Abstract 1327: Targeting the RB-pathway in sarcoma: Utility of CDK4/6 inhibitors

Proceedings: AACR Annual Meeting 2014; April 5-9, 2014; San Diego, CA Background: Sarcomas are rare neoplasms of mesenchymal origin. Survival rates have not improved with standard chemotherapy; however, molecular insights have posed new drug targets. Aberrations in the RB-pathway, including loss of RB, CDK4 and cyclin D1 overexpression and p16 loss, have been identified in sarcomas. PD 03322991 (PD) is a second generation CDK4/6 inhibitor with the potential to reactivate the RB-pathway and suppress cell cycle progression. We evaluated the efficacy of PD in a panel of sarcoma cell lines. Methods: Sarcoma cell lines (HT-1080, SK-LMS-1 and SaOS-2) were evaluated for RB-pathway alterations using western blot. Cells were treated with PD for 1, 2, 4, 6 and 8 days to determine IC50 values by MTT assay. Following the same treatment protocol at IC50 concentrations of PD, cells were harvested for cell cycle analysis, western blot and annexin cell death assay. Results: HT-1080 and SK-LMS-1 were RB-positive with high expression of phospho-RB; SaOS-2 was RB-negative. CDK4/6 were uniformly expressed. HT-1080 and SK-LMS-1 had p16 loss while SaOS-2 overexpressed cyclin D1. PD was effective in all cell lines. Increasing treatment duration led to reduction of IC50 values. PD caused marked G1 arrest as early as day 1 in RB-positive HT-1080 (86.2% vs 74.2% [vehicle]) and SK-LMS-1 (91.4% vs 67.8% [vehicle]) but not RB-negative SaOS-2 (52.9-59.4% vs 57.0% [vehicle]). Annexin assay demonstrated minimal apoptosis with PD (HT-1080: 1.4-3.0% vs 41.2% [control], SK-LMS-1: 0.6-12.0% vs 23.4% [control], SaOS-2: 3.6-7.3% vs 78.7% [control]). Conclusions: PD appears to have a cytostatic rather than cytotoxic effect in sarcoma. RB-negativity does not preclude PD efficacy but may require higher IC50 values. In RB-negative sarcomas, PD does not cause cell cycle arrest suggesting an alternate mechanism from reactivation of the RB-pathway is present. PD may serve as a potential therapy for sarcoma and should be studied in combination with other targeted therapies that can take advantage of its ability to cause G1 arrest. View this table: Citation Format: Ashleigh M. Francis, Jason P. Carey, Angela Alexander, Khandan Keyomarsi, Kelly K. Hunt. Targeting the RB-pathway in sarcoma: Utility of CDK4/6 inhibitors. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1327. doi:10.1158/1538-7445.AM2014-1327