Pilot investigation on long-term subcutaneous microdialysis: proof of principle in humans

Microdialysis of Voriconazole and its N-Oxide Metabolite: Amalgamating Knowledge of Distribution and Metabolism Processes in Humans

PURPOSE

Voriconazole is an essential antifungal drug whose complex pharmacokinetics with high interindividual variability impedes effective and safe therapy. By application of the minimally-invasive sampling technique microdialysis, interstitial space fluid (ISF) concentrations of VRC and its potentially toxic N-oxide metabolite (NO) were assessed to evaluate target-site exposure for further elucidating VRC pharmacokinetics.

METHODS

Plasma and ISF samples of a clinical trial with an approved VRC dosing regimen were analyzed for VRC and NO concentrations. Concentration-time profiles, exposure assessed as area-under-the-curve (AUC) and metabolic ratios of four healthy adults in plasma and ISF were evaluated regarding the impact of multiple dosing and CYP2C19 genotype.

RESULTS

VRC and NO revealed distribution into ISF with AUC values being ≤2.82- and 17.7-fold lower compared to plasma, respectively. Intraindividual variability of metabolic ratios was largest after the first VRC dose administration while interindividual variability increased with multiple dosing. The CYP2C19 genotype influenced interindividual differences with a maximum 6- and 24-fold larger AUCNO/AUCVRC ratio between the intermediate and rapid metabolizer in plasma and ISF, respectively. VRC metabolism was saturated/auto-inhibited indicated by substantially decreasing metabolic concentration ratios with increasing VRC concentrations and after multiple dosing.

CONCLUSION

The feasibility of the simultaneous microdialysis of VRC and NO in vivo was demonstrated and provided new quantitative insights by leveraging distribution and metabolism processes of VRC in humans. The exploratory analysis suggested substantial dissimilarities of VRC and NO pharmacokinetics in plasma and ISF. Ultimately, a thorough understanding of target-site pharmacokinetics might contribute to the optimization of personalized VRC dosing regimens.

Microdialysis of Drug and Drug Metabolite: a Comprehensive In Vitro Analysis for Voriconazole and Voriconazole N-oxide

PURPOSE

Voriconazole is a therapeutically challenging antifungal drug associated with high interindividual pharmacokinetic variability. As a prerequisite to performing clinical trials using the minimally-invasive sampling technique microdialysis, a comprehensive in vitro microdialysis characterization of voriconazole (VRC) and its potentially toxic N-oxide metabolite (NO) was performed.

METHODS

The feasibility of simultaneous microdialysis of VRC and NO was explored in vitro by investigating the relative recovery (RR) of both compounds in the absence and presence of the other. The dependency of RR on compound combination, concentration, microdialysis catheter and study day was evaluated and quantified by linear mixed-effects modeling.

RESULTS

Median RR of VRC and NO during individual microdialysis were high (87.6% and 91.1%). During simultaneous microdialysis of VRC and NO, median RR did not change (87.9% and 91.1%). The linear mixed-effects model confirmed the absence of significant differences between RR of VRC and NO during individual and simultaneous microdialysis as well as between the two compounds (p > 0.05). No concentration dependency of RR was found (p = 0.284). The study day was the main source of variability (46.3%) while the microdialysis catheter only had a minor effect (4.33%). VRC retrodialysis proved feasible as catheter calibration for both compounds.

CONCLUSION

These in vitro microdialysis results encourage the application of microdialysis in clinical trials to assess target-site concentrations of VRC and NO. This can support the generation of a coherent understanding of VRC pharmacokinetics and its sources of variability. Ultimately, a better understanding of human VRC pharmacokinetics might contribute to the development of personalized dosing strategies.

Evaluation of local bioavailability of metronidazole from topical formulations using dermal microdialysis: Preliminary study in a Yucatan mini-pig model

Dermal microdialysis (dMD) can measure the rate and extent to which a topically administered active pharmaceutical ingredient (API) becomes available in the dermis. Using multiple test-sites on the same subject, and replicate probes at each test-site, it is feasible to compare the cutaneous pharmacokinetics of an API from different topical dermatological drug products in parallel on the same subject with this technique. This study design would help to reduce variability. However, there are technical considerations related to the dMD experimental methods that must be characterized and optimized to ensure that an in vivo dMD study is selective, sensitive, discriminating, and reproducible. The goals of this study were to assess: the minimum distance required between test-sites to prevent cross-talk between probes due to potential lateral-diffusion; the sensitivity of the dMD method to detect differences in the local concentration of metronidazole (MTZ) among single escalating doses; the ability to discriminate between the two different formulations; and the stability of the dMD-probes over 48 h. Results indicate that lateral-diffusion and systemic redistribution of the API following topical application of the drug product were negligible, thus MTZ measured by dMD can be selectively attributed to the dermal bioavailability of the API from the applied topical dose. The dMD methodology was able to detect differences in the bioavailability of MTZ from the cream compared to the gel when applied at the same dose, as well as among different doses of the same formulation over a 48-hour sampling duration; therefore, the method is sensitive. The percentage loss of D₃-MTZ from the probe compared to its original concentration in the perfusate indicates that the probe performance was stable over the 48 h.

Quantification of microdialysis related variability in humans: Clinical trial design recommendations

OBJECTIVE

Target-site concentrations obtained via the catheter-based minimally invasive microdialysis technique often exhibit high variability. Catheter calibration is commonly performed via retrodialysis, in which a transformation factor, termed relative recovery (RR), is determined. Leveraging RR values from a rich data set of a very large clinical microdialysis study, promised to contribute critical insight into the origin of the reportedly high target-site variability. The present work aimed (i) to quantify and explain variability in RR associated with the patient (including non-obese vs. obese) and the catheter, and (ii) to derive recommendations on the design of future clinical microdialysis studies.

METHODS

A prospective, age- and sex-matched parallel group, single-centre trial in non-obese and obese patients (BMI=18.7-86.9 kg/m²) was performed. 1-3 RR values were obtained in the interstitial fluid of the subcutaneous fat tissue in one catheter per upper arm of 120 patients via the retrodialysis method (n_RR=1008) for a panel of drugs (linezolid, meropenem, tigecycline, cefazolin, fosfomycin, piperacillin and acetaminophen). A linear mixed-effects model was developed to quantify the different types of variability in RR and to explore the association between RR and patient body size descriptors.

RESULTS

Estimated RR was highest for acetaminophen (69.7%, 95%CI=65.0% to 74.3%) and lowest for piperacillin (40.4%, 95%CI=34.6% to 46.0%). The linear mixed-effects modelling analysis showed that variability associated with the patient (σ=15.9%) was the largest contributor (46.7%) to overall variability, whereas the contribution of variability linked to the catheter (σ=5.55%) was ~1/6 (16.8%). The relative contribution of residual unexplained variability (σ=12.0%, including intracatheter variability) was ~1/3 (36.4%). The limits of agreement of repeated RR determinations in a single catheter ranged from 0.694-1.64-fold (linezolid) to 0.510-3.02-fold (cefazolin). Calculated fat mass affected RR, explaining the observed lower RR in obese (ΔRR_mean= -29.7% relative reduction) versus non-obese patients (p<0.001); yet only 15.8% of interindividual variability was explained by this effect. No difference in RR was found between catheters implanted into the left or right arm (p=0.732).

CONCLUSIONS

Three recommendations for clinical microdialysis trial design were derived: 1) High interindividual variability underscored the necessity of measuring individual RR per patient. 2) The low relative contribution of intercatheter variability to overall variability indicated that measuring RR with a single catheter per patient is sufficient for reliable catheter calibration. 3) The wide limits of agreement from multiple RR in the same catheter implied an uncertainty of a factor of two in target-site drug concentration estimation necessitating to perform catheter calibration (retrodialysis sampling) multiple times per patient. To allow routine clinical use of microdialysis, research efforts should aim at further understanding and minimising the method-related variability. Optimised study designs in clinical trials will ultimately yield more informative microdialysis data and increase our understanding of this valuable sampling technique to derive target-site drug exposure.

[Anti-infective treatment in obesity-"just double it?"]

Adaequate antibiotic therapy is crucial for successful anti-infective therapy. In addition to the choice of the right antibiotic and the duration of therapy, the dose also plays a decisive role. Obesity has an influence on the pharmacokinetics of antibiotics, which can lead to underdosing if previous weight-independent dosing regimes are used. It is therefore necessary to carry out systematic measurements of concentrations in obese patients. Since pharmacokinetic differences between plasma and the interstitial fluid of different target tissues have been observed for different antibiotics, the measurement is also necessary in the target tissue. The technique of microdialysis is best suited for this purpose as it allows concentrations to be measured continuously in the target tissue.

Novel insights into the complex pharmacokinetics of voriconazole: a review of its metabolism

Voriconazole, a second-generation triazole frequently used for the prophylaxis and treatment of invasive fungal infections, undergoes complex metabolism mainly involving various (polymorphic) cytochrome P450 enzymes in humans. Although high inter- and intraindividual variability in voriconazole pharmacokinetics have been observed and the therapeutic range for this compound is relatively narrow, the metabolism of voriconazole has not been fully elucidated yet. The available literature data investigating the multiple different pathways and metabolites are extremely unbalanced and thus the absolute or relative contribution of the different pathways and enzymes involved in the metabolism of voriconazole remains uncertain. Furthermore, other factors such as nonlinear pharmacokinetics caused by auto-inhibition or -induction and polymorphisms of the metabolizing enzymes hinder safe and effective voriconazole dosing in clinical practice and have not yet been studied sufficiently. This review aimed at amalgamating the available literature on the pharmacokinetics of voriconazole in vitro and in vivo, with a special focus on metabolism in adults and children, in order to congregate an overall landscape of the current body of knowledge and identify knowledge gaps, opening the way towards further research in order to foster the understanding, towards better therapeutic dosing decisions.

High voriconazole target-site exposure after approved sequence dosing due to nonlinear pharmacokinetics assessed by long-term microdialysis

Voriconazole, a broad-spectrum antifungal drug used to prevent and treat invasive fungal infections, shows complex pharmacokinetics and is primarily metabolised by various CYP enzymes. An adequate unbound antibiotic concentration-time profile at the target-site of an infection is crucial for effective prophylaxis or therapy success. Therefore, the aim was to evaluate the pharmacokinetics of voriconazole after the approved sequence dosing in healthy volunteers in interstitial space fluid, assessed by microdialysis, and in plasma. Moreover, potential pharmacogenetic influences of CYP2C19 polymorphisms on pharmacokinetics were investigated. The prospective, open-labelled, uncontrolled long-term microdialysis study included 9 healthy male individuals receiving the approved sequence dosing regimen for voriconazole. Unbound voriconazole concentrations were sampled over 84 h in interstitial space fluid of subcutaneous adipose tissue and in plasma and subsequently quantified via high-performance liquid chromatography. For pharmacokinetic data analysis, non-compartmental analysis was used. High interindividual variability in voriconazole concentration-time profiles was detected although dosing was adapted to body weight for the first intravenous administrations. Due to nonlinear pharmacokinetics, target-site exposure of voriconazole in healthy volunteers was found to be highly comparable to plasma exposure, particularly after multiple dosing. Regarding the CYP2C19 genotype-predicted phenotype, the individuals revealed a broad spectrum, ranging from poor to rapid metaboliser status. A strong relation between CYP2C19 genotype-predicted phenotype and voriconazole clearance was identified.

mPEG-PLA/TPGS mixed micelles via intranasal administration improved the bioavailability of lamotrigine in the hippocampus

Purpose

This study aimed to develop a novel methoxy poly(ethylene glycol)-poly(lactide) (mPEG-PLA)/D-α-tocopherol polyethylene glycol 1000 succinate (TPGS) mixed micelle drug delivery system to improve lamotrigine (LTG) distribution in the hippocampus.

Methods

LTG-loaded mPEG-PLA/TPGS mixed micelles and LTG-loaded mPEG-PLA micelles were formulated, and their characteristics, particle size, surface morphology, and release behavior in vitro were researched. Then, a microdialysis sampling technique coupled with two validated chromatographic systems was developed for the continuous measurement of the protein-unbound form of LTG in the rat plasma and hippocampus after administering two kinds of micelles and LTG solution intranasally.

Results

The drug loading and mean size of LTG-loaded micelles and LTG-loaded mixed micelles prepared with optimal formulation were 36.44%±0.14%, 39.28%±0.26%, 122.9, and 183.5 nm, respectively, with a core–shell structure. The cumulative release rate in vivo of LTG-loaded mixed micelles was 84.21% at 24 hours and showed more sustained release while that of LTG-loaded micelles was 80.61% at 6 hours. The T_max and area under concentration-time curve from zero to time of last quantifiable concentration of LTG solution, LTG-loaded micelles, and LTG-loaded mixed micelles were 55, 35, and 15 minutes and about 5,384, 16,500, and 25,245 (min⋅μg)/L in the hippocampus, respectively.

Conclusion

The results revealed that LTG-loaded mPEG-PLA/TPGS mixed micelles enhanced the absorption of LTG at the nasal cavity and reduced the efflux of LTG in the brain, suggesting that the function of TPGS inhibited P-glycoprotein and LTG-loaded mPEG-PLA/TPGS mixed micelles had the potential to overcome refractory epilepsy.

Utility of Microdialysis in Infectious Disease Drug Development and Dose Optimization

Adequate drug penetration to a site of infection is absolutely imperative to ensure sufficient antimicrobial treatment. Microdialysis is a minimally invasive, versatile technique, which can be used to study the penetration of an antiinfective agent in virtually any tissue of interest. It has been used to investigate drug distribution and pharmacokinetics in variable patient populations, as a tool in dose optimization, a potential utility in therapeutic drug management, and in the study of biomarkers of disease progression. While all of these applications have not been fully explored in the field of antiinfectives, this review provides an overview of how microdialysis has been applied in various phases of drug development, a focus on the specific applications in the subspecialties of infectious disease (treatment of bacterial, fungal, viral, parasitic, and mycobacterial infections), and developing applications (biomarkers and therapeutic drug management).

Voriconazole Enhances the Osteogenic Activity of Human Osteoblasts In Vitro through a Fluoride-Independent Mechanism

Periostitis, which is characterized by bony pain and diffuse periosteal ossification, has been increasingly reported with prolonged clinical use of voriconazole. While resolution of clinical symptoms following discontinuation of therapy suggests a causative role for voriconazole, the biological mechanisms contributing to voriconazole-induced periostitis are unknown. To elucidate potential mechanisms, we exposed human osteoblasts in vitro to voriconazole or fluconazole at 15 or 200 μg/ml (reflecting systemic or local administration, respectively), under nonosteogenic or osteogenic conditions, for 1, 3, or 7 days and evaluated the effects on cell proliferation (reflected by total cellular DNA) and osteogenic differentiation (reflected by alkaline phosphatase activity, calcium accumulation, and expression of genes involved in osteogenic differentiation). Release of free fluoride, vascular endothelial growth factor (VEGF), and platelet-derived growth factor (PDGF) was also measured in cell supernatants of osteoblasts exposed to triazoles, with an ion-selective electrode (for free fluoride) and enzyme-linked immunosorbent assays (ELISAs) (for VEGF and PDGF). Voriconazole but not fluconazole significantly enhanced the proliferation and differentiation of osteoblasts. In contrast to clinical observations, no increases in free fluoride levels were detected following exposure to either voriconazole or fluconazole; however, significant increases in the expression of VEGF and PDGF by osteoblasts were observed following exposure to voriconazole. Our results demonstrate that voriconazole can induce osteoblast proliferation and enhance osteogenic activity in vitro. Importantly, and in contrast to the previously proposed mechanism of fluoride-stimulated osteogenesis, our findings suggest that voriconazole-induced periostitis may also occur through fluoride-independent mechanisms that enhance the expression of cytokines that can augment osteoblastic activity.

Role of microdialysis in pharmacokinetics and pharmacodynamics: current status and future directions

Diagnostic and therapeutic decisions in medical practice are still generally based on blood concentrations of drugs and/or biomolecules despite the knowledge that biochemical events and pharmacological effects usually take place in tissue rather than in the bloodstream. Microdialysis is a semi-invasive technique that is able to measure concentrations of the free, active drug or endogenous compounds in almost all human tissues and organs. It is currently being used to monitor brain metabolic processes and quantify tissue biomarkers, and determine transdermal drug distribution and tissue pharmacokinetics, confirming its importance as a widely used sampling technique in clinical drug monitoring and drug development as well as therapy and disease follow-up, contributing to rationalizing drug dosing regimens and influencing the clinical decision-making process.

Cerebral microdialysis in glioma studies, from theory to application

Despite recent advances in the treatment of solid tumors, there are few effective treatments for malignant gliomas due to the infiltrative nature, and the protective shield of blood-brain barrier or blood-tumor barriers that restrict the passage of chemotherapy drugs into the brain. Imaging techniques, such as PET and MRI, have allowed the assessment of tumor function in vivo, but they are indirect measures of activity and do not easily allow continuous repeated evaluations. Because the biology of glioma on a cellular and molecular level is fairly unknown, especially in relation to various treatments, the development of novel therapeutic approaches to this devastating condition requires a strong need for a deeper understanding of the tumor's pathophysiology and biochemistry. Cerebral microdialysis, a probe-based sampling technique, allows a discrete volume of the brain to be sampled for neurochemical analysis of neurotransmitters, metabolites, biomarkers, and chemotherapy drugs, which has been employed in studying brain tumors, and is significant for improving the treatment of glioma. In this review, the current concepts of cerebral microdialysis for glioma are elucidated, with a special emphasis on its application to neurochemistry and pharmacokinetic studies.

Role of microdialysis in pharmacokinetics and pharmacodynamics: current status and future directions

Diagnostic and therapeutic decisions in medical practice are still generally based on blood concentrations of drugs and/or biomolecules despite the knowledge that biochemical events and pharmacological effects usually take place in tissue rather than in the bloodstream. Microdialysis is a semi-invasive technique that is able to measure concentrations of the free, active drug or endogenous compounds in almost all human tissues and organs. It is currently being used to monitor brain metabolic processes and quantify tissue biomarkers, and determine transdermal drug distribution and tissue pharmacokinetics, confirming its importance as a widely used sampling technique in clinical drug monitoring and drug development as well as therapy and disease follow-up, contributing to rationalizing drug dosing regimens and influencing the clinical decision-making process.