Synthesis and biologic evaluation of ⁶⁴Cu-labeled rhenium-cyclized αS-MSH peptide analog using a cross-bridged cyclam chelator

Early detection of cutaneous melanoma is essential, as prognosis with metastatic melanoma is poor. Previous studies showed that ⁶⁴Cu-DOTA- ReCCMSH(Arg¹¹) (DOTA is 1,4,7,10-tetraazacyclododecane-N,N′, N″,N‴-tetraacetic acid), a cyclic analog of α-melanocyte- stimulating hormone (α-MSH), has the potential for the detection of malignant melanoma using PET. However, ⁶⁴Cu-DOTA-ReCCMSH(Arg ¹¹) demonstrated high background in nontarget tissues due to the in vivo instability of the Cu-DOTA moiety. CBTE2A (CBTE2A is 4,11- bis(carboxymethyl)-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane) has been shown to be a more stable copper chelate with improved in vivo stability, resulting in an improvement in clearance from nontarget tissues. The goal of this study was to conjugate CBTE2A to the α-MSH targeting ReCCMSH(Arg¹¹) peptide for labeling to ⁶⁴Cu and to investigate whether the increased metal-chelate stability with CBTE2A would improve imaging quality. Methods: The recyclized peptide CBTE2A-ReCCMSH(Arg¹¹) was synthesized using a solid-phase peptide synthesizer followed by rhenium cyclization. In vivo characteristics of ⁶⁴Cu-CBTE2A-ReCCMSH(Arg¹¹) were examined with small-animal PET and acute biodistribution studies in B16/F1 tumor-bearing mice. Results: Biodistribution studies showed high and rapid receptor-mediated tumor uptake with values similar to those reported for ⁶⁴Cu- and ⁸⁶Y-labeled DOTA-ReCCMSH(Arg¹¹). Nontarget organ concentration for ⁶⁴Cu-CBTE2A-ReCCMSH(Arg ¹¹) was considerably lower than that of the ⁶⁴Cu-DOTA analog, resulting in significantly higher tumor-to-nontarget tissue ratios. Compared with ⁸⁶Y-DOTA-ReCCMSH( Arg¹¹), ⁶⁴Cu-CBTE2A-ReCCMSH(Arg¹¹) demonstrated increased tumor retention and kidney clearance. Small-animal PET images showed that the tumor could be clearly visualized at all time points (0.5-24 h). Conclusion: Our data suggest the superior stability of the ⁶⁴Cu-CBTE2A moiety compared with ⁶⁴Cu-DOTA, making ⁶⁴Cu-CBTE2A-ReCCMSH(Arg ¹¹) an ideal candidate for the PET of malignant melanoma.