(2-[¹⁸F]fluoropropionyl-(d)phe¹)-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 radiopharmaceutical for quantitative in vivo mapping of somatostatin receptors. [¹⁸F]-fluoroacylation is achieved with n.c.a. 2-[¹⁸F]fluoropropionic acid 4-nitrophenylester which is reacted with ε{lunate}-Boc-Lys⁵-octreotide. After deprotection the desired N^α-[¹⁸F]fluoropropionylated octreotide ([¹⁸F]SDZ 223-228) is obtained. Final HPLC purification gives rise to radiochemical yields of 65 ± 5% based on the fluoroacylation agent. Binding experiments using rat cortex membranes indicate an affinity for somatostatin receptors of pK_i = 8.6 ± 0.2. The biological activity of this SRIF analog is demonstrated by the inhibition of growth hormone release from cultured pituitary cells. The pIC₅₀ in this test system is 8.75, indicating full biological activity. Biodistribution studies with NMRI mice show predominantly renal excretion, rapid blood clearance and only negligible bone activity, i.e. formation of free fluoride.