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Development of Novel 68Ga- and 18F-Labeled GnRH-I Analogues with High GnRHR-Targeting Efficiency

A large majority of tumors of the reproductive system express the gonadotropin releasing hormone receptor (GnRHR). Blockade and activation of this receptor with various antagonistic and agonistic analogues of native GnRH-I (pGlu1-His2-Trp3-Ser4-Tyr5-Gly6-Leu7-Arg8-Pro9-Gly10-NH2), respectively, has...

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Published in:Bioconjugate chemistry 2008-06, Vol.19 (6), p.1256-1268
Main Authors: Schottelius, Margret, Berger, Sebastian, Poethko, Thorsten, Schwaiger, Markus, Wester, Hans-Jürgen
Format: Article
Language:English
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Summary:A large majority of tumors of the reproductive system express the gonadotropin releasing hormone receptor (GnRHR). Blockade and activation of this receptor with various antagonistic and agonistic analogues of native GnRH-I (pGlu1-His2-Trp3-Ser4-Tyr5-Gly6-Leu7-Arg8-Pro9-Gly10-NH2), respectively, has shown efficient suppression of tumor growth. In this study, the GnRH-receptor system has been evaluated with respect to its suitability as a target for in vivo peptide receptor targeting using radiolabeled GnRH-analogues, and in parallel, new 18F- and 68Ga-labeled GnRH analogues have been developed. In vitro radioligand binding assays performed with various GnRHR-expressing human cell lines using [125I]Triptorelin (d-Trp6-GnRH-I) as the standard radioligand revealed a very low level of GnRH receptor expression on the cell surface. Generally, total cellular activity was very low (∼3% of the applied activity), and only a small fraction (max. 40%) of cell-associated activity could be attributed to receptor-specific radioligand binding/internalization. However, substitution of fetal calf serum by NU serum in the culture medium led to increased and stable GnRHR-expression, especially in the ovarian cancer cell line EFO-27, thus allowing for a stable experimental setup for the evaluation of the new radiolabeled GnRH-I analogues. The new radiolabeled GnRH-I analogues developed in this study were all based on the d-Lys6-GnRH-I-scaffold. For 68Ga-labeling, the latter was coupled with DOTA at d-Lys6. To allow 18F-labeling via chemoselective oxime formation, d-Lys6-GnRH-I was also conjugated with Ahx (aminohexanoic acid) or β-Ala, which in turn was coupled with Boc-aminooxyacetic acid. 18F-labeling via oxime formation with 4-[18F]fluorobenzaldehyde was performed using the Boc-protected precursors. Receptor affinities of [68Ga]DOTA-GnRH-I, d-Lys6-Ahx([18F]FBOA)-GnRH-I, and d-Lys6-βAla([18F]FBOA)-GnRH-I (FBOA = fluorobenzyloxime acetyl) were determined using GnRHR-membrane preparations, and internalization efficiency of the new radioligands was determined in EFO-27 cells. Both quantities were highest for d-Lys6-Ahx([18F]FBOA)-GnRH-I (IC50 = 0.50 ± 0.08 nM vs 0.13 ± 0.08 nM for Triptorelin; internalization: 86 ± 16% of the internal reference [125I]Triptorelin), already substantially reduced in the case of the -βAla([18F]FBOA)-derivative (IC50 = 0.86 ± 0.13 nM; internalization: 42 ± 3% of [125I]Triptorelin), while the [68Ga]DOTA-analogue showed almost complete loss of binding
ISSN:1043-1802
1520-4812
DOI:10.1021/bc800058k