Abstract 3043: Targeting CDK4/6 inhibitor resistance in relapsed osteosarcoma via PI3 Kinase inhibition

Osteosarcoma (OS) is the most common primary bone malignancy in children as well as in adolescents and young adults (AYA). Approximately 35% of OS patients develop metastases and relapse after first-line treatment emphasizing the need for new therapies. Our objective was to use patient -omics analyses to prioritize models of relapsed pediatric and AYA OS to investigate an unexplored combination therapy. Genomic data from pediatric and AYA patients enrolled in the Precision Genomics program at Riley Hospital for Children, Indiana University Health indicate that dysregulation of cyclin-dependent kinases 4 and 6 (CDK4/6) is one of the top actionable signatures. The cyclin D‐CDK4/6 complex regulates retinoblastoma protein (RB)-E2F transcription factor interactions. Upon cyclin D-CDK4/6-mediated RB phosphorylation, the RB-E2F complex dissociates, and cell cycle progression ensues. CDK4/6 inhibitors (CDK4/6i) typically induce cell cycle arrest rather than cell death and can activate compensatory pathways such as PI3K. Moreover, aberrant PI3K activation has been reported in OS. Our hypothesis is that inhibition of CDK4/6 and PI3K pathways will be efficacious and well-tolerated in RB1-proficient (RB+) OS models exhibiting hyperactivation of the cyclin D-CDK4/6 complex. Cell growth response to CDK4/6i (Palbociclib or Abemaciclib) and a PI3K/mTOR inhibitor (PI3K/mTORi, Voxtalisib) was evaluated in RB+ OS cell lines and an RB+ patient-derived xenograft (PDX)-derived xenoline (TT2-77). Combination index and Bliss independence analyses indicated that CDK4/6i+PI3K/mTORi resulted in additive to synergistic inhibition of growth in RB+ OS cell lines at clinically relevant concentrations. In the TT2-77 PDX, whole genome sequencing indicated that the original OS biopsy and the TT2-77 PDX generated from a resection sample harbor signatures associated with CDK4/6 pathway up-regulation. Global and phosphoproteome analysis were conducted on TT2 PDX tumors from NOD/SCIDγnull mice treated with vehicle vs. Palbociclib for 5 days. Modulation of 6226 phosphopeptides and 3870 total proteins were observed and included downregulation of phosphopeptides and total protein levels of CDK1 as well as DNA replication proteins in Palbociclib-treated TT2-77 PDX mice. Furthermore, TT2-77 PDX mice were treated with Palbociclib daily (10-120 mg/kg) for 3 weeks. TT2-77 PDX growth was completely blocked by high dose Palbociclib. While all mice survived, clinical observation criteria indicated that this dose was not optimal. In TT2-77 PDX mice treated with 50 mg/kg Palbociclib, tumor growth significantly decreased compared to vehicle (p Citation Format: Farinaz Barghi, Pankita H. Pandya, M. Reza Saadatzadeh, Khadijeh Bijangi-Vishehsaraei, Barbara J. Bailey, Erika A. Dobrota, Courtney Young, Melissa A. Trowbridge, Kathy Coy, Anthony L. Sinn, Harlan E. Shannon, Jamie L. Renbarger, Karen E. Pollok. Targeting CDK4/6 inhibitor resistance in relapsed osteosarcoma via PI3 Kinase inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 3043.