Insertion and extension of acyclic, dideoxy, and ara nucleotides by herpesviridae, human alpha and human beta polymerases. A unique inhibition mechanism for 9-(1,3-dihydroxy-2-propoxymethyl)guanine triphosphate

Cell Death, by Any Other Name…

Studies trying to understand cell death, this ultimate biological process, can be traced back to a century ago. Yet, unlike many other fashionable research interests, research on cell death is more alive than ever. New modes of cell death are discovered in specific contexts, as are new molecular pathways. But what is "cell death", really? This question has not found a definitive answer yet. Nevertheless, part of the answer is irreversibility, whereby cells can no longer recover from stress or injury. Here, we identify the most distinctive features of different modes of cell death, focusing on the executive final stages. In addition to the final stages, these modes can differ in their triggering stimulus, thus referring to the initial stages. Within this framework, we use a few illustrative examples to examine how intercellular communication factors in the demise of cells. First, we discuss the interplay between cell-cell communication and cell death during a few steps in the early development of multicellular organisms. Next, we will discuss this interplay in a fully developed and functional tissue, the gut, which is among the most rapidly renewing tissues in the body and, therefore, makes extensive use of cell death. Furthermore, we will discuss how the balance between cell death and communication is modified during a pathological condition, i.e., colon tumorigenesis, and how it could shed light on resistance to cancer therapy. Finally, we briefly review data on the role of cell-cell communication modes in the propagation of cell death signals and how this has been considered as a potential therapeutic approach. Far from vainly trying to provide a comprehensive review, we launch an invitation to ponder over the significance of cell death diversity and how it provides multiple opportunities for the contribution of various modes of intercellular communication.

Resistance to a Nucleoside Analog Antiviral Drug from More Rapid Extension of Drug-Containing Primers

Nucleoside analogs are mainstays of antiviral therapy. Although resistance to these drugs hinders their use, understanding resistance can illuminate mechanisms of the drugs and their targets. Certain nucleoside analogs, such as ganciclovir (GCV), a leading therapy for human cytomegalovirus (HCMV), contain the equivalent of a 3'-hydoxyl moiety, yet their triphosphates can terminate genome synthesis (nonobligate chain termination). For ganciclovir, chain termination is delayed until incorporation of the subsequent nucleotide, after which viral polymerase idling (repeated addition and removal of incorporated nucleotides) prevents extension. Here, we investigated how an alanine-to-glycine substitution at residue 987 (A987G), in conserved motif V in the thumb subdomain of the catalytic subunit (Pol) of HCMV DNA polymerase, affects polymerase function to overcome delayed chain termination and confer ganciclovir resistance. Steady-state enzyme kinetic studies revealed no effects of this substitution on incorporation of ganciclovir-triphosphate into DNA that could explain resistance. We also found no effects of the substitution on Pol's exonuclease activity, and the mutant enzyme still exhibited idling after incorporation of GCV and the subsequent nucleotide. However, despite extending normal DNA primers similarly to wild-type enzyme, A987G Pol more rapidly extended ganciclovir-containing DNA primers, thereby overcoming chain termination. The mutant Pol also more rapidly extended RNA primers, a previously unreported activity for HCMV Pol. Structural analysis of related Pols bound to primer-templates provides a rationale for these results. These studies uncover a new drug resistance mechanism, potentially applicable to other nonobligate chain-terminating nucleoside analogs, and shed light on polymerase functions.IMPORTANCE While resistance to antiviral drugs can hinder their clinical use, understanding resistance mechanisms can illuminate how these drugs and their targets act. We studied a substitution in the human cytomegalovirus (HCMV) DNA polymerase that confers resistance to a leading anti-HCMV drug, ganciclovir. Ganciclovir is a nucleoside analog that terminates DNA replication after its triphosphate and the subsequent nucleotide are incorporated. We found that the substitution studied here results in an increased rate of extension of drug-containing DNA primers, thereby overcoming termination, which is a new mechanism of drug resistance. The substitution also induces more rapid extension of RNA primers, a function that had not previously been reported for HCMV polymerase. Thus, these results provide a novel resistance mechanism with potential implications for related nucleoside analogs that act against established and emerging viruses, and shed light on DNA polymerase functions.

Cidofovir, a New Agent with Potent Anti-Herpesvirus Activity

Cidofovir is a potent, broad spectrum antiviral agent with activity in vitro and in vivo against cytomegalovirus and other members of the herpesvirus family, as well as certain other DNA viruses. After uptake into cells it is converted enzymatically to cidofovir diphosphate, a structural analogue of deoxycytidine triphosphate, which selectively inhibits viral DNA polymerases relative to host cell polymerases. Cross-resistance to cidofovir is not usually seen with human cytomegalovirus isolates that are foscarnet-resistant, or isolates that are ganciclovir-resistant due to a deficiency in ganciclovir phosphorylation. Cross-resistance is seen, however, with isolates that are ganciclovir resistant due to polymerase mutations. A prolonged elimination phase seen in vivo, correlates with a long intracellular half-life seen in vitro and allows for efficacy in animal models of virus infection with infrequent dosing or prophylaxis. Clinical studies of intravenous cidofovir in cytomegalovirus retinitis in patients with AIDS are claimed to show delay of retinitis progression with maintenance doses given once every 2 weeks.

Potency and Stereoselectivity of Cyclopropavir Triphosphate Action on Human Cytomegalovirus DNA Polymerase

Cyclopropavir (CPV) is a promising antiviral drug against human cytomegalovirus (HCMV). As with ganciclovir (GCV), the current standard for HCMV treatment, activation of CPV requires multiple steps of phosphorylation and is enantioselective. We hypothesized that the resulting CPV triphosphate (CPV-TP) would stereoselectively target HCMV DNA polymerase and terminate DNA synthesis. To test this hypothesis, we synthesized both enantiomers of CPV-TP [(+) and (-)] and investigated their action on HCMV polymerase. Both enantiomers inhibited HCMV polymerase competitively with dGTP, with (+)-CPV-TP exhibiting a more than 20-fold lower apparent Ki than (-)-CPV-TP. Moreover, (+)-CPV-TP was a more potent inhibitor than GCV-TP. (+)-CPV-TP also exhibited substantially lower apparent Km and somewhat higher apparent kcat values than (-)-CPV-TP and GCV-TP for incorporation into DNA by the viral polymerase. As is the case for GCV-TP, both CPV-TP enantiomers behaved as nonobligate chain terminators, with the polymerase terminating DNA synthesis after incorporation of one additional nucleotide. These results elucidate how CPV-TP acts on HCMV DNA polymerase and help explain why CPV is more potent against HCMV replication than GCV.

Assessment of the Human Cytomegalovirus UL97 Gene for Identification of Resistance to Ganciclovir in Iranian Immunosuppressed Patients

Background

Human cytomegalovirus (HCMV) infections are a major cause of morbidity and mortality among immunocompromised patients. Prolonged antiviral therapy is a cause of mutation and drug resistance in the HCMV genome.

Objectives

The aim of this study was to identify resistance to ganciclovir (GCV) in Iranian immunosuppressed patients at two different stages of the disease: early (before GCV is initiated) and late (after six months of GCV therapy).

Patients and Methods

In this study, 87 specimens from Iranian patients were amplified using nested PCR amplification of the UL97 gene. Sequence analyses of products were performed for identifying the mutated codons.

Results

The present study show that the most frequent GCV-resistant mutations occurred in codons A594V (26.43%), H520Q (18.39%), and M460V (13.79%), consequently occurring at a low frequency in the L595S (2.29%), E596G (1.14%), and Del 594 (1.14%) codons, and with intermediate frequency in the C592G (10.34%), M460I (9.19%), and C603W (6.89%) codons. We describe for the first time a new GCV-resistance mutation, the deletion of codon 594, in the UL97 gene of Iranian HCMV patients after GCV therapy, following renal transplantation.

Conclusions

The findings of the present study can be utilized to detect GCV resistance patterns among Iranian immunocompromised patients and to treat HCMV infections accordingly.

Targeted Adenoviral Vector Demonstrates Enhanced Efficacy for In Vivo Gene Therapy of Uterine Leiomyoma

BACKGROUND

Gene therapy is a potentially effective non-surgical approach for the treatment of uterine leiomyoma. We demonstrated that targeted adenovirus vector, Ad-SSTR-RGD-TK/GCV, was highly effective in selectively inducing apoptosis and inhibiting proliferation of human leiomyoma cells in vitro while sparing normal myometrial cells.

STUDY DESIGN

An in-vivo study, to compare efficacy and safety of modified adenovirus vector Ad-SSTR-RGD-TK/GCV versus untargeted vector for treatment of leiomyoma.

MATERIALS AND METHODS

Female nude mice were implanted with rat leiomyoma cells subcutaneously. Then mice were randomized into three groups. Group 1 received Ad-LacZ (marker gene), Group 2 received untargeted Ad-TK, and Group 3 received the targeted Ad-SSTR-RGD-TK. Tumors were measured weekly for 4 weeks. Then mice were sacrificed and tissue samples were collected. Evaluation of markers of apoptosis, proliferation, extracellular matrix, and angiogenesis was performed using Western Blot & Immunohistochemistry. Statistical analysis was done using ANOVA. Dissemination of adenovirus was assessed by PCR.

RESULTS

In comparison with the untargeted vector, the targeted adenoviral vector significantly shrank leiomyoma size (P < 0.05), reduced expression of proliferation marker (PCNA) (P < 0.05), induced expression of apoptotic protein, c-PARP-1, (P < 0.05) and inhibited expression of extracellular matrix-related genes (TGF beta 3) and angiogenesis-related genes (VEGF & IGF-1) (P < 0.01). There were no detectable adenovirus in tested tissues other than leiomyoma lesions with both targeted and untargeted adenovirus.

CONCLUSION

Targeted adenovirus, effectively reduces tumor size in leiomyoma without dissemination to other organs. Further evaluation of this localized targeted strategy for gene therapy is needed in appropriate preclinical humanoid animal models in preparation for a future pilot human trial.

Mechanism of ganciclovir-induced chain termination revealed by resistant viral polymerase mutants with reduced exonuclease activity

Many antiviral and anticancer drugs are nucleoside analogs that target polymerases and cause DNA chain termination. Interestingly, ganciclovir (GCV), the first line of therapy for human cytomegalovirus (HCMV) infections, induces chain termination despite containing the equivalent of a 3'-hydroxyl group. Certain HCMV GCV resistance (GCV(r)) mutations, including ones associated with treatment failures, result in substitutions in the 3'-5' exonuclease (Exo) domain of the catalytic subunit of the viral DNA polymerase (Pol). To investigate how these mutations confer resistance, we overexpressed and purified wild-type (WT) HCMV Pol and three GCV(r) Exo mutants. Kinetic studies provided little support for resistance being due to effects on Pol binding or incorporation of GCV-triphosphate. The mutants were defective for Exo activity on all primer templates tested, including those with primers terminating with GCV, arguing against the mutations increasing excision of the incorporated drug. However, although the WT enzyme terminated DNA synthesis after incorporation of GCV-triphosphate and an additional nucleotide (N+1), the Exo mutants could efficiently synthesize DNA to the end of such primer templates. Notably, the Exo activity of WT Pol rapidly and efficiently degraded N+2 primer templates to N+1 products that were not further degraded. On N+1 primer templates, WT Pol, much more than the Exo mutants, converted the incoming deoxynucleoside triphosphate to its monophosphate, indicative of rapid addition and removal of incorporated nucleotides ("idling"). These results explain how GCV induces chain termination and elucidate a previously unidentified mechanism of antiviral drug resistance.

Adenovirus-Mediated Suicide SCLC Gene Therapy Using the Increased Activity of the hTERT Promoter by the MMRE and SV40 Enhancer

Telomerase is a ribonucleoprotein complex of which the function is to add telomeric repeats to chromosomal ends. Telomerase consists of two essential components, the telomerase RNA template (hTR) and the catalytic subunit (hTERT). hTERT is expressed only in cells and tissues positive for telomerase activity, i.e., tumor or stem cells. The aim of this study was to use increased telomerase promoter activity in small-cell lung cancer (SCLC) gene therapy. The hTERT promoter and Myc-Max response elements (MMRE) in pGL3-Control vector containing SV40 enhancer resulted in strong expression of the luciferase gene only in telomerase positive and myc overexpressing SCLC cell line but not in normal human cell line. To investigate the possibility of the utilization of the MMRE, hTERT promoter, and SV40 enhancer in targeted SCLC gene therapy, adenovirus vector expressing HSV-TK controlled by the MMRE, hTERT promoter, and SV40 enhancer for the induction of telomerase positive and myc-overexpressing cancer specific cell death was constructed. SCLC cells infected with Ad-MMRE-hT-TK-enh were significantly suppressed and induced apoptosis more than those of Ad-hT-TK or Ad-hT-TK-enh infected cells. Telomerase and c-myc are activated in 60 approximately 80% of SCLC, so the increased activity of telomerase promoter can be used for targeted SCLC gene therapy. These results show that the MMRE, hTERT promoter, and SV40 enhancer can be used in SCLC targeted cancer gene therapy.

Activity of the Human Telomerase Catalytic Subunit (hTERT) Gene Promoter Could Be Increased by the SV40 Enhancer

Telomerase is a ribonucleoprotein complex of which the function is to add telomeric repeats to chromosomal ends. Telomerase consists of two essential components, the telomerase RNA template (hTR) and the catalytic subunit (hTERT). hTERT is expressed only in cells and tissues positive for telomerase activity, i.e., tumor and fetal cells. The aim of this study is to test the increased telomerase promoter activity for cancer gene therapy in adenovirus vector. We cloned the hTERT promoter in place of the SV40 promoter in the pGL3-contol vector to be increased by the SV40 enhancer sequences, resulting in strong expression of luc+ only in telomerase positive cancer cells. Then we transfected the constructed plasmid into a normal human cell line and several cancer cell lines. Through these experiments, we identified the selective and increased expression of the luciferase gene controlled by the hTERT promoter and the SV40 enhancer in the telomerase positive cancer cell lines. To investigate the possibility of utilizing the hTERT promoter and the SV40 enhancer in targeted cancer gene therapy, we constructed an adenovirus vector expressing HSV-TK controlled by the hTERT promoter and the SV40 enhancer for the induction of specific telomerase positive cancer cell death. NSCLC cells infected by Ad-hT-TK-enh were more significantly suppressed and induced apoptosis than those infected by Ad-hT-TK. Telomerase is activated in 80 approximately 90% of cancers, so adenovirus with increasing telomerase promoter activity might be used for targeted cancer gene therapy using suicide genes. These results show that the hTERT promoter and the SV40 enhancer might be used for targeted cancer gene therapy.

Targeting adenoviral vectors for enhanced gene therapy of uterine leiomyomas

STUDY QUESTION

Is targeted adenovirus vector, Ad-SSTR-RGD-TK (Adenovirus -human somatostatin receptor subtype 2- arginine, glycine and aspartate-thymidine kinase), given in combination with ganciclovir (GCV) against immortalized human leiomyoma cells (HuLM) a potential therapy for uterine fibroids?

SUMMARY ANSWER

Ad-SSTR-RGD-TK/GCV, a targeted adenovirus, effectively reduces cell growth in HuLM cells and to a significantly greater extent than in human uterine smooth muscle cells (UtSM).

WHAT IS KNOWN ALREADY

Uterine fibroids (leiomyomas), a major cause of morbidity and the most common indication for hysterectomy in premenopausal women, are well-defined tumors, making gene therapy a suitable and potentially effective non-surgical approach for treatment. Transduction of uterine fibroid cells with adenoviral vectors such as Ad-TK/GCV (herpes simplex virus thymidine kinase gene) decreases cell proliferation.

STUDY DESIGN, SIZE, DURATION

An in vitro cell culture method was set up to compare and test the efficacy of a modified adenovirus vector with different multiplicities of infection in two human immortalized cell lines for 5 days.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Immortalized human leiomyoma cells and human uterine smooth muscle cells were infected with different multiplicities of infection (MOI) (5-100 plaque-forming units (pfu)/cell) of a modified Ad-SSTR-RGD-TK vector and subsequently treated with GCV. For comparison, HuLM and UtSM cells were transfected with Ad-TK/GCV and Ad-LacZ/GCV. Cell proliferation was measured using the CyQuant assay in both cell types. Additionally, western blotting was used to assess the expression of proteins responsible for regulating proliferation and apoptosis in the cells.

MAIN RESULTS AND THE ROLE OF CHANCE

Transduction of HuLM cells with Ad-SSTR-RGD-TK/GCV at 5, 10, 50 and 100 pfu/cell decreased cell proliferation by 28, 33, 45, and 84%, respectively (P < 0.05) compared with untransfected cells, whereas cell proliferation in UtSM cells transfected with the same four MOIs of Ad-SSTR-RGD-TK/GCV compared with that of untransfected cells was decreased only by 8, 23, 25, and 28%, respectively (P < 0.01). Western blot analysis showed that, in comparison with the untargeted vector Ad-TK, Ad-SSTR-RGD-TK/GCV more effectively reduced expression of proteins that regulate the cell cycle (Cyclin D1) and proliferation (PCNA, Proliferating Cell Nuclear Antigen), and it induced expression of the apoptotic protein BAX, in HuLM cells.

LIMITATIONS, REASONS FOR CAUTION

Results from this study need to be replicated in an appropriate animal model before testing this adenoviral vector in a human trial.

WIDER IMPLICATIONS OF THE FINDINGS

Effective targeting of gene therapy to leiomyoma cells enhances its potential as a non-invasive treatment of uterine fibroids.

Turning an antiviral into an anticancer drug: nanoparticle delivery of acyclovir monophosphate

Anti-herpes simplex virus (HSV) drug acyclovir (ACV) is phosphorylated by the viral thymidine kinase (TK), but not the cellular TK. Phosphorylated ACV inhibits cellular DNA synthesis and kills the infected cells. We hypothesize that ACV monophosphate (ACVP), which is an activated metabolite of ACV, should be efficient in killing cells independent of HSV-TK. If so, ACVP should be a cytotoxic agent if properly delivered to the cancer cells. The Lipid/Calcium/Phosphate (LCP) nanoparticles (NPs) with a membrane/core structure were used to encapsulate ACVP to facilitate the targeted delivery of ACVP to the tumor. The LCP NPs showed entrapment efficiency of ~70%, the nano-scaled particle size and positive zeta potential. Moreover, ACVP-loaded LCP NPs (A-LCP NPs) exhibited concentration-dependent cytotoxicity against H460 cells and increased S-phase arrest. More importantly, a significant reduction of the tumor volume over 4 days following administration (p<0.05-0.005) of A-LCP NPs, suggests excellent in vivo efficacy. Whereas, two free drugs (ACV and ACVP) and blank LCP NPs showed little or no therapeutic effect. It was also found that the high efficacy of A-LCP NPs was associated with the ability to induce dramatic apoptosis of the tumor cells, as well as significantly inhibit tumor cell proliferation and cell cycle progression. In conclusion, with the help of LCP NPs, monophosphorylation modification of ACV can successfully modify an HSV-TK-dependent antiviral drug into an anti-tumor drug.

DNA polymerase beta ribonucleotide discrimination: insertion, misinsertion, extension, and coding

DNA polymerases must select nucleotides that preserve Watson-Crick base pairing rules and choose substrates with the correct (deoxyribose) sugar. Sugar discrimination represents a great challenge because ribonucleotide triphosphates are present at much higher cellular concentrations than their deoxy-counterparts. Although DNA polymerases discriminate against ribonucleotides, many therapeutic nucleotide analogs that target polymerases have sugar modifications, and their efficacy depends on their ability to be incorporated into DNA. Here, we investigate the ability of DNA polymerase beta to utilize nucleotides with modified sugars. DNA polymerase beta readily inserts dideoxynucleoside triphosphates but inserts ribonucleotides nearly 4 orders of magnitude less efficiently than natural deoxynucleotides. The efficiency of ribonucleotide insertion is similar to that reported for other DNA polymerases. The poor polymerase-dependent insertion represents a key step in discriminating against ribonucleotides because, once inserted, a ribonucleotide is easily extended. Likewise, a templating ribonucleotide has little effect on insertion efficiency or fidelity. In contrast to insertion and extension of a ribonucleotide, the chemotherapeutic drug arabinofuranosylcytosine triphosphate is efficiently inserted but poorly extended. These results suggest that the sugar pucker at the primer terminus plays a crucial role in DNA synthesis; a 3'-endo sugar pucker facilitates nucleotide insertion, whereas a 2'-endo conformation inhibits insertion.

Engineering of a chimeric RB69 DNA polymerase sensitive to drugs targeting the cytomegalovirus enzyme

Detailed structural and biochemical studies with the human cytomegalovirus (HCMV UL54) DNA polymerase are hampered by difficulties to obtain this enzyme in large quantities. The crystal structure of the related RB69 DNA polymerase (gp43) is often used as a model system to explain mechanisms of inhibition of DNA synthesis and drug resistance. However, here we demonstrate that gp43 is approximately 400-fold less sensitive to the pyrophosphate analog foscarnet, when compared with UL54. The RB69 enzyme is also able to discriminate against the nucleotide analog inhibitor acyclovir. In contrast, the HCMV polymerase is able to incorporate this compound with similar efficiency as observed with its natural counterpart. In an attempt to identify major determinants for drug activity, we replaced critical regions of the nucleotide-binding site of gp43 with equivalent regions of the HCMV enzyme. We show that chimeric gp43-UL54 enzymes that contain residues of helix N and helix P of UL54 are resensitized against foscarnet and acyclovir. Changing a region of three amino acids of helix N showed the strongest effects, and changes of two segments of three amino acids in helix P further contributed to the reversal of the phenotype. The engineered chimeric enzyme can be produced in large quantities and may therefore be a valuable surrogate system in drug development efforts. This system may likewise be used for detailed structural and biochemical studies on mechanisms associated with drug action and resistance.

Gene therapy of benign gynecological diseases

Gene therapy is the introduction of genetic material into patient's cells to achieve therapeutic benefit. Advances in molecular biology techniques and better understanding of disease pathogenesis have validated the use of a variety of genes as potential molecular targets for gene therapy based approaches. Gene therapy strategies include: mutation compensation of dysregulated genes; replacement of defective tumor-suppressor genes; inactivation of oncogenes; introduction of suicide genes; immunogenic therapy and antiangiogenesis based approaches. Preclinical studies of gene therapy for various gynecological disorders have not only shown to be feasible, but also showed promising results in diseases such as uterine leiomyomas and endometriosis. In recent years, significant improvement in gene transfer technology has led to the development of targetable vectors, which have fewer side-effects without compromising their efficacy. This review provides an update on developing gene therapy approaches to treat common gynecological diseases such as uterine leiomyoma and endometriosis.

Antimicrobial strategies: inhibition of viral polymerases by 3'-hydroxyl nucleosides

Over the past 20 years, nucleoside analogues have constituted an arsenal of choice in the fight against HIV, hepatitis B and C viruses, and herpesviruses. Classical antiviral nucleosides such as zidovudine act as obligate chain terminators. Once incorporated as monophosphates into the viral nucleic acid, they immediately block the progression of the polymerase as a result of their lack of a reactive 3'-hydroxyl (3'-OH) group. This review explores beyond the paradigm of obligate chain termination, from a structural and a mechanistic perspective, the strategy of inhibiting viral polymerases (RNA- and DNA-dependant) with nucleoside analogues containing a 3'-OH group. Depending on their mechanism of action, these molecules typically fall into the following three categories: (i) delayed chain terminators; (ii) pseudo-obligate chain terminators; or (iii) mutagenic nucleosides. Delayed chain terminators (i.e. penciclovir, cidofovir and entecavir) block the polymerase at an internal position within the viral nucleic acid, whereas R7128 and the 4'C substituted nucleosides do not permit subsequent incorporation events. Ribavirin, 5-hydroxydeoxycytidine and KP1461 are not chain terminators. Instead, they inhibit viral replication after mispairing with the template base, resulting in random mutations that are often lethal. Finally, brivudine, clevudine and other L-nucleosides have unique or yet to be defined mechanisms of inhibition.

Sleeping Beauty-mediated suicide gene therapy of hepatocellular carcinoma

The aim of this study was to use gene therapy via the Sleeping Beauty (SB) system to increase telomerase promoter activity to target hepatocellular carcinoma (HCC). In previous studies, we identified selective and increased expression of luciferase and suicide genes controlled by the hTERT (human telomerase reverse transcriptase) promoter and the SV40 enhancer in telomerase-positive cancer cell lines. Because telomerase is activated in about 80% of HCCs, it is likely that increasing the activity of the telomerase promoter with a suicide gene will effectively eradicate HCCs. We found that the telomerase promoter mediated SB system can efficiently insert transgene into HCC genomes. Also, telomerase promoter activity was increased using a SB vector expressing suicide gene HSV-TK (herpes simplex virus thymidine kinase) controlled by the hTERT promoter and a SV40 enhancer for the induction of telomerase-positive cancer-specific cell death. HCC cell lines transfected with pT.hTp.HSV-tk.Con with active helper plasmid and ganciclovir (GCV) significantly inhibited cancer cell growth. These results indicate that Sleeping Beauty transposon mediated suicide gene expression can be used in HCC-targeted cancer gene therapy.

Use of base-modified duplex-stabilizing deoxynucleoside 5'-triphosphates to enhance the hybridization properties of primers and probes in detection polymerase chain reaction

Several base-modified duplex-stabilizing deoxyribonucleoside 5'-triphosphates (dNTPs) have been evaluated as agents for enhancing the hybridization properties of primers and probes in real-time polymerase chain reaction (PCR). It was shown that pyrimidines substituted at the 5-position with bromine or iodine atoms and methyl or propynyl groups are incorporated into PCR amplicons by Taq DNA polymerase as efficiently as natural dNTPs. The dNTP of 2-aminoadenosine was incorporated somewhat less efficiently than dATP but still supported PCR. Incorporation of these modified nucleotides into the amplified DNA represents a simple and inexpensive way to stabilize duplexes of primers and probes and is particularly effective in improving the amplification and detection of A/T-rich sequences. This technology permits the use of higher PCR annealing temperatures or alternatively a reduction in the length of the oligonucleotide components. Examples of successful application in TaqMan and Scorpion real-time detection assays are provided. Limits of the approach are identified and discussed. For example, application of the 5-bromo and 5-iodo derivatives may be limited to relatively G/C-rich DNA targets and, in particular, to those lacking long runs of adenylate and/or thymidylate. Simultaneous use of base-modified analogues of dATP and dTTP should be avoided in PCR due to "overstabilization" of the amplicon.

Eudragit® microparticles as a possible tool for ophthalmic administration of acyclovir

This paper describes the production and characterization of polyacrylic polymer (Eudragit RL, RS and NE) microparticles by spray drying method. Microparticles were designed for ophthalmic administration of acyclovir. Microparticle morphology was characterized by optical and electron microscopy. The release kinetics of the drug from microspheres were determined by a dialysis method. The spray drying method described allows the production of microparticles with acceptable encapsulation efficiency and appropriate dimensional characteristics for ophthalmic administration. Release profile data indicate that acyclovir is released from microparticles in a controlled manner. In addition the release pattern of the drug is influenced by the type of Eudragit used for microparticle production. Moreover the plaque reduction efficiency of acyclovir containing microparticles (except for RS/NE microspheres) is comparable to that displayed by the free drug. Finally our results suggest that acyclovir containing microparticles could represent an interesting system for the release of this antiviral drug at the eye site.

Hydroxyurea induces bystander cytotoxicity in cocultures of herpes simplex virus thymidine kinase-expressing and nonexpressing HeLa cells incubated with ganciclovir

Suicide gene therapy with the herpes simplex virus thymidine kinase (HSV-TK) cDNA and ganciclovir can elicit cytotoxicity to transgene-expressing and nonexpressing bystander cells via transfer of ganciclovir phosphates through gap junctions. HeLa cells do not exhibit bystander cytotoxicity, although we showed recently that they transfer low levels of ganciclovir phosphates to bystander cells. Here, we attempted to induce bystander cytotoxicity using hydroxyurea, an inhibitor of ribonucleotide reductase, to decrease the endogenous dGTP pool, which should lessen competition with ganciclovir triphosphate for DNA incorporation. Addition of hydroxyurea to cocultures of HSV-TK-expressing and bystander cells synergistically increased ganciclovir-mediated cytotoxicity to both cell populations while producing primarily an additive effect in cultures of 100% HSV-TK-expressing cells. Whereas HSV-TK-expressing cells in coculture were approximately 50-fold less sensitive to ganciclovir compared with cultures of 100% HSV-TK-expressing cells, addition of hydroxyurea restored ganciclovir sensitivity. Quantification of deoxynucleoside triphosphate pools showed that hydroxyurea decreased dGTP pools without significantly affecting ganciclovir triphosphate levels. Although hydroxyurea significantly increased the ganciclovir triphosphate:dGTP value for 12 to 24 hours in HSV-TK-expressing and bystander cells from coculture (1.4- to 4.9-fold), this value was increased for <12 hours (2.5-fold) in 100% HSV-TK-expressing cells. These data suggest that the prolonged increase in the ganciclovir triphosphate:dGTP value in cells in coculture resulted in synergistic cytotoxicity. Compared with enhancement of bystander cytotoxicity through modulation of gap junction intercellular communication, this strategy is superior because it increased cytotoxicity to both HSV-TK-expressing and bystander cells in coculture. This approach may improve clinical efficacy.

Potent antiviral activity of north-methanocarbathymidine against Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV) infection is a prerequisite for the development of Kaposi's sarcoma (KS). Blocking lytic KSHV replication may hinder KS tumorigenesis. Here, we report potent in vitro anti-KSHV activity of 2'-exo-methanocarbathymidine [North-methanocarbathymidine (N-MCT)], a thymidine analog with a pseudosugar ring locked in the northern conformation, which has previously been shown to block the replication of herpes simplex virus types 1 and 2. N-MCT inhibited KSHV virion production in lytically induced KSHV-infected BCBL-1 cells with a substantially lower 50% inhibitory concentration (IC50) than those of cidofovir (CDV) and ganciclovir (GCV) (IC50, mean +/- standard deviation: 0.08 +/- 0.03, 0.42 +/- 0.07, and 0.96 +/- 0.49 microM for N-MCT, CDV, and GCV, respectively). The reduction in KSHV virion production was accompanied by a corresponding decrease in KSHV DNA levels in the N-MCT-treated BCBL-1 cells, indicating that the compound blocked lytic KSHV DNA replication. A time- and dose-dependent accumulation of N-MCT-triphosphate (TP) was demonstrated in lytically induced BCBL-1 cells, while uninfected cells showed virtually no accumulation. The levels of N-MCT-TP were significantly decreased in the presence of 5'-ethynylthymidine, a potent inhibitor of herpesvirus thymidine kinase, resulting in the abrogation of anti-KSHV activity of N-MCT. N-MCT-TP more effectively blocked in vitro DNA synthesis by KSHV DNA polymerase with an IC50 of 6.24 +/- 0.08 microM (mean +/- standard deviation) compared to CDV-diphosphate (14.70 +/-2.47 microM) or GCV-TP (24.59 +/- 5.60 microM). Taken together, N-MCT is a highly potent and target-specific anti-KSHV agent which inhibits lytic KSHV DNA synthesis through its triphosphate metabolite produced in KSHV-infected cells expressing a virally encoded thymidine kinase.

GCV phosphates are transferred between HeLa cells despite lack of bystander cytotoxicity

The role of gap junctional intercellular communication (GJIC) in bystander killing with herpes simplex virus thymidine kinase (HSV-TK) and ganciclovir (GCV) was evaluated in U251 cells expressing a dominant-negative connexin 43 cDNA (DN14), and in HeLa cells, reportedly devoid of connexin protein. These cell lines both exhibited 0% GJIC when assayed by Lucifer Yellow fluorescent dye microinjection. Bystander cytotoxicity was still apparent in 50:50 cocultures of DN14 and HSV-TK-expressing U251 cells, but not in 50:50 cocultures of HeLa cells. However, the sensitivity of HeLa HSV-TK-expressing cells to GCV decreased nearly 100-fold (IC90=109 microM) when cocultured with bystander cells compared to results in 100% cultures of HSV-TK-expressing cells (IC90=1.2 microM). A more sensitive flow cytometry technique to measure GJIC over 24 h revealed that the DN14 and HeLa cells exhibited detectable levels of communication (29 and 23%, respectively). Transfer of phosphorylated GCV to HeLa bystander cells occurred within 4 h after drug addition, and GCV triphosphate (GCVTP) accumulated to 213+/-84 pmol/10(6) cells after 24 h. In addition, GCVTP levels were decreased in HSV-TK-expressing cells in coculture (867+/-33 pmol/10(6) cells) compared to 100% cultures of HSV-TK-expressing cells (1773+/-188 pmol/10(6) cells). The half-life of GCVTP in the HSV-TK-expressing cells was approximately four times that measured in the bystander cells (12.3 and 3.1 h, respectively). These data suggest that the lack of bystander cytotoxicity in HeLa cocultures is due to low transfer of phosphorylated GCV and a rapid half-life of GCVTP in the bystander cells. Thus, GCV phosphate transfer to non-HSV-TK-expressing bystander cells may mediate either bystander cell killing or sparing of HSV-TK-positive cells, depending upon the cell specific drug metabolism.

Polymerization of the triphosphates of AraC, 2′,2′-difluorodeoxycytidine (dFdC) and OSI-7836 (T-araC) by human DNA polymerase α and DNA primase

OSI-7836 (4'-thio-araC, T-araC) is a nucleoside analogue that shows efficacy against solid tumor xenograft models. We examined how the triphosphates of OSI-7836 (T-araCTP), cytarabine (araCTP), and gemcitabine (dFdCTP) affected the initiation of new DNA strands by the pol alpha primase complex. Whereas dFdCTP very weakly inhibited primase, both T-araCTP and araCTP potently inhibited this enzyme. Primase polymerized T-araCTP and araCTP more readily than its natural substrate, CTP, and incorporation resulted in strong chain termination. dFdCTP, araCTP, and T-araCTP inhibited pol alpha competitively with respect to dCTP. When exogenously added primentemplates were used, pol alpha incorporated all three analogues into DNA, and incorporation caused either weak chain termination (dFdCTP), strong termination (araCTP), or extremely strong termination (T-araC). Furthermore, pol alpha polymerized T-araCTP only nine-fold less well than dCTP, whereas it polymerized araCTP and dFdCTP 24- and 83-fold less well, respectively. The presence of these three analogues in the template strand resulted in significant pausing by pol alpha, although the site and severity of pausing varied between the analogues. During the elongation of primase-synthesized primers, a reaction that is thought to mimic the normal sequence of events during the initiation of new DNA strands, pol alpha polymerized all three compounds. However, incorporation of araCTP and dFdCTP resulted in minimal chain termination, while incorporation of T-araCTP still caused extremely strong termination. The implications of these results with respect to how these compounds affect cells are discussed.

Evaluation of combined gene regulatory elements for transcriptional targeting of suicide gene expression to malignant melanoma

Selective killing of tumors can be achieved by targeting the transcription of suicide genes via specific DNA control elements to malignant cells. Three different enhancer-promoter systems were constructed and evaluated for their capability to direct gene expression to melanoma. Two tissue-specific (tyrosine and MIA) promoters and one weak viral promoter were fused to multiple tandem copies of a melanocyte-specific enhancer element. Reporter gene assays revealed a maximum increase in transcription by combining each promoter with 3-4 copies of the enhancer and demonstrated that all enhancer-promoter combinations exhibited tissue-specific activity. Though this activity was still significantly less than that of the strong but unspecific cytomegalovirus (CMV) promoter. In contrast, when those combinations were employed to drive the expression of two suicide genes, encoding the diptheria toxin A chain (DT-A) and the prodrug-activating herpes simplex virus thymidine kinase (TK), respectively, only those constructs in which transcription was under control of tissue-specific promoter elements mediated selective killing of melanoma cells. This killing was in the range of cell death induced by CMV promoter activity. Our data indicate that the enhancer/tyrosinase and enhancer/MIA promoter constructs but not the viral promoter constructs can provide a valuable tool for selective suicide gene expression in melanoma.

Comparative kinetics of nucleotide analog incorporation by vent DNA polymerase

Comparative kinetic and structural analyses of a variety of polymerases have revealed both common and divergent elements of nucleotide discrimination. Although the parameters for dNTP incorporation by the hyperthermophilic archaeal Family B Vent DNA polymerase are similar to those previously derived for Family A and B DNA polymerases, parameters for analog incorporation reveal alternative strategies for discrimination by this enzyme. Discrimination against ribonucleotides was characterized by a decrease in the affinity of NTP binding and a lower rate of phosphoryl transfer, whereas discrimination against ddNTPs was almost exclusively due to a slower rate of phosphodiester bond formation. Unlike Family A DNA polymerases, incorporation of 9-[(2-hydroxyethoxy)methyl]X triphosphates (where X is adenine, cytosine, guanine, or thymine; acyNTPs) by Vent DNA polymerase was enhanced over ddNTPs via a 50-fold increase in phosphoryl transfer rate. Furthermore, a mutant with increased propensity for nucleotide analog incorporation (Vent(A488L) DNA polymerase) had unaltered dNTP incorporation while displaying enhanced nucleotide analog binding affinity and rates of phosphoryl transfer. Based on kinetic data and available structural information from other DNA polymerases, we propose active site models for dNTP, ddNTP, and acyNTP selection by hyperthermophilic archaeal DNA polymerases to rationalize structural and functional differences between polymerases.

Differential mutation patterns in thymidine kinase and DNA polymerase genes of herpes simplex virus type 1 clones passaged in the presence of acyclovir or penciclovir

A total of 21 clones of acyclovir (ACV)-resistant (ACV(r)) herpes simplex virus type 1 (HSV-1) and 23 clones of penciclovir (PCV)-resistant (PCV(r)) HSV-1, emerging during serial passages in the presence of ACV or PCV, were isolated under conditions excluding contamination of resistant mutants in the starting virus culture, and their mutations in the thymidine kinase (TK) and DNA polymerase (DNA Pol) genes were analyzed comparatively. Mutations in the TK genes from ACV(r) mutants consisted of 50% single nucleotide substitutions and 50% frameshift mutations, while the corresponding figures for the PCV(r) mutants were 4 and 96%, respectively (P < 0.001). Eight of the 21 ACV(r) clones, but none of the 23 PCV(r) clones, had mutations in DNA Pol. Only nucleotide substitution(s) could be detected in the DNA Pol gene, as the gene is essential for virus replication. Therefore, the results for the DNA Pol mutants are concordant with those for the TK mutants in that a single nucleotide substitution was commonly observed in the ACV(r), but not in the PCV(r), mutants. These results clearly point to differential mutation patterns between ACV(r) and PCV(r) HSV-1 clones.

Comparative analysis of DNA breakage, chromosomal aberrations and apoptosis induced by the anti-herpes purine nucleoside analogues aciclovir, ganciclovir and penciclovir

Nucleoside analogues have been used in antiviral therapy and suicide cancer gene therapy. Therefore, it is of importance to compare their potential cytotoxic and genotoxic action. Using metabolically competent CHO cells expressing the thymidine kinase gene of herpes simplex virus type 1 (CHO-HSVtk cells) as a model system, the induction of DNA breaks was compared with the induction of structural chromosomal aberrations and apoptosis/necrosis after exposure to the anti-herpes nucleoside analogues aciclovir (ACV), ganciclovir (GCV) and penciclovir (PCV). After continuous treatment of CHO-HSVtk cells with the drugs, LD(10) in a colony-forming assay was 50, 0.5 and 1 microM for ACV, GCV and PCV, respectively, with GCV to be the most potent agent as determined at a given dose level. There was a remarkable difference in the activity of the agents to kill HSVtk expressing and non-expressing cells: the difference in cellular sensitivity of HSVtk(+) versus HSVtk(-) cells at LD(10) level was 7-fold for ACV, 60-fold for GCV and 400-fold for PCV. The drugs were shown to be strong inducers of apoptosis that was analysed as to concentration- and time-dependence; they induced to only very low extent necrosis. The agents were also highly potent in the induction of DNA single-strand breaks (SSBs) and double-strand breaks (DSBs) (as measured by single cell gel electrophoresis (SCGE)) and chromosomal aberrations. Although PCV induced DNA DSBs with a kinetics and frequency similar to that of GCV, it caused mostly condensation defects instead of "typical" structural chromosomal aberrations. For the drugs used, the frequency of apoptotic cells and the induction of abnormal mitoses appear to be related indicating genotoxic effects induced by the agents to be involved in cell killing due to apoptosis.

Metabolic pathways of N-methanocarbathymidine, a novel antiviral agent, in native and herpes simplex virus type 1 infected Vero cells

N-methanocarbathymidine ((N)-MCT), a thymidine analog incorporating a pseudosugar with a fixed Northern conformation, exhibits potent antiherpetic activity against herpes simplex virus types 1 (HSV-1) and 2 (HSV-2). This study contrasts the metabolic pathway of (N)-MCT and the well-known antiherpetic agent ganciclovir (GCV) in HSV-1-infected and uninfected Vero cells. Treatment of HSV-1 infected Vero cells immediately after viral infection with (N)-MCT profoundly inhibited the development of HSV-1 infection. Using standard plaque reduction assay to measure viral infection, (N)-MCT showed a potency greater than that of ganciclovir (GCV), the IC50s were 0.02 and 0.25 microM for (N)-MCT and GCV, respectively. (N)-MCT showed no cytotoxic effect on uninfected Vero cells (CC50>100 microM). Dose and time dependence studies showed high levels of (N)-MCT-triphosphate ((N)-MCT-TP), and GCV-triphosphate (GCV-TP) in HSV-1-infected cells incubated with (N)-MCT or GCV, respectively. In contrast, uninfected cells incubated with (N)-MCT showed elevated levels of (N)-MCT-monophosphate only, while low levels of mono, di- and triphosphates of GCV were found following incubation with GCV. Although the accumulation rate of (N)-MCT and GCV phosphates in HSV-1-infected cells were similar, the decay rate of (N)-MCT-TP was slower than that of GCV-TP. These results suggest that: (1) the antiviral activity of (N)-MCT against herpes viruses is mediated through its triphosphate metabolite; (2) in contrast to GCV, the diphosphorylation of (N)-MCT in HSV-1- infected cells is the rate limiting step; (3) (N)-MCT-TP accumulates rapidly and has a long half-life in HSV-1-infected cells; and (4) HSV-tk catalyzed the mono, and diphosphorylation of (N)-MCT while monophosphorylating GCV only. These results provide a biochemical rational for the highly selective and effective inhibition of HSV-1 by (N)-MCT.

Gene therapy for head and neck cancer

Head and neck cancer, because of its anatomic accessibility and poor overall survival rate, has become a frequent target of novel gene therapy intervention strategies. Viral and nonviral vectors have been used to transfer a variety of tumor suppressor genes, suicide genes, and immunologic genes into head and neck cancer cells in both the laboratory and clinical setting. Gene therapy as an isolated treatment modality will probably not replace standard treatment modalities in the management of head and neck cancer. It seems likely, however, that gene transfer will find its way into the multidisciplinary care of the head and neck cancer patient, where novel treatments are combined with standard therapies in order to maximize tumor response.

Multi-log cytotoxicity of carbocyclic 2'-deoxyguanosine in HSV-TK-expressing human tumor cells

Ganciclovir (GCV) is widely used as a prodrug for selective activation in tumor cells expressing herpes simplex virus thymidine kinase (HSV-TK) because of its ability to induce multi-log cytotoxicity to HSV-TK-expressing as well as nonexpressing bystander cells. We now report that another substrate for HSV-TK, D-carbocyclic 2'-deoxyguanosine (CdG), induces multi-log cytotoxicity in HSV-TK-expressing and bystander cells at concentrations <or=3 microM. We have compared the cytotoxicity and cell cycle effects of CdG to that observed with GCV in two human tumor cell lines. The results demonstrated that cytotoxicity of CdG was similar to that of GCV in both U251 glioblastoma and SW620 colon carcinoma cells that stably expressed HSV-TK. In addition, CdG induced a potent bystander effect in both cell types in co-cultures consisting of HSV-TK-expressing and nonexpressing bystander (lacZ-expressing) cells at ratios of 50:50 or 10:90. Selectivity for HSV-TK-expressing compared to lacZ-expressing cells was similar for CdG and GCV in the U251 cells, however CdG was less selective than GCV in the SW620 cell lines. Despite their ability to induce multi-log cytotoxicity at similar concentrations, CdG and GCV exhibited differential effects on cell cycle progression. Cells incubated with 1 microM CdG for 24 hr accumulated in S-phase and G(2)/M after drug washout, and the majority of cells died prior to cell division. This contrasts with the delayed effects of 1 microM GCV that were not evident until after cell division when cells attempted S-phase for the second time. Thus, CdG is a potent cytotoxic agent that merits further investigation to determine whether it will be therapeutically effective in enzyme-prodrug therapy with HSV-TK.

Differential cytotoxicity and bystander effect of the rabbit cytochrome P450 4B1 enzyme gene by two different prodrugs: implications for pharmacogene therapy

The time course of cytotoxicity induction and the bystander effect of the rabbit cytochrome P450 4B1 (cyp4B1)/4-ipomeanol (4-IM) or 2-aminoanthracene (2-AA) pharmacogene therapy systems were investigated and compared with the herpes simplex virus type 1 thymidine kinase/ganciclovir (HSV-tk/GCV) system. Experiments were performed in rat 9L gliosarcoma cells stably expressing cyp4B1 (9L-4B1), HSV-tk (9L-tk), or their egfp (enhanced green fluorescent protein) fusion genes. Cyp4B1-mediated activation of 2-AA showed a high cell killing efficiency within only 48 hours with an onset after already 15 minutes of prodrug exposure. Residual 9L-4B1 cells were mostly damaged sublethally upon 2-AA treatment showing an S phase arrest by cell cycle analysis. 4-IM treatment of 9L-4B1 cells generated an overall weaker cell killing, especially after prodrug exposure times of less than 48 hours. Residual cells surviving 4-IM treatment showed a G2/M arrest and restarted proliferation after prodrug treatment was stopped. HSV-tk/GCV pharmacogene therapy resulted in a slower cytotoxicity induction than cyp4B1/2-AA treatment with a significantly lower cell killing efficiency after 24 and 48 hours. HSV-tk/GCV-mediated cytotoxicity was widely similar to the cytotoxicity induced by cyp4B1/4-IM with the exception of a continuous 48-hour prodrug exposure where 4-IM treatment showed a significantly higher cell killing rate. Cells surviving HSV-tk/GCV suicide gene therapy were not viable and showed an S-phase arrest. Whereas HSV-tk/GCV induced a strong bystander effect, only moderate bystander cell death depending on cell-to-cell contact was demonstrated in 9L/9L-4B1 cocultures upon 2-AA treatment and was even absent with 4-IM, thereby contrasting with earlier reports. The absence of a strong bystander effect may limit, on one hand, the overall utility of the cyp4B1 systems for cancer gene therapy. On the other hand, the weak bystander effect together with the fast induction of cytotoxicity may provide marked advantages for the use of the cyp4B1 systems as biosafety enhancers for gene marking or replacement studies and donor lymphocyte infusions after allogeneic bone marrow transplantation.

Advances in gene therapy for malignant melanoma

BACKGROUND

The recent developments in the field of gene transfer have advanced the use of gene therapy as a novel strategy against a variety of human malignancies. Due to its unique set of characteristics, melanoma represents a suitable target for the clinical translation of the different gene transfer approaches recently developed. The goal of gene therapy targeted to melanoma cells is to introduce "suicide" genes, to transfer tumor suppressor genes, to inactivate aberrant oncogene expression, or to introduce genes encoding immunologically relevant molecules. Gene therapy targeted to the host's immune cells has been developed as an additional strategy to redirect immune responses against melanoma.

METHODS

The authors reviewed the published gene transfer studies in experimental models, as well as the results of gene therapy clinical trials for patients with melanoma.

RESULTS

Clinical trials have shown the feasibility and safety of gene therapy against malignant melanoma. Although no major successes have been reported, the positive results observed in some patients support the potential for gene therapy in the management of this disease.

CONCLUSIONS

Gene therapy of melanoma using current gene transfer approaches is feasible and safe. Better vector technology as well as increased understanding of the "bystander effect" triggered by gene transfer approaches would provide the tools to validate gene therapy as an effective modality of treatment for malignant melanoma.

Antiviral therapy for cytomegalovirus infections in pediatric patients

Appreciation of the spectrum of illness caused by cytomegalovirus (CMV) infections has increased markedly during the past 2 decades. The number of immunosuppressed patients also has increased during the same time period, reflecting the central tenet that CMV disease is most severe in this patient population. Fortunately, antiviral therapies with activity against CMV also have been identified during this same time course, and they include ganciclovir, foscamet, and cidofovir. Although all 3 of these therapies can have significant toxicities associated with them, nonetheless they are employed with relative frequency to treat potentially life-threatening CMV disease. Ganciclovir is the first-line compound used, followed by foscarnet and cidofovir. This article summarizes those CMV infections that require antiviral therapy and outlines therapeutic options for each.

The Haemophilus ducreyi cytolethal distending toxin induces cell cycle arrest and apoptosis via the DNA damage checkpoint pathways

The cytolethal distending toxins (CDTs) induce cell cycle arrest by a mechanism still not well characterized. We demonstrate that the effect of the Haemophilus ducreyi CDT (HdCDT) is cell type-specific: B cell lines underwent apoptosis, epithelial cells and keratinocytes arrested exclusively in G(2), whereas normal fibroblasts arrested both in G(1) and G(2). We studied normal keratinocytes and fibroblasts, which are relevant for understanding the pathogenicity of H. ducreyi. The response to HdCDT resembles the checkpoint response activated by ionizing radiation. Both responses were characterized by an early induction of the p53 gene and the cyclin-dependent kinase inhibitor p21 in fibroblasts, and activation of the chk2 kinase in epithelial cells. In the Ataxia Telangiectasia-mutated gene (ATM)-deficient lymphoblastoid cell lines, intoxication was significantly delayed compared with ATM wild type cells, and was associated with a slower kinetic of p53 stabilization, suggesting that the early response to HdCDT is ATM-dependent. Activation of ATM-dependent pathways was further confirmed by the ability of caffeine to partially override the HdCDT-mediated cell cycle arrest. Our data shed new light on the mechanism of action of this novel family of bacterial toxins, limiting the target candidates to DNA or molecules directly involved in activation of checkpoint responses.

Mutation of Gln125 to Asn selectively abolishes the thymidylate kinase activity of herpes simplex virus type 1 thymidine kinase

The broad substrate specificity of herpes simplex virus type 1 (HSV-1) thymidine kinase (TK) has provided the basis for selective antiherpetic therapy and, more recently, suicide gene therapy for the treatment of cancer. We have now constructed an HSV-1 TK mutant enzyme, in which an asparagine (N) residue is substituted for glutamine (Q) at position 125, and have evaluated the effect of this amino acid change on enzymatic activity. In marked contrast with wild-type HSV-1 TK, which displays both thymidine kinase and thymidylate kinase activities, the HSV-1 TK(Q125N) mutant was unable to phosphorylate pyrimidine nucleoside monophosphates but retained significant phosphorylation activity for thymidine and a series of antiherpetic pyrimidine and purine nucleoside analogs. The abrogation of HSV-1 TK-associated thymidylate kinase activity resulted in a 100-fold accumulation of the monophosphate form of (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU) in osteosarcoma cells transfected with the HSV-1 TK(Q125N) gene compared with osteosarcoma cells expressing wild-type HSV-1 TK. BVDU monophosphate accumulation gave rise to a much greater inhibition of cellular thymidylate synthase in HSV-1 TK(Q125N) gene-transfected cells than wild-type HSV-1 TK gene-transfected osteosarcoma tumor cells without significantly changing the cytostatic potency of BVDU for the HSV-1 TK gene-transfected tumor cells. Accordingly, the presence of the Q125N mutation in HSV-1 TK gene-transfected tumor cells was found to result in a multilog decrease in the cytostatic activity of those pyrimidine nucleoside analogs that in their monophosphate form do not have marked affinity for thymidylate synthase [i.e., 1-beta-D-arabinofuranosylthymine and (E)-5-(2-bromovinyl)-1-beta-D-arabinofuranosyluracil].

Genetic risks of antiviral nucleoside analogues--a survey

The available informations on the genotoxic effects in experimental systems of the antiherpesvirus nucleosides aciclovir, penciclovir, ganciclovir, brivudine and cidofovir as well as of the antiretrovirals zidovudine (AZT), lamivudine, zalcitabine (ddC), didanosine and stavudine are reviewed. Furthermore, data on carcinogenic activity of these drugs in laboratory rodents are compiled. Most nucleoside analogue antivirals induce chromosomal aberrations but are inactive in gene mutation assays. Carcinogenicity findings in mice and rats are variable but clearly positive for AZT and ddC. The possible mechanisms by which these agents may cause damage in the genetic information are still largely hypothetical, and experimental findings do not permit relevant extrapolations to the situation in man. There is no conclusive evidence that any of the drugs caused tumours in humans. The use of nucleoside analogues in antiviral therapy remains a pragmatic option that seems justified by risk/benefit assessment.

Selective abolishment of pyrimidine nucleoside kinase activity of herpes simplex virus type 1 thymidine kinase by mutation of alanine-167 to tyrosine

Herpes simplex virus type 1 (HSV-1) encodes a thymidine kinase (TK) that markedly differs from mammalian nucleoside kinases in terms of substrate specificity. It recognizes both pyrimidine 2'-deoxynucleosides and a variety of purine nucleoside analogs. Based on a computer modeling study and in an attempt to modify this specificity, an HSV-1 TK mutant enzyme containing an alanine-to-tyrosine mutation at amino acid position 167 was constructed. Compared with wild-type HSV-1 TK, the purified mutant HSV-1 TK(A167Y) enzyme was heavily compromised in phosphorylating pyrimidine nucleosides such as (E)-5-(2-bromovinyl)-2'-deoxyuridine and the natural substrate dThd, whereas its ability to phosphorylate the purine nucleoside analogs ganciclovir (GCV) and lobucavir was only reduced approximately 2-fold. Moreover, a markedly decreased competition of natural pyrimidine nucleosides (i.e., thymidine) with purine nucleoside analogs for phosphorylation by HSV-1 TK(A167Y) was observed. Human osteosarcoma cells transduced with the wild-type HSV-1 TK gene were extremely sensitive to the cytostatic effects of antiherpetic pyrimidine [i.e., (E)-5-(2-bromovinyl)-2'-deoxyuridine] and purine (i.e., GCV) nucleoside analogs. Transduction with the HSV-1 TK(A167Y) gene sensitized the osteosarcoma cells to a variety of purine nucleoside analogs, whereas there was no measurable cytostatic activity of pyrimidine nucleoside analogs. The unique properties of the A167Y mutant HSV-1 TK may give this enzyme a therapeutic advantage in an in vivo setting due to the markedly reduced dThd competition with GCV for phosphorylation by the HSV-1 TK.

Gene therapy for head and neck cancer

OBJECTIVES/HYPOTHESIS

New treatment methods are needed for head and neck cancer to improve survival without increasing morbidity. Gene therapy is a potential method of improving patient outcome. Progress in gene therapy for cancer is reviewed with emphasis on the limitations of vector technology and treatment strategies. Given the current technological vector limitations in transmitting the therapeutic genes, treatments that require the fewest number of cells to be altered by the new gene are optimal. Therefore an immune-based gene therapy strategy was selected in which the tumors were transfected with the gene for an alloantigen, human leukocyte antigen (HLA)-B7, a class I major histocompatibility complex (MHC). This would restore an antigen presentation mechanism in the tumor to induce an antitumor response. This gene therapy strategy was tested in patients with advanced, unresectable head and neck cancer.

STUDY DESIGN

Prospective trial.

METHODS

Twenty patients with advanced head and neck cancer who had failed conventional therapy and did not express HLA-B7 were treated with gene therapy using a lipid vector by direct intratumoral injection. The gene therapy product contained the HLA-B7 gene and the beta2-microglobulin gene, which permits complete expression of the class I MHC at the cell surface. Patients were assessed for any adverse effects, for changes in tumor size, for time to disease progression, and for survival. Biopsy specimens were assessed for pathological response, HLA-B7 expression, apoptosis, cellular proliferation, CD-8 cells, granzyme, and p53 status.

RESULTS

There were no adverse effects from the gene therapy. At 16 weeks after beginning gene therapy, four patients had a partial response and two patients had stable disease. Two of the tumors completely responded clinically, but tumor was still seen on pathological examination. The time to disease progression in the responding patients was 20 to 80 weeks. The median survival in patients who completed gene therapy was 54 weeks, compared with 21 weeks in patients whose tumors progressed after the first cycle of treatment. One patient survived for 106 weeks without any additional therapy. HLA-B7 was demonstrated in the treated tumors, and increased apoptosis was seen in the responding tumors.

CONCLUSION

Significant advances have been made in the field of gene therapy for cancer. Alloantigen gene therapy has had efficacy in the treatment of cancer and can induce tumor responses in head and neck tumors. Alloantigen gene therapy has significant potential as an adjunctive treatment of head and neck cancer.

Gene therapy for lung cancer

The advances that have been made over the past decade in the field of gene transfer as well as in the fields of immunology and the molecular biology of tumorigenesis have brought to reality the possibility of using gene transfer as an anti-cancer treatment modality. The published results of clinical trials using this approach to date have been very limited, and a considerable amount of work still needs to be done in order to make this an effective treatment modality. However the developments that have occurred in the past several years indicate that this modality will become efficacious in the foreseeable future.

Gene therapy for carcinoma of the breast: Genetic toxins

Gene therapy was initially envisaged as a potential treatment for genetically inherited, monogenic disorders. The applications of gene therapy have now become wider, however, and include cardiovascular diseases, vaccination and cancers in which conventional therapies have failed. With regard to oncology, various gene therapy approaches have been developed. Among them, the use of genetic toxins to kill cancer cells selectively is emerging. Two different types of genetic toxins have been developed so far: the metabolic toxins and the dominant-negative class of toxins. This review describes these two different approaches, and discusses their potential applications in cancer gene therapy.

Prodrug activation gene therapy and external beam irradiation in the treatment of prostate cancer

OBJECTIVES

Gene therapy may represent a new avenue for the development of multimodal treatment for men with locally advanced prostate cancer. This study explores the potential benefits of combining adenovirus-mediated (ADV) herpes simplex virus thymidine kinase gene (HSV-tk) transduction and ganciclovir (GCV) therapy with external beam radiation therapy (XRT) to enhance the therapeutic efficacy of each treatment alone.

METHODS

ADV/HSV-tk-transduced mouse prostate cancer cells, RM-1, were irradiated as single-cell suspensions at escalating doses in a cesium source (4.4 Gy/min). HSV-tk-expressing cells were randomized to receive varying doses and varying chronologies of GCV therapy in relation to XRT to fully evaluate potential cooperative activities. End points were determined in a clonogenic assay by counting colonies with greater than 50 cells 7 days after replating. The potential role of apoptosis as a mediator of enhanced cell killing was addressed by a TUNEL assay 12 and 24 hours after therapy.

RESULTS

Neither ADV infection nor GCV alone affected XRT killing. However, the combination of ADV/HSV-tk+GCV plus XRT maintained the 1 log of cell kill from gene therapy alone through escalating doses of radiation. Radiation sensitization was noted at higher doses of radiation (8.8 Gy or more). Although decreasing the GCV dose had a profound negative influence on HSV-tk+GCV-mediated killing, combination therapy continued to maintain the degree of HSV-tk+GCV killing through escalating doses of XRT in an additive fashion but did not result in radiosensitization. Changing the chronology of GCV exposure in relation to XRT did not significantly alter the additive activities of combination therapy. Studies of apoptosis noted a doubling of apoptotic activity with HSV-tk+GCV compared with HSV-tk+PBS with or without XRT. However, there was no significant change in apoptotic activity in combination therapy over HSV-tk+GCV alone within the 24-hour period after GCV exposure.

CONCLUSIONS

The combination of ADV/HSV-tk+GCV and XRT appears to result in at least additive, and with higher doses of radiation, synergistic killing activities, indicating a potential usefulness of this treatment strategy for patients with prostate cancer.

Herpes simplex virus thymidine kinase/ganciclovir-induced apoptosis involves ligand-independent death receptor aggregation and activation of caspases

Suicide gene therapy systems such as the herpes simplex thymidine kinase/ganciclovir system (TK/GCV) may kill cancer cells by apoptosis through as yet undefined mechanisms. Here we show that TK/GCV treatment induces p53 accumulation and increases cell surface expression of CD95 and tumor necrosis factor receptor, which is likely to involve p53-mediated translocation of CD95 to the cell surface. TK/GCV-induced apoptosis involves CD95-L-independent CD95 aggregation leading to the formation of a Fas-associated death domain protein (FADD) and caspase-8-containing, death-inducing signaling complex. Dominant negative FADD, the caspase-8 inhibitor zIETD-fmk [Z-Ile-Glu(OMe)-Thr-Asp(OMe)-fluoromethylketone], and zVAD-fmk (Z-Val-Ala-Asp-fluoromethylketone) partially abrogate TK/GCV-induced apoptosis. In addition to apoptosis induction, TK/GCV treatment strongly sensitizes for CD95-L-, TNF-, and TNF-related, apoptosis-inducing, ligand (TRAIL)-induced cell death in constitutively resistant cells. These findings may be used to increase the efficacy of TK/GCV and other suicide gene therapy systems for the treatment of cancer.

Exonucleases and the incorporation of aranucleotides into DNA

The polymerization of nucleotide analogs into DNA is a common strategy used to inhibit DNA synthesis in rapidly dividing tumor cells and viruses. The mammalian DNA polymerases catalyze the insertion of the arabinofuranosyl analogs of dNTPs (aranucleotides) into DNA efficiently, but elongate from the 3' aranucleotides poorly. Slow elongation provides an opportunity for exonucleases to remove aranucleotides. The exonuclease activity associated with DNA polymerase delta removes araCMP from 3' termini with the same efficiency that it removes a paired 3' deoxycytosine suggesting that the proofreading exonucleases associated with DNA polymerases might remove aranucleotides inefficiently. A separate 30 kDa exonuclease has been purified from mammalian cells that removes araCMP from 3' termini. The activity of this enzyme in the cell could remove aranucleotides from 3' termini of DNA and decrease the efficacy of the analogs. Inhibition analysis of the purified exonuclease shows that this enzyme is inhibited by thioinosine monophosphate (TIMP) with a Ki = 17 microM. When high TIMP levels are generated in HL-60 cells, incorporation of araC in DNA is increased about 16-fold relative to total DNA synthesis. This increased araC in DNA is likely a result of exonuclease inhibition in the cell. Thus, exonucleases in cells might play an important role in removing aranucleotides inserted by DNA polymerases.

Ablation of tumor cells in vivo by direct injection of HSV-thymidine kinase retroviral vector and ganciclovir therapy

The introduction of therapeutic genes into proliferating tumor cells in vivo by direct intralesional injection of retroviral vectors can provide an effective and valuable approach for the treatment of a variety of solid tumor types. Efficient transduction of tumor cells in situ by direct injection was demonstrated using a retroviral vector containing the beta-galactosidase (beta-gal) gene. Ablation therapy in vivo was demonstrated using a retroviral vector containing the Herpes simplex virus thymidine kinase gene (HSV-TK) to deliver the TK gene into the murine colorectal tumor cell line CT26. Ablation of CT26 tumor cells in situ was achieved by directly injecting high-titer HSV-TK retroviral vector preparations into the site of tumor cell inoculation followed by intraperitoneal (i.p.) delivery of ganciclovir (GCV). This gene therapy strategy demonstrated a markedly lower rate of tumor progression, with several complete regressions, compared to animals in control groups. We also demonstrated that resistance to subsequent challenges with unmodified CT26 cells and an enhanced cellular immune response is associated with tumor regression in immunocompetent animals. Our results demonstrate the feasibility of direct in situ administration of HSV-TK retroviral vectors for the treatment of cancer and suggest that a cellular immune response may be elicited by this therapy.

Effects of nucleoside analog incorporation on DNA binding to the DNA binding domain of the GATA-1 erythroid transcription factor

We investigate here the effects of the incorporation of the nucleoside analogs araC (1-beta-D-arabinofuranosylcytosine) and ganciclovir (9-[(1,3-dihydroxy-2-propoxy)methyl] guanine) into the DNA binding recognition sequence for the GATA-1 erythroid transcription factor. A 10-fold decrease in binding affinity was observed for the ganciclovir-substituted DNA complex in comparison to an unmodified DNA of the same sequence composition. AraC substitution did not result in any changes in binding affinity. 1H-15N HSQC and NOESY NMR experiments revealed a number of chemical shift changes in both DNA and protein in the ganciclovir-modified DNA-protein complex when compared to the unmodified DNA-protein complex. These changes in chemical shift and binding affinity suggest a change in the binding mode of the complex when ganciclovir is incorporated into the GATA DNA binding site.

Metabolism and metabolic actions of 6-methylpurine and 2-fluoroadenine in human cells

Activation of purine nucleoside analogs by Escherichia coli purine nucleoside phosphorylase (PNP) is being evaluated as a suicide gene therapy strategy for the treatment of cancer. Because the mechanisms of action of two toxic purine bases, 6-methylpurine (MeP) and 2-fluoroadenine (F-Ade), that are generated by this approach are poorly understood, mechanistic studies were initiated to learn how these compounds differ from agents that are being used currently. The concentration of F-Ade, MeP, or 5-fluorouracil required to inhibit CEM cell growth by 50% after a 4-hr incubation was 0.15, 9, or 120 microM, respectively. F-Ade and MeP were also toxic to quiescent MRC-5, CEM, and Balb 3T3 cells. Treatment of CEM, MRC-5, or Balb 3T3 cells with either F-Ade or MeP resulted in the inhibition of protein, RNA, and DNA syntheses. CEM cells converted F-Ade and MeP to F-ATP and MeP-ribonucleoside triphosphate (MeP-R-TP), respectively. The half-life for disappearance of HeP-ribonucleoside triphosphate from CEM cells was approximately 48 hr, whereas the half-lives of F-ATP and ATP were approximately 5 hr. Both MeP and F-Ade were incorporated into the RNA and DNA of CEM cells. These studies indicated that the mechanisms of action of F-Ade and MeP were quite different from those of other anticancer agents, and suggested that the generation of these agents in tumor cells by E. coli PNP could result in significant advantages over those generated by either herpes simplex virus thymidine kinase or E. coli cytosine deaminase. These advantages include a novel mechanism of action resulting in toxicity to nonproliferating and proliferating tumor cells and the high potency of these agents during short-term treatment.