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Molecules 2019, 24(6), 1015; https://doi.org/10.3390/molecules24061015

Comparing Gly11/dAla11-Replacement vs. the in-Situ Neprilysin-Inhibition Approach on the Tumor-targeting Efficacy of the 111In-SB3/111In-SB4 Radiotracer Pair

1
Molecular Radiopharmacy, INRASTES, NCSR “Demokritos”, 15310 Athens, Greece
2
Department of Radiology, Erasmus MC, 3015 GD Rotterdam, The Netherlands
3
Cytrotron Rotterdam BV, Erasmus MC, 3015 GD Rotterdam, The Netherlands
*
Author to whom correspondence should be addressed.
Academic Editor: Licia Uccelli
Received: 31 January 2019 / Revised: 9 March 2019 / Accepted: 11 March 2019 / Published: 13 March 2019
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Abstract

Background: The GRPR-antagonist 68Ga-SB3 visualized prostate cancer lesions in animal models and in patients. Switching radiometal from 68Ga to 111In impaired tumor targeting in mice, but coinjection of the neprilysin (NEP)-inhibitor phosphoramidon (PA) stabilized 111In-SB3 in circulation and remarkably increased tumor uptake. We herein report on the biological profile of 111In-SB4: 111In-[dAla11]SB3. Methods: The biological responses of 111In-SB3/SB4 were compared in PC-3 cells and animal models. Results: Gly11/dAla11-replacement deteriorated GRPR-affinity (SB4 IC50: 10.7 ± 0.9 nM vs. SB3 IC50: 4.6 ± 0.3 nM) and uptake in PC-3 cells (111In-SB4: 1.3 ± 0.4% vs. 111In-SB3 16.2 ± 0.8% at 1 h). 111In-SB4 was more stable than 111In-SB3, but PA-coinjection stabilized both radiotracers in peripheral mice blood. Unmodified 111In-SB3 showed higher uptake in PC-3 xenografts (8.8 ± 3.0%ID/g) vs. 111In-SB4 (3.1 ± 1.1%ID/g) at 4 h pi. PA-coinjection improved tumor uptake, with 111In-SB3 still showing superior tumor targeting (38.3 ± 7.9%ID/g vs. 7.4 ± 0.3%ID/g for 111In-SB4). Conclusions: Replacement of Gly11 by dAla11 improved in vivo stability, however, at the cost of GRPR-affinity and cell uptake, eventually translating into inferior tumor uptake of 111In-SB4 vs. unmodified 111In-SB3. On the other hand, in-situ NEP-inhibition turned out to be a more efficient and direct strategy to optimize the in vivo profile of 111In-SB3, and potentially other peptide radiotracers. View Full-Text
Keywords: GRPR-antagonist; bombesin-like radioligand; tumor targeting; tumor imaging; neprilysin-inhibition; phosphoramidon; in vivo stability GRPR-antagonist; bombesin-like radioligand; tumor targeting; tumor imaging; neprilysin-inhibition; phosphoramidon; in vivo stability
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Lymperis, E.; Kaloudi, A.; Kanellopoulos, P.; de Jong, M.; Krenning, E.P.; Nock, B.A.; Maina, T. Comparing Gly11/dAla11-Replacement vs. the in-Situ Neprilysin-Inhibition Approach on the Tumor-targeting Efficacy of the 111In-SB3/111In-SB4 Radiotracer Pair. Molecules 2019, 24, 1015.

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