Uptake and therapeutic effectiveness of 125I- and 211At-methylene blue for pigmented melanoma in an animal model system

The investigations concerning a targeted radiotherapy for pigmented melanoma with a radiolabeled phenothiazine derivative, 3,7-(dimethylamino)phenazathionium chloride [methylene blue (MTB)], were continued using melanotic and amelanotic sublines of B16 melanoma. Two radionuclides, 125I and 211At, emitting Auger electrons and alpha particles, respectively, replaced 35S previously studied since their biological effectiveness is significantly higher. In vitro autoradiography revealed a selective accumulation of methylene blue labeled with either of the radioisotopes in pigmented melanoma cells but its absence in nonpigmented cells. Treatments with [125I]MTB and [211At]MTB were performed both in vitro and in vivo, with their effectiveness determined by lung clonogenic assay. [125I]MTB proved to be relatively ineffective when incorporated into melanosomes distributed in the cytoplasm, i.e., too far away from the genome. Conspicuous therapeutic effects were achieved with [211At]MTB for pigmented melanoma only. 211At itself did not affect either of the investigated sublines of B16 melanoma confirming once again the high affinity of methylene blue to melanin. Calculations of cumulative radiation doses from [211At]MTB deposited in melanotic melanoma tumors and pigmented normal organs which would be at a particular risk led to the conclusion that [211At]MTB could be used for a highly selective and very efficient targeted radiotherapy of pigmented melanomas without damaging normal tissues.