Sandostatin labeled with 99mTc: in vitro stability, in vivo validity and comparison with 111In-DTPA-octreotide

Sandostatin, a synthetic octapeptide analog of a native hormone somatostatin, was labeled with a commonly available, inexpensive radionuclide, 99mTc, and evaluated for its suitability for in vivo imaging. Labeling was accomplished by reduction of the cystine bridge, which provided two sulfhydryl groups for chelation with 99mTc. The complex was examined for thermodynamic stability in vitro and in experimental animals. Receptor specificity of the complex was determined using rat brain cortex membrane rich in somatostatin receptors, and its tissue distribution was studied in nude mice bearing human prostate cancer. In these studies, 99mTc-labeled oxytocin, a nonspecific peptide with similar molecular weight, served as a control and 111In-DTPA-octreotide served as a standard. The labeling method was simple, did not require protecting and deprotecting functional groups and yields were high (ca. 70%). The in vitro and in vivo stability was excellent, and Kd values were in the nanomolar range, similar to those of 111In-DTPA-octreotide. At 24 hours post-injection, the tumor uptake for 99mTc-Sandostatin, expressed as percent of injected dose per gram (% ID/g), was higher, but the tumor/blood and tumor/muscle ratios were lower than those for 111In-DTPA-octreotide. This agent, with its improved target-to-nontarget ratios, should prove to be of value for imaging somatostatin receptor-positive tumors.