Introduction of an Arginine linker reduced the renal uptake of Tc-99m-labeled Arg-Ala-Asp-conjugated alpha-MSH peptide

Objectives ^99mTc-RAD-Lys-(Arg¹¹)CCMSH displayed high melanoma uptake (19.91 ± 4.02% ID/g at 2 h post-injection) in B16/F1 melanoma-bearing C57 mice in our previous report. However, the renal uptake was extremely high (92.97 ± 21.72% ID/g at 2 h post-injection). Thus, it is highly desirable to reduce the renal uptake. The purpose of this study was to examine whether the replacement of the Lys linker with an Arg linker could decrease the renal uptake of ^99mTc-RAD-Arg-(Arg¹¹)CCMSH.

Methods The RAD motif {cyclic(Arg-Ala-Asp-DTyr-Asp)} was coupled to [Cys^3,4,10, D-Phe⁷, Arg¹¹]α-MSH_3-13 {(Arg¹¹)CCMSH} through the Arg linker (substituting the Lys linker) to generate the novel RAD-Arg-(Arg¹¹)CCMSH peptide. The receptor binding affinity of RAD-Arg-(Arg¹¹)CCMSH was determined in B16/F1 melanoma cells. The melanoma targeting and pharmacokinetic properties of ^99mTc-RAD-Arg-(Arg¹¹)CCMSH were determined in B16/F1 melanoma-bearing C57 mice.

Results IC₅₀ value of RAD-Arg-(Arg¹¹)CCMSH was 0.22 nM in B16/F1 melanoma cells. ^99mTc-RAD-Arg-(Arg¹¹)CCMSH exhibited rapid and high tumor uptake (17.98 ± 4.96% ID/g at 2 h post-injection) in B16/F1 melanoma-bearing C57 mice. As compared to ^99mTc-RAD-Lys-(Arg¹¹)CCMSH, the replacement of the Lys linker with an Arg linker dramatically decreased the renal uptake of ^99mTc-RAD-Arg-(Arg¹¹)CCMSH by 68, 62, 73 and 64% at 0.5, 2, 4 and 24 h post-injection, respectively. Flank B16/F1 melanoma lesions were clearly imaged at 2 h post-injection using ^99mTc-RAD-Arg-(Arg¹¹)CCMSH as an imaging probe.

Conclusions High melanoma uptake and decreased renal uptake of ^99mTc-RAD-Arg-(Arg¹¹)CCMSH suggests that the replacement of the Lys linker with an Arg linker may be useful in reducing the renal uptake of other ^99mTc-RXD-Arg-(Arg¹¹)CCMSH peptides in future studies.