Preparation of n.c.a. 6-[18F]fluoro-L-tryptophan using copper-mediated radiofluorination

Preparation of n.c.a. 6-[18F]fluoro-L-tryptophan using copper-mediated radiofluorinationObjectives: The essential amino acid tryptophan is involved in various physiological processes. Besides protein synthesis, tryptophan is the precursor for serotonin and kynurenine. An upregulated utilization of tryptophan in tumor cells was reported (1). The degradation product kynurenine was described as an important factor in tumor growth and immune suppression (2). Accordingly, labelled tryptophan could enable to trace alterations of tryptophan uptake in regions of serotonergic neurons (3). Until now, [18F]fluorotryptophan was labelled using the Balz-Schiemann reaction (4) providing insufficient radiochemical yields. A recently published isotopic exchange reaction (5) appears to engender challenges in automation that are preventing their routine use. In the last few years, several innovative 18F-fluorination methods have been published (6-8). The copper-mediated radiofluorination method exhibits several major advantages like sufficient RCYs, bench-stable precursors and mild reaction conditions and was therefore chosen in this study. First, the synthesis of an appropriate aryl boronic ester was developed. The labelling step and hydrolysis were optimized with regard to automation.Methods: An appropriate precursor for 18F-labelling was designed starting from bromo-indole. The Schöllkopf’s auxiliary (9) was introduced within five steps. The pinacol boronate ester was inserted using a Suzuki-Miyaura coupling reaction. Figure 1: Synthesis scheme for the pinacol ester tryptophan derivative 3.In preliminary studies indole derivatives with the pinacol ester in different position were used as model compounds. The highest yields were obtained at 6-position of the indole motif. Besides reaction time and temperature, different reaction conditions like the type of nucleophilicity enhancer, temperature, time and solvents were examined. The subsequent deprotection step was optimized with regard to acid, reaction time and temperature. Figure 2: Radiosynthesis of 6-[18F]fluoro-L-tryptophan.Results: The synthesis of the appropriate precursor for copper-mediated radiofluorination was achieved within six steps and an overall yield of 37 %. The optimized radiofluorination conditions are as follows: precursor and Cu(OTf)2(py)4 were solved in sulfolane/acetonitrile were given on dried tetraethylammonium [18F]fluoride (TEA[18F]F) and heated for 20 min at 110 °C. The radiochemical conversion (RCC) protected [18F]4 was 52±10 % determined by radioTLC. Purification of [18F]4 from residual [18F]fluoride and copper was achieved by using a silica cartridge. Highest RCC for deprotection yields were obtained using 48 % hydrobromic acid for 25 min at 165°C giving 36.1±5 % RCC.The total radio synthesis of 6-[18F]fluoro-L-tryptophan was carried out within 120 min including HPLC purification (Figure 3) with an overall radiochemical yield of 13±4 %. The radiochemical purity was more than 99 %, with an enantiomeric excess of 89 % and a specific activity of 280 GBq/µmol. Automation of the reaction as well as the biological evaluation of the radiotracer is in progress. Figure 3: Radioanalysis of HPLC purified 6-[18F]fluoro-L-tryptophan.Conclusion: The herein reported method allows to obtain n.c.a. 6-[18F]fluoro- L-tryptophan in high RCYs. Accordingly, the potential of this tracer could be evaluated in different applications like tumor detection or psychological disorders. Furthermore, an automated synthesis unit equipped with a two-reactor-system is in progres