Abstract 4295: AZ’5576, a selective CDK9 inhibitor, demonstrates in vitro and in vivo activity in diverse preclinical models of non-Hodgkin lymphoma
Authors: Cidado Justin , Proia Theresa , Gregory Gareth , Todorovski Izabela , Boiko Scott , Martin Maryann San , Johnstone Ricky , Drew Lisa 
Published: July 2017

Abstract
Cyclin-dependent kinase 9 (Cdk9) is a serine/threonine kinase that regulates elongation of transcription through phosphorylation of RNA polymerase II at serine 2 (pSer2-RNAPII). Transient inhibition of Cdk9 results in modulation of genes with short-lived transcripts and labile proteins, thereby representing a potential therapeutic opportunity in tumors dependent upon oncogenes fitting these criteria. Mcl1, an anti-apoptotic protein that has been linked to increased survival and chemotherapy resistance in various cancers, and Myc, a proto-oncogenic transcription factor that coordinates diverse transcription programs and is over-expressed, amplified, or translocated in many cancers, are two such oncogenes.
AZ’5576 is a potent, highly selective, and orally bioavailable inhibitor of Cdk9 that inhibits Cdk9 enzyme activity with an IC50 &lt;5nM and decreases phosphorylation of Ser2-RNAPII in cells with an IC50 of 96nM. In sensitive cell lines, short-term treatment with AZ’5576 led to a rapid dose- and time-dependent decrease in pSer2-RNAPII with concomitant loss of Mcl1 and Myc mRNA and protein, resulting in the cleavage and activation of caspase 3 and loss of cell viability. Using cell potency and magnitute of caspase activation to define sensitivity, we previously reported that AZ’5576 demonstrates broad in vitro and in vivo activity across hematological malignancies. Initially focusing on acute myeloid leukemia (AML) and multiple myeloma (MM), we have now expanded our scope to encompass non-Hodgkin lymphoma (NHL), which includes diffuse large B-cell lymphoma (DLBCL), Burkitt’s lymphoma (BL), mantle cell lymphoma (MCL), and Waldenstrom macroglobulinemia (WM).
In vitro screening assays reveal about half (19/35) of NHL cell lines undergo cell death in response to AZ’5576, demonstrating a potency &lt;520nM and a maximum caspase activation &gt;55% after 6 hour treatment. Combining AZ’5576 with a BTK inhibitor further enhances cell death compared to that observed with either single agent alone in BTK inhibitor-sensitive ABC-DLBCL cell lines. Consistent with the in vitro data, significant anti-tumor activity was associated with short-term reduction of pSer2-RNAPII tumor levels after intermittent dosing of AZ’5576 in a mouse xenograft model of ABC-DLBCL cell line OCILY10. Similarly, combining AZ’5576 with a BTK inhibitor in vivo drove tumors into regression (combination=199% TGI) compared to the partial TGI from either single agent alone (AZ’5576=79% TGI, Acalabrutinib=58% TGI) in the OCILY10 xenograft model. Finally, in an agressive Eµ-Myc transgenic mouse model of B-cell lymphoma, AZ’5576 treatment results in potent induction of apoptosis in vitro and a greater than 50 day increase in median overall survival in vivo.
Together, these results highlight the therapeutic potential for selective CDK9 inhibition in the treatment of patients with non-Hodgkin lymphoma.
Citation Format: Justin Cidado, Theresa Proia, Gareth Gregory, Izabela Todorovski, Scott Boiko, Maryann San Martin, Ricky Johnstone, Lisa Drew. AZ’5576, a selective CDK9 inhibitor, demonstrates in vitro and in vivo activity in diverse preclinical models of non-Hodgkin lymphoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4295. doi:10.1158/1538-7445.AM2017-4295