[en] Radiochemists/radiopharmacists, involved in the preparation of radiopharmaceuticals are regularly confronted with the requirement of continuous high quality productions in their day-to-day business. One of these requirements is high specific or molar activity of the radiotracer in order to avoid e.g. receptor saturation and pharmacological or even toxic effects of the applied tracer for positron emission tomography. In the case of ¹¹C-labeled radiotracers, the reasons for low molar activity are manifold and often the search for potential ¹²C-contaminations is time-consuming.

[en] In drug development, biomarkers for cerebral applications have a lower success rate compared to cardiovascular drugs or tumor therapeutics. One reason is the missing blood brain barrier penetration, caused by the tracer's interaction with efflux transporters such as the P-gp (MDR1 or ABCB1). Aim of this study was the development of a reliable model to measure the interaction of radiotracers with the human efflux transporter P-gp in parallel to the radiolabeling process. LigandTracer® Technology was used with the wildtype cell line MDCKII and the equivalent cell line overexpressing human P-gp (MDCKII-hMDR1). The method was evaluated based on established PET tracers with known interaction with the human P-gp transporter and in nanomolar concentration (15 nM). [¹¹C]SNAP-7941 and [¹⁸F]FE@SNAP were used as P-gp substrates by comparing the real-time model with an uptake assay and μPET images. [¹¹C]DASB [¹¹C]Harmine, [¹⁸F]FMeNER,[¹⁸F]FE@SUPPY and [¹¹C]Me@HAPTHI were used as tracers without interactions with P-gp in vitro. However, [¹¹C]Me@HAPTHI shows a significant increase in SUV levels after blocking with Tariquidar. The developed real-time kinetic model uses directly PET tracers in a compound concentration, which is reflecting the in vivo situation. This method may be used at an early stage of radiopharmaceutical development to measure interactions to P-gp before conducting animal experiments.

[en] 

Background

[¹¹C]erlotinib has been proposed as a PET tracer to visualize the mutational status of the epidermal growth factor receptor (EGFR) in cancer patients. For clinical use, a stable, reproducible and high-yielding radiosynthesis method is a prerequisite. In this work, two production schemes for [¹¹C]erlotinib applied in a set of preclinical and clinical studies, starting from either [¹¹C]CH₄ or [¹¹C]CO₂, are presented and compared in terms of radiochemical yields, molar activities and overall synthesis time. In addition, a time-efficient RP-HPLC method for quality control is presented, which requires not more than 1 min.

Results

[¹¹C]erlotinib was reliably produced applying both methods with decay-corrected radiochemical yields of 13.4 ± 6.2% and 16.1 ± 4.9% starting from in-target produced [¹¹C]CO₂ and [¹¹C]CH₄, respectively. Irradiation time for the production of [¹¹C]CO₂ was higher in order to afford final product amounts sufficient for patient application. Overall synthesis time was comparable, mostly attributable to adaptions in the semi-preparative HPLC protocol. Molar activities were 1.8-fold higher for the method starting from [¹¹C]CH₄ (157 ± 68 versus 88 ± 57 GBq/μmol at the end of synthesis).

Conclusions

This study compared two synthetic protocols for the production of [¹¹C]erlotinib with in-target produced [¹¹C]CO₂ or [¹¹C]CH₄. Both methods reliably yielded sufficiently high product amounts for preclinical and clinical use.

[en] The decision whether an in-house produced short-lived radiopharmaceutical can be applied in-vivo is based on (1) the fulfilment of all quality criteria; (2) the availability of enough radioactivity for subsequent imaging; and (3) a molar activity (MA) above the set limits to guarantee safe administration without competing occupancy of the non-radioactive compound; and (4) an activity concentration, which is high enough for the application in certain preclinical studies. Hence, time reduction can be of major importance to increase final product yields, MA and activity concentrations. Usually, optimization in this respect only focuses on the radiotracer preparation steps but especially quality control (QC) is rarely even mentioned. Therefore, aim of this work is the establishment of optimized conditions for chromatographic analysis using HPLC within the QC to enable a significant time reduction, which then directly leads to an increase in available amount of radioactive product as well as MA at the time of application.

[en] Introduction: Due to its involvement in a variety of pathologies (obesity, diabetes, gut inflammation and depression), the melanin concentrating hormone receptor 1 (MCHR1) is a new target for the treatment of these lifestyle diseases. We previously presented the radiosynthesis of [¹¹C]SNAP-7941, the first potential PET tracer for the MCHR1. Methods: We herein present its in vitro and in vivo evaluation, including binding affinity, plasma stability, stability against liver mircrosomes and carboxylesterase, lipohilicity, biodistribution, in vivo metabolism and small-animal PET. Results: [¹¹C]SNAP-7941 evinced high stability against liver microsomes, carboxylesterase and in human plasma. The first small-animal PET experiments revealed a 5 fold increased brain uptake after Pgp/BCRP inhibition. Therefore, it can be assumed that [¹¹C]SNAP-7941 is a Pgp/BCRP substrate. No metabolites were found in brain. Conclusion: On the basis of these experiments with healthy rats, the suitability of [¹¹C]SNAP-7941 for the visualisation of central and peripheral MCHR1 remains speculative

[en] The intention for the present study was to implement a microfluidic set-up for N-¹¹C-methylations in a flow-through microreactor device with [¹¹C]DASB as model-compound and [¹¹C]CH₃I and [¹¹C]CH₃OTf, respectively, as ¹¹C-methylation agents. Due to an observed “aging” effect of the ¹¹C-methylation agents' solution, this goal was not achieved. Nevertheless, based on these observations, the time consumption for the vessel-based routine production of [¹¹C]DASB was reduced (34±1 min) and RCY was increased to 45.1±4.6% (EOB; 5.2±0.95 GBq EOS). - Highlights: ► Aging effect of ¹¹C-methylation agents observed. ► Microfluidic set-up for remote ¹¹C-methylations questionable. ► Vessel-based approach was ameliorated. ► Radiochem. yield of [¹¹C]DASB was increased to 45.1±4.6% (EOB; 5.2±0.95 GBq EOS).

[en] Aim of this work was the implementation of a generalized in-loop synthesis for ¹¹C-carboxylations and subsequent ¹¹C-acylations on the TRACERlab FxC Pro platform. The set-up was tested using [carbonyl-¹¹C]WAY-100635 and, for the first time, [¹¹C]-(+)-PHNO. Its general applicability could be demonstrated and both [carbonyl-¹¹C]WAY-100635 and [¹¹C]-(+)-PHNO were prepared with high reliability and satisfying outcome. - Highlights: • Generalized method for in-loop ¹¹C-carboxylations implemented. • Grignard reactions successfully tested. • First in-loop procedure for [¹¹C]-(+)PHNO established. • Satisfactory synthesis outcome for both [carbonyl-¹¹C]WAY-100635 and [¹¹C]-(+)PHNO. • No distillation for purification of intermediate required

[en] Introduction: Recently, first applications of microfluidic principles for radiosyntheses of positron emission tomography compounds were presented, but direct comparisons with conventional methods were still missing. Therefore, our aims were (1) the set-up of a microfluidic procedure for the preparation of the recently developed adenosine A₃-receptor tracers [¹⁸F]FE-SUPPY [5-(2-[¹⁸F]fluoroethyl)2,4-diethyl-3-(ethylsulfanylcarbonyl) -6-phenylpyridine-5-carboxylate] and [¹⁸F]FE-SUPPY:2 [5-ethyl-2,4-diethyl-3-((2-[¹⁸F]fluoroethyl)sulfanylcarbonyl) -6-phenylpyridine-5-carboxylate] and (2) the direct comparison of reaction conditions and radiochemical yields of the no-carrier-added nucleophilic substitution with [¹⁸F]fluoride between microfluidic and conventional methods. Methods: For the determination of optimal reaction conditions within an Advion NanoTek synthesizer, 5-50 μl of precursor and dried [¹⁸F]fluoride solution were simultaneously pushed through the temperature-controlled reactor (26^oC-180^oC) with defined reactant bolus flow rates (10-50 μl/min). Radiochemical incorporation yields (RCIYs) and overall radiochemical yields for large-scale preparations were compared with data from conventional batch-mode syntheses. Results: Optimal reaction parameters for the microfluidic set-up were determined as follows: 170^oC, 30-μl/min pump rate per reactant (reaction overall flow rate of 60 μl/min) and 5-mg/ml precursor concentration in the reaction mixture. Applying these optimized conditions, we observed a significant increase in RCIY from 88.2% to 94.1% (P<.0001, n≥11) for [¹⁸F]FE-SUPPY and that from 42.5% to 95.5% (P<.0001, n≥5) for [¹⁸F]FE-SUPPY:2 using microfluidic instead of conventional heating. Precursor consumption was decreased from 7.5 and 10 mg to 1 mg per large-scale synthesis for both title compounds, respectively. Conclusion: The direct comparison of radiosyntheses data applying a conventional method and a microfluidic approach revealed a significant increase of RCIY using the microfluidic approach.