Malignant melanoma is the most aggressive form of skin cancer. The early detection of primary melanoma tumors and metastases using non-invasive PET imaging determines the outcome of this disease. Previous studies have shown that benzamide derivatives (e.g. procainamide) conjugated with PET radionuclides specifically bind to melanin pigment of melanoma tumors. 68Ga chelating agents can have high influence on physiological properties of 68Ga labeled bioactive molecules, as was experienced during the application of HBED-CC on PSMA ligand. The aim of this study was to assess this concept in the case of the melanin specific procaindamide (PCA) and to compare the melanin specificity of 68Ga-labeled PCA using HBED-CC and NODAGA chelators under in vitro and in vivo conditions.
Procainamide (PCA) was conjugated with HBED-CC and NODAGA chelators and was labeled with Ga-68. The melanin specificity of 68Ga-HBED-CC-PCA and 68Ga-NODAGA-PCA was investigated in vitro and in vivo using amelanotic (MELUR and A375) and melanin containing (B16-F10) melanoma cell lines. Tumor-bearing mice were prepared by subcutaneous injection of B16-F10, MELUR and A375 melanoma cells into C57BL/6 and SCID mice. 21 ± 2 days after tumor cell inoculation and 90 min after intravenous injection of the 68Ga-labelledlabeled radiopharmacons whole body PET/MRI scans were performed.
68Ga-NODAGA-PCA and 68Ga-HBED-CC-PCA were produced with excellent radiochemical purity (98%). In vitro experiments demonstrated that after 30 and 90 min incubation time 68Ga-NODAGA-PCA uptake of B16-F10 cells was significantly (p ≤ 0.01) higher than the 68Ga-HBED-CC-conjugated PCA accumulation in the same cell line. Furthermore, significant difference (p ≤ 0.01 and 0.05) was found between the uptake of melanin negative and positive cell lines using 68Ga-NODAGA-PCA and 68Ga-HBED-CC-PCA. In vivo PET/MRI studies using tumor models revealed significantly (p ≤ 0.01) higher 68Ga-NODAGA-PCA uptake (SUVmean: 0.46 ± 0.05, SUVmax: 1.96 ± 0.25, T/M ratio: 40.7 ± 4.23) in B16-F10 tumors in contrast to 68Ga-HBED-CC-PCA where the SUVmean, SUVmax and T/M ratio were 0.13 ± 0.01, 0.56 ± 0.11 and 11.43 ± 1.24, respectively.
Melanin specific PCA conjugated with NODAGA chelator showed higher specific binding properties than conjugated with HBED-CC. The chemical properties of the bifunctional chelators used for 68Ga-labeling of PCA determine the biological behaviour of the probes. Due to the high specificity and sensitivity 68Ga-labeled PCA molecules are promising radiotracers in melanoma imaging.