Ruthenium-based complexes have been suggested as promising anticancer drugs exhibiting reduced general toxicity compared to platinum-based drugs. In particular, Ru(& eta;(6)-arene)(PTA)Cl-2 (PTA = 1,3,5-triaza-7-phosphaadamantane), or RAPTA, complexes have demonstrated efficacy against breast cancer by suppressing metastasis, tumorigenicity, and inhibiting the replication of the human tumor suppressor gene BRCA1. However, RAPTA compounds have limited cytotoxicity, and therefore comparatively high doses are required. This study explores the activity of a series of RAPTA-like ruthenium(ii) arene compounds against MCF-7 and MDA-MB-231 breast cancer cell lines and Ru(& eta;(6)-toluene)(PPh3)(2)Cl was identified as a promising candidate. Notably, [Ru(& eta;(6)-toluene)(PPh3)(2)Cl]Cl was found to be remarkably stable and highly cytotoxic, and selective to breast cancer cells. The minor groove of DNA was identified as a relevant target.