Melanoma targeting with [99mTc(N)(PNP3)]-labeled α-melanocyte stimulating hormone peptide analogs: Effects of cyclization on the radiopharmaceutical properties

The purpose of this study was to evaluate the effect of cyclization on the biological profile of a [99mTc(N)(PNP3)]-labeled α-melanocyte stimulating hormone peptide analog. A lactam bridge-cyclized H-Cys-Ahx-βAla3-c[Lys4-Glu-His-D-Phe-Arg-Trp-Glu10]-Arg11-Pro-Val-NH2 (NAP-NS2) and the corresponding linear H-Cys-Ahx-βAla-Nle-Asp-His-D-Phe-Arg-Trp-Gly-NH2 (NAP-NS1) peptide were synthetized, characterized by ESI-MS spectroscopy and their melanocortin-1 receptor (MC1R) binding affinity was determined in B16/F10 melanoma cells. The consistent [99mTc(N)(PNP3)]-labeled compounds were readily obtained in high specific activity and their stability and biological properties were assessed. As an example, the chemical identity of [99mTc(N)(NAP-NS1)(PNP3)]+ was confirmed by carrier added experiments supported by radio/UV HPLC analysis combined with ESI(+)-MS. Compared with the linear peptide, cyclization negatively affected the biological properties of NAP-NS2 peptide by reducing its binding affinity for MC1R and by decreasing the overall excretion rate of the corresponding [99mTc(N)(PNP3)]-labeled peptide from the body as well as its in vivo stability. [99mTc(N)(NAP-NS1)(PNP3)]+ was evaluated for its potential as melanoma imaging probe in murine melanoma model. Data from in vitro and in vivo studies on B16/F10 melanoma model of [99mTc(N)(NAP-NS1)(PNP3)]+ clearly evidenced that the radiolabeled linear peptide keeps its biological properties up on the conjugation to the [99mTc(N)(PNP3)]-building block. The progressive increase of the tumor-to-nontarget ratios over the time indicates a quite stable interaction between the radio-complex and the MC1R.