Efficacy of combination lurbinectedin (LURBI) plus doxorubicin (DOX) from the phase 1B softtissue sarcoma (STS) lead-in to a randomized phase 2 trial in leiomyosarcoma (LMS)

11507 Background: Single agent or combination chemotherapy, typically Dox- or gemcitabine-based, are standard 1st- and 2nd-line therapies in patients (pts) with metastatic LMS; however, objective response rates (ORR), progression-free, and overall survival (PFS/OS) remain inadequate. Lurbi (PharmaMar S.A. / Jazz Pharmaceuticals) binds to DNA inhibiting transcription and inducing double strand breaks leading to apoptosis and delaying S/G2 progression. In a prior pilot study, we showed Lurbi+Dox is well-tolerated, with signs of activity, particularly in LMS. We designed this phase 1b to optimize dosing to lead into a randomized (1:1) phase 2 trial of Dox+/-Lurbi in anthracycline-naïve LMS. Herein we provide updated efficacy and tolerability data for this Phase 1b cohort. Methods: Pts > 18 yrs with locally advanced/metastatic, unresectable non-GIST STS (Phase 1b only) w/o prior anthracycline or Lurbi/trabectedin, ECOG PS < 3, RECIST 1.1 measurable, and normal organ function were eligible. The phase 1b followed a 3+3 design. Dosing included fixed Lurbi (3.2 mg/m 2 d1) with two Dox dose levels (DL; DL1: 25 mg/m 2 d1; DL2 25 mg/m 2 d1+8). All pts received GCSF prophylaxis. Tumor assessments were every 2 cycles. DL1 was chosen as RP2D and the phase 2 trial began accrual in Aug 2022 aiming to enroll 50 LMS pts randomized 1:1 (stratified by uterine LMS [uLMS]] v. other LMS) with PFS as primary endpoint. Pts progressing on Dox are allowed to cross to Lurbi monotherapy. Results: 10 patients were enrolled in Phase 1b. Histologies included 5 LMS (4 uLMS) and 1 each of myxofibrosarcoma (myxFS), undifferentiated pleomorphic sarcoma (UPS), dedifferentiated liposarcoma (DDLPS), endometrial stromal sarcoma (ESS), and solitary fibrous tumor (SFT). With a median follow-up of 344 days, 5/10 pts remain on treatment with median PFS of 357 days (95%CI 175-ND). 6 pts demonstrated partial response (PR; 5 confirmed), 3 with stable disease (SD), and 1 with progressive disease (PD, Table). Median time to PR was 81 days (range 46-207). Duration of response was over 132 days in all responders with 3 still on treatment. Treatment-related AEs were typical for Dox/Lurbi, including nausea, fatigue, and reversible LFT elevations/cytopenias. Updated safety and response data will be provided at the time of the meeting. Conclusions: Lurbi+Dox is well-tolerated with efficacy in STS including 6/10 pts achieving PR, 9/10 pts staying on treatment for >120d, and 6/10 pts >200d. The randomized phase 2 study is currently enrolling. Clinical trial information: NCT05099666 . [Table: see text]