Abstract 1268: Early proliferative response via [18]FLT-PET/CT and serum TK1 activity are predictive biomarkers for synergistic effect of sequential cell cycle targeted therapy in sarcoma

Abstract Introduction: Sarcomas are rare, aggressive tumors that make up a higher percentage of overall cancer mortality in children and young adults. Primary management of sarcoma patients includes surgical resection with adjuvant or neoadjuvant radiation/chemotherapy. Even with aggressive multimodal treatment regimens, patients with high-grade sarcomas experience suboptimal prognosis and advanced disease is almost universally fatal. Innovative biomarker driven targeted therapies are in urgent need to improve survival in these patients. Discoveries in molecular and genetic basis of sarcoma have revealed new targets that deserve consideration. Inactivation of retinoblastoma (Rb) pathway, which causes unabated cell proliferation, through overexpression of cyclin D1 and/or CDK4/6 has been detected in the majority of sarcomas even when wild-type Rb protein is expressed. Hence, inhibition of cell cycle progression through reactivation of the tumor suppressor function of Rb by inhibiting CDK4/6 activity poses a potential avenue for new therapeutic options. Methods: A panel of 4 sarcoma cell lines and 4 patient derived xenograft (PDX) models were used: 2 with an intact Rb pathway (Rb+ve, p16-ve) and 2 with an altered Rb pathway (Rb low/-ve, p16+ve). CDK4/6 inhibitor, abemaciclib, was used to reactivate the Rb pathway followed by treatment with gemcitabine or WEE1-kinase-inhibitor, to target the enriched S and G2/M phase cells post-abemaciclib removal. 18F-FLT PET imaging and serum thymidine kinase 1 (TK1) were used to monitor changes in cell proliferation and treatment response in both in vitro and in vivo. Changes in cell cycle biomarkers in response to treatment were also interrogated. Results: Rb+ve sarcoma cell lines were sensitive to the anti-proliferative activity of abemaciclib leading to their reversible G1 arrest, while Rb null cells did not arrest in any phase of cell cycle in response to CDK4/6 inhibition. Following abemciclib removal, Rb+ve models (but not Rb-ve) showed significant increase in 18F-FLT biotracer uptake concomitant with elevated serum TK1 and enrichment of cells in S phase. The enrichment of cells in S phase maximized therapeutic efficacy of S-phase or G2/M targeting agents leading to synergistic cell killing in Rb+ve cell lines and PDX models. Conclusion: Our study presents an innovative treatment strategy inhibiting the Rb pathway followed by S or G2/M pathway sequentially resulting in synergistic cell killing in those cells/tumors with wild-type Rb. We also incorporate real-time PET imaging and serum TK1 as non-invasive indicators of response to the treatment strategy. The study with PDX models also identified biomarkers of cell proliferation to be used in conjunction with PET imaging to provide the pre-clinical rationale to translate our findings into clinical trials for this aggressive disease. Citation Format: Tuyen Duong Nguyen, Marc A. Pina, Yan Wang, Cansu Karakas, Tuyen N. Bui, Osama Mawlawi, Alexander J. Lazar, Davis R. Ingram, Peter J. Hougton, Kelly K. Hunt, Khandan Keyomarsi. Early proliferative response via [18]FLT-PET/CT and serum TK1 activity are predictive biomarkers for synergistic effect of sequential cell cycle targeted therapy in sarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1268.