Molecular approaches to chemo-radiotherapy

Applications of tissue-specific and cancer-selective gene promoters for cancer diagnosis and therapy

Current treatment of solid tumors with standard of care chemotherapies, radiation therapy and/or immunotherapies are often limited by severe adverse toxic effects, resulting in a narrow therapeutic index. Cancer gene therapy represents a targeted approach that in principle could significantly reduce undesirable side effects in normal tissues while significantly inhibiting tumor growth and progression. To be effective, this strategy requires a clear understanding of the molecular biology of cancer development and evolution and developing biological vectors that can serve as vehicles to target cancer cells. The advent and fine tuning of omics technologies that permit the collective and spatial recognition of genes (genomics), mRNAs (transcriptomics), proteins (proteomics), metabolites (metabolomics), epiomics (epigenomics, epitranscriptomics, and epiproteomics), and their interactomics in defined complex biological samples provide a roadmap for identifying crucial targets of relevance to the cancer paradigm. Combining these strategies with identified genetic elements that control target gene expression uncovers significant opportunities for developing guided gene-based therapeutics for cancer. The purpose of this review is to overview the current state and potential limitations in developing gene promoter-directed targeted expression of key genes and highlights their potential applications in cancer gene therapy.

Feasibility of Bone Marrow Mesenchymal Stem Cell-Mediated Synthetic Radiosensitive Promoter-Combined Sodium Iodide Symporter for Radiogenetic Ovarian Cancer Therapy

Ovarian cancer is the most lethal gynecological cancer, most patients relapse within 12-24 months, and eventually die, especially platinum-resistant patients. Gene therapy has been one of the most potential methods for tumor treatment. Bone marrow mesenchymal stem cells (BMSCs) have been used for systemic delivery of therapeutic genes to solid tumors. Sodium iodide symporter (NIS) is an intrinsic membrane glycoprotein and can concentrate ¹³¹I, which is important for radionuclide therapy and nuclear medicine imaging in recent years. However, the rapid iodine efflux has become a bottleneck for NIS-mediated radionuclide gene therapy. Our previous studies found that the early growth response-1 (Egr1) promoter containing CC(A/T)6GG (CArG) elements had an ¹³¹I radiation-positive feedback effect on the NIS gene. Other research showed the synthesized Egr1 promoter containing four CArG elements, E4, was nearly three times as sensitive as the Egr1 promoter. In our study, BMSC-E4-NIS was engineered to express NIS under the control of E4 promoter using lentivirial vectors. After BMSC-E4-NIS implantation, no tumors were seen in BALB/c nude mice and BMSC-E4-NIS did not promote the growth of SKOV3 tumor. BMSCs migrated toward ovarian cancer samples in chemotaxis assays and to ovarian tumors in mice. Using micro-single-photon emission computed tomography/computed tomography (SPECT/CT) imaging, we found that E4 promoter produced a notable increase in ¹²⁵I uptake after ¹³¹I irradiation, the radionuclide uptake is almost three and six times more than Egr1 and cytomegalovirus (CMV) promoters. These studies confirmed the feasibility of using BMSCs as carriers for lentivirus-mediated E4-NIS gene therapy for ovarian cancer. Further research on BMSC-E4-NIS gene therapy for ovarian cancer in vivo will also be carried on, and if successful, this might provide a new adjuvant therapeutical option for platinum-resistant ovarian cancer patients and provide a new method for dynamic evaluation of curative effect.

Overexpression of SENP1 reduces the stemness capacity of osteosarcoma stem cells and increases their sensitivity to HSVtk/GCV

Osteosarcoma stem cells are able to escape treatment with conventional chemotherapeutic drugs, as the majority of them are in a quiescent state. Recent reports have suggested that small ubiquitin-like modifiers (SUMOs) serve important roles in the maintenance of cancer stem cell stemness. Therefore, a potential strategy to increase the effectiveness of chemotherapeutic agents is to interfere with SUMO modification of proteins associated with the maintenance of stemness in osteosarcoma stem cells. The present study revealed a significant decrease in the expression of SUMO1 specific peptidase 1 (SENP1) in osteosarcoma tissues and osteosarcoma cell lines, and SENP1 expression was much lower in osteosarcoma stem cells than in non-cancer stem cells. Further experiments indicated that the low levels of SENP1 were essential for maintenance of stemness in osteosarcoma stem cells. Overexpression of SENP1 resulted in a marked decrease in the maintenance of stemness, but only slightly induced apoptosis of osteosarcoma cells, which is crucial to reduce the side effects of drugs on normal precursor cells. Finally, SENP1 overexpression led to a significant increase in the sensitivity of osteosarcoma stem cells to the herpes simplex virus 1 thymidine kinase gene in combination with ganciclovir in vitro and in vivo. In conclusion, the present study described a novel method to increase the sensitivity of osteosarcoma stem cells to chemotherapeutic drugs. Notably, this approach may significantly reduce the required dose of conventional chemotherapeutic drugs and reduce side effects.

Radiobiological Response of Cervical Cancer Cell Line in Low Dose Region: Evidence of Low Dose Hypersensitivity (HRS) and Induced Radioresistance (IRR)

BACKGROUND

Purpose of the present study was to examine the response of cervical cancer cell line (HeLa cell line) to low dose radiation using clonogenic assay and mathematical modeling of the low dose response by Joiner's induced repair model.

MATERIALS AND METHODS

Survival of HeLa cells following exposure to single and fractionated low doses of γ (gamma)-ray, 6 MV, and 15 MV photon was measured by clonogenic assay.

RESULTS

HeLa cell line demonstrated marked low dose response consisting of an area of HRS and IRR in the dose region of <1 Gy. The two gradients of the low dose region (αs and αr) were distinctly different with a transition dose (Dc) of 0.28-0.40 cGy.

CONCLUSION

HeLa cell line demonstrates marked HRS and IRR with distinct transition dose. This may form the biological basis of the clinical study to investigate the chemo potentiating effect of low dose radiation in cervical cancer.

CArG-driven GADD45α activated by resveratrol inhibits lung cancer cells

We report anticarcinogenic effects of suicide gene therapy that relies on the use of resveratrol-responsive CArG elements from the Egr-1 promoter to induce GADD45α. In A549 lung cancer cells, endogenous GADD45α was not induced upon resveratrol treatment. Therefore, induction of exogenous GADD45α resulted in growth inhibition. Resveratrol transiently induced Egr-1 through ERK/JNK-ElK-1. Hence, we cloned natural or synthetic Egr-1 promoter upstream of GADD45α cDNA to create a suicide gene therapy vector. Since natural promoter may have antagonized effects, we tested synthetic promoter that contains either five, six or nine repeats of CArG elements essential in the Egr-1 promoter to drive the expression of GADD45α upon resveratrol treatment. Further analysis confirmed that both synthetic promoter and natural Egr-1 promoter were able to “turn on” the expression of GADD45α when combined with resveratrol, and subsequently led to suppression of cell proliferation and apoptosis.

Gene-directed enzyme prodrug therapy

As one targeting strategy of prodrug delivery, gene-directed enzyme prodrug therapy (GDEPT) promises to realize the targeting through its three key features in cancer therapy-cell-specific gene delivery and expression, controlled conversion of prodrugs to drugs in target cells, and expanded toxicity to the target cells' neighbors through bystander effects. After over 20 years of development, multiple GDEPT systems have advanced into clinical trials. However, no GDEPT product is currently marketed as a drug, suggesting that there are still barriers to overcome before GDEPT becomes a standard therapy. In this review, we first provide a general introduction of this prodrug targeting strategy. Then, we utilize the four most thoroughly studied systems to illustrate components, mechanisms, preclinical and clinical results, and further development directions of GDEPT. These four systems are herpes simplex virus thymidine kinase/ganciclovir, cytosine deaminase/5-fluorocytosine, cytochrome P450/oxazaphosphorines, and nitroreductase/CB1954 system. Later, we focus our discussion on bystander effects including local and distant bystander effects. Lastly, we discuss carriers that are used to deliver genes for GDEPT including virus carriers and non-virus carriers. Among these carriers, the stem cell-based gene delivery system represents one of the newest carriers under development, and may brought about a breakthrough to the gene delivery issue of GDEPT.

Radiation quality-dependent bystander effects elicited by targeted radionuclides

The efficacy of radiotherapy may be partly dependent on indirect effects, which can sterilise malignant cells that are not directly irradiated. However, little is known of the influence of these effects in targeted radionuclide treatment of cancer. We determined bystander responses generated by the uptake of radioiodinated iododeoxyuridine ([*I]IUdR) and radiohaloanalogues of meta-iodobenzylguanidine ([*I]MIBG) by noradrenaline transporter (NAT) gene-transfected tumour cells. NAT specifically accumulates MIBG. Multicellular spheroids that consisted of 5% of NAT-expressing cells, capable of the active uptake of radiopharmaceutical, were sterilised by treatment with 20 kBqmL(-1) of the alpha-emitter meta-[211At]astatobenzylguanidine ([211At]MABG). Similarly, in nude mice, retardation of the growth of tumour xenografts containing 5% NAT-positivity was observed after treatment with [131I]MIBG. To determine the effect of subcellular localisation of radiolabelled drugs, we compared the bystander effects resulting from the intracellular concentration of [131I]MIBG and [131I]IUdR (low linear energy transfer (LET) beta-emitters) as well as [123I]MIBG and [123I]IUdR (high LET Auger electron emitters). [*I]IUdR is incorporated in DNA whereas [*I]MIBG accumulates in extranuclear sites. Cells exposed to media from [131I]MIBG- or [131I]IUdR-treated cells demonstrated a dose-response relationship with respect to clonogenic cell death. In contrast, cells receiving media from cultures treated with [123I]MIBG or [123I]IUdR exhibited dose-dependent toxicity at low dose but elimination of cytotoxicity with increasing radiation dose (i.e. U-shaped survival curves). Therefore radionuclides emitting high LET radiation may elicit toxic or protective effects on neighbouring untargeted cells at low and high dose respectively. It is concluded that radiopharmaceutical-induced bystander effects may depend on LET of the decay particles but are independent of site of intracellular concentration of radionuclide.

Gene therapy via inducible nitric oxide synthase: a tool for the treatment of a diverse range of pathological conditions

Nitric oxide (NO(.)) is a reactive nitrogen radical produced by the NO synthase (NOS) enzymes; it affects a plethora of downstream physiological and pathological processes. The past two decades have seen an explosion in the understanding of the role of NO(.) biology, highlighting various protective and damaging modes of action. Much of the controversy surrounding the role of NO(.) relates to the differing concentrations generated by the three isoforms of NOS. Both calcium-dependent isoforms of the enzyme (endothelial and neuronal NOS) generate low-nanomolar/picomolar concentrations of NO(.). By contrast, the calcium-independent isoform (inducible NOS (iNOS)) generates high concentrations of NO(.), 2-3 orders of magnitude greater. This review summarizes the current literature in relation to iNOS gene therapy for the therapeutic benefit of various pathological conditions, including various states of vascular disease, wound healing, erectile dysfunction, renal dysfunction and oncology. The available data provide convincing evidence that manipulation of endogenous NO(.) using iNOS gene therapy can provide the basis for future clinical trials.

The radiation-inducible pE9 promoter driving inducible nitric oxide synthase radiosensitizes hypoxic tumour cells to radiation

Driving high-level transgene expression in a tumour-specific manner remains a key requirement in the development of cancer gene therapy. We have previously demonstrated the strong anticancer effects of generating abnormally high levels of intracellular NO(*) following the overexpression of the inducible nitric oxide synthase (iNOS) gene. Much of this work has focused on utilizing exogenously activated promoters, which have been primarily induced using X-ray radiation. Here we further examine the potential of the pE9 promoter, comprising a combination of nine CArG radio-responsive elements, to drive the iNOS transgene. Effects of X-ray irradiation on promoter activity were compared in vitro under normoxic conditions and various degrees of hypoxia. The pE9 promoter generated high-level transgene expression, comparable with that achieved using the constitutively driven cytomegalovirus promoter. Furthermore, the radio-resistance of radiation-induced fibrosarcoma-1 (RIF-1) mouse sarcoma cells exposed to 0.1 and 0.01% O(2) was effectively eliminated following transfection with the pE9/iNOS construct. Significant inhibition of tumour growth was also observed in vivo following direct intratumoural injection of the pE9/iNOS construct compared to empty vector alone (P<0.001) or to a single radiation dose of 10 Gy (P<0.01). The combination of both therapies resulted in a significant 4.25 day growth delay compared to the gene therapy treatment alone (P<0.001). In summary, we have demonstrated the potential of the pE9/iNOS construct for reducing radio-resistance conferred by tumour cell hypoxia in vitro and in vivo, with greater tumour growth delay observed following the treatment with the gene therapy construct as compared with radiotherapy alone.

Hypoxia imaging-directed radiation treatment planning

Increasing evidence supports the role of the tumor microenvironment in modulating cancer behavior. Tissue hypoxia, an important and common condition affecting the tumor microenvironment, is well established as a resistance factor in radiotherapy. Increasing evidence points to the ability of hypoxia to induce the expression of gene products, which confer aggressive tumor behavior and promote broad resistance to therapy. These factors suggest that determining the presence or absence of tumor hypoxia is important in planning cancer therapy. Recent advances in PET hypoxia imaging, conformal radiotherapy, and imaging-directed radiotherapy treatment planning now make it possible to perform hypoxia-directed radiotherapy. We review the biological aspects of tumor hypoxia and PET imaging approaches for measuring tumor hypoxia, along with methods for conformal radiotherapy and image-guided treatment, all of which provide the underpinnings for hypoxia-directed therapy. As a case example, we review emerging data on PET imaging of hypoxia to direct radiotherapy.

Plasmid engineering for controlled and sustained gene expression for nonviral gene therapy

Gene therapy requires the introduction of genetic material in diseased cells with the aim of treating or ultimately curing a disease. Since the start of gene therapy clinical trials in 1990, gene therapy has proven to be possible, but studies to date have highlighted the difficulty of achieving efficient, specific, and long-term transgene expression. Efforts to improve gene therapy strategies over the past years were mainly aimed at solving the problem of delivery, without paying much attention to the optimization of the expression cassette. With the current understanding of the eukaryotic transcription machinery and advanced molecular biology techniques at our disposition, it has now become possible to create custom-made transgene expression cassettes optimized for gene therapy applications. In this review, we will discuss several strategies that have been explored to improve the level and duration of transgene expression, to increase control over expression, or to restrict transgene expression to specific cell types or tissues. Although still in its infancy, such strategies will eventually lead to improvement of nonviral gene therapy and expansion of the range of possible therapeutic applications.

A novel chimeric promoter that is highly responsive to hypoxia and metals

To develop a potent hypoxia-inducible promoter, we evaluated the usefulness of chimeric combinations of the (Egr-1)-binding site (EBS) from the Egr-1 gene, the metal-response element (MRE) from the metallothionein gene, and the hypoxia-response element (HRE) from the phosphoglycerate kinase 1 gene. In transient transfection assays, combining three copies of HRE (3 x HRE) with either EBS or MRE significantly increased hypoxia responsiveness. When a three-enhancer combination was tested, the EBS-MRE-3 x HRE (E-M-H) gave a hypoxia induction ratio of 69. The expression induced from E-M-H-pGL3 was 2.4-fold higher than that induced from H-pGL3 and even surpassed the expression from a human cytomegalovirus promoter-driven vector. The high inducibility of E-M-H was confirmed by validation studies in different cells and by expressing other cDNAs. Gel shift assays together with functional overexpression studies suggested that increased levels of hypoxia-inducible factor 1alpha, metal transcription factor-1 and Egr-1 may be associated with the high inducibility of the E-M-H chimeric promoter. E-M-H was also induced by hypoxia mimetics such as Co2+ and deferoxamine (DFX) and by hydrogen peroxide. Gene expression from the E-M-H was reversible as shown by the reduced expression of the transgene upon removal of inducers such as hypoxia and DFX. In vivo evaluation of the E-M-H in ischemic muscle revealed that erythropoietin secretion and luciferase and LacZ expression were significantly higher in the E-M-H group than in a control or H group. With its high induction capacity and versatile means of modulation, this novel chimeric promoter should find wide application in the treatment of ischemic diseases and cancer.

Expression of Egr-1 gene and its correlation with the oncogene proteins in non-irradiated and irradiated esophageal squamous cell carcinoma

We study the expression of early growth response gene-1 (Egr-1 gene) in non-irradiated and irradiated human esophageal cancer tissues, and its relationship with the expression of C-fos, C-jun onco-proteins as well as Egr-1 target gene proteins P53, Rb and Bax expression. In situ hybridization (ISH) and immunohistochemistry (IHC) were used respectively to detect Egr-1 mRNA, Egr-1, C-fos, C-jun, P53, Rb and Bax proteins in 80 surgically resected non-irradiated and irradiated tumor specimens of esophageal squamous cell carcinoma. Egr-1 gene mRNA and Bax protein were located in the cytoplasm, whereas Egr-1, C-fos, C-jun, P53, Rb proteins were located in the nuclei. Egr-1 was expressed in nine out of 40 cases (22.5%) of non-irradiated and 23 of 40 cases (57.5%) of irradiated tumor specimens. No correlation was found between Egr-1 gene expression and C-fos, C-jun onco-proteins expression, neither was any correlation disclosed between Egr-1 gene expression with its target gene protein expression. Patients who underwent radiotherapy with Egr-1 overexpressed in their cancer tissue had better prognosis. Radiotherapy up-regulates Egr-1 expression in esophageal carcinoma. Egr-1 overexpression may be a potential radiation response gene marker and may play an important role in prognosis of esophageal squamous cell carcinoma.

Evaluation of a synthetic CArG promoter for nitric oxide synthase gene therapy of cancer

Nitric oxide synthase gene therapy has been shown to be effective at inducing apoptosis in experimental tumours and sensitizing them to radiotherapy. We have also shown that expression of inducible nitric oxide synthase (iNOS) can be effectively restricted to the tumour volume by the use of the radiation inducible promoter (WAF1) to drive the transgene in clinically relevant protocols. A synthetic construct (pE9), incorporating nine radiosensitive CArG elements from the Egr1 promoter, has recently been developed for cancer gene therapy. We have now investigated basal gene expression of transgenes driven by this promoter to assess its suitability for use in iNOS gene therapy protocols in vivo. Transfection of human microvascular endothelial cells (HMEC-1) with pE9iNOS, using a cationic lipid vector, resulted in progressively increasing (<5-fold) levels of iNOS protein expression up to 8 h after transfection. Transfection of an ex vivo rat artery preparation with pE9iNOS caused 83% inhibition of response to the vasoconstrictor phenylephrine (PE). CMViNOS transfection also reduced response to PE, but by only 52%. A single injection of 25 microg of pE9iNOS DNA in a lipid vector into the centre of a murine sarcoma (RIF1) induced iNOS protein expression by four-fold and increased nitrite concentration eight-fold. This caused a 7-day delay in tumour growth and was more effective than the constitutive CMV-driven construct. Our data suggest that generation of NO*, as a result of iNOS overexpression, is capable of further activating the E9 promoter, through a positive feedback loop, yielding stronger and sustained levels of NO*. This pE9iNOS combination may, therefore, be particularly useful in an anticancer gene therapy strategy as its antitumour effect in vivo was clearly superior to that of the strong constitutive promoter, CMV.

Regional Radiochemotherapy Using In Situ Hydrogel

PURPOSE

To evaluate the feasibility of regional radiochemotherapy of mammary tumors using in situ hydrogel loaded with cisplatin (CDDP) and rhenium-188 ((188)Re).

METHODS

Sodium alginate (SA) and calcium chloride were used to create a hydrogel for delivery of CDDP and (188)Re. In vitro studies were performed to evaluate cytotoxic effects of (188)Re-hydrogel and sustained-release ability of the CDDP-hydrogel. Tumor-bearing rats were injected with (188)Re-hydrogel (0.5-1 mCi/rat), (188)Re-perrhenate (0.5-1 mCi/rat, intratumoral, I.T.), CDDP-hydrogel (3 mg/kg), and (188)Re-hydrogel loaded with CDDP (3 mg/kg body weight, 0.5-1 mCi/rat), respectively, and groups receiving (188)Re were imaged at 24 and 48 h postinjection. Tumor volume, body weight, imaging, and kidney function were assessed as required for each group.

RESULTS

Successful formation of the hydrogel was demonstrated by cytotoxic effects of (188)Re-hydrogel and slow release of CDDP-hydrogel in vitro. Tumor volume measurements showed significant delay in tumor growth in treated vs. control groups with minimal variation in normal kidney function for the CDDP-hydrogel group. Scintigraphic images indicated localization of (188)Re-hydrogel in the tumor site up to 48 h postinjection.

CONCLUSIONS

Our data demonstrate the feasibility of using hydrogel for delivery of chemotherapeutics and radiation locally. This technique may have applications involving other contrast modalities as well as treatment in cases where tumors are inoperable.

Imaging hypoxia and angiogenesis in tumors

There is a clear need in cancer treatment for a noninvasive imaging assay that evaluates the oxygenation status and heterogeneity of hypoxia and angiogenesis in individual patients. Such an assay could be used to select alternative treatments and to monitor the effects of treatment. Of the several methods available, each imaging procedure has at least one disadvantage. The limited quantitative potential of single-photon emission CT and MR imaging always limits tracer imaging based on these detection systems. PET imaging with FMISO and Cu-ATSM is ready for coordinated multicenter trials, however, that should move aggressively forward to resolve the debate over the importance of hypoxia in limiting response to cancer therapy. Advances in radiation treatment planning, such as intensity-modulated radiotherapy, provide the ability to customize radiation delivery based on physical conformity. With incorporation of regional biologic information, such as hypoxia and proliferating vascular density in treatment planning, imaging can create a biologic profile of the tumor to direct radiation therapy. Presence of widespread hypoxia in the tumor benefits from a systemic hypoxic cell cytotoxin. Angiogenesis is also an important therapeutic target. Imaging hypoxia and angiogenesis complements the efforts in development of antiangiogenesis and hypoxia-targeted drugs. The complementary use of hypoxia and angiogenesis imaging methods should provide the impetus for development and clinical evaluation of novel drugs targeted at angiogenesis and hypoxia. Hypoxia imaging brings in information different from that of FDG-PET but it will play an important niche role in oncologic imaging in the near future. FMISO, radioiodinated azamycin arabinosides, and Cu-ATSM are all being evaluated in patients. The Cu-ATSM images show the best contrast early after injection but these images are confounded by blood flow and their mechanism of localization is one step removed from the intracellular O2 concentration. FMISO has been criticized as inadequate because of its clearance characteristics, but its uptake after 2 hours is probably the most purely reflective of regional PO2 at the time the radiopharmaceutical is used. The FMISO images show less contrast than those of Cu-ATSM because of the lipophilicity and slower clearance of FMISO but attempts to increase the rate of clearance led to tracers whose distribution is contaminated by blood flow effects. For single-photon emission CT the only option is radioiodinated azamycin arabinosides, because the technetium agents are not yet ready for clinical evaluation. Rather than develop new and improved hypoxia agents, or even quibbling about the pros and cons of alternative agents, the nuclear medicine community needs to convince the oncology community that imaging hypoxia is an important procedure that can lead to improved treatment outcome.

Chemotherapy increases transgene expression in leukemic cells

BACKGROUND

Patients with acute myeloid leukemia (AML) often obtain complete remission with chemotherapy but the majority of patients relapse. Combining chemotherapy and gene therapy appears to be a promising approach; however, the effects of chemotherapy on transgene expression in leukemic cells have not yet been investigated.

METHODS

DA1-3b leukemic cells were transfected with pCDNA3 plasmids carrying GM-CSF or LacZ cDNA. The leukemic K562 cell line and primary cultured AML cells were transduced with an Ad5.CMV-LacZ adenoviral vector. Cells were then incubated with various concentrations of daunorubicin (DNR) and cytosine arabinoside (Ara-C), and expression of the transgene was measured. Murine DA1-3b-pCDNA3/LacZ leukemic cells were also injected into syngeneic C3H/Hej mice.

RESULTS

In the cells carrying pCDNA3, DNR and Ara-C dramatically increased expression of the LacZ and GM-CSF transgenes. Over-expression depended on drug dose and was due to increased transcription. Enhancement was also observed in K562 cells and in some primary cultured AML samples transduced with the Ad5.CMV-LacZ adenovirus. Addition of N-acetyl-L-cysteine inhibited the over-expression, suggesting that reactive oxygen species were involved in activating the CMV promoter. In the A549 lung carcinoma cell line transduced with Ad5.CMV-LacZ, Ara-C had only a minor effect, and DNR had a detrimental effect, suggesting that expression depends on cell type. In vivo experiments in which mice received DA1-3b-pCDNA3/LacZ leukemic cells, and were then treated with Ara-C, also showed increased transgene expression in these leukemic cells.

CONCLUSIONS

In leukemic cells, chemotherapeutic agents can induce over-expression of transgenes. This suggests a promising combined strategy for the treatment of acute leukemia.

Combined-modality treatment of solid tumors using radiotherapy and molecular targeted agents

PURPOSE

Molecular targeted agents have been combined with radiotherapy (RT) in recent clinical trials in an effort to optimize the therapeutic index of RT. The appeal of this strategy lies in their potential target specificity and clinically acceptable toxicity.

DESIGN

This article integrates the salient, published research findings into the underlying molecular mechanisms, preclinical efficacy, and clinical applicability of combining RT with molecular targeted agents. These agents include inhibitors of intracellular signal transduction molecules, modulators of apoptosis, inhibitors of cell cycle checkpoints control, antiangiogenic agents, and cyclo-oxygenase-2 inhibitors.

RESULTS

Molecular targeted agents can have direct effects on the cytoprotective and cytotoxic pathways implicated in the cellular response to ionizing radiation (IR). These pathways involve cellular proliferation, DNA repair, cell cycle progression, nuclear transcription, tumor angiogenesis, and prostanoid-associated inflammation. These pathways can also converge to alter RT-induced apoptosis, terminal growth arrest, and reproductive cell death. Pharmacologic modulation of these pathways may potentially enhance tumor response to RT though inhibition of tumor repopulation, improvement of tumor oxygenation, redistribution during the cell cycle, and alteration of intrinsic tumor radiosensitivity.

CONCLUSION

Combining RT and molecular targeted agents is a rational approach in the treatment of solid tumors. Translation of this approach from promising preclinical data to clinical trials is actively underway.

Transcriptional Targeting in Cancer Gene Therapy

Cancer gene therapy has been one of the most exciting areas of therapeutic research in the past decade. In this review, we discuss strategies to restrict transcription of transgenes to tumour cells. A range of promoters which are tissue-specific, tumour-specific, or inducible by exogenous agents are presented. Transcriptional targeting should prevent normal tissue toxicities associated with other cancer treatments, such as radiation and chemotherapy. In addition, the specificity of these strategies should provide improved targeting of metastatic tumours following systemic gene delivery. Rapid progress in the ability to specifically control transgenes will allow systemic gene delivery for cancer therapy to become a real possibility in the near future.

Prodrugs for Gene-Directed Enzyme-Prodrug Therapy (Suicide Gene Therapy)

This review focuses on the prodrugs used in suicide gene therapy. These prodrugs need to satisfy a number of criteria. They must be efficient and selective substrates for the activating enzyme, and be metabolized to potent cytotoxins preferably able to kill cells at all stages of the cell cycle. Both prodrugs and their activated species should have good distributive properties, so that the resulting bystander effects can maximize the effectiveness of the therapy, since gene transduction efficiencies are generally low. A total of 42 prodrugs explored for use in suicide gene therapy with 12 different enzymes are discussed, particularly in terms of their physiocochemical properties. An important parameter in determining bystander effects generated by passive diffusion is the lipophilicity of the activated form, a property conveniently compared by diffusion coefficients (log P for nonionizable compounds and log D(7) for compounds containing an ionizable centre). Many of the early antimetabolite-based prodrugs provide very polar activated forms that have limited abilities to diffuse across cell membranes, and rely on gap junctions between cells for their bystander effects. Several later studies have shown that more lipophilic, neutral compounds have superior diffusion-based bystander effects. Prodrugs of DNA alkylating agents, that are less cell cycle-specific than antimetabolites and more effective against noncycling tumor cells, appear in general to be more active prodrugs, requiring less prolonged dosing schedules to be effective. It is expected that continued studies to optimize the bystander effects and other properties of prodrugs and the activated species they generate will contribute to improvements in the effectiveness of suicide gene therapy.

Novel gene therapy approach for nasopharyngeal carcinoma

This article will provide an overview on the status of cancer gene therapy, focussed specifically on its potential application in nasopharyngeal carcinoma (NPC). The concepts and strategies behind the design of therapeutic targets such as p53, p16, and death genes will be described. One of the major challenges in cancer gene therapy is tumor-specific expression of therapeutic genes, and a transcriptional targeting approach will be reviewed, in reference to NPC. Specifically, the ability to exploit the presence of Epstein-Barr virus (EBV) will be emphasized. The currently available preclinical data on genetic therapeutic approaches for NPC will be reviewed, and an outline for its future role in management of NPC, in conjunction with existing cytotoxic modalities of ionizing radiation and chemotherapy will be provided.