SBRT Combined With Reduced-Dose Anthracycline Chemotherapy and Toripalimab as Neoadjuvant Therapy for Localized High-Risk Undifferentiated Pleomorphic Sarcoma

Overview

This is a prospective, single-center, single-arm, open-label phase II exploratory study to evaluate the efficacy, safety, and immune remodeling effects of stereotactic body radiotherapy (SBRT) combined with reduced-dose anthracycline chemotherapy and the PD-1 inhibitor toripalimab as neoadjuvant therapy, followed by postoperative stratified maintenance therapy, in patients with localized high-risk undifferentiated pleomorphic sarcoma (UPS). A Simon two-stage design will be used, with 39 patients planned (stage 1: 13 patients; 35 patients in total, including a 10% dropout rate). Treatment consists of SBRT (8-10 Gy in 5 fractions) followed by toripalimab (240 mg intravenously every 3 weeks, 3 cycles in the neoadjuvant phase) combined with liposomal doxorubicin (30-40 mg/m² intravenously every 3 weeks, 2 cycles in the neoadjuvant phase). After response evaluation, radical surgery is performed. Postoperative stratified maintenance therapy is delivered according to pathological response: patients achieving major pathologic response (MPR) or hyalinized pathologic response (HPR) receive toripalimab monotherapy (240 mg every 3 weeks, up to 14 cycles), while those not achieving MPR/HPR receive toripalimab combined with liposomal doxorubicin (2-4 cycles). The primary endpoint is the rate of major pathologic response (MPR). Secondary endpoints include the rate of hyalinized pathologic response (HPR), radiologic objective response rate (ORR), R0 resection rate, limb salvage rate, and safety.

Undifferentiated pleomorphic sarcoma (UPS) accounts for 5-10% of soft tissue sarcomas. For localized high-risk UPS of the extremities, the standard of care is preoperative radiotherapy plus radical surgery; however, even with R0 resection, the risk of distant metastasis is 30-40% and 2-year disease-free survival is approximately 50%. The SARC028 study showed an objective response rate of 23% in UPS with pembrolizumab, and SU2C-SARC032 demonstrated that adding perioperative pembrolizumab to preoperative radiotherapy improved 2-year DFS (67% vs 52%, HR 0.61). Preclinical and clinical data suggest that stereotactic body radiotherapy (SBRT) with high dose per fraction better synergizes with immune checkpoint blockade than conventional fractionation; sparing the tumor-draining lymph nodes is critical for preserving immune priming. This phase II study evaluates the triplet neoadjuvant regimen of SBRT, reduced-dose anthracycline chemotherapy, and toripalimab followed by postoperative stratified maintenance in patients with localized high-risk UPS. The fixed treatment schedule is: weeks 1-3 SBRT (8-10 Gy in 5 fractions, on alternate days or consecutively) plus first dose of toripalimab; week 4 first chemotherapy cycle plus second dose of toripalimab; weeks 5-6 recovery; week 7 second chemotherapy cycle plus third dose of toripalimab; weeks 8-9 recovery and response assessment; weeks 10-12 radical surgery. Postoperative maintenance begins within 4-8 weeks after surgery. Patients achieving MPR or HPR receive toripalimab monotherapy 240 mg every 3 weeks for up to 14 cycles; patients not achieving MPR/HPR receive toripalimab for 14 cycles combined with liposomal doxorubicin (2-4 cycles; 2 cycles for HPR \>30%). The sample size uses Simon's two-stage Minimax design with one-sided alpha of 0.05 and 80% power, assuming P0 = 60% and P1 = 80%. The first stage enrolls 13 patients (stop if \<=8 responders); the second stage expands to 35 patients total (declare regimen ineffective if \<=25 responders overall). Allowing for a 10% dropout rate, the planned total enrollment is 39 patients. Efficacy assessment: pathologic response (MPR defined as \<=10% residual viable tumor cells; HPR defined as \>30% hyalinization) is evaluated using a standardized pathology SOP, with confirmation by an independent central pathology laboratory. Radiologic response is evaluated by RECIST v1.1 every 12 weeks. Safety: adverse events are graded per NCI CTCAE v5.0; all adverse events are recorded up to at least 100 days after the last dose. Major wound complications are monitored within 90 days after surgery. Follow-up: visits at 30 days and 84 days after surgery, then every 3 months for survival; imaging every 12 weeks in the first year, every 24 weeks in the second year, and every 6-12 months in years 3-5; ctDNA and peripheral blood immune monitoring every 3 months for at least 2 years.