Objective

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, such as 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 [1]. We will report further optimization of this series with a focus on pharmacokinetic and physicochemical properties suitable for an intravenous agent with short target engagement. This work led to the identification of AZD4573, a potent inhibitor of CDK9 with fast-off binding kinetics and high selectivity versus other kinases, including other CDK family kinases. AZD4573 exhibits a short half-life in multiple preclinical species 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. 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. [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)