Purpose: To develop a tumor growth inhibition model for adult diffuse low-grade gliomas (LGG) able to describe tumor size evolution in patients treated with chemotherapy or radiotherapy.Experimental Design: Using longitudinal mean tumor diameter (MTD) data from 21 patients treated with first-line procarbazine, 1-(2-chloroethyl)-3-cyclohexyl-l-nitrosourea, and vincristine (PCV) chemotherapy, we formulated a model consisting of a system of differential equations, incorporating tumorspecific and treatment-related parameters that reflect the response of proliferative and quiescent tumor tissue to treatment. The model was then applied to the analysis of longitudinal tumor size data in 24 patients treated with first-line temozolomide (TMZ) chemotherapy and in 25 patients treated with first-line radiotherapy.Results: The model successfully described the MTD dynamics of LGG before, during, and after PCV chemotherapy. Using the same model structure, we were also able to successfully describe the MTD dynamics in LGG patients treated with TMZ chemotherapy or radiotherapy. Tumor-specific parameters were found to be consistent across the three treatment modalities. The model is robust to sensitivity analysis, and preliminary results suggest that it can predict treatment response on the basis of pretreatment tumor size data.Conclusions: Using MTD data, we propose a tumor growth inhibition model able to describe LGG tumor size evolution in patients treated with chemotherapy or radiotherapy. In the future, this model might be used to predict treatment efficacy in LGG patients and could constitute a rational tool to conceive more effective chemotherapy schedules.

The pharmacokinetics and pharmacodynamics of drugs are significantly altered in the burn patient, and the burn patient population shows wide inter- and intraindividual variation in drug handling. Burn injury evolves in two phases. The first phase corresponds to the burn shock, which occurs during the first 48 hours after thermal injury. In this phase, hypovolaemia, oedema, hypoalbuminaemia and a low glomerular filtration rate are observed, which result in a slower rate of drug distribution and lower renal clearance. The second phase (beyond 48 hours after injury) is a hyperdynamic state with high blood flow in the kidneys and liver, an increased alpha1-acid-glycoprotein level and loss of the drug with exudate leakage. As a result, protein binding, drug distribution and clearance may be altered. Because of the alteration in these variables, wide intraindividual variation of pharmacokinetic parameters occurs depending upon the time since thermal injury and fluid resuscitation. Interindividual variations may be correlated with the percentage of the body surface area that is burnt, creatinine clearance, albuminaemia or the alpha1-acid-glycoprotein level. A number of important variations in pharmacodynamic parameters have been described, but their mechanisms are poorly understood. From a practical point of view, for the subpopulation of burn patients who eliminate drugs extremely rapidly, higher doses and/or shorter dosing intervals are required to avoid treatment inefficacy. Drug concentration measurements help to take into account interindividual variability. However, adaptation of doses based on Bayesian methods is frequently not possible because the distribution of pharmacokinetic parameters is poorly characterized in this population. Methods based only on individual data or on a surrogate marker for efficacy may be used to optimize the dosing regimen in this population.

The pharmacokinetics and pharmacodynamics of drugs are different in adult and paediatric populations, the latter being particularly heterogeneous. These differences in pharmacokinetics and pharmacodynamics justify specific studies but raise a number of ethical and practical issues. The main practical difficulties to circumvent while performing clinical studies in children are the invasiveness of the procedures and the obstacles to patient recruitment. The invasiveness related to pain/anxiety and blood loss precludes the performance of classical pharmacokinetic studies in children in many instances, particularly in neonates and infants. Population approaches, which rely on pharmacokinetic-pharmacodynamic modelling, are particularly appealing in paediatric populations because these models can cope with sparse data. The relevance of population approaches to investigation of the dose-concentration-effect relationships and to qualitative/quantitative assessment of factors that may explain interindividual variability has already been emphasized. The aims of this review are to summarize the currently available literature on population pharmacokinetic-pharmacodynamic studies in children and to discuss a number of recent methodological developments that may facilitate the evaluation of drugs in this population by alleviating invasiveness and, subsequently, facilitating recruitment of patients. The present survey confirms that population approaches in paediatrics have already reached a large audience and that they are mostly used for analysis of sparse data. However, pharmacokinetic-pharmacodynamic studies in children are still scarce. New classes of models may extend the scope of the use of population models in paediatrics. Kinetic-pharmacodynamic models, where use of the term 'kinetic' rather than 'pharmacokinetic' emphasizes the absence of pharmacokinetic data, are indirect models where the (unobserved) drug kinetics are described by a single compartment involving a single rate constant. These models, which alleviate the need for blood samples used for the measurement of drug concentration, may be very useful in paediatric studies. Physiological and physiopathological models also have potential applications but require further development. Because the number of measurements in a single individual needs to be limited in children, it is crucial to optimize the design of the experiment in order to avoid inaccurate and unreliable results. In this review, formal optimization and simulation to evaluate a design are presented, and specific problems raised by the application of these techniques in paediatrics are addressed. Finally, the related technique of clinical trial simulation and its applications are presented and discussed.

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After completing this course, the reader will be able to:1. Describe the profile of severe toxicities in patients treated with sorafenib.2. Summarize the pharmacokinetics of sorafenib-induced toxicities.3. Identify predictive factors for early and delayed toxicities in patients treated with sorafenib.This article is available for continuing medical education credit at CME.TheOncologist.com. CME CME ABSTRACT Background. Sorafenib displays major interpatient pharmacokinetic variability. It is unknown whether the pharmacokinetics of sorafenib influence its toxicity.Methods. We analyzed the severity and kinetics of sorafenib-induced toxicities in unselected consecutive patients with cancer, as well as their relationship with biolog-

BackgroundIdentifying predictive biomarkers of drug response is of key importance to improve therapy management and drug selection in cancer therapy. To date, the influence of drug exposure and pharmacogenetic variants on sorafenib-induced toxicity remains poorly documented. The aim of this pharmacokinetic/pharmacodynamic (PK/PD) study was to investigate the relationship between early toxicity and drug exposure or pharmacogenetic variants in unselected adult outpatients treated with single-agent sorafenib for advanced solid tumors.MethodsToxicity was recorded in 54 patients on days 15 and 30 after treatment initiation and sorafenib exposure was assessed in 51 patients. The influence of polymorphisms in CYP3A5, UGT1A9, ABCB1 and ABCG2 was examined in relation to sorafenib exposure and toxicity. Clinical characteristics, drug exposure and pharmacogenetic variants were tested univariately for association with toxicities. Candidate variables with p<0.1 were analyzed in a multivariate analysis.ResultsGender was the sole parameter independently associated with sorafenib exposure (p = 0.0008). Multivariate analysis showed that increased cumulated sorafenib (AUCcum) was independently associated with any grade ≥3 toxicity (p = 0.037); UGT1A9 polymorphism (rs17868320) with grade ≥2 diarrhea (p = 0.015) and female gender with grade ≥2 hand-foot skin reaction (p = 0.018). Using ROC curve, the threshold AUCcum value of 3,161 mg/L.h was associated with the highest risk to develop any grade ≥3 toxicity (p = 0.018).ConclusionIn this preliminary study, increased cumulated drug exposure and UGT1A9 polymorphism (rs17868320) identified patients at high risk for early sorafenib-induced severe toxicity. Further PK/PD studies on larger population are warranted to confirm these preliminary results.

BackgroundHydroxyurea (HU) is the first approved pharmacological treatment of sickle cell anemia (SCA). The objectives of this study were to develop population pharmacokinetic(PK)-pharmacodynamic(PD) models for HU in order to characterize the exposure-efficacy relationships and their variability, compare two dosing regimens by simulations and develop some recommendations for monitoring the treatment.MethodsThe models were built using population modelling software NONMEM VII based on data from two clinical studies of SCA adult patients receiving 500-2000 mg of HU once daily. Fetal hemoglobin percentage (HbF%) and mean corpuscular volume (MCV) were used as biomarkers for response. A sequential modelling approach was applied. Models were evaluated using simulation-based techniques. Comparisons of two dosing regimens were performed by simulating 10000 patients in each arm during 12 months.ResultsThe PK profiles were described by a bicompartmental model. The median (and interindividual coefficient of variation (CV)) of clearance was 11.6 L/h (30%), the central volume was 45.3 L (35%). PK steady-state was reached in about 35 days. For a given dosing regimen, HU exposure varied approximately fivefold among patients. The dynamics of HbF% and MCV were described by turnover models with inhibition of elimination of response. In the studied range of drug exposures, the effect of HU on HbF% was at its maximum (median Imax was 0.57, CV was 27%); the effect on MCV was close to its maximum, with median value of 0.14 and CV of 49%. Simulations showed that 95% of the steady-state levels of HbF% and MCV need 26 months and 3 months to be reached, respectively. The CV of the steady-state value of HbF% was about 7 times larger than that of MCV. Simulations with two different dosing regimens showed that continuous dosing led to a stronger HbF% increase in some patients.ConclusionsThe high variability of response to HU was related in part to pharmacokinetics and to pharmacodynamics. The steady-state value of MCV at month 3 is not predictive of the HbF% value at month 26. Hence, HbF% level may be a better biomarker for monitoring HU treatment. Continuous dosing might be more advantageous in terms of HbF% for patients who have a strong response to HU.Trial RegistrationThe clinical studies whose data are analysed and reported in this work were not required to be registered in France at their time. Both studies were approved by local ethics committees (of Mondor Hospital and of Kremlin-Bicetre Hospital) and written informed consent was obtained from each patient.