Heavy metal chelator TPEN attenuates fura-2 fluorescence changes induced by cadmium, mercury and methylmercury. TPEN, a cell-permeable chelator for heavy metal cations with a low affinity for Ca ²⁺ . In cells stimulated with 10 or 30 μM cadmium chloride, the addition of TPEN at 3 hr after exposure significantly decreases the elevated fura-2 fluorescence ratio to the basal levels within 10 min (119.6±2.4% or 109±1.5% decrease in ΔRatio (F340/F380) induced by 10 or 30 μM cadmium chloride, respectively), suggesting that a cadmium chloride-induced increase in the fura-2 fluorescence ratio is dependent on an increase in intracellular heavy metal cations but not intracellular Ca ²⁺ . TPEN is a metal chelator, which targets colon cancer cells through redox cycling of copper. TPEN reduces cell viability in a dose- and time-dependent manner. TPEN-induced cell death is also dependent on the redox cycling of copper since the copper chelator neocuproine inhibited DNA damage and reduced pChk1, γ-H2AX, and ATM protein expression. Cell death by low TPEN concentrations, involved ATM/ATR signaling in all 3 cell lines, since pre-incubation with specific inhibitors of ATM and DNA-PK led to the recovery of cells from TPEN-induced DNA damage.