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This study will compare the efficacy and safety of unesbulin plus dacarbazine versus placebo plus dacarbazine in participants with unresectable or metastatic, relapsed or refractory LMS who have received at least 1 prior line of systemic therapy.

The primary objective of this study is to determine the maximum tolerated dose (MTD) and recommended phase 2 dose (RP2D) of unesbulin in combination with dacarbazine for the treatment of advanced LMS and determine the overall safety profile of unesbulin in combination with dacarbazine.
This study will employ the time-to-event continual reassessment method (TITE-CRM) for dose finding. Treatment will be initiated at dose level 2 (DL2) (Dacarbazine 1000 milligrams per square meter [mg/m^2] intravenously [IV] every 21 days in combination with unesbulin 200 milligrams [mg] orally twice weekly) for the first participant. This dose level represents the investigator's best assessment of the MTD based on available toxicity data for both agents. For subsequent participants, the dose level at which treatment is initiated will be selected based on the TITE-CRM using the most up to date dose-limiting toxicity (DLT) information from all participants previously treated. To enroll additional participants at the RP2D, the study is amended to include an expansion cohort of up to 12 participants (some of whom could be ongoing participants who reconsent).
Treatment will continue for each participant until evidence of unacceptable toxicity, disease progression, or treatment discontinuation for another reason.

The goal of this study is to evaluate the safety of the study drug PTC596 (Unesbulin) taken in combination with radiotherapy (RT) when given to pediatric patients newly diagnosed with High-Grade Glioma (HGG) including diffuse intrinsic pontine glioma (DIPG).
The main aims of the study are to:
* Find the safe dose of the study drug PTC596that can be given without causing serious side effects.
* Find out the amount of drug that enters blood (in all patients) and tumor (in patients who receive drug prior to a planned surgery for removal of their brain tumor)
During the first cycle (6-7weeks), patients will receive drug orally twice a week in combination with daily RT. During subsequent cycles (4 weeks each), they will receive only the study drug orally twice a week.
Funding Source - FDA OOPD