Targeting of Indium 111-labeled Bivalent Hapten to Human Melanoma Mediated by Bispecific Monoclonal Antibody Conjugates: Imaging of Tumors Hosted in Nude Mice

Abstract

Antibody conjugates were prepared by coupling F(ab′)₂ or Fab′ fragments of an antibody specific for the human high molecular weight-melanoma associated antigen to Fab′ fragments of an antibody specific for indium-diethylenetriaminepentaacetate complexes. Monovalent and bivalent haptens were synthesized by reacting the dipeptide tyrosyl-lysine with diethylenetriaminepentaacetic cyclic anhydride. In vitro, the antibody conjugate mediated binding of the ¹¹¹In-labeled haptens to melanoma cells. In vivo, it allowed specific localization of the haptens in A375 tumors. The bivalent hapten exhibited much higher efficiency at targeting ¹¹¹In onto cells, both in vitro and in vivo. Antibody conjugate and hapten doses (2 µg and 1 pmol, respectively) and the delay between antibody conjugate and tracer injections (24 h) were adjusted to maximize tumor uptake (4% injected dose/g) and tumor to normal tissue contrast (>3) obtained 3 h after injection of the ¹¹¹In-labeled bivalent hapten. This two-step technique, when compared to direct targeting of ¹¹¹In-labeled F(ab′)₂ fragments, provided lower localization of injected activity into the tumor (× 0.25), but higher tumor/tissue ratios, especially with respect to liver (× 7), spleen (× 8), and kidneys (× 10). In addition, high contrast images were obtained within 3 hours, instead of days. Thus, antibody conjugate-mediated targeting of small bivalent haptens, labeled with short half-life isotopes, is proposed as a general method for improving tumor radioimmunolocalization.