Efficacy of the combination of MEK and CDK4/6 inhibitors in vitro and in vivo in KRAS mutant colorectal cancer models

// Michael S. Lee 1, 2 , Timothy L. Helms 3 , Ningping Feng 5 , Jason Gay 5 , Qing Edward Chang 5 , Feng Tian 4 , Ji Y. Wu 4 , Carlo Toniatti 5, 6 , Timothy P. Heffernan 5, 6 , Garth Powis 7 , Lawrence N. Kwong 3 , Scott Kopetz 4 1 Division of Hematology and Oncology, Department of Medicine, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA 2 Lineberger Comprehensive Cancer Center, Chapel Hill, NC, USA 3 Department of Genomic Medicine, University of Texas MD Anderson Cancer Center, Houston, TX, USA 4 Department of Gastrointestinal Medical Oncology, University of Texas MD Anderson Cancer Center, Houston, TX, USA 5 Institute for Applied Cancer Science, University of Texas MD Anderson Cancer Center, Houston, TX, USA 6 Center for Co-Clinical Trials, University of Texas MD Anderson Cancer Center, Houston, TX, USA 7 Sanford-Burnham Medical Research Institute, La Jolla, CA, USA Correspondence to: Lawrence N. Kwong, email: lkwong@mdanderson.org Scott Kopetz, email: skopetz@mdanderson.org Keywords: KRAS, NRAS, MEK inhibitor, CDK4/6 inhibitor Received: December 28, 2015      Accepted: April 16, 2016      Published: May 04, 2016 ABSTRACT Purpose: Though the efficacy of MEK inhibitors is being investigated in KRAS -mutant colorectal cancers (CRC), early clinical trials of MEK inhibitor monotherapy did not reveal significant antitumor activity. Resistance to MEK inhibitor monotherapy developed through a variety of mechanisms converging in ERK reactivation. Since ERK increases cyclin D expression and increases entry into the cell cycle, we hypothesized that the combination of MEK inhibitors and CDK4/6 inhibitors would have synergistic antitumor activity and cause tumor regression in vivo . Results: The combination of MEK and CDK4/6 inhibitors synergistically inhibited cancer cell growth in vitro and caused tumor regression in vivo in cell line and patient-derived xenograft models. Combination therapy markedly decreased levels of phosphorylated ribosomal protein S6 both in vitro and in vivo and decreased Ki67 staining in vivo . Experimental Design: We performed in vitro proliferation, colony formation, apoptosis, and senescence assays, and Western blots, on a panel of 11 KRAS mutant CRC cell lines treated with the MEK inhibitor MEK162, the CDK4/6 inhibitor palbociclib, or the combination. We also treated 4 KRAS mutant CRC cell line and patient-derived xenografts with the MEK inhibitor trametinib, the CDK4/6 inhibitor palbociclib, or the combination, and performed immunohistochemical and reverse phase protein array analysis. Conclusions: Combined inhibition of both MEK and CDK4/6 is effective in preclinical models of KRAS mutant CRC and justifies a planned phase II clinical trial in patients with refractory KRAS -mutant CRC. Efficacy of the combination of MEK and CDK4/6 inhibitors in vitro and in vivo in KRAS mutant colorectal cancer models.