(P 133) INVESTIGATION OF COMBINING MDM2 INHIBITORS WITH DR5 AGONISTS FOR BONE SARCOMA TREATMENT

Objective: Death Receptor 5 (DR5) is expressed on the surface of primary bone sarcoma cells. Enhanced
activation of DR5 has been achieved with novel DR5 agonists (1). INBRX-109 is currently being
investigated in a randomized, blinded, placebo-controlled, in patients with unresectable or metastatic conventional chondrosarcoma
(ClinicalTrials.gov Identifier: NCT04950075), which is an area of unmet need (2). Combining agonists
of DR5 with an MDM2 inhibitor (MDM2i) such as RG7388 could have enhanced effects.

1. Gamie Z, et al., Targeting Death Receptor 5 (DR5) for the imaging and treatment of primary bone
and soft tissue tumors: an update of the literature. Frontiers in molecular biosciences.
2024;11:1384795.
2. Laitinen MK, et al., Enhancing surgical margins: A key to survival in pelvic chondrosarcoma.
European Journal of Surgical Oncology. 2024:109491.

Methods: Western blotting for total protein (p53, p21, MDM2) and flow cytometry for surface DR5 expression
after RG7388 exposure to the SJSA-1 osteosarcoma (known to have MDM2 amplification), SW1353 chondrosarcoma and HT1080
dedifferentiated chondrosarcoma cell lines. RT-PCR for DR5 transcript levels. Cytotoxicity assays
following MDM2i +/- DR5 agonist application using Cell Counting Kit-8 (CCK-8) and Sulforhodamine B
(SRB) assays.

Results: RG7388 increased DR5 mRNA transcript levels in the SJSA-1 osteosarcoma cell line. Total DR5
protein expression was enhanced in the SJSA-1 cell line. DR5 surface expression increased in all the cell
lines. A dose-dependent cytotoxic effect was found in SJSA-1 and HT1080 cell lines, and increased caspase 3-7 for the SJSA-
1 cell line.

Conclusion: RG7388 is cytotoxic and upregulates DR5 in bone sarcoma cell lines. Combining this agent with novel
DR5 agonists may enhance cell killing and potentially survival outcomes. The phase 2 clinical trial of INBRX-109 in unresectable or metastatic conventional chondrosarcoma patients (ClinicalTrials.gov Identifier: NCT04950075) has an estimated primary completion date of September 2025. Depending on the result, it may be that characterisation of patients for tissue DR5 level of expression and combination therapy with an MDM2i is a potential line of investigation to be incorporated into further trials and help enhance the outcomes.