∙ Patients may be included in the study if they meet all of the following criteria:

• Age ≥ 16 years. Patients under 18 years, should be fully grown (proof of fused growth plates).

• Patients with histological evidence of stage IV metastatic high-grade leiomyosarcoma (grade 2 - 3 according to the Federation Nationale des Centres de Lutte Contre le Cancer (FNCLCC) grading system) not amenable to curative treatment with surgery or radiotherapy.

• Patients must have at least one unidimensionally measurable lesion by computed tomography as defined by Response Evaluation Criteria In Solid Tumors (RECIST) v.1.1. If only 1 lesion is present at screening this lesion should not have been irradiated during previous treatments.

• Life expectancy of at least 3 months in the judgment of the investigator.

• ECOG ≤ 1.

• Documented negative test for HIV, HBV and HCV. For HBV serology, the determination of HBsAg and anti-HBcAg-Ab is required. In patients with serology documenting previous exposure to HBV, negative serum HBV-DNA is required. For HCV: HCV-RNA or HCV antibody test. Subjects with a positive test for HCV antibody but no detection of HCV-RNA indicating no current infection are eligible.

• Female patients: negative serum pregnancy test for women of childbearing potential (WOCBP)\* within 14 days of starting treatment. WOCBP must agree to use, from the screening to six months following the last study drug administration, highly effective contraception methods, as defined by the Recommendations for contraception and pregnancy testing in clinical trials issued by the Head of Medicine Agencies' Clinical Trial Facilitation Group (www.hma.eu/ctfg.html) and which include, for instance, progesterone-only or combined (estrogen- and progesterone-containing) hormonal contraception associated with inhibition of ovulation, intrauterine devices, intrauterine hormone-releasing systems, bilateral tubal occlusion, vasectomized partner or sexual abstinence.

• Male patients: Male subjects able to father children must agree to use two acceptable methods of contraception throughout the study (e.g., condom with spermicidal gel). Double-barrier contraception is required.

• Informed consent signed and dated to participate in the study.

⁃ Willingness and ability to comply with the scheduled visits, treatment plan, laboratory tests and other study procedures.

‣ Women of childbearing potential are defined as females who have experienced menarche, are not postmenopausal (12 months with no menses without an alternative medical cause) and are not permanently sterilized (e.g., tubal occlusion, hysterectomy, bilateral oophorectomy or bilateral salpingectomy).