Liposomally entrapped second antibody improves tumour imaging with radiolabelled (first) antitumour antibody.

The usefulness of antibodies directed against tumour products for localisation and therapy of cancer is limited by the large proportion of administered antibody which remains non-specifically in the circulation and extravascular space. Liposomally entrapped second antibody (LESA) directed against the first (antitumour) antibody has been used to accelerate clearance of non-tumour-bound first antibody without affecting its clearance from the tumour. Intravenously administered LESA binds the first antibody and LESA is cleared by the reticuloendothelial system. LESA accelerated clearance of radiolabelled antibody directed against carcinoembryonic antigen (CEA) from the circulation in four of five patients with gastrointestinal cancer, enhancing gamma-camera imaging of the tumour in three of them. These results suggest that LESA can improve the sensitivity and specificity of tumour imaging with radiolabelled antitumour antibody. This strategy may also have the potential to improve the therapeutic ratio of some toxic agents linked to antitumour antibody.