We have shown that novel hybrid pyrrolidinedione–thiazolidinone molecules Les-6287, Les-6294, and Les-6328 possessed cytotoxicity towards human breast carcinoma cells of MCF-7, T-47D, MDA-MB-231, and HCC1954 lines and murine breast cancer 4T1 cells that differ in their genotype (depending on ER, PR, and HER2 expression). This evidence concerns the gene ERBB2 and breast carcinoma.