Radioiodine labelling of a small chemotactic peptide, utilizing two different prosthetic groups: A comparative study

摘要

The use of iodobenzoates as pre-labelled prosthetic groups in the radioiodination of peptides is becoming increasingly popular. The utilization of an iodovinyl ester unit as an alternative conjugation agent for the preparation of a radioiodinated peptide conjugate (peptide-[~(123)I]l-PEA) was investigated in this study. A pre-labelled unit, containing a tetrafluorophenyl ester group, was purified on a small C18 column and the dimethylformamide eluate was used directly in the conjugation step. Similar methodology was applied for a comparative radiosynthesis of an iodobenzoate analogue (peptide-[~(123)I]IB), using a succinimidyl ester. The influences of various reaction parameters on conjugation yields were investigated while maintaining a fixed amount of peptide. At high activity levels (more than 200 MBq) the conjugation yield of peptide-[~(123)I]l-PEA was very sensitive to relatively large reaction volumes and increased amount of base. By optimizing these parameters, the formation of radiochemical impurities was minimized. Under similar conditions, peptide-[~(123)I]l-PEA formed much faster than peptide-[~(123)I]IB. Sep-Pak C18 purification afforded conjugates with radiochemical purities both in excess of 98% and free from unreacted peptide. Recovered conjugation yields of peptide-[~(123)I]l-PEA in 50% ethanol were in excess of 60%, while those for peptide-[~(123)I]IB were less than 40%.