Author Manuscript Published OnlineFirst on September 17, 2020; DOI: 10.1158/1078-0432.CCR-20-1696 Author manuscripts have been peer reviewed and accepted for publication but have not yet been edited. TITLE: Pediatric PK/PD Phase I Trial of Pexidartinib in Relapsed and Refractory Leukemias and Solid Tumors Including Neurofibromatosis Type I related Plexiform Neurofibromas AUTHORS: Lauren H. Boal1,2,5,6, John Glod1,6, Melissa Spencer1, Miki Kasai1, Joanne Derdak1, Eva Dombi1, Mark Ahlman3, Daniel W. Beury1, Melinda S. Merchant1 , Christianne Persenaire1, David J. Liewehr4, Seth M. Steinberg4, Brigitte C. Widemann1, Rosandra N. Kaplan1 1Pediatric Oncology Branch, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 2Center for Cancer and Blood Disorders, Children’s National Medical Center, Washington, District of Columbia 3 Radiology and Imaging Sciences, Clinical Center, National Institutes of Health, Bethesda, Maryland 4Biostatistics and Data Management Section, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 5Current address Massachusetts General Hospital for Children, Harvard Medical School, Boston, Massachusetts 6Authors contributed equally to this work Running title: Pexidartinib in refractory pediatric tumors Corresponding Author: Rosandra N. Kaplan National Cancer Institute/National Institutes of Health 10 Center Drive, Bethesda, MD 20817 [email protected] Page 1 Downloaded from clincancerres.aacrjournals.org on September 23, 2021. © 2020 American Association for Cancer Research. Author Manuscript Published OnlineFirst on September 17, 2020; DOI: 10.1158/1078-0432.CCR-20-1696 Author manuscripts have been peer reviewed and accepted for publication but have not yet been edited. Phone: 240-383-6697 Conflict of Interest Disclosures: The authors have no conflicts of interest to disclose STATEMENT OF TRANSLATIONAL RELEVANCE (120-150 words): Despite aggressive multimodal therapy, many high risk pediatric solid tumor patients develop metastatic progression and succumb to their disease. Novel treatment approaches targeting both tumor intrinsic pathways as well as elements of the metastatic microenvironment including immune suppressive myeloid cells such as tumor associated macrophages (TAMs) may be a promising strategy for improving outcomes. Colony Stimulating Factor 1 Receptor (CSF-1R) signaling is important in macrophage biology including impacting myeloid cell mobilization, migration, survival, and proliferation. Inhibition of CSF-1R through pexidartinib, an oral inhibitor of tyrosine kinases including CSF-1R, KIT, and FLT3, may decrease tumor progression by suppressing the effects of TAMs and other monocyte-derived populations in the tumor microenvironment. We conducted the first pediatric phase I trial of pexidartinib to study its safety profile, pharmacokinetics, and recommended pediatric phase II dose. The trial resulted in establishment of a safe, and feasible phase 2 dosing regimen and potential biomarkers for pediatric patients. ABSTRACT Purpose: Simultaneously targeting the tumor and tumor microenvironment (TME) may hold promise in treating children with refractory solid tumors. Pexidartinib, an oral inhibitor of tyrosine kinases including Colony Stimulating Factor 1 Receptor (CSF-1R), KIT, and FLT3, is FDA approved in adults with tenosynovial giant cell tumor (TGCT). A phase I trial was conducted in pediatric and young adult patients (pts) with refractory leukemias or solid tumors including neurofibromatosis type 1 (NF1) related Page 2 Downloaded from clincancerres.aacrjournals.org on September 23, 2021. © 2020 American Association for Cancer Research. Author Manuscript Published OnlineFirst on September 17, 2020; DOI: 10.1158/1078-0432.CCR-20-1696 Author manuscripts have been peer reviewed and accepted for publication but have not yet been edited. plexiform neurofibromas (PN). Materials and Methods: A rolling-six design with dose levels (DL) of 400 mg/m2, 600 mg/m2, and 800 mg/m2 once daily for 28 day cycles (C) was used. Response was assessed at regular intervals. PK and population PK were analyzed during C1. Results: Twelve pts (4 per DL, 9 evaluable) enrolled on the dose escalation phase and four patients enrolled in the expansion cohort: median (lower, upper quartile) age 16 (14, 16.5) years. No dose-limiting toxicities (DLT) were observed. PK appeared linear over three DLs. PK modeling and simulation determined a weight based recommended phase 2 dose (RP2D). Two pts had stable disease and 1 pt with peritoneal mesothelioma (C49+) had a sustained partial response 67% RECIST reduction. PD markers included a rise in plasma macrophage colony stimulating factor levels and a decrease in absolute monocyte count . Conclusions: Pexidartinib in pediatric pts was well tolerated at all DL tested, achieved target inhibition and resulted in a weight based RPD2 dose. Page 3 Downloaded from clincancerres.aacrjournals.org on September 23, 2021. © 2020 American Association for Cancer Research. Author Manuscript Published OnlineFirst on September 17, 2020; DOI: 10.1158/1078-0432.CCR-20-1696 Author manuscripts have been peer reviewed and accepted for publication but have not yet been edited. INTRODUCTION Despite improvement in therapies for children and young adults with cancer many patients develop metastasis and progressive disease and there is a growing appreciation for the tumor cell extrinsic regulation in this process1-3. Targeting microenvironment dependencies may be a promising direction for improving survival in pediatric solid tumor patients. Agents that have both a direct anti-tumor effect and reform the tumor microenvironment are especially attractive. Pexidartinib (PLX3397) is an oral small molecule inhibitor of class III protein tyrosine kinases including CSF-1R, KIT, and oncogenic FLT3 kinase4,5. In addition to direct tumor targeting, pexidartinib acts on solid tumors by inhibiting FLT3 kinase and KIT on myeloid progenitor cells and CSF-1R signaling which is important in the mobilization, migration, survival, and proliferation of monocytes and macrophages. Myeloid cells are the most abundant immune cell within many tumors and often increase in number during metastatic progression6,7. The microenvironment of many pediatric solid tumors is rich in immune suppressive tumor associated macrophages (TAMs)8-10, and inhibition of CSF-1R may interfere with their development or function11- 13. Although the full picture of the diversity and differential function of myeloid cells in pediatric malignancies is incomplete, there is growing evidence that a heterogeneous population of myeloid cells regulate progression of many diverse pediatric cancers including myeloid cells that promote cancer growth progression, and regulate immune suppression in osteosarcoma, soft tissue sarcomas and rhabdoid tumors 14-18. The myeloid cell populations within the tumor microenvironment in pediatric cancers can hold both pro-tumorigenic and anti-tumor functions19. Polarization of monocytes and neutrophils to an immune suppressive phenotype or to antigen presentation and phagocytic roles is seen Page 4 Downloaded from clincancerres.aacrjournals.org on September 23, 2021. © 2020 American Association for Cancer Research. Author Manuscript Published OnlineFirst on September 17, 2020; DOI: 10.1158/1078-0432.CCR-20-1696 Author manuscripts have been peer reviewed and accepted for publication but have not yet been edited. in different myeloid cell populations and may vary in myeloid cell populations in different cell states depending on the signals within the local microenvironment. Evolving evidence suggests that classical monocytes or monocytic myeloid derived suppressor cells that express high levels of CSF1R can establish a supportive environment that promotes cancer cell survival, therapeutic resistance in pediatric leukemia, gliomas and neuroblastoma similar to findings in adult carcinomas14,19,20. Targeting any one particular signaling axis may not be sufficient to dramatically alter the myeloid component of the tumor microenvironment, but inhibition of the CSF1-CSF1R axis holds promise to limit the CSF1R high expressing M2 macrophage and monocytic myeloid derived suppressor cells (MDSCs) which are associated with enhanced inflammation and angiogenesis, diminished tumor specific T cell responses and increased tumor invasion and metastasis11,15,20. Diminishing CSF-CSF1R signaling may tip the balance in favor of M1 macrophages that can induce anti-tumor T cell responses and phagocytosis of stressed and dying tumor cells. Neurofibromatosis type 1 (NF1) related plexiform neurofibromas (PN) contain abundant TAMs, mast cells and NF1 -/- Schwann cells and this microenvironment produces high levels of stem cell factor I (scf-1) and IL34, the ligands for KIT and CSF-1R respectively21-24. Inhibition of CSF-1R and KIT in NF1 related PN may decrease tumor progression25. In refractory leukemias, FLT3 and KIT inhibition may be beneficial through a direct effect on neoplastic cells. KIT is overexpressed in up to 80% of acute myelogenous leukemia (AML) 26-28 and FLT3 and FLT3 ligand are increased in several pediatric leukemias, with aberrant expression in more than 90% of AML including leukemia stem cells29and nearly 100% of B-cell acute lymphoblastic leukemia30. In regard to both acute myeloid leukemia and acute lymphoblastic leukemia, especially in the recurrent setting, the leukemia cells may be regulated by a myeloid immune suppressive bone