Abstract: The subject of the invention are new mitochondrially targeted E/Z isomers of aliphatic triphenylphosphonium derivatives of tamoxifen where the aliphatic chain is alkyl or alkenyl, and their corresponding tertiary amine salts and/or their mixture (MitoTAX). Alkyl triphenylphosphonium derivatives of tamoxifen have the general formula (I), where n=8 to 12 and where Z is selected from the group of organic salts or inorganic salts. Alkenyl triphenylphosphonium derivatives of tamoxifen have the general formula IA, where n=6 to 10 and where Z has the above mentioned meaning. These compounds are applicable for the treatment of neoplastic disease, especially those with high HER2 protein levels. The drug for the treatment of neoplastic diseases according to the invention contains at least one E/Z isomer of aliphatic triphenylphosphonium derivatives of tamoxifen of the general formula (I) and/or IA or their corresponding salts of tertiary amine.