Sensitive measurement of positron emitters eluted from HPLC

Abstract

For sensitive analysis of the radioactive-metabolite in human PET, a radio-HPLC system coupled to a newly designed positron detector was constructed. The detector had the advantages of low noise level (1.7±1.0 cpm) and high sensitivity (32±1%) due to coincidence counting and large BGO crystals. Furthermore, the detector was easy to move, since a pair of the BGO housings coupled to photomultipliers was effectively arranged in parallel and a HPLC cell with different volume could be inserted between the BGO housing. This radio-HPLC system was useful for analyzing samples with low radioactivity. It was applied to the measurement of [¹¹C]FLB457 in plasma, having high affinity and high selectivity with dopamine D2 receptors. Extremely low radioactivity of [¹¹C]FLB457 (2500 dpm) could be analyzed by using the radio-HPLC system. The performance of this detector was compared with those of commercially available systems that had been used as sensitive detectors for 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).