This application is prepared in response to the funding opportunity: NCI Pediatric In Vivo Testing Program (U01), RFA-CA-20-034 to renew our existing PPTC UO1 grant. Specifically, we propose to continue the in vivo testing program for central nervous system (CNS) tumors using our panel of patient derived orthotopic xenograft (PDOX) models. Brain tumor is the leading cause of cancer-related death in children. One of the challenges in clinical drug development is how to effectively prioritize drug candidates to ensure clinical success in cancer patients. However, efforts in identifying new anti-cancer agents for that are most likely to be effective in the clinic have been blocked for many years due to the lack of clinically relevant and molecularly accurate model system. Fortunately, we have established a panel of 150 PDOX models of pediatric brain tumors through direct injection of patient tumor specimens into the brains of SCID mice. These PDOX models are shown to have replicated the histopathology and major genetic abnormalities of the original patient tumors even during serial sub-transplantations in vivo in mouse brains. They not only represent different clinical stage (i.e., at diagnosis, relapse and terminal/autopsy) but also replicate a broad spectrum of the newly identified molecular subtypes of nearly all types of pediatric brain tumors. The xenograft tumor cells can also be cryopreserved for sustained and on-demand supply of tumorigenic PDOX cells. This capacity combined with our optimized surgical procedure, with which we can implant up to 260 mice per day, makes it possible for us to test multiple (e.g., 6-10) drugs per year for every tumor type. Our objective is therefore to make use of this unique panel of PDOX models to examine therapeutic efficacy of new agents and to analyze mechanisms of action and therapy resistance in high grade glioma, medulloblastoma, ependymoma, DIPG and ATRT. Our hypothesis is that these patient-specific PDOX tumors will respond to anti-cancer therapies similarly to the corresponding human primary tumors, and the effective agents identified through this system would have better chances of clinical success. To test this hypothesis, we will perform a series of in vitro and in vivo assays to achieve the following aims: 1) to identify genetically accurate candidate PDOX models that bear the therapeutic target(s) of new investigational drugs through data mining of our mouse model molecular characterization databases; 2) to select the most responsive models through functional in vitro screening to determine time- and dose-responses; 3) to demonstrate therapeutic efficacy of new investigational drugs in multiple target-bearing PDOX models; and 4) to perform detailed analysis of cellular and molecular mechanisms of cell killing as well as the causes of therapy resistance both in vitro and in vivo. Our novel panel of PDOX mouse models represents a broad spectrum of genetic abnormalities of pediatric CNS tumors. All ...