Abstract LB-224: Tyrosine phosphorylation of p27Kip1 associates with Palbociclib responsiveness in breast cancer
Authors: Blain Stacy W. , Gottesman Susan R.S. , Somma Jonathan , Tsiperson Vladislav , McGuinness Julia , Ingham Matthew , Kalinsky Kevin , Schwartz Gary , Tress Elina , Hibshoosh Hanina , Kahila Mohamed Mh , Samra Bachar Samra , Taiwo Evelyn , Xie Steve 
Published: August 2019

Abstract
PURPOSE: Cdk4 inhibitors (CDK4i), such as palbociclib, are approved in combination with hormonal therapy as a front line treatment for metastatic HR+, Her2- breast cancer. However, despite these advances, many patients demonstrate either de novo or acquired resistance. This highlights the need to identify biomarkers to pinpoint patients who would respond to this therapy. In Rb+ tumors, which comprise the majority of HR+ breast cancers, Ki67, Cyclin D, cdk4, and p16 levels do not identify responsive subgroups. Cyclin D or cdk4 levels themselves are not reliable because tyrosine phosphorylated p27Kip1 (pY88) is required for activation of the cyclin D-cdk4 (DK4) complex. We have previously shown that the level of pY88 correlates with cdk4 activity: the more pY88 detected, the greater the specific activity of cdk4 (Patel, et. al. MCR, 2018). We developed dual immunohistochemical staining for p27 and pY88 and demonstrated that pY88 status, which is negative in benign breast epithelium, stratified tumors across all hormone receptor groups (Gottesman, et.al, MCR, 2018). Lack of pY88 suggested that DK4 was inactive, and that these samples would not have the active cdk4 target. We have grown tumor resection material in explant culture, treated with palbociclib, and stained with Ki67 as a marker of response. Explants from the no pY88 group were non-responsive, while explants from the low or high pY88 group responded to drug. Thus, we hypothesized the pY88-p27 status may serve as a biomarker for patients that can respond to cdk4i therapy.
RESULTS: We analyzed paraffin-embedded archival breast cancer biopsies from a 13 patient cohort of HR+, Her2- patients who had received Palbociclib/Letrozole in the front line metastatic setting. 62% of these patients progressed within 8 months of treatment, while 38% were on drug for 12-38 months. Using our staining method, we stratified patients into three groups based on pY88 status: 23% had no staining (Group 0), 38% had a low percentage of pY88+ cells (Group 1), and 38% had a high percentage of pY88+ cells (Group 2, &gt;30% of cells pY88+). We then compared the time until disease progression, when they were removed from drug, for each group. Patients with a pY88 score of 0 had disease progression within 6-8 months, suggesting that they exhibited primary resistance. Patients with a pY88 score of 2 also showed disease progression within 6 months. This suggested that the Y88 high group may have too much active cdk4 target to respond to pharmacological drug dose, a hypothesis supported by our breast cancer cell line data. However, patients with a low pY88 score were all still on drug after 12+ months with either reduced tumor or stable disease. Additional patients continue to be recruited.
CONCLUSION: Our data suggest that pY88-p27 status, as a surrogate marker for cdk4 activity, associates with responsiveness to CDK4i treatment. Clinical use of the pY88 biomarker may identify patients responsive or resistant to Cdk4 targeting drugs.
Citation Format: Stacy W. Blain, Susan R.S. Gottesman, Jonathan Somma, Vladislav Tsiperson, Julia McGuinness, Matthew Ingham, Kevin Kalinsky, Gary Schwartz, Elina Tress, Hanina Hibshoosh, Mohamed Mh Kahila, Bachar Samra Samra, Evelyn Taiwo, Steve Xie. Tyrosine phosphorylation of p27Kip1 associates with Palbociclib responsiveness in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-224.