Modulation of cyclophosphamide-based cytochrome P450 gene therapy using liver P450 inhibitors

Moringa Oleifera Seed Extract Concomitantly Supplemented with Chemotherapy Worsens Tumor Progression in Mice with Triple Negative Breast Cancer and Obesity

Triple negative breast cancer (TNBC) is an aggressive and highly metastatic breast cancer subtype with limited treatment options. Obesity and insulin resistance are associated with a worse prognosis in those with TNBC. Moringa oleifera (moringa) is a tropical edible plant used for both food and medicinal purposes and found to have anti-obesity and anti-cancer effects in vitro and in preclinical models. The anti-cancer effects of moringa seed extract alone and in combination with chemotherapy were evaluated in immunocompromised female mice with diet-induced obesity bearing MDA-MB-231-derived xenograft tumors. Moringa supplementation protected against high-fat diet- and chemotherapy-induced increases in fasting glucose and improved insulin sensitivity. Moringa supplementation alone did not attenuate tumor growth relative to chemotherapy alone, and in combination worsened tumor progression. Moringa supplementation alone reduced angiogenesis, but this effect was abrogated in combination with chemotherapy. Moringa supplementation may be an effective strategy to improve metabolic health in mice with obesity and TNBC and reduce angiogenesis in tumors, but may have a negative interaction when used as a concurrent complementary therapy. Caution should be taken when considering the consumption of moringa seed extracts while receiving chemotherapy for breast cancer treatment. Further investigations of alternative timings of moringa therapy are warranted.

Molecular modeling approaches to address drug-metabolizing enzymes (DMEs) mediated chemoresistance: a review

Resistance against clinically approved anticancer drugs is the main roadblock in cancer treatment. Drug metabolizing enzymes (DMEs) that are capable of metabolizing a variety of xenobiotic get overexpressed in malignant cells, therefore, catalyzing drug inactivation. As evident from the literature reports, the levels of DMEs increase in cancer cells that ultimately lead to drug inactivation followed by drug resistance. To puzzle out this issue, several strategies inclusive of analog designing, prodrug designing, and inhibitor designing have been forged. On that front, the implementation of computational tools can be considered a fascinating approach to address the problem of chemoresistance. Various research groups have adopted different molecular modeling tools for the investigation of DMEs mediated toxicity problems. However, the utilization of these in-silico tools in maneuvering the DME mediated chemoresistance is least considered and yet to be explored. These tools can be employed in the designing of such chemotherapeutic agents that are devoid of the resistance problem. The current review canvasses various molecular modeling approaches that can be implemented to address this issue. Special focus was laid on the development of specific inhibitors of DMEs. Additionally, the strategies to bypass the DMEs mediated drug metabolism were also contemplated in this report that includes analogs and pro-drugs designing. Different strategies discussed in the review will be beneficial in designing novel chemotherapeutic agents that depreciate the resistance problem.

1-Aminobenzotriazole: A Mechanism-Based Cytochrome P450 Inhibitor and Probe of Cytochrome P450 Biology

1-Aminobenzotriazole (1-ABT) is a pan-specific, mechanism-based inactivator of the xenobiotic metabolizing forms of cytochrome P450 in animals, plants, insects, and microorganisms. It has been widely used to investigate the biological roles of cytochrome P450 enzymes, their participation in the metabolism of both endobiotics and xenobiotics, and their contributions to the metabolism-dependent toxicity of drugs and chemicals. This review is a comprehensive evaluation of the chemistry, discovery, and use of 1-aminobenzotriazole in these contexts from its introduction in 1981 to the present.

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.

A Comparison of the In Vitro Inhibitory Effects of Thelephoric Acid and SKF-525A on Human Cytochrome P450 Activity

Thelephoric acid is an antioxidant produced by the hydrolysis of polyozellin, which is isolated from Polyozellus multiplex. In the present study, the inhibitory effects of polyozellin and thelephoric acid on 9 cytochrome P450 (CYP) family members (CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4) were examined in pooled human liver microsomes (HLMs) using a cocktail probe assay. Polyozellin exhibited weak inhibitory effects on the activities of all 9 CYPs examined, whereas thelephoric acid exhibited dose- and time-dependent inhibition of all 9 CYP isoforms (IC50 values, 3.2-33.7 μM). Dixon plots of CYP inhibition indicated that thelephoric acid was a competitive inhibitor of CYP1A2 and CYP3A4. In contrast, thelephoric acid was a noncompetitive inhibitor of CYP2D6. Our findings indicate that thelephoric acid may be a novel, non-specific CYP inhibitor, suggesting that it could replace SKF-525A in inhibitory studies designed to investigate the effects of CYP enzymes on the metabolism of given compounds.

Toward a new age of cellular pharmacokinetics in drug discovery

Pharmacokinetics, pharmacology, and toxicology are the major determinants of the success or failure of candidates during drug development. Because inappropriate pharmacokinetics often leads to inefficacy, even toxicity, pharmacokinetics studies have been regarded as crucial components in drug preclinical and clinical research. However, new data increasingly reveal that drug concentrations in plasma or tissues cannot totally explain the efficacy of drug on the target organ. For most drugs that interact with targets localized in cells, intracellular penetration, accumulation, distribution, and elimination are important parameters governing the efficacy in the target cells. So, there is a pressing need to clarify the cellular pharmacokinetics and thus evaluate the efficacy of drugs in the target cells. This review provides a general overview regarding current knowledge about cellular pharmacokinetics in some specific cells and also summarizes the factors that can influence cellular pharmacokinetics. It concludes by discussing potential strategies for optimizing cellular pharmacokinetics and advocating that global cellular pharmacokinetics studies be conducted in future research toward improving drug efficacy.

Oral glutamine protects against cyclophosphamide-induced cardiotoxicity in experimental rats through increase of cardiac glutathione

OBJECTIVE

This study evaluated the effects of supplemental oral glutamine (GLN) on acute cardiotoxicity of cyclophosphamide (CPA) in experimental rats. The dose-related cardiotoxicity of CPA is associated with a rapid decrease in cardiac glutathione (GSH) and oxidative cardiac injury. GLN is a rate-limiting precursor for GSH synthesis during periods of oxidative and other types of stress when it becomes a conditionally essential amino acid.

METHODS

Forty-four male Fischer 344 rats were randomized into two groups to receive 1 g.kg(-1).d(-1) of GLN or glycine by gavage. After 2 d of prefeeding, each of these groups was further randomized into three subgroups to receive intraperitoneally a lethal dose of CPA (450 mg/kg), a sublethal dose of CPA (200 mg/kg), or saline (controls). Twenty-four hours later all six groups of rats were sacrificed and blood GLN was measured. Cardiac tissue was examined for histopathologic alterations: GSH and oxidized GSH concentrations.

RESULTS

The results showed that dietary GLN decreased cardiac necrosis and maintained normal cardiac GSH levels. Elevated cardiac GSH levels in the GLN group correlated with increased arterial GLN levels. GLN protected against the acute cardiotoxic effects of CPA and significantly improved the short-term survival after lethal and sublethal doses of CPA.

CONCLUSION

These data suggest that GLN may protect against CPA-related cardiac injury through maintenance of cardiac GSH metabolism.

Collaboration between hepatic and intratumoral prodrug activation in a P450 prodrug-activation gene therapy model for cancer treatment

Presently, we investigate the mechanisms whereby intratumoral expression of a cyclophosphamide-activating hepatic cytochrome P450 gene enhances therapeutic activity when cyclophosphamide is given on an every 6-day (metronomic) schedule. In P450-deficient 9L gliosarcomas grown in severe combined immunodeficient mice, metronomic cyclophosphamide substantially decreased tumor microvessel density and induced a approximately 70% loss of endothelial cells that began after the second cyclophosphamide treatment. These responses were accompanied by increased expression of the endogenous angiogenesis inhibitor thrombospondin-1 in tumor-associated host cells but by decreased expression in 9L tumor cells. These antiangiogenic responses preceded tumor regression and are likely key to the therapeutic activity of metronomic cyclophosphamide. Unexpectedly, 9L/2B11 tumors, grown from 9L cells infected with retrovirus encoding the cyclophosphamide-activating P450 2B11, exhibited antiangiogenic responses very similar to 9L tumors. This indicates that the tumor endothelial cell population is well exposed to liver-activated cyclophosphamide metabolites and that intratumoral P450 confers limited additional anti-endothelial cell bystander activity. In contrast, an increase in apoptosis, which preceded the antiangiogenic response, was substantially enhanced by intratumoral P450 2B11 expression. 9L/2B11 tumor regression was accompanied by an overall loss of tumor cellularity and by substantial enlargement of remaining P450-immunoreactive tumor cells as the number of P450-positive tumor cell decreased and the P450 protein content declined with cyclophosphamide treatment. We conclude that metronomic cyclophosphamide regresses P450-expressing tumors by two independent but complementary mechanisms: increased tumor cell killing via intratumoral P450-catalyzed prodrug activation, coupled with strong antiangiogenic activity, which is primarily associated with hepatic prodrug activation.

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.

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.

Oxidation and rearrangements of flavanones by mammalian cytochrome P450

To clarify the metabolic pathways of flavanones in mammals, the metabolism of (+/-)-flavanone and (+/-)-4'-methoxyflavanone by rat liver microsomes and recombinant human P450s in which structural changes are readily identifiable were examined. The beta-nicotinamide adenine dinucleotide phosphate (NADPH)-dependent formation of flavone plus (+/-)-2,3-trans-flavanonol and of 4'-methoxyflavone plus (+/-)-2,3-trans-4'-methoxyflavanonol, respectively, by rat liver microsomes was observed. The same metabolites were generated by recombinant human P450s in addition to the formation of isoflavone from (+/-)-flavanone. The kinetic isotope effects in these reactions were examined using deuterated (+/-)-flavanone and (+/-)-4'-methoxyflavanone. There was a strong isotope effect in the production of flavanonols, but the isotope effect in the production of flavones was small. The results indicated that the P450-mediated conversion of (+/-)-flavanone and of (+/-)-4'-methoxyflavanone to the corresponding metabolites proceeded via abstraction of a hydrogen radical from the C-2- or C-3-position of the flavanone skeleton. The antioxidant properties of flavanone and its metabolites were examined by measuring superoxide-scavenging activity in a xanthine-xanthine oxidase-cytochrome c system. (+/-)-2,3-trans-Flavanonol had higher activity than that of other flavonoids. Flavanones are metabolized by mammalian P450s, providing important information relevant to the metabolism and pharmacological action of dietary flavanones.

Tumoral drug metabolism: overview and its implications for cancer therapy

Drug-metabolizing enzymes (DME) in tumors are capable of biotransforming a variety of xenobiotics, including antineoplastics, resulting in either their activation or detoxification. Many studies have reported the presence of DME in tumors; however, heterogeneous detection methodology and patient cohorts have not generated consistent, firm data. Nevertheless, various gene therapy approaches and oral prodrugs have been devised, taking advantage of tumoral DME. With the need to target and individualize anticancer therapies, tumoral processes such as drug metabolism must be considered as both a potential mechanism of resistance to therapy and a potential means of achieving optimal therapy. This review discusses cytotoxic drug metabolism by tumors, through addressing the classes of the individual DME, their relevant substrates, and their distribution in specific malignancies. The limitations of preclinical models relative to the clinical setting and lack of data on the changes of DME with disease progression and host response will be discussed. The therapeutic implications of tumoral drug metabolism will be addressed-in particular, the role of DME in predicting therapeutic response, the activation of prodrugs, and the potential for modulation of their activity for gain are considered, with relevant clinical examples. The contribution of tumoral drug metabolism to cancer therapy can only be truly ascertained through large-scale prospective studies and supported by new technologies for tumor sampling and genetic analysis such as microarrays. Only then can efforts be concentrated in the design of better prodrugs or combination therapy to improve drug efficacy and individualize therapy.

In Vitro Inhibitory Effect of 1-Aminobenzotriazole on Drug Oxidations in Human Liver Microsomes: a Comparison with SKF-525A

1-Aminobenzotriazole (ABT) is extensively used as a non-specific cytochrome P450 (CYP) inhibitor. In this study, the inhibitory effect of ABT on CYP-dependent drug oxidations was investigated in human liver microsomes (HLM) and compared with that of SKF-525A, another non-specific inhibitor. The following probe activities for human CYP isoforms were determined using pooled HLM: phenacetin O-deethylation (CYP1A2); diclofenac 4'-hydroxylation (CYP2C9); S-mephenytoin 4'-hydroxylation, (CYP2C19); bufuralol 1'-hydroxylation (CYP2D6); chlorzoxazone 6-hydroxylation (CYP2E1); midazolam 1'-hydroxylation, nifedipine oxidation, and testosterone 6beta-hydroxylation (CYP3A). ABT had the strongest inhibitory effect on the CYP3A-dependent drug oxidations and the weakest effect on the diclofenac 4'-hydroxylation. SKF-525A potently inhibited the bufuralol 1'-hydroxylation, but weakly inhibited chlorzoxazone 6-hydroxylation. The inhibitory effects of ABT and SKF-525A were increased by preincubation in some probe reactions, and this preincubation effect was greater in ABT than in SKF-525A. The remarkable IC50 shift (> 10 times) by preincubation with ABT was observed on the phenacetin O-deethylation, chlorzoxazone 6-hydroxylation, and midazolam 1'-hydroxylation. In conclusion, ABT and SKF-525A had a wide range of IC50 values in inhibiting the drug oxidations by HLM with and without preincubation.

Effects of cytochrome P450 inhibitors on agonist-induced Ca2+ responses and production of NO and PGI2 in vascular endothelial cells

Production of endothelium-dependent vascular relaxing factors, such as nitric oxide (NO) and prostaglandin I2 (PGI2), and endothelium-derived hyperpolarizing factor (EDHF), is regulated in part by changes in intracellular Ca2+ concentration ([Ca2+]i) in vascular endothelial cells (ECs). Cytochrome P450 (CYP), shown to mediate endothelium-dependent hyperpolarization via epoxyeicosatrienoic acids, is one of the candidates for EDHF. In this study we tested the hypotheses that CYP might be involved in EC Ca2+ signaling and that CYP activity might be linked with production of vasodilating factors other than EDHF. To this end, structurally different CYP inhibitors including SKF 525A, econazole and miconazole were tested on primary cultured porcine aortic endothelial cells. Intracellular Ca2+ concentration was measured using the fluorescent Ca2+ indicator fura-2/AM. Bradykinin (BK, 10 nM) and thapsigargin (TG 1 microM) provoked large biphasic increases in [Ca2+], which consist of Ca2+ release from intracellular stores and transplasmalemmal Ca2+ entry. SKF 525A dose-dependently (30-100 microM) inhibited BK- and TG-stimulated Ca2+ entry, but not intracellular Ca2+ store release. Econazole (10 microM) and miconazole (10 microM) had the same effect as SKF 525A on the Ca2+ entry. SKF 525A also dose-dependently inhibited BK-stimulated production of NO and PGI2, assessed by measuring cGMP and 6-keto-PGF(1alpha) concentration. These data suggest that, in addition to its regulation of EDHF production, CYP also contributes to the regulation of other endothelium-dependent vasorelaxing factors by modifying EC Ca2+ signaling.

In Vitro Inhibitory Effect of 1-Aminobenzotriazole on Drug Oxidations Catalyzed by Human Cytochrome P450 Enzymes: A Comparison with SKF-525A and Ketoconazole

1-Aminobenzotriazole (ABT) is widely used as a non-specific inhibitor of animal cytochrome P450 (CYP). In the present study, the inhibitory effect of ABT was investigated on drug oxidations catalyzed by human CYP isoforms. This inhibitory effect was compared with that of SKF-525A, another non-specific inhibitor, and ketoconazole, a potent inhibitor of CYP3A. Bacurovirus-expressed recombinant human CYP isoforms were used as an enzyme source. The specific activities for human CYP isoforms are: phenacetin O-deethylation, for CYP1A2; diclofenac 4'-hydroxylation, for CYP2C9; S-mephenytoin 4'-hydroxylation, for CYP2C19; bufuralol 1'-hydroxylation, for CYP2D6; chlorzoxazone 6-hydroxylation, for CYP2E1; testosterone 6beta-hydroxylation, nifedipine oxidation, and midazolam 1'-hydroxylation, for CYP3A4. ABT inhibited both CYP1A2-dependent activity (Ki=330 microM) and CYP2E1-dependent activity (Ki=8.7 microM). In contrast, SKF-525A weakly inhibited CYP1A2-dependent activities (46% inhibition at 1200 microM) and CYP2E1-dependent activities (65% inhibition at 1000 microM). ABT exhibited the highest Ki value for CYP2C9-dependent diclofenac 4'-hydroxylation among those determined by this assay (Ki=3500 microM). Moreover, SKF-525A showed strong inhibition of CYP2D6-dependent bufuralol 1'-hydroxylation (Ki=0.043 microM). Ketoconazole inhibited all tested drug oxidations, however, its inhibitory effect on CYP1A2-dependent activities was very weak (50% inhibition at 120 microM). ABT, SKF-525A, and ketoconazole showed different selectivity and had a wide range of Ki values for the drug oxidations catalyzed by human CYP enzymes. Therefore, we conclude that inhibitory studies designed to predict the contribution of CYP enzymes to the metabolism of certain compounds should be performed using multiple CYP inhibitors, such as ABT, SKF-525A, and ketoconazole.

Prodrugs for Gene-Directed Enzyme-Prodrug Therapy (Suicide Gene Therapy)

This review focuses on the prodrugs used in suicide gene therapy. These prodrugs need to satisfy a number of criteria. They must be efficient and selective substrates for the activating enzyme, and be metabolized to potent cytotoxins preferably able to kill cells at all stages of the cell cycle. Both prodrugs and their activated species should have good distributive properties, so that the resulting bystander effects can maximize the effectiveness of the therapy, since gene transduction efficiencies are generally low. A total of 42 prodrugs explored for use in suicide gene therapy with 12 different enzymes are discussed, particularly in terms of their physiocochemical properties. An important parameter in determining bystander effects generated by passive diffusion is the lipophilicity of the activated form, a property conveniently compared by diffusion coefficients (log P for nonionizable compounds and log D(7) for compounds containing an ionizable centre). Many of the early antimetabolite-based prodrugs provide very polar activated forms that have limited abilities to diffuse across cell membranes, and rely on gap junctions between cells for their bystander effects. Several later studies have shown that more lipophilic, neutral compounds have superior diffusion-based bystander effects. Prodrugs of DNA alkylating agents, that are less cell cycle-specific than antimetabolites and more effective against noncycling tumor cells, appear in general to be more active prodrugs, requiring less prolonged dosing schedules to be effective. It is expected that continued studies to optimize the bystander effects and other properties of prodrugs and the activated species they generate will contribute to improvements in the effectiveness of suicide gene therapy.

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.

Novel treatments and therapies in development for pancreatic cancer

Until recently, 5-fluorouracil was the most widely used treatment for non-resectable pancreatic cancer. This treatment, however, only resulted in a median survival time of approximately 4 months. In the last few years, gemcitabine has rapidly become the new treatment benchmark, due more to its superior clinical benefit rather than to it conferring an increased median survival (approximately 5-6 months). Thus, the outlook for patients with pancreatic cancer is still relatively bleak. A number of new treatment options are presently being investigated. Some of these are combination therapies involving gemcitabine and other chemotherapeutic agents or radiation. Other novel treatment strategies are also already being evaluated in clinical studies. Some of the more promising treatments in development are discussed and evaluated in this article.

Targeting tumors with non-viral gene delivery systems

Targeting therapeutic genes to tumors is an attractive concept in curing malignant diseases. Systemic gene delivery systems are needed for therapeutic applications in which the target cells are not directly accessible, and which can only be reached via the systemic route. Recent developments in the field of non-viral gene delivery have shown that, based on (poly)cationic carrier molecules, DNA can be efficiently targeted to tumors via the bloodstream. Tailor-made synthetic vectors can be used to achieve predominant gene expression in tumor tissue. Therapeutic concepts based, for example, on suicide genes or cytokines, showed encouraging results in preclinical and also in first clinical evaluations.