Alwin D. R. Huitema

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.

Clinical Pharmacokinetics of Therapeutic Monoclonal Antibodies

Monoclonal antibodies (mAbs) have been used in the treatment of various diseases for over 20 years and combine high specificity with generally low toxicity. Their pharmacokinetic properties differ markedly from those of non-antibody-type drugs, and these properties can have important clinical implications. mAbs are administered intravenously, intramuscularly or subcutaneously. Oral administration is precluded by the molecular size, hydrophilicity and gastric degradation of mAbs. Distribution into tissue is slow because of the molecular size of mAbs, and volumes of distribution are generally low. mAbs are metabolized to peptides and amino acids in several tissues, by circulating phagocytic cells or by their target antigen-containing cells. Antibodies and endogenous immunoglobulins are protected from degradation by binding to protective receptors (the neonatal Fc-receptor [FcRn]), which explains their long elimination half-lives (up to 4 weeks). Population pharmacokinetic analyses have been applied in assessing covariates in the disposition of mAbs. Both linear and nonlinear elimination have been reported for mAbs, which is probably caused by target-mediated disposition. Possible factors influencing elimination of mAbs include the amount of the target antigen, immune reactions to the antibody and patient demographics. Bodyweight and/or body surface area are generally related to clearance of mAbs, but clinical relevance is often low. Metabolic drug-drug interactions are rare for mAbs. Exposure-response relationships have been described for some mAbs. In conclusion, the parenteral administration, slow tissue distribution and long elimination half-life are the most pronounced clinical pharmacokinetic characteristics of mAbs.

Piraña and PCluster: A modeling environment and cluster infrastructure for NONMEM

Pharmacokinetic-pharmacodynamic modeling using non-linear mixed effects modeling (NONMEM) is a powerful yet challenging technique, as the software is generally accessed from the command line. A graphical user interface, Piraña, was developed that offers a complete modeling environment for NONMEM, enabling both novice and advanced users to increase efficiency of their workflow. Piraña provides features for the management and creation of model files, the overview of modeling results, creation of run reports and handling of datasets and output tables, and the running of custom R scripts on model output. Through the secure shell (SSH) protocol, Piraña can also be used to connect to Linux clusters (SGE, MOSIX) for distribution of workload. Modeling with NONMEM is computationally burdensome, which may be alleviated by distributing runs to computer clusters. A solution to this problem is offered here, called PCluster. This platform is easy to set up, runs in standard network environments, and can be extended with additional nodes if needed. The cluster supports the modeling toolkit Perl speaks NONMEM (PsN), and can include dedicated or non-dedicated PCs. A daemon script, written in Perl, was designed to run in the background on each node in the cluster, and to manage job distribution. The PCluster can be accessed from Piraña, and both software products have extensively been tested on a large academic network. The software is available under an open-source license.

Drug interactions between antiretroviral drugs and comedicated agents.

HIV-infected individuals usually receive a wide variety of drugs in addition to their antiretroviral drug regimen. Since both non-nucleoside reverse transcriptase inhibitors and protease inhibitors are extensively metabolised by the cytochrome P450 system, there is a considerable potential for pharmacokinetic drug interactions when they are administered concomitantly with other drugs metabolised via the same pathway. In addition, protease inhibitors are substrates as well as inhibitors of the drug transporter P-glycoprotein, which also can result in pharmacokinetic drug interactions. The nucleoside reverse transcriptase inhibitors are predominantly excreted by the renal system and may also give rise to interactions.This review will discuss the pharmacokinetics of the different classes of antiretroviral drugs and the mechanisms by which drug interactions can occur. Furthermore, a literature overview of drug interactions is given, including the following items when available: coadministered agent and dosage, type of study that is performed to study the drug interaction, the subjects involved and, if specified, the type of subjects (healthy volunteers, HIV-infected individuals, sex), anti-retroviral drug(s) and dosage, interaction mechanism, the effect and if possible the magnitude of interaction, comments, advice on what to do when the interaction occurs or how to avoid it, and references.This discussion of the different mechanisms of drug interactions, and the accompanying overview of data, will assist in providing optimal care to HIV-infected patients.

Quantification of protease inhibitors and non-nucleoside reverse transcriptase inhibitors in dried blood spots by liquid chromatography–triple quadrupole mass spectrometry

A bioanalytical method for the determination of most commonly prescribed protease inhibitors (atazanavir, darunavir, lopinavir and ritonavir) and non-nucleoside reverse transcriptase inhibitors (efavirenz and nevirapine) was developed and validated according to FDA guidelines. In brief, dried blood spots were punched out of a collection paper with a 0.25 in. diameter punch. The analytes were extracted from the punched-out disc using a mixture of acetonitrile, methanol and 0.2M zinc sulphate in water (1:1:2, v/v/v) containing the internal standards dibenzepine, 13C6-efavirenz and D5-saquinavir. 20 microL of the extract was injected onto the reversed-phase C18 column (150 mm x 2.0 mm) for separation from endogenous compounds and the analytes were quantified using a triple quadrupole mass spectrometer. The analytical run time was only 10 min. Validated concentration ranges covered the ranges encountered in routine clinical practice. The assay was linear over the concentration ranges tested (0.1-20 mg/L for atazanavir, lopinavir, nevirapine and efavirenz and 0.05-10 mg/L for darunavir and ritonavir). Accuracies and inter- and intra-run precisions at all levels ranged from 96.2 to 113.9% and 3.1 to 13.3%, respectively. Analytes in dried blood spots were stable for at least 7 days at 30 degrees C. The method enabled patient-friendly sample collection, easy and cheap sample shipment and non-hospital based sampling for therapeutic drug monitoring and pharmacokinetic studies.

Pharmacokinetics of Miltefosine in Old World Cutaneous Leishmaniasis Patients

ABSTRACT The pharmacokinetics of miltefosine in leishmaniasis patients are, to a great extent, unknown. We examined and characterized the pharmacokinetics of miltefosine in a group of patients with Old World (Leishmania major) cutaneous leishmaniasis. Miltefosine plasma concentrations were determined in samples taken during and up to 5 months after the end of treatment from 31 Dutch military personnel who contracted cutaneous leishmaniasis in Afghanistan and were treated with 150 mg miltefosine/day for 28 days. Samples were analyzed with a validated liquid chromatography-tandem mass spectrometry assay with a lower limit of quantification (LLOQ) of 4 ng/ml. Population pharmacokinetic modeling was performed with nonlinear mixed-effect modeling, using NONMEM. The pharmacokinetics of miltefosine could best be described by an open two-compartment disposition model, with a first elimination half-life of 7.05 days and a terminal elimination half-life of 30.9 days. The median concentration in the last week of treatment (days 22 to 28) was 30,800 ng/ml. The maximum duration of follow-up was 202 days after the start of treatment. All analyzed samples contained a concentration above the LLOQ. Miltefosine is eliminated from the body much slower than previously thought and is therefore still detectable in human plasma samples taken 5 to 6 months after the end of treatment. The presence of subtherapeutic miltefosine concentrations in the blood beyond 5 months after treatment might contribute to the selection of resistant parasites, and moreover, the measures for preventing the teratogenic risks of miltefosine treatment should be reconsidered.

Pharmacogenetic Screening of CYP3A and ABCB1 in Relation to Population Pharmacokinetics of Docetaxel

Purpose: Despite the extensive clinical experience with docetaxel, unpredictable interindividual variability in efficacy and toxicity remain important limitations associated with the use of this anticancer drug. Large interindividual pharmacokinetic variability has been associated with variation in toxicity profiles. Genetic polymorphisms in drug-metabolizing enzymes and drug transporters could possibly explain the observed pharmacokinetic variability. The aim of this study was therefore to investigate the influence of polymorphisms in the CYP3A and ABCB1 genes on the population pharmacokinetics of docetaxel. Experimental Design: Whole blood samples were obtained from patients with solid tumors and treated with docetaxel to quantify the exposure to docetaxel. DNA was collected to determine polymorphisms in the CYP3A and ABCB1 genes with DNA sequencing. A population pharmacokinetic analysis of docetaxel was done using nonlinear mixed-effect modeling. Results: In total, 92 patients were assessable for pharmacokinetic analysis of docetaxel. A three-compartmental model adequately described the pharmacokinetics of docetaxel. Several polymorphisms in the CYP3A and ABCB1 genes were found, with allele frequencies of 0.54% to 48.4%. The homozygous C1236T polymorphism in the ABCB1 gene (ABCB1*8) was significantly correlated with a decreased docetaxel clearance (−25%; P = 0.0039). No other relationships between polymorphisms and pharmacokinetic variables reached statistical significance. Furthermore, no relationship between haplotypes of CYP3A and ABCB1 and the pharmacokinetics could be identified. Conclusions: The polymorphism C1236T in the ABCB1 gene was significantly related to docetaxel clearance. Our current finding may provide a meaningful tool to explain interindividual differences in docetaxel treatment in daily practice.

Pharmacokinetics and Pharmacokinetic Variability of Heroin and its Metabolites: Review of the Literature

This article reviews the pharmacokinetics of heroin after intravenous, oral, intranasal, intramuscular and rectal application and after inhalation in humans, with a special focus on heroin maintenance therapy in heroin dependent patients. In heroin maintenance therapy high doses pharmaceutically prepared heroin (up to 1000 mg/day) are prescribed to chronic heroin dependents, who do not respond to conventional interventions such as methadone maintenance treatment. Possible drug-drug interactions with the hydrolysis of heroin into 6-monoacetylmorphine and morphine, the glucuronidation of morphine and interactions with drug transporting proteins are described. Since renal and hepatic impairment is common in the special population of heroin dependent patients, specific attention was paid on the impact of renal and hepatic impairment. Hepatic impairment did not seem to have a clinically relevant effect on the pharmacokinetics of heroin and its metabolites. However, some modest effects of renal impairment have been noted, and therefore control of the creatinine clearance during heroin-assisted treatment seems recommendable.

Low Nevirapine Plasma Concentrations Predict Virological Failure in an Unselected HIV-1-Infected Population

ObjectiveTo assess the relationship between plasma nevirapine concentrations and plasma HIV-1 RNA response in HIV-1-infected patients.DesignAn observational cohort analysis.MethodsPlasma samples were obtained on a routine basis from 189 patients receiving nevirapine 200mg twice daily, and plasma nevirapine concentrations were measured with reversed phase high performance liquid chromatography. Patients were divided into two groups based on plasma nevirapine concentrations below (and equal to) or above 3 mg/L. The association between steady-state nevirapine concentrations and plasma HIV-1 RNA was determined by multi-variate analysis.ResultsOut of 189 patients, 13 (7%) had low nevirapine plasma concentrations and 176 patients had concentrations above 3 mg/L. In total, 22 (12%) patients showed virological failure and 8 patients (4%) discontinued nevirapine because of adverse effects. The risk of failure in patients with nevirapine plasma concentrations ≤3mg/L was increased (relative risk 5.0, 95% CI 1.8–13.7). Rashes and liver enzyme elevations each occurred in 8% of patients.ConclusionLow nevirapine drug concentrations are predictive of virological failure.

Influence of polymorphisms of drug metabolizing enzymes (CYP2B6, CYP2C9, CYP2C19, CYP3A4, CYP3A5, GSTA1, GSTP1, ALDH1A1 and ALDH3A1) on the pharmacokinetics of cyclophosphamide and 4-hydroxycyclophosphamide.

Purpose The anticancer agent, cyclophosphamide, is metabolized by cytochrome P450 (CYP), glutathione S-transferase (GST) and aldehyde dehydrogenase (ALDH) enzymes. Polymorphisms of these enzymes may affect the pharmacokinetics of cyclophosphamide and thereby its toxicity and efficacy. The purpose of this study was to evaluate the effects of known allelic variants in the CYP2B6, CYP2C9, CYP2C19, CYP3A4, CYP3A5, GSTA1, GSTP1, ALDH1A1 and ALDH3A1 genes on the pharmacokinetics of the anticancer agent, cyclophosphamide, and its active metabolite 4-hydroxycyclophosphamide. Experimental design A cohort of 124 Caucasian patients received a high-dose chemotherapy combination consisting of cyclophosphamide (4–6 g/m2), thiotepa (320–480 mg/m2) and carboplatin (area under the curve 13–20 mg×min/ml) as intravenous infusions over 4 consecutive days. Genomic DNA was analysed using PCR and sequencing. Liquid chromatography–tandem mass spectrometry was used to measure plasma concentrations of cyclophosphamide and 4-hydroxycyclophosphamide. The relationship between allelic variants and the elimination pharmacokinetic parameters noninducible cyclophosphamide clearance (CLnonind), inducible cyclophosphamide clearance (CLind) and elimination rate constant of 4-hydroxycyclophosphamide (k4OHCP) were evaluated using nonlinear mixed effects modelling. Results The interindividual variability in the noninducible cyclophosphamide clearance, inducible cyclophosphamide clearance and 4-hydroxycyclophosphamide clearance was 23, 27 and 31%, respectively. No effect of the allelic variants investigated on the clearance of cyclophosphamide or 4-hydroxycyclophosphamide could be demonstrated. Conclusion This study indicates that the presently evaluated variant alleles in the CYP2B6, CYP2C9, CYP2C19, CYP3A4, CYP3A5, GSTA1, GSTP1, ALDH1A1 and ALDH3A1 genes do not explain the interindividual variability in cyclophosphamide and 4-hydroxycyclophosphamide pharmacokinetics and are, probably, not the cause of the observed variability in toxicity.