Targeting angiogenesis for radioimmunotherapy with a ¹⁷⁷Lu-labeled antibody

Increased angiogenesis is a marker of aggressiveness in many cancers. Targeted radionuclide therapy of these cancers with angiogenesis-targeting agents may curtail this increased blood vessel formation and slow the growth of tumors, both primary and metastatic. CD105, or endoglin, has a primary role in angiogenesis in a number of cancers, making this a widely applicable target for targeted radioimmunotherapy. The anti-CD105 antibody, TRC105 (TRACON Pharmaceuticals), was conjugated with DTPA for radiolabeling with ¹⁷⁷Lu (t_1/2 6.65 days). Balb/c mice were implanted with 4T1 mammary carcinoma cells, and five study groups were used: ¹⁷⁷Lu only, TRC105 only, ¹⁷⁷Lu-DTPA-IgG (a nonspecific antibody), ¹⁷⁷Lu-DTPA-TRC105 low-dose, and ¹⁷⁷Lu-DTPA-TRC105 high-dose. Toxicity of the agent was monitored by body weight measurements and analysis of blood markers. Biodistribution studies of ¹⁷⁷Lu-DTPA-TRC105 were also performed at 1 and 7 days after injection. Ex vivo histology studies of various tissues were conducted at 1, 7, and 30 days after injection of high-dose ¹⁷⁷Lu-DTPA-TRC105. Biodistribution studies indicated steady uptake of ¹⁷⁷Lu-DTPA-TRC105 in 4T1 tumors between 1 and 7 days after injection (14.3 ± 2.3%ID/g and 11.6 ± 6.1%ID/g, respectively; n = 3) and gradual clearance from other organs. Significant inhibition of tumor growth was observed in the high-dose group, with a corresponding significant increase in survival (p < 0.001, all groups). In most study groups (all except the nonspecific IgG group), the body weights of the mice did not decrease by more than 10%, indicating the safety of the injected agents. Serum alanine transaminase levels remained nearly constant indicating no damage to the liver (a primary clearance organ of the agent), and this was confirmed by ex vivo histological analyses. ¹⁷⁷Lu-DTPA-TRC105, when administered at a sufficient dose, is able to curtail tumor growth and provide a significant survival benefit without off-target toxicity. Thus, this targeted agent could be used in combination with other treatment options to slow tumor growth allowing the other agents to be more effective. (orig.)