Molecular Imaging and Radionuclide Therapy of Melanoma Targeting the Melanocortin 1 Receptor

Diagnosis of Melanoma with 61Cu-Labeled PET Tracer

Until the recent years, substances containing radioactive 61Cu were strongly considered as potential positron-emitting radiopharmaceuticals for use in positron emission tomography (PET) applications; however, due to their suitably long half-life, and generator-independent and cost-effective production, they seem to be economically viable for human imaging. Since malignant melanoma (MM) is a major public health problem, its early diagnosis is a crucial contributor to long-term survival, which can be achieved using radiolabeled α-melanocyte-stimulating hormone analog NAPamide derivatives. Here, we report on the physicochemical features of a new CB-15aneN5-based Cu(II) complex ([Cu(KFTGdiac)]-) and the ex vivo and in vivo characterization of its NAPamide conjugate. The rigid chelate possesses prompt complex formation and suitable inertness (t1/2 = 18.4 min in 5.0 M HCl at 50 °C), as well as excellent features in the diagnosis of B16-F10 melanoma tumors (T/M(SUVs) (in vivo): 12.7, %ID/g: 6.6 ± 0.3, T/M (ex vivo): 22).

Preclinical evaluation of MC1R targeting theranostic pair [155Tb]Tb-crown-αMSH and [161Tb]Tb-crown-αMSH

BACKGROUND

Targeted radionuclide therapy is established as a highly effective strategy for the treatment of metastatic tumors; however, the co-development of suitable imaging companions to therapy remains significant challenge. Theranostic isotopes of terbium (149Tb, 152Tb, 155Tb, 161Tb) have the potential to provide chemically identical radionuclidic pairs, which collectively encompass all modes of nuclear decay relevant to nuclear medicine. Herein, we report the first radiochemistry and preclinical studies involving 155Tb- and 161Tb-labeled crown-αMSH, a small peptide-based bioconjugate suitable for targeting melanoma.

METHODS

155Tb was produced via proton induced spallation of Ta targets using the isotope separation and acceleration facility at TRIUMF with isotope separation on-line (ISAC/ISOL). The radiolabeling characteristics of crown-αMSH with 155Tb and/or 161Tb were evaluated by concentration-dependence radiolabeling studies, and radio-HPLC stability studies. LogD7.4 measurements were obtained for [161Tb]Tb-crown-αMSH. Competitive binding assays were undertaken to determine the inhibition constant for [natTb]Tb-crown-αMSH in B16-F10 cells. Pre-clinical biodistribution and SPECT/CT imaging studies of 155Tb and 161Tb labeled crown-αMSH were undertaken in male C57Bl/6 J mice bearing B16-F10 melanoma tumors to evaluate tumor specific uptake and imaging potential for each radionuclide.

RESULTS

Quantitative radiolabeling of crown-αMSH with [155Tb]Tb3+ and [161Tb]Tb3+ was demonstrated under mild conditions (RT, 10 min) and low chelator concentrations; achieving high molar activities (23-29 MBq/nmol). Radio-HPLC studies showed [161Tb]Tb-crown-αMSH maintains excellent radiochemical purity in human serum, while gradual metabolic degradation is observed in mouse serum. Competitive binding assays showed the high affinity of [natTb]Tb-crown-αMSH toward MC1R. Two different methods for preparation of the [155Tb]Tb-crown-αMSH radiotracer were investigated and the impacts on the biodistribution profile in tumor bearing mice is compared. Preclinical in vivo studies of 155Tb- and 161Tb- labeled crown-αMSH were performed in parallel, in mice bearing B16-F10 tumors; where the biodistribution results showed similar tumor specific uptake (6.06-7.44 %IA/g at 2 h pi) and very low uptake in nontarget organs. These results were further corroborated through a series of single-photon emission computed tomography (SPECT) studies, with [155Tb]Tb-crown-αMSH and [161Tb]Tb-crown-αMSH showing comparable uptake profiles and excellent image contrast.

CONCLUSIONS

Collectively, our studies highlight the promising characteristics of [155Tb]Tb-crown-αMSH and [161Tb]Tb-crown-αMSH as theranostic pair for nuclear imaging (155Tb) and radionuclide therapy (161Tb).

5-Hydroxypyrroloindoline Affords Tryptathionine and 2,2'-bis-Indole Peptide Staples: Application to Melanotan-II

With peptides increasingly favored as drugs, natural product motifs, namely the tryptathionine staple, found in amatoxins and phallotoxins, and the 2,2'-bis-indole found in staurosporine represent unexplored staples for unnatural peptide macrocycles. We disclose the efficient condensation of a 5-hydroxypyrroloindoline with either a cysteine-thiol or a tryptophan-indole to form a tryptathionine or 2-2'-bis-indole staple. Judicious use of protecting groups provides for chemoselective stapling using α-MSH, which provides a basis for investigating both chemoselectivity and affinity. Both classes of stapled peptides show nanomolar Ki's, with one showing a sub-nanomolar Ki value.

Tuning the Softness of the Pendant Arms and the Polyazamacrocyclic Backbone to Chelate the 203Pb/212Pb Theranostic Pair

A series of macrocyclic ligands were considered for the chelation of Pb2+: 1,4,7,10-tetrakis[2-(methylsulfanyl)ethyl]-1,4,7,10-tetraazacyclododecane (DO4S), 1,4,7-tris[2-(methylsulfanyl)ethyl]-1,4,7,10-tetraazacyclododecane (DO3S), 1,4,7-tris[2-(methylsulfanyl)ethyl]-10-acetamido-1,4,7,10-tetraazacyclododecane (DO3SAm), 1,7-bis[2-(methylsulfanyl)ethyl]-1,4,7,10-tetraazacyclododecane-4,10-diacetic acid (DO2A2S), 1,5,9-tris[2-(methylsulfanyl)ethyl]-1,5,9-triazacyclododecane (TACD3S), 1,4,7,10-tetrakis[2-(methylsulfanyl)ethyl]-1,4,7,10-tetrazacyclotridecane (TRI4S), and 1,4,8,11-tetrakis[2-(methylsulfanyl)ethyl]-1,4,8,11-tetrazacyclotetradecane (TE4S). The equilibrium, the acid-mediated dissociation kinetics, and the structural properties of the Pb2+ complexes formed by these chelators were examined by UV-Visible and nuclear magnetic resonance (NMR) spectroscopies, combined with potentiometry and density functional theory (DFT) calculations. The obtained results indicated that DO4S, DO3S, DO3SAm, and DO2A2S were able to efficiently chelate Pb2+ and that the most suitable macrocyclic scaffold for Pb2+ is 1,4,7,10-tetrazacyclododecane. NMR spectroscopy gave insights into the solution structures of the Pb2+ complexes, and 1H-207Pb interactions confirmed the involvement of S and/or O donors in the metal coordination sphere. Highly fluxional solution behavior was discovered when Pb2+ was coordinated to symmetric ligands (i.e., DO4S and DO2A2S) while the introduction of structural asymmetry in DO3S and DO3SAm slowed down the intramolecular dynamics. The ligand ability to chelate [203Pb]Pb2+ under highly dilute reaction conditions was explored through radiolabeling experiments. While DO4S and DO3S possessed modest performance, DO3SAm and DO2A2S demonstrated high complexation efficiency under mild reaction conditions (pH = 7, 5 min reaction time). The [203Pb]Pb2+ complexes' integrity in human serum over 24 h was appreciably good for [203Pb][Pb(DO4S)]2+ (80 ± 5%) and excellent for [203Pb][Pb(DO3SAm)]2+ (93 ± 1%) and [203Pb][Pb(DO2A2S)] (94 ± 1%). These results reveal the promise of DO2A2S and DO3SAm as chelators in cutting-edge theranostic [203/212Pb]Pb2+ radiopharmaceuticals.

61Cu-Labelled radiodiagnostics of melanoma with NAPamide-targeted radiopharmaceutical

Malignant melanoma is a major public health problem with an increasing incidence and mortality in the Caucasian population due to its significant metastatic potential. The early detection of this cancer type by imaging techniques like positron emission tomography acts as an important contributor to the long-term survival. Based on literature data, the radio labelled alpha-MSH analog NAPamide molecule is an appropriate diagnostic tool for the detection of melanoma tumors. Inspired by these facts, a new radiotracer, the [⁶¹Cu]Cu-KFTG-NAPamide has been synthesized to exploit the beneficial features of the positron emitter ⁶¹Cu and the melanoma specificity of the NAPamide molecule. In this work, we report a new member of the CB-15aneN₅ ligand family (KFTG) as the chelator for ⁶¹Cu(II) complexation. On the basis of the thorough physico-chemical characterization, the rigid [Cu(KFTG)]⁺ complex exhibits fast complex formation (t_1/2 = 155 s at pH 5.0 and 25 °C) and high inertness (t_1/2 = 2.0 h in 5.0 M HCl at 50 °C) as well as moderate superoxide dismutase activity (IC₅₀ = 2.3 μM). Furthermore, the [⁶¹Cu]Cu-KFTG-NAPamide possesses outstanding features in the diagnostics of B16-F10 melanoma tumors by PET imaging: (T/M(SUVs) (in vivo): appr. 14, %ID/g: 7 ± 1 and T/M (ex vivo): 315 ± 24 at 180 min).

Preparation, biological characterization and preliminary human imaging studies of 68Ga-DOTA-IBA

Purpose

In this study, DOTA-IBA was radiolabeled with ⁶⁸Ga and we determined the optimum labelling conditions and assessed the biological properties of ⁶⁸Ga-DOTA-IBA. We investigated the biodistribution of ⁶⁸Ga-DOTA-IBA in normal animals and undertook PET/CT imaging in humans. Finally, we explored the feasibility ⁶⁸Ga-DOTA-IBA as a bone imaging agent and demonstrated its potential for the therapeutic release of ¹⁷⁷Lu/²²⁵Ac-DOTA-IBA.

Methods

The controlled variables method was used to assess the impact of variables on the radiochemical purity of ⁶⁸Ga-DOTA-IBA. The biological properties of ⁶⁸Ga-DOTA-IBA were investigated.⁶⁸Ga-DOTA-IBA micro-PET/CT imaging was performed on animals. Volunteers were recruited for ⁶⁸Ga-DOTA-IBA imaging and data were compared to ^99mTc-MDP imaging studies to calculate the target to non-target ratio (T/NT) of the lesions.

Results

The prepared ⁶⁸Ga-DOTA-IBA had a radiochemical purity of >97% and demonstrated good biological properties with a good safety profile in normal mice. PET/CT imaging of the animals showed rapid blood clearance with high contrast between the bone and stroma. Human imaging showed that ⁶⁸Ga-DOTA-IBA could detect more lesions compared to ^99mTc-MDP and had a higher targeted to untargeted ratio.

Conclusions

⁶⁸Ga-DOTA-IBA is an osteophilic radiopharmaceutical that can be synthesized using a simple labelling method. ⁶⁸Ga-DOTA-IBA has high radiochemical purity and is stable in vitro stability. It is rapidly cleared from the blood, has low toxicity and has strong targeting to the bone with long retention times. We also found that it is rapidly cleared in non-target tissues and has high contrast on whole-body bone imaging. ⁶⁸Ga-DOTA-IBA PET/CT has potential as a novel bone imaging bone modality in patients with metastatic disease.

Late-Stage Functionalization with Cysteine Staples Generates Potent and Selective Melanocortin Receptor-1 Agonists

In this work, cysteine staples were used as a late-stage functionalization strategy to diversify peptides and build conjugates targeting the melanocortin G-protein-coupled receptors [melanocortin receptor-1 (MC1R) and MC3R-MC5R]. Monocyclic and bicyclic agonists based on sunflower trypsin inhibitor-1 were used to generate a selection of stapled peptides that were evaluated for binding (pK_i) and functional activation (pEC₅₀) of the melanocortin receptor subtypes. Stapled peptides generally had improved activity, with aromatic stapled peptides yielding selective MC1R agonists, including a xylene-stapled peptide (2) with an EC₅₀ of 1.9 nM for MC1R and >150-fold selectivity for MC3R and MC4R. Selected stapled peptides were further functionalized with linkers and payloads, generating a series of conjugated peptides with potent MC1R activity, including one pyridazine-functionalized peptide (21) with picomolar activity at MC1R (K_i 58 pM; EC₅₀ < 9 pM). This work demonstrates that staples can be used as modular synthetic tools to tune potency and selectivity in peptide-based drug design.

Melanocortin 1 Receptor Agonists Based on a Bivalent, Bicyclic Peptide Framework

We have designed a new class of highly potent bivalent melanocortin receptor ligands based on the nature-derived bicyclic peptide sunflower trypsin inhibitor 1 (SFTI-1). Incorporation of melanotropin pharmacophores in each of the two turn regions of SFTI-1 resulted in substantial gains in agonist activity particularly at human melanocortin receptors 1 and 3 (hMC1R/hMC3R) compared to monovalent analogues. In in vitro binding and functional assays, the most potent molecule, compound 6, displayed low picomolar agonist activity at hMC1R (pEC₅₀ > 10.3; EC₅₀ < 50 pM; pK_i: 10.16 ± 0.04; K_i: 69 ± 5 pM) and is at least 30-fold more selective for this receptor than for hMC3R, hMC4R, or hMC5R. The results are discussed in the context of structural homology models of hMCRs in complex with the developed bivalent ligands.

Selective Cyclized α-Melanocyte-Stimulating Hormone Derivative with Multiple N-Methylations for Melanoma Imaging with Positron Emission Tomography

In this study, we designed and evaluated a novel α-melanocyte-stimulating hormone derivative with four N-methylations for melanocortin 1 receptor-targeted melanoma imaging with positron emission tomography (PET). The resulting peptide, DOTA-Pip-Nle⁴-Cyclo[Asp⁵-N-Me-His⁶-d-Phe⁷-N-Me-Arg⁸-N-Me-Trp⁹-N-Me-Lys¹⁰]αMSH_4-10-NH₂ (CCZ01099), showed high receptor selectivity, greatly improved stability, and rapid internalization. [⁶⁸Ga]Ga-CCZ01099 showed clear tumor visualization and excellent tumor-to-normal tissue contrast with PET imaging in a preclinical melanoma model. Therefore, CCZ01099 is a promising compound for imaging and potentially radioligand therapy for melanoma.

18F-Labeled Cyclized α-Melanocyte-Stimulating Hormone Derivatives for Imaging Human Melanoma Xenograft with Positron Emission Tomography

Since metastatic melanoma is deadly, early diagnosis thereof is crucial for managing the disease. We recently developed α-melanocyte-stimulating hormone (αMSH) derivatives, [⁶⁸Ga]Ga-CCZ01048 and [¹⁸F]CCZ01064, that target the melanocortin 1 receptor (MC1R) for mouse melanoma imaging. In this study, we aim to evaluate [¹⁸F]CCZ01064 as well as a novel dual-ammoniomethyl-trifluoroborate (AmBF₃) derivative, [¹⁸F]CCZ01096, for targeting human melanoma xenograft using μPET imaging. The peptides were synthesized on solid phase using Fmoc chemistry. Radiolabeling was achieved in a one-step ¹⁸F-¹⁹F isotope-exchange reaction. μPET imaging and biodistribution studies were performed in NSG mice bearing SK-MEL-1 melanoma xenografts. The MC1R density on the SK-MEL-1 cell line was determined to be 972 ± 154 receptors/cell (n = 4) via saturation assays. Using [¹⁸F]CCZ01064, moderate tumor uptake (3.05 ± 0.47%ID/g) and image contrast were observed at 2 h post-injection. Molar activity was determined to play a key role. CCZ01096 with two AmBF₃ motifs showed comparable sub-nanomolar binding affinity to MC1R and much higher molar activity. This resulted in improved tumor uptake (6.46 ± 1.42%ID/g) and image contrast (tumor-to-blood and tumor-to-muscle ratios were 30.6 ± 5.7 and 85.7 ± 11.3, respectively) at 2 h post-injection. [¹⁸F]CCZ01096 represents a promising αMSH-based μPET imaging agent for human melanoma and warrants further investigation for potential clinical translation.

Radiochemical and radiopharmacological characterization of a 64 Cu-labeled α-MSH analog conjugated with different chelators

Radiolabeled α-melanocyte-stimulating hormone (α-MSH) derivatives have a high potential for diagnosis and treatment of melanoma, because of high specificity and binding affinity to the melanocortin-1 receptor (MC1R). Hence, the α-MSH-derived peptide NAP-NS1 with a β-Ala linker (ε-Ahx-β-Ala-Nle-Asp-His-D-Phe-Arg-Trp-Gly-NH₂ ) was conjugated to different chelators: either to NOTA (p-SCN-Bn-1,4,7-triazacyclononane-1,4,7-triacetic acid), to a hexadentate bispidine carbonate derivative (dimethyl-9-(((4-nitrophenoxy)carbonyl)oxy)-2,4-di(pyridin-2-yl)-3,7-bis(pyridin-2-ylmethyl)-3,7-diazabicyclo[3.3.1]nonane-1,5-dicarboxylate), or to DMPTACN (p-SCN-Ph-bis(2-pyridyl-methyl)-1,4,7-triaza-cyclononane), labeled with ⁶⁴ Cu, and investigated in terms of radiochemical and radiopharmacological properties. For the three ⁶⁴ Cu-labeled conjugates negligible transchelation, suitable buffer and serum stability, as well as appropriate water solubility, was determined. The three conjugates exhibited high binding affinity (low nanomolar range) in murine B16F10, human MeWo, and human TXM13 cells. The B_max values of [⁶⁴ Cu]Cu-bispidine-NAP-NS1 ([⁶⁴ Cu]Cu-2) and [⁶⁴ Cu]Cu-DMPTACN-NAP-NS1 ([⁶⁴ Cu]Cu-3) were higher than those of [⁶⁴ Cu]Cu-NOTA-NAP-NS1 ([⁶⁴ Cu]Cu-1), implying that different charged chelate units might have an impact on binding capacity. Preliminary in vivo biodistribution studies suggested the main excretion pathway of [⁶⁴ Cu]Cu-1 and [⁶⁴ Cu]Cu-3 to be renal, while that of [⁶⁴ Cu]Cu-2 seemed to be both renal and hepatobiliary. An initial moderate uptake in the kidney decreased clearly after 60 minutes. All three ⁶⁴ Cu-labeled conjugates should be considered for further in vivo investigations using a suitable xenograft mouse model.

The pharmacological properties of 3-arm or 4-arm DOTA constructs for conjugation to α-melanocyte-stimulating hormone analogues for melanoma imaging

Background

Although a 3-arm DOTA construct, which has three carboxylic acids, h has been applied for conjugation to many peptides, we investigated if a 4-arm DOTA construct conjugated to peptides improves chemical properties for melanoma imaging of the melanocortin 1 receptor compared to 3-arm DOTA-conjugated peptides.

Methods

Specific activities, radiolabeling efficiencies, and partition coefficients were evaluated using ¹¹¹In-labeled 3-arm and 4-arm DOTA-α-melanocyte-stimulating hormone (MSH). For assessment of MC1-R affinity and accumulation in tumor cells in vitro, B16-F1 melanoma and/or 4T1 breast cancer cells were incubated with ¹¹¹In-labeled 3-arm and 4-arm DOTA-α-MSH with and without α-MSH as a substrate. The stability was evaluated using mouse liver homogenates and plasma. Biological distribution and whole-body single photon emission computed tomography imaging of ¹¹¹In-labeled 3-arm and 4-arm DOTA-α-MSH were obtained using B16-F1 melanoma-bearing mice.

Results

Specific activities and radiolabeling efficiencies of both radiotracers were about 1.2 MBq/nM and 90–95%, respectively. The partition coefficients were −0.28 ± 0.03 for ¹¹¹In-labeled 3-arm DOTA-α-MSH and −0.13 ± 0.04 for ¹¹¹In-labeled 4-arm DOTA-α-MSH. Although accumulation was significantly inhibited by α-MSH in B16-F1 cells, the inhibition rate of ¹¹¹In-labeled 4-arm DOTA-α-MSH was lower than that of ¹¹¹In-labeled 3-arm DOTA-α-MSH. ¹¹¹In-labeled 4-arm DOTA-α-MSH was taken up early into B16-F1 cells and showed higher accumulation than ¹¹¹In-labeled 3-arm DOTA-α-MSH after 10 min of incubation. Although these stabilities were relatively high, the stability of ¹¹¹In-labeled 4-arm DOTA-α-MSH was higher than that of ¹¹¹In-labeled 3-arm DOTA-α-MSH. Regarding biological distribution, ¹¹¹In-labeled 4-arm DOTA-α-MSH showed significantly lower average renal accumulation (1.38-fold) and significantly higher average melanoma accumulation (1.32-fold) than ¹¹¹In-labeled 3-arm DOTA-α-MSH at all acquisition times. ¹¹¹In-labeled 4-arm DOTA-α-MSH showed significantly higher melanoma-to-kidney, melanoma-to-blood, and melanoma-to-muscle ratios than ¹¹¹In-labeled 3-arm DOTA-α-MSH.

Conclusions

The 4-arm DOTA construct has better chemical properties for peptide radiotracers than the 3-arm DOTA construct.

Less Exploited GPCRs in Precision Medicine: Targets for Molecular Imaging and Theranostics

Precision medicine relies on individually tailored therapeutic intervention taking into account individual variability. It is strongly dependent on the availability of target-specific drugs and/or imaging agents that recognize molecular targets and patient-specific disease mechanisms. The most sensitive molecular imaging modalities, Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET), rely on the interaction between an imaging radioprobe and a target. Moreover, the use of target-specific molecular tools for both diagnostics and therapy, theranostic agents, represent an established methodology in nuclear medicine that is assuming an increasingly important role in precision medicine. The design of innovative imaging and/or theranostic agents is key for further accomplishments in the field. G-protein-coupled receptors (GPCRs), apart from being highly relevant drug targets, have also been largely exploited as molecular targets for non-invasive imaging and/or systemic radiotherapy of various diseases. Herein, we will discuss recent efforts towards the development of innovative imaging and/or theranostic agents targeting selected emergent GPCRs, namely the Frizzled receptor (FZD), Ghrelin receptor (GHSR-1a), G protein-coupled estrogen receptor (GPER), and Sphingosine-1-phosphate receptor (S1PR). The pharmacological and clinical relevance will be highlighted, giving particular attention to the studies on the synthesis and characterization of targeted molecular imaging agents, biological evaluation, and potential clinical applications in oncology and non-oncology diseases. Whenever relevant, supporting computational studies will be also discussed.

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. In vitro competition binding assays showed subnanomolar inhibition constant ( K_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.