Preliminary results of a first-in-human phase 1b (aCCeleR8-001) study of S-531011, a humanized anti-CCR8 monoclonal antibody, in patients with advanced solid tumors.

3076 Background: C-C motif chemokine receptor 8 (CCR8) is selectively upregulated in tumor-infiltrating regulatory T cells (TI-Tregs) in multiple cancers, inhibiting anti-tumor activity of the host immune system. S-531011, a humanized IgG1 monoclonal antibody, is anticipated to deplete CCR8-positive TI-Tregs, restoring anti-tumor immunity without inducing auto-immunity. Methods: An aCCeleR8-001 study is a Phase 1b/2, multicenter, open-label study of S-531011 which consists of Phase 1b Dose Escalation part (Parts A-1 and A-2) and Phase 2 Dose Expansion part. The safety/tolerability, pharmacokinetic (PK), pharmacodynamic, and anti-tumor activity of S-531011 as monotherapy and in combination with pembrolizumab (Merck & Co., Inc.) were evaluated in patients with various types of locally advanced or metastatic solid tumors. S-531011 monotherapy was administered at 8, 24, 80, 240, 800, or 1600 mg/kg intravenously every 3 weeks (Q3W) in Part A-1, whereas patients in Part A-2 received S-531011 at 80, 240, 800, or 1600 mg/kg in combination with pembrolizumab 200 mg/kg Q3W. The data were analyzed when all patients in Dose Escalation cohorts completed the dose-limiting toxicity (DLT) observation period. Results: As of the data cutoff date (30 Sep 2024), 40 and 35 patients were enrolled in Parts A-1 and A-2, respectively. No DLTs were reported at any dose level and the maximum administered dose of S-531011 was 1600 mg in both parts. One patient reported an infusion-related reaction in Part A. Immune-related adverse events (irAEs) were reported in two patients (5.0%; Grade 1/2 only) in Part A-1, whereas 15 irAEs were reported in 10 patients (28.6%; including four Grade 3 irAEs in four patients) in Part A-2. PK of S-531011 was approximately dose proportional with a terminal elimination half-life of 10 to 12 days regardless of dose level. CCR8 receptors in PBMCs were occupied at doses of 80 mg or higher. PK/CCR8 receptor occupancy modeling analysis indicated that > 90% of receptors in tumor tissues were occupied in the range of 80 to 800 mg. Multiplex immunohistochemistry analysis demonstrated proof of mechanism as evidenced by CCR8-positive Treg depletion in tumor tissue at doses of 24 mg or higher. Among 62 evaluable patients dosed at 80 to 1600 mg in Part A, four patients (6.5%) had confirmed partial response, three of whom had colorectal cancer (CRC). Twenty patients (32.3%) had disease control for ≥ 6 weeks. Response rate was not correlated with dose (80 to 1600 mg). Following a comprehensive data review, tentative recommended Phase 2 doses were determined to be 80 to 800 mg in both parts. Conclusions: S-531011 was well tolerated up to 1600 mg as monotherapy and in combination with pembrolizumab. A higher response rate in patients with CRC warrants further exploration of this tumor type in Phase 2 Dose Expansion part. Phase 2 CRC cohorts are currently ongoing. Clinical trial information: NCT05101070 .