PET Imaging of Melanoma Using Melanin-Targeted Probe

Synthesis and Preclinical Evaluation of a 68Ga-Labeled Pyridine-Based Benzamide Dimer for Malignant Melanoma Imaging

Benzamide (BZA), a small molecule that can freely cross cell membranes and bind to melanin, has served as an effective targeting group for melanoma theranostics. In this study, a novel pyridine-based BZA dimer (denoted as H-2) was labeled with ⁶⁸Ga ([⁶⁸Ga]Ga-H-2) for positron emission tomography (PET) imaging of malignant melanomas. [⁶⁸Ga]Ga-H-2 was obtained with high radiochemical yield (98.0 ± 2.0%) and satisfactory radiochemical purity (>95.0%). The specificity and affinity of [⁶⁸Ga]Ga-H-2 were confirmed in melanoma B16F10 cells and in vivo PET imaging of multiple tumor models (B16F10 tumors, A375 melanoma, and lung metastases). Monomeric [⁶⁸Ga]Ga-H-1 was prepared as a control radiotracer to verify the effects of the molecular structure on pharmacokinetics. The values of the lipid-water partition coefficient of [⁶⁸Ga]Ga-H-2 and [⁶⁸Ga]Ga-H-1 demonstrated hydrophilicity with log P = -2.37 ± 0.07 and -2.02 ± 0.09, respectively. PET imaging and biodistribution showed a higher uptake of [⁶⁸Ga]Ga-H-2 in B16F10 primary and metastatic melanomas than that in A375 melanomas. However, the relatively low uptake of monomeric [⁶⁸Ga]Ga-H-1 in B16F10 tumors and high accumulation in nontarget organs resulted in poor PET imaging quality. This study demonstrates the synthesis and preclinical evaluation of the novel pyridine-based BZA dimer [⁶⁸Ga]Ga-H-2 and indicates that the dimer tracer has promising applications in malignant melanoma-specific PET imaging because of its high uptake and long-time retention in malignant melanoma.

Translating Molecules into Imaging—The Development of New PET Tracers for Patients with Melanoma

Melanoma is a deadly disease that often exhibits relentless progression and can have both early and late metastases. Recent advances in immunotherapy and targeted therapy have dramatically increased patient survival for patients with melanoma. Similar advances in molecular targeted PET imaging can identify molecular pathways that promote disease progression and therefore offer physiological information. Thus, they can be used to assess prognosis, tumor heterogeneity, and identify instances of treatment failure. Numerous agents tested preclinically and clinically demonstrate promising results with high tumor-to-background ratios in both primary and metastatic melanoma tumors. Here, we detail the development and testing of multiple molecular targeted PET-imaging agents, including agents for general oncological imaging and those specifically for PET imaging of melanoma. Of the numerous radiopharmaceuticals evaluated for this purpose, several have made it to clinical trials and showed promising results. Ultimately, these agents may become the standard of care for melanoma imaging if they are able to demonstrate micrometastatic disease and thus provide more accurate information for staging. Furthermore, these agents provide a more accurate way to monitor response to therapy. Patients will be able to receive treatment based on tumor uptake characteristics and may be able to be treated earlier for lesions that with traditional imaging would be subclinical, overall leading to improved outcomes for patients.