BACKGROUND: Desmoplastic small round cell tumor (DSRCT) and synovial sarcoma (SySa) are rare cancers primarily affecting adolescents and young adults. Prognosis is generally poor, particularly upon metastasis, due to limited efficacy of chemotherapies and the lack of molecular mechanism-based approaches. Recently, overexpression of somatostatin receptors (SSTR) 2, 3, and 5 was described in high proportions of DSRCT and SySa. Given the efficacy of somatostatin analogs in other SSTR-expressing malignancies, we will evaluate pasireotide as maintenance therapy in patients with locally advanced or metastatic DSRCT and SySa.

TRIAL DESIGN: PAMSARC (Pasireotide as Maintenance Treatment With Monthly Deep Intramuscular Injection in SSTR2/3/5-Expressing SySa and DSRCT) is an open-label, multicentric, single-arm phase II trial evaluating pasireotide maintenance therapy in adolescents and adults with locally advanced or metastatic SySa or DSRCT who have achieved stable disease or partial response after completion of chemotherapy. Molecular eligibility is determined by SSTR2/3/5 expression via RNA analysis performed within the precision oncology programs MASTER and INFORM. Due to pasireotide's regulatory approval being limited to adults, enrollment follows a "3 + 3 staggered" approach for both adults and adolescents. Treatment consists of pasireotide administered as intragluteal depot injection every 28±3 days. The primary endpoint is progression-free survival; secondary and exploratory analyses include overall survival, patient-reported outcomes, and disease monitoring via liquid biopsies. The sample size calculation assumes exponential data, with 90 % power to detect a hazard ratio of 0.5, requiring 28 participants with an expected 22 events during the study. Sensitivity analyses are planned for both age groups and disease types.

TRIAL REGISTRATION NUMBERS: EUCT: 2024-511935-86-00-00; ClinicalTrials.gov: NCT06456359.