Effective metabolic targeting of human osteosarcoma cells in vitro and in orthotopic nude-mouse models with recombinant methioninase

Background: Methionine dependence may be the only known general metabolic defect in cancer. In order to exploit methionine dependence for therapy, our laboratory previously cloned L-methionine α-deamino-γ-mercaptomethane lyase [EC 4.4.1.11]) (recombinant methioninase [rMETase]), which was subsequently tested in mouse models of various types of human tumors. The present study aimed to investigate the efficacy of rMETase on human osteosarcoma cells in vitro and in vivo. Materials and Methods: Human osteosarcoma cell lines 143B, HOS and SOSN2 were tested in vitro for survival during a 72-h exposure to rMETase using the WST-8 assay. Halfmaximal inhibitory concentrations were calculated for in vitro efficacy experiments. 143B cells were orthotopically transplanted into the tibia of nude mice. Mouse models were randomized into the following groups 1 week after transplantation: Group 1, untreated control; Group 2, cisplatinum (CDDP) [intraperitoneal (i.p.) injection at 6 mg/kg weekly, for 3 weeks], positive control; Group 3, rMETase, 100 units/mouse i.p. daily, for 21 days. Tumor sizes and body weight were measured with calipers and a digital balance once per week, respectively. Results: rMETase significantly inhibited osteosarcoma cell growth, in a dose-dependent manner, in vitro. Both CDDP and rMETase treatment significantly inhibited tumor volume compared to untreated control mice at 5 weeks after initiation. Tumor volumes were as follows: Group 1, untreated, control: 1808.2 ± 344 mm3; Group 2, CDDP: 1102.2 ± 316 mm3, p=0.0008 compared to untreated control; Group 3, rMETase: 884.8 ± 361 mm3, p=0.0001 compared to untreated control. There were no animal deaths in any group. The body weight of mice was not significantly different between any group. Conclusion: rMETase showed promising efficacy against osteosarcoma, a recalcitrant tumor type. Future studies will investigate the efficacy of rMETase on patient-derived orthotopic xenograft (PDOX) models of osteosarcoma as a bridge to testing rMETase in the clinic.