Preparation and chromatographic purification of 125I-[Tyr0]-somatostatin-14 for the use in radioimmunoassay and receptor-binding experiments.

[Tyr0]-somatostatin-14 (SST-14) was converted to the corresponding radiolabeled 125I-[Tyr0]-SST-14 derivative by use of the chloramine-T technique. After solid-phase extraction (SPE) of the crude radiopeptide with microfine silica gel and desorption with acetone-water-acetic acid 39:40:1, the label was subjected to size exclusion chromatography (SEC) on Sephadex G-25 fine. Fractions attributable to the target compound were collected and investigated with respect to their specific as well as non-specific binding to a specific anti-somatostatin antibody. All fractions exhibiting an optimum ratio of specific versus non-specific binding were pooled and lyophilized for their further use in both radioimmunoassay (RIA) measurements of somatostatin like immunoreactivity (SLI) and receptor-binding experiments. A specific activity of approx. 1.6 x 10(6) Ci/M was calculated for the radiolabel prepared in this manner. Approximately 85-90% of radioactivity attributable to labeled [Tyr0]-SST species was incorporated into the desired mono-iodinated component. When 125I-[Tyr0]-SST-14 was purified by reversed-phase high performance liquid chromatography (RP-HPLC) using isocratic elution with 0.1 M triethylammonium formate (TEAF) buffer of pH 2.2 in 22% acetonitrile after prior SPE, specific binding decreases to about 80% compared with the value obtained for the radiopeptide subjected to SEC. Nevertheless, RP-HPLC proves as an efficient tool for rapid purity control of 125I-[Tyr0]-somatostatin-14 samples at different storage time intervals.