Abstract 2624: Dual PLK1 and BRD4 inhibition has synergistic therapeutic effects against high-risk rhabdomyosarcoma

Concomitant inhibition of multiple therapeutic targets is an established strategy to improve the durability of clinical responses to targeted therapies. Rhabdomyosarcoma is the most common childhood soft tissue solid tumor and cure rates for patients with high-risk rhabdomyosarcoma remain dismal. Resistance to intensive chemotherapy is common, and few molecular therapies have been introduced clinically. High-risk rhabdomyosarcoma is driven by expression of the oncogenic fusion protein PAX3-FOXO1, which acts as a transcription factor. Recently, many efforts have been made to identify pharmacological means of inhibiting PAX3-FOXO1, given its central role in rhabdomyosarcoma cell line survival. The BET protein BRD4 has been shown to be required for transcription of PAX3-FOXO1 and its target genes and pharmacological inhibition via BET protein inhibitors exhibits anti-rhabdomyosarcoma effects. Phosphorylation of PAX3-FOXO1 by the polo like kinase 1 (PLK1) increases the stability of the oncoprotein. Based on these previous observations, we hypothesized that simultaneous inhibition of BRD4 and PLK1 could synergistically target PAX3-FOXO1 stability and activity, leading to increased anti-rhabdomyosarcoma effects. Indeed, combination treatment with PLK1 and BRD4 inhibitors exhibited synergistic effects specifically against PAX3-FOXO1-driven rhabdomyosarcoma cell lines. Dual PLK1-BRD4 inhibition suppressed proliferation and induced apoptosis in rhabdomyosarcoma cell lines at low nanomolar concentrations. Our results demonstrate a new strategy towards achieving dual synergistic targeting of BRD4 and PLK1 with a single agent which may lead to clinical translation for therapy of refractory rhabdomyosarcoma. Citation Format: Natalie Timme, Shuai Liu, Hailemichael Yosief, Heathcliff Dorado García, Ian MacArthur, Annabell Szymansky, Constantin Dohna, Georg Seifert, Patrick Hundsdörfer, Andrej Lissat, Angelika Eggert, Johannes Schulte, Wei Zhang, Anton Henssen. Dual PLK1 and BRD4 inhibition has synergistic therapeutic effects against high-risk rhabdomyosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2624.