An Innovative Pharmacometric Approach for the Simultaneous Analysis of Frequency, Duration and Severity of Migraine Events.

PURPOSE: To explore the use of a multistate repeated, time-to-categorical event model describing the frequency, severity and duration of migraines. METHODS: Subject level data from patients in placebo arms from two efficacy trials for migraine-preventive treatments were used. Models were developed using NONMEM 7.3. A survival model was combined with an ordered categorical model to form the repeated-time-to-start of categorical migraine event model, which simultaneously described the time-to-start of migraines and the severity of the starting migraine event. This was linked to a repeated-time-to-end of migraine event model with different hazard functions depending on the severity of the ongoing migraine event. Model performance was internally and externally qualified. RESULTS: The successfully qualified model showed that patients responding to placebo had a reduction in migraine incidence rate, and a decreased proportion of severe migraines. There was an increase in moderate migraine duration, an increased proportion of mild migraines and a reduction in proportion of severe migraines. Age was related to migraine duration. CONCLUSIONS: The model represents an innovative framework for clinical trial modeling and simulation, and successfully describes placebo effect in migraine prevention. This approach can be adapted to investigate exposure-response relationship of drugs and can also be implemented in other therapeutic areas where the rate, duration and severity of disease episodes are relevant to trial outcomes.

Impact of the US FDA "Biopharmaceutics Classification System" (BCS) Guidance on Global Drug Development.

The FDA guidance on application of the biopharmaceutics classification system (BCS) for waiver of in vivo bioequivalence (BE) studies was issued in August 2000. Since then, this guidance has created worldwide interest among biopharmaceutical scientists in regulatory agencies, academia, and industry toward its implementation and further expansion. This article describes how the review implementation of this guidance was undertaken at the FDA and results of these efforts over last dozen years or so across the new, and the generic, drug domains are provided. Results show that greater than 160 applications were approved, or tentatively approved, based on the BCS approach across multiple therapeutic areas; an additional significant finding was that at least 50% of these approvals were in the central nervous system (CNS) area. These findings indicate a robust utilization of the BCS approach toward reducing unnecessary in vivo BE studies and speeding up availability of high quality pharmaceutical products. The article concludes with a look at the adoption of this framework by regulatory and health policy organizations across the globe, and FDA's current thinking on areas of improvement of this guidance.

Exposure-response analyses of blood pressure and heart rate changes for methylphenidate in healthy adults.

The aim of the study was to evaluate the exposure-response (E-R) relationships of blood pressure (BP) and heart rate (HR) changes in healthy adults taking methylphenidate (MPH). Intensive time profiles of BP and HR from healthy adults in placebo and MPH treatment arms of seven clinical trials from the FDA internal database were utilized for this analysis. The analysis model contains a circadian component for placebo effect and an E-R component to describe drug effect. Internal validation was performed using goodness-of-fit plots and visual predictive check. A meta-database based on a systemic literature search was constructed and used for external validation of the developed models. We found that circadian models could quantify the time profiles of BP/HR in placebo arms. Linear models could describe the correlations between MPH concentrations, and BP/HR changes. The BP and HR changes were highly dependent on the shapes of MPH pharmacokinetic (PK) profiles without an apparent time delay. MPH has the greatest effect on HR, followed by systolic BP, and diastolic BP. Internal validation revealed that the developed models could adequately describe the circadian rhythms of HR and BP in placebo arms and the E-R relationships of MPH. External validation showed the models had good predictive capability of the literature data. In conclusion, the developed models adequately characterized the circadian rhythm and the MPH induced effects on BP and HR. The changes in BP and HR were highly correlated with MPH blood levels with no apparent delay. The time courses of BP and HR are similar to the MPH PK profiles. As a result, the immediate-release formulation may yield larger maximum BP and HR effect than the extended-release formulation under similar dose.

Pediatric Extrapolation Approach for U.S. Food and Drug Administration Approval of Brexpiprazole in Patients Aged 13 to 17 Years with Schizophrenia.

A pharmacokinetic (PK) bridging approach was successfully employed to support the dosing regimen and approval of brexpiprazole in pediatric patients aged 13-17 years with schizophrenia. Brexpiprazole was approved in 2015 for the treatment of schizophrenia and the adjunctive treatment of major depressive disorder in adults based on efficacy and safety data from clinical trials. On January 13, 2020, the US Food and Drug Administration issued a general advice letter to sponsors highlighting the acceptance of efficacy extrapolation of certain atypical antipsychotics from adult patients to pediatric patients considering the similarity in disease and exposure-response relationships. Brexpiprazole is the first atypical antipsychotic approved in pediatrics using this approach. The PK data available from pediatric patients aged 13-17 years have shown high variability due to the limited number of PK evaluable subjects, which limits a robust estimation of differences between adult and pediatric patients. The PK model-based approach was thus utilized to evaluate the appropriateness of the dosing regimen by comparing PK exposures in pediatric patients aged 13-17 years with exposures achieved in adults at the approved doses. In addition to exposure matching, safety data from a long-term open-label clinical study in pediatric patients informed the safety profile in pediatric patients. This report illustrates the potential of leveraging previously collected efficacy, safety, and PK data in adult patients to make a regulatory decision in pediatric patients for the indication of schizophrenia.

Clinical Pharmacology in Drug Development for Rare Diseases in Neurology: Contributions and Opportunities.

Several challenges are associated with rare disease drug development in neurology. In this article, we summarize the US Food and Drug Administration's experience with clinical drug development for rare neurological diseases and discuss clinical pharmacology's critical contributions to drug development for rare diseases. We used publicly available information to identify and screen drug products approved for rare neurological indications between 1983 and 2019. We highlighted cases in which clinical pharmacology contributed to the evidence of drug efficacy, dose selection for pivotal clinical trials, dose optimization based on intrinsic and extrinsic factors, pharmacokinetic bridging for formulations, and efficacy bridging across different racial groups. Fifty-one approved drug products were identified since the introduction of the Orphan Drug Act in 1983. Interestingly, the number of approvals in the last few years increased significantly, probably due to advances in genomic research and targeted drug modalities. Evaluation of dose selection in patient populations showed that in 52% of cases, the sponsors did not evaluate efficacy for more than one or two dose levels throughout the development program. Clinical pharmacology studies to evaluate the effect of intrinsic or extrinsic factors were adequately characterized in most of the applications. With the expansion of model informed drug development applications, (e.g., quantitative systems pharmacology and deep learning neural network models), the role and impact of clinical pharmacology is expected to grow exponentially in the next decade and enhance the development of novel treatment modalities for neurological rare diseases.

Extrapolation of Efficacy from Adults to Pediatric Patients of Drugs for Treatment of Partial Onset Seizures: A Regulatory Perspective.

The US Food and Drug Administration (FDA) has concluded that the efficacy of drugs approved for the treatment of partial onset seizures (POS) in adults can be extrapolated to pediatric patients 1 month of age and above and that independent efficacy trials in this pediatric population are no longer needed. This paper focuses on the dosing, pharmacokinetic (PK), exposure-response, and clinical information that were leveraged from the approved drugs for the treatment of POS to conduct analyses that supported extrapolation of efficacy in pediatric patients. Clinical data from trials for eight drugs (levetiracetam, oxcarbazepine, topiramate, lamotrigine, gabapentin, perampanel, tiagabine, and vigabatrin) approved in both adults and pediatric patients for the treatment of POS were analyzed. Comparisons of exposures at approved doses, placebo response, and model-based exposure-response relationships were performed. Based on disease similarity, similar response to intervention, and similar exposure-response relationships in adults and pediatric patients, it was concluded that extrapolation of efficacy in pediatric patients aged 1 month and above is acceptable. PK analysis to determine pediatric dose and regimens that provide drug exposure similar to that known to be effective in adult patients with POS will be required, along with long-term open-label safety data in pediatric patients.

Shortened Positive and Negative Symptom Scale as an Alternate Clinical Endpoint for Acute Schizophrenia Trials: Analysis from the US Food & Drug Administration.

Objective: To evaluate the performance of the individual Positive and Negative Symptom Scale (PANSS) items, and to assess the feasibility of using a shortened version of the PANSS as an alternative regulatory endpoint for evaluating the efficacy of drugs to treat schizophrenia. Design: Data from 32 randomized, placebo-controlled, multiregional trials from eight atypical antipsychotic programs (N=14,219) submitted to the US Food and Drug Administration were used in the analyses. Item response theory analysis on baseline PANSS item scores was used to identify the best performing items of the PANSS to derive the shortened, or modified, PANSS (mPANSS). Concordance rates of mPANSS total with the PANSS total trial results at week 6 were examined, and implications of using mPANSS on trial sample size evaluated. Results: Five of the positive items, six of the negative items, and eight of the general items were assessed as sensitive to describe the underlying symptom severity and comprise mPANSS. The overall concordance rate between mPANSS and total PANSS results at week 6 was 97.6%. Using mPANSS resulted in a 32% reduction in samples size relative to using total PANSS. Conclusions: Based on this research, mPANSS may be considered a potential alternative clinical endpoint for acute schizophrenia trials. However, it will need psychometric validation before it can be fully implemented in clinical trials in place of total PANSS. If such implementation occurs, the development of new drugs for schizophrenia, a public health imperative, may be considerably improved.

Extrapolation of Efficacy and Dose Selection in Pediatrics: A Case Example of Atypical Antipsychotics in Adolescents With Schizophrenia and Bipolar I Disorder.

Pediatric labeling information for novel atypical antipsychotics can be significantly delayed as the result of time lag between initial drug approval in adults and the completion of pediatric clinical trials. This delay can lead health care providers to rely on limited evidence-based literature to make critical therapeutic decisions for pediatric patients. Effective and scientifically justified dosing recommendations are needed to improve treatment outcomes in pediatric patients with schizophrenia and bipolar I disorder. Extrapolation-based drug development strategies rely on leveraging prior data to reduce evidentiary requirements for newer data in establishing drug efficacy. On January 13, 2020, the US Food and Drug Administration (FDA) released a general advice letter to sponsors highlighting the acceptance of extrapolating efficacy of atypical antipsychotics to pediatric patients. This review provides insight into the FDA's justification for extrapolating efficacy from adult to pediatric patients and provides a rationale for dose selection in pediatric patients with schizophrenia and bipolar I disorder.

Use of Partial Area Under the Curve in Bioavailability or Bioequivalence Assessments: A Regulatory Perspective.

Peak drug concentration (Cmax ) and total exposure, such as area under the concentration-time curve (AUC) from time zero to infinity may be insufficient for assessing relative bioavailability (BA) or bioequivalence (BE) among two products in cases where rapid onset of action or controlled duration of effect is needed to ensure similar drug efficacy. Regulatory agencies have recommended the use of partial AUC (pAUC) as an additional exposure measure for relative BA or BE assessments. The pAUC metric describes pharmacokinetic profiles with the focus on quantification of exposures over specific time intervals to support the determination of relative BA or BE for these drug products in relation to respective reference products. The principles and rationales for using pAUCs are included in the US Food and Drug Administration (FDA)'s general BA or BE guidances. Specific pAUC recommendations are also reflected in product-specific guidances for generic drug development published by the FDA. Rationales for the use of pAUCs in relative BA or BE assessments are based on drug-specific and product-specific considerations. This white paper introduces the general framework, including rationales for pAUC recommendations, and provides an overview of the current status, challenges, and the FDA considerations on the use of pAUC for relative BA or BE assessments in the United States.

The Trend of Increasing Placebo Response and Decreasing Treatment Effect in Schizophrenia Trials Continues: An Update From the US Food and Drug Administration.

OBJECTIVE: Concerns of increasing placebo response and declining treatment effect in acute schizophrenia trials have been reported for new drug applications (NDAs) submitted to the US Food and Drug Administration (FDA) during an 18-year period from 1991 through January 2009 (ie, the pre-2009 period). Current exploratory analyses provide an update in the trends observed in placebo response, treatment effect, and dropout rates for NDAs submitted from February 2009 to 2015 (ie, the post-2009 period). DATA SOURCES: Clinical trial data from all acute schizophrenia trials that were submitted as part of NDAs to the US FDA during a 24-year period from 1991 to 2015. STUDY SELECTION: Aggregate trial-level efficacy data from multicenter, multiregional, randomized, placebo-controlled, 4- to 8-week, fixed- and flexible-dose trials in adult schizophrenia patients were compiled. There were 12 NDAs pre-2009 (32 trials, N = 11,567) and 3 NDAs post-2009 (14 trials, N = 6,434). DATA EXTRACTION: Baseline demographic and disease variables and scores on the Positive and Negative Syndrome Scale (PANSS) were summarized and compared for the two time periods (pre-2009 and post-2009). The primary efficacy measure was mean change from baseline to endpoint in total PANSS score obtained by last-observation-carried-forward analysis. Regional differences in placebo response and treatment effect were explored for the two time periods based on baseline patient characteristics, sample size, and dropout rates. RESULTS: Trials were predominantly multiregional (10/14; 71%) during the post-2009 period compared to the pre-2009 period (11/32; 34%). The overall trial success rates were 57% (8/14) and 78% (25/32) during the post-2009 and pre-2009 periods, respectively. Comparing the pre-2009 and post-2009 periods, the mean placebo response (change from baseline in PANSS score) increased from -6.4 to -10.5 and the mean treatment effect (drug response - placebo response) declined from -8.6 to -5.8 , with substantial differences observed especially in North American trials. In North American trials, placebo response increased from -4.3 (pre-2009) to -8.5 (post-2009), and treatment effect decreased from -9.0 (pre-2009) to -3.4 (post-2009). The difference in placebo response (pre- and post-2009: -10.0 and -11.3 ) and treatment effect (pre and post-2009: -8.1 and -6.4 ) in multiregional trials for the two time periods remained minimal. Baseline disease severity remained similar in the pre- and post-2009 time periods, with PANSS scores ranging between 85 and 100. Trials with higher mean baseline PANSS scores tended to show higher treatment effect irrespective of the time period and region. Post-2009, dropout rates were higher (55%) in North American trials compared with 33% in multiregional trials, similar to the pre-2009 trend. CONCLUSIONS: The continuing trend of increasing placebo responses and decreasing treatment effects in schizophrenia trials over the 24-year period does remain of great concern, especially with respect to North American trials. However, given the current global nature of drug development, close attention to trial conduct and reexamination of design elements for future trials may be warranted.

Association of End Point Definition and Randomized Clinical Trial Duration in Clinical Trials of Schizophrenia Medications.

Importance: Facilitating the development of safe and effective medications for schizophrenia is a public health imperative. Objectives: To evaluate the association of shortening randomized clinical trial (RCT) duration with the modification of the Positive and Negative Syndrome Scale (PANSS) for the design of RCTs of medications for schizophrenia and to offer perspective on an alternative regulatory pathway to the historically accepted trial duration and response assessment. Data Sources: A database was created consisting of clinical trial data from 32 placebo-controlled RCTs of 8 atypical antipsychotic drugs approved by the US Food and Drug Administration (FDA) between January 1, 2001, and December 31, 2015. The database included information on total and individual PANSS item ratings, demographic characteristics, disposition, and adverse events (AEs). Study Selection: All clinical trials submitted to 8 new drug applications of atypical antipsychotic drugs were selected. Data Extraction and Synthesis: Quality control checks were performed to ensure that the collected data were consistent with the reported results of each trial. Data were collected from March 15, 2015, to September 30, 2015. Data analysis was conducted from October 1, 2015, to June 20, 2016. Main Outcomes and Measures: The following analyses were performed: (1) longitudinal assessment of mean change from baseline in total PANSS score, (2) correlation analyses between change from baseline in total PANSS score at week 6 and earlier time points, (3) concordance analyses of outcomes across trials between week 6 and earlier time points using total PANSS and modified PANSS, and (4) analyses of time course of treatment-emergent AEs. Results: The final database contained data from 14 219 participants enrolled in 32 drug trials; 9805 of 14 219 participants (69.0%) were male and were either white (7183 [50.5%]) or black (4346 [30.6%]) individuals. The mean (SD) age during treatment was 38.9 (10.9) years, and the mean (SD) age at schizophrenia diagnosis was 25 (8.5) years. Statistically significant separation between treatment response and placebo response was observed after 1 week of treatment. The overall concordance rate across treatment groups steadily increased from week 1 to week 4 (68.0% for week 1, 74.0% for week 2, 83.0% for week 3, and 93.0% for week 4). Trends in AE occurrence were evident by week 1 and percentage of AEs were similar across weeks 3, 4, and 6. The overall concordance rate between change from baseline in the modified PANSS score and change from baseline in the total PANSS score was 93.0% (80 of 86 treatment groups) at week 4 and 97.7% (84 of 86 treatment groups) at week 6. Shortening the trial duration to 4 weeks increased the required sample size to 502 participants. Using the modified PANSS as the end point, the sample size for a 4-week trial was 402 participants and 296 participants for a 6-week trial. Conclusions and Relevance: Findings from this analysis suggest that there is the potential to streamline the design of schizophrenia drug clinical trials. Trial sponsors may consider incorporating these strategies and are encouraged to consult with the FDA early in the drug development process.

Application of Clinical Pharmacology Principles in Drug Development of Modified-Release Products: Leveraging Exposure-Response Information to Support Approval.

The development of modified-release (MR) drug products aims to address a clinical need such as improving patient compliance. There are multiple pathways and development strategies for the registration and approval of MR products. The development strategy of an MR product is usually dependent on the availability and pharmacokinetic/pharmacodynamics (PK/PD) characteristics of the reference drug product, that is, an immediate-release (IR) product or a reference MR. Compared with a reference IR product, an MR product is likely to have a different PK profile over the least common dosing time due to unequal dosing intervals. In case of differences in PK profiles between the MR product and the reference product, confirmatory efficacy and safety studies may be needed to support registration. In some cases, however, a thorough clinical PK/PD characterization may provide sufficient basis to support the approval of the proposed MR product without the need for additional safety and efficacy studies. This article summarizes the US Food and Drug Administration experience and the regulatory considerations supporting the approval of MR products in the past 6 years and discusses cases in which clinical pharmacology and PK/PD information were leveraged to support approval without the need for additional clinical studies. Details of all these cases are available in the public domain. In 2 cases a well-characterized exposure-response relationship provided sufficient justification that differences in the shape of the PK profiles were not clinically relevant. In the remaining 3 cases a thorough characterization of the PK profile along with a risk-based approach provided bases for approval.

Assessment of Similarity in Antipsychotic Exposure-Response Relationships in Clinical Trials Between Adults and Adolescents With Acute Exacerbation of Schizophrenia.

Despite agreement that early-onset schizophrenia is continuous with the adult-onset form, quantitative relationships between antipsychotic exposure and clinical response are relatively unexplored in adolescents, compared to adults. Clinical efficacy data from second-generation antipsychotic development programs (N = 5951 adults and N = 1035 adolescents ranging from 12 to 17 years old) were collected from available new drug applications submitted to the US Food and Drug Administration from 1993 to 2017. The developed disease-drug trial models adequately predicted the longitudinal trend in total positive and negative syndrome scale scores in both adults and adolescents using a Weibull placebo response, time-delayed drug effect, and a Weibull structural dropout model. Maximum drug effect was similar between the two populations and was estimated to be between a range of 5% to 11% in adults and 5% to 7% in adolescents. Half maximal effective concentration parameter estimates also indicated similar exposure-response relationships in adults and adolescents across all 4 antipsychotics. Simulated adolescent data using final model parameter estimates from the adult model were in agreement with adolescent observations. This analysis confirms similarity in exposure-response for efficacy and could expedite the development of second-generation antipsychotics for adolescents.

Pharmacometric Bridging Approach for U.S. Food and Drug Administration Approval and Identification of Topiramate Dosing Regimen for Pediatric Patients 2-9 Years of Age With Epilepsy.

This study was performed to identify an efficacious dosing regimen for U.S. Food and Drug Administration approval of topiramate for initial monotherapy in pediatric patients aged 2-9 years diagnosed with partial onset seizures and primary generalized tonic-clonic seizures using a pharmacometric bridging approach. The approval of topiramate in monotherapy of epilepsy for adult and pediatric patients (10-15 years) was based on efficacy and safety data from clinical trials. Our analysis showed that exposure-response relationship was similar between adult and pediatric patients (6-15 years) treated with topiramate as monotherapy for epilepsy. Specific dosing in pediatric patients 2-9 years of age was derived and included in the simulations by matching predicted exposures in pediatric patients (2-9 years) to a range of exposures observed in adult and pediatric patients (6-9 years) in a previously conducted clinical trial. The analysis allowed for U.S. Food and Drug Administration approval of topiramate for initial monotherapy in pediatric patients (2-9 years).

A Proposal of Conducting Bioequivalence Trials with Gastric pH Modulators for Two Oral Formulations Demonstrating Different Dissolution Profiles at Elevated pH.

In this paper, a special case for bioequivalence evaluation of oral formulations is discussed. Drug formulations with different forms of active moieties (e.g., free base and salt) may yield different dissolution characteristics and, thus, differ in absorption at elevated gastric pH. However, routine bioequivalence trials using subjects with normal gastric pH (i.e., ~ 1) may fail to identify these differences because dissolution/absorption profiles of the two formulations at normal gastric pH are similar. In the case of palbociclib, it is confirmedthat the free base and salt formulations showed different absorption in patients with different gastric pH. Significant reduction in drug absorption was observed only in patients with elevated gastric pH using free base formulation. The discovery that the free base had significantly reduced absorption hinged on the inclusion of enough patients with elevated gastric pH to detect a difference in a bioequivalence trial. This raises a concern, as demonstrated through simulation, that dissolution/absorption differences in other formulations could be missed in routine bioequivalence trials. Aside from differences in active pharmaceutical ingredients (APIs), other factors, such as changes in excipients or manufacturing methods, may also lead to exposure differences between formulations at elevated gastric pH. For formulations containing different forms of the same active moiety or the same API and showing different dissolution profiles at elevated pH (i.e., pH ~ 4-6.8), evaluation of bioequivalence with gastric pH modulators (e.g., a H2 blocker) in addition to routine bioequivalence assessments may help to ensure therapeutic equivalence in patients with elevated gastric pH.

A Quantitative Justification of Similarity in Placebo Response Between Adults and Adolescents With Acute Exacerbation of Schizophrenia in Clinical Trials.

Early-onset schizophrenia, or "adolescent schizophrenia," has a global incidence ranging up to 4% of all schizophrenia cases. Clinical data from antipsychotic programs were collected from new drug applications submitted to the US Food and Drug administration from 1993 to 2015. A placebo response-dropout model was developed to describe the time course of total positive and negative syndrome scale (PANSS) scores in adults and adolescents. The final model in both populations suggested that patients with higher baseline scores exhibited a greater absolute reduction from baseline. Higher baseline total PANSS, enrollment in US trials, and increases or small improvements in total PANSS were found to be predictors of dropout in both populations. Simulated adolescent data using the final adult placebo response model resembled the observed adolescent data. By confirming similar changes in disease symptomology during an acute exacerbation, efficient regulatory pathways for adolescents can be facilitated by using the extrapolation paradigm.

An integrated clinical pharmacology approach for deriving dosing recommendations in a regulatory setting: review of recent cases in psychiatry drugs.

Clinical pharmacology as an interdisciplinary science is unique in its capacity and the diversity of the methods and approaches it can provide to derive dosing recommendations in various subpopulations. This article illustrates cases where an integrated clinical pharmacology approach was used to derive dosing recommendations for psychiatry drugs within regulatory settings. The integrated approach is based on the view that once a drug is shown to be effective in the general population, it is reasonable to take into consideration other relevant findings and the use of alternative scientific tools and analysis to derive dosing recommendations in specific populations. The method provides useful means to solve the challenges of the paucity of available data and lead to clear dosing instructions. This in turn expands the benefits of any given drug to all individuals in which the drug is likely to be effective.

Improving pediatric dosing through pediatric initiatives: what we have learned.

OBJECTIVE: The goal was to review the impact of pediatric drug studies, as measured by the improvement in pediatric dosing and other pertinent information captured in the drug labeling. METHODS: We reviewed the pediatric studies for 108 products submitted (July 1998 through October 2005) in response to a Food and Drug Administration written request for pediatric studies, and the subsequent labeling changes. We analyzed the dosing modifications and focused on drug clearance as an important parameter influencing pediatric dosing. RESULTS: The first 108 drugs with new or revised pediatric labeling changes had dosing changes or pharmacokinetic information (n = 23), new safety information (n = 34), information concerning lack of efficacy (n = 19), new pediatric formulations (n = 12), and extended age limits (n = 77). A product might have had > or = 1 labeling change. We selected specific examples (n = 16) that illustrate significant differences in pediatric pharmacokinetics. CONCLUSIONS: Critical changes in drug labeling for pediatric patients illustrate that unique pediatric dosing often is necessary, reflecting growth and maturational stages of pediatric patients. These changes provide evidence that pediatric dosing should not be determined by simply applying weight-based calculations to the adult dose. Drug clearance is highly variable in the pediatric population and is not readily predictable on the basis of adult information.