N-isopropyl-4-[123I]iodoamphetamine (123I-lMp) products:: a difference in radiochemical purity, unmetabolized fraction, and octanol extraction fraction in arterial blood and regional brain uptake in rats

N-isopropyl-4-[I-123]iodoamphetamine (I-123- IMP) is a lipophilic compound utilized for cerebral blood flow (CBF) measurement with single photon emission computed tomography (SPECT). Two different I-123-IMP products (IMPA and IMPB) are commercially available. We examined the radiochemical purity, unmetabolized fraction, and octanol extraction fraction in arterial blood, and the regional brain uptake of IMPA and IMPB in a rat model. IMPB (96.4%+/-0.08%, P < 0.05) showed significantly higher radiochemical purity than IMPA (95.5% +/- 0.20%). The mean unmetabolized fraction in arterial blood taken at 10 min after intravenous administration of IMPB (69.50% +/- 4.4%, P < 0.01) was significantly higher than that of IMPA (59.6% +/- 2.6%). The mean octanol extraction fraction of IMP, (75.0% +/- 1.3%, P < 0.01) was also significantly higher than that of IMPA (67.2% +/- 0.8%). The mean levels of radioactivity in arterial blood sampled at 10 min after injection and mean regional brain radioactivity (cerebral cortices, basal ganglia, brain stem, and cerebellum) at 10-12min after injection were not significantly different between IMPA and IMPB. The present study indicates differences in the radiochemical purity and the unmetabolized and octanol extraction fraction in arterial blood between the two commercially available I-123-IMp products. The appropriate octanol extraction fractions for IMPA and IMPB should be determined in humans and employed for quantitative CBF measurement in clinical SPECT.