(2-[F-18]FLUOROPROPIONYL-(D)PHE(1))-OCTREOTIDE, A POTENTIAL RADIOPHARMACEUTICAL FOR QUANTITATIVE SOMATOSTATIN RECEPTOR IMAGING WITH PET - SYNTHESIS, RADIOLABELING, IN-VITRO VALIDATION AND BIODISTRIBUTION IN MICE

Octreotide is labeled with fluorine-18 as a potential radiopharmaceuti cal for quantitative in vivo mapping of somatostatin receptors. [F-18] -fluoroacylation is achieved with n.c.a. 2-[F-18]fluoropropionic acid 4-nitro-phenylester which is reacted with epsilon-Boc-Lys(5)-octreotid e. After deprotection the desired N(a)lpha-[F-18]fluoro-propionylated octreotide ([F-18]SDZ 223-228) is obtained. Final HPLC purification gi ves rise to radiochemical yields of 65 +/- 5% based on the fluoroacyla tion agent. Binding experiments using rat cortex membranes indicate an affinity for somatostatin receptors of pK(i) = 8.6 +/- 0.2. The biolo gical activity of this SRIF analog is demonstrated by the inhibition o f growth hormone release from cultured pituitary cells. The pIC(50) in this test system is 8.75, indicating full biological activity. Biodis tribution studies with NMRI mice show predominantly renal excretion, r apid blood clearance and only negligible bone activity, i.e. formation of free fluoride.