Analysis of [¹¹C]Choline Using High Pressure Ion Chromatography System (HPIC) with Autosampler

Objectives: Solid-phase extraction (SPE) based [¹¹C]Choline synthesis simplifies the production process and eliminates the use of HPLC for the final purification. It requires passing the radiochemical identity and purity criterion of [¹¹C]Choline, as well as residual precursor dimethylethanolamine (DMAE) concentration tests prior to the release for clinic use. Traditionally, these tests of [¹¹C]Choline in 0.9% NaCl solution were analyzed by high-pressure liquid chromatography (HPLC) or ion chromatography. Thanks to the advance of ion chromatography technology, in this study, we reported a new method for [¹¹C]Choline analysis using high pressure ion chromatography (HPIC) with autosampler, which significantly improves the efficiency of multiple injections and reduces staff radiation exposure. Briefly, we firstly determined HPIC system suitability, and then used three batches of [¹¹C]Choline to validate the HPIC system.

Methods: DMAE, Choline and [¹¹C]Choline solutions were analyzed by a HPIC-5000+ dual system (Thermo Fisher) equipped with an autosampler, a conductivity detector, a radio-detector, and an analytical cation column (IC-PAK M/D, 3.9 x 150 mm, Waters). Injection loop volume was 25 µL and the mobile phase was 5 mM HCl solution at a flowrate of 1 mL/min. A CCRS suppressor (CCRS 500, 4 mm, Thermo Fisher) was applied with 25 mM TBAOH regenerant solution at a flow rate of 3 mL/min. HPIC system suitability including limit of blank (LoB = mean_blank + 1.645[asterisk]SD_blank), limit of detection (LoD = mean_blank + 10[asterisk]SD_blank) and limit of quantitation (LoQ)[1], linearity, accuracy and precision were evaluated. LoB, LoD and LoQ were determined with repeated injections of blank or low concentration samples. The HPIC linearity, accuracy and precision were assayed by analyzing sequential diluted DMAE or Choline standard solutions (0.01, 0.1, 1, 10, 20, 40, 60, 80 and 100 µg/mL). To validate the HPIC system for [¹¹C]Choline analysis, three batches of [¹¹C]Choline were produced by [¹¹C]MeI methylation of loop preloaded DMAE precursor (50 µL) on GE Tracerlab FX2-MeI and Tracerlab FX2-M units. Quality control [¹¹C]Choline samples were injected to HPIC system for evaluation.

Results: Valid HPIC system suitability was acquired with processing of repeated sample injections. The LoB, LoD and LoQ of HPIC were calculated as 0.0015 µg/mL, 0.0057 µg/mL and 10 µg/mL for DMAE, and 0.0011 µg/mL, 0.0046 µg/mL and 1 µg/mL for Choline, respectively. Linearity trendline were calculated with the series of diluted samples (DMAE: y = 0.0351x + 0.0931, R² = 0.9935; Choline: y = 0.0463x - 0.0121, R² = 1). The accuracy of 100 ± 3% was obtained in the range of 1-100 µg/mL Choline solution. The precisions for both DMAE and Choline were <1% coefficient of variation (COV) in the accuracy ranges. In [¹¹C]Choline analysis, the radiochemical identity of [¹¹C]Choline was confirmed with the retention time of 9.63 ± 0.01 min, showing a 1.8% delay corresponding to nonradioactive Choline standard (9.46 ± 0.02 min; 10 µg/mL). The radiochemical purities were observed to be >99% in all three batches of [¹¹C]Choline validation. The DMAE and nonradioactive Choline concentrations were 16.8 ± 1.5 µg/mL (acceptance criterion: <75 µg/mL) and 3.6 ± 0.5 µg/mL.

Conclusions: A validation analysis of [¹¹C]Choline using HPIC with autosampler was established. System suitability tests demonstrated this method was appropriate for [¹¹C]Choline analysis. As a result, [¹¹C]Choline radiochemical purity and identity, as well as the residual of DMAE in three batches products met the release criterion.

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