• Patients with histologically documented diagnosis of leiomyosarcoma

• Patients with diagnosis of unresectable or metastatic leiomyosarcoma

• Patients who received at least on previous systemic treatment with anthracycline-based chemotherapy.

• Patients suitable to receive gemcitabine or trabectedin therapy.

• Measurable or evaluable disease with RECIST 1.1 criteria.

• Evidence of progression according RECIST 1.1 during the 6 months before study entry.

• Age ≥18 years

• Eastern Cooperative Oncology Group (ECOG) Performance Status ≤ 2

• All previous anticancer treatments must have completed ≥ 3 weeks prior to first dose of study drug.

⁃ The patient has resolution of adverse events, with the exception of alopecia, and of all clinically significant toxic effects of prior loco-regional therapy, surgery, radiotherapy or systemic anticancer therapy to ≤ Grade 1, by National Cancer Institute - Common Terminology Criteria for Adverse Events (NCI-CTCAE) Version 5.0

⁃ Adequate bone marrow, liver and renal function

⁃ Left Ventricular Ejection Fraction ≥ 50% and/or above lower institutional limit of normality.

⁃ Female patients of child-bearing potential must have negative pregnancy test within 7 days before initiation each cycle of chemotherapy.

⁃ No history of arterial and/or venous thromboembolic event within the previous 12 months.

⁃ The patient or legal representative must be able to read and understand the informed consent form and must have been willing to give written informed consent prior to any study specific procedure. The subject may also provide an optional consent for the biological/translational sub-study associated.