Samarium-153 chelate localization in malignant melanoma

153Sm, a radiolanthanide of half life 46.27 h, has a gamma emission of 0.103 MeV which is well suited to imaging, it is also a moderate energy beta emitter and tumour localization of various 153Sm chelates was evaluated in B16 murine melanoma to assess their endoradiotherapeutic potential. 153Sm was prepared from enriched 152Sm in the Australian Nuclear Science and Technology Organization reactor. 153Sm chelates were prepared from 153Sm-chloride and their chromatographic behaviour characterized. Tumour and organ uptake of 153Sm-chloride, 153Sm-citrate and the 153Sm chelates, DTPA, HEDTA, HIDA, BZ, PBH, PIH and NTA were measured at 1, 6, 24 and 48 h after intravenous administration to C57 black mice bearing either melanotic or amelanotic B16 melanoma of mean size 0.75 cm3. Histopathological examination of the tumours at each passaging assured comparability of the degree of melanogenesis and the absence of necrosis. 153Sm-chloride was immobile on chromatography and the rapid hepatic accumulation of both 153Sm-chloride and 153Sm-citrate was attributed to in vivo formation of a colloid. In contrast, 153Sm-DTPA, moving at the solvent front on chromatography, showed no reticuloendothelial accumulation in vivo and was rapidly excreted by the kidneys without tumour uptake. The other 153Sm chelates were of intermediate stability and all localized in both melanotic and amelanotic tumours, although to a significantly lesser degree than 67Ga-citrate. The relatively high 153Sm-HIDA activity in liver and 153Sm-NTA activity in bone impaired tumour definition, but on imaging of all the 153Sm chelates only 153Sm-DTPA failed to demonstrate the B16 melanoma and the best tumour delineation was obtained using 153Sm-HEDTA.