Optimization of polymyxin B regimens for the treatment of carbapenem-resistant organism nosocomial pneumonia: a real-world prospective study

This prospective study was conducted at two intensive care units (ICU) between January 2020 and December 2021 in the Second Xiangya Hospital of Central South University (Changsha, China). Patients were included if (a) age ≥ 18 years; (b) diagnosed with nosocomial pneumonia that developed more than 48 h after admission; (c) at least two consecutive samples on different days (time interval at least 24 h) showed the presence of CRO from bronchial secretions or bronchoalveolar lavage samples; (d) received intravenous polymyxin B treatment over 3 days. The exclusion criteria were as follows: (a) concomitant lung cancer with obstructive pneumonitis or cystic fibrosis; (b) solid organ transplantation; (c) hematologic malignancies and hematopoietic cell transplant recipients; (d) receiving renal replacement therapy. HAP was defined according to the 2016 clinical practice guidelines of the Infectious Diseases Society of America and the American Thoracic Society [21]. Determination of carbapenem susceptibility of CRO was followed by the European Committee on Antimicrobial Susceptibility Testing (EUCAST). Updated EUCAST Clinical Breakpoints of polymyxin B were sensitivity (S ≤ 2 mg/L) and drug resistance (R > 2 mg/L). The antimicrobial susceptibility testing was performed using the VITEK-2 Compact system with VITEK cards (0.5–16 mg/L for colistin) (bioMérieux, France). The following information was extracted from the electronic medical records: demographic and co-morbidity profiles, clinical and microbiological features of the infections, and the antimicrobial treatment regimens. Creatinine clearance (CrCL) was calculated using the Cockcroft–Gault equation. The endpoint was clinical efficacy and 30-day all-cause mortality. Assessment of clinical efficacy was conducted at the end of treatment, and 30-day mortality was recorded from the start of polymyxin B treatment. This prospective study was approved by the Ethics Committee of the Second Xiangya Hospital, Central South University. Informed consent was obtained from all patients or legal representatives of the patients (No. ChiCTR1900022231).

Polymyxin B was given to all patients empirically as a loading dose of 100–200 mg followed by a maintenance dose of 40–100 mg every 12 h for at least 3 days. The infusion time was at least 1 h. Aerosol delivery of polymyxin B (25 mg or 50 mg twice daily) was using a vibrating mesh nebulizer, synchronized with the inspiratory cycle of the ventilator. Two to six blood samples (2 mL) were randomly collected immediately before the seventh dose of polymyxin B and at 0, 1, 2, 4, 6, 8 and 10 h after the end of infusion. The supernatant was immediately stored at − 80 °C until analysis.

An established high-performance liquid chromatography-tandem mass spectrometry (HPLC–MS/MS) was used to measure the concentrations of polymyxin B₁ and polymyxin B₂ as described previously by the authors’ laboratory (The total concentration of polymyxin B = [polymyxin B₁ concentration/polymyxin B₁ molecular + polymyxin B₂ concentration/polymyxin B₂ molecular]*total polymyxin B molecular) [22]. The interday precision was < 12%, the intraday precision was < 9%, and the accuracy ranged from 96.1 to 110.4%. The limit of quantification (LLOD) was 0.03 mg/L, and all of the polymyxin B concentrations detected were over LLOD.

The Phoenix NLME program (version 8.1. Pharsight, A Certara Company, USA) with the method of first-order conditional estimation-extended least square method (FOCE-ELS) was used to develop the population PK model by analyzing polymyxin B concentration. The objective function value (OFV), goodness-of-fit plots and the reasonable of population PK parameters were used to selection of the structure model. The stepwise covariate modeling (SCM) approach was used to test the covariate model in this analysis; age, sex, body weight, alanine aminotransferase (ALT), aspartate aminotransferase (AST), total bilirubin (TBIL), direct bilirubin (DBIL), serum albumin (ALB) and CrCL were evaluated as the covariates. The SCM consists of a forward selection step (the criterion is p < 0.05 for ΔOFV decreased ≥ 3.84) and a backward elimination step (the criterion is p < 0.001 for ΔOFV increased ≥ 10.83). In addition, the goodness-of-fit plots were used to assess the validity of the population PK model, the prediction-corrected visual predictive check (pcVPC) was used to assess the predictive performance of the key models. The bootstrap method was used to assess the accuracy.

The PK/PD indices included AUC_ss,24 h, AUC_ss,24 h/MIC, the peak and trough concentration at steady state (C_max,ss and C_min,ss), C_max,ss /MIC and C_min,ss /MIC. AUC_ss,24 h, C_max,ss and C_min,ss were calculated based on the empirical Bayesian (EBEs). For those patients with more than one baseline pathogen, AUC_ss,24 h/MIC, C_max,ss /MIC and C_min,ss /MIC evaluations were based on the pathogen with the highest MIC value.

Clinical outcomes were classified as clinical success (CS) and clinical failure (CF), and were assessed by two physicians. CS was defined as a composite of survival; hemodynamic stability; body temperature < 38 °C, improved biochemistry indicators of infection; stable or improved PaO₂/FiO₂ ratio. Additionally, for patients with bacteremia, microbiological cure (no growth of the initial isolate in blood cultures) must be achieved by the end of the treatment [23‐25]. Patients who did not meet all above criteria were classified as CF. 30-day all-cause mortality was recorded from the start of polymyxin B treatment.

Variables potentially related to clinical efficacy and 30-day all-cause mortality were assessed, including: demographics, co-morbidities, clinical conditions, dosage regimen and concentration of polymyxin B. To develop receiver operating characteristic (ROC) curves, the PK/PD indices AUC_ss,24 h/MIC, C_max,ss/MIC and C_min,ss/MIC were used as predictors of clinical efficacy. The area under the diagnostic curve (AUC_ROC) was calculated to evaluate the correlation of the above parameters with clinical efficacy and 30-day all-cause mortality. Youden index of the ROC curves was calculated by "sensitivity + specificity-1", and the values corresponding to the maximum Youden index is the optimal cutoff point value of the PK/PD indices.

Based on the final population PK models, the plasma concentration-time profile of 1,000 individuals was simulated. The dosages were selected according to the most commonly used regimens in clinical practice. The regimens were 100–200 mg loading dose followed by 75–150 mg every 12 h. The infusion time was set to 2 h.

Statistical analysis was performed with SPSS 24.0 (SPSS, IBM Company, Chicago, IL, USA) software. Continuous variables are presented as the mean ± standard deviation (SD) if normally distributed and were compared using Student’s t tests. The median and interquartile range (IQR) are presented for abnormally distributed data, and the Mann–Whitney U test was used. Categorical variables are expressed as counts and percentages, and the chi-square test or Fisher’s exact test was used. Spearman’s rank correlation coefficient (r) was used to analyze the correlation between C_min,ss, C_max,ss and AUC_ss,24 h. Univariate analysis was performed for all variables to identify possible predictors for clinical efficacy. Variables with a p < 0.05 were entered into the multivariate logistic regression models. A forward stepwise (likelihood ratio) method was performed to determine the predictors using a significance level of 0.05 for entry and 0.10 for removal from the model. 30-day all-cause mortality was evaluated with Cox regression model. P < 0.05 was considered statistically significant.

During the study period, 132 patients (≥ 18 years) received polymyxin B therapy. Among them, 11 patients were non-HAP, five patients had no pathogenic microorganism result, four patients were solid transplant recipients, four patients received renal replacement and three patients received polymyxin B treatment ≤ 3 days. Thus, 105 patients were eventually enrolled. The demographic characteristics of all patients are summarized in Table 1. The median APACHE II score of these patients was 18 (IQR: 12, 25), and the rate of sepsis was 39.0% (41/105). The most common pathogenic bacteria were Acinetobacter baumannii (N = 86; 81.9%), followed by Klebsiella pneumoniae (N = 42; 40.0%) and Pseudomonas aeruginosa (N = 14; 13.3%). For most of our CRO stains, MIC values of polymyxin B were 1 mg/L, and ≤ 0.5 mg/L for three isolates of Klebsiella pneumoniae. MIC₅₀ and MIC₉₀ of polymyxin B were 1 mg/L for Acinetobacter baumannii, Klebsiella pneumoniae and Pseudomonas aeruginosa. The loading dose of polymyxin B was 100 mg (IQR: 100,150 mg). The median polymyxin B daily dose was 2.3 mg/kg (IQR: 2.0, 2.9 mg/kg) with a duration of 12 days (IQR: 9, 16).

The population PK model was developed based on 295 plasma concentrations obtained from 105 patients, each patient on average contributed three clinical samples. A two-compartment model fully described the data, and no covariate was statistically significant to PK parameters. A proportional error model was used to evaluate the residual variability. In addition, the shrinkage of the clearance between central compartment and peripheral compartment (CLd) and volume in peripheral compartment (Vp) were more than 50%, so the inter-individual variability (IIV) of CLd and Vp were fixed 0. Due to the highly correlation between the clearance in central compartment (CL) and the volume distribution in central compartment (Vc), the omega block was applied to improve the accuracy of IIV in each patient, and the estimated covariance between CL and Vc is 100%.

The goodness-of-fit plots in the final model are shown in Fig. 1. The plots were shown that the structure of the final model was not biased and that the model was acceptable. The result of pcVPC is presented in Fig. 2. Most concentrations were within the 90% CIs, indicating that the final model had a good description of the original data. The bootstrap results are shown in Table 2, which indicating qualified precision for the final population PK models. In the final model, the typical values of CL, Vc, CLd, and Vp were 1.56 L/h, 12.5 L, 2.41 L/h, and 29.9 L, respectively. The final PK model equations were as follows: \(\mathrm{CL}\left(L/h\right)=1.56*\mathrm{exp}(\eta \mathrm{CL})\), \(\mathrm{Vc}\left(L\right)=12.5*\mathrm{exp}(\eta \mathrm{Vc})\), \(\mathrm{CLd}\left(L/h\right)=2.41\), \(\mathrm{Vp}(L)=29.9\). The results of final PPK model are shown in Table 2.

The median values of C_min,ss, C_max,ss and AUC_ss,24 h were 1.9 mg/L (IQR:1.4, 2.7 mg/L), 5.9 mg/L (4.6, 7.1 mg/L) and 67.2 mg·h/L (IQR:54.8, 84.2 mg·h/L), respectively. Spearman’s rank correlation analysis showed that AUC_ss,24 h were positively correlated C_min,ss, C_max,ss (Additional file 1: Fig. S1a, Additional file 2: Fig. S1b).

Among the 105 patients, the clinical success rate was 66.7% (70/105). Patients in the clinical success group were more likely to combine with inhaled polymyxin B (40.0% VS 17.1%, p = 0.012) than patients in the clinical failure group. The dosage (daily, total), duration and plasma concentrations (C_min,ss, C_max,ss, AUC_ss,24 h, C_min,ss/MIC, C_max,ss/MIC and AUC_ss,24 h/MIC) of polymyxin B were significantly higher (p < 0.05) in the clinical success group than in the clinical failure group (Table 3). Based on these results, a total of 12 possible risk factors were identified by the univariate analysis. These 12 variables (p < 0.05) were used to develop a multivariate logistic regression model. Finally, AUC_ss,24 h/MIC (AOR = 0.97, 95% CI 0.95–0.99, p = 0.009), daily dose (AOR = 0.98, 95% CI 0.97–0.99, p = 0.028), and combination of inhaled polymyxin B (AOR = 0.32, 95% CI 0.11–0.94, p = 0.039) were included in the final model. A ROC curve was used to calculate the discriminatory power of the model (AUC = 0.795) (Fig. 3). In addition, considering that AUC_ss,24 h and dose are directly correlated, the relationship between efficacy and dose normalized-AUC_ss,24 h/MIC was tested using the Covariate-Adjusted Residuals method to remove this intrinsic correlation (AOR = 0.62, 95% CI 0.38–1.01, p = 0.056).

In addition, the ROC curves support that the AUC_ss,24 h/MIC (AUC_ROC = 0.719) was superior to the other two PK/PD indices for the prediction of clinical efficacy. When the Youden index was the largest (1.401), the corresponding optimal cutoff point value of AUC_ss,24 h/MIC was 66.9. The predictive sensitivity and the specificity of this value were 65.6% and 77.4%, respectively (Fig. 4).

Subgroup analysis showed that combination of inhaled polymyxin B was a significant predictor for the clinical efficacy (OR: 0.21, 95% CI: 0.06–0.79, p = 0.022) of the 51 patients with AUC_ss,24 h/MIC < 66.9. The clinical success rate was significantly higher in the intravenous (IV) plus inhaled (IH) group (71.4% vs. 35.1%, p = 0.017). However, of the 54 patients with AUC_ss,24 h/MIC ≥ 66.9, the combination of inhaled polymyxin B was not a significant predictor of the clinical efficacy (OR: 0.46, 95% CI: 0.09–2.46, p = 0.365), the clinical success rate in IV + IH group (90.0%) was higher than that in IV group (82.3%), but there was no difference between the two groups (p = 0.279) (Fig. 5).

The 30-day all-cause mortality was 26.7% (28/105). None of the investigated PK/PD indices was significantly associated with 30-day mortality, and patients with higher APACHEII score, shorter treatment duration and clinical failure were associated with higher mortality in Cox regression model (Additional file 3: Table S1). ROC analysis was not conducted for mortality since no PK/PD index correlated with it was found in Cox regression model.

Our PK/PD analysis results showed that AUC_ss,24 h/MIC over 66.9 was associated with better clinical efficacy. Therefore, this cutoff value was adopted as PK/PD target and the probability of target attainment (PTA) was estimated for seven different MICs ranging from 0.125 to 8 mg/L on day 1 and day 4, respectively.

For an MIC less than 0.5 mg/L, the PTAs of all simulated regimens were greater than 90% on Day 4; When the MIC value was less than 1 mg/L, regimens from 100 to 150 mg every 12 h achieved the target exposure of AUC_ss,24 h/MIC over 66.9. None of the simulated regimens achieved adequate target attainment at the current CLSI and EUCAST breakpoint of 2 mg/L. Furthermore, we found that a loading dose made it possible to achieve the target PTA on Day 1, indicating that the loading dose is essential for polymyxin B treatments (Fig. 6).