Interim results of a phase II trial of first line retifanlimab (R) plus gemcitabine and docetaxel (GD) in patients (pts) with advanced soft tissue sarcoma (STS)

11518 Background: GD is an alternative to doxorubicin in the front line setting in advanced STS. In the Phase I portion of this study, the PD-1 antibody R, plus GD, was generally well tolerated. We hypothesized that R+GD would be more effective than historical controls treated with GD alone. Methods: This is an ongoing open-label single-center study of R+GD in pts with treatment-naïve unresectable or metastatic high-grade STS. G (900 mg/m 2 ) is administered on days 1 and 8 and D (75 mg/m 2 ) on day 8, in 21-day cycles. R (375 mg flat dose) is administered on day 1 starting in cycle 2 and continued as monotherapy ‘maintenance’ after completion of 6 cycles of GD. Up to fifty pts can accrue into five histology-specific cohorts of 10 pts each. The primary endpoint is to estimate the progression-free survival (PFS) rate at 24 weeks. Secondary endpoints included safety, best overall response rate (RR) by RECIST 1.1, disease control rate (DCR), and duration of response (DOR). Subgroup analyses to evaluate all endpoints within each histology-specific cohort are preplanned. An early stopping rule for excessive toxicity will halt accrual if a prespecified severe adverse event rate is surpassed. Results: As of January 11, 2023, 43 pts were enrolled and treated with R+GD. The leiomyosarcoma (LMS), undifferentiated pleomorphic sarcoma (UPS)/myxofibrosarcoma (MFS), dedifferentiated liposarcoma (DDLPS), and other STS cohorts (n = 10 pts each) were fully accrued. Three of 10 angiosarcoma (AS) pts were enrolled. Median age was 59.5 (range 21 – 81) and 27 (63%) were male. Two DDLPS patients were not yet evaluable for response. Of 41 evaluable pts, the best overall RR was 22% (95% confidence interval [CI]) 11 – 38). Confirmed responses were seen in UPS/MFS (n = 4), LMS (2), AS (2), and follicular dendritic cell sarcoma (1). Three pts (LMS, ossifying fibromyxoid tumor, AS) had an unconfirmed PR, for a best RR of 29% (95% CI 16 – 46). Median PFS was 32.7 weeks (95% CI 26.4 – not estimable [NE]) and median DOR was 24 weeks (95% CI 15 – NE). Safety was evaluated in all 43 pts and the early stopping rule was not triggered. Eighteen (42%) pts had at least one Grade (Gr) 3 or 4 treatment-related adverse event (TRAE). The most common ( > 5%) were anemia (16%), neutropenia (9%), febrile neutropenia (7%), lung infection (7%), and leukopenia (7%). Seven pts (16%) had pneumonitis: one Gr 1, four Gr 2, and two Gr 3. Six pts (14%) stopped treatment due to toxicity, including five with pneumonitis. Conclusions: The median PFS of R+GD appears promising compared to historical controls, although the primary endpoint analysis is pending completion of accrual. There was a higher incidence of pneumonitis with the combination compared to GD alone. Future studies of this combination will need to carefully consider the benefits and risks after the final efficacy and safety analyses are performed. Clinical trial information: NCT04577014 .