Polylysine-Gd-DTPAn and polylysine-Gd-DOTAn coupled to anti-CEA F(ab')2 fragments as potential immunocontrast agents. Relaxometry, biodistribution, and magnetic resonance imaging in nude mice grafted with human colorectal carcinoma.

RATIONALE AND
OBJECTIVES: Immunocontrast agents used for magnetic resonance imaging require antibodies that preserve the immunoreactivity while containing a high number of chelated paramagnetic ions.
METHODS: Anti-CEA F(ab')2 fragments were coupled to polylysine-Gd-DOTA and polylysine-Gd-DTPA. A paramagnetic load as high as n = 24 to 28 metal ions per antibody was reached.
RESULTS: The immunoreactivity of the gadolinium (Gd)-labeled anti-CEA F(ab')2 immunoconjugates was 80% to 85%. Compared with that of commercial chelates, the relaxivity (R1) increase is as follows: Gd-DTPA < Gd-DOTA < Gd-H2O < PL-Gd-DTPA24-28 < PL-Gd-DTPA24-28 F(ab')2 < PL-Gd-DOTA24-28 < PL-Gd-DOTA24-28 F(ab')2. 1H nuclear magnetic relaxation dispersion data of immunoconjugates showed that the high relaxivity enhancement was the result of a reduction of the molecular tumbling rate. Twenty-four hours after intravenous injection of 50 micrograms (1 mumol Gd/kg) of Gd-labeled immunoconjugates to nude mice grafted with human colorectal carcinoma LS 174T, the tumor uptake was 10% to 15%, resulting in an increase of R1 of up to 15% to 20% versus noninjected mice. No difference was found between PL-Gd-DTPA24-28 F(ab')2 and PL-Gd-DOTA24-28 F(ab')2 immunoconjugates for tumor, liver, and kidney uptake. A high signal intensity of tumor was observed in 50% of the tested mice.