Chelation treatments using daily subcutaneous injections of <tex-math>${\rm ZnNa}_{3}\text{-}{\rm DTPA}$</tex-math> were begun 2 wk after the intravenous administration of 0.3 μCi/kg241 Am citrate to six female beagles averaging 505 days of age. Retention of americium in the liver and in nonliver tissue (mainly skeleton) was followed serially in the living dogs by a combination of total-body and partial-body counting. During the first 13 mo of DTPA therapy, the removal of241 Am from two dogs given one DTPA injection each day of 0.027 or 0.034 mmole/kg was similar to the fraction removed from two other dogs given a similar total of 0.035 or 0.037 mmole/kg/day in five fractionated injections each day. Increasing the daily amount of DTPA to 0.36 and 5.0 mmole/kg/day in the two remaining dogs only slightly increased the removal of241 Am. At the beginning of chelation therapy, all six dogs averaged 43% of the injected241 Am in the liver and 46% in nonliver tissue.241 Am retention in liver at 2 mo of therapy was about 2% of pretreatment liver retention, decreased to roughly 1% of pretreatment retention by 5 mo of DTPA administration, and was undetectable at 13 months. Nonliver retention at 2 mo was 53% of pretreatment retention in nonliver tissue, at 5 mo averaged about 40%, and had decreased to 27% of pretreatment values by 13 mo of chelation therapy. In contrast, for beagles of our colony not treated with DTPA,241 Am in the liver and in nonliver tissue exhibited a biological half-time in the order of 10 yr.

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