Antithrombin III: Biodistribution in Healthy Volunteers

Summary

Five healthy volunteers were injected intravenously with 73-90 uCi purified human ¹³¹I-Antithrombin III (AT III), specific biological activity 5.6 U/mg. The tracer data were analysed using a three compartment model. The plasma radioactivity half life was 66.2 ± 1.2 (sem) h, the fractional catabolic rate constant of the plasma pool was 0.025 ± 0.002 (sem) h^-1. These data were comparable with those described in the literature. Because of the difficulty in translating the mathematical analysis of various compartments into the biological model, biodistribiition was monitored by a gamma camera linked to a DEC PDF 11/34 computer system. Dynamic and static images were obtained at fixed time intervals following the injection of ¹³¹I-AT III.

Whole body scanning at intervals between the time of injection (t=0)and t=24.5 h showed ¹³¹I-AT III distribution over the heart, lungs, liver, spleen and great vessels. Dynamic scanning was performed over the heart, spleen and liver. Overlayed frames in the first ten minutes after the ¹³¹I-AT III injection showed the following radioactivity expressed as percentage of the injected dose; 5.9% ± 0.3(sem) over the heart, 10.6% ± 0.9 (sem) over the liver and 1.1% ⊥ 0.1(sem) over the spleen.

A slower decline of the radioactivity between t = 0 and t = 24 h; (19%) was measured over the liver compared with the radioactivity disappearance over the heart region. This shows, in combination with the fact that the radioactivity disappearance over the heart was identical with the radioactivity decline measured in the plasma samples that retention of ¹³¹I-AT III occurred in the liver. Heparin iv injected 6 h after the ¹³¹I-AT III injection in two volunteers induced a sharp increase of radioactivity over the liver region during the scanning demonstrating that heparin enhances the ¹³¹I-AT III uptake in the liver.

Keywords

Antithrombin III biodistribution - Healthy volunteers

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