Selective Deposition of Radium in Normal and Neoplastic Melanocytes

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Abstract
After a single intravenous injection of radium in beagles, marked intracellular deposition of the nuclide was demonstrated in the iridiocytes of the tapetum, the melanocytes of the ocular vascular tunic, and the melanocytes of a malignant melanoma. Autoradiographic studies established that most of the Ra226 is localized in formed melanin particles; thus the deposition site is dominantly cytoplasmic. At the higher dose levels (3 to 10 [mu]c of Ra226 /kg), the ocular burden produced degeneration and necrosis of all iridiocytes and many melanocytes. This was manifest grossly as marked depigmentation of the choroid and iris and complete loss of the tapetum. The lower dose level (0.05 to 0.35 [mu]c of Ra226/kg) produced hypermelanosis without significant hyperplasia. The latent periods were related to the respective injected dose levels. Selective deposition of Ra226 in the pigmented melanocytes of a canine malignant melanoma after injection with 25.1 [mu]c of Ra226 per kilogram resulted in marked involution beginning 9 days postinjection. By 35 days after injection, the tumor was less than one-third its preinjection size. Regression of the metastases occurred concurrently with the primary site. Alpha-particle track counting indicated that the concentrations of Ra226 in melanocytes and/or melanophores of the melanoma ranged from 40 to 630 [mu]c of Ra226/kg. Death was related to hematopoietic depression. Thus, there is a tissue other than bone in which selective deposition of radium occurs. The data suggest, that radium, given intravascularly, might have therapeutic value in the treatment of heavily pigmented metastatic melanomas.

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