Advances in cancer gene therapy

The First Approved Gene Therapy Product for Cancer Ad-p53 (Gendicine): 12 Years in the Clinic

Gendicine (recombinant human p53 adenovirus), developed by Shenzhen SiBiono GeneTech Co. Ltd., was approved in 2003 by the China Food and Drug Administration (CFDA) as a first-in-class gene therapy product to treat head and neck cancer, and entered the commercial market in 2004. Gendicine is a biological therapy that is delivered via minimally invasive intratumoral injection, as well as by intracavity or intravascular infusion. The wild-type (wt) p53 protein expressed by Gendicine-transduced cells is a tumor suppressor that is activated by cellular stress, and mediates cell-cycle arrest and DNA repair, or induces apoptosis, senescence, and/or autophagy, depending upon cellular stress conditions. Based on 12 years of commercial use in >30,000 patients, and >30 published clinical studies, Gendicine has exhibited an exemplary safety record, and when combined with chemotherapy and radiotherapy has demonstrated significantly higher response rates than for standard therapies alone. In addition to head and neck cancer, Gendicine has been successfully applied to treat various other cancer types and different stages of disease. Thirteen published studies that include long-term survival data showed that Gendicine combination regimens yield progression-free survival times that are significantly longer than standard therapies alone. Although the p53 gene is mutated in >50% of all human cancers, p53 mutation status did not significantly influence efficacy outcomes and long-term survival rate for Ad-p53-treated patients. To date, Shenzhen SiBiono GeneTech has manufactured 41 batches of Gendicine in compliance with CFDA QC/QA requirements, and 169,571 vials (1.0 × 10¹² vector particles per vial) have been used to treat patients. No serious adverse events have been reported, except for vector-associated transient fever, which occurred in 50-60% of patients and persisted for only a few hours. The manufacturing accomplishments and clinical experience with Gendicine, as well as the understanding of its cellular mechanisms of action and implications, could provide valuable insights for the international gene therapy community and add valuable data to promote further developments and advancements in the gene therapy field.

Cancer-specific targeting of an adenovirus-delivered herpes simplex virus thymidine kinase suicide gene using translational control

Two technical hurdles, gene delivery and target specificity, have hindered the development of effective cancer gene therapies. In order to circumvent the problem of tumor specificity, the suicide gene, HSV-1 thymidine kinase (HSV-Tk), was modified with a complex 5' upstream-untranslated region (5'-UTR) that limits efficient translation to cells expressing high levels of the translation initiation factor, eIF4E. Since previous studies have shown that most tumor cells express elevated levels of eIF4E, tumor-specific gene delivery was optimized by incorporation of the 5'-UTR-modified suicide gene (HSV-UTk) into an adenovirus vector (Ad-CMV-UTk). The efficacy of this novel approach of targeting suicide gene expression and limiting cytotoxicity by means of translational restriction was tested in vitro with the use of the human breast cancer cell lines (MCF-7, MDA-MB435, and ZR-75-1). As controls, normal MCF10A, HMEC, and HMSC cell lines that express relatively low levels of eIF4E were used. Real-time reverse-transcription polymerase chain reaction (RT-PCR) was used to quantify HSV-Tk mRNA for cells infected with Ad-CMV-UTk as well as with Ad-CMV-Tk (a control adenovirus in which HSV-Tk is not regulated at the level of translation). Translation of HSV-Tk in the Ad-infected cells was measured by Western blot analysis. In addition, cytotoxicity was determined following treatment with the pro-drug ganciclovir (GCV) using an MTT viability assay. Finally, microPET imaging was used to assess cancer cell-specific expression of HSV-Tk and expression in normal tissues in vivo after intraperitoneal injection of Ad-CMV-Tk or Ad-CMV-UTk. These data collectively showed enhanced cancer cell-specific gene expression and reduced normal tissue gene expression for the Ad-HSV-UTk compared to the Ad-CMV-Tk, leading to increased cancer cell-enhanced GCV cytotoxicity. These results indicate that translational targeting of suicide gene expression in tumor cells in vitro and in vivo is effective and may provide a platform for enhanced cancer gene therapy specificity.

Gene-directed enzyme prodrug therapy: a review of enzyme/prodrug combinations

Gene-directed enzyme prodrug therapy (GDEPT) is a promising, new, two-step treatment for cancer chemotherapy. In the first step, the gene for a foreign enzyme is administered and is directed to the tumour, where it is expressed by the use of specific promoters. In the second step, injected prodrugs are activated by the foreign enzyme. The design and synthesis of prodrugs able to undergo enzymatic activation in such systems is an essential component. This review focuses on the requirements which must be fulfilled by the components of GDEPT systems in order for this therapy to be considered a realistic possibility. A special emphasis is placed on the description of the prodrugs used in GDEPT protocols and the requirement for a bystander effect is also discussed.

Radiosynthesis of 4-[(2-chloroethyl)(2-[(11)C]ethyl)amino]-phenoxycarbonyl-l-glutamic acid a half mustard prodrug as a potential probe for imaging antibody- and gene-directed enzyme prodrug therapy with positron emission tomography

The potential antibody directed prodrug therapy half-mustard prodrug 4-[(2-chloroethyl)(2-ethyl)amino]-phenoxycarbonyl-L-glutamic acid was synthesised by reductive alkylation of 4-[(2-chloroethyl)amino]-phenoxycarbonyl-L-glutamic acid using acetaldehyde. 4-[(2-chloroethyl)[(11)C](2-ethyl)amino]phenoxycarbonyl-L-glutamic acid was synthesized with 18-22% decay corrected radiochemical yield in 45 min from EOB by reductive alkylation of 4-[(2-chloroethyl)amino]-phenoxycarbonyl-L-glutamic acid using [(11)C]acetaldehyde. [(11)C]Acetaldehyde was prepared in 60% decay corrected radiochemical yield by oxidation of [(11)C]ethanol over heated copper oxide. The radiosynthesis of [(11)C]ethanol was re-examined and optimized. 4-[(2-chloroethyl)(2-ethyl)amino]-phenoxycarbonyl-L-glutamic acid was found to have affinity for carboxypeptidase G2; the K(m) and V(max) were 99.4-115.9 microM (n=3) and 3.6-5.0 microM/min, respectively, at a carboxypeptidase G2 concentration of 0.0247 U/ml.

The emerging fields of suicide gene therapy and virotherapy

Gene therapy is defined as a technology aimed at modifying the genetic component of cells for therapeutic benefit. 'Suicide genes' can be introduced into cancer cells to make them more sensitive to chemotherapeutics or toxins. Chemotherapeutic suicide gene therapy approaches are known as gene-directed enzyme prodrug therapy or gene-prodrug activation therapy. Other approaches include replacement gene therapy, antisense strategies and induction of resistance to normal cells. All gene therapy strategies share a common component, which is the need for a selective delivery vehicle or vector with tumor-targeting capabilities. This need has led to the in-depth investigation of viruses as new vectors for gene therapy.

Prospects for viral-based strategies enhancing the anti-tumor effects of ionizing radiation

Ionizing radiation (IR) has been extensively used to treat a variety of solid tumors to improve local control and overall survival in patients. Gene therapy strategies represent one experimental direction to improve radiocurability. These gene therapy strategies include (1) replacement of mutated or deleted tumor-suppressor genes, (2) delivery of prodrugs, (3) transduction of genes under the control of radiation-inducible promoters, and (4) genetically engineered viruses that replicate preferentially in tumor cells after IR. Although any one of these viral-based gene therapy approaches is unlikely to succeed independently, experimental results suggest that clinically important antitumor can be achieved when these strategies are combined with IR. Several of these strategies are currently being or soon will be evaluated in clinical trials. This review focuses on molecular mechanisms and potential clinical application of these viral-based gene therapy strategies to improve the therapeutic index of IR.

Regressions of established breast carcinoma xenografts by carboxypeptidase G2 suicide gene therapy and the prodrug CMDA are due to a bystander effect

The role of the bystander effect in the treatment of a human breast carcinoma xenograft was studied by suicide gene therapy with carboxypeptidase G2 (CPG2) and CMDA. Cells expressing enzymatically active surface-tethered bacterial CPG2 [stCPG2(Q)3] were mixed with control beta-galactosidase (beta-Gal)-expressing cells to give stCPG2(Q)3:beta-Gal ratios of, respectively: group 1, 0:100; group 2, 10:90; group 3, 50:50; and group 4, 100:0. Four days after injection of the cells into nude mice, the prodrug 4-[(2-chloroethyl)(2-mesyloxyethyl)amino]benzoyl-L-glutamic acid (CMDA) was administered. Tumor growth delay correlated well with the levels of stCPG2(Q)3 expression: group 1, 0 day delay; group 2, 10 days; group 3, 16 days; and group 4, 90 days. Similarly, the number of cures was strongly correlated to the levels of stCPG2(Q)3 activity: group 1, zero of six cured; group 2, one of six cured; group 3, three of six cured and group 4, four of six cured. There was a good correlation between CPG2 enzyme activity in the tumors and the number of cures. The majority of cells from groups 2 and 3 were apoptotic whereas those from group 1 were not, indicating a substantial bystander effect in the tumors. These results suggest that a bystander effect plays a major role in suicide gene therapy regimens with stCPG2(Q)3 and CMDA.

Strategies for enhancing viral-based gene therapy using ionizing radiation

Many gene-therapy strategies under investigation aim to increase the efficacy of current cancer-treatment regimens. Promising results have been obtained in the laboratory and early clinical trials using viral-based motifs specifically designed to enhance the efficacy of ionizing radiation or chemotherapy. These strategies fall into two general categories: replication-incompetent viral shuttle vectors for the delivery of specific genes encoding a chemo/radiation modulator and attenuated replication-competent viruses with proposed replicative advantages in tumor cells. In this review, we discuss the rational, molecular mechanisms, and clinical application of these strategies with particular focus on recent research applying these viral-based strategies to improve the therapeutic index of ionizing radiation.

Adenovirus-mediated E2F-1 gene transfer efficiently induces apoptosis in melanoma cells

BACKGROUND

E2F-1 is a transcription factor that stimulates cellular proliferation and cell cycle progression from G(1) to S-phase. Somewhat paradoxically, E2F-1 also has the properties of a tumor suppressor. Overexpression of E2F-1 has been shown to induce apoptosis in some cancer cells. In the current study, the effect of adenovirus-mediated E2F-1 gene transfer on human melanoma cell growth was investigated.

METHODS

Two human melanoma cell lines, SK-MEL-28 (wild-type p53) and SK-MEL-2 (mutant p53), were treated by mock infection, infection with a control vector expressing the beta-galactosidase gene (Ad5CMV-LacZ), or infection with a vector expressing E2F-1 (Ad5CMV-E2F-1) at a multiplicity of infection of 100. Cell proliferation and viability were determined by WST-1 assay and trypan blue exclusion, respectively. Apoptosis was assessed by cell flow cytometry and confirmed by cell morphology, in situ terminal deoxynucleotidyl nick end labeling assay, and poly(ADP-ribose) polymerase cleavage assay.

RESULTS

Marked overexpression of E2F-1 was evident in both cell lines 24 hours after infection with Ad5CMVE2F-1 by Western blot analysis. E2F-1 overexpression resulted in growth inhibition and rapid loss of cell viability. Overexpression of E2F-1 also resulted in premature S-phase entry and G(2) arrest at 24 hours followed by apoptotic cell death at 48 hours. After Ad5CMVE2F-1 infection, expression of Bax and Bak was unchanged, whereas Mcl-1 levels decreased markedly. In SK-MEL-28 cells, Bcl-XL levels also declined after E2F-1 expression. Bcl-2 was undetectable in SK-MEL-28 cells but was increased in SK-MEL-2 cells in response to E2F-1 overexpression.

CONCLUSIONS

Adenovirus-mediated E2F-1 gene transfer efficiently induces widespread apoptosis in human melanoma cells. E2F-1 overexpression induced apoptosis in cell lines containing wild-type and mutant p53, suggesting that this effect does not require wild-type p53 function. Anti-apoptotic proteins of the Bcl-2 family, notably Mcl-1 and Bcl-XL, may be involved in mediating the response to E2F-1. These data suggest that adenovirus-mediated E2F-1 gene therapy may be effective in the treatment of melanoma.

A cell surface tethered enzyme improves efficiency in gene-directed enzyme prodrug therapy

The potential for expressing the bacterial enzyme carboxypeptidase G2 (CPG2) tethered to the outer surface of mammalian cells was examined for use in gene-directed enzyme prodrug therapy. The affinity of CPG2 for the substrate methotrexate was unaffected by three mutations required to prevent N-linked glycosylation. Breast carcinoma MDA MB 361 cells expressing CPG2 internally showed only a very modest increase in sensitivity to the prodrug CMDA because the prodrug did not enter the cells. Cells expressing surface-tethered CPG2, however, became 16-24-fold more sensitive to CMDA and could mount a good bystander effect. Systemic administration of CMDA to mice bearing established xenografts of the transfected cells led to sustained tumor regressions or cures.