(P 64) OSTEOSARCOMA. PATIENT-DERIVED XENOGRAFTS MODELS TO RECAPITULATE PATIENTS' CHEMORESISTANCE AND METASTATIC DISEASE

Objective: Osteosarcoma (OS) is the most common malignant bone tumor in the pediatric and young adult’s population. The introduction of neo-adjuvant polychemotherapy in the 1980s was a massive success for patients with localized OS, improving the 5-year-overall survival rate to a plateau of 70%. However, the most challenging characteristic of OS is its ability to metastasize, especially in the lungs, despite the high-dose cytotoxic chemotherapy. Metastasis at diagnosis (30% of OS) or after relapse of localized OS is the leading cause of death in OS patients. Almost no improvement has been introduced for these patients in the last three decades. There are few spontaneous metastatic models in OS, and most transplantation models (patient-derived orthotopic xenografts, PDOX) are focused on the primary disease. This paucity of human-derived relapse and metastatic models contributes to the slow advances in patient care.

Methods: We wanted to develop clinically relevant OS PDOXs from OS patients' biopsies, starting from tumor samples naïves of any treatment. The PDOXs models were engineered to express the Luciferase and then implanted orthotopically in immunocompromised mice. We applied fractionated high-dose chemotherapy to tumor-bearing mice using Doxorubicin, Cisplatin, and Methotrexate (MAP regimen). At the tumor volume cut-off of 0.5cm3, the hindlimb was amputated, and the mouse was monitored by BLI until the appearance of metastatic disease progression before being euthanized. Primary, relapses, and metastases are then profiled by next-generation sequencing at the bulk and single-cell levels.

Results: Similar to what is observed in OS patients, MAP chemotherapy significantly reduces tumor growth in our models. But we can also observe that not all the PDOXs OS tumors react the same at the treatment, some tumors respond well at first before becoming chemotherapy resistant. In both groups, monitoring these mice revealed the presence of macro-metastases in the lungs by BLI months after the implantation and amputation. Therefore, MAP chemotherapy does not entirely prevent metastatic spreading but tends to reduce the number of macroscopic metastases in the MAP arm (26%) compared to the vehicle arm (55%). MAP chemotherapy-treated mice have better overall survival and even achieve complete cure when compared to mice treated with surgery alone.

Conclusion: The OS chemotherapy resistance and metastatic dissemination can be modeled in vivo using neoadjuvant polychemotherapy in murine patient-derived orthotopic xenograft. We were able to generate local and distant relapses resistant to the MAP chemotherapy, samples that are being molecularly profiled by RNAseq (bulk and single-cell) and Whole Exome Sequencing to identify the most promising candidates' driver genes of the metastatic and chemo-resistant OS. These models, which recapitulate the OS patients' clinical history, will allow us to understand better the metastatic spreading of primary OS under chemotherapy treatment.