GALLIUM-67 GALLIUM-68-[DFO]-OCTREOTIDE - A POTENTIAL RADIOPHARMACEUTICAL FOR PET IMAGING OF SOMATOSTATIN RECEPTOR-POSITIVE TUMORS - SYNTHESIS AND RADIOLABELING IN-VITRO AND PRELIMINARY IN-VIVO STUDIES/

When labeled with gamma-emitting radionuclides, somatostatin analogs h ave the potential to localize somatostatin receptor-positive tumors us ing gamma camera scintigraphy. We present a somatostatin analog, [DFO] -octreotide (SDZ 216-927), that comprises desferrioxamine B coupled to octreotide via a succinyl tinker. This conjugate can be labeled with either Ga-67 for gamma scintigraphy or Ga-68 for PET imaging. The Ga-6 7-labeled conjugate is stable in vitro to autoradiolysis over a 24-hr period. Methods: Rats bearing a somatostatin receptor-positive pancrea tic islet cell tumor were injected with 20 MBq of Ga-67[DFO]-octreotid e (33 GBq Ga-67/mu mole). Results: After 1 hr, the accumulation of Ga- 67[DFO]-octreotide was 0.38 +/- 0.08 %ID/g and the tumor-to-nontumor r atios for blood, muscle, river and intestine were 2.5, 7.4, 1.9 and 1. 6, respectively. PET studies with Ga-68[DFO]-octreotide recorded a ver y rapid accumulation at the tumor and a subsequent residence half-life of about 6 hr. Conclusion: Gallium-68-[DFO]-octreotide can be used in PET studies to diagnose receptor-positive tumors such as gastroentero -pancreatic, small-cell lung and breast tumors.