MEK Inhibitor and a PDL1 Inhibitor Patients With Locally Advanced and/or Metastatic Soft Tissue Sarcoma

Inclusion Criteria: I1. Male or female patients aged of at least : \- 12 years on day of signing informed consent. I2. Histologically-confirmed diagnosis of soft tissue sarcomas, confirmed by a pathologist from RRePS Network, among the 2 cohorts: * Rhabdomyosarcomas (RMS). * Malign Peripheral Nerve Sheath Tumors (MPNST). I3. Availability of a representative formalin-fixed paraffin-embedded (FFPE) primary and/or metastatic tumor tissue with an associated pathology report for molecular prescreening i.e. either an archival block or a dedicated freshly collected de novo tumor biopsy. I4. Documented MAPK pathway status and known Tumor Mutational Burden (TMB) before C1D1. I5. Previous treatment with anthracycline-based chemotherapy (in the neoadjuvant, adjuvant or metastatic setting). Note: this criteria not mandatory for rhabdomyosarcoma. I6. Previous treatment by at least one line of chemotherapy in the advanced/metastatic setting before C1D1. I7. Documented radiological disease progression as per RECIST V1.1 before C1D1. I8. At least one measurable lesion according to RECIST v1.1 before C1D1. I9. Mandatory for adult patients only - Presence of at least one tumor lesion visible by medical imaging and accessible to repeatable percutaneous sampling that permits core needle biopsy without unacceptable risk of a significant procedural complications, and suitable for retrieval of 4 cores using a 16-gauge diameter needle or larger. I10. Performance status: * Karnofsky perfo…