A phase I/II clinical trial in localized prostate cancer of an adenovirus expressing nitroreductase with CB1954 [correction of CB1984]

Smart nanocarriers for enzyme-activated prodrug therapy

Exogenous enzyme-activated prodrug therapy (EPT) is a potential cancer treatment strategy that delivers non-human enzymes into or on the surface of the cell and subsequently converts a non-toxic prodrug into an active cytotoxic substance at a specific location and time. The development of several pharmacological pairs based on EPT has been the focus of anticancer research for more than three decades. Numerous of these pharmacological pairs have progressed to clinical trials, and a few have achieved application in specific cancer therapies. The current review highlights the potential of enzyme-activated prodrug therapy as a promising anticancer treatment. Different microbial, plant, or viral enzymes and their corresponding prodrugs that advanced to clinical trials have been listed. Additionally, we discuss new trends in the field of enzyme-activated prodrug nanocarriers, including nanobubbles combined with ultrasound (NB/US), mesoscopic-sized polyion complex vesicles (PICsomes), nanoparticles, and extracellular vesicles (EVs), with special emphasis on smart stimuli-triggered drug release, hybrid nanocarriers, and the main application of nanotechnology in improving prodrugs.

Recent advances in nano-based drug delivery systems for treatment of liver cancer

Liver cancer is one of the aggressive primary tumors as evident by high rate of incidence and mortality. Conventional treatments (e.g. chemotherapy) suffer from various drawbacks including wide drug distribution, low localized drug concentration, and severe off-site toxicity. Therefore, they cannot satisfy the mounting need for safe and efficient cancer therapeutics, and alternative novel strategies are needed. Nano-based drug delivery systems (NDDSs) are among these novel approaches that can improve the overall therapeutic outcomes. NDDSs are designed to encapsulate drug molecules and target them specifically to liver cancer. Thus, NDDSs can selectively deliver therapeutic agents to the tumor cells and avoid distribution to off-target sites which should improve the safety profile of the active agents. Nonetheless, NDDSs should be well designed, in terms of the preparing materials, nanocarriers structure, and the targeting strategy, in order to accomplish these objectives. This review discusses the latest advances of NDDSs for cancer therapy with emphasis on the aforementioned essential design components. The review also entails the challenges associated with the clinical translation of NDDSs, and the future perspectives towards next-generation NDDSs.

Prodrug activation by 4,4'-bipyridine-mediated aromatic nitro reduction

Unleashing prodrugs through nitro-reduction is a promising strategy in cancer treatment. In this study, we present a unique bioorthogonal reaction for aromatic nitro reduction, mediated by 4,4'-bipyridine. The reaction is a rare example of organocatalyst-mediated bioorthogonal reaction. This bioorthogonal reaction demonstrates broad substrate scope and proceeds at low micromolar concentrations under biocompatible conditions. Our mechanistic study reveals that water is essential for the reaction to proceed at biorelevant substrate concentrations. We illustrate the utility of our reaction for controlled prodrug activation in mammalian cells, bacteria, and mouse models. Furthermore, a nitro-reduction-annulation cascade is developed for the synthesis of indole derivatives in living cells.

Programming tumor evolution with selection gene drives to proactively combat drug resistance

Most targeted anticancer therapies fail due to drug resistance evolution. Here we show that tumor evolution can be reproducibly redirected to engineer therapeutic opportunity, regardless of the exact ensemble of pre-existing genetic heterogeneity. We develop a selection gene drive system that is stably introduced into cancer cells and is composed of two genes, or switches, that couple an inducible fitness advantage with a shared fitness cost. Using stochastic models of evolutionary dynamics, we identify the design criteria for selection gene drives. We then build prototypes that harness the selective pressure of multiple approved tyrosine kinase inhibitors and employ therapeutic mechanisms as diverse as prodrug catalysis and immune activity induction. We show that selection gene drives can eradicate diverse forms of genetic resistance in vitro. Finally, we demonstrate that model-informed switch engagement effectively targets pre-existing resistance in mouse models of solid tumors. These results establish selection gene drives as a powerful framework for evolution-guided anticancer therapy.

Structural Evaluation of a Nitroreductase Engineered for Improved Activation of the 5-Nitroimidazole PET Probe SN33623

Bacterial nitroreductase enzymes capable of activating imaging probes and prodrugs are valuable tools for gene-directed enzyme prodrug therapies and targeted cell ablation models. We recently engineered a nitroreductase (E. coli NfsB F70A/F108Y) for the substantially enhanced reduction of the 5-nitroimidazole PET-capable probe, SN33623, which permits the theranostic imaging of vectors labeled with oxygen-insensitive bacterial nitroreductases. This mutant enzyme also shows improved activation of the DNA-alkylation prodrugs CB1954 and metronidazole. To elucidate the mechanism behind these enhancements, we resolved the crystal structure of the mutant enzyme to 1.98 Å and compared it to the wild-type enzyme. Structural analysis revealed an expanded substrate access channel and new hydrogen bonding interactions. Additionally, computational modeling of SN33623, CB1954, and metronidazole binding in the active sites of both the mutant and wild-type enzymes revealed key differences in substrate orientations and interactions, with improvements in activity being mirrored by reduced distances between the N5-H of isoalloxazine and the substrate nitro group oxygen in the mutant models. These findings deepen our understanding of nitroreductase substrate specificity and catalytic mechanisms and have potential implications for developing more effective theranostic imaging strategies in cancer treatment.

Structure and Dynamics of Three Escherichia coli NfsB Nitro-Reductase Mutants Selected for Enhanced Activity with the Cancer Prodrug CB1954

Escherichia coli NfsB has been studied extensively for its potential for cancer gene therapy by reducing the prodrug CB1954 to a cytotoxic derivative. We have previously made several mutants with enhanced activity for the prodrug and characterised their activity in vitro and in vivo. Here, we determine the X-ray structure of our most active triple and double mutants to date, T41Q/N71S/F124T and T41L/N71S. The two mutant proteins have lower redox potentials than wild-type NfsB, and the mutations have lowered activity with NADH so that, in contrast to the wild-type enzyme, the reduction of the enzyme by NADH, rather than the reaction with CB1954, has a slower maximum rate. The structure of the triple mutant shows the interaction between Q41 and T124, explaining the synergy between these two mutations. Based on these structures, we selected mutants with even higher activity. The most active one contains T41Q/N71S/F124T/M127V, in which the additional M127V mutation enlarges a small channel to the active site. Molecular dynamics simulations show that the mutations or reduction of the FMN cofactors of the protein has little effect on its dynamics and that the largest backbone fluctuations occur at residues that flank the active site, contributing towards its broad substrate range.

Engineering the Escherichia coli Nitroreductase NfsA to Create a Flexible Enzyme-Prodrug Activation System

Bacterial nitroreductase enzymes that can efficiently convert nitroaromatic prodrugs to a cytotoxic form have numerous applications in targeted cellular ablation. For example, the generation of cytotoxic metabolites that have low bystander potential (i.e., are largely confined to the activating cell) has been exploited for precise ablation of specific cell types in animal and cell-culture models; while enzyme-prodrug combinations that generate high levels of bystander cell killing are useful for anti-cancer strategies such as gene-directed enzyme-prodrug therapy (GDEPT). Despite receiving substantial attention for such applications, the canonical nitroreductase NfsB from Escherichia coli has flaws that limit its utility, in particular a low efficiency of conversion of most prodrugs. Here, we sought to engineer a superior broad-range nitroreductase, E. coli NfsA, for improved activity with three therapeutically-relevant prodrugs: the duocarmycin analogue nitro-CBI-DEI, the dinitrobenzamide aziridine CB1954 and the 5-nitroimidazole metronidazole. The former two prodrugs have applications in GDEPT, while the latter has been employed for targeted ablation experiments and as a precise 'off-switch' in GDEPT models to eliminate nitroreductase-expressing cells. Our lead engineered NfsA (variant 11_78, with the residue substitutions S41Y, L103M, K222E and R225A) generated reduced metabolites of CB1954 and nitro-CBI-DEI that exhibited high bystander efficiencies in both bacterial and 2D HEK-293 cell culture models, while no cell-to-cell transfer was evident for the reduced metronidazole metabolite. We showed that the high bystander efficiency for CB1954 could be attributed to near-exclusive generation of the 2-hydroxylamine reduction product, which has been shown in 3D cell culture to cause significantly greater bystander killing than the 4-hydroxylamine species that is also produced by NfsB. We similarly observed a high bystander effect for nitro-CBI-DEI in HCT-116 tumor spheroids in which only a small proportion of cells were expressing variant 11_78. Collectively, our data identify variant 11_78 as a broadly improved prodrug-activating nitroreductase that offers advantages for both targeted cellular ablation and suicide gene therapy applications.

The YfkO Nitroreductase from Bacillus Licheniformis on Gold-Coated Superparamagnetic Nanoparticles: Towards a Novel Directed Enzyme Prodrug Therapy Approach

The bacterial nitroreductase NfnB has been the focus of a great deal of research for its use in directed enzyme prodrug therapy in combination with the nitroreductase prodrug CB1954 with this combination of enzyme and prodrug even entering clinical trials. Despite some promising results, there are major limitations to this research, such as the fact that the lowest reported Km for this enzyme far exceeds the maximum dosage of CB1954. Due to these limitations, new enzymes are now being investigated for their potential use in directed enzyme prodrug therapy. One such enzyme that has proved promising is the YfkO nitroreductase from Bacillus Licheniformis. Upon investigation, the YfkO nitroreductase was shown to have a much lower Km (below the maximum dosage) than that of NfnB as well as the fact that when reacting with the prodrug it produces a much more favourable ratio of enzymatic products than NfnB, forming more of the desired 4-hydroxylamine derivative of CB1954.

Repurposing 18F-FMISO as a PET tracer for translational imaging of nitroreductase-based gene directed enzyme prodrug therapy

Nitroreductases (NTR) are a family of bacterial enzymes used in gene directed enzyme prodrug therapy (GDEPT) that selectively activate prodrugs containing aromatic nitro groups to exert cytotoxic effects following gene transduction in tumours. The clinical development of NTR-based GDEPT has, in part, been hampered by the lack of translational imaging modalities to assess gene transduction and drug cytotoxicity, non-invasively. This study presents translational preclinical PET imaging to validate and report NTR activity using the clinically approved radiotracer, ¹⁸F-FMISO, as substrate for the NTR enzyme.

Methods: The efficacy with which ¹⁸F-FMISO could be used to report NfsB NTR activity in vivo was investigated using the MDA-MB-231 mammary carcinoma xenograft model. For validation, subcutaneous xenografts of cells constitutively expressing NTR were imaged using ¹⁸F-FMISO PET/CT and fluorescence imaging with CytoCy5S, a validated fluorescent NTR substrate. Further, examination of the non-invasive functionality of ¹⁸F-FMISO PET/CT in reporting NfsB NTR activity in vivo was assessed in metastatic orthotopic NfsB NTR expressing xenografts and metastasis confirmed by bioluminescence imaging. ¹⁸F-FMISO biodistribution was acquired ex vivo by an automatic gamma counter measuring radiotracer retention to confirm in vivo results. To assess the functional imaging of NTR-based GDEPT with ¹⁸F-FMISO, PET/CT was performed to assess both gene transduction and cytotoxicity effects of prodrug therapy (CB1954) in subcutaneous models.

Results:¹⁸F-FMISO retention was detected in NTR⁺ subcutaneous xenografts, displaying significantly higher PET contrast than NTR^- xenografts (p < 0.0001). Substantial ¹⁸F-FMISO retention was evident in metastases of orthotopic xenografts (p < 0.05). Accordingly, higher ¹⁸F-FMISO biodistribution was prevalent ex vivo in NTR⁺ xenografts. ¹⁸F-FMISO NfsB NTR PET/CT imaging proved useful for monitoring in vivo NTR transduction and the cytotoxic effect of prodrug therapy.

Conclusions:¹⁸F-FMISO NfsB NTR PET/CT imaging offered significant contrast between NTR⁺ and NTR^- tumours and effective resolution of metastatic progression. Furthermore, ¹⁸F-FMISO NfsB NTR PET/CT imaging proved efficient in monitoring the two steps of GDEPT, in vivo NfsB NTR transduction and response to CB1954 prodrug therapy. These results support the repurposing of ¹⁸F-FMISO as a readily implementable PET imaging probe to be employed as companion diagnostic test for NTR-based GDEPT systems.

Prodrugs and prodrug-activated systems in gene therapy

The inclusion of genes that control cell fate (so-called suicide, or kill-switch, genes) into gene therapy vectors is based on a compelling rationale for the safe and selective elimination of aberrant transfected cells. Prodrug-activated systems were developed in the 1980s and 1990s and rely on the enzymatic conversion of non-active prodrugs to active metabolites that lead to cell death. Although considerable effort and ingenuity has gone into vector design for gene therapy, less attention has been directed at the efficacy or associated adverse effects of the prodrug systems employed. In this review, we discuss prodrug systems employed in clinical trials and consider their role in the field of gene therapy. We highlight potential drawbacks associated with the use of specific prodrugs, such as systemic toxicity of the activated compound, the paucity of data on biodistribution of prodrugs, bystander effects, and destruction of genetically modified cells, and how these can inform future advances in cell therapies.

Combination of Subtherapeutic Doses of Tretazicar and Liposomal Amphotericin B Suppresses and Cures Leishmania major-Induced Cutaneous Lesions in Murine Models

Cutaneous leishmaniasis (CL) is the most common form of leishmaniasis affecting human populations, yet CL remains largely ignored in drug discovery programs. CL causes disfiguring skin lesions and often relapses after "clinical cure" using existing therapeutics. To expand the pool of anti-CL lead candidates, we implemented an integrated screening platform comprising three progressive Leishmania parasite life cycle forms. We identified tretazicar (CB1954, 5-(aziridin-1-yl)-2,4-dinitrobenzamide) as a potent inhibitor of Leishmania parasite viability across multiple Leishmania species, which translated into complete and prolonged in vivo suppression of CL lesion formation in BALB/c mice when used as a monotherapy and which was superior to liposomal amphotericin B. In addition, oral twice a day administration of tretazicar healed the majority of existing Leishmania major (L. major) cutaneous lesions. In drug combination studies, there was a strong potentiation when subtherapeutic doses of liposomal amphotericin B and tretazicar were simultaneously administered. This drug combination decreased L. major lesion size in mice earlier than individual monotherapy drug treatments and maintained all animals lesion free for up to 64 days after treatment cessation. In contrast, administration of subtherapeutic doses of tretazicar or amphotericin B as monotherapies resulted in no or partial lesion cures, respectively. We propose that tretazicar should be explored as a component of a systemic CL combination therapy and potentially for other diseases where amphotericin B is a first line therapy.

Enhanced expression of coxsackievirus and adenovirus receptor in lipopolysaccharide-induced inflammatory macrophages is through TRIF-dependent innate immunity pathway

AIMS

Inflammatory macrophages have been proposed as a therapeutic target for joint disorders caused by inflammation. This study aimed to investigate the expression and regulation of coxsackievirus-adenovirus receptor (CAR) in lipopolysaccharide (LPS)-stimulated inflammatory macrophages whereby to evaluate the feasibility of virus-directed enzyme prodrug therapy (VDEPT).

MAIN METHODS

Macrophage cell lines (RAW264.7 and J774A.1) and primary macrophage cells derived from rat spleen were used to evaluate the expression of CAR protein or CAR mRNA. Specific inhibitors for TLR4 pathway were used to investigate the regulation of CAR expression. CAR expression in rat joints was documented by immunohistochemistry. Conditionally replicating adenovirus, CRAd-EGFP(PS1217L) or CRAd-NTR(PS1217H6), and non-replicating adenovirus CTL102 were used to transduce genes for enhanced green fluorescent protein (EGFP) or nitroreductase (NTR), respectively. The expression of EGFP, NTR, and the toxicity induced by CB1954 activation were evaluated.

KEY FINDINGS

The in vitro experiments revealed that CAR upregulation was mediated through the TLR4/TRIF/IRF3 pathway in LPS-stimulated inflammatory macrophage RAW264.7 and J774A.1 cells. The inflammatory RAW264.7 cells upregulated CAR expression following LPS stimulation, leading to higher infectability, increased NTR expression, and enhanced sensitization to CB1954. In animal experiments, the induction of CAR expression was observed in the CD68-expressing primary macrophages and in the CD68-expressing macrophages within joints following LPS stimulation.

SIGNIFICANCE

In conclusion, we report an enhanced CAR expression in inflammatory macrophages in vitro and in vivo through the immune response elicited by LPS. Thus, the TLR4/TRIF/IRF3 pathway of macrophages, when activated, could facilitate the therapeutic application of adenovirus-mediated VDEPT.

Microbial nitroreductases: A versatile tool for biomedical and environmental applications

Nitroreductases, enzymes found mostly in bacteria and also in few eukaryotes, use nicotinamide adenine dinucleotide (NADH) or nicotinamide adenine dinucleotide phosphate (NADPH) as a cofactor for their activity and metabolize an enormous list of a diverse nitro group-containing compounds. Nitroreductases that are capable of metabolizing nitroaromatic and nitro heterocyclic compounds have drawn great attention in recent years owing to their biotechnological, biomedical, environmental, and human impact. These enzymes attracted medicinal chemists and pharmacologists because of their prodrug selectivity for activation/reduction of nitro compounds that wipe out pathogens/cancer cells, leaving the host/normal cells unharmed. It is applied in diverse fields of study like prodrug activation in treating cancer and leishmaniasis, designing fluorescent probes for hypoxia detection, cell imaging, ablation of specific cell types, biodegradation of nitro-pollutants, and interpretation of mutagenicity of nitro compounds. Keeping in view the immense prospects of these enzymes and a large number of research contributions in this area, the present review encompasses the enzymatic reaction mechanism, their role in antibiotic resistance, hypoxia sensing, cell imaging, cancer therapy, reduction of recalcitrant nitro chemicals, enzyme variants, and their specificity to substrates, reaction products, and their applications.

Evolutionary aspects of the Viridiplantae nitroreductases

Background

Nitroreductases are a family of evolutionarily related proteins catalyzing the reduction of nitro-substituted compounds. Nitroreductases are widespread enzymes, but nearly all modern research and practical application have been concentrated on the bacterial proteins, mainly nitroreductases of Escherichia coli. The main aim of this study is to describe the phylogenic distribution of the nitroreductases in the photosynthetic eukaryotes (Viridiplantae) to highlight their structural similarity and areas for future research and application.

Results

This study suggests that homologs of nitroreductase proteins are widely presented also in Viridiplantae. Maximum likelihood phylogenetic tree reconstruction method and comparison of the structural models suggest close evolutional relation between cyanobacterial and Viridiplantae nitroreductases.

Conclusions

This study provides the first attempt to understand the evolution of nitroreductase protein family in Viridiplantae. Our phylogeny estimation and preservation of the chloroplasts/mitochondrial localization indicate the evolutional origin of the plant nitroreductases from the cyanobacterial endosymbiont. A defined high level of the similarity on the structural level suggests conservancy also for the functions. Directions for the future research and industrial application of the Viridiplantae nitroreductases are discussed.

Evaluation of two xenobiotic reductases from Pseudomonas putida for their suitability for magnetic nanoparticle-directed enzyme prodrug therapy as a novel approach to cancer treatment

Directed enzyme prodrug therapy (DEPT) is a cancer chemotherapy strategy in which bacterial enzymes are delivered to a cancer site before prodrug administration, resulting in prodrug activation at the cancer site and more localized treatment. A major limitation to DEPT is the poor effectiveness of the most studied enzyme for the CB1954 prodrug, NfnB from Escherichia coli, at concentrations suitable for human use. Much research into finding alternative enzymes to NfnB has resulted in the identification of the Xenobiotic reductases, XenA and XenB, which have been shown in the literature to reduce environmentally polluting nitro‐compounds. In this study, they were assessed for their potential use in cancer prodrug therapy strategies. Both proteins were cloned into the pET28a+ expression vector to give the genetically modified proteins XenA‐his and XenB‐his, of which only XenB‐his was active when tested with CB1954. XenB‐his was further modified to include a cysteine‐tag to facilitate direct immobilization on to a gold surface for future magnetic nanoparticle DEPT (MNDEPT) treatments and was named XenB‐cys. When tested using high‐performance liquid chromatography (HPLC), XenB‐his and XenB‐cys both demonstrated a preference for reducing CB1954 at the 4‐nitro position. Furthermore, XenB‐his and XenB‐cys successfully induced cell death in SK‐OV‐3 cells when combined with CB1954. This led to XenB‐cys being identified as a promising candidate for use in future MNDEPT treatments.

Directed evolution of the B. subtilis nitroreductase YfkO improves activation of the PET-capable probe SN33623 and CB1954 prodrug

OBJECTIVES

To use directed evolution to improve YfkO-mediated reduction of the 5-nitroimidazole PET-capable probe SN33623 without impairing conversion of the anti-cancer prodrug CB1954.

RESULTS

Two iterations of error-prone PCR, purifying selection, and FACS sorting in a DNA damage quantifying GFP reporter strain were used to identify three YfkO variants able to sensitize E. coli host cells to at least 2.4-fold lower concentrations of SN33623 than the native enzyme. Two of these variants were able to be purified in a functional form, and in vitro assays revealed these were twofold and fourfold improved in k_cat/K_M with SN33623 over wild type YfkO. Serendipitously, the more-active variant was also nearly fourfold improved in k_cat/K_M versus wild type YfkO in converting CB1954 to a genotoxic drug.

CONCLUSIONS

The enhanced activation of the PET imaging probe SN33623 and CB1954 prodrug exhibited by the lead evolved variant of YfkO offers prospects for improved enzyme-prodrug therapy.

Extracellular Vesicle-Mediated In Vitro Transcribed mRNA Delivery for Treatment of HER2+ Breast Cancer Xenografts in Mice by Prodrug CB1954 without General Toxicity

Prodrugs are harmless until activated by a bacterial or viral gene product; they constitute the basis of gene-delivered prodrug therapies called GDEPT, which can kill tumors without major side effects. Previously, we utilized the prodrug CNOB (C₁₆H₇CIN₂O₄; not clinically tested) and enzyme HChrR6 in GDEPT to generate the drug MCHB (C₁₆H₉CIN₂O₂) in tumors. Extracellular vesicles (EVs) were used for directed gene delivery and HChrR6 mRNA as gene. Here, the clinical transfer of this approach is enhanced by: (i) use of CB1954 (tretazicar) for which safe human dose is established; HChrR6 can activate this prodrug. (ii) EVs delivered in vitro transcribed (IVT) HChrR6 mRNA, eliminating the potentially harmful plasmid transfection of EV producer cells we utilized previously; this has not been done before. IVT mRNA loading of EVs required several steps. Naked mRNA being unstable, we ensured its prodrug activating functionality at each step. This was not possible using tretazicar itself; we relied instead on HChrR6's ability to convert CNOB into MCHB, whose fluorescence is easily visualizable. HChrR6 mRNA-translated product's ability to generate fluorescence from CNOB vicariously indicated its competence for tretazicar activation. (iii) Systemic IVT mRNA-loaded EVs displaying an anti-HER2 single-chain variable fragment ("IVT EXO-DEPTs") and tretazicar caused growth arrest of human HER2⁺ breast cancer xenografts in athymic mice. As this occurred without injury to other tissues, absence of off-target mRNA delivery is strongly indicated. Many cancer sites are not amenable for direct gene injection, but current GDEPTs require this. In circumventing this need, a major advance in GDEPT applicability has been accomplished.

Cell-Penetrating Peptides as a Tool for the Cellular Uptake of a Genetically Modified Nitroreductase for use in Directed Enzyme Prodrug Therapy

Directed enzyme prodrug therapy (DEPT) involves the delivery of a prodrug-activating enzyme to a solid tumour site, followed by the subsequent activation of an administered prodrug. One of the most studied enzyme-prodrug combinations is the nitroreductase from Escherichia coli (NfnB) with the prodrug CB1954 [5-(aziridin-1-yl)-2,4-dinitro-benzamide]. One of the major issues faced by DEPT is the ability to successfully internalize the enzyme into the target cells. NfnB has previously been genetically modified to contain cysteine residues (NfnB-Cys) which bind to gold nanoparticles for a novel DEPT therapy called magnetic nanoparticle directed enzyme prodrug therapy (MNDEPT). One cellular internalisation method is the use of cell-penetrating peptides (CPPs), which aid cellular internalization of cargo. Here the cell-penetrating peptides: HR9 and Pep-1 were tested for their ability to conjugate with NfnB-Cys. The conjugates were further tested for their potential use in MNDEPT, as well as conjugating with the delivery vector intended for use in MNDEPT and tested for the vectors capability to penetrate into cells.

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.

Magnetic Functionalized Nanoparticles for Biomedical, Drug Delivery and Imaging Applications

Medicine is constantly looking for new and improved treatments for diseases, which need to have a high efficacy and be cost-effective, creating a large demand on scientific research to discover such new treatments. One important aspect of any treatment is the ability to be able to target only the illness and not cause harm to another healthy part of the body. For this reason, metallic nanoparticles have been and are currently being extensively researched for their possible medical uses, including medical imaging, antibacterial and antiviral applications. Superparamagnetic metal nanoparticles possess properties that allow them to be directed around the body with a magnetic field or directed to a magnetic implant, which opens up the potential to conjugate various bio-cargos to the nanoparticles that could then be directed for treatment in the body. Here we report on some of the current bio-medical applications of various metal nanoparticles, including single metal nanoparticles, functionalized metal nanoparticles, and core-shell metal nanoparticles using a core of Fe₃O₄ as well as synthesis methods of these core-shell nanoparticles.

Hypoxia-targeted drug delivery

Hypoxia is a state of low oxygen tension found in numerous solid tumours. It is typically associated with abnormal vasculature, which results in a reduced supply of oxygen and nutrients, as well as impaired delivery of drugs. The hypoxic nature of tumours often leads to the development of localized heterogeneous environments characterized by variable oxygen concentrations, relatively low pH, and increased levels of reactive oxygen species (ROS). The hypoxic heterogeneity promotes tumour invasiveness, metastasis, angiogenesis, and an increase in multidrug-resistant proteins. These factors decrease the therapeutic efficacy of anticancer drugs and can provide a barrier to advancing drug leads beyond the early stages of preclinical development. This review highlights various hypoxia-targeted and activated design strategies for the formulation of drugs or prodrugs and their mechanism of action for tumour diagnosis and treatment.

Time dependent HPLC analysis of the product ratio of enzymatically reduced prodrug CB1954 by a modified and immobilised nitroreductase

Directed enzyme prodrug therapy is a chemotherapy strategy that utilises prodrug-activating enzymes to activate prodrugs at the tumour location, thus reducing off-target effects. The most commonly investigated enzyme for use with the CB1954 prodrug is the NfnB nitroreductase from E. coli. Literature states that CB1954 is reduced by NfnB at the 2- or 4-position at a 1:1 ratio; deviation from this ratio has been observed in the literature, but not further investigated. The kinetic parameters for the genetically-modified enzymes; NfnB-his, NfnB-cys and AuNP-NfnB-cys were assessed and HPLC analysis was used to determine the hydroxylamine product ratios formed when reacted with CB1954. Time-dependent HPLC studies were carried out to assess how this ratio changes over time. It was shown that the hydroxylamine ratio formed by the reduction of CB1954 by a nitroreductase changes over time and that this change in ratio relates directly to the kinetics of the reaction. Thus, the hydroxylamine ratio measured using HPLC at a given time point was not a true indication of the preference of the nitroreductase enzymes during catalysis. These results question how nitroreductases are evaluated in terms of the hydroxylamine ratio and it is suspected that this phenomenon may also apply to other enzyme/prodrug combinations.

Evaluating the abilities of diverse nitroaromatic prodrug metabolites to exit a model Gram negative vector for bacterial-directed enzyme-prodrug therapy

Gene-directed enzyme-prodrug therapy (GDEPT) employs tumour-tropic vectors including viruses and bacteria to deliver a genetically-encoded prodrug-converting enzyme to the tumour environment, thereby sensitising the tumour to the prodrug. Nitroreductases, able to activate a range of promising nitroaromatic prodrugs to genotoxic metabolites, are of great interest for GDEPT. The bystander effect (cell-to-cell transfer of activated prodrug metabolites) has been quantified for some nitroaromatic prodrugs in mixed multilayer human cell cultures, however while these provide a good model for viral DEPT (VDEPT) they do not inform on the ability of these prodrug metabolites to exit bacterial vectors (relevant to bacterial-DEPT (BDEPT)). To investigate this we grew two Escherichia coli strains in co-culture; an activator strain expressing the nitroreductase E. coli NfsA and a recipient strain containing an SOS-GFP DNA damage responsive gene construct. In this system, induction of GFP by reduced prodrug metabolites can only occur following their transfer from the activator to the recipient cells. We used this to investigate five clinically relevant prodrugs: metronidazole, CB1954, nitro-CBI-DEI, and two dinitrobenzamide mustard prodrug analogues, PR-104A and SN27686. Consistent with the bystander efficiencies previously measured in human cell multilayers, reduced metronidazole exhibited little bacterial cell-to-cell transfer, whereas nitro-CBI-DEI was passed very efficiently from activator to recipient cells post-reduction. However, in contrast with observations in human cell multilayers, the nitrogen mustard prodrug metabolites were not effectively passed between the two bacterial strains, whereas reduced CB1954 was transferred efficiently. Using nitroreductase enzymes that exhibit different biases for the 2- versus 4-nitro substituents of CB1954, we further showed that the 2-nitro reduction products exhibit substantially higher levels of bacterial cell-to-cell transfer than the 4-nitro reduction products, consistent with their relative bystander efficiencies in human cell culture. Overall, our data suggest that prodrugs may differ in their suitability for VDEPT versus BDEPT applications and emphasise the importance of evaluating an enzyme-prodrug partnership in an appropriate context for the intended vector.

Anti-HER2 scFv-Directed Extracellular Vesicle-Mediated mRNA-Based Gene Delivery Inhibits Growth of HER2-Positive Human Breast Tumor Xenografts by Prodrug Activation

This paper deals with specific targeting of the prodrug/enzyme regimen, CNOB/HChrR6, to treat a serious disease, namely HER2⁺ human breast cancer with minimal off-target toxicity. HChrR6 is an improved bacterial enzyme that converts CNOB into the cytotoxic drug MCHB. Extracellular vesicles (EV) were used for mRNA-based HchrR6 gene delivery: EVs may cause minimal immune rejection, and mRNA may be superior to DNA for gene delivery. To confine HChrR6 generation and CNOB activation to the cancer, the EVHB chimeric protein was constructed. It contains high-affinity anti-HER2 scFv antibody (ML39) and is capable of latching on to EV surface. Cells transfected with EVHB-encoding plasmid generated EVs displaying this protein ("directed EVs"). Transfection of a separate batch of cells with the new plasmid, XPort/HChrR6, generated EVs containing HChrR6 mRNA; incubation with pure EVHB enabled these to target the HER2 receptor, generating "EXO-DEPT" EVs. EXO-DEPT treatment specifically enabled HER2-overexpressing BT474 cells to convert CNOB into MCHB in actinomycin D-independent manner, showing successful and specific delivery of HChrR6 mRNA. EXO-DEPTs-but not undirected EVs-plus CNOB caused near-complete growth arrest of orthotopic BT474 xenografts in vivo, demonstrating for the first time EV-mediated delivery of functional exogenous mRNA to tumors. EXO-DEPTs may be generated from patients' own dendritic cells to evade immune rejection, and without plasmids and their potentially harmful genetic material, raising the prospect of clinical use of this regimen. This approach can be used to treat any disease overexpressing a specific marker. Mol Cancer Ther; 17(5); 1133-42. ©2018 AACR.

Limb-bud and Heart Overexpression Inhibits the Proliferation and Migration of PC3M Cells

Background: The limb-bud and heart gene (LBH) was discovered in the early 21st century and is specifically expressed in the mouse embryonic limb and heart development. Increasing evidences have indicated that LBH not only plays an important role in embryo development, it is also closely correlated with the occurance and progression of many tumors. However, its function in prostate cancer (PCa) is still not well understood. Here, we explored the effects of LBH on the proliferation and migration of the PCa cell line PC3M. Methods: LBH expression in tissues and cell lines of PCa was detected by immunohistochemistry and Western blotting. Lentivirus was used to transduct the LBH gene into the PC3M cells. Stable LBH-overexpressing PC3M-LBH cells and PC3M-NC control cells were obtained via puromycin screening. Cell proliferation was examined using the 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) assay. Cell cycle distribution and apoptosis rate were investigated using flow cytometry. Cell migration was studied using the Transwell assay. Results: LBH expression level was down-regulated in 3 different PCa cell lines, especially in PC3M cells, compared with the normal prostate epithelial cells(RWPE-1). Cell lines of LBH-upregulated PC3M-LBH and PC3M-NC control were successfully constructed. Significantly increased LBH expression level and decreased cyclin D1 and cyclin E2 expression level was found in PC3M-LBH cells as compared to the PC3M-NC cells. The overexpression of LBH significantly inhibited PC3M cell proliferation in vitro and tumor growth in nude mice. LBH overexpression in PC3M cell, also induced cell cycle G0/G1 phase arrest and decreased the migration of PC3M cells. Conclusions: Our results reveal that LBH expression is down-regulated in the tissue and cell lines of PCa. LBH overexpression inhibits PC3M cell proliferation and tumor growth by inducing cell cycle arrest through down-regulating cyclin D1and cyclin E2 expression. LBH might be a therapeutic target and potential diagnostic marker in PCa.

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.

Enzyme/Prodrug Systems for Cancer Gene Therapy

The use of enzyme/prodrug system has gained attention because it could help improve the efficacy and safety of conventional cancer chemotherapies. In this approach, cancer cells are first transfected with a gene that can express an enzyme with ability to convert a non-toxic prodrug into its active cytotoxic form. As a result, the activated prodrug could kill the transfected cancer cells. Despite the significant progress of different suicide gene therapy protocols in preclinical studies and early clinical trials, none has reached the clinic due to several shortcomings. These include slow prodrug-drug conversion rate, low transfection/transduction efficiency of the vectors and nonspecific toxicity/immunogenicity related to the delivery systems, plasmid DNA, enzymes and/or prodrugs. This mini review aims at providing an overview of the most widely used enzyme/prodrug systems with emphasis on reporting the results of the recent preclinical and clinical studies.

Azoreductases in drug metabolism

Azoreductases are flavoenzymes that have been characterized in a range of prokaryotes and eukaryotes. Bacterial azoreductases are associated with the activation of two classes of drug, azo drugs for the treatment of inflammatory bowel disease and nitrofuran antibiotics. The mechanism of reduction of azo compounds is presented; it requires tautomerisation of the azo compound to a quinoneimine and provides a unifying mechanism for the reduction of azo and quinone substrates by azoreductases. The importance of further work in the characterization of azoreductases from enteric bacteria is highlighted to aid in the development of novel drugs for the treatment of colon related disorders. Human azoreductases are known to play a crucial role in the metabolism of a number of quinone-containing cancer chemotherapeutic drugs. The mechanism of hydride transfer to quinones, which is shared not only between eukaryotic and prokaryotic azoreductases but also the wider family of NAD(P)H quinone oxidoreductases, is outlined. The importance of common single nucleotide polymorphisms (SNPs) in human azoreductases is described not only in cancer prognosis but also with regard to their effects on the efficacy of quinone drug-based cancer chemotherapeutic regimens. This highlights the need to screen patients for azoreductase SNPs ahead of treatment with these regimens.

LINKED ARTICLES

This article is part of a themed section on Drug Metabolism and Antibiotic Resistance in Micro-organisms. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.14/issuetoc.

Utilizing native fluorescence imaging, modeling and simulation to examine pharmacokinetics and therapeutic regimen of a novel anticancer prodrug

Background

Success of cancer prodrugs relying on a foreign gene requires specific delivery of the gene to the cancer, and improvements such as higher level gene transfer and expression. Attaining these objectives will be facilitated in preclinical studies using our newly discovered CNOB-GDEPT, consisting of the produrg: 6-chloro-9-nitro-5-oxo-5H-benzo-(a)-phenoxazine (CNOB) and its activating enzyme ChrR6, which generates the cytotoxic product 9-amino-6-chloro-5H-benzo[a]phenoxazine-5-one (MCHB). MCHB is fluorescent and can be noninvasively imaged in mice, and here we investigated whether MCHB fluorescence quantitatively reflects its concentration, as this would enhance its reporter value in further development of the CNOB-GDEPT therapeutic regimen. PK parameters were estimated and used to predict more effective CNOB administration schedules.

Methods

CNOB (3.3 mg/kg) was injected iv in mice implanted with humanized ChrR6 (HChrR6)-expressing 4T1 tumors. Fluorescence was imaged in live mice using IVIS Spectrum, and quantified by Living Image 3.2 software. MCHB and CNOB were quantified also by LC/MS/MS analysis. We used non-compartmental model to estimate PK parameters. Phoenix WinNonlin software was used for simulations to predict a more effective CNOB dosage regimen.

Results

CNOB administration significantly prolonged mice survival. MCHB fluorescence quantitatively reflected its exposure levels to the tumor and the plasma, as verified by LC/MS/MS analysis at various time points, including at a low concentration of 2 ng/g tumor. The LC/MS/MS data were used to estimate peak plasma concentrations, exposure (AUC_0-24), volume of distribution, clearance and half-life in plasma and the tumor. Simulations suggested that the CNOB-GDEPT can be a successful therapy without large increases in the prodrug dosage.

Conclusion

MCHB fluorescence quantifies this drug, and CNOB can be effective at relatively low doses. MCHB fluorescence characteristics will expedite further development of CNOB-GDEPT by, for example, facilitating specific gene delivery to the tumor, its prolonged expression, as well as other attributes necessary for successful gene-delivered enzyme prodrug therapy.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-016-2508-6) contains supplementary material, which is available to authorized users.

Oncolytic adenovirus-mediated therapy for prostate cancer

Prostate cancer is a leading cause of cancer-related death and morbidity in men in the Western world. Tumor progression is dependent on functioning androgen receptor signaling, and initial administration of antiandrogens and hormone therapy (androgen-deprivation therapy) prevent growth and spread. Tumors frequently develop escape mechanisms to androgen-deprivation therapy and progress to castration-resistant late-stage metastatic disease that, in turn, inevitably leads to resistance to all current therapeutics, including chemotherapy. In spite of the recent development of more effective inhibitors of androgen–androgen receptor signaling such as enzalutamide and abiraterone, patient survival benefits are still limited. Oncolytic adenoviruses have proven efficacy in prostate cancer cells and cause regression of tumors in preclinical models of numerous drug-resistant cancers. Data from clinical trials demonstrate that adenoviral mutants have limited toxicity to normal tissues and are safe when administered to patients with various solid cancers, including prostate cancer. While efficacy in response to adenovirus administration alone is marginal, findings from early-phase trials targeting local-ized and metastatic prostate cancer suggest improved efficacy in combination with cytotoxic drugs and radiation therapy. Here, we review recent progress in the development of multimodal oncolytic adenoviruses as biological therapeutics to improve on tumor elimination in prostate cancer patients. These optimized mutants target cancer cells by several mechanisms including viral lysis and by expression of cytotoxic transgenes and immune-stimulatory factors that activate the host immune system to destroy both infected and noninfected prostate cancer cells. Additional modifications of the viral capsid proteins may support future systemic delivery of oncolytic adenoviruses.

Advancing Clostridia to Clinical Trial: Past Lessons and Recent Progress

Most solid cancers contain regions of necrotic tissue. The extent of necrosis is associated with poor survival, most likely because it reflects aggressive tumour outgrowth and inflammation. Intravenously injected spores of anaerobic bacteria from the genus Clostridium infiltrate and selectively germinate in these necrotic regions, providing cancer-specific colonisation. The specificity of this system was first demonstrated over 60 years ago and evidence of colonisation has been confirmed in multiple tumour models. The use of "armed" clostridia, such as in Clostridium Directed Enzyme Prodrug Therapy (CDEPT), may help to overcome some of the described deficiencies of using wild-type clostridia for treatment of cancer, such as tumour regrowth from a well-vascularised outer rim of viable cells. Successful preclinical evaluation of a transferable gene that metabolises both clinical stage positron emission tomography (PET) imaging agents (for whole body vector visualisation) as well as chemotherapy prodrugs (for conditional enhancement of efficacy) would be a valuable early step towards the prospect of "armed" clostridia entering clinical evaluation. The ability to target the immunosuppressive hypoxic tumour microenvironment using CDEPT may offer potential for synergy with recently developed immunotherapy strategies. Ultimately, clostridia may be most efficacious when combined with conventional therapies, such as radiotherapy, that sterilise viable aerobic tumour cells.

Progress and problems with the use of suicide genes for targeted cancer therapy

Among various gene therapy methods for cancer, suicide gene therapy attracts a special attention because it allows selective conversion of non-toxic compounds into cytotoxic drugs inside cancer cells. As a result, therapeutic index can be increased significantly by introducing high concentrations of cytotoxic molecules to the tumor environment while minimizing impact on normal tissues. Despite significant success at the preclinical level, no cancer suicide gene therapy protocol has delivered the desirable clinical significance yet. This review gives a critical look at the six main enzyme/prodrug systems that are used in suicide gene therapy of cancer and familiarizes readers with the state-of-the-art research and practices in this field. For each enzyme/prodrug system, the mechanisms of action, protein engineering strategies to enhance enzyme stability/affinity and chemical modification techniques to increase prodrug kinetics and potency are discussed. In each category, major clinical trials that have been performed in the past decade with each enzyme/prodrug system are discussed to highlight the progress to date. Finally, shortcomings are underlined and areas that need improvement in order to produce clinical significance are delineated.

Knockdown of PSCA induces EMT and decreases metastatic potentials of the human prostate cancer DU145 cells

Background

Prostate stem cell antigen (PSCA) expression has been shown to correlate with prostatic carcinogenesis and prostate cancer (PCa) progression. The underlying mechanisms for these processes are currently unknown. Epithelial to mesenchymal transition (EMT) has been associated with the invasiveness and the distant metastasis of PCa. In this study, we investigated the effects of knocking down the PSCA on the cell migration, the invasiveness, and the EMT of the PCa cell line DU145 in vitro and in vivo.

Methods

Four target sequences of the small hairpin RNA for PSCA were designed, and the best effect knockdown sequence shRNA#1 was screened to construct the stable transfected DU145 cell line (DU145 shRNA#1), the scramble sequence was also designed to construct the stable transfected DU145 cell line(DU145 scramble). Cell migration and invasion were studied using Transwell assay. Quantitative RT-PCR, Western blot (WB) were used to quantify PSCA, E-cadherin, β-catenin, Vimentin, Fibronectin expression in DU145, DU145 scramble, DU145 shRNA#1 in vitro and in vivo. RT-PCR, immunofluorescent staining were used to quantify PSCA, E-cadherin, and Vimentin expression in vitro. EMT-related genes Snail, Slug, and Twist, were quantified by quantitative RT-PCR in vitro.

Results

The constructed stable knockdown of the PSCA in the DU145 cell had a silencing effect up to 90.5 %. DU145 shRNA#1 became scattered from the tightly packed colonies. It was associated with decreased cell migration and invasion. There was also an increased Vimentin and Fibronectin expression, an inhibited E-cadherin and β-catenin expression at both the mRNA and the protein levels when compared to the DU145 and the DU145 scramble in vitro and vivo. Furthermore, with the exception of the Snail, the expression of EMT-related Slug and Twist genes were upregulated.

Conclusions

Our data indicated that knockdown of PSCA induced EMT and reduced metastatic potentials of the DU145 cells, suggesting that PSCA played an important role in prostatic carcinogenesis and progression.

Rapid fluorescence detection of hypoxic microenvironments by nitro-benzyl conjugated chitosan nanoparticles encapsulating hydrophobic fluorophores

Hypoxia-responsive chitosan nanoparticles synthesized by the conjugation of nitro-benzyl derivatives into chitosan polymers and the subsequent self-assembly of them with hydrophobic fluorophores, were successfully applied for the rapid determination of the hypoxic status of lung carcinoma cells (A549) within 30 min.

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.

Identification of novel nitroreductases from Bacillus cereus and their interaction with the CB1954 prodrug

Directed enzyme prodrug therapy is a form of cancer chemotherapy in which bacterial prodrug-activating enzymes, or their encoding genes, are directed to the tumour before administration of a prodrug. The prodrug can then be activated into a toxic drug at the tumour site, reducing off-target effects. The bacterial nitroreductases are a class of enzymes used in this therapeutic approach and although very promising, the low turnover rate of prodrug by the most studied nitroreductase enzyme, NfnB from Escherichia coli (NfnB_Ec), is a major limit to this technology. There is a continual search for enzymes with greater efficiency, and as part of the search for more efficient bacterial nitroreductase enzymes, two novel enzymes from Bacillus cereus (strain ATCC 14579) have been identified and shown to reduce the CB1954 (5-(aziridin-1-yl)-2,4-dinitrobenzamide) prodrug to its respective 2'-and 4'-hydroxylamine products. Both enzymes shared features characteristic of the nitro-FMN-reductase superfamily including non-covalently associated FMN, requirement for the NAD(P)H cofactor, homodimeric, could be inhibited by Dicoumarol (3,3'-methylenebis(4-hydroxy-2H-chromen-2-one)), and displayed ping pong bi bi kinetics. Based on the biochemical characteristics and nucleotide alignment with other nitroreductase enzymes, one enzyme was named YdgI_Bc and the other YfkO_Bc. Both B. cereus enzymes had greater turnover for the CB1954 prodrug compared with NfnB_Ec, and in the presence of added NADPH cofactor, YfkO_Bc had superior cell killing ability, and produced mainly the 4'-hydroxylamine product at low prodrug concentration. The YfkO_Bc was identified as a promising candidate for future enzyme prodrug therapy.

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.

Inhibition of human diffuse large B-cell lymphoma growth by JC polyomavirus-like particles delivering a suicide gene

Background

Diffuse large B-cell lymphoma (DLBCL) is one of the most common types of aggressive B-cell non-Hodgkin lymphoma. About one-third of patients are either refractory to the treatment or experience relapse afterwards, pointing to the necessity of developing other effective therapies for DLBCL. Human B-lymphocytes are susceptible to JC polyomavirus (JCPyV) infection, and JCPyV virus-like particles (VLPs) can effectively deliver exogenous genes to susceptible cells for expression, suggesting the feasibility of using JCPyV VLPs as gene therapy vectors for DLBCL.

Methods

The JCPyV VLPs packaged with a GFP reporter gene were used to infect human DLBCL cells for gene delivery assay. Furthermore, we packaged JCPyV VLPs with a suicide gene encoding thymidine kinase (TK) to inhibit the growth of DLBCL in vitro and in vivo.

Results

Here, we show that JCPyV VLPs effectively entered human germinal center B-cell-like (GCB-like) DLBCL and activated B-cell-like (ABC-like) DLBCL and expressed the packaged reporter gene in vitro. As measured by the MTT assay, treatment with tk-VLPs in combination with gancyclovir (GCV) reduced the viability of DLBCL cells by 60%. In the xenograft mouse model, injection of tk-VLPs through the tail vein in combination with GCV administration resulted in a potent 80% inhibition of DLBCL tumor nodule growth.

Conclusions

Our results demonstrate the effectiveness of JCPyV VLPs as gene therapy vectors for human DLBCL and provide a potential new strategy for the treatment of DLBCL.

MLF1 interacting protein: a potential gene therapy target for human prostate cancer?

Here, we investigated the role of one gene that has been previously associated with human prostate carcinoma cells-myelodysplasia/myeloid leukemia factor 1 interacting protein (MLF1IP)-in order to better ascertain its role in human prostate carcinogenesis. The prostate cancer cell line PC-3 was lentivirally transfected to silence endogenous MLF1IP gene expression, which was confirmed by real-time quantitative PCR (RT-qPCR). Cellomics ArrayScan VTI imaging and MTT assays were conducted to assess cell proliferation. Cell cycle phase arrest and apoptosis were assayed by flow cytometry. Colony formation was assessed by fluorescence microscopy. MLF1IP gene expression was also analyzed by RT-qPCR in sixteen prostate cancer tissue samples and six healthy control prostate tissue samples from human patients. Cell proliferation was significantly inhibited in MLF1IP-silenced cells relative to control cells. G1 phase, S and G2/M phase cell counts were not significantly changed in MLF1IP-silenced cells relative to control cells. Apoptosis was significantly increased in MLF1IP-silenced cells, while MLF1IP-silenced cells displayed a significantly reduced number of cell colonies, compared to control cells. The 16 human prostate cancer tissue samples revealed no clear upregulation or downregulation in MLF1IP gene expression. MLF1IP significantly promotes prostate cancer cell proliferation and colony formation and significantly inhibits apoptosis without affecting cell cycle phase arrest. Further study is required to conclusively determine whether MLF1IP is upregulated in human prostate cancer tumors and to determine the precise cellular mechanism(s) for MLF1IP in prostate carcinogenesis.

Gene-directed enzyme prodrug therapy

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

Targeted prodrug approaches for hormone refractory prostate cancer

Due to the propensity of relapse and resistance with prolonged androgen deprivation therapy (ADT), there is a growing interest in developing non-hormonal therapeutic approaches as alternative treatment modalities for hormone refractory prostate cancer (HRPC). Although the standard treatment for HRPC consists of a combination of ADT with taxanes and anthracyclines, the clinical use of chemotherapeutics is limited by systemic toxicity stemming from nondiscriminatory drug exposure to normal tissues. In order to improve the tumor selectivity of chemotherapeutics, various targeted prodrug approaches have been explored. Antibody-directed enzyme prodrug therapy (ADEPT) and gene-directed enzyme prodrug therapy (GDEPT) strategies leverage tumor-specific antigens and transcription factors for the specific delivery of cytotoxic anticancer agents using various prodrug-activating enzymes. In prostate cancer, overexpression of tumor-specific proteases such as prostate-specific antigen (PSA) and prostate-specific membrane antigen (PSMA) is being exploited for selective activation of anticancer prodrugs designed to be activated through proteolysis by these prostate cancer-specific enzymes. PSMA- and PSA-activated prodrugs typically comprise an engineered high-specificity protease peptide substrate coupled to a potent cytotoxic agent via a linker for rapid release of cytotoxic species in the vicinity of prostate cancer cells following proteolytic cleavage. Over the past two decades, various such prodrugs have been developed and they were effective at inhibiting prostate tumor growth in rodent models; several of these prodrug approaches have been advanced to clinical trials and may be developed into effective therapies for HRPC.