Abstract 465: The preclinical characterization of TST003, a first-in-class mAb targeting gremlin-1 overexpressed by cancer-associated fibroblasts and tumor cells
Authors: Sun Di , Guo Huanhuan , Cheng Chaping , Li Hongjun , Yao Xinlai , Lu Shuang , Ding Jie , Zhang Shenjie , Zhou Shijie , Yu Steven , Gu Yi , Qian Xueming 
Published: April 2023

Abstract
Cancer associated fibroblast (CAF) is a rich source of factors for immune suppression and promotes cancer progression and metastasis via epithelial to mesenchymal transition (EMT). It is well known that tumors with mesenchymal phenotype have significantly poor prognosis and reduced response to checkpoint inhibitors. Targeting CAF becomes an increasingly attractive cancer therapeutic approach. Gremlin-1 is a member of the TGFβ superfamily, expressed by CAFs and tumor cells, and known to play a key role in EMT transition, cancer cell proliferation and stroma maintenance. Gremlin-1 overexpression in CAF or tumor cells often correlates with poor clinical prognosis in multiple cancers including prostate, pancreatic, gastric cancer etc. Here we describe the characterization of TST003 (14E3), a novel humanized IgG1 monoclonal antibody targeting Gremlin-1. TST003 bound to human Gremlin-1 with high affinity and high selectivity, and blocked Gremlin-1 binding to BMP2/4 with an EC50 of 3.68 nM to BMP2 and 4.53 nM to BMP4 respectively. TST003 could dose-dependently reverse Gremlin-1 inhibition of the BMP4-mediated Smad1/5/9 phosphorylation in tumor cells as measured by Western blot. In an ex vivo assay using prostate cancer patient-derived organoids (PDOs), TST003 inhibited the growth of 9 out of 12 human PDOs significantly. TST003 also demonstrated its single agent tumor growth inhibition (TGI&gt;50%) at 10 mg/kg in a human prostate cancer PC3 xenograft model on castrated male nude mice and a human colorectal cancer patient derived xenograft (PDX) mouse model in the presence of human PBMC. Both tumor models expressed Gremlin-1 as detected by IHC analysis. In addition, a significantly better anti-tumor activity was observed when TST003 combined with an anti-VEGFR2 antibody in this CRC PDX (MSS, Kras G12D, PD-L1 negative) model. The safety profiles of TST003 were characterized in the single and repeated dose toxicology studies integrated with safety pharmacology, local tolerance, and in vitro hemolysis study, TCR study, and cytokine release study. TST003 was well tolerated in cynomolgus monkeys without noteworthy abnormalities following a single dose or repeated doses. In summary, TST003 is a first-in-class therapeutic mAb targeting Gremlin-1 overexpressed by CAFs and tumor cells. Our preclinical characterization results provided the rationale for on-going clinical evaluation of TST003 in patients with advanced solid tumors with high unmet medical need either as monotherapy or in combination with SoC.
Citation Format: Di Sun, Huanhuan Guo, Chaping Cheng, Hongjun Li, Xinlai Yao, Shuang Lu, Jie Ding, Shenjie Zhang, Shijie Zhou, Steven Yu, Yi Gu, Xueming Qian. The preclinical characterization of TST003, a first-in-class mAb targeting gremlin-1 overexpressed by cancer-associated fibroblasts and tumor cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 465.