Inclusion Criteria: Screening Phase

          -  Participants with recurrent, refractory, or progressive medulloblastoma.

          -  Age ≥ 1 year and < 25 years at the time of screening.

          -  Participants and/or guardian can understand and is willing to sign a written informed
             consent document according to institutional guidelines.

        Exclusion Criteria: Screening Phase

          -  Previous exposure to any CHK1 inhibitor.

          -  Participants with a history of clinically significant, uncontrolled heart disease
             and/or repolarization abnormalities.

          -  Participants with any history of QTc prolongation (i.e. QTc interval of > 480 msec).

        Inclusion Criteria: Strata A and B

          -  Participant must be ≥1 year and <25 years of age at time of screening.

          -  Participant must have recurrent, progressive or refractory Group 3/Group 4 or SHH
             medulloblastoma (per central pathology confirmation of primary tissue and/or relapsed
             tissue). Central pathology review previously completed at St. Jude Children's Research
             Hospital using equivalent methods can be used for enrollment. Note: Group 3/Group 4
             may be referred to as Non-WNT Non-SHH (NWNS) in pathology reports. Medulloblastoma
             patients with indeterminate molecular subgroup after central pathology review are
             eligible for enrollment on stratum A.

          -  Participant must have measurable or evaluable disease as defined in the protocol.

          -  Participant must have received their last dose of myelosuppressive anticancer
             chemotherapy at least 3 weeks prior to study enrollment.

          -  Participants must have had their last fraction of radiation (including CSI) at least 4
             weeks prior to study enrollment. Participants who received radiation therapy for
             palliation must have had their last fraction of radiation at least 2 weeks prior to
             study enrollment.

             -- Note: Participants must have relapsed with recurrent, progressive or refractory
             disease after any prior radiation therapy that is not considered palliative.
             Palliative radiation therapy is defined as local small port RT to alleviate and/or
             palliate symptoms. (CSI, whole brain RT, large field/port RT, or large field/port
             multilevel spinal RT will not be considered palliative at any dose.)

          -  Participant who are receiving corticosteroids must be on a stable or decreasing dose
             for at least 1 week prior to enrollment with no plans for escalation.

          -  Participant must have a Lansky (≤ 16 years) or Karnofsky (> 16 years) performance
             score of ≥50 and, in the opinion of the investigator, a minimum life expectancy of at
             least 6 weeks.

             -- Note: Participants who are unable to walk because of paralysis, but who are up in a
             wheelchair, will be considered ambulatory for the purpose of assessing the performance
             score.

          -  Participant must have adequate bone marrow and organ function as defined as:

               -  ANC ≥ 1.0 x 10^9/L without growth factor support within 7 days

               -  Platelet count ≥ 75x 10^9/L without support of a platelet transfusion within 7
                  days

               -  Hemoglobin ≥8.0 g/dL without support of a blood transfusion within 7 days

               -  Potassium, total calcium (corrected for serum albumin), magnesium, sodium and
                  phosphorus within institutional normal limits or corrected to within normal
                  limits with supplements before first dose of study medication

               -  Serum creatinine ≤ the maximum serum creatinine based on age/gender: Age: 1 to <
                  2 years; maximum serum creatinine (mg/dL): 0.6 (male, female); Age: 2 to < 6
                  years; maximum serum creatinine (mg/dL): 0.8 (male, female); Age: 6 to < 10
                  years; maximum serum creatinine (mg/dL): 1 (male, female); Age: 10 to < 13 years;
                  maximum serum creatinine (mg/dL): 1.2 (male, female); Age: 13 to < 16 years;
                  maximum serum creatinine (mg/dL): 1.5 (male), 1.4 (female); Age :≥ 16 years;
                  maximum serum creatinine (mg/dL): 1.7 (male), 1.4 (female).

               -  Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 2.5 x ULN.
                  For the purposes of this study the ULN of ALT and AST is 45 U/L.

               -  Total bilirubin ≤ ULN; or if > ULN then direct bilirubin ≤ 1.5 x ULN

          -  Female participants of childbearing age must have a negative pregnancy test at the
             time of enrollment.

          -  Participants of childbearing or child fathering potential must be willing to use
             medically acceptable form of birth control during treatment and for 16 weeks after
             stopping treatment.

          -  Participants and/or guardian have the ability to understand and the willingness to
             sign a written informed consent document according to institutional guidelines.

        Exclusion Criteria: Strata A and B

          -  Participant who is receiving any other investigational agents.

          -  Participants with other clinically significant medical disorders (i.e. serious
             infections or significant cardiac, pulmonary, hepatic, psychiatric, or other organ
             dysfunction) that could compromise their ability to tolerate protocol therapy or would
             interfere with the study procedures or results.

          -  Participant with a history of clinically significant, uncontrolled heart disease
             and/or repolarization abnormalities as documented by a standard 12-lead ECG.

          -  Shortening fraction of <27% by ECHO or ejection fraction of <50% by gated radionuclide
             study.

          -  Prior history of QTc prolongation or QTc interval of > 480 msec.

          -  Female participants who are breastfeeding a child.

          -  Participants are excluded if unable to comply with guidelines listed in appendix I.