The introduced [(68)Ga]NODAGA-RGD combines easy accessibility with high stability and good imaging properties making it an interesting alternative to the (18)F-labelled RGD peptides currently used for imaging α(v)β(3) expression.

Within the last years 89Zr has attracted considerable attention as long-lived radionuclide for positron emission tomography (PET) applications. So far desferrioxamine B (DFO) has been mainly used as bifunctional chelating system. Fusarinine C (FSC), having complexing properties comparable to DFO, was expected to be an alternative with potentially higher stability due to its cyclic structure. In this study, as proof of principle, various FSC-RGD conjugates targeting αvß3 integrins were synthesized using different conjugation strategies and labeled with 89Zr. In vitro stability, biodistribution, and microPET/CT imaging were evaluated using [89Zr]FSC-RGD conjugates or [89Zr]triacetylfusarinine C (TAFC). Quantitative 89Zr labeling was achieved within 90 min at room temperature. The distribution coefficients of the different radioligands indicate hydrophilic character. Compared to [89Zr]DFO, [89Zr]FSC derivatives showed excellent in vitro stability and resistance against transchelation in phosphate buffered saline (PBS), ethylenediaminetetraacetic acid solution (EDTA), and human serum for up to 7 days. Cell binding studies and biodistribution as well as microPET/CT imaging experiments showed efficient receptor-specific targeting of [89Zr]FSC-RGD conjugates. No bone uptake was observed analyzing PET images indicating high in vivo stability. These findings indicate that FSC is a highly promising chelator for the development of 89Zr-based PET imaging agents.

PurposeThis study was designed to determine safety, tolerability, and radiation burden of a [68Ga]NODAGA-RGD-PET for imaging integrin αvβ3 expression in patients with hepatocellular carcinoma (HCC) and liver cirrhosis. Moreover, metabolic stability and biokinetic data were compiled.MethodsAfter injection of 154–184 MBq [68Ga]NODAGA-RGD three consecutive PET/CT scans were acquired starting 8.3 ± 2.1, 36.9 ± 2.8, and 75.1 ± 3.4 min after tracer injection. For metabolite analysis, blood and urine samples were analyzed by HPLC. For dosimetry studies, residence time VOIs were placed in the corresponding organs. The OLINDA/EXM program was used to estimate the absorbed radiation dose.ResultsThe radiopharmaceutical was well tolerated and no drug-related adverse effects were observed. No metabolites could be detected in blood (30 and 60 min p.i.) and urine (60 min p.i.). [68Ga]NODAGA-RGD showed rapid and predominantly renal elimination. Background radioactivity in blood, intestine, lung, and muscle tissue was low (%ID/l 60 min p.i. was 0.56 ± 0.43, 0.54 ± 0.39, 0.22 ± 0.05, and 0.16 ± 0.8, respectively). The calculated effective dose was 21.5 ± 5.4 μSv/MBq, and the highest absorbed radiation dose was found for the urinary bladder wall (0.26 ± 0.09 mSv/MBq). No increased uptake of the tracer was found in HCC compared with the background liver tissue.Conclusions[68Ga]NODAGA-RGD uptake in the HCCs lesions was not sufficient to use this tracer for imaging these tumors. [68Ga]NODAGA-RGD was well tolerated and metabolically stable. Due to rapid renal excretion, background radioactivity was low in most of the body, resulting in low radiation burden and indicating the potential of [68Ga]NODAGA-RGD PET for non-invasive determination of integrin αvβ3 expression.

Over the last years Gallium-68 (68Ga) has received tremendous attention for labeling of radiopharmaceuticals for positron emission tomography (PET). 68Ga labeling of biomolecules is currently based on bifunctional chelators containing aminocarboxylates (mainly DOTA and NOTA). We have recently shown that cyclic peptide siderophores have very good complexing properties for 68Ga resulting in high specific activities and excellent metabolic stabilities, in particular triacetylfusarinine-C (TAFC). We postulated, that, starting from its deacetylated form (Fusarinine-C (FSC)) trimeric bioconjugates are directly accessible to develop novel targeting peptide based 68Ga labeled radiopharmaceuticals. As proof of principle we report on the synthesis and 68Ga-radiolabeling of a trimeric FSC-RGD conjugate, [68Ga]FSC-(RGD)3, targeting αvβ3 integrin, which is highly expressed during tumor-induced angiogenesis.Synthesis of the RGD peptide was carried out applying solid phase peptide synthesis (SPPS), followed by the coupling to the siderophore [Fe]FSC via in situ activation using HATU/HOAt and DIPEA. Subsequent demetalation allowed radiolabeling of FSC-(RGD)3 with 68Ga. The radiolabeling procedure was optimized regarding peptide amount, reaction time, temperature as well buffer systems. For in vitro evaluation partition coefficient, protein binding, serum stability, αvβ3 integrin binding affinity, and tumor cell uptake were determined. For in vitro tests as well as for the biodistribution studies αvβ3 positive human melanoma M21 and αvβ3 negative M21-L cells were used.[68Ga]FSC-(RGD)3 was prepared with high radiochemical yield (> 98%). Distribution coefficient was − 3.6 revealing a hydrophilic character, and an IC50 value of 1.8 ± 0.6 nM was determined indicating a high binding affinity for αvβ3 integrin. [68Ga]FSC-(RGD)3 was stable in PBS (pH 7.4), FeCl3- and DTPA-solution as well as in fresh human serum at 37 °C for 2 hours. Biodistribution assay confirmed the receptor specific uptake found in vitro. Uptake in the αvβ3 positive tumor was 4.3% ID/g 60 min p.i. which was 3-fold higher than the monomeric [68Ga]NODAGA-RGD. Tumor to blood ratio of approx. 8 and tumor to muscle ratio of approx. 7 were observed. [68Ga]FSC-(RGD)3 serves as an example for the feasibility of a novel class of bifunctional chelators based on cyclic peptide siderophores and shows excellent targeting properties for αvβ3 integrin in vivo for imaging tumor-induced neovascularization.