Effects of the amino acid linkers on the melanoma-targeting and pharmacokinetic properties of ¹¹¹In-labeled lactam bridge-cyclized α-MSH peptides

The purpose of this study was to examine the profound effects of the amino acid linkers on the melanoma-targeting and pharmacokinetic properties of ¹¹¹In-labeled lactam bridge-cyclized DOTA-[X]-CycMSH_hex {1,4,7,10-tetraazacyclododecane-1,4,7,10- tetraacetic acid-[X]-c[Asp-His-DPhe- Arg-Trp-Lys]-CONH2; X = GGNle, GENle, or NleGE; GG = -Gly-Gly- and GE = -Gly-Glu-} peptides. Methods: Three novel peptides (DOTA-GGNle- CycMSH _hex, DOTA-GENle-CycMSH_hex, and DOTA-NleGECycMSH _hex) were designed and synthesized. The melanocortin-1 (MC1) receptor-binding affinities of the peptides were determined in B16/F1 melanoma cells. The melanoma-targeting and pharmacokinetic properties of ¹¹¹In-DOTA-GGNle-CycMSH_hex and ¹¹¹In-DOTA- GENle-CycMSH_hex were determined in B16/F1 melanoma-bearing C57 mice. Results: DOTA-GGNle- CycMSH_hex and DOTA-GENle-CycMSH_hex displayed 2.1 and 11.5 nM MC1 receptor-binding affinities, whereas DOTA-NleGECycMSH_hex showed 873.4 nM MC1 receptor-binding affinity. The introduction of the -GG- linker maintained high melanoma uptake while decreasing kidney and liver uptake of ¹¹¹In-DOTAGGNle- CycMSH _hex. The tumor uptake of ¹¹¹In-DOTA-GGNle- CycMSH _hex was 19.05 ± 5.04 and 18.6 ± 3.56 percentage injected dose per gram at 2 and 4 h after injection, respectively. ¹¹¹In-DOTA-GGNle-CycMSH_hex exhibited 28%, 32%, and 42% less kidney uptake than ¹¹¹In-DOTA-Nle-CycMSH_hex we reported previously, and 61%, 65%, and 68% less liver uptake than ¹¹¹In-DOTA-Nle-CycMSH_hex at 2, 4, and 24 h after injection, respectively. Conclusion: The amino acid linkers exhibited profound effects on the melanoma-targeting and pharmacokinetic properties of the ¹¹¹In-labeled lactam bridge-cyclized a-melanocyte-stimulating hormone peptides. Introduction of the -GG- linker maintained high melanoma uptake while reducing kidney and liver uptake of ¹¹¹In-DOTA-GGNle- CycMSH _hex, highlighting its potential as an effective imaging probe for melanoma detection, as well as a therapeutic peptide for melanoma treatment when labeled with a therapeutic radionuclide.