Abstract 4427: LS007, a potent and orally bioavailable CDK9 inhibitor, suppresses the growth of triple-negative and estrogen receptor-positive breast cancer

Abstract

Abstract Treatment of breast cancer represents a major therapeutic challenge, and thus identification of new targeted therapy is of paramount importance. Anti-apoptotic proteins, BCL2 and MCL1, and oncoprotein c-MYC are implicated in evasion of apoptosis and resistance to chemotherapy in both triple-negative breast cancer (TNBC) and estrogen receptor-positive breast cancer (ER+ BC). Transcription of these oncogenic factors requires cyclin dependent kinase 9 (CDK9). CDK9 phosphorylates the C-terminal domain (CTD) of RNA polymerase II (RNAPII) and acts as a rate-limiting step of transcription. Thus, targeting CDK9 can reduce the expression of these proteins effectively and simultaneously, presenting a promising therapeutic strategy for breast cancer. Utilizing extensive medicinal chemistry and biochemical assays, we previously identified and developed LS007 as an orally bioavailable (F = 56% in cynomolgus monkey at 4 mg/kg), and nanomolar inhibitor of CDK9 (Ki = 4 nM). In vitro, inhibition of CDK9-mediated phosphorylation of RNAPII (pRNAPIIS2) by LS007 resulted in downregulation of BCL2, MCL1 and c-MYC and activation of PARP cleavage indicating apoptosis in breast cancer cells (TNBC: MDA-MB-231, Hs578T and ER+ BC: MCF-7, T47D). In vivo, LS007 was well tolerated in mice after oral administration (MTD = 50 mg/kg QD and 100 mg/kg Q3D). In MDA-MB-231 TNBC xenograft, by daily oral dosing at 25 mg/kg and 50 mg/kg LS007 significantly reduced tumor growth (p < 0.001) with T/C of 53 % and 35 %, respectively, on day 21, when compared to the vehicle dosed group. Similarly, in MCF-7 ER+ BC xenograft model, oral dosing of LS007 at 25 mg/kg QD, 50 mg/kg Q2D and 75 mg/kg Q3D showed significant tumor growth inhibition with T/C of 50 %, 60 % and 47 %, respectively, and increased in animal survival (p < 0.001) compared to vehicle control. It was safe in the xenograft models as there was no significant body weight loss or other overt toxicities. In addition, histological analysis of major organs (liver, kidney, heart, bone marrow and intestine) of the mice receiving LS007 showed no drug related toxicities. These results support for development of LS007 towards the clinical trials for the treatment of TNBC and ER+ BC. Citation Format: Muhammed Hamidur Rahaman, Yinghui Zhang, Md Saiful Islam, Gary Heinemann, Abel Tesfaye Anshabo, Hugo Albrecht, Robert Milne, Shudong Wang. LS007, a potent and orally bioavailable CDK9 inhibitor, suppresses the growth of triple-negative and estrogen receptor-positive breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4427.