Nicholas Butowski1, Howard Colman1, John F De Groot1, Antonio M Omuro1, Lakshmi Nayak1, Patrick Y Wen1, Timothy F Cloughesy1, Adhirai Marimuthu1, Sam Haidar1, Arie Perry1, Jason Huse1, Joanna Phillips1, Brian L West1, Keith B Nolop1, Henry H Hsu1, Keith L Ligon1, Annette M Molinaro1, Michael Prados1. 1. University of California at San Francisco, San Francisco, California (N.B., A.P., J.P., A.M.M, M.P.); Huntsman Cancer Institute, University of Utah, Salt Lake City, Utah (H.C.); The University of Texas MD Anderson Cancer Center, Houston, Texas (J.F.D.G.); Department of Neurology, Memorial Sloan Kettering Cancer Hospital, New York, New York (A.M.O.); Center for Neuro-Oncology, Dana-Farber/Brigham and Women's Cancer Center, Boston, Massachusetts (L.N., P.Y.W., S.H., K.L.L.); UCLA Medical Center, Los Angeles, California (T.F.C.); Plexxikon Inc., Berkeley, California (A.M., B.L.W., K.B.N., H.H.H.); Department of Pathology, Brigham and Women's Hospital, Boston, Massachusetts (K.L.L.); Department of Pathology, Memorial Sloan Kettering Cancer Hospital, New York, New York (J.H.).
Abstract
BACKGROUND: The colony stimulating factor 1 receptor (CSF1R) ligands, CSF1 and interleukin-34, and the KIT ligand, stem cell factor, are expressed in glioblastoma (GB). Microglia, macrophages, blood vessels, and tumor cells also express CSF1R, and depletion of the microglia reduces tumor burden and invasive capacity. PLX3397 is an oral, small molecule that selectively inhibits CSF1R and KIT, penetrates the blood-brain barrier in model systems, and represents a novel approach for clinical development. METHODS: We conducted a phase II study in patients with recurrent GB. The primary endpoint was 6-month progression-free survival (PFS6). Secondary endpoints included overall survival response rate, safety, and plasma/tumor tissue pharmacokinetics. Exploratory endpoints included pharmacodynamic measures of drug effect in blood and tumor tissue. RESULTS: A total of 37 patients were enrolled, with 13 treated prior to a planned surgical resection (Cohort 1) and 24 treated without surgery (Cohort 2). PLX3397 was given at an oral dose of 1000 mg daily and was well tolerated. The primary efficacy endpoint of PFS6 was only 8.6%, with no objective responses. Pharmacokinetic endpoints revealed a median maximal concentration (Cmax) of 8090 ng/mL, with a time to attain Cmax of 2 hour in plasma. Tumor tissue obtained after 7 days of drug exposure revealed a median drug level of 5500 ng/g. Pharmacodynamic changes included an increase in colony stimulating factor 1 and reduced CD14(dim)/CD16+ monocytes in plasma compared with pretreatment baseline values. CONCLUSION: PLX3397 was well tolerated and readily crossed the blood-tumor barrier but showed no efficacy. Additional studies are ongoing, testing combination strategies and potential biomarkers to identify patients with greater likelihood of response.
BACKGROUND: The colony stimulating factor 1 receptor (CSF1R) ligands, CSF1 and interleukin-34, and the KIT ligand, stem cell factor, are expressed in glioblastoma (GB). Microglia, macrophages, blood vessels, and tumor cells also express CSF1R, and depletion of the microglia reduces tumor burden and invasive capacity. PLX3397 is an oral, small molecule that selectively inhibits CSF1R and KIT, penetrates the blood-brain barrier in model systems, and represents a novel approach for clinical development. METHODS: We conducted a phase II study in patients with recurrent GB. The primary endpoint was 6-month progression-free survival (PFS6). Secondary endpoints included overall survival response rate, safety, and plasma/tumor tissue pharmacokinetics. Exploratory endpoints included pharmacodynamic measures of drug effect in blood and tumor tissue. RESULTS: A total of 37 patients were enrolled, with 13 treated prior to a planned surgical resection (Cohort 1) and 24 treated without surgery (Cohort 2). PLX3397 was given at an oral dose of 1000 mg daily and was well tolerated. The primary efficacy endpoint of PFS6 was only 8.6%, with no objective responses. Pharmacokinetic endpoints revealed a median maximal concentration (Cmax) of 8090 ng/mL, with a time to attain Cmax of 2 hour in plasma. Tumor tissue obtained after 7 days of drug exposure revealed a median drug level of 5500 ng/g. Pharmacodynamic changes included an increase in colony stimulating factor 1 and reduced CD14(dim)/CD16+ monocytes in plasma compared with pretreatment baseline values. CONCLUSION:PLX3397 was well tolerated and readily crossed the blood-tumor barrier but showed no efficacy. Additional studies are ongoing, testing combination strategies and potential biomarkers to identify patients with greater likelihood of response.
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