Radiation dosimetry of [123I]-CLINDE

1446 Objectives [123I]-CLINDE is an iodine-123 labelled imidazopyridine with selective uptake by peripheral benzodiazepine receptors (PBR IC50 = 1.7 nM, CBR >400) which may be useful for SPECT imaging of the brain. The aim of this preclinical study is to estimate the radiation dose to humans based on biokinetics in a rat model. Methods [123I]-CLINDE (0.75 MBq) was administered to adult SD rats intravenously by the tail vein. The rats were sacrificed between 5 mins to 48 h p.i. Organs and tissues were collected and activity concentrations were measured in a gamma counter. The % injected dose per gram of wet tissue (%ID/g) was calculated against a standard and corrected for activity in the tail. The %ID/g values from rats were converted to %ID per organ of a human model by a mass extrapolation method. Doses were estimated using OLINDA.EXM v1.1 with assumed fractions and half-times for renal and hepatobiliary clearance. Results Early tracer uptake was seen in liver, spleen, kidney, lungs, heart, thyroid, pancreas and adrenals, which over 48h increased in adrenals and thyroid and gradually cleared from other organs. The thyroid, urinary bladder, colon and adrenals received the highest doses. The effective dose was 0.043 mSv/MBq injected. Conclusions The rat model gives a good first approximation to dose, similar to the ICRP estimate of 0.05mSv/MBq for generic 123I-labelled brain receptor substances. A study in humans would clarify the biological clearance rates which are a significant factor in dose estimation