Developmental pharmacogenetics of immunosuppressants in pediatric organ transplantation

Treatment optimization of maintenance immunosuppressive agents in pediatric renal transplant recipients

Introduction: Graft survival in pediatric kidney transplant patients has increased significantly within the last three decades, correlating with the discovery and utilization of new immunosuppressants as well as improvements in patient care. Despite these developments in graft survival for patients, there is still improvement needed, particularly in long-term care in pediatric patients receiving grafts from deceased donor patients. Maintenance immunosuppressive therapies have narrow therapeutic indices and are associated with high inter-individual and intra-individual variability.Areas covered: In this review, we examine the impact of pharmacokinetic variability on renal transplantation and its association with age, genetic polymorphisms, drug-drug interactions, drug-disease interactions, renal insufficiency, route of administration, and branded versus generic drug formulation. Pharmacodynamics are outlined in terms of the mechanism of action for each immunosuppressant, potential adverse effects, and the utility of pharmacodynamic biomarkers.Expert opinion: Acquiring abetter quantitative understanding of immunosuppressant pharmacokinetics and pharmacodynamic components should help clinicians implement treatment regimens to maintain the balance between therapeutic efficacy and drug-related toxicity.

The first study in pediatric: Population pharmacokinetics of sirolimus and its application in Chinese children with immune cytopenia

The narrow therapeutic index and large inter-individual variability in sirolimus pharmacokinetics (PK) make therapeutic drug monitoring (TDM) necessary. Factors responsible for PK variability are not well understood, and published PK studies do not include pediatric patients with immune cytopenia. The objective of this study was to characterize the PK of sirolimus in pediatric patients with immune cytopenia and to develop a population PK model in Chinese children and evaluate its utility for dose individualization. A total of 27 children with either acquired or congenital immune cytopenia aged 8.16 ± 3.60 years (range: 1-15 years) were included. TDM data for sirolimus were collected. The population PK model of sirolimus was described using the nonlinear mixed-effects modeling (Phoenix NLME 1.3 software) approach. Covariate analysis was applied to select candidate factors associated with PK parameters. The final model was validated using bootstrap (1000 runs) and visual predictive check (VPC) method. A one-compartment model with first-order absorption and elimination was developed. The outcome parameters were as follows: apparent clearance (CL/F) 5.63 L/h, apparent distribution volume (V/F) 144.16 L. Inter-individual variabilities for CL/F and V/F were 3.53% and 7.27%, respectively. The intra-individual variability of proportional error model was 22.45%. The covariate test found that body weight and total bilirubin were strongly associated with clearance; however, we did not find the relevance between the covariate and volume of distribution of sirolimus. Personalized dosage regimens were provided based on Bayesian method. The oral dose should be adjusted according to weight and total bilirubin. This is the first study to describe a population PK model of sirolimusin pediatric patients with immune cytopenia. Population pharmacokinetic (PPK) model-based dose individualization of sirolimus and the design of future clinical studies in children will be facilitated by the developed model in this study.

Population pharmacokinetics of cyclosporine in Chinese children receiving hematopoietic stem cell transplantation

Cyclosporine (CsA) is characterized by a narrow therapeutic window and high interindividual pharmacokinetic variability, particularly in juvenile patients. The aims of this study were to build a population pharmacokinetic model of CsA in Chinese children with hematopathy who received allogeneic hematopoietic stem cell transplantation (allo-HSCT) and to identify covariates affecting CsA pharmacokinetics. A total of 86 Chinese children aged 8.4 ± 3.8 years (range 1.1–16.8 years) who received allo-HSCT were enrolled. Whole blood samples were collected before allo-HSCT. Genotyping was performed using an Agena MassARRAY system. A total of 1010 trough plasma concentration values of CsA and clinical data were collected. The population pharmacokinetic model of CsA was constructed using nonlinear mixed-effects modeling (NONMEM) software. The stability and performance of the final model were validated using bootstrapping and normalized prediction distribution errors. We showed that a one-compartment model with first-order elimination adequately described the pharmacokinetics of CsA. The typical values for clearance (CL) and volume of distribution (V) were 42.3 L/h and 3100 L, respectively. Body weight, postoperative days, CYP3A4*1 G genotype, estimated glomerular filtration rate and coadministration of triazole antifungal drugs were identified as significant covariates for CL. Weight and postoperative days were significant covariates for the V of CsA. Our model can be adopted to optimize the CsA dosing regimen for Chinese children with hematopathy receiving allo-HSCT.

Early and Accurate Prediction of Clinical Response to Methotrexate Treatment in Juvenile Idiopathic Arthritis Using Machine Learning

Background and Aims: Accurately predicting the response to methotrexate (MTX) in juvenile idiopathic arthritis (JIA) patients before administration is the key point to improve the treatment outcome. However, no simple and reliable prediction model has been identified. Here, we aimed to develop and validate predictive models for the MTX response to JIA using machine learning based on electronic medical record (EMR) before and after administering MTX.

Materials and Methods: Data of 362 JIA patients with MTX mono-therapy were retrospectively collected from EMR between January 2008 and October 2018. DAS44/ESR-3 simplified standard was used to evaluate the MTX response. Extreme gradient boosting (XGBoost), support vector machine (SVM), random forest (RF), and logistic regression (LR) algorithms were applied to develop and validate models with 5-fold cross-validation on the randomly split training and test set. Data of 13 patients additionally collected were used for external validation.

Results: The XGBoost screened out the optimal 10 pre-administration features and 6 mix-variables. The XGBoost established the best model based on the 10 pre-administration variables. The performances were accuracy 91.78%, sensitivity 90.70%, specificity 93.33%, AUC 97.00%, respectively. Similarly, the XGBoost developed a better model based on the 6 mix-variables, whose performances were accuracy 94.52%, sensitivity 95.35%, specificity 93.33%, AUC 99.00%, respectively.

Conclusion: Based on common EMR data, we developed two MTX response predictive models with excellent performance in JIA using machine learning. These models can predict the MTX efficacy early and accurately, which provides powerful decision support for doctors to make or adjust therapeutic scheme before or after treatment.

CYP3A5 genotype affects time to therapeutic tacrolimus level in pediatric kidney transplant recipients

BACKGROUND

Optimal management of immunosuppression in kidney transplantation requires a delicate balance of efficacy and toxicity. Tacrolimus (TAC) dose requirements are significantly impacted by genetic variation in CYP3A5 polymorphisms, however the impact that genotype has on clinical outcomes in the pediatric kidney transplant population remains unclear.

METHODS

We evaluated a retrospective cohort of 98 pediatric kidney transplant recipients. The primary exposure was CYP3A5 genotype, which classified each recipient into the expresser (at least one CYP3A5*1 allele) or non-expresser group (only CYP3A5*3 alleles). The primary outcome was time to achieve a steady therapeutic TAC concentration. Secondary outcomes include incidence of early allograft rejection and calcineurin inhibitor (CNI) nephrotoxicity during the first year post-transplant.

RESULTS

The study cohort included 55 (56%) expressers and 43 (44%) non-expressers of the CYP3A5*1 allele. Expressers had a significantly longer time to achieve a steady therapeutic TAC concentration than non-expressers (log rank, P = 0.03). Expressers had a trend for higher incidence of early allograft rejection (29.1% vs 16.3%, log rank, P = 0.16). Early biopsy-proven CNI nephrotoxicity was seen in 60% of recipients, with no differences in the rate between expressers and non-expressers.

CONCLUSIONS

Pediatric kidney transplant recipients with the CYP3A5*1 allele (expressers) take a longer time to achieve therapeutic TAC levels than those with the CYP3A5*3 allele (non-expressers). However, we observed no significant differences in acute rejection or CNI nephrotoxicity based on CYP3A5 genotype. Thus CYP3A5 genotype was not observed to have an immediate impact on early transplant outcomes.

Clinical aspects of tacrolimus use in paediatric renal transplant recipients

The calcineurin inhibitor tacrolimus, cornerstone of most immunosuppressive regimens, is a drug with a narrow therapeutic window: underexposure can lead to allograft rejection and overexposure can result in an increased incidence of infections, toxicity and malignancies. Tacrolimus is metabolised in the liver and intestine by the cytochrome P450 3A (CYP3A) isoforms CYP3A4 and CYP3A5. This review focusses on the clinical aspects of tacrolimus pharmacodynamics, such as efficacy and toxicity. Factors affecting tacrolimus pharmacokinetics, including pharmacogenetics and the rationale for routine CYP3A5*1/*3 genotyping in prospective paediatric renal transplant recipients, are also reviewed. Therapeutic drug monitoring, including pre-dose concentrations and pharmacokinetic profiles with the available "reference values", are discussed. Factors contributing to high intra-patient variability in tacrolimus exposure and its impact on clinical outcome are also reviewed. Lastly, suggestions for future research and clinical perspectives are discussed.

A randomized clinical trial of age and genotype-guided tacrolimus dosing after pediatric solid organ transplantation

BACKGROUND

Tacrolimus pharmacokinetics are influenced by age and CYP3A5 genotype with CYP3A5 expressors (CYP3A5*1/*1 or *1/*3) being fast metabolizers. However, the benefit of genotype-guided dosing in pediatric solid organ transplantation has been understudied.

OBJECTIVE

To determine whether age and CYP3A5 genotype-guided starting dose of tacrolimus result in earlier attainment of therapeutic drug concentrations.

SETTING

Single hospital-based transplant center.

METHODS

This was a randomized, semi-blinded, 30-day pilot trial. Between 2012 and 2016, pediatric patients listed for solid organ transplant were consented and enrolled into the study. Participants were categorized as expressors, CYP3A5*1/*1 or CYP3A5*1/*3, and nonexpressors, CYP3A5*3/*3. Patients were stratified by age (≤ or > 6 years) and randomized (2:1) after transplant to receive genotype-guided (n = 35) or standard (n = 18) starting dose of tacrolimus for 36-48 hours and were followed for 30 days.

RESULTS

Median age at transplant in the randomized cohort was 2.1 (0.75-8.0) years; 24 (45%) were male. Participants in the genotype-guided arm achieved therapeutic concentrations earlier at a median (IQR) of 3.4 (2.5-6.6) days compared to those in the standard dosing arm of 4.7 (3.5-8.6) days (P = 0.049), and had fewer out-of-range concentrations [OR (95% CI) = 0.60 (0.44, 0.83), P = 0.002] compared to standard dosing, with no difference in frequency of adverse events between the two groups.

CONCLUSIONS

CYP3A5 genotype-guided dosing stratified by age resulted in earlier attainment of therapeutic tacrolimus concentrations and fewer out-of-range concentrations.

The Effect of Weight and CYP3A5 Genotype on the Population Pharmacokinetics of Tacrolimus in Stable Paediatric Renal Transplant Recipients

BACKGROUND

The aim of this study was to develop a population pharmacokinetic model of tacrolimus in paediatric patients at least 1 year after renal transplantation and simulate individualised dosage regimens.

PATIENTS AND METHODS

We included 54 children with median age of 11.1 years (range 3.8-18.4 years) with 120 pharmacokinetic profiles performed over 2 to 4 h. The pharmacokinetic analysis was performed using the non-linear mixed-effects modelling software (NONMEM(®)). The impact of covariates including concomitant medications, age, the cytochrome P450 (CYP) CYP3A5*3 gene and the adenosine triphosphate binding cassette protein B1 (ABCB1) 3435 C→T gene polymorphism on tacrolimus pharmacokinetics was analysed. The final model was externally validated on an independent dataset and dosing regimens were simulated.

RESULTS

A two-compartment model adequately described tacrolimus pharmacokinetics. Apparent oral clearance (CL/F) was associated with weight (allometric scaling) but not age. Children with lower weight and CYP3A5 expressers required higher weight-normalised tacrolimus doses. CL/F was inversely associated with haematocrit (P < 0.05) and γ-glutamyl transpeptidase (γGT) (P < 0.001) and was increased by 45 % in carriers of the CYP3A5*1 allele (P < 0.001). CL/F was not associated with concomitant medications. Dose simulations show that a daily tacrolimus dose of 0.2 mg/kg generates a pre-dose concentration (C 0) ranging from 5 to 10 µg/L depending on the weight and CYP3A5 polymorphism. The median area under the plasma concentration-time curve (AUC) corresponding with a tacrolimus C 0 of 4-8 µg/L was 97 h·µg/L (interquartile range 80-120).

CONCLUSIONS

In patients beyond the first year after transplantation, there is a cumulative effect of CYP3A5*1 polymorphism and weight on the tacrolimus C 0. Children with lower weight and carriers of the CYP3A5*1 allele have higher weight-normalised tacrolimus dose requirements.

The compelling case for therapeutic drug monitoring of mycophenolate mofetil therapy

We have reviewed current evidence on the therapeutic drug monitoring (TDM) of mycophenolic acid (MPA) in relationship to drug efficacy and safety. The relationship between actual MPA exposure and mycophenolate mofetil (MMF) dose has been shown to be weak in children and adolescents. The TDM of MPA exposure should ideally be performed using full pharmacokinetic profiles or limited sampling strategies. Recent evidence has provided some rationale for using the post-dose trough level as a single measure. In terms of short-term efficacy, there is strong evidence that a MPA area under the time-concentration curve of >30 mg × h/L reduces acute rejection episodes early after renal transplantation, and there is evolving evidence that aiming for the same exposure over the long term may be a viable strategy to reduce the formation of donor-specific antibodies. Strong evidence also supports the existence of important drug interactions and age/developmental dependent differences in drug metabolism that may necessitate the need for TDM of MMF therapy. Based on these findings and given the substantial inter- and intra-patient variability of MPA exposure, it would appear that MMF therapy should be subject to TDM to avoid over- and under-dosing. This may be a viable strategy to reduce treatment-emergent adverse events and to increase the effective pediatric transplant survival rates.

Clinical efficacy and pharmacokinetics of tacrolimus in children with steroid-resistant nephrotic syndrome

BACKGROUND

Tacrolimus has gained acceptance in the management of steroid-resistant nephrotic syndrome (SRNS) in children. Due to limited data, therapeutic range is extrapolated from pediatric renal transplant recipients. This study was designed to assess therapeutic efficacy of tacrolimus in children with SRNS and its correlation with inter-dose area under concentration curve (AUC0-12 h) and trough concentration (C0).

METHODS

Pre dose, 0.5, 1.0, 1.5, 2.0, 2.5, 3, 4, 8, and 12 h after drug administration blood samples were collected in 25 children who were on tacrolimus for a minimum of 3 months and AUC0-12 h was calculated.

RESULTS

There was an 80% (20/25) response rate with 64% (16/25) children achieving complete remission. Median C0 in remission was higher than in relapse group (2.95 ng/ml, versus 1.20 ng/ml, p = 0.005). Median AUC0-12 h in remission was higher compared to those in relapse group (79.75 versus 35.15 μg × h/l; p = 0.025). Maximum concentration after drug administration (Cmax) among the groups was not significantly different. There was a significant correlation between C0 and AUC0-12 h (r = 0.79); and Cmax and AUC0-12 h (r = 0.84). Five patients had a rise in serum creatinine, of which four were still proteinuric and had lower C0 and AUC0-12 h. No other adverse effect was noted.

CONCLUSIONS

Tacrolimus had beneficial clinical response in SRNS. Target C0 and AUC0-12 h level for treatment remission was higher than those in relapse in children with SRNS but was lower than required in transplant recipient.

Choosing the right dose of tacrolimus

Choosing the right dose of tacrolimus 'adapted to each individual patient' is a central question after transplantation. The pharmacokinetic behaviour of tacrolimus in paediatric patients is significantly influenced by clinical factors growth and maturation, as well as genetic factors. Large interindividual variability and narrow therapeutic index make dosage individualisation mandatory in children. CYP3A5 expressers require a 1.8-fold higher tacrolimus dose than non-expressers. A visual patient-tailored dosing chart, taking into consideration the child's weight, recent haematocrit level and CYP3A5 genotype, was developed based on a population pharmacokinetic-pharmacogenetic model, and can be used routinely to individualise tacrolimus starting dose. Area under the concentration-time curve-based dosage adaptation through limited sampling strategy and Bayesian estimation is more reliable than trough concentration. Therapeutic drug monitoring and dosage adaptation can be included in routine post-transplantation consultation and should be considered in the urgent situations (eg, rejection, adverse event, lack of compliance, change of coadministration drug with potential drug-drug interaction and other situations).

Rapid Resolution of Tacrolimus Intoxication–Induced AKI With a Corticosteroid and Phenytoin

Objective: To report a novel approach to the management of tacrolimus intoxication that leads to rapid normalization of serum tacrolimus concentrations. Case Summary: A 9-year-old female renal transplant recipient developed a severe tacrolimus intoxication as a result of prolonged diarrhea, which resulted in acute kidney injury, severe dehydration, and neurological symptoms. We used a combination of intravenous steroids and intravenous phenytoin to normalize the tacrolimus level from 32 to 5 ng/mL in less than 24 hours, with complete resolution of symptoms and signs. Discussion: Tacrolimus intoxication is a rare event but may result in life-threatening complications. Treatment recommendations beyond holding the drug and enzyme induction with phenytoin or phenobarbital are elusive. This approach leads to a relatively slow normalization of the tacrolimus level over 72 hours. The authors hypothesized that additional induction of the p-glycoprotein through steroids was synergistic. Conclusions: The combination of phenytoin and a corticosteroid may be an effective approach that leads to rapid normalization of severely elevated tacrolimus levels.

Dosage individualization in children: integration of pharmacometrics in clinical practice

BACKGROUND

Children are in a continuous and dynamically changing state of growth and development. A thorough understanding of developmental pharmacokinetics (PK) and pharmacodynamics (PD) is required to optimize drug therapy in children.

DATA SOURCES

Based on recent publications and the experience of our group, we present an outline on integrating pharmacometrics in pediatric clinical practice to develop evidence-based personalized pharmacotherapy.

RESULTS

Antibiotics in septic neonates and immunosuppressants in pediatric transplant recipients are provided as proof-of-concept to demonstrate the utility of pharmacometrics in clinical practice. Dosage individualization based on developmental PK-PD model has potential benefits of improving the efficacy and safety of drug therapy in children.

CONCLUSION

The pharmacometric technique should be better developed and used in clinical practice to personalize drug therapy in children in order to decrease variability of drug exposure and associated risks of overdose or underdose.

Pediatric perspective on pharmacogenomics

The advances in high-throughput genomic technologies have improved the understanding of disease pathophysiology and have allowed a better characterization of drug response and toxicity based on individual genetic make up. Pharmacogenomics is being recognized as a valid approach used to identify patients who are more likely to respond to medication, or those in whom there is a high probability of developing severe adverse drug reactions. An increasing number of pharmacogenomic studies are being published, most include only adults. A few studies have shown the impact of pharmacogenomics in pediatrics, highlighting a key difference between children and adults, which is the contribution of developmental changes to therapeutic responses across different age groups. This review focuses on pharmacogenomic research in pediatrics, providing examples from common pediatric conditions and emphasizing their developmental context.

Management of acute rejection in paediatric liver transplantation

The success of paediatric liver transplantation is attributed to improved surgical techniques and the advent of calcineurin inhibitor-based immunosuppression. Acute rejection (AR) rarely results in graft loss with calcineurin inhibitor immunosuppressive regimens, and the advent of newer agents like interleukin (IL)-2 receptor antibodies. The latter have the benefit of reducing the incidence of AR further and may be of use in patients who are susceptible to recurrent AR, were retransplanted for graft rejection or are in a steroid-sparing regimen. A total of 60 % of all paediatric liver transplants result in AR; however, there is a 75 % response rate to initial steroid therapy. Steroid therapy remains the mainstay of initial AR management, coupled with an increase in baseline immunosuppression. Steroid-resistant rejection (SRR), previously an immediate indication for potent anti-lymphocyte preparations, is now effectively treated with chimeric or humanised IL-2 receptor monoclonal antibodies. Recurrent AR can be treated by adding adjuvant immunosuppressive agents such as mycophenolate mofetil (MMF) or sirolimus. Studies have also demonstrated the efficacy of MMF as rescue therapy for SRR. Anti-lymphocyte preparations such as anti-thymocyte globulin (ATG) and OKT3 are rarely used in SRR but may be of use as rescue therapy for severe SRR. The challenges of the management of AR remain in the management of recurrent AR and SRR. We discuss the pathogenesis, diagnosis and management of AR, including prevention, and specific management of AR and SRR based on current evidence and our own experience at the King's College Paediatric Liver, Gastroenterology and Nutrition Centre in London.

Population pharmacokinetics of ciclosporin in Chinese children with aplastic anemia: effects of weight, renal function and stanozolol administration

AIM

To develop a population pharmacokinetic model for the immunosuppressant ciclosporin in Chinese children with aplastic anemia and to identify covariates influencing ciclosporin pharmacokinetics.

METHODS

A total of 102 children with either acquired or congenital aplastic anemia aged 8.8±3.6 years (range 0.9-17.6 years) were included. Therapeutic drug monitoring (TDM) data for ciclosporin were collected. The population pharmacokinetic model of ciclosporin was described using the nonlinear mixed-effects modeling (NONMEM) VI software. The final model was validated using bootstrap and normalized prediction distribution errors.

RESULTS

A one-compartment model with first-order absorption and elimination was developed. The estimated CL/F was 15.1, which was lower than those of children receiving stem cell or kidney transplant reported in the West (16.9-29.3). The weight normalized CL/F was 0.45 (range: 0.27-0.70) Lh(-1)·kg(-1). The covariate analysis identified body weight, serum creatinine and concomitant administration of the anabolic steroid stanozolol as individual factors influencing the CL/F of ciclosporin.

CONCLUSION

Our model could be used to optimize the ciclosporin dosing regimen in Chinese children with aplastic anemia.

Pediatric transplantation: opportunities for pharmacogenomics and genomics

Heterogeneity is the rule among pediatric heart transplant recipients. Patients vary in age, size, organ maturity, immune system maturity and underlying disease etiology, which can all influence post-transplant outcomes. Overall, the survival of pediatric transplant recipients continues to improve and the goal remains long-term survival of the primary graft and mitigation of long-term complications and adverse events. The evolving fields of pharmacogenomics and genomics have the potential to revolutionize and personalize the care of pediatric transplant recipients, and although clinical validation in a pediatric cohort is lacking, many of these technologies are becoming more readily available. We discuss genotype-guided dosing of immunosuppressant medications and other commonly used medications after transplantation, the influence of donor and recipient genotype on risk of post-transplant complications, genotype-guided selection of therapies to treat complications, and the use of next-generation sequencing for noninvasive detection of graft rejection.

Pharmacogenetic determinant of the drug interaction between tacrolimus and omeprazole

A 17-year-old adolescent with acute nephrotoxicity had CYP3A4-5, CYP2C19, and ABCB1 genotyping performed to understand a suspected drug interaction between tacrolimus and omeprazole. The determinant role of individual pharmacogenetic profile in the occurrence of tacrolimus nephrotoxicity is presented and discussed.

Pharmacogenomics in pediatric rheumatology

PURPOSE OF REVIEW

Despite major advancements in therapeutics, variability in drug response remains a challenge in both adults and children diagnosed with rheumatic disease. The genetic contribution to interindividual variability has emerged as a promising avenue of exploration; however, challenges remain in making this knowledge relevant in the clinical realm.

RECENT FINDINGS

New genetic associations in patients with rheumatic disease have been reported for disease modifying antirheumatic drugs, antimetabolites and biologic drugs. However, many of these findings are in need of replication, and few have taken into account the concept of ontogeny, specific to pediatrics.

SUMMARY

In the current era in which we practice, genetic variation will undoubtedly contribute to variability in therapeutic response and may be a factor that will ultimately impact individualized care. However, preliminary studies have shown that there are many hurdles that need to be overcome as we explore pharmacogenomic associations specifically in the field of pediatric rheumatology.

Population pharmacokinetics and maximum a posteriori probability Bayesian estimator of abacavir: application of individualized therapy in HIV-infected infants and toddlers

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

Abacavir is used to treat HIV infection in both adults and children. The recommended paediatric dose is 8 mg kg(-1) twice daily up to a maximum of 300 mg twice daily. Weight was identified as the central covariate influencing pharmacokinetics of abacavir in children.

WHAT THIS STUDY ADDS

A population pharmacokinetic model was developed to describe both once and twice daily pharmacokinetic profiles of abacavir in infants and toddlers. Standard dosage regimen is associated with large interindividual variability in abacavir concentrations. A maximum a posteriori probability Bayesian estimator of AUC(0-) (t) based on three time points (0, 1 or 2, and 3 h) is proposed to support area under the concentration-time curve (AUC) targeted individualized therapy in infants and toddlers.

AIMS

To develop a population pharmacokinetic model for abacavir in HIV-infected infants and toddlers, which will be used to describe both once and twice daily pharmacokinetic profiles, identify covariates that explain variability and propose optimal time points to optimize the area under the concentration-time curve (AUC) targeted dosage and individualize therapy.

METHODS

The pharmacokinetics of abacavir was described with plasma concentrations from 23 patients using nonlinear mixed-effects modelling (NONMEM) software. A two-compartment model with first-order absorption and elimination was developed. The final model was validated using bootstrap, visual predictive check and normalized prediction distribution errors. The Bayesian estimator was validated using the cross-validation and simulation-estimation method.

RESULTS

The typical population pharmacokinetic parameters and relative standard errors (RSE) were apparent systemic clearance (CL) 13.4 () h−1 (RSE 6.3%), apparent central volume of distribution 4.94 () (RSE 28.7%), apparent peripheral volume of distribution 8.12 () (RSE14.2%), apparent intercompartment clearance 1.25 () h−1 (RSE 16.9%) and absorption rate constant 0.758 h−1 (RSE 5.8%). The covariate analysis identified weight as the individual factor influencing the apparent oral clearance: CL = 13.4 × (weight/12)1.14. The maximum a posteriori probability Bayesian estimator, based on three concentrations measured at 0, 1 or 2, and 3 h after drug intake allowed predicting individual AUC0–t.

CONCLUSIONS

The population pharmacokinetic model developed for abacavir in HIV-infected infants and toddlers accurately described both once and twice daily pharmacokinetic profiles. The maximum a posteriori probability Bayesian estimator of AUC(0-) (t) was developed from the final model and can be used routinely to optimize individual dosing.

Population pharmacokinetics and pharmacogenetics of once daily prolonged-release formulation of tacrolimus in pediatric and adolescent kidney transplant recipients

BACKGROUND AND OBJECTIVES

Tacrolimus(PR) is a new prolonged-release once-daily formulation of the calcineurin inhibitor tacrolimus, currently used in adult transplant patients. As there are no pharmacokinetic data available in pediatric kidney transplant recipients, the aims of this study were to develop a population pharmacokinetic model of tacrolimus(PR) in pediatric and adolescent kidney transplant recipients and to identify covariates that have a significant impacts on tacrolimus(PR) pharmacokinetics, including CYP3A5 polymorphism.

METHODS

Pharmacokinetic samples were collected from 22 pediatric kidney transplant patients. Population pharmacokinetic analysis was performed using NONMEM. Pharmacogenetic analysis was performed on the CYP3A5 gene.

RESULTS

The pharmacokinetic data were best described by a one-compartment model with first order absorption and lag-time. The weight normalized oral clearance CL/F [CL/F/ (weight/70)(0.75)] was lower in patients with CYP3A5 3/3 as compared to patients with the CYP3A5 1/3 (32.2 ± 10.1 vs. 53.5 ± 20.2 L/h, p = 0.01).

CONCLUSIONS

The population pharmacokinetic model of tacrolimus(PR) was developed and validated in pediatric and adolescent kidney transplant patients. Body weight and CYP3A5 polymorphism were identified as significant factors influencing pharmacokinetics. The developed model could be useful to optimize individual pediatric tacrolimus (PR) dosing regimen in routine clinical practice.