Bioreductive GDEPT using cytochrome P450 3A4 in combination with AQ4N

The importance of hypoxia in radiotherapy for the immune response, metastatic potential and FLASH-RT

PURPOSE

Hypoxia (low oxygen) is a common feature of solid tumors that has been intensely studied for more than six decades. Here we review the importance of hypoxia to radiotherapy with a particular focus on the contribution of hypoxia to immune responses, metastatic potential and FLASH radiotherapy, active areas of research by leading women in the field.

CONCLUSION

Although hypoxia-driven metastasis and immunosuppression can negatively impact clinical outcome, understanding these processes can also provide tumor-specific vulnerabilities that may be therapeutically exploited. The different oxygen tensions present in tumors and normal tissues may underpin the beneficial FLASH sparing effect seen in normal tissue and represents a perfect example of advances in the field that can leverage tumor hypoxia to improve future radiotherapy treatments.

The Evolution and Future of Targeted Cancer Therapy: From Nanoparticles, Oncolytic Viruses, and Oncolytic Bacteria to the Treatment of Solid Tumors

While many classes of chemotherapeutic agents exist to treat solid tumors, few can generate a lasting response without substantial off-target toxicity despite significant scientific advancements and investments. In this review, the paths of development for nanoparticles, oncolytic viruses, and oncolytic bacteria over the last 20 years of research towards clinical translation and acceptance as novel cancer therapeutics are compared. Novel nanoparticle, oncolytic virus, and oncolytic bacteria therapies all start with a common goal of accomplishing therapeutic drug activity or delivery to a specific site while avoiding off-target effects, with overlapping methodology between all three modalities. Indeed, the degree of overlap is substantial enough that breakthroughs in one therapeutic could have considerable implications on the progression of the other two. Each oncotherapeutic modality has accomplished clinical translation, successfully overcoming the potential pitfalls promising therapeutics face. However, once studies enter clinical trials, the data all but disappears, leaving pre-clinical researchers largely in the dark. Overall, the creativity, flexibility, and innovation of these modalities for solid tumor treatments are greatly encouraging, and usher in a new age of pharmaceutical development.

Targeting Hypoxia: Hypoxia-Activated Prodrugs in Cancer Therapy

Hypoxia is an important characteristic of most solid malignancies, and is closely related to tumor prognosis and therapeutic resistance. Hypoxia is one of the most important factors associated with resistance to conventional radiotherapy and chemotherapy. Therapies targeting tumor hypoxia have attracted considerable attention. Hypoxia-activated prodrugs (HAPs) are bioreductive drugs that are selectively activated under hypoxic conditions and that can accurately target the hypoxic regions of solid tumors. Both single-agent and combined use with other drugs have shown promising antitumor effects. In this review, we discuss the mechanism of action and the current preclinical and clinical progress of several of the most widely used HAPs, summarize their existing problems and shortcomings, and discuss future research prospects.

Extrahepatic cytochrome P450 epoxygenases: pathophysiology and clinical significance in human gastrointestinal cancers

Cytochrome P450 (CYP) epoxygenases, a multi-gene superfamily of heme-containing enzymes, are commonly known to metabolize endogenous arachidonic acid (AA) to epoxyeicosatrienoic acids (EETs). The role of CYPs is mostly studied in liver drugs metabolism, cardiac pathophysiology, and hypertension fields. Particularly, the biological functions of these enzymes have increasingly attracted a growing interest in cancer biology. Most published studies on CYPs in cancer have been limited to their role as drug metabolizing systems. The activity of these enzymes may affect drug pharmacokinetics and bioavailability as well as exogenous compounds turnover. Some CYP isoforms are selectively highly expressed in tumors, suggesting a potential mechanistic role in promoting resistance to chemotherapy. Majority of drugs elicit their effects in extrahepatic tissues whereby their metabolism can significantly determine treatment outcome. Nonetheless, the role of extrahepatic CYPs is not fully understood and targeting these enzymes as effective anti-cancer therapies are yet to be developed. This review article summarizes an up-to-date body of information from published studies on CYP enzymes expression levels and pathophysiological functions in human normal and malignant gastrointestinal (GI) tract tissues. Specifically, we reviewed and discussed the current research initiatives by emphasizing on the clinical significance and the pathological implication of CYPs in GI malignancies of esophagus, stomach, and colon.

Human Family 1-4 cytochrome P450 enzymes involved in the metabolic activation of xenobiotic and physiological chemicals: an update

This is an overview of the metabolic activation of drugs, natural products, physiological compounds, and general chemicals by the catalytic activity of cytochrome P450 enzymes belonging to Families 1-4. The data were collected from > 5152 references. The total number of data entries of reactions catalyzed by P450s Families 1-4 was 7696 of which 1121 (~ 15%) were defined as bioactivation reactions of different degrees. The data were divided into groups of General Chemicals, Drugs, Natural Products, and Physiological Compounds, presented in tabular form. The metabolism and bioactivation of selected examples of each group are discussed. In most of the cases, the metabolites are directly toxic chemicals reacting with cell macromolecules, but in some cases the metabolites formed are not direct toxicants but participate as substrates in succeeding metabolic reactions (e.g., conjugation reactions), the products of which are final toxicants. We identified a high level of activation for three groups of compounds (General Chemicals, Drugs, and Natural Products) yielding activated metabolites and the generally low participation of Physiological Compounds in bioactivation reactions. In the group of General Chemicals, P450 enzymes 1A1, 1A2, and 1B1 dominate in the formation of activated metabolites. Drugs are mostly activated by the enzyme P450 3A4, and Natural Products by P450s 1A2, 2E1, and 3A4. Physiological Compounds showed no clearly dominant enzyme, but the highest numbers of activations are attributed to P450 1A, 1B1, and 3A enzymes. The results thus show, perhaps not surprisingly, that Physiological Compounds are infrequent substrates in bioactivation reactions catalyzed by P450 enzyme Families 1-4, with the exception of estrogens and arachidonic acid. The results thus provide information on the enzymes that activate specific groups of chemicals to toxic metabolites.

Hypoxia-active nanoparticles used in tumor theranostic

Hypoxia is a hallmark of malignant tumors and often correlates with increasing tumor aggressiveness and poor treatment outcomes. Therefore, early diagnosis and effective killing of hypoxic tumor cells are crucial for successful tumor control. There has been a surge of interdisciplinary research aimed at developing functional molecules and nanomaterials that can be used to noninvasively image and efficiently treat hypoxic tumors. These mainly include hypoxia-active nanoparticles, anti-hypoxia agents, and agents that target biomarkers of tumor hypoxia. Hypoxia-active nanoparticles have been intensively investigated and have demonstrated advanced effects on targeting tumor hypoxia. In this review, we present an overview of the reports published to date on hypoxia-activated prodrugs and their nanoparticle forms used in tumor-targeted therapy. Hypoxia-responsive nanoparticles are inactive during blood circulation and normal physiological conditions but are activated by hypoxia once they extravasate into the hypoxic tumor microenvironment. Their use can enhance the efficiency of tumor chemotherapy, radiotherapy, fluorescence and photoacoustic intensity, and other imaging and therapeutic strategies. By targeting the broad habitats of tumors, rather than tumor-specific receptors, this strategy has the potential to overcome the problem of tumor heterogeneity and could be used to design diagnostic and therapeutic nanoparticles for a broad range of solid tumors.

How Have Cancer Clinical Trial Eligibility Criteria Evolved Over Time?

Knowledge reuse of cancer trial designs may benefit from a temporal understanding of the evolution of the target populations of cancer studies over time. Therefore, we conducted a retrospective analysis of the trends of cancer trial eligibility criteria between 1999 and 2014. The yearly distributions of eligibility concepts for chemicals and drugs, procedures, observations, and medical conditions extracted from free-text eligibility criteria of 32,000 clinical trials for 89 cancer types were analyzed. We identified the concepts that trend upwards or downwards in all or selected cancer types, and the concepts that show anomalous trends for some cancers. Later, concept trends were studied in a disease-specific manner and illustrated for breast cancer. Criteria trends observed in this study are also validated and interpreted using evidence from the existing medical literature. This study contributes a method for concept trend analysis and original knowledge of the trends in cancer clinical trial eligibility criteria.

Radiogenic Therapy: Novel Approaches for Enhancing Tumor Radiosensitivity

Radiotherapy (RT) is a well established modality for treating many forms of cancer. However, despite many improvements in treatment planning and delivery, the total radiation dose is often too low for tumor cure, because of the risk of normal tissue damage. Gene therapy provides a new adjunctive strategy to enhance the effectiveness of RT, offering the potential for preferential killing of cancer cells and sparing of normal tissues. This specificity can be achieved at several levels including restricted vector delivery, transcriptional targeting and specificity of the transgene product. This review will focus on those gene therapy strategies that are currently being evaluated in combination with RT, including the use of radiation sensitive promoters to control the timing and location of gene expression specifically within tumors. Therapeutic transgenes chosen for their radiosensitizing properties will also be reviewed, these include:

gene correction therapy, in which normal copies of genes responsible for radiation-induced apoptosis are transfected to compensate for the deletions or mutated variants in tumor cells (p53 is the most widely studied example). enzymes that synergize the radiation effect, by generation of a toxic species from endogenous precursors ( e.g., inducible nitric oxide synthase) or by activation of non toxic prodrugs to toxic species ( e.g., herpes simplex virus thymidine kinase/ganciclovir) within the target tissue. conditionally replicating oncolytic adenoviruses that synergize the radiation effect. membrane transport proteins ( e.g., sodium iodide symporter) to facilitate uptake of cytotoxic radionuclides.

The evidence indicates that many of these approaches are successful for augmenting radiation induced tumor cell killing with clinical trials currently underway.

Radiation enhances the therapeutic effect of Banoxantrone in hypoxic tumour cells with elevated levels of nitric oxide synthase

Banoxantrone (AQ4N) is a prototype hypoxia selective cytotoxin that is activated by haem containing reductases such as inducible nitric oxide synthase (iNOS). In the present study, we evaluate whether elevated levels of iNOS in human tumour cells will improve their sensitivity to AQ4N. Further, we examine the potential of radiation to increase cellular toxicity of AQ4N under normoxic (aerobic) and hypoxic conditions. We employed an expression vector containing the cDNA for human iNOS to transfect human fibrosarcoma HT1080 tumour cells. Alternatively, parental cells were exposed to a cytokine cocktail to induce iNOS gene expression and enzymatic activity. The cells were then treated with AQ4N alone and in combination with radiation in the presence or absence of the iNOS inhibitor N-methyl-L‑arginine. In parental cells, AQ4N showed little difference in toxicity under hypoxic verses normoxic conditions. Notably, cells with upregulated iNOS activity showed a significant increase in sensitivity to AQ4N, but only under conditions of reduced oxygenation. When these cells were exposed to the combination of AQ4N and radiation, there was much greater cell killing than that observed with either modality alone. In the clinical development of hypoxia selective cytotoxins it is likely they will be used in combination with radiotherapy. In the present study, we demonstrated that AQ4N can selectively kill hypoxic cells via an iNOS-dependent mechanism. This hypoxia-selective effect can be augmented by combining AQ4N with radiation without increasing cytotoxicity to well‑oxygenated tissues. Collectively, these results suggest that targeting hypoxic tumours with high levels of iNOS with a combination of AQ4N and radiotherapy could be a useful clinical therapeutic strategy.

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.

Probing cytochrome P450-mediated activation with a truncated azinomycin analogue

Selective cytochrome P450 bioactivation of truncated azinomycin.

Impact of tumor blood flow modulation on tumor sensitivity to the bioreductive drug banoxantrone

We investigated the hypoxia-dependent cytotoxicity of AQ4N (banoxantrone) using a panel of 13 cancer cell lines and studied its relationship to the expression of the quinone reductase DT-diaphorase (NQO1), which is widely found in cancer cells. We also investigated pharmacologic treatments that increase tumor hypoxia in vivo and their impact on AQ4N chemosensitivity in a solid tumor xenograft model. AQ4N showed ≥ 8-fold higher cytotoxicity under hypoxia than normoxia in cultures of 9L rat gliosarcoma and H460 human non-small-cell lung carcinoma cells but not for 11 other human cancer cell lines. DT-diaphorase protein levels and AQ4N chemosensitivity were poorly correlated across the cancer cell line panel, and AQ4N chemosensitivity was not affected by DT-diaphorase inhibitors. The vasodilator hydralazine decreased tumor perfusion and increased tumor hypoxia in 9L tumor xenografts, and to a lesser extent in H460 tumor xenografts. However, hydralazine did not increase AQ4N-dependent antitumor activity. Combination of AQ4N with the angiogenesis inhibitor axitinib, which increases 9L tumor hypoxia, transiently increased antitumor activity but with an increase in host toxicity. These findings indicate that the capacity to bioactivate AQ4N is not dependent on DT-diaphorase and is not widespread in cultured cancer cell lines. Moreover, the activation of AQ4N cytotoxicity in vivo requires tumor hypoxia that is more extensive or prolonged than can readily be achieved by vasodilation or by antiangiogenic drug treatment.

Evaluation of bioreductive activation of anticancer drugs idarubicin and mitomycin C by NADH-cytochrome b5 reductase and cytochrome P450 2B4

This study attempted to investigate the ability of microsomal NADH-cytochrome b5 reductase and cytochrome P450 2B4 to reductively activate idarubicin and mitomycin C. In vitro plasmid DNA damage experiments and assays using purified hepatic enzymes were employed to examine their respective roles in the metabolic activation of anticancer drugs. Mitomycin C was found to be not a good substrate for microsomal b5 reductase unlike P450 reductase. It produced low amounts of strand breaks in DNA when incubated with b5 reductase and its one-electron reduction by purified enzyme was found as negligible. Our findings revealed that P450 reductase-mediated metabolism of idarubicin resulted in a large increase in single-strand DNA breaks, whereas, b5 reductase neither catalyzed the reduction of idarubicin nor mediated the formation of DNA damage in the presence of idarubicin. The reconstitution studies, on the other hand, have identified rabbit liver CYP2B4 isozyme as being a potential candidate enzyme for reductive bioactivation of idarubicin and mitomycin C. Thus, the present novel findings strongly suggest that while b5 reductase could not play a key role in the cytotoxic and/or antitumor effects of idarubicin and mitomycin C, CYP2B4 could potentiate their activity in combination with P450 reductase.

Prodrugs for improving tumor targetability and efficiency

As the mainstay in the treatment of various cancers for several decades, chemotherapy is successful but still faces challenges including non-selectivity and high toxicity. Improving the selectivity is therefore a critical step to improve the therapeutic efficacy of chemotherapy. Prodrug is one of the most promising approaches to increase the selectivity and efficacy of a chemotherapy drug. The classical prodrug approach is to improve the pharmaceutical properties (solubility, stability, permeability, irritation, distribution, etc.) via a simple chemical modification. This review will focus on various targeted prodrug designs that have been developed to increase the selectivity of chemotherapy drugs. Various tumor-targeting ligands, transporter-associated ligands, and polymers can be incorporated in a prodrug to enhance the tumor uptake. Prodrugs can also be activated by enzymes that are specifically expressed at a higher level in tumors, leading to a selective anti-tumor effect. This can be achieved by conjugating the enzyme to a tumor-specific antibody, or delivering a vector expressing the enzyme into tumor cells.

Design of anticancer prodrugs for reductive activation

Anticancer prodrugs designed to target specifically tumor cells should increase therapeutic effectiveness and decrease systemic side effects in the treatment of cancer. Over the last 20 years, significant advances have been made in the development of anticancer prodrugs through the incorporation of triggers for reductive activation. Reductively activated prodrugs have been designed to target hypoxic tumor tissues, which are known to overexpress several endogenous reductive enzymes. In addition, exogenous reductive enzymes can be delivered to tumor cells through fusion with tumor-specific antibodies or overexpressed in tumor cells through gene delivery approaches. Many anticancer prodrugs have been designed to use both the endogenous and exogenous reductive enzymes for target-specific activation and these prodrugs often contain functional groups such as quinones, nitroaromatics, N-oxides, and metal complexes. Although no new agents have been approved for clinical use, several reductively activated prodrugs are in various stages of clinical trial. This review mainly focuses on the medicinal chemistry aspects of various classes of reductively activated prodrugs including design principles, structure-activity relationships, and mechanisms of activation and release of active drug molecules.

The HIF-1-active microenvironment: an environmental target for cancer therapy

Solid tumors possess unique microenvironments that are exposed to chronic hypoxic conditions, so-called tumor hypoxia. Although more than half a century has passed since it was suggested that tumor hypoxia correlated with bad treatment outcomes and contributed to the recurrence of cancer, no fundamental solution to this problem has yet been found. Hypoxia-inducible factor HIF-1 is the main transcription factor that regulates the cellular response to hypoxia. It induces various genes, whose function is strongly associated with the malignant alteration of the entire tumor. The cellular changes induced by HIF-1 are extremely important therapeutic targets of cancer therapy, particularly in the therapy against refractory cancers. Therefore targeting strategies to overcome the HIF-1-active microenvironment are important for cancer therapy.

The hypoxia-activated ProDrug AQ4N penetrates deeply in tumor tissues and complements the limited distribution of mitoxantrone

Hypoxic tumor cells are likely to be resistant to conventional chemotherapy, in large part because many anticancer drugs are unable to penetrate into poorly oxygenated tumor tissue. Here, we used quantitative immunofluorescence to study the distribution of mitoxantrone and AQ4N in tumor tissue. AQ4N is a prodrug activated under hypoxic conditions to AQ4, which is structurally similar to mitoxantrone and inhibits topoisomerase II. We characterized the penetration of mitoxantrone and AQ4N/AQ4 through multilayered cell cultures (MCC) and in relation to blood vessels and hypoxic regions in human tumor xenografts. We also studied tumor growth delay after treatment with each agent alone and with the combination. In both MCC and xenografts, mitoxantrone is taken up by proximal cells and penetrates slowly to distant regions. In contrast, AQ4N rapidly penetrates MCC and tumor tissue in vivo, and AQ4N (or its reduced form AQ4) is detected at high concentration within hypoxic regions. The targeting of mitoxantrone to oxygenated regions and AQ4N/AQ4 to hypoxic tumor regions results in effective drug exposure over the entire tumor after combined treatment and increases tumor growth delay compared with either drug alone. The combination of a clinically used anticancer drug with limited tissue penetration and a structurally related drug activated in regions of tumor hypoxia is an effective strategy to overcome chemoresistance due to the tumor microenvironment. This study supports clinical evaluation of AQ4N in combination with conventional anticancer agents, such as mitoxantrone.

A phase 1 open-label, accelerated dose-escalation study of the hypoxia-activated prodrug AQ4N in patients with advanced malignancies

PURPOSE

AQ4N is a novel prodrug that is selectively bioreduced to AQ4, a topoisomerase II inhibitor, in hypoxic tumor. This study assessed the maximum tolerated dose and pharmacokinetics of AQ4N when administered weekly in patients with advanced cancers.

EXPERIMENTAL DESIGN

AQ4N was administered as a 30-minute i.v. infusion on days 1, 8, and 15 of a 28-day cycle in eight dose cohorts ranging from 12 to 1,200 mg/m(2). Accelerated titration design was used and the maximum tolerated dose was defined as the highest dose at which fewer than two of six patients had a dose-limiting toxicity.

RESULTS

Sixteen patients were treated with cumulative doses of AQ4N ranging from 61.6 through 9,099.1 mg/m(2). A single patient per cohort was treated up to 384 mg/m(2) without toxicities. At 1,200 mg/m(2), two of five patients experienced a dose-limiting toxicity (grade 5 respiratory failure and grade 3 fatigue). Five cohort assigned patients were treated without toxicity at 768 mg/m(2), establishing this dose as the maximum tolerated dose. Among the most common adverse events observed were fatigue (38%), diarrhea (31%), nausea (25%), vomiting (25%), and anorexia (13%). Anticipated blue coloration of body fluids or skin was observed in all patients. The pharmacokinetics of AQ4N were dose proportional over all doses studied. Three patients experienced stable disease, including a patient with collecting duct renal cancer stable for 25 months.

CONCLUSION

AQ4N is well tolerated when administered weekly on a 3-of-4-week schedule at 768 mg/m(2). Further combination studies investigating the safety and efficacy of AQ4N are ongoing.

Potentiation of methoxymorpholinyl doxorubicin antitumor activity by P450 3A4 gene transfer

Preclinical and clinical studies of CYP gene-directed enzyme prodrug therapy have been focused on anticancer prodrugs activated by CYP2B enzymes, which have low endogenous expression in human liver; however, the gene therapeutic potential of CYP3A enzymes, which are highly expressed in human liver, remains unknown. This study investigated methoxymorpholinyl doxorubicin (MMDX; nemorubicin), a novel CYP3A-activated anticancer prodrug. Retroviral transfer of CYP3A4 increased 9L gliosarcoma cell chemosensitivity to MMDX 120-fold (IC(50)=0.2 nM in 9L/3A4 cells). In CHO cells, overexpression of P450 reductase in combination with CYP3A4 enhanced chemosensitivity to MMDX, and to ifosfamide, another CYP3A4 prodrug, 11- to 23-fold compared with CYP3A4 expression alone. CYP3A4 expression and MMDX chemosensitivity were increased in human lung (A549) and brain (U251) tumor cells infected with replication-defective adenovirus encoding CYP3A4. Coinfection with Onyx-017, a replication-conditional adenovirus that coamplifies and coreplicates the Adeno-3A4 virus, led to large increases in CYP3A4 RNA but only modest increases in CYP3A4 protein and activity. MMDX induced remarkable growth delay of 9L/3A4 tumors, but not the P450-deficient parental 9L tumors, in immunodeficient mice administered low-dose MMDX either intravenous or by direct intratumoral (i.t.) injection (60 microg kg(-1), every 7 days x 3). Notably, the i.t. route was substantially less toxic to the mouse host. No antitumor activity was observed with intraperitoneal MMDX treatment, suggesting a substantial hepatic first pass effect, with activated MMDX metabolites formed in the liver having poor access to the tumor site. These studies demonstrate that human CYP3A4 has strong potential for MMDX prodrug-activation therapy and suggest that endogenous tumor cell expression of CYP3A4, and not hepatic CYP3A4 activity, is a key determinant of responsiveness to MMDX therapy in cancer patients in vivo.

Enhancement of intratumoral cyclophosphamide pharmacokinetics and antitumor activity in a P450 2B11-based cancer gene therapy model

The therapeutic utility of cytochrome P450-based enzyme prodrug therapy is well established by preclinical studies and in initial clinical trials. The underlying premise of this gene therapy is that intratumoral P450 expression leads to in situ activation of anticancer P450 prodrugs, such as cyclophosphamide (CPA), with intratumoral accumulation of its activated 4-OH metabolite. In mice bearing 9L gliosarcomas expressing the CPA 4-hydroxylase P450 2B6, enhanced tumor apoptosis was observed 48 h after CPA treatment; however, intratumoral 4-OH-CPA levels were indistinguishable from those of P450-deficient tumors, indicating that the bulk of activated CPA is derived from hepatic metabolism. In contrast, in 9L tumors expressing P450 2B11, a low K(m) CPA 4-hydroxylase, intratumoral 4-OH-CPA levels were higher than in blood, liver and P450-deficient tumors. Intratumoral 4-OH-CPA increased dose-dependently, without saturation at 140 mg kg(-1) CPA, suggesting restricted tumor cell permeation of the parent drug. To circumvent this problem, CPA was administered by direct intratumoral injection, which increased the maximum concentration and area under the curve of drug concentration x time (AUC) of intratumoral 4-OH-CPA by 1.8- and 2.7-fold, respectively. An overall 3.9-fold increase in intratumoral 4-OH-CPA AUC, and in antitumor activity, was obtained when CPA release to systemic circulation was delayed using the slow-release polymer poloxamer 407 as vehicle for intratumoral CPA delivery. These findings highlight the advantage of gene therapy strategies that combine low K(m) P450 prodrug activation enzymes with slow, localized release of P450 prodrug substrates.

Bioreductive drugs: from concept to clinic

One of the key issues for radiobiologists is the importance of hypoxia to the radiotherapy response. This review addresses the reasons for this and primarily focuses on one aspect, the development of bioreductive drugs that are specifically designed to target hypoxic tumour cells. Four classes of compound have been developed since this concept was first proposed: quinones, nitroaromatics, aliphatic and heteroaromatic N-oxides. All share two characteristics: (1) they require hypoxia for activation and (2) this activation is dependent on the presence of specific reductases. The most effective compounds have shown the ability to enhance the anti-tumour efficacy of agents that kill better-oxygenated cells, i.e. radiation and standard cytotoxic chemotherapy agents such as cisplatin and cyclophosphamide. Tirapazamine (TPZ) is the most widely studied of the lead compounds. After successful pre-clinical in vivo combination studies it entered clinical trial; over 20 trials have now been reported. Although TPZ has enhanced some standard regimens, the results are variable and in some combinations toxicity was enhanced. Banoxantrone (AQ4N) is another agent that is showing promise in early phase I/II clinical trials; the drug is well tolerated, is known to locate in the tumour and can be given in high doses without major toxicities. Mitomycin C (MMC), which shows some bioreductive activation in vitro, has been tested in combination trials. However, it is difficult to assign the enhancement of its effects to targeting of the hypoxic cells because of the significant level of its hypoxia-independent toxicity. More specific analogues of MMC, e.g. porfiromycin and apaziquone (EO9), have had variable success in the clinic. Other new drugs that have good pre-clinical profiles are PR 104 and NLCQ-1; data on their clinical safety/efficacy are not yet available. This paper reviews the pre-clinical data and discusses the clinical studies that have been reported.

Selective Tumor Targeting by the Hypoxia-Activated Prodrug AQ4N Blocks Tumor Growth and Metastasis in Preclinical Models of Pancreatic Cancer

PURPOSE

The antitumor activities and pharmacokinetics of the hypoxia-activated cytotoxin AQ4N and its metabolites were assessed in several preclinical models of pancreatic cancers.

EXPERIMENTAL DESIGN

The cytotoxic effects of AQ4N prodrug and its bioreduced form, AQ4, were tested against multiple human tumor cell lines using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays. Nude mice bearing s.c. or orthotopically implanted human BxPC-3 or Panc-1 tumor cells were treated with AQ4N. Tumor growth inhibition, time to progression/end point, and liver metastasis were evaluated in treatment versus control groups. Plasma and tumor levels of AQ4N and its metabolites were quantitated by liquid chromatography-tandem mass spectrometry.

RESULTS

In contrast to AQ4N, the bioreduced AQ4 metabolite displayed potent cytotoxicity in many human tumor lines, including those derived from human pancreatic adenocarcinomas. Single-agent administration of AQ4N significantly delayed tumor growth, progression, and survival in a manner comparable with gemcitabine in multiple pancreatic tumor models in vivo. Survival increases were accompanied by a reduction in incidence and spread of liver metastasis. Quantitation of AQ4N and its metabolites in tumor-bearing mice showed that the prodrug is rapidly cleared from the circulation by 24 h and neither of the bioreduced metabolites was detected in plasma. In contrast, AQ4N readily penetrated BxPC-3 tumors and the cytotoxic AQ4 metabolite rapidly accumulated in tumor tissues at high levels in a dose-dependent fashion.

CONCLUSION

AQ4N undergoes rapid and selective conversion into the potent antineoplastic metabolite AQ4 in tumors in vivo and provides proof of principle for the use of hypoxia-activated prodrugs in the treatment against pancreatic cancers.

Suicide genes for cancer therapy

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

Tumor hypoxia in cancer therapy

Human solid tumors are invariably less well-oxygenated than the normal tissues from which they arose. This so-called tumor hypoxia leads to resistance to radiotherapy and anticancer chemotherapy as well as predisposing for increased tumor metastases. In this chapter, we examine the resistance of tumors to radiotherapy produced by hypoxia and, in particular, address the question of whether this resistance is the result of the physicochemical free radical mechanism that produces resistance to radiation killing of cells in vitro. We conclude that a major part of the resistance, though perhaps not all, is the result of the physicochemical free radical mechanism of the oxygen effect in sensitizing cells to ionizing radiation. However, in modeling studies used to evaluate the effect of fractionated irradiation on tumor response, it is essential to consider the fact that the tumor cells are at a wide range of oxygen concentrations, not just at the extremes of oxygenated and hypoxic. Prolonged hypoxia of the tumor tissue also leads to necrosis, and necrotic regions are also characteristic of solid tumors. These two characteristics--hypoxia and necrosis--represent clear differences between tumors and normal tissues and are potentially exploitable in cancer treatment. We discuss strategies for exploiting these differences. One such strategy is to use drugs that are toxic only under hypoxic conditions. The second strategy is to take advantage of the selective induction under hypoxia of the hypoxia-inducible factor (HIF)-1. Gene therapy strategies based on this strategy are in development. Finally, tumor hypoxia can be exploited using live obligate anaerobes that have been genetically engineered to express enzymes that can activate nontoxic prodrugs into toxic chemotherapeutic agents.

Cancer chemotherapy and drug metabolism

Drug-metabolizing enzymes and drug transporters are key determinants of the pharmacokinetics and pharmacodynamics of many antineoplastic agents. Metabolism and transport influence the cytotoxic effects of antineoplastic agents in target tumor cells and normal host tissues. This article summarizes several state-of-the-art approaches to enhancing the effectiveness and safety of cancer therapy based on recent developments in our understanding of antineoplastic drug metabolism and transport. Advances in four interrelated research areas presented at a recent symposium sponsored by the Division for Drug Metabolism of the American Society for Pharmacology and Experimental Therapeutics (Experimental Biology 2004; Washington D.C., April 17-21, 2004) are discussed: 1) interactions of anthracyclines with drug-metabolizing enzymes; 2) use of hypoxia-selective gene-directed enzyme prodrug therapy (GDEPT) in combination with bioreductive prodrugs; 3) synergy between glutathione conjugation and conjugate efflux in conferring resistance to electrophilic toxins; and 4) use of cytochromes P450 as prodrug-activating enzymes in GDEPT strategies. A clear theme emerged from this symposium: drug metabolism and transport processes can be modulated and exploited in ways that may offer distinct therapeutic advantages in the management of patients with cancer.

Activation of oxazaphosphorines by cytochrome P450: Application to gene-directed enzyme prodrug therapy for cancer

Cancer chemotherapeutic prodrugs, such as the oxazaphosphorines cyclophosphamide and ifosfamide, are metabolized by liver cytochrome P450 enzymes to yield therapeutically active, cytotoxic metabolites. The effective use of these prodrugs is limited by host toxicity associated with the systemic distribution of cytotoxic metabolites formed in the liver. This problem can, in part, be circumvented by implementation of cytochrome P450 gene-directed enzyme prodrug therapy (P450 GDEPT), a prodrug activation strategy for cancer treatment that augments tumor cell exposure to cytotoxic drug metabolites generated locally by a prodrug-activating cytochrome P450 enzyme. P450 GDEPT has been exemplified in preclinical rodent and human tumor models, where chemosensitivity to a P450 prodrug can be greatly increased by introduction of a prodrug-activating P450 gene. Further enhancement of the efficacy of P450-based gene therapy can be achieved: by co-expression of P450 with the flavoenzyme NADPH-P450 reductase, which provides electrons required for P450 metabolic activity; by metronomic (anti-angiogenic) scheduling of the prodrug; by localized delivery of the prodrug to the tumor; and by combination with anti-apoptotic factors, which slow the death of the P450 'factory' cells and thereby enhance the bystander cytotoxic response. P450 GDEPT has several important features that make it a clinically attractive strategy for cancer treatment. These include: the substantial bystander cytotoxicity of P450 prodrugs such as cyclophosphamide and ifosfamide; the ability to use human P450 genes and thereby avoid an immune response to the therapeutic gene; the use of well-established conventional chemotherapeutic prodrugs, as well as bioreductive drugs activated by P450/P450 reductase in a hypoxic tumor environment; and the potential to decrease systemic exposure to active drug metabolites by selective inhibition of hepatic P450 activity. Recent advances in this area of research are reviewed, and two proof-of-concept clinical trials that highlight the utility of this strategy are discussed.

Monitoring drug-protein interaction

A variety of therapeutic drugs can undergo biotransformation via Phase I and Phase II enzymes to reactive metabolites that have intrinsic chemical reactivity toward proteins and cause potential organ toxicity. A drug-protein adduct is a protein complex that forms when electrophilic drugs or their reactive metabolite(s) covalently bind to a protein molecule. Formation of such drug-protein adducts eliciting cellular damages and immune responses has been a major hypothesis for the mechanism of toxicity caused by numerous drugs. The monitoring of protein-drug adducts is important in the kinetic and mechanistic studies of drug-protein adducts and establishment of dose-toxicity relationships. The determination of drug-protein adducts can also provide supportive evidence for diagnosis of drug-induced diseases associated with protein-drug adduct formation in patients. The plasma is the most commonly used matrix for monitoring drug-protein adducts due to its convenience and safety. Measurement of circulating antibodies against drug-protein adducts may be used as a useful surrogate marker in the monitoring of drug-protein adducts. The determination of plasma protein adducts and/or relevant antibodies following administration of several drugs including acetaminophen, dapsone, diclofenac and halothane has been conducted in clinical settings for characterizing drug toxicity associated with drug-protein adduct formation. The monitoring of drug-protein adducts often involves multi-step laboratory procedure including sample collection and preliminary preparation, separation to isolate or extract the target compound from a mixture, identification and determination. However, the monitoring of drug-protein adducts is often difficult because of short half-lives of the protein adducts, sampling problem and lack of sensitive analytical techniques for the protein adducts. Currently, chromatographic (e.g. high performance liquid chromatography) and immunological methods (e.g. enzyme-linked immunosorbent assay) are two major techniques used to determine protein adducts of drugs in patients. The present review highlights the importance for clinical monitoring of drug-protein adducts, with an emphasis on methodology and with a further discussion of the application of these techniques to individual drugs and their target proteins.

From bench to bedside for gene-directed enzyme prodrug therapy of cancer

Gene therapy of cancer offers the possibility of a targeted treatment that destroys tumors and metastases, but not normal tissues. In gene-directed enzyme prodrug therapy (GDEPT), or suicide gene therapy, the gene encoding an enzyme is delivered to tumor cells, followed by administration of a prodrug, which is converted locally to a cytotoxin by the enzyme. The producer cells as well as surrounding bystanders are subsequently killed. Promising results have meant that suicide gene therapy has reached multicenter phase III clinical trials. This review will discuss the development, efficiency, mode of action and pharmacokinetics of seven GDEPT systems in vitro and in vivo. We will review the latest data of those systems in clinical trials (herpes simplex virus thymidine kinase/gancyclovir, bacterial cytosine deaminase/5-fluorocytosine, bacterial nitroreductase/CB1954 and cytochrome P450/cyclophosphamide), as well as the development of more recent and experimental systems which are not yet in clinical trials (P450 reductase/tirapazamine, carboxypeptidase/CMDA, horseradish peroxidase/indole-3-acetic acid or paracetamol and others).

The Japanese experiences with hypoxia-targeting pharmacoradiotherapy: from hypoxic cell sensitisers to radiation-activated prodrugs

Tumour hypoxia is a negative factor in cancer radiotherapy. In order to overcome the problem, various pharmacotherapies have been investigated as an adjunct to radiotherapy. The use of hypoxic cell sensitisers is a classical strategy, and many new compounds have been developed and investigated. Development of more efficient compounds than those currently available seems difficult and clinical studies to prove the efficacy of the existing compounds are encouraged, especially in combination with radiosurgery, intraoperative radiotherapy, and interstitial irradiation, in which a single high dose of radiation is used. Following the advent of hypoxic cell sensitisers, hypoxic cytotoxins have become available. Among them, tirapazamine has already gained success when combined with cisplatin in non-small cell lung cancer. The beneficial effect of tirapazamine when combined with radiation needs to be determined. As a third-generation compound in this field, antitumour prodrugs that are activated by irradiation under hypoxic conditions via one-electron reduction have been proposed. Prodrugs of 5-fluorouracil and 5-fluoro-2'-deoxyuridine have shown in vivo as well as in vitro activity. Although clinical evaluation of the compounds is not warranted due to a relatively low in vivo effect, this strategy appears promising if the prodrug design can be applied to more potent agents that shall be developed in future.

A cytochrome P450 2B6 meditated gene therapy strategy to enhance the effects of radiation or cyclophosphamide when combined with the bioreductive drug AQ4N

BACKGROUND

AQ4N is metabolised in hypoxic cells by cytochrome P450s (CYPs) to the cytotoxin AQ4. Most solid tumours are known to contain regions of hypoxia whereas levels of CYPs have been found to vary considerably. Enhancement of CYP levels may be obtained using gene-directed enzyme prodrug therapy (GDEPT). We have therefore examined the potential of a CYP2B6-mediated GDEPT strategy to enhance the anti-tumour effect of the combination of AQ4N with radiation or cyclophosphamide (CPA).

METHODS

In vitro and in vivo transient transfection of human CYP2B6 +/- CYP reductase (CYPRED) was investigated in RIF-1 mouse tumours. Efficacy in vitro was assessed using the alkaline comet assay (ACA). In vivo, the time to reach 4x the treatment volume (quadrupling time; VQT) was used as the end point.

RESULTS

When CYP2B6 was transfected into RIF-1 cells and treated with AQ4N under hypoxic conditions there was a significant increase in DNA damage (measured by the ACA) compared with non-transfected cells. In vivo, a single intra-tumoural injection of a CYP2B6 vector construct significantly enhanced tumour growth delay in combination with AQ4N (100 mg/kg) and 10 Gy X-rays. AQ4N (100 mg/kg) and CPA (100 mg/kg) with CYP2B6 and CYPRED also enhanced tumour growth delay; this effect became significant when the schedule was repeated 14 days later (p = 0.0197).

CONCLUSIONS

The results show the efficacy of a CYP2B6-mediated GDEPT strategy for bioreduction of AQ4N; this may offer an additional approach to target radiation- and chemo-resistant hypoxic tumours that should enhance overall tumour control.

Bioreduction activated prodrugs of camptothecin: molecular design, synthesis, activation mechanism and hypoxia selective cytotoxicity

Several water-soluble derivatives (CPT3, CPT3a-d) of camptothecin (CPT) were synthesized, among which CPT3 bearing an N,N'-dimethyl-1-aminoethylcarbamate side-chain was further conjugated with reductively eliminating structural units of indolequinone, 4-nitrobenzyl alcohol and 4-nitrofuryl alcohol to produce novel prodrugs of camptothecin (CPT4-6). All CPT derivatives were of lower cytotoxicity than their parent compound of CPT. In contrast, CPT4 and CPT6 showed higher hypoxia selectivity of cytotoxicity towards tumor cells than CPT. A mechanism by which a representative prodrug CPT4 is activated in the presence of DT-diaphorase to release CPT was also discussed. The bioreduction activated CPT prodrugs including CPT4 and CPT6 are identified to be promising for application to the hypoxia targeting tumor chemotherapy.

Use of replication-conditional adenovirus as a helper system to enhance delivery of P450 prodrug-activation genes for cancer therapy

Cytochrome P450 (CYP) gene transfer sensitizes tumor xenografts to anticancer prodrugs such as cyclophosphamide (CPA) without a detectable increase in host toxicity. Optimal prodrug activation is achieved when a suitable P450 gene (e.g., human CYP2B6) is delivered in combination with NADPH-cytochrome P450 reductase (P450R), which encodes the flavoenzyme P450 reductase. We sought to improve this gene therapy by coordinated delivery and expression of P450 and P450R on a single bicistronic vector using an internal ribosomal entry site (IRES) sequence. Retrovirus encoding a CYP2B6-IRES-P450R expression cassette was shown to induce strong P450-dependent CPA cytotoxicity in a population of infected 9L gliosarcoma cells. Adeno-P450, a replication-defective, E1/E3 region-deleted adenovirus engineered to express CYP2B6-IRES-P450R, induced intracellular CPA 4-hydroxylation, and CPA cytotoxicity, in a broad range of human cancer cell lines. However, limited Adeno-P450 gene transfer and CPA chemosensitization was seen with certain human tumor cells, notably PC-3 prostate and HT-29 colon cancer cells. Remarkable improvements could be obtained by coinfecting the tumor cells with Adeno-P450 in combination with Onyx-017, an E1b-55k gene-deleted adenovirus that selectively replicates in p53 pathway-deficient cells. Substantial increases in gene expression were observed during the early stages of viral infection, reflecting an apparent coamplification of the Adeno-P450 genome, followed by enhanced viral spread at later stages, as demonstrated in cultured tumor cells, and in A549 and PC-3 solid tumor xenografts grown in scid mice. This combination of the replication-defective Adeno-P450 with a replication-conditional and tumor cell-targeted helper adenovirus dramatically improved the low gene transfer observed with some human tumor cell lines and correspondingly increased tumor cell-catalyzed CPA 4-hydroxylation, CPA cytotoxicity, and in vivo antitumor activity in a PC-3 tumor xenograft model. The use of tumor-selective, replicating adenovirus to promote the spread of replication-defective gene therapy vectors, such as Adeno-P450, substantially increases the therapeutic potential of adenoviral delivery systems, and should lead to increased activity and enhanced tumor selectivity of cytochrome P450 and other gene-directed enzyme prodrug therapies.

Cytochrome P450 enzymes: Novel options for cancer therapeutics

The concept of overexpression of individual forms of cytochrome P450 enzymes in tumor cells is now becoming well recognized. Indeed, a growing body of research highlights the overexpression of P450s, particularly CYP1B1, in tumor cells as representing novel targets for anticancer therapy. The purpose of this review is to outline the novel therapeutic options and opportunities arising from both enhanced endogenous expression of cytochrome P450 in tumors and cytochrome P450-mediated gene therapy.

Transcriptional Targeting in Cancer Gene Therapy

Cancer gene therapy has been one of the most exciting areas of therapeutic research in the past decade. In this review, we discuss strategies to restrict transcription of transgenes to tumour cells. A range of promoters which are tissue-specific, tumour-specific, or inducible by exogenous agents are presented. Transcriptional targeting should prevent normal tissue toxicities associated with other cancer treatments, such as radiation and chemotherapy. In addition, the specificity of these strategies should provide improved targeting of metastatic tumours following systemic gene delivery. Rapid progress in the ability to specifically control transgenes will allow systemic gene delivery for cancer therapy to become a real possibility in the near future.