Improvement of Pharmacokinetics of Radioiodinated Tyr3-Octreotide by Conjugation with Carbohydrates

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Abstract
Among a variety of other factors, the clearance kinetics and routes of excretion of radiopharmaceuticals are of crucial importance for early and high tumor/background ratios and thus signal intensity in diagnostic imaging by single photon emission tomography (SPECT) or positron emission tomography (PET). To overcome the unfavorable pharmacokinetics of radiohalogenated octreotide analogues, we evaluated three carbohydrated conjugates of Tyr3-octreotide (TOC). Glucose ([125I]Gluc-TOC), maltose ([125I]Malt-TOC), and maltotriose ([125I]Mtr-TOC) derivatives of [125I]TOC were synthesized via Maillard reaction and subsequent radioiodination. In cells transfected with sst1-sst5, I-Malt-TOC, and I-Mtr-TOC show sst-subtype binding profiles similar to I-TOC with high affinity for sst2. Comparative biodistribution studies 10, 30, and 60 min pi in nude mice bearing rat pancreatic tumor xenografts showed fast blood clearance for all glycosylated derivatives. Due to their markedly increased hydrophilicity, [125I]Gluc-TOC and [125I]Malt-TOC were mainly cleared via the kidneys, which led to a significant decrease in activity accumulation in liver and intestine (5.3 and 1.4 versus 10.6%ID/g for [125I]TOC in the liver, 1.7 and 1.0 versus 3.8%ID/g for [125I]TOC in the intestine 60 min pi). For all compounds, hydrophilicity and uptake in liver and intestines correlate. Uptake of the carbohydrate conjugates in the kidney was comparable. Compared to the parent compound, the accumulation of the carbohydrated compounds in sst-rich tissues (pancreas, adrenals) was increased by a factor of 1.5−3.5. While tumor uptake of [125I]TOC (6.7 ± 2.6%ID/g), [125I]Malt-TOC (5.3 ± 1.9%ID/g), and [125I]Mtr-TOC (4.9 ± 2.2%ID/g) at 30 min postinjection was comparable, accumulation of [125I]Gluc-TOC was significantly increased (10.1 ± 2.8%ID/g at 30 min pi). Somatostatin receptor specificity of tumor uptake was confirmed by pretreatment, competition, and displacement experiments in vivo using 0.8 mg TOC/kg and γ-camera imaging. Glycosylation proved to be a powerful tool for the development of high affinity sst ligands with excellent excretion profiles and improved tumor accumulation.

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