Abstract 2799: Pharmacodynamic biomarkers for TGFBR1 inhibition in oncology
Authors: Heinz Richard E. , Allred Curtis A. , Kircher David A. , Matsumura Yuta , Franz Bettina , Ye Zhizhou , Lee Jinny , Whatcott Clifford J. , Foulks Jason M. , Siddiqui Adam , Warner Steven L. 
Published: April 2023

Transforming growth factor-beta (TGF-ß) is a master regulatory cytokine with pleotropic effects important in tumor development. TGF-ß1 binds TGF-ß receptor 2, which recruits and phosphorylates TGF-ß receptor 1 (TGFBR1). TGFBR1 in turn phosphorylates SMAD2 and SMAD3. Phosphorylated SMAD2/3 (pSMAD2/3) complexes with SMAD4 to translocate to the nucleus and enact transcriptional programs. In late-stage tumors, TGF-ß signaling is oncogenic, promoting growth, invasion, immune suppression, tissue remodeling, and epithelial to mesenchymal transition (EMT).

In an effort to counter the oncogenic effects of TGF-ß signaling, Sumitomo Pharma Oncology, Inc. is developing TP-6379, an investigational, orally available, small molecule, selective TGFBR1 inhibitor. TP-6379 has a biochemical IC50 of 1.07 nM and has been shown to reduce pSMAD2 in Rhabdomyosarcoma cells with an IC50 of 108 nM. A dose dependent increase of TP-6379 was observed in plasma and tumors of A549 tumor bearing athymic nude mice (10, 50, 75, and 100 mg/kg PO). In another study using the same mouse model, pSMAD2 has shown an inverse correlation with pharmacokinetics showing &gt; 90% inhibition for up to 4 hours (75 mg/kg PO).

Pharmacodynamic (PD) biomarkers are critical for determining biological activity, but procuring serial tumor biopsies for PD analysis is challenging for many tumor types. We hypothesize that pSMAD2 can be measured in non-tumor tissues as a surrogate for activity within the tumor. Surrogate tissues, such as peripheral blood mononuclear cells (PBMCs), skin punches, and even circulating tumor cells (CTCs) are much less invasive and can be sampled over multiple timepoints. We observed a measurable decrease of PD biomarkers pSMAD2/3 in PBMCs and skin after treatment with TP-6379 using a bead-based immunoassay (Luminex). Healthy human donor PBMCs treated ex vivo for 2 hours with up to 1 µM TP-6379 showed a 96% reduction in pSMAD2/3 signal. Although TP-6379 was still active in reducing pSMAD2 in PBMCs when treated in whole blood for 2 hours, it required around 30-fold more drug to achieve a similar effect, possibly due to serum protein binding. Skin samples from in vivo dosed tumor bearing mice also show significant inhibition of pSMAD2/3, which correlated with significant pSMAD2/3 inhibition in tumors from both EMT6 and 4T1 syngeneic triple-negative breast cancer mouse tumor models. We confirmed that the Luminex assay is sensitive enough to reliably measure pSMAD2/3 in healthy human doner skin punches down to 3 mm in diameter. Lastly, CTCs from breast cancer patients’ whole blood treated ex vivo for 24 hours with as low as 1 µM TP-6379 showed a decrease in pSMAD2 and EMT marker SNAI1 by as much as 96% and 86%, respectively, when measured by immunofluorescence.

In summary, our research has shown that pSMAD2/3 and EMT marker SNAI1 are valid PD biomarkers for TP-6379 therapy. We propose that these biomarkers would be best measured clinically using PBMCs, CTCs and/or skin punches.

Citation Format: Richard E. Heinz, Curtis A. Allred, David A. Kircher, Yuta Matsumura, Bettina Franz, Zhizhou Ye, Jinny Lee, Clifford J. Whatcott, Jason M. Foulks, Adam Siddiqui, Steven L. Warner. Pharmacodynamic biomarkers for TGFBR1 inhibition in oncology [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 2799.