A precise thymalfasin-regulated PRaG regimen for advanced refractory solid tumors: An open-label, prospective, multicenter clinical study (PRaG 5.0 study).
Authors: Kong Yuehong , Xu Meiling , Li Shicheng , Chen Rongzheng , Zhao Xiangrong , Zhang Junjun , Xing Pengfei , Zhang Liyuan 
Published: June 2024

e14637 Background: PRaG regimen, which consisted of anti-PD-1 therapy, radiotherapy and GM-CSF, had a positive efficacy and tolerable safety for advanced refractory solid tumors who were heavily treated. However, severe lymphocytopenia was one of the major hurdles for immunotherapy. Thymalfasin has been wildly used as an adjuvant therapy for multiple carcinomas for elevating and sustaining the abundance of T lymphocyte. An exploratory phase II study was conducted to assess the efficacy of a meticulously thymalfasin-controlled PRaG regimen in advanced solid tumors (NCT05790447). Methods: Subjects with advanced solid tumors, having progressed beyond at least first-line chemotherapy, were eligible and treated with thymalfasin at three doses, dependent on the baseline absolute T lymphocyte count. Following seven days of thymalfasin treatment in patients with low baseline lymphocytes, they received at least two cycles of the PRaG regimen until either no suitable lesions remained for irradiation or the normal tissue tolerance dose was reached. When completing PRaG cycles, patients were maintained PD-1 inhibitor and thymalfasin until disease progression or intolerable adverse events. The primary endpoint was the overall response rate (ORR) according to RECIST 1.1. Peripheral blood samples were collected for subsequent translational research. Results: As of January 31, 2024, six patients were enrolled in the study and completed at least one tumor assessment. The ORR was 33.3%, and the disease control rate was 50.0% (2 patients with partial response (PR), one patient with stable disease (SD), and three patients with progressive disease). Four patients underwent single-cell RNA sequencing (scRNA-seq) analyses of frozen peripheral blood mononuclear cells (PBMCs) derived from before and after two cycles of therapy. Early-activated CD8+T cells, the intermediate state between the naïve-like and the effector-memory subsets, increased post-treatment, especially in three responders with PR and SD after two cycles of PRaG and thymalfasin, while a CD8+ terminally exhausted (Tex) cluster showed a decreasing trend in these responders. Seventy-six differential genes were identified in early-activated CD8+T cells, mainly enriched in activation of immune response, myeloid leukocyte activation, and T cell receptor signaling pathways (gene ontology). CCL3, PTPRC, NFKBIA, FOSB, RELB, and TGFB1 were vital genes in the gene network analysis. Conclusions: The preliminary results of the PRaG 5.0 regimen revealed a manageable safety profile and encouraging efficacy. A loading dose of thymalfasin modulation appeared to increase in early-activated CD8+T cell proportions, which might correlate with favorable responses. These results should be validated in a larger cohort of patients, and the underlying mechanisms warrant further exploration. Clinical trial information: NCT05790447 .