Radioiodinated rhodamine-123: preparation and preliminary evaluation as an agent for tumor scintigraphy.

Mitochondria specific dye, rhodamine-123 (Rh-123), has been labeled with radioactive iodine and tissue distribution of the radiolabeled product has been studied in mice. Transplantable KHT sarcoma and a spontaneous adenomammary carcinoma served as tumor models. The FAB mass spectra of iodinated Rh-123 indicated that mono and di-iodo products were formed which, as shown by other in vitro tests, were positively charged, and were heavily taken up by the mitochondria of living cells in culture. In animals, it was observed that, initially the radioactivity was taken up by all major organs from which it cleared rapidly including that from the KHT sarcoma; but not from the spontaneous adenomammary carcinoma. As a result, the spontaneous tumors retained much higher radioactivity than the equal weight of blood (x4.3) or muscle (x9.5), and were unequivocally detectable by external scintigraphy. The mitochondria selectivity and the specificity of the radioiodinated Rh-123 for the tumors of epithelial origin are exciting and warrant further studies of its use in diagnosis and therapy.