Cyclin-dependent kinase 9 (CDK9) is a serine/threonine kinase that regulates elongation of transcription through phosphorylation of RNA polymerase II at serine 2 (p-Ser2-RNAPII). Transient inhibition of CDK9 results in reduced protein levels for genes that have short half-lives of transcripts and proteins, thus presenting a potential therapeutic opportunity in tumors dependent upon oncogenes fitting such criteria. One example is Mcl-1, an anti-apoptotic protein that plays a key role in cancer cell survival. A potent and selective CDK9 inhibitor having appropriate physical properties and pharmacokinetics (intravenous administration and short t1/2) would enable short yet tuneable target engagement, allowing high flexibility in order to optimize the efficacy / tolerability balance in the clinic. We previously reported the identification of AZ5576 from an amidopyridine series, as a potent, highly selective and orally bioavailable preclinical inhibitor of CDK9. Here we report further optimization of this series with a focus on pharmacokinetic and physicochemical properties suitable for an intravenous agent with short target engagement. We discuss the Structure Activity Relationships (SAR) and Structure Property Relationships (SPR) in this series, specifically increasing human metabolic clearance (in order to achieve short half-life) and solubility whilst improving potency. This work led to the identification of AZD4573, a potent inhibitor of CDK9 (IC50 of <0.004 μM) with fast-off binding kinetics (t1/2 16 min) and high selectivity versus other kinases, including other CDK family kinases. AZD4573 exhibits a short half-life in multiple preclinical species (less than one hour in rat, dog and monkey) and good solubility for intravenous administration. Short-term treatment with AZD4573 led to a rapid dose- and time-dependent decrease in cellular pSer2-RNAPII, resulting in activation of caspase 3 and cell apoptosis in a broad range of haematological cancer cell lines (e.g. caspase activation EC50 0.0137 μM in an acute myeloid leukemia model MV4-11). Correspondingly, in vivo efficacy was demonstrated in xenograft models derived from multiple haematological tumours (e.g. regression at 15 mg/kg twice weekly in MV4-11 xenografts). These results support AZD4573 as a clinical candidate for the treatment of haematological malignancies (first disclosure of the structure at this meeting).

1 Cidado J et al, AZ5576, a novel, potent and selective CDK9 inhibitor, induces rapid cell death and achieves efficacy in multiple preclinical hematological models, AACR poster presentation, 3572 (2016)

Citation Format: Bernard Barlaam, Chris De Savi, Lisa Drew, Andrew D. Ferguson, Douglas Ferguson, Chungang Gu, Janet Hawkins, Alexander W. Hird, Michelle L. Lamb, Nichole O'Connell, Kurt Pike, Theresa Proia, Maryann San Martin, Melissa M. Vasbinder, Jeff Varnes, Jianyan Wang, Wenlin Shao. Discovery of AZD4573, a potent and selective inhibitor of CDK9 that enables transient target engagement for the treatment of hematologic malignancies [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 1650.