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A quantitatively-driven evaluation of existing clinical data and associated knowledge to accelerate drug discovery and development is a highly valuable approach across therapeutic areas, but remains underutilized. This is especially the case for rare diseases for which development is particularly challenging. The current work outlines an organizational framework to support a quantitatively-based reverse translation approach to clinical development. This approach was applied to characterize predictors of the trajectory of cognition in Hunter syndrome (Mucopolysaccharidosis Type II; MPS-II), a rare X-linked lysosomal storage disorder, highly heterogeneous in its course. Specifically, we considered ways to refine target populations based on age, cognitive status, and biomarkers, that is, cerebrospinal fluid glycosaminoglycans (GAG), at trial entry. Data from a total of 138 subjects (age range 2.5 to 10.1 years) from Takeda-sponsored internal studies and external natural history studies in MPS-II were included. Quantitative analyses using mixed-effects models were performed to characterize the relationships between neurocognitive outcomes and potential indicators of disease progression. Results revealed a specific trajectory in cognitive development across age with an initial progressive phase, followed by a plateau between 4 and 8 years and then a variable declining phase. Additionally, results suggest a faster decline in cognition among subjects with lower cognitive scores or with higher cerebrospinal fluid GAG at enrollment. These results support differences in the neurocognitive course of MPS-II between distinct groups of patients based on age, cognitive function, and biomarker status at enrollment. These differences should be considered when designing future clinical trials., (© 2024 The Authors. Clinical and Translational Science published by Wiley Periodicals LLC on behalf of American Society for Clinical Pharmacology and Therapeutics.)

Purpose: The objectives of this study were to develop a population pharmacokinetic model of methotrexate (MTX) and its primary metabolite 7-hydroxymethotrexate (7OHMTX) in children with brain tumors, to identify the sources of pharmacokinetic variability, and to assess whether MTX and 7OHMTX systemic exposures were related to toxicity., Methods: Patients received 2.5 or 5 g/m 2 MTX as a 24-hour infusion and serial samples were analyzed for MTX and 7OHMTX by an LC-MS/MS method. Pharmacokinetic parameters were estimated using nonlinear mixed-effects modeling. Demographics, laboratory values, and genetic polymorphisms were considered as potential covariates to explain the pharmacokinetic variability. Association between MTX and 7OHMTX systemic exposures and MTX-related toxicities were explored using random intercept logistic regression models., Results: The population pharmacokinetics of MTX and 7OHMTX were adequately characterized using two-compartment models in 142 patients (median 1.91 y; age range 0.09 to 4.94 y) in 513 courses. The MTX and 7OHMTX population clearance values were 4.6 and 3.0 l/h/m 2 , respectively. Baseline body surface area and estimated glomerular filtration rate were significant covariates on both MTX and 7OHMTX plasma disposition. Pharmacogenetic genotypes were associated with MTX pharmacokinetic parameters but had only modest influence. No significant association was observed between MTX or 7OHMTX exposure and MTX-related toxicity., Conclusions: MTX and 7OHMTX plasma disposition were characterized for the first time in young children with brain tumors. No exposure-toxicity relationship was identified in this study, presumably due to aggressive clinical management which led to a low MTX-related toxicity rate., Competing Interests: Declaration of Competing Interest Authors declare no conflict of interest., (Copyright © 2023. Published by Elsevier B.V.)

Introduction: The unbound brain extracelullar fluid (brain ECF ) to plasma steady state partition coefficient, K p,uu,BBB , values provide steady-state information on the extent of blood-brain barrier (BBB) transport equilibration, but not on pharmacokinetic (PK) profiles seen by the brain targets. Mouse models are frequently used to study brain PK, but this information cannot directly be used to inform on human brain PK, given the different CNS physiology of mouse and human. Physiologically based PK (PBPK) models are useful to translate PK information across species., Aim: Use the LeiCNS-PK3.0 PBPK model, to predict brain extracellular fluid PK in mice., Methods: Information on mouse brain physiology was collected from literature. All available connected data on unbound plasma, brain ECF PK of 10 drugs (cyclophosphamide, quinidine, erlotonib, phenobarbital, colchicine, ribociclib, topotecan, cefradroxil, prexasertib, and methotrexate) from different mouse strains were used. Dosing regimen dependent plasma PK was modelled, and Kpuu,BBB values were estimated, and provided as input into the LeiCNS-PK3.0 model to result in prediction of PK profiles in brain ECF ., Results: Overall, the model gave an adequate prediction of the brain ECF PK profile for 7 out of the 10 drugs. For 7 drugs, the predicted versus observed brain ECF data was within two-fold error limit and the other 2 drugs were within five-fold error limit., Conclusion: The current version of the mouse LeiCNS-PK3.0 model seems to reasonably predict available information on brain ECF from healthy mice for most drugs. This brings the translation between mouse and human brain PK one step further., (© 2023. The Author(s).)

Background: Topotecan, an antitumor drug with systemic exposure (SE)-dependent activity against many pediatric tumors has wide interpatient pharmacokinetic variability, making it challenging to attain the desired topotecan SE. The study objectives were to update our topotecan population pharmacokinetic model, to evaluate the feasibility of determining individual topotecan clearance using a single blood sample, and to apply this approach to topotecan data from a neuroblastoma trial to explore exposure-response relationships., Procedure: Our previous population pharmacokinetic and covariate model was updated using data from 13 clinical pediatric studies. A simulation-based Bayesian analysis was performed to determine if a single blood sample could be sufficient to estimate individual topotecan clearance. Following the Bayesian approach, single pharmacokinetic samples collected from a Children's Oncology Group Phase III clinical trial (ANBL0532; NCT0056767) were analyzed to estimate individual topotecan SE. Associations between topotecan SE and toxicity or early response were then evaluated., Results: The updated population model included the impact of patient body surface area (BSA), age, and renal function on topotecan clearance. The Bayesian analysis with the updated model and single plasma samples showed that individual topotecan clearance values were estimated with good precision (mean absolute prediction error ≤16.2%) and low bias (mean prediction error ≤7.2%). Using the same approach, topotecan SE was derived in patients from ANBL0532. The exposure-response analysis showed an increased early response after concomitant cyclophosphamide and topotecan up to a topotecan SE of 45 h ng/mL., Conclusions: A simple single-sample approach during topotecan therapy could guide dosing for patients, resulting in more patients reaching target attainment., (© 2023 Wiley Periodicals LLC.)

The promise of viral vector-based gene therapy (GT) as a transformative paradigm for treating severely debilitating and life-threatening diseases is slowly coming to fruition with the recent approval of several drug products. However, they have a unique mechanism of action often necessitating a tortuous clinical development plan. Expertise in such complex therapeutic modality is still fairly limited in this emerging class of adeno-associated virus (AAV) vector-based gene therapies. Because of the irreversible mode of action and incomplete understanding of genotype-phenotype relationship and disease progression in rare diseases careful considerations should be given to GT product's benefit-risk profile. In particular, special attention needs to be paid to safe dose selection, reliable dose exposure response (including clinically relevant endpoints), or creative approaches in study design targeting small patient populations during clinical development. We believe that quantitative tools encompassed within model-informed drug development (MIDD) framework fits quite well in the development of such novel therapies, as they enable us to benefit from the totality of data approach in order to support dose selection as well as optimize clinical trial designs, end point selection, and patient enrichment. In this thought leadership paper, we provide our collective experiences, identify challenges, and suggest areas of improvement in applications of modeling and innovative trial design in development of AAV-based GT products and reflect on the challenges and opportunities for incorporating MIDD tools and more in rational development of these products., (© 2023 The Authors. Clinical Pharmacology & Therapeutics © 2023 American Society for Clinical Pharmacology and Therapeutics.)

Background: Recurrent atypical teratoid/rhabdoid tumor (AT/RT) is, most often, a fatal pediatric malignancy with limited curative options., Methods: We conducted a phase II study of Aurora kinase A inhibitor alisertib in patients aged <22 years with recurrent AT/RT. Patients received alisertib once daily (80 mg/m2 as enteric-coated tablets or 60 mg/m2 as liquid formulation) on Days 1-7 of a 21-day cycle until progressive disease (PD) occurred. Alisertib plasma concentrations were measured in cycle 1 on Days 1 (single dose) and 7 (steady state) and analyzed with noncompartmental pharmacokinetics. Trial efficacy end point was ≥10 participants with stable disease (SD) or better at 12 weeks., Results: SD (n = 8) and partial response (PR) (n = 1) were observed among 30 evaluable patients. Progression-free survival (PFS) was 30.0% ± 7.9% at 6 months and 13.3% ± 5.6% at 1 year. One-year overall survival (OS) was 36.7% ± 8.4%. Two patients continued treatment for >12 months. PFS did not differ by AT/RT molecular groups. Neutropenia was the most common adverse effect (n = 23/30, 77%). The 22 patients who received liquid formulation had a higher mean maximum concentration (Cmax) of 10.1 ± 3.0 µM and faster time to Cmax (Tmax = 1.2 ± 0.7 h) than those who received tablets (Cmax = 5.7 ± 2.4 µM, Tmax = 3.4 ± 1.4 h)., Conclusions: Although the study did not meet predetermined efficacy end point, single-agent alisertib was well tolerated by children with recurrent AT/RT, and SD or PR was observed in approximately a third of the patients., (© The Author(s) 2022. Published by Oxford University Press on behalf of the Society for Neuro-Oncology. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

The brain penetration of methotrexate (MTX) and its metabolite 7-hydroxymethotrexate (7OHMTX) was characterized in non-tumor bearing mice and mice bearing orthotopic Group 3 medulloblastoma. Plasma pharmacokinetic studies and cerebral and ventricular microdialysis studies were performed in animals dosed with 200 or 1000 mg/kg MTX by IV bolus. Plasma, brain/tumor extracellular fluid (ECF) and lateral ventricle cerebrospinal fluid (CSF) MTX and 7OHMTX concentration-time data were analyzed by validated LC-MS/MS methods and modeled using a population-based pharmacokinetic approach and a hybrid physiologically-based model structure for the brain compartments. Brain penetration was similar for MTX and 7OHMTX and was not significantly different between non-tumor and tumor bearing mice. Overall, mean (±SD) model-derived unbound plasma to ECF partition coefficient K p,uu were 0.17 (0.09) and 0.17 (0.12) for MTX and 7OHMTX, respectively. Unbound plasma to CSF K p,uu were 0.11 (0.06) and 0.18 (0.09) for MTX and 7OHMTX, respectively. The plasma and brain model were scaled to children using allometric principles and pediatric physiological parameters. Model-based simulations were adequately overlaid with digitized plasma and CSF lumbar data collected in children receiving different MTX systemic infusions. This model can be used to further explore and optimize methotrexate dosing regimens in children with brain tumors., Competing Interests: Declaration of competing interest The author declares no conflict of interest. All authors have approved the final manuscript., (Copyright © 2022 The Japanese Society for the Study of Xenobiotics. Published by Elsevier Ltd. All rights reserved.)

IWR-1-endo, a small molecule that potently inhibits the Wnt/β-catenin signaling pathway by stabilizing the AXIN2 destruction complex, can inhibit drug efflux at the blood−brain barrier. To conduct murine cerebral microdialysis research, validated, sensitive, and reliable liquid chromatography−tandem mass spectrometry (LC-MS/MS) methods were used to determine IWR-1-endo concentration in the murine plasma and brain microdialysate. IWR-1-endo and the internal standard (ISTD) dabrafenib were extracted from murine plasma and microdialysate samples by a simple solid-phase extraction protocol performed on an Oasis HLB µElution plate. Chromatographic separation was executed on a Kinetex C18 (100A, 50 × 2.1 mm, 4 µm particle size) column with a binary gradient of water and acetonitrile, each having 0.1% formic acid, pumped at a flow rate of 0.6 mL/min. Detection by mass spectrometry was conducted in the positive selected reaction monitoring ion mode by monitoring mass transitions 410.40 > 344.10 (IWR-1-endo) and 520.40 > 307.20 (ISTD). The validated curve range of IWR-1-endo was 5−1000 ng/mL for the murine plasma method (r2 ≥ 0.99) and 0.5−500 ng/mL for the microdialysate method (r2 ≥ 0.99). The lower limit of quantification (LLOQ) was 5 ng/mL and 0.5 ng/mL for the murine plasma and microdialysate sample analysis method, respectively. Negligible matrix effects were observed in murine plasma and microdialysate samples. IWR-1-endo was extremely unstable in murine plasma. To improve the stability of IWR-1-endo, pH adjustments of 1.5 were introduced to murine plasma and microdialysate samples before sample storage and processing. With pH adjustment of 1.5 to the murine plasma and microdialysate samples, IWR-1-endo was stable across several tested conditions such as benchtop, autosampler, freeze−thaw, and long term at −80 °C. The LC-MS/MS methods were successfully applied to a murine pharmacokinetic and cerebral microdialysis study to characterize the unbound IWR-1-endo exposure in brain extracellular fluid and plasma.

Group3 (G3) medulloblastoma (MB) is one of the deadliest forms of the disease for which novel treatment is desperately needed. Here we evaluate ribociclib, a highly selective CDK4/6 inhibitor, with gemcitabine in mouse and human G3MBs. Ribociclib central nervous system (CNS) penetration was assessed by in vivo microdialysis and by IHC and gene expression studies and found to be CNS-penetrant. Tumors from mice treated with short term oral ribociclib displayed inhibited RB phosphorylation, downregulated E2F target genes, and decreased proliferation. Survival studies to determine the efficacy of ribociclib and gemcitabine combination were performed on mice intracranially implanted with luciferase-labeled mouse and human G3MBs. Treatment of mice with the combination of ribociclib and gemcitabine was well tolerated, slowed tumor progression and metastatic spread, and increased survival. Expression-based gene activity and cell state analysis investigated the effects of the combination after short- and long-term treatments. Molecular analysis of treated versus untreated tumors showed a significant decrease in the activity and expression of genes involved in cell-cycle progression and DNA damage response, and an increase in the activity and expression of genes implicated in neuronal identity and neuronal differentiation. Our findings in both mouse and human patient-derived orthotopic xenograft models suggest that ribociclib and gemcitabine combination therapy warrants further investigation as a treatment strategy for children with G3MB., (©2022 The Authors; Published by the American Association for Cancer Research.)

Background: Genomic aberrations in the cell cycle and PI3K/Akt/mTOR pathways have been reported in diffuse intrinsic pontine glioma (DIPG) and high-grade glioma (HGG). Dual inhibition of CDK4/6 and mTOR has biologic rationale and minimal overlapping toxicities. This study determined the recommended phase 2 dose (RP2D) of ribociclib and everolimus following radiotherapy in children with DIPG and HGG., Methods: Patients were enrolled according to a Rolling-6 design and received ribociclib and everolimus once daily for 21 and 28 days, respectively. All patients with HGG and biopsied DIPG were screened for retinoblastoma protein presence by immunohistochemistry. Pharmacokinetics were analyzed., Results: Nineteen patients enrolled (median age: 8 years [range: 2-18]). Three patients enrolled at each dose level 1 and 2 without dose-limiting toxicities (DLT). Thirteen patients were enrolled at dose level 3, with one patient experiencing a DLT (grade 3 infection). One patient came off therapy before cycle 9 due to cardiac toxicity. The most common grade 3/4 toxicities were neutropenia (33%), leucopenia (17%), and lymphopenia (11%). Steady-state everolimus exposures in combination were 1.9 ± 0.9-fold higher than single-agent administration. Median overall survival for 15 patients with DIPG was 13.9 months; median event-free survival for four patients with HGG was 10.5 months. Two longer survivors had tumor molecular profiling identifying CDKN2A/B deletion and CDK4 overexpression., Conclusion: The combination of ribociclib and everolimus following radiotherapy in children with newly diagnosed DIPG and HGG was well tolerated, with a RP2D of ribociclib 170 mg/m 2 and everolimus 1.5 mg/m 2 . Results will inform a molecularly guided phase II study underway to evaluate efficacy., (© The Author(s) 2022. Published by Oxford University Press, the Society for Neuro-Oncology and the European Association of Neuro-Oncology.)

Purpose: Crenolanib, an oral inhibitor of platelet-derived growth factor receptor, was evaluated to treat children and young adults with brain tumors. Crenolanib population pharmacokinetics and covariate influence were characterized in this patient population., Methods: Patients enrolled on this phase I study (NCT01393912) received oral crenolanib once daily. Serial single-dose and steady-state serum pharmacokinetic samples were collected and analyzed using a validated LC-ESI-MS/MS method. Population modeling and covariate analysis evaluating demographics, laboratory values, and comedications were performed. The impact of significant covariates on crenolanib exposure was further explored using model simulations., Results: Crenolanib serum concentrations were analyzed for 55 patients (2.1-19.2 years-old) and best fitted with a linear two-compartment model, with delayed absorption modeled with a lag time. A typical patient [8-year-old, body surface area (BSA) 1 m 2 ] had an apparent central clearance, volume, and absorption rate of 41 L/h, 54.3 L, and 0.19 /h, respectively. Patients taking acid reducers (histamine H 2 antagonists or proton pump inhibitors) concomitantly exhibited about 2- and 1.7-fold lower clearance and volume (p < 0.0001 and p = 0.018, respectively). Crenolanib clearance increased with BSA (p < 0.0001), and absorption rate decreased with age (p < 0.0001). Model simulations showed cotreatment with an acid reducer was the only covariate significantly altering crenolanib exposure and supported the use of BSA-based crenolanib dosages vs flat-dosages for this population., Conclusions: Crenolanib pharmacokinetics were adequately characterized in children and young adults with brain tumors. Despite marked increased drug exposure with acid reducer cotreatment, crenolanib therapy was well tolerated. No dosing adjustments are recommended for this population., (© 2022. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Background: Platelet-derived growth factor receptor (PDGFR) signaling has been directly implicated in pediatric high-grade gliomagenesis. This study evaluated the safety and tolerability of crenolanib, a potent, selective inhibitor of PDGFR-mediated phosphorylation, in pediatric patients with high-grade glioma (HGG)., Methods: We used a rolling-6 design to study the maximum tolerated dose (MTD) of once-daily crenolanib administered during and after focal radiation therapy in children with newly diagnosed diffuse intrinsic pontine glioma (DIPG) (stratum A) or with recurrent/progressive HGG (stratum B). Pharmacokinetics were studied during the first cycle at the first dose and at steady state (day 28). Alterations in PDGFRA were assessed by Sanger or exome sequencing and interphase fluorescence in situ hybridization or single nucleotide polymorphism arrays., Results: Fifty evaluable patients were enrolled in the 2 strata, and an MTD of 170 mg/m 2 was established for both. Dose-limiting toxicities were primarily liver enzyme elevations and hematologic count suppression in both strata. Crenolanib AUC 0-48h and C MAX did not differ significantly for crushed versus whole-tablet administration. Overall, PDGFRA alterations were observed in 25% and 30% of patients in stratum A and B, respectively. Neither crenolanib therapy duration nor survival outcomes differed significantly by PDGFRA status, and overall survival of stratum A was similar to that of historical controls., Conclusions: Children tolerate crenolanib well at doses slightly higher than the established MTD in adults, with a toxicity spectrum generally similar to that in adults. Studies evaluating intratumoral PDGFR pathway inhibition in biomarker-enriched patients are needed to evaluate further the clinical utility of crenolanib in this population., (© The Author(s) 2021. Published by Oxford University Press, the Society for Neuro-Oncology and the European Association of Neuro-Oncology.)

Purpose: Crizotinib, a potent oral tyrosine kinase inhibitor, was evaluated in combination with dasatinib in a phase 1 trial (NCT01644773) in children with progressive or recurrent high-grade and diffuse intrinsic pontine gliomas (HGG and DIPG). This study aimed to characterize the pharmacokinetics of crizotinib in this population and identify significant covariates., Methods: Patients (N = 36, age range 2.9-21.3 years) were treated orally once or twice-daily with 100-215 mg/m 2 crizotinib and 50-65 mg/m 2 dasatinib. Pharmacokinetic studies were performed for crizotinib alone after the first dose and at steady state, and for the drug combination at steady state. Crizotinib plasma concentrations were measured using a validated LC-MS/MS method. Population modeling was performed (Monolix) and the impact of factors including patient demographics and co-medications were investigated on crizotinib pharmacokinetics., Results: Crizotinib concentrations were described with a linear two-compartment model and absorption lag time. Concomitant dasatinib and overweight/obese status significantly influenced crizotinib pharmacokinetics, resulting in clinically relevant impact (> 20%) on drug exposure. Crizotinib mean apparent clearance (CL/F) was 66.7 L/h/m 2 after single-dose and decreased to 26.5 L/h/m 2 at steady state when given alone, but not when combined with dasatinib (mean 60.8 L/h/m 2 ). Overweight/obese patients exhibited lower crizotinib CL/F and apparent volume V 1 /F (mean 46.2 L/h/m 2 and 73.3 L/m 2 ) compared to other patients (mean 75.5 L/h/m 2 and 119.3 L/m 2 , p < 0.001)., Conclusion: A potential pharmacokinetic interaction was observed between crizotinib and dasatinib in children with HGG and DIPG. Further, crizotinib exposure was significantly higher in overweight/obese patients, who may require a dosing adjustment., (© 2021. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Sorafenib improves outcomes in adult hepatocellular carcinoma; however, hand foot skin reaction (HFSR) is a dose limiting toxicity of sorafenib that limits its use. HFSR has been associated with sorafenib systemic exposure. The objective of this study was to use modeling and simulation to determine whether using pharmacokinetically guided dosing to achieve a predefined sorafenib target range could reduce the rate of HFSR. Sorafenib steady-state exposures (area under the concentration curve from 0 to 12-h [AUC 0->12 h ]) were simulated using published sorafenib pharmacokinetics at either a fixed dosage (90 mg/m 2 /dose) or a pharmacokinetically guided dose targeting an AUC 0->12 h between 20 and 55 h µg/ml. Dosages were either rounded to the nearest quarter of a tablet (50 mg) or capsule (10 mg). A Cox proportional hazard model from a previously published study was used to quantify HFSR toxicity. Simulations showed that in-target studies increased from 50% using fixed doses with tablets to 74% using pharmacokinetically guided dosing with capsules. The power to observe at least 4 of 6 patients in the target range increased from 33% using fixed dosing with tablets to 80% using pharmacokinetically guided with capsules. The expected HFSR toxicity rate decreased from 22% using fixed doses with tablets to 16% using pharmacokinetically guided dosing with capsules. The power to observe less than 6 of 24 studies with HFSR toxicity increased from 51% using fixed dosing with tablets to 88% using pharmacokinetically guided with capsules. Our simulations provide the rationale to use pharmacokinetically guided sorafenib dosing to maintain effective exposures that potentially improve tolerability in pediatric clinical trials., (© 2021 The Authors. Clinical and Translational Science published by Wiley Periodicals LLC on behalf of the American Society for Clinical Pharmacology and Therapeutics.)

Background: Pediatric low-grade gliomas (pLGGs) are the most common childhood brain tumor. Progression-free survival (PFS) is much lower than overall survival, emphasizing the need for alternative treatments. Sporadic (without neurofibromatosis type 1) optic pathway and hypothalamic gliomas (OPHGs) are often multiply recurrent and cause significant visual deficits. Recently, there has been a prioritization of functional outcomes., Methods: We present results from children with recurrent/progressive OPHGs treated on a PBTC (Pediatric Brain Tumor Consortium) phase II trial evaluating efficacy of selumetinib (AZD6244, ARRY-142886) a MEK-1/2 inhibitor. Stratum 4 of PBTC-029 included patients with sporadic recurrent/progressive OPHGs treated with selumetinib at the recommended phase II dose (25mg/m2/dose BID) for a maximum of 26 courses., Results: Twenty-five eligible and evaluable patients were enrolled with a median of 4 (1-11) previous therapies. Six of 25 (24%) had partial response, 14/25 (56%) had stable disease, and 5 (20%) had progressive disease while on treatment. The median treatment courses were 26 (2-26); 14/25 patients completed all 26 courses. Two-year PFS was 78 ± 8.5%. Nineteen of 25 patients were evaluable for visual acuity which improved in 4/19 patients (21%), was stable in 13/19 (68%), and worsened in 2/19 (11%). Five of 19 patients (26%) had improved visual fields and 14/19 (74%) were stable. The most common toxicities were grade 1/2 CPK elevation, anemia, diarrhea, headache, nausea/emesis, fatigue, AST and ALT increase, hypoalbuminemia, and rash., Conclusions: Selumetinib was tolerable and led to responses and prolonged disease stability in children with recurrent/progressive OPHGs based upon radiographic response, PFS, and visual outcomes., (© The Author(s) 2021. Published by Oxford University Press on behalf of the Society for Neuro-Oncology. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

JQ1, is a cell-permeable small-molecule inhibitor of bromodomain and extra-terminal protein (BET) function with reportedly good CNS penetration, however, unbound and pharmacologically active CNS JQ1 exposures have not been characterized. Additionally, no quantitative bioanalytical methods for JQ1 have been described in the literature to support the CNS penetration studies. In the present article, we discuss the development and validation of a sensitive and reliable liquid chromatography-tandem mass spectrometry (LC-MS/MS) quantitative methods to determine JQ1 in mouse plasma and brain microdialysate. JQ1 and the internal standard, dabrafenib (ISTD), were extracted from plasma and microdialysate samples using a simple solid phase extraction protocol performed on an Oasis HLB μElution plate. Chromatographic separation of JQ1 and ISTD was achieved on a reversed phase C 12 analytical column with gradient elution profile of mobile phases (MP A: water containing 0.1 % formic acid and MP B: acetonitrile containing 0.1 % formic acid) at a flow rate of 0.6 mL/min. The mass spectrometric detection was performed in the positive MRM ion mode by monitoring the transitions 457.40 > 341.30 (JQ1) and 520.40 > 307.20 (ISTD). The calibration curves demonstrated good linearities over the concentration range of 5-1000 ng/mL for the mouse plasma method (r 2 ≥ 0.99) and 0.5-500 ng/mL for the microdialysate method (r 2 ≥ 0.99). The experimental limit of quantification obtained was 5 and 0.5 ng/mL for the mouse plasma and microdialysate method, respectively, with the coefficient of variation less than 10 % for the analyte peak area. All the other validation parameters, including intra-and inter-day accuracy and precision, matrix effect, selectivity, carryover effect, and stability, were within the USFDA bioanalytical guidelines acceptance limits. The LC-MS/MS method was successfully applied to a mouse pharmacokinetic and cerebral microdialysis study to characterize the unbound JQ1 exposure in brain extracellular fluid and plasma., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Radiation therapy (RT) is currently the standard treatment for diffuse intrinsic pontine glioma (DIPG), the most common cause of death in children with brain cancer. A pharmacodynamic model was developed to describe the radiation-induced tumor shrinkage and overall survival in mice bearing DIPG. CD1-nude mice were implanted in the brain cortex with luciferase-labeled patient-derived orthotopic xenografts of DIPG (SJDIPGx7 H3F3A WT / K27 M and SJDIPGx37 H3F3A K27M / K27M ). Mice were treated with image-guided whole-brain RT at 1 or 2 Gy/fraction 5-days-on 2-days-off for a cumulative dose of 20 or 54 Gy. Tumor progression was monitored with bioluminescent imaging (BLI). A mathematical model describing BLI and overall survival was developed with data from mice receiving 2 Gy/fraction and validated using data from mice receiving 1 Gy/fraction. BLI data were adequately fitted with a logistic tumor growth function and a signal distribution model with linear radiation-induced killing effect. A higher tumor growth rate in SJDIPGx37 versus SJDIPGx7 xenografts and a killing effect decreasing with higher tumor baseline (p < 0.0001) were identified. Cumulative radiation dose was suggested to inhibit the tumor growth rate according to a Hill function. Survival distribution was best described with a Weibull hazard function in which the hazard baseline was a continuous function of tumor BLI. Significant differences were further identified between DIPG cell lines and untreated versus treated mice. The model was adequately validated with mice receiving 1 Gy/fraction and will be useful in guiding future preclinical trials incorporating radiation and to support systemic combination therapies with RT., (© 2021 The Authors. CPT: Pharmacometrics & Systems Pharmacology published by Wiley Periodicals LLC on behalf of American Society for Clinical Pharmacology and Therapeutics.)

Purpose: Genomic aberrations in cell cycle and PI3K pathways are commonly observed in pediatric brain tumors. This study determined the MTD/recommended phase II dose (RP2D) of ribociclib and everolimus and characterized single-agent ribociclib concentrations in plasma and tumor in children undergoing resection., Patients and Methods: Patients were enrolled in the phase I study according to a rolling 6 design and received ribociclib and everolimus daily for 21 and 28 days, respectively. Surgical patients received ribociclib at the pediatric RP2D (350 mg/m 2 ) for 7-10 days preoperatively followed by enrollment on the phase I study. Pharmacokinetics were analyzed for both cohorts., Results: Sixteen patients were enrolled on the phase I study (median age, 10.3 years; range, 3.9-20.4) and 6 patients in the surgical cohort (median age, 11.4 years; range: 7.2-17.1). Thirteen patients were enrolled at dose level 1 without dose-limiting toxicities (DLT). Two of the 3 patients at dose level 2 experienced DLTs (grade 3 hypertension and grade 4 alanine aminotransferase). The most common grade 3/4 toxicities were lymphopenia, neutropenia, and leukopenia. The RP2D of ribociclib and everolimus was 120 and 1.2 mg/m 2 for 21 and 28 days, respectively. Steady-state everolimus exposures with ribociclib were 2.5-fold higher than everolimus administered alone. Ribociclib plasma, tumor concentrations, and cerebrospinal fluid (CSF) samples were collected. The mean tumor-to-plasma ratio of ribociclib was 19.8 (range, 2.22-53.4)., Conclusions: Ribociclib and everolimus were well-tolerated and demonstrated pharmacokinetic properties similar to those in adults. Potential therapeutic ribociclib concentrations could be achieved in CSF and tumor tissue, although interpatient variability was observed., (©2021 American Association for Cancer Research.)

Tacrolimus is a calcineurin inhibitor used to prevent acute graft versus host disease in adult patients receiving allogeneic hematopoietic stem cell transplantation (HCT). Previous population pharmacokinetic (PK) models have been developed in solid organ transplant, yet none exists for patients receiving HCT. The primary objectives of this study were to (1) use a previously published population PK model in adult patients who underwent kidney transplant and apply it to allogeneic HCT; (2) evaluate model-predicted tacrolimus steady-state trough concentrations and simulations in patients receiving HCT; and (3) evaluate covariates that affect tacrolimus PK in allogeneic HCT. A total of 252 adult patients receiving allogeneic HCT were included in the study. They received oral tacrolimus twice daily (0.03 mg/kg) starting 3 days prior to transplant. Data for these analyses included baseline clinical and demographic data, genotype data for single nucleotide polymorphisms in CYP3A4/5 and ABCB1, and the first tacrolimus steady-state trough concentration. A dosing simulation strategy based on observed trough concentrations (rather than model-based predictions) resulted in 12% more patients successfully achieving tacrolimus trough concentrations within the institutional target range (5-10 ng/ml). Stepwise covariate analyses identified HLA match and conditioning regimen (myeloablative vs. reduced intensity) as significant covariates. Ultimately, a previously published tacrolimus population PK model in kidney transplant provided a platform to help establish a model-based dose adjustment strategy in patients receiving allogenic HCT, and identified HCT-specific covariates to be considered for future prospective studies. Study Highlights WHAT IS THE CURRENT KNOWLEDGE ON THE TOPIC? Tacrolimus is a cornerstone immunosuppressant used in patients who undergo organ transplantations. However, because of its narrow therapeutic index and wide interpatient pharmacokinetic (PK) variability, optimizing its dose is crucial to maximize efficacy and minimize tacrolimus-induced toxicities. Prior to this study, no tacrolimus population PK models have been developed for adult patients receiving allogeneic hematopoietic stem cell transplantation (HCT). Therefore, research effort was warranted to develop a population PK model that begins to propose more precision tacrolimus dosing and begins to address both a clinical and scientific gap in this patient population. WHAT QUESTION DID THIS STUDY ADDRESS? The study addressed whether there is value in utilizing the observed tacrolimus steady-state trough concentrations from patients receiving allogeneic HCT within the context of a pre-existing population PK model developed for kidney transplant. The study also addressed whether there are clinically relevant covariates specific to adult patients receiving allogeneic HCT. WHAT DOES THIS STUDY ADD TO OUR KNOWLEDGE? Inclusion of a single steady-state tacrolimus trough concentration is beneficial to model predictions. The dosing simulation strategy based on observed tacrolimus concentration, rather than the model-predicted concentration, resulted in more patients achieving the target range at first steady-state collection. Future studies should evaluate HLA matching and myeloablative conditioning versus reduced intensity conditioning regimens as covariates. These data and model-informed dose adjustments should be included in future prospective studies. This research could also serve as a template as to how to assess the utility of prior information for other disease settings. HOW MIGHT THIS CHANGE CLINICAL PHARMACOLOGY OR TRANSLATIONAL SCIENCE? The M2 model fitting method and D2 dosing simulation method can be applied to other clinical pharmacology studies where only a single steady-state trough concentration is available per patient in the presence of a previously published population PK model., (© 2021 The Authors. Clinical and Translational Science published by Wiley Periodicals LLC on behalf of the American Society for Clinical Pharmacology and Therapeutics.)

Background: Disruption of cell-cycle regulators is a potential therapeutic target for brain tumors in children and adolescents. The aim of this study was to determine the maximum tolerated dose (MTD) and describe toxicities related to palbociclib, a selective cyclin-dependent kinase 4/6 (CDK4/6) inhibitor in pediatric patients with progressive/refractory brain tumors with intact retinoblastoma protein., Methods: Palbociclib was administered orally starting at 50 mg/m 2 daily for the first 21 days of a 28-day course. Dose escalation was according to the Rolling-6 statistical design in less heavily (stratum I) and heavily pretreated (stratum II) patients, and MTD was determined separately for each group. Pharmacokinetic studies were performed during the first course, and pharmacodynamic studies were conducted to evaluate relationships between drug levels and toxicities., Results: A total of 21 patients were enrolled on stratum I and 14 patients on stratum II. The MTD for both strata was 75 mg/m 2 . Palbociclib absorption (mean T max between 4.9 and 6.6 h) and elimination (mean half-life between 11.3 and 19.5 h) were assessed. The most common toxicity was myelosuppression. Higher palbociclib exposure was associated with grade 3/4 neutropenia and leukopenia. Dose limiting toxicities included grade 4 neutropenia and grade 3 thrombocytopenia and dehydration. No patients had an objective response to palbociclib therapy., Conclusions: Palbociclib was safely administered to children and adolescents at a dosage of 75 mg/m 2 for 21 consecutive days followed by seven days of rest in both strata. Future studies will establish its optimal utilization in pediatric patients with brain tumors., (© 2021 Wiley Periodicals LLC.)

Selumetinib, a highly specific mitogen-activated protein kinase 1/2 inhibitor, is approved for children older than 2 years of age with neurofibromatosis 1 who have inoperable plexiform neurofibromas. By selectively binding to mitogen-activated protein kinase 1/2 proteins, selumetinib can arrest the mitogen-activated protein kinase/extracellular signal-regulated kinase signaling pathway that regulates critical cellular responses. Selumetinib has shown promising results as a single agent or in combination with conventional chemotherapy and other targeted therapies both preclinically and clinically, in multiple cancers including pediatric low-grade glioma, non-small cell lung cancer, and melanoma, among others. The pharmacokinetic profiles of selumetinib and its active metabolite N-desmethyl selumetinib have been well characterized in both adults and children. Both compounds exhibited rapid absorption and mean terminal elimination half-lives of about 7.5 h, with minimal accumulation at steady state. Three population pharmacokinetic models have been developed in adults and children, characterizing large inter- and intra-patient variabilities, and identifying significant covariates including food intake on selumetinib absorption, weight metrics, age, co-administration of cytochrome modulators, and Asian ethnicity on selumetinib apparent oral clearance. The most common side effects associated with selumetinib are dermatologic, gastrointestinal toxicities, and fatigue. Most toxicities are mild or moderate, generally tolerated and manageable. Cardiovascular and ocular toxicities remain less frequent but can be potentially more severe and require close monitoring. Overall, selumetinib exhibits a favorable safety profile and pharmacokinetic properties, with promising activity in multiple solid tumors, supporting current and further evaluation in combination with conventional chemotherapy and other targeted agents.

Silmitasertib (CX-4945) as a potent and selective inhibitor of CK2 exhibited promising in vitro and in vivo anti-cancer activity. An assay employing cation-exchange solid phase extraction (SPE) followed by LC-MS/MS analysis was successfully developed and validated for the quantitation of silmitasertib in human plasma, brain tissue, and human cerebrospinal fluid (CSF). Reverse phase chromatographic separation was achieved using Synergi™ hydro-RP column (4 μm, 75 × 2.0 mm) and gradient elution with 5 mM ammonium formate aqueous solution (pH 6.5) as mobile phase A and 0.1% formic acid in acetonitrile as mobile phase B. Multiple reaction monitoring (MRM) transition of m/z 350.2 → 223.2 and m/z 316.2 → 223.2 were chosen for detection of silmitasertib and internal standard (CX-4786) respectively. Since silmitasertib concentration in patient plasma is expected to be in a wide range due to the study design, two calibration curves with range 0.2-125 ng/ml and 32-20,000 ng/ml were established. A different curve ranging from 2 to 40 ng/g was used for measurement of silmitasertib in brain tissue, while another calibration curve ranging from 0.2 to 20 ng/ml was established for CSF. All these calibration curves corresponding to different matrices showed good linearity (R 2  > 0.99) over the concentration range. This assay demonstrated excellent precision below 15% and accuracies between 85% and 115% within-day and between-day for all the concentration levels in each matrix. This assay was also validated for each matrix for selectivity, sensitivity, matrix effects, recovery, and stability. We applied the validated method to the analysis of plasma silmitasertib for a clinical study., Competing Interests: Declaration of Competing Interest The authors declared that there is no conflict of interest., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Background: Talazoparib combined with irinotecan and temozolomide demonstrated efficacy in a murine Ewing sarcoma model. Based on these data, we conducted a phase I trial of talazoparib and irinotecan with/without temozolomide in paediatric patients with recurrent/refractory solid malignancies., Patients and Methods: Cohorts of 3-6 patients with recurrent/refractory solid malignancies received escalating doses of oral talazoparib and intravenous irinotecan (arm A) and oral talazoparib, oral temozolomide and intravenous irinotecan (arm B) in a 3 + 3 design. Talazoparib was administered on days 1-6, and intravenous irinotecan and oral temozolomide were administered on days 2-6, of a 21-day course. Serum for talazoparib and irinotecan pharmacokinetics was obtained during course 1. UGT1A1 polymorphism and Schlafen family member 11 (SLFN11) immunohistochemical staining were performed., Results: Forty-one patients (20 males; median age, 14.6 years; 24 with recurrent disease) were evaluable for dose escalation. Twenty-nine and 12 patients were treated on arm A and arm B, respectively, for a total of 208 courses. The most common diagnosis was Ewing sarcoma (53%). The most common ≥grade III haematologic toxicities in arms A and B included neutropenia (78% and 31%, respectively) and thrombocytopenia (42% and 31%, respectively). In arms A and B, febrile neutropenia (24% and 14%, respectively) and diarrhoea (21% and 7%, respectively) were the most common ≥grade III non-hematologic toxicities. Six patients (Ewing sarcoma [5 patients] and synovial sarcoma [1 patient]) had a response (1 with a complete response, 5 with a partial response). The objective response rates were 10.3% (arm A) and 25% (arm B). Pharmacokinetic testing demonstrated no evidence of drug-drug interaction between talazoparib and irinotecan. UGT1A1 was not related to response. SLFN11 positivity was associated with best response to therapy., Conclusions: The combination of talazoparib and irinotecan with/without temozolomide is feasible and active in Ewing sarcoma, and further investigation is warranted., Competing Interests: Conflict of interest statement The authors declare no potential conflict of interest., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Background: The prognosis for children with recurrent solid tumors generally is poor. Targeting mammalian target of rapamycin (mTOR) and vascular endothelial growth factor A with everolimus and bevacizumab, respectively, synergistically improves progression-free survival and is well tolerated in adults with solid tumors., Methods: In the current phase 1 study, a total of 15 children with recurrent or refractory solid tumors were treated with bevacizumab and everolimus to establish the maximum tolerated dose, toxicity, and preliminary antitumor response (ClinicalTrials.gov identifier NCT00756340). The authors also evaluated everolimus-mediated inhibition of the mTOR pathway in the peripheral blood mononuclear cells of treated patients., Results: Tumors predominantly were soft tissue and/or bone sarcomas (8 cases) and brain tumors (5 cases). The first 2 patients enrolled at dose level 1 (10 mg/kg of bevacizumab and 4 mg/m 2 of everolimus) experienced dose-limiting toxicities (DLTs). The next 5 patients were enrolled at dose level 0 (8 mg/kg of bevacizumab and 4 mg/m 2 of everolimus), and DLTs occurred in 2 patients. The authors then modified the protocol to permit expansion of dose 0, and 8 additional patients were added, with no DLTs reported. Of all the patients, stable disease occurred in 4 patients (30.8%; median, 2 courses), and progressive disease occurred in 9 patients (69.2%). Overall survival was 0.59 years (95% CI, 0.24-1.05 years). The mTOR biomarker phospho-4EBP1 Thr/37/46 significantly decreased from baseline to day 27 in peripheral blood mononuclear cells (P = .045). Phospho-AKT levels also decreased from those at baseline., Conclusions: The maximum tolerated dose of cotreatment with bevacizumab and everolimus was 8 mg/kg of bevacizumab and 4 mg/m 2 of everolimus in a 4-week cycle for children with recurrent solid tumors., (© 2020 American Cancer Society.)

Purpose: To characterize the population pharmacokinetics of cyclophosphamide, active 4-hydroxy-cyclophosphamide (4OH-CTX), and inactive carboxyethylphosphoramide mustard (CEPM), and their associations with hematologic toxicities in infants and young children with brain tumors. To use this information to provide cyclophosphamide dosing recommendations in this population., Patients and Methods: Patients received four cycles of a 1-hour infusion of 1.5 g/m 2 cyclophosphamide. Serial samples were collected to measure cyclophosphamide, 4OH-CTX, and CEPM plasma concentrations. Population pharmacokinetic modeling was performed to identify the patient characteristics influencing drug disposition. Associations between drug exposures and metrics reflecting drug-induced neutropenia, erythropenia, and thrombocytopenia were investigated. A Bayesian approach was developed to predict 4OH-CTX exposure using only cyclophosphamide and CEPM plasma concentrations., Results: Data from 171 patients (0.07-4.9 years) were adequately fitted by a two-compartment (cyclophosphamide) and one-compartment model (metabolites). Young infants (<6 months) exhibited higher mean 4OH-CTX exposure than did young children (138.4 vs. 107.2 μmol/L·h, P < 0.0001). No genotypes exhibited clinically significant influence on drug exposures. Worse toxicity metrics were significantly associated with higher 4OH-CTX exposures. Dosing simulations suggested decreased cyclophosphamide dosage to 1.2 g/m 2 for young infants versus 1.5 g/m 2 for children to attain similar 4OH-CTX exposure. Bayesian-modeled 4OH-CTX exposure predictions were precise (mean absolute prediction error 14.8% ± 4.2%) and had low bias (mean prediction error 4.9% ± 5.1%)., Conclusions: A 4OH-CTX exposure-toxicity association was established, and a decreased cyclophosphamide dosage for young infants was suggested to reduce toxicity in this population. Bayesian modeling to predict 4OH-CTX exposure may reduce clinical processing-related costs and provide insights into further exposure-response associations., (©2019 American Association for Cancer Research.)

Aims: No population pharmacokinetic studies of high-dose methotrexate (HDMTX) have been conducted in infants with brain tumours, which are a vulnerable population. The aim of this study was to evaluate HDMTX disposition in these children to provide a rational basis for MTX dosing., Methods: Patients received 4 monthly courses of HDMTX (5 g/m 2 or 2.5 g/m 2 for infants aged ≤31 days) as a 24-h infusion. Serial samples were analysed for MTX by an enzyme immunoassay method. Pharmacokinetic parameters were estimated using nonlinear mixed effects population modelling. Demographics, concomitant medications and genetic polymorphisms were considered as pharmacokinetic covariates while MTX exposure and patient age were considered as covariates for Grade 3 and 4 toxicities., Results: The population pharmacokinetics of HDMTX were estimated in 178 patients (age range 0.02-4.7 years) in 648 courses. The population clearance and volume were 90 mL/min/m 2 and 14.4 L/m 2 , respectively. Significant covariates on body surface area adjusted MTX clearance included estimated glomerular filtration rate and co-treatment with dexamethasone or vancomycin. No significant association was observed between MTX toxicity and MTX exposure, patient age, leucovorin dosage or duration. MTX clearance in infants ≤31 days at enrolment was 44% lower than in older infants, but their incidence of toxicity was not higher since they also received a lower MTX dosage., Conclusions: By aggressively following institutional clinical guidelines, HDMTX-related toxicities were low, and using covariates from the population pharmacokinetic model enabled the calculation of a rational dosage for this patient population for future clinical trials., (© 2019 The British Pharmacological Society.)

Prexasertib (LY2606368) is a potent and selective small molecule inhibitor of cell-cycle checkpoint CHK1 and CHK2 protein kinases and is currently under clinical evaluation for treatment of pediatric malignancies. As a candidate therapy for pediatric Group 3 medulloblastoma (G3MB), prexasertib CNS penetration was evaluated in mice using cerebral microdialysis and pharmacokinetic modeling. A plasma pharmacokinetic study with a population-based design was performed in CD1 nude mice bearing G3MB orthotopically implanted in the brain and receiving a single dose of prexasertib (10 mg/kg, subcutaneously) to characterize prexasertib disposition and to establish a limited plasma sampling model for the microdialysis studies. The microdialysis studies were performed in both non-tumor bearing mice and in mice bearing G3MB receiving 10 mg/kg prexasertib subcutaneously, for up to 24 h post-dose. Plasma and extracellular fluid (ECF) concentrations were quantified using validated LC MS/MS methods, and analyzed using a population pharmacokinetic model. Model-derived prexasertib tumor/ECF to plasma partition coefficient K p,uu (ratio of tumor/brain ECF to unbound plasma AUC 0-24 h ) was significantly greater in G3MB tumor-bearing mice (0.17 ± 0.08) compared to non-tumor bearing mice (0.09 ± 0.04, p = 0.04). A pharmacodynamic study was then performed in mice bearing G3MB (20 mg/kg, IV) to evaluate prexasertib-induced target engagement after a single dose. Phosphorylated CHK1 serine 345 (pCHK1 S345), phosphorylated Histone 2A variant (γ-H2AX), and cleaved caspase-3 were quantified in mouse G3MB tumor tissues by immunohistochemistry at different time points up to 24 h post-dose. The induction of pCHK1 S345 and γ-H2AX peaked at 2 h after the dose and was elevated above baseline for at least 6 h, reflecting relevant CHK1 inhibition and DNA damage. Cleaved caspase-3 levels increased at 24 h suggesting initiation of cell apoptosis. Adequate unbound prexasertib exposure reached the brain tumor site relative to target engagement in G3MB tumor bearing mice at a clinically relevant dosage. These results support further preclinical and clinical development of prexasertib to treat children with medulloblastoma., (Copyright © 2019 Elsevier B.V. All rights reserved.)

A liquid chromatography electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) method was developed and validated to measure GDC-0084 in human plasma and cerebrospinal fluid (CSF). Reverse-phase chromatography with gradient elution was performed using a C 18 column (50 × 2.0 mm, 3 μm). Solid-phase extraction of plasma and CSF was employed to give excellent recovery. MS detection was performed with positive ion screening in multiple reaction monitoring mode. The precursor to the product ions (Q1 → Q3) selected for GDC-0084 and GDC-0084-d 6 were 383.2 → 353.2 and 389.2 → 353.2, respectively. A separate calibration curve was established for human plasma and CSF. Both calibration curves, ranging from 0.2 to 200 ng/mL, were linear and had acceptable intra- and inter-day precision and accuracy. The lower limit of quantitation and limit of detection for GDC-0084 in human plasma were 0.2 ng/mL (signal/noise ≥47) and 0.005 ng/mL (signal/noise ≥3.5), respectively, and for GDC-0084 in human CSF were 0.2 ng/mL (signal/noise ≥19.7) and 0.04 ng/mL (signal/noise ≥7.2). This method was successfully applied to analyze serial plasma samples obtained from children with diffuse intrinsic pontine gliomas and other midline gliomas who participated in pharmacokinetic studies as part of a phase I clinical trial of GDC-0084., (© 2019 John Wiley & Sons, Ltd.)

Purpose: Erlotinib (Tarceva ® ), a potent small molecule inhibitor of the epidermal growth factor receptor tyrosine kinase, has been evaluated to treat infants and children with primary brain tumors. The pharmacokinetics of erlotinib and its primary metabolite OSI-420 were characterized and exposure-safety associations were investigated., Methods: This analysis involved patients enrolled in two clinical studies and receiving oral erlotinib once daily as part of treatment. Single-dose and steady-state erlotinib and OSI-420 plasma concentrations were assayed using HPLC-MS/MS methods. Population pharmacokinetic modeling and univariate covariate analysis evaluating demographic, clinical and selected CYP3A5, CYP3A4, ABCB1, and ABCG2 genotypes were performed. Associations between erlotinib and OSI-420 pharmacokinetics, and with toxicities (diarrhea and skin rash) occurring post-dose were explored., Results: Data from 47 patients (0.7-19 years old) were collected and best fitted by one-compartment linear models. Erlotinib and OSI-420 apparent clearances (CL/F and CL m /F m ) were higher in patients < 5 years compared to older patients (mean CL/F: 6.8 vs 3.6 L/h/m 2 , and mean CL m /F m : 79 vs 38 L/h/m 2 , p < 0.001), and were 1.62-fold and 1.73-fold higher in males compared to females (p < 0.01). Moreover, CL/F was 1.53-fold higher in wild-type patients than in patients heterozygous or homozygous mutant for ABCG2 rs55930652 (p < 0.05). Most of the toxicities reported were grade 1. No associations were found between drug pharmacokinetics and drug-induced toxicities., Conclusions: Erlotinib therapy was well tolerated by pediatric patients with primary brain tumors. No dosing adjustments based on age or patient characteristics are recommended for this patient population.

Purpose: Ribociclib, an orally bioavailable small-molecule CDK4/6 inhibitor is currently undergoing evaluation to treat pediatric central nervous system (CNS) tumors. However, it is crucial that it penetrates the brain and tumor. Thus, the objectives of the present study were to derive a clinically relevant mouse dosage for cerebral microdialysis studies, and to characterize ribociclib CNS penetration in non-tumor bearing mice and in mice bearing DIPGx7 (glioma) cortical allograft tumors., Methods: A plasma pharmacokinetic study of ribociclib (100 mg/kg, orally) was performed in CD1 nude mice bearing glioma cortical allografts to obtain initial plasma pharmacokinetic parameters and to derive D-optimal plasma sampling time-points for microdialysis studies. Using a cerebral microdialysis technique, the extracellular fluid (ECF) disposition of ribociclib was evaluated after a single oral ribociclib dose (100 mg/kg) in non-tumor bearing mice and in mice bearing glioma cortical allografts. A one-compartment plasma model with absorption and ECF compartments were fit to plasma and ECF concentration-time data using a nonlinear mixed effects modeling approach (NONMEM 7.2)., Results: The mean unbound ribociclib plasma exposure (6812 ng/ml*h) was similar to that observed clinically at recommended dosages in adults. The median ribociclib ECF to plasma partition coefficient (K p,uu ) in non-tumor bearing and glioma mice was 0.10 and 0.07, respectively, and was not statistically different (t test, p = 0.19)., Conclusions: The CNS penetration observed was encouraging enough to move ribociclib forward with preclinical efficacy studies in models of pediatric brain tumors.

Aims: Tacrolimus has been associated with notable extrarenal adverse effects (AEs), which are unpredictable and impact patient morbidity. The association between model-predicted tacrolimus exposure metrics and standardized extrarenal AEs in stable renal transplant recipients was investigated and a limited sampling strategy (LSS) was developed to predict steady-state tacrolimus area under the curve over a 12-h dosing period (AUC ss,0-12h )., Methods: All recipients receiving tacrolimus and mycophenolic acid ≥6 months completed a 12-h cross-sectional observational pharmacokinetic-pharmacodynamic study. Patients were evaluated for the presence of individual and composite gastrointestinal, neurological, and aesthetic AEs during the study visit. The associations between AEs and tacrolimus exposure metrics generated from a published population pharmacokinetic model were investigated using a logistic regression analysis in NONMEM 7.3. An LSS was determined using a Bayesian estimation method with the same patients., Results: Dose-normalized tacrolimus AUC ss,0-12h and apparent clearance were independently associated with diarrhoea, dyspepsia, insomnia and neurological AE ratio. Dose-normalized tacrolimus maximum concentration was significantly correlated with skin changes and acne. No AE associations were found with trough concentrations. Using limited sampling at 0, 2h; 0, 1, 4h; and 0, 1, 2, 4h provided a precise and unbiased prediction of tacrolimus AUC (root mean squared prediction error < 10%), which was not well characterized using trough concentrations only (root mean squared prediction error >15%)., Conclusions: Several AEs (i.e. diarrhoea, dyspepsia, insomnia and neurological AE ratio) were associated with tacrolimus dose normalized AUC ss,0-12h and clearance. Skin changes and acne were associated with dose-normalized maximum concentrations. To facilitate clinical implementation, a LSS was developed to predict AUC ss,0-12h values using sparse patient data to efficiently assess projected immunosuppressive exposure and potentially minimize AE manifestations., (© 2018 The British Pharmacological Society.)

Tacrolimus, a calcineurin inhibitor, is a common immunosuppressant prescribed after organ transplantation and has notable inter- and intrapatient pharmacokinetic variability. The sources of variability have been investigated using population pharmacokinetic modeling over the last 2 decades. This article provides an updated synopsis on published nonlinear mixed-effects analyses developed for tacrolimus in transplant recipients. The objectives were to establish a detailed overview of the current data and to investigate covariate relationships determined by the models. Sixty-three published analyses were reviewed, and data regarding the study design, modeling approach, and resulting findings were extracted and summarized. Most of the studies investigated tacrolimus pharmacokinetics in adult and pediatric renal and liver transplants after administration of the immediate-release formulation. Model structures largely depended on the study sampling strategy, with ∼50% of studies developing a 1-compartment model using trough concentrations and a 2-compartment model with delayed absorption from intensive sampling. The CYP3A5 genotype, as a covariate, consistently impacted tacrolimus clearance, and dosing adjustments were required to achieve similar drug exposure among patients. Numerous covariates were identified as sources of interindividual variability on tacrolimus pharmacokinetics with limited consistency across these studies, which may be the result of the study designs. Additional analyses are required to further evaluate the potential impact of these covariates and the clinical implementation of these models to guide tacrolimus dosing recommendations. This article may be useful for guiding the design of future population pharmacokinetic studies and provides recommendations for the selection of an existing optimal model to individualize tacrolimus therapy., (© 2018, The American College of Clinical Pharmacology.)

Purpose: Cyclophosphamide is widely used to treat children with medulloblastoma; however, little is known about its brain penetration. We performed cerebral microdialysis to characterize the brain penetration of cyclophosphamide (130 mg/kg, IP) and its metabolites [4-hydroxy-cyclophosphamide (4OH-CTX) and carboxyethylphosphoramide mustard (CEPM)] in non-tumor bearing mice and mice bearing orthotopic Group 3 medulloblastoma., Methods: A plasma pharmacokinetic study was performed in non-tumor-bearing CD1- nude mice, and four cerebral microdialysis studies were performed in non-tumor-bearing (M1 and M3) and tumor- bearing mice (M2 and M4). Plasma samples were collected up to 6-hours post-dose, and extracellular fluid (ECF) samples were collected over 60-minute intervals for 24-hours post-dose. To stabilize and quantify 4OH-CTX, a derivatizing solution was added in blood after collection, and either directly in the microdialysis perfusate (M1 and M2) or in ECF collection tubes (M3 and M4). Plasma/ECF cyclophosphamide and CEPM, and 4OH-CTX concentrations were separately measured using different LC-MS/MS methods., Results: All plasma/ECF concentrations were described using a population-based pharmacokinetic model. Plasma exposures of cyclophosphamide, 4OH-CTX, and CEPM were similar across studies (mean AUC=112.6, 45.6, and 80.8 µmol∙hr/L). Hemorrhage was observed in brain tissue when the derivatizing solution was in perfusate compared with none when in collection tubes, which suggested potential sample contamination in studies M1 and M2. Model-derived unbound ECF to plasma partition coefficients (Kp,uu) were calculated to reflect CNS penetration of the compounds. Lower cyclophosphamide Kp,uu was obtained in tumor-bearing mice versus non-tumor bearing mice (mean 0.15 versus 0.22, p=0.019). No differences in Kp,uu were observed between these groups for 4OH- CTX and CEPM (overall mean 0.10 and 0.07)., Conclusions: Future studies will explore potential mechanisms at the brain-tumor barrier to explain lower cyclophosphamide brain penetration in tumor-bearing mice. These results will be used to further investigate exposure-response relationships in medulloblastoma xenograft models.

Tacrolimus exhibits inter-patient pharmacokinetic variability attributed to CYP3A5 isoenzymes and the efflux transporter, P-glycoprotein. Most black renal transplant recipients require higher tacrolimus doses compared to whites to achieve similar troughs when race-adjusted recommendations are used. An established guideline provides tacrolimus genotype dosing recommendations based on CYP3A5*1(W/T) and loss of protein function variants: CYP3A5*3 (rs776746), CYP3A5*6 (rs10264272), CYP3A5*7 (rs41303343) and may provide more comprehensive race-adjusted dosing recommendations. Our objective was to develop a tacrolimus population pharmacokinetic model evaluating demographic, clinical, and genomic factors in stable black and white renal transplant recipients. A secondary objective investigated race-based tacrolimus regimens and genotype-specific dosing. Sixty-seven recipients receiving oral tacrolimus and mycophenolic acid ≥6 months completed a 12-hour pharmacokinetic study. CYP3A5*3,*6,*7 and ABCB1 1236C>T, 2677G>T/A, 3435C>T polymorphisms were characterized. Patients were classified as extensive, intermediate, and poor metabolizers using a novel CYP3A5*3*6*7 metabolic composite. Modeling and simulation was performed with computer software (NONMEM 7.3, ICON Development Solutions; Ellicott City, Maryland). A 2-compartment model with first-order elimination and absorption with lag time best described the data. The CYP3A5*3*6*7 metabolic composite was significantly associated with tacrolimus clearance (P value < .05), which was faster in extensive (mean: 45.0 L/hr) and intermediate (29.5 L/hr) metabolizers than poor metabolizers (19.8 L/hr). Simulations support CYP3A5*3*6*7 genotype-based tacrolimus dosing to enhance general race-adjusted regimens, with dose increases of 1.5-fold and 2-fold, respectively, in intermediate and extensive metabolizers for comparable exposures to poor metabolizers. This model offers a novel approach to determine tacrolimus dosing adjustments that maintain comparable therapeutic exposure between black and white recipients with different CYP3A5 genotypes., (© 2018, The American College of Clinical Pharmacology.)

Purpose: Flotetuzumab (MGD006 or S80880) is a bispecific molecule that recognizes CD3 and CD123 membrane proteins, redirecting T cells to kill CD123-expressing cells for the treatment of acute myeloid leukemia. In this study, we developed a mathematical model to characterize MGD006 exposure-response relationships and to assess the impact of its immunogenicity in cynomolgus monkeys. Experimental Design: Thirty-two animals received multiple escalating doses (100-300-600-1,000 ng/kg/day) via intravenous infusion continuously 4 days a week. The model reflects sequential binding of MGD006 to CD3 and CD123 receptors. Formation of the MGD006/CD3 complex was connected to total T cells undergoing trafficking, whereas the formation of the trimolecular complex results in T-cell activation and clonal expansion. Activated T cells were used to drive the peripheral depletion of CD123-positive cells. Anti-drug antibody development was linked to MGD006 disposition as an elimination pathway. Model validation was tested by predicting the activity of MGD006 in eight monkeys receiving continuous 7-day infusions. Results: MGD006 disposition and total T-cell and CD123-positive cell profiles were well characterized. Anti-drug antibody development led to the suppression of T-cell trafficking but did not systematically abolish CD123-positive cell depletion. Target cell depletion could persist after drug elimination owing to the self-proliferation of activated T cells generated during the first cycles. The model was externally validated with the 7-day infusion dosing schedule. Conclusions: A translational model was developed for MGD006 that features T-cell activation and expansion as a key driver of pharmacologic activity and provides a mechanistic quantitative platform to inform dosing strategies in ongoing clinical studies. Clin Cancer Res; 24(11); 2631-41. ©2018 AACR ., (©2018 American Association for Cancer Research.)