Zur trägerarmen Radiofluorierung von Peptiden und Proteinen über prosthetische Gruppen

Abstract

Two complementary methods, $^{18}$F-fluoroacylation and $^{18}$F-fluoroamidation, were studied for n.c.a. labeling of peptides and proteins. Nucleophilic $^{18}$F-fluorination of suitable protected carboxylic acids such as small aliphatic $\alpha$-bromo acetic- or propionic acid esters or p-trimethylammonium triflatesubstituted benzoic acid esters was the common first step in the fluoroacylation methods. Following deprotection, formation of imidazolides, succinimide esters orp-nitrophenyl esters as reactive intermediates was investigated. In view of an automated synthesis for the production of $^{18}$F-fluoroacylation agents as keyintermediates, a route to p-nitrophenylesters via the corresponding $^{18}$F-fluorinated acid chloride was also developed. The reactivity of the $^{18}$F-labeled acylation agents towards a wide range of amines with different sterical hindrance and basicities was compared. Even with low reactive aniline derivatives almost quantitative formation of the corresponding $^{18}$F-fluorinated amides was observed. For pharmacolgical studies using positron emission tomography (PET), thesomatostatin analog octreotide was selectively $^{18}$F-fluoroacylated at the N-terminus of the cyclic octapeptide using the $\epsilon$-Lys-Boc protected precursor. Binding studies with the non radioactive fluoropropionylated standard compound and rat cortex membran es revealed high affinity (pK$_{i}$=8.6) to the somatostatin receptor and almost unchanged biological activity compared to the native octreotide. As alternative to $^{18}$F-fluoroacylation the inverse reaction, using $^{18}$F-fluorinated amines and unlabeled acylation agents, was investigated. For this purpose halogenated, Boc-protected amines were used as precursors in the n.c.a. nucleophilic fluorination step. 3-[$^{18}$F]Fluoropropylamine proved to be optimal for $^{18}$F-fluoroamidation with respect to radiochemical yield and reactivity towards acylation agents. Thus suitable derivatives of the vitamin Biotin could be labeled with high radiochemical yields using $^{18}$F-fluoroacylation as weil as $^{18}$F-fluoroamidation. Both methods led to labeled compounds with full biological activity as shown by their binding ability to the protein avidin. Avidin itself could be labeled using the $^{18}$F-fluoroacylation method. In this case affinity chromatography revealed preservation ofthe biological activity of the $^{18}$F-labeled compound