Assessment of the effects of immunogenicity on the pharmacokinetics, efficacy and safety of tildrakizumab.

BACKGROUND: We evaluated antidrug antibody (ADA) development in patients with chronic plaque psoriasis from three clinical trials of tildrakizumab, a humanized anti-interleukin-23p19 monoclonal antibody (P05495, reSURFACE 1 and reSURFACE 2). OBJECTIVES: To determine the effects of immunogenicity on the pharmacokinetics, efficacy and safety of tildrakizumab. METHODS: In 1400 (weeks 12-16) and 780 (weeks 52-64) evaluable patients randomized to tildrakizumab 100 or 200 mg, treatment-emergent ADA-positive (TE-POS) patients were identified and characterized for neutralizing antibodies (NAbs). Pharmacokinetics, safety and efficacy were evaluated by ADA status. RESULTS: In patients treated with tildrakizumab 100 or 200 mg continuously, < 7% were inconclusive at 52-64 weeks. In long-term data through 52-64 weeks, the incidence of TE-POS was 6·5% (100 mg) and 8·2% (200 mg) and the incidence of TE-POS NAb-POS was 2·5% (100 mg) and 3·2% (200 mg). TE-POS NAb-POS patients had modestly increased median tildrakizumab clearance (36·5%) compared with ADA-NEG patients. Percentage Psoriasis Area and Severity Index improvements in TE-POS NAb-POS vs. ADA-NEG patients on continuous treatment through week 52 were 76% (n = 10) vs. 91% (n = 342) for 100 mg and 77% (n = 12) vs. 87% (n = 299) for 200 mg. The incidence of potential immunogenicity-related adverse events did not indicate a clear trend in any positive ADA patient category compared with ADA-NEG patients through weeks 52-64. The effects of ADA on pharmacokinetics, efficacy and safety at 12-16 weeks were also summarized. CONCLUSIONS: ADA development with tildrakizumab treatment for 52-64 weeks was low; around 3% of patients developed TE-POS NAb-POS ADAs and showed lower serum concentrations and corresponding reduced efficacy. No relationship between ADAs and safety was observed. What's already known about this topic? Unwanted immune responses - for example immunogenicity and antidrug antibodies (ADAs) - have been observed with therapeutic monoclonal antibodies and can affect efficacy and safety. Tildrakizumab is a humanized monoclonal antibody targeting interleukin-23 and is currently approved for patients with plaque psoriasis. What does this study add? ADA development in tildrakizumab-treated patients with psoriasis over 52 weeks was low. The small proportion of patients who had treatment-emergent ADAs and had neutralizing antibodies experienced lower serum tildrakizumab concentrations and reduced efficacy. No relationship between ADAs and safety events was observed.

Good Practices in Model-Informed Drug Discovery and Development: Practice, Application, and Documentation.

This document was developed to enable greater consistency in the practice, application, and documentation of Model-Informed Drug Discovery and Development (MID3) across the pharmaceutical industry. A collection of "good practice" recommendations are assembled here in order to minimize the heterogeneity in both the quality and content of MID3 implementation and documentation. The three major objectives of this white paper are to: i) inform company decision makers how the strategic integration of MID3 can benefit R&D efficiency; ii) provide MID3 analysts with sufficient material to enhance the planning, rigor, and consistency of the application of MID3; and iii) provide regulatory authorities with substrate to develop MID3 related and/or MID3 enabled guidelines.

Effect of clarithromycin on the pharmacokinetics and pharmacodynamics of pimozide in healthy poor and extensive metabolizers of cytochrome P450 2D6 (CYP2D6).

BACKGROUND: The use of pimozide is associated with prolongation of the QT interval and fatal ventricular arrhythmia. We recently reported 2 fatal cases in patients taking pimozide and clarithromycin and we have shown that clarithromycin inhibits CYP3A-mediated metabolism of pimozide in vitro. In this study, we examined the effect of clarithromycin on pimozide pharmacokinetics and QT interval changes in a total of 12 healthy subjects (7 men and 5 women), documented as extensive metabolizers or poor metabolizers of CYP2D6. METHODS: In a randomized, double-blind placebo-controlled crossover design, subjects were given a single 6-mg oral dose of pimozide after 5 days of treatment with clarithromycin (500 mg twice a day) or a placebo pill. Blood samples were obtained before and for 96 hours after pimozide administration, and plasma pimozide and clarithromycin concentrations were measured by HPLC. Electrocardiograms for the analysis of the QTc intervals were recorded immediately before each blood sample. RESULTS: Pimozide significantly lengthened QTc interval in the first 20 hours in both the placebo-treated groups (delta QTcmax = 13.3 +/- 5.3 ms; P = .003) and clarithromycin-treated groups (delta QTcmax = 15.7 +/- 9.5 ms; P = .005) compared with baseline values. This is consistent with an effect of the parent drug. Clarithromycin caused a significant increase in the peak plasma concentration (P = .015), terminal elimination half-life (P = .003), and area under the plasma concentration-time curve (P = .024) and a decrease in the clearance (P = .029) of pimozide. Mean QTcmax observed within 20 hours of pimozide administration was significantly greater in the clarithromycin-treated group (23.8 +/- 12.2 ms; P = .0397) than in the placebo-treated group (16.8 +/- 6 ms). There was no significant effect of CYP2D6 or gender on the pharmacokinetics or pharmacodynamics of pimozide. CONCLUSIONS: A single 6-mg oral dose of pimozide resulted in measurable QT interval changes. Clarithromycin inhibited CYP3A-mediated pimozide metabolism and the resulting elevation in plasma concentrations may increase the risk of pimozide cardiotoxicity.

Modulation of the cytochrome P450-mediated metabolism of ifosfamide by ketoconazole and rifampin.

BACKGROUND: The autoinducible metabolic transformation of the anticancer agent ifosfamide involves activation through 4-hydroxyifosfamide to the ultimate cytotoxic ifosforamide mustard and deactivation to 2- and 3-dechloroethylifosfamide with concomitant release of the neurotoxic chloroacetaldehyde. Activation is mediated by cytochrome P450 (CYP) 3A4 and deactivation by CYP3A4 and CYP2B6. The aim of this study was to investigate modulation of the CYP-mediated metabolism of ifosfamide with ketoconazole, a potent inhibitor of CYP3A4, and rifampin (INN, rifampicin), an inducer of CYP3A4/CYP2B6. METHODS: In a double-randomized, 2-way crossover study a total of 16 patients received ifosfamide 3 g/m(2) per 24 hours intravenously, either alone or in combination with 200 mg ketoconazole twice daily (1 day before treatment and 3 days of concomitant administration) or 300 mg rifampin twice daily (3 days before treatment and 3 days of concomitant administration). Plasma pharmacokinetics and urinary excretion of ifosfamide, 2- and 3-dechloroethylifosfamide, and 4-hydroxyifosfamide were assessed in both courses. Data analysis was performed with a population pharmacokinetic model with a description of autoinduction of ifosfamide. RESULTS: Rifampin increased the clearance of ifosfamide at the start of therapy at 102%. The fraction of ifosfamide metabolized to the dechloroethylated metabolites was increased, whereas exposure to the metabolites was decreased as a result of increased elimination. The fraction metabolized and the exposure to 4-hydroxyifosfamide were not significantly influenced. Ketoconazole did not affect the fraction metabolized or the exposure to the dechloroethylated metabolites, whereas both parameters were reduced with 4-hydroxyifosfamide. CONCLUSIONS: Coadministration of ifosfamide with ketoconazole or rifampin did not produce changes in the pharmacokinetics of the parent or metabolites that may result in an increased benefit of ifosfamide therapy.

Population pharmacokinetics of ifosfamide and its dechloroethylated and hydroxylated metabolites in children with malignant disease: a sparse sampling approach.

OBJECTIVE: To assess the feasibility of a sparse sampling approach for the determination of the population pharmacokinetics of ifosfamide, 2- and 3-dechloroethyl-ifosfamide and 4-hydroxy-ifosfamide in children treated with single-agent ifosfamide against various malignant tumours. DESIGN: Pharmacokinetic assessment followed by model fitting. PATIENTS: The analysis included 32 patients aged between 1 and 18 years receiving a total of 45 courses of ifosfamide 1.2, 2 or 3 g/m2 in 1 or 3 hours on 1, 2 or 3 days. METHODS: A total of 133 blood samples (median of 3 per patient) were collected. Plasma concentrations of ifosfamide and its dechloroethylated metabolites were determined by gas chromatography. Plasma concentrations of 4-hydroxy-ifosfamide were measured by high-performance liquid chromatography. The models were fitted to the data using a nonlinear mixed effects model as implemented in the NONMEM program. A cross-validation was performed. RESULTS: Population values (mean +/- standard error) for the initial clearance and volume of distribution of ifosfamide were estimated at 2.36 +/- 0.33 L/h/m2 and 20.6 +/- 1.6 L/m2 with an interindividual variability of 43 and 32%, respectively. The enzyme induction constant was estimated at 0.0493 +/- 0.0104 L/h2/m2. The ratio of the fraction of ifosfamide metabolised to each metabolite to the volume of distribution of that metabolite, and the elimination rate constant, of 2- and 3-dechloroethyl-ifosfamide and 4-hydroxy-ifosfamide were 0.0976 +/- 0.0556, 0.0328 +/- 0.0102 and 0.0230 +/- 0.0083 m2/L and 3.64 +/- 2.04, 0.445 +/- 0.174 and 7.67 +/- 2.87 h(-1), respectively. Interindividual variability of the first parameter was 23, 34 and 53%, respectively. Cross-validation indicated no bias and minor imprecision (12.5 +/- 5.1%) for 4-hydroxy-ifosfamide only. CONCLUSIONS: We have developed and validated a model to estimate ifosfamide and metabolite concentrations in a paediatric population by using sparse sampling.

Clinical pharmacokinetics and pharmacodynamics of ifosfamide and its metabolites.

This review discusses several issues in the clinical pharmacology of the antitumour agent ifosfamide and its metabolites. Ifosfamide is effective in a large number of malignant diseases. Its use, however, can be accompanied by haematological toxicity, neurotoxicity and nephrotoxicity. Since its development in the middle of the 1960s, most of the extensive metabolism of ifosfamide has been elucidated. Identification of specific isoenzymes responsible for ifosfamide metabolism may lead to an improved efficacy/toxicity ratio by modulation of the metabolic pathways. Whether ifosfamide is specifically transported by erythrocytes and which activated ifosfamide metabolites play a key role in this transport is currently being debated. In most clinical pharmacokinetic studies, the phenomenon of autoinduction has been observed, but the mechanism is not completely understood. Assessment of the pharmacokinetics of ifosfamide and metabolites has long been impaired by the lack of reliable bioanalytical assays. The recent development of improved bioanalytical assays has changed this dramatically, allowing extensive pharmacokinetic assessment, identifying key issues such as population differences in pharmacokinetic parameters, differences in elimination dependent upon route and schedule of administration, implications of the chirality of the drug and interpatient pharmacokinetic variability. The mechanisms of action of cytotoxicity, neurotoxicity, urotoxicity and nephrotoxicity have been pivotal issues in the assessment of the pharmacodynamics of ifosfamide. Correlations between the new insights into ifosfamide metabolism, pharmacokinetics and pharmacodynamics will rationalise the further development of therapeutic drug monitoring and dose individualisation of ifosfamide treatment.

Reduction of cyclophosphamide bioactivation by thioTEPA: critical sequence-dependency in high-dose chemotherapy regimens.

PURPOSE: Cyclophosphamide and thioTEPA are frequently used simultaneously in high-dose chemotherapy regimens. During a pharmacokinetic study of 31 courses in 20 patients of cyclophosphamide and its activated metabolite 4-hydroxycyclophosphamide given in the combination cyclophosphamide thioTEPA carboplatin, a sharp decrease in 4-hydroxycyclophosphamide concentration was observed immediately after the start of the thioTEPA infusion. A drug-drug interaction was suspected. This putative interaction was investigated in this study. METHODS: Possible sequence dependency, due to inhibition of the formation of 4-hydroxycyclophosphamide by thioTEPA, was investigated by altering the sequence of infusion in three patients (four courses) receiving high-dose chemotherapy with cyclophosphamide (1,000 or 1,500 mg/m2 per day), thioTEPA (80 or 120 mg/m2 per day) and carboplatin (265 or 400 mg/m2 per day) in short infusions for four consecutive days. The pharmacokinetics of cyclophosphamide and 4-hydroxycyclophosphamide were established. Possible inhibition of the metabolism of cyclophosphamide and thioTEPA was investigated in human microsomes. RESULTS: A striking sequence dependency of the pharmacokinetics of 4-hydroxycyclophosphamide was observed. Administration of thioTEPA 1 h prior to cyclophosphamide resulted in decreased Cmax (-62%) and AUC (-26%) values of 4-hydroxycyclophosphamide compared to those of thioTEPA administered 1 h after cyclophosphamide. In human microsomes an inhibition of the conversion of cyclophosphamide to 4-hydroxycyclophosphamide by thioTEPA was observed at clinically relevant concentrations with an IC50 of 23 microM. No inhibition of the formation of TEPA by cyclophosphamide was observed. CONCLUSIONS: ThioTEPA strongly inhibits the bioactivation of cyclophosphamide and this may decrease both efficacy and toxicity. Our results seriously question the practice of the simultaneous continuous infusion of cyclophosphamide and thioTEPA and suggest that the sequencing and scheduling of these two agents in high-dose chemotherapy regimens may be of critical importance.

Population pharmacokinetics of ifosfamide and its 2- and 3-dechloroethylated and 4-hydroxylated metabolites in resistant small-cell lung cancer patients.

The aim of this study was to develop a population pharmacokinetic model that could describe the pharmacokinetics of ifosfamide. 2- and 3-dechloroethylifosfamide and 4-hydroxyifosfamide, and calculate their plasma exposure and urinary excretion. A group of 14 patients with small-cell lung cancer received a 1-h intravenous infusion of 2.0 or 3.0 g/m2 ifosfamide over 1 or 2 days in combination with 175 mg/m2 paclitaxel and carboplatin at AUC 6. The concentration-time profiles of ifosfamide were described by an ifosfamide concentration-dependent development of autoinduction of ifosfamide clearance. Metabolite compartments were linked to the ifosfamide compartment enabling description of the concentration-time profiles of 2- and 3-dechloroethylifosfamide and 4-hydroxyifosfamide. The Bayesian estimates of the pharmacokinetic parameters were used to calculate the systemic exposure to ifosfamide and its metabolites for the four ifosfamide schedules. Fractionation of the dose over 2 days resulted increased metabolite formation, especially of 2-dechloroethylifosfamide, probably due to increased autoinduction. Renal recovery was only minor with 6.6% of the administered dose excreted unchanged and 9.8% as dechloroethylated metabolites. In conclusion, ifosfamide pharmacokinetics were described with an ifosfamide concentration-dependent development of autoinduction and allowed estimation of the population pharmacokinetics of the metabolites of ifosfamide. Fractionation of the dose resulted in increased exposure to 2-dechloroethylifosfamide, probably due to increased autoinduction.

Prediction of clinical irrelevance of PK differences in atorvastatin using PK/PD models derived from literature-based meta-analyses.

To support the development of a fixed-dose combination (FDC) of ezetimibe and atorvastatin for the treatment of dyslipidemia, bioequivalence (BE) studies were conducted across a combined dose range (10/10, 10/20, 10/40, and 10/80 mg of ezetimibe/atorvastatin). In the BE trials, all parameters met traditional BE bounds except for atorvastatin peak plasma concentration (Cmax) at two intermediate doses. Literature-based metadata analysis predicted that the observed difference in Cmax between an ezetimibe+atorvastatin FDC and coadministration of these agents translates directly into a non-clinically significant change of <1.2% absolute difference in the percentage lowering of low-density-lipoprotein cholesterol . Both FDC doses were confirmed to be clinically equivalent to coadministration in the subsequent clinical equivalence trials. These data suggest that modeling of dose-response relationships may be useful in predicting clinical equivalence, lowering cost/timelines through effective powering of studies, and predicting the effectiveness of new dosage formulations without the need for additional clinical efficacy trials in regulatory settings.

Implementation of quantitative and systems pharmacology in large pharma.

Quantitative and systems pharmacology concepts and tools are the foundation of the model-informed drug development paradigm at Merck for integrating knowledge, enabling decisions, and enhancing submissions. Rigorous prioritization of modeling and simulation activities has enabled key drug development decisions and led to a high return on investment through significant cost avoidance. Critical factors for the successful implementation, examples on impact on decision making with associated return of investment, and drivers for continued success are discussed.

Modeling and simulation of the time course of asenapine exposure response and dropout patterns in acute schizophrenia.

Modeling and simulation were utilized to characterize the efficacy dose response of sublingual asenapine in patients with schizophrenia and to understand the outcomes of six placebo-controlled trials in which placebo responses and dropout rates varied. The time course of total Positive and Negative Syndrome Scale (PANSS) scores was characterized for placebo and asenapine treatments in a pharmacokinetic-pharmacodynamic model in which the asenapine effect was described by an E(max) model, increasing linearly over the 6-week study period. A logistic regression model described the time course of dropouts, with previous PANSS value being the most important predictor. The last observation carried forward (LOCF) time courses were well described in simulations from the combined PANSS + dropout model. The observed trial outcomes were successfully predicted for all the placebo arms and the majority of the treatment arms. Although simulations indicated that the post hoc probability of success of the performed trials was low to moderate, these analyses demonstrated that 5 and 10 mg twice-daily (b.i.d.) doses of asenapine have similar efficacy.

Planning for an adaptive design: a case study in COPD.

We discuss the practical and clinical considerations encountered when planning a Phase IIa trial in chronic obstructive pulmonary disease (COPD). Various adaptive strategies for reducing the cost of the trial and the statistical implications of these are explored. Use of the EAST software to evaluate the properties of the study designs with one or more interim analyses for futility, efficacy or either is described. We emphasize the rationale for choosing between alternative designs and the relationship between the clinical and statistical considerations.

Sensitive assay for pimozide in human plasma using high-performance liquid chromatography with fluorescence detection: application to pharmacokinetic studies.

A method is described for the measurement of pimozide in human plasma using HPLC with fluorescence detection. The method is specific and sensitive in the range of concentrations seen in human plasma after conventional dosing (1-15 ng/ml) with a limit of quantification of 1 ng/ml. The calibration curves are linear for concentrations between 1 and 50 ng/ml. Within-day and inter-day coefficients of variation are less than 7.4% and 15.5%, respectively, at three concentrations of pimozide (2, 10 and 20 ng/ml). Intra-day and inter-day bias are less than 18.5% and 12.5%, respectively. A pharmacokinetic study conducted in a healthy volunteer administered 6 mg of pimozide orally demonstrates the utility of this method.

Identification and characterization of human cytochrome P450 isoforms interacting with pimozide.

Using human liver microsomes (HLMs) and recombinant human cytochrome P450 (CYP450) isoforms, we identified the major route of pimozide metabolism, the CYP450 isoforms involved, and documented the inhibitory effect of pimozide on CYP450 isoforms. Pimozide was predominantly N-dealkylated to 1,3-dihydro-1-(4-piperidinyl)-2H-benzimidazol-2-one (DHPBI). The formation rate of DHPBI showed biphasic kinetics in HLMs, which suggests the participation of at least two activities. These were characterized as high-affinity (K(m1) and Vmax1) and low-affinity (K(m2) and Vmax2) components. The ratio of Vmax1 (14 pmol/min/mg protein)/K(m1) (0.73 microM) was 5.2 times higher than the ratio of Vmax2 (244 pmol/min/mg protein)/K(m2) (34 microM). K(m2) was 91 times higher than K(m1). The formation rate of DHPBI from 25 microM pimozide in nine human livers correlated significantly with the catalytic activity of CYP3A (Spearman r = 0.79, P = .028), but not with other isoforms. Potent inhibition of DHPBI formation from 10 microM pimozide was observed with ketoconazole (88%), troleandomycin (79%), furafylline (48%) and a combination of furafylline and ketoconazole (96%). Recombinant human CYP3A4 catalyzed DHPBI formation from 10 microM pimozide at the highest rate (V = 2.2 +/- 0.89 pmol/min/pmol P450) followed by CYP1A2 (V = 0.23 +/- 0.08 pmol/min/pmol P450), but other isoforms tested did not. The K(m) values derived with recombinant CYP3A4 and CYP1A2 were 5.7 microM and 36.1 microM, respectively. Pimozide itself was a potent inhibitor of CYP2D6 in HLMs when preincubated for 15 min (Ki = 0.75 +/- 0.98 microM) and a moderate inhibitor of CYP3A (Ki = 76.7 +/- 34.5 microM), with no significant effect on other isoforms tested. Our results suggest that pimozide metabolism is catalyzed mainly by CYP3A, but CYP1A2 also contributes. Pimozide metabolism is likely to be subject to interindividual variability in CYP3A and CYP1A2 expression and to drug interactions involving these isoforms. Pimozide itself may inhibit the metabolism of drugs that are substrates of CYP2D6.

Pharmacokinetic-pharmacodynamic modeling of the electroencephalogram effect of synthetic opioids in the rat: correlation with the interaction at the mu-opioid receptor.

The purpose of our investigation was to characterize the relationships between the pharmacodynamics of synthetic opioids in vivo and the interaction at the mu-opioid receptor. The pharmacokinetics and pharmacodynamics were determined in vivo after a single i.v. infusion of 3.14 mg/kg alfentanil (A), 0.15 mg/kg fentanyl (F) or 0.030 mg/kg sufentanil (S) in rats. Amplitudes in the 0.5 to 4.5 Hz frequency band of the electroencephalogram (EEG) was used as pharmacodynamic endpoint. The EEG effect intensity was related to the (free) concentration in blood (A) or in a hypothetical effect compartment (F, S) on basis of the sigmoidal Emax pharmacodynamic model. The interaction at the mu-opioid receptor was determined in vitro on basis of the displacement of [3H]-naloxone binding in washed rat brain membranes. The value of the sodium shift was used as a measure of in vitro intrinsic efficacy. For the EEG effect the in vivo potencies based on free drug concentrations (EC50,u) were 4.62 +/- 0.66 ng/ml (A), 0.69 +/- 0.05 ng/ml (F) and 0.29 +/- 0.06 ng/ml (S). In the receptor binding studies the affinities at the mu-opioid receptor (Kl) were 47.4 +/- 6.6 nM (A), 8.6 +/- 4.1 nM (F) and 2.8 +/- 0.2 nM (S). For each opioid the ratio between EC50,u and Kl was the same with a value of 0.23-0.25, indicating the existence of receptor reserve for the EEG effect. The intrinsic activity (Emax) of the three opioids in vivo was similar with values of 111 +/- 10 microV (A), 89 +/- 11 microV (F) and 104 +/- 4 microV (S). However, the values of the sodium shift varied between 2.8 (S) and 19.1 (A). Further analysis of the in vivo pharmacodynamic data on basis of an operational model of agonism provided evidence for a large receptor reserve, which explains why compounds with different values of the sodium shift all behave as full agonists in vivo.

Simultaneous determination of N,N',N"-triethylenethiophosphoramide, cyclophosphamide and some of their metabolites in plasma using capillary gas chromatography.

A sensitive assay for the simultaneous determination of N,N',N"-triethylenethiophosphoramide (thioTEPA), its metabolite N,N',N"-triethylenephosphoramide (TEPA), cyclophosphamide (CP) and its metabolite 2-dechloroethylcyclophosphamide (2-DCE-CP) in plasma has been developed and validated. The analytes were determined using gas chromatography with nitrogen/phosphorus selective detection after liquid-liquid extraction with chloroform using 100 microl of plasma. Diphenylamine (for TEPA, thioTEPA and 2-DCE-CP) and imipramine (for CP) were used as internal standards. The limits of quantitation for thioTEPA, TEPA, CP and 2-DCE-CP were 5, 5, 50 and 250 ng/ml, respectively. Linear calibration curves were observed over two decades of concentration. Accuracy, within-day and between-day precision were less than 13% for all analytes. Stability of the analytes proved to be satisfactory for at least 1 month, stored at -70 degrees C. Analysis of samples obtained from patients receiving cyclophosphamide, thioTEPA and carboplatin in a high-dose regimen demonstrated the applicability of the assay.

Population pharmacokinetics and exploratory pharmacodynamics of ifosfamide and metabolites after a 72-h continuous infusion in patients with soft tissue sarcoma.

OBJECTIVE: The population pharmacokinetics and pharmacodynamics of the cytostatic agent ifosfamide and its main metabolites 2- and 3-dechloroethylifosfamide and 4-hydroxyifosfamide were assessed in patients with soft tissue sarcoma. METHODS: Twenty patients received 9 or 12 g/m2 ifosfamide administered as a 72-h continuous intravenous infusion. The population pharmacokinetic model was built in a sequential manner, starting with a covariate-free model and progressing to a covariate model with the aid of generalised additive modelling. RESULTS: The addition of the covariates weight, body surface area, albumin, serum creatinine, serum urea, alkaline phosphatase and lactate dehydrogenase improved the prediction errors of the model. Typical pretreatment (mean +/- SEM) initial clearance of ifosfamide was 3.03 +/- 0.18 l/h with a volume of distribution of 44.0 +/- 1.8 l. Autoinduction, dependent on ifosfamide levels, was characterised by an induction half-life of 11.5 +/- 1.0 h with 50% maximum induction at 33.0 +/- 3.6 microM ifosfamide. Significant pharmacokinetic-pharmacodynamic relationships (P = 0.019) were observed between the exposure to 2- and 3-dechloroethylifosfamide and orientational disorder, a neurotoxic side-effect. No pharmacokinetic-pharmacodynamic relationships between exposure to 4-hydroxyifosfamide and haematological toxicities could be observed in this population.

Determination of ifosfamide, 2- and 3-dechloroethyifosfamide using gas chromatography with nitrogen-phosphorus or mass spectrometry detection.

A comparison was made between methods for determining ifosfamide (IF), 2- (2DCE) and 3-dechloroethylifosfamide (3DCE) using gas chromatography with nitrogen-phosphorus detection (GC-NPD) versus positive ion electron-impact ion-trap mass spectrometry (GC-MS2). Sample pretreatment involved liquid-liquid extraction with ethyl acetate after adding trofosfamide as internal standard and alkalinization. The GC-NPD was linear, specific, and sensitive for all analytes in the range of 0.0500-100 microg/mL with lower limits of quantification (LLQ) of 0.0500 microg/mL using a 50-microgL plasma sample. The GC-MS2 was linear, specific, and sensitive for IF, 2DCE, and 3DCE in the ranges of 0.250-100, 0.500-25.0, and 0.500-25.0 microg/mL, respectively, with LLQs of 0.250, 0.500, and 0.500 microg/mL. The ranges of accuracy, within-day precision, and between-day precision for analysis of all compounds with GC-NPD did not exceed 93.3% to 105.4%, 8.0% and 9.8%, respectively. The ranges of accuracy, within-day precision, and between-day precision for analysis of all compounds with GC-MS2 did not exceed 86.5% to 99.0%, 9.0% and 12.7%, respectively. In conclusion, GC-NPD proved to be superior to GC-MS2 in sensitivity, detection range, accuracy, and precisions. Therefore GC-NPD is the method of choice for fast un-derivatized determination of IF, 2DCE, and 3DCE in human plasma, and it can readily be used for clinical pharmacokinetic studies and routine monitoring of IF-treated patients in a hospital setting.

Influence of dose and infusion duration on pharmacokinetics of ifosfamide and metabolites.

The anticancer drug ifosfamide is a prodrug requiring activation through 4-hydroxyifosfamide to ifosforamide mustard, to exert cytotoxicity. Deactivation of ifosfamide leads to 2- and 3-dechloroethylifosfamide and the release of potentially neurotoxic chloracetaldehyde. The aim of this study was to quantify and to compare the pharmacokinetics of ifosfamide, 2- and 3-dechloroethylifosfamide, 4-hydroxyifosfamide, and ifosforamide mustard in short (1-4 h), medium (24-72 h), and long infusion durations (96-240 h) of ifosfamide. An integrated population pharmacokinetic model was used to describe the autoinducible pharmacokinetics of ifosfamide and its four metabolites in 56 patients. The rate by which autoinduction of the metabolism of ifosfamide developed was found to be significantly dependent on the infusion schedule. The rate was 52% lower with long infusion durations compared with short infusion durations. This difference was, however, comparable with its interindividual variability (22%) and was, therefore, considered to be of minor clinical importance. Autoinduction caused a less than proportional increase in the area under the ifosfamide plasma concentration-time curve (AUC) and more than proportional increase in metabolite exposure with increasing ifosfamide dose. During long infusion durations dose-corrected exposures (AUC/D) were significantly decreased for ifosfamide and increased for 3-dechloroethylifosfamide compared with short infusion durations. No differences in dose-normalized exposure to ifosfamide and metabolites were observed between short and medium infusion durations. This study demonstrates that the duration of ifosfamide infusion influences the exposure to the parent and its metabolite 3-dechloroethylifosfamide. The observed dose and infusion duration dependence should be taken into account when modeling ifosfamide metabolism.