Inclusion Criteria:

          1. Signed consent of an Institutional Review Board (IRB)-approved informed consent form
             (ICF) and Health Insurance Portability and Accountability Act (HIPAA) authorization
             for release of personal health information (if appropriate).

          2. Willingness and ability to comply with scheduled visits, drug administration plan,
             laboratory tests, other study procedures, and study restrictions.

          3. Disease Status including all of the following:

               1. Histological or cytological confirmation of LMS arising at any anatomic site.

               2. Advanced (metastatic) or locally advanced unresectable disease.

               3. Ineligible for other high-priority national or institutional study.

               4. Measurable disease per RECIST v1.1 criteria.

             Demographics:

          4. Age greater than or equal to (>/=) 18

          5. Male and Female

             Performance Status:

          6. Eastern Cooperative Oncology Group (ECOG) performance status 0-1.

             Hematopoietic:

          7. Absolute neutrophil count (ANC) count >/= 1,500/cubic millimeters (mm^3) without the
             use of growth factors in the past 7 days;

          8. Platelet count >=100,000/mm^3 without platelet transfusion in the past 5 days;

          9. Hemoglobin >=9 grams per deciliter (g/dL) (packed red blood cell transfusion is
             allowed).

             Hepatic:

         10. Bilirubin lesser than (<) upper limit of normal (ULN);

         11. Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) <1.5 times ULN;

         12. Participants with liver metastases may be enrolled.

             Pulmonary:

         13. Participants with well-controlled asthma (for example Use of rescue medications <2
             times/week over the last 12 months) or Chronis Obstructive Pulmonary Disease (COPD)
             (for example no exacerbations over the prior 3 months) may be enrolled.

             Renal:

         14. Creatinine <1.5 times normal, or creatinine clearance greater than (>) 45 milliliters
             per minute (mL/min).

             Prior Therapies:

         15. Toxicity from prior therapies recovered to Grade lesser than or equal to (<=) 1 or
             participant's baseline, except for alopecia. In addition, endocrinopathies associated
             with prior immunotherapy based treatments which are well controlled on replacement
             medication are not exclusionary

         16. Chemotherapy:

             a. Up to and inclusive of 4 prior systemic cytotoxic oncology therapy regimens for
             metastatic, locally recurrent, or unresectable LMS. Note: prior treatment with
             non-cytotoxic therapy regimens (for example targeted therapies, hormonal therapies, or
             tyrosine kinase inhibitors) are not considered cytotoxic oncology therapies.

             Surgery:

         17. At least 4 weeks since prior surgery and recovered in opinion of investigator.

             Other:

         18. Capable of swallowing oral medication.

         19. Females of childbearing potential must have a negative pregnancy test within 7 days
             prior to being registered for protocol therapy.

         20. Males and females of childbearing potential must be willing to use an effective method
             of contraception (hormonal or barrier method of birth control; abstinence) from the
             time consent is signed until 90 days after treatment discontinuation. Note: The
             Definition of effective contraception will be based on the judgement of the Principal
             Investigator (PI) or Designee.

        Exclusion Criteria:

        Participants meeting any of the following criteria will not be eligible for enrollment:

          1. Received any systemic anticancer therapy including investigational agents <=3 weeks
             prior to initiation of study treatment. Additionally, Participants may have not
             received radiation </= 3 weeks prior to initiation of study treatment.

          2. Co-existing active infection or any co-existing medical condition likely to interfere
             with study procedures, including:

             a. Significant cardiovascular disease (New York Heart Association Class III or IV
             cardiac disease), myocardial infarction within the past 6 months, unstable angina,
             congestive heart failure requiring therapy, unstable arrhythmia or a need for
             anti-arrhythmic therapy, or evidence of ischemia on electrocardiogram (ECG), marked
             baseline prolongation of QT/QTc (corrected QT interval) interval, for example,
             repeated demonstration of a QTc interval >500 milliseconds (msec) (Long QT Syndrome
             [congenital]).

          3. Known human immunodeficiency virus (HIV), hepatitis B virus (HBV), or hepatitis C
             virus (HCV) positivity.

          4. History of solid organ transplantation.

             Therapeutics:

          5. Known or suspected allergy or immediate or delayed hypersensitivity to PTC596 or
             dacarbazine or any agent given in this study.

             Gastrointestinal:

          6. Bowel obstruction, malabsorption, or other contraindication to oral medication.

          7. Gastrointestinal disease or other condition that could affect absorption.

          8. Active peptic ulcer disease.

          9. Inflammatory bowel disease (including ulcerative colitis and Crohn's disease),
             diverticulitis, cholecystitis, symptomatic cholangitis or appendicitis.

         10. Any condition that impairs participant's ability to swallow oral medications.

             Wounds /Surgery:

         11. Serious non-healing wound, ulcer, or bone fractures.

         12. Major surgery, open biopsy or significant traumatic injury which has not recovered in
             the opinion of the investigator, within 28 days of registration for protocol therapy.

         13. Mucosal or internal bleeding.

             Concomitant Medications:

         14. Concomitant strong CYP1A2 inhibitors (like selective serotonin reuptake inhibitor
             [SSRI] agents fluvoxamine and fluoxetine) should be avoided. CYP1A2 inhibitors may
             inhibit the conversion of dacarbazine to its active metabolite and may increase the
             exposure of PTC596.

             Other:

         15. Prior malignancy that required treatment or has shown evidence of recurrence (except
             for non-melanoma skin cancer or adequately treated cervical carcinoma in situ) during
             the 5 years prior to initiation. Cancer treated with curative intent more than 5 years
             previously and without evidence of recurrence is not an exclusion.

         16. Known coagulopathy or bleeding diathesis. Participants on anti-coagulation should be
             monitored closely and International Normalized Ratio (INR) within normal range.

         17. Prior or ongoing clinically significant illness, medical or psychiatric condition,
             medical history, physical findings, ECG findings, or laboratory abnormality that, in
             the investigator's opinion, could affect the safety of the participant, or alter the
             absorption, distribution, metabolism, or excretion of the study drugs, or could impair
             the assessment of study results.

         18. History of brain metastases or leptomeningeal disease at any time in participant's
             history, including treated central nervous system (CNS) disease which is clinically
             and radiographically stable.