MULTISTEP TUMOR TARGETING IN NUDE-MICE USING BISPECIFIC ANTIBODIES AND A GALLIUM CHELATE SUITABLE FOR IMMUNOSCINTIGRAPHY WITH POSITRON EMISSION TOMOGRAPHY

To improve tumor:tissue ratios in immunoscintigraphy, a three-step tar geting method has been developed, The reagents used were (a) a radioac tive, low molecular weight chelate prepared from ionic gallium and a p henolic polyaminocarboxylic acid, which can be labeled either with the single-photon emitter Ga-67 or with the short-lived positron emitter Ga-68 (t1/2 = 68 min); (b) a bispecific monoclonal antibody (bs-mAb) s ynthesized from the F(ab)(2) fragment of the 1.1ASML antibody specific for the glycoprotein CD44v associated with a rat pancreas carcinoma c ell line and the F(ab') fragment of an antibody specific for the galli um chelate; and (c) the nonradioactive gallium chelate covalently coup led to transferrin, which served as a high molecular weight blocker to prevent binding of the radioactive gallium chelate to bs-mAbs in the circulation. Targeting experiments in tumor-bearing nude mice with dif ferent doses of bs-mAbs, blocker, and Ga-67 chelate were adjusted to m aximize tumor to tissue contrasts and tumor uptake, Compared with the biodistribution of the I-131-labeled, native 1.1ASML antibody 24 h pos tinjection, a schedule using 100 pmol bs-mab 24 h later 100 pmol block er, 15 min later 16 pmol Ga-67 chelate, 1 h later examination, increas ed tumor:blood and tumor: liver ratios by a factor of 5 while keeping the localization of radioactivity in the tumor constant (10.1% injecte d dose/g), High-contrast images using either Ga-67 or Ga-68 were obtai ned within 1 h, The targeting method described enables the use of the short-lived positron emitter Ga-68 and thus allows the combination of an improved immunoscintigraphy and positron emission tomography.