Sensitive measurement of positron emitters eluted from HPLC
Introduction
Positron emission tomography (PET) has played an important role for diagnosing tumors and clarifying the brain functions (Fowler et al., 1999). However, the method still needs further improvements in the production and quality control of PET radiopharmaceuticals, metabolite analysis and so on, to overcome the difficulties caused by the short half-lives (20, 10, 2 and 110 min for 11C, 13N, 15O and 18F, respectively) of the radionuclides used in PET and by low radioactivity concentration in a human body after administration. For example, the concentration of the injected PET radiopharmaceuticals in blood will decrease quickly by decay, dilution in the body and absorption into organs. In the case of [11C]Ro15-1788 or [11C]raclopride, its blood concentration rapidly decreases after injection into the human body, being about 4 kBq/ml after about 5 min (Swahn et al (1989), Swahn et al (1992)). On the other hand, recently, three-dimensional PET has been used, and this has allowed a remarkable decrease of the administration dose of radioactivity (1/3–1/10) (Sossi et al., 1998; Trébossen et al., 1998). These situations obliged us to measure extremely low concentrations of radioactivity in blood in metabolite analysis.
Labeled compounds and their metabolites have been analyzed mainly by radio-HPLC and radio-TLC (Eckelman et al., 1998; Ishiwata et al., 1998; Irie et al., 1996). Radio-HPLC has been widely used because (1) the high sensitivity due to a large sample load, (2) the high resolution, and (3) real-time monitoring by connecting the HPLC outlet to multiple detectors arranged in series (such as UV, ECD, or radioactivity detectors). The following characteristics are desirable for an HPLC radioactivity detector used in radioactive-metabolite analysis in plasma: (1) high sensitivity, (2) low noise, (3) on-line monitoring, (4) easy shielding, (5) small dimensions, (6) easy displacement of a flow cell to another with a different volume, (7) no absorption of samples in a flow cell, and (8) portability. Some commercially available radioactivity detectors give fairly high sensitivity and have been used in metabolite analysis of radiotracers. But they still need further improvement from the viewpoints of the above criteria. Here, we have developed a radioactivity detector coupled to an HPLC system to meet the above criteria and have assessed its physical characteristics such as detection efficiency, linearity and detection limit. In addition, we have applied the system to metabolite analysis at very low concentrations of radioactivity and compared the performance with those obtained with commercially available detectors: (1) RADIOMATIC 150TR (“150TR”; PACKARD Instrument, Tokyo)), (2) RADIO ANALYZER RLC-701 (“RLC-701”; Aloka, Tokyo), and (3) γ-counter (“A5530”; PACKARD Instrument, Tokyo).
Section snippets
Preparation of samples
As a [11C]-labeled compound, [11C]FLB457 (Halldin et al., 1995) used for measurement of dopamine D2 receptors was produced by a well-known method (Suzuki et al., 1985). The radionuclidic purity of the product was evaluated by measurement using a dose calibrator (IGC-3, Aloka, Tokyo), which had been previously calibrated with 137Cs, at 2-min intervals for 3 h from immediately after production. In addition, 7 h after synthesis, 11 Bq of [11C]FLB457 (100 μl) was placed in a polyethylene bag (50×50 mm,
Evaluation of the detector performance
Decay curves obtained by monitoring of [11C]FLB457 for 7 h using a dose calibrator and the detector showed no contamination with radionuclides of a short or long half-life. Impure radionuclides were removed, probably by the chemical separation and purification in the synthesis process.
Fig. 2 shows the time course of the radioactivity of [11C]FLB457 measured by the present detector. The detection limit was estimated to be 0.3 Bq according to the method described in the experimental section.
Fig. 3
Acknowledgements
The authors wish to thank Mr. H. Hirayama (Oyokoken) and Miss J. Noguchi (SHI Accelerator Services) for fruitful discussions. Special thanks are also given to Mr. T. Kawakami (NIRS), Mr. N. Nengaki (SHI Accelerator Services) and Mr. A. Masuda (Tokyo Nuclear Service) for their technical support in 11C-production.
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