Biokinetics, metabolism and PET imaging with Cu-64 and Ga-68 labeled ghrelin analogues in rats

Ghrelin is a gastric peptide involved in food intake control, and growth hormone release. Besides its well-defined orexigenic role, ghrelin is likely involved in tumor development and growth. Therefore, non-invasive imaging agents for determining ghrelin receptors are desirable. However, ghrelin receptors are expressed in low level making detection via imaging difficult. The labeling using activated esters like 4-(F-18-fluoro)benzoyl-succinimide (2) was not possible due to the existence of three primary amino groups per peptide. Therefore, human ghrelin1-28 [Lys16(NODAGA)]-ghrelin1-28 and a short ghrelin inverse agonist NODAGA-KwFwLL-NH2 were synthesised on solid-phase. Subsequently, radiolabeling yielded Cu-64- and Ga-68-NODAGA-peptides in high radiochemical purity (>98%) with a specific activity >10 GBq/Gmol. For basic radiopharmacological characterization biodistribution, small animal PET, and metabolite analysis in arterial blood plasma were carried out in Wistar rats with all 4 radiotracers. The ghrelin analogues were in high amounts taken up by the kidneys. The Cu-64- and Ga-68-NODAGA-KwFwLL-NH2 were for the most part hepathobilliary eliminated. The metabolite analysis showed the well-known des acyl ghrelin as the main metabolite in vivo reaching 90% in blood plasma after one hour. On the other site the NODAGAKwFwLL-NH2 peptides were highly stabile; only 5% metabolites were observed after one hour in rat plasma. Interestingly, the biodistribution and kinetics were similar for the pairs of peptide labeled ether with Cu-64 or Ga-68. This shows the small effect of the different ionization of the Ga-68 (neutral) or Cu-64 (one negative charge) NODAGA complexes on the biodistribution. The blood clearance of these peptides was relative low, with about 50% of starting amount after one hour. The high stability and low blood clearance are important prerequisites for further studies of ghrelin receptor imaging. These results indicate that further in vivo evaluation of radiolabeled ghrelin inverse agonists as potential PET tracers for ghrelin receptors is warranted.

Reference: (1) Holst, B., et al. (2003) High constitutive signaling of the ghrelin receptor--identification of a potent inverse agonist. Mol Endocrinol 17, 2201-10. (2) Bergmann, R., et al. (2002) Biodistribution and catabolism of (18)F-labeled neurotensin(8-13) analogs. Nucl Med Biol 29, 61-72.

Acknowledgement: This project was partially supported by FP7 project “GIPIO”, Project Reference: 223057