Abstract LB-214: pY88-p27Kip1 status acts as a biomarker to determine responsiveness to cdk4 inhibitor therapy

PURPOSE: Cdk4 targeting drugs (cdk4i), such as Palbociclib, are approved in combination with Estrogen modulation therapy for metastatic ER/PR+, Her2- breast cancer. However, there are no biomarkers to pinpoint patients who would respond to this type of therapy. 20-40% of metastatic ER/PR+, Her2- patients exhibit primary resistance to cdk4i therapy, highlighting the need for a companion diagnostic for cdk4i use. Rb- tumors appear resistant to cdk4i, but this is an infrequent event in HR+ breast cancer. In Rb+ tumors, Ki67, Cyclin D, cdk4, or p16 do not appear to stratify responsive and non-responsive subgroups. The levels of cyclin D or cdk4 themselves may not be reliable measures of responsiveness, due to the fact that a third protein, p27Kip1, is required for activation of the cyclin D-cdk4 (DK4) complex. Tyrosine (Y) phosphorylation of p27 on residue Y88 activates the DK4-p27 ternary complex, and the level of pY88-p27 correlates with cdk4 activity and Palbociclib responsiveness in tissue culture cells. We hypothesized the pY88-p27 status may serve as a biomarker for patients that are responsive to cdk4i therapy. RESULTS: We developed a dual immunohistochemistry assay for p27 and pY88, which we used to analyze paraffin-embedded, archival breast cancer tumor samples. We used non-cancerous material obtained from core needle biopsies as non-neoplastic (control) and found that while strong p27 staining was detected (brown) in normal epithelial cells, all benign epithelium was negative for pY88 (pink staining). By examining a cohort of pathologically identical patients (ER/PR+, Her2- with similar Ki67 levels and grades), we were able to stratify them into three groups based on pY88 status: 21% had no pY88 staining (Group 0), 26% had a low percentage of pY88+ cells (Group 1), and 52% had very high pY88 staining (Group 2, >25% of cells pY88+). Similar groupings were detected in material analyzed from Her2+ or Triple Negative breast cancer patients. Lack of pY88 staining in Group 0 patients suggested that DK4 was not active, and these patients would not respond to Palbociclib, while those in Group 1 or 2, with some active DK4, would respond. To test this hypothesis, following informed consent, we stratified patients who were scheduled to undergo mastectomy or lumpectomy based on pY88 status. Post surgery fresh tumor material was grown in explant culture, followed by treatment with Palbociclib. 48 h. later samples were formalin-fixed, paraffin-embedded and stained with Ki67 as a marker of proliferation. We found that the explant material obtained from Group 0 patients was non-responsive to Palbociclib, while material obtained from Group 1 and 2 patients responded to Palbociclib-mediated inhibition in a statistically significant manner. CONCLUSION: Our data suggest that pY88-p27 status, as a surrogate marker for cdk4 activity, determined responsiveness to Palbociclib treatment in explant culture. Use of the pY88 biomarker may aid in the expansion of cdk4i therapy into other breast cancer subgroups, where currently these therapies are not approved. Citation Format: Stacy W. Blain, Susan R. Gottesman, Jonathan Somma, Lisa Dresner, Vladislav Tsiperson. pY88-p27Kip1 status acts as a biomarker to determine responsiveness to cdk4 inhibitor therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr LB-214.