SYNTHESIS AND QUALITY ASSESSMENT OF THE POSITRON EMITTER RADIOTRACER 2-[18F]-FLUOROETHYLCHOLINE, A [18F]-LABELLED CHOLINE ANALOGUE FOR TUMOURS IMAGING

Abstract

Introduction: [18F]-labelled choline analogues, such as 2-[18F]-Fluoroethylcholine ( [18F]-FECH ), have suggested to be the new generation of choline derivatives for the imaging of prostate and brain tumours. In fact, tumour cells with enhanced proliferation rate exhibit an improved choline uptake due to the increased membrane phospholipids biosynthesis. The aim of this dissertation is the development and validation of a reliable automatic synthesis of [18F]-FECH. Moreover, in order to assure a safe employment of the radiotracer in a research clinical trial, particular attention was paid to establish the proper quality controls required. Materials and Methods: [18F]-FECH was synthesized with varying synthetic approaches (one step and two steps reaction, preparative HPLC or solid phase extraction purification) and by using two different automatic synthesizers (FXF-N Tracer Lab Synthesizer and Mx Tracer Lab). In the first step, cyclotron produced [18F]-Fluoride was reacted for 5 minutes at 80°C, with a 20 mg/ml solution of 1,2- bis(tosyloxy)ethane (diOTsEt) in acetonitrile in order to obtain the [18F]-EtOTs intermediate. The reaction was performed in anhydrous condition and in presence of tetrabutylammonium (TBA) as transfer phase catalyst. In the second step, without any further purification, [18F]-EtOTs was reacted for 10 minutes at 100° C in the same reactor, with a 88.6 mg/ml solution of N,N-dimethylethanolamine (DMAE) in acetonitrile. The reaction mixture was then purified by SPE or HPLC and then eluted with 16 ml of physiological solution. The effect of different precursors (Br2Et, diNosEt and BrOtsEt) on the reactions was also assessed but with lower success. Quality controls on the final product were performed by means of TLC, GC and HPLC equipped with conductimetric, UV and radiometric detectors. Sterility and absence of bacterial endotoxin were assessed by following the European Pharmacopeia guidelines. Results: [18F]-FECH was prepared with a yield between 17±2 and 41 ±2 % uncorrected for decay in a time ranging between 33 and 55 minutes, depending on the synthetic approach and on the synthesizer used. Radiochemical purity was always greater than 99 % with both free [18F]-fluoride and [18F]-EtOTs less than 0.5 % of the total radioactivity. The DMAE concentration always resulted less than 15 mg/L and was detected with either GC or HPLC with conductimetric detection. The main chemical impurity was due to dimethylmorpholinium (dMM), a cationic by-product derived from the direct reaction and cyclization of diOTsEt and DMAE. The dMM concentration was detected by HPLC with conductimetric detection and always resulted less than 180 mg/L. The absence of tosylated by-products or free OTs- anions was assessed by HPLC with UV detection. Residual solvents and TBA concentrations respected the limits fixed for an injectable radiopharmaceutical preparation and were detected by means of GC in the same analysis conditions used for DMAE. All the sample analyzed were sterile. Conclusions: [18F]-FECH radiopharmaceutical was synthesized with an high yield and radiochemical purity by using a automatic synthesizers. The complete elimination of the dMM by-product from the final solution was not possible by means of a solid phase extraction as a purification method. A research clinical trial was activated and, up to now, more than 150 patients were safely examined