Nanotechnology- A Promising Approach for Suicide Gene Therapy

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Cancer is one of the world’s most dreadful diseases and the battle against cancer continues till date. Suicide gene therapy for cancer is one of the best approaches for annihilation of cancer. In brief, suicide gene codes for an enzyme which converts a nontoxic prodrug into toxic metabolites and subsequently mediates death of host cells itself on account of which it is named “suicide” gene therapy. These suicide gene when constitutively expressed by the cells not only mediates death of host cells but also inflicts strong bystander effects on neighboring cells by predisposing them to toxic downstream metabolites. Due to such advantages, they manifest minimal systemic toxicity and are also effective against many drug resistance cancer cells. Among all existing suicide genes, Cytosine Deaminase (CD) and Herpes Simplex Virus-thymidine kinase (HSVtk) have shown promising results initially and has been investigated extensively since long. The HSVtk enzyme initially phosphorylates the prodrug Ganciclovir (GCV) to its monophosphate form, which is subsequently phosphorylated again by endogenous cellular kinase to generate nucleotide analogs (di- and triphosphate forms of GVC). Triphosphate form of GCV is then readily incorporated into DNA during the course of DNA synthesis and acts as a chain terminator to prevent further DNA synthesis, which ultimately induces cell death.

The therapeutic efficacy of HSVtk suicide gene therapy is often limited by cell-to-cell contact which is a prerequisite for transport of downstream metabolic byproducts of ganciclovir to neighboring cells so as to attain bystander-killing effect. As an outcome of such drawbacks, HSVtk suicide gene does not seem to be effective against different cell types. In contrary to this, Cytosine Deaminase (CD) efficiently converts prodrug 5-Fluorocytosine (5- FC) into therapeutically active anticancer agent 5-Fluorouracil (5- FU), which subsequently permeates across the cell membrane to mediate bystander killing effects on adjacent neighboring cells. Thus, 5-FC/CD system attains suicide gene therapy much more efficiently as compared to other counterparts. Although 5-FC/CD system attains better therapeutic outcomes, it is ineffective against 5-FC resistant cancer cells and thus its anticancer potential could not be generalized for all cancer types. In order to overcome such drawback, Gopinath et al. have designed Cytosine Deaminase-Uracil Phosphoribosyltransferase (CD-UPRT) bifunctional suicide gene construct in which Uracil Phosphoribosyltransferase (UPRT) acts upon product of CD i.e. 5-FU and converts it further into other toxic metabolites.

The therapeutic effect of suicide genes can be enhanced by combinatorial approaches. In combination therapy, two or more drugs with similar or different mode of action are employed to realize synergistic anticancer therapeutic potentials. Such synergistic anticancer potential of combination of radiation therapy and 5-FC/ CD plus UPRT gene therapy was demonstrated by Kambara et al. against malignant gliomas [8]. Apart from this, the combination therapy also provides scope for exploiting radio sensitizing properties of 5-FU and by stander effects during the course of treatment. Many research groups have reported the use of suicide gene in combination with chemotherapy and radiation to enhance the therapeutic effect and to overcome the drug resistance. Gopinath et al. were the first to report the applications of silver nanoparticles for synergizing the therapeutic effect of suicide gene. They have also reported the synergistic therapeutic effect of suicide gene with anticancer drug curcumin. One of the major challenging tasks in suicide gene therapy is lack of suitable vectors for targeted delivery of suicide gene to cancer cells. The application of such DNAbased therapeutics is largely limited due to poor cellular uptake, degradation by serum nucleases and rapid renal clearance following systemic administration. In addition to these, organ specific targeted DNA therapy has been a major challenge to overcome off-target gene therapy. In order to circumvent these limitations, numerous organ specific targeted nanocarriers have been developed recently for systemic administration.