Evaluation of novel 99mTc(I)-labeled homobivalent α-melanocyte-stimulating hormone analogs for melanocortin-1 receptor targeting.

Aiming to apply the multivalency concept to melanoma imaging, we have assessed the in vivo melanocortin type 1 receptor (MC1R)-targeting properties of (99m)Tc(I)-labeled homobivalent peptide conjugates which contain copies of the α-melanocyte-stimulating hormone (α-MSH) analog [Ac-Nle(4), Asp(5), D-Phe(7), Lys(11)]α-MSH4-11 separated by linkers of different length (L(2) nine atoms and L(3) 14 atoms). The MC1R-binding affinity of L(2) and L(3) is significantly higher than that of the monovalent conjugate L(1). Metallation of these conjugates yielded the complexes fac-M(CO)(3)(k(3)-L) (M is (99m)Tc/Re; 1/1a, L is L(1); 2/2a, L is L(2); 3/3a, L is L(3)), with IC(50) values in the subnanomolar and nanomolar range. The MC1R-mediated internalization of 2 and 3 is higher than that of 1 in B16F1 melanoma cells. Biodistribution studies in melanoma-bearing mice have shown low nonspecific accumulation with a tumor uptake that correlates with IC(50) values. However, no correlation between tumor uptake and valency was found. Nevertheless, 2 displayed the highest tumor retention, and the best tumor to nontarget organ ratios.