A phase Ia/Ib, dose-escalation/expansion study of the MDM2-p53 antagonist BI 907828 in patients (pts) with solid tumors: Safety and efficacy in patients with dedifferentiated liposarcoma (DDLPS)

11554 Background: Inactivation of p53 is a key mechanism which promotes tumor survival and proliferation. p53 inactivation can be due to mutations in TP53 or downregulation of wild-type (wt) p53 by the key negative regulator mouse double-minute 2 (MDM2). BI 907828 is a highly potent MDM2–p53 antagonist which has demonstrated preclinical antitumor activity, particularly in TP53-wt MDM2-amplified DDLPS models. In this phase I study (NCT03449381), BI 907828 monotherapy is being evaluated in pts with advanced solid tumors, including DDLPS. During dose escalation (phase Ia), BI 907828 had a manageable safety profile and the selected recommended dose for expansion (RDE) was 45 mg q3w. Here we report safety data in all pts who received the RDE of 45 mg q3w and present efficacy data in the subgroup of pts with DDLPS. Methods: In Phase Ia, pts received one of two BI 907828 dosing schedules: Arm A, day 1 of 21-day cycles (q3w); Arm B, days 1 and 8 of 28-day cycles. During Phase Ib (dose expansion), pts were enrolled to Cohort 1 ( TP53wt, MDM2-amplified sarcoma) or Cohort 2 (other TP53wt, MDM2-amplified solid tumors). Here we focus on Cohort 1 (received BI 907828 45 mg q3w). The primary endpoint (phase Ib) was progression-free survival (PFS). Secondary endpoints/objectives included overall response rate (ORR) and grade ≥3 treatment-related AEs (TRAEs). Results: As of December 22, 2022, a total of 137 pts had been enrolled; 72 (52.6%) were male, 77 (56.2%)/59 (43.1%) had ECOG PS 0/1, and the median number of prior systemic therapies was 2 (range, 0–11). In the 73 pts who received the RDE of 45 mg q3w, the most common any-grade TRAEs were nausea (74.0%) and fatigue (61.6%). 33 (45.2%) pts had grade ≥3 TRAEs; the most common were thrombocytopenia (23.3%), neutropenia (21.9%), anemia (11.0%), and leukopenia (11.0%). 22 (30.1%) pts had TRAEs leading to dose reductions and 6 (8.2%) had TRAEs leading to treatment discontinuation. 20 pts (27.4%) had serious AEs; the most common were nausea, pulmonary embolism (each 4.1%), sepsis, small intestine obstruction, vomiting, thrombocytopenia, and pyrexia (each 2.7%). 50/73 pts who received 45 mg q3w had advanced DDLPS; all had MDM2-amplified disease. Of the 42 evaluable DDLPS pts, 8 achieved a confirmed PR (ORR 19.0%; locally assessed) and a further 29 achieved a best response of stable disease (including 2 with unconfirmed PR), giving a disease control rate of 88.1%. Preliminary median PFS was 8.1 months. Conclusions: BI 907828 demonstrated a manageable safety profile overall and encouraging preliminary efficacy was seen in pts with advanced MDM2-amplified DDLPS; Phase Ib is ongoing. BI 907828 is also being evaluated versus doxorubicin as first-line treatment for pts with advanced DDLPS in the ongoing Phase II/III Brightline-1 study (NCT05218499), for which the FDA has granted a Fast Track Designation. Clinical trial information: NCT03449381 .