Synthesis and Evaluation of Glycosylated Octreotate Analogues Labeled with Radioiodine and ²¹¹At via a Tin Precursor

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^α-(1-deoxy-d-fructosyl)-N^ε-(3-iodobenzoyl)-Lys⁰-octreotate (GIBLO) and the corresponding tin precursor N^α-(1-deoxy-d-fructosyl)-N^ε-[(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^α-(1-deoxy-d-fructosyl)-N^ε-(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^α-(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.