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JUEL-3602
Schirbel, Andreas
Zur Synthese trägerarmer PET-Radiotracer mit Kohlenstoff-11
110 S., 1998



Carbon-11 offers the unique possibility of authentic labelling of molecules as radioindicators for non invasive and quantitative determination of physiological functions via positron emission tomography (PET). Therefore, the goal of this thesis was to synthesize of different n.c.a. 11C-labelled pharmaceuticals for in vivo distribution studies with PET.

For the determination of the pharmacokinetics of [1-11C]acetate in porcine myocardium during prolonged ischemia, n.c.a. [1-11C]acetate was synthesized via carboxylation of methylmagnesium bromide with in target produced n.c.a. [11C]CO2 with a radiochemical yield (RCY) of 68 ± 7 %. The fast (18 min) and reliable radiosynthesis allowed for repeated tracer administration at short intervals (< 20 min).

In order to study the pharmacokinetics and metabolism of acetylsalicylic acid (Aspirin®) in humans, [1-11C]acetylsalicylic acid, acetyl-[carboxy-11C]salicylic acid and [carboxy-11C]salicylic acid were prepared. N.c.a. [1-11C]acetylsalicylic acid was synthesized via the reaction of [1-11C]acetylchloride with salicylic acid salts. The use of the silver salt proved to be superior to the sodium salt and resulted in radiochemical yields of 32 ± 5 %. Base-line (clean) separation of the labelled product was achieved using radio-HPLC.

With regard to the preparation of n.c.a. [carboxy-11C]salicylic acid, several protected and unprotected phenol derivates were metallated and subsequently carboxylated using n.c.a. [11C]CO2. Best results (87 ± 3 % RCY) could be achieved with 2-(methoxymethoxy)-phenylmagnesium iodide as a precursor and subsequent quantitative cleavage of the MOM-group. Acetylation of n.c.a. [carboxy-11C] salicylic acid to acetyl-[carboxy-11C]salicylic acid was performed using acetylchloride in CH2Cl2 with a radiochemical yield of 65 ± 4 %.

Lubeluzole, a new cerebroprotective compound for use in ischemic stroke, was labelled via 11C-methylation of the corresponding desmethyl compound. The main reaction parameters like concentration of precursor, time, temperature, solvent and added base were investigated. Under optimized conditions a RCY of 82 ± 3 % was obtained at room temperature within one minute. A labelled side product was identified as (endo-methyl)-lubeluzole and its formation was suppressed by the choice of DMSO as the most suitable solvent.

Furthermore, a new method for the radiosynthesis of n.c.a. [11C]ureas was developed which avoided the use of the highly toxic [11C]COCl2 (phosgene). Starting from the in target produced n.c.a. [11C]CO2 and subsequent dehydration of intermediately formed [11C]carbamates, an efficient one-pot synthetic procedure could be established. The model compound aniline could efficiently be converted with n.c.a. [11C]CO2 into the corresponding carbamate salt and subsequently be dehydrated with phosphorylchloride to give [11C]phenylisocyanate. In contrast to reactions under stoichiometric conditions, the labelled isocyanate was isolated in relatively low RCY (16 ± 4 %) because it reacted in situ with the excess of aniline to the corresponding [11C]urea (65 ± 4 % RCY). Correspondingly, the excess of the dehydrating reagent phosphorylchloride caused a consecutive reaction, thereby generating N,N´-diphenyl[11C]carbodiimide as a side product, although in low RCY (< 5 %). Therefore, the isocyanate and the urea cannot be prevented from further reaction under n.c.a. reaction conditions. Also, attempts to isolate and analyze the carbamate formed in the first reaction step failed due to its high reactivity and water sensitivity. Hence, optimization of the individual reaction parameters of the consecutive steps was performed by determining the radiochemical yields of the isocyanate, the urea and the carbodiimide. Optimization of the reaction parameters showed that the temperature of the carbamate formation was the critical factor for obtaining a high RCY.

11C-Carbonylation of benzylamine and phenethylamine resulted in 80 - 90 % and 20 - 30 % RCY, respectively. In both cases, no labelled isocyanates were isolated due to the high nucleophilicity of the amines.

For possible synthesis of several radiopharmaceuticals containing the benzimidazolone moiety, the 11C-carbonylation was performed with ortho diaminobenzene as a model compound. The desired cyclized product was obtained in 20 - 30 % RCY, while a competing reaction yielded the open-chain urea with about 40 % RCY.

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