Molecular Biotechnology: Principles and Applications - Glick, B., Pasternak, J. 2002

Molecular Biotechnology of Microbiological Systems
Gene Therapy
Activation of a Drug Precursor (Prodrug)

Despite the widespread use of surgery, radiotherapy, and Chemotherapy, malignant neoplasms remain one of the leading causes of death, making The Development of novel therapeutic strategies highly relevant. One such approach relies on the eradication of proliferating tumor Cells using ganciclovir [GCV; 9-(1,3-dihydroxy-2-propoxymethyl)guanine], an activated product of the Herpes simplex virus thymidine kinase (HSVtk) Gene. This method involves the in vivo Transduction or transfection of tumor cells with the HSVtk gene under the control of an active promoter, followed by ganciclovir administration a few days later. The viral thymidine kinase phosphorylates ganciclovir to form ganciclovir monophosphate. Host Cell Kinases exhibit minimal affinity for ganciclovir itself, yet readily add phosphate groups to its monophosphate derivative, yielding ganciclovir triphosphate. This compound inhibits DNA polymerase, halts DNA Synthesis, and induces the death of proliferating cells. Furthermore, through intercellular contacts, ganciclovir triphosphate can diffuse into untransfected neighboring tumor cells, leading to their destruction as well. A single HSVtk-expressing tumor cell can eliminate up to 10 unmodified cells, a phenomenon known as the "bystander effect".

A gene that induces the death of its host cell under specific conditions is referred to as a "suicide gene," whereas the term "prodrug" designates an inactive form of a therapeutic agent that is activated via another component of the therapeutic system. Although various other prodrug–activator gene combinations have been developed, the GCV–HSVtk system remains the most widely utilized.

The efficacy of the GCV–HSVtk system has been demonstrated in numerous preclinical studies. However, Phase I clinical trials involving patients with terminal Cancer failed to show tumor regression. This was likely because the HSVtk gene was transduced into too few tumor cells and, despite the bystander effect, proved insufficient to suppress tumor growth. Currently, researchers are developing novel approaches to enhance transduction efficiency and deliver the HSVtk gene throughout the entire tumor volume.

Combined approaches employing two distinct gene systems have also been developed for cancer Gene Therapy. One such strategy integrates GCV–HSVtk therapy with gene immunotherapy (Fig. 21.12). A fraction of the tumor cells is transduced with the HSVtk gene, while another fraction is transduced with a cloned cDNA (or gene) encoding a cytokine. Cytokines (such as interleukin-2, interleukin-12, and others) act as signaling molecules that mobilize immune cells and stimulate the Immune Response. Evidence indicates that tumor Proteins released from cells destroyed by suicide gene therapy interact with immune cells recruited to the tumor site by the cytokine, thereby triggering an anti-tumor immune response. In addition, anti-tumor Antibodies enter the bloodstream and circulate systemically, preventing The formation of metastases.

This gene therapy approach has been tested experimentally by implanting colon cancer cells—separately transduced with the HSVtk gene and a cytokine gene—into the livers of experimental animals. Administration of ganciclovir halted tumor growth in the Liver. Moreover, subsequent injection of untransduced tumor cells into other Tissues of these animals failed to induce tumor formation. In contrast, control animals under identical conditions developed tumors at all sites where untransduced colon cancer cells were injected. Despite these highly promising results, prior to initiating clinical trials of suicide gene therapy or its various combinatorial strategies, it is essential to determine which tumor types are susceptible to such Treatment and to evaluate potential side effects.