Increased teniposide clearance with concomitant anticonvulsant therapy

Pharmacogenetics of the Central Nervous System-Toxicity and Relapse Affecting the CNS in Pediatric Acute Lymphoblastic Leukemia

Simple Summary

Despite recent improvements in cure rates, pediatric acute lymphoblastic leukemia (ALL) patients remain at risk to develop relapse disease or suffer from therapy-associated side effects. Over 5% of adverse events appear in the central nervous system (CNS) and can impact survival or quality of life of the patients. Inherited genetic variations are possible predictive factors for these adverse events. This retrospective study aimed to investigate if inherited genetic variations in genes encoding drug-metabolizing enzymes and drug transporters localized in the blood-brain barrier are predictive for CNS events. Our results suggest that certain ABCB1, ABCG2 and GSTP1 gene polymorphisms influence CNS toxicity and CNS relapse. A more effective drug-clearance could lead to less toxicity but contribute to a higher chance of relapse and vice versa. Genetic variants in ABCB1, ABCG2 or GSTP1 genes are promising candidates for personalized medicine.

Abstract

Despite improving cure rates in childhood acute lymphoblastic leukemia (ALL), therapeutic side effects and relapse are ongoing challenges. These can also affect the central nervous system (CNS). Our aim was to identify germline gene polymorphisms that influence the risk of CNS events. Sixty single nucleotide polymorphisms (SNPs) in 20 genes were genotyped in a Hungarian non-matched ALL cohort of 36 cases with chemotherapy related acute toxic encephalopathy (ATE) and 544 controls. Five significant SNPs were further analyzed in an extended Austrian-Czech-NOPHO cohort (n = 107 cases, n = 211 controls) but none of the associations could be validated. Overall populations including all nations’ matched cohorts for ATE (n = 426) with seizure subgroup (n = 133) and posterior reversible encephalopathy syndrome (PRES, n = 251) were analyzed, as well. We found that patients with ABCB1 rs1045642, rs1128503 or rs2032582 TT genotypes were more prone to have seizures but those with rs1045642 TT developed PRES less frequently. The same SNPs were also examined in relation to ALL relapse on a case-control matched cohort of 320 patients from all groups. Those with rs1128503 CC or rs2032582 GG genotypes showed higher incidence of CNS relapse. Our results suggest that blood-brain-barrier drug transporter gene-polymorphisms might have an inverse association with seizures and CNS relapse.

Seizures in patients with cancer

Seizures are common in patients with cancer and either result from brain lesions, paraneoplastic syndromes, and complications of cancer treatment or are provoked by systemic illness (metabolic derangements, infections). Evaluation should include a tailored history, neurologic examination, laboratory studies, neuroimaging, and electroencephalogram. In unprovoked seizures, antiepileptic drug (AED) treatment is required, and a nonenzyme-inducing AED is preferred. Treatment of the underlying cancer with surgery, chemotherapy, and radiation therapy also can help reduce seizures. Benzodiazepines are useful in the treatment of both provoked seizures and breakthrough epileptic seizures and as first-line treatment for status epilepticus. Counseling for safety is an important component in the care of a patient with cancer who has seizures. Good seizure management can be challenging but significantly improves the quality of life during all phases of care, including end-of-life care.

Pharmacokinetic interaction between mitotane and etoposide in adrenal carcinoma: a pilot study

Background The combination of mitotane and platinum-etoposide chemotherapy is a front-line treatment in metastatic adrenocortical carcinoma (ACC), although this regimen shows limited efficacy. Pharmacokinetic drug-drug interaction between mitotane, a strong CYP3A4 inducer, and etoposide, which is a substrate of CYP3A4, may contribute to chemoresistance. The aim of this pilot study was to assess the pharmacokinetic interaction between mitotane and etoposide in ACC patients. Methods Five consecutive ACC patients treated with platinum etoposide (120-150 mg/m2 day 1-2-3 at cycle 1), with or without concomitant mitotane, were included. In the absence of limiting toxicity, a dose escalation of etoposide was proposed since cycle 2. Plasma etoposide concentrations were measured using liquid chromatography at 0, 4 and 24 h after each infusion. Clearance and area under the curve (AUC) of etoposide were determined at each cycle. Results Patients received two to six chemotherapy cycles, in association with mitotane (N = 4) or after mitotane discontinuation (N = 1). Etoposide clearance was two-fold higher with concomitant mitotane (4.95 L/h) than after mitotane discontinuation (2.53 L/h, P = 0.014), and 2.5-fold higher than that in reference population not treated with mitotane (1.81 L/h). Etoposide dose escalation was performed in four patients under mitotane, resulting in two minor tumor responses and one severe toxicity (febrile aplasia) at dose of 300 mg/m2/day. Tumor response was associated with higher etoposide AUC (267.3 vs 188.8 mg.h/L, P = 0.04). Conclusion A drug-drug interaction between mitotane and etoposide may contribute to the low efficacy of platinum-etoposide chemotherapy. This pilot study suggests further a potential benefit of increasing etoposide dose in ACC patients receiving mitotane.

A guide to clinically relevant drug interactions in oncology

This paper presents an overview of new information on clinically relevant drug-drug interactions, particular focuses on negative drug interactions in oncology. We have generated a concise table of drug-drug interactions that provides a synopsis of the clinical outcome of the interaction along with a recommendation for management. We have also generated other tables that describe specific interactions with methotrexate and dosing guidelines for cytotoxic drugs in the presence of renal or hepatic dysfunction. Since warfarin is one of the non-anticancer drugs that is commonly used in cancer patients for the treatment and prevention of venous thromboembolism, its interactions with other anticancer drugs that have been reported in literatures were also reviewed in this paper. In general, drug interactions observed in cancer patients may be categorized into pharmacokinetic, pharmacodynamic and pharmaceutic interactions. Pharmacokinetic interactions involve one drug altering the absorption, distribution, metabolism, or excretion of another drug. Interpatient variability in the pharmacokinetic profile of many anticancer agents often complicates the predictability of the antitumor response and toxicities. Among four pharmacokinetic characteristics, drug interactions involving hepatic metabolism is probably the most common and important mechanism responsible for oncologic drug interactions. For example, several anticancer drugs including taxanes, vinca alkaloids, and irinotecan are known to be metabolized by cytochrome CYP3A4. Enzyme-inducing anticonvulsants have been shown to significantly decrease the plasma levels of these anticancer drugs, thereby compromising the anti-tumor effects. N ephrotoxicity or changes in hepatic function caused by some anticancer drugs (e.g., cisplatin, asparaginase) may also have an impact on the pharmacokinetics of the interacting agents. Pharmacodynamic interactions may occur when two or more drugs acting at a common receptor-binding site impact on the pharmacologic action of the object drug, without influencing the pharmacokinetics of each interacting agent. In clinical setting, a decrease of antitumor efficacy was observed in breast cell lines when gemcitabine or vinorelbine were used in combination with paclitaxel. On the other hand, a decreased incidence of thrombocytopenia was seen in patients receiving combination of carboplatin and palcitaxel compared to those receiving carboplatin alone. The third type of drug-drug interaction is known as pharmaceutic interaction. When one drug may alter the physical or chemical compatibility of another drug that utlimately leads to a change in appearance of the solution or a decrease of effectiveness of the drug due to drug inactivation or degradation.

Seizures and cancer: drug interactions of anticonvulsants with chemotherapeutic agents, tyrosine kinase inhibitors and glucocorticoids

Patients with cancer commonly experience seizures. Combined therapy with anticonvulsant drugs (AEDs) and chemotherapeutic drugs or tyrosine kinase inhibitors carries inherent risks on drug-drug interactions (DDIs). In this review, pharmacokinetic studies of AEDs with chemotherapeutic drugs, tyrosine kinase inhibitors, and glucocorticoids are discussed, including data on maximum tolerated dose, drug clearance, elimination half-life, and organ exposure. Enzyme-inducing AEDs (EIAEDs) cause about a 2-fold to 3-fold faster clearance of concurrent chemotherapeutic drugs metabolized along the same pathway, including cyclophosphamide, irinotecan, paclitaxel, and teniposide, and up to 4-fold faster clearance with the tyrosine kinase inhibitors crizotinib, dasatinib, imatinib, and lapatinib. The use of tyrosine kinase inhibitors, particularly imatinib and crizotinib, may lead to enzyme inhibition of concurrent therapy. Many of the newer generation AEDs do not induce or inhibit drug metabolism, but they can alter enzyme activity by other drugs including AEDs, chemotherapeutics and tyrosine kinase inhibitors. Glucocorticoids can both induce and undergo metabolic change. Quantitative data on changes in drug metabolism help to apply the appropriate dose regimens. Because the large individual variability in metabolic activity increases the risks for undertreatment and/or toxicity, we advocate routine plasma drug monitoring. There are insufficient data available on the effects of tyrosine kinase inhibitors on AED metabolism.

The role of nuclear receptors in pharmacokinetic drug–drug interactions in oncology

Drug-drug interactions can have a major impact on treatment outcome in cancer patients. These patients are at high risk of such interactions, because they are treated with combinations of multiple cytotoxic anticancer drugs or hormonal agents often co-administered with prophylactic antiemetics and analgesics to provide palliation. Interactions between drugs can affect the pharmacokinetics of concomitantly administered chemotherapeutic agents. Especially, due to the specific properties of anticancer drugs, such as a narrow therapeutic index and steep dose-toxicity curve, small pharmacokinetic changes can have significant clinical consequences like decreased therapeutic efficacy or increased toxicity. An important mechanism that underlies these interactions is the induction of enzymes or efflux transporters involved in the biotransformation and clearance of anticancer drugs. Several nuclear receptors, like the pregnane X receptor (PXR), constitutively androstane receptor (CAR), have been shown to regulate induction. Activation of these receptors will lead to induction of important enzymes like cytochrome P450 3A4 (CYP3A4), which is involved in the biotransformation of more than 50% of all clinically used drugs. Therefore, concomitant administration of agents that activate PXR will affect the pharmacokinetics of drugs that are substrate for PXRs target genes, which include CYP3A4 and MDR-1. Understanding of the molecular mechanisms that underlie enzyme induction and the identification of (new) drugs involved in pharmacokinetic drug-drug interactions may contribute to the predictability of drug-drug interactions and eventually help to develop safer anticancer regimens.

Pharmacokinetic considerations in the treatment of CNS tumours

Despite aggressive therapy, the majority of primary and metastatic brain tumour patients have a poor prognosis with brief survival periods. This is because of the different pharmacokinetic parameters of systemically administered chemotherapeutic agents between the brain and the rest of the body. Specifically, before systemically administered drugs can distribute into the CNS, they must cross two membrane barriers, the blood-brain barrier (BBB) and blood-cerebrospinal fluid (CSF) barrier (BCB). To some extent, these structures function to exclude xenobiotics, such as anticancer drugs, from the brain. An understanding of these unique barriers is essential to predict when and how systemically administered drugs will be transported to the brain. Specifically, factors such as physiological variables (e.g. blood flow), physicochemical properties of the drug (e.g. molecular weight), as well as influx and efflux transporter expression at the BBB and BCB (e.g. adenosine triphosphate-binding cassette transporters) determine what compounds reach the CNS. A large body of preclinical and clinical research exists regarding brain penetration of anticancer agents. In most cases, a surrogate endpoint (i.e. CSF to plasma area under the concentration-time curve [AUC] ratio) is used to describe how effectively agents can be transported into the CNS. Some agents, such as the topoisomerase I inhibitor, topotecan, have high CSF to plasma AUC ratios, making them valid therapeutic options for primary and metastatic brain tumours. In contrast, other agents like the oral tyrosine kinase inhibitor, imatinib, have a low CSF to plasma AUC ratio. Knowledge of these data can have important clinical implications. For example, it is now known that chronic myelogenous leukaemia patients treated with imatinib might need additional CNS prophylaxis. Since most anticancer agents have limited brain penetration, new pharmacological approaches are needed to enhance delivery into the brain. BBB disruption, regional administration of chemotherapy and transporter modulation are all currently being evaluated in an effort to improve therapeutic outcomes. Additionally, since many chemotherapeutic agents are metabolised by the cytochrome P450 3A enzyme system, minimising drug interactions by avoiding concomitant drug therapies that are also metabolised through this system may potentially enhance outcomes. Specifically, the use of non-enzyme-inducing antiepileptic drugs and curtailing nonessential corticosteroid use may have an impact.

Camptothecin and podophyllotoxin derivatives: inhibitors of topoisomerase I and II - mechanisms of action, pharmacokinetics and toxicity profile

Camptothecins represent an established class of effective agents that selectively target topoisomerase I by trapping the catalytic intermediate of the topoisomerase I-DNA reaction, the cleavage complex. The water-soluble salt camptothecin-sodium - introduced in early trials in the 1960s - was highly toxic in animals, whereas the semisynthetic derivatives irinotecan and topotecan did not cause haemorrhagic cystitis because of their higher physicochemical stability and solubility at lower pH values. Myelosuppression, neutropenia and, to a lesser extent, thrombocytopenia are dose-limiting toxic effects of topotecan. In contrast to the structurally-related topotecan, irinotecan is a prodrug which has to be converted to SN-38, its active form. SN-38 is inactivated by conjugation, thus patients with Gilbert's syndrome and other forms of genetic glucuronidation deficiency are at an increased risk of irinotecan-induced adverse effects, such as neutropenia and diarrhoea. The cytotoxic mechanism of podophyllotoxin is the inhibition of topoisomerase II. Common adverse effects of etoposide include dose-limiting myelosuppression. Hypersensitivity reactions are more common with etoposide and teniposide than with etoposide phosphate because the formulations of the former contain sensitising solubilisers. Leukopenia and thrombocytopenia occur in 65% and 80%, respectively, of patients after administration of conventional doses of teniposide. Anorexia, vomiting and diarrhoea are generally of mild severity after administration of conventional doses of topoisomerase II inhibitors. Clinical pharmacokinetic studies have revealed substantial interindividual variabilities regarding the area under the concentration-time curve values and steady-state concentrations for all drugs reviewed in this article. Irinotecan, etoposide and teniposide are degraded via complex metabolic pathways. In contrast, topotecan primarily undergoes renal excretion. Regarding etoposide and teniposide, the extent of catechol formation over time during drug metabolism may be associated with a higher risk for secondary malignancies.

Supportive Care of Brain Tumor Patients

The supportive care of patients who have brain tumors consists mainly of the treatment of brain edema, seizures, venous thromboembolism, and cognitive dysfunction. Each of these complications may occur in patients who have primary or metastatic brain tumors. The development of any of these complications significantly increases the morbidity and mortality associated with brain tumors. Effective treatment is usually possible, however, and can result in an improved quality of life for these patients.

Seizures and brain tumors

Seizures commonly occur in people with brain tumors. They may be the presenting symptom of a brain tumor, or develop some time after tumor diagnosis. The risk of seizures is greatest when the tumors have a central location, slow growth rate, and when multiple lesions are present. Interactions between anti-epileptic drugs (AEDs), chemotherapeutic agents, and corticosteroids increase the complexity and challenge in managing seizures, and drugs that do not interfere with the cytochrome P-450 enzyme complex and have low protein binding may be preferable. The comparative efficacy and side effects of the various AEDs are not established in brain tumors, so drug choice relies on both the theoretical advantages of pharmacokinetic properties and clinical judgment. Prophylactic anticonvulsant treatment is not advisable in brain tumor patients who have not experienced seizures.

Medical management of patients with brain tumors

The most common medical problems in brain tumor patients include the management of seizures, peritumoral edema, medication side effects, venous thromboembolism (VTE), fatigue and cognitive dysfunction. Despite their importance, there are relatively few studies specifically addressing these issues. There is increasing evidence that brain tumor patients who have not had a seizure do not benefit from prophylactic antiepileptic medications. Patients on corticosteroids are at greater risk of Pneumocystis jerovecii pneumonia and may benefit from prophylactic therapy. There is also growing evidence suggesting that anticoagulation may be more effective than inferior vena cava IVC) filtration devices for treating VTE in brain tumor patients and the risk of hemorrhage with anticoagulation is relatively small. Low-molecular weight heparin may be more effective than coumadin. Medications such as modafinil and methylphenidate have assumed an increasing role in the treatment of fatigue, while donepezil and memantine may be helpful with memory loss.

Recent advances in the treatment of malignant astrocytoma

Malignant gliomas, including the most common subtype, glioblastoma multiforme (GBM), are among the most devastating of neoplasms. Their aggressive infiltration in the CNS typically produces progressive and profound disability--ultimately leading to death in nearly all cases. Improvement in outcome has been elusive despite decades of intensive clinical and laboratory research. Surgery and radiotherapy, the traditional cornerstones of therapy, provide palliative benefit, while the value of chemotherapy has been marginal and controversial. Limited delivery and tumor heterogeneity are two fundamental factors that have critically hindered therapeutic progress. A novel chemoradiotherapy approach, consisting of temozolomide administered concurrently during radiotherapy followed by adjuvant systemic temozolomide, has recently demonstrated a meaningful, albeit modest, improvement in overall survival for newly diagnosed GBM patients. As cell-signaling alterations linked to the development and progression of gliomas are being increasingly elucidated, targeted therapies have rapidly entered preclinical and clinical evaluation. Responses to therapies that function via DNA damage have been associated with specific mediators of resistance that may also be subject to targeted therapies. Other approaches include novel locoregional delivery techniques to overcome barriers of delivery. The simultaneous development of multiple advanced therapies based on specific tumor biology may finally offer glioma patients improved survival.

Pharmacokinetic studies in children with cancer

We reviewed the current status of our knowledge of pharmacokinetics and pharmacodynamics of some anti-neoplastic drugs, used in the treatment of childhood cancer. Extrapolation of data from pharmacokinetic studies in adults to the paediatric population is often not feasible. Specific studies in children are needed. Of all reviewed anti-neoplastic drugs methotrexate appears to be most extensively studied. Methotrexate pharmacokinetics is correlated with toxicity and response to therapy, and it has been shown that individualized adaptive dosing of methotrexate is correlated with a better response to therapy without increasing toxicity in children with ALL and osteosarcoma. Of most of the other reviewed anti-neoplastic drugs it is demonstrated that pharmacokinetics is correlated with toxicity, and of some drugs a relationship of pharmacokinetics with response to therapy is demonstrated as well. In case of cytarabine, etoposide, and teniposide, individualized dosing also appears to be feasible. However, there is no evidence that this strategy improves response to therapy. Specifically data on pharmacokinetic and pharmacodynamic correlations and effect of pharmacokinetically guided, individualized dosing are important for the design of optimal cancer chemotherapy for individual patients. Unfortunately for a considerable number of anti-neoplastic drugs these specific data are lacking in children and future research is needed.

Interactions Between Antiretrovirals and Antineoplastic Drug Therapy

Despite the established impact of highly active antiretroviral therapy (HAART) in reducing HIV-related morbidity and mortality, malignancy remains an important cause of death. Patients who receive the combination of cancer chemotherapy and HAART may achieve better response rates and higher rates of survival than patients who receive antineoplastic therapy alone. However, the likelihood of drug interactions with combined therapy is high, since protease inhibitors (PIs) and non-nucleoside reverse transcriptase inhibitors (NNRTIs) are substrates and potent inhibitors or inducers of the cytochrome P450 (CYP) system. Since many antineoplastic drugs are also metabolised by the CYP system, coadministration with HAART could result in either drug accumulation and possible toxicity, or decreased efficacy of one or both classes of drugs. Although formal, prospective pharmacokinetic interaction studies are not available in most instances, it is possible to infer the nature of drug interactions based on the metabolic fates of these agents. Paclitaxel and docetaxel are both metabolised by the CYP system, although differences exist in the nature of the isoenzymes involved. Case reports describing adverse consequences of concomitant taxane-antiretroviral therapy exist. Although other confounding factors may have been present, these cases serve as reminders of the vigilant monitoring necessary when taxanes and HAART are coadministered. Similarly, vinca alkaloids are substrates of CYP3A4 and are, thus, vulnerable to PI- or NNRTI-mediated changes in their pharmacokinetics. Interactions with the alkylating agents cyclophosphamide and ifosfamide are complicated as a result of the involvement of the CYP3A4 and CYP2B6 isoenzymes in both the metabolic activation of these drugs and the generation of potentially neurotoxic metabolites. Existing data regarding the metabolic fate of the anthracyclines doxorubicin and daunorubicin suggest that clinically detrimental interactions would not be expected with coadministered HAART. Commonly used endocrine therapies are largely substrates of the CYP system and may, therefore, be amenable to modulation by concomitant HAART. In addition, tamoxifen itself has been associated with reduced concentrations of both anastrozole and letrozole, raising the concern that similar inducing properties may adversely affect the outcome of PI- or NNRTI-based therapy. Similarly, dexamethasone is both a substrate and concentration-dependent inducer of CYP3A4; enhanced corticosteroid pharmacodynamics may result with CYP3A4 inhibitors, while the efficacy of concomitant HAART may be compromised with prolonged dexamethasone coadministration. Since PIs and NNRTIs may also induce or inhibit the expression of P-glycoprotein, the potential for additional interactions to arise via modulation of this transporter also exists. Further research delineating the combined safety and pharmacokinetics of antiretrovirals and antineoplastic therapy is necessary.

Carboplatin and teniposide as third-line chemotherapy in patients with recurrent oligodendroglioma or oligoastrocytoma: a phase II study

BACKGROUND

This study was a phase II study of third-line chemotherapy with carboplatin plus teniposide in patients with recurrent oligodendroglioma.

PATIENTS AND METHODS

Patients with oligodendroglioma progressive or recurrent after surgery, radiotherapy and chemotherapy with PCV (lomustine/procarbazine/vincristine) and temozolomide were treated with 350 mg/m(2) carboplatin on day 1, and 50 mg/m(2) teniposide on days 1-3, every 4 weeks.

RESULTS

Response and toxicity were evaluated in all 23 patients enrolled in the study. Two had partial response [8.6%; 95% confidence interval (CI) 1.8% to 28.6%] and 12 stable disease (52.17%; 95% CI 30% to 73%). Median time to progression was 19 weeks (95% CI 11.4-35.0), and 34.8% of the patients (95% CI 20.0% to 61.0%) had progression-free survival at 6 months. Median survival time was 60.7 weeks (95% CI 39.8 to not achieved) and 51% of the patients (95% CI 33.5% to 79.7%) were alive at 12 months. A total of 103 cycles were administered (on average 4.4 per patient; range 1-9). Toxicity was mild and mainly hematological, with grade 4 neutropenia and grade 4 thrombocytopenia in two (8.6%) and three patients (13%), respectively.

CONCLUSIONS

Although the response rate of combined carboplatin and teniposide chemotherapy in heavily pretreated oligodendroglial tumors is moderate, the toxicity is manageable, and delay of progression in responders or stable patients may still confer a relevant clinical benefit.

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.

Interactions between antiepileptic and chemotherapeutic drugs

Cancer and epilepsy commonly co-occur, and concomitant administration of antiepileptic (AEDS) and chemotherapeutic drugs (CTDs) is necessary in many cases. Many drugs are metabolised by the hepatic cytochrome P450 (CYP) isoenzyme system, and coadministration of AEDs and CTDs can lead to clinically relevant interactions by induction or inhibition of enzymes in shared metabolic pathways. These interactions can cause insufficient tumour and seizure control or lead to unforeseen toxicity. Enzyme-inducing AEDs reduce the effects of taxanes, vinca alkaloids, methotrexate, teniposide, and camptothecin analogues. Inhibition of the metabolism of nitrosoureas or etoposide by valproic acid can lead to CTD toxicity. Poor seizure control may result from the combinations of phenytoin with cisplatin or corticosteroids, and valproic acid with methotrexate. Increased toxicity of AEDs can occur when phenytoin is combined with 5-fluorouracil. Use of enzyme-inducing AEDs should be avoided in patients with cancer, particularly in association with chemotherapy. Generally, valproic acid-although not free from interactions-would be the agent of first choice. Some of the newer AEDs not metabolised by the P450 system may prove to be good alternatives.

Medical management of patients with brain tumors

The most common medical problems in brain tumor patients involve the management of seizures, peritumoral edema, medication side effects, and venous thromboembolism. Despite the importance of these issues, there are very few studies specifically addressing them. Nonetheless, there has been some progress in recent years. There is increasing evidence that brain tumor patients who have not had a seizure do not benefit from prophylactic antiepileptic medications. Patients taking corticosteroids are at greater risk of Pneumocystis carinii pneumonia and may benefit from prophylactic therapy. There is also growing evidence suggesting that anticoagulation may be more effective than inferior vena cava filtration devices for treating venous thromboembolism in brain tumor patients and that the risk of hemorrhage with anticoagulation is relatively small.

The importance of drug interactions in epilepsy therapy

Long-term antiepileptic drug (AED) therapy is the reality for the majority of patients diagnosed with epilepsy. One AED will usually be sufficient to control seizures effectively, but a significant proportion of patients will need to receive a multiple AED regimen. Furthermore, polytherapy may be necessary for the treatment of concomitant disease. The fact that over-the-counter drugs and nutritional supplements are increasingly being self-administered by patients also must be considered. Therefore the probability of patients with epilepsy experiencing drug interactions is high, particularly with the traditional AEDs, which are highly prone to drug interactions. Physicians prescribing AEDs to patients with epilepsy must, therefore, be aware of the potential for drug interactions and the effects (pharmacokinetic and pharmacodynamic) that can occur both during combination therapy and on drug discontinuation. Although pharmacokinetic interactions are numerous and well described, pharmacodynamic interactions are few and usually concluded by default. Perhaps the most clinically significant pharmacodynamic interaction is that of lamotrigine (LTG) and valproic acid (VPA); these drugs exhibit synergistic efficacy when coadministered in patients with refractory partial and generalised seizures. Hepatic metabolism is often the target for pharmacokinetic drug interactions, and enzyme-inducing drugs such as phenytoin (PHT), phenobarbitone (PB), and carbamazepine (CBZ) will readily enhance the metabolism of other AEDs [e.g., LTG, topiramate (TPM), and tiagabine (TGB)]. The enzyme-inducing AEDs also enhance the metabolism of many other drugs (e.g., oral contraceptives, antidepressants, and warfarin) so that therapeutic efficacy of coadministered drugs is lost unless the dosage is increased. VPA inhibits the metabolism of PB and LTG, resulting in an elevation in the plasma concentrations of the inhibited drugs and consequently an increased risk of toxicity. The inhibition of the metabolism of CBZ by VPA results in an elevation of the metabolite CBZ-epoxide, which also increases the risk of toxicity. Other examples include the inhibition of PHT and CBZ metabolism by cimetidine and CBZ metabolism by erythromycin. In recent years, a more rational approach has been taken with regard to metabolic drug interactions because of our enhanced understanding of the cytochrome P450 system that is responsible for the metabolism of many drugs, including AEDs. The review briefly discusses the mechanisms of drug interactions and then proceeds to highlight some of the more clinically relevant drug interactions between AEDs and between AEDs and non-AEDs. Understanding the fundamental principles that contribute to a drug interaction may help the physician to better anticipate a drug interaction and allow a graded and planned therapeutic response and, therefore, help to enhance the management of patients with epilepsy who may require treatment with polytherapy regimens.

Cytotoxic chemotherapy: advances in delivery, pharmacology, and testing

Adjuvant treatment of malignant gliomas, the most common types of primary brain tumors, with intravenous (iv) chemotherapy has not significantly improved survival for patients with these forms of cancer. A major factor in the failure of iv chemotherapy is the blood-brain barrier (BBB), a physiologic impediment to the delivery of cytotoxic chemotherapeutic drugs to the central nervous system (CNS). Intra-arterial and intrathecal infusion, blood-brain barrier disruption, high-dose chemotherapy, intratumoral administration, and convection-enhanced delivery are methods developed to overcome the BBB. Although some of these methods may increase the local concentration-time profile, improvement in clinical outcomes has yet to be definitively established. New methods for assessment of drug delivery to the brain tumor will assume increasing importance in the study of new cytotoxic chemotherapeutic drugs for these types of cancer. Pharmacokinetic studies are critical components of these clinical trials and allow assessment of drug delivery to the CNS and brain tumor. Additionally, pharmacokinetic studies will remain an important component of early clinical trials, particularly for identifying significant drug interactions involving the various supporting medications that are typically used in this patient population.

Adverse effect of anticonvulsants on efficacy of chemotherapy for acute lymphoblastic leukaemia

BACKGROUND

Many antileukaemic agents or their metabolites are inactivated by liver enzymes. Most anticonvulsant drugs induce drug-metabolising enzymes and thereby increase the clearance of anticancer agents. We investigated whether anticonvulsants compromise the efficacy of cancer chemotherapy.

METHODS

We identified whom of 716 children treated consecutively for acute lymphoblastic leukaemia at a single academic hospital in the USA between 1984 and 1994 received treatment for 30 days or longer with anticonvulsants (phenytoin, phenobarbital, carbamazepine, or a combination) at the same time as antileukaemic therapy. Cox's proportional-hazards models were used to assess the prognostic significance of anticonvulsants on event-free survival and risk of haematological and central-nervous-system (CNS) relapse, with stratification for treatment protocol.

FINDINGS

40 (5.6%) of 716 patients received anticonvulsants. Use of these drugs was associated with age over 10 years (p=0.003), non-hyperdiploid leukaemia (p=0.031), and T-cell immunophenotype (p=0.022). After adjustment for age and ploidy, anticonvulsant therapy was significantly related to worse event-free survival (hazard ratio 2.67 [95% CI 1.50-4.76]; p=0.0009), haematological relapse (3.40 [1.69-6.88]; p=0.0006), and CNS relapse (2.90 [1.01-8.28]; p=0.047) among the 566 patients with B-lineage leukaemia. No such associations were seen among the 114 patients with T-cell leukaemia (p=0.61, 0.35, and 0.53, respectively). Faster clearance of teniposide (p=0.0001) and methotrexate (p=0.051), but not cytarabine (p=0.26) was found among patients receiving anticonvulsants.

INTERPRETATION

Long-term anticonvulsant therapy increases the systemic clearance of several antileukaemic agents and is associated with lower efficacy of chemotherapy. Alternatives to enzyme-inducing anticonvulsants should be prescribed for patients receiving chemotherapy for acute lymphoblastic leukaemia.

Clinical applications of anticancer drugs targeted to topoisomerase II

Agents which 'poison' the enzyme topoisomerase II, have proven to be useful drugs for cancer treatment. Six antineoplastic drugs, which target topoisomerase II (doxorubicin, daunorubicin, idarubicin, mitoxantrone, etoposide and teniposide) are currently approved for clinical use in the United States. In this paper, the strategies and goals of cancer chemotherapy are summarized for the non-clinician. The use, pharmacology and toxicity of each of the six currently approved topoisomerase II inhibiting agents are reviewed.

Autoregulation of 5-fluorouracil metabolism

5-Fluorouracil (5-FU) is a commonly used anticancer agent for the treatment of gastrointestinal, head and neck, and breast tumours. This study determined the influence of 5-FU on dihydropyrimidine dehydrogenase (DPD) activity, the enzyme responsible for its in vivo degradation. DPD activity was measured in mononuclear cells obtained prior to and after the administration of 5-FU in 20 patients with colorectal cancer. Following the results from the human studies, DPD activity was measured in Sprague-Dawley rat liver up to 72 h after administration of 5-FU 200 mg/kg as a single injection. Total liver P450 content and the production of testosterone metabolites (indicative of CYP3A activity) were also analysed to determine the specificity of 5-FU-associated alteration in rat liver metabolism. Human mononuclear cell DPD activity decreased by a median of 38.7% following the administration of 5-FU (P = 0.001). 5-FU-induced alterations in rat liver DPD were also observed, with the lowest activity occurring 48 h after injection (50% of control activity; P = 0.009). Rat liver DPD activity returned to near control values by 72 h postinjection. Rat liver total P450 content and CYP3A activity were not significantly different in 5-FU treated or control tissues. Thus, 5-FU demonstrates autoregulation of its metabolism through inhibition of DPD activity. Although this inhibition appears to be specific for DPD, the mechanism for enzyme inhibition is not clear. These findings may aid in the design of 5-FU treatment regimens and provide the basis for further studies into the regulation of DPD.

Clinically relevant drug-drug interactions in oncology

Although anticancer agents are one of the most toxic classes of medication prescribed today, there is relatively little information available about clinically relevant drug-drug interactions. Pharmacokinetic drug interactions have been described, including alterations in absorption, catabolism, and excretion. For example, an increased bioavailability of 6-mercaptopurine has been observed when combined with either allopurinol or methotrexate, leading to increased toxicity in some patients. Induction of etoposide or teniposide clearance by anticonvulsants has also been described, resulting in a lower systemic exposure and risk for lower anticancer activity. Alterations in elimination of methotrexate has been observed with probenecid, presumably through competition for renal secretion. There are also several examples of pharmacodynamic interactions. The combination of 5-fluorouracil plus folinic acid results in more efficient inhibition of thymidylate synthase, a finding which is now utilized routinely in the treatment of colorectal cancer. Improvements in the in vitro and early clinical testing now allow a relatively high degree of prediction of potential clinical drug interactions, prior to observations of untoward drug effects. In conclusion, drug interactions among commonly used anticancer agents have been identified. Their clinical significance can have more impact than many other classes of medications due to the narrow therapeutic index of antineoplastic agents and the potential for lethal side-effects. It is only through prospective, preclinical and early clinical evaluation that the presence of clinically significant drug interactions can be identified and the information used to provide better therapy for this significant health problem.

Anticonvulsant usage is associated with an increased risk of procarbazine hypersensitivity reactions in patients with brain tumors

BACKGROUND

Procarbazine usage in brain tumors has a high incidence of hypersensitivity reactions compared with its use in other malignancies. Procarbazine oxidation to a reactive intermediate is enhanced by phenobarbital. Patients with primary brain tumors would have a preferential exposure to anticonvulsants compared to patients with other malignancies.

OBJECTIVE

To determine whether anticonvulsant exposure is associated with procarbazine hypersensitivity reactions in patients with primary brain tumors.

METHODS

This retrospective cohort study included 83 patients with primary brain tumors who were treated with procarbazine between 1981 and 1996 at a university hospital-based regional oncology center. Data were extracted by chart review. The data collected included age, sex, race, tumor type, smoking, alcohol usage, and all concomitant medications, as well as creatinine, aspartate aminotransferase, total bilirubin, and anticonvulsant serum levels. Anticonvulsant exposure was determined by the presence of detectable serum levels. Cases of procarbazine hypersensitivity reactions were identified through a review of progress notes.

RESULTS

There were 20 patients with procarbazine hypersensitivity reactions. A significant association between the exposure to anticonvulsants and the development of procarbazine hypersensitivity reactions was found (p = 0.05). In addition, there was a significant dose-response association between the development of procarbazine hypersensitivity and the presence of therapeutic anticonvulsant serum levels (p = 0.03).

CONCLUSIONS

Concomitant exposure to anticonvulsants is associated with procarbazine hypersensitivity reactions, possibly though a reactive intermediate generated by CYP3A isoform induction. All patients in this cohort received enzyme-inducing anticonvulsants. New anticonvulsants devoid of this property are available. These data support trials that use these newer agents for the prophylaxis of seizures in patients with brain tumors who are to receive procarbazine.

The role of human cytochrome P450 enzymes in the metabolism of anticancer agents: implications for drug interactions

1. Little information is available about the pharmacokinetic interactions of anticancer drugs in man. However, clinically significant drug interactions do occur in cancer chemotherapy, and it is likely that important interactions have not been recognized. 2. Specific cytochrome P450 (CYP) enzymes have been recently shown to be involved in the metabolism of several essential anticancer agents. In particular, enzymes of the CYP3A subfamily play a role in the metabolism of many anticancer drugs, including epipodophyllotoxins, ifosphamide, tamoxifen, taxol and vinca alkaloids. CYP3A4 has been shown to catalyse the activation of the prodrug ifosphamide, raising the possibility that ifosphamide could be activated in tumour tissues containing this enzyme. 3. As examples of recently found, clinically significant interactions, cyclosporin considerably increases plasma doxorubicin and etoposide concentrations. Although cyclosporin and calcium channel blockers may influence the pharmacokinetics of certain anticancer agents by inhibiting their CYP3A mediated metabolism, it is more likely that these P-glycoprotein inhibitors inhibit P-glycoprotein mediated drug elimination. 4. Appropriate caution should be exercised when combining P-glycoprotein inhibitors and potential CYP3A inhibitors with cancer chemotherapy.

Successful treatment of acute lymphoblastic leukemia in a child with cystic fibrosis

A 3 1/2 year old girl with cystic fibrosis who underwent successful treatment for acute lymphoblastic leukemia remains in complete remission 36 months after diagnosis. We also report high clearance rates of three antineoplastic agents in this patient. Drug doses were adjusted to achieve optimal systemic exposure.

Disposition of total and unbound etoposide following high-dose therapy

Total and unbound etoposide pharmacokinetics were studied in 16 adult patients (median age, 34 years; range, 18-61 years) undergoing autologous bone marrow transplantation for advanced lymphoma after receiving high-dose etoposide (35-60 mg/kg) as a single intravenous infusion. Pretreatment values for mean serum albumin and total bilirubin were 3.0 +/- 0.4 g/dl and 0.5 +/- 0.4 mg/dl, respectively. Etoposide plasma concentrations and protein binding (%unbound) were determined by high-performance liquid chromatography (HPLC) and equilibrium dialysis, respectively. Pharmacokinetic parameters for unbound and total etoposide were calculated by nonlinear regression analysis using a two-compartment model. The mean (+/- SD) parameters for total etoposide included: clearance (CL), 31.8 +/- 17.7 ml min-1 m-2; volume of distribution (Vss), 11.5 +/- 5.9 l/m2, and terminal half-life (t1/2 beta), 7.2 +/- 3.7 h. Mean unbound CL was 209.6 +/- 62.7 ml min-1 m-2 and %unbound was 16% +/- 5%. The mean etoposide %unbound was inversely related to serum albumin (r2 = 0.45, P = 0.0043). The mean %unbound at the end of the etoposide infusion was higher than that at the lowest measured concentration (21% vs 13%, respectively; P = 0.017), suggesting that concentration-dependent binding may occur after high etoposide doses. The median total CL was higher in patients with serum albumin concentrations of < or = 3.0 g/dl than in those with levels of > 3.0 g/dl (34.6 vs 23.5 ml min-1 m-2, P = 0.05). Total CL was directly related to %unbound (r2 = 0.61, P = 0.0004). Unbound CL was unrelated to either serum albumin or %unbound. These results demonstrate that hypoalbuminemia is independently associated with an increased etoposide %unbound and rapid total CL after the administration of high-dose etoposide. Unbound CL in hypoalbuminemic patients is unchanged in the presence of normal total bilirubin values.