199Au production for use as a radiolabel of gold cluster immunoconjugates

199Au is an attractive radionuclide for radioimmunotherapy due to its 3.14 day half-life. Intermediate energy β- emission (β-max 295 keV), and imageable gamma ray (158 keV, 37%). A method has been developed to covalently link stable clusters of gold atoms to monoclonal antibodies, offering the possibility of delivering a greater amount of radiolabel per antibody than previously attainable. We have investigated an alternative production route to higher specific activity gold, 198Pt(n,γ)199Pt β- → 199Au. Enriched 198Pt targets would be irradiated to produce short-lived 199Pt (t1/2 = 30.8 min) which decays by β- emission to 199Au. Calculations show that bombarding 100 mg of 198Pt for seven days in the HFBR at a flux of 8.2 x 1014 n/cm2-sec would yield 1.5 Ci of 199Au with a theoretical specific activity of 209 Ci/mg. Carrier-free radiochemical separations of 199Au from platinum targets have also been studied using targets of 20-60 mg of natural platinum metal (7.2% 198Pt). The results for three Au/Pt radiochemical separation procedures adapted from the literature are reported here. One scheme is based on the selective extraction of bromoauric acid, HAuBr4, into isopropyl ether. Another shows the results of a method devised from anion exchange in mixed solvent media, acetone/HC1, and acetone/HNO3. In addition, an extraction chromatography study employing tri-n-butyl phosphate (TBP) is summarized. The extraction chromatography with TBP has been shown to yield the highest separation factor and is the fastest and most reliable procedure for remote operation. In conclusion, these studies have shown that 199Au can be produced with high specific activity and high radionuclidic purity for antibody labeling with gold clusters. (author)