The bipositive
ions and
anions, with few exceptions, indicated a low
tumor uptake rate. On the other hand, compounds of Hg, Au and Bi, which have a strong binding power to
protein, showed a high
tumor uptake rate. As Hg2+, Au+ and Bi3+ are soft
acids according to the classification of
Lewis acids, it was thought that these
ions would bind strongly to soft bases (R-SH, R-S-) present in
tumor tissue. For many hard
acids such as 85Sr2+, 67Ga3+, 181Hf4+, and 95Nb5+,
tumor uptake rates are shown as a function of ionic potentials (valency/ionic radii) of the
metal ions. Considering the present data and previously reported results, it was presumed that hard
acids of trivalence, quadrivalence and pentavalence would replace
calcium in the
calcium salts of hard bases (
calcium salts of
acid mucopolysaccharides, etc.). Ionic potentials of alkaline metals and Tl were small, but the
tumor-uptake rate of these elements indicated various values. As Ge and Sb are bound by covalent bonds to
chloride, GeCl4 and SbCl3 behaved differently from many metallic compounds in
tumor tissue.