Background

[PubMed]

The ¹¹¹In-labeled 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-conjugated monomeric [Tyr³]octreotide, abbreviated as ¹¹¹In-monomeric [Tyr³]octreotide, is an octreotide-based agent developed by Yim et al. for somatostatin receptor (SSTR)–targeted imaging and radionuclide therapy (1).

The human SSTR family is a group of G-protein–coupled receptors with five members (SSTR1–SSTR5). All receptor members have seven α-helical transmembrane domains and possess a highly conserved sequence motif (YANSCANPI/VLY) in the seventh topology, which serves as a signature sequence for this family (2-4). Overall, there is 39%–57% sequence identity among the members, with the highest homology between SSTR1 and SSTR4, and among SSTR2, SSTR3, and SSTR5, respectively. The two groups of receptors also differ in their interactions with somatostatin (SST) and its analogs (1, 3-5). SSTR2, SSTR3, and SSTR5 have a high affinity for octreotide and seglitide, whereas SSTR1 and SSTR4 exhibit a very low affinity for them. SSTRs are distributed widely in cells both in the nervous system and periphery, and they have been shown to be overexpressed in a large number of malignancies, with particularly high density in neuroendocrine tumors (2, 6, 7).

As the targets of SST radiopharmaceuticals, SSTRs are of considerable clinical relevance for tumor imaging and radionuclide therapy (2, 8, 9). Because the native SST has a very short biological half-life (<2 min), various analogs have been synthesized, including octreotide, lanreotide, vapreotide, and their derivatives (4, 9, 10). In general, these analogs have a high affinity to SSTR2, somewhat lower affinity to SSTR3 and SSTR5, and almost no affinity to SSTR1 and SSTR4. Treatment with the radiolabeled analogs results in reduced hormonal overproduction and symptomatic relief in most patients with neuroendocrine tumors; however, it is much less successful in tumor size reduction (11, 12). Imaging with ¹¹¹In-, ⁹⁰Y-, or ¹⁷⁷Lu-labeled [DOTA⁰,Tyr³]octreotide analogs has proven the usefulness of octreotide conjugates in the diagnosis and staging of neuroendocrine tumors (7-9, 13).

There is growing interest in the development of polyvalent ligands for dual imaging and therapeutic purposes, considering that polyvalent ligands may have a higher binding affinity than monovalent analogs (1). Yim et al., synthesized mono-, di-, and tetrameric [Tyr³]octreotide conjugates with a two-stage metal-free ligation procedure (1). The investigators characterized these conjugates and compared them with ¹¹¹In-labeled [DOTA⁰,Tyr³]octreotide ([¹¹¹In-DOTA⁰,Tyr³]octreotide), a chemically engineered and long-acting analog. [¹¹¹In-DOTA⁰,Tyr³]octreotide has been applied for imaging and treatment in patients with neuroendocrine tumors (9, 11, 12). Yim et al. have shown that the ¹¹¹In-labeled monomeric and dimeric [Tyr³]octreotides, but not the ¹¹¹In-labeled tetrameric [Tyr³]octreotide, have a similar binding affinity for SSTRs and a longer tumor retention time when compared with [¹¹¹In-DOTA⁰,Tyr³]octreotide (1). This chapter describes the results obtained with ¹¹¹In-monomeric [Tyr³]octreotide.

Synthesis

[PubMed]

DOTA-Conjugated monomeric [Tyr³]octreotide was synthesized with a two-stage ligation procedure that combined the Cu(I)-catalyzed “click” reaction between dendrimeric alkynes and peptidic azides and the subsequent copper-free thio acid/azide “sulfo-click” amidation reaction between the obtained dendrimeric peptide thio acids and a DOTA-derived sulfonyl azide (1). The ¹¹¹In-labeling was achieved in NH₄OAc buffer (pH 5.5) with ¹¹¹InCl₃ by heating the reaction mixture for 10–15 min at 95ºC. The radiochemical yield of the ¹¹¹In-monomeric [Tyr³]octreotide was >98% with a specific activity of 13.6 GBq/μmol (0.37 Ci/μmol). Its lipophilicity, expressed as log(D) value, was −2.20 ± 0.17, 25-fold higher than that of [¹¹¹In-DOTA⁰,Tyr³]octreotide (−3.59 ± 0.14). This could be attributed to the aromatic character of the dendrimer-derived linker moiety.

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

Binding affinity of the monomeric [Tyr³]octreotide for SSTRs was determined using SSTR2-expressing AR42J tumor cells in a competitive binding assay (1). [¹¹¹In-DOTA⁰,Tyr³]octreotide was used as a radiotracer, and commercially available [DOTA⁰,Tyr³]octreotide was included as a competing ligand. The monomeric [Tyr³]octreotide had a 50% inhibition concentration (IC₅₀) of 1.32 nM, better than that of the reference [DOTA⁰,Tyr³]octreotide (IC₅₀ = 2.45 nM). The IC₅₀ values for the dimeric and tetrameric [Tyr³]octreotide conjugates were 2.45 nM and 14.0 nM, respectively. These results showed a decreased binding affinity with increased valency. The hypothesized increase in receptor affinity due to multimerization was not observed, especially in the case of tetrameric [Tyr³]octreotide, which may be attributed to the limited water solubility of the [Tyr³]octreotide conjugates. The IC₅₀ values for all ¹¹¹In-labeled conjugates improved by a factor of 1.4–2.9, compared to unlabeled counterparts.

Animal Studies

Human Studies

[PubMed]

No references are currently available.

References

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