Metformin for Brain Repair in Children With Cranial-Spinal Radiation for Medulloblastoma

Inclusion criteria: Survivors will be included if they:

1. Have been treated with cranial or cranial-spinal radiation,
2. Are between 5 and 21 years of age at time of consent, and
3. Either declare English as their native language or have had at least two years of schooling in English at the time of their baseline assessment.
4. Have been diagnosed with a brain tumour requiring treatment with cranial or cranial-spinal radiation at least 2 years ago, is not receiving active treatment and no more than 15 years may have elapsed between treatment with cranial-spinal radiation and time of the trial. Survivors with a shunt will be included in the trial, but will need to be identified prior to study enrollment to discuss any specific considerations for imaging.
5. Meet criteria for adequate organ function requirements:

   1. Adequate renal function defined as: Creatinine clearance or radioisotope glomerular filtration rate (GFR) \> 70ml/min/1.73 m2 or serum creatinine based on age/gender as follows:

      Maximum Serum Creatinine Level (mg/dL)

      5 to \< 10 years: Male = 1; Female = 1

      10 to \< 13 years: Male = 1.2; Female = 1.2

      13 to \< 16 years: Male = 1.5; Female = 1.4

      ≥ 16 years: Male = 1.7; Female = 1.4
   2. Adequate liver function defined as:

   Total bilirubin \< 1.5 x upper limit of normal (ULN) for age, and,

   serum glutamate oxaloacetate transaminase (SGOT) (AST) or serum glutamate pyruvate transaminase (SGPT) (ALT) \< 3 x upper limit of normal (ULN) for age.
6. Females of childbearing potential must have a negative pregnancy test result and must agree to use a medically acceptable method of contraception throughout the entire study period and for 30 days after the last dose of study drug.
7. Informed consent will be obtained from the participants and/or their legal guardians by study team members authorized to consent for this study.

Exclusion criteria: Survivors will be excluded if they

1. Are receiving palliative care.
2. Are unable to participate in neuro-imaging without sedation as this is the primary outcome measure for the trial.
3. Are unable to swallow tablets.
4. Are unstable and/or insulin-dependent (Type 1) diabetic patients.
5. Have acute or chronic metabolic acidosis and/or lactic acidosis.
6. Any female patient or partner who has reached menarche and male patients who are not willing to use an effective method of contraception.
7. Patient who is pregnant or lactating and does not agree to stop breastfeeding while receiving trial treatment.
8. Have a history of renal disease or renal dysfunction e.g., as suggested by elevated serum creatinine levels (see 5.a. Inclusion criteria) or abnormal creatinine clearance.
9. Have a history of congestive heart failure requiring pharmacologic treatment.
10. Have a known hypersensitivity to metformin hydrochloride.