Abstract 5719: Targeting CDK4/6 and BET proteins for the treatment of RB+ and RB- osteosarcoma
Authors: Malko Rada , Pandya Pankita H. , Shannon Harlan E. , Bailey Barbara J. , Dobrota Erika A. , Young Courtney , Barghi Farinaz , Riyahi Nikan , Justice Ryli , Trowbridge Melissa A. , Coy Kathryn L. , Sinn Anthony L. , Bijangi-Vishehsaraei Khadijeh , Ferguson Micheal J. , Saadatzadeh M. Reza , Pollok Karen E. 
Published: March 2024

Abstract
Osteosarcoma (OS) is the most common malignant bone cancer characterized by heterogeneity and genomic instability. Treatment and survival of patients have not improved in several decades, making it urgent to develop safe and effective therapies. Dysregulation of cell cycle regulators CDKN2A, cyclins, and CDK4/6 represent high-risk signatures in aggressive OS and can be targeted via CDK4/6 inhibitors (CDK4/6i). However, monotherapy often fails due to resistance from speculated RB loss as well as hyperactivation of compensatory signaling pathways in RB proficient cancers. In addition, &gt;70% of OS patients have RB loss which is viewed as a biomarker of therapeutic response that negates the use of CDK4/6i. However, more research is needed in RB-deficient preclinical models to clearly define and validate the utility of RB as a biomarker for patient stratification. Based on our screening data, we hypothesize that combining CDK4/6i with bromodomain and extraterminal domain inhibitors (BETi) will potentiate anti-tumorigenic effects regardless of RB status in OS. BETi creates imbalances between transcription and replication which worsens oncogenic stress. In OS cell lines, combination index and Bliss analysis indicated additive-to-synergistic growth inhibition using CDK4/6i (abemaciclib or palbociclib) and BETi (AZD5153). Dual therapy also resulted in increased apoptosis at clinically relevant concentrations in an OS xenoline (TT2) compared to monotherapy. To determine potential for emergence of therapeutic resistance, an in-vitro longitudinal growth delay assay was established. While single agent therapies resulted in transient decreases in cell numbers over time, combination CDK4/6i+BETi resulted in cell death (p&lt;0.05). To determine the effect of RB loss on therapeutic response to CDK4/6i, CRISPR mediated RB-knockout (KO) clones were developed in 143B human and K7M2 murine OS cell lines. RB KO cells exhibited sensitivity to CDK4/6i irrespective of RB status. However, levels of CDK4/6i required for 50% growth inhibition in RB KO were 1-3-fold higher using palbociclib, with negligible differences observed with abemaciclib in RB KO vs WT clones. To determine if therapeutic response is achievable in-vivo, a single-agent dosing study was conducted using palbociclib (40mg/kg, SID) and AZD5153 (1mg/kg, BID) in patient-derived xenograft (PDX) model derived from a metastatic lesion in the pelvis (TT2 PDX) harboring a CDK4/6 hyperactivation signature and monoallelic RB loss. Tumor growth reduction and survival was significantly higher in monotherapy groups compared to vehicle. However, one week after dosing was stopped, survival rates based on tumor volume endpoint were 40% and 100% for palbociclib and AZD5153 respectively. In-vivo efficacy of dual CDK46i+BETi is underway. This data provides rationale for further study of novel therapeutic options which may expand the clinical utility of CDK4/6i regardless of RB status.
Citation Format: Rada Malko, Pankita H. Pandya, Harlan E. Shannon, Barbara J. Bailey, Erika A. Dobrota, Courtney Young, Farinaz Barghi, Nikan Riyahi, Ryli Justice, Melissa A. Trowbridge, Kathryn L. Coy, Anthony L. Sinn, Khadijeh Bijangi-Vishehsaraei, Micheal J. Ferguson, M. Reza Saadatzadeh, Karen E. Pollok. Targeting CDK4/6 and BET proteins for the treatment of RB+ and RB- osteosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5719.