Modification of the structure of a metallopeptide: synthesis and biological evaluation of (111)In-labeled DOTA-conjugated rhenium-cyclized alpha-MSH analogues.

Rhenium-cyclized CCMSH analogues are novel melanoma-targeting metallopeptides with high tumor uptake, long tumor retention, and low background in normal tissues, which make these metallopeptides an ideal structural motif for designing novel melanoma-targeting agents. ReCCMSH has been derivatized with a 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelate so that it can be labeled with a wide variety of radionuclides for imaging and therapeutic applications. This study involved optimization of the in vivo biological properties of DOTA-ReCCMSH (S), through modification of the structure of the metallopeptide. Several DOTA-ReCCMSH analogues, Ac-Lys(DOTA)-ReCCMSH (4) DOTA-ReCCMSH(Arg(11)) (6), DOTA-ReCCMSH-OH (8), and DOTA-ReCCMSH-Asp-OH (10), were synthesized using solid phase peptide synthesis followed by rhenium cyclization. The IC(50) values of the metallopeptides were determined through competitive binding assays against (125)I-(Tyr(2))-NDP. Radiolabeling of the DOTA-rhenium-cyclized peptides with (111)In was carried out in NH(4)OAc (0.1 M; pH 5.5)-buffered solution for 30 min at 70 degrees C. The stability of the radiolabeled complexes was evaluated in 0.01 M, pH 7.4, phosphate-buffered saline/0.1% bovine serum albumin solution. After separation of the radiolabeled peptide from the unlabeled peptide by reverse phase high-performance liquid chromatography, the biodistribution of the radiolabeled complex was performed in C57 mice bearing B16/F1 murine melanoma tumors. All radiolabeled complexes showed fast blood clearance (2 h postinjection (pi): (111)In-S, 0.07 +/- 0.03% ID/g; (111)In-4, 0.09 +/- 0.06% ID/g; (111)In-6, 0.21 +/- 0.08% ID/g; (111)In-8, 0.11 +/- 0.10% ID/g; and (111)In-10, 0.05 +/- 0.03% ID/g), and their clearance was predominantly through the urine (4 h pi: 93.5 +/- 1.7, 87.8 +/- 6.5, 89.8 +/- 4.2, 93.3 +/- 1.1, and 93.8 +/- 1.8 (% ID) for (111)In-labeled S, 4, 6, 8, and 10, respectively). Tumor uptake values of 9.45 +/- 0.90, 6.01 +/- 2.36, 17.41 +/- 5.61, 9.27 +/- 0.68, and 7.32 +/- 2.09 (% ID/g) for (111)In-labeled S, 4, 6, 8, and 10, respectively, were observed at 4 h pi. The kidney uptake was 9.27 +/- 2.65% ID/g for (111)In-S, 19.02 +/- 2.63% ID/g for (111)In-4, 7.37 +/- 1.13% ID/g for (111)In-6, 8.70 +/- 0.88% ID/g for (111)In-8, and 8.13 +/- 1.47% ID/g for (111)In-10 at 4 h pi. Complex 6 showed high melanoma uptake and lower kidney uptake than the corresponding Lys(11) analogues, supporting 6 for further investigations as a potential therapeutic radiopharmaceutical.