Various embodiments of this invention relate generally to targeted activation and delivery of therapeutic drugs to cells that produce prostate specific antigen (PSA), prostate specific membrane antigen (PSMA) or human glandular kallikrein (hK2). Various further embodiments relate more specifically to PSMA-specific peptide prodrugs that become activated to yield therapeutic drugs. Further aspects of various embodiments of the present invention also relate to methods and compositions for treating or preventing cancers and methods and compositions for detecting and/or imaging cancers.