Pieter Colin

Development and validation of a fast and uniform approach to quantify β-lactam antibiotics in human plasma by solid phase extraction-liquid chromatography-electrospray-tandem mass spectrometry.

Monitoring of plasma antibiotic concentrations is necessary for individualization of antimicrobial chemotherapy dosing in special patient populations. One of these special populations of interest are the post-bariatric surgery patients. Until today, little is known on the effect of this procedure on drug disposition and efficacy. Therefore, close monitoring of antimicrobial plasma concentrations in these patients is warranted. A fast and uniform ultra-high-performance liquid chromatography (UPLC) method with tandem mass spectrometric detection (MS/MS) has been developed and qualified for the simultaneous quantification of β-lactam antibiotics in human plasma. Compounds included in this multi-component analysis are: amoxicillin, ampicillin, phenoxymethylpenicillin, piperacillin, cefuroxime, cefadroxil, flucloxacillin, meropenem, cefepime, ceftazidime, tazobactam, linezolid and cefazolin. After spiking of five different stable isotope labelled internal standards, plasma samples were prepared for UPLC-MS/MS analysis by mixed-mode solid phase extraction. The developed method was proven to be free of (relative) matrix effects and proved to be reliable for the quantification of 12 out of 13 β-lactam antibiotics. As a proof of concept the method has been applied to plasma samples obtained from a healthy volunteer treated with amoxicillin. The analytical method is suitable for use in a therapeutic drug monitoring setting, providing the clinician with reliable measurements on β-lactam antibiotic plasma concentrations in a timely manner.

Oral bioavailability of moxifloxacin after Roux-en-Y gastric bypass surgery

OBJECTIVES Roux-en-Y gastric bypass surgery is the most commonly performed procedure for the treatment of morbid obesity. This anatomical alteration may affect the absorption and consequently the bioavailability of oral drugs. This study aims to investigate the oral bioavailability of moxifloxacin in 12 healthy volunteers who underwent gastric bypass surgery. PATIENTS AND METHODS In this randomized crossover study, each subject received two single standard doses of 400 mg of moxifloxacin orally or intravenously administered on two occasions separated by a washout period of 1 week. Serial venous blood samples were drawn up to 72 h after dosing and moxifloxacin plasma levels were measured by a validated HPLC method with fluorescence detection. [clinicaltrials.gov database (identifier: NCT01130922).] RESULTS After oral dosing, moxifloxacin plasma concentrations reached a maximum (C(max)) of 3.38 ± 1.41 mg/L after 1.75 h (0.75-4.00). After intravenous dosing, C(max) and T(max) were 4.53 ± 1.43 mg/L and 1.03 h (0.75-2.50), respectively. The mean areas under the plasma concentration time curve extrapolated to infinity (AUC(∞)) were 46.2 ± 1.4 mg · h/L after oral dosing and 52.3 ± 1.3 mg · h/L after intravenous dosing, resulting in a mean oral bioavailability of 88.32% [90% confidence interval (CI) 85.64%-91.08%]. CONCLUSIONS This study confirms that exposure to moxifloxacin is equivalent for oral and intravenous administration of 400 mg dosages in healthy volunteers who underwent gastric bypass surgery. But these exposures were more than 50% higher than those described for subjects without gastric bypass. This may suggest a higher enterohepatic recirculation of moxifloxacin after gastric bypass.

Propofol Breath Monitoring as a Potential Tool to Improve the Prediction of Intraoperative Plasma Concentrations

IntroductionMonitoring of drug concentrations in breathing gas is routinely being used to individualize drug dosing for the inhalation anesthetics. For intravenous anesthetics however, no decisive evidence in favor of breath concentration monitoring has been presented up until now. At the same time, questions remain with respect to the performance of currently used plasma pharmacokinetic models implemented in target-controlled infusion systems. In this study, we investigate whether breath monitoring of propofol could improve the predictive performance of currently applied, target-controlled infusion models.MethodsBased on data from a healthy volunteer study, we developed an addition to the current state-of-the-art pharmacokinetic model for propofol, to accommodate breath concentration measurements. The potential of using this pharmacokinetic (PK) model in a Bayesian forecasting setting was studied using a simulation study. Finally, by introducing bispectral index monitor (BIS) measurements and the accompanying BIS models into our PK model, we investigated the relationship between BIS and predicted breath concentrations.Results and DiscussionWe show that the current state-of-the-art pharmacokinetic model is easily extended to reliably describe propofol kinetics in exhaled breath. Furthermore, we show that the predictive performance of the a priori model is improved by Bayesian adaptation based on the measured breath concentrations, thereby allowing further treatment individualization and a more stringent control on the targeted plasma concentrations during general anesthesia. Finally, we demonstrated concordance between currently advocated BIS models, relying on predicted effect-site concentrations, and our new approach in which BIS measurements are derived from predicted breath concentrations.

Physiology-Based IVIVE Predictions of Tramadol from in Vitro Metabolism Data

ABSTRACTPurposeTo predict the tramadol in vivo pharmacokinetics in adults by using in vitro metabolism data and an in vitro-in vivo extrapolation (IVIVE)-linked physiologically-based pharmacokinetic (PBPK) modeling and simulation approach (Simcyp®).MethodsTramadol metabolism data was gathered using metabolite formation in human liver microsomes (HLM) and recombinant enzyme systems (rCYP). Hepatic intrinsic clearance (CLintH) was (i) estimated from HLM corrected for specific CYP450 contributions from a chemical inhibition assay (model 1); (ii) obtained in rCYP and corrected for specific CYP450 contributions by study-specific intersystem extrapolation factor (ISEF) values (model 2); and (iii) scaled back from in vivo observed clearance values (model 3). The model-predicted clearances of these three models were evaluated against observed clearance values in terms of relative difference of their geometric means, the fold difference of their coefficients of variation, and relative CYP2D6 contribution.ResultsModel 1 underpredicted, while model 2 overpredicted the total tramadol clearance by −27 and +22%, respectively. The CYP2D6 contribution was underestimated in both models 1 and 2. Also, the variability on the clearance of those models was slightly underpredicted. Additionally, blood-to-plasma ratio and hepatic uptake factor were identified as most influential factors in the prediction of the hepatic clearance using a sensitivity analysis.ConclusionIVIVE-PBPK proved to be a useful tool in combining tramadol’s low turnover in vitro metabolism data with system-specific physiological information to come up with reliable PK predictions in adults.

A Model Based Analysis of IPEC Dosing of Paclitaxel in Rats

ABSTRACTPurposeA strong pharmacokinetic rational exists for the use of (Hyperthermic) Intraperitoneal Perioperative Chemotherapy in peritoneal carcinomatosis. However, controversy remains regarding the optimal treatment strategies. Paclitaxel is believed to be a good compound for IPEC treatment because of its favourable pharmacokinetic properties.MethodsRat experiments were set up to gain insight in PTX’s pharmacokinetics and pharmacodynamics after IPEC treatment with Taxol®. Afterwards a Pharmacokinetic—Pharmacodynamic model was developed, that concurrently describes plasma and tumour exposure post IPEC dosing. Moreover, the developed model adequately describes the time-course of tumour apoptosis as well as the treatment effect on tumour volume.ResultsWe show that the complex absorption processes underlying PTX absorption from the peritoneal cavity post IPEC dosing, give rise to a markedly non-linear dose response relationship. Furthermore, we show that, in order to optimize treatment efficiency whilst concurrently minimizing the possibility of systemic toxicities, lowering the dose and extending exposure to the cytotoxic solution is the way forward.ConclusionsBased on the close resemblance between tumour exposure in our animal model and tumour exposure in patients treated under similar conditions, we hypothesise that, according to our findings in the rat, in the treatment of PC using IPEC administration of PTX, less is truly more.

Development and validation of an enzyme-linked immunosorbent assay for the quantification of cytochrome 3A4 in human liver microsomes

Little is known about the influence of hepatic pathologies on cytochrome P450 (CYP) mediated drug metabolism in children. The determination of the abundance of the different isoforms in pediatric microsomes may provide valuable information on the mechanisms of possible changes in activity. Until now, western blotting was mostly used for abundance measurements, but this technique only provides semi-quantitative data. Therefore, this study aimed to develop and validate an indirect ELISA for the quantification of the most important CYP isoform, CYP3A4, in human liver microsomes, using commercially available reagents. Samples, calibrators and validation samples were diluted to a final concentration of 10 μg microsomal protein/ml. A polyclonal antibody raised against the full length human protein was used as primary antibody; horseradish peroxidase conjugated secondary antibodies for detection. The assay was validated for sensitivity, working range and calibration, accuracy and precision. Amounts of CYP3A4 between 2 and 300 pmol/mg microsomal protein could be quantified with a 5-parameter logistics function with 1/x weighting factor. Coefficients of variation of intra and inter assay variability were between 9.54 and 13.98% (16.34% at LLOQ), and between 10.51 and 14.55% (19.44% at LLOQ), respectively. The relative error (%RE) varied between -5.96 and 6.68% (11.53% at LLOQ), and the total error between 11.93 and 21.23% (30.97% at LLOQ). The cross-reactivity of the method with human CYP2E1 showed to have no significant effect on the accuracy of the results. Successful analysis of five samples from an ongoing study demonstrated the usefulness of the method.

Community pharmacists' evaluation of potentially inappropriate prescribing in older community-dwelling patients with polypharmacy: observational research based on the GheOP³S tool

Background In this study, we aimed to (i) determine the prevalence of potentially inappropriate prescribing (PIP) in community‐dwelling older polypharmacy patients using the Ghent Older Peoples Prescriptions community‐Pharmacy Screening (GheOP3S) tool, (ii) identify the items that account for the highest proportion of PIP and (iii) identify the patient variables that may influence the occurrence of PIP. Additionally, pharmacist‐physician contacts emerging from PIP screening with the GheOP3S tool and feasibility of the GheOP3S tool in daily practice were evaluated. Methods A prospective observational study was carried out between December 2013 and July 2014 in 204 community pharmacies in Belgium. Patients were eligible if they were (i) ≥70 years, (ii) community‐dwelling, (iii) using ≥5 chronic drugs, (iv) a regular visitor of the pharmacy and (v) understanding Dutch or French. Community pharmacists used a structured interview to obtain demographic data and medication use and subsequently screened for PIP using the GheOP3S tool. A Poisson regression was used to investigate the association between different covariates and the number of PIP. Results In 987 (97%) of 1016 included patients, 3721 PIP items were detected (median of 3 per patient; inter quartile range: 2‐5). Most frequently involved with PIP are drugs for the central nervous system such as hypnosedatives, antipsychotics and antidepressants. Risk factors for a higher PIP prevalence appeared to be a higher number of drugs (30% extra PIPs per 5 extra drugs), female gender (20% extra PIPs), higher body mass index (BMI, 20% extra PIPs per 10‐unit increase in BMI) and poorer functional status (30% extra PIPs with 6‐point increase). The feasibility of the GheOP3S tool was acceptable although digitalization of the tool would improve implementation. Despite detecting at least one PIP in 987 patients, only 39 physicians were contacted by the community pharmacists to discuss the items. Conclusion A high prevalence of PIP in community‐dwelling older polypharmacy patients in Belgium was detected which urges for interventions to reduce PIP.

What about confidence intervals?: A word of caution when interpreting PTA simulations

OBJECTIVES In the field of antimicrobial chemotherapy, readers are increasingly confronted with population pharmacokinetic models and the ensuing simulation results with the purpose to improve the efficiency of currently used therapeutic regimens. One such type of analysis is Monte Carlo (MC) simulations in support of dose selection. At the moment, results of these MC simulations consist of predictions for the typical individual/population only. The uncertainty associated with the parameters, from which the simulations are derived, is completely ignored. Here, we highlight the importance of and the need to include parameter uncertainty in PTA simulations. METHODS Using MC simulation with parameter uncertainty, we estimated CIs around PTA curves. The added benefit of this approach was illustrated using, on the one hand, a population pharmacokinetic model developed in-house for a β-lactam antibiotic and, on the other hand, results from a previously published PTA analysis. RESULTS Our examples illustrate that proper clinical decision-making requires more than the typical PTA curve. Therefore, authors should be encouraged to provide an estimate of the uncertainty along with their simulations and to take this into account when interpreting the results. We feel that CIs around PTA curves provide this information in a comprehensive manner without requiring advanced knowledge on the underlying modelling approaches from the reader. CONCLUSIONS We believe that this approach should be advocated by all stakeholders in antibiotic stewardship programmes to safeguard the quality of clinical decision-making in the future.

Zwitterionic hydrophilic interaction liquid chromatography-tandem mass spectrometry with HybridSPE-precipitation for the determination of intact cisplatin in human plasma

Cisplatin is a first-line chemotherapeutic for the treatment of a wide variety of cancers since its discovery in the 1960s. Although various techniques have been reported for the measurement of total platinum in biological matrices, such as inductively coupled plasma-mass spectrometry and derivatization procedures, a specific, sensitive and robust assay for the quantification of intact cisplatin is still lacking. Therefore, we present a rapid, selective, sensitive, and reliable UHPLC-MS/MS based method for the determination of intact cisplatin in human plasma in support of a Phase II clinical trial. The optimal chromatographic behavior of cisplatin was achieved on a Syncronis HILIC column (50 × 2.1mm, 1.7µm, zwitterionic stationary phase). The retention behavior of cisplatin on this zwitterion-based stationary phase was well described by an adsorptive interaction model. A simple sample preparation based on protein precipitation combined with the removal of phospholipids by HybridSPE-precipitation was developed. The method was proven to be free of a relative matrix effect. The assay was validated within a range of 20 - 10,000ng/mL using 100μL of plasma sample. The intra and inter-day precisions were all less than 7.6%, and none of the bias was greater than 13.1%, thus corroborating that the developed method is precise and accurate. As a proof of concept, the assay has been successfully applied to plasma samples obtained from different patients who were enrolled in the Phase II trial and were treated with cisplatin.

Dexmedetomidine pharmacokinetic–pharmacodynamic modelling in healthy volunteers: 1. Influence of arousal on bispectral index and sedation

Background Dexmedetomidine, a selective α 2 -adrenoreceptor agonist, has unique characteristics, such as maintained respiratory drive and production of arousable sedation. We describe development of a pharmacokinetic-pharmacodynamic model of the sedative properties of dexmedetomidine, taking into account the effect of stimulation on its sedative properties. Methods In a two-period, randomized study in 18 healthy volunteers, dexmedetomidine was delivered in a step-up fashion by means of target-controlled infusion using the Dyck model. Volunteers were randomized to a session without background noise and a session with pre-recorded looped operating room background noise. Exploratory pharmacokinetic-pharmacodynamic modelling and covariate analysis were conducted in NONMEM using bispectral index (BIS) monitoring of processed EEG. Results We found that both stimulation at the time of Modified Observers Assessment of Alertness/Sedation (MOAA/S) scale scoring and the presence or absence of ambient noise had an effect on the sedative properties of dexmedetomidine. The stimuli associated with MOAA/S scoring increased the BIS of sedated volunteers because of a transient 170% increase in the effect-site concentration necessary to reach half of the maximal effect. In contrast, volunteers deprived of ambient noise were more resistant to dexmedetomidine and required, on average, 32% higher effect-site concentrations for the same effect as subjects who were exposed to background operating room noise. Conclusions The new pharmacokinetic-pharmacodynamic models might be used for effect-site rather than plasma concentration target-controlled infusion for dexmedetomidine in clinical practice, thereby allowing tighter control over the desired level of sedation. Clinical trial registration NCT01879865.