Dose-on-demand of diverse ¹⁸F-fluorocholine derivatives through a two-step microfluidic approach

Abstract

Introduction

The validation and confirmation of clinical usefulness of new and known positron emission tomography (PET) tracers require stable production routes and simple and robust radiochemical procedures. Microfluidic technologies are regarded as an approach that could allow an unprecedented flexibility and productivity in PET radiopharmaceutical research. In this work, we will show how a commercially available microfluidic system can be used for a sequential and repeatable radiosynthesis of three different fluorocholine analogues currently under investigation as tumor tracers.

Methods

Advion microfluidic system was used for performing the synthesis and purification of [¹⁸F]fluoromethyl, [¹⁸F]fluoroethyl or [¹⁸F]fluoropropyl choline employing a two-step approach, starting from the corresponding alkyl-ditosylate and reacting the [¹⁸F]fluorotosylate obtained in the first step with neat dimethylethanolamine. The purification was obtained using a recyclable SPE cartridge set.

Results

The three products, fluoromethylcholine, fluoroethylcholine and fluoropropylcholine, were obtained in good to optimum yields (22%–54% decay corrected) with a 15-min procedure. The production could be restarted several times for producing each one of the tracers without decrease in yields and purities, in accordance with a dose-on-demand (DOD) approach. The final products were formulated in isotonic saline solution.

Conclusion

The described approach gives a proof of principle of the enhanced productivity obtainable using a microfluidic approach; in particular, the possibility to produce the reported tracers in a DOD fashion following a homogeneous synthetic and purification approach will foster further studies on the clinical evaluation of the best fluorocholine analogue for prostate cancer imaging without biasing for differences in radiochemical approach.

Introduction

Positron emission tomography (PET) represents a powerful molecular imaging technique largely used for diagnosis and therapy control but also in pharmaceutical and medicinal chemistry research. The scientific community is therefore very active in searching new tracers, mainly labeled with ¹¹C and ¹⁸F, which could act as biomarkers with high prognostic value.

Cancer is the biggest field of both diagnostic and research applications of PET. Within this context, prostate cancer has high prevalence in adult males; therefore, efficient in vivo molecular imaging agents, possibly having relationship to prognosis and therapy outcome, are required. ¹¹C-labeled choline [1], [2] and acetate [3], [4] have been proposed as candidate tracers for selective imaging of prostate cancer. However, the use of these tracers is limited by the 20-min half-life of ¹¹C, and therefore, these are utilizable only in centers with an on-site cyclotron. ¹⁸F has a longer half-life and hence would allow the preparation of fluorinated analogues having a longer span of use, including the shipment to centers which are distant from the production site. However, introduction of a fluorine atom in such molecules may change its biological acceptance; this is the case of fluoroacetate [5] which has a different fate than acetate, but may not be the case of fluorinated analogues of choline. Indeed, several groups have reported on the synthesis of fluorinated choline analogues [6], [7], [8], [9], [10], such as fluoromethylcholine (FMC), fluoroethylcholine (FEC) and fluoropropylcholine (FPC). Some of these fluorinated compounds have already been used in human studies with variable success in diagnosing prostatic tumor and metastases [11]. In general, FMC is considered a better analogue of choline than FEC and FPC [6]. However, a head-to-head comparison of all these tracers vs. ¹¹C-choline is not currently available, and quantitative relative determination of biodistribution parameters including urinary excretion would help to further characterize their validation as imaging tracers and possibly as biomarkers of prostate cancer [12], [13], [14], [15].

The aim of this work is to report on a microfluidic approach that allows the synthesis, using a single experimental platform and a common procedure, of one or more of the ¹⁸F-fluorinated cholines previously mentioned and to provide further evidence that this approach could lead to a dose-on-demand (DOD) preparation of each of them for their easy testing and comparison in tumor imaging.

The use of microfluidics is receiving increasing attention in radiochemistry for the possibility to obtain expeditious and reliable syntheses of PET radiotracers [16], [17], [18], [19], [20]. We describe here the use of microfluidics for the preparation of fluorinated cholines and demonstrate the ability of this approach in reducing the quantity of starting radioactivity, precursors and solvents to what is just sufficient to obtain, at the end of the process, a single dose of tracer and include the possibility to promptly restart the line and have access to an additional production run which may be either of the same or of a different tracer. The possibility to dramatically reduce reaction time and reagent consumption is well in line with the limits imposed by the short half-lives of PET radionuclides and with the advantage that may be obtained by lowering the amount of chemical ingredients to be used in the synthesis. This DOD approach may be extended to produce different tracers, further to the ¹⁸F-fluorocholines reported here, from the same batch of starting radioactivity, thus reducing personnel exposure while allowing maximum flexibility to respond to the users' schedule.

These features are also important in basic radiopharmaceutical research, where ready availability of repeated productions of the same tracer or even of different radiotracers is often an advantage during in vivo pharmacological screening and/or metabolic investigation of novel radiotracers [21], [22], [23], [24].