Radiolanthanide-Labeled Monoclonal Antibody CC49 for Radioimmunotherapy of Cancer:  Biological Comparison of DOTA Conjugates and 149Pm, 166Ho, and 177Lu

The radiolanthanides 149Pm, 166Ho, and 177Lu have decay characteristics suitable for radioimmunotherapy (RIT) of cancer. N-Hydroxysulfosuccinimidyl DOTA (DOTA-OSSu) and methoxy-DOTA (MeO-DOTA) were conjugated to the anti-TAG-72 monoclonal antibody CC49 for radiolabeling with 149Pm, 166Ho, and 177Lu. While both DOTA conjugates could be labeled to high specific activity with 177Lu, MeO-DOTA afforded superior conjugate stability, radiolabeling, and radiochemical purity. Pilot biodistributions in nude mice bearing LS174T human colon carcinoma xenografts demonstrated that MeO-DOTA afforded higher tumor uptake and lower kidney retention of 177Lu than DOTA-OSSu. The in vitro stability of 149Pm-, 166Ho-, and 177Lu-MeO-DOTA−CC49 was evaluated using serum and hydroxyapatite assays. Serum stability of radiolanthanide-labeled MeO-DOTA−CC49 followed a trend based on the coordination energies of the radiometals, with 177Lu showing the highest stability after 96 to 168 h at 37 °C. In contrast, MeO-DOTA−CC49 labeled with all three radiolanthanides was >92% stable to hydroxyapatite challenge for 168 h at 37 °C. Comprehensive biodistributions of 149Pm-, 166Ho-, and 177Lu-MeO-DOTA−CC49 were obtained in LS174T-bearing nude mice. Maximum tumor uptakes were 100.0% ID/g for 149Pm at 96 h, 69.5% ID/g for 166Ho at 96 h, and 132.4% ID/g for 177Lu at 168 h. Normal organ uptakes were generally low, except in the liver, spleen, and kidney at early time points. By 96 to 168 h postinjection, nontarget organ uptake decreased to approximately 7% ID/g (kidney), 12% ID/g (spleen), and 20% ID/g (liver) for each radiolanthanide. When labeled with 149Pm, 166Ho, and 177Lu, MeO-DOTA−CC49 has potential for RIT of colorectal cancer and other carcinomas.