Structural modification on the Lys linker enhanced tumor to kidney uptake ratios of ^99mTc-labeled RGD-conjugated α-MSH hybrid peptides

The purpose of this study was to examine whether the structural modification on the positively charged Lys linker could reduce the kidney uptake of ^99mTc-labeled Arg-Gly-Asp (RGD)-conjugated α-melanocyte stimulating hormone (α-MSH) hybrid peptides. The RGD motif {cyclic(Arg-Gly-Asp-d-Tyr-Asp)} was coupled to [Cys^3,4,10, d-Phe ⁷, Arg¹¹]α-MSH_3-13 {(Arg ¹¹)CCMSH} through a neutral glycine linker to eliminate the positively charged amino side chain of the Lys linker or without a linker to delete the Lys linker. The receptor binding affinity of RGD-Gly-(Arg ¹¹)CCMSH and RGD-(Arg¹¹)CCMSH was determined in B16/F1 melanoma cells. The melanoma targeting and imaging properties of ^99mTc-RGD-Gly-(Arg¹¹)CCMSH and ^99mTc-RGD- (Arg¹¹)CCMSH were determined in B16/F1 melanoma-bearing C57 mice. The structural modification on the Lys linker retained a low nanomolar receptor binding affinity of RGD-Gly-(Arg¹¹)CCMSH and RGD-(Arg ¹¹)CCMSH (1.5 and 1.0 nM, respectively). The structural modification on the Lys linker dramatically decreased the renal uptake of ^99mTc-RGD-Gly-(Arg¹¹)CCMSH and ^99mTc-RGD- (Arg¹¹)CCMSH by 79% and 77% at 4 h postinjection compared to ^99mTc-RGD-Lys-(Arg¹¹)CCMSH. ^99mTc-RGD-(Arg ¹¹)CCMSH displayed a higher melanoma uptake (16.12 ± 3.09% ID/g) than ^99mTc-RGD-Gly-(Arg¹¹)CCMSH (11.50 ± 1.01% ID/g) at 2 postinjection. The tumor uptake of ^99mTc-RGD- (Arg¹¹)CCMSH was 1.4 times the tumor uptake of ^99mTc-RGD- Gly-(Arg¹¹)CCMSH at 2 postinjection. A dramatically enhanced tumor-to-kidney uptake ratio of ^99mTc-RGD-(Arg¹¹)CCMSH suggests that ¹⁸⁸Re-RGD-(Arg¹¹)CCMSH may behave in a similar fashion warranting future evaluation for melanoma treatment.