Development of a Ga-68 labeled PET tracer with short linker for prostate-specific membrane antigen (PSMA) targeting.

Glu-Urea-Lys (GUL) derivatives have been reported as prostate-specific membrane antigen (PSMA) agent. We developed derivatives of GUL conjugated with NOTA or DOTA via a thiourea linker and tested their feasibility as PSMA imaging agents after labeling with 68Ga. NOTA-GUL and DOTA-GUL were synthesized and labeled with 68Ga using generator-eluted 68GaCl3 in 0.1 M HCl in the presence of 1 M NaOAc at pH 5.5. The stabilities of 68Ga-labeled compounds in human serum were tested at 37.5 °C. A competitive binding assay was performed using the PSMA-positive prostate cancer cell line 22Rv1 and [125I]MIP-1072 (PSMA-specific binding agent) as a tracer. Biodistribution and micro-PET studies were performed using 22Rv1-xenograft BALB/c nude mice. The radiolabeling efficiency of NOTA-GUL (>99%) was higher than that of DOTA-GUL (92%). The IC50 of Ga-NOTA-GUL was 18.3 nM. In the biodistribution study, tumor uptake of 68Ga-NOTA-GUL (5.40% ID/g) was higher than that of 68Ga-DOTA-GUL (4.66% ID/g) at 1 h. Tumor/muscle and tumor/blood uptake ratios of 68Ga-NOTA-GUL (31.8 and 135, respectively) were significantly higher than those of 68Ga-DOTA-GUL (16.1 and 31.1, respectively). The tumor/kidney uptake ratio of 68Ga-NOTA-GUL was 3.4-fold higher than that of 68Ga-DOTA-GUL. 68Ga-NOTA-GUL showed specific uptake to PSMA positive tumor xenograft and was blocked by co-injection of the cold ligand. In conclusion, we successfully synthesized 68Ga-NOTA-GUL and 68Ga-DOTA-GUL for prostate cancer imaging. 68Ga-NOTA-GUL showed better radiochemical and biodistribution results. 68Ga-NOTA-GUL may be a promising PSMA targeting radiopharmaceutical.

Development of a complementary PET/MR dual-modal imaging probe for targeting prostate-specific membrane antigen (PSMA).

We tried to develop a dual-modal PET/MR imaging probe using a straightforward one-pot method by encapsulation with specific amphiphiles. In this study, iron oxide (IO) nanoparticles were encapsulated with three amphiphiles containing PEG, DOTA and the prostate-specific membrane antigen (PSMA)-targeting ligand in aqueous medium. The diameter of the prepared nanoparticle DOTA-IO-GUL was 11.01±1.54nm. DOTA-IO-GUL was labeled with (68)Ga in high efficiency. The DOTA-IO-GUL showed a dose-dependent binding to LNCaP (PSMA positive) cells via a competitive binding study against (125)I-labeled MIP-1072 (PSMA-targeting agent). Additionally, PET and MR imaging results showed PSMA selective uptake by only 22Rv1 (PSMA positive) but not PC-3 (PSMA negative) in dual-tumor xenograft mouse model study. MR imaging showed high resolution, and PET imaging enabled quantification and confirmation of the specificity. In conclusion, we have successfully developed the specific PSMA-targeting IO nanoparticle, DOTA-IO-GUL, as a dual-modality probe for complementary PET/MR imaging. FROM THE CLINICAL EDITOR: The combination of using Positron Emission Tomography (PET) and computed tomography (CT) in clinical practice is now the norm. With advances in technology, the next step would be to develop combined PET and Magnetic Resonance (MR) dual-imaging. In this article, the authors described their positive study on the development of a dual-modal PET/MR imaging probe using a prostate cancer model.

Synthesis of 68Ga-labeled DOTA-nitroimidazole derivatives and their feasibilities as hypoxia imaging PET tracers.

The imaging of hypoxia is important for therapeutic decision making in various diseases. (68)Ga is an important radionuclide for positron emission tomography (PET), and its usage is increasing, due to the development of the (68)Ge/(68)Ga-generator. In the present study, the authors synthesized two nitroimidazole derivatives by conjugating nitroimidazole and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) via an amide bond (4) and a thiourea bond (5). Both derivatives were labeled with (68)Ga with high labeling efficiency and were stable after labeling. The low partition coefficients (logP) of (68)Ga-4 (-4.6) and (68)Ga-5 (-4.5) demonstrated the hydrophilic natures of the derivatives, and both showed higher uptake in cancer cell lines cultured under hypoxic condition than under normoxic condition. However, (68)Ga-5 showed higher liver uptake than (68)Ga-4 in a biodistribution study due to higher lipophilicity. In an animal PET study, (68)Ga-4 showed higher standard uptake values (SUV) in tumors than (68)Ga-5 in mice xenografted with CT-26 mouse colon cancer cells.

Preparation of a promising angiogenesis PET imaging agent: 68Ga-labeled c(RGDyK)-isothiocyanatobenzyl-1,4,7-triazacyclononane-1,4,7-triacetic acid and feasibility studies in mice.

UNLABELLED: Arg-Gly-Asp (RGD) derivatives have been labeled with various radioisotopes for the imaging of angiogenesis in ischemic tissue, in which alpha(v)beta(3) integrin plays an important role. In this study, cyclic Arg-Gly-Asp-D-Tyr-Lys [c(RGDyK)] was conjugated with 2-(p-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid (SCN-Bz-NOTA) and then labeled with (68)Ga. The labeled RGD so produced was subjected to an in vitro binding assay and in vivo biodistribution and PET studies. METHODS: A mixture of SCN-Bz-NOTA (660 nmol) and c(RGDyK) (600 nmol) in 0.1 M sodium carbonate buffer (pH 9.5) was allowed to react for 20 h at room temperature in the dark for thiourea bond formation. The conjugate obtained was purified by semipreparative high-performance liquid chromatography (HPLC). The purified c(RGDyK)-SCN-Bz-NOTA (NOTA-RGD) was then labeled with (68)Ga from a (68)Ge/(68)Ga generator and purified by semipreparative HPLC. A competitive binding assay for c(RGDyK) and NOTA-RGD was performed with (125)I-c(RGDyK) as a radioligand and alpha(v)beta(3) integrin-coated plates as a solid phase. (68)Ga-NOTA-RGD (0.222 MBq/100 microL) was injected, through a tail vein, into mice with hind limb ischemia and into mice bearing human colon cancer SNU-C4 xenografts. Biodistribution and imaging studies were performed at 1 and 2 h after injection. RESULTS: The labeling of NOTA-RGD with (68)Ga was straightforward. The K(i) values of c(RGDyK) and NOTA-RGD were 1.3 and 1.9 nM, respectively. In the biodistribution study, the mean +/- SD uptake of (68)Ga-NOTA-RGD by ischemic muscles was 1.6+/-0.2 percentage injected dose per gram (%ID/g); this uptake was significantly blocked by cold c(RGDyK) to 0.6+/-0.3 %ID/g (P<0.01). Tumor uptake was 5.1+/-1.0 %ID/g, and the tumor-to-blood ratio was 10.3+/-4.8. Small-animal PET revealed rapid excretion through the urine and high levels of tumor and kidney uptake. CONCLUSION: Stable (68)Ga-NOTA-RGD was obtained in a straightforward manner at a high yield and showed a high affinity for alpha(v)beta(3) integrin, specific uptake by angiogenic muscles, a high level of uptake by tumors, and rapid renal excretion. (68)Ga-NOTA-RGD was found to be a promising radioligand for the imaging of angiogenesis.

Differential receptor targeting of liver cells using 99mTc-neoglycosylated human serum albumins.

Neolactosyl human serum albumin (LSA) targets asialoglycoprotein receptor and shows high liver uptake due to accumulation in hepatocytes. Although neomannosyl human serum albumin (MSA) also shows high liver uptake, it has been reported to be taken up by Kupffer cells and endothelial cells. We compared the biological properties of LSA and MSA. 99mTc-LSA and 99mTc-MSA biodistribution in mice were investigated after intravenous injection. In vivo localization of rhodaminisothiocyanate (RITC)-LSA and fluoresceineisothiocyanate (FITC)-MSA were investigated in mouse liver. Excretion routes of 99mTc-LSA and 99mTc-MSA metabolites were examined. Both 99mTc-LSA and 99mTc-MSA showed high liver uptakes. RITC-LSA was taken up by hepatocytes whereas FITC-MSA was taken up by Kupffer cells and endothelial cells. 99mTc-MSA showed higher spleen and kidney uptakes than 99mTc-LSA. 99mTc-LSA metabolites excreted in urine and feces accounted for 44.4 and 50.0% of 99mTc-LSA injected, respectively, while 99mTc-MSA metabolites accounted for 51.5 and 10.3%, respectively. In conclusion, LSA is specifically taken up by hepatcytes while MSA by Kupffer cells and endothelial cells. After taken up by the liver, LSA is metabolized by the hepatocytes and then excreted through both the hepatobiliary tract and kidney, whereas MSA is metabolized by Kupffer cells and endoghelial cells and then excreted mainly through the kidney.

An improved method of 18F peptide labeling: hydrazone formation with HYNIC-conjugated c(RGDyK).

Radiolabeled alpha(v)beta(3)-integrin antagonists are increasingly investigated as a means of imaging angiogenesis. Several methods of labeling alpha(v)beta(3)-integrin binding peptide with (18)F have been reported recently. In the present study, we devised a straightforward means for labeling Arg-Gly-Asp (RGD) peptide with (18)F via hydrazone formation between c(RGDyK)-hydrazinonicotinic acid (HYNIC) (3) and 4-[(18)F]-fluorobenzaldehyde ([(18)F]4). The resulting reaction mixture was purified by HPLC to give 4'-[(18)F]-fluorobenzylidenehydrazone-6-nicotinamide-c(RGDyK) ([(18)F]5). The conjugation efficiency of 3 and 4 to form [(18)F]5 was 95.2%, and the radiochemical purity of [(18)F]5 after purification was >99%. The specific activity of [(18)F]5 estimated by radio-HPLC was 20.5 GBq/mumol (end of synthesis). Competitive binding assay of c(RGDyK) (1) and 5 was performed using [(125)I]iodo-c(RGDyK) as a radioligand, and K(i) values were found to be 2.8 and 21.7 nM, respectively. For the biodistribution study, the angiogenic mouse model was established by inducing unilateral ischemia on the left hindlimbs of ICR mice after femoral artery ablation. Seven days after inducing ischemia, [(18)F]5 was administered to the mice through the tail vein. Ischemic muscle uptake of [(18)F]5 was significantly higher than that of normal muscle (P<.01). Specific uptake was confirmed by coinjection of 1 with [(18)F]5. Here, we successfully labeled RGD peptide with (18)F via hydrazone formation between 3 and 4, resulting to [(18)F]5. [(18)F]5 was found to have high affinity for alpha(v)beta(3)-integrin and to accumulate specifically in ischemic hindlimb muscle of mice. We suggest that (18)F labeling via formation of hydrazone between HYNIC peptide and [(18)F]4 is a useful method for labeling c(RGDyK), which can be applied for imaging angiogenesis.

Development of 99mTc-neomannosyl human serum albumin (99mTc-MSA) as a novel receptor binding agent for sentinel lymph node imaging.

BACKGROUND AND AIM: Various mannose receptor-binding agents, for example 99mTc-diethylenetriaminepentaacetic acid (DTPA)-mannosyl-polymer, have been developed for sentinel lymph node (SLN) imaging. In order to simplify the synthesis and labelling procedure and to improve the biological properties, we developed a novel mannose receptor-binding agent, 99mTc-neomannosyl human serum albumin (99mTc-MSA), for SLN imaging. METHODS: MSA was synthesized by conjugating mannopyranosylphenylisothiocyanate to human serum albumin (HSA). After reducing MSA with beta-mercaptoethanol and PD-10 column purification, a medronate solution containing stannous fluoride was added, divided into aliquots and freeze-dried. Reduced MSA was labelled with 99mTc-pertechnetate solution. The stability was checked for 24 h at 37 degrees C in human serum. The biodistribution of 99mTc-MSA in mice was investigated by intravenous injection through the tail vein and subcutaneous injection into the foot pad. The biodistributions of 99mTc-HSA and 99mTc-antimony sulphur colloid (99mTc-ASC) were also investigated for comparison. Dynamic whole-body images were obtained for 30 min after subcutaneous injection into the rats' foot pads. RESULTS: The number of mannose molecules conjugated per MSA was 15.9. The number of thiol groups produced was 19.4 per MSA after reduction with beta-mercaptoethanol. Labelling yields were always higher than 97%. 99mTc-MSA was stable for 24 h at 37 degrees C in human serum. The biodistribution in mice after intravenous injection showed high liver uptake (50.7+/-5.5% and 42.7+/-3.7% injected dose per gram at 10 and 60 min, respectively). 99mTc-MSA and 99mTc-ASC showed high accumulation in the lymph nodes after subcutaneous injection, whereas 99mTc-HSA and Tc-tin colloid did not, in both biodistribution and imaging studies. CONCLUSIONS: We have successfully developed a novel 99mTc-MSA for lymphoscintigraphy. The results of animal studies show that 99mTc-MSA has promising properties for SLN imaging.