Phase 1b portion of the ACTION-1 phase 1b/3 trial of RYZ101 in gastroenteropancreatic neuroendocrine tumors (GEP-NET) progressing after <sup>177</sup> Lu somatostatin analogue (SSA) therapy: Safety and efficacy findings.
Authors: Jonathan R. Strosberg, Michael Morris, Gary A. Ulaner, Daniel M. Halperin, Samuel H. Mehr, Daneng Li, Heloisa P. Soares, Lowell Brian Anthony, Sandy Diana Kotiah, Heather Jacene, Margot ET Tesselaar, Pamela L. Kunz, Denis Vasconcelos Ferreira, Joanne Li, Kimberly Ma, Jessica Rearden, Susan Moran, Thomas A. Hope, Simron Singh
Journal: Journal of Clinical Oncology (2025)

661 Background: RYZ101 ( 225 Ac-DOTATATE) is an alpha-emitting radiopharmaceutical in development for SSTR2+ solid tumors. Alpha-particles have a shorter path length/higher linear energy transfer than beta-particles, causing more frequent double-strand DNA breaks and potentially improving the therapeutic index. ACTION-1 (NCT05477576) is a two-part, global, randomized, controlled, open-label, phase 1b/3 trial of RYZ101 in advanced, well-differentiated SSTR+ GEP-NETs progressing after 177 Lu-SSA therapy. Herein, we report updated results from the phase 1b portion of the trial. Methods: The phase 1b portion of the ACTION-1 trial had a dose de-escalation/Bayesian optimal interval design with boundaries based on a dose-limiting toxicity (DLT) rate of 25%. Patients received RYZ101 IV every 8 weeks for 4 cycles. Planned dose levels (n=6/level): Level 0 (starting dose) 120 kBq/kg; Level 1 90 kBq/kg; Level 2 60 kBq/kg. DLTs were assessed for 56 days after the first RYZ101 dose. Treatment-emergent adverse events (TEAEs) were graded by NCI-CTCAE v5.0. A Data Review Committee oversaw dose de-escalation decisions/safety data. Tumor response was assessed locally by RECIST v1.1. Results: Seventeen patients received at least one dose of RYZ101 at 120 kBq/kg (4 doses: 15 patients; 2 doses: 2 patients; median 8.3 MBq). Baseline patient characteristics: median age 63 years; male (n=11); ECOG PS 0/1 (n=10/7); primary tumor site GI/pancreas (n=12/5). As of 14 December 2023, the most frequent TEAEs were anemia (58.8%), nausea (58.8%), and fatigue (52.9%). Serious adverse events (SAEs) were observed in 6 patients (35.3%, none were treatment-related); grade ≥3 TEAEs occurred in 9 patients (5 were treatment-related, 29.4%). No TEAEs led to treatment discontinuation. Four patients had TEAEs leading to dose modification, dose hold, and/or dose delays. There was one grade 5 TEAE of hepatic failure that was deemed unrelated to the study drug. The confirmed objective response rate was 29.4% (n=5; 1 complete response, 4 partial responses). One additional patient had an unconfirmed partial response at the time of data cutoff, which was subsequently confirmed. Seven patients (41.2%) had stable disease and 3 (17.6%) had progressive disease. The median duration of response was not estimable (95% CI 9.26 months, not estimable), and the median progression-free survival was not estimable (95% CI 12.16 months, not estimable). Conclusions: RYZ101 was well tolerated, and a fixed dose of 10.2 MBq was declared the recommended phase 3 dose. Initial data suggest promising efficacy and a manageable safety profile. Part 2 (phase 3) of the ACTION-1 trial is enrolling and will compare RYZ101 at 10.2 MBq every 8 weeks for 4 cycles with standard of care in patients with advanced SSTR2+ GEP-NETs progressing after 177 Lu-labeled SSAs. Clinical trial information: NCT05477576 .