[99mTc]Oxotechnetium(V) Complexes of Amine-Amide-Dithiol Chelates with Dialkylaminoalkyl Substituents as Potential Diagnostic Probes for Malignant Melanoma

[99mTc]oxotechnetium(V) complexes of amine-amide-dithiol (AADT) chelates containing tertiary amine substituents were synthesized and shown to have affinity for melanoma. For complexation the AADT-CH2[CH2]nNR2 (n = 1, 2; R = Et, n-Bu) ligand was mixed with a [99mTc]oxotechnetium(V)-glucoheptonate precursor to make the AADT-[99mTc]oxotechnetium(V) complexes in nearly quantitative yield. Structurally analogous nonradioactive oxorhenium(V) complexes were also synthesized and characterized. In vitro sigma-receptor affinity measurements indicate these complexes to possess sigma-affinity in the low micromolar range with K(i) values in the 7.8-26.1 and 0.18-2.3 microM range for the sigma1- and sigma2-receptors, respectively. In vitro cell uptake of the 99mTc complexes in intact B16 murine melanoma cells at 37 degrees C after a 60-min incubation ranged from 12% for complex 2 (n = 1, R = n-Bu) to 68% for complex 4 (n = 2, R = n-Bu). In vivo evaluation of complexes 1-Tc-4-Tc in the C57Bl/B16 mouse melanoma model demonstrated significant tumor localization. Complex 1-Tc (n = 1, R = Et) displayed an in vivo tumor uptake of 7.6% ID/g at 1 h after administration with initial melanoma/blood (M/B), melanoma/spleen (M/S), and melanoma/lung (M/L) ratios >4; these ratios increased to 10.8, 10.1, and 7.3, respectively, at 6 h. While complex 3-Tc (n = 3, R = Et) had an initial tumor uptake of 3.7% ID/g 1 h after administration with M/B, M/S, and M/L ratios >2, a greater tumor retention and slightly faster clearance from nontumor-containing organs resulted in M/B, M/S, and M/L ratios of 19.1, 19.1, and 12.7, respectively, at 6 h. The high tumor uptake and significant tumor/nontumor ratios indicate that such small technetium-99m-based molecular probes can be developed as in vivo diagnostic agents for melanoma and its metastases.