Abstract 5984: CDK9 inhibition as a potential therapeutic strategy in Ewing sarcoma
Authors: Matsumura Yuta , Heinz Richard E. , Allred Curtis A. , Siddiqui Adam , Foulks Jason M. , Warner Steven L. 
Published: April 2023

Abstract
Ewing sarcoma (EWS) is the second most common bone and soft tissue sarcoma in children and adolescents and accounts for approximately 2% of all childhood cancer diagnoses. EWS is a highly aggressive cancer: the overall 5-year survival rate for localized and metastatic EWS are 67 and 38%, respectively. Each year, approximately 200 children and adolescents in the United States are diagnosed with EWS. Genetically, nearly all EWS have chromosomal translocations in which a member of the FET gene family is fused with an ETS transcription factor, with 85% of cases being EWSR1-FLI1. Cyclin-dependent kinase 9 (CDK9) is a serine/threonine protein kinase involved in transcription elongation through phosphorylation and activation of RNA polymerase II (RPB1) and increases the levels of key oncogenic genes like myeloid cell leukemia-1 (MCL-1) and c-Myc, which are reported to be involved in chemoresistance. CDK9 may be a viable target in transcriptionally addicted cancers including EWS driven by oncogenic fusion genes like EWSR1-FLI1. TP-1287 is an investigational orally delivered phosphate pro-drug of alvocidib, a potent CDK9 inhibitor. We hypothesized that alvocidib, the pharmaceutically active form of TP-1287, may show activity in EWS cells as a single agent and in combination with standard of care agents. Alvocidib demonstrated the suppression of downstream targets of CDK9 including c-Myc in EWS cell lines after 6-hr treatment. In cell viability assays using CellTiter-Glo, alvocidib resulted in IC50 values of 68 - 125 nM in EWS cell lines after 3-day treatment and 27 - 160 nM in EWS cells from patient derived xenograft (PDX) models, after 6-day treatment. MCL-1 amplified EWS cells were observed significantly more sensitive to alvocidib than MCL-1 diploid EWS cells in the EWS cells from PDX models (p=0.004). Further bioinformatic analysis to predict alvocidib sensitivity is currently underway. Alvocidib demonstrated an additive inhibitory response in combination with topotecan, phosphoramide mustard, a biologically active metabolite of cyclophosphamide, and palifosfamide, a biologically active metabolite of ifosfamide, in EWS cell lines after 3-day treatment. Alvocidib treatment showed reduction of RAD51 which is involved in DNA damage response (DDR), suggesting the activity of alvocidib on DDR may contribute to this additive response. In an in vivo preclinical study using the A-673 model, TP-1287 (2.5 mg/kg, QD) showed tumor growth inhibition (TGI). In vivo combination studies with standard of care agents using EWS xenograft models are currently ongoing. In summary, alvocidib has shown activity inhibiting the growth of EWS cells as a single agent and an additive response in combination with EWS standard of care agents in EWS cell lines. TP-1287 may be a potential therapeutic option for the current regimen in EWS. TP-1287 is being investigated for solid tumors including EWS (clinicaltrials.gov, NCT03604783).
Citation Format: Yuta Matsumura, Richard E. Heinz, Curtis A. Allred, Adam Siddiqui, Jason M. Foulks, Steven L. Warner. CDK9 inhibition as a potential therapeutic strategy in Ewing sarcoma. [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 5984.