Parameters ruling optimization of radiolabelling of polyamino polycarboxylated functionalized peptide derivatives: a case study report.

Parameters studies revealed that successful labeling of DTPA-Neurotensin(8-13) analogues depend on several physico-chemical parameters. The pH of the reaction mixture seemed to be the most critical parameter for obtaining high labeling yields; quantitative radiolabelling was only guaranteed at a pH between 4.2 and 5.5. At a pH of 4.5, metal ion contaminants originating from peptide synthesis and purification procedures were shown not to effect radiolabelling. Nevertheless, proper reducing agents were included in a proposed Kit labeling procedure in order to avoid potential competition from trivalent metal ion contamination, and thus guarantee successful 111In-complexation. The complexing capacity of DTPA for radioactive In(3+) strongly depends upon the pH. As a consequence, labeling yields must be expressed as [[K(ass) x alpha(4) x [DTPA-NT](0)/(1+ K(ass) x alpha(4) x [DTPA-NT](0))], to where K(ass) is the association constant and alpha(4) is a pH dependent correction factor of the association constant.