Pharmacokinetics and dosimetry of cobalt-55 and cobalt-57

The isotopes Co-55 and Co-57 have been evaluated for PET and SPECT imaging in several clinical brain studies. For clinical application of cobalt, it is important to know the delivered radiation dose. The biodistribution of Co-55 in both rat and humans after intravenous (bolus)-administration was studied. Based on pharmacokinetic data, radiation dose calculations according to the MIRD system are presented, By combining present measurements with literature data on (CoCl2)-Co-60, the radiation dose delivered by (CoCl2)-Co-56 (T-1/2 78.8 days) and (CoCl2)-Co-57 (T-1/2 = 270 days) could be assessed. Methods: Whole-body Co-PET was performed in two healthy volunteers and one rat after intravenous injection of 37 and 3.7 MBq (1 resp, 0.1 mCl) Co-55, respectively. Blood samples were withdrawn during 300 min in humans. In seven rats the Co-55-biodistribution was determined by postmortem analysis. The residence time of the liver (critical organ) was determined in rats and humans. Blood partition-data of Co-55 were assessed resulting in basic pharmacokinetic data in humans. Based on these kinetic data, radiation dose was calculated using the MIRD protocol. Results: In both the humans and the rat, the liver and bladder retained the highest fractions of Co-55 (about 50% resp. 40% of the administered dose), The liver residence time in humans was 8.6 hr. The free fraction Co-55 in the human plasma was at maximum 12%. The total-body mean transit time was 152 min. The volume of the central compartment = 2.8 liter and the steady-state distribution volume = 48 liter. Conclusion: From these results, according to the WHO recommendations for class II studies, 22.2 MBq (0.6 mCi) Co-55 and 14.8 MBq (0.4 mCi) Co-57 (excluding any radionuclide contamination) can be used.