Radioimmunotargeting of Human Rhabdomyosarcoma Using Monoclonal Antibody 8H9

Although metastatic rhabdomyosarcoma (RMS) is chemotherapy and radiotherapy responsive, few patients are cured. 8H9, a murine IgG₁ monoclonal antibody, recognizes a unique cell surface tumor antigen broadly distributed on neuroectodermal, epithelial, and mesenchymal tumors, including RMS. We now report on the in vitro characterization of radiolabeled 8H9 and its in vivo immunotargeting potential in mice with subcutaneous human RMS. Saturation-binding studies carried out to determine ¹²⁵I-8H9 affinity to the RMS cell line HTB82 demonstrated that ¹²⁵I-8H9 had a K_d of 10.3nM with an estimated 115,000 binding sites on every HTB82 cell. ¹²⁵I-8H9 was retained on the cell surface without significant internalization. Biodistribution of ¹²⁵I-8H9 was studied in athymic mice bearing HTB82 xenografts. Following intravenous injection of 4.44MBq of ¹²⁵I-8H9, selective tumor uptake was evident 4 to 172 hours after injection. Average tumor uptake was 7.0 ± 1.8, 11.5 ± 3.9, 15.1 ± 3.7, and 5.4 ± 1.2% injected dose per gram at 4, 24, 48, and 172 hours, respectively. Mean tumor: tissue ratios were maximal at 172 hours(for lung, 4, kidney, 6, liver, 7, spleen, 11, femur, 14, muscle, 18, and brain, 48). Established RMS xenografts treated with a single injection of 18.5 MBq ¹³¹I-8H9 were significantly suppressed compared to controls. Radiolabeled 8H9 effectively targeted RMS xenografts and may have a potential clinical role in radioimmunotherapy.