Progress and problems with the use of suicide genes for targeted cancer therapy

Abstract

Among various gene therapy methods for cancer, suicide gene therapy attracts a special attention because it allows selective conversion of non-toxic compounds into cytotoxic drugs inside cancer cells. As a result, therapeutic index can be increased significantly by introducing high concentrations of cytotoxic molecules to the tumor environment while minimizing impact on normal tissues. Despite significant success at the preclinical level, no cancer suicide gene therapy protocol has delivered the desirable clinical significance yet. This review gives a critical look at the six main enzyme/prodrug systems that are used in suicide gene therapy of cancer and familiarizes readers with the state-of-the-art research and practices in this field. For each enzyme/prodrug system, the mechanisms of action, protein engineering strategies to enhance enzyme stability/affinity and chemical modification techniques to increase prodrug kinetics and potency are discussed. In each category, major clinical trials that have been performed in the past decade with each enzyme/prodrug system are discussed to highlight the progress to date. Finally, shortcomings are underlined and areas that need improvement in order to produce clinical significance are delineated.

Keywords: Bystander effect; Cancer Gene therapy; Cytosine deaminase; Enzyme prodrug; GDEPT; Ganciclovir; Nitroreductase; Thymidine kinase.

Figure 1

Schematic representation of the two step process in suicide gene therapy. In step 1, with the help of a vector the cancer cells are transduced by suicide genes resulting in expression of an enzyme. In step 2, prodrug is administered which can be converted into its cytotoxic form by the enzyme and kill not only the transduced cells but also the neighboring ones.

Figure 2

Mechanism of action for ganciclovir (GCV). HSVTK phosphorylates thymidine to thymidine monophosphate (thymidine-MP) which undergoes more phosphopylation steps by cell endogenous kinases. The final product, thymidine triphosphate (thymidine-TP) is one of the building blocks of DNA structure. GCV competes with thymidine and gets phosphorylated to ganciclovir-MP first and then ganciclovir-TP. Ganciclovir-TP blocks DNA elongation by inhibiting DNA polymerase.

Figure 3

Deamination of 5-FC by cytosine deaminase (CD). The product, 5-FU is converted to 5-FUMP and eventually 5-FUTP which blocks RNA synthesis. Other byproducts such as 5-FdUMP and 5-FdUTP block DNA synthesis after incorporation into DNA structure.

Figure 4

Activation of CB1954 by bacterial nitroreductase. After activation, both 4-hydroxylamine and 2-hydroxylamine metabolites cause DNA damage.

Figure 5

Activation of CMDA and ZD2767P to their toxic metabolites by bacterial carboxypeptidase G2 (CPG2).

Figure 6

The two-step process which produces the toxic metabolites, phosphoramide mustard and acrolein from cyclophosphamide. CYP enzyme super family catalyzes the first step whereas the second step is a self-immolative reaction.

Figure 7

The conversion of 6-methylpurine deoxyriboside to 6 -methylpurine by bacterial purine nucleoside phosphorylase.