In preclinical mouse models of prostate cancer, α-(alpha)-amanitin conjugated to an antibody directed against prostate-specific membrane antigen (PSMA; FOLH1; GCPII) showed high antitumoral activity and caused complete remission at single i.v. doses of 150 μg/kg of toxin, with no more than marginal weight loss in treated animals. Also, amanitin-based antibody-drug conjugates using an anti-Her2 antibody such as trastuzumab showed high antitumor activity in a series of models of preclinical oncology designed to establish efficacy of the trial drug in the treatment of HER2+ breast cancer. Alpha-amanitin is highly active in drug-resistant cells, independent of the status of expression of multi-drug resistant transporters because of its hydrophilic structure.