Monoclonal Antibodies

Monoclonal antibodies (McAb) are used successfully in nuclear medicine for localization of tumors but an initial attempt at extending this use to MRI with paramagnetic (Gd3+) labeled antibodies was unsuccessful because of the estimated 800-fold lesser sensitivity of MRI. This problem can be addressed in several ways: 1) increasing the number of paramagnetic ions attached to the McAb; 2) attaching several paramagnetic ions to a macromolecule that in turn is attached to a McAb; 3) using more antibodies or those with an affinity to many antigenic sites per cell or both; and 4) using a superparamagnetic particle attached to the McAb. Implanted human colon carcinoma tumors in mice have been successfully imaged by using monoclonal antibodies with a large number of Gd-DTPA molecules attached.

Additional studies report the use of very small magnetite particles coated with McAb. The magnetite cores are 10-20 nm in diameter with a total particle diameter of 20-32 nm. The magnetic moment of these superparamagnetic particles is about 1000 times that of comparable paramagnetic particles. This allows the use of 1-10 nmol concentrations of the McAb coated magnetite particles. Mixed success has been obtained in rodents with implanted neuroblastoma and human colon carcinoma.