A Phase 1 Study of the EZH2 Inhibitor Tazemetostat in Pediatric Subjects With Relapsed or Refractory INI1-Negative Tumors or Synovial Sarcoma

Inclusion Criteria:

1. Age (at the time of consent/assent): ≥6 months to ≤18 years

- Cohort 4 only: ≥10 years to ≤18 years

2. Performance Status:

- If <12 years of age: Lanksy Performance Status >50%

- If ≥12 years of age: Karnofsky Performance Status >50% NOTE: If subject is unable
to walk due to paralysis, but is mobile in a wheelchair, subject is considered to
be ambulatory for the purpose of assessing their performance status.

3. Has provided written informed consent

4. Has a life expectancy of >3 months

5. Has relapsed or refractory disease and no standard treatment options as determined by
locally or regionally available standards of care and treating physician's discretion

6. Is ineligible or inappropriate for other treatment regimens known to have effective
potential

7. Has a documented local diagnostic pathology of original biopsy confirmed by a Clinical
Laboratory Improvement Amendments (CLIA)/College of American Pathologists (CAP) or
equivalent laboratory certification

8. Has all prior treatment (i.e., chemotherapy, immunotherapy, radiotherapy) related
clinically significant toxicities resolve to ≤ Grade 1 per CTCAE, version 4.03 or are
clinically stable and not clinically significant, at time of enrollment

9. Has completed a prior therapy (ies) according to the criteria below:

- Other investigational study agent (any medicinal product that is not approved in
the country of treatment for any indication, adult or pediatric) (At least 30
days or five half-lives, whichever is longer, since last dose prior to the first
dose of tazemetostat)

- Chemotherapy: cytotoxic (At least 14 days since last dose of chemotherapy prior
to first dose of tazemetostat)

- Chemotherapy: nitrosoureas (At least 6 weeks since last dose of nitrosoureas
prior to first dose of tazemetostat)

- Chemotherapy: non-cytotoxic (e.g., small molecule inhibitor) (At least 14 days
since last dose of non-cytotoxic chemotherapy prior to first dose of
tazemetostat)

- Monoclonal antibody (ies) (At least 28 days since the last dose of any monoclonal
antibody prior to first dose of tazemetostat)

- Immunotherapy (e.g., tumor vaccine) At least 6 weeks since last dose of
immunotherapy agent(s) prior to first dose of tazemetostat)

- Radiotherapy (RT) (At least 14 days from last local site RT prior to first dose
of tazemetostat/At least 21 days from stereotactic radiosurgery prior to first
dose of tazemetostat/At least 12 weeks from craniospinal, ≥ 50% radiation of
pelvis, or total body irradiation prior to first dose of tazemetostat)

- Hematopoietic growth factor (At least 14 days from last dose of hematopoietic
growth factor prior to first dose of tazemetostat)

- Hematopoietic cell transplantation (At least 60 days from infusion of
hematopoietic cells prior to first dose of tazemetostat)

10. Has adequate hematologic (bone marrow and coagulation factors), renal and hepatic
function as defined by criteria below:

- Hematologic (BM Function):

- Hemoglobin ≥ 8 g/dL

- Platelets ≥100,000/mm^3 (≥100 x 10^9/L)

- ANC ≥1,000/mm^3 (≥1.0 x 10^9/L)

- Hematologic (Coagulation Factors):

- INR/ PTd ≤1.5 ULN

- PTT ≤1.5 ULN

- Fibrinogen ≥0.75 LLN

- Renal Function (creatinine clearance or serum creatinine):

- Calculated creatinine clearance ≥50 mL/min/1.73m^2

- Serum creatinine 6 months to 1 year: male 0.6 mg/dL (53 µmol/L) female 0.5
mg/dL (44 µmol/L)

- Serum creatinine 1 to < 2 years: male 0.6 mg/dL (53 µmol/L) female 0.6 mg/dL
(53 µmol/L)

- Serum creatinine 2 to < 6 years: male 0.8 mg/dL (71 µmol/L) female 0.8 mg/dL
(71 µmol/L)

- Serum creatinine 6 to <10 years: male 1 mg/dL (88 µmol/L) female 1 mg/dL (88
µmol/L)

- Serum creatinine 10 to <13 years: male 1.2 mg/dL (106 µmol/L) female 1.2
mg/dL (106 µmol/L)

- Serum creatinine 13 to <16 years: male 1.5 mg/dL (133 µmol/L) female 1.4
mg/dL (125 µmol/L)

- Serum creatinine ≥16 years: male 1.7 mg/dL (150 µmol/L) female 1.4 mg/dL
(125 µmol/L)

- Hepatic Function:

- Total bilirubin <1.5 x ULN

- ALT or AST <3 x ULN Eligibility can be determined by either total or
conjugated bilirubin

11. For subjects with CNS involvement: Subjects must have deficits that are stable for a
minimum of 14 days prior to enrollment, or seizures that are stable, not increasing in
frequency or severity and controlled on current anti-seizure medication(s) for a
minimum of 7 days prior to enrollment

NOTE: Subjects with leptomeningeal disease or brian tumors with positive cerebral
spinal fluid cytology are eligible for this study. Subjects may receive
glucocorticoids (at stable or tapering dose) to control CNS symptoms prior to
enrollment; however, subjects should receive a stable or tapering dose for at least 7
days prior to enrollment.

12. Has a shortening fraction of >27% or an ejection fraction of ≥50% by echocardiogram or
multi-gated acquisition scan and New York Heart Association Class<2

13. Has a QT interval corrected by Fridericia's formula (QTcF) ≤450 msec

14. Is able to swallow and retain orally administered medication and does not have any
uncontrolled gastrointestinal (GI) condition such as nausea, vomiting, or diarrhea, or
any clinically significant GI abnormalities that may alter absorption such as
malabsorption syndromes, hereditary fructose intolerance, glucose-galactose
malabsorption, sucrose-isomaltase insufficiency, or major resection of stomach and/or
bowels NOTE: Nasogastric and gastrostomy tube administration of the oral suspension
formulation of study drug is permitted.

15. Has sufficient tumor tissue (slides or blocks) available for central confirmatory
testing of immunohistochemistry and/or cytogenetics/fluorescence in situ hybridization
(FISH) and/or deoxyribonucleic acid mutation analysis (required for study entry but
enrollment based on local results)

16. Is willing and able to comply with all aspects of the protocol as judged by
Investigator

17. For female subjects of childbearing potential: Subject must:

- Have a negative beta-human chorionic gonadotropin (β-hCG) pregnancy test at time
of Screening and within 72 hours prior to planned first dose of tazemetostat
(urine or serum test is acceptable however, positive urine tests must be
confirmed with serum testing), and

- Agree to use effective contraception, as defined in Section 8.5.11 start of
Screening until 30 days following the last dose of study treatment and have a
male partner who uses a condom, or

- Practice true abstinence (when this is in line with the preferred and usual
lifestyle of the subject, see Section 8.5.11, or

- Have a male partner who is vasectomized with confirmed azoospermia

18. For male subjects with a female partner of childbearing potential: Subject must:

- Be vasectomized or

- Agree to use condoms as defined in Section 8.5.11 from first dose of tazemetostat
until 30 days following the last dose of tazemetostat, or

- Have a female partner who is NOT of childbearing potential

For Dose Escalation Only:

To be eligible for enrollment in dose escalation, a subject must meet ALL of the following
criteria in addition to the inclusion criteria listed above for all subjects:

1. Has evaluable disease as defined as lesions that can be accurately measured at least
in one dimension by radiographic examination or physical examination and other lesions
such as bone lesions, leptomeningeal disease, ascites, hepatosplenomegaly from
disease.

2. Has one of the following histologically confirmed tumors: (NOTE: Evidence of
diagnostic pathology of original biopsy confirmed by a CLIA/CAP certified laboratory
must be available)

- Rhabdoid tumor:

- ATRT

- MRT

- RTK

- Selected tumors with rhabdoid features

- NI1-negative tumor:

- Epithelioid sarcoma

- Epithelioid malignant peripheral nerve sheath tumor

- Extraskeletal myxoid chondrosarcoma

- Myoepithelial carcinoma

- Renal medullary carcinoma

- Other INI1-negative malignant tumors (e.g., dedifferentiated chordoma) with
Sponsor approval

- Synovial sarcoma with SS18-SSX rearrangement (NOTE: Evidence of diagnostic
pathology of original biopsy confirmed by a CLIA/CAP certified laboratory must be
available)

3. For subjects with ATRT, MRT, RTK, or selected tumors with rhabdoid features only: the
following test results must be available: Morphology and immunophenotypic panel
consistent with rhabdoid tumor and Loss of INI1 or SMARCA4 confirmed by IHC, or
Molecular confirmation of tumor bi-allelic INI1 or SMARCA4 loss/mutation when INI1 or
SMARCA4 IHC is equivocal or unavailable

4. For subjects with INI1 negative tumor only:

the following test results must be available: Morphology and immunophenotypic panel
consistent with INI1-negative tumors, and Loss of INI1 confirmed by IHC, or Molecular
confirmation of tumor bi-allelic INI1 loss/mutation when INI1 IHC is equivocal or
unavailable

5. For subjects with synovial sarcoma only:

The following test results must be available:

Morphology consistent with synovial sarcoma, and Cytogenetics or FISH and/or molecular
confirmation (e.g., DNA sequencing) of SS18 rearrangement t(X;18)(p11;q11)

For Dose Expansion Only:

Note: To be eligible for enrollment in Dose Expansion, a subject must meet ALL of the
following criteria in addition to the inclusion criteria for ALL subjects listed above

1. Has measurable disease

2. Has one of the following histologically confirmed tumors:

- Cohort 1 - ATRT

- Cohort 2 - MRT/RTK/selected tumors with rhabdoid features

- Cohort 3 - INI-negative tumors:

- Epithelioid sarcoma

- Epithelioid malignant peripheral nerve sheath tumor

- Extraskeletal myxoid chondrosarcoma(EMC)

- Myoepithelial carcinoma

- Renal medullary carcinoma

- Chordoma (poorly differentiated or de-differentiated)

- Other INI1-negative malignant tumors (e.g., dedifferentiated chordoma) with
Sponsor approval

- Cohort 4 - Tumor types eligible for Cohorts 1 through 3 or synovial sarcoma with
SS18-SSX rearrangement To be eligible for enrollment in Dose Expansion, a subject
must meet All of the following criteria in addition to the inclusion criteria for
All subjects listed above.

NOTE: Evidence of diagnostic pathology of original biopsy confirmed by a CLIA/CAP or
other Sponsor-approved certified laboratory must be available.

3. For subjects with ATRT/MRT/RTK only - have the following test results available:

- Morphology and immunophenotypic panel consistent with rhabdoid tumor, and

- Loss of INI1 or SMARCA4 confirmed by IHC, or

- Molecular confirmation of tumor bi-allelic INI1 or SMARCA4 loss/mutation when
INI1 or SMARCA4 IHC is equivocal or unavailable

4. For subjects with INI1-negative tumors only: The following test results must be
available:

- Morphology and immunophenotypic panel consistent with INI1-negative tumors, and

- Loss of INI1 confirmed by IHC, or

- Molecular confirmation of tumor bi-allelic INI1 loss/mutation when INI1 IHC is
equivocal or unavailable

5. For subjects with synovial sarcoma only: The following test results must be available:

- Morphology consistent with synovial sarcoma, and

- Cytogenetics or FISH and/or molecular confirmation (e.g., deoxyribonucleic acid
[DNA] sequencing) of SS18 rearrangement t(X;18)(p11;q11)

6. For subjects to be enrolled in Cohort 4: Able to swallow and retain orally
administered tablets

Exclusion Criteria:

1. Has had prior exposure to tazemetostat or other inhibitor(s) of EZH2

2. Is being actively treated for another concurrent malignancy or is less than five years
from completion of treatment for another malignancy

3. Has participated in another interventional clinical study and received investigational
drug within 30 days or 5 half-lives, whichever is longer, prior to the planned first
dose of tazemetostat

4. Has had major surgery within 2 weeks prior to enrollment NOTE: Minor surgery (e.g.,
minor biopsy of extracranial site, central venous catheter placement, shunt revision)
is permitted within 2 weeks prior to enrollment.

5. Has thrombocytopenia, neutropenia, or anemia of Grade ≥3 (per CTCAE 4.03 criteria) or
any prior history of myeloid malignancies, including myelodysplastic syndrome (MDS).
Has abnormalities known to be associated with MDS (e.g. del 5q, chr 7 abn) and MPN
(e.g. JAK2 V617F) observed in cytogenetic testing and DNA sequencing.

Note: Bone marrow aspirate/biopsy will be conducted following abnormal peripheral
blood smear morphology assessment conducted by central lab. Cytogenetic testing and
DNA sequencing will be conducted following an abnormal result of bone marrow
aspirate/biopsy.

6. Has a prior history of T-LBL/T-ALL.

7. Has clinically active heart disease including prolonged corrected QTcF (>450 msec)

8. Is currently taking any prohibited medication(s)

9. Is unwilling to exclude grapefruit juice, Seville oranges and grapefruit from the diet
and all foods that contain those fruits from time of enrollment to while on study

10. Has an active infection requiring systemic treatment

11. Is immunocompromised (i.e. congenital immunodeficiencies), including subjects known
history of infection with human immunodeficiency virus (HIV)

12. Has known history of chronic infection with hepatitis B virus (hepatitis B surface
antigen positive) or hepatitis C virus (detectable HCV RNA)

13. Has had a symptomatic venous thrombosis within the 14 days prior to study enrollment
NOTE: Subjects with a history of a deep vein thrombosis 14 days prior to study
enrollment who are on anticoagulation therapy with low molecular weight heparin are
eligible for this study

14. For subjects with CNS involvement (primary tumor or metastatic disease): Have any
active bleeding, or new intratumoral hemorrhage of more than punctate size on
Screening MRI obtained within 14 days of starting study drug,or known bleeding
diathesis or treatment with anti-platelet or anti-thrombotic agents 15.15. Has known
hypersensitivity to any of the components of tazemetostat or other inhibitor(s) of
EZH2, or hypersensitivity to Ora-sweet or methylparaben

16. Has an uncontrolled intercurrent illness including, but not limited to, uncontrolled
infection, or psychiatric illness/social situations that would limit compliance with study
requirements 17. For female subjects of childbearing potential: Is pregnant or nursing For
male subjects: Is unwilling to adhere to contraception criteria from time of enrollment in
study to at least 30 days after last dose of tazemetostat.