Abstract P2-16-20: Biomarkers to predict response to the CDK 4/6 inhibitor, palbociclib (PD 0332991) in a single-agent phase II trial in advanced breast cancer

Abstract

Abstract Background: The Cyclin D1-CDK4/6 complex is critical in regulating the G1/S checkpoint and phosphorylation of retinoblastoma protein (Rb); palbociclib is a highly-selective CDK 4/6 inhibitor. CCND1, the gene encoding Cyclin D1, is amplified in 15% of breast tumors; p16, the endogenous inhibitor of the complex is lost in up to half of breast tumors. We hypothesized that breast tumors containing either alteration or a high proliferative rate would have enhanced sensitivity to palbociclib. We conducted a single-agent, phase II trial of palbociclib in patients with advanced breast cancer (UPCC03909). In this trial, the clinical benefit rate (partial response [PR] + stable disease ≥6 months [6mSD]) was 17% (DeMichele, ASCO, 2013). The current analysis was a secondary endpoint to determine whether Rb expression, p16 loss, Ki-67 index or CCND1 amplification predicted response in the phase II trial. Methods: Enrollment on UPCC03909 required archival tumor collection from either primary tumor or metastatic lesions. Fresh frozen paraffin-embedded (FFPE) tumor sections were tested for expression of Rb, Ki-67 and p16 by immunohistochemistry (IHC), and CCND1 amplification by fluorescence in situ hybridization. Categorical variables based on% tumor staining and intensity scores (negative defined as 0 or 1+ intensity staining, equivocal defined as 2+ staining or 3+ in <30%, or positive defined as 3+ staining intensity in 30%) were generated for Rb and p16 nuclear staining. Fisher's Exact test was used to test for associations between nominal factors, the Cochran-Mantel-Hænszel test for associations between ordinal factors, and the Cox proportional hazard model for association of progression-free survival (PFS) with biomarkers. Results: Thirty-seven patients were enrolled; response evaluation is available in 36/37. All had FFPE tumor available for Rb, 33/37 (89%)for CCND1 amplification and 29/37 (78%) for Ki-67 and p16 analyses. For complete results see Table 1. Conclusions: These results provide preliminary evidence that breast tumors more likely to respond to palbociclib may have higher Rb nuclear expression, lower Ki67 indices and/or loss of p16. Larger studies are needed to confirm these results. Table 1: Biomarker Summary and ResponseBiomarkerBiomarker CategoizationFrequency (%)Response Rate PR+6mSD/total (%)PFS Hazard Ratio (95%CI)Overall Population (n = 37) 6/36 (17%) Ki-67 (n = 29)≤ 10 (n = 20)69%4/19 (21%)1.00 > 10 (n = 9)31%1/9 (11%)1.20 (0.50, 2.88)Rb (n = 37)Nuclear Score: Negative (n = 11)30%0/11 (0%)1.00 Equivocal (n = 15)40%2/14 (14.3%)0.72 (0.28, 1.84) Positive (n = 11)30%4/11 (36.4%)0.71 (0.28, 1.80)p16 (n = 29)Loss or Low Expression (n = 16)55%4/16 (25%)1.00 Moderate or High Expression (n = 13)45%1/12 (8.3%)1.08 (0.47, 2.49)CCND1 Amplification (n = 34)Non-amplified (n = 25)88%4/25 (16%)1.00 Amplified (n = 9)12%1/8 (12.5%)1.06 (0.46, 2.4) Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P2-16-20.