Abstract IA08: Reprogramming human cancer cells in CDK4/6 inhibitor therapy
Authors: Chen-Kiang Selina , Liberto Maurizio Di , Vijay Priyanka , Chiron David , Huang Xiangao , Ely Scott , Elemento Olivier , Mason Christopher , Cantley Lewis , Leonard John P. , Martin Peter 
Published: November 2016

Abstract
CDK4 and CDK6 drive cell cycle progression through early G1 and are frequently deregulated in human cancer. Selective inhibition of CDK4/6 with palbociclib (PD 0332991) has demonstrated exciting clinical efficacy in diverse human cancers. However, the mechanism that discriminates sensitivity from resistance to targeting CDK4/6 remains obscure.
Mantle cell lymphoma (MCL) is an incurable non-Hodgkin's lymphoma where deregulated CDK4 activity and cyclin D1 expression underlies unrestrained proliferation and disease progression. In a phase I clinical trial in recurrent MCL, we demonstrated that palbociclib alone produced a durable clinical response with a favorable toxicity profile. To investigate the mechanism for targeting CDK4/6, we have now shown in primary cancer cells that 1) inhibition of CDK4/6 leads to early G1 arrest that requires Rb, the CDK4/6 substrate; 2) prolonged early G1 arrest (pG1) sensitizes Rb-proficient cancer cells to killing by diverse clinically-relevant agents; and 3) pG1 sensitization stems from continuous expression of genes scheduled for early G1 only. This leads to an imbalance in genes expression, which is exacerbated in synchronous progression to S phase (pG1-S) after palbociclib withdrawal, due to incomplete restoration of cell cycle-coupled gene expression.
In a hypothesis-driven phase Ib clinical trial, targeting CDK4 with palbociclib in sequential combination with the proteasome inhibitor bortezoimb was well tolerated (n=16). It exhibited a durable palbociclib dose-dependent clinical activity, including one complete remission for over 3 years with only one progression at the optimal dose combination (n=6). Longitudinal integrative whole transcriptome and whole exome sequencing of tumor cells isolated from serial lymph node biopsies demonstrated that palbociclib initially induced pG1 in MCL cells of all patients, regardless of copy number variation or mutation (ATM, p53). Cell cycle control by palbociclib is thus initially intact in MCL, but is insufficient to predict the clinical response.
As predicted, only genes programmed for early G1 were expressed pG1, concurrent PI3K inactivation in primary MCL cells. However, &lt;1% of the 1400 genes suppressed (not programmed) in pG1 in clinically-responding patients were activated in non-responding patients. These genes were critical for redox homeostasis, suggesting that PI3K inactivation and redox stress mediates pG1 sensitization.
This study represents the first investigation of genes that discriminate sensitivity from resistance in targeting CDK4/CDK6 in human cancer, through integrative longitudinal analysis of whole exome and whole transcriptome sequencing in concert with protein expression analysis and functional studies. Selective inhibition of CDK4 induces pG1 in all MCL patients, which apparently reprograms MCL for clinical response to bortezomib through PI3K inactivation and suppression of genes for redox homeostasis.
Citation Format: Selina Chen-Kiang, Maurizio Di Liberto, Priyanka Vijay, David Chiron, Xiangao Huang, Scott Ely, Olivier Elemento, Christopher Mason, Lewis Cantley, John P. Leonard, Peter Martin. Reprogramming human cancer cells in CDK4/6 inhibitor therapy. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Cancer Cell Cycle - Tumor Progression and Therapeutic Response; Feb 28-Mar 2, 2016; Orlando, FL. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(11_Suppl):Abstract nr IA08.