Nonlinear pharmacokinetics of piperacillin in healthy volunteers--implications for optimal dosage regimens

Model-informed dose optimization for prophylactic piperacillin-tazobactam in perioperative pediatric critically ill patients

Piperacillin/tazobactam (PTZ) is frequently prescribed during the perioperative period as prophylaxis in critically ill patients. Current international guidelines recommend that the pediatric intraoperative dosing regimen for PTZ be 90-112.5 mg/kg (80-100 mg/kg as piperacillin [PIP]) administered every 2 hours (Q2H). Concerns have been raised not only about the risk of nephrotoxicity due to elevated PIP exposure but also regarding the practicality of adhering to a 2-h dosing interval in clinical settings. To address these concerns, we employed population pharmacokinetic (PK) modeling and simulation approaches to optimize PTZ dosing regimens in pediatric intraoperative patients. PIP plasma concentration data were obtained from 34 patients using an opportunistic sampling strategy. A two-compartment model was found to adequately describe the PK data. Creatinine clearance was identified as a significant covariate on clearance. The inclusion of inter-occasion variability significantly improved model fit. Simulations across body weights of 10-70 kg and creatinine clearance of 20-130 mL/min/1.73 m2 demonstrated that 6-15 mg/kg Q2H, or a 10 mg/kg loading dose followed by 1.0-2.75 mg/kg/h continuous infusion would achieve free PIP concentrations being above the minimum inhibitory concentration (MIC) for 100% of the dosing interval (100% fT >1× MIC). For achieving 100% fT >4× MIC, 25-55 mg/kg Q2H or a 20 mg/kg loading dose followed by 3.25-9.25 mg/kg/h continuous infusion was derived. The model-informed simulations indicated that both lower Q2H doses and continuous infusion methods are clinically viable options and potentially resolve current clinical challenges during intraoperative dosing.

A Pooled Pharmacokinetic Analysis for Piperacillin/Tazobactam Across Different Patient Populations: From Premature Infants to the Elderly

BACKGROUND AND OBJECTIVES

The pharmacokinetics (PK) of piperacillin/tazobactam (PIP/TAZ) is highly variable across different patient populations and there are controversies regarding non-linear elimination as well as the fraction unbound of PIP (fUNB_PIP). This has led to a plethora of subgroup-specific models, increasing the risk of misusing published models when optimising dosing regimens. In this study, we aimed to develop a single model to simultaneously describe the PK of PIP/TAZ in diverse patient populations and evaluate the current dosing recommendations by predicting the PK/pharmacodynamics (PD) target attainment throughout life.

METHODS

Population PK models were separately built for PIP and TAZ based on data from 13 studies in various patient populations. In the development of those single-drug models, postnatal age (PNA), postmenstrual age (PMA), total body weight (TBW), height, and serum creatinine (SCR) were tested as covariates. Subsequently, a combined population PK model was established and the correlations between the PK of PIP and TAZ were tested. Monte Carlo simulations were performed based on the final combined model to evaluate the current dosing recommendations.

RESULTS

The final combined model for PIP/TAZ consisted of four compartments (two for each drug), with covariates including TBW, PMA, and SCR. For a 70-kg, 35-year-old patient with SCR of 0.83 mg L-1, the PIP values for V1, CL, V2 and Q2 were 10.4 L, 10.6 L h-1, 11.6 L and 15.2 L h-1, respectively, and the TAZ values were 10.5 L, 9.58 L h-1, 13.7 L and 16.8 L h-1, respectively. The CL for both drugs show maturation in early life, reaching 50% at 54.2 weeks PMA. With advancing age, CL of TAZ declines to 50% at 61.6 years PMA, whereas CL of PIP declines more slowly, reaching 50% at 89.1 years PMA. The fUNB_PIP was estimated as 64.5% and non-linear elimination was not supported by our data. The simulation results indicated considerable differences in PK/PD target attainment for different patient populations under current recommended dosing regimens.

CONCLUSIONS

We developed a combined population PK model for PIP/TAZ across a broad range of patients covering the extremes of patient characteristics. This model can be used as a robust a priori model for Bayesian forecasting to achieve individualised dosing. The simulations indicate that adjustments based on the allometric theory as well as maturation and decline of CL of PIP may help the current dosing recommendations to provide consistent target attainment across patient populations.

Population pharmacokinetic rationale for intravenous contezolid acefosamil followed by oral contezolid dosage regimens

Contezolid is a novel oxazolidinone antibiotic with a promising safety profile. Oral contezolid and its intravenous (IV) prodrug contezolid acefosamil (CZA) are in development for treatment of diabetic foot and acute bacterial skin and skin structure infections (ABSSSI). The prodrug CZA is converted to active contezolid via intermediate MRX-1352. This study aimed to provide the pharmacokinetic rationale for safe, effective, and flexible dosage regimens with initial IV CZA followed by oral contezolid. We simultaneously modeled plasma concentrations from 110 healthy volunteers and 74 phase 2 patients with ABSSSI via population pharmacokinetics (using the importance sampling estimation algorithm), and optimized dosage regimens by Monte Carlo simulations. This included data on MRX-1352, contezolid, and its metabolite MRX-1320 from 66 healthy volunteers receiving intravenous CZA (150-2400 mg) for up to 28 days, and 74 patients receiving oral contezolid [800 mg every 12 h (q12h)] for 10 days. The apparent total clearance for 800 mg oral contezolid with food was 16.0 L/h (23.4% coefficient of variation) in healthy volunteers and 17.7 L/h (53.8%) in patients. CZA was rapidly converted to MRX-1352, which subsequently transformed to contezolid. The proposed dosage regimen used an IV CZA 2000 mg loading dose with 1000 mg IV CZA q12h as maintenance dose(s), followed by 800 mg oral contezolid q12h (with food). During each 24-h period, Monte Carlo simulations predicted this regimen to achieve consistent areas under the curve of 91.9 mg·h/L (range: 76.3-106 mg·h/L) under all scenarios. Thus, this regimen was predicted to reliably achieve efficacious contezolid exposures independent of timing of switch from IV CZA to oral contezolid.IMPORTANCEThis study provides the population pharmacokinetic rationale for the dosage regimen of the intravenous (IV) prodrug contezolid acefosamil (CZA) followed by oral contezolid. We developed the first integrated population model for the pharmacokinetics of the MRX-1352 intermediate prodrug, active contezolid, and its main metabolite MRX-1320 based on data from three clinical studies in healthy volunteers and phase 2 patients. The proposed regimen was predicted to reliably achieve efficacious contezolid exposures independent of timing of switch from IV CZA to oral contezolid.

Predicting Volume of Distribution in Neonates: Performance of Physiologically Based Pharmacokinetic Modelling

The volume of distribution at steady state (Vss) in neonates is still often estimated through isometric scaling from adult values, disregarding developmental changes beyond body weight. This study aimed to compare the accuracy of two physiologically based pharmacokinetic (PBPK) Vss prediction methods in neonates (Poulin & Theil with Berezhkovskiy correction (P&T+) and Rodgers & Rowland (R&R)) with isometrical scaling. PBPK models were developed for 24 drugs using in-vitro and in-silico data. Simulations were done in Simcyp (V22) using predefined populations. Clinical data from 86 studies in neonates (including preterms) were used for comparison, and accuracy was assessed using (absolute) average fold errors ((A)AFEs). Isometric scaling resulted in underestimated Vss values in neonates (AFE: 0.61), and both PBPK methods reduced the magnitude of underprediction (AFE: 0.82-0.83). The P&T+ method demonstrated superior overall accuracy compared to isometric scaling (AAFE of 1.68 and 1.77, respectively), while the R&R method exhibited lower overall accuracy (AAFE: 2.03). Drug characteristics (LogP and ionization type) and inclusion of preterm neonates did not significantly impact the magnitude of error associated with isometric scaling or PBPK modeling. These results highlight both the limitations and the applicability of PBPK methods for the prediction of Vss in the absence of clinical data.

Predictive Performance of Physiologically Based Pharmacokinetic Modelling of Beta-Lactam Antibiotic Concentrations in Adipose, Bone, and Muscle Tissues

Physiologically based pharmacokinetic (PBPK) models consist of compartments representing different tissues. As most models are only verified based on plasma concentrations, it is unclear how reliable associated tissue profiles are. This study aimed to assess the accuracy of PBPK-predicted beta-lactam antibiotic concentrations in different tissues and assess the impact of using effect site concentrations for evaluation of target attainment. Adipose, bone, and muscle concentrations of five beta-lactams (piperacillin, cefazolin, cefuroxime, ceftazidime, and meropenem) in healthy adults were collected from literature and compared with PBPK predictions. Model performance was evaluated with average fold errors (AFEs) and absolute AFEs (AAFEs) between predicted and observed concentrations. In total, 26 studies were included, 14 of which reported total tissue concentrations and 12 unbound interstitial fluid (uISF) concentrations. Concurrent plasma concentrations, used as baseline verification of the models, were fairly accurate (AFE: 1.14, AAFE: 1.50). Predicted total tissue concentrations were less accurate (AFE: 0.68, AAFE: 1.89). A slight trend for underprediction was observed but none of the studies had AFE or AAFE values outside threefold. Similarly, predictions of microdialysis-derived uISF concentrations were less accurate than plasma concentration predictions (AFE: 1.52, AAFE: 2.32). uISF concentrations tended to be overpredicted and two studies had AFEs and AAFEs outside threefold. Pharmacodynamic simulations in our case showed only a limited impact of using uISF concentrations instead of unbound plasma concentrations on target attainment rates. The results of this study illustrate the limitations of current PBPK models to predict tissue concentrations and the associated need for more accurate models. SIGNIFICANCE STATEMENT: Clinical inaccessibility of local effect site concentrations precipitates a need for predictive methods for the estimation of tissue concentrations. This is the first study in which the accuracy of PBPK-predicted tissue concentrations of beta-lactam antibiotics in humans were assessed. Predicted tissue concentrations were found to be less accurate than concurrent predicted plasma concentrations. When using PBPK models to predict tissue concentrations, this potential relative loss of accuracy should be acknowledged when clinical tissue concentrations are unavailable to verify predictions.

Comparison of Piperacillin and Tazobactam Pharmacokinetics in Critically Ill Patients with Trauma or with Burn

Critical illness caused by burn and sepsis is associated with pathophysiologic changes that may result in the alteration of pharmacokinetics (PK) of antibiotics. However, it is unclear if one mechanism of critical illness alters PK more significantly than another. We developed a population PK model for piperacillin and tazobactam (pip-tazo) using data from 19 critically ill patients (14 non-burn trauma and 5 burn) treated in the Military Health System. A two-compartment model best described pip-tazo data. There were no significant differences found in the volume of distribution or clearance of pip-tazo in burn and non-burn patients. Although exploratory in nature, our data suggest that after accounting for creatinine clearance (CrCl), doses would not need to be increased for burn patients compared to trauma patients on consideration of PK alone. However, there is a high reported incidence of augmented renal clearance (ARC) in burn patients and pharmacodynamic (PD) considerations may lead clinicians to choose higher doses. For critically ill patients with normal kidney function, continuous infusions of 13.5-18 g pip-tazo per day are preferable. If ARC is suspected or the most stringent PD targets are desired, then continuous infusions of 31.5 g pip-tazo or higher may be required. This approach may be reasonable provided that therapeutic drug monitoring is enacted to ensure pip-tazo levels are not supra-therapeutic.

An Integral Pharmacokinetic Analysis of Piperacillin and Tazobactam in Plasma and Urine in Critically Ill Patients

BACKGROUND AND OBJECTIVES

Although dose optimization studies have been performed for piperacillin and tazobactam separately, a combined integral analysis is not yet reported. As piperacillin and tazobactam pharmacokinetics are likely to show correlation, a combined pharmacokinetic model should be preferred to account for this correlation when predicting the exposure. Therefore, the aim of this study was to describe the pharmacokinetics and evaluate different dosing regimens of piperacillin and tazobactam in critically ill patients using an integral population pharmacokinetic model in plasma and urine.

METHODS

In this observational study, a total of 39 adult intensive care unit patients receiving piperacillin-tazobactam as part of routine clinical care were included. Piperacillin and tazobactam concentrations in plasma and urine were measured and analyzed using non-linear mixed-effects modeling. Monte Carlo simulations were performed to predict the concentrations for different dosing strategies and different categories of renal function.

RESULTS

A combined two-compartment linear pharmacokinetic model for both piperacillin and tazobactam was developed, with an output compartment for the renally excreted fraction. The addition of 24-h urine creatinine clearance significantly improved the model fit. A dose of 12/1.5 g/24 h as a continuous infusion is sufficient to reach a tazobactam concentration above the target (2.89 mg/L) and a piperacillin concentration above the target of 100% f T_>1×MIC (minimum inhibitory concentration [MIC] ≤ 16 mg/L). To reach a target of 100% f T_>5×MIC with an MIC of 16 mg/L, piperacillin doses of up to 20 g/24 h are inadequate. Potential toxic piperacillin levels were reached in 19.6% and 47.8% of the population with a dose of 12 g/24 h and 20 g/24 h, respectively.

CONCLUSIONS

A regular dose of 12/1.5 g/24 h is sufficient in > 90% of the critically ill population to treat infections caused by Escherichia coli and Klebsiella pneumoniae with MICs ≤ 8 mg/L. In case of infections caused by Pseudomonas aeruginosa with an MIC of 16 mg/L, there is a fine line between therapeutic and toxic exposure. Dosing guided by renal function and therapeutic drug monitoring could enhance target attainment in such cases.

CLINICALTRIALS

GOV IDENTIFIER

NCT03738683.

Population Pharmacokinetic Model of Piperacillin in Critically Ill Patients and Describing Interethnic Variation Using External Validation

Objectives: This study aimed to develop a piperacillin population PK model for critically ill Brazil-ian patients and describe interethnic variation using an external validation. Methods: Plasma samples were obtained from 24 ICU patients during the fifth day of piperacillin treatment and assayed by HPLC-UV. Population pharmacokinetic modelling was conducted using Pmetrics. Empiric dose of 4 g IV 6- and 8-hourly were simulated for 50 and 100% fT > MIC and the probabil-ity of target attainment (PTA) and the fractional target attainment (FTA) determined. Results: A two-compartment model was designed to describe the pharmacokinetics of critically ill Brazillian patients. Clearance and volume of distribution were (mean ± SD) 3.33 ± 1.24 L h−1 and 10.69 ± 4.50 L, respectively. Creatinine clearance was positively correlated with piperacillin clearance and a high creatinine clearance was associated with lower values of PTA and FTA. An external vali-dation was performed using data from two different ethnic ICU populations (n = 30), resulting in acceptable bias and precision. Conclusion: The primary pharmacokinetic parameters obtained from critically ill Brazilian patients were similar to those observed in studies performed in critically ill patients of other ethnicities. Based on our results, the use of dose adjustment based on creati-nine clearance is required in Brazilian patients.

Population pharmacokinetics of continuous infusion of piperacillin/tazobactam in very elderly hospitalized patients and considerations for target attainment against Enterobacterales and Pseudomonas aeruginosa

Continuous infusion (CI) piperacillin/tazobactam is frequently used to treat infections in very elderly patients. This study aimed to conduct a population pharmacokinetic analysis of CI piperacillin/tazobactam, and to identify optimal dosages for safe and effective probability of target attainment (PTA) against Enterobacterales and Pseudomonas aeruginosa. Non-linear mixed-effects modelling was performed with Pmetrics. Monte Carlo simulations assessed the steady-state concentration (Css) of increasing piperacillin/tazobactam regimens (from 2.25 to 18 g daily by continuous infusion). Permissible doses were defined as those associated with <10% probability of Css >157.2 mg/L. PTA at the pharmacodynamic targets of free plasma steady-state concentration (fCss)/minimum inhibitory concentration (MIC) ≥1 and ≥4 and cumulative fraction of response (CFR) against EUCAST MIC distribution were also calculated. A total of 141 patients (median age 85 years) provided 217 plasma piperacillin Css. Most patients (55.2%) had hospital-acquired pneumonia and intra-abdominal infections. A one-compartment pharmacokinetic model with parallel linear and Michaelis-Menten elimination best described piperacillin data. Creatinine clearance (CL_CR) was the covariate retained by the model. Pharmacokinetic estimates were 6.05 L/h for clearance and 3.39 mg/L for the Michaelis-Menten constant. Permissible doses were up to 4.5, 9, 11.25 and 13.5 g daily by continuous infusion for patients with CL_CR of 0-19, 20-39, 40-59 and 60-79 mL/min/1.73 m², respectively. At the clinical breakpoint of 8 mg/L, the permissible doses only achieved optimal PTA for fCss/MIC ≥1 in patients with CL_CR 20-79 mL/min/1.73 m². Optimal CFRs with the permissible doses were only attained against Escherichia coli and Proteus mirabilis. Permissible dosages and CL_CR should be considered for prescribing CI piperacillin/tazobactam in very elderly patients.

Piperacillin-Tazobactam in Intensive Care Units: A Review of Population Pharmacokinetic Analyses

Piperacillin-tazobactam is a potent β-lactam/β-lactamase inhibitor antibiotic commonly prescribed in the intensive care unit setting. Admitted patients often show large variability in treatment response due to multiple pathophysiological changes present in this population that alter the drug's pharmacokinetics. This review summarizes the population pharmacokinetic models developed for piperacillin-tazobactam and provides comprehensive data on current dosing strategies while identifying significant covariates in critically ill patients. A literature search on the PubMed database was conducted, from its inception to July 2020. Relevant articles were retained if they met the defined inclusion/exclusion criteria. A total of ten studies, published between 2009 and 2020, were eligible. One- and two-compartment models were used in two and eight studies, respectively. The lowest estimated piperacillin clearance value was 3.12 L/h, and the highest value was 19.9 L/h. The estimations for volume of distribution varied between 11.2 and 41.2 L. Tazobactam clearance values ranged between 5.1 and 6.78 L/h, and tazobactam volume of distribution values ranged between 17.5 and 76.1 L. The most frequent covariates were creatinine clearance and body weight, each present in four studies. Almost all studies used an exponential approach for the interindividual variability. The highest variability was observed in piperacillin central volume of distribution, at a value of 75.0%. Simulations showed that continuous or extended infusion methods performed better than intermittent administration to achieve appropriate pharmacodynamic targets. This review synthesizes important pharmacokinetic elements for piperacillin-tazobactam in an intensive care unit setting. This will help clinicians better understand changes in the drug's pharmacokinetic parameters in this specific population.

Pharmacokinetic and Pharmacodynamic Optimization of Antibiotic Therapy in Cystic Fibrosis Patients: Current Evidences, Gaps in Knowledge and Future Directions

Antibiotic therapy is one of the main treatments for cystic fibrosis (CF). It aims to eradicate bacteria during early infection, calms down the inflammatory process, and leads to symptom resolution of pulmonary exacerbations. CF can modify both the pharmacokinetic (PK) and pharmacodynamic (PD) profiles of antibiotics, therefore specific PK/PD endpoints should be determined in the context of CF. Currently available data suggest that optimal PK/PD targets cannot be attained in sputum with intravenous aminoglycosides. Continuous infusion appears preferable for β-lactam antibiotics, but optimal concentrations in sputum are unlikely to be reached, with some possible exceptions such as meropenem and ceftolozane. Usual doses are likely suboptimal for fluoroquinolones and linezolid, whereas daily doses of 45-60 mg/kg and 200 mg could be convenient for vancomycin and doxycycline, respectively. Weekly azithromycin doses of 22-30 mg/kg could also be appropriate for its anti-inflammatory effect. The difficulty with achieving optimal concentrations supports the use of combined treatments and the inhaled administration route, as very high local concentrations, concomitantly with low systemic exposure, can be obtained with the inhaled route for aminoglycosides, colistin, and fluoroquinolones, thus minimizing the risk of toxicity.

Estimation of piperacillin clearance with different glomerular filtration rate formulas in critically ill children

AIMS

Glomerular filtration rate (GFR) is difficult to assess in critically ill children using gold standard method and alternatives are needed. This study aimed to determine the most accurate GFR estimation formula for assessing piperacillin clearance in critically ill children, using a published piperacillin pharmacokinetics (PK) population model.

METHODS

All children hospitalized in the paediatric intensive care unit of a single institution who were receiving piperacillin were included. PK were described using the nonlinear mixed effect modelling software MONOLIX. In the initial PK model, GFR was estimated according to the Schwartz 1976 formula. We evaluated a set of 12 additional validated formulas, developed using plasma creatinine and/or cystatin C concentrations, in the building model to assess the lowest between-subject variability for piperacillin clearance.

RESULTS

We included 20 children with a median (range) postnatal age of 1.9 (0.1-19) years, body weight of 12.5 (3.5-69) kg. Estimated GFR according to the Schwartz 1976 formula was 160.5 (38-315) mL min^-1 1.73 m^-2 . Piperacillin clearance was best predicted by the Bouvet combined formula.

CONCLUSION

The combined Bouvet formula was the most accurate GFR estimation formula for assessing piperacillin clearance in critically ill children.

Time above the MIC of Piperacillin-Tazobactam as a Predictor of Outcome in Pseudomonas aeruginosa Bacteremia

Pseudomonas aeruginosa bacteremia is an infection associated with a high mortality rate. Piperacillin-tazobactam is a β-lactam-β-lactamase inhibitor combination that is frequently used for the management of Pseudomonas aeruginosa infections. The pharmacokinetic-pharmacodynamic index associated with in vitro maximal bacterial killing for piperacillin-tazobactam is the percentage of the time between doses at which the free fraction concentration remains above the MIC (%fT _>MIC). However, the precise %fT _>MIC target associated with improved clinical outcomes is unknown. The aim of this study was to investigate the correlation between the survival of patients with Pseudomonas aeruginosa bacteremia and the threshold of the piperacillin-tazobactam %fT _>MIC This retrospective study included all adult patients hospitalized over an 82-month period with Pseudomonas aeruginosa bacteremia and treated with piperacillin-tazobactam. Patients with a polymicrobial infection or those who died within 72 h of the time of collection of a sample for culture were excluded. The %fT _>MIC of piperacillin-tazobactam associated with in-hospital survival was derived using classification and regression tree analysis. After screening 270 patients, 78 were eligible for inclusion in the study; 18% died during hospitalization. Classification and regression tree analysis identified a %fT _>MIC of >60.68% to be associated with improved survival, and this remained statistically significant after controlling for clinical covariates (odds ratio = 7.74, 95% confidence interval = 1.32 to 45.2). In conclusion, the findings recommend dosing of piperacillin-tazobactam with the aim of achieving a pharmacodynamic target %fT _>MIC of at least 60% in these patients.

Saturable elimination of piperacillin in critically ill patients: implications for continuous infusion

The study aimed to evaluate saturation of piperacillin elimination in critically ill adult patients. Seventeen critically ill adult patients received continuous and intermittent infusion of piperacillin/tazobactam. Piperacillin plasma concentrations (n = 217) were analysed using population pharmacokinetic (PopPK) modelling. Post-hoc simulations were performed to evaluate the type I error rate associated with the study. Unseen data were used to validate the final model. The mean error (ME) and root mean square error (RMSE) were calculated as a measure of bias and imprecision, respectively. A PopPK model with parallel linear and non-linear elimination best fitted the data. The median and 95% confidence interval (CI) for the model parameters drug clearance (CL), volume of central compartment (V), volume of peripheral compartment (V_p) and intercompartmental clearance (Q) were 9 (7.69-11) L/h, 6.18 (4.93-11.2) L, 11.17 (7.26-12) L and 15.61 (12.66-23.8) L/h, respectively. The Michaelis-Menten constant (K_m) and the maximum elimination rate for Michaelis-Menten elimination (V_max) were estimated without population variability in the model to avoid overfitting and inflation of the type I error rate. The population estimates for K_m and V_max were 37.09 mg/L and 353.57 mg/h, respectively. The bias (ME) was -20.8 (95% CI -26.2 to -15.4) mg/L, whilst imprecision (RMSE) was 49.2 (95% CI 41.2-56) mg/L. In conclusion, piperacillin elimination is (partially) saturable. Moreover, the population estimate for K_m lies within the therapeutic window and therefore saturation of elimination should be accounted for when defining optimum dosing regimens for piperacillin in critically ill patients.

Novel Population Pharmacokinetic Approach to Explain the Differences between Cystic Fibrosis Patients and Healthy Volunteers via Protein Binding

The pharmacokinetics in patients with cystic fibrosis (CF) has long been thought to differ considerably from that in healthy volunteers. For highly protein bound β-lactams, profound pharmacokinetic differences were observed between comparatively morbid patients with CF and healthy volunteers. These differences could be explained by body weight and body composition for β-lactams with low protein binding. This study aimed to develop a novel population modeling approach to describe the pharmacokinetic differences between both subject groups by estimating protein binding. Eight patients with CF (lean body mass [LBM]: 39.8 ± 5.4kg) and six healthy volunteers (LBM: 53.1 ± 9.5kg) received 1027.5 mg cefotiam intravenously. Plasma concentrations and amounts in urine were simultaneously modelled. Unscaled total clearance and volume of distribution were 3% smaller in patients with CF compared to those in healthy volunteers. After allometric scaling by LBM to account for body size and composition, the remaining pharmacokinetic differences were explained by estimating the unbound fraction of cefotiam in plasma. The latter was fixed to 50% in male and estimated as 54.5% in female healthy volunteers as well as 56.3% in male and 74.4% in female patients with CF. This novel approach holds promise for characterizing the pharmacokinetics in special patient populations with altered protein binding.

Utilization of data below the analytical limit of quantitation in pharmacokinetic analysis and modeling: promoting interdisciplinary debate

Traditionally, bioanalytical laboratories do not report actual concentrations for samples with results below the LOQ (BLQ) in pharmacokinetic studies. BLQ values are outside the method calibration range established during validation and no data are available to support the reliability of these values. However, ignoring BLQ data can contribute to bias and imprecision in model-based pharmacokinetic analyses. From this perspective, routine use of BLQ data would be advantageous. We would like to initiate an interdisciplinary debate on this important topic by summarizing the current concepts and use of BLQ data by regulators, pharmacometricians and bioanalysts. Through introducing the limit of detection and evaluating its variability, BLQ data could be released and utilized appropriately for pharmacokinetic research.

Using Population Pharmacokinetic Modeling and Monte Carlo Simulations To Determine whether Standard Doses of Piperacillin in Piperacillin-Tazobactam Regimens Are Adequate for the Management of Febrile Neutropenia

Changes in the pharmacokinetics of piperacillin in febrile neutropenic patients have been reported to result in suboptimal exposures. This study aimed to develop a population pharmacokinetic model for piperacillin and perform dosing simulation to describe optimal dosing regimens for hematological malignancy patients with febrile neutropenia. Concentration-time data were obtained from previous prospective observational pharmacokinetic and interventional therapeutic drug monitoring studies. Nonparametric population pharmacokinetic analysis and Monte Carlo dosing simulations were performed with the Pmetrics package for R. A two-compartment model, with between-subject variability for clearance (CL), adequately described the data from 37 patients (21 males, age of 59 ± 12 years [means ± standard deviations] and weight of 77 ± 16 kg). Parameter estimates were CL of 18.0 ± 4.8 liters/h, volume of distribution of the central compartment of 14.3 ± 7.3 liters, rate constant for piperacillin distribution from the central to peripheral compartment of 1.40 ± 1.35 h^-1, and rate constant for piperacillin distribution from the peripheral to central compartment of 4.99 ± 7.81 h^-1 High creatinine clearance (CL_CR) was associated with reduced probability of target attainment (PTA). Extended and continuous infusion regimens achieved a high PTA of >90% for an unbound concentration of piperacillin remaining above the MIC (fT_>MIC) of 50%. Only continuous regimens achieved >90% PTA for 100% fT_>MIC when CL_CR was high. The cumulative fraction of response (FTA, for fractional target attainment) was suboptimal (<85%) for conventional regimens for both empirical and directed therapy considering 50% and 100% fT_>MIC FTA was maximized with prolonged infusions. Overall, changes in piperacillin pharmacokinetics and the consequences on therapeutic dosing requirements appear similar to those observed in intensive care patients. Guidelines should address the altered dosing needs of febrile neutropenic patients exhibiting high CL_CR or with known/presumed infections from high-MIC bacteria.

In vivo pharmacodynamics of piperacillin/tazobactam: implications for antimicrobial efficacy and resistance suppression with innovator and generic products

Recent studies have shown that the pharmacodynamic (PD) index driving the efficacy of β-lactam/β-lactamase inhibitor combinations such as ceftazidime/avibactam and ceftolozane/tazobactam is the percentage of time the free inhibitor concentration is above a threshold (fT_>threshold). However, data with piperacillin/tazobactam (TZP) are scarce. Here we aimed to assess the relationship between fT_>threshold and TZP antibacterial efficacy by a population pharmacokinetic study in mice and dose-effect experiments in a neutropenic murine thigh infection model with two isogenic strains of Escherichia coli differentially expressing TEM-1 β-lactamase. We also explored the dynamics of resistance selection with the innovator and a non-equivalent generic, extrapolated the results to the clinic by Monte Carlo simulation of standard TZP doses, and estimated the economic impact of generic-selected resistance. The fT_>threshold index described well the efficacy of TZP versus E. coli, with threshold values from 0.5 mg/L to 2 mg/L and mean exposures of 42% for stasis and 56% for 1 log₁₀ kill. The non-equivalent generic required a longer exposure (fT_>threshold 33%) to suppress resistance compared with the innovator (fT_>threshold 22%), leading to a higher frequency of resistance selection in the clinical simulation (16% of patients with the generic vs. 1% with the innovator). Finally, we estimated that use of TZP generics in a scenario of 25% therapeutic non-equivalence would result in extra expenses approaching US$1 billion per year in the USA owing to selection of resistant micro-organisms, greatly offsetting the savings gained from generic substitution and further emphasising the need for demonstrated and not assumed therapeutic equivalence.

[Pharmacokinetics and pharmacodynamics of antibiotics in intensive care]

Optimized dosage regimens of antibiotics have remained obscure since their introduction. During the last two decades pharmacokinetic(PK)-pharmacodynamic(PD) relationships, originally established in animal experiments, have been increasingly used in patients. The action of betalactams is believed to be governed by the time the plasma concentration is above the minimum inhibitory concentration (MIC). Aminoglycosides act as planned when the peak concentration is a multiple of the MIC and vancomycin seems to work best when the area under the plasma vs. time curve (AUC) to MIC has a certain ratio. Clinicians should be aware that these relationships can only be an indication in which direction dosing should go. Larger studies with sufficiently high numbers of patients and particularly severely sick patients are needed to prove the concepts. In times where all antibiotics can be measured with new technologies, the introduction of therapeutic drug monitoring (TDM) is suggested for ICUs (Intensive Care Unit). The idea of a central lab for TDM of antibiotics such as PEAK (Paul Ehrlich Antibiotika Konzentrationsmessung) is supported.

Doripenem population pharmacokinetics and dosing requirements for critically ill patients receiving continuous venovenous haemodiafiltration

OBJECTIVES

Doripenem is a newer carbapenem with little data available to guide effective dosing during renal replacement therapy in critically ill patients. The objective of this study was to determine the population pharmacokinetics of doripenem in critically ill patients undergoing continuous venovenous haemodiafiltration (CVVHDF) for acute kidney injury (AKI).

METHODS

This was an observational pharmacokinetic study in 12 infected critically ill adult patients with AKI undergoing CVVHDF and receiving 500 mg of doripenem intravenously every 8 h as a 60 min infusion. Serial blood samples were taken on 2 days of treatment and used for population pharmacokinetic analysis with S-ADAPT.

RESULTS

The median (IQR) age was 62 (53-71) years, the median (IQR) weight was 77 (67-96) kg and the median (IQR) APACHE II score was 29 (19-32). The median blood, dialysate and replacement fluid rates were 200, 1000 and 1000 mL/h, respectively. A two-compartment linear model with doripenem clearance described by CVVHDF, renal or non-renal mechanisms was most appropriate. The mean value for total doripenem clearance was 4.46 L/h and volume of distribution was 38.0 L. Doripenem clearance by CVVHDF was significantly correlated with the replacement fluid flow rate and accounted for ∼30%-37% of total clearance. A dose of 500 mg intravenously every 8 h achieved favourable pharmacokinetic/pharmacodynamics for all patients up to an MIC of 4 mg/L.

CONCLUSIONS

This is the first paper describing the pharmacokinetics/pharmacodynamics of doripenem in critically ill patients with AKI receiving CVVHDF. A dose of 500 mg intravenously every 8 h was appropriate for our CVVHDF settings for infections caused by susceptible bacteria.

Population pharmacokinetic analysis of piperacillin in burn patients

Piperacillin in combination with tazobactam, a β-lactamase inhibitor, is a commonly used intravenous antibiotic for the empirical treatment of infection in intensive care patients, including burn patients. The purpose of this study was to develop a population pharmacokinetic (PK) model for piperacillin in burn patients and to predict the probability of target attainment (PTA) using MICs and concentrations simulated from the PK model. Fifty burn patients treated with piperacillin-tazobactam were enrolled. Piperacillin-tazobactam was administered via infusion for approximately 30 min at a dose of 4.5 g (4 g piperacillin and 0.5 g tazobactam) every 8 h. Blood samples were collected just prior to and at 1, 2, 3, 4, and 6 h after the end of the infusion at steady state. The population PK model of piperacillin was developed using NONMEM. A two-compartment first-order elimination PK model was finally chosen. The covariates included were creatinine clearance (CLCR), day after burn injury (DAI), and sepsis. The final PK parameters were clearance (liters/h) (equal to 16.6 × [CLCR/132] + DAI × [-0.0874]), central volume (liters) (equal to 25.3 + 14.8 × sepsis [0 for the absence or 1 for the presence of sepsis]), peripheral volume (liters) (equal to 16.1), and intercompartmental clearance (liters/h) (equal to 0.636). The clearance and volume of piperacillin were higher than those reported in patients without burns, and the terminal half-life and PTA decreased with the increased CLCR. Our PK model suggests that higher daily doses or longer durations of infusion of piperacillin should be considered, especially for burn patients with a CLCR of ≥ 160 ml/min.

A pharmacometric approach to investigate the impact of methylxanthine abstinence and caffeine consumption on CYP1A2 activity

This study aimed to investigate the impact of methylxanthine abstinence (MA) periods on CYP1A2 activity in individuals with varying levels of caffeine consumption through development of a population pharmacokinetic model of caffeine and its major metabolite paraxanthine. This study developed and evaluated a mixed-effects pharmacokinetic model for caffeine and paraxanthine concentration-time data derived from a sequential single-dose cross-over study in healthy male volunteers (n = 30) who received oral 100 mg caffeine doses. Participants received caffeine with and without a MA period. Participants were classified as low (0-100 mg/d), medium (100-200 mg/d), or high (>200 mg/d) caffeine consumers (LCCs, MCCs, or HCCs, respectively). All caffeine and paraxanthine concentration-time data were simultaneously modeled. Caffeine pharmacokinetics was described by a two-compartment model with first-order absorption and two first-order elimination pathways. Paraxanthine was described by a one-compartment model with first-order absorption and elimination. Among LCCs (n = 16) and MCCs (n = 9), there was no difference in the mean (95% confidence interval) total apparent caffeine clearance (CL) between the MA period [LCCs: 6.88 (5.61-8.16 l/h); MCCs: 10.09 (7.57-12.60 l/h)] versus the no MA period [LCCs: 6.22 (4.97-7.46 l/h); MCCs: 9.68 (7.12-12.24 l/h)]. The mean CL among HCCs (n = 5) was considerably higher in the MA period [10.48 (5.62-15.33 l/h)] compared with the no MA period [6.30 (3.40-9.20 l/h)] (P < 0.05). The decrease in CL in the no MA period among HCC appears to be due to alternative caffeine elimination pathways, rather than CYP1A2.

Population pharmacokinetics of piperacillin and tazobactam in critically ill patients undergoing continuous renal replacement therapy: application to pharmacokinetic/pharmacodynamic analysis

OBJECTIVES

To evaluate the pharmacokinetics of piperacillin/tazobactam in critically ill patients undergoing continuous renal replacement therapy (CRRT) and to assess the success of the therapy against susceptible bacteria.

PATIENTS AND METHODS

Sixteen patients undergoing CRRT with different degrees of renal function were included in the study. Blood and ultrafiltrate samples were drawn after administration of piperacillin/tazobactam (4/0.5 g) every 4, 6 or 8 h. The data were analysed by a population approach using NONMEM 7.2. The probability of target attainment (PTA) of maintaining free piperacillin levels above the MIC during the entire dosing interval was estimated by simulation of intermittent and continuous infusions.

RESULTS

The pharmacokinetics of piperacillin and tazobactam were best described by two-compartment models where the elimination of both drugs was conditioned by renal [dependent on creatinine clearance (CLCR)], non-renal and extracorporeal clearances. A 20 min infusion of piperacillin/tazobactam administered every 6 h provided high PTAs against MICs ≤ 32 mg/L in patients with severe renal failure. In patients with normal or moderate renal function PTAs ≥ 90% were only obtained up to MICs ≤ 8 mg/L with short infusions. However, simulating continuous infusion, higher probabilities of success were obtained against MICs of 32 and 16 mg/L when CLCR was 50 and 100 mL/min, respectively.

CONCLUSIONS

Population pharmacokinetic models have been developed and validated for piperacillin and tazobactam. Based on the pharmacokinetic/pharmacodynamic analysis, dosing recommendations are given considering the residual renal function of the patient and the MIC for the isolated bacteria.

Quantifying subpopulation synergy for antibiotic combinations via mechanism-based modeling and a sequential dosing design

Quantitative modeling of combination therapy can describe the effects of each antibiotic against multiple bacterial populations. Our aim was to develop an efficient experimental and modeling strategy that evaluates different synergy mechanisms using a rapidly killing peptide antibiotic (nisin) combined with amikacin or linezolid as probe drugs. Serial viable counts over 48 h were obtained in time-kill experiments with all three antibiotics in monotherapy against a methicillin-resistant Staphylococcus aureus USA300 strain (inoculum, 10(8) CFU/ml). A sequential design (initial dosing of 8 or 32 mg/liter nisin, switched to amikacin or linezolid at 1.5 h) assessed the rate of killing by amikacin and linezolid against nisin-intermediate and nisin-resistant populations. Simultaneous combinations were additionally studied and all viable count profiles comodeled in S-ADAPT and NONMEM. A mechanism-based model with six populations (three for nisin times two for amikacin) yielded unbiased and precise (r = 0.99, slope = 1.00; S-ADAPT) individual fits. The second-order killing rate constants for nisin against the three populations were 5.67, 0.0664, and 0.00691 liter/(mg · h). For amikacin, the maximum killing rate constants were 10.1 h(-1) against its susceptible and 0.771 h(-1) against its less-susceptible populations, with 14.7 mg/liter amikacin causing half-maximal killing. After incorporating the effects of nisin and amikacin against each population, no additional synergy function was needed. Linezolid inhibited successful bacterial replication but did not efficiently kill populations less susceptible to nisin. Nisin plus amikacin achieved subpopulation synergy. The proposed sequential and simultaneous dosing design offers an efficient approach to quantitatively characterize antibiotic synergy over time and prospectively evaluate antibiotic combination dosing strategies.

Population pharmacokinetics of piperacillin at two dose levels: influence of nonlinear pharmacokinetics on the pharmacodynamic profile

Piperacillin in combination with tazobactam is one of the most commonly used intravenous antibiotics. There is evidence for a possible saturable elimination of piperacillin. Therefore, the saturable elimination and its impact on the choice of optimal dosage regimens were quantified. In a randomized crossover study, 10 healthy volunteers received 1,500 mg and 3,000 mg of piperacillin as 5-min intravenous infusion. Population pharmacokinetics based on plasma and urine data were determined utilizing NONMEM and S-ADAPT. Probabilities of target attainment (PTAs) were compared for different models and dosage regimens, based on the target time of the non-protein-bound concentration above the MIC of at least 50% of the dosing interval. Total clearance of piperacillin was 18% (geometric mean ratio, 90% confidence interval, 11 to 24%) lower (P < 0.01), and renal clearance was 24% (9 to 37%) lower (P = 0.02) at the high compared to the low dose. The final model included first-order nonrenal elimination and parallel first-order and mixed-order renal elimination. Nonrenal clearance was 5.44 liter/h (coefficient of variation, 18%), first-order renal clearance was 4.42 liter/h (47%), and the maximum elimination rate of mixed-order renal elimination was 219 mg/h (84%), with a Michaelis-Menten constant of 36.1 mg/liter (112%). Compared to models with saturable elimination, a linear model predicted up to 10% lower population PTAs for high-dose short-term infusions (6 g every 8 h) and up to 4% higher population PTAs for low-dose continuous infusions (6 g/day). While renal elimination of piperacillin was saturable at therapeutic concentrations, the extent of saturation of nonrenal clearance was small. The influence of saturable elimination on PTAs for clinically relevant dosage regimens was relatively small.

Population pharmacokinetics of extended-infusion piperacillin-tazobactam in hospitalized patients with nosocomial infections

While extended infusions of piperacillin-tazobactam (TZP) are increasingly used in practice, the effect of infusion on the pharmacokinetic (PK) profile of TZP has not been widely assessed. To assess its effect on the pharmacokinetic profile of TZP, seven serum samples were collected from 11 hospitalized patients who received 3.375 g TZP intravenously for 4 h every 8 h. Population pharmacokinetic models were fit to the PK data utilizing first-order, Michaelis-Menten (MM), and parallel first-order/MM clearance. A population PK model with first-order clearance was fit to the tazobactam PK data. Monte Carlo simulations (MCSs) were used to determine the most effective administration schedule to ensure that free piperacillin concentrations were above the MIC for at least 50% of the dosing interval (50% fT>MIC) and to quantify the extent of the nonlinear clearance. The model incorporating parallel linear/MM clearance best fit the piperacillin PK data. The MCSs demonstrated that approximately 50% of the administered piperacillin is cleared by the nonlinear clearance mechanism. The results of the MCSs also revealed that more intensive TZP extended infusion dosing schemes (3.375 to 4.5 g intravenously [3-h infusion] every 6 h) than those commonly used in clinical practice were needed to maximize the 50% fT>MIC for MICs of ≥8 mg/liter. This study suggests that extended infusion of TZP is the most effective method of administration for patients with nosocomial infections. Due to the hyperclearance nature of the hospitalized patient populations studied, more intensive TZP dosing regimens may be needed to maximize fT>MIC in certain hospitalized populations.