P-287 Preclinical evaluation of alpelisib (PI3K inhibitor) and its synergistic effect in combination with ribociclib (CDK 4/6 inhibitor) in colorectal cancer

Abstract

Multiple activating genetic mutations in the phosphatidylinositol-3 kinase (PI3K) and the mitogen-activated protein kinase (MAPK) pathways have been implicated in the development of resistance to anti-cancer therapies. Ribociclib has limited activity as a single agent in colorectal cancer (CRC). However, combining ribociclib with targeted therapies of the MAPK and PI3K pathways may be a promising treatment strategy in CRC. Previous in vitro studies in our lab have demonstrated a clear and highly synergistic antiproliferative effect of the combination of palbocicib and gedatolisib in multiple CRC cell lines, regardless of background mutational status. We explored the in vitro efficacy of drug combinations (ribociclib and alpelisib (R+A)) in four CRC cell lines with different mutational status; CACO-2 (PIK3CA/KRAS wildtype), LS-1034 (KRAS mutated), SNU-C4 (PIK3CA mutated), and DLD-1 (PIK3CA/KRAS mutated) cells. Drug combination index was calculated using calcusyn biosoft software. IC50s for alpelisib (A) and ribociclib (R) calculated for all four cells lines. CACO2 (PIK3CA/KRAS wildtype) cell lines were resistant to ribociclib (IC50 >15μM). The LS1034, SNUC4 and DLD-1 cells were sensitive to ribociclib (IC50: DLD1=0.4014μM; LS1034=1.126μM; SNUC4=0.259μM). The cell lines had varying sensitivity to alpelisib (IC50s: LS1034=313nM; SNUC4=441nM; DLD-1=1307nM; CACO-2=1547nM). The Caco-2 wild-type cell line was relatively resistant to both drugs, in particular ribociclib (IC50 >15μM). Drug combination analysis showed that combination of R+A has a synergistic anti-proliferative effect in all CRC cell lines tested. The combination of R+A is highly synergistic in LS1034 cells which harbour a KRAS mutation (CI=0.16). The combination of R+A is also synergistic in DLD-1 cells which have co-occurring KRAS and PIK3CA mutations (CI=0.78), as well as in SNUC4 (PIK3CA-mutated; CI=0.49) and CACO-2 (wild-type; CI=0.28) cell lines. A synergistic response to treatment with the combination of R+A is seen in all cell lines. We will further investigate the combination of ribociclib and alpelisib in vivo in CRC animal models. The aim of this project is to justify a phase I clinical trial of ribociclib and alpelisib in CRC.