Inclusion Criteria:

- Age

- Patients must be 12 months of age and 30 years of age at the time of study
entry for patients diagnosed with DIPG.

- Patients must be 12 months of age and 21 years of age at the time of study
entry for patients diagnosed with HGG.

- RB status

- Diagnostic stereotactic biopsy: Patients diagnosed with DIPG may choose to have
a stereotactic biopsy prior to starting radiation therapy.

- Toxicities related to biopsy must have resolved prior to proceeding with
radiation therapy.

- Screening for Rb applies to all patients with available tissue except for
patients diagnosed with DIPG and bi-thalamic tumors.

- If resection occurred at an outside institution, eligibility and treatment MRI
evaluations in addition to Rb testing must be completed at CCHMC.

- Tumor

- Diffuse Intrinsic Pontine Glioma (DIPG) Patients with newly diagnosed diffuse
intrinsic pontine gliomas (DIPGs), defined as tumors with a pontine epicenter
and diffuse intrinsic involvement of the pons, are eligible without histologic
confirmation.

- Patients with brainstem tumors that do not meet these criteria or not considered
to be typical intrinsic pontine gliomas will only be eligible if the tumors are
biopsied and proven to be an anaplastic astrocytoma, glioblastoma multiforme,
gliosarcoma, anaplastic mixed glioma or fibrillary astrocytoma.

- Note: Patients with juvenile pilocytic astrocytoma, pilomyxoid astrocytoma,
gangliogliomas, or other mixed gliomas without anaplasia are not eligible.

- Patients with diffuse intrinsic brain stem glioma (DIPG) can be enrolled without
the need for available tumor tissue for RB protein status confirmation.

- Bi-thalamic tumors, biopsied and noted to have intact RB. Bi-thalamic tumors
that are not biopsied will be eligible to enroll on the DIPG/bi-thalamic
non-biopsied arm.

- Patients with disseminated disease are not eligible, and MRI of spine must be
performed prior to enrollment if the treating physician suspects disseminated
disease.

- High-grade Glioma (HGG)

- Patients must have had histologically verified anaplastic astrocytoma,
glioblastoma multiforme, high grade glioma, NOS or gliosarcoma AND RB positive
noted on immunohistochemistry. Patients with primary spinal cord tumors are
eligible.

- Patients with a diagnosis of oligodendroglioma or oligoastrocytoma are not
eligible.

- Performance Status Karnofsky > 50 % for patients >16 years of age or Lansky > 50 for
patients < 16 years of age. Patients 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.

- Prior Therapy Patients must have not received any prior therapy other than surgery,
radiation and/or steroids.

- Radiation therapy requirements

- Patients with DIPG who received radiation therapy should have received radiation
therapy at the current standard dose of 54 Gy total. Radiation therapy will be
interrupted for acute toxicities per the treating physician.

- Patients diagnosed with HGG will receive radiation therapy at the current
standard dose of 59.4 Gy total. Radiation therapy will be interrupted for acute
toxicities per the treating physician.

- Timing of Radiation

- Non-biopsied DIPG or bi-thalamic tumors 30 days post radiographic diagnosis

- DIPG with stereotactic biopsy 37 days post radiographic diagnosis and at least
14 days post biopsy

- HGG 37 days post surgery and at least 14 days post biopsy or resection.

- Organ Function

- Patients must have normal organ and marrow function documented within 14 days of
study entry and within 7 days of the start of treatment of Ribociclib as defined
below:

- Absolute neutrophil count 1000mm3

- Platelets 75,000/mm3 (unsupported for 7 days)

- Hemoglobin 9g/dl (unsupported) for 7 days

- Total bilirubin 3 times upper limit of normal (ULN) for age

- ALT (SGPT) 2.5 X ULN for age

- Albumin 2 g/dl

- Creatinine clearance or radioisotope GFR 70mL/min/1.73m2 or serum creatinine
based on age/gender.

- Pregnancy Status Female patients of childbearing potential, must not be pregnant or
breast-feeding. Female patients of childbearing potential must have a negative serum
or urine pregnancy test.

- Pregnancy Prevention Patients of childbearing or child fathering potential must use a
highly effective method of contraception throughout the study while taking the drug
and for 21 days after stopping treatment

- Female patients with infants must agree not to breastfeed their infants while on this
study.

- Informed Consent Signed informed consent according to institutional guidelines must
be obtained. Assent, when appropriate, will be obtained according to institutional
guidelines.

Exclusion Criteria:

- Concurrent Illness Patients with any clinically significant unrelated systemic
illness (serious infections or significant cardiac, pulmonary, hepatic or other organ
dysfunction) that would compromise the patient's ability to tolerate protocol therapy
or would likely interfere with the study procedures or results.

- Inability to Participate Patients with inability to return for follow-up visits or
obtain follow-up studies required to assess toxicity to therapy.

- Received a radiosensitizer or any additional adjuvant therapy during radiation
therapy.

- Seizures Patients who are currently receiving enzyme inducing anti-epileptic drugs
that are known strong inducers or inhibitors of CYP3A4/5 (EIAEDs). Patients with a
history of seizures and maintained on an anti-epileptic drug that is not a strong
inducers or inhibitor of CYP3A4/5 are eligible.

- Patient has a known hypersensitivity to Ribociclib or any of its excipients.

- Clinically significant active cardiac disease, uncontrolled heart disease and/or
history of cardiac dysfunction including any of the following

- History of acute coronary syndromes (including myocardial infarction, unstable
angina, coronary artery bypass grafting, coronary angioplasty, or stenting) or
symptomatic pericarditis within 12 months prior to screening

- History of documented congestive heart failure (New York Heart Association
functional classification III-IV)

- Documented cardiomyopathy

- Patient has a Left Ventricular Ejection Fraction (LVEF) < 50 % as determined by
Multiple Gated acquisition (MUGA) scan or echocardiogram (ECHO) at screening

- History of any cardiac arrhythmias, e.g., ventricular, supraventricular, nodal
arrhythmias, or conduction abnormality within 12 months of screening

- Long QT syndrome or family history of idiopathic sudden death or congenital long
QT syndrome, or any of the following:

- Risk factors for Torsades de Pointe (TdP) including uncorrected hypokalemia or
hypomagnesemia, history of cardiac failure, or history of clinically
significant/symptomatic bradycardia.

- Concomitant use of medication(s) with a known risk to prolong the QT interval
and/or known to cause Torsades de Pointe that cannot be discontinued (within 5
half-lives or 7 days prior to starting study drug) or replaced by safe
alternative medication

- Hypertension defined below:

- Patients 2-17 years of age must have a blood pressure that is 95th percentile
for age, height and gender at the time of enrollment.

- Patients who are 18 years of age must have a systolic blood pressure that is >
160 or diastolic < 90 mm of Hg at the time of enrollment

- Note: If a BP reading prior to enrollment is above the 95th percentile for age,
height and gender, it is to be rechecked and documented to be 95th percentile
for age, height and gender prior to patient enrollment.

- Inability to determine the QTcF interval on the ECG (i.e. unreadable or not
interpretable) or QTcF > 450 msec (using Fridericia's correction). All as determined
by screening ECG (using the mean QTcF of triplicate ECGs)

- Patient is currently receiving any of the following medications and cannot be
discontinued 7 days prior to starting study drug Ribociclib:

- Known strong inducers or inhibitors of CYP3A4/5, including grapefruit,
grapefruit hybrids, pummelos, star-fruit, and Seville oranges

- Medications that have a narrow therapeutic window and are predominantly
metabolized through CYP3A4/5

- Medications that have a known risk to prolong the QT interval or induce Torsades
de Pointes

- Herbal preparations/medications, dietary supplements.

- Patient is currently receiving warfarin or other coumadin-derived anticoagulant for
treatment, prophylaxis or otherwise. Therapy with heparin, low molecular weight
heparin (LMWH) or fondaparinux is allowed

- Patient has a history of non-compliance to medical regimen.

- Known need for major surgery within 14 days of the first dose of Ribociclib.
Gastrostomy, insertion of a G tube, Ventriculo-peritoneal shunt, endoscopic
ventriculostomy and central venous access are NOT considered major surgery. (Tumor
biopsy is not considered as a major surgery).

- Patients with diagnostic imaging consistent with DIPG and evidence of hemorrhage on
MRI will be excluded from the option to have a biopsy obtained, however may proceed
with eligibility for the non-biopsy arm.

Minimum Eligible Age: 12 Months

Maximum Eligible Age: 30 Years

Eligible Gender: Both