The radiochemical dipyrrolidinedithiocarbamato-<tex-math>${}^{212}{\rm Pb}({\rm II})[{}^{212}{\rm Pb}({\rm PDC})_{2}]$</tex-math> is synthesized and its effects on colony formation in cultured Chinese hamster V79 cells are investigated. The cellular uptake, biological retention, subcellular distribution and cytotoxicity of the radiocompound are determined. The <tex-math>${}^{212}{\rm Pb}$</tex-math> is taken up quickly by the cells, reaching saturation levels in 1.25 h. When the cells are washed, the intracellular activity is retained with a biological half-life of 11.6 h. Gamma-ray spectroscopy indicates that the <tex-math>${}^{212}{\rm Pb}$</tex-math> daughters (212 Bi,212 Po and <tex-math>${}^{208}{\rm Tl}$</tex-math>) are in secular equilibrium within the cell. About 72% of the cellular activity localizes in the cell nucleus, of which 35% is bound specifically to nuclear DNA. The mean cellular uptake required to achieve 37% survival is 0.35 mBq of <tex-math>${}^{212}{\rm Pb}$</tex-math> per cell, which delivers a dose of 1.0 Gy to the cell nucleus when the recoil energy of212 Bi and212 Po decays is ignored and 1.7 Gy when recoil is included. The corresponding RBE values compared to acute external137 Cs γ rays at 37% survival are 4.0 and 2.3, respectively. The chemical <tex-math>${\rm Pb}({\rm PDC})_{2}$</tex-math> is not chemotoxic at the concentrations used in this study. Because the β-particle emitter <tex-math>${}^{212}{\rm Pb}$</tex-math> decays to the α-particle-emitting daughters212 Bi and212 Po, these studies provide information on the biological effects of α-particle decays that occur in the cell nucleus. Our earlier studies with cells of the same cell line using210 Po (emits 5.3 MeV α particle) localized predominantly in the cytoplasm resulted in an RBE of 6. These earlier results for210 Po, along with the present results for <tex-math>${}^{212}{\rm Pb}$</tex-math>, suggest that the recoil energy associated with the212 Bi and212 Po daughter nuclei plays little or no role in imparting biological damage to critical targets in the cell nucleus.

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