Cytochrome P450 gene-directed enzyme prodrug therapy (GDEPT) for cancer

Designing cytochrome P450 enzymes for use in cancer gene therapy

Cancer is a significant global socioeconomic burden, as millions of new cases and deaths occur annually. In 2020, almost 10 million cancer deaths were recorded worldwide. Advancements in cancer gene therapy have revolutionized the landscape of cancer treatment. An approach with promising potential for cancer gene therapy is introducing genes to cancer cells that encode for chemotherapy prodrug metabolizing enzymes, such as Cytochrome P450 (CYP) enzymes, which can contribute to the effective elimination of cancer cells. This can be achieved through gene-directed enzyme prodrug therapy (GDEPT). CYP enzymes can be genetically engineered to improve anticancer prodrug conversion to its active metabolites and to minimize chemotherapy side effects by reducing the prodrug dosage. Rational design, directed evolution, and phylogenetic methods are some approaches to developing tailored CYP enzymes for cancer therapy. Here, we provide a compilation of genetic modifications performed on CYP enzymes aiming to build highly efficient therapeutic genes capable of bio-activating different chemotherapeutic prodrugs. Additionally, this review summarizes promising preclinical and clinical trials highlighting engineered CYP enzymes' potential in GDEPT. Finally, the challenges, limitations, and future directions of using CYP enzymes for GDEPT in cancer gene therapy are discussed.

Microbial cytosine deaminase is a programmable anticancer prodrug mediating enzyme: antibody, and gene directed enzyme prodrug therapy

Cytosine deaminase (CDA) is a non-mammalian enzyme with powerful activity in mediating the prodrug 5-fluorcytosine (5-FC) into toxic drug 5-fluorouracil (5-FU), as an alternative directed approach for the traditional chemotherapies and radiotherapies of cancer. This enzyme has been frequently reported and characterized from various microorganisms. The therapeutic strategy of 5-FC-CDA involves the administration of CDA followed by the prodrug 5-FC injection to generate cytotoxic 5-FU. The antiproliferative activity of CDA-5-FC elaborates from the higher activity of uracil pathway in tumor cells than normal ones. The main challenge of the therapeutic drug 5-FU are the short half-life, lack of selectivity and emergence of the drug resistance, consistently to the other chemotherapies. So, mediating the 5-FU to the tumor cells by CDA is one of the most feasible approaches to direct the drug to the tumor cells, reducing its toxic effects and improving their pharmacokinetic properties. Nevertheless, the catalytic efficiency, stability, antigenicity and targetability of CDA-5-FC, are the major challenges that limit the clinical application of this approach. Thus, exploring the biochemical properties of CDA from various microorganisms, as well as the approaches for localizing the system of CDA-5-FC to the tumor cells via the antibody directed enzyme prodrug therapy (ADEPT) and gene directed prodrug therapy (GDEPT) were the objectives of this review. Finally, the perspectives for increasing the therapeutic efficacy, and targetability of the CDA-5-FC system were described.

Bioactive three-dimensional silk composite in vitro tumoroid model for high throughput screening of anticancer drugs

HYPOTHESIS

Modeling three-dimensional (3D) in vitro culture systems recapitulating spatiotemporal characteristics of native tumor-mass has shown tremendous potential as a pre-clinical tool for drug screening. However, their applications in clinical settings are still limited due to inappropriate recapitulation of tumor topography, culture instability, and poor durability of niche support.

EXPERIMENTS

Here, we have fabricated a bio-active silk composite scaffold assimilating tunable silk from Bombyx mori and - arginine-glycine-aspartate (RGD) rich silk from Antheraea assama to provide a better 3D-matrix for breast (MCF 7) and liver (HepG2) tumoroids. Cellular mechanisms underlying physiological adaptations in 3D constructs and subsequent drug responses were compared with conventional monolayer and multicellular spheroid culture.

FINDINGS

Silk composite matrix assists prolonged growth and high metabolic activity (Cytochrome P450 reductase) in breast and liver 3D-tumoroids. Enhanced stemness expression (Cell surface adhesion receptor; CD44, Aldehyde dehydrogenase 1) and epithelial-mesenchymal-transition markers (E-cadherin, Vimentin) at transcript and protein levels demonstrate that bio-active matrix-assisted 3D environment augmenting metastatic potential in tumoroids. Together, enhanced secretion of Transforming growth factor β (TGFβ), anchorage-independency, and colony-forming potential of cells in the 3D-tumoroids further corroborates the aggressive behavior of cells. Moreover, the multilayered 3D-tumoroids exhibit decreased sensitivity to some known anticancer drugs (Doxorubicin and Paclitaxel). In conclusion, the bio-active silk composite matrix offers an advantage in developing robust and sustainable 3D tumoroids for a high-throughput drug screening platform.

Circulating Extracellular Vesicles Containing Xenobiotic Metabolizing CYP Enzymes and Their Potential Roles in Extrahepatic Cells Via Cell-Cell Interactions

The cytochrome P450 (CYP) family of enzymes is known to metabolize the majority of xenobiotics. Hepatocytes, powerhouses of CYP enzymes, are where most drugs are metabolized into non-toxic metabolites. Additional tissues/cells such as gut, kidneys, lungs, blood, and brain cells express selective CYP enzymes. Extrahepatic CYP enzymes, especially in kidneys, also metabolize drugs into excretable forms. However, extrahepatic cells express a much lower level of CYPs than hepatocytes. It is possible that the liver secretes CYP enzymes, which circulate via plasma and are eventually delivered to extrahepatic cells (e.g., brain cells). CYP circulation likely occurs via extracellular vesicles (EVs), which carry important biomolecules for delivery to distant cells. Recent studies have revealed an abundance of several CYPs in plasma EVs and other cell-derived EVs, and have demonstrated the role of CYP-containing EVs in xenobiotic-induced toxicity via cell–cell interactions. Thus, it is important to study the mechanism for packaging CYP into EVs, their circulation via plasma, and their role in extrahepatic cells. Future studies could help to find novel EV biomarkers and help to utilize EVs in novel interventions via CYP-containing EV drug delivery. This review mainly covers the abundance of CYPs in plasma EVs and EVs derived from CYP-expressing cells, as well as the potential role of EV CYPs in cell–cell communication and their application with respect to novel biomarkers and therapeutic interventions.

Thyroid hormones induce doxorubicin chemosensitivity through enzymes involved in chemotherapy metabolism in lymphoma T cells

Thyroid hormones (THs) – 3,3′,5-triiodo-L-thyronine (T3) and L-thyroxine (T4) – are important regulators of the metabolism and physiology of most normal tissues. Cytochrome P450 family 3A members are drug metabolizing enzymes involved in the activation and detoxification of several drugs. CYP3A4 is the major enzyme involved in the metabolism of chemotherapeutic drugs. In this work, we demonstrate that THs induce a significant increase in CYP3A4 mRNA levels, protein expression and metabolic activity through the membrane receptor integrin αvβ3 and the activation of signalling pathways through Stat1 and NF-κB. We reasoned that TH-induced CYP3A4 modulation may act as an important regulator in the metabolism of doxorubicin (Doxo). Experiments in vitro demonstrated that in CYP3A4-knocked down cells, no TH-mediated chemosensitivity to Doxo was observed. We also found that THs modulate these functions by activating the membrane receptor integrin αvβ3. In addition, we showed that the thyroid status can modulate CYP450 mRNA levels in tumor and liver tissues, and the tumor volume in response to chemotherapy in vivo. In fact, Doxo treatment in hypothyroid mice was associated with lower tumors, displaying lower levels of CYP enzymes, than euthyroid mice. However, higher mRNA levels of CYP enzymes were found in livers from Doxo treated hypothyroid mice respect to control. These results present a new mechanism by which TH could modulate chemotherapy response. These findings highlight the importance of evaluating thyroid status in patients during application of T-cell lymphoma therapeutic regimens.

Semiconducting Polymer Nanobiocatalysts for Photoactivation of Intracellular Redox Reactions

An organic semiconducting polymer nanobiocatalyst (SPNB) composed of a semiconducting polymer core conjugated with microsomal cytochrome P450 (CYP) has been developed for photoactivation of intracellular redox. The core serves as the light-harvesting unit to initiate photoinduced electron transfer (PET) and facilitate the regeneration of dihydronicotinamide adenine dinucleotide phosphate (NADPH), while CYP is the catalytic center for intracellular redox. Under light irradiation, the semiconducting core can efficiently catalyze the generation of NADPH with a turnover frequency (TOF) 75 times higher than the reported nanosystems, ensuring the supply of the cofactor for intracellular redox. SPNB-mediated intracellular redox thus can be efficiently activated by light in living cells to convert the model substrate and also to trigger the bioactivation of anticancer drugs. This study provides an organic nanobiocatalytic system that allows light to remotely control intracellular redox in living systems.

Effect of Cyclophosphamide Treatment on Central and Effector Memory T Cells in Mice

Immunological memory is a key feature of adaptive immunity. It provides the organism with long-lived and robust protection against infection. The important question is whether cyclophosphamide (CP), as immunosuppressive agent used in cancer therapy and in some autoimmune diseases, may act on the memory T-cell population. We investigated the effect of CP on the percentage of central memory T cells (T_CM) and effector memory T cells (T_EM) in the mouse model of CP-induced immunosuppression (8-10-week-old male C57BL/6 mice CP treated for 7 days at the daily dose of 50 μg/g body weight [bw], manifested the best immunosuppression status, as compared to lower doses of CP: 10 or 20 μg/g bw). The CP induced a significant decrease in the percentage of CD8⁺ (T_CM), compared to nonimmunosuppressed mice. This effect was not observed in the case of CD4⁺ T_CM population. The percentage of gated T_EM with CD4 and CD8 phenotype was significantly decreased in CP-treated mice, as compared to the control ones. Taken together, the above data indicate that CP-induced immunosuppression in mice leads to a reduction in the abundance of central memory cells possessing preferentially CD8⁺ phenotype as well as to a reduction in the percentage of effector memory cells (splenocytes both CD4⁺ and CD8⁺), compared to the cells from nonimmunosuppressed mice. These findings in mice described in this article may contribute to the understanding of the complexity of the immunological responses in humans and extend research on the impact of the CP model of immunosuppression in mice and memory T-cell populations.

The Use of Gene Therapy in Cancer Research and Treatment

Gene therapy involves identifying a gene of interest and then manipulating the expression of this gene through a variety of techniques. Here we specifically address gene therapy's role in cancer research. This paper will encompass thoroughly investigated techniques such as cancer vaccines and suicide gene therapy and the latest advancements in and applications of these techniques. It will also cover newer techniques such as Antisense Oligonucleotides and small interfering RNAs and how these technologies are being developed and used. The use of gene therapy continues to expand in cancer research and has an integral role in the advancement of cancer treatment.

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.

Pharmacokinetic strategies to improve drug penetration and entrapment within solid tumors

Despite the discovery of a large number of anticancer agents, cancer still remains among the leading causes of death since the middle of the twentieth century. Solid tumors possess a high degree of genetic instability and emergence of treatment resistance. Tumor resistance has emerged for almost all approved anticancer drugs and will most probably emerge for newly discovered anticancer agents as well. The use of pharmacokinetic approaches to increase anticancer drug concentrations within the solid tumor compartment and prolong its entrapment might diminish the possibility of resistance emergence at the molecular pharmacodynamic level and might even reverse tumor resistance. Several novel treatment modalities such as metronomic therapy, angiogenesis inhibitors, vascular disrupting agents and tumor priming have been introduced to improve solid tumor treatment outcomes. In the current review we will discuss the pharmacokinetic aspect of these treatment modalities in addition to other older treatment modalities, such as extracellular matrix dissolving agents, extracellular matrix synthesis inhibitors, chemoembolization and cellular efflux pump inhibition. Many of these strategies showed variable degrees of success/failure; however, reallocating these modalities based on their influence on the intratumoral pharmacokinetics might improve their understanding and treatment outcomes.

Activation of the anticancer drugs cyclophosphamide and ifosfamide by cytochrome P450 BM3 mutants

Cyclophosphamide (CPA) and ifosfamide (IFA) are widely used anticancer agents that require metabolic activation by cytochrome P450 (CYP) enzymes. While 4-hydroxylation yields DNA-alkylating and cytotoxic metabolites, N-dechloroethylation results in the generation of neuro- and nephrotoxic byproducts. Gene-directed enzyme prodrug therapies (GDEPT) have been suggested to facilitate local CPA and IFA bioactivation by expressing CYP enzymes within the tumor cells, thereby increasing efficacy. We screened bacterial CYP BM3 mutants, previously engineered to metabolize drug-like compounds, for their ability to catalyze 4-hydroxylation of CPA and IFA. Two CYP BM3 mutants showed very rapid initial bioactivation of CPA and IFA, followed by a slower phase of product formation. N-dechloroethylation by these mutants was very low (IFA) to undetectable (CPA). Using purified CYP BM3 as an extracellular bioactivation tool, cytotoxicity of CPA and IFA metabolism was confirmed in U2OS cells. This novel application of CYP BM3 possibly provides a clean and catalytically efficient alternative to liver microsomes or S9 for the study of CYP-mediated drug toxicity. To our knowledge, the observed rate of CPA and IFA 4-hydroxylation by these CYP BM3 mutants is the fastest reported to date, and might be of potential interest for CPA and IFA GDEPT.

Adenoviral vectors for prodrug activation-based gene therapy for cancer

Cancer cell heterogeneity is a common feature - both between patients diagnosed with the same cancer and within an individual patient's tumor - and leads to widely different response rates to cancer therapies and the potential for the emergence of drug resistance. Diverse therapeutic approaches have been developed to combat the complexity of cancer, including individual treatment modalities designed to target tumor heterogeneity. This review discusses adenoviral vectors and how they can be modified to replicate in a cancer-specific manner and deliver therapeutic genes under multi-tiered regulation to target tumor heterogeneity, including heterogeneity associated with cancer stem cell-like subpopulations. Strategies that allow for combination of prodrug-activation gene therapy with a novel replication-conditional, heterogeneous tumor-targeting adenovirus are discussed, as are the benefits of using adenoviral vectors as tumor-targeting oncolytic vectors. While the anticancer activity of many adenoviral vectors has been well established in preclinical studies, only limited successes have been achieved in the clinic, indicating a need for further improvements in activity, specificity, tumor cell delivery and avoidance of immunogenicity.

Chemotherapy pro-drug activation by biocatalytic virus-like nanoparticles containing cytochrome P450

This work shows, for the first time, the encapsulation of a highly relevant protein in the biomedical field into virus-like particles (VLPs). A bacterial CYP variant was effectively encapsulated in VLPs constituted of coat protein from cowpea chlorotic mottle virus (CCMV). The catalytic VLPs are able to transform the chemotherapeutic pro-drug, tamoxifen, and the emerging pro-drug resveratrol. The chemical nature of the products was identified, confirming similar active products than those obtained with human CYP. The enzymatic VLPs remain stable after the catalytic reaction. The potential use of these biocatalytic nanoparticles as targeted CYP carriers for the activation of chemotherapy drugs is discussed.

Prodrugs: a challenge for the drug development

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

Mechanisms of resistance to endocrine therapy in breast cancer: focus on signaling pathways, miRNAs and genetically based resistance

Breast cancer is the most frequent malignancy diagnosed in women. Approximately 70% of breast tumors express the estrogen receptor (ER). Tamoxifen and aromatase inhibitors (AIs) are the most common and effective therapies for patients with ERα-positive breast cancer. Alone or combined with chemotherapy, tamoxifen significantly reduces disease progression and is associated with more favorable impact on survival in patients. Unfortunately, endocrine resistance occurs, either de novo or acquired during the course of the treatment. The mechanisms that contribute to hormonal resistance include loss or modification in the ERα expression, regulation of signal transduction pathways, altered expression of specific microRNAs, balance of co-regulatory proteins, and genetic polymorphisms involved in tamoxifen metabolic activity. Because of the clinical consequences of endocrine resistance, new treatment strategies are arising to make the cells sensitive to tamoxifen. Here, we will review the current knowledge on mechanisms of endocrine resistance in breast cancer cells. In addition, we will discuss novel therapeutic strategies to overcome such resistance. Undoubtedly, circumventing endocrine resistance should help to improve therapy for the benefit of breast cancer patients.

The role of constitutive androstane receptor in oxazaphosphorine-mediated induction of drug-metabolizing enzymes in human hepatocytes

PURPOSE

To investigate the roles of the constitutive androstane receptor (CAR) in cyclophosphamide (CPA)- and ifosfamide (IFO)-mediated induction of hepatic drug-metabolizing enzymes (DME).

METHODS

Induction of DMEs was evaluated using real-time RT-PCR and Western blotting analysis in human primary hepatocyte (HPH) cultures. Activation of CAR, pregnane X receptor (PXR), and aryl hydrocarbon receptor by CPA and IFO was assessed in cell-based reporter assays in HepG2 cells and/or nuclear translocation assays in HPHs.

RESULTS

CYP2B6 reporter activity was significantly enhanced by CPA and IFO in HepG2 cells co-transfected with CYP2B6 reporter plasmid and a chemical-responsive human CAR variant (CAR1 + A) construct. Real-time RT-PCR and Western blotting analysis in HPHs showed that both CPA and IFO induced the expressions of CYP2B6 and CYP3A4. Notably, treatment of HPHs with CPA but not IFO resulted in significant nuclear accumulation of CAR, which represents the initial step of CAR activation. Further studies in HPHs demonstrated that selective inhibition of PXR by sulforaphane preferentially repressed IFO- over CPA-mediated induction of CYP2B6.

CONCLUSION

These results provide novel insights into the differential roles of CAR in the regulation of CPA- and IFO-induced DME expression and potential drug-drug interactions.

Adenoviral delivery of pan-caspase inhibitor p35 enhances bystander killing by P450 gene-directed enzyme prodrug therapy using cyclophosphamide+

Background

Cytochrome P450-based suicide gene therapy for cancer using prodrugs such as cyclophosphamide (CPA) increases anti-tumor activity, both directly and via a bystander killing mechanism. Bystander cell killing is essential for the clinical success of this treatment strategy, given the difficulty of achieving 100% efficient gene delivery in vivo using current technologies. Previous studies have shown that the pan-caspase inhibitor p35 significantly increases CPA-induced bystander killing by tumor cells that stably express P450 enzyme CYP2B6 (Schwartz et al, (2002) Cancer Res. 62: 6928-37).

Methods

To further develop this approach, we constructed and characterized a replication-defective adenovirus, Adeno-2B6/p35, which expresses p35 in combination with CYP2B6 and its electron transfer partner, P450 reductase.

Results

The expression of p35 in Adeno-2B6/p35-infected tumor cells inhibited caspase activation, delaying the death of the CYP2B6 "factory" cells that produce active CPA metabolites, and increased bystander tumor cell killing compared to that achieved in the absence of p35. Tumor cells infected with Adeno-2B6/p35 were readily killed by cisplatin and doxorubicin, indicating that p35 expression is not associated with acquisition of general drug resistance. Finally, p35 did not inhibit viral release when the replication-competent adenovirus ONYX-017 was used as a helper virus to facilitate co-replication and spread of Adeno-2B6/p35 and further increase CPA-induced bystander cell killing.

Conclusions

The introduction of p35 into gene therapeutic regimens constitutes an effective approach to increase bystander killing by cytochrome P450 gene therapy. This strategy may also be used to enhance other bystander cytotoxic therapies, including those involving the production of tumor cell toxic protein products.

Hypoxia--implications for pharmaceutical developments

Cells sense oxygen availability using not only the absolute value for cellular oxygen in regard to its energetic and metabolic functions, but also the gradient from the cell surface to the lowest levels in the mitochondria. Signals are used for regulatory purposes locally as well as in the generation of cellular, tissue, and humoral remodeling. Lowered oxygen availability (hypoxia) is theoretically important in the consideration of pharmacology because (1) hypoxia can alter cellular function and thereby the therapeutic effectiveness of the agent, (2) therapeutic agents may potentiate or protect against hypoxia-induced pathology, (3) hypoxic conditions may potentiate or mitigate drug-induced toxicity, (4) hypoxia may alter drug metabolism and thereby therapeutic effectiveness, and (5) therapeutic agents might alter the relative coupling of blood flow and energy metabolism in an organ. The prototypic biochemical effect of hypoxia is related to its known role as a cofactor in a number of enzymatic reactions, e.g., oxidases and oxygenases, which are affected independently from the bioenergetic effect of low oxygen on energetic functions. The cytochrome P-450 family of enzymes is another example. Here, there is a direct effect of oxygen availability on the conformation of the enzyme, thereby altering the metabolism of drug substrates. Indirectly, the NADH/NAD+ ratio is increased with 10% inspired oxygen, leading not only to reduced oxidation of ethanol but also to reduction of azo- and nitro-compounds to amines and disulfides to sulfhydryls. With chronic hypoxia, many of these processes are reversed, suggesting that hypoxia induces the drug-metabolizing systems. Support for this comes from observations that hypoxia can induce the hypoxic inducible factors which in turn alters transcription and function of some but not all cytochrome P-450 isoforms. Hypoxia is identified as a cofactor in cancer expression and metastatic potential. Thus, the effects of hypoxia play an important role in pharmacology, and the signaling pathways that are affected by hypoxia could become new targets for novel therapy or avenues for prevention.

Engineering cytochrome P450 biocatalysts for biotechnology, medicine and bioremediation

IMPORTANCE OF THE FIELD

Cytochrome P450 enzymes comprise a superfamily of heme monooxygenases that are of considerable interest for the: i) synthesis of novel drugs and drug metabolites; ii) targeted cancer gene therapy; iii) biosensor design; and iv) bioremediation. However, their applications are limited because cytochrome P450, especially mammalian P450 enzymes, show a low turnover rate and stability, and require a complex source of electrons through cytochrome P450 reductase and NADPH.

AREAS COVERED IN THIS REVIEW

In this review, we discuss the recent progress towards the use of P450 enzymes in a variety of the above-mentioned applications. We also present alternate and cost-effective ways to perform P450-mediated reaction, especially using peroxides. Furthermore, we expand upon the current progress in P450 engineering approaches describing several recent examples that are utilized to enhance heterologous expression, stability, catalytic efficiency and utilization of alternate oxidants.

WHAT THE READER WILL GAIN

The review provides a comprehensive knowledge in the design of P450 biocatalysts for potentially practical purposes. Finally, we provide a prospective on the future aspects of P450 engineering and its applications in biotechnology, medicine and bioremediation.

TAKE HOME MESSAGE

Because of its wide applications, academic and pharmaceutical researchers, environmental scientists and healthcare providers are expected to gain current knowledge and future prospects of the practical use of P450 biocatalysts.

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.

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.

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.

Enhanced antitumor activity of P450 prodrug-based gene therapy using the low Km cyclophosphamide 4-hydroxylase P450 2B11

Gene therapy using the prodrug-activating enzyme P450 2B6 has shown substantial promise in preclinical and initial clinical studies with the P450 prodrugs cyclophosphamide and ifosfamide. We sought to optimize this therapy using the canine P450 enzyme 2B11, which activates cyclophosphamide and ifosfamide with Km of 80 to 160 micromol/L, approximately 10- to 20-fold lower than the Km of P450 2B6. Retrovirus encoding a P450 2B11-internal ribosome entry signal-P450 reductase expression cassette induced marked cyclophosphamide and ifosfamide cytotoxicity toward 9L gliosarcoma cells and exhibited an impressive bystander killing effect at micromolar prodrug concentrations, where P450 2B6 displayed low activity. Adeno-2B11, a replication-defective, E1/E3 region-deleted adenovirus engineered to coexpress P450 2B11 and P450 reductase, dramatically increased tumor cell-catalyzed cyclophosphamide 4-hydroxylation and cytotoxicity compared with Adeno-2B6 and effected strong bystander killing at low (20 micromol/L) cyclophosphamide concentrations. Further increases in cyclophosphamide cytotoxicity were obtained in several human cancer cell lines, including a 4-hydroperoxycyclophosphamide-resistant MCF-7 breast cancer cell line, when Adeno-2B11 was combined with Onyx-017, an E1b-55-kDa gene-deleted, tumor cell-replicating adenovirus that coamplifies and facilitates tumor cell spread of Adeno-2B11. To evaluate the therapeutic effect of P450 2B11 expression in vivo, 9L gliosarcoma cells transduced with P450-expressing retrovirus were grown as solid s.c. tumors in immunodeficient mice. Cyclophosphamide treatment on a metronomic, 6-day repeating schedule led to full regression of 9L/2B11 tumors but not P450-deficient control tumors, resulting in a tumor-free period lasting up to approximately 100 days. 9L/2B6 tumors regressed more slowly and exhibited a tumor-free period of only 21 to 39 days. Thus, P450 gene-directed enzyme prodrug therapy can be greatly improved by using the low Km P450 enzyme 2B11, which catalyzes intratumoral activation of cyclophosphamide and ifosfamide at pharmacologically relevant drug concentrations.

Effects of hypoxia and limited diffusion in tumor cell microenvironment on bystander effect of P450 prodrug therapy

Cytochrome P450 (CYP) enzyme 2B1 metabolizes the anticancer prodrug cyclophosphamide (CPA) to 4-hydroxy-CPA, which decomposes to the cytotoxic metabolites acrolein and phosphoramide mustard. We have evaluated the bystander cytotoxicity of CPA in combination with CYP2B1 gene-directed enzyme prodrug therapy using a cell culture-based agarose overlay technique. This method mimics the tumor microenvironment by limiting the diffusion of metabolites and by reducing the oxygen concentration to levels similar to those found in solid tumors. Under these conditions, the CYP activity of CYP2B1-expressing tumor cells was decreased by 80% compared to standard aerobic conditions. Despite this decrease in metabolic activity, a potent bystander effect was observed, resulting in up to 90% killing by CPA of a tumor cell population comprised of only approximately 20% CYP-expressing tumor cells. Similarly, transient transfection of a small fraction ( approximately 14%) of a human hepatoma Huh7 cell population with a CYP2B1 expression plasmid followed by short-term treatment with CPA (5 h) led to an eradication of 95% of the cells. No such bystander effect was observed without the agarose overlay. These findings suggest that the agarose overlay technique is very useful as an in vitro test system for investigation of the bystander effect of CYP/CPA and other enzyme/prodrug combinations under conditions that mimic the hypoxic conditions present in solid tumors in vivo.

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.

Metabolism and transport of oxazaphosphorines and the clinical implications

The oxazaphosphorines including cyclophosphamide (CPA), ifosfamide (IFO), and trofosfamide represent an important group of therapeutic agents due to their substantial antitumor and immuno-modulating activity. CPA is widely used as an anticancer drug, an immunosuppressant, and for the mobilization of hematopoetic progenitor cells from the bone marrow into peripheral blood prior to bone marrow transplantation for aplastic anemia, leukemia, and other malignancies. New oxazaphosphorines derivatives have been developed in an attempt to improve selectivity and response with reduced toxicity. These derivatives include mafosfamide (NSC 345842), glufosfamide (D19575, beta-D-glucosylisophosphoramide mustard), NSC 612567 (aldophosphamide perhydrothiazine), and NSC 613060 (aldophosphamide thiazolidine). This review highlights the metabolism and transport of these oxazaphosphorines (mainly CPA and IFO, as these two oxazaphosphorine drugs are the most widely used alkylating agents) and the clinical implications. Both CPA and IFO are prodrugs that require activation by hepatic cytochrome P450 (CYP)-catalyzed 4-hydroxylation, yielding cytotoxic nitrogen mustards capable of reacting with DNA molecules to form crosslinks and lead to cell apoptosis and/or necrosis. Such prodrug activation can be enhanced within tumor cells by the CYP-based gene directed-enzyme prodrug therapy (GDEPT) approach. However, those newly synthesized oxazaphosphorine derivatives such as glufosfamide, NSC 612567 and NSC 613060, do not need hepatic activation. They are activated through other enzymatic and/or non-enzymatic pathways. For example, both NSC 612567 and NSC 613060 can be activated by plain phosphodiesterase (PDEs) in plasma and other tissues or by the high-affinity nuclear 3'-5' exonucleases associated with DNA polymerases, such as DNA polymerases and epsilon. The alternative CYP-catalyzed inactivation pathway by N-dechloroethylation generates the neurotoxic and nephrotoxic byproduct chloroacetaldehyde (CAA). Various aldehyde dehydrogenases (ALDHs) and glutathione S-transferases (GSTs) are involved in the detoxification of oxazaphosphorine metabolites. The metabolism of oxazaphosphorines is auto-inducible, with the activation of the orphan nuclear receptor pregnane X receptor (PXR) being the major mechanism. Oxazaphosphorine metabolism is affected by a number of factors associated with the drugs (e.g., dosage, route of administration, chirality, and drug combination) and patients (e.g., age, gender, renal and hepatic function). Several drug transporters, such as breast cancer resistance protein (BCRP), multidrug resistance associated proteins (MRP1, MRP2, and MRP4) are involved in the active uptake and efflux of parental oxazaphosphorines, their cytotoxic mustards and conjugates in hepatocytes and tumor cells. Oxazaphosphorine metabolism and transport have a major impact on pharmacokinetic variability, pharmacokinetic-pharmacodynamic relationship, toxicity, resistance, and drug interactions since the drug-metabolizing enzymes and drug transporters involved are key determinants of the pharmacokinetics and pharmacodynamics of oxazaphosphorines. A better understanding of the factors that affect the metabolism and transport of oxazaphosphorines is important for their optional use in cancer chemotherapy.

Use of directed evolution of mammalian cytochromes P450 for investigating the molecular basis of enzyme function and generating novel biocatalysts

Directed evolution has been successfully applied to the design of industrial biocatalysts for enhanced catalytic efficiency and stability, and for examining the molecular basis of enzyme function. Xenobiotic-metabolizing mammalian cytochromes P450 with their catalytic versatility and broad substrate specificity offer the possibility of widespread applications in industrial synthesis, medicine, and bioremediation. However, the requirement for NADPH-cytochrome P450 reductase, often cytochrome b5, and an expensive cofactor, NADPH, complicates the design of mammalian P450 enzymes as biocatalysts. Recently, Guengerich and colleagues have successfully performed directed evolution of P450s 1A2 and 2A6 initially by using colony-based colorimetric and genotoxicity screening assays, respectively, followed by in vitro fluorescence-based activity screening assays. More recently, our laboratory has developed a fluorescence-based in vitro activity screening assay system for enhanced catalytic activity of P450s 2B1 and 3A4. The studies indicate an important role of amino acid residues outside of the active site, which would be difficult to target by other methods. The approach can now be expanded to design these as well as new P450s using more targeted substrates of environmental, industrial, and medical importance.

Insights into oxazaphosphorine resistance and possible approaches to its circumvention

The oxazaphosphorines cyclophosphamide, ifosfamide and trofosfamide remain a clinically useful class of anticancer drugs with substantial antitumour activity against a variety of solid tumors and hematological malignancies. A major limitation to their use is tumour resistance, which is due to multiple mechanisms that include increased DNA repair, increased cellular thiol levels, glutathione S-transferase and aldehyde dehydrogenase activities, and altered cell-death response to DNA damage. These mechanisms have been recently re-examined with the aid of sensitive analytical techniques, high-throughput proteomic and genomic approaches, and powerful pharmacogenetic tools. Oxazaphosphorine resistance, together with dose-limiting toxicity (mainly neutropenia and neurotoxicity), significantly hinders chemotherapy in patients, and hence, there is compelling need to find ways to overcome it. Four major approaches are currently being explored in preclinical models, some also in patients: combination with agents that modulate cellular response and disposition of oxazaphosphorines; antisense oligonucleotides directed against specific target genes; introduction of an activating gene (CYP3A4) into tumor tissue; and modification of dosing regimens. Of these approaches, antisense oligonucleotides and gene therapy are perhaps more speculative, requiring detailed safety and efficacy studies in preclinical models and in patients. A fifth approach is the design of novel oxazaphosphorines that have favourable pharmacokinetic and pharmacodynamic properties and are less vulnerable to resistance. Oxazaphosphorines not requiring hepatic CYP-mediated activation (for example, NSC 613060 and mafosfamide) or having additional targets (for example, glufosfamide that also targets glucose transport) have been synthesized and are being evaluated for safety and efficacy. Characterization of the molecular targets associated with oxazaphosphorine resistance may lead to a deeper understanding of the factors critical to the optimal use of these agents in chemotherapy and may allow the development of strategies to overcome resistance.

Cytochrome p450 profile of colorectal cancer: identification of markers of prognosis

PURPOSE

The cytochromes P450 (P450) are a multigene family of enzymes with a central role in the oxidative metabolism of a wide range of xenobiotics, including anticancer drugs, carcinogens, and endogenous compounds. The purpose of this study was to define the P450 profile of colorectal cancer and establish the prognostic significance of expression of individual P450s in colorectal cancer.

EXPERIMENTAL DESIGN

Immunohistochemistry for a panel of 23 P450s was done on a colorectal cancer tissue microarray consisting of 264 primary colorectal cancers, 91 lymph node metastasis, and 10 normal colorectal samples. The intensity of immunoreactivity in each sample was established by light microscopy.

RESULTS

The most frequently expressed form of P450 in normal colon was CYP3A4. In primary colorectal cancer, several P450s (CYP1B1, CYP2S1, CYP2U1, CYP3A5, and CYP51) were present at a significantly higher level of intensity compared with normal colon. P450 expression was also detected in lymph node metastasis and the presence of several P450s (CYP1B1, CYP2A/2B, CYP2F1, CYP4V2, and CYP39) in the lymph node metastasis strongly correlated with their presence in corresponding primary tumors. The presence of strong CYP51 (log-rank = 12.11, P = 0.0005) or strong CYP2S1 (log-rank = 6.72, P = 0.0095) immunoreactivity were associated with poor prognosis. CYP51 was also an independent marker of prognosis (P = 0.009).

CONCLUSIONS

The expression of individual P450s has been established in colorectal cancer. Several P450s show increased expression in colorectal cancer. High expression of CYP51 or CYP2S1 were associated with poor prognosis and CYP51 is an independent marker of prognosis.

Enantioselective metabolism and cytotoxicity of R-ifosfamide and S-ifosfamide by tumor cell-expressed cytochromes P450

The anticancer prodrug ifosfamide (IFA) contains a chiral phosphorous atom and is administered in the clinic as a racemic mixture of R-IFA and S-IFA. Hepatic cytochrome P450 (P450) enzymes exhibit enantioselective preferences in the metabolism of R-IFA and S-IFA; however, the impact of this selectivity on P450-dependent anticancer activity is not known. Presently, the metabolism and cytotoxicity of R-IFA and S-IFA were determined in 9L gliosarcoma and Chinese hamster ovary tumor cells expressing an IFA-activating P450 enzyme and by in vitro steady-state kinetic analysis using cDNA-expressed P450 enzymes. Tumor cells expressing P450 enzyme CYP3A4 were the most sensitive to R-IFA cytotoxicity, whereas tumor cells expressing CYP2B1 or CYP2B6 were most sensitive to cyclophosphamide (CPA), an isomer of IFA. Correspondingly, CYP3A4-expressing cells and cDNA-expressed CYP3A4 metabolized R-IFA to yield the active, 4-hydroxylated metabolite at a 2- to 3-fold higher rate than they metabolized S-IFA or CPA. CYP2B cells and cDNA-expressed CYP2B enzymes metabolized CPA almost exclusively by 4-hydroxylation, whereas R-IFA and S-IFA were substantially converted to inactive, N-dechloroethylated metabolites. Further investigation revealed that CYP3A1, a rat enzyme, exhibited superior kinetic properties compared with the human enzyme CYP3A4, with R-IFA and S-IFA both metabolized with high catalytic efficiency by 4-hydroxylation and with a K(m) value of 200 microM, approximately 5-fold lower than CYP3A4. Based on these kinetic parameters and metabolic profiles, R-IFA is expected to exert greater anticancer activity than S-IFA or CPA against tumors that express CYP3A enzymes, whereas tumors expressing CYP2B enzymes may be more sensitive to CPA treatment.

Directed Evolution of Mammalian Cytochrome P450 2B1

Cytochrome P450 2B1 has been subjected to directed evolution to investigate the role of amino acid residues outside of the active site and to engineer novel, more active P450 catalysts. A high throughput screening system was developed to measure H(2)O(2)-supported oxidation of the marker fluorogenic substrate 7-ethoxy-4-trifluoromethylcoumarin (7-EFC). Random mutagenesis by error-prone polymerase chain reaction and activity screening were optimized using the L209A mutant of P450 2B1 in an N-terminally modified construct with a C-terminal His tag (P450 2B1dH). Two rounds of mutagenesis and screening and one subcloning step yielded the P450 2B1 quadruple mutant V183L/F202L/L209A/S334P, which demonstrated a 6-fold higher k(cat) than L209A. Further random or site-directed mutagenesis did not improve the activity. When assayed in an NADPH-supported reconstituted system, V183L/L209A demonstrated lower 7-EFC oxidation than L209A. Therefore, F202L/L209A/S334P was generated, which showed a 2.5-fold higher k(cat)/K(m) for NADPH-dependent 7-EFC oxidation than L209A. F202L/L209A/S334P also showed enhanced catalytic efficiency with 7-benzyloxyresorufin, benzphetamine, and testosterone, and a 10-fold increase in stereoselectivity for testosterone 16alpha-versus 16beta-hydroxylation compared with 2B1dH. Enhanced catalytic efficiency of F202L/L209A/S334P was also retained in the full-length P450 2B1 background with 7-EFC and testosterone as substrates. Finally, the individual mutants were tested for metabolism of the anti-cancer prodrugs cyclophosphamide and ifosfamide. Several of the mutants showed increased metabolism via the therapeutically beneficial 4-hydroxylation pathway, with L209A/S334P showing 2.8-fold enhancement of k(cat)/K(m) with cyclophosphamide and V183L/L209A showing 3.5-fold enhancement with ifosfamide. Directed evolution can thus be used to enhance P450 2B1 catalytic efficiency across a panel of substrates and to identify functionally important residues distant from the active site.

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.

Role of Cytochrome P450 Activity in the Fate of Anticancer Agents and in Drug Resistance

Although activity of cytochrome P450 isoenzymes (CYPs) plays a major role in the fate of anticancer agents in patients, there are relatively few clinical studies that evaluate drug metabolism with therapeutic outcome. Nevertheless, many clinical reports in various non-oncology fields have shown the dramatic importance of CYP activity in therapeutic efficacy, safety and interindividual variability of drug pharmacokinetics. Moreover, variability of drug metabolism in the liver as well as in cancer cells must also be considered as a potential factor mediating cancer resistance. This review underlines the role of drug metabolism mediated by CYPs in pharmacokinetic variability, drug resistance and safety. As examples, biotransformation pathways of tamoxifen, paclitaxel and imatinib are reviewed. This review emphasises the key role of therapeutic drug monitoring as a complementary tool of investigation to in vitro data. For instance, pharmacokinetic data of anticancer agents have not often been published within subpopulations of patients who show ultra-rapid, extensive or poor metabolism (e.g. due to CYP2D6 and CYP2C19 genotypes). Besides kinetic variability in the systemic circulation, induction of CYP activity may participate in creating drug resistance by speeding up the cancer agent degradation specifically in the target cells. For one cancer agent, various mechanisms of resistance are usually identified within different cell clones. This review also tries to emphasise that drug resistance mediated by CYP activity in cancer cells should be taken into consideration to a greater degree. The unequivocal identification of the metabolising enzymes involved in clinical conditions will eventually allow improvement and individualisation of anticancer agent therapy, i.e. drug dosage and selection. In addition, a more complete understanding of the metabolism of anticancer agents will assist in the prediction of drug-drug interactions, as anticancer agent combinations are becoming more prevalent.

Antitumor Activity of Methoxymorpholinyl Doxorubicin: Potentiation by Cytochrome P450 3A Metabolism

Methoxymorpholinyl doxorubicin (MMDX) is a novel liver cytochrome P450 (P450)-activated anticancer prodrug whose toxicity toward cultured tumor cells can be potentiated up to 100-fold by incubation with liver microsomes and NADPH. In the present study, a panel of human liver microsomes activated MMDX with potentiation ratios directly correlated to the CYP3A-dependent testosterone 6beta-hydroxylase activity of each liver sample. Microsome-activated MMDX exhibited nanomolar IC(50) values in growth-inhibition assays of human tumor cell lines representing multiple tissues of origin: lung (A549 cells), brain (U251 cells), colon (LS180 cells), and breast (MCF-7 cells). Analysis of individual cDNA-expressed CYP3A enzymes revealed that rat CYP3A1 and human CYP3A4 activated MMDX more efficiently than rat CYP3A2 and that human P450s 3A5 and 3A7 displayed little or no activity. MMDX cytotoxicity was substantially increased in Chinese hamster ovary cells after stable expression of CYP3A4 in combination with P450 reductase. CYP3A activation of MMDX abolished the parent drug's residual cross-resistance in a doxorubicin-resistant MCF-7 cell line that overexpresses P-glycoprotein. CYP3A-activated MMDX displayed a comparatively high intrinsic stability, with a t(1/2) of approximately 5.5 h at 37 degrees C. MMDX was rapidly activated by CYP3A at low ( approximately 1-5 nM) prodrug concentrations, with 100% tumor cell kill obtained after as short as a 2-h exposure to the activated metabolite. These findings demonstrate that MMDX can be activated by CYP3A metabolism to a potent, long-lived, and cell-permeable cytotoxic metabolite and suggest that this anthracycline prodrug may be used in combination with CYP3A4 in a P450 prodrug activation-based gene therapy for cancer treatment.

Aryl hydrocarbon receptor activation of an antitumor aminoflavone: Basis of selective toxicity for MCF-7 breast tumor cells

Aminoflavone (4H-1-benzopyran-4-one, 5-amino-2-(4-amino-3-fluorophenyl)-6,8-difluoro-7-methyl; NSC 686288) demonstrates differential antiproliferative activity in the National Cancer Institute's anticancer drug screen. We demonstrate here that MCF-7 human breast cancer cells are sensitive to aminoflavone both in vitro and when grown in vivo as xenografts in athymic mice. As previous work has indicated that aminoflavone requires metabolic activation by cytochrome P450 1A1 (CYP1A1), we investigated the effect of aminoflavone on CYP1A1 expression and on the aryl hydrocarbon receptor (AhR), a transcriptional regulator of CYP1A1. In aminoflavone-sensitive but not aminoflavone-resistant cells, the drug caused a 100-fold induction of CYP1A1 mRNA and a corresponding increase in ethoxyresorufin-O-deethylase activity. An AhR-deficient variant of the MCF-7 breast carcinoma, AHR100, with diminished CYP1A1 inducibility, exhibits cellular resistance to aminoflavone and is refractory to CYP1A1 mRNA induction by the drug. The increase in CYP1A1 mRNA in the aminoflavone-sensitive MCF-7 breast tumor cell results from transcriptional activation of xenobiotic-responsive element (XRE)–controlled transcription. Aminoflavone treatment causes a translocation of the AhR from the cytoplasm to the nucleus with subsequent formation of AhR-XRE protein DNA complexes. In contrast to the aminoflavone-sensitive MCF-7 cells, the resistant cell lines (MDA-MB-435, PC-3, and AHR100) demonstrated constitutive nuclear localization of AhR. Additionally, aminoflavone failed to induce ethoxyresorufin-O-deethylase activity, CYP1A1 transcription, AhR-XRE complex formation, and apoptosis in aminoflavone-resistant cells. These results suggest that the cytotoxicity of aminoflavone in a sensitive breast tumor cell line is the result of the engagement of AhR-mediated signal transduction.

Activation of the anticancer prodrugs cyclophosphamide and ifosfamide: identification of cytochrome P450 2B enzymes and site-specific mutants with improved enzyme kinetics

Cyclophosphamide (CPA) and ifosfamide (IFA) are oxazaphosphorine anticancer prodrugs metabolized by two alternative cytochrome P450 (P450) pathways, drug activation by 4-hydroxylation and drug inactivation by N-dechloroethylation, which generates the neurotoxic and nephrotoxic byproduct chloroacetaldehyde. CPA and IFA metabolism catalyzed by P450s 2B1, 2B4, 2B5, and seven site-specific 2B1 mutants was studied in a reconstituted Escherichia coli expression system to identify residues that contribute to the unique activities and substrate specificities of these enzymes. The catalytic efficiency of CPA 4-hydroxylation by rat P450 2B1 was 10- to 35-fold higher than that of rabbit P450 2B4 or 2B5. With IFA, approximately 50% of metabolism proceeded via N-dechloroethylation for 2B1 and 2B4, whereas CPA N-dechloroethylation corresponded to only approximately 3% of total metabolism (2B1) or was absent (2B4, 2B5). Improved catalytic efficiency of CPA and IFA 4-hydroxylation was obtained upon substitution of 2B1 Ile-114 by Val, and replacement of Val-363 by Leu or Ile selectively suppressed CPA N-dechloroethylation >or=90%. P450 2B1-V367A, containing the Ala replacement found in 2B5, exhibited only approximately 10% of wild-type 2B1 activity for both substrates. Canine P450 2B11, which has Val-114, Leu-363, and Val-367, was therefore predicted to be a regioselective CPA 4-hydroxylase with high catalytic efficiency. Indeed, P450 2B11 was 7- to 8-fold more active as a CPA and IFA 4-hydroxylase than 2B1, exhibited a highly desirable low K(m) (80-160 microM), and catalyzed no CPA N-dechloroethylation. These findings provide insight into the role of specific P450 2B residues in oxazaphosphorine metabolism and pave the way for gene therapeutic applications using P450 enzymes with improved catalytic activity toward these anticancer prodrug substrates.

Sensitivity of renal cell carcinoma to aminoflavone: role of CYP1A1

PURPOSE

The aminoflavone analogue (AF) exhibits antitumor activity in vitro, particularly against neoplastic cells of renal origin. We identified cellular correlates of responsiveness to AF in continuous human tumor renal cell carcinoma lines and in tumor cell isolates, termed renal carcinoma cell strains, from patients with clear cell and papillary renal neoplasms.

MATERIALS AND METHODS

In vitro antiproliferative activity of AF was evaluated using the sulforhodamine B protein dye assay. In vivo antitumor activity of the drug was determined in mice bearing xenografts. Covalent binding of AF/metabolite(s) was assessed following exposure of cells to AF for 16 hours. CYP1A1 and CYP1B1 mRNA and apoptosis were quantitated by reverse transcriptase-polymerase chain reaction and enzyme-linked immunosorbent assay, respectively.

RESULTS

AF produced total growth inhibition in vitro in 3 of 6 human tumor renal cell lines at concentrations of 90 to 400 nM. In vivo treatment of mice bearing xenografts of the Caki-1 renal cell carcinoma, sensitive to AF in vitro, resulted in significant antitumor activity, including tumor-free survivors. Studies in 13 renal cell strains isolated from patients with clear cell (9) or papillary (4) renal cell carcinoma indicated that 3 of 4 papillary strains were sensitive to AF compared with 2 of 9 clear cell strains. AF sensitive renal cell lines and strains exhibited induction of CYP1A1 and CYP1B1 gene expression, increased covalent binding of AF metabolite(s) and apoptosis.

CONCLUSIONS

AF has noteworthy antitumor activity against certain human tumor renal cell lines in vitro and in vivo, which correlates with drug metabolism to covalently binding metabolites after CYP1A1 and CYP1B1 gene expression. We hypothesize that it leads to apoptosis induction. AF sensitive renal cell strains are predominantly of the papillary histological type. These results are limited by the small numbers of cell lines and cell strains but they are suggestive of the need for further testing in larger collections of cell strains.

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.

Bystander cell killing spreading from endothelial to tumor cells in a three-dimensional multicellular nodule model after Escherichia coli nitroreductase gene delivery

Tumor cells are elusive targets for standard anticancer chemotherapy due to their heterogeneity and genetic instability. On the other hand, proliferating host endothelial cells (ECs) are genetically stable and have a low mutational rate. Thus, antiangiogenic therapy directed against tumor's ECs should, in principle, improve the efficacy of antitumor therapy by inducing little or no drug resistance. Here we present a gene-directed enzyme prodrug therapy (GDEPT) strategy for targeting the tumor vasculature, using the Escherichia coli nitroreductase (ntr) gene delivery associated with the treatment with the prodrug CB1954. In a first time we demonstrated the ability of the ntr/CB1954 system to induce an apoptotic-mediated cell death on monolayer cultures of human umbilical vein ECs (HUV-EC-C). Then, when ntr-transfected HUV-EC-C cells (HUV-EC-C/ntr(+)) were associated in a three-dimensional (3-D) multicellular nodule model with untransfected B16-F10 murine melanoma cell line, we observed a CB1954-mediated bystander cell killing effect from endothelial to neighboring melanoma cells. To our knowledge, this is the first report indicating that GDEPT-based antiangiogenic targeting may be an effective approach for cancer treatment relied on the spreading of the bystander effect from endothelial to tumor cells.

Sustained P450 expression and prodrug activation in bolus cyclophosphamide-treated cultured tumor cells. Impact of prodrug schedule on P450 gene-directed enzyme prodrug therapy

Cytochrome P450-based gene therapy can substantially increase the sensitivity of tumor cells to P450-activated cancer chemotherapeutic prodrugs such as cyclophosphamide (CPA) without increasing host toxicity. While the role of 4-OH-CPA, the primary active metabolite of CPA, in eliciting tumor cell death is well established, the effect of 4-OH-CPA exposure on the capacity of P450-expressing tumor cells for continued metabolism and activation of CPA has not been investigated. The present study addresses this question and characterizes the impact of CPA dose and treatment schedule on the ability of P450-expressing tumor cells to sustain prodrug activation over time. 9L gliosarcoma cells expressing human P450 2B6 and treated with CPA in a continuous manner exhibited a time- and CPA dose-dependent decrease in P450-catalyzed CPA 4-hydroxylase activity. This decrease reflects a selective, 4-OH-CPA-induced loss of cellular P450 protein content. By contrast, when the P450-expressing tumor cells were treated with CPA as a single 8 hours exposure, cellular CPA 4-hydroxylase activity and P450 protein expression were substantially prolonged when compared to continuous prodrug treatment. This schedule-dependent effect of CPA was influenced by the level of P450 protein expressed in the tumor cells. At high P450 protein and activity levels, which could be achieved by culturing the tumor cells at high cell density, net production and release of 4-OH-CPA into the culture media was increased substantially. This increase fully offset the decline in CPA 4-hydroxylase activity as the tumor cells underwent CPA-induced apoptotic death. These findings demonstrate the impact of CPA dose and treatment schedule on the efficacy of P450 gene-directed enzyme prodrug therapy, with bolus CPA treatment being compatible with sustained expression of P450 protein and maintenance of P450-dependent prodrug activation by the target tumor tissue.

Identification of novel enzyme-prodrug combinations for use in cytochrome P450-based gene therapy for cancer

Gene-directed enzyme prodrug therapy can be used to increase the therapeutic activity of anti-cancer prodrugs that undergo liver cytochrome P450 (CYP)-catalyzed prodrug to active drug conversion. The present report describes a cell-culture-based assay to identify CYP gene-CYP prodrug combinations that generate bystander cytotoxic metabolites and that may potentially be useful for CYP-based gene therapy for cancer. A panel of rat liver microsomes, comprising distinct subsets of drug-inducible hepatic CYPs, was evaluated for prodrug activation in a four-day 9L gliosarcoma cell growth inhibition assay. A strong NADPH- and liver microsome-dependent increase in 9L cytotoxicity was observed for the CYP prodrugs cyclophosphamide, ifosfamide, and methoxymorpholinyl doxorubicin (MMDX) but not with three other CYP prodrugs, procarbazine, dacarbazine, and tamoxifen. MMDX activation was potentiated approximately 250-fold by liver microsomes from dexamethasone-induced rats (IC(50) (MMDX) approximately 0.1nM), suggesting that dexamethasone-inducible CYP3A enzymes contribute to activation of this novel anthracycline anti-tumor agent. This CYP3A dependence was verified in studies using liver microsomes from uninduced male and female rats and by using the CYP3A-selective inhibitors troleandomycin and ketoconazole. These findings highlight the advantages of using cell culture assays to identify novel CYP prodrug-CYP gene combinations that are characterized by production of cell-permeable, cytotoxic metabolites and that may potentially be incorporated into CYP-based gene therapies for cancer treatment.

Cytochrome P450-based cancer gene therapy: current status

Results from a number of preclinical studies have demonstrated that a P450-based gene-directed enzyme prodrug therapy (GDEPT) strategy for the treatment of cancer is both safe and efficacious. This strategy has now moved forward into the clinic. At least two different approaches using different delivery methods (retroviral vector MetXia [Oxford BioMedica] and encapsulated P450 expressing cells), different cytochrome P450 isoforms (human CYP2B6 versus rat CYP2B1) and different prodrugs (cyclophosphamide [CPA] versus ifosfamide [IFA]) have concluded Phase I/II clinical trial with encouraging results. In the future, P450-based GDEPT can potentially be further enhanced by improved vectors for P450 gene delivery and disease-targeted promoters for focused gene expression at the target site. In addition, there is scope for developing synthetic P450s and their respective prodrugs to improve both enzyme kinetics and the profile of the active moiety.