LLC1 is an amiloride derivative with cytotoxicity against breast cancer cells, particularly those resistant to treatment. The IC 50 values of LLC1 for MCF7, MCF7 MX-100, MCF7 TS, MCF7 TR-1, and MCF7 TR-5 are 13, 12, 25, 26, and 19 mM, respectively. LLC1 shows potential for research in the field of cancer therapy.