In vivo sensitization of ovarian tumors to chemotherapy by expression of E. coli purine nucleoside phosphorylase in a small fraction of cells

Thymidylate synthase disruption to limit cell proliferation in cell therapies

Stem and progenitor cells hold great promise for regenerative medicine and gene therapy approaches. However, transplantation of living cells entails a fundamental risk of unwanted growth, potentially exacerbated by CRISPR-Cas9 or other genetic manipulations. Here, we describe a safety system to control cell proliferation while allowing robust and efficient cell manufacture, without any added genetic elements. Inactivating TYMS, a key nucleotide metabolism enzyme, in several cell lines resulted in cells that proliferate only when supplemented with exogenous thymidine. Under supplementation, TYMS-/--pluripotent stem cells proliferate, produce teratomas, and successfully differentiate into potentially therapeutic cell types such as pancreatic β cells. Our results suggest that supplementation with exogenous thymidine affects stem cell proliferation, but not the function of stem cell-derived cells. After differentiation, postmitotic cells do not require thymidine in vitro or in vivo, as shown by the production of functional human insulin in mice up to 5 months after implantation of stem cell-derived pancreatic tissue.

Perspectives and challenges in developing small molecules targeting purine nucleoside phosphorylase

As a cytosolic enzyme involved in the purine salvage pathway metabolism, purine nucleoside phosphorylase (PNP) plays an important role in a variety of cellular functions but also in immune system, including cell growth, apoptosis and cancer development and progression. Based on its T-cell targeting profile, PNP is a potential target for the treatment of some malignant T-cell proliferative cancers including lymphoma and leukemia, and some specific immunological diseases. Numerous small-molecule PNP inhibitors have been developed so far. However, only Peldesine, Forodesine and Ulodesine have entered clinical trials and exhibited some potential for the treatment of T-cell leukemia and gout. The most recent direction in PNP inhibitor development has been focused on PNP small-molecule inhibitors with better potency, selectivity, and pharmacokinetic property. In this perspective, considering the structure, biological functions, and disease relevance of PNP, we highlight the recent research progress in PNP small-molecule inhibitor development and discuss prospective strategies for designing additional PNP therapeutic agents.

Evaluating antitumor activity of Escherichia coli purine nucleoside phosphorylase against head and neck patient-derived xenografts

BACKGROUND

Purine nucleoside phosphorylase (PNP) gene transfer represents a promising approach to treatment of head and neck malignancies. We tested recombinant adenovirus already in phase I/II clinical testing and leading-edge patient-derived xenografts (PDX) as a means to optimize this therapeutic strategy.

METHODS

Our experiments investigated purine base cytotoxicity, PNP enzyme activity following treatment of malignant tissue, tumor mass regression, viral receptor studies, and transduction by tropism-modified adenovirus.

RESULTS

Replication deficient vector efficiently transduced PDX cells and mediated significant anticancer effect following treatment with fludarabine phosphate in vivo. Either 6-methylpurine or 2-fluoroadenine (toxic molecules generated by the PNP approach) ablated head and neck cancer cell proliferation. High levels of adenovirus-3 specific receptors were detected in human tumor models, and vector was evaluated that utilizes this pathway.

CONCLUSIONS

Our studies provide the scientific foundation necessary to improve PNP prodrug cleavage and advance a new treatment for head and neck cancer.

Suicide gene strategies applied in ovarian cancer studies

Ovarian cancer represents the most lethal gynecological malignancy among women in developed countries. Despite the recent innovations, the improvements in the 5-year survival rate have been insufficient and the management of this disease still remains a challenge. The fact that the majority of patients experience recurrent or resistant disease have substantiated the necessity of an innovative treatment. Among various strategies investigated, the recent strides made in gene delivery techniques have made gene therapy, including suicide gene strategies, a potential alternative for treating ovarian cancer. Various suicide gene candidates, which are capable of promoting cancer cell apoptosis directly after its entry or indirectly by prodrug administration, can be separated into three systems using enzyme-coding, toxin or pro-apoptotic genes. With this review, we aim to provide an overview of different suicide genes depending on therapeutic strategies, the vectors used to deliver these transgenes specifically to malignant cells, and the combined treatments of these genes with various therapeutic regimens.

Targeting purine metabolism in ovarian cancer

Purine, an abundant substrate in organisms, is a critical raw material for cell proliferation and an important factor for immune regulation. The purine de novo pathway and salvage pathway are tightly regulated by multiple enzymes, and dysfunction in these enzymes leads to excessive cell proliferation and immune imbalance that result in tumor progression. Maintaining the homeostasis of purine pools is an effective way to control cell growth and tumor evolution, and exploiting purine metabolism to suppress tumors suggests interesting directions for future research. In this review, we describe the process of purine metabolism and summarize the role and potential therapeutic effects of the major purine-metabolizing enzymes in ovarian cancer, including CD39, CD73, adenosine deaminase, adenylate kinase, hypoxanthine guanine phosphoribosyltransferase, inosine monophosphate dehydrogenase, purine nucleoside phosphorylase, dihydrofolate reductase and 5,10-methylenetetrahydrofolate reductase. Purinergic signaling is also described. We then provide an overview of the application of purine antimetabolites, comprising 6-thioguanine, 6-mercaptopurine, methotrexate, fludarabine and clopidogrel. Finally, we discuss the current challenges and future opportunities for targeting purine metabolism in the treatment-relevant cellular mechanisms of ovarian cancer.

Intratumoral generation of 2-fluoroadenine to treat solid malignancies of the head and neck

This report describes treatment of locoregional head and neck squamous cell carcinoma (HNSCC) by an innovative, experimental strategy involving generation of a robust anti-cancer agent (2-fluoroadenine [F-Ade]) following transduction by Escherichia coli purine nucleoside phosphorylase (PNP) in a small number of tumor cells. F-Ade works by a unique mechanism of action (ablation of RNA and protein synthesis) and confers tumor regressions of otherwise refractory HNSCC in human subjects. Clinical studies have now advanced to a pivotal (registration-directed) trial involving locoregional HNSCC, with plans to begin subject enrollment late in 2018. The present review is the first to summarize use of PNP in the context of HNSCC, and provides background regarding this emerging anti-cancer approach.

Use of E. coli Purine Nucleoside Phosphorylase in the Treatment of Solid Tumors

BACKGROUND

The selective expression of non-human genes in tumor tissue to activate non-toxic compounds (Gene Directed Prodrug Enzyme Therapy, GDEPT) is a novel strategy designed for killing tumor cells in patients with little or no systemic toxicity. Numerous non-human genes have been evaluated, but none have yet been successful in the clinic.

METHODS

Unlike human purine nucleoside phosphorylase (PNP), E. coli PNP accepts adenine containing nucleosides as substrates, and is therefore able to selectively activate non-toxic purine analogs in tumor tissue. Various in vitro and in vivo assays have been utilized to evaluate E. coli PNP as a potential activating enzyme.

RESULTS

We and others have demonstrated excellent in vitro and in vivo anti-tumor activity with various GDEPT strategies utilizing E. coli PNP to activate purine nucleoside analogs. A phase I clinical trial utilizing recombinant adenoviral vector for delivery of E. coli PNP to solid tumors followed by systemic administration of fludarabine phosphate (NCT01310179; IND# 14271) has recently been completed. In this trial, significant anti-tumor activity was demonstrated with negligible toxicity related to the therapy. The mechanism of cell kill (inhibition of RNA and protein synthesis) is distinct from all currently used anticancer drugs and all experimental compounds under development. The approach has demonstrated excellent ability to kill neighboring tumor cells that do not express E. coli PNP, is active against non-proliferating and proliferating tumors cells (as well as tumor stem cells, stroma), and is therefore very effective against solid tumors with a low growth fraction.

CONCLUSION

The unique attributes distinguish this approach from other GDEPT strategies and are precisely those required to mediate significant improvements in antitumor therapy.

6-Methylpurine derived sugar modified nucleosides: Synthesis and evaluation of their substrate activity with purine nucleoside phosphorylases

6-Methylpurine (MeP) is cytotoxic adenine analog that does not exhibit selectivity when administered systemically, and could be very useful in a gene therapy approach to cancer treatment involving Escherichia coli PNP. The prototype MeP releasing prodrug, 9-(β-d-ribofuranosyl)-6-methylpurine, MeP-dR has demonstrated good activity against tumors expressing E. coli PNP, but its antitumor activity is limited due to toxicity resulting from the generation of MeP from gut bacteria. Therefore, we have embarked on a medicinal chemistry program to identify non-toxic MeP prodrugs that could be used in conjunction with E. coli PNP. In this work, we report on the synthesis of 9-(6-deoxy-β-d-allofuranosyl)-6-methylpurine (3) and 9-(6-deoxy-5-C-methyl-β-d-ribo-hexofuranosyl)-6-methylpurine (4), and the evaluation of their substrate activity with several phosphorylases. The glycosyl donors; 1,2-di-O-acetyl-3,5-di-O-benzyl-α-d-allofuranose (10) and 1-O-acetyl-3-O-benzyl-2,5-di-O-benzoyl-6-deoxy-5-C-methyl-β-d-ribohexofuran-ose (15) were prepared from 1,2:5,6-di-O-isopropylidine-α-d-glucofuranose in 9 and 11 steps, respectively. Coupling of 10 and 15 with silylated 6-methylpurine under Vorbrüggen glycosylation conditions followed conventional deprotection of the hydroxyl groups furnished 5'-C-methylated-6-methylpurine nucleosides 3 and 4, respectively. Unlike 9-(6-deoxy-α-l-talo-furanosyl)-6-methylpurine, which showed good substrate activity with E. coli PNP mutant (M64V), the β-d-allo-furanosyl derivative 3 and the 5'-di-C-methyl derivative 4 were poor substrates for all tested glycosidic bond cleavage enzymes.

6-Methylpurine derived sugar modified nucleosides: Synthesis and in vivo antitumor activity in D54 tumor expressing M64V-Escherichia coli purine nucleoside phosphorylase

Impressive antitumor activity has been observed with fludarabine phosphate against tumors that express Escherichia coli purine nucleoside phosphorylase (PNP) due to the liberation of 2-fluoroadenine in the tumor tissue. 6-Methylpurine (MeP) is another cytotoxic adenine analog that does not exhibit selectivity when administered systemically, and could be very useful in a gene therapy approach to cancer treatment involving E. coli PNP. The prototype MeP releasing prodrug 9-(2-deoxy-β-d-ribofuranosyl)-6-methylpurine (1) [MeP-dR] has demonstrated good activity against tumors expressing E. coli PNP, but its antitumor activity is limited due to toxicity resulting from the generation of MeP from gut bacteria. Therefore, we have embarked on a medicinal chemistry program to identify a combination of non-toxic MeP prodrugs and non-human adenosine glycosidic bond cleaving enzymes. The two best MeP-based substrates with M64V-E coli PNP, a mutant which was engineered to tolerate modification at the 5'-position of adenosine and its analogs, were 9-(6-deoxy-α-l-talofuranosyl)-6-methylpurine (3) [methyl(talo)-MeP-R] and 9-(α-l-lyxofuranosyl)6-methylpurine (4) [lyxo-MeP-R]. The detailed synthesis methyl(talo)-MeP-R and lyxo-MeP-R, and the evaluation of their substrate activity with 4 enzymes not normally associated with cancer patients is described. In addition, we have determined the intraperitoneal pharmacokinetic (ip-PK) properties of methyl(talo)-MeP-R and have determined its in vivo bystander activity in mice bearing D54 tumors that express M64V PNP. The observed good in vivo bystander activity of [methyl(talo)-MeP-R/M64V-E coli PNP combination suggests that these agents could be useful for the treatment of cancer.

Phase I dose-escalating trial of Escherichia coli purine nucleoside phosphorylase and fludarabine gene therapy for advanced solid tumors

BACKGROUND

The use of Escherichia coli purine nucleoside phosphorylase (PNP) to activate fludarabine has demonstrated safety and antitumor activity during preclinical analysis and has been approved for clinical investigation.

PATIENTS AND METHODS

A first-in-human phase I clinical trial (NCT 01310179; IND 14271) was initiated to evaluate safety and efficacy of an intratumoral injection of adenoviral vector expressing E. coli PNP in combination with intravenous fludarabine for the treatment of solid tumors. The study was designed with escalating doses of fludarabine in the first three cohorts (15, 45, and 75 mg/m(2)) and escalating virus in the fourth (10(11)-10(12) viral particles, VP).

RESULTS

All 12 study subjects completed therapy without dose-limiting toxicity. Tumor size change from baseline to final measurement demonstrated a dose-dependent response, with 5 of 6 patients in cohorts 3 and 4 achieving significant tumor regression compared with 0 responsive subjects in cohorts 1 and 2. The overall adverse event rate was not dose-dependent. Most common adverse events included pain at the viral injection site (92%), drainage/itching/burning (50%), fatigue (50%), and fever/chills/influenza-like symptoms (42%). Analysis of serum confirmed the lack of systemic exposure to fluoroadenine. Antibody response to adenovirus was detected in two patients, suggesting that neutralizing immune response is not a barrier to efficacy.

CONCLUSIONS

This first-in-human clinical trial found that localized generation of fluoroadenine within tumor tissues using E. coli PNP and fludarabine is safe and effective. The pronounced effect on tumor volume after a single treatment cycle suggests that phase II studies are warranted.

CLINICALTRIALSGOV IDENTIFIER

NCT01310179.

Prodrugs: a challenge for the drug development

It is estimated that about 10% of the drugs approved worldwide can be classified as prodrugs. Prodrugs, which have no or poor biological activity, are chemically modified versions of a pharmacologically active agent, which must undergo transformation in vivo to release the active drug. They are designed in order to improve the physicochemical, biopharmaceutical and/or pharmacokinetic properties of pharmacologically potent compounds. This article describes the basic functional groups that are amenable to prodrug design, and highlights the major applications of the prodrug strategy, including the ability to improve oral absorption and aqueous solubility, increase lipophilicity, enhance active transport, as well as achieve site-selective delivery. Special emphasis is given to the role of the prodrug concept in the design of new anticancer therapies, including antibody-directed enzyme prodrug therapy (ADEPT) and gene-directed enzyme prodrug therapy (GDEPT).

Pseudomonas aeruginosa NfsB and nitro-CBI-DEI--a promising enzyme/prodrug combination for gene directed enzyme prodrug therapy

BACKGROUND

The nitro-chloromethylbenzindoline prodrug nitro-CBI-DEI appears a promising candidate for the anti-cancer strategy gene-directed enzyme prodrug therapy, based on its ability to be converted to a highly cytotoxic cell-permeable derivative by the nitroreductase NfsB from Escherichia coli. However, relative to some other nitroaromatic prodrugs, nitro-CBI-DEI is a poor substrate for E. coli NfsB. To address this limitation we evaluated other nitroreductase candidates from E. coli and Pseudomonas aeruginosa.

FINDINGS

Initial screens of candidate genes in the E. coli reporter strain SOS-R2 identified two additional nitroreductases, E. coli NfsA and P. aeruginosa NfsB, as being more effective activators of nitro-CBI-DEI than E. coli NfsB. In monolayer cytotoxicity assays, human colon carcinoma (HCT-116) cells transfected with P. aeruginosa NfsB were >4.5-fold more sensitive to nitro-CBI-DEI than cells expressing either E. coli enzyme, and 23.5-fold more sensitive than untransfected HCT-116. In three dimensional mixed cell cultures, not only were the P. aeruginosa NfsB expressing cells 540-fold more sensitive to nitro-CBI-DEI than pure cultures of untransfected HCT-116, the activated drug that they generated also displayed an unprecedented local bystander effect.

CONCLUSION

We posit that the discrepancy in the fold-sensitivity to nitro-CBI-DEI between the two and three dimensional cytotoxicity assays stems from loss of activated drug into the media in the monolayer cultures. This emphasises the importance of evaluating high-bystander GDEPT prodrugs in three dimensional models. The high cytotoxicity and bystander effect exhibited by the NfsB_Pa/nitro-CBI-DEI combination suggest that further preclinical development of this GDEPT pairing is warranted.

Enzymes to die for: exploiting nucleotide metabolizing enzymes for cancer gene therapy

Suicide gene therapy is an attractive strategy to selectively destroy cancer cells while minimizing unnecessary toxicity to normal cells. Since this idea was first introduced more than two decades ago, numerous studies have been conducted and significant developments have been made to further its application for mainstream cancer therapy. Major limitations of the suicide gene therapy strategy that have hindered its clinical application include inefficient directed delivery to cancer cells and the poor prodrug activation capacity of suicide enzymes. This review is focused on efforts that have been and are currently being pursued to improve the activity of individual suicide enzymes towards their respective prodrugs with particular attention to the application of nucleotide metabolizing enzymes in suicide cancer gene therapy. A number of protein engineering strategies have been employed and our discussion here will center on the use of mutagenesis approaches to create and evaluate nucleotide metabolizing enzymes with enhanced prodrug activation capacity and increased thermostability. Several of these studies have yielded clinically important enzyme variants that are relevant for cancer gene therapy applications because their utilization can serve to maximize cancer cell killing while minimizing the prodrug dose, thereby limiting undesirable side effects.

Tumor-targeting Salmonella typhimurium, a natural tool for activation of prodrug 6MePdR and their combination therapy in murine melanoma model

The PNP/6-methylpurine 2'-deoxyriboside (6MePdR) system is an efficient gene-directed enzyme prodrug therapy system with significant antitumor activities. In this system, Escherichia coli purine nucleoside phosphorylase (ePNP) activates nontoxic 6MePdR into potent antitumor drug 6-methylpurine (6MeP). The Salmonella typhimurium PNP (sPNP) gene has a 96-% sequence homology in comparison with ePNP and also has the ability to convert 6MePdR to 6MeP. In this study, we used tumor-targeting S. typhimurium VNP20009 expressing endogenous PNP gene constitutively to activate 6MePdR and a combination treatment of bacteria and prodrug in B16F10 melanoma model. The conversion of 6MePdR to 6MeP by S. typhimurium was analyzed by HPLC and the enzyme activity of sPNP was confirmed by in vitro (tetrazolium-based colorimetric assay) MTT cytotoxicity assay. After systemic administration of VNP20009 to mice, the bacteria largely accumulated and specifically delivered endogenous sPNP in the tumor. In comparison with VNP20009 or 6MePdR treatment alone, combined administration of VNP20009 followed by 6MePdR treatment significantly delayed the growth of B16F10 tumor and increased the CD8(+) T-cell infiltration. In summary, our results demonstrated that the combination therapy of S. typhimurium and prodrug 6MePdR is a promising strategy for cancer therapy.

Suicide gene therapy in cancer: where do we stand now?

Suicide gene therapy is based on the introduction into tumor cells of a viral or a bacterial gene, which allows the conversion of a non-toxic compound into a lethal drug. Although suicide gene therapy has been successfully used in a large number of in vitro and in vivo studies, its application to cancer patients has not reached the desirable clinical significance. However, recent reports on pre-clinical cancer models demonstrate the huge potential of this strategy when used in combination with new therapeutic approaches. In this review, we summarize the different suicide gene systems and gene delivery vectors addressed to cancer, with particular emphasis on recently developed systems and associated bystander effects. In addition, we review the different strategies that have been used in combination with suicide gene therapy and provide some insights into the future directions of this approach, particularly towards cancer stem cell eradication.

Purine Nucleoside Phosphorylase mediated molecular chemotherapy and conventional chemotherapy: a tangible union against chemoresistant cancer

BACKGROUND

Late stage Ovarian Cancer is essentially incurable primarily due to late diagnosis and its inherent heterogeneity. Single agent treatments are inadequate and generally lead to severe side effects at therapeutic doses. It is crucial to develop clinically relevant novel combination regimens involving synergistic modalities that target a wider repertoire of cells and lead to lowered individual doses. Stemming from this premise, this is the first report of two- and three-way synergies between Adenovirus-mediated Purine Nucleoside Phosphorylase based gene directed enzyme prodrug therapy (PNP-GDEPT), docetaxel and/or carboplatin in multidrug-resistant ovarian cancer cells.

METHODS

The effects of PNP-GDEPT on different cellular processes were determined using Shotgun Proteomics analyses. The in vitro cell growth inhibition in differentially treated drug resistant human ovarian cancer cell lines was established using a cell-viability assay. The extent of synergy, additivity, or antagonism between treatments was evaluated using CalcuSyn statistical analyses. The involvement of apoptosis and implicated proteins in effects of different treatments was established using flow cytometry based detection of M30 (an early marker of apoptosis), cell cycle analyses and finally western blot based analyses.

RESULTS

Efficacy of the trimodal treatment was significantly greater than that achieved with bimodal- or individual treatments with potential for 10-50 fold dose reduction compared to that required for individual treatments. Of note was the marked enhancement in apoptosis that specifically accompanied the combinations that included PNP-GDEPT and accordingly correlated with a shift in the expression of anti- and pro-apoptotic proteins. PNP-GDEPT mediated enhancement of apoptosis was reinforced by cell cycle analyses. Proteomic analyses of PNP-GDEPT treated cells indicated a dowregulation of proteins involved in oncogenesis or cancer drug resistance in treated cells with accompanying upregulation of apoptotic- and tumour- suppressor proteins.

CONCLUSION

Inclusion of PNP-GDEPT in regular chemotherapy regimens can lead to significant enhancement of the cancer cell susceptibility to the combined treatment. Overall, these data will underpin the development of regimens that can benefit patients with late stage ovarian cancer leading to significantly improved efficacy and increased quality of life.

Advancement and prospects of tumor gene therapy

Gene therapy is one of the most attractive fields in tumor therapy. In past decades, significant progress has been achieved. Various approaches, such as viral and non-viral vectors and physical methods, have been developed to make gene delivery safer and more efficient. Several therapeutic strategies have evolved, including gene-based (tumor suppressor genes, suicide genes, antiangiogenic genes, cytokine and oxidative stress-based genes) and RNA-based (antisense oligonucleotides and RNA interference) approaches. In addition, immune response-based strategies (dendritic cell- and T cell-based therapy) are also under investigation in tumor gene therapy. This review highlights the progress and recent developments in gene delivery systems, therapeutic strategies, and possible clinical directions for gene therapy.

Enhanced efficiency of prodrug activation therapy by tumor-selective replicating retrovirus vectors armed with the Escherichia coli purine nucleoside phosphorylase gene

Gene transfer of the Escherichia coli purine nucleoside phosphorylase (PNP) results in potent cytotoxicity after administration of the prodrug fludarabine phosphate (F-araAMP). Here, we have tested whether application of this strategy in the context of replication-competent retrovirus (RCR) vectors, which can achieve highly efficient tumor-restricted transduction as well as persistent expression of transgenes, would result in effective tumor inhibition, or, alternatively, would adversely affect viral replication. We found that RCR vectors could achieve high levels of PNP expression concomitant with the efficiency of their replicative spread, with significant cell killing activity in vitro and potent therapeutic effects in vivo. In U-87 xenograft models, replicative spread of the vector resulted in progressive transmission of the PNP transgene, as evidenced by increasing PNP enzyme activity with time after vector inoculation. On F-araAMP administration, high efficiency gene transfer of PNP by the RCR vector resulted in significant suppression of tumor growth and extended survival time. As the RCR mediates stable integration of the PNP gene and continuous expression, an additional round of F-araAMP administration resulted in further survival benefit. RCR-mediated PNP suicide gene therapy thus represents a highly efficient form of intracellular chemotherapy, and may achieve effective antitumor activity with less systemic toxicity.

Regioselective metalation of 6-methylpurines: synthesis of fluoromethyl purines and related nucleosides for suicide gene therapy of cancer

Metalation of 6-methyl-9-(tetrahydro-2H-pyran-2-yl)purine (10) with lithiating agents of varying basicities such as n-BuLi and LiHMDS in THF at -78 degrees C resulted in metalation at both of the 6-CH(3) moiety and the 8-CH position, irrespective of the molar equivalence of the base. On the other hand, a regioselective metalation at the 6-CH(3) moiety of 10 was observed with NaHMDS or KHMDS, under similar conditions. Treatment of the potassium salts of 10 and of the protected riboside derivative 6-methyl-9-(beta-D-2,3,5-tri-O-tert-butyldimethylsilylribofuranosyl)purine (22) with N-fluorobenzenesulfonamide (NFSI) at -78 degrees C gave the corresponding 6-fluoromethylpurine derivatives 11 and 23, respectively, in good yields. Deprotection of 11 and 23 under standard conditions gave 6-fluoromethylpurine (6-FMeP, 3) and 6-fluoromethyl-9-(beta-D-ribofuranosyl)purine (6-FMePR, 4), respectively, in high yield. Both 3 and 4 demonstrated cytotoxic activity against CCRF-CEM cells in culture. 6-FMePR is a good substrate for E. coli purine nucleoside phosphorylase (E. coli PNP) with a comparable substrate activity to that of the parent nucleoside, 6-methyl-9-(beta-D-ribofuranosyl)purine (6-MePR, 21). The cytotoxic activity of 6-FMeP along with the substrate activity of 6-FMePR with E. coli PNP meet the fundamental requirements for using 6-FMeP as a potential toxin in PNP/prodrug based cancer gene therapy.

Development of gene therapy in association with clinically used cytotoxic deoxynucleoside analogues

The clinical use of cytotoxic deoxynucleoside analogues is often limited by resistance mechanisms due to enzymatic deficiency, or high toxicity in nontumor tissues. To improve the use of these drugs, gene therapy approaches have been proposed and studied, associating clinically used deoxynucleoside analogues such as araC and gemcitabine and suicide genes or myeloprotective genes. In this review, we provide an update of recent results in this area, with particular emphasis on human deoxycytidine kinase, the deoxyribonucleoside kinase from Drosophila melanogaster, purine nucleoside phosphorylase from Escherichia coli, and human cytidine deaminase. Data from literature clearly show the feasibility of these systems, and clinical trials are warranted to conclude on their use in the treatment of cancer patients.

Expression of the reporter genes lacZ and EGFP in human glioblastoma cells using Listeria monocytogenes as vector for gene transfer

OBJECTIVE

Glioblastoma has become a model target for viral gene therapy approaches. To circumvent some of the inherent problems of viral vectors, we examined the feasibility of Listeria monocytogenes for foreign gene expression in glioblastoma cells.

METHODS

The internalin-dependent uptake of L. monocytogenes into human glioma cell lines and into 22 cell cultures obtained during neurosurgery was studied by in vitro infection assays. Internalization rates of wild-type L. monocytogenes were compared with a deletion mutant and to four strains with plasmid-based recomplementation of the internalin A/B operon. For gene transfer experiments, plasmids encoding lacZ or EGFP under control of the cytomegalovirus promotor were transfected into Listeriae. Incubation times in cell culture were studied regarding infection of cells, intracellular listerial replication and reporter gene translation. Gene transfer efficiency was determined by beta-galactosidase activity and by fluorescence microscopy.

RESULTS

The internalin-defective strain (DeltainlAB2) showed less uptake compared with the wild type. An increased invasion rate was observed in strain DeltainlAB2 + inlB(PinlA) overexpressing internalin B; complementation of the entire operon (DeltainlAB2 + inlAB(PinlA)) revealed a synergistic effect. For gene transfer experiments into glioblastoma cells, the pathogenic strain 4bL312 was employed, and efficiencies up to 5% were achieved.

DISCUSSION

Internalins A and B are major determinants for listerial uptake into neuroepithelial tumors. The L. monocytogenes vector system enables foreign gene expression in glioblastoma cells. Ongoing research deals with optimization of transfection efficiencies, with the use of less pathogenic substrains and the cloning of suicide gene vectors.

Suicide gene/prodrug therapy using salmonella-mediated delivery of Escherichia coli purine nucleoside phosphorylase gene and 6-methoxypurine 2'-deoxyriboside in murine mammary carcinoma 4T1 model

Attenuated salmonella have been reported to be capable of both selectively growing in tumors and expressing exogenous genes for tumor-targeted therapy. As 6-methoxypurine 2'-deoxyriboside (MoPdR) is similar to 6-methylpurine 2'-deoxyriboside in structure, we aimed to evaluate the antitumoral effect of the Escherichia coli purine nucleoside phosphorylase (ePNP) gene, using an attenuated salmonella-mediated delivery system, in combination with MoPdR. A novel mutant serovar Typhimurium (SC36) was used to carry the pEGFP-C1-ePNP vector that contains an enhanced green fluorescent protein and an ePNP gene under the control of the cytomegalovirus promoter. The function of the ePNP expression vector was confirmed in vitro using the enzymic conversion of MoPdR into methoxypurine. We also observed a high bystander effect induced by the ePNP/MoPdR system with a very low proportion (1%) of ePNP-positive cells and 5 microg/mL MoPdR, although the growth of parental cells was affected appreciably by MoPdR. The killing effect and increased apoptosis induced by SC36 carrying the ePNP expression vector (SC/ePNP) were detected by cytotoxicity assay and propidium iodide staining flow cytometry analysis, in combination with MoPdR. SC/ePNP was given orally to mice bearing mammary carcinomas, and its antitumor effect was evaluated. SC/ePNP plus MoPdR significantly inhibited tumor growth by approximately 86.6-88.7% and prolonged the survival of tumor-hosting mice. Our data support the view that MoPdR combined with the ePNP gene could be used in gene-directed enzyme prodrug therapy. Attenuated salmonella could be a promising strategy to improve ePNP/MoPdR bystander killing due to its preferential accumulation and anticancer activity in tumors.

Synergistic antitumor efficacy of suicide/ePNP gene and 6-methylpurine 2'-deoxyriboside via Salmonella against murine tumors

Some anaerobes and facultative anaerobes have been used in tumor-specific gene therapy by reason of their selective growth in tumors. In this work, we aimed to evaluate the anticancer efficacy of attenuated Salmonella typhimurium as a carrier to deliver the Escherichia coli purine nucleoside phosphorylase (ePNP) gene for GDEPT (gene-directed enzyme-prodrug therapy). A live attenuated purine-auxotrophic strain of S. typhimurium (SC36) was used to carry the pEGFP-C1-ePNP vector that contains a green fluorescent protein (GFP) and an ePNP gene under the control of the human cytomegalovirus (CMV) promoter. The function of the ePNP expression vector was confirmed in vitro using the enzymic conversion of 6-methylpurine 2'-deoxyriboside (MePdR) into 6-methylpurine. We also observed a high bystander effect induced by the ePNP/MePdR system with a very low proportion (1%) of ePNP-positive cells. The killing effect and increased apoptosis induced by SC/ePNP (SC36 carrying the ePNP expression vector) infection were detected by cytotoxicity assay and PI staining flow cytometry analysis, in combination with MePdR administration. Furthermore, SC/ePNP was administered orally into mice bearing melanomas or pulmonary tumors, and its anti-tumor effect was evaluated. When the tumor was huge (500 mm(3)) at the beginning of MePdR administration, SC/ePNP plus MepdR significantly inhibited tumor growth by about 59-80% and prolonged survival of mice. Complete tumor regression and long-term cure were achieved by MePdR administration, even when the tumor was large (100 mm(3)) at the beginning of MePdR treatment. Our data support a hopeful view that tumor-targeting SC36 could improve antitumor efficacy of the ePNP/MePdR system due to its preferential accumulation and anticancer activity in tumors.

Targeting gene therapy for hepatocarcinoma cells with the E. coli purine nucleoside phosphorylase suicide gene system directed by a chimeric alpha-fetoprotein promoter

For hepatocarcinoma (HCC) gene therapy, the tumoricidal efficacy and selective expression of therapeutic gene remain two major challenges. The Escherichia coli (E. coli) purine nucleoside phosphorylase (PNP)/9-(2-deoxy-beta-dribofuranosyl)-6-methylpurine (MeP-dR) suicide gene system exhibits excellent anti-tumor effects, indicating this system directed by a HCC-specific promoter would offer a possibility of targeting gene therapy for HCC. To test this hypothesis, here, we prepared a plasmid (p[HRE]AF/PNP) containing the E. coli PNP/MeP-dR system and a chimeric human alpha-fetoprotein (AFP) promoter, [HRE]AF. We introduced this plasmid into AFP-positive and low-AFP-generating human HCC cells, and evaluated its therapeutic effects on both human HCC cell lines. In the presence of hypoxia, the E. coli PNP gene directed by the [HRE]AF promoter were HCC-specifically expressed in two human HCC cell lines and, moreover, the [HRE]AF-PNP/MeP-dR therapy would yield significant and selective cytotoxicity in both AFP-positive and low-AFP-generating HCC cells. Our findings suggest the [HRE]AF-PNP/MeP-dR therapy has worthy potentialities as an effective strategy for targeting therapy of AFP-positive, and especially AFP-negative or low-AFP-generating HCC.

Prodrug converting enzyme gene delivery by L. monocytogenes

Background

Listeria monocytogenes is a highly versatile bacterial carrier system for introducing protein, DNA and RNA into mammalian cells. The delivery of tumor antigens with the help of this carrier into tumor-bearing animals has been successfully carried out previously and it was recently reported that L. monocytogenes is able to colonize and replicate within solid tumors after local or even systemic injection.

Methods

Here we report on the delivery of two prodrug converting enzymes, purine-deoxynucleoside phosphorylase (PNP) and a fusion protein consisting of yeast cytosine deaminase and uracil phosphoribosyl transferase (FCU1) into cancer cells in culture by L. monocytogenes. Transfer of the prodrug converting enzymes was achieved by bacterium mediated transfer of eukaryotic expression plasmids or by secretion of the proteins directly into the host cell cytosol by the infecting bacteria.

Results

The results indicate that conversion of appropriate prodrugs to toxic drugs in the cancer cells occured after both procedures although L. monocytogenes-mediated bactofection proved to be more efficient than enzyme secretion 4T1, B16 and COS-1 tumor cells. Exchanging the constitutively P_CMV-promoter with the melanoma specific P_4xTETP-promoter resulted in melanoma cell-specific expression of the prodrug converting enzymes but reduced the efficiencies.

Conclusion

These experiments open the way for bacterium mediated tumor specific activation of prodrugs in live animals with tumors.

Lymphoma chemovirotherapy: CD20-targeted and convertase-armed measles virus can synergize with fludarabine

Combination chemotherapy regimen incorporating CD20 antibodies are commonly used in the treatment of CD20-positive non-Hodgkin's lymphoma (NHL). Fludarabine phosphate (F-araAMP), cyclophosphamide, and CD20 antibodies (Rituximab) constitute the FCR regimen for treating selected NHL, including aggressive mantle cell lymphoma (MCL). As an alternative to the CD20 antibody, we generated a CD20-targeted measles virus (MV)-based vector. This vector was also armed with the prodrug convertase purine nucleoside phosphorylase (PNP) that locally converts the active metabolite of F-araAMP to a highly diffusible substance capable of efficiently killing bystander cells. We showed in infected cells that early prodrug administration controls vector spread, whereas late administration enhances cell killing. Control of spread by early prodrug administration was also shown in an animal model: F-araAMP protected genetically modified mice susceptible to MV infection from a potentially lethal intracerebral challenge. Enhanced oncolytic potency after extensive infection was shown in a Burkitt's lymphoma xenograft model (Raji cells): After systemic vector inoculation, prodrug administration enhanced the therapeutic effect synergistically. In a MCL xenograft model (Granta 519 cells), intratumoral (i.t.) vector administration alone had high oncolytic efficacy: All mice experienced complete but temporary tumor regression, and survival was two to four times longer than that of untreated mice. Cells from MCL patients were shown to be sensitive to infection. Thus, synergy of F-araAMP with a PNP-armed and CD20-targeted MV was shown in one lymphoma therapy model after systemic vector inoculation.

An Immunocompetent Murine Model for Oncolysis with an Armed and Targeted Measles Virus

An immunocompetent model is required to test therapeutic regimens for clinical trials with the oncolytic measles virus (MV). Toward developing this model, a retargeted MV that enters murine colon adenocarcinoma cells forming tumors in syngeneic C57BL/6 mice was generated. Since MV infection tends to be less efficient in murine than in human cells, the targeted virus was also armed with the prodrug convertase, purine nucleoside phosphorylase (PNP), and named MV-PNP-antiCEA. We have shown before that in cultured cells, infection with this virus activated the prodrug, 6-methylpurine-2'-deoxyriboside (MeP-dR), causing extensive cytotoxicity. When injected intratumorally (IT), MV-PNP-antiCEA inhibited subcutaneous tumor growth marginally, but subsequent administration of the prodrug enhanced the oncolytic effect. Systemic delivery of MV-PNP-antiCEA alone had no substantial oncolytic effects, but in combination with the prodrug it was therapeutic, revealing synergistic effects between virus and prodrug. Immunosuppression with cyclophosphamide (CPA) retarded the appearance of MV neutralizing antibodies and enhanced oncolytic efficacy: survival was 100%, with 9 out of 10 animals going into complete remission. This immunocompetent murine model facilitates the testing of therapeutic regimens for clinical trials.

Suicide genes for cancer therapy

The principle of using suicide genes for gene directed enzyme prodrug therapy (GDEPT) of cancer has gained increasing significance during the 20 years since its inception. The astute application of suitable GDEPT systems should permit tumour ablation in the absence of off-target toxicity commonly associated with classical chemotherapy, a hypothesis which is supported by encouraging results in a multitude of pre-clinical animal models. This review provides a clear explanation of the rationale behind the GDEPT principle, outlining the advantages and limitations of different GDEPT strategies with respect to the roles of the bystander effect, the immune system and the selectivity of the activated prodrug in contributing to their therapeutic efficacy. An in-depth analysis of the most widely used suicide gene/prodrug combinations is presented, including details of the latest advances in enzyme and prodrug optimisation and results from the most recent clinical trials.

Design and evaluation of 5'-modified nucleoside analogs as prodrugs for an E. coli purine nucleoside phosphorylase mutant

Our studies have led to the identification of an E. coli PNP mutant (M64V) that is able to cleave numerous 5'-modified nucleoside analogs with much greater efficiency than the wild-type enzyme. The biological activity of the three best substrates of this mutant (9-[6-deoxy-alpha-L-talofuranosyl]-6-methylpurine (methyl(talo)-MeP-R), 9-[6-deoxy-alpha-L-talofuranosyl]-2-F-adenine, and 9-[alpha-L-lyxofuranosyl]-2-F-adenine) were evaluated so that we can optimally utilize these compounds. Our results indicated that the mechanism of toxicity of methyl(talo)-MeP-R to mice was due to its cleavage to MeP by a bacterial enzyme, and that the toxicity of the two F-Ade analogs was due to their cleavage to F-Ade by mammalian methylthioadenosine phosphorylase.

Transcriptional tumor-selective promoter targeting of E. coli purine nucleoside phosphorylase for pancreatic cancer suicide gene therapy

BACKGROUND

Pancreatic cancer remains a rapidly fatal disease. Suicide gene therapy has been shown to be an effective tool for pancreatic tumor cell destruction, but a cell-specific gene delivery is required to limit host toxicity. The objective of this study was both to design recombinant vectors in which the suicide gene E. coli purine nucleoside phosphorylase (ePNP) is under the control of either CEA or MUC1 promoter sequences and to investigate on experimental pancreatic carcinomas the selective killing effects of the conditional ePNP/prodrug (MePdR) system.

METHODS

Transcriptional activities of CEA and MUC1 promoter sequences were analyzed using luciferase reporter gene constructions. Thereafter, recombinant vectors expressing ePNP under control of the most promising pCEA and pMUC1 sequences were designed and used to establish stable tumor cell transfectants from two human pancreatic cell lines, respectively tumor-marker positive (BxPc3) or negative (Panc-1), then applied for in vitro and in vivo experiments.

RESULTS

Transient experiments indicated that CEA and MUC1 promoter sequences confer specificity while preserving high transcriptional activities. The MePdR treatment induced a high in vitro cytotoxicity on the sole CEA- and MUC1-producing cell lines (i.e. BxPc3-CEA and -MUC1/ePNP). In the same way, prodrug treatment induced a significant tumor regression on the sole tumor-marker-positive BxPc3 xenografts, whilst the Panc1-CEA and -MUC1/ePNP tumors were not affected.

CONCLUSIONS

These data confirm and extend the antitumor efficacy of the ePNP/MePdR killing system and demonstrate the feasibility of the transcriptional targeting strategy under tumor marker promoter control and thereby a preferential killing of CEA- and MUC1-producing pancreatic tumor cells. Thus, efficient in vivo gene delivery and transcriptional targeting constitute the major future clinical challenge for a selective pancreatic cancer suicide gene strategy.

Antibiotic-mediated chemoprotection enhances adaptation of E. coli PNP for herpes simplex virus-based glioma therapy

The E. coli PNP suicide gene sensitizes solid tumors to nucleoside prodrugs, such as 6-methylpurine-2'-deoxyriboside (MeP-dR). In this study using lentiviral, MuLv, and HSV-based gene transfer, we quantified thresholds for inhibition of tumor growth and bystander killing by E. coli PNP and tested the role of intestinal flora in this process. Regressions of human glioma tumors following retroviral transduction exhibited dose dependence on both the level of PNP expression and the dose of MeP-dR administered, including strong tumor inhibition when 90-99% bystander cells comprised the tumor mass. A replication competent, non-neurovirulent herpes simplex virus (HSV) deficient in both copies of the gamma-1 34.5 gene was next engineered to express E. coli PNP under the egr-1 promoter (HSV-PNP). HSV-PNP injected intratumorally (17 million pfu/0.05 ml) in nude mice bearing 300 mg human glioma flank tumors produced a delay in tumor growth (approximately 24 days delay to one doubling). MeP-dR treatment after antibiotic therapy (to eliminate enteric flora encoding PNP enzymes) resulted in antitumor enhancement, with arrest of tumor growth (delay to doubling >50 days). Bystander killing of the magnitude described here has been difficult to accomplish with other suicide genes, such as HSV-tk or cytosine deaminase. The results establish a model for applying E. coli PNP to HSV treatment of glioma.

Experimental studies onPNP suicide gene therapy of hepatoma

To investigate the killing effect of PNP/MeP-dR suicide gene system on hepatoma cells, pcDNA3.0/PNP, an eukaryotic expression vector harboring E. coli PNP gene, was transfected into human hepatoma HepG2 cells by liposome-mediated method. A HepG2 cell line with stable PNP gene expression, HepG2/PNP, was established with presence of G418 selection. The cell growth curves were determined with trypan blue staining. The sensitivity of HepG2/PNP to MeP-dR and bystander effects were assayed by MTT and FCM methods. The enzymatic activity of the product of PNP gene was determined by HPLC method. The cytotoxic effects of MeP-dR on HepG2/PNP cells were obvious (IC50 = 4.5 micromol/L) and all HepG2/PNP cells were killed 4 days after the treatment with 100 micromol/L MeP-dR. In mixed cultures containing increasing percentages of HepG2/PNP cells, total population killing was demonstrated when HepG2/PNP cells accounted for as few as 5% of all HepG2 cells 8 days after the treatment with 100 micromol MeP-dR. High-pressure liquid chromatography (HPLC) demonstrated that the PNP enzyme could convert MeP-dR into 6-MP. PNP/MeP-dR suicide gene system had an advantage over traditional suicide gene systems for hepatoma gene therapy. Our e results suggest that high-level bystander effects of this system result in significant anti-tumor responses to hepatoma gene therapy, especially in vivo.

Neuroblastoma-specific cytotoxicity mediated by the Mash1-promoter and E. coli purine nucleoside phosphorylase

BACKGROUND

Neuroblastoma is derived from cells of neural crest origin and often expresses the transcription factor human achaete-scute homolog 1 (HASH1). The aim of this study was to selectively kill neuroblastoma cells by expressing the suicide gene E. coli purine nucleoside phosphorylase (PNP) under the control of the Mash1 promoter, the murine homolog of HASH1.

PROCEDURE

The E. coli PNP gene regulated by the Mash1 promoter was cloned into an expression vector and transfected into neuroblastoma and non-neuroblastoma cell lines. After addition of the prodrug M2-fluoroadenine 9-beta-D-arabinofuranoside (F-araA) the cell-specific toxicity was examined. To optimize the cell specific activity, different sizes of the Mash1 promoter were analyzed in neuroblastoma cell lines and compared with the activity in non-neuroblastoma cells.

RESULTS

Estimated as the percentages of CMV enhancer-promoter, the activity was significantly higher in the neuroblastoma cells, ranging from 17 to 58% when the shortest and the most active promoter was measured. The non-neuroblastoma cells yielded only 1-6% of the CMV promoter activity. When the shortest Mash1 promoter was combined with the E. coli PNP gene the cytotoxicity was 65% in the neuroblastoma cells with low cell death in the non-neuroblastoma cell lines, relative to the cytotoxicity where the E.coli PNP gene was regulated by the strong but non-specific CMV enhancer-promoter.

CONCLUSIONS

We show here that the Mash1 promoter regulating the PNP gene confers a cell-type selective toxicity in neuroblastoma cell lines. These results indicate the feasibility to use the Mash1 promoter for regulating E.coli PNP expression in gene-directed enzyme prodrug therapy (GDEPT) of neuroblastoma.

Excellent in vivo bystander activity of fludarabine phosphate against human glioma xenografts that express the escherichia coli purine nucleoside phosphorylase gene

Escherichia coli purine nucleoside phosphorylase (PNP) expressed in tumors converts relatively nontoxic prodrugs into membrane-permeant cytotoxic compounds with high bystander activity. In the present study, we examined tumor regressions resulting from treatment with E. coli PNP and fludarabine phosphate (F-araAMP), a clinically approved compound used in the treatment of hematologic malignancies. We tested bystander killing with an adenoviral construct expressing E. coli PNP and then more formally examined thresholds for the bystander effect, using both MuLv and lentiviral vectoring. Because of the importance of understanding the mechanism of bystander action and the limits to this anticancer strategy, we also evaluated in vivo variables related to the expression of E. coli PNP (level of E. coli PNP activity in tumors, ectopic expression in liver, percentage of tumor cells transduced in situ, and accumulation of active metabolites in tumors). Our results indicate that F-araAMP confers excellent in vivo dose-dependent inhibition of bystander tumor cells, including strong responses in subcutaneous human glioma xenografts when 95 to 97.5% of the tumor mass is composed of bystander cells. These findings define levels of E. coli PNP expression necessary for antitumor activity with F-araAMP and demonstrate new potential for a clinically approved compound in solid tumor therapy.

Enzyme-catalyzed activation of anticancer prodrugs

The rationale fo the development of prodrugs relies upon delivery of higher concentrations of a drug to target cells compared to administration of the drug itself. In the last decades, numerous prodrugs that are enzymatically activated into anti-cancer agents have been developed. This review describes the most important enzymes involved in prodrug activation notably with respect to tissue distribution, up-regulation in tumor cells and turnover rates. The following endogenous enzymes are discussed: aldehyde oxidase, amino acid oxidase, cytochrome P450 reductase, DT-diaphorase, cytochrome P450, tyrosinase, thymidylate synthase, thymidine phosphorylase, glutathione S-transferase, deoxycytidine kinase, carboxylesterase, alkaline phosphatase, beta-glucuronidase and cysteine conjugate beta-lyase. In relation to each of these enzymes, several prodrugs are discussed regarding organ- or tumor-selective activation of clinically relevant prodrugs of 5-fluorouracil, axazaphosphorines (cyclophosphamide, ifosfamide, and trofosfamide), paclitaxel, etoposide, anthracyclines (doxorubicin, daunorubicin, epirubicin), mercaptopurine, thioguanine, cisplatin, melphalan, and other important prodrugs such as menadione, mitomycin C, tirapazamine, 5-(aziridin-1-yl)-2,4-dinitrobenzamide, ganciclovir, irinotecan, dacarbazine, and amifostine. In addition to endogenous enzymes, a number of nonendogenous enzymes, used in antibody-, gene-, and virus-directed enzyme prodrug therapies, are described. It is concluded that the development of prodrugs has been relatively successful; however, all prodrugs lack a complete selectivity. Therefore, more work is needed to explore the differences between tumor and nontumor cells and to develop optimal substrates in terms of substrate affinity and enzyme turnover rates fo prodrug-activating enzymes resulting in more rapid and selective cleavage of the prodrug inside the tumor cells.

Identification and characterization of a novel gene of grouper iridovirus encoding a purine nucleoside phosphorylase

Purine nucleoside phosphorylase (PNP) is a key enzyme in the purine salvage pathway. It catalyses the reversible phosphorolysis of purine (2′-deoxy)ribonucleosides to free bases and (2′-deoxy)ribose 1-phosphates. Here, a novel piscine viralPNPgene that was identified from grouper iridovirus (GIV), a causative agent of an epizootic fish disease, is reported. This putative GIVPNPgene encodes a protein of 285 aa with a predicted molecular mass of 30 332 Da and shows high similarity to the humanPNPgene. Northern and Western blot analyses of GIV-infected grouper kidney (GK) cells revealed that PNP expression increased in cells with time from 6 h post-infection. Immunocytochemistry localized GIV PNP in the cytoplasm of GIV-infected host cells. PNP–EGFP fusion protein was also observed in the cytoplasm of PNP–EGFP reporter construct-transfected GK and HeLa cells. From HPLC analysis, the recombinant GIV PNP protein was shown to catalyse the reversible phosphorolysis of purine nucleosides and could accept guanosine, inosine and adenosine as substrates. In conclusion, this is the first report of a viral PNP with enzymic activity.

Designer gene therapy using an Escherichia coli purine nucleoside phosphorylase/prodrug system

Activation of prodrugs by Escherichia coli purine nucleoside phosphorylase (PNP) provides a method for selectively killing tumor cells expressing a transfected PNP gene. This gene therapy approach requires matching a prodrug and a known enzymatic activity present only in tumor cells. The specificity of the method relies on avoiding prodrug cleavage by enzymes already present in the host cells or the intestinal flora. Using crystallographic and computer modeling methods as guides, we have redesigned E. coli PNP to cleave new prodrug substrates more efficiently than does the wild-type enzyme. In particular, the M64V PNP mutant cleaves 9-(6-deoxy-alpha-L-talofuranosyl)-6-methylpurine with a kcat/Km over 100 times greater than for native E. coli PNP. In a xenograft tumor experiment, this compound caused regression of tumors expressing the M64V PNP gene.

Construction of two vectors harboring PNP gene under control of two different promoters and their expression

AIM: To construct two expression vectors carrying PNP gene under a cytomegalovirus (CMV) promoter and a hybrid a-fetoprotein (AFP) tissue-specific promoter, and to detect and analyze their expression in different cell lines.

METHODS: [HRE]AF promoter was inserted into pcDNA3.0 vector, and a recombinant vector controlled by the hybrid AFP promoter, p[HRE]AF, was constructed. Inserting PNP gene into pcDNA3.0 and p[HRE]AF vectors separately, two PNP gene expression vectors driven by two different promoters, pcDNA3.0/PNP and p[HRE]AF /PNP, were constructed by using recombinant DNA techniques. The recombinants were analyzed and identified by restriction enzymes, PCR and sequencing. pcDNA3.0/PNP and p[HRE]AF/PNP were transfected into human hepatocellular carcinoma cell lines by liposome-mediated method. The expression of PNP gene in four different cell lines was detected by RT-PCR method.

RESULTS: All target fragments were separately cloned into corresponding vectors. We detected the expression of PNP gene under the control of CMV promoter in all cell lines, and the tissue-specific expression of PNP gene under the control of [HRE]AF promoter in AFP positive and negative hepatocellular carcinoma cell line was positive.

CONCLUSION: Two expression vectors harboring PNP gene are novel effective vectors for human hepatocellular carcinoma gene therapy, and p[HRE]AF/PNP is a target-expressing vector in AFP positive, especially in AFP negative hepatocellular carcinoma cells.

Suicide gene/prodrug therapy for pancreatic adenocarcinoma by E. coli purine nucleoside phosphorylase and 6-methylpurine 2'-deoxyriboside

OBJECTIVE

Recent advances in diagnostics, staging, and therapy for pancreatic cancer have not resulted in significant improvements in long-term survival, and development of new approaches is particularly urgent. The use of prodrug-activating genes is a possible approach for cancer gene therapy. The aim of this study was to evaluate the efficacy of Escherichia coli purine nucleoside phosphorylase (ePNP) on pancreatic tumors. ePNP activates the prodrug 6-methylpurine deoxyribose (MePdR) into methyl purine (MeP), which is highly toxic to dividing and nondividing cells.

METHODS

A recombinant pCAG-ePNP vector was constructed and used to establish pancreatic cancer cells expressing constitutively ePNP (ePNP+). The ePNP/MePdR system effects were tested in vitro on HA-RPC (rat) and BxPC3 (human) pancreatic cancer cell lines and then in vivo on tumors established in nude mice with BxPC3 ePNP+ cells.

RESULTS

MePdR treatment of ePNP+ tumor cells induced cytotoxic and antiproliferative effects in a concentration-dependent manner with a 100% cell death since 5 x 10 mol/L. Bystander effect was strong in vitro as more than 50% of tumor cells were killed by MePdR with only 1%-2% of ePNP+ cells. In vivo, tumor growth was completely abolished with a prodrug treatment initiated 2 days after tumor cell inoculation, and mice remained tumor free. In addition, even if MePdR treatment was applied to large tumors, tumors significantly regressed.

CONCLUSION

These preliminary results support the therapeutic potential of the MePdR/ePNP system, which induces a highly cytotoxic effect with a potent bystander effect on pancreatic tumors.

Gene therapy for ovarian cancer: progress and potential

Gene therapy remains a promising therapeutic modality for ovarian cancer. Yet much work remains to be done to see gene therapy realize its full potential in elucidating the complex genetic interactions of delivered genes within target cancer cells and in the development of improved vector systems. Because most neoplasms involve multiple mutations, the targeting of a single mutation is unlikely to achieve total tumor control: gene therapy strategies that target multiple cellular processes or invoke various antitumor approaches need to be investigated. Additionally, current vector systems do not transduce ovarian cancer cells efficiently and are hampered by immune responses that further limit their efficacy. Additionally, limitations in vector specificity lead to transduction of normal cells and subsequent toxicity. Investigators are developing refinements to current gene therapy approaches that would address these limitations and that are soon to be incorporated into clinical trials. It is hoped that these advances will lead to improvements in the therapeutic index for ovarian cancer gene therapy and provide another effective therapeutic tool for this deadly disease.

Polyethylene glycol-conjugated adenosine phosphorylase: development of alternative enzyme therapy for adenosine deaminase deficiency

Purine nucleoside phosphorylase had previously been engineered to accept 6-amino substituted purine nucleosides by two active site substitutions, Asn243Asp; Lys244Gln. In the present study, recombinant adenosine phosphorylase (AP) has been conjugated to branched polyethylene glycol (PEG) polymers of approximately 42.5 kDa. Matrix-assisted laser desorption/ionization analysis and SDS acrylamide electrophoresis analysis indicated a subunit composition of greater than 205 kDa consistent with the conjugation of as many as four PEG molecules per AP subunit. The PEG-conjugated enzyme retained greater than 90% of the native catalytic activity. Administration of the enzyme to mice demonstrated the PEG-AP to have a 67-fold increased plasma half-life compared to the native enzyme, 65.1+/-2.9 h versus 57.8+/-1.1 min, respectively. PEG-AP was principally confined to the plasma with minimal activity detected in tissues and of these spleen had the greatest activity and essentially no activity was found in urine. PEG-AP has retained activity with inosine and its injection into PNP-deficient mice resulted in a 2.7-fold increase in urine urate. AP was also shown to protect human CEM cells in culture from the toxic effects of 2'-deoxyadenosine. These studies provide evidence for consideration of PEG-AP as an alternative enzyme therapy for the inherited deficiency of adenosine deaminase.

A long-acting suicide gene toxin, 6-methylpurine, inhibits slow growing tumors after a single administration

We have demonstrated antitumor activity against refractory human glioma and pancreatic tumors with 6-methylpurine (MeP) using either a suicide gene therapy strategy to selectively release 6-methylpurine in tumor cells or direct intratumoral injection of 6-methylpurine itself. A single i.p. injection in mice of the prodrug 9-beta-D-[2-deoxyribofuranosyl]-6-methylpurine (MeP-dR; 134 mg/kg) caused sustained regression lasting over 70 days of D54 (human glioma) tumors transduced with the Escherichia coli purine nucleoside phosphorylase (PNP), and a single intratumoral injection of 6-methylpurine (5-10 mg/kg) elicited prolonged delays of the growth of D54 tumors and CFPAC human pancreatic carcinoma. Because the D54 tumor doubling time is >15 days, the experiments indicate that prodrug activation by E. coli PNP engenders destruction of both dividing and nondividing tumor compartments in vivo and, therefore, address a fundamental barrier that has limited the development of suicide gene strategies in the past. A prolonged retention time of 6-methylpurine metabolites in tumors was noted in vivo (T(1/2) >24 h compared with a serum half-life of <1 h). By high-pressure liquid chromatography, metabolites of [(3)H]MeP-dR were 5- to 6-fold higher in tumors expressing E. coli PNP. These experiments point to new endpoints for monitoring E. coli PNP suicide gene therapy, including intratumoral enzymatic activity, in situ (intratumoral) prodrug conversion, and tumor regressions after direct injection of a suicide gene toxin. The findings also help explain the strong in vivo bystander killing mechanism ascribed by several laboratories to E. coli PNP in the past.

Cancer gene therapy

Cancer gene therapy is the transfer of genetic material to the cells of an individual with the goal of eradicating cancer cells, both in the primary tumor and metastases. Cancer gene therapy strategies exploit our expanding knowledge of the genetic basis of cancer, thereby allowing rationally targeted interventions at the molecular level. The successful implementation of cancer gene therapy in the clinic awaits the development of vectors capable of specific and efficient gene delivery to cancer cells. The first clinical applications of cancer gene therapy are likely to be in combination with conventional therapies, such as radiotherapy and immunotherapy.

Antitumor activity of 2-fluoro-2'-deoxyadenosine against tumors that express Escherichia coli purine nucleoside phosphorylase

The selective expression of Escherichia coli purine nucleoside phosphorylase (PNP) in solid tumors has been successfully used to activate two purine nucleoside analogs [9-(2-deoxy-beta-D-ribofuranosyl)-6-methylpurine (MeP-dR) and 9-beta-D-arabinofuranosyl-2-fluoroadenine (F-araA)] resulting in lasting tumor regressions and cures. E. coli PNP also cleaves 2-fluoro-2'-deoxyadenosine (F-dAdo) to 2-F-adenine, which is the toxic purine analog liberated from F-araA that has high bystander activity and is active against nonproliferating tumor cells. As F-dAdo is 3000 times better than F-araA as a substrate for E. coli PNP, we have evaluated its antitumor activity against D54 gliomas that express E. coli PNP and have characterized its in vivo metabolism in order to better understand its mechanism of action with respect to the other two agents. Like MeP-dR and F-araA-5'-monophosphate (F-araAMP, a prodrug of F-araA), treatment of mice bearing D54 tumors that express E. coli PNP with F-dAdo resulted in excellent antitumor activity. Although F-dAdo was as active as MeP-dR and better than F-araAMP, it was not dramatically better than either compound because of its short plasma half-life and the limited activation of F-adenine to toxic metabolites. Regardless, these results indicated that F-dAdo was also an excellent prodrug for use with gene vectors that deliver E. coli PNP to tumor cells.

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.

Development of a melanoma-specific adenovirus

Concerns regarding the hepatotoxicity of adenovirus for cancer gene therapy have led to attempts to engineer viruses for tissue-specific gene expression and tissue-specific replication. The Tyrex2 (a tandem murine melanocyte-specific enhancer) system was used to express luciferase, purine nucleoside phosphorylase (PNP), and the essential adenoviral gene E1A. In nonmelanoma cell lines, the CMV promoter/enhancer (CMV p/e) was 969 times stronger than the Tyrex2 construct, whereas in melanoma cells it was only 2.6 times stronger. An adenovirus with Tyrex2 regulating PNP (Ad2Tyr2-PNP) was tested for cytotoxicity. In melanoma cells, treatment with Ad2Tyr2-PNP plus the prodrug 6-methylpurine deoxyriboside (6-MPDR) resulted in 90% cytotoxicity by day 4. In non-melanoma cell lines, only the CMV p/e resulted in significant cytotoxicity. We compared the intrinsic E1A promoter/enhancer (E1A p/e) system with the melanoma-specific constructs and found that the Tyrex2 system achieved higher levels of luciferase than the E1A p/e in all melanoma lines tested. In non-melanoma cell lines, the E1A p/e is 12.4 times stronger than the Tyrex2 construct. Tyrex2 was then used to regulate adenoviral E1A expression to construct a melanoma-specific replicating adenovirus. We were unable to achieve selective replication. As E1A is a known transactivator of the adenovirus major late promoter (MLP), we studied the ability of the MLP to express a transgene in the context of tissue-selective E1A expression. We were able to demonstrate high levels of luciferase activity with this construct; however, selectivity was lost. Melanoma-specific adenovirus expression was achieved with the Tyrex2 construct, and this led to melanoma-specific cytotoxicity by the potent PNP suicide gene. Selective melanoma-specific replication was not successful. The MLP may be a useful promoter in the context of a tissue-specific replicating adenovirus.