Population pharmacokinetics III: design, analysis, and application of population pharmacokinetic Studies

Application of machine learning techniques in population pharmacokinetics/pharmacodynamics modeling

Population pharmacokinetics/pharmacodynamics (pop-PK/PD) consolidates pharmacokinetic and pharmacodynamic data from many subjects to understand inter- and intra-individual variability due to patient backgrounds, including disease state and genetics. The typical workflow in pop-PK/PD analysis involves the determination of the structure model, selection of the error model, analysis based on the base model, covariate modeling, and validation of the final model. Machine learning is gaining considerable attention in the medical and various fields because, in contrast to traditional modeling, which often assumes linear or predefined relationships, machine learning modeling learns directly from data and accommodates complex patterns. Machine learning has demonstrated excellent capabilities for prescreening covariates and developing predictive models. This review introduces various applications of machine learning techniques in pop-PK/PD research.

Population pharmacokinetic dosimetry model using imaging data to assess variability in pharmacokinetics of 177 Lu-PSMA-617 in prostate cancer patients

Studies to evaluate and optimize [177 Lu]Lu-PSMA treatment focus primarily on individual patient data. A population pharmacokinetic (PK) dosimetry model was developed to explore the potential of using imaging data as input for population PK models and to characterize variability in organ and tumor uptake of [177 Lu]Lu-PSMA-617 in patients with low volume metastatic prostate cancer. Simulations were performed to identify the effect of dose adjustments on absorbed doses in salivary glands and tumors. A six-compartment population PK model was developed, consisting of blood, salivary gland, kidneys, liver, tumor, and a lumped compartment representing other tissue (compartment 1-6, respectively), based on data from 10 patients who received [177 Lu]Lu-PSMA-617 (2 cycles, ~ 3 and ~ 6 GBq). Data consisted of radioactivity levels (decay corrected) in blood and tissues (9 blood samples and 5 single photon emission computed tomography/computed tomography scans). Observations in all compartments were adequately captured by individual model predictions. Uptake into salivary glands was saturable with an estimated maximum binding capacity (Bmax ) of 40.4 MBq (relative standard error 12.3%) with interindividual variability (IIV) of 59.3% (percent coefficient of variation [CV%]). IIV on other PK parameters was relatively minor. Tumor volume was included as a structural effect on the tumor uptake rate constant (k15 ), where a two-fold increase in tumor volume resulted in a 1.63-fold increase in k15 . In addition, interoccasion variability on k15 improved the model fit (43.5% [CV%]). Simulations showed a reduced absorbed dose per unit administered activity for salivary glands after increasing radioactivity dosing from 3 to 6 GBq (0.685 Gy/GBq vs. 0.421 Gy/GBq, respectively). All in all, population PK modeling could help to improve future radioligand therapy research.

Phenylbutazone pharmacokinetics in southern white rhinoceros (Ceratotherium simum simum) after oral administration

Southern white rhinoceros (Ceratotherium simum simum) frequently develop painful conditions, such as traumatic injuries or osteoarthritis, necessitating the administration of pain-relieving medications. One of the preferred treatments is the nonsteroidal anti-inflammatory drug phenylbutazone because of the availability of oral formulations and the familiarity of its use in horses. For the main study, a single oral dose of phenylbutazone at 2 mg/kg was administered to healthy adult rhinoceros (n = 33) housed at six North American zoological institutions. Each rhinoceros had up to four blood samples collected under voluntary behavioural restraint at up to four predetermined time points (0, 1, 1.5, 2, 3, 4, 6, 8, 10, 24, 30 and 48 h). Drug analysis was performed by high-performance liquid chromatography. The population pharmacokinetic parameters were calculated with nonlinear mixed-effects modelling, and analysis showed a peak concentration (C_MAX ) of 3.8 µg/ml at 1.8 h and an elimination half-life of 9 h. The concentrations achieved were similar to what has been reported for horses and were within the half maximal effective concentration for horses for at least 10 h. A multi-dose trial in five rhinoceros receiving 2 mg/kg orally once daily for five days found mild accumulation at a predicted factor of 1.2. This study represents the first pharmacokinetic data of phenylbutazone in any rhinoceros species.

Concept and utility of population pharmacokinetic and pharmacokinetic/pharmacodynamic models in drug development and clinical practice

Due to frequent clinical trial failures and consequently fewer new drug approvals, the need for improvement in drug development has, to a certain extent, been met using model-based drug development. Pharmacometrics is a part of pharmacology that quantifies drug behaviour, treatment response and disease progression based on different models (pharmacokinetic - PK, pharmacodynamic - PD, PK/PD models, etc.) and simulations. Regulatory bodies (European Medicines Agency, Food and Drug Administration) encourage the use of modelling and simulations to facilitate decision-making throughout all drug development phases. Moreover, the identification of factors that contribute to variability provides a basis for dose individualisation in routine clinical practice. This review summarises current knowledge regarding the application of pharmacometrics in drug development and clinical practice with emphasis on the population modelling approach.

Population Pharmacokinetics of Clotting Factor Concentrates and Desmopressin in Hemophilia

Hemophilia A and B are bleeding disorders caused by a deficiency of clotting factor VIII and IX, respectively. Patients with severe hemophilia (< 0.01 IU mL⁻¹) and some patients with moderate hemophilia (0.01–0.05 IU mL⁻¹) administer clotting factor concentrates prophylactically. Desmopressin (d-amino d-arginine vasopressin) can be applied in patients with non-severe hemophilia A. The aim of administration of factor concentrates or desmopressin is the prevention or cessation of bleeding. Despite weight-based dosing, it has been demonstrated that factor concentrates still exhibit considerable pharmacokinetic variability. Population pharmacokinetic analyses, in which this variability is quantified and explained, are increasingly performed in hemophilia research. These analyses can assist in the identification of important patient characteristics and can be applied to perform patient-tailored dosing. This review aims to present and discuss the population pharmacokinetic analyses that have been conducted to develop population pharmacokinetic models describing factor levels after administration of factor VIII or factor IX concentrates or d-amino d-arginine vasopressin. In total, 33 publications were retrieved from the literature. Two approaches were applied to perform population pharmacokinetic analyses, the standard two-stage approach and non-linear mixed-effect modeling. Using the standard two-stage approach, four population pharmacokinetic models were established describing factor VIII levels. In the remaining 29 analyses, the non-linear mixed-effect modeling approach was applied. NONMEM was the preferred software to establish population pharmacokinetic models. In total, 18 population pharmacokinetic analyses were conducted on the basis of data from a single product. From all available population pharmacokinetic analyses, 27 studies also included data from pediatric patients. In the majority of the population pharmacokinetic models, the population pharmacokinetic parameters were allometrically scaled using actual body weight. In this review, the available methods used for constructing the models, key features of these models, patient population characteristics, and established covariate relationships are described in detail.

The Steps to Therapeutic Drug Monitoring: A Structured Approach Illustrated With Imatinib

Pharmacometric methods have hugely benefited from progress in analytical and computer sciences during the past decades, and play nowadays a central role in the clinical development of new medicinal drugs. It is time that these methods translate into patient care through therapeutic drug monitoring (TDM), due to become a mainstay of precision medicine no less than genomic approaches to control variability in drug response and improve the efficacy and safety of treatments. In this review, we make the case for structuring TDM development along five generic questions: 1) Is the concerned drug a candidate to TDM? 2) What is the normal range for the drug's concentration? 3) What is the therapeutic target for the drug's concentration? 4) How to adjust the dosage of the drug to drive concentrations close to target? 5) Does evidence support the usefulness of TDM for this drug? We exemplify this approach through an overview of our development of the TDM of imatinib, the very first targeted anticancer agent. We express our position that a similar story shall apply to other drugs in this class, as well as to a wide range of treatments critical for the control of various life-threatening conditions. Despite hurdles that still jeopardize progress in TDM, there is no doubt that upcoming technological advances will shape and foster many innovative therapeutic monitoring methods.

Pharmacokinetic-Pharmacodynamic Modeling in Pediatric Drug Development, and the Importance of Standardized Scaling of Clearance

Pharmacokinetic/pharmacodynamic (PKPD) modeling is important in the design and conduct of clinical pharmacology research in children. During drug development, PKPD modeling and simulation should underpin rational trial design and facilitate extrapolation to investigate efficacy and safety. The application of PKPD modeling to optimize dosing recommendations and therapeutic drug monitoring is also increasing, and PKPD model-based dose individualization will become a core feature of personalized medicine. Following extensive progress on pediatric PK modeling, a greater emphasis now needs to be placed on PD modeling to understand age-related changes in drug effects. This paper discusses the principles of PKPD modeling in the context of pediatric drug development, summarizing how important PK parameters, such as clearance (CL), are scaled with size and age, and highlights a standardized method for CL scaling in children. One standard scaling method would facilitate comparison of PK parameters across multiple studies, thus increasing the utility of existing PK models and facilitating optimal design of new studies.

Misidentification Subtype of Alzheimer's Disease Psychosis Predicts a Faster Cognitive Decline

The presence of psychosis is associated with a more rapid decline in Alzheimer's disease (AD), but the impact of paranoid (persecutory delusions) and misidentification (misperceptions and/or hallucinations) subtypes of psychosis on the speed of decline in AD is still unclear. We analyzed data on Alzheimer's Disease Neuroimaging Initiative 2 participants with late mild cognitive impairment or AD, and we described individual trajectories of Alzheimer's Disease Assessment Scale-Cognitive Subscale scores using a semimechanistic logistic model with a mixed effects-based approach, which accounted for dropout and adjusted for baseline Mini Mental State Examination scores. The covariate model included psychosis subtypes, age, gender, education, medications, and Apolipoprotein E epsilon 4 (Apo-e ε4 genotype). We found that the Alzheimer's Disease Assessment Scale-Cognitive Subscale rate of increase was doubled in misidentification (β_{r,misid_subtype}  = 0.63, P = 0.031) and mixed (both subtypes; β_{r,mixed_subtype}  = 0.70, P = 0.003) when compared with nonpsychotic (or paranoid) patients, suggesting that the misidentification subtype may represent a distinct AD sub-phenotype associated with an accelerated pathological process.

Model-informed pediatric development applied to bilastine: Analysis of the clinical PK data and confirmation of the dose selected for the target population

Bilastine is a non-sedating second-generation H₁ antihistamine approved for treatment of allergic rhinoconjunctivitis (AR) and urticaria (U) in adults at the oral (p.o.) dose of 20 mg once daily (OD). Optimal attributes can be anticipated for its clinical use in pediatrics due to its favorable safety and tolerability and age-independent PD profile. The aim of this work was to characterize bilastine PK in children through population modeling of data from a limited sampling confirmatory clinical trial in children with AR or U. The objective was also to ascertain whether the proposed dose (10 mg/day) in the target pediatric subset aged 2-<12 years matches the systemic exposure seen in adults at the 20 mg/day dose. A popPK model characterizing bilastine PK behavior in children aged from 4 to <12 years treated with 10 mg oral bilastine daily was successfully developed and qualified. No relationship was found between bilastine PK and age or weight; stopping rules pre-stablished to finalize the trial, i.e., model completeness and no dependence of exposure on decreasing age, were thus fulfilled. On a second step, the popPK model in children was linked to the PD model in adults assuming the same PD as described in adults and used to compare the PD outcome between both populations. Finally, an allometric scaling method and a physiological approximation were used to evaluate the suitability of the selected dose in the youngest children, showing that children from 2 years were deemed to belong to the same population as well. The achievement of comparable PK (i.e., within the range) to that observed in adults after the therapeutic dose of 20 mg, together with the achievement of similar PD and additional integrative analysis, served to confirm the validity of the 10 mg daily dose for the target pediatric subset (2 to <12 years).

Mathematical modeling and simulation in animal health. Part III: Using nonlinear mixed-effects to characterize and quantify variability in drug pharmacokinetics

A common feature of human and veterinary pharmacokinetics is the importance of identifying and quantifying the key determinants of between-patient variability in drug disposition and effects. Some of these attributes are already well known to the field of human pharmacology such as bodyweight, age, or sex, while others are more specific to veterinary medicine, such as species, breed, and social behavior. Identification of these attributes has the potential to allow a better and more tailored use of therapeutic drugs both in companion and food-producing animals. Nonlinear mixed effects (NLME) have been purposely designed to characterize the sources of variability in drug disposition and response. The NLME approach can be used to explore the impact of population-associated variables on the relationship between drug administration, systemic exposure, and the levels of drug residues in tissues. The latter, while different from the method used by the US Food and Drug Administration for setting official withdrawal times (WT) can also be beneficial for estimating WT of approved animal drug products when used in an extralabel manner. Finally, NLME can also prove useful to optimize dosing schedules, or to analyze sparse data collected in situations where intensive blood collection is technically challenging, as in small animal species presenting limited blood volume such as poultry and fish.

Therapeutic D2/3 receptor occupancies and response with low amisulpride blood concentrations in very late-onset schizophrenia-like psychosis (VLOSLP)

OBJECTIVE

Antipsychotic drug sensitivity in very late-onset schizophrenia-like psychosis (VLOSLP) is well documented, but poorly understood. This study aimed to investigate blood drug concentration, D2/3 receptor occupancy and outcome in VLOSLP during open amisulpride prescribing, and compare this with Alzheimer's disease (AD).

METHODS

Blood drug concentration, prolactin, symptoms and extrapyramidal side-effects (EPS) were serially assessed during dose titration. [¹⁸ F]fallypride imaging was used to quantify D2/3 receptor occupancy. Average steady-state amisulpride concentration (Caverage, ng/ml) was estimated by incorporating pharmacokinetic (PK) data into an existing population PK model (25 AD participants, 20 healthy older people).

RESULTS

Eight patients (target 20) were recruited (six women; 76 + - 6 years; six treatment compliant; five serially sampled; three with paired imaging data). Mean + - SD symptom reduction was 74 ± 12% (50-100 mg/day; 92.5 + -39.4 ng/ml). Mild EPS emerged at 96 ng/ml (in AD, severe EPS, 50 mg/day, 60 ng/ml). In three participants, imaged during optimal treatment (50 mg/day; 41-70 ng/ml), caudate occupancy was 44-59% (58-74% in AD across a comparable Caverage).

CONCLUSIONS

Despite the small sample size, our findings are highly relevant as they suggest that, as in AD, 50 mg/day amisulpride is associated with >40% occupancy and clinically relevant responses in VLOSLP. It was not possible to fully characterise concentration-occupancy relationships in VLOSLP, and it is thus unclear whether the greater susceptibility of those with AD to emergent EPS was accounted for by increased central drug access. Further investigation of age- and diagnosis-specific threshold sensitivities is warranted, to guide amisulpride prescribing in older people, and therapeutic drug monitoring studies offer a potentially informative future approach. Copyright © 2017 John Wiley & Sons, Ltd.

Therapeutic window of dopamine D2/3 receptor occupancy to treat psychosis in Alzheimer's disease

See Caravaggio and Graff-Guerrero (doi:10.1093/awx023) for a scientific commentary on this article.Antipsychotic drugs, originally developed to treat schizophrenia, are used to treat psychosis, agitation and aggression in Alzheimer's disease. In the absence of dopamine D2/3 receptor occupancy data to inform antipsychotic prescribing for psychosis in Alzheimer's disease, the mechanisms underpinning antipsychotic efficacy and side effects are poorly understood. This study used a population approach to investigate the relationship between amisulpride blood concentration and central D2/3 occupancy in older people with Alzheimer's disease by combining: (i) pharmacokinetic data (280 venous samples) from a phase I single (50 mg) dose study in healthy older people (n = 20, 65-79 years); (ii) pharmacokinetic, 18F-fallypride D2/3 receptor imaging and clinical outcome data on patients with Alzheimer's disease who were prescribed amisulpride (25-75 mg daily) to treat psychosis as part of an open study (n = 28; 69-92 years; 41 blood samples, five pretreatment scans, 19 post-treatment scans); and (iii) 18F-fallypride imaging of an antipsychotic free Alzheimer's disease control group (n = 10, 78-92 years), to provide additional pretreatment data. Non-linear mixed effects modelling was used to describe pharmacokinetic-occupancy curves in caudate, putamen and thalamus. Model outputs were used to estimate threshold steady state blood concentration and occupancy required to elicit a clinically relevant response (>25% reduction in scores on delusions, hallucinations and agitation domains of the Neuropsychiatric Inventory) and extrapyramidal side effects (Simpson Angus Scale scores > 3). Average steady state blood levels were low (71 ± 30 ng/ml), and associated with high D2/3 occupancies (65 ± 8%, caudate; 67 ± 11%, thalamus; 52 ± 11%, putamen). Antipsychotic clinical response occurred at a threshold concentration of 20 ng/ml and D2/3 occupancies of 43% (caudate), 25% (putamen), 43% (thalamus). Extrapyramidal side effects (n = 7) emerged at a threshold concentration of 60 ng/ml, and D2/3 occupancies of 61% (caudate), 49% (putamen) and 69% (thalamus). This study has established that, as in schizophrenia, there is a therapeutic window of D2/3 receptor occupancy for optimal treatment of psychosis in Alzheimer's disease. We have also shown that occupancies within and beyond this window are achieved at very low amisulpride doses in Alzheimer's disease due to higher than anticipated occupancies for a given blood drug concentration. Our findings support a central pharmacokinetic contribution to antipsychotic sensitivity in Alzheimer's disease and implicate the blood-brain barrier, which controls central drug access. Whether high D2/3 receptor occupancies are primarily accounted for by age- or disease-specific blood-brain barrier disruption is unclear, and this is an important future area of future investigation, as it has implications beyond antipsychotic prescribing.

Quantifying the Exposure of Tapentadol Extended Release in Japanese Patients with Cancer Pain and Bridging Tapentadol Pharmacokinetics Across Populations Using a Modeling Approach

BACKGROUND AND OBJECTIVES

Tapentadol extended release (ER) is approved for the management of chronic and acute pain in adults. There has been no report of tapentadol ER pharmacokinetics in subjects with cancer pain. This analysis investigated tapentadol ER pharmacokinetics in Japanese patients with cancer pain and bridged it with the pharmacokinetics in Japanese healthy subjects and Caucasian patients with cancer pain.

METHODS

Nonlinear mixed-effect pharmacokinetic modeling was conducted based on pooled tapentadol ER concentration data collected in five Phase 1 studies from 138 Japanese and Korean healthy subjects and in two Phase 3 studies from 215 Japanese and Korean subjects with cancer pain. Expected tapentadol exposure in subjects with different characteristics was assessed via simulation. Tapentadol ER exposures in Caucasian populations were compared with those in corresponding Japanese populations.

RESULTS

Tapentadol ER pharmacokinetics in Japanese cancer-pain patients were adequately described by a time-invariant, one-compartment disposition model with two input functions and first-order elimination. Weight, age, and albumin were identified as statistically significant covariates, but do not warrant dose adjustment. Comparable pharmacokinetics were shown between Japanese healthy and Caucasian healthy subjects, and between Japanese cancer-pain patients and Caucasian cancer-pain patients.

CONCLUSION

The apparent differences in the estimated individual pharmacokinetic parameters in Japanese healthy subjects and Japanese cancer-pain patients taking tapentadol ER were explained by covariates incorporated in a unified pharmacokinetic model. Population modeling was essential in this cross-population bridging analysis.

A Longitudinal HbA1c Model Elucidates Genes Linked to Disease Progression on Metformin

One-third of type-2 diabetic patients respond poorly to metformin. Despite extensive research, the impact of genetic and nongenetic factors on long-term outcome is unknown. In this study we combine nonlinear mixed effect modeling with computational genetic methodologies to identify predictors of long-term response. In all, 1,056 patients contributed their genetic, demographic, and long-term HbA1c data. The top nine variants (of 12,000 variants in 267 candidate genes) accounted for approximately one-third of the variability in the disease progression parameter. Average serum creatinine level, age, and weight were determinants of symptomatic response; however, explaining negligible variability. Two single nucleotide polymorphisms (SNPs) in CSMD1 gene (rs2617102, rs2954625) and one SNP in a pharmacologically relevant SLC22A2 gene (rs316009) influenced disease progression, with minor alleles leading to less and more favorable outcomes, respectively. Overall, our study highlights the influence of genetic factors on long-term HbA1c response and provides a computational model, which when validated, may be used to individualize treatment.

Understanding and applying pharmacometric modelling and simulation in clinical practice and research

Understanding the dose–concentration–effect relationship is a fundamental component of clinical pharmacology. Interpreting data arising from observations of this relationship requires the use of mathematical models; i.e. pharmacokinetic (PK) models to describe the relationship between dose and concentration and pharmacodynamic (PD) models describing the relationship between concentration and effect. Drug development requires several iterations of pharmacometric model‐informed learning and confirming. This includes modelling to understand the dose–response in preclinical studies, deriving a safe dose for first‐in‐man, and the overall analysis of Phase I/II data to optimise the dose for safety and efficacy in Phase III pivotal trials. However, drug development is not the boundary at which PKPD understanding and application stops. PKPD concepts will be useful to anyone involved in the prescribing and administration of medicines for purposes such as determining off‐label dosing in special populations, individualising dosing based on a measured biomarker (personalised medicine) and in determining whether lack of efficacy or unexpected toxicity maybe solved by adjusting the dose rather than the drug. In clinical investigator‐led study design, PKPD can be used to ensure the optimal dose is used, and crucially to define the expected effect size, thereby ensuring power calculations are based on sound prior information. In the clinical setting the most likely people to hold sufficient expertise to advise on PKPD matters will be the pharmacists and clinical pharmacologists. This paper reviews fundamental PKPD principles and provides some real‐world examples of PKPD use in clinical practice and applied clinical research.

Children in clinical trials: towards evidence-based pediatric pharmacotherapy using pharmacokinetic-pharmacodynamic modeling

INTRODUCTION

In pediatric pharmacotherapy, many drugs are still used off-label, and their efficacy and safety is not well characterized. Different efficacy and safety profiles in children of varying ages may be anticipated, due to developmental changes occurring across pediatric life.

AREAS COVERED

Beside pharmacokinetic (PK) studies, pharmacodynamic (PD) studies are urgently needed. Validated PKPD models can be used to derive optimal dosing regimens for children of different ages, which can be evaluated in a prospective study before implementation in clinical practice. Strategies should be developed to ensure that formularies update their drug dosing guidelines regularly according to the most recent advances in research, allowing for clinicians to integrate these guidelines in daily practice. Expert commentary: We anticipate a trend towards a systems-level approach in pediatric modeling to optimally use the information gained in pediatric trials. For this approach, properly designed clinical PKPD studies will remain the backbone of pediatric research.

A population approach to characterise amisulpride pharmacokinetics in older people and Alzheimer's disease

INTRODUCTION

Current prescribing guidelines for the antipsychotic amisulpride are based largely on pharmacokinetic (PK) studies in young adults, and there is a relative absence of data on older patients, who are at greatest risk of developing adverse events.

METHODS

This study aimed to develop a population PK model for amisulpride specifically in older people, by combining data from a richly sampled phase 1, single (50 mg) dose study in healthy older people (n = 20, 65-79 years), with a clinical dataset obtained during off label, low-dose (25-75 mg daily) amisulpride prescribing in older people with Alzheimer's disease (AD) (n = 25, 69-92 years), as part of an observational study.

RESULTS

After introducing a scaling factor based on body weight, age accounted for 20 % of the inter-individual variability in drug clearance (CL), resulting in a 54 % difference in CL between those aged 65 and those aged 85 years, and higher blood concentrations in older patients.

DISCUSSION

These findings argue for the consideration of age and weight-based dose stratification to optimise amisulpride prescribing in older people, particularly in those aged 85 years and above.

Establishing Good Practices for Exposure-Response Analysis of Clinical Endpoints in Drug Development

This tutorial aims at promoting good practices for exposure–response (E-R) analyses of clinical endpoints in drug development. The focus is on practical aspects of E-R analyses to assist modeling scientists with a process of performing such analyses in a consistent manner across individuals and projects and tailored to typical clinical drug development decisions. This includes general considerations for planning, conducting, and visualizing E-R analyses, and how these are linked to key questions.

Population pharmacokinetics of high-dose methotrexate after intravenous administration in Chinese osteosarcoma patients from a single institution

BACKGROUND

High-dose methotrexate (HD-MTX) with folinic acid (leucovorin) rescue is the gold standard therapy in the treatment of osteosarcoma. The plasma concentration of MTX is closely related to efficacy and toxicity. There are large individual differences. Many authors have described the pharmacokinetic (PK) profile of MTX regarding osteosarcoma under a variety of circumstances. However, no data concerning Chinese osteosarcoma patient PKs using the nonlinear mixed effects models (NONMEM) have been previously reported. The goals of this study were to establish the population pharmacokinetics (PPK) of HD-MTX treatment in Chinese osteosarcoma patients, and to explore the influence of patient covariates and between-occasion variability on drug disposition.

METHODS

An intravenous HD-MTX solution (10 g/m 2 ) was given 274 times to 148 osteosarcoma patients. MTX plasma concentrations were measured at 0, 6, 12, 24, 48 and 72 h after commencement of the infusion, and the fluorescence polarization immunoassay was used to determine MTX plasma concentrations. The PPK model and parameters were estimated using NONMEM software. The effects of fixed-effect factors were evaluated, and the final regression model was obtained.

RESULTS

The following population parameters were obtained using a two-compartment model: CL1 (clearance of central compartment): (CL1 ) = CL1TV × [1 - θ CL1- MTXNUM × MTXNUM] × [1 - θ CL1- CrCl1 × (CrCl1 - 1.89)] × e ηCL1i (L/h). V1 (central volume): (V1)i = V1TV × e ηV1i (L). CL2 (clearance of peripheral compartment): (CL2)i = CL2TV × [1 - θCL2 - BODY AREA × (body area - 1.62)] × e ηCL2i (L/h). V2 (peripheral compartment): (V2 )i = V2TV × [1 - θ V2-bodyarea × (bodyarea-1.62)] × e ηV2i (L). The PPK parameters (RSD%) were CL1, V1, CL2 and V2 with values of 6.20 L/h (8.48%), 19.6 L (extremely small), 0.0172 L/h (50.9%) and 0.515 L (39.1%), respectively. Creatinine clearance and the number of methotrexate chemotherapy cycles before MTX infusion had a significant effect on the CL1, and body surface area had a significant effect on the CL2 and the V2 (P < 0. 01).

CONCLUSIONS

A good fit was derived for the PPK. The model could be used to provide guidance for MTX treatment and reduce adverse effects.

Pharmacokinetic/pharmacodynamic modelling approaches in paediatric infectious diseases and immunology

Pharmacokinetic/pharmacodynamic (PKPD) modelling is used to describe and quantify dose-concentration-effect relationships. Within paediatric studies in infectious diseases and immunology these methods are often applied to developing guidance on appropriate dosing. In this paper, an introduction to the field of PKPD modelling is given, followed by a review of the PKPD studies that have been undertaken in paediatric infectious diseases and immunology. The main focus is on identifying the methodological approaches used to define the PKPD relationship in these studies. The major findings were that most studies of infectious diseases have developed a PK model and then used simulations to define a dose recommendation based on a pre-defined PD target, which may have been defined in adults or in vitro. For immunological studies much of the modelling has focused on either PK or PD, and since multiple drugs are usually used, delineating the relative contributions of each is challenging. The use of dynamical modelling of in vitro antibacterial studies, and paediatric HIV mechanistic PD models linked with the PK of all drugs, are emerging methods that should enhance PKPD-based recommendations in the future.

Chain length of dietary alkylresorcinols affects their in vivo elimination kinetics in rats

Two phenolic acids, 3,5-dihydroxybenzoic acid (DHBA) and 3-(3,5-dihydroxyphenyl)- propanoic acid (DHPPA), are the major metabolites of cereal alkylresorcinols (ARs). Like their precursors, AR metabolites have been suggested as biomarkers for intake of whole-grain wheat and rye and as such could aid the understanding of diet-disease associations. This study estimated and compared pharmacokinetic parameters of ARs and their metabolites in rats and investigated differences in metabolite formation after ingestion of different AR homologs. Rats were i.v. infused for 30 min with 2, 12, or 23 μmol/kg DHBA or DHPPA or orally given the same amounts of the AR homologs, C17:0 and C25:0. Repeated plasma samples, obtained from rats for 6 h (i.v.) or 36 h (oral), were simultaneously analyzed for ARs and their metabolites by GC-mass spectrometry. Pharmacokinetic parameters were estimated by population-based compartmental modeling and noncompartmental calculation. A 1-compartment model best described C25:0 pharmacokinetics, whereas C17:0 and AR metabolites best fitted 2-compartment models. Combined models for simultaneous prediction of AR and metabolite concentration were more complex, with less reliable estimates of pharmacokinetic parameters. Although the AUC of C17:0 was lower than that of C25:0 (P < 0.05), the total amount and composition of AR metabolites did not differ between rats given C17:0 or C25:0. The elimination half-life of ARs and their metabolites increased with length of the side chain (P-trend < 0.001) and ranged from 1.2 h (DHBA) to 8.8 h (C25:0). The formation of AR metabolites was slower than their elimination, indicating that the rate of AR metabolism and not excretion of DHBA and DHPPA determines their plasma concentrations in rats.

Bayesian inference for generalized linear mixed model based on the multivariate t distribution in population pharmacokinetic study

This article provides a fully bayesian approach for modeling of single-dose and complete pharmacokinetic data in a population pharmacokinetic (PK) model. To overcome the impact of outliers and the difficulty of computation, a generalized linear model is chosen with the hypothesis that the errors follow a multivariate Student t distribution which is a heavy-tailed distribution. The aim of this study is to investigate and implement the performance of the multivariate t distribution to analyze population pharmacokinetic data. Bayesian predictive inferences and the Metropolis-Hastings algorithm schemes are used to process the intractable posterior integration. The precision and accuracy of the proposed model are illustrated by the simulating data and a real example of theophylline data.

Population pharmacokinetics and pharmacogenetics of alcohol in Chinese and Indians in Singapore

WHAT IS KNOWN AND OBJECTIVE

  Interindividual variability in alcohol pharmacokinetics is influenced by a number of factors, including polymorphisms in genes mediating alcohol pharmacology, ethnicity, sex and body size. Several studies have evaluated the population pharmacokinetics of alcohol from breath alcohol measures. None of these studies, however, have evaluated ethnicity and alcohol-metabolizing enzyme genotypes as covariates in their population pharmacokinetic modelling. We aimed to develop a population pharmacokinetic model using clinical and genetic factors and to identify covariates that influenced interindividual variability in alcohol clearance and volume of distribution.

METHODS

  Hundred and eighty healthy subjects (90 Chinese and 90 Indians; 45 males and 45 females from each ethnic group) ingested a vodka-orange juice mixture to simulate social drinking. Subjects were genotyped for the ADH1B (Arg48His), ALDH2 (Glu504Lys) and CYP2E1 (c.-1293G>C and c.-1053C>T) polymorphisms. A base pharmacokinetic model was developed using the nonmem software (NONMEM Project Group, University of California, San Francisco, San Francisco, CA, USA) to determine the alcohol clearance and volume of distribution. The model was extended to include covariates that influenced the between-subject variability.

RESULTS AND DISCUSSION

  Body weight and sex significantly influenced absorption rate and volume of distribution of alcohol. Body weight and ADH1B Arg48His polymorphism significantly influenced alcohol clearance. The Michaelis-Menten elimination rate (Vmax ) was decreased by 10% in homozygous ADH1B*1/*1 subjects. Ethnicity was not determined to be a significant covariate in the final population pharmacokinetic model.

WHAT IS NEW AND CONCLUSION

  Gender and body weight were covariates that contributed most to explaining the observed interindividual alcohol pharmacokinetic variability. Of the four SNPs examined in this study, only ADH1B Arg48His polymorphism had a significant, though modest, effect on the pharmacokinetics of alcohol.

Limited-sampling strategies for anti-infective agents: systematic review

BACKGROUND

Area under the concentration-time curve (AUC) is a pharmacokinetic parameter that represents overall exposure to a drug. For selected anti-infective agents, pharmacokinetic-pharmacodynamic parameters, such as AUC/MIC (where MIC is the minimal inhibitory concentration), have been correlated with outcome in a few studies. A limited-sampling strategy may be used to estimate pharmacokinetic parameters such as AUC, without the frequent, costly, and inconvenient blood sampling that would be required to directly calculate the AUC.

OBJECTIVE

To discuss, by means of a systematic review, the strengths, limitations, and clinical implications of published studies involving a limited-sampling strategy for anti-infective agents and to propose improvements in methodology for future studies.

METHODS

The PubMed and EMBASE databases were searched using the terms "anti-infective agents", "limited sampling", "optimal sampling", "sparse sampling", "AUC monitoring", "abbreviated AUC", "abbreviated sampling", and "Bayesian". The reference lists of retrieved articles were searched manually. Included studies were classified according to modified criteria from the US Preventive Services Task Force.

RESULTS

Twenty studies met the inclusion criteria. Six of the studies (involving didanosine, zidovudine, nevirapine, ciprofloxacin, efavirenz, and nelfinavir) were classified as providing level I evidence, 4 studies (involving vancomycin, didanosine, lamivudine, and lopinavir-ritonavir) provided level II-1 evidence, 2 studies (involving saquinavir and ceftazidime) provided level II-2 evidence, and 8 studies (involving ciprofloxacin, nelfinavir, vancomycin, ceftazidime, ganciclovir, pyrazinamide, meropenem, and alpha interferon) provided level III evidence. All of the studies providing level I evidence used prospectively collected data and proper validation procedures with separate, randomly selected index and validation groups. However, most of the included studies did not provide an adequate description of the methods or the characteristics of included patients, which limited their generalizability.

CONCLUSIONS

Many limited-sampling strategies have been developed for anti-infective agents that do not have a clearly established link between AUC and clinical outcomes in humans. Future studies should first determine if there is an association between AUC monitoring and clinical outcomes. Thereafter, it may be worthwhile to prospectively develop and validate a limited-sampling strategy for the particular anti-infective agent in a similar population.

PKgraph: an R package for graphically diagnosing population pharmacokinetic models

Population pharmacokinetic (PopPK) modeling has become increasing important in drug development because it handles unbalanced design, sparse data and the study of individual variation. However, the increased complexity of the model makes it more of a challenge to diagnose the fit. Graphics can play an important and unique role in PopPK model diagnostics. The software described in this paper, PKgraph, provides a graphical user interface for PopPK model diagnosis. It also provides an integrated and comprehensive platform for the analysis of pharmacokinetic data including exploratory data analysis, goodness of model fit, model validation and model comparison. Results from a variety of modeling fitting software, including NONMEM, Monolix, SAS and R, can be used. PKgraph is programmed in R, and uses the R packages lattice, ggplot2 for static graphics, and rggobi for interactive graphics.

Computational opioid prescribing: a novel application of clinical pharmacokinetics

We implemented a pharmacokinetics-based mathematical modeling technique using algebra to assist pre-scribers with point-of-care opioid dosing. We call this technique computational opioid prescribing (COP). Because population pharmacokinetic parameter values are needed to estimate drug dosing regimen designs for individual patients using COP, and those values are not readily available to prescribers because they exist scattered in the vast pharmacology literature, we estimated the population pharmacokinetic parameter values for 12 commonly prescribed opioids from various sources using the bootstrap resampling technique. Our results show that opioid dosing regimen design, evaluation, and modification is feasible using COP. We conclude that COP is a new technique for the quantitative assessment of opioid dosing regimen design evaluation and adjustment, which may help prescribers to manage acute and chronic pain at the point-of-care. Potential benefits include opioid dose optimization and minimization of adverse opioid drug events, leading to potential improvement in patient treatment outcomes and safety.

Interpreting population pharmacokinetic-pharmacodynamic analyses - a clinical viewpoint

The population analysis approach is an important tool for clinical pharmacology in aiding the dose individualization of medicines. However, due to their statistical complexity the clinical utility of population analyses is often overlooked. One of the key reasons to conduct a population analysis is to investigate the potential benefits of individualization of drug dosing based on patient characteristics (termed covariate identification). The purpose of this review is to provide a tool to interpret and extract information from publications that describe population analysis. The target audience is those readers who are aware of population analyses but have not conducted the technical aspects of an analysis themselves. Initially we introduce the general framework of population analysis and work through a simple example with visual plots. We then follow-up with specific details on how to interpret population analyses for the purpose of identifying covariates and how to interpret their likely importance for dose individualization.

Plasma and intracellular population pharmacokinetic analysis of tenofovir in HIV-1-infected patients

The relationships among the dose of tenofovir disoproxil fumarate (TDF), tenofovir (TFV) plasma concentrations, and intracellular TFV diphosphate (TFV-DP) concentrations are poorly understood. Our objective was to characterize TFV and TFV-DP relationships. Data were pooled from two studies in HIV-infected persons (n = 55) on stable antiretroviral therapy. TFV and TFV-DP were measured with validated liquid chromatography/tandem mass spectrometry (LC/MS/MS) methods. Nonlinear mixed effects modeling (NONMEM 7) was used to develop the population model and explore the influence of covariates on TFV. A sequential analysis approach was utilized. A two-compartment model with first-order absorption best described TFV PK (FOCEI). An indirect stimulation of response model best described TFV-DP, where formation of TFV-DP was driven by plasma TFV concentration. Final plasma population estimates were as follows: absorption rate constant, 1.03 h(-1); apparent clearance (CL/F), 42 liters/h (33.5% interindividual variability [IIV]); intercompartment clearance, 181 liters/h; apparent central distribution volume (Vc/F), 273 liters (64.8% IIV); and apparent peripheral distribution volume (Vp/F), 440 liters (46.5% IIV). Creatinine clearance was the most significant covariate on CL/F and Vc/F. The correlation between CL/F and Vc/F was 0.553. The indirect response model for TFV-DP resulted in estimates of the maximal intracellular concentration (E(max)), the TFV concentration producing 50% of E(max) (EC(50)), and the intracellular elimination rate constant (k(out)) of 300 fmol/10(6) cells (82% IIV), 100 ng/ml (106% IIV), and 0.008 h(-1), respectively. The estimated k(out) gave an 87-h TFV-DP half-life. A predictive check assessment indicated satisfactory model performance. This model links formation of TFV-DP with plasma TFV concentrations and should facilitate more informed investigations of TFV clinical pharmacology.

PKreport: report generation for checking population pharmacokinetic model assumptions

BACKGROUND

Graphics play an important and unique role in population pharmacokinetic (PopPK) model building by exploring hidden structure among data before modeling, evaluating model fit, and validating results after modeling.

RESULTS

The work described in this paper is about a new R package called PKreport, which is able to generate a collection of plots and statistics for testing model assumptions, visualizing data and diagnosing models. The metric system is utilized as the currency for communicating between data sets and the package to generate special-purpose plots. It provides ways to match output from diverse software such as NONMEM, Monolix, R nlme package, etc. The package is implemented with S4 class hierarchy, and offers an efficient way to access the output from NONMEM 7. The final reports take advantage of the web browser as user interface to manage and visualize plots.

CONCLUSIONS

PKreport provides 1) a flexible and efficient R class to store and retrieve NONMEM 7 output, 2) automate plots for users to visualize data and models, 3) automatically generated R scripts that are used to create the plots; 4) an archive-oriented management tool for users to store, retrieve and modify figures, 5) high-quality graphs based on the R packages, lattice and ggplot2. The general architecture, running environment and statistical methods can be readily extended with R class hierarchy. PKreport is free to download at http://cran.r-project.org/web/packages/PKreport/index.html.

The role of population PK-PD modelling in paediatric clinical research

Children differ from adults in their response to drugs. While this may be the result of changes in dose exposure (pharmacokinetics [PK]) and/or exposure response (pharmacodynamics [PD]) relationships, the magnitude of these changes may not be solely reflected by differences in body weight. As a consequence, dosing recommendations empirically derived from adults dosing regimens using linear extrapolations based on body weight, can result in therapeutic failure, occurrence of adverse effect or even fatalities. In order to define rational, patient-tailored dosing schemes, population PK-PD studies in children are needed. For the analysis of the data, population modelling using non-linear mixed effect modelling is the preferred tool since this approach allows for the analysis of sparse and unbalanced datasets. Additionally, it permits the exploration of the influence of different covariates such as body weight and age to explain the variability in drug response. Finally, using this approach, these PK-PD studies can be designed in the most efficient manner in order to obtain the maximum information on the PK-PD parameters with the highest precision. Once a population PK-PD model is developed, internal and external validations should be performed. If the model performs well in these validation procedures, model simulations can be used to define a dosing regimen, which in turn needs to be tested and challenged in a prospective clinical trial. This methodology will improve the efficacy/safety balance of dosing guidelines, which will be of benefit to the individual child.

Interrelations between plasma caffeine concentrations and neurobehavioural effects in healthy volunteers: model analysis using NONMEM

The objective was to develop a population pharmacokinetic-pharmacodynamic model of caffeine's psychomotor effects in healthy, non-habitual users of caffeine. Twenty Chinese males each received a single dose of 250 mg of caffeine orally. Plasma concentrations of caffeine were determined at various times within 24 h after dosing. The subjects' psychomotor performance was evaluated before and at various times after dosing by a test battery consisting of oculomotor assessment (saccadic velocity) as well as the computerised Swedish Performance Evaluation System. Nonlinear mixed-effects modelling to analyse the pharmacokinetic-pharmacodynamic relationships was performed using NONMEM. Model robustness was assessed by a nonparametric bootstrap. The results showed that caffeine caused significant improvements in psychomotor functioning. The time course of these effects was best described by pharmacokinetic/pharmacodynamic models involving an effect compartment. The transfer half-lives between plasma and effect site for different domains of psychomotor functioning were in the range 24.8-49.5 min. Evaluation of the final models showed close agreement between pairs of bootstrapped and final model parameter estimates (all differences<10%). These results provided the first suggestive evidence that caffeine effects on psychomotor performance occur after some time delay relative to changes in plasma caffeine concentration. The models for the neurobehavioural tests provided similar transfer half-lives between plasma and effect site.

Population pharmacokinetics of enrofloxacin and its metabolite ciprofloxacin in chicken based on retrospective data, incorporating first-pass metabolism

A population pharmacokinetic (PPK) model for enrofloxacin and its metabolite ciprofloxacin in chicken based on retrospective data was developed. Plasma concentrations of enrofloxacin and its metabolite ciprofloxacin were determined in blood samples from chicken administered either enrofloxacin via oral and intravenous routes or ciprofloxacin via intravenous injection. The disposition of enrofloxacin and ciprofloxacin was described simultaneously by an integrated mathematic model. Two compartments were used to describe the enrofloxacin and ciprofloxacin disposition profiles. The formation of ciprofloxacin was through the central compartment of enrofloxacin. The integrated model was estimated with nonlinear mixed effects model (NONMEM). The total clearance of enrofloxacin (CLEN) and ciprofloxacin (CLCP) was 0.613 L/h and 1.15 L/h, respectively. Correlation between CLEN, the central compartment volume of distribution for enrofloxacin (V2) and CLCP was estimated. After intravenous administration of enrofloxacin, the transformation rate of enrofloxacin to ciprofloxacin was 0.429 L/h. The bioavailability factor after oral administration was 0.926, and 12.6% of enrofloxacin after oral administration was transformed to ciprofloxacin via first-pass effect. Pharmacodynamic (PD) evaluation was performed using area under concentration time curve of active moiety from 0 to 24 h and MIC collected from literature. This study is the first one to use PPK method to investigate parent drug and its metabolite disposition and PDs using an integrated model in veterinary medicine.

Measuring the glomerular filtration rate in obese individuals without overt kidney disease

BACKGROUND

Identifying methods to accurately measure the glomerular filtration rate (GFR) in obese individuals without kidney overt kidney disease is necessary to understanding the pathophysiology and natural history of obesity-related kidney disease.

METHODS

Using a cross-sectional design, iohexol clearance and disposition was measured, an optimal sampling schedule was identified, and the reliability of GFR-estimating methods was described in 29 obese individuals with normal serum creatinine levels. Iohexol disposition was measured using population pharmacokinetics. The agreement with GFR-estimating equations was assessed by intraclass coefficients.

RESULTS

Mean age was 44 ± 10 years, body mass index 45 ± 10, creatinine 0.7 ± 0.2 mg/dl (62 ± 18 μmol/l) , and cystatin C 0.83 ± 0.18 mg/dl (8.3 ± 1.8 mg/l). Iohexol disposition fit a two-compartment model and 5 sampling windows were identified over a 4-hour period to optimize model accuracy and minimize blood draws. Precision was not compromised with this sampling design. Neither creatinine nor cystatin C were linearly correlated with the measured GFR though cystatin C was independent of body composition. Agreement was fair to poor between the measured GFR and GFR-estimating equations.

CONCLUSION

This study offers a rigorous method to study obesity-related kidney disease and improve upon suboptimal GFR-estimating methods.

Retrospective population pharmacokinetic/pharmacodynamic analysis of pyridostigmine, a cholinesterase inhibitor, in Chinese males

Objectives

We have characterised the population pharmacokinetics-pharmacodynamics of pyridostigmine given as pyridostigmine bromide.

Methods

Over three days 50 healthy Chinese male subjects each received seven doses of 30 mg pyridostigmine bromide orally (3 times 10 mg every 8 h). Plasma concentrations of pyridostigmine and red blood cell acetylcholinesterase (AChE) activity were determined at various times within the eight hours after the first and the seventh doses. The resulting pharmacokinetic data were fitted to a single compartment open model with first-order absorption and elimination. The pharmacodynamics were modelled using an inhibitory Emax model. The potential influence of demographic and biological covariates on the model parameters was investigated. Nonlinear mixed effects modelling was performed using NONMEM.

Key findings

The apparent clearance and volume of distribution as well as absorption rate constant of plasma pyridostigmine were estimated to be 136 1/h, 130 1 and 0.68 1/h, respectively. The maximum red blood cell AChE activity decrease (Emax) and plasma pyridostigmine concentration producing 50% of this reduction (EC50) were estimated to be 9.32 AChE units per gram haemoglobin and 51.9 ng/ml, respectively. None of the tested covariates were found to be correlated with any of the model parameters. Dosing simulations suggested that 30 mg repeated every six hours might be needed to achieve steady-state trough percentage inhibition above the recommended 10% in healthy Chinese males.

Conclusions

The pharmacokinetics and the effects of pyridostigmine on red blood cell AChE activity were described using a mixed effects model. For Chinese males, the dosing interval may have been shorter than that recommended for the Caucasian population. Additional studies are needed to confirm these findings.

Population pharmacokinetics of acyclovir in children and young people with malignancy after administration of intravenous acyclovir or oral valacyclovir

Acyclovir is effective in the prevention and treatment of herpes simplex virus (HSV) and varicella-zoster virus (VZV) infections. The aim of this study was to characterize the population pharmacokinetics of acyclovir observed following treatment with intravenous acyclovir and oral valacyclovir (valaciclovir) in young people with malignancy. Plasma acyclovir concentration-time data were collected from 43 patients (age range, 9 months to 20 years) who had been given multiple doses of acyclovir (5 mg/kg of body weight) and/or valacyclovir (10 mg/kg). Nonlinear mixed-effect modeling was employed to analyze acyclovir population pharmacokinetics and identify influential covariates. Simulations (n = 1,000) were conducted to explore the ability of the current doses to maintain acyclovir concentrations above the recommended 50% inhibitory concentration for HSV or VZV (0.56 mg/liter or 1.125 mg/liter, respectively) for more than 12 h. A one-compartment pharmacokinetic model with first-order elimination best described the acyclovir concentration-time data. The population mean estimates for clearance (CL), volume of distribution (V), absorption rate (k(a)), and bioavailability (F) were 3.55 liters/h, 7.36 liters, 0.63 h(-1), and 0.60, respectively. Inclusion of body weight and estimated creatinine CL (CL(CR)) in the final model reduced the interindividual variabilities in CL and V from 61% to 24% and from 75% to 36%, respectively. Simulations revealed that with the use of the current doses, maximal efficacy can be achieved in over 45% of patients weighing 25 to 50 kg and with CL(CR) levels of 2.0 to 4.0 liters/h/m(2), but only in a much smaller proportion of patients, with low weights (10 kg) and high CL(CR)s (5.5 liters/h/m(2)), suggesting that higher doses are required for this subgroup. This validated population pharmacokinetic model for acyclovir may be used to develop dosing guidelines for safe and effective antiviral therapy in young people with malignancy.

Nonnucleoside reverse transcriptase inhibitor pharmacokinetics in a large unselected cohort of HIV-infected women

BACKGROUND

Small intensive pharmacokinetic (PK) studies of medications in early-phase trials cannot identify the range of factors that influence drug exposure in heterogenous populations. We performed PK studies in large numbers of HIV-infected women on nonnucleoside reverse transcriptase inhibitors (NNRTIs) under conditions of actual use to assess patient characteristics that influence exposure and evaluated the relationship between exposure and response.

METHODS

Two hundred twenty-five women on NNRTI-based antiretroviral regimens from the Women's Interagency HIV Study were enrolled into 12-hour or 24-hour PK studies. Extensive demographic, laboratory, and medication covariate data were collected before and during the visit to be used in multivariate models. Total NNRTI drug exposure was estimated by area under the concentration-time curves.

RESULTS

Hepatic inflammation and renal insufficiency were independently associated with increased nevirapine exposure in multivariate analysis: crack cocaine, high fat diets, and amenorrhea were associated with decreased levels (n = 106). Higher efavirenz exposure was seen with increased transaminase, albumin levels, and orange juice consumption; tenofovir use, increased weight, being African American, and amenorrhea were associated with decreased exposure (n = 119). With every 10-fold increase in nevirapine or efavirenz exposure, participants were 3.3 and 3.6 times likely to exhibit virologic suppression, respectively. Patients with higher drug exposure were also more likely to report side effects on therapy.

CONCLUSIONS

Our study identifies and quantitates previously unrecognized factors modifying NNRTI exposure in the "real-world" setting. Comprehensive PK studies in representative populations are feasible and may ultimately lead to dose optimization strategies in patients at risk for failure or adverse events.

Population pharmacokinetics of high-dose methotrexate after intravenous administration in pediatric patients with osteosarcoma

The goal of this study was to establish the population pharmacokinetics (PK) of high-dose methotrexate (HD-MTX) treatment in children with osteosarcoma and to explore the influence of patient covariates and between-occasion variability on drug disposition. Patient covariates and concentration-time data were collected. PK data analysis from 209 HD-MTX cycles from 14 patients was performed using the population approach (NONMEM V). Internal and external validations were performed to confirm the model. PK of methotrexate was best described by a 2-compartment open PK model with first-order elimination from the central compartment. Between-subject variability (BSV) was included in total plasma clearance (CL) and in central compartment distribution volume (V1) [coefficient of variation (CV) 11.9% and 8.9%, respectively]. The CV of BSV in the residual error was 25.5%. Between-occasion variability was only retained for CL (CV 8.2%). RE consisted of a proportional error of 41.6%. Age and body weight in CL and body weight in V1 were identified as the appropriate covariates. The final estimates of total CL and V1 were given by the equations CL = 88.5.(AGE/15) + 27.4 x (WGT/50) L/d and V1 = 11.0 + 5.6 x (WGT/50) L, respectively. Internal validation results showed that the 95% confidence interval covered all the observed MTX concentrations. Mean bias and precision of the individual predicted concentrations, calculated in a validation dataset, resulted in -1.36% and 19.71%, respectively. A population PK model was developed for HD-MTX in children with osteosarcoma. Validation studies confirmed the suitability of the model for further dose individualization by means of a Bayesian approach.

Overview of model-building strategies in population PK/PD analyses: 2002-2004 literature survey

AIMS

A descriptive survey of published population pharmacokinetic and/or pharmacodynamic (PK/PD) analyses from 2002 to 2004 was conducted and an evaluation made of how model building was performed and reported.

METHODS

We selected 324 articles in Pubmed using defined keywords. A data abstraction form (DAF) was then built comprising two parts: general characteristics including article identification, context of the analysis, description of clinical studies from which the data arose, and model building, including description of the processes of modelling. The papers were examined by two readers, who extracted the relevant information and transmitted it directly to a MySQL database, from which descriptive statistical analysis was performed.

RESULTS

Most published papers concerned patients with severe pathology and therapeutic classes suffering from narrow therapeutic index and/or high PK/PD variability. Most of the time, modelling was performed for descriptive purposes, with rich rather than sparse data and using NONMEM software. PK and PD models were rarely complex (one or two compartments for PK; E(max) for PD models). Covariate testing was frequently performed and essentially based on the likelihood ratio test. Based on a minimal list of items that should systematically be found in a population PK-PD analysis, it was found that only 39% and 8.5% of the PK and PD analyses, respectively, published from 2002 to 2004 provided sufficient detail to support the model-building methodology.

CONCLUSIONS

This survey allowed an efficient description of recent published population analyses, but also revealed deficiencies in reporting information on model building.

Population pharmacokinetics of tafenoquine during malaria prophylaxis in healthy subjects

The population pharmacokinetics of tafenoquine were studied in Australian soldiers taking tafenoquine for malarial prophylaxis. The subjects (476 males and 14 females) received a loading dose of 200 mg tafenoquine base daily for 3 days, followed by a weekly dose of 200 mg tafenoquine for 6 months. Blood samples were collected from each subject after the last loading dose and then at weeks 4, 8, and 16. Plasma tafenoquine concentrations were determined by liquid chromatography-tandem mass spectrometry. Population modeling was performed with NONMEM, using a one-compartment model. Typical values of the first-order absorption rate constant (K(a)), clearance (CL/F), and volume of distribution (V/F) were 0.243 h(-1), 0.056 liters/h/kg, and 23.7 liters/kg, respectively. The intersubject variability (coefficient of variation) in CL/F and V/F was 18% and 22%, respectively. The interoccasion variability in CL/F was 18%, and the mean elimination half-life was 12.7 days. A positive linear association between weight and both CL/F and V/F was found, but this had insufficient impact to warrant dosage adjustments. Model robustness was assessed by a nonparametric bootstrap (200 samples). A degenerate visual predictive check indicated that the raw data mirrored the postdose concentration-time profiles simulated (n = 1,000) from the final model. Individual pharmacokinetic estimates for tafenoquine did not predict the prophylactic outcome with the drug for four subjects who relapsed with Plasmodium vivax malaria, as they had similar pharmacokinetics to those who were free of malaria infection. No obvious pattern existed between the plasma tafenoquine concentration and the pharmacokinetic parameter values for subjects with and without drug-associated moderate or severe adverse events. This validated population pharmacokinetic model satisfactorily describes the disposition and variability of tafenoquine used for long-term malaria prophylaxis in a large cohort of soldiers on military deployment.

Estimation of pharmacokinetic parameters based on the patient-adjusted population data

Population pharmacokinetic data, adjusted for patient characteristics, are recommended for the design of initial dosage regimens of some therapeutically monitored drugs in patients for whom patient-specific data are not available. However, despite widespread use by clinicians such as pharmacists, a clear understanding of the principles of population pharmacokinetics, including data collection and analysis and its limitations, is often lacking. This article describes the 2 main methods of obtaining population kinetic data, namely the two-stage method and nonlinear mixed effect model, and their applications to the pharmacokinetic-based design of dosage regimens. Additionally, some numerical examples are provided to assist the reader in understanding the material. The author uses these tools in a pharmacokinetics course taught to PharmD students.