Study of Selinexor and Doxorubicin in Advanced Soft Tissue Sarcomas

Inclusion Criteria: * Written informed consent * Patient must be willing and able to comply with scheduled visits, treatment plan, laboratory tests and other study procedures. * Age ≥ 18 years. * Patients must have histologically confirmed locally advanced/unresectable or metastatic soft tissue sarcoma. * Patients must have not received prior doxorubicin. * Patient must show evidence of progressive disease on study entry or newly diagnosed patients with de novo metastatic measurable disease * Patient must have measureable disease as defined by RECIST 1.1. * Eastern Cooperative Oncology Group (ECOG) performance status of 0-1 * Adequate hematopoietic function * Adequate hepatic function: * Adequate renal function * Adequate cardiac function13 * Patients must agree to use methods of contraception as a agreed upon by the patient and study doctor Exclusion Criteria: * Patient is pregnant or lactating * Radiation, chemotherapy, immunotherapy, any other systemic anticancer therapy, or participation in an investigational anti-cancer study ≤3 weeks prior to initiation of therapy. * Major surgery within 4 weeks before initiation of therapy * Unstable cardiovascular function * Active, ongoing or uncontrolled active infection within one week prior to first dose. * Malignancies other than disease under study within 2 years prior to Cycle 1, Day 1. * Known to be HIV seropositive * Known active hepatitis A, B, or C infection; or known to be positive for hepatitis C virus (HCV) RNA or hepatitis B virus (HBV) surface antigen (HBsAg) * Patients with active CNS malignancy. * Patients with any gastrointestinal dysfunctions that could interfere with the absorption of Selinexor or patients with significantly diseased or obstructed gastrointestinal tract or uncontrolled vomiting or diarrhea. * Inability or unwillingness to take supportive medications such as anti-nausea and anti anorexia agents. * In the opinion of the Investigator, patients who are significantly below their ideal body weight * Serious psychiatric or medical conditions that could interfere with treatment * Concurrent therapy with approved or investigational anticancer therapeutic agents * Any condition that, in the opinion of the Investigator, would interfere with evaluation of the study regimen or interpretation of patient safety or study results