Carbohydration of N-terminus and substitution of a
threonine for the
threoninol residue at the C-terminus of Tyr3-octreotide (TOC) has resulted in improved pharmacokinetics and
tumor targeting of its radioiodinated derivatives. Yet, these
peptides are very susceptible to in vivo deiodination due to the similarity of
monoiodotyrosine (MIT) to
thyroid hormone. The goal of this work was to develop octreotate analogues containing both a
sugar moiety and a nontyrosine prosthetic group on which a radioiodine or 211At can be introduced. Solid-phase synthesis and subsequent modifications delivered an iodo standard of the target
peptide N(alpha)-(1-deoxy-D-fructosyl)-N(epsilon)-(3-iodobenzoyl)-Lys0-octreotate (GIBLO) and the corresponding
tin precursor N(alpha)-(1-deoxy-D-fructosyl)-N(epsilon)-[(3-tri-n-butylstannyl)benzoyl]-Lys0-octreotate (GTBLO). GIBLO displaced [125I]TOC from
somatostatin receptor subtype 2 (SSTR2)-positive AR42J rat pancreatic
tumor cell membranes with an IC50 of 0.46 +/- 0.05 nM suggesting that GIBLO retained affinity to SSTR2. GTBLO was radiohalogenated to [131I]GIBLO and N(alpha)-(1-deoxy-D-fructosyl)-N(epsilon)-(3-[211At]astatobenzoyl)-Lys0-octreotate ([211At]GABLO) in 21.2 +/- 4.9% and 46.8 +/- 9.5% radiochemical yields, respectively. From a paired-label internalization assay using D341 Med
medulloblastoma cells, the maximum specific internalized radioactivity from [131I]GIBLO was 1.78 +/- 0.8% of input dose compared to 9.67 +/- 0.43% for N(alpha)-(1-deoxy-D-fructosyl)-[125I]iodo-Tyr3-octreotate ([125I]I-Gluc-TOCA). Over a 4 h period, the extent of internalization of [131I]GIBLO and [211At]GABLO was similar in this cell line. In D341 Med murine subcutaneous xenografts, the uptake of [125I]I-Gluc-TOCA at 0.5, 1 and 4 h was 21.5 +/- 4.0% ID/g, 18.8 +/- 7.7% ID/g, and 0.9 +/- 0.4% ID/g, respectively. In comparison, these values for [131I]GIBLO were 6.9 +/- 1.2% ID/g, 4.7 +/- 1.4% ID/g, and 0.8 +/- 0.5% ID/g. Both in vitro and in vivo catabolism studies did not suggest the severance of the lys0 along with its appendages from the
peptide. Taken together, although GIBLO maintained affinity to SSTR2, its
tumor uptake both in vitro and in vivo was substantially lower than that of I-Gluc-TOCA suggesting other factors such as net charge and overall geometry of the
peptide may be important.