The study objectives are to assess the potential for PLX3397 to improve the efficacy of standard of care radiation therapy (RT) + temozolomide in patients with newly diagnosed glioblastoma (GBM).

Study drug will be administered twice daily for 7 days prior to the initiation of RT (radiation therapy)and will continue twice daily during the course of RT. The RT schedule will be once daily for 5 days per week for 6 weeks (total radiation dose of 60 Gy. Oral temozolomide will be administered once daily (7 days per week) for the duration of RT. Four weeks after the completion of the course of RT, patients will be started on once-daily adjuvant temozolomide (Day 1-5 of a 28 day cycle) and PLX3397 twice daily (28 days of a 28 day cycle) for up to 12 cycles in the absence of progressive disease or unacceptable toxicities. After discontinuation of study drug, patients will continue to be followed for OS every 6 months. For patients participating in the run-in Phase 1b of the study, intra patient dose escalation will be permitted after a RP2D has been established. The Phase 2 portion of the study will enroll patients to be treated with PLX3397 at RP2D.

For the Phase 1b portion of the study, 2 cohorts (800 mg/day and 1000 mg/day) are planned to be enrolled at approximately 7-10 sites. Each cohort will consist of approximately 7 patients. Therefore, a minimum of 14 patients are planned to be enrolled in Phase 1b. Additional patients may be required for replacement patients, or if lower dose cohorts (600 mg or 400 mg) are required. For the Phase 2 portion of the study, enrollment is planned to include approximately 44 patients.

Inclusion Criteria:

• Male or female patients ≥18 years old.
• Histologically confirmed definitive GBM or gliosarcoma by partial or complete surgical resection (i.e. not by biopsy only) within 5 weeks prior to PLX3397 administration (C1D1). Tumor must have a supratentorial component. For all patients, availability of a surgical paraffin tumor block sufficient to generate at least 20 unstained slides; or, if a paraffin tumor block is unavailable, at least 20 unstained slides.
• The patient must have recovered from the effects of surgery, post-operative infection, and other complications before study registration.
• A diagnostic contrast-enhanced MRI or CT scan of the brain must be performed preoperatively and postoperatively prior to the initiation of radiotherapy, within 28 days (preferably 14 days) prior to C1D1.
• Patients unable to undergo MR imaging because of non-compatible devices can be enrolled, provided pre- and post-operative contrast-enhanced CT scans are obtained and are of sufficient quality.
• Patients must receive RT at the participating institution.
• Women of child-bearing potential must have a negative pregnancy test within 14 days of initiation of dosing and must agree to use an acceptable method of birth control while on study drug and for 3 months after the last dose. Women of non-childbearing potential may be included if they are either surgically sterile or have been postmenopausal for ≥1 year. Men of child-bearing potential must also agree to use an acceptable method of birth control while on study drug, and for 3 months after the last dose.
• Karnofsky performance status of ≥70.
• Adequate hematologic, hepatic, and renal function (absolute neutrophil count ≥1.5x 109/L, Hgb >10 g/dL, platelet count ≥100 x 109/L, AST/ALT ≤2.5x ULN, creatinine ≤1.5x ULN).
• Willing and able to provide written informed consent prior to any study related procedures and to comply with all study requirements.

Exclusion Criteria:

• Evidence of recurrent GBM or metastases detected outside of the cranial vault.
• Investigational drug use within 28 days of the first dose of PLX3397 or concurrently.
• Prior use of Gliadel wafers or any other intratumoral or intracavitary treatment.
• Prior radiation or chemotherapy for glioblastoma or glioma.
• Prior chemotherapy or radiosensitizers for cancer of the head and neck (except for T1 glottic cancer) that would result in an overlap of radiation fields.
• Prior allergic reaction to temozolomide.
• History of Grade 2 (CTCAE v4) or greater acute intracranial hemorrhage.
• Active cancer (either concurrent or within the last 3 years) that requires non-surgical therapy (e.g. chemotherapy or radiation therapy), with the exception of surgically treated basal or squamous cell carcinoma of the skin, melanoma in-situ, or carcinoma in-situ of the cervix.
• Chronic active hepatitis B or C.
• Refractory nausea and vomiting, malabsorption, biliary shunt, or significant bowel resection that would preclude adequate absorption of study drug.
• Patients with serious illnesses, uncontrolled infection, medical conditions, or other medical history including abnormal laboratory results, which in the investigator's opinion would be likely to interfere with a patient's participation in the study, or with the interpretation of the results.
• Women of child-bearing potential who are pregnant or breast feeding.
• At Screening QTcF ≥450 msec for males and ≥470 msec for females.

De-identified individual participant data (IPD) and applicable supporting clinical trial documents may be available upon request at https://vivli.org/. In cases where clinical trial data and supporting documents are provided pursuant to our company policies and procedures, Daiichi Sankyo will continue to protect the privacy of our clinical trial participants. Details on data sharing criteria and the procedure for requesting access can be found at this web address: https://vivli.org/ourmember/daiichi-sankyo/

Studies for which the medicine and indication have received European Union (EU) and United States (US), and/or Japan (JP) marketing approval on or after 01 January 2014 or by the US or EU or JP Health Authorities when regulatory submissions in all regions are not planned and after the primary study results have been accepted for publication.

Formal request from qualified scientific and medical researchers on IPD and clinical study documents from clinical trials supporting products submitted and licensed in the United States, the European Union and/or Japan from 01 January 2014 and beyond for the purpose of conducting legitimate research. This must be consistent with the principle of safeguarding study participants' privacy and consistent with provision of informed consent.