Alpha-emitting bismuth cyclohexylbenzyl DTPA constructs of recombinant humanized anti-CD33 antibodies: Pharmacokinetics, bioactivity, toxicity and chemistry

The alpha-particle-emitting radionuclides have several physical characteris tics that make them attractive candidates for radioimmunotherapy: (a) high linear energy transfer; (b) short path lengths (50-80 mu m); and (c) limite d ability of cells to repair damage to DNA. This article describes the phar macokinetic, bioactivity, toxicity and chemical characteristics of cr-parti cle-emitting, Bi-213 and Bi-212 radiometal conjugated HuM195 (anti-CD33) co nstructs. Conjugation of HuM195 to SCN-CHX-A-DTPA resulted in the attachmen t of up to 10 chelating ligand molecules per antibody. Results: Radiolabeli ng efficiency of the CHX-A-DTPA-HuM195 construct with 213Bi was 78% +/- 10% (n = 46) after 10 min at specific activities of up to 1110 MBq/mg. The imm unoreactivity of the Bi-213-labeled CHX-A-DTPA-HuM195 construct was 84% +/- 10% (n = 28) and was independent of the specific activity, The bismuth-lab eled CHX-A-DTPA-HuM195 construct was rapidly internalized into the cell in a time-dependent manner ranging from 50% at 1 h to 65% at 24 h. Bi-205/Bi-2 06-labeled constructs were stable for at least 2 d in vitro in the presence of human serum at 37 degrees C. After injection into mice, there was no up take or toss of bismuth to mouse tissues, which do not express CD33, or to the kidney, which has avidity for free bismuth. Mice injected intraperitone ally with doses of (Bi-213)CHX-A-DTPA-HuM195 ranging from 18.5 to 740 MBq/k g showed no toxicity, but at 2590 MBq/kg, two of the three mice died within 2 wk and a third mouse showed significant reductions in white blood cell c ounts. Mice injected intraveneously with doses of (Bi-213)CHX-A-DTPA-HuM195 up to 370 MBq/kg exhibited little toxicity, but 666 MBq/kg was above the M TD for mice. Leukemia cell killing in vitro with bismuth-labeled HuM195 sho wed dose- and specific activity-dependent killing of CD33+ HL60 cells; appr oximately 50% killing was observed when two bismuth atoms (50 fM radiolabel ed antibody) were initially bound onto the target cell surface. Conclusion: Alpha-emitting antibodies are among the most potent cytotoxic agents known , yet are specific and appear safe in vivo. The physical and biochemical ch aracteristics of the Bi-213 isotope and its generation, as well as the bioc hemistry of the Bi-213-labeled CHX-A-DTPA-HuM195 construct, make it possibl e to use the constructs safely and feasibly in humans at therapeutic levels .