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Radioiodinated methylene blue--a promising agent for melanoma scintigraphy: labelling, stability and in vitro uptake by melanoma cells.

Melanoma is a tumor of continuously increasing incidence for which new methods of imaging and targeted therapy are widely sought. Radioiodinated methylene blue is a promising tracer, showing selective uptake in human pigmented melanoma cells. We performed 131I-labeling of the tracer using 1% methylene blue injection United States Pharmacopeia (USP) and 131I sodium iodide. For quality control, a Merck high performance liquid chromatography (HPLC) system was used. We developed a new HPLC procedure using 0.1% trifluoroacetic acid, 90% acetonitrile and 10% water as solvent for isocratic elution of the tracer and applied a TLC method using ITLC-SG strips and the same solvent. The stability of the preparation was studied for 15 min, 3 h and 6 h. In order to evaluate the potential relevance of 131I-labeled methylene blue for melanoma detection, the in vitro uptake of 131I-methylene blue was investigated in SK-MEL 28 and 518A2 melanoma cells. Time and a temperature influence on uptake of 1311 methylene blue by these two melanoma cells were investigated. The radiochemical purity obtained by the HPLC method was 99.97 +/- 0.08% (n=8), while that by the TLC method was 99.88 +/- 0.16% (n=8). This indicates the excellent agreement between these two methods. The stability was persistent over 6 h and amounted to 99.75% +/- 0.21% (n=8). The uptake of 131I methylene blue was time and temperature dependent by both melanoma cells lines. The net cellular uptake on incubation at 37 degrees C of 131I methylene blue by SK-MEL 28 cells was high at 56.3-61.8% and that by 518A2-cells was 36.3-56.0%. Uptake by these cells was also investigated at 22 degrees C. The uptake by both cell types was also high at this temperature, but lower than that at 37 degrees C, amounting to 45.0-51.7% and 25.6-36.3%, respectively. Due to its easy handling and quite high uptake by melanoma cells, we expect that this tracer could be successfully used in routine application for melanoma imaging or eventual radiotherapy.