18 Suicide Genes: Gene Therapy Applications Using Enzyme/Prodrug Strategies

Abstract

A major focus of experimental gene-transfer research is an effort to develop efficient technologies to confer selectable traits to cells. Genetic negative-selection systems have found wide applications ranging from the generation of “knockout” animals used to study the role of individual genes on development; and morphogenesis, to use in clinical trials in humans for serious disorders including AIDS, cancer, and graft-versus-host disease. The largest preclinical and therapeutic experience with negative selective gene transfer is the application of “suicide” gene/prodrug therapy for the treatment of cancer and related conditions.

The basic components of a suicide gene/prodrug system include a gene encoding an enzyme (“suicide” gene), usually of viral or prokaryotic origin that is capable of converting an otherwise nontoxic prodrug into an active toxin that causes cell death. In the absence of the prodrug, expression of the suicide gene should be innocuous, with no toxicity or other effects on normal cellular metabolism. The enzyme should have a high catalytic constant (K_cat) of the prodrug to allow rapid activation and it should be able to function efficiently at very low substrate concentrations (low K_m) (Connors 1995). The ideal suicide gene is one with no cellular homolog; however, this is not an absolute requirement provided the prodrug is not activated to any significant degree by the native cellular enzyme.

Any potentially useful suicide gene/prodrug combination is constrained by the currently available gene-transfer systems (Zhang and Russell 1996; Wilson 1997). Genetically engineered retroviruses and adenoviruses, the most frequently used gene-transfer vectors, have size limitations...