Purpose: The aim of this study was to evaluate the applicability of [(99m)Tc]Demotensin 5 and 6 [Formula: see text] in the targeted diagnostic imaging of neurotensin subtype 1 receptor (NTS1-R)-expressing tumours.
Methods: Labelling of Demotensin 5 and 6 with (99m)Tc was conducted by brief incubation with (99m)TcO(4) (-), SnCl(2) and citrate anions in alkaline medium at ambient temperature. Affinities of conjugates for the NTS1-R were determined by competition binding experiments in WiDr cell membranes using [(125)I-Tyr(3)]NT as the radioligand. Saturation binding assays were conducted for [(99m)Tc/(99g)Tc]Demotensin 6 in WiDr cell membranes. Internalisation of [(99m)Tc]Demotensin 5 and 6 was studied at 37 degrees C in WiDr cells. Biodistribution of [(99m)Tc]Demotensin 5 and 6 was performed in female Swiss nu/nu mice bearing human WiDr xenografts.
Results: Unlabelled conjugates showed a high affinity for the human NTS1-R (Demotensin 5 IC(50)=0.03+/-0.01 nM; Demotensin 6 IC(50)=0.08+/-0.02 nM), while high affinity was also exhibited by (radio)metallated [(99m)Tc/(99g)Tc]Demotensin 6 (K (d)=0.13+/-0.01 nM). [(99m)Tc]Demotensin 5 and 6 internalised rapidly and specifically in WiDr cells. After injection in WiDr tumour-bearing mice, radiopeptides, and especially the doubly stabilised [(99m)Tc]Demotensin 6, showed NTS1-R-mediated uptake in the intestines and in the implanted tumour (4.30+/-0.45%ID/g at 1 h post injection) and rapid renal excretion from non-target tissues into the urine.
Conclusion: [(99m)Tc]Demotensin 6 shows a favourable preclinical profile and further testing in patients is warranted to monitor its eventual applicability as a radiotracer in the diagnostic imaging of NTS1-R-positive tumours.