Radioiodinated Exendin-4 Is Superior to the Radiometal-Labelled Glucagon-Like Peptide-1 Receptor Probes Overcoming Their High Kidney Uptake

Abstract

GLP-1 receptors are ideal targets for preoperative imaging of benign insulinoma and for quantifying the beta cell mass. The existing clinical tracers targeting GLP-1R are all agonists with low specific activity and very high kidney uptake. In order to solve those issues we evaluated GLP-1R agonist Ex-4 and antagonist Ex(9–39) radioiodinated at Tyr40 side by side with [Nle14,Lys40(Ahx-DOTA-68Ga)NH2]Ex-4 (68Ga-Ex-4) used in the clinic. The Kd, Bmax, internalization and binding kinetics of [Nle14,125I-Tyr40-NH2]Ex-4 and [Nle14,125I-Tyr40-NH2]Ex(9–39) were studied in vitro using Ins-1E cells. Biodistribution and imaging studies were performed in nude mice bearing Ins-1E xenografts. In vitro evaluation demonstrated high affinity binding of the [Nle14,125I-Tyr40-NH2]Ex-4 agonist to the Ins-1E cells with fast internalization kinetics reaching a plateau after 30 min. The antagonist [Nle14,125I-Tyr40-NH2]Ex(9–39) did not internalize and had a 4–fold higher Kd value compared to the agonist. In contrast to [Nle14,125I-Tyr40-NH2]Ex(9–39), which showed low and transient tumor uptake, [Nle14,125I-Tyr40-NH2]Ex-4 demonstrated excellent in vivo binding properties with tumor uptake identical to that of 68Ga-Ex-4, but substantially lower kidney uptake resulting in a tumor-to-kidney ratio of 9.7 at 1 h compared to 0.3 with 68Ga-Ex-4. Accumulation of activity in thyroid and stomach for both peptides, which was effectively blocked by irenat, confirms that in vivo deiodination is the mechanism behind the low kidney retention of iodinated peptides. The 124I congener of [Nle14,125I-Tyr40-NH2]Ex-4 demonstrated a similar favourable biodistribution profile in the PET imaging studies in contrast to the typical biodistribution pattern of [Nle14,Lys40(Ahx-DOTA-68Ga)NH2]Ex-4. Our results demonstrate that iodinated Ex-4 is a very promising tracer for imaging of benign insulinomas. It solves the problem of high kidney uptake of the radiometal-labelled tracers by improving the tumor-to-kidney ratio measured for [Nle14,Lys40(Ahx-DOTA-68Ga)NH2]Ex-4 by 32 fold.

Pancreatic imaging using an antibody fragment targeting the zinc transporter type 8: a direct comparison with radio-iodinated Exendin-4

“…A surprising finding is the lower pancreas uptake of [ 125 I]Exendin-4 in our study in comparison with another 125 I-labeled Exendin-4 reported by Läppchen et al [ 32 ]. This group have successfully labeled Exendin-4 with 125 I, and the uptake of the tracer, [Nle 14 , 125 I-Tyr 40 -NH 2 ]Ex-4, in the pancreas was 25.3 ± 4.2%IA/g at the same 1-h time point [ 32 ]. It is important to mention that Exendin-4 was labeled using two different methods in these studies.…”

The somatostatin receptor 2 antagonist 64Cu-NODAGA-JR11 outperforms 64Cu-DOTA-TATE in a mouse xenograft model

“…All cell experiments were performed in a human embryonic kidney HEK-293 cell line stably expressing human sst2 receptors (HEK-hsst2) (a gift from Prof. Schulz, University of Jena) plated in 6-well plates in triplicates (10 6 cells/well). The saturation binding experiments were performed with radioligand ( 64/nat Cu-NODAGA-JR11 and 64/nat Cu-DOTA-TATE) concentrations ranging from 0.5–75 nM and 0.5–90 nM, respectively, for 2 hours at +4ºC as described previously [ 24 ]. The K d and Bmax values were calculated using GraphPad.…”

Exendin‐4 analogs in insulinoma theranostics

“…For both indium‐111 and technetium‐99m exendin‐based tracers, tumor‐to‐kidney ratios do increase over time, and therefore, authors suggest that additional late scans should be made in case of negative scans early after injection, as has been suggested for longer‐lived positron‐emitting radionuclides such as zirconium‐89 as well . The use of radioiodinated exendin‐based tracers should also be considered to reduce renal accumulation …”

“…[ 125 I]I‐BH‐exendin(9‐39) exhibited high uptake in murine pancreas, and the antagonist [Lys 27 ( 125 I]I‐BH‐exendin(9‐39)NH 2 demonstrated tumor uptake that was comparable with the agonists but with a substantially reduced kidney uptake . In contrast, the antagonist [Nle 14 ,[ 125 I]I‐Tyr 40 ‐NH 2 ]exendin(9‐39) showed lower affinity and lower internalization than the more favorable agonist [Nle 14 ,[ 125 I]I‐Tyr 40 ‐NH 2 ]exendin‐4 …”