DOSIMETRY OF FRACTIONATED ADMINISTRATION OF I-125 LABELED-ANTIBODY ATEXPERIMENTAL RADIOIMMUNOTARGETING

BACKGROUND. Radiotherapy of solid tumors is preferably performed in fr actionated doses. Conversely, radioimmunotherapy with nuclide-carrying antibodies delivers a continuously decreasing low dose rate during a longer time period after a single injection. In the current study, the same total amount of I-125-labeled anticytokeratin monoclonal antibod y (MoAb) was administrated in one, three, or ten injections and the do simetry was evaluated. METHODS. Three groups of nude mice (10 mice eac h) with HeLa Hep 2 xenografts were injected with 1 x 100 mu g/22.2 meg abecquerel (MBq), 3 x 33 mu g/7.4 MBq, and 10 x 10 mu g/2.22 MBq I-125 -labeled TS1 MoAb, respectively. The mice were examined scintigraphica lly over a 54-day period (total number of radio immuno-scintigraphies (RISs) = approximately 700) and doses to tumor and normal tissues were estimated according to the medical internal radiation dose formalism. RESULTS. A single bolus injection caused higher tumor uptake, tumor d ose, and tumor to nontumor dose ratio than administration of the same total dose of antibody and radioactivity in three or ten separate inje ctions. The single bolus injection caused a tenfold higher tumor uptak e (% injected dose, or ID) compared with the group receiving ten injec tions. This caused a tumor dose of 17 gray to the group receiving a si ngle bolus injection. CONCLUSIONS. in this antigen target system, a si ngle injection of a large amount of antibody was found to be more. eff icient than the same antibody dose subdivided into three or ten fracti ons. It was concluded that not only the radioactivity but also the amo unt of antibody per fraction should be considered when determining opt imal fractionated radioimmunotherapy. (C) 1997 American Cancer Society .