Melanoma Imaging Using ¹⁸F‑Labeled α‑Melanocyte-Stimulating Hormone Derivatives with Positron Emission Tomography

Melanocortin 1 receptor (MC1R) is specifically expressed in the majority of melanomas, a leading cause of death related to skin cancers. Accurate staging and early detection is crucial in managing melanoma. Based on the α-melanocyte-stimulating hormone (αMSH) sequence, MC1R-targeted peptides have been studied for melanoma imaging, predominately for use with single-photon emission computed tomography, with few attempts made for positron emission tomography (PET). ¹⁸F is a commonly used PET isotope due to readily available cyclotron production, pure positron emission, and a favorable half-life (109.8 min). In this study, we aim to design and evaluate αMSH derivatives that enable radiolabeling with ¹⁸F for PET imaging of melanoma. We synthesized three imaging probes based on the structure of Nle⁴-cyclo­[Asp⁵-His-d-Phe⁷-Arg-Trp-Lys¹⁰]-NH₂ (Nle-CycMSH_hex), with a Pip linker (CCZ01064), an Acp linker (CCZ01070), or an Aoc linker (CCZ01071). ¹⁸F labeling was enabled by an ammoniomethyl-trifluoroborate (AmBF₃) moiety. <i>In vitro</i> competition binding assays showed subnanomolar inhibition constant (<i>K</i>_i) values for all three peptides. The ¹⁸F radiolabeling was performed via a one-step ¹⁸F–¹⁹F isotope exchange reaction that resulted in high radiochemical purity (>95%) and good molar activity (specific activity) ranging from 40.7 to 66.6 MBq/nmol. All three ¹⁸F-labeled peptides produced excellent tumor visualization with PET imaging in C57BL/6J mice bearing B16-F10 tumors. The tumor uptake was 7.80 ± 1.77, 5.27 ± 2.38, and 5.46 ± 2.64% injected dose per gram of tissue (%ID/g) for [¹⁸F]­CCZ01064, [¹⁸F]­CCZ01070, and [¹⁸F]­CCZ01071 at 1 h post-injection (p.i.), respectively. Minimal background activity was observed except for kidneys at 4.99 ± 0.20, 4.42 ± 0.54, and 13.55 ± 2.84%ID/g, respectively. The best candidate [¹⁸F]­CCZ01064 was further evaluated at 2 h p.i., which showed increased tumor uptake at 11.96 ± 2.31%ID/g and further reduced normal tissue uptake. Moreover, a blocking study was performed for CCZ01064 at 1 h p.i., where tumor uptake was significantly reduced to 1.97 ± 0.60%ID/g, suggesting the tumor uptake was receptor mediated. In conclusion, [¹⁸F]­CCZ01064 showed high tumor uptake, low normal tissue uptake, and fast clearance and is therefore a suitable and promising candidate for PET imaging of melanoma

Imaging Bradykinin B1 Receptor with ⁶⁸Ga-Labeled [des-Arg¹⁰]Kallidin Derivatives: Effect of the Linker on Biodistribution and Tumor Uptake

Bradykinin B1 receptor (B1R) that is overexpressed in cancers but minimally expressed in normal healthy tissues represents an attractive biomarker for the development of cancer imaging agents. The goal of this study was to evaluate the effect of different linkers on the pharmacokinetics and tumor uptake of a B1R-targeting radio-peptide sequence, ⁶⁸Ga-DOTA-linker-Lys-Arg-Pro-Hyp-Gly-Cha-Ser-Pro-Leu. Four peptides, SH01078, P03034, P04115, and P04168, with 6-aminohexanoic acid, 9-amino-4,7-dioxanonanoic acid, Gly-Gly, and 4-amino-(1-carboxymethyl)­piperidine, respectively, as the linker were synthesized and evaluated. In vitro competition binding assays showed that the <i>K</i>_i values of SH01078, P03034, P04115, and P04168 were 27.8 ± 4.9, 16.0 ± 1.9, 11.4 ± 2.5, and 3.6 ± 0.2 nM, respectively. Imaging and biodistribution studies were performed in mice bearing both B1R-positive HEK293T::hB1R and B1R-negative HEK293T tumors. All tracers showed mainly renal excretion with excellent tumor visualization and minimal background activity except for kidneys and bladder. The average uptake of ⁶⁸Ga-labeled SH01078, P03034, and P04115 in HEK293T::hB1R tumor was similar (1.96–2.17%ID/g) at 1 h postinjection. ⁶⁸Ga-P04168 generated higher HEK293T::hB1R tumor uptake (4.15 ± 1.13%ID/g) and lower background activity, leading to a >2-fold improvement in HEK293T::hB1R tumor-to-background (HEK293T tumor, blood, muscle, and liver) contrasts over those of ⁶⁸Ga-labeled SH01078, P03034, and P04115. Our results indicate that the choice of linker affects binding affinity, pharmacokinetics, and tumor targeting. The use of the cationic 4-amino-(1-carboxymethyl)­piperidine linker improved tumor visualization, and the resulting ⁶⁸Ga-P04168 might be promising for clinical application for imaging B1R-expressing tumors with positron emission tomography

<i>p</i>‑NO₂–Bn–H₄neunpa and H₄neunpa–Trastuzumab: Bifunctional Chelator for Radiometalpharmaceuticals and ¹¹¹In Immuno-Single Photon Emission Computed Tomography Imaging

Potentially nonadentate (N₅O₄) bifunctional chelator <i>p</i>-SCN–Bn–H₄neunpa and its immunoconjugate H₄neunpa–trastuzumab for ¹¹¹In radiolabeling are synthesized. The ability of <i>p</i>-SCN–Bn–H₄neunpa and H₄neunpa–trastuzumab to quantitatively radiolabel ¹¹¹InCl₃ at an ambient temperature within 15 or 30 min, respectively, is presented. Thermodynamic stability determination with In³⁺, Bi³⁺, and La³⁺ resulted in high conditional stability constant (<i>p</i>M) values. In vitro human serum stability assays have demonstrated both ¹¹¹In complexes to have high stability over 5 days. Mouse biodistribution of [¹¹¹In]­[In­(<i>p</i>-NO₂–Bn–neunpa)]⁻, compared to that of [¹¹¹In]­[In­(<i>p</i>-NH₂–Bn–CHX-A″–diethylenetriamine pentaacetic acid (DTPA))]^2–, at 1, 4, and 24 h shows fast clearance of both complexes from the mice within 24 h. In a second mouse biodistribution study, the immunoconjugates ¹¹¹In-neunpa–trastuzumab and ¹¹¹In–CHX-A″–DTPA–trastuzumab demonstrate a similar distribution profile but with slightly lower tumor uptake of ¹¹¹In-neunpa–trastuzumab compared to that of ¹¹¹In–CHX-A″–DTPA–trastuzumab. These results were also confirmed by immuno-single photon emission computed tomography (immuno-SPECT) imaging in vivo. These initial investigations reveal the acyclic bifunctional chelator <i>p</i>-SCN–Bn–H₄neunpa to be a promising chelator for ¹¹¹In (and other radiometals) with high in vitro stability and also show H₄neunpa–trastuzumab to be an excellent ¹¹¹In chelator with promising biodistribution in mice