LAUSR

Background: FDA-approved CDK 4/6 inhibitors (CDK4/6i) including palbociclib, abemaciclib, and ribociclib have demonstrated clinical benefit in treating hormone receptor-positive (HR+) breast cancer. However, whether there is differential sensitivity to each therapy in vivo is unknown. In addition, whether these inhibitors demonstrate activity towards other tumor types is unclear. To better understand potential differences in CDK4/6i activity, we compared activity of each in a panel of 100 HR+/- breast cancer PDX models. To identify additional tumor types sensitive to CDK4/6i, we screened single agent palbociclib in an additional panel of 350 PDX models representing solid and hematologic malignancies. FFPE samples were collected from control and treatment groups in all studies at endpoint, and %T/C values in each model were calculated and in breast studies compared. Methods: START PDX models were established and validated as previously described and further characterized using IHC, WES, RNAseq, and drug sensitivity studies. For each CDK4/6i study, models were implanted SC in immune-deficient mice and studies initiated at 200-300 mm3 (n=1-3/grp). CDK4/6i agents were administered orally once daily at 50-75 mg/kg for a minimum of twenty-eight days or until study completion. Models tested with palbociclib included breast, lung, ovary, pancreas, head/neck, renal, gastric, uterine, colorectal, melanoma, and various hematologic malignancies. Study endpoints included tumor volume and time from treatment initiation with %T/C values and tumor regression reported at study completion; a %T/C of ≤ 20% versus control was considered sensitive. Partial tumor regression (PR) (%PR=1-99%) and complete tumor regression (CR) (%CR=100%) values versus Day 0 tumor volume of treated groups were also reported. Results: HR+/- breast models demonstrated variable sensitivity to the three CDK4/6i with HR+ models established from chemo-naive patients reporting greatest tumor growth inhibition (TGI). Two of four HR+ ESR1 mutant models reported modest sensitivity to CDK4/6i (ST941Y537S and ST2535D538G), while ST2177Y537S was sensitive and ST2056Y537S refractory to the three therapies. Evaluation of palbociclib in additional indications reported notable TGI versus control in several indications with the following percentage of tested models reporting sensitivity: Lung: 10%, Ovary: 15%, Pancreas: 30%, Head/Neck: 35%, Renal: 45%, Gastric: 60%, Uterine: 20%, Colorectal: 45%, Melanoma: 40% and Hematologic: 15%. In addition, several indications reported tumor regressions including one CR (lung: ST1748) and partial responses in ovary, pancreas, head/neck, renal, colorectal, and melanoma. Conclusion: We evaluated and compared three FDA-approved CDK4/6i therapies in a panel of HR+/- breast PDX models and identified models with differential responses. In addition, we screened single agent palbociclib in a panel of 350 PDX models representing solid and hematologic malignancies and identified several sensitive models in multiple indications, including some with partial or complete tumor regressions. Citation Format: Lizette Gamez, Tomoyuki Mashimo, Alyssa Moriarty, Kyriakos Papadopoulos, Drew Rasco, Amita Patnaik, Amy Lang, Ronald Drengler, Lon Smith, Michael J Wick. PDX-based screen to evaluate and compare approved CDK4/6 inhibitors in breast cancer and determine palbociclib efficacy in additional tumor types [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr A008. doi:10.1158/1535-7163.TARG-19-A008