Proceedings of the Think Tank for Osteosarcoma Medical Advisory Board

Presentations

Case history. A 38-year-old Asian male was diagnosed in January 2023 with a high-grade conventional osteosarcoma of T11 spine. He underwent surgical resection of a 4.5 cm tumor, which extended to the margins, followed by stereotactic body radiation therapy (SBRT) at a dose of 40 Gy, four cycles of doxorubicin and cisplatin, and two cycles of doxorubicin, ifosfamide/mesna (AIM). To date, the patient is in remission, six months after completion of adjuvant chemotherapy.

Research on osteosarcoma metabolism and the poor prognosis indicated by high phosphoglycerate dehydrogenase (PHGDH) expression levels in osteosarcoma tumors was presented (4). Inhibition of PHGDH in osteosarcoma attenuates cellular proliferation and activates mTORC1; therefore, PHGDH inhibition will sensitize osteosarcoma to mTORC1 inhibition. Preclinical justification for the use of PHGDH inhibitors with non-rapalog mTORC1 inhibition in osteosarcoma was presented. The combination of the metabolic inhibitors NCT-503, Perhexiline and ALPI3MT55 inhibited growth of UTSW2035 cells the most. Metabolism presents a ripe opportunity for therapeutic development, as it is primarily based on enzymes that can be inhibited. Dietary interventions with Ketogenic, and Serine/Glycine and Cysteine- free diets would need to be validated in clinical trials. The use of cabozantinib to prevent recurrence was also discussed but was not voted on due to its toxic effects.

Osteosarcoma is characterized by a highly heterogeneous and chaotic genome (5). There are no canonical translocations nor primary genetic drivers. As a result, development of precision targeted therapies for osteosarcoma will be difficult. Molecular profiling is important to help elucidate the complex genome and potential oncogenic drivers and passenger mutations. Novel CDK inhibitors that target the G₁ cell cycle may be promising combinatorial agents. A personalized approach with combinatorial strategies is the most realistic avenue for improving outcomes of upfront and recurrence management.

Research on chimeric antigen receptor-T (CAR-T) cell therapy with a number of molecular targets including B7-H3 (Cell Determinant 276), Chondroitin sulfate PG 4 (CSP64), and disialoganglioside-2 (GD2) was presented as rescue therapy in the event of recurrence/metastasis. The B7-H3/CD276 antigen is an immune checkpoint antigen that promotes pro-tumorigenic functions, such as tumor migration, invasion, and metastases. (CSPG4) is a surface component of immature cells and marker of aggressive phenotype across many cancer types. In preclinical studies, CAR-T, suberoylanilide hydroxamic acid (SAHA)+ B7-H3 mice had 100% survival at 1 year. B7-H3, CSP64, and CCNG1 are highly expressed in cancer stem cells (6, 7). B7-H3, CSP64, and CCNG1 expression levels are enhanced in subject’s tumor according to the BostonGene RNA sequence analysis. Therefore, combination therapies with DeltaRex-G and B7-H3/CSP64 CAR-T cell therapy could increase the chance of tumor eradication and cure for this subject and for similar cases of osteosarcoma. Since DeltaRex-G kills IL-2 stimulated primary human T cells in vitro, DeltaRex-G may reduce the severity of cytokine release syndrome caused by CAR-T cell therapy while also reducing tumor burden.

The use of immune checkpoint inhibitors has not been shown to be effective in advanced osteosarcoma and therefore was not recommended. Additionally, immunotherapy is associated with immune-related side effects, which could be significant. In the EURAMOS trial, there was no significant difference in event-free survival (EFS) between methotrexate/doxorubicin/cisplatin (MAP) and MAP+ Interferon when given as adjuvant therapy, despite significant toxicity (8). Additional options could be a one-time dose of bacillus Calmette-Guérin (BCG) vaccine or muramyl tripeptide phosphatidylethanolamine (L-MTPPE). The use of L-MTPPE showed an 8% absolute increase in EFS with tolerable side effects compared to chemotherapy and interferon. Expanded access for L-MTPPE (NCT04571229) is open for eligible subjects with osteosarcoma whose tumor has been completely removed and who are in complete remission (9).

Targeting the G₁ cell cycle checkpoint. DeltaRex-G is an off-the-shelf tumor-targeted gene therapy product encoding a CCNG1 inhibitor that arrests the cell cycle in G₀-G₁ phase (10). Consequently, DeltaRex-G kills not only proliferating cancer cells but also stroma producing fibroblasts and associated blood supply with minimal, if any, side effects. DeltaRex-G also acts as an immune modulator because reducing stroma breaks the barrier for entry of chemotherapy, immunotherapy, targeted therapy, and CAR-T cell therapies. CCNG1 expression is enhanced in all osteosarcoma tumors tested and DeltaRex-G has gained expanded access as a platform therapy for an intermediate size population of advanced osteosarcoma and soft tissue sarcoma (NCT04091295). In a Phase 2 study using DeltaRex-G monotherapy in chemo-resistant metastatic osteosarcoma (n=17 evaluable patients), there were 4 partial response, 8 stable disease by positron emission tomography, overall response rate 23.5%, disease control rate 70.6% with grade 1 fatigue and photophobia and no grade 2 adverse events (11). In Phase 1 and 2 studies of 98 patients, DeltaRex-G induced an 8.2% 10-to-15-year long-term survivorship in subjects with chemo-resistant Stage 4 osteosarcoma (n=3), mPNST (n=1), pancreatic adenocarcinoma (n=1), breast cancer (n=2), and B cell lymphoma (n=1). Three of eight patients received Deltarex-G with Reximmune-C, a tumor targeted retrovector encoding a GM-CSF gene. There were no reported second malignancies or delayed adverse events in the long-term survivors. DeltaRex-G gained FDA orphan drug status for osteosarcoma and soft tissue sarcoma in the USA in 2009, fast track designation for pancreatic cancer in the USA in 2009, and accelerated approval for all solid malignancies in the Philippines in 2007 (12).

Genetic engineering of innate immunocompetent cells, such as gamma delta T cells, natural killer (NK) cells and NKT cells have shown promise in preclinical trials and are in clinical testing (13). A major advantage of these cells is their potential use as off-the-shelf therapies. Preclinical testing has shown 100% survival of neuroblastoma with gamma delta T cells and NKT cells. CAR-T engineering shows predicted benefits in killing cancer cells, and co-expression with IL-15 and IL-12 has shown improvements in potency. Expression of bispecific T cell engagers (bites) is a promising alternative to CART strategies, as expression of bites can activate non-modified T cells. Clinical testing of gamma delta T cells administered to the resected cavity of glioblastoma is showing promise in a phase I trial. A major issue in the advancement of immunotherapies is the time required for vector and cell manufacturing. There is potential opportunity to partner with select companies that can provide guaranteed access to GMP manufacturing of osteosarcoma product candidates.

A number of repurposed drugs and naturopathic therapies and their mechanisms of action were also presented (14). The proposed repurposed regimen consists of nine drugs designed to attack malignant cells via different molecular pathways known to be important in osteosarcoma, thus limiting the potential for treatment resistance. In many cases there is neither in vitro nor in vivo evidence of benefit of these drugs in osteosarcoma. Naturopathic therapies are backed by significant in vitro evidence of efficacy; however, the panel pointed out that the micromolar concentrations used in these studies are not achievable in humans. The advantages and disadvantages of Mistletoe were discussed. Notably, a 12-month clinical trial from a single center in Italy showed remarkable efficacy of subcutaneous Mistletoe, with 5/9 patients with recurrent disease being disease-free at the end of the study. The consensus was that the safety and efficacy of repurposed and naturopathic therapies should be tested in a mouse model of osteosarcoma.

It would be beneficial to obtain an extensive profile from different parts of the tumor sample using genomics, proteomics, metabolomics, and immunohistochemistry (IHC) analysis for PDL1 and Her2 (15).

For the purpose of preventing recurrence, DeltaRex-G was the only treatment that was voted upon by all participants and gained favorable recommendations from 10 of 12 attendees with one agreeing only if it would not take away from other patients with more advanced disease. Of those who opposed, one recommended its use only in a metastatic setting and one recommended that a Phase 2 randomized study of 60-80 high-risk osteosarcoma subjects be conducted, which may lead to accelerated approval of DeltaRex-G in the USA.