AT7519, a Potent Multi-Targeted CDK Inhibitor, Is Active in CLL Patient Samples Independent of Stage
Authors: Lock Vicky , Cooke Laurence , Yule Murray , Thompson Neil T , Croce K. Della , Lyons John F , Squires Matthew S , Mahadevan Daruka 
Published: April 2020

Abstract
Cyclin Dependent Kinases (CDKs) play a central role in the eukaryotic cell cycle. The activation of these kinases is modulated by the expression and binding of their regulatory cyclin partners. Their key role in cell cycle progression, coupled to evidence that pathways leading to their activation are deregulated in a number of human cancers makes them attractive therapeutic targets. More recently the role of CDKs 7, 8 and 9 in the regulation of transcription has been explored. CDK9 has been shown to play a role in the regulation of transcription via phosphorylation of RNA polymerase II (RNA pol II). The outcome of transcriptional inhibition via CDK9 exhibits significant variation between cell lines. B-Cell lymphoproliferative disorders, including CLL, rely on the expression of transcripts with a short half-life such as Mcl-1, Bcl-2 and XIAP for survival. In vitro studies have demonstrated that compounds with transcriptional inhibitory effects are effective pro-apoptotic agents in models of this disease.
AT7519 is a potent inhibitor of cyclin dependent kinases 1, 2 and 9 and is currently in early phase clinical development. These studies profile the mechanism of action of AT7519 on CLL cells isolated from patients. Primary cell samples were isolated from a total of 15 patients with CLL with various stages of disease (8 Stage 0, 0/I or II and 7 Stage IV) and who were either treatment naïve or had received a variety of prior therapies. Patient samples were characterised for cytogenetic abnormalities (11q, 17p and 13q deletion or trisomy 12) as well IgVH mutation and ZAP70 expression.
AT7519 was shown to induce apoptosis (by MTS, morphology and PARP cleavage) in these samples at concentrations of 100–700nM. AT7519 appears equally effective at inhibiting the survival of CLL cells harbouring a variety of mutations including those representative of patients that fall within poorer prognosis treatment groups. The amount of AT7519 required to induce cell death in 50% of the CLL cell population increased as exposure time was decreased but significant cell death was obtained at doses approximating to 1uM following 4–6h of treatment. These doses are equivalent to exposures achieved in ongoing AT7519 clinical studies indicating that cytotoxic doses can be achieved in patients on well tolerated schedules.
The mechanism of AT7519 cytotoxic effects was investigated by western blotting for a variety of cell cycle and apoptotic markers following incubation with compound. Short term treatments (4–6h) resulted in inhibition of phosphorylation of the transcriptional marker RNA pol II and the downregulation of the anti-apoptotic protein Mcl-1. Additional antiapoptotic proteins including XIAP and Bcl-2 remained unchanged. The reduction in Mcl-1 protein levels was associated with an increase in the apoptotic marker cleaved PARP.
No inhibition of cell cycle markers such as phospho-retinoblastoma protein was observed in the same samples suggesting that the cytotoxic effects of AT7519 in CLL patient samples is due to its transcriptional activity alone.
Together the data suggest AT7519 offers a promising treatment strategy for patients with advanced B-cell leukemia and lymphoma.