Oral-recombinant Methioninase Lowers the Effective Dose and Eliminates Toxicity of Cisplatinum for Primary Osteosarcoma of the Mammary Gland in a Patient-derived Orthotopic Xenograft Mouse Model

Background/Aim: Primary osteosarcoma of the mammary gland is a very rare disease, accounting for under 1% of all mammary gland malignancies. There is no established first-line treatment, and prognosis is poor compared to conventional breast cancer. We previously demonstrated the efficacy of cisplatinum and eribulin in a patient-derived orthotopic xenograft (PDOX) mouse model of primary breast osteosarcoma. However, these drugs show significant clinical toxicity. All cancers are addicted to methionine (Hoffman effect). In the present study, we determined whether methionine restriction with oral recombinant methioninase (o-rMETase) would lower the effective dose of cisplatinum in a PDOX model of primary osteosarcoma of the mammary gland, thereby reducing its toxicity. Materials and Methods: Mouse PDOX models of primary osteosarcoma of the breast were randomized into the following groups: control; cisplatinum (weekly at 3 or 6 mg/kg); twice-daily o-rMETase; or o-rMETase combined with 3 mg/kg cisplatinum, with treatment for 2 weeks. Results: Cisplatinum at 6 mg/kg significantly inhibited breast osteo-sarcoma growth compared with the untreated control and mice treated with 3 mg/kg cisplatinum (p=0.01 and 0.009, respectively). There was no significant difference in tumor growth between mice treated with cisplatinum at 3 mg/kg and the control (p=0.16). Combination therapy with cisplatinum at 3 mg/kg and twice daily o-rMETase regressed the osteosarcoma of the mammary gland (p=0.009), similar to the inhibition by cisplatinum at 6 mg/kg alone. Cisplatinum at 6 mg/kg caused a significant loss of mouse body weight, compared to the control (p=0.02). There was no significant body-weight loss with the combination therapy of o-rMETase and cisplatinum at 3 mg/kg, compared to the untreated control. Conclusion: o-rMETase halved the effective dose of cisplatinum, thereby eliminating cisplatinum toxicity, demonstrating a future clinical strategy for therapy of osteosarcoma of the breast.