A family of lipophilic, cationic Au(I) complexes of N-heterocyclic carbenes (NHCs) have been designed as new mitochondria-targeted antitumor agents that combine both selective mitochondrial accumulation and selective thioredoxin reductase inhibition properties within a single molecule. Two-step ligand exchange reactions with cysteine (Cys) and selenocysteine (Sec) occur with release of the NHC ligands. At physiological pH the rate constants for the reactions with Sec are 20- to 80-fold higher than those with Cys. The complexes are selectively toxic to two highly tumorigenic breast cancer cell lines and not to normal breast cells, and the degree of selectivity and potency are optimized by modification of the substituent on the simple imidazolium salt precursor. The lead compound is shown to accumulate in mitochondria of cancer cells, to cause cell death through a mitochondrial apoptotic pathway and to inhibit the activity of thioredoxin reductase (TrxR) but not the closely related and Se-free enzyme glutathione reductase.