Noninvasive optical imaging of nitroreductase gene-directed enzyme prodrug therapy system in living animals

Hemicyanine-Based Near-Infrared Fluorescence Off-On Probes for Imaging Intracellular and In Vivo Nitroreductase Activity

Nitroreductase (NTR) has the ability to activate nitro group-containing prodrugs and decompose explosives; thus, the evaluation of NTR activity is specifically important in pharmaceutical and environmental areas. Numerous studies have verified effective fluorescent methods to detect and image NTR activity; however, near-infrared (NIR) fluorescence probes for biological applications are lacking. Thus, in this study, we synthesized novel NIR probes (NIR-HCy-NO₂ 1-3) by introducing a nitro group to the hemicyanine skeleton to obtain fluorescence images of NTR activity. Additionally, this study was also designed to propose a different water solubility and investigate the catalytic efficiency of NTR. NIR-HCy-NO₂ inherently exhibited a low fluorescence background due to the interference of intramolecular charge transfer (ICT) by the nitro group. The conversion from the nitro to amine group by NTR induced a change in the absorbance spectra and lead to the intense enhancement of the fluorescence spectra. When assessing the catalytic efficiency and the limit of detection (LOD), including NTR activity imaging, it was demonstrated that NIR-HCy-NO₂ 1 was superior to the other two probes. Moreover, we found that NIR-HCy-NO₂ 1 reacted with type I mitochondrial NTR in live cell imaging. Conclusively, NIR-HCy-NO₂ demonstrated a great potential for application in various NTR-related fields, including NTR activity for cell imaging in vivo.

A Highly Selective Fluorescent Probe for the Detection of Nitroreductase Based on a Naphthalimide Scaffold

The development of fluorescent probes for nitroreductase (NTR) has received intense attention because of its biological significance and wide application. In this work, a novel fluorescent probe for the detection of NTR in aqueous solution was designed and synthesized on a 1,8-naphthalimide scaffold. In the presence of NTR and nicotinamide adenine dinucleotide (NADH) under physiological conditions, the probe was converted into a 4-hydroxy-1,8-naphthalimide derivative and exhibited a sharp fluorescence enhancement at 550 nm, with a high selectivity for NTR over various analytes. The detection limit for NTR was determined to be 9.8 ng/ml by this probe. Due to its low signal background, this probe showed > 70-fold fluorescence enhancement. Theoretical calculations revealed that the reason for the fluorescence quenching of this probe is the photoinduced electron transfer (PET) from both the nitrobenzene and morpholine groups to the naphthalimide fluorophore.

SP94-Targeted Triblock Copolymer Nanoparticle Delivers Thymidine Kinase-p53-Nitroreductase Triple Therapeutic Gene and Restores Anticancer Function against Hepatocellular Carcinoma in Vivo

Gene-directed enzyme-prodrug therapy (GDEPT) is a promising approach for cancer therapy, but it suffers from poor targeted delivery in vivo. Polyethylenimine (PEI) is a cationic polymer efficient in delivering negatively charged nucleic acids across cell membranes; however, it is highly toxic in vivo. Hence, we efficiently reduced PEI toxicity without compromising its transfection efficiency by conjugating it with poly(d,l-lactic-co-glycolic acid) (PLGA) and poly(ethylene glycol) (PEG) as triblock copolymers through a multistep synthetic process. The synthesized nanoparticles showed efficient delivery of loaded nucleic acids to tumor cells in vitro and in vivo in mice. We used this nanoparticle to deliver a rationally engineered thymidine kinase (TK)-p53-nitroreductase (NTR) triple therapeutic gene against hepatocellular carcinoma (HCC), where p53 tumor suppressor gene is mutated in more than 85% of cancers. TK-p53-NTR triple gene therapy restores p53 function and potentiates cancer cell response to delivered prodrugs (ganciclovir (GCV) and CB1954). We used SP94 peptide-functionalized PLGA-PEG-PEI nanoparticles for the optimal delivery of TK-p53-NTR therapeutic gene in vivo. The nanoparticles prepared from the conjugated polymer showed high loading efficiency for the DNA and markedly enhanced TK-NTR-mediated gene therapy upon the simultaneous coexpression of p53 by the concurrent rescue of the endogenous apoptotic pathway in HCC cells of both p53-mutant and wild-type phenotypes in vitro. In vivo delivery of TK-p53-NTR genes by SP94-targeted PLGA-PEG-PEI NP in mice resulted in a strong expression of suicide genes selectively in tumors, and subsequent administration of GCV and CB1954 led to a decline in tumor growth, and established a superior therapeutic outcome against HCC. We demonstrate a highly efficient approach that exogenously supplements p53 to enable synergy with the outcome of TK-NTR suicide gene therapy against HCC.

Microvesicle-Mediated Delivery of Minicircle DNA Results in Effective Gene-Directed Enzyme Prodrug Cancer Therapy

An emerging approach for cancer treatment employs the use of extracellular vesicles, specifically exosomes and microvesicles, as delivery vehicles. We previously demonstrated that microvesicles can functionally deliver plasmid DNA to cells and showed that plasmid size and sequence, in part, determine the delivery efficiency. In this study, delivery vehicles comprised of microvesicles loaded with engineered minicircle (MC) DNA that encodes prodrug converting enzymes developed as a cancer therapy in mammary carcinoma models. We demonstrated that MCs can be loaded into shed microvesicles with greater efficiency than their parental plasmid counterparts and that microvesicle-mediated MC delivery led to significantly higher and more prolonged transgene expression in recipient cells than microvesicles loaded with the parental plasmid. Microvesicles loaded with MCs encoding a thymidine kinase (TK)/nitroreductase (NTR) fusion protein produced prolonged TK-NTR expression in mammary carcinoma cells. In vivo delivery of TK-NTR and administration of prodrugs led to the effective killing of both targeted cells and surrounding tumor cells via TK-NTR-mediated conversion of codelivered prodrugs into active cytotoxic agents. In vivo evaluation of the bystander effect in mouse models demonstrated that for effective therapy, at least 1% of tumor cells need to be delivered with TK-NTR-encoding MCs. These results suggest that MC delivery via microvesicles can mediate gene transfer to an extent that enables effective prodrug conversion and tumor cell death such that it comprises a promising approach to cancer therapy.

Engineering Escherichia coli NfsB To Activate a Hypoxia-Resistant Analogue of the PET Probe EF5 To Enable Non-Invasive Imaging during Enzyme Prodrug Therapy

Gene-directed enzyme prodrug therapy (GDEPT) uses tumor-tropic vectors to deliver prodrug-converting enzymes such as nitroreductases specifically to the tumor environment. The nitroreductase NfsB from Escherichia coli (NfsB_Ec) has been a particular focal point for GDEPT and over the past 25 years has been the subject of several engineering studies seeking to improve catalysis of prodrug substrates. To facilitate clinical development, there is also a need to enable effective non-invasive imaging capabilities. SN33623, a 5-nitroimidazole analogue of 2-nitroimidazole hypoxia probe EF5, has potential for PET imaging exogenously delivered nitroreductases without generating confounding background due to tumor hypoxia. However, we show here that SN33623 is a poor substrate for NfsB_Ec. To address this, we used assay-guided sequence and structure analysis to identify two conserved residues that block SN33623 activation in NfsB_Ec and close homologues. Introduction of the rational substitutions F70A and F108Y into NfsB_Ec conferred high levels of SN33623 activity and enabled specific labeling of E. coli expressing the engineered enzyme. Serendipitously, the F70A and F108Y substitutions also substantially improved activity with the anticancer prodrug CB1954 and the 5-nitroimidazole antibiotic prodrug metronidazole, which is a potential biosafety agent for targeted ablation of nitroreductase-expressing vectors.

Recent progress in the imaging detection of enzyme activities in vivo

Enzymatic activities are important for normal physiological processes and are also critical regulatory mechanisms for many pathologies. Identifying the enzyme activities in vivo has considerable importance in disease diagnoses and monitoring of the physiological metabolism. In the past few years, great strides have been made towards the imaging detection of enzyme activity in vivo based on optical modality, MRI modality, nuclear modality, photoacoustic modality and multifunctional modality. This review summarizes the latest advances in the imaging detection of enzyme activities in vivo reported within the past years, mainly concentrating on the probe design, imaging strategies and demonstration of enzyme activities in vivo. This review also highlights the potential challenges and the further directions of this field.

Tailoring the properties of a hypoxia-responsive 1,8-naphthalimide for imaging applications

Sensing hypoxia in tissues and cell models can provide insights into its role in disease states and cell development. Fluorescence imaging is a minimally-invasive method of visualising hypoxia in many biological systems. Here we present a series of improved bioreductive fluorescent sensors based on a nitro-naphthalimide structure, in which selectivity, photophysical properties, toxicity and cellular uptake are tuned through structural modifications. This new range of compounds provides improved probes for imaging and monitoring hypoxia, customised for a range of different applications. Studies in monolayers show the different reducing capabilities of hypoxia-resistant and non-resistant cell lines, and studies in tumour models show successful staining of the hypoxic region.

Ultrasound-guided delivery of thymidine kinase-nitroreductase dual therapeutic genes by PEGylated-PLGA/PIE nanoparticles for enhanced triple negative breast cancer therapy

AIM

Triple-negative breast cancer (TNBC) is an aggressive breast cancer subtype. Since no targeted therapy is available, gene-directed enzyme prodrug therapy (GDEPT) could be an attractive strategy for treating TNBC.

MATERIALS & METHODS

Polyethylene glycol (PEG)ylated-poly(lactic-co-glycolic acid)/polyethyleneimine nanoparticles (PLGA/PEI NPs) were synthesized and complexed with TK-NTR fusion gene. Ultrasound (US) and microbubble (MB) mediated sonoporation was used for efficient delivery of the TK-NTR-DNA-NP complex to TNBC tumor in vivo for cancer therapy. Therapeutic effect was evaluated by treating TNBC cells in vitro and tumor xenograft in vivo by using prodrugs ganciclovir (GCV) and CB1954.

RESULTS

TNBC cells treated with GCV/CB1954 prodrugs after transfection of TK-NTR-DNA by PEGylated-PLGA/PEI NP resulted in high apoptotic-index. US-MB image-guided delivery of TK-NTR-DNA-NP complex displayed significant expression level of TK-NTR protein and showed tumor reduction when treated with GCV/CB1954 prodrugs in TNBC xenograft in vivo.

CONCLUSION

US-MB image-guided delivery of TK-NTR gene by PEGylated-PLGA/PEI NPs could be a potential prodrug therapy for TNBC in the clinic.

Designer bacteria as intratumoural enzyme biofactories

Bacterial-directed enzyme prodrug therapy (BDEPT) is an emerging form of treatment for cancer. It is a biphasic variant of gene therapy in which a bacterium, armed with an enzyme that can convert an inert prodrug into a cytotoxic compound, induces tumour cell death following tumour-specific prodrug activation. BDEPT combines the innate ability of bacteria to selectively proliferate in tumours, with the capacity of prodrugs to undergo contained, compartmentalised conversion into active metabolites in vivo. Although BDEPT has undergone clinical testing, it has received limited clinical exposure, and has yet to achieve regulatory approval. In this article, we review BDEPT from the system designer's perspective, and provide detailed commentary on how the designer should strategize its development de novo. We report on contemporary advancements in this field which aim to enhance BDEPT in terms of safety and efficacy. Finally, we discuss clinical and regulatory barriers facing BDEPT, and propose promising approaches through which these hurdles may best be tackled.

Antitumor Synergism and Enhanced Survival with a Tumor Vasculature-Targeted Enzyme Prodrug System, Rapamycin, and Cyclophosphamide

Mutant cystathionine gamma-lyase was targeted to phosphatidylserine exposed on tumor vasculature through fusion with Annexin A1 or Annexin A5. Cystathionine gamma-lyase E58N, R118L, and E338N mutations impart nonnative methionine gamma-lyase activity, resulting in tumor-localized generation of highly toxic methylselenol upon systemic administration of nontoxic selenomethionine. The described therapeutic system circumvents systemic toxicity issues using a novel drug delivery/generation approach and avoids the administration of nonnative proteins and/or DNA required with other enzyme prodrug systems. The enzyme fusion exhibits strong and stable in vitro binding with dissociation constants in the nanomolar range for both human and mouse breast cancer cells and in a cell model of tumor vascular endothelium. Daily administration of the therapy suppressed growth of highly aggressive triple-negative murine 4T1 mammary tumors in immunocompetent BALB/cJ mice and MDA-MB-231 tumors in SCID mice. Treatment did not result in the occurrence of negative side effects or the elicitation of neutralizing antibodies. On the basis of the vasculature-targeted nature of the therapy, combinations with rapamycin and cyclophosphamide were evaluated. Rapamycin, an mTOR inhibitor, reduces the prosurvival signaling of cells in a hypoxic environment potentially exacerbated by a vasculature-targeted therapy. IHC revealed, unsurprisingly, a significant hypoxic response (increase in hypoxia-inducible factor 1 α subunit, HIF1A) in the enzyme prodrug-treated tumors and a dramatic reduction of HIF1A upon rapamycin treatment. Cyclophosphamide, an immunomodulator at low doses, was combined with the enzyme prodrug therapy and rapamycin; this combination synergistically reduced tumor volumes, inhibited metastatic progression, and enhanced survival. Mol Cancer Ther; 16(9); 1855-65. ©2017 AACR.

Targeting assay of a fusion protein applied in enzyme prodrug therapy

Tumor growth and metastasis are dependent on angiogenesis. The overexpression of integrin α_vβ₃ on angiogenic vessels and on numerous malignant human tumor cells suggests that these labeled ligands of integrin are potentially suitable for molecular imaging and in targeted therapy of tumors. In previous studies, we added a β-lactamase variant with reduced immunogenicity to the cyclic peptide RGD4C, resulting in the fusion protein RGD4CβL, which is suitable for use in targeted enzyme prodrug therapy (TEPT), a promising treatment for tumors. The targeting of the aforementioned fusion protein serves an important role in TEPT. In the present study, RGD4CβL was labeled with ¹²⁵I and the targeting effect on integrin-positive tumors was evaluated. The results demonstrated that the ¹²⁵I-RGD4CβL protein exhibited high levels of accumulation at the tumor site and rapid renal clearance, which revealed the potency and efficiency of RGD4CβL in TEPT.

Combined antitumor activity of the nitroreductase/CB1954 suicide gene system and γ-rays in HeLa cells in vitro

Escherichia coli nitroreductase (NTR) may convert the prodrug CB1954 (5-(aziridin-1-yl)-2,4-dinitrobenzamide) into a bifunctional alkylating agent, which may lead to DNA crosslinks and the apoptosis of cancer cells. NTR/CB1954 has been demonstrated to be an effective gene therapy in cancer cells. The present study examined whether the NTR/CB1954 suicide gene system had cytotoxic effects on HeLa cells and may improve the radiosensitivity of HeLa cells to γ-rays. It was observed that the NTR/CB1954 suicide gene system exerted marked cytotoxic effects on HeLa cells. The combined therapeutic effects of NTR/CB1954 and γ-rays on HeLa cells demonstrated a synergistic effect. CB1954 at concentrations of 12.5 and 25 µmol/l increased the sensitization enhancement ratio of HeLa cells to 1.54 and 1.66, respectively. Therefore, when compared with monotherapy, the combined therapy of NTR/CB1954 and γ-rays may increase the apoptotic rate and enhance the radiosensitivity of HeLa cells. The combined therapy of γ-ray radiation and the NTR/CB1954 suicide gene system may be a novel and potent therapeutic method for the treatment of cervical carcinoma.

Bioreductive fluorescent imaging agents: applications to tumour hypoxia

The development of new optical chemosensors for various reductases presents an ideal approach to visualise areas of tissue hypoxia.

Nitroreductase gene-directed enzyme prodrug therapy: insights and advances toward clinical utility

This review examines the vast catalytic and therapeutic potential offered by type I (i.e. oxygen-insensitive) nitroreductase enzymes in partnership with nitroaromatic prodrugs, with particular focus on gene-directed enzyme prodrug therapy (GDEPT; a form of cancer gene therapy). Important first indications of this potential were demonstrated over 20 years ago, for the enzyme-prodrug pairing of Escherichia coli NfsB and CB1954 [5-(aziridin-1-yl)-2,4-dinitrobenzamide]. However, it has become apparent that both the enzyme and the prodrug in this prototypical pairing have limitations that have impeded their clinical progression. Recently, substantial advances have been made in the biodiscovery and engineering of superior nitroreductase variants, in particular development of elegant high-throughput screening capabilities to enable optimization of desirable activities via directed evolution. These advances in enzymology have been paralleled by advances in medicinal chemistry, leading to the development of second- and third-generation nitroaromatic prodrugs that offer substantial advantages over CB1954 for nitroreductase GDEPT, including greater dose-potency and enhanced ability of the activated metabolite(s) to exhibit a local bystander effect. In addition to forging substantial progress towards future clinical trials, this research is supporting other fields, most notably the development and improvement of targeted cellular ablation capabilities in small animal models, such as zebrafish, to enable cell-specific physiology or regeneration studies.

Real time detection of live microbes using a highly sensitive bioluminescent nitroreductase probe

A highly sensitive and selective nitroreductase probe, showing a rapid and strong bioluminescence enhancement (>100-fold in 5 minutes), and its initial application in the real time detection of both Gram positive and Gram negative live bacteria and monitoring of their growth has been reported.

Development of a Bioluminescent Nitroreductase Probe for Preclinical Imaging

Bacterial nitroreductases (NTRs) have been widely utilized in the development of novel antibiotics, degradation of pollutants, and gene-directed enzyme prodrug therapy (GDEPT) of cancer that reached clinical trials. In case of GDEPT, since NTR is not naturally present in mammalian cells, the prodrug is activated selectively in NTR-transformed cancer cells, allowing high efficiency treatment of tumors. Currently, no bioluminescent probes exist for sensitive, non-invasive imaging of NTR expression. We therefore developed a "NTR caged luciferin" (NCL) probe that is selectively reduced by NTR, producing light proportional to the NTR activity. Here we report successful application of this probe for imaging of NTR in vitro, in bacteria and cancer cells, as well as in vivo in mouse models of bacterial infection and NTR-expressing tumor xenografts. This novel tool should significantly accelerate the development of cancer therapy approaches based on GDEPT and other fields where NTR expression is important.

Prodrug enzymes and their applications in image-guided therapy of cancer: tracking prodrug enzymes to minimize collateral damage

Many cytotoxic therapies are available to kill cancer cells. Unfortunately, these also inflict significant damage on normal cells. Identifying highly effective cancer treatments that have minimal or no side effects continues to be a major challenge. One of the strategies to minimize damage to normal tissue is to deliver an activating enzyme that localizes only in the tumor and converts a nontoxic prodrug to a cytotoxic agent locally in the tumor. Such strategies have been previously tested but with limited success due in large part to the uncertainty in the delivery and distribution of the enzyme. Imaging the delivery of the enzyme to optimize timing of the prodrug administration to achieve image-guided prodrug therapy would be of immense benefit for this strategy. Here, we have reviewed advances in the incorporation of image guidance in the applications of prodrug enzymes in cancer treatment. These advances demonstrate the feasibility of using clinically translatable imaging in these prodrug enzyme strategies.

Noninvasive theranostic imaging of HSV1-sr39TK-NTR/GCV-CB1954 dual-prodrug therapy in metastatic lung lesions of MDA-MB-231 triple negative breast cancer in mice

Metastatic breast cancer is an obdurate cancer type that is not amenable to chemotherapy regimens currently used in clinic. There is a desperate need for alternative therapies to treat this resistant cancer type. Gene-Directed Enzyme Prodrug Therapy (GDEPT) is a superior gene therapy method when compared to chemotherapy and radiotherapy procedures, proven to be effective against many types of cancer in pre-clinical evaluations and clinical trials. Gene therapy that utilizes a single enzyme/prodrug combination targeting a single cellular mechanism needs significant overexpression of delivered therapeutic gene in order to achieve therapy response. Hence, to overcome this obstacle we recently developed a dual therapeutic reporter gene fusion that uses two different prodrugs, targeting two distinct cellular mechanisms in order to achieve effective therapy with a limited expression of delivered transgenes. In addition, imaging therapeutic reporter genes offers additional information that indirectly correlates gene delivery, expression, and functional effectiveness as a theranostic approach. In the present study, we evaluate the therapeutic potential of HSV1-sr39TK-NTR fusion dual suicide gene therapy system that we recently developed, in MDA-MB-231 triple negative breast cancer lung-metastatic lesions in a mouse model. We compared the therapeutic potential of HSV1-sr39TK-NTR fusion with respective dual prodrugs GCV-CB1954 with HSV1-sr39TK/GCV and NTR/CB1954 single enzyme prodrug system in this highly resistant metastatic lesion of the lungs. In vitro optimization of dose and duration of exposure to GCV and CB1954 was performed in MDA-MB-231 cells. Drug combinations of 1 μg/ml GCV and 10 μM CB1954 for 3 days was found to be optimal regimen for induction of significant cell death, as assessed by FACS analysis. In vivo therapeutic evaluation in animal models showed a complete ablation of lung metastatic nodules of MDA-MB-231 triple negative breast cancer cells following two consecutive doses of a combination of GCV (40 mg/kg) and CB1954 (40 mg/kg) administered at 5 day intervals. In contrast, the respective treatment condition in animals expressing HSV1-sr39TK or NTR separately, showed minimal or no effect on tumor reduction as measured by bioluminescence (tumor mass) and [¹⁸F]-FHBG microPET (TK expression) imaging. These highlight the strong therapeutic effect of the dual fusion prodrug therapy and its use in theranostic imaging of tumor monitoring in living animals by multimodality molecular imaging.

Visualizing cellular interactions with a generalized proximity reporter

Interactions among neighboring cells underpin many physiological processes ranging from early development to immune responses. When these interactions do not function properly, numerous pathologies, including infection and cancer, can result. Molecular imaging technologies, especially optical imaging, are uniquely suited to illuminate complex cellular interactions within the context of living tissues in the body. However, no tools yet exist that allow the detection of microscopic events, such as two cells coming into close proximity, on a global, whole-animal scale. We report here a broadly applicable, longitudinal strategy for probing interactions among cells in living subjects. This approach relies on the generation of bioluminescent light when two distinct cell populations come into close proximity, with the intensity of the optical signal correlating with relative cellular location. We demonstrate the ability of this reporter strategy to gauge cell-cell proximity in culture models in vitro and then evaluate this approach for imaging tumor-immune cell interactions using a murine breast cancer model. In these studies, our imaging strategy enabled the facile visualization of features that are otherwise difficult to observe with conventional imaging techniques, including detection of micrometastatic lesions and potential sites of tumor immunosurveillance. This proximity reporter will facilitate probing of numerous types of cell-cell interactions and will stimulate the development of similar techniques to detect rare events and pathological processes in live animals.

Nitroreductase, a near-infrared reporter platform for in vivo time-domain optical imaging of metastatic cancer

The ability to visualize reporter gene expression in vivo has revolutionized all facets of biologic investigation and none more so than imaging applications in oncology. Near-infrared reporter gene imaging may facilitate more accurate evaluation of chemotherapeutic response in preclinical models of orthotopic and metastatic cancers. We report the development of a cell permeable, quenched squarine probe (CytoCy5S), which is reduced by Escherichia coli nitroreductase (NTR), resulting in a near-infrared fluorescent product. Time-domain molecular imaging of NTR/CytoCy5S reporter platform permitted noninvasive monitoring of disease progression in orthotopic xenografts of disseminated leukemia, lung, and metastatic breast cancer. This methodology facilitated therapeutic evaluation of NTR gene-directed enzymatic prodrug therapy with conventional metronidazole antibiotics. These studies show NTR/CytoCy5S as a near-infrared gene reporter system with broad preclinical and prospective clinical applications within imaging, and gene therapy, of cancer.

Dual-therapeutic reporter genes fusion for enhanced cancer gene therapy and imaging

Two of the successful gene-directed enzyme prodrug therapies include herpes simplex virus-thymidine kinase (HSV1-TK) enzyme-ganciclovir prodrug and the Escherichia coli nitroreductase (NTR) enzyme-CB1954 prodrug strategies; these enzyme-prodrug combinations produce activated cytotoxic metabolites of the prodrugs capable of tumor cell death by inhibiting DNA synthesis and killing quiescent cells, respectively. Both these strategies also affect significant bystander cell killing of neighboring tumor cells that do not express these enzymes. We have developed a dual-combination gene strategy, where we identified HSV1-TK and NTR fused in a particular orientation can effectively kill tumor cells when the tumor cells are treated with a fusion HSV1-TK-NTR gene- along with a prodrug combination of GCV and CB1954. In order to determine whether the dual-system demonstrate superior therapeutic efficacy than either HSV1-TK or NTR systems alone, we conducted both in vitro and in vivo tumor xenograft studies using triple negative SUM159 breast cancer cells, by evaluating the efficacy of cell death by apoptosis and necrosis upon treatment with the dual HSV1-TK genes-GCV-CB1954 prodrugs system, and compared the efficiency to HSV1-TK-GCV and NTR-CB1954. Our cell-based studies, tumor regression studies in xenograft mice, histological analyses of treated tumors and bystander studies indicate that the dual HSV1-TK-NTR-prodrug system is two times more efficient even with half the doses of both prodrugs than the respective single gene-prodrug system, as evidenced by enhanced apoptosis and necrosis of tumor cells in vitro in culture and xenograft of tumor tissues in animals.

Emerging implications of nonmammalian cytosine deaminases on cancer therapeutics

Nonmammalian cytosine deaminases (CDs) have been investigated for last 30 years in the context of cancer therapy. The therapeutic effect of CD is based on its ability to catalyze the conversion of nontoxic prodrug 5-fluorocytosine (5FC) into the anticancer drug 5-fluorouracil (5FU) by deamination of the number 4 carbon of 5FC. This deamination property of CD has been explored to develop innovative therapeutic approach for treatment of cancer. A general overview is needed for the identification of efficient cytosine deaminases for potential use in cancer therapy. In this review, we have discussed about nonmammalian CDs for a variety of prodrug gene/enzyme therapy applications with several recent examples. Finally, we have provided a prospective on the future aspects of CDs and their applications in cancer therapy.