Data from Combined Blockade of MEK and CDK4/6 Pathways Induces Senescence to Improve Survival in Pancreatic Ductal Adenocarcinoma

Abstract

<div>Abstract<p>Activating <i>KRAS</i> mutations, a defining feature of pancreatic ductal adenocarcinoma (PDAC), promote tumor growth in part through the activation of cyclin-dependent kinases (CDK) that induce cell-cycle progression. p16<sup>INK4a</sup> (p16), encoded by the gene <i>CDKN2A</i>, is a potent inhibitor of CDK4/6 and serves as a critical checkpoint of cell proliferation. Mutations in and subsequent loss of the p16 gene occur in PDAC at a rate higher than that reported in any other tumor type and results in Rb inactivation and unrestricted cellular growth. Therefore, strategies targeting downstream RAS pathway effectors combined with CDK4/6 inhibition (CDK4/6i) may have the potential to improve outcomes in this disease. Herein, we show that expression of p16 is markedly reduced in PDAC tumors compared with normal pancreatic or pre-neoplastic tissues. Combined MEK inhibition (MEKi) and CDK4/6i results in sustained downregulation of both ERK and Rb phosphorylation and a significant reduction in cell proliferation compared with monotherapy in human PDAC cells. MEKi with CDK4/6i reduces tumor cell proliferation by promoting senescence-mediated growth arrest, independent of apoptosis <i>in vitro</i>. We show that combined MEKi and CDK4/6i treatment attenuates tumor growth in xenograft models of PDAC and improves overall survival over 200% compared with treatment with vehicle or individual agents alone in <i>Ptf1a<sup>cre/+</sup></i>;<i>LSL-KRAS<sup>G12D/+</sup></i>;<i>Tgfbr2<sup>flox/flox</sup></i> (PKT) mice. Histologic analysis of PKT tumor lysates reveal a significant decrease in markers of cell proliferation and an increase in senescence-associated markers without any significant change in apoptosis. These results demonstrate that combined targeting of both MEK and CDK4/6 represents a novel therapeutic strategy to synergistically reduce tumor growth through induction of cellular senescence in PDAC.</p></div>