Modification of the Structure of a Metallopeptide:  Synthesis and Biological Evaluation of ¹¹¹In-Labeled DOTA-Conjugated Rhenium-Cyclized α-MSH Analogues

Abstract

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.

64Cu-Labeled Alpha-Melanocyte-Stimulating Hormone Analog for MicroPET Imaging of Melanocortin 1 Receptor Expression

“…Several α-MSH analogs which can recognize the MC1R have been labeled with different radionuclides for melanoma detection and radiotherapy (9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(26)(27)(28). The most promising melanoma-targeting peptides are linear peptide NAPamide (16) and rhenium-cyclized CCMSH peptides (9,11,15).…”

“…The most promising melanoma-targeting peptides are linear peptide NAPamide (16) and rhenium-cyclized CCMSH peptides (9,11,15). NAPamide is a short peptide with only eight amino acid residues.…”

“…Better chelation systems for 64 Cu such as TETA (1,4,8,11tetraazacyclotetradecane-N,N',N",N"'-tetraacetic acid) or cross-bridged cyclam ligands may improve the metal-chelate stability and subsequently improving the biodistribution (34)(35). Similar to other radiometal-labeled peptides (9,11,17,21,30), high renal activity accumulation and retention of 64 Cu-DOTA-NAPamide is also observed in both tumor models ( Figure 3A and B). This could possibly be decreased by the use of L-or D-lysine without affecting tumor attention (9,15) in order to achieve a better kidney to normal tissue ratio and reduce radiation dose to the tested subject.…”

203Pb-Labeled α-Melanocyte–Stimulating Hormone Peptide as an Imaging Probe for Melanoma Detection

“…However, 90 Y-and 111 Inlabeled monoclonal antibody and peptide showed differences in their biologic properties (8,9,18), which raised some concerns about the validity of using 111 In-labeled conjugates as imaging surrogates for their 90 Y-labeled conjugates. The atomic radius of 90 Y fits nearly perfectly into the cavity of DOTA, whereas 111 In has a smaller atomic radius than that of 90 Y.…”

“…However, in the cases mentioned earlier, similar but not chemically identical radiometals were used. Even though the radiometals may have similar coordination chemistries, small differences among radionuclides can result in different pharmacokinetics (8,9). Therefore, it is desirable to radiolabel the targeting com-pound with 2 chemically identical radioisotopes to extract the maximum benefit from the matched-pair approach to radiopharmaceutical design.…”