Labeling, stability and biodistribution studies of 99mTc-cyclized Tyr3-octreotate derivatives

Abstract

Introduction-To probe the interplay between radiotracer stability and somatostatin receptor affinity, Tyr 3 -octreotate and six variations of its peptide sequence, for which the Re-cyclized products were previously reported, were radiolabeled with 99m Tc and investigated for their in vitro stability.Methods-Radiolabeling of the peptides was effected by ligand exchange from 99m Tcglucoheptonate, and the desired products were purified by radio-RP-HPLC. The in vitro stability in phosphate-buffered saline, mouse serum, and cysteine solutions at physiological temperature and pH for all seven 99m Tc-cyclized peptides was determined by radio-RP-HPLC and radio-TLC. Normal CF-1 mouse biodistribution studies were performed for three of the 99m Tc-cyclized peptides.Results-Based on the fully characterized Re-cyclized peptide analogues, four 99m Tccoordination motifs were proposed for the 99m Tc-cyclized peptides. Technetium-99m-cyclized Tyr 3 -octreotate derivatives with N 2 S 2 metal coordination modes and large metal ring sizes were susceptible to oxidation and loss of 99m Tc in the form of 99m TcO 4 − , as evidenced by their instability in the various solutions under physiological conditions (15-58% intact at 24 h). As anticipated, the addition of a third cysteine to the sequence stabilized the 99m Tc metal coordination, and peptides with NS 3 coordination modes remained >85% intact out to 24 h. No significant differences were observed in the biodistribution studies performed with three peptides of varying stabilities.Conclusions-Improvements in stability were not sufficient to outweigh the low somatostatin receptor affinity for the peptides in this study. Further improvements in the peptide sequence and/ or metal coordination are needed to result in a radiodiagnostic/radiotherapeutic pair for targeting the somatostatin receptor.

Synthesis, Characterization, and Initial Biological Evaluation of [99mTc]Tc‐Tricarbonyl‐labeled DPA‐α‐MSH Peptide Derivatives for Potential Melanoma Imaging

“…This occurrence has been previously reported by Radford et al . for another class of [ 99m Tc]Tc‐DPA labeled peptides for whose the addition of a radioprotective agent such as gentisic acid and an adjustment of the pH to 7.4 was necessary to improve their stability . High challenge stability confirmed that the coordination between [ 99m Tc]Tc‐tricarbonyl and DPA apparently is sufficient for in vivo use as it has been described earlier …”

Synthesis and evaluation of a 99mTc tricarbonyl-labeled somatostatin receptor-targeting antagonist peptide for imaging of neuroendocrine tumors

“…Though 99m Tc has relatively low decay energetics, we have observed radiolysis in other peptide-based complexes labeled with 99m Tc. [18] The described stability procedure was modified by the addition of GA to the incubation vial and pH readjustment to 7.4 using a 1 M NaHCO3 solution. The histidine/GA results demonstrated a marked improvement in stability, with 99m TcL-sst2-ANT remaining highly intact (≥99%) through 24 hours, supporting the radiolysis hypothesis.…”

Technetium‐99m radiochemistry for pharmaceutical applications

“…In another effort, modifications in the peptide sequence of the somatostatin‐receptor‐targeted octreotate led to the preparation of cyclized 99m Tc analogues. In these radioconjugates, the disulfide bridge between two distant cysteine residues of the peptide was prereduced and the thiol groups generated were made available for oxotechnetium cyclized complexes …”

“…In these radioconjugates, the disulfide bridge between two distant cysteine residues of the peptide was prereduced and the thiol groups generated were made available for oxotechnetium cyclized complexes. 124 Finally, the "integrated" approach, where the metal-chelate part forms the pharmacophore moiety, can be applied ( Figure 10). In this approach, careful design of the chelator must be applied to mimic the structure of the bioactive molecule after complexation of the radiometal.…”