Abstract P255: Alvocidib synergizes with BRD4 inhibitors to improve cytotoxity in an AML cell line (2021)

Abstract The super enhancer complex (SEC) is a group of transcription regulatory proteins that coordinate the expression of genetic programs which determine cell identity and drive diseases such as cancer. In acute myeloid leukemia (AML), SECs have been shown to turn on transcriptional programs that drive tumorigenesis and progression. SEC-regulated transcription begins as Cyclin-Dependent Kinase 9 (CDK9) and Cyclin T1 are recruited from an inhibitory complex by bromodomain 4 (BRD4) and brought to the transcriptional start site of genes. CDK9 phosphorylates RNA polymerase II, releasing it from the SEC and leading to transcriptional elongation and gene expression. Alvocidib is a potent CDK9 inhibitor with clinical activity in AML from multiple Phase II studies. Additionally, BRD4 inhibitors have shown early promise in clinical studies with a focus on AML. Starting with the observation of close association of CDK9 and BRD4 and the central role of the SEC in AML, we previously showed that CDK9 inhibitors combined with bromodomain inhibitors inhibited the SEC more effectively than either of these compounds alone. In this study, we sought to quantify the synergy between alvocidib and each of the BRD4 inhibitors, OTX015, IBET762, and CPI-0610, in the MV-4-11 AML cell line. After determining the single agent IC50 value of each agent, 6x6 matrix combination assays were used to determine synergy between alvocidib and OTX015, IBET762, or CPI-0610. The matrix combination assays consisted of concentrations of each : 4x IC50, 2x IC50, 1x IC50, 0.5x IC50, 0.25x IC50, and 0.125x IC50 value. In the combination assays, the cells were incubated with alvocidib for 48 h, then a BRD4 inhibitor was added, and the cells were incubated with alvocidib and the BRD4 inhibitor for a further 48 h. Combination index (CI) values were calculated using the Chou-Talalay method, where CI values less than 1 indicate synergism and lower CI values indicating stronger synergism. Synergistic interactions were observed in MV-4-11 cells for all three combinations of alvocidib plus a BRD4 inhibitor. For the combination of alvocidib and OTX015, synergy was observed for 81% (29/36) of the tested combinations, with the strongest synergy (CI = 0.155) observed at 20 nM alvocidib and 1.472 µM OTX015. For the combination of alvocidib and IBET762, synergy was observed for 75% (27/36) of the tested combinations, with the strongest synergy (CI = 0.144) observed at 10 nM alvocidib and 6.288 µM IBET762. For the combination of alvocidib and CPI-0610, synergy was observed for 72% (26/36) of the tested combinations, with the strongest synergy (CI = 0.101) observed at 10 nM alvocidib and 5.312 µM CPI-0610. These results suggest that combination strategies might be rationally designed to improve tumor targeting effects while reducing adverse side effects. Clinical studies utilizing these combination strategies are the next steps to further explore this approach. Citation Format: Sal Sommakia, Ethika Tyagi, Yuta Matsumura, Jason M. Foulks, Steven L. Warner. Alvocidib synergizes with BRD4 inhibitors to improve cytotoxity in an AML cell line [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P255.