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 ¹⁵³Smchloride 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.