Abstract 3887: Reversible inhibition of CDK4/CDK6 sensitizes hematological tumor cells for cytotoxic killing by inducing sequential G1 arrest and synchronous S phase entry that enhances the intrinsic apoptosis pathway
Authors: Ho Phoebe Joy , Huang Xiangao , Ely Scott , Chadburn Amy , Martin Peter , Moore Malcolm A.S , Leonard John , Niesvizky Ruben , Liberto Maurizio Di , Chen-Kiang Selina 
Published: February 2011

Abstract
Dysregulation of the cell cycle is a hallmark of cancer. However, targeting the cell cycle in cancer has only been modestly successful since broad-spectrum cyclin-dependent kinase (CDK) inhibitors lack specificity and are highly toxic. The critical importance of controlling CDK4/CDK6 in cancer treatment is further exemplified by recent evidence of prominent CDK4/CDK6 dysregulation in human cancers, including breast cancer, metastatic lung adenocarcinoma, glioblastoma, mantle cell lymphoma (MCL) and multiple myeloma (MM).
To advance targeting the cell cycle in cancer, we have developed a novel strategy to both inhibit the cell cycle and enhance cytotoxic killing in tumor cells using PD 0332991(PD), the only known CDK4/CDK6-specific inhibitor that is also reversible, potent and orally bio-available. We demonstrated that inhibition of CDK4/CDK6 with PD leads to a complete early G1 arrest in primary MM and MCL cells (IC50 60nM) in the absence of apoptosis and upon release of the G1 block, synchronous cell cycle progression to S phase in proliferating MM and MCL cell lines. Prolonged early G1 arrest enhances cytotoxic killing by diverse clinically relevant drugs at low dose, including the proteasome inhibitor bortezomib, and this is dramatically augmented during synchronous S phase entry. It is caspase-dependent, triggered by mitochondrial membrane depolarization and activation of the intrinsic apoptosis pathway. Kinetics studies of cell cycle-coupled gene expression at RNA and protein levels further revealed that expression of upstream pro-apoptotic BH3-domain only Bim and Noxa and their regulation by cytotoxic drugs are cell cycle-dependent. Bim is upregulated in early G1 arrest to neutralize the anti-apoptotic MCL-1 and Bcl-2, whereas Noxa is silenced in G1 but dramatically upregulated in S phase, greater when combined with bortezomib. Silencing Bim expression and Noxa expression by shRNA interference further confirmed that Bim and Noxa critically mediate cell cycle control of cytotoxic killing in MM and MCL, Bim in prolonged G1 arrest and Noxa in S phase.
In vivo, timely administration and discontinuation of PD treatment induces sequential early G1 arrest and synchronous S phase progression in human myeloma xenografts, leading to synergistic tumor suppression in response to bortezomib. As PD is orally bio-available, specific and low in toxicity, this strategy has been implemented in the first phase I/II clinical trial targeting CDK4/CDK6 with PD in combination with bortezomib and dexamethasone in MM with promising results. Collectively, our in vitro and in vivo results demonstrate, for the fist time, that targeting CDK4/CDK6 in combination with cytotoxic killing is a promising mechanism-based strategy to target both the cell cycle and cell survival in MM, MCL and potentially other cancers.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3887.