Key Inclusion Criteria:

          1. Ability to understand and willingness to provide written informed consent to
             participate in this study

          2. ≥12 years of age

          3. Diagnosis:

               1. Maintenance treatment cohort: Histologic or cytologic diagnosis of Ewing's
                  sarcoma, including non-resected, locally advanced or metastatic disease

               2. Salvage treatment cohort: Non-resected, locally advanced or metastatic sarcoma
                  (including Ewing's sarcoma) for which there is no standard of care treatment and
                  no curative other option

          4. Radiographic disease assessment within 35 days prior to enrollment and planned
             treatment start with study drug

          5. Prior treatment:

               1. Maintenance treatment cohort : 1-3 prior lines of systemic treatment (including
                  current treatment)

               2. Salvage cohort: Any number of prior treatments

          6. Maintenance treatment cohort only: Patient completed current line of treatment
             (systemic, surgery, radiation) prior to enrollment, without disease progression as
             compared to baseline AND has achieved at least one of the following

               1. CR in response to current second or third line treatment

               2. PR in response to current line of treatment (after at least 4 cycles, if
                  treatment included systemic therapy)

               3. SD in response to current line of treatment (after at least 4 cycles, if
                  treatment included systemic therapy)

          7. Measurable disease, except for patients in Cohort A who have achieved CR at the
             conclusion of current 2nd or 3rd line of treatment

          8. ECOG performance status 0-2

          9. Adequate organ function defined as all laboratory parameters ≤ Grade 2 NCI CTCAE
             criteria

         10. Patients must be able to swallow and retain whole capsules

        Key Exclusion Criteria:

          1. Systemic anticancer agents within 14 days prior to treatment on study

          2. Major surgery within 30 days

          3. Prior treatment with SM-88

          4. Any screening laboratory, electrocardiogram (ECG), other clinical finding, comorbidity
             or clinical history that, in the opinion of the investigator, indicates an
             unacceptable risk for patient to participate in the study or would limit patient's
             ability to comply with study requirements

          5. History of any drug allergies or significant adverse reactions to any of the
             components of SM-88

          6. History of light sensitive diseases for which methoxsalen would be contraindicated

          7. Current or anticipated treatment with a contraindicated medication

          8. Evidence of viral infections including human immunodeficiency virus (HIV), hepatitis
             B, and hepatitis C