DOTA-[Tyr3]-Octreotide Acid

Metformin is the most widely prescribed drug for type 2 diabetes. Chemically, metformin is a hydrophilic base that functions as an organic cation, suggesting that it may have the capacity to inhibit the tubular reabsorption of peptide radiotracers. The purpose of this study was to investigate whether metformin could reduce renal uptake of peptidyl radiotracers and serve as a radioprotective agent for peptide receptor radionuclide therapy (PRRT). Methods: We used two radiolabeled peptides: a 68Ga-labeled cyclic (TNYL-RAW) peptide (68Ga-NOTA-c(TNYL-RAW) (NOTA: 1,4,7 triazacyclononane-1,4,7-trisacetic acid) targeting EphB4 receptors and an 111In- or 64Cu-labeled octreotide (111In/64Cu-DOTA-octreotide) (DOTA: 1,4,7,10 triazacyclododecane-1,4,7,10-tetraacetic acid) targeting somatostatin receptors. Each radiotracer was injected intravenously into normal Swiss mice or tumor-bearing nude mice in the presence or absence of metformin administered intravenously or orally. Micropositron emission tomography or microsingle-photon emission computed tomography images were acquired at different times after radiotracer injection, and biodistribution studies were performed at the end of the imaging session. To assess the radioprotective effect of metformin on the kidneys, normal Swiss mice received two doses of 111In-DOTA-octreotidein the presence or absence of metformin, and renal function was analyzed via blood chemistry and histology. Results: Intravenous injection of metformin with 68Ga-NOTA-c(TNYL-RAW) or 111In-DOTA-octreotide reduced the renal uptake of the radiotracer by 60% and 35%, respectively, compared to uptake without metformin. These reductions were accompanied by greater uptake in the tumors for both radiolabeled peptides. Moreover, the renal uptake of 111In-DOTA-octreotide was significantly reduced when metformin was administered via oral gavage. Significantly more radioactivity was recovered in the urine collected over a period of 24 h after intravenous injection of 64Cu-DOTA-octreotide in mice that received oral metformin than in mice that received vehicle. Finally, coadministration of 111In-DOTA-octreotide with metformin mitigated radio-nephrotoxicity. Conclusion: Metformin inhibits kidney uptake of peptidyl radiotracers, protecting the kidney from nephrotoxicity. Further studies are needed to elucidate the mechanisms of these finding and to optimize mitigation of radiation-induced damage to kidney in PRRT.

Diabetes mellitus (DM) is a metabolic disorder characterized by hyperglycemia, and the loss or dysfunction of pancreatic beta cells has been reported before the appearance of clinical symptoms and hyperglycemia. To evaluate beta cell mass (BCM) for improving the detection and treatment of DM at earlier stages, we focused on somatostatin receptors that are highly expressed in the pancreatic beta cells, and developed a positron emission tomography (PET) probe derived from octreotide, a metabolically stable somatostatin analog. Octreotide was conjugated with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), a chelating agent, and labeled with (68)Gallium ((68)Ga). After intravenous injection of (68)Ga-DOTA-octreotide, a 90-min emission scan of the abdomen was performed in normal and DM model rats. The PET studies showed that (68)Ga-DOTA-octreotide radioactivity was highly accumulated in the pancreas of normal rats and that the pancreatic accumulation was significantly reduced in the rats administered with an excess amount of unlabeled octreotide or after treatment with streptozotocin, which was used for the chemical induction of DM in rats. These results were in good agreement with the ex vivo biodistribution data. These results indicated that the pancreatic accumulation of (68)Ga-DOTA-octreotide represented specific binding to the somatostatin receptors and reflected BCM. Therefore, PET imaging with (68)Ga-DOTA-octreotide could be a potential tool for evaluating BCM.

In vivo somatostatin receptor scintigraphy using Octreoscan is a valuable method for the visualisation of human endocrine tumours and their metastases. Recently, several new, alternative somatostatin radioligands have been synthesised for diagnostic and radiotherapeutic use in vivo. Since human tumours are known to express various somatostatin receptor subtypes, it is mandatory to assess the receptor subtype affinity profile of such somatostatin radiotracers. Using cell lines transfected with somatostatin receptor subtypes sst1, sst2, sst3, sst4 and sst5, we have evaluated the in vitro binding characteristics of labelled (indium, yttrium, gallium) and unlabelled DOTA-[Tyr3]-octreotide, DOTA-octreotide, DOTA-lanreotide, DOTA-vapreotide, DTPA-[Tyr3]-octreotate and DOTA-[Tyr3]-octreotate. Small structural modifications, chelator substitution or metal replacement were shown to considerably affect the binding affinity. A marked improvement of sst2 affinity was found for Ga-DOTA-[Tyr3]-octreotide (IC50 2.5 nM) compared with the Y-labelled compound and Octreoscan. An excellent binding affinity for sst2 in the same range was also found for In-DTPA-[Tyr3]-octreotate (IC50 1.3 nM) and for Y-DOTA-[Tyr3]-octreotate (IC50 1.6 nM). Remarkably, Ga-DOTA-[Tyr3]-octreotate bound at sst2 with a considerably higher affinity (IC50 0.2 nM). An up to 30-fold improvement in sst3 affinity was observed for unlabelled or Y-labelled DOTA-octreotide compared with their Tyr3-containing analogue, suggesting that replacement of Tyr3 by Phe is crucial for high sst3 affinity. Substitution in the octreotide molecule of the DTPA by DOTA improved the sst3 binding affinity 14-fold. Whereas Y-DOTA-lanreotide had only low affinity for sst3 and sst4, it had the highest affinity for sst5 among the tested compounds (IC50 16 nM). Increased binding affinity for sst3 and sst5 was observed for DOTA-[Tyr3]-octreotide, DOTA-lanreotide and DOTA-vapreotide when they were labelled with yttrium. These marked changes in subtype affinity profiles are due not only to the different chemical structures but also to the different charges and hydrophilicity of these compounds. Interestingly, even the coordination geometry of the radiometal complex remote from the pharmacophoric amino acids has a significant influence on affinity profiles as shown with Y-DOTA versus Ga-DOTA in either [Tyr3]-octreotide or [Tyr3]-octreotate. Such changes in sst affinity profiles must be identified in newly designed radiotracers used for somatostatin receptor scintigraphy in order to correctly interpret in vivo scintigraphic data. These observations may represent basic principles relevant to the development of other peptide radioligands.