2550 - Safety and Efficacy of Combination PRaG Regimen with Dynamic and Precise Regulation of Thymalfasin Dose Based on T Lymphocyte Count for the Treatment of Chemo-Refractory Metastatic Solid Tumors: An Op

Y. Kong, M. Xu, S. Li, R. Chen, J. Zhang, P. Xing, and Z. LiYuan; Center for Cancer Diagnosis and Treatment, The Second Af?liated Hospital of Soochow University, Suzhou, China

Purpose/Objective(s): Our previous study demonstrated that the triple combination therapy consisting of anti-PD-1 therapy,hypofractionated Radiotherapy(HFRT) and GM-CSF(PRaG regimen), exhibited positive efficacy and tolerable safety for advanced refractory solid tumors patients. Thymalfasin, commonly used as adjuvant therapy for various carcinomas to enhance and sustain T lymphocyte levels, could potentially augment the efficacy of PRaG therapy. An exploratory phase II study aimed at evaluating the efficacy of a meticulously controlled thymalfasin-supplemented PRaG regimen in advanced solid tumors (NCT05790447). Materials/

Methods: Patients with advanced solid tumors that had progressed after at least one line of chemotherapy were eligible. They were treated with thymalfasin in three doses, based on the absolute T lymphocyte count in each cycle. After a seven-day loading dose of thymalfasin for patients with low baseline lymphocytes, they underwent at least two cycles of the PRaG regimen until no suitable lesions remained for irradiation or the dose reached normal tissue tolerance. After completing PRaG cycles, patients continued with a PD-1 inhibitor and thymalfasin until disease progression or intolerable side effects occurred. The primary endpoint was the overall response rate (ORR) according to RECIST 1.1. Peripheral blood was collected for further translational research.

Results: As of January 31, 2024, six patients had enrolled and completed at least one tumor assessment. The ORR was 33.3%, with a disease control rate of 50.0% (2 patients with partial response (PR), one with stable disease (SD), and three with progressive disease). Four patients underwent single-cell RNA sequencing (scRNA-seq) of peripheral blood mononuclear cells (PBMCs) before and after two cycles of therapy. Early-activated CD8⁺T effector (Teff) cells, an intermediate state between naïve-like and effector-memory subsets, increased post-treatment, especially in the three responders (PR and SD) after two cycles of PRaG and thymalfasin. A CD8⁺terminally-exhausted T cells (Tex) cluster decreased in these responders. Seventy-six differential genes were identified in early-activated CD8⁺Teff cells, primarily involved in immune response activation, myeloid leukocyte activation, and T cell receptor signaling pathways (gene ontology). Six key genes were screened from 76 differential genes by gene network analysis were CCL3, PTPRC, NFKBIA, FOSB, RELB and TGFB1.

Conclusion: The preliminary results indicate the PRaG 5.0 regimen is well tolerated with acceptable toxicity. Our data suggest that HFRT can enhance the anti-tumor effects of anti-PD-1 therapy and thymalfasin and GM-CSF demonstrate a synergistic efficacy. The activation of early-activated CD8⁺Teff cells may correlate with favorable responses, but further research in a larger patient cohort is necessary to confirm these findings.