Thank you for stating the following financial disclosure:

[The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.].

At this time, please address the following queries:

a. Please clarify the sources of funding (financial or material support) for your study. List the grants or organizations that supported your study, including funding received from your institution.

b. State what role the funders took in the study. If the funders had no role in your study, please state: “The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.”

c. If any authors received a salary from any of your funders, please state which authors and which funders.

d. If you did not receive any funding for this study, please state: “The authors received no specific funding for this work.”

Please include your amended statements within your cover letter; we will change the online submission form on your behalf.

� We received no specific funding for this work. We include this statement, “The authors received no specific funding for this work” within my cover letter. Thanks.

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: Thank you for the opportunity to your manuscript.

Thank you very much for your positive assessment of our research and for all the informative advice.

Line 48-49 - Reword to "Vancomycin is an important treatment option...", as saying it's "an important treatment of choice" implies there are multiple choices.

� I changed the sentence as you recommended.

Line 50-51, “Vancomycin is an important treatment option for invasive MRSA infections.”

Line 52 - Vancomycin is concentration dependent, not independent.

� Thank you for your comment. Raybak, et al. mentioned in a review paper in 2006 as follows; “Vancomycin is concentration-independent antibiotic” (Clin Infect Dis 2006;42:S35-S39). Larsson et al. also suggested that the administration of stepwise increasing clinical concentrations (range, 5–40 mg/L) resulted in no appreciable difference in killing even though the postantibiotic effect of vancomycin is dependent on the concentration (J Antimicrob Chemother. 1996;38:589-97). However, as you said, the serum concentration of vancomycin is used as one of the important indicators of vancomycin PK/PD. Considering this sentence may be confusing to readers, I deleted the sentence of “vancomycin is a concentration-independent antibiotic” in the revised manuscript.

Line 55 - This is true when MIC is 1 or less than 1

� Thank you for your comment. I corrected the typo as follows;

Line 56-57, “Although there is an assumption that a vancomycin C trough of > 15 µg/mL when the MIC is ≤ 1 µg/mL,”

Line 61 - Ctrough is correlated if MIC is 1 or less but otherwise it is correlated. Trough of 15-20 will achieve AUC/MIC > 400 for S. aureus > 90% of the time if MIC is 1 or less. The problem is the "MIC creep" in which AUC/MIC > 400 is more difficult to achieve. I suggest reviewing Pai et al. Advanced Drug Delivery Reviews. 2014;77:55-57 and Alsutan et al. Peditr Infect Dis J. 2018;37:880-885.

� Thank you for sharing important points and references. As mentioned, this situation is only applicable to MRSA with MIC ≤ 1 µg/mL which is determined by broth microdilution (BMD). Therefore, it is stated that it is meaningful only in the case of MIC ≤ 1 µg/mL as follow;

Line 56-58, vancomycin C trough of > 15 µg/mL when the MIC is ≤ 1 µg/mL can be used as a good surrogate marker for AUC/MIC ratio of ≥ 400 with the MIC determined by broth microdilution (BMD) for achieving clinical effectiveness in adults [4]

Line 61-63, “vancomycin C trough 7–10 μg/mL could predict the achievement of AUC/MIC > 400 at a dose of 15 mg/kg/dose every 6 hours in the case of patients with normal renal function, if the MIC ≤ 1 µg/mL [6]”

As with the opinion of another reviewer, the ASHP report released in 2020 has also been added as a major reference along with the two papers above.

Line 106 - Why did you not define renal toxicity as also a need to adjust the vancomycin dose based on SCr? Or even a clinical definition of AKI? Would have been more useful to capture a more inclusive definition/data.

� Thank you for comments. Most commonly used definition of vancomycin associated AKI is an increase in the serum creatinine level of 0.5mg/dL, or a 50% increase from baseline in consecutive daily readings, or a decreased in calculated creatinine CL(CLcr) of 50% from baseline on 2 consecutive days in the absence of a alternative explanation (Raybak, et al. ASHP report 2020).

Since this study included young infants, the initial Cr value may be low, for example, if the serum Cr is doubled to 0.2 mg/dL from 0.1 mg/dL, it is too much to consider as AKI. According to your comments, the definition of AKI is also added in the manuscript as follows: “AKI was defined by an increase in the serum creatinine level of 0.5 mg/dL.” If AKI was defined as a rise of 0.5 mg/dL or more in serum Cr, AKI occurred in 3 out of 73 people in this study. In addition, the previously mentioned “renal toxicity” was modified to a “clinically significant nephrotoxicity”.

Line 113-117, Acute kidney injury (AKI) was defined as an increase in serum creatinine levels by 0.5 mg/dL (Raybak, et al. ASHP report 2020). The clinically significant renal toxicity of vancomycin was defined as an additional need for renal replacement therapy during vancomycin treatment or the need to replace vancomycin with an alternate antibiotic because of serum creatinine elevation.

Line 125 - Did you calculate power? It is unclear to me which outcome is your primary outcome.

� Thank you for comments. This study was not based on data collected prospectively, but was a retrospective study of pediatric MRSA bacteremia during the last 8 years, when vancomycin TDM was actively performed. Therefore, statistical power was not accurately calculated in advance.

� According to your comments, I clarified the primary outcome as follows:

Line 100-101, The primary outcome of this study is to clarify the relationship between the initial C trough and AUC/MIC, and the potential impact of the variables on clinical and microbiological outcomes.

Line 176 - The phenomenon of "MIC creep" could have skewed your data. There is now a higher proportion of "susceptible" S. aureus isolates that have an MIC of 2 versus 0.5 or 1 and increased clinical failure.

� I agree with your opinion. In this study, the persistent and recurrent bacteremia were observed more frequently in the group with vancomycin MIC 2.0 μg/mL although there was no statistical significance. With the MIC being a component of the vancomycin AUC/MIC targeted surrogate for efficacy, it is important to be aware of vancomycin susceptibility patterns for MRSA. In our institute, the proportion of S. aureus isolates with MIC 1 μg/mL or greater than 1 μg/mL, which was determined by MicroScan WalkAway showed an increasing trend. Table 2, which shows the annual trend of the vancomycin MIC of MRSA isolates, is also added in the revised manuscript.

Line 179 - You mention the creep here; how are you defining it?

� Thanks for pointing out. I have used inappropriate terms in the previous manuscript because vancomycin MIC “creep” can be defined as a gradual increase in the central tendency of the vancomycin MIC for the dominant wild-type population (Sader, et al. AAC 2009). Furthermore, an increase in the proportion of strains for which the vancomycin MIC is >1 or >1.5 ug/mL may be caused by either extensive use of vancomycin or more likely when dealing with S. aureus, the dissemination of a clone or clones with less susceptibility to vancomycin. Thus, it is important to clearly distinguish MIC creep from an increased occurrence of specified epidemic clones for which vancomycin MIC are elevated.

� The following sentences in the previous manuscript were deleted to minimize confusion.

Line 188, “Although no definite creep of vancomycin MIC was observed”

Line 349-350 “In addition, there was no significant vancomycin MIC creep phenomenon during this study period”

� Instead of the term “MIC creep”, the following sentences were added in the revised manuscript.

Line 193-195, During the study period, the proportion of MRSA isolates with vancomycin MIC � 0.5 μg/mL showed a decreasing trend; 30.8% (2010-2012), 3.3% (2013–2015), and 5.9% (2016–2018) (P for trend =0.246).

Line 342-350 Our study suggested that persistent or recurrent bacteremia tended to occur more often in the group with vancomycin MIC 2.0 μg/mL compared to those with MIC ≤ 1.0 μg/mL although there was no statistical significance. Even though it is difficult to say clearly that vancomycin MIC creep was observed during this study, the proportion of MRSA isolates with vancomycin MIC 1.0 μg/mL or greater than 1.0 μg/mL showed an increasing trend. However, it is important to clearly distinguish MIC creep from an increased occurrence of specified epidemic clones for which vancomycin MIC are elevated [26]. So, it is necessary to monitor long-term changes in vancomycin MIC and the increase of specific clones with higher MIC.

213 - Make sure you have %s in the parentheses where appropriate throughout the entire chart.

� I checked the numbers and %s in tables.

Line 222 - Why did those whose vancomycin MIC was 1 or less have more persistent bacteremia? What confounders were at play? Dose differences?

� Sorry for the confusion. In the revised manuscript, it was revised and described as follow:

� Line 267-270 Compared to the group with vancomycin MIC ≤ 1.0 μg/mL, persistent bacteremia at 48–72 hours and recurrent MRSA bacteremia were more frequently observed in those with vancomycin MIC 2.0 μg/mL although there was no statistical significance (33.3% vs 60.0%; P=0.105, and 16.1% vs 40.0%; P=0.077, respectively).

In addition, I corrected errors of numbers in table 3 in the revised manuscript.

Line 226 - This is an important finding. I would recommend also stratifying by > 80 mg/kg, as doses this high would warrant a switch to a different antibiotic. I would also stratify AUC > 400 attainment by < 60, 60-80, and > 80 mg/kg/day.

� In this study, only the initial dose of vancomycin was analyzed, and no initial dose was given more than 80mg/kg a day. So, no further analysis could proceed.

Line 273 - You reference a study showing AUC/MIC > 300 can be used as a marker of clinical efficacy, but the standard is > 400. A value of 300-400 would be subtherapeutic and even though you found an initial AUC/MIC > 300 is superior to < 300, you did not (as far as I can tell) address the role of the MIC. What is important is correlating AUC with MIC, dose, trough, and bacteremia clearance.

� Thank you for mentioning a good point. According to your advice, I modified it as follows: many parts in the discussion

� Line 329- 337, Although a recent study in adult patients showed that AUC/MIC of > 300 could be used as a surrogate marker for favorable clinical outcomes [10], a value of AUC/MIC 300-400 might be subtherapeutic, because AUC/MIC > 400 has been used as a standard surrogate marker for predicting the effectiveness of vancomycin use [18]. In this study, initial vancomycin AUC/MIC < 300 could be used as a predictor of persistent MRSA bacteremia at 48–72 hrs, and AUC/MIC was a more reliable pharmacokinetic parameter predicting favorable microbiological outcomes compared to C trough. However, given that the initial AUC/MIC > 400 was only achieved in 33% in this study, the role of MIC should be addressed in correlating AUC with MIC, dose, C trough, and bacteremia clearance.

Line 293 - Why would you have to draw and peak and trough 2 hours apart for this method?

� Sorry for confusion. I modified the previous sentence as follows:

Line 326, it is not always feasible to draw blood from young children at least two times per day

Line 297 - You mention there was no difference between MC Line 300 - You state there is no MIC creep; how are you defining this? 60% of patients with MIC 2 had persistent bacteremia. I would say that the fact that initial AUC > 400 was only achieved 33% of time could be due to MIC creep.

� Thanks a lot for a good point. According to your advice, I modified lots of part in the discussion in the revised manuscript as follows.

� Line 329-344, Although a recent study in adult patients showed that AUC/MIC of > 300 could be used as a surrogate marker for favorable clinical outcomes [10], a value of AUC/MIC 300-400 might be subtherapeutic, because AUC/MIC > 400 has been used as a standard surrogate marker for predicting the effectiveness of vancomycin use [18]. In this study, initial vancomycin AUC/MIC < 300 could be used as a predictor of persistent MRSA bacteremia at 48–72 hrs, and AUC/MIC was a more reliable pharmacokinetic parameter predicting favorable microbiological outcomes compared to C trough. However, given that the initial AUC/MIC > 400 was only achieved in 33% in this study, the role of MIC should be addressed in correlating AUC with MIC, dose, C trough, and bacteremia clearance.

There is a controversy about a limited activity of vancomycin against MRSA isolates with a MIC at the high end of the susceptibility range [22, 23]; vancomycin MIC ≥ 1.5 μg/mL is associated with treatment failure in MRSA bacteremia [24]. In other study, however, the clinical outcomes of MRSA bacteremia were not different between vancomycin MIC < 2.0 μg/mL or 2.0 μg/mL [25]. Our study suggested that persistent or recurrent bacteremia tended to occur more often in the group with vancomycin MIC 2.0 μg/mL compared to those with MIC ≤ 1.0 μg/mL although there was no statistical significance.

� In addition, the previous sentence of “Clinical and microbiologic outcomes were not influenced by vancomycin MIC.” was deleted in the revised manuscript.

Reviewer #2: Yoo and colleagues have evaluated vancomycin PK/PD parameters in the treatment of pediatric MRSA bacteremia with regards to clinical and microbiological outcomes. In a retrospective single center cohort study over 8 years, they found that there was a weak relationship between trough concentration and AUC/MIC, and that initial AUC/MIC <300 was associated with persistent MRSA bacteremia at 48-72 hours. Despite the long study period, there were only 73 children in the final analysis and even fewer children who experienced the outcomes of interest. Given the dearth in PK/PD and clinical outcome data in pediatric S. aureus bacteremia, the authors are to be commended for their research, and show that the published AUC/MIC target of 400 for adults is also reasonable for children.

� Thank you for your positive evaluation of our research. The revised version is supplemented by reflecting the informative opinions you mentioned. Thank you again.

Comments:

Lines 72-73:

The authors proposed to study Ctrough and AUC/MIC for optimal dosing to achieve favorable clinical and microbiological outcomes. However this really is a study of initial Ctrough or AUC/MIC, rather than a study of PK/PD parameters over time during a treatment course. It maybe that subsequent Ctrough or AUC/MIC are also important in outcomes, but this has not been evaluated. This should be highlighted in the introduction and in the discussion.

� Thanks for your comments. To avoid misinterpretation, the expression “initial” is marked in front of both the Ctrough and the AUC used in this study. In addition, the following sentences as you mentioned were added as one of the drawbacks in this study.

� Line 378-382, Finally, this is a study of initial Ctrough or AUC/MIC, rather than a study of PK/PD parameters over time during a treatment course. Given the clinical significance of subsequent Ctrough or AUC/MIC, a delicate analysis of the relationship between PK/PD parameters and clinical/microbiological outcomes throughout the treatment period will be essential.

Line 96:

Vancomycin MIC was performed using MicroScan. The establishment of AUC/MIC targets were established using BMD, and there are known method-based differences that lead to differences in MIC results. Was there any consideration for performing BMD?

� Thank you for mentioning a good point. In considering the differences between these methods of MIC testing, I described the following:

� Line 350-354, Additionally, there is considerable variability in MIC results between the susceptibility testing methods [27, 28], and the current critical PK/PD index was accepted with the MIC determined by BMD [14]. Because our institution perform MIC testing using automated systems, MicroScan WalkAway, it may be helpful to consider performing BMD in future studies to minimize method-dependent differences in MIC results.

� Although I could not analyze BMD results further in this study, I will try to conduct BMD analysis together in future studies. Thank you.

Lines 108-109:

The definition of renal toxicity included … “the need to replace vancomycin with an alternate antibiotic because of serum creatinine elevation”. Although no patients experienced renal toxicity, the latter definition is very vague. How much serum creatinine elevation was required to meet this definition? Did it differ between treating clinicians? This may underestimate the true incidence of renal toxicity.

� Thank you for the comments. It was also mentioned in the response to Reviewer 1`s advice, the following definitions of acute kidney injury were added

� Line 113-117, Acute kidney injury (AKI) was defined as an increase in serum creatinine levels by 0.5 mg/dL [14]. The clinically significant renal toxicity of vancomycin was defined as an additional need for renal replacement therapy during vancomycin treatment or the need to replace vancomycin with an alternate antibiotic because of serum creatinine elevation.

� Line 185-187, Although AKI occurred in 3 out of 73 in this study, there was no clinically significant renal toxicity associated with vancomycin use.

Line 114:

One of the clinical outcomes was “recurrence of MRSA bacteremia”. The definition for this outcome does not appear until the footnote of Table 2. Suggest insert into the Methods section here.

� Thanks for the comments. The following was added to the related content method section.

� Line 108-110, Recurrent MRSA bacteremia was defined as another MRSA bacteremia event 1 month after MRSA bacteremia resolution.

Lines 142-148, 164-170, 171-172:

These paragraphs are simply repeating information that is found in Table 1. There is no need to repeat data in the text that are also found in the Table. Suggest re-work text/table so that there is no repetition of material.

� Thanks for the comments. In order to reduce overlapping content and make it clearer, the following sentences were deleted in the revised manuscript.

� In the previous manuscript (line 152-157) “including congenital heart disease (28.8%, 21 out of 73), malignancy (16.4%, 12 out of 73), multiple anomalies (6.8%, 5 out of 73), chronic lung disease (5.5%, 4 out of 73), neurologic disease (5.5%, 4 out of 73), and prematurity (4.1%, 3 out of 73). Most of the MRSA bacteremia cases (98.6%; 72 out of 73) were healthcare-associated infections and indwelling medical devices were involved in 79.5% (58 out of 73).”

Lines 187-191:

The authors have reported mean Ctrough and AUC/MIC values. Were these normally distributed? Looking at the mean and range distributions provided, I suspect the data are non-parametric, so median values might be better.

Also, it is not clear that mean +/- SD are reported because SD has not been indicated in the text.

� Thank you for the comments. I modified to median value and interquartile range instead of mean+/- standard deviation in the revised manuscript.

Lines 271-280:

The authors have referenced predominantly adult literature for PK/PD targets. The updated vancomycin consensus guideline has been released (Rybak Am J Health Syst Pharm 2020) and now includes pediatric sections. Suggest that this is included in discussion and references, rather than the superceded 2009 version. This updated guideline also recommends similar AUC/MIC targets to the adult population.

� At the time of first submission of this paper, this guideline was not published. However, at the time of receiving this review opinion, this major guideline was released, so these contents were reflected and added to the reference.

Table 1:

Please also include number of patients with 30-day mortality, not just percentage (to be consistent with formatting of the rest of the table). What was the rate of recurrent bacteremia?

=> I corrected the numbers of fatal cases and denominators of clinical outcomes after reviewing the medical record again. One out of 73 patients with MRSA bacteremia was transferred to another hospital during treatment, allowing analysis of mortality and recurrence rates in a total of 72 patients: all-cause 30-day mortality was 9.7% (7 out of 72). Recurrent MRSA bacteremia occurred in 19.4% (14/72).

Table 2: (Table 3 in the revised manuscript)

Although it is not summarized elsewhere, the number of children with recurrent bacteremia was 14. From the denominator population of 62 children (presumably there were incomplete/missing data for the remaining 11 children), this is 23%. Additionally, there were 27 children with persistent bacteremia (37%). These are extraordinarily high rates of recurrent and persistent bacteremia. Do the authors have any explanation for these high rates?

� Thank you for the comments. The denominators of recurrent bacteremia and fatality was modified clinical results in Table 3 in the revised manuscript.

� According to your advice and comments, it was described in the discussion section as follows;

Line 362-367, The recurrence rate was relatively high in this study, although it was not possible to clearly distinguish between recurrence or reinfection of MRSA bacteremia because genotyping analysis using pulsed field gel electrophoresis or multilocus sequence typing was not performed. This might be explained by a high proportion of patients with severe underlying disease and long-term hospitalization, and clinician`s preference for salvage therapy for catheter associated infection.

Table 3: (Table 4 in the revised manuscript)

Presentation of the adjusted multivariable model for persistent MRSAB and mortality should only include the final parsimonious model (ie. only variables with p<0.05).

Note that the small numbers of children with these outcomes (27 for persistent SAB and 9 for mortality) makes multivariable analysis challenging due to the low event rate. This should be added to the discussion.

=> Thanks for your advice. According to your comments, I modified the table in the revised manuscprit. The variables included in this analysis are limited to age, bacaaterial co-infection, presence of invasive device, presence of primary focus, initial C trough, and initial AUC/MIC. Except for the variables of initial C trough and initial AUC/MIC, the other variables did not show statistical significance in the univariate analysis.

The sentence you mentioned was added to the discussion.

Line 369-370, the low event rate of clinical outcome including mortality makes multivariable analysis challenging.

Thanks again for all your comments.

Minor errors:

Line 126: Fischer should be spelt Fisher

� Thank you. I corrected the typo.

Table 2:

The numbers in each group of AUC/MIC category should also be presented (as has been done for Ctrough)

The numbers in each group (persistent MRSAB, recurrent MRSAB, mortality) should be presented in the top columns.

� As you mentioned, I wrote down the numbers in each category. Thank you.

Figure 1:

Both axes should have a unit of measurement presented.

� Thanks. Units of measure are written on x-axis (ug/mL)

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Submitted filename: Response to Reviewers_2020 12 22_3 .docx

Click here for additional data file.

27 Jan 2021

PONE-D-20-22107R1

Impact of Initial Vancomycin Pharmacokinetic/Pharmacodynamic Parameters on the Clinical and Microbiological Outcomes of Methicillin-Resistant Staphylococcus aureus Bacteremia in Children

PLOS ONE

Dear Dr. Lee,

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

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Reviewer #2: All comments have been addressed

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Reviewer #2: Yes

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Reviewer #2: Yes

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Reviewer #1: Thank you for addressing my comments. It is a pleasure to review your updated manuscript.

1. Abstract, line 32-33: Rephrase to make clear C trough was measured X times within the time period of Jan. 2010 to March 2018.

2. Abstract, line 36: Make clear that the 47.5% you are referring to is for mechanically ventilated patients (vs ICU admissions). Adding a comma after "intensive care unit stay" should help to clarify that.

3. Abstract, line 37 and 45: "mortality" used more commonly than "fatality"

4. Abstract, line 41, 44, 47: In general, I am noticing "hours" is abbreviated "hrs" or "hr". This should be consistent throughout; I would probably just keep the word spelled out as "hours" in your manuscript text.

5. Introduction, line 51: "major health concerns" is very broad/indirect. One of the major health concerns for what? Pediatrics? Hospitalized patients?

6. Introduction, line 56: cite

7. Introduction, line 58: You say "studies" but only give one citation

8. Discussion, page 20, line 4: "good relationship" should be changed to "positive correlation" or something else more objective.

9. Discussion, page 20, line 5: "good linear relationship" - same comment as above

10. Discussion, page 21, line 10: Please use quotations and define "MIC creep" when first introduced in your manuscript.

11. Discussion, page 21, line 11: Use >/= consistently vs switching to spelling out "greater than". Also, "MIC creep" is when MIC is > 1, not equal to or greater than 1, and still reported as susceptible.

12. Discussion, page 21, line 23-24: Rephrase; "could expose them to adverse effects such as nephrotoxicity or ototoxicity" or something similar that implies the potential only (and ototoxicity should be mentioned)

Thank you for addressing my previous comments so thoroughly. I am grateful for the opportunity to review this manuscript again.

Reviewer #2: I am satisfied that the authors have made revisions according to previous reviewer comments submitted.

**********

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Reviewer #1: No

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10 Feb 2021

Reviewer #1: Thank you for addressing my comments. It is a pleasure to review your updated manuscript.

1. Abstract, line 32-33: Rephrase to make clear C trough was measured X times within the time period of Jan. 2010 to March 2018.

� Thank you for the comments. Only 89% (65/73) of the patients measured Ctrough at least twice. To clarify, I modified the sentence as follows;

C trough was measured at least one time within the time period of January 2010 to March 2018.

2. Abstract, line 36: Make clear that the 47.5% you are referring to is for mechanically ventilated patients (vs ICU admissions). Adding a comma after "intensive care unit stay" should help to clarify that.

� To make it clear, I changed the sentence as follows;

Previously, “Severe cases required intensive care unit stay and mechanical ventilation occurred in 47.5% (35/73)

In this revised one, “Severe clinical outcomes leading to intensive care unit stay and/or use of mechanical ventilation occurred in 47.5% (35/73)

3. Abstract, line 37 and 45: "mortality" used more commonly than "fatality"

=> Thanks for the comments. Throughout the text, we have modified the term fatality by substituting the term mortality.

4. Abstract, line 41, 44, 47: In general, I am noticing "hours" is abbreviated "hrs" or "hr". This should be consistent throughout; I would probably just keep the word spelled out as "hours" in your manuscript text.

� According to the opinion, it was unified and revised in “hours”. Thanks.

5. Introduction, line 51: "major health concerns" is very broad/indirect. One of the major health concerns for what? Pediatrics? Hospitalized patients?

=> Thanks for the comments. To make it clear, it has been modified as follows;

In the previous manuscript, “Methicillin-resistant Staphylococcus aureus (MRSA) is one of the major health concerns,”

In the revised one, “Methicillin-resistant Staphylococcus aureus (MRSA) is one of the major health concerns in both healthcare-associated and community-acquired infections in children,”

6. Introduction, line 56: cite

� Reference 1 was added after the sentence as follows; Vancomycin is an important treatment option for invasive MRSA infections [1].

7. Introduction, line 58: You say "studies" but only give one citation

=> Reference 3 is a review paper on PK/PD of vancomycin. Thanks.

8. Discussion, page 20, line 4: "good relationship" should be changed to "positive correlation" or something else more objective.

� I changed it to “positive correlation” instead of “good relationship”.

9. Discussion, page 20, line 5: "good linear relationship" - same comment as above

� I changed it to “positive correlation” instead of “good linear relationship”

10. Discussion, page 21, line 10: Please use quotations and define "MIC creep" when first introduced in your manuscript.

� According to your comments, I modified the previous sentences and use as follows;

Even though it is difficult to say clearly that vancomycin “MIC creep” was observed during this study, which was defined as a gradual increase in the central tendency of the vancomycin MIC above 1 or 1.5 μg/mL for the dominant wild-type population [26],

11. Discussion, page 21, line 11: Use >/= consistently vs switching to spelling out "greater than". Also, "MIC creep" is when MIC is > 1, not equal to or greater than 1, and still reported as susceptible.

� Thanks for the points. I modified the sentence as follows;

there was no increased trend of the proportion of MRSA isolates with vancomycin MIC > 1.0 μg/mL.

12. Discussion, page 21, line 23-24: Rephrase; "could expose them to adverse effects such as nephrotoxicity or ototoxicity" or something similar that implies the potential only (and ototoxicity should be mentioned)

=> Thank you for the comments. We changed the previous sentence as follows;

In the previous manuscript, “In addition, targeting vancomycin trough levels > 15 µg/mL in pediatrics would expose them to unnecessary adverse events of nephrotoxicity.”

In the revised one, “In addition, targeting vancomycin trough levels > 15 µg/mL in pediatrics could expose them to adverse effects such as nephrotoxicity or ototoxicity.”

Thank you for addressing my previous comments so thoroughly. I am grateful for the opportunity to review this manuscript again.

� We sincerely appreciate your meticulous review of our manuscript and great expert opinion.

Reviewer #2: I am satisfied that the authors have made revisions according to previous reviewer comments submitted.

� Thank you for reviewing our manuscript and for your informative expert opinion. Thank you again.

Submitted filename: Response to reviewers_2021 02 10.docx

Click here for additional data file.

12 Feb 2021

Impact of Initial Vancomycin Pharmacokinetic/Pharmacodynamic Parameters on the Clinical and Microbiological Outcomes of Methicillin-Resistant Staphylococcus aureus Bacteremia in Children

PONE-D-20-22107R2

Dear Dr. Lee,

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Taeok Bae

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

24 Mar 2021

PONE-D-20-22107R2

Impact of Initial Vancomycin Pharmacokinetic/Pharmacodynamic Parameters on the Clinical and Microbiological Outcomes of Methicillin-Resistant Staphylococcus aureus Bacteremia in Children

Dear Dr. Lee:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

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PLOS ONE