Biokinetics of ruthenium isotopes in humans and its dependence on chemical speciation

Tracer studies in healthy volunteers using stable isotopes of ruthenium were performed with the objective of extending the limited knowledge about the biokinetics of the radionuclides of this element. Tracer solutions were prepared starting from metallic ruthenium powders enriched in either Ru or Ru (isotopic enrichment ca. 98%). Tracer administration was followed by the collection of blood and urine samples at fixed times. Some investigations were conducted by means of the double tracer technique, consisting in the simultaneous administration of two tracers, one orally and the other intravenously. Tracer concentrations in biological samples were determined by means of proton activation analysis. Blood clearance and urinary excretion were noticeably affected by the degree of complexation of ruthenium in the administered solution. After injection of solutions characterized by a high degree of complexation between ruthenium and citrate, the disappearance from plasma was faster than with poorly complexed ruthenium. Accordingly, the percentage promptly eliminated into urine was about 50% for highly complexed ruthenium and 20% in the case of low complexation. The fraction absorbed from the intestine into the circulation was lower than the current ICRP estimate of 0.05 for nearly all chemical forms investigated. Only in the case of high complexation degree the absorbed fraction was significantly higher (0.108±0.022) than the ICRP value.