The invention described herein pertains to drug delivery conjugates for targeted therapy. The invention described herein relates to methods of treating PSMA expressing cancers with a PSMA ligand-tubulysin compound. The invention described herein also relates to methods of treating PSMA-expressing cancers with a PSMA ligand-tubulysin compound in patients where stable disease results after treatment with the PSMA ligand-tubulysin compound.