Human metabolism of orally administered radioactive cobalt chloride

This study investigated the human gastrointestinal uptake (f₁) and subsequent whole-body retention of orally administered inorganic radioactive cobalt. Of eight adult volunteers aged between 24 and 68 years, seven were given solutions of ⁵⁷Co (T_1/2 = 272 d) containing a stable cobalt carrier, and six were given carrier-free ⁵⁸Co (T_1/2 = 71 d). The administered activities ranged between 25 and 103 kBq. The observed mean f₁, based on 6 days accumulated urinary excretion sampling and whole-body counting, was 0.028 ± 0.0048 for carrier-free ⁵⁸Co, and 0.016 ± 0.0021 for carrier-associated ⁵⁷Co. These values were in reasonable agreement with values reported from previous studies involving a single intake of inorganic cobalt. The time pattern of the total retention (including residual cobalt in the GI tract) included a short-term component with a biological half-time of 0.71 ± 0.03 d (average ± 1 standard error of the mean for the two nuclides), an intermediate component with a mean half-time of 32 ± 8.5 d, and a long-term component (observed in two volunteers) with half-times ranging from 80 to 720 d for the two isotopes. From the present data we conclude that for the short-lived ⁵⁷Co and ⁵⁸Co, more than 95% of the internal absorbed dose was delivered within 7 days following oral intake, with a high individual variation influenced by the transit time of the unabsorbed cobalt through the gastro-intestinal tract. - Highlights: • Gastrointestinal (GI) uptake, f₁, of inorganic radioactive cobalt varies between 0.007 and 0.043 among humans. • Mean f₁ among human volunteers was found to be 0.022 ± 0.003. • More than 95% of the estimated internal absorbed dose occurs within 7 d for the short-lived radionuclides ⁵⁷Co and ⁵⁸Co. • More than 60% of the time integrated absorbed dose from ingestion of ⁶⁰Co occurs within 7 d. • Residence time of cobalt in the human GI-tract strongly influences the time integrated internal absorbed dose