Targeting somatostatin receptors: preclinical evaluation of novel 18F-fluoroethyltriazole-Tyr3-octreotate analogs for PET.

Abstract

The incidence and prevalence of gastroenteropancreatic neuroendocrine tumors has been increasing over the past 3 decades. Because of high densities of somatostatin receptors (sstr)--mainly sstr-2--on the cell surface of these tumors, (111)In-diethylenetriaminepentaacetic acid-octreotide scintigraphy has become an important part of clinical management. (18)F-radiolabeled analogs with suitable pharmacokinetics would permit PET with more rapid clinical protocols. We compared the affinity in vitro and tissue pharmacokinetics by PET of 5 structurally related (19)F/(18)F-fluoroethyltriazole-Tyr(3)-octreotate (FET-TOCA) analogs: FET-G-polyethylene glycol (PEG)-TOCA, FETE-PEG-TOCA, FET-G-TOCA, FETE-TOCA, and FET-βAG-TOCA to the recently described (18)F-aluminum fluoride NOTA-octreotide ((18)F-AIF-NOTA-OC) and the clinical radiotracer (68)Ga-DOTATATE. All (19)F-fluoroethyltriazole-Tyr(3)-octreotate compounds retained high agonist binding affinity to sstr-2 in vitro (half-maximal effective concentration, 4-19 nM vs. somatostatin at 5.6 nM). Dynamic PET showed that incorporation of PEG linkers, exemplified by (18)F-FET-G-PEG-TOCA and (18)F-FETE-PEG-TOCA, reduced uptake in high sstr-2-expressing AR42J pancreatic cancer xenografts. (18)F-FET-βAG-TOCA showed the lowest nonspecific uptake in the liver. Tumor uptake increased in the order (68)Ga-DOTATATE < (18)F-AIF-NOTA ≤ (18)F-FET-βAG-TOCA < (18)F-FET-G-TOCA. The uptake of (18)F-FET-βAG-TOCA was specific: a radiolabeled scrambled peptide, (18)F-FET-βAG-[W-c-(CTFTYC)K], did not show tumor uptake; there was lower uptake of (18)F-FET-βAG-TOCA in AR42J xenografts when mice were pretreated with 10 mg of unlabeled octreotide per kilogram; and there was low uptake of (18)F-FET-βAG-TOCA in low sstr-2-expressing HCT116 xenografts. We have developed novel fluoroethyltriazole-Tyr(3)-octreotate radioligands that combine high specific binding with rapid target localization and rapid pharmacokinetics for high-contrast PET. (18)F-FET-βAG-TOCA and (18)F-FET-G-TOCA are candidates for future clinical evaluation.