Abstract 3063: Mcl-1 dependency is a predictive biomarker for apoptotic induction by short-term dinaciclib (SCH 727965) treatment

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL Dinaciclib (SCH 727965) is a potent and selective inhibitor of the cyclin-dependent kinases (CDKs) 1, 2, 5 and 9 undergoing clinical testing against a range of solid and hematologic malignancies. From preclinical studies, more than 140 cell lines have been profiled for dinaciclib response in long-term (>72 hrs) viability or clonogenicity assays with >97% of the lines showing IC50 β25 nM. This uniformly low nM potency is likely due to repression of both cell cycle progression and transcription through inhibition of CDK1/2 and CDK9, respectively. CDK9 phosphorylation of the RNA pol II (RNAPII) at Ser2 and 5 is required for transcriptional initiation and elongation. We and others have observed rapid CDK9-dependent effects on cells after short-term dinaciclib exposure, including loss of RNAPII Ser2 phosphorylation followed by rapid elimination of the short half-life, pro-survival protein Mcl-1. Since a cancer cell's ability to avoid apoptosis is dependent on the balance of several Bcl-2 antiapoptotic family members, including Bcl-2, Bcl-xL and Mcl-1, we hypothesized that Mcl-1 dependent cell lines would be more sensitive to dinaciclib treatment. Moreover, we anticipated that this differential sensitivity could be discriminated from longer-term inhibitory cell cycle effects by conducting short-term dinaciclib exposure assays. Here we report the activity of dinaciclib to induce apoptosis on a panel of 25 human solid tumor cell lines with varying levels of Mcl-1 dependency. Mcl-1 dependency in solid tumor cell lines has been reported to correlate with the Mcl-1 to Bcl-xL mRNA ratio or the level of Mcl-1 gene amplification. Cell viability was assessed after an 18 hour, 100 nM dinaciclib treatment while target engagement and induction of apoptosis was determined after 8 hours. With one exception, all cell lines showed potent CDK9 target engagement as determined by loss of RNAPII Ser2 phosphorylation and corresponding reduced Mcl-1 protein levels. We observed that loss of cell viability, measured by ATP content, directly correlated with the Mcl-1/Bcl-xL mRNA ratio. A dramatic increase in PARP cleavage was also observed in cell lines with the highest Mcl-1/Bcl-xL mRNA ratio. Furthermore, the extent of PARP cleavage correlated with levels of caspase-3/7 activity. Bcl-2 levels did not significantly impact the dinaciclib response. These data provide a rationale for utilizing Mcl-1 dependency as a predictive biomarker for dinaciclib anti-cancer response in solid tumor malignancies. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3063. doi:1538-7445.AM2012-3063