Enantioselective induction of cyclophosphamide metabolism by phenytoin

Candidate Proficiency Test Chemicals to Address Industrial Chemical Applicability Domains for in vitro Human Cytochrome P450 Enzyme Induction

Cytochrome P450 (CYP) enzymes play a key role in the metabolism of both xenobiotics and endogenous chemicals, and the activity of some CYP isoforms are susceptible to induction and/or inhibition by certain chemicals. As CYP induction/inhibition can bring about significant alterations in the level of in vivo exposure to CYP substrates and metabolites, CYP induction/inhibition data is needed for regulatory chemical toxicity hazard assessment. On the basis of available human in vivo pharmaceutical data, a draft Organisation for Economic Co-operation and Development Test Guideline (TG) for an in vitro CYP HepaRG test method that is capable of detecting the induction of four human CYPs (CYP1A1/1A2, 2B6, and 3A4), has been developed and validated for a set of pharmaceutical proficiency chemicals. However to support TG adoption, further validation data was requested to demonstrate the ability of the test method to also accurately detect CYP induction mediated by industrial and pesticidal chemicals, together with an indication on regulatory uses of the test method. As part of "GOLIATH", a European Union Horizon-2020 funded research project on metabolic disrupting chemical testing approaches, work is underway to generate supplemental validated data for an additional set of chemicals with sufficient diversity to allow for the approval of the guideline. Here we report on the process of proficiency chemical selection based on a targeted literature review, the selection criteria and considerations required for acceptance of proficiency chemical selection for OECD TG development (i.e. structural diversity, range of activity, relevant chemical sectors, global restrictions etc). The following 13 proposed proficiency chemicals were reviewed and selected as a suitable set for use in the additional validation experiments: tebuconazole, benfuracarb, atrazine, cypermethrin, chlorpyrifos, perfluorooctanoic acid, bisphenol A, N,N-diethyl-m-toluamide, benzo-[a]-pyrene, fludioxonil, malathion, triclosan, and caffeine. Illustrations of applications of the test method in relation to endocrine disruption and non-genotoxic carcinogenicity are provided.

Inhibition and induction of CYP enzymes in humans: an update

The cytochrome P450 (CYP) enzyme family is the most important enzyme system catalyzing the phase 1 metabolism of pharmaceuticals and other xenobiotics such as herbal remedies and toxic compounds in the environment. The inhibition and induction of CYPs are major mechanisms causing pharmacokinetic drug–drug interactions. This review presents a comprehensive update on the inhibitors and inducers of the specific CYP enzymes in humans. The focus is on the more recent human in vitro and in vivo findings since the publication of our previous review on this topic in 2008. In addition to the general presentation of inhibitory drugs and inducers of human CYP enzymes by drugs, herbal remedies, and toxic compounds, an in-depth view on tyrosine-kinase inhibitors and antiretroviral HIV medications as victims and perpetrators of drug–drug interactions is provided as examples of the current trends in the field. Also, a concise overview of the mechanisms of CYP induction is presented to aid the understanding of the induction phenomena.

Validation of in vitro methods for human cytochrome P450 enzyme induction: Outcome of a multi-laboratory study

CYP enzyme induction is a sensitive biomarker for phenotypic metabolic competence of in vitro test systems; it is a key event associated with thyroid disruption, and a biomarker for toxicologically relevant nuclear receptor-mediated pathways. This paper summarises the results of a multi-laboratory validation study of two in vitro methods that assess the potential of chemicals to induce cytochrome P450 (CYP) enzyme activity, in particular CYP1A2, CYP2B6, and CYP3A4. The methods are based on the use of cryopreserved primary human hepatocytes (PHH) and human HepaRG cells. The validation study was coordinated by the European Union Reference Laboratory for Alternatives to Animal Testing of the European Commission's Joint Research Centre and involved a ring trial among six laboratories. The reproducibility was assessed within and between laboratories using a validation set of 13 selected chemicals (known human inducers and non-inducers) tested under blind conditions. The ability of the two methods to predict human CYP induction potential was assessed. Chemical space analysis confirmed that the selected chemicals are broadly representative of a diverse range of chemicals. The two methods were found to be reliable and relevant in vitro tools for the assessment of human CYP induction, with the HepaRG method being better suited for routine testing. Recommendations for the practical application of the two methods are proposed.

Enantioselectivity in the Metabolism of Cyclophosphamide in Patients With Multiple or Systemic Sclerosis

The aim of this study was to evaluate the enantioselective pharmacokinetics of cyclophosphamide and its metabolites 4-hydroxycyclophosphamide and carboxyethylphosphoramide mustard in patients with systemic or multiple sclerosis. Patients with systemic sclerosis (n = 10) or multiple sclerosis (n = 10), genotyped for the allelic variants of CYP2C9*2 and CYP2C9*3 and of the CYP2B6 G516T polymorphism, were treated with 50 mg cyclophosphamide/kg daily for 4 days. Serial blood samples were collected up to 24 hours after administration of the last cyclophosphamide dose. Cyclophosphamide, 4-hydroxycyclophosphamide, and carboxyethylphosphoramide enantiomers were analyzed in plasma samples using liquid chromatography-tandem mass spectrometry coupled to chiral column Chiralcel OD-R or Chiralpak AD-RH. Cytokines IL-2, IL-4, IL-6, IL-8, IL-10, IL- 12p70, IL-17, TNF-α, and INT-δ in the plasma samples collected before cyclophosphamide infusion were analyzed by Milliplex MAP human cytokine/chemokine. Pharmacokinetic parameters showed higher plasma concentrations of (S)-(-)-cyclophosphamide (AUC 215.0 vs 186.2 μg·h/mL for multiple sclerosis patients and 219.1 vs 179.2 μg·h/mL for systemic sclerosis patients) and (R)-4-hydroxycyclophosphamide (AUC 5.6 vs 3.7 μg·h/mL for multiple sclerosis patients and 6.3 vs 5.6 μg·h/mL for systemic sclerosis patients) when compared to their enantiomers in both groups of patients, whereas the pharmacokinetics of the carboxyethylphosphoramide metabolite was not enantioselective. Cytokines' plasma concentrations were similar between multiple and systemic sclerosis groups. The pharmacokinetics of cyclophosphamide is enantioselective in patients with systemic sclerosis and multiple sclerosis, with higher plasma concentrations of the (S)-(-)-cyclophosphamide enantiomer due to the preferential formation of the (R)-4-hydroxycyclophosphamide metabolite.

Pharmacokinetics of cyclophosphamide enantiomers in patients with breast cancer

PURPOSE

Adjuvant chemotherapy with cyclophosphamide (CYC) is used for the treatment of breast cancer. CYC is used as a racemic mixture, although preclinical data have demonstrated differences in the efficacy and toxicity of its enantiomers, with (S)-(-)-CYC exhibiting a higher therapeutic index. The present study investigated the enantioselectivity and influence of CYP2B6, CYP2C9, CYP2C19, and CYP3A on the kinetic disposition of CYC in patients with breast cancer.

METHODS

Fifteen patients previously submitted to removal of the tumor and treated with racemic CYC (900 or 1,000 mg/m(2)) and epirubicin were included in the study. The in vivo activity of CYP3A was evaluated using midazolam as a marker drug. Serial blood samples were collected up to 24 h after administration of the first cycle of CYC.

RESULTS

The kinetic disposition of CYC was enantioselective in patients with breast cancer, with plasma accumulation of the (S)-(-)-CYC enantiomer (AUC 195.0 vs. 174.8 μg h/mL) due to the preferential clearance of the (R)-(+)CYC enantiomer (5.1 vs. 5.7 L/h). Clearance of either CYC enantiomer did not differ between the CYP2B6, CYP2C9, and CYP2C19 genotypes or as a function of the in vivo activity of CYP3A evaluated by midazolam clearance.

CONCLUSIONS

The pharmacokinetics of CYC is enantioselective in patients with breast cancer concomitantly treated with epirubicin and ondansetron. Genotyping or phenotyping did not contribute to adjustment of the CYC dose regimen in patients included in this study.

Cyclophosphamide and cancer: golden anniversary

Cyclophosphamide remains one of the most successful and widely utilized antineoplastic drugs. Moreover, it is also a potent immunosuppressive agent and the most commonly used drug in blood and marrow transplantation (BMT). It was initially synthesized to selectively target cancer cells, although the hypothesized mechanism of tumor specificity (activation by cancer cell phosphamidases) transpired to be irrelevant to its activity. Nevertheless, cyclophosphamide's unique metabolism and inactivation by aldehyde dehydrogenase is responsible for its distinct cytotoxic properties. Differential cellular expression of aldehyde dehydrogenase has an effect on the anticancer therapeutic index and immunosuppressive properties of cyclophosphamide. This Review highlights the chemistry, pharmacology, clinical toxic effects and current clinical applications of cyclophosphamide in cancer and autoimmune disorders. We also discuss the development of high-dose cyclophosphamide for BMT and the treatment of autoimmune diseases.

Pharmacokinetics of once-daily IV busulfan as part of pretransplantation preparative regimens: a comparison with an every 6-hour dosing schedule

In pretransplantation therapy busulfan is typically given every 6 hours. We infused busulfan once daily at 130 mg/m2 for 4 days, performing pharmacokinetic analyses on plasma concentration-time data (n = 60 patients) on days 1, 3, and/or 4. Mean (percent coefficient of variation) maximum concentration, volume of distribution, half-life, and clearance were 3.6 microg/mL (13.8%), 22.6 L/m2 (20.2%), 2.73 hours (27.5%), and 109 mL/min/m2 (26%), respectively. The mean (percent coefficient of variation) and median daily areas under the curve were 4873 (21.8%) and 4871 microM x minute. Intrapatient variability in day-to-day estimated clearance was <20%, without day-to-day drug accumulation. The pharmokinetic parameters were compared with those from 47 patients given intravenous busulfan at approximately 0.8 mg/kg (approximately 32 mg/m2) every 6 hours. We conclude that there is (1) a dose proportionality based on mean and median areas under the curve, (2) unchanged estimated clearance with a 4-fold increase in dose and a 2.5-fold difference in dosing rate, (3) negligible variability in dose-to-dose pharmacokinetics and negligible interdose accumulation with once-daily administration, and (4) no change in pharmokinetic parameter(s) with concomitant use of imidazole antifungals, oral contraceptives, or phenytoin. In summary, intravenous busulfan has highly predictable, linear pharmacokinetics from 32 mg/m2 (approximately 0.8 mg/kg) to 130 mg/m2 (approximately 3.2 mg/kg). Further, once-daily intravenous busulfan dosing is convenient, favoring its more widespread application.

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.

Clinical pharmacokinetics of cyclophosphamide

Cyclophosphamide is an extensively used anticancer and immunosuppressive agent. It is a prodrug undergoing a complicated process of metabolic activation and inactivation. Technical difficulties in the accurate determination of the cyclophosphamide metabolites have long hampered the assessment of the clinical pharmacology of this drug. As these techniques are becoming increasingly available, adequate description of the pharmacokinetics of cyclophosphamide and its metabolites has become possible. There is incomplete understanding on the role of cyclophosphamide metabolites in the efficacy and toxicity of cyclophosphamide therapy. However, relationships between toxicity (cardiotoxicity, veno-occlusive disease) and exposure to cyclophosphamide and its metabolites have been established. Variations in the balance between metabolic activation and inactivation of cyclophosphamide owing to autoinduction, dose escalation, drug-drug interactions and individual differences have been reported, suggesting possibilities for optimisation of cyclophosphamide therapy. Knowledge of the pharmacokinetics of cyclophosphamide, and possibly monitoring the pharmacokinetics of cyclophosphamide in individuals, may be useful for improving its therapeutic index.

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.

Human constitutive androstane receptor mediates induction of CYP2B6 gene expression by phenytoin

Compared with its rodent orthologs, little is known about the chemical specificity of human constitutive androstane receptor (hCAR) and its regulation of hepatic enzyme expression. Phenytoin (PHY), a widely used antiepileptic drug, is a potent inducer of CYP2B6 in primary human hepatocytes, but does not activate human pregnane X receptor (PXR) significantly in cell-based transfection assays at the same concentrations associated with potent induction of CYP2B6. Based on this observation, we hypothesized that PHY may be a selective activator of hCAR. In primary human hepatocytes, expression of CYP2B6 reporter genes containing phenobarbital-responsive enhancer module (PBREM) or PBREM/xenobiotic-responsive enhancer module (XREM) response elements were activated up to 14- and 28-fold, respectively, by 50 microm PHY. By contrast, parallel experiments in HepG2 cell lines co-transfected with an hPXR expression vector did not show increased reporter activity. These results indicated that a PXR-independent pathway, which is retained in primary hepatocytes, is responsible for PHY induction of CYP2B6. Further experiments revealed that PHY effectively translocates hCAR from the cytoplasm into the nucleus in both primary human hepatocytes and CAR(-/-) mice. Compared with vehicle controls, PHY administration significantly increased CYP2B6 reporter gene expression, when this reporter construct was delivered together with hCAR expression vector into CAR(-/-) mice. However, PHY did not increase reporter gene expression in CAR(-/-) mice in the absence of hCAR vector, implying that CAR is essential for mediating PHY induction of CYP2B6 gene expression. Taken together, these observations demonstrate that, in contrast to most of the known CYP2B6 inducers, PHY is a selective activator of CAR in humans.

REGULATION OF CYP2B6 IN PRIMARY HUMAN HEPATOCYTES BY PROTOTYPICAL INDUCERS

The objectives of this study were to evaluate the ability of 14 compounds, which differentially activate human pregnane X receptor (hPXR), to induce CYP2B6 expression and to compare CYP2B6 and CYP3A4 concentration- and time-dependent induction by select inducers. Three primary human hepatocyte preparations were treated daily for 3 days with three concentrations of all compounds. Additional concentration- and/or time-response studies were conducted with clotrimazole, phenytoin, phenobarbital, and rifampin in six preparations. CYP2B6 and CYP3A4 protein and activities were assessed by Western blotting, bupropion hydroxylation, and testosterone 6beta-hydroxylation, respectively. To evaluate hPXR activation by the 14 compounds, reporter gene assays were conducted using Huh7 cells cotransfected with hPXR and a CYP2B6 (NR1)5-LUC reporter plasmid. Clotrimazole, phenobarbital, rifampin, and ritonavir strongly induced CYP2B6 and activated hPXR; dexamethasone t-butylacetate and sulfinpyrazone induced CYP2B6 weakly and activated hPXR moderately; paclitaxel strongly activated hPXR but did not increase CYP2B6 expression; carbamazepine and phenytoin moderately or strongly increased CYP2B6 expression but weakly activated hPXR; and dexamethasone, methotrexate, probenecid, sulfadimidine, and troleandomycin demonstrated weak or negligible effects on CYP2B6 and hPXR. EC50 values for CYP2B6 and CYP3A4 induction by clotrimazole, phenobarbital, phenytoin, and rifampin were strongly correlated (r2 = 0.99) and were statistically indistinguishable for clotrimazole, phenytoin, and rifampin. Kinetic constants governing time-dependent induction by phenobarbital and rifampin were also similar between CYP2B6 and CYP3A4. These results indicate that CYP2B6 is highly inducible by known CYP3A4 inducers and suggest that hPXR is a major determinant of CYP2B6-inducible expression for many, but not all, compounds evaluated in this study.

Gabapentin to control seizures in children undergoing cancer treatment

Hepatic clearance of chemotherapy drugs is increased by many antiepilepsy drugs. At our institution, new-onset seizures in children on chemotherapy are treated with gabapentin, a nonhepatic enzyme inducer. The charts of all children given gabapentin for seizures were reviewed. At a median follow-up of 34 months, seizures were controlled in 74% of 50 children given gabapentin monotherapy as initial treatment: 91% of the leukemia group, 57% of the brain tumor group, and 75% of the other tumor group. Seizures were controlled in 49% of 59 children in whom gabapentin was added to other antiepilepsy drugs: 43% of the leukemia group, 53% of the brain tumor group, and 50% of the other tumor group. More than one seizure at presentation, focal neurologic deficits, high-dose methotrexate, brain irradiation, and T2-weighted signal abnormality around the brain tumor cavity predicted uncontrolled seizures. Only 8 children (7%) reported adverse effects, and the drug was discontinued in two. Gabapentin effectively controls seizures in children receiving chemotherapy and is well tolerated.

Clinically important drug interactions in epilepsy: interactions between antiepileptic drugs and other drugs

Antiepileptic drugs (AEDs) are commonly prescribed for long periods, up to a lifetime, and many patients will require treatment with other agents for the management of concomitant or intercurrent conditions. When two or more drugs are prescribed together, clinically important interactions can occur. Among old-generation AEDs, carbamazepine, phenytoin, phenobarbital, and primidone are potent inducers of hepatic enzymes, and decrease the plasma concentration of many psychotropic, immunosuppressant, antineoplastic, antimicrobial, and cardiovascular drugs, as well as oral contraceptive steroids. Most new generation AEDs do not have clinically important enzyme inducing effects. Other drugs can affect the pharmacokinetics of AEDs; examples include the stimulation of lamotrigine metabolism by oral contraceptive steroids and the inhibition of carbamazepine metabolism by certain macrolide antibiotics, antifungals, verapamil, diltiazem, and isoniazid. Careful monitoring of clinical response is recommended whenever a drug is added or removed from a patient's AED regimen.

Recent applications of stereoselective chromatography

Some recent applications of stereoselective chromatography in the fields of clinical pharmacy, drug analysis, food, and natural products are reviewed. The review is documented with up-to-date literature, which will assist further expansion of research in these areas.

Developmental therapeutics in childhood cancer. A perspective from the Children's Oncology Group and the US Food and Drug Administration

Drug development in pediatric oncology has been reviewed, concentrating on overall development issues and COG studies of cytotoxic compounds. A variety of interesting molecules with more specific targeting are becoming available. The challenges that remain include the availability of such compounds for pediatric trial and their study in a timely fashion, and the subsequent incorporation of the new agents into more up-front regimens, with the ultimate shared goal of curing more children with less toxicity.

Metabolism and pharmacokinetics of oxazaphosphorines

The 2 most commonly used oxazaphosphorines are cyclophosphamide and ifosfamide, although other bifunctional mustard analogues continue to be investigated. The pharmacology of these agents is determined by their metabolism, since the parent drug is relatively inactive. For cyclophosphamide, elimination of the parent compound is by activation to the 4-hydroxy metabolite, although other minor pathways of inactivation also play a role. Ifosfamide is inactivated to a greater degree by dechloroethylation reactions. More robust assay methods for the 4-hydroxy metabolites may reveal more about the clinical pharmacology of these drugs, but at present the best pharmacodynamic data indicate an inverse relationship between plasma concentration of parent drug and either toxicity or antitumour effect. The metabolism of cyclophosphamide is of particular relevance in the application of high dose chemotherapy. The activation pathway of metabolism is saturable, such that at higher doses (greater than 2 to 4 g/m2) a greater proportion of the drug is eliminated as inactive metabolites. However, both cyclophosphamide and ifosfamide also act to induce their own metabolism. Since most high dose regimens require a continuous infusion or divided doses over several days, saturation of metabolism may be compensated for, in part, by auto-induction. Although a quantitative distinction may be made between the cytochrome P450 isoforms responsible for the activating 4-hydroxylation reaction and those which mediate the dechloroethylation reactions, selective induction of the activation pathway, or inhibition of the inactivating pathway, has not been demonstrated clinically. Mathematical models to describe and predict the relative contributions of saturation and autoinduction to the net activation of cyclophosphamide have been developed. However, these require careful validation and may not be applicable outside the exact regimen in which they were derived. A further complication is the chiral nature of these 2 drugs, with some suggestion that one enantiomer may have a favourable profile of metabolism over the other. That the oxazaphosphorines continue to be the subject of intensive investigation over 30 years after their introduction into clinical practice is partly because of their antitumour activity. Further advances in analytical and molecular pharmacological techniques may further optimise their use and allow rational design of more selective analogues.