The integrity of the disulfide bond in a cyclic somatostatin analog during 99mTc complexation reactions

Radiopharmaceutical development of a freeze-dried kit formulation for the preparation of [99mTc-EDDA-HYNIC-D-Phe1, Tyr3]-octreotide, a somatostatin analog for tumor diagnosis

[(99m)Tc-EDDA-HYNIC-D-Phe(1),Tyr(3)]-Octreotide ((99m)Tc-EDDA/HYNIC-TOC) is a promising new radiopharmaceutical with the potential to replace [(111)In-DTPA-D-Phe(1)]-Octreotide ((111)In-DTPA-OCT) as the radiopharmaceutical for somatostatin receptor scintigraphy due to the advantage of improved image quality, lower radiation dose for the patient, and daily availability. Here we describe the development of a freeze-dried kit formulation based on the Tricine/EDDA exchange labeling approach for the preparation of this radiopharmaceutical in a clinical setting. Three parameters were of major importance to achieve a suitable formulation with a radiochemical purity (RCP) >90%: addition of bulking agent, the pH of the freeze-drying solution, and the content of stannous chloride. The final formulation consisted of 20 mg Tricine, 10 mg EDDA, 50 mg Mannitol, 20 microg SnCl(2). 2H(2)O, and 20 microg [HYNIC-D-Phe(1), Tyr(3)]-Octreotide (HYNIC-TOC). Radiolabeling was performed by addition of 0.2 M Na(2)HPO(4) to adjust the pH to 6-7, followed by 0.5-2 GBq (99m)Tc sodium pertechnetate, in a total volume of 2 mL and incubation for 10 min in a boiling water bath. Mean RCP values of 10 batches showed values >90% over a storage period of up to 1 year, a high stability up to 24 h of the final preparation, and retained biological activity. The developed kit formulation forms the basis for further clinical evaluation of this promising new radiopharmaceutical.

Receptor-Mediated Tumor Targeting with Radiopeptides. Part 1. General Concepts and Methods: Applications to Somatostatin Receptor-Expressing Tumors

Radiolabeled peptides have become important tools in nuclear oncology, both as diagnostics and more recently also as therapeutics. They represent a distinct sector of the molecular targeting approach, which in many areas of therapy will implement the old "magic bullet" concept by specifically directing the therapeutic agent to the site of action. In this three-part review, we present a comprehensive overview of the literature on receptor-mediated tumor targeting with the different radiopeptides currently studied. Part I summarizes the general concepts and methods of targeting, the selection of radioisotopes, chelators, and the criteria of peptide ligand development. Then, the >400 studies on the application to somatostatin/somatostatin-release inhibiting factor receptor-mediated tumor localization and treatment will be reviewed, demonstrating that peptide radiopharmaceuticals have gained an important position in clinical medicine.

Synthesis and preliminary evaluation of Tc-99m-labeled somatostatin analog (RC-160) using "3+1" mixed ligand approach

The success of (111)In-pentetreotide as a cancer-imaging agent has given impetus to the search for other peptide-based radiopharmaceuticals. The labeling with Tc-99m has become even more attractive because of the ready availability and near ideal physical properties. Additionally, the kinetics of the peptide-receptor interactions favors the radiolabeling with technetium-99m. A somatostatin analog RC-160 has been labeled with Tc-99m using the "3+1" mixed ligand approach utilizing the NNS/S coordination sites. The ternary complex was formed in greater than 95% within 30 min by simultaneous reduction and complexation of technetium-99m pertechnetate. The Tc-99m and the surrogate rhenium complexes showed similar chromatographic behavior. The complex was evaluated by in vitro receptor binding studies carried out on HTB-121 breast cancer cell line and biodistribution studies performed in normal mice. Our findings suggest that RC-160 can be labeled by the mixed ligand approach with the complex retaining its biological activity and warrants further studies.

Plasma protein binding of (99m)Tc-labeled hydrazino nicotinamide derivatized polypeptides and peptides

6-Hydrazinopyridine-3-carboxylic acid (HYNIC) constitutes one of the most attractive reagents to prepare (99m)Tc-labeled polypeptides and peptides of various molecular weights in combination with two tricine molecules as coligands. Indeed, (99m)Tc-HYNIC-conjugated IgG showed biodistribution of radioactivity similar to that of (111)In-DTPA-conjugated IgG. However, recent studies indicated significant plasma protein binding when the (99m)Tc labeling procedure was expanded to low molecular weight peptides. In this study, pharmacokinetics of (99m)Tc-HYNIC-conjugated IgG, Fab and RC160 using tricine were compared with their radioiodinated counterparts to evaluate this (99m)Tc-labeling method. In mice, [(99m)Tc](HYNIC-IgG)(tricine)(2) and [(99m)Tc](HYNIC-Fab)(tricine)(2) showed persistent localization of radioactivity in tissues when compared with their (125)I-labeled counterparts. [(99m)Tc](HYNIC-IgG)(tricine)(2) eliminated from the blood at a rate similar to that of (125)I-labeled IgG, while [(99m)Tc](HYNIC-Fab)(tricine)(2) showed significantly slower clearance of the radioactivity than (125)I-labeled Fab. On size-exclusion HPLC analyses, little changes were observed in radiochromatograms after incubation of [(99m)Tc](HYNIC-IgG)(tricine)(2) in murine plasma. However, [(99m)Tc](HYNIC-Fab)(tricine)(2) and [(99m)Tc](HYNIC-RC160)(tricine)(2) demonstrated significant increases in the radioactivity in higher molecular weight fractions in plasma. Formation of higher molecular weight species was reduced when [(99m)Tc](HYNIC-RC160)(tricine)(2) was stabilized with nicotinic acid (NIC) to generate [(99m)Tc](HYNIC-RC160)(tricine)(NIC). [(99m)Tc](HYNIC-RC160)(tricine)(NIC) also demonstrated significantly faster clearance of the radioactivity from the blood than [(99m)Tc](HYNIC-RC160)(tricine)(2). These findings suggested that one of the tricine coligands in (99m)Tc-HYNIC-labeled (poly)peptides would be replaced with plasma proteins to generate higher molecular weight species that exhibit slow blood clearance. In addition, the molecular sizes of parental peptides played an important role in the progression of the exchange reaction of one of the tricine coligands with plasma proteins.