Target Validation and Efficacy of Metformin in Patients With Posterior Fossa Group A (PFA) Ependymoma

This is a multi-site study of the pharmacodynamic effects and efficacy of metformin in children and young adults with recurrent or progressive Posterior Fossa Group A (PFA) ependymoma.

Inclusion Criteria: 1. Participants must have recurrent or progressive posterior fossa A (PFA) ependymoma following surgery AND radiation treatment (RT). 2. Participants must have a diagnosis of PFA ependymoma. Any number of previous recurrences are permissible provided the participant meets other enrollment criteria. 3. Participants must have adequate tumor tissue available from initial diagnosis or from pre- trial enrollment. Formalin-fixed paraffin-embedded (FFPE) material (1 full block) should be provided. If FFPE material is not available, 10 unstained slides with an accompanying hematoxylin and eosin (H\&E) report should be provided. Target Validation (TV) Phase: o Participants are candidates to undergo elective surgery for removal of all or a portion of their recurrent/progressive tumor. Efficacy Phase: * Participant must have measurable disease; this will be defined as lesions that can be accurately measured in two dimensions (longest diameter to be recorded) The size threshold is met if both in-plane diameters are ≥10 mm or both in-plane diameters are at least two times the MRI slice thickness, plus the interslice gap with a minimum size of no less than double the slice thickness on MRI. * Participants with an isolated local progression of the tumor following RT (or stereotactic radiosurgery, SRS) must be \> 6 months from completion of RT to the lesion to rule out pseudo progression or must have tissue confirmation of progression prior to enrollment. * Previously irradiated lesions are considered non-measurable except in cases of documented progression of the lesion since the completion of radiation therapy as outlined above. 4. Prior Therapy: Participants must not be receiving metformin for other medical indications or previous exposure to metformin following their diagnosis of PFA ependymoma. However, participants treated on the TV phase, but did not continue onto maintenance therapy will be allowed to enroll on the efficacy phase with future recurrences or progression of their disease. 5. Age: 1 -39 years at the time of enrollment. 6. Performance Score: Karnofsky \>= 50 for participants \> 16 years of age and Lansky \>=50 for participants \= 1000/mm\^3. 2. Platelet count \>=100,000/mm\^3 (transfusion independent, defined as not receiving platelet transfusions for at least 7 days prior to enrollment). 3. Serum creatinine \< 1.5 Upper Limit normal (ULN) based on age and gender 4. Total bilirubin \