Flora T. Musuamba

Limited sampling models and Bayesian estimation for mycophenolic acid area under the curve prediction in stable renal transplant patients co-medicated with ciclosporin or sirolimus.

Background and ObjectiveMycophenolate mofetil is a prodrug of mycophenolic acid (MPA), an immunosuppressive agent used in combination with corticosteroids and calcineurin inhibitors or sirolimus for the prevention of acute rejection after solid organ transplantation. Although MPA has a rather narrow therapeutic window and its pharmacokinetics show considerable intra- and interindividual variability, dosing guidelines recommend a standard dosage regimen of 0.5–1.0g twice daily in adult renal, liver and cardiac transplant recipients. The main objective ofthe present study was to develop a method to predict the MPA area under the plasma concentration-time curve during one 12-hour dosing interval (AUC12) by using multiple linear regression models and maximum aposteriori (MAP) Bayesian estimation methods in patients co-medicated with ciclosporin or sirolimus, aiming to individualize the dosage regimen of mycophenolate mofetil.Patients and MethodsPharmacokinetic profiles of MPA and mycophenolic acid glucuronide (MPAG), the main metabolite of MPA, were obtained from 40 stable adult renal allograft recipients on three different occasions: the day before switching from ciclosporin to sirolimus co-medication (±7.4 months posttransplantation; period I) and at 60 days and 270 days after the switch (periods II and III). Blood samples for determination of MPA and MPAG concentrations in plasma were taken at 0 hours (pre-dose) and at 0.33, 0.66, 1.25, 2, 4, 6, 8 and 12 hours after oral intake ofmycophenolate mofetil. The MPA AUC12 was calculated by the trapezoidal method (the observed AUC12). Patients were randomly divided into (i) a model-building test group (n = 27); and (ii) a model-validation group (n = 13). Multiple linear regression models were developed, based on three sampling times after drug administration. Subsequently, a population pharmacokinetic model describing MPA and MPAG plasma concentrations was developed using nonlinear mixed-effects modelling and a Bayesian estimator based on the population pharmacokinetic model was used to predict the MPA AUC12 based on three sampling times taken within 2 hours following dosing.ResultsFifty-two percent ofthe observed AUC12 values (three periods) in the 40 patients receiving a fixed dose of mycophenolate mofetil 750mg twice daily were outside the recommended therapeutic range (30–60 mg • h/mL). The failure of the standard dose to yield an AUC12 value within the therapeutic range was especially pronounced during the first study period. Of the multiple linear regression models that were tested, the equation based on the 0-hour (pre-dose), 0.66- and 2-hour sampling times showed the best predictive performance in the validation group: r2 = 0.79, relative root mean square error (rRMSE)= 14% and mean relative prediction error (MRPE) = 0.9%. The pharmacokinetics of MPA and MPAG were best described by a two-compartment model with first-order absorption and elimination for MPA, plus a compartment for MPAG, also including a gastrointestinal compartment and enterohepatic cycling in the case of sirolimus co-medication. The ratio of aminotransferase liver enzymes (AST and ALT) and the glomerular filtration rate significantly influenced MPA glucuronidation and MPAG renal excretion, respectively. Bayesian estimation ofthe MPA AUC12 based on 0-, 1.25- and 2-hour sampling times predicted the observed AUC12 values ofthe patients in the validation group, with the following predictive performance characteristics: r2 = 0.93, rRMSE= 12.4% and MRPE = -0.4%.ConclusionUse ofthe developed multiple linear regression equation and Bayesian estimator, both based on only three blood sampling times within 2 hours following a dose of mycophenolate mofetil, allowed an accurate prediction ofa patient’s MPA AUC12 for therapeutic drug monitoring and dose individualization. These findings should be validated in a randomized prospective trial.

Validation of a Liquid Chromatography-Mass Spectrometric Assay for Tacrolimus in Peripheral Blood Mononuclear Cells.

As a potential alternative to whole-blood tacrolimus (TAC) monitoring, a sensitive and selective method was developed for quantifying this immunosuppressant in human peripheral blood mononuclear cell population (PBMCs). These cells, expected to be a more specific biological matrix than whole blood to reflect pharmacological efficacy, could be promising for TAC therapeutic drug monitoring (TDM). The assay was developed using liquid chromatography-tandem mass spectrometry (LC-MS/MS). PBMCs are isolated from 7 mL whole blood by centrifugation over Ficoll gradient density and washed twice with phosphate-buffered saline at 4°C. Harvested cells were suspended within 1.5 mL of phosphate-buffered saline. Cell counts were performed to express and normalize TAC amount per 106 cells. TAC was extracted by a liquid-liquid extraction in basic medium (NH4OH) with 1-chlorobutane, and ascomycin was used as internal standard. After evaporation of the supernatant under nitrogen, the residue was reconstituted in methanol (MeOH). Compounds were eluted on a C18 column by a mixture of acetonitrile/water (90/10, vol/vol) containing 0.1% formic acid and 2 mmol/L of ammonium acetate. TAC and internal standard were monitored by detecting specific ammoniated product ions using multiple reaction monitoring acquisition mode in electrospray positive ionization. This method was fully validated in the range of 0.01-5 ng/mL. Limit of detection and of quantification are 0.005 and 0.01 ng/mL, respectively. Intra-assay and interassay recoveries ranged from 89.2% to 114.3% and 85.3% to 103.9%, respectively. Intra-assay and interassay imprecisions ranged from 9.3% to 12% and 10.7% to 12.2%, respectively, across the analytical range. Mean TAC extraction efficiency was 80.9% ± 8.3%. Matrix effects were minimal with <8% ion suppression. This method is currently applied in clinical research protocols and allows the measurement of small intracellular amounts of TAC down to 0.006 ng per 106 PBMCs in kidney-transplanted recipients. This method could be a new potential tool for TAC TDM, providing new perspectives for optimizing immunosuppressive therapy. Further studies should be conducted to fully evaluate the benefit of intracellular TAC concentrations in refinement of TDM strategies for TAC to ensure optimal clinical outcomes.

Population pharmacokinetic modeling and optimal sampling strategy for Bayesian estimation of amikacin exposure in critically ill septic patients

Because the sepsis-induced pharmacokinetic (PK) modifications need to be considered in aminoglycoside dosing, the present study aimed to develop a population PK model for amikacin (AMK) in severe sepsis and to subsequently propose an optimal sampling strategy suitable for Bayesian estimation of the drug PK parameters. Concentration-time profiles for AMK were obtained from 88 critically ill septic patients during the first 24 hours of antibiotic treatment. The population PK model was developed using a nonlinear mixed effects modeling approach. Covariate analysis included demographic data, pathophysiological characteristics, and comedication. Optimal sampling times were selected based on a robust Bayesian design criterion. Taking into account clinical constraints, a two-point sampling approach was investigated. A two-compartment model with first-order elimination best fitted the AMK concentrations. Population PK estimates were 19.2 and 9.34 L for the central and peripheral volume of distribution and 4.31 and 2.21 L/h for the intercompartmental and total body clearance. Creatinine clearance estimated using the Cockcroft-Gault equation was retained in the final model. The two optimal sampling times were 1 hour and 6 hours after onset of the drug infusion. Predictive performance of individual Bayes estimates computed using the proposed optimal sampling strategy was reported: mean prediction errors were less than 5% and root mean square errors were less than 30%. The present study confirmed the significant influence of the creatinine clearance on the PK disposition of AMK during the first hours of treatment in critically ill septic patients. Based on the population estimates, an optimal sampling strategy suitable for Bayesian estimation of the drug PK parameters was developed, meeting the need of clinical practice.

The revised EMA guideline for the investigation of bioequivalence for immediate release oral formulations with systemic action

On August 1, 2010, a revised guidance regarding bioequivalence (BE) assessment for the approval of innovator (bridging studies, variations, line extensions) and generic medicinal products in the EU came into effect (EMA Guideline on the Investigation of Bioequivalence, CPMP/EWP/QWP/1401/98 Rev. 1/Corr**, London, 20 January 2010). This guideline specifies the requirements for BE assessment for immediate release oral dosage forms with systemic action. Compared to the previous BE guideline of the EMA, clearer guidance is now given on several topics including BE assessment of highly variable drugs/drug products (HVDs/HVDPs), the use of metabolite data, acceptance criteria for narrow therapeutic index drugs (NTIDs), BCS-based biowaivers, and dose strength to be used in case of application for marketing authorization of several strengths. However, the health authorities of the various EU member states do not necessarily apply the same rules as far as substitution and switchability between medicinal products are concerned. Moreover, differences still exist between the BE guidelines of the major health authorities (FDA, EMA, NIHC, ...) on topics such as HVDs/HVDPs, NTIDs and BCS-based biowaivers. Global harmonization should be the next logical step to guarantee accessibility to safe and efficacious drug products for patients in all parts of the world.

Modelled target attainment after meropenem infusion in patients with severe nosocomial pneumonia: the PROMESSE study

OBJECTIVES The objective of this study was to propose an optimal treatment regimen of meropenem in critically ill patients with severe nosocomial pneumonia. PATIENTS AND METHODS Among 55 patients in intensive care treated with 1 g of meropenem every 8 h for severe nosocomial pneumonia, 30 were assigned to intermittent infusion (II; over 0.5 h) and 25 to extended infusion (EI; over 3 h) groups. Based on plasma and epithelial lining fluid (ELF) concentrations determined at steady-state, pharmacokinetic modelling and Monte Carlo simulations were undertaken to assess the probability of attaining drug concentrations above the MIC for 40%-100% of the time between doses (%T > 1-fold and 4-fold MIC), for 1 or 2 g administered by either method. RESULTS Penetration ratio, measured by the ELF/plasma ratio of AUCs, was statistically higher in the EI group than in the II group (mean ± SEM: 0.29 ± 0.030 versus 0.20 ± 0.033, P = 0.047). Considering a maximum susceptibility breakpoint of 2 mg/L, all dosages and modes of infusions achieved 40%-100% T > 1-fold MIC in plasma, but none did so in ELF, and only the 2 g dose over EI achieved 40%-100% T > 4-fold MIC in plasma. CONCLUSIONS The optimum regimen to treat severe nosocomial pneumonia was 2 g of meropenem infused over 3 h every 8 h. This regimen achieved the highest pharmacodynamic targets both in plasma and in ELF.

Prediction of paraquat exposure and toxicity in clinically ill poisoned patients: a model based approach.

AIMS Paraquat poisoning is a medical problem in many parts of Asia and the Pacific. The mortality rate is extremely high as there is no effective treatment. We analyzed data collected during an ongoing cohort study on self-poisoning and from a randomized controlled trial assessing the efficacy of immunosuppressive therapy in hospitalized paraquat-intoxicated patients. The aim of this analysis was to characterize the toxicokinetics and toxicodynamics of paraquat in this population. METHODS A non-linear mixed effects approach was used to perform a toxicokinetic/toxicodynamic population analysis in a cohort of 78 patients. RESULTS The paraquat plasma concentrations were best fitted by a two compartment toxicokinetic structural model with first order absorption and first order elimination. Changes in renal function were used for the assessment of paraquat toxicodynamics. The estimates of toxicokinetic parameters for the apparent clearance, the apparent volume of distribution and elimination half-life were 1.17 l h(-1) , 2.4 l kg(-1) and 87 h, respectively. Renal function, namely creatinine clearance, was the most significant covariate to explain between patient variability in paraquat clearance.This model suggested that a reduction in paraquat clearance occurred within 24 to 48 h after poison ingestion, and afterwards the clearance was constant over time. The model estimated that a paraquat concentration of 429 μg l(-1) caused 50% of maximum renal toxicity. The immunosuppressive therapy tested during this study was associated with only 8% improvement of renal function. CONCLUSION The developed models may be useful as prognostic tools to predict patient outcome based on patient characteristics on admission and to assess drug effectiveness during antidote drug development.

Empirical models for dosage optimization of four beta-lactams in critically ill septic patients based on therapeutic drug monitoring of amikacin.

OBJECTIVES The study aims to develop empirical models able to predict the pharmacokinetics (PK) of four beta-lactams using the amikacin (AMK) therapeutic drug monitoring (TDM), in order to optimize their dosage regimens. DESIGN AND METHODS 69 critically ill septic patients were included. All received a first dose of AMK combined with piperacillin/tazobactam, ceftazidime, cefepime or meropenem. A multivariate analysis was performed to predict the beta-lactam PK using AMK PK parameters estimated from TDM and using pathophysiological variables. RESULTS An optimal prediction model was identified for each PK parameter of each beta-lactam. The best predictor of each model was one of the AMK PK parameters estimated from TDM. Other variables included colloid solution, renal and hepatic biomarkers, age and body weight. CONCLUSION PK of the four beta-lactams could be easily and rapidly predicted in critically ill septic patients using the AMK TDM. These predictions could improve the beta-lactam dosages in clinical practice.

Influence of donor–recipient CYP3A4/5 genotypes, age and fluconazole on tacrolimus pharmacokinetics in pediatric liver transplantation: a population approach

AIM To characterize the effect of donor and recipient CYP3A4, CYP3A5 and ABCB1 genotypes as well as relevant patient characteristics on tacrolimus pharmacokinetics in pediatric liver transplantation. PATIENTS & METHODS Data from 114 pediatric liver transplant recipients were retrospectively collected during the first 3 months following transplantation. Population pharmacokinetic analysis was performed using nonlinear mixed effects modeling, including characterization of influential covariates. RESULTS A two-compartment model with first order elimination best fitted the data. Estimates of apparent volume of the central compartment, intestinal clearance, hepatic clearance and intercompartmental clearance were 79 l, 0.01 l/h, 10.9 l/h and 105 l/h, respectively. Time post-transplantation, recipient age, donor CYP3A5 and CYP3A4 genotypes and fluconazole administration significantly influenced tacrolimus apparent clearance while bodyweight influenced volume of distribution. CONCLUSION The proposed model displayed acceptable fitting performances and enabled identification of statistically significant and clinically relevant covariates on tacrolimus pharmacokinetics in the early pediatric post liver transplantation period.

Statistical tools for dose individualization of mycophenolic acid and tacrolimus co-administered during the first month after renal transplantation

AIM To predict simultaneously the area under the concentration-time curve during one dosing interval [AUC(0,12 h)] for mycophenolic acid (MPA) and tacrolimus (TAC), when concomitantly used during the first month after transplantation, based on common blood samples. METHODS Data were from two different sources, real patient pharmacokinetic (PK) profiles from 65 renal transplant recipients and 9000 PK profiles simulated from previously published models on MPA or TAC in the first month after transplantation. Multiple linear regression (MLR) and Bayesian estimation using optimal samples were performed to predict MPA and TAC AUC(0,12 h) based on two concentrations. RESULTS The following models were retained: AUC(0,12 h) = 16.5 + 4.9 × C1.5 + 6.7 × C3.5 (r(2) = 0.82, rRMSE = 9%, with simulations and r(2) = 0.66, rRMSE = 24%, with observed data) and AUC(0,12 h) = 24.3 + 5.9 × C1.5 + 12.2 × C3.5 (r(2) = 0.94, rRMSE = 12.3%, with simulations r(2) = 0.74, rRMSE = 15%, with observed data) for MPA and TAC, respectively. In addition, bayesian estimators were developed including parameter values from final models and values of concentrations at 1.5 and 3.5 h after dose. Good agreement was found between predicted and reference AUC(0,12 h) values: r(2) = 0.90, rRMSE = 13% and r(2) = 0.97, rRMSE = 5% with simulations for MPA and TAC, respectively and r(2) = 0.75, rRMSE = 11% and r(2) = 0.83, rRMSE = 7% with observed data for MPA and TAC, respectively. CONCLUSION Statistical tools were developed for simultaneous MPA and TAC therapeutic drug monitoring. They can be incorporated in computer programs for patient dose individualization.

Feasibility of the Video-Assisted Thyroid Surgery Under Hypnosis Associated to local Anesthesia

Background: Video-assisted thyroidectomy (VAT) is feasible and safe compared to conventional surgery. Thyroidectomy under hypnosis associated to local anesthesia (HYLA) as an alternative to general anesthesia (GA) has been shown to be effective. However, its combination with VAT has not yet been reported. The aim of the study is to describe the feasibility of VAT under HYLA as a complete minimally invasive approach and evaluate its safety. Methods: Out of 130 consecutive patients referred for thyroidectomy and selected for VAT, 50 patients opted voluntarily for HYLA. Safety and feasibility were considered primary endpoints. Results: Twenty eight patients benefited from a total thyroidectomy (56%). The remaining patients underwent thyroid lobectomy. Median operating time was 102.5minutes (range 70-177) and 92.5minutes (range 51-143) for total thyroidectomy and lobectomy, respectively. Median time in operating room was 146.5minutes (range101-222) and 133minutes (range 96-178) for total thyroidectomy and lobectomy, respectively. Conversion from VAT to a conventional surgical approach occurred in 4 patients (8%). No conversion to GA was required. Hemodynamic parameter measurements were stable during surgery. No permanent hypocalcemia or vocal cord palsy were observed. Ninety eight percent of the patients required just one overnight stay in hospital. Conclusion: The current brief report demonstrates that the combination of VAT and HYLA is feasible without the risk of additional surgical morbidity.