177Lu-antibody conjugates for single-cell kill of B-lymphoma cells in vitro and for therapy of micrometastases in vivo

Antibodies (Abs) conjugated to 177Lu, a relatively low-energy β-particle emitter, were evaluated in vitro for their cytotoxic activity and in vivo for their therapeutic activity against disseminated B-cell lymphoma xenografts in SCID mice. 177Lu was compared with other β-particle emitters (131I and 90Y), and also with emitters of low-energy electrons (LEEs, meaning Auger and conversion electrons of <50 keV). The Abs used reacted with CD20, CD74 or HLA-DR, and the target cell was the Raji B lymphoma. Like the other β-particle emitters, 177Lu was a potent and specific toxic agent in vitro, when conjugated to Abs recognizing high-density antigens. It appeared to be slightly less potent than 131I per decay, but this difference was relatively small, and would not be a major factor in the selection of the optimal radionuclide for clinical use. The nonspecific toxicity from 177Lu was less than from 90Y, but 177Lu still produced greater nonspecific toxicity in vitro than LEE emitters. The maximum tolerated dose (MTD) of 177Lu-anti-CD74 in SCID mice was 1.81 MBq (49 μCi)/mouse. When this dose was administered on day 5 after tumor inoculation, significant protection was obtained, but considerably less than the protection obtained in previous experiments with LEE emitters 111In and 67Ga. In conclusion, 177Lu has advantages over other available β-particle emitters as a therapeutic agent, but its efficacy in the treatment of micrometastases seems to be less than that of LEE emitters, due to greater nonspecific toxicity. This conclusion, however, may not apply to therapy of macroscopic tumors.