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

The Adaptation of MCF-7 Breast Cancer Spheroids to the Chemotherapeutic Doxorubicin: The Dynamic Role of Phase I Drug Metabolizing Enzymes

Background/Objectives: Drug resistance (DR) is a major challenge in cancer therapy, contributing to approximately 90% of cancer-related deaths. While alterations in drug metabolism are known to be key drivers of DR, their role-particularly in the early stages of acquired chemoresistance-remains understudied. Phase I drug-metabolizing enzymes (DMEs), especially cytochrome P450s (CYPs), significantly influence the metabolic fate of chemotherapeutic agents, directly affecting drug response. This study aimed to investigate the role of Phase I DMEs in the early metabolic adaptation of breast cancer (BC) MCF-7 cells to doxorubicin (DOX). Methods: Four types of spheroids were generated from MCF-7 cells that were either DOX-sensitive (DOXS) or adapted to low concentrations of the chemotherapeutic agent (DOXA 25, 35, and 45 nM). The expression levels of 92 Phase I DMEs and the activities of specific CYP isoforms were assessed in both DOXS and DOXA spheroids. Results: A total of twenty-four DMEs, including fifteen CYPs and nine oxidoreductases, were found to be differentially expressed in DOXA spheroids. Pathway analysis identified key roles for the differentially expressed DMEs in physiologically relevant pathways, including the metabolism of drugs, arachidonic acid, retinoic acid, and vitamin D. Conclusions: The deconvolution of these pathways highlights a highly dynamic process driving early-stage DOX resistance, with a prominent role of CYP3A-dependent metabolism in DOX adaptation. Our findings provide valuable insights into the underlying molecular mechanisms driving the early adaptation of MCF-7 cells to DOX exposure.

Discovery of anti-colon cancer agents targeting wild-type and mutant p53 using computer-aided drug design

Mutations in the p53 gene are common and occur in over 50% of all cancers, as it is involved in DNA damage repair, cell cycle regulation and apoptosis. Moreover, the p53 gene is mutated in 70% of colon cancers. Therefore, the development of drugs to combat this mutation requires urgent attention. With this in mind, in silico drug design approaches were applied on quinoline derivatives with anticancer activity. In 3D-QSAR study, steric, electrostatic, hydrophobic and H-bond acceptor fields (SEHA) play an important role in prediction and design of new colon cancer compounds. Indeed, the two best CoMSIA/SEHA models with (Q2 = 0.737, R2 = 0.914, R pred 2 = 0.720) and (Q2 = 0.738, R2 = 0.919, R pred 2 = 0.739) show good prediction of human colon carcinoma HCT 116 (p53+/+) and (p53-/-) activities, respectively. Furthermore, the predictive ability and robustness of these models were tested by several validation methods. Molecular docking analyses reveal crucial interactions with the active sites of the p53 protein in both wild type and mutant. Based on these theoretical studies, we designed 10 new compounds with good anticancer activity potential, which were evaluated using ADMET properties. Molecular dynamics simulations were performed to confirm the detailed binding mode of the docking results. Finally, the MM-GBSA based on molecular dynamics simulation confirmed that the designed compounds were able to form stable hydrogen bonding interactions with the crucial residues, which are essential to overcome the p53 mutation in colon cancer.Communicated by Ramaswamy H. Sarma.

The Complex Dynamic of Phase I Drug Metabolism in the Early Stages of Doxorubicin Resistance in Breast Cancer Cells

The altered activity of drug metabolism enzymes (DMEs) is a hallmark of chemotherapy resistance. Cytochrome P450s (CYPs), mainly CYP3A4, and several oxidoreductases are responsible for Phase I metabolism of doxorubicin (DOX), an anthracycline widely used in breast cancer (BC) treatment. This study aimed to investigate the role of Phase I DMEs involved in the first stages of acquisition of DOX-resistance in BC cells. For this purpose, the expression of 92 DME genes and specific CYP-complex enzymes activities were assessed in either sensitive (MCF-7 parental cells; MCF-7/DOX^S) or DOX-resistant (MCF-7/DOX^R) cells. The DMEs genes detected to be significantly differentially expressed in MCF-7/DOX^R cells (12 CYPs and eight oxidoreductases) were indicated previously to be involved in tumor progression and/or chemotherapy response. The analysis of CYP-mediated activities suggests a putative enhanced CYP3A4-dependent metabolism in MCF-7/DOX^R cells. A discrepancy was observed between CYP-enzyme activities and their corresponding levels of mRNA transcripts. This is indicative that the phenotype of DMEs is not linearly correlated with transcription induction responses, confirming the multifactorial complexity of this mechanism. Our results pinpoint the potential role of specific CYPs and oxidoreductases involved in the metabolism of drugs, retinoic and arachidonic acids, in the mechanisms of chemo-resistance to DOX and carcinogenesis of BC.

The Synergistic Antitumor Effect of Tanshinone IIA Plus Adriamycin on Human Hepatocellular Carcinoma Xenograft in BALB/C Nude Mice and Their Influences on Cytochrome P450 CYP3A4 In Vivo

OBJECTIVE

Hepatocellular carcinoma is one of the most common diseases that seriously threaten human life and health. In this study, we evaluated the inhibitory effect of tanshinone IIA (Tan IIA) combined with adriamycin (ADM) on human hepatocellular carcinoma and developed a platform to assess the function if Chinese herbal ingredients combined with chemotherapy drugs have synergistic antitumor effects in vivo.

METHODS

Established animal model of human hepatocarcinoma HepG2 cell in nude mice. Mice were divided into model control group, Tan IIA group, ADM group, and Tan IIA + ADM group. The changes from general condition, weight, tumor volume, and inhibition rate were observed. The data were gathered from serum AST level and histopathological changes. The content and activity of cytochrome P450 were determined by spectrophotometric analysis. CYP3A4 protein expression was analyzed by western blotting. The binding model crystal structure of Tan IIA and ADM with pregnane X receptor (PXR) was evaluated by Discovery Studio 2.1.

RESULTS

A combination of Tan IIA with ADM could improve life quality by relieving ADM toxicity, decreasing tumor volume, declining serum AST level, and improving liner pathological section in tumor-bearing mice. The inhibitory rates of Tan IIA, ADM, and cotreatment were 32.77%, 60.96%, and 73.18%, respectively. The Tan IIA group significantly enhanced the content of cytochrome b5, P450, and erythromycin-N-demethylase activity. CYP3A4 protein expression was enhanced obviously by the Tan IIA + ADM group. Virtual molecular docking showed that both Tan IIA and ADM could be stably docked with the same binding site of PXR but different interactions.

CONCLUSIONS

Tan IIA in combination with ADM could improve the life quality in tumor-bearing mice and enhance the antitumor effect. The Tan IIA group increased the concentration of cytochrome P450 enzymes and activity. Combined Tan IIA with ADM could upregulate the CYP3A4 protein expression and make relevant interaction with protein PXR by virtual docking.

Enhancement of Tumor Cell Death by Combining gef Gene Mediated Therapy and New 1,4-Benzoxazepin-2,6-Dichloropurine Derivatives in Breast Cancer Cells

New treatment modalities are urgently needed to better manage advanced breast cancer. Combination therapies are usually more effective than monotherapy. In this context, the use of cyclic and acyclic O,N-acetals derivative compounds in combination with the suicide gef gene shown a potent anti-tumor activity and represent a new generation of anticancer agents. Here, we evaluate the use of the gef gene to promote and increase the anti-tumor effect of cyclic and acyclic O,N-acetals purine derivatives and elucidate their mechanisms of action. Among all compounds tested, those with a nitro group and a cyclic pattern structures (FC-30b2, FC-29c, and bozepinib) are the most benefited from the gef gene effect. These compounds, in combination with gef gene, were able to abolish tumor cell proliferation with a minimal dose leading to more effective and less toxic chemotherapy. The effect of this combined therapy is triggered by apoptosis induction which can be found deregulated in the later stage of breast cancer. Moreover, the combined therapy leads to an increase of cell post-apoptotic secondary necrosis that is able to promote the immunogenicity of cancer cells leading to a successful treatment. This data suggests that this novel combination therapy represents a promising candidate for breast cancer treatment.

Effects of traditional herbal formulae on human CYP450 isozymes

OBJECTIVE

To assess the effects of traditional herbal formulae Sijunzi Decoction (, Sagunja-tang, SJZD), Siwu Decoction (, Samul-tang, SWD), Bawu Decoction (, Palmul-tang, BWD) and Shiquan Dabu Decoction (, Sipjeondaebo-tang, SDD) on the activities of human cytochrome P450 (CYP450), a drug-metabolizing enzyme.

METHODS

Herbal formula water extracts were filtered and lyophilized after the powder extracts were dissolved in distilled water. The activities of major human CYP450 isozymes (CYP3A4, CYP2C19, CYP2D6 and CYP2E1) were measured using in vitro fluorescence-based enzyme assays. The inhibitory effects of the herbal formulas on the activities of CYP450 were characterized as half maximal inhibition concentration (IC₅₀) values.

RESULTS

All the tested herbal formulae inhibited CYP2C19 activity (IC₅₀: SJZD, 83.28 μg/mL; SWD, 235.54 μg/mL; BWD, 166.82 μg/mL; SDD, 178.19 μg/mL); SJZD (IC₅₀ = 196.46 μg/mL), SWD (IC₅₀ = 333.42 μg/mL) and SDD (IC₅₀ = 163.42 μg/mL) inhibited CYP2E1-mediated metabolism; whereas BWD exhibited comparatively weak inhibition of CYP2E1 (IC₅₀ = 501.78 μg/mL). None of the four herbal formulas significantly affected CYP3A4 or CYP2D6.

CONCLUSIONS

These results suggest that SJZD, SWD, BWD and SDD could potentially inhibit the metabolism of co-administered synthetic drugs whose primary route of elimination is via CYP2C19. In addition, clinically relevant pharmacokinetic interactions could occur when SJZD, SWD or SDD is co-administered with drugs metabolized by CYP2E1. Our findings provide information for the safety and effective clinical use of these four classic herbal formulas.

Role of pharmacogenetics of drug-metabolizing enzymes in treating osteosarcoma

INTRODUCTION

Drug-metabolizing enzymes (DMEs) biotransform several toxins and xenobiotics in both tumor and normal cells, resulting in either their detoxification or their activation. Since DMEs also metabolize several chemotherapeutic drugs, they can significantly influence tumor response to chemotherapy and susceptibility of normal tissues to collateral toxicity of anticancer treatments.

AREAS COVERED

This review discusses the pharmacogenetics of DMEs involved in the metabolism of drugs which constitute the backbone of osteosarcoma (OS) chemotherapy, highlighting what is presently known for this tumor and their possible impact on the modulation of future treatment approaches.

EXPERT OPINION

Achieving further insight into pharmacogenetic markers and biological determinants related to treatment response in OS may ultimately lead to individualized treatment regimens, based on a combination of genotype and tumor characteristics of each patient.

Probing cytochrome P450-mediated activation with a truncated azinomycin analogue

Selective cytochrome P450 bioactivation of truncated azinomycin.

The anticancer drug ellipticine activated with cytochrome P450 mediates DNA damage determining its pharmacological efficiencies: studies with rats, Hepatic Cytochrome P450 Reductase Null (HRN™) mice and pure enzymes

Ellipticine is a DNA-damaging agent acting as a prodrug whose pharmacological efficiencies and genotoxic side effects are dictated by activation with cytochrome P450 (CYP). Over the last decade we have gained extensive experience in using pure enzymes and various animal models that helped to identify CYPs metabolizing ellipticine. In this review we focus on comparison between the in vitro and in vivo studies and show a necessity of both approaches to obtain valid information on CYP enzymes contributing to ellipticine metabolism. Discrepancies were found between the CYP enzymes activating ellipticine to 13-hydroxy- and 12-hydroxyellipticine generating covalent DNA adducts and those detoxifying this drug to 9-hydroxy- and 7-hydroellipticine in vitro and in vivo. In vivo, formation of ellipticine-DNA adducts is dependent not only on expression levels of CYP3A, catalyzing ellipticine activation in vitro, but also on those of CYP1A that oxidize ellipticine in vitro mainly to the detoxification products. The finding showing that cytochrome b5 alters the ratio of ellipticine metabolites generated by CYP1A1/2 and 3A4 explained this paradox. Whereas the detoxification of ellipticine by CYP1A and 3A is either decreased or not changed by cytochrome b5, activation leading to ellipticine-DNA adducts increased considerably. We show that (I) the pharmacological effects of ellipticine mediated by covalent ellipticine-derived DNA adducts are dictated by expression levels of CYP1A, 3A and cytochrome b5, and its own potency to induce these enzymes in tumor tissues, (II) animal models, where levels of CYPs are either knocked out or induced are appropriate to identify CYPs metabolizing ellipticine in vivo, and (III) extrapolation from in vitro data to the situation in vivo is not always possible, confirming the need for these animal models.

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.

Ellipticine oxidation and DNA adduct formation in human hepatocytes is catalyzed by human cytochromes P450 and enhanced by cytochrome b5

Ellipticine is an antineoplastic agent considered a pro-drug, the pharmacological and genotoxic effects of which are dependent on cytochrome P450 (CYP)- and/or peroxidase-mediated activation to covalent DNA adducts. We investigated whether ellipticine-DNA adducts are formed in human hepatic microsomes and human hepatocytes. We then identified which human CYPs oxidize ellipticine to metabolites forming DNA adducts and the effect of cytochrome b(5) on this oxidation. 13-Hydroxyellipticine, the metabolite forming the major ellipticine-DNA adduct, was generated mainly by CYP3A4 and 1A1, followed by CYP2D6>2C19>1B1>1A2>2E1 and >2C9. Cytochrome b(5) increased formation of this metabolite by human CYPs, predominantly by CYP1A1, 3A4, 1A2 and 2C19. Formation of 12-hydroxyellipticine is generated mainly by CYP2C19, followed by CYP2C9>3A4>2D6>2E1 and >2A6. Other CYPs were less active (CYP2C8 and 2B6) or did not oxidize ellipticine to this metabolite (CYP1A1, 1A2 and 1B1). CYP2D6 was the most efficient enzyme generating ellipticine N(2)-oxide. CYP3A4 and 1A1 in the presence of cytochrome b(5) are mainly responsible for bioactivation of ellipticine to DNA adduct 1 (formed by ellipticine-13-ylium from 13-hydroxyellipticine), while 12-hydroxyellipticine generated during the CYP2C19-mediated ellipticine oxidation is the predominant metabolite forming ellipticine-12-ylium that generates ellipticine-DNA adduct 2. These ellipticine-DNA adducts were also generated by human hepatic microsomes and in primary human hepatocytes exposed to ellipticine. Ellipticine is toxic to these hepatocytes, decreasing their viability; the IC(50) value of ellipticine in these cells was 0.7 μM. In liver CYP3A4 is the predominant ellipticine activating CYP species, which is expected to result in efficient metabolism after oral ingestion of ellipticine in humans.

EET signaling in cancer

Inflammation and angiogenesis in the tumor microenvironment are increasingly implicated in tumorigenesis. Endogenously produced lipid autacoids, locally acting small-molecule mediators, play a central role in inflammation and tissue homeostasis. These lipid mediators, collectively referred to as eicosanoids, have recently been implicated in cancer. Although eicosanoids, including prostaglandins and leukotrienes, are best known as products of arachidonic acid metabolism by cyclooxygenases and lipoxygenases, arachidonic acid is also a substrate for another enzymatic pathway, the cytochrome P450 (CYP) system. This eicosanoid pathway consists of two main branches: ω-hydroxylases which converts arachidonic acid to hydroxyeicosatetraenoic acids (HETEs) and epoxygenases which converts it to four regioisomeric epoxyeicosatrienoic acids (EETs; 5,6-EET, 8,9-EET, 11,12-EET, and 14,15-EET). EETs regulate inflammation and vascular tone. The bioactive EETs are produced predominantly in the endothelium and are mainly metabolized by soluble epoxide hydrolase to less active dihydroxyeicosatrienoic acids. EET signaling was originally studied in conjunction with inflammatory and cardiovascular disease. Arachidonic acid and its metabolites have recently stimulated great interest in cancer biology. To date, most research on eicosanoids in cancer has focused on the COX and LOX pathways. In contrast, the role of cytochrome P450-derived eicosanoids, such as EETs and HETEs, in cancer has received little attention. While CYP epoxygenases are expressed in human cancers and promote human cancer metastasis, the role of EETs (the direct products of CYP epoxygenases) in cancer remains poorly characterized. In this review, the emerging role of EET signaling in angiogenesis, inflammation, and cancer is discussed.

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.

Cytochrome P450-derived eicosanoids: the neglected pathway in cancer

Endogenously produced lipid autacoids are locally acting small molecule mediators that play a central role in the regulation of inflammation and tissue homeostasis. A well-studied group of autacoids are the products of arachidonic acid metabolism, among which the prostaglandins and leukotrienes are the best known. They are generated by two pathways controlled by the enzyme systems cyclooxygenase and lipoxygenase, respectively. However, arachidonic acid is also substrate for a third enzymatic pathway, the cytochrome P450 (CYP) system. This third eicosanoid pathway consists of two main branches: ω-hydroxylases convert arachidonic acid to hydroxyeicosatetraenoic acids (HETEs) and epoxygenases convert it to epoxyeicosatrienoic acids (EETs). This third CYP pathway was originally studied in conjunction with inflammatory and cardiovascular disease. Arachidonic acid and its metabolites have recently stimulated great interest in cancer biology; but, unlike prostaglandins and leukotrienes the link between cytochome P450 metabolites and cancer has received little attention. In this review, the emerging role in cancer of cytochrome P450 metabolites, notably 20-HETE and EETs, are discussed.

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.

Gef gene therapy enhances the therapeutic efficacy of doxorubicin to combat growth of MCF-7 breast cancer cells

PURPOSE

The potential use of combined therapy is under intensive study including the association between classical cytotoxic and genes encoding toxic proteins which enhanced the antitumour activity. The main aim of this work was to evaluate whether the gef gene, a suicide gene which has a demonstrated antiproliferative activity in tumour cells, improved the antitumour effect of chemotherapeutic drugs used as first-line treatment in the management of advanced breast cancer.

METHODS

MCF-7 human breast cancer cells were transfected with gef gene using pcDNA3.1-TOPO expression vector. To determine the effect of the combined therapy, MCF-7 transfected and non-transfected cells were exposed to paclitaxel, docetaxel and doxorubicin at different concentrations. The growth-inhibitory effect of gef gene and/or drugs was assessed by MTT assay. Apoptosis modulation was determined by flow cytometric analysis, DNA fragmentation and morphological analysis. Multicellular tumour spheroids (MTS) from MCF-7 cells were used to confirm effectiveness of combined therapy (gef gene and drug).

RESULTS

Our results demonstrate that combined therapy gef gene/drugs (paclitaxel, docetaxel or doxurubicin) caused a decrease in cell viability. However, only the gef-doxorubicin (10 microM) combination induced a greater enhancement in the antitumour activity in MCF-7 cells. Most importantly, this combined strategy resulted in a significant synergistic effect, thus allowing lower doses of the drug to be used to achieve the same therapeutic effect. These results were confirmed using MTS in which volume decrease with combined therapy was greater than obtained using the gene therapy or chemotherapy alone, or the sum of both therapies.

CONCLUSIONS

The cytotoxic effect of gef gene in breast cancer cells enhances the chemotherapeutic effect of doxorubicin. This therapeutic approach has the potential to overcome some of the major limitations of conventional chemotherapy, and may therefore constitute a promising strategy for future applications in breast cancer therapy.

Targeting drug-metabolizing enzymes for effective chemoprevention and chemotherapy

The primary focus of chemoprevention research is the prevention of cancer using pharmacological, biological, and nutritional interventions. Chemotherapeutic approaches that have been used successfully for both the prevention and treatment of a number of human malignancies have arisen from the identification of specific agents and appropriate molecular targets. Although drug-metabolizing enzymes have historically been targeted in attempts to block the initial, genotoxic events associated with the carcinogenic process, emerging evidence supports the idea that manipulating drug-metabolizing enzymes may also be an effective strategy to be used for treating tumor progression, invasion, and, perhaps, metastasis. This report summarizes a symposium that presents some recent progress in this area. One area of emphasis is the development of a CYP17 inhibitor for treatment of prostate cancer that may also have androgen-independent anticancer activity at higher concentrations. A second focus is the use of a mouse model to investigate the effects of aryl hydrocarbon receptor and Cyp1b1 status and chemopreventative agents on transplacental cancer. A third area of focus is the phytochemical manipulation of not only cytochrome P450 (P450) enzymes but also phase II inflammatory and antioxidant enzymes via the nuclear factor-erythroid 2-related factor 2 pathway to block tumor progression. A final highlight is the use of prodrugs activated by P450 enzymes to halt tumor growth and considerations of dosing schedule and targeted delivery of the P450 transgene to tumor tissue. In addition to highlighting recent successes in these areas, limitations and areas that should be targeted for further investigation are discussed.