Abstract CT009: Results of a dose- and regimen-finding Phase Ib study of HDM201 in combination with ribociclib in patients with locally advanced or metastatic liposarcoma

Background: Well-differentiated/dedifferentiated liposarcoma (WDLPS/DDLPS) is characterized by a consistent coamplification of human double minute 2 homolog (HDM2) and cyclin-dependent kinase 4 (CDK4) which inactivates the tumor suppressor pathways p53 and Rb, respectively. HDM201 is a selective inhibitor of the p53-HDM2 interaction and ribociclib is a CDK4/6 inhibitor. Preclinical studies suggested a synergy in in vitro and in vivo models of WDLPS/DDLPS and both agents have demonstrated single-agent clinical activity in solid tumors. We aim to determine the optimal dose and regimen of HDM201 + ribociclib and to assess the preliminary antitumor activity of this combination in patients (pts) with liposarcoma. Methods: In this multicenter, open-label, Phase Ib ongoing study, pts with locally advanced or metastatic liposarcoma that had unequivocally progressed on, or despite prior systemic therapy were treated orally with HDM201 + ribociclib. Three treatment regimens were explored: Regimen (Reg) 1 (HDM201 + ribociclib daily for the first 2 wks [QD 1st 2 wks] in a 4-wk cycle), Reg 4 (HDM201 every 3 wks + ribociclib QD 1st 2 wks in a 3-wk cycle), and Reg 5 (HDM201 every 4 wks + ribociclib QD 1st 2 wks in a 4-wk cycle). Results: As of Nov 21, 2017, 74 pts received HDM201 + ribociclib (Reg 1 n=26; Reg 4 n=29; Reg 5 n=19); 12 pts (6 each in Reg 4 and 5) were still receiving treatment. Ten pts (Reg 1 n=3; Reg 4 n=6; Reg 5 n=1) discontinued treatment due to adverse events (AEs) and 2 pts (1 each in Reg 4 and 5) died. The most common AE of any grade, regardless of cause, reported across regimens (Reg 1; Reg 4; Reg 5) was nausea (81%; 76%; 63%) which was mainly Grade 1/2 and not dose limiting. Common Grade 3/4 AEs regardless of cause included neutropenia (39%; 52%; 42%), thrombocytopenia (35%; 45%; 42%), anemia (27%; 17%; 21%), leukopenia (27%; 28%; 37%), and lymphopenia (15%; 14%; 21%). Dose-limiting toxicities were reported in 16 pts (2; 9; 5) and all except 1 (prolonged QT) were hematologic (including neutropenia [n=5], thrombocytopenia [n=4], febrile neutropenia, and anemia [n=2 each]). Partial responses were observed in 3 (4%) pts (2 in Reg 4; 1 in Reg 5). Stable disease was achieved by 36 (49%) pts (11 in Reg 1; 16 in Reg 4; 9 in Reg 5). The median progression-free survival (PFS) was 2.7 mo (95% confidence interval [CI]: 1.9-8.2 mo) in Reg 1, 4.8 mo (95% CI: 3.9 mo-not reached) in Reg 4, and 2.1 mo (95% CI: 1.4 mo-not reached) in Reg 5. Conclusions: HDM201 + rib