Efficacy of Dual HER‐2 and CDK‐4/6 Signaling Blockade at Overcoming Primary HER2 Therapy Resistance in Breast Cancer
Authors: Franco Yesenia , Perez Luis , Mody Hardik , Ande Anusha , Fleisher Brett , Ait‐Oudhia Sihem 
Published: June 2021

ObjectivesTo evaluate a novel combination therapy of lapatinib (LAP), 5‐fluorouracil (5‐FU), and abemaciclib (ABE) at overcoming primary resistance to epidermal growth factor receptor 2 (HER2) therapy in HER2‐positive breast cancer that is refractory to trastuzumab, an anti‐HER2 monoclonal antibody, treatment.MethodsConcentration‐responses profiles for LAP, 5‐FU, ABE, and combinations were determined over 72 hours of treatment exposure to large range of drugs concentrations on JIMT ‐ 1 cells, a HER2‐positive breast cancer cell line that is refractory to trastuzumab treatment. The inhibition of cellular viability was measured at 0, 24, 48, and 72 hours using a CCK‐8 cellular viability kit for single, double, and triple combinations of agents. The drug‐drug interactions of LAP with 5‐FU, LAP with ABE, and 5‐FU with ABE at various concentrations were determined by calculating the combination indexes (CI) using CompuSyn software. The inhibitory concentrations of each agent leading to 50% of maximal effect (IC50) were determined by fitting the observed cellular viability responses to an inhibitory Hill function. Mathematical modeling was performed using Monolix Software (2016R1). All estimated parameters were determined as mean±SE. Statistical significance between various treatments arms including controls were assessed with one‐way analysis of variance followed by Tukey test using GraphPad Prism Version 5.ResultsThe IC50 for LAP, 5‐FU and ABE were 7.71±0.4, 19.44±6.79, and 3.32±0.45 μM. Comparison of cellular viability responses following 48 and 72 hours exposures to combinations of drugs revealed that the regimens LAP+ABE and LAP+ABE+5‐FU caused significant declines relative to their respective single agents with P<0.001 at 48 hours post‐drug exposure and P<0.001 for LAP+ABE[±5‐FU] vs. LAP as well as LAP+ABE+5‐FU vs. 5‐FU, and P<0.05 for LAP+ABE[±5‐FU] vs. ABE after 72 hours post drug exposure. In contrast, cell viabilities of double combinations consisting of 5‐FU with either LAP or ABE were not significantly different from the respective single agents (P > 0.05). The addition of 5‐FU to the combination of LAP with ABE did not cause further reduction in JIMT‐1 cellular viability response compared to LAP+ABE (P > 0.05). The CI analysis of cell viability data at 48 h post drugs exposures showed that the combination of LAP with ABE is strongly synergistic (CI = 0.15), whereas 5‐FU with ABE is antagonistic (CI = 1.26).ConclusionAmong the three single agents evaluated on JIMT‐1 cells, ABE is the most cytotoxic with the lowest IC50 followed by LAP. Whereas, 5‐FU displayed only a cytostatic effect. Combination therapy of ABE with LAP is synergistic and more efficacious than with addition of 5‐FU. Our study suggests that ABE+LAP overcomes HER2 therapy resistance in vitro. Further studies evaluating changes in key protein levels relating to apoptosis and cell cycle progression over time are ongoing.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.