Prodrug activation enzymes in cancer gene therapy

Among the broad array of genes that have been evaluated for tumor therapy, those encoding prodrug activation enzymes are especially appealing as they directly complement ongoing clinical chemotherapeutic regimes. These enzyme s can activate prodrugs that have low inherent toxicity using both bacteria l and yeast enzymes, or enhance prodrug activation by mammalian enzymes. Th e general advantage of the farmer is the large therapeutic index that can b e achieved, and of the latter, the non-immunogenicity (supporting longer pe riods of prodrug activation) and the fact that the prodrugs will continue t o have some efficacy after transgene expression is extinguished. This revie w article describes 13 different prodrug activation schemes developed over the last 15 years, two of which - activation of ganciclovir by viral thymid ine kinase and activation of 5-fluorocytosine to 5-fluorouracil - are curre ntly being evaluated in clinical trials. Essentially all of these prodrug a ctivation enzymes mediate toxicity through disruption of DNA replication, w hich occurs at differentially high rates in tumor cells compared with most normal cells. In cancer gene therapy, vectors target delivery of therapeuti c genes to tumor cells, in contrast to the use of antibodies in antibody-di rected prodrug therapy. Vector targeting is usually effected by direct inje ction into the tumor mass or surrounding tissues, but the efficiency of gen e delivery is usually low. Thus it is important that the activated drug is able to act on non-transduced tumor cells. This bystander effect may requir e cell-to-cell contact or be mediated by facilitated diffusion or extracell ular activation to target neighboring tumor cells. Effects at distant sites are believed to be mediated by the immune system, which can be mobilized t o recognize tumor antigens by prodrug-activated gene therapy. Prodrug activ ation schemes can be combined with each other and with other treatments, su ch as radiation, in a synergistic manner. Use of prodrug wafers for intratu moral drug activation and selective permeabilization of the tumor vasculatu re to prodrugs and vectors should further increase the value of this new th erapeutic modality. Copyright (C) 2000 John Wiley & Sons, Ltd.