Melanocortin 1 Receptor Targeted Imaging of Melanoma With Gold Nanocages and Positron Emission Tomography

Abstract

Purpose: Melanoma is a lethal skin cancer with unmet clinical needs for targeted imaging and therapy. Nanoscale materials conjugated with targeting components have shown great potential to improve tumor delivery efficiency while minimizing undesirable side effects in vivo. Herein, we proposed to develop targeted nanoparticles for melanoma theranostics.

Method: In this work, gold nanocages (AuNCs) were conjugated with α-melanocyte-stimulating hormone (α-MSH) peptide and radiolabeled with ⁶⁴Cu for melanocortin 1 receptor-(MC1R) targeted positron emission tomography (PET) in a mouse B16/F10 melanoma model.

Results: Their controlled synthesis and surface chemistry enabled well-defined structure and radiolabeling efficiency. In vivo pharmacokinetic evaluation demonstrated comparable organ distribution between the targeted and nontargeted AuNCs. However, micro-PET/computed tomography (CT) imaging demonstrated specific and improved tumor accumulation via MC1R-mediated delivery. By increasing the coverage density of α-MSH peptide on AuNCs, the tumor delivery efficiency was improved.

Conclusion: The controlled synthesis, sensitive PET imaging, and optimal tumor targeting suggested the potential of targeted AuNCs for melanoma theranostics.

Keywords: gold nanocage; melanocortin 1 receptor; melanoma; positron emission tomography; α-melanocyte-stimulating hormone.

Figure 1.

Scheme for the preparation of AuNC-PEG-MSH.

Figure 2.

Characterization of gold nanocages. A, Transmission electron microscopy image of AuNC-PEG-MSH. B, UV-Vis spectra of unmodified gold nanocages (AuNCs), AuNCs-PEG, and AuNCs-PEG-MSH showing similar LSPR peaks. C, Plots of temperatures as a function of irradiation time for suspension of H₂O, AuNCs, AuNCs-PEG, AuNCs-PEG-MSH. The concentration (0.025 nmol/L) of each suspension had the same maximum extinction intensity. The laser power density was 1.1 W/cm³. D, Instant radio-thin layer chromatogram (iTLC) of purified ⁶⁴Cu-AuNCs-PEG-MSH.

Figure 3.

In vivo biodistribution of ⁶⁴Cu-AuNCs-PEG-MSH and ⁶⁴Cu-AuNCs-PEG in wild-type C57BL/6 mice at 4 (top panel) and 24 hours (bottom panel) (n = 4/group) postintravenous injection.

Figure 4.

A, Representative positron emission tomography/computed tomography (PET/CT) images of ⁶⁴Cu-AuNCs-PEG-MSH, ⁶⁴Cu-AuNCs-PEG, and competitive receptor blocking in mice bearing B16/F10 melanoma at 24 hours postinjection. The blocking study was carried by coinjection of nonradiolabeled AuNCs-PEG-MSH in excess (molar ratio of AuNCs-PEG-MSH to ⁶⁴Cu-AuNCs-PEG-MSH at 40:1; yellow Arrow T, Tumor). B, Quantitative tumor uptakes of ⁶⁴Cu-AuNCs-PEG-MSH, ⁶⁴Cu-AuNCs-PEG, and blocking studies at 24 hours. (n = 4/group). C, Tumor-to-muscle ratio of ⁶⁴Cu-AuNCs-PEG-MSH and ⁶⁴Cu-AuNCs-PEG at 24 hours (n = 4 / group). *P < .05, **P < .001.

Figure 5.

A, Representative positron emission tomography/computed tomography (PET/CT) images of ⁶⁴Cu-AuNCs-PEG-MSH with high loading of α-melanocyte-stimulating hormone (α-MSH) peptide and competitive receptor blocking studies in mice-bearing B16/F10 melanoma at 24 and 48 hours postinjection. The competitive blocking studies were carried with coinjection of non-radiolabeled AuNCs-PEG-MSH conjugated with a high loading of α-MSH peptide in excess amount (molar ratio of AuNCs-PEG-MSH: ⁶⁴Cu-AuNCs-PEG-MSH = 40:1; T, tumor). B, Quantitative tumor uptake analysis at 24 and 48 hours (n = 4/group). C, Tumor-to-muscle ratio of ⁶⁴Cu-AuNCs-PEG-MSH with high α-MSH peptide at 24 and 48 hours (n = 4/group). *P < .005, **P < .001.

Figure 6.

Hematoxylin and eosin (H&E) staining of B16/F10 tumor tissue showing large and polynucleated tumor cells. Immunohistochemical staining of MC1R receptor in B16/F10 tumor. Tumor cells are stained with MC1R (blue) and counterstained with nuclear fast red (pink). The brown color indicates melanin expression (marked by arrows). All panels are at ×400.