Missing Selectivity of Targeted 4 beta-Phorbol Prodrugs Expected to be Potential Chemotherapeutics

Abstract

Targeting cytotoxic 4 beta-phorbol esters toward cancer tissue was attempted by conjugating a 4 beta-pborbol derivative with substrates for the proteases prostate-specific antigen (PSA) and prostate-specific membrane antigen (PSMA) expressed in cancer tissue. The hydrophilic peptide moiety was hypothesized to prevent penetration of the prodrugs into cells and prevent interaction with PKC. Cleavage of the peptide in cancer tumors was envisioned to release lipophilic cytotoxins, which subsequently penetrate into cancer cells. The 4 beta-phorbol esters were prepared from 4 beta-phorbol isolated from Croton tiglium seeds, while the peptides were prepared by solid-phase synthesis. Cellular assays revealed activation of PKC by the prodrugs and efficient killing of both peptidase positive as well as peptidase negative cells. Consequently no selectivity for enzyme expressing cells was found.