Melanoma is a
cancer with a rising incidence, and metastatic disease is almost always lethal. We investigated the feasibility of targeting
melanin, an intracellular melanocyte pigment, to deliver cytotoxic radiation to human
melanoma cells in vivo by using a
melanin-binding mAb (6D2). Nude mice bearing MNT1 pigmented human
melanoma tumors were treated with mAb 6D2 labeled with 1.5 mCi (1 Ci = 37 GBq) of the beta-emitter 188-Rhenium (188Re) and manifested inhibition of
tumor growth and prolonged survival. mAb 6D2 bound
tumor melanin and demonstrated no crossreactivity with normal melanized tissues in black mice. The mechanism of
melanin targeting involved Ab binding to extracellular
melanin released during
tumor cell turnover or to dying cells with permeable membranes. In this approach, the cytotoxic radiation emanating from labeled Ab bound to
melanin is presumably delivered by "crossfire" effect to the adjacent viable
tumor cells. Our results establish the feasibility of targeting
melanin released from dead
melanoma cells in
tumors with radiolabeled Abs to achieve a
therapeutic effect. In contrast to conventional
tumor antigens,
melanin is insoluble, resistant to degradation, and can be expected to accumulate in targeted tissues, suggesting that the efficacy of
therapy could increase with each subsequent treatment cycle.