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

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 Tyr(40) side by side with [Nle(14),Lys(40)(Ahx-DOTA-(68)Ga)NH(2)]Ex-4 ((68)Ga-Ex-4) used in the clinic. The Kd, Bmax, internalization and binding kinetics of [Nle(14),(125)I-Tyr(40)-NH(2)]Ex-4 and [Nle(14),(125)I-Tyr(40)-NH(2)]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 [Nle(14),(125)I-Tyr(40)-NH(2)]Ex-4 agonist to the Ins-1E cells with fast internalization kinetics reaching a plateau after 30 min. The antagonist [Nle(14),(125)I-Tyr(40)-NH(2)]Ex(9–39) did not internalize and had a 4–fold higher K(d) value compared to the agonist. In contrast to [Nle(14),(125)I-Tyr(40)-NH(2)]Ex(9–39), which showed low and transient tumor uptake, [Nle(14),(125)I-Tyr(40)-NH(2)]Ex-4 demonstrated excellent in vivo binding properties with tumor uptake identical to that of (68)Ga-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 (68)Ga-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 (124)I congener of [Nle(14),(125)I-Tyr(40)-NH(2)]Ex-4 demonstrated a similar favourable biodistribution profile in the PET imaging studies in contrast to the typical biodistribution pattern of [Nle(14),Lys(40)(Ahx-DOTA-(68)Ga)NH(2)]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 [Nle(14),Lys(40)(Ahx-DOTA-(68)Ga)NH(2)]Ex-4 by 32 fold.