Recombinant Metallothionein-Conjugated Streptavidin Labeled with ¹⁸⁸Re and ^99mTc

Consideration is now being given to the use of avidin (or streptavidin) and biotin for radiotherapy of tumor. Accordingly, the goal of this study was to radiolabel a mouse metallothionein-streptavidin fusion protein with ¹⁸⁸Re and to compare its properties to those of the same fusion protein radiolabeled with 99mTc. A recombinant metallothionein-streptavidin fusion protein was radiolabeled by transchelation with ^99mTc- and ¹⁸⁸Re-glucoheptonate. Labeling efficiency, which was not optimized for either radionuclide, was approximately 60% for ^99mTc and 20% for ¹⁸⁸Re. Radiochemical purity was demonstrated by size exclusion HPLC both by nearly quantitative shifts of the ¹⁸⁸Re label to higher molecular weight upon the addition of biotinylated antibody and by the absence of a shift with biotin-saturated ¹⁸⁸Re-metallothionein-streptavidin. Stability of the labels in 37 °C serum was evaluated by comparing the HPLC radiochromatograms of serum samples both before and after the addition of biotinylated antibody. The ¹⁸⁸Re label behaved like ^99mTc in that the same peaks were evident, including one prominent peak due to labeled cysteine. Recoveries during HPLC analysis of serum samples showed that oxidation rates to perrhenate and pertechnetate were identical. However, instability to cysteine challenge was greater for ¹⁸⁸Re; for example, the loss of label to cysteine after 24 h under one set of conditions was 41% for ¹⁸⁸Re and 22% with ^99mTc. Analysis by HPLC of liver and kidney homogenates from mice administered the labeled antibodies were qualitatively and, in large measure, quantitatively independent of label. Biodistributions at 5 h in normal mice were statistically identical between the two labels in blood and in most tissues. In conclusion, streptavidin may be radiolabeled with radiorhenium using recombinant mouse metallothionein as a bifunctional chelator, and under one set of labeling conditions at least, ¹⁸⁸Re showed similar in vitro and in vivo behavior to that of ^99mTc labeled to the same fusion protein.