Targeted alpha therapy in vivo: direct evidence for single cancer cell kill using 149Tb-rituximab.

This study demonstrates high-efficiency sterilisation of single cancer cells in a SCID mouse model of leukaemia using rituximab, a monoclonal antibody that targets CD20, labelled with terbium-149, an alpha-emitting radionuclide. Radio-immunotherapy with 5.5 MBq labelled antibody conjugate (1.11 GBq/mg) 2 days after an intravenous graft of 5.10(6) Daudi cells resulted in tumour-free survival for >120 days in 89% of treated animals. In contrast, all control mice (no treatment or treated with 5 or 300 micro g unlabelled rituximab) developed lymphoma disease. At the end of the study period, 28.4%+/-4% of the long-lived daughter activity remained in the body, of which 91.1% was located in bone tissue and 6.3% in the liver. A relatively high daughter radioactivity concentration was found in the spleen (12%+/-2%/g), suggesting that the killed cancer cells are mainly eliminated through the spleen. This promising preliminary in vivo study suggests that targeted alpha therapy with (149)Tb is worthy of consideration as a new-generation radio-immunotherapeutic approach.

The influence of EDTMP-concentration on the biodistribution of radio-lanthanides and 225-Ac in tumor-bearing mice. The ISOLDE Collaboration.

High-resolution gamma spectroscopy was applied to measure simultaneously the biodistribution of carrier-free radionuclides of several lanthanides (141Ce, 145Sm, 149Gd, 167Tm) and 225Ac in tumor-bearing nude mice. Mixtures of the radiotracers were injected in solutions containing different concentrations of EDTMP (ethylenediaminetetramethylenephosphonic acid). The strong dependence of liver uptake on the ionic radius of the radio-lanthanides was confirmed for all tracers used. The ratios of radioactivity concentrated in tumour that concentrated in liver are strongly influenced by the EDTMP concentration, reaching values close to 10 for Tm, 3 for Sm, and 1 for Ac. The optimal EDTMP concentrations, giving highest tumor-to-liver ratios of enrichment, were between 1 and 10 mM for 100 microL injected volume for the animal model used in this experiment. In radionuclide therapy using EDTMP as ligands, close control of ligand concentration will be necessary.

Site-specific modification of a fragment of a chimeric monoclonal antibody using reverse proteolysis.

We propose a novel method for the site-specific labeling of antibodies under mild conditions and give as an example the modification of an F(ab')2-like fragment of the chimeric monoclonal antibody B72.3. The F(ab')2-like fragment was produced by the action of the protease lysyl endopeptidase. Reverse proteolysis, catalyzed by the same enzyme, was then used to attach carbohydrazide specifically to the carboxyl termini of the heavy chains of the fragment. Finally, a radiolabeled chelator possessing an aldehyde group was conjugated to the modified fragment through a hydrazone linkage. The resulting site-specifically labeled F(ab')2-like fragment was characterized by gel electrophoresis and by enzymic digestion. It was found to possess immunoreactivity equivalent to that of the unmodified F(ab')2-like fragment as determined by immunofluorescence and ELISA (enzyme-linked immunosorbent assay) techniques. The advantages and disadvantages of this labeling method, which appear to be of quite general applicability, are discussed.