In vivo microdialysis in pharmacological studies of antibacterial agents in the brain

Sample preparation techniques for protein binding measurement in radiopharmaceutical approaches: A short review

Plasma protein binding (PPB) measurement is a key step in radiopharmaceutical studies for the development of positron emission tomography (PET) radioligands. PPB refers to the binding degree of a radioligand, radiotracer, or drug to blood plasma proteins or tissues after administration into the body. Several techniques have been successfully developed and applied for PPB measurement of PET radioligands. However, there is room for progress among these techniques in relation to duration time, adaptability with nonpolar radioligands, in vivo measurement, specificity, and selectivity. This mini review gives a brief overview of advances, limitations, and prospective applications of commercially-available PPB methods.

Microdialysis Monitoring in Clinical Traumatic Brain Injury and Its Role in Neuroprotective Drug Development

Injuries to the central nervous system continue to be vast contributors to morbidity and mortality; specifically, traumatic brain injury (TBI) is the most common cause of death during the first four decades of life. Several modalities are used to monitor patients suffering from TBI in order to prevent detrimental secondary injuries. The microdialysis (MD) technique, introduced during the 1990s, presents the treating physician with a robust monitoring tool for brain chemistry in addition to conventional intracranial pressure monitoring. Nevertheless, some limitations remain, such as limited spatial resolution. Moreover, while there have been several attempts to develop new potential pharmacological therapies in TBI, there are currently no available drugs which have shown clinical efficacy that targets the underlying pathophysiology, despite various trials investigating a plethora of pharmaceuticals. Specifically in the brain, MD is able to demonstrate penetration of the drug through the blood-brain barrier into the brain extracellular space at potential site of action. In addition, the downstream effects of drug action can be monitored directly. In the future, clinical MD, together with other monitoring modalities, can identify specific pathological substrates which require tailored treatment strategies for patients suffering from TBI.

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).

LC–MS/MS assay of ropinirole in rat biological matrices: elimination of lysoglycerophosphocholines-based matrix effect

Aim: To adequately support PK evaluation of ropinirole in rats following intranasal administration, it was desirable to determine ropinirole concentrations in rat plasma, brain homogenate and microdialysate. Results & methodology: A robust LC–MS/MS method has been developed for the determination of ropinirole in biological samples. Liquid–liquid extraction using ethyl acetate eliminated matrix effect due to significantly reduced levels of lysoglycerophosphocholines. The assay was fully validated with dynamic ranges of 0.01–20 ng/ml for plasma and brain homogenate samples and 0.1–200 ng/ml for microdialysate samples. Conclusion: The proposed method was accurate and precise for the quantification of ropinirole in biological samples and was successfully applied to a microdialysis study of ropinirole in rats.

Consensus statement from the 2014 International Microdialysis Forum

Microdialysis enables the chemistry of the extracellular interstitial space to be monitored. Use of this technique in patients with acute brain injury has increased our understanding of the pathophysiology of several acute neurological disorders. In 2004, a consensus document on the clinical application of cerebral microdialysis was published. Since then, there have been significant advances in the clinical use of microdialysis in neurocritical care. The objective of this review is to report on the International Microdialysis Forum held in Cambridge, UK, in April 2014 and to produce a revised and updated consensus statement about its clinical use including technique, data interpretation, relationship with outcome, role in guiding therapy in neurocritical care and research applications.

Use of cassette dosing to enhance the throughput of rat brain microdialysis studies

This study was designed to increase the throughput of rat brain microdialysis studies by administration of compounds as a cassette as opposed to discrete study. Eight compounds (carbamazepine, citalopram, desmethylclozapine, diphenhydramine, gabapentin, metoclopramide, naltrexone, and stavudine) were selected and administered as an intravenous bolus dose at 0.5-3.3 mg/kg each followed by an intravenous infusion at 1 mg/kg per hour for 6 hours in rats in a cassette or discrete dosing. The dialysate, plasma, brain, and cerebrospinal fluid were collected and analyzed using liquid chromatography-tandem mass spectrometry. The microdialysis probe recovery was determined by an in vitro gain method. The recovery between the cassette and discrete dosing was similar, with an average of 1.0 ± 0.10-fold difference. The stavudine interstitial fluid (ISF) concentration, as measured by brain microdialysis, was below the low limit of quantitation and was excluded from the analyses. The ratios of ISF concentration to unbound plasma concentration were within 2-fold for six of the remaining seven compounds, with an average of 0.92 ± 0.51-fold difference between the cassette and discrete methods. The ratios of ISF concentration to unbound brain concentration, as measured by the brain homogenate method, were also similar, with a 1.1 ± 0.7-fold difference. In addition, the ratios of ISF to cerebrospinal fluid concentrations were similar, with a 1.5 ± 0.6-fold difference. The results from this study support the use of a cassette dosing approach to enhance the throughput of rat brain microdialysis studies in drug discovery.

Applications of the pharmacokinetic/pharmacodynamic (PK/PD) analysis of antimicrobial agents

The alarming increase of resistance against multiple currently available antibiotics is leading to a rapid lose of treatment options against infectious diseases. Since the antibiotic resistance is partially due to a misuse or abuse of the antibiotics, this situation can be reverted when improving their use. One strategy is the optimization of the antimicrobial dosing regimens. In fact, inappropriate drug choice and suboptimal dosing are two major factors that should be considered because they lead to the emergence of drug resistance and consequently, poorer clinical outcomes. Pharmacokinetic/pharmacodynamic (PK/PD) analysis in combination with Monte Carlo simulation allows to optimize dosing regimens of the antibiotic agents in order to conserve their therapeutic value. Therefore, the aim of this review is to explain the basis of the PK/PD analysis and associated techniques, and provide a brief revision of the applications of PK/PD analysis from a therapeutic point-of-view. The establishment and reevaluation of clinical breakpoints is the sticking point in antibiotic therapy as the clinical use of the antibiotics depends on them. Two methodologies are described to establish the PK/PD breakpoints, which are a big part of the clinical breakpoint setting machine. Furthermore, the main subpopulations of patients with altered characteristics that can condition the PK/PD behavior (such as critically ill, elderly, pediatric or obese patients) and therefore, the outcome of the antibiotic therapy, are reviewed. Finally, some recommendations are provided from a PK/PD point of view to enhance the efficacy of prophylaxis protocols used in surgery.

Physiologically based pharmacokinetic modelling of drug penetration across the blood-brain barrier--towards a mechanistic IVIVE-based approach

Predicting the penetration of drugs across the human blood-brain barrier (BBB) is a significant challenge during their development. A variety of in vitro systems representing the BBB have been described, but the optimal use of these data in terms of extrapolation to human unbound brain concentration profiles remains to be fully exploited. Physiologically based pharmacokinetic (PBPK) modelling of drug disposition in the central nervous system (CNS) currently consists of fitting preclinical in vivo data to compartmental models in order to estimate the permeability and efflux of drugs across the BBB. The increasingly popular approach of using in vitro-in vivo extrapolation (IVIVE) to generate PBPK model input parameters could provide a more mechanistic basis for the interspecies translation of preclinical models of the CNS. However, a major hurdle exists in verifying these predictions with observed data, since human brain concentrations can't be directly measured. Therefore a combination of IVIVE-based and empirical modelling approaches based on preclinical data are currently required. In this review, we summarise the existing PBPK models of the CNS in the literature, and we evaluate the current opportunities and limitations of potential IVIVE strategies for PBPK modelling of BBB penetration.

Cerebral microdialysis in clinical studies of drugs: pharmacokinetic applications

The ability to deliver drug molecules effectively across the blood-brain barrier into the brain is important in the development of central nervous system (CNS) therapies. Cerebral microdialysis is the only existing technique for sampling molecules from the brain extracellular fluid (ECF; also termed interstitial fluid), the compartment to which the astrocytes and neurones are directly exposed. Plasma levels of drugs are often poor predictors of CNS activity. While cerebrospinal fluid (CSF) levels of drugs are often used as evidence of delivery of drug to brain, the CSF is a different compartment to the ECF. The continuous nature of microdialysis sampling of the ECF is ideal for pharmacokinetic (PK) studies, and can give valuable PK information of variations with time in drug concentrations of brain ECF versus plasma. The microdialysis technique needs careful calibration for relative recovery (extraction efficiency) of the drug if absolute quantification is required. Besides the drug, other molecules can be analysed in the microdialysates for information on downstream targets and/or energy metabolism in the brain. Cerebral microdialysis is an invasive technique, so is only useable in patients requiring neurocritical care, neurosurgery or brain biopsy. Application of results to wider patient populations, and to those with different pathologies or degrees of pathology, obviously demands caution. Nevertheless, microdialysis data can provide valuable guidelines for designing CNS therapies, and play an important role in small phase II clinical trials. In this review, we focus on the role of cerebral microdialysis in recent clinical studies of antimicrobial agents, drugs for tumour therapy, neuroprotective agents and anticonvulsants.

Determination of gemcitabine and its metabolite in extracellular fluid of rat brain tumor by ultra performance liquid chromatography-tandem mass spectrometry using microdialysis sampling after intralesional chemotherapy

The cytotoxic agent Gemcitabine (2',2'-difluoro-2'-deoxycytidine) has been proved to be effective in the treatment of malignant gliomas. A rapid, sensitive and specific ultra performance liquid chromatography with tandem mass spectrometry (UPLC-MS/MS) assay using microdialysis sampling was developed and validated to quantify gemcitabine and its major metabolite 2',2'-difluoro-2'-deoxyuridine (dFdU) in Sprague-Dawley rat bearing 9L glioma. Microdialysis probes were surgically implanted into the area of rat brain tumor in the striatal hemisphere, and artificial cerebrospinal fluid was used as a perfusion medium. The samples were analyzed directly by UPLC-MS/MS after the addition of 5-bromouracil as an internal standard (IS). Separation was achieved on Agilent SB-C(18) (50 mm × 2.1mm I.D., 1.8 μm) column at 40 °C using an isocratic elution method with acetonitrile and 0.1% formic acid (4:96, v/v) at a flow rate of 0.2 mL/min. Detection was performed using electrospray ionization in positive ion selected reaction monitoring mode by monitoring the following ion transitions m/z 264.0→112.0 (gemcitabine), m/z 265.1→113.0 (dFdU) and m/z 190.9→173.8 (IS). The calibration curves of gemcitabine and dFdU were linear in the concentration range of 0.66-677.08 ng/mL and 0.31-312.00 ng/mL, respectively. The lower limit of quantification of gemcitabine and dFdU were 0.66 ng/mL and 0.31 ng/mL, respectively. The lower limit of detection of gemcitabine and dFdU were calculated to be 0.2 ng/mL and 0.1 ng/mL, respectively. All the validation data, such as intra- and inter-day precision, accuracy, selectivity and stability, were within the required limits. The validated method was simple, precise and accurate, which was successfully employed to determinate the concentrations of gemcitabine and dFdU in the extracellular fluid of rat brain tumor.