AAPS-FDA workshop white paper: microdialysis principles, application and regulatory perspectives

Doxorubicin concentrations in bone tumour-relevant tissues after bolus and continuous infusion: a randomized porcine microdialysis study

PURPOSE

Doxorubicin is a widely used chemotherapeutic drug that can be administered intravenously as both a bolus infusion and a continuous infusion. The latter is believed to lower the risk of cardiotoxicity, which is a critical long-term complication of doxorubicin treatment. The local tissue concentrations of doxorubicin will be reflected in both treatment efficacy and toxicity, but very limited information is available. The aim of this study was to measure the concentration of doxorubicin after continuous and bolus infusion in tissue compartments around a typical location of a bone tumour.

METHODS

Sixteen pigs (female, Danish Landrace, mean weight 77 kg) were randomized into two groups of eight. Both groups received an intravenous infusion of 150 mg doxorubicin; Group 1 received a bolus infusion (10-15 min) and Group 2 received a continuous infusion (6 h). Before infusion, microdialysis catheters were placed intravenously and in four bone tumour-relevant tissue compartments (cancellous bone, subcutaneous tissue, synovial fluid of the knee joint and muscle tissue). Sampling was done (n = 15) over 24 h, and venous blood samples were collected as a reference.

RESULTS

Area under the concentration-time curve (AUC0-24 h) for plasma (total concentration) was significantly different between the two groups, while peak drug concentration (Cmax) was significantly higher in two compartments (plasma and synovial fluid of the knee joint) in Group 1 compared to Group 2. Overall, the unbound tissue concentrations were extremely low with values below 0.20 µg/mL.

CONCLUSION

The pharmacokinetic profile for doxorubicin in the investigated tissues is very similar when comparing bolus and 6 h continuous infusion.

Metabolome analyses of skin dialysate: Insights into skin interstitial fluid biomarkers

BACKGROUND

Metabolites in biofluids can serve as biomarkers for diagnosing diseases and monitoring body conditions. Among the available biofluids, interstitial fluid (ISF) in the skin has garnered considerable attention owing to its advantages, which include inability to clot, easy access to the skin, and possibility of incorporating wearable devices. However, the scientific understanding of skin ISF composition is limited.

OBJECTIVE

In this study, we aimed to compare metabolites between skin dialysate containing metabolites from the skin ISF and venous blood (plasma) samples, both collected under resting states.

METHODS

We collected forearm skin dialysate using intradermal microdialysis alongside venous blood (plasma) samples from 12 healthy young adults. We analyzed these samples using capillary electrophoresis-fourier transform mass spectrometry-based metabolomics (CE-FTMS).

RESULTS

Significant positive correlations were observed in 39 metabolites between the skin dialysate and plasma, including creatine (a mitochondrial disease biomarker), 1-methyladenosine (an early detection of cancer biomarker), and trimethylamine N-oxide (a posterior predictor of heart failure biomarker). Based on the Human Metabolome Technologies database, we identified 12 metabolites unique to forearm skin dialysate including nucleic acids, benzoate acids, fatty acids, amino acids, ascorbic acid, 3-methoxy-4-hydroxyphenylethyleneglycol (an Alzheimer's disease biomarker), and cysteic acid (an acute myocardial infarction biomarker).

CONCLUSION

We show that some venous blood biomarkers may be predicted from skin dialysate or skin ISF, and that these fluids may serve as diagnostic and monitoring tools for health and clinical conditions.

Have We Neglected to Study Target-Site Drug Exposure in Children? A Systematic Review of the Literature

BACKGROUND AND OBJECTIVE

Drug dosing should ideally be based on the drug concentrations at the target site, which, for most drugs, corresponds to the tissue. The exact influence of growth and development on drug tissue distribution is unclear. This systematic review compiles the current knowledge on the tissue distribution of systemically applied drugs in children, with the aim to identify priorities in tissue pharmacokinetic (PK) research in this population.

METHODS

A systematic literature search was performed in the MEDLINE and Embase databases.

RESULTS

Forty-two relevant articles were identified, of which 71% investigated antibiotics, while drug classes from the other studies were anticancer drugs, antifungals, anthelmintics, sedatives, thyreostatics, immunomodulators, antiarrhythmics, and exon skipping therapy. The majority of studies (83%) applied tissue biopsy as the sampling technique. Tonsil and/or adenoid tissue was most frequently examined (70% of all included patients). The majority of studies had a small sample size (median 9, range 1-93), did not include the youngest age categories (neonates and infants), and were of low reporting quality. Due to the heterogeneous data from different study compounds, dosing schedules, populations, and target tissues, the possibility for comparison of PK data between studies was limited.

CONCLUSION

The influence of growth and development on drug tissue distribution continues to be a knowledge gap, due to the paucity of tissue PK data in children, especially in the younger age categories. Future research in this field should be encouraged as techniques to safely investigate drug tissue disposition in children are available.

Microdialysis as a sampling tool for the chemotherapeutic agent Doxorubicin

Doxorubicin is a chemotherapeutic agent used for more than fifty years to treat a great variety of cancers in both children and adults. Despite hereof, pharmacokinetic knowledge is almost solely based on systemic plasma concentrations. Microdialysis is a catheter-based pharmacokinetic sampling tool enabling simultaneous target site sampling of unbound molecules of interest. The aim of this study was to thoroughly evaluate the feasibility of applying microdialysis for sampling of Doxorubicin in both in vitro experiments and an in vivo setting. Doxorubicin relative recovery by gain and by loss was tested for different catheter types, perfusion fluids, concentrations and collection vials. Adsorption tests revealed polystyrene/santoprene vials to be the biggest contributor of unwanted adsorption between Doxorubicin and the microdialysis equipment, and confirmed LoBind Eppendorf tubes to be a suitable alternative. The methodological combination of polyamide membranes, saline as perfusion fluid and LoBind Eppendorf sampling tubes demonstrated no statistically significant differences for relative recovery by gain and by loss, and the relative recovery was also found to be concentration independent. We conclude, that a proper microdialysis set-up can be used to collect samples containing concentrations of the chemotherapeutic drug Doxorubicin in vitro and in vivo, which encourage future pharmacokinetic studies to evaluate current treatment regimens to find the most effective and least toxic anti-neoplastic treatment for the patients.

Opportunities and challenges in the diagnostic utility of dermal interstitial fluid

The volume of interstitial fluid (ISF) in the human body is three times that of blood. Yet, collecting diagnostically useful ISF is more challenging than collecting blood because the extraction of dermal ISF disrupts the delicate balance of pressure between ISF, blood and lymph, and because the triggered local inflammation further skews the concentrations of many analytes in the extracted fluid. In this Perspective, we overview the most meaningful differences in the make-up of ISF and blood, and discuss why ISF cannot be viewed generally as a diagnostically useful proxy for blood. We also argue that continuous sensing of small-molecule analytes in dermal ISF via rapid assays compatible with nanolitre sample volumes or via miniaturized sensors inserted into the dermis can offer clinically advantageous utility, particularly for the monitoring of therapeutic drugs and of the status of the immune system.

Microdialysis as a safe and feasible method to study target-site piperacillin-tazobactam disposition in septic piglets and children

OBJECTIVES

Knowledge on the tissue penetration of piperacillin-tazobactam in children with sepsis is lacking. In this study, the feasibility and performance of microdialysis experiments were explored in septic piglets and children as part of a translational research project.

METHODS

Multiple-day microdialysis investigations were performed in muscle tissue of 22 piglets (of which 11 were septic) and 6 children with sepsis. An in vitro experiment preceded the (pre)clinical trials to derive optimal experimental settings and calibration technique. Linear mixed-effects models quantified the impact of sepsis on relative recovery (RR) and intercatheter, interindividual, interoccasion, and residual variability.

RESULTS

In vivo microdialysis was well tolerated in piglets and children, with no significant adverse events reported. Using identical experimental settings, lower RR values were recorded in healthy and septic piglets (range: piperacillin, 17.2-29.1% and tazobactam, 23.5-29.1%) compared with the in vitro experiment (piperacillin, 43.3% and tazobactam, 55.3%), and there were unacceptably low values in children with sepsis (<10%). As a result, methodological changes were made in the pediatric trial. Realistic tissue concentration-time curves were derived in piglets and children. In piglets, sepsis reduced the RR. The greatest contributors to RR variability were residual (>40%) and interoccasion (>30%) variability. The internal standard method was the preferred calibration technique in both piglets and children.

CONCLUSIONS

Microdialysis is a safe and applicable method for the measurement of tissue drug concentrations in piglets and children. This study demonstrated the impact of experimental settings, sepsis, and target population on individual RR.

Blood-brain barrier dysfunction in intensive care unit

The central nervous system is characterized by a peculiar vascularization termed blood-brain barrier (BBB), which regulates the exchange of cells and molecules between the cerebral tissue and the whole body. BBB dysfunction is a life-threatening condition since its presence corresponds to a marker of severity in most diseases encountered in the intensive care unit (ICU). During critical illness, inflammatory response, cytokine release, and other phenomena activating the brain endothelium contribute to alterations in the BBB and increase its permeability to solutes, cells, nutrients, and xenobiotics. Moreover, patients in the ICU are often old, with underlying acute or chronic diseases, and overly medicated due to their critical condition; these factors could also contribute to the development of BBB dysfunction. An accurate diagnostic approach is critical for the identification of the mechanisms underlying BBB alterations, which should be rapidly managed by intensivists. Several methods were developed to investigate the BBB and assess its permeability. Nevertheless, in humans, exploration of the BBB requires the use of indirect methods. Imaging and biochemical methods can be used to study the abnormal passage of molecules through the BBB. In this review, we describe the structural and functional characteristics of the BBB, present tools and methods for probing this interface, and provide examples of the main diseases managed in the ICU that are related to BBB dysfunction.

Population Pharmacokinetic Modeling of Free Plasma and Free Brain Concentrations of Ceftaroline in Healthy and Methicillin-Resistant Staphylococcus aureus-Infected Wistar Rats

A population pharmacokinetic model was developed to describe alterations in ceftaroline brain disposition caused by meningitis in healthy and methicillin-resistant Staphylococcus aureus (MRSA)-infected rats. Blood and brain microdialysate samples were obtained after a single bolus dose of ceftaroline fosamil (20 mg/kg) administered intravenously. Plasma data were modeled as one compartment, and brain data were added to the model as a second compartment, with bidirectional drug transport between plasma and brain (Qin and Qout). The cardiac output (CO) of the animals showed a significant correlation with the relative recovery (RR) of plasma microdialysis probes, with animals with greater CO presenting lower RR values. The Qin was approximately 60% higher in infected animals, leading to greater brain exposure to ceftaroline. Ceftaroline brain penetration was influenced by MRSA infection, increasing from 17% (Qin/Qout) in healthy animals to 27% in infected animals. Simulations of a 2-h intravenous infusion of 50 mg/kg every 8 h achieved >90% probability of target attainment (PTA) in plasma and brain for the modal MRSA MIC (0.25 mg/L), suggesting that the drug should be considered an option for treating central nervous system infections.

Concentrations of Co-Administered Meropenem and Vancomycin in Spinal Tissues Relevant for the Treatment of Pyogenic Spondylodiscitis-An Experimental Microdialysis Study

Co-administration of meropenem and vancomycin has been suggested as a systemic empirical antibiotic treatment of pyogenic spondylodiscitis. The aim of this study was, in an experimental porcine model, to evaluate the percentage of an 8-h dosing interval of co-administered meropenem and vancomycin concentrations above the relevant minimal inhibitory concentrations (MICs) (%T>MIC) in spinal tissues using microdialysis. Eight female pigs (Danish Landrace breed, weight 78-82 kg) received a single-dose bolus infusion of 1000 mg of meropenem and 1000 mg vancomycin simultaneously before microdialysis sampling. Microdialysis catheters were applied in the third cervical (C3) vertebral cancellous bone, the C3-C4 intervertebral disc, paravertebral muscle, and adjacent subcutaneous tissue. Plasma samples were obtained for reference. The main finding was that for both drugs, the %T>MICs were highly reliant on the applied MIC target, but were heterogeneous across all targeted tissues, ranging from 25-90% for meropenem, and 10-100% for vancomycin. For both MIC targets, the highest %T>MIC was demonstrated in plasma, and the lowest %T>MIC was demonstrated in the vertebral cancellous bone for meropenem, and in the intervertebral disc for vancomycin. When indicated, our findings may suggest a more aggressive dosing approach of both meropenem and vancomycin to increase the spinal tissue concentrations to treat the full spectrum of potentially encountered bacteria in a spondylodiscitis treatment setting.

Comparison of Intravenous Microdialysis and Standard Plasma Sampling for Monitoring of Vancomycin and Meropenem Plasma Concentrations-An Experimental Porcine Study

Microdialysis is a catheter-based method suitable for dynamic sampling of unbound antibiotic concentrations. Intravenous antibiotic concentration sampling by microdialysis has several advantages and may be a superior alternative to standard plasma sampling. We aimed to compare concentrations obtained by continuous intravenous microdialysis sampling and by standard plasma sampling of both vancomycin and meropenem in a porcine model. Eight female pigs received 1 g of both vancomycin and meropenem, simultaneously over 100 and 10 min, respectively. Prior to drug infusion, an intravenous microdialysis catheter was placed in the subclavian vein. Microdialysates were collected for 8 h. From a central venous catheter, plasma samples were collected in the middle of every dialysate sampling interval. A higher area under the concentration/time curve and peak drug concentration were found in standard plasma samples compared to intravenous microdialysis samples, for both vancomycin and meropenem. Both vancomycin and meropenem concentrations obtained with intravenous microdialysis were generally lower than from standard plasma sampling. The differences in key pharmacokinetic parameters between the two sampling techniques underline the importance of further investigations to find the most suitable and reliable method for continuous intravenous antibiotic concentration sampling.

Piperacillin Steady State Concentrations in Target Tissues Relevant for PJI Treatment—A Randomized Porcine Microdialysis Study Comparing Continuous Infusion with Intermittent Short-Term Infusion

(1) Introduction: Piperacillin is a common antibiotic choice in the treatment of periprosthetic joint infections (PJI) caused by Pseudomonas aeruginosa. The aim of this study was to assess and compare the time with free piperacillin concentration above the minimum inhibitory concentration (fT > MIC) at steady state in target tissues relevant for PJI treatment following continuous and intermittent short-term infusion. (2) Methods: 16 pigs were randomized to receive either continuous or intermittent short-term infusion of piperacillin. Steady state piperacillin concentrations were assessed using microdialysis in tibial cortical bone, tibial cancellous bone, synovial fluid of the knee joint, and subcutaneous tissue. MIC-targets of 4, 8, 16, and 64 mg/L were applied. Plasma samples were obtained as reference. (3) Results: Continuous infusion resulted in longer fT > MIC for MIC targets of 4 mg/L and 8 mg/L compared to intermittent short-term infusion in all compartments with the exception of tibial cortical bone. For the MIC-target of 16 mg/L, continuous infusion resulted in a longer fT > MIC in all compartments except for the bone compartments. No differences between groups were seen when applying a MIC-target of 64 mg/L. (4) Conclusions: An aggressive dosing strategy may be necessary to obtain sufficient piperacillin concentrations in all bone compartments, particularly if more aggressive targets are applied. Based on the present study, continuous infusion should be considered in the treatment of PJI.

A Novel Microfluidic-Based OMC-PEDOT-PSS Composite Electrochemical Sensor for Continuous Dopamine Monitoring

Fast and precise analysis techniques using small sample volumes are required for next-generation clinical monitoring at the patient's bedside, so as to provide the clinician with relevant chemical data in real-time. The integration of an electrochemical sensor into a microfluidic chip allows for the achievement of real-time chemical monitoring due to the low consumption of analytes, short analysis time, low cost, and compact size. In this work, dopamine, used as a model, is an important neurotransmitter responsible for controlling various vital life functions. The aim is to develop a novel serpentine microfluidic-based electrochemical sensor, using a screen-printed electrode for continuous dopamine detection. The developed sensor employed the composite of ordered mesoporous carbon (OMC) and poly (3,4 ethylenedioxythiophene)-poly (styrene sulfonate) (PEDOT-PSS). The performance of a microfluidic, integrated with the sensor, was amperometrically evaluated using a computer-controlled microfluidic platform. The microfluidic-based dopamine sensor exhibited a sensitivity of 20.2 ± 0.6 μA μmol L^-1, and a detection limit (LOD) of 21.6 ± 0.002 nmol L^-1, with high selectivity. This microfluidic-based electrochemical sensor was successfully employed to determine dopamine continuously, which could overcome the problem of sensor fouling with more than 90% stability for over 24 h. This novel microfluidic sensor platform provides a powerful tool for the development of a continuous dopamine detection system for human clinical application.

Next generation genetically encoded fluorescent sensors for serotonin

We developed a family of genetically encoded serotonin (5-HT) sensors (sDarken) on the basis of the native 5-HT1A receptor and circularly permuted GFP. sDarken 5-HT sensors are bright in the unbound state and diminish their fluorescence upon binding of 5-HT. Sensor variants with different affinities for serotonin were engineered to increase the versatility in imaging of serotonin dynamics. Experiments in vitro and in vivo showed the feasibility of imaging serotonin dynamics with high temporal and spatial resolution. As demonstrated here, the designed sensors show excellent membrane expression, have high specificity and a superior signal-to-noise ratio, detect the endogenous release of serotonin and are suitable for two-photon in vivo imaging.

Accelerator mass spectrometry for quantification of micro- and therapeutic dose diclofenac in microdialysis samples

Background: Microdialysis sampling after drug microdosing may provide tissue pharmacokinetic data early in clinical drug development. However, low administered doses and small sample volumes pose an analytical challenge, particularly for highly protein-bound drugs. Materials & methods: Carbon-14 [¹⁴C]diclofenac was used as a model drug to assess the technical and analytical feasibility of in vivo microdialysis after microdose administration in an in vitro setup. Results: [¹⁴C]diclofenac dialysate concentrations were accurately quantified with accelerator MS. [¹⁴C]diclofenac dialysate recoveries were similar in the presence and absence of therapeutic diclofenac concentrations but were considerably decreased when albumin was added to the immersion solution, suggesting high protein binding. Conclusion: These results demonstrate the feasibility of combining microdosing and microdialysis to assess tissue pharmacokinetics.

Cefuroxime concentrations in the anterior and posterior column of the lumbar spine - an experimental porcine study

BACKGROUND CONTEXT

Surgical site infection following spine surgery is associated with increased morbidity and mortality. Perioperative antibiotic prophylaxis is a key factor in lowering the risk of acquiring an infection. Previous studies have assessed perioperative cefuroxime concentrations in the anterior column of the cervical spine with an anterior surgical approach. However, the majority of surgeries are performed in the posterior column and many surgeries involve the lumbar spine.

PURPOSE

The objective of this study was to compare the perioperative tissue concentrations of cefuroxime in the anterior and posterior column during lumbar spine surgery with a posterior surgical approach.

STUDY DESIGN

In vivo experimental pharmacokinetic study of cefuroxime concentrations in an acute preclinical porcine model.

METHODS

The lumbar vertebral column was exposed from L1 to L5 in 8 female pigs. Microdialysis catheters were placed for sampling in the anterior column (vertebral body) and posterior column (posterior arch) within the same vertebra (L5). Cefuroxime (1.5 g) was administered intravenously. Microdialysates and plasma samples were continuously obtained over 8 hours. Cefuroxime concentrations were quantified by Ultra High Performance Liquid Chromatography Tandem Mass Spectrometry. The primary endpoint was the time above the cefuroxime clinical breakpoint minimal inhibitory concentration (T>MIC) for Staphylococcus aureus of 4 µg/mL. The secondary endpoint was tissue penetration (AUC_tissue/AUC_plasma).

RESULTS

Mean T>MIC 4 µg/mL (95% confidence interval) was 123 min (105-141) in plasma, 97 min (79-115) in the anterior column and 93 min (75-111) in the posterior column. Tissue penetration (95% confidence interval) was incomplete for both the anterior column 0.48 (0.40-0.56) and posterior column 0.40 (0.33-0.48).

CONCLUSIONS

T>MIC was comparable between the anterior and posterior column. Mean cefuroxime concentrations decreased below the clinical breakpoint minimal inhibitory concentration for S. aureus of 4 µg/mL after 123 minutes (plasma), 97 minutes (anterior column) and 93 minutes (posterior column). This is shorter than the duration of most lumbar spine surgeries, and therefore alternative dosing regimens should be considered in posterior open lumbar spine surgeries lasting more than 1.5 hours.

CLINICAL SIGNIFICANCE

Open lumbar spine surgery often involves extensive soft tissue dissection, stripping and retraction of the paraspinal muscles which may impair the local blood flow exposing the lumbar vertebra to postoperative infections. A single intravenous administration of 1.5 g cefuroxime only provided sufficient prophylactic target tissue concentrations in the vertebra of the lumbar spine for up to 1.5 hours.

Bedside interpretation of cerebral energy metabolism utilizing microdialysis in neurosurgical and general intensive care

The microdialysis technique was initially developed for monitoring neurotransmitters in animals. In 1995 the technique was adopted to clinical use and bedside enzymatic analysis of glucose, pyruvate, lactate, glutamate and glycerol. Under clinical conditions microdialysis has also been used for studying cytokines, protein biomarkers, multiplex proteomic and metabolomic analyses as well as for pharmacokinetic studies and evaluation of blood-brain barrier function. This review focuses on the variables directly related to cerebral energy metabolism and the possibilities and limitations of microdialysis during routine neurosurgical and general intensive care. Our knowledge of cerebral energy metabolism is to a large extent based on animal experiments performed more than 40 years ago. However, the different biochemical information obtained from various techniques should be recognized. The basic animal studies analyzed brain tissue homogenates while the microdialysis technique reflects the variables in a narrow zone of interstitial fluid surrounding the probe. Besides the difference of the volume investigated, the levels of the biochemical variables differ in different compartments. During bedside microdialysis cerebral energy metabolism is primarily reflected in measured levels of glucose, lactate and pyruvate and the lactate to pyruvate (LP) ratio. The LP ratio reflects cytoplasmatic redox-state which increases instantaneously during insufficient aerobic energy metabolism. Cerebral ischemia is characterized by a marked increase in intracerebral LP ratio at simultaneous decreases in intracerebral levels of pyruvate and glucose. Mitochondrial dysfunction is characterized by a moderate increase in LP ratio at a very marked increase in cerebral lactate and normal or elevated levels of pyruvate and glucose. The patterns are of importance in particular for interpretations in transient cerebral ischemia. A new technique for evaluating global cerebral energy metabolism by microdialysis of the draining cerebral venous blood is discussed. In experimental studies it has been shown that pronounced global cerebral ischemia is reflected in venous cerebral blood. Jugular bulb microdialysis has been investigated in patients suffering from subarachnoid hemorrhage, during cardiopulmonary bypass and resuscitation after out of hospital cardiac arrest. Preliminary results indicate that the new technique may give valuable information of cerebral energy metabolism in clinical conditions when insertion of an intracerebral catheter is contraindicated.

Plasma and Interstitial Fluid Pharmacokinetics of Prophylactic Cefazolin in Elective Bariatric Surgery Patients

Guidelines for surgical prophylactic dosing of cefazolin in bariatric surgery vary in terms of recommended dose. This study aimed to describe the plasma and interstitial fluid (ISF) cefazolin pharmacokinetics in patients undergoing bariatric surgery and to determine an optimum dosing regimen. Abdominal subcutaneous ISF concentrations (measured using microdialysis) and plasma samples were collected at regular time points after administration of cefazolin 2 g intravenously. Total and unbound cefazolin concentrations were assayed and then modeled using Pmetrics. Monte Carlo dosing simulations (n = 5,000) were used to define cefazolin dosing regimens able to achieve a fractional target attainment (FTA) of >95% in the ISF suitable for the MIC for Staphylococcus aureus in isolates of ≤2 mg · L^-1 and for a surgical duration of 4 h. Fourteen patients were included, with a mean (standard deviation [SD]) bodyweight of 148 (35) kg and body mass index (BMI) of 48 kg · m^-2. Cefazolin protein binding ranged from 14 to 36% with variable penetration into ISF of 58% ± 56%. Cefazolin was best described as a four-compartment model including nonlinear protein binding. The mean central volume of distribution in the final model was 18.2 (SD 3.31) L, and the mean clearance was 32.4 (SD 20.2) L · h^-1. A standard 2-g dose achieved an FTA of >95% for all patients with BMIs ranging from 36 to 69 kg · m^-2. A 2-g prophylactic cefazolin dose achieves appropriate unbound plasma and ISF concentrations in obese and morbidly obese bariatric surgery patients.

Unbound Brain-to-Plasma Partition Coefficient, Kp,uu,brain-a Game Changing Parameter for CNS Drug Discovery and Development

PURPOSE

More than 15 years have passed since the first description of the unbound brain-to-plasma partition coefficient (Kp,uu,brain) by Prof. Margareta Hammarlund-Udenaes, which was enabled by advancements in experimental methodologies including cerebral microdialysis. Since then, growing knowledge and data continue to support the notion that the unbound (free) concentration of a drug at the site of action, such as the brain, is the driving force for pharmacological responses. Towards this end, Kp,uu,brain is the key parameter to obtain unbound brain concentrations from unbound plasma concentrations.

METHODS

To understand the importance and impact of the Kp,uu,brain concept in contemporary drug discovery and development, a survey has been conducted amongst major pharmaceutical companies based in Europe and the USA. Here, we present the results from this survey which consisted of 47 questions addressing: 1) Background information of the companies, 2) Implementation, 3) Application areas, 4) Methodology, 5) Impact and 6) Future perspectives.

RESULTS AND CONCLUSIONS

From the responses, it is clear that the majority of the companies (93%) has established a common understanding across disciplines of the concept and utility of Kp,uu,brain as compared to other parameters related to brain exposure. Adoption of the Kp,uu,brain concept has been mainly driven by individual scientists advocating its application in the various companies rather than by a top-down approach. Remarkably, 79% of all responders describe the portfolio impact of Kp,uu,brain implementation in their companies as 'game-changing'. Although most companies (74%) consider the current toolbox for Kp,uu,brain assessment and its validation satisfactory for drug discovery and early development, areas of improvement and future research to better understand human brain pharmacokinetics/pharmacodynamics translation have been identified.

Cutaneous Pharmacokinetic Approaches to Compare Bioavailability and/or Bioequivalence for Topical Drug Products

The evaluation of bioequivalence (BE) involves comparing the test product to its reference product in a study whose fundamental scientific principles allow inferring of the clinical performance of the products. Several test methods have been discussed and developed to evaluate topical bioavailability (BA) and BE. Pharmacokinetics-based approaches characterize the rate and extent to which an active ingredient becomes available at or near its site of action in the skin. Such methodologies are considered to be among the most accurate, sensitive, and reproducible approaches for determining the BA or BE of a product.

Microdosing as a Potential Tool to Enhance Clinical Development of Novel Antibiotics: A Tissue and Plasma PK Feasibility Study with Ciprofloxacin

Background and Objective

In microdose studies, drug pharmacokinetics is measured in humans after administration of subtherapeutic doses. While previous microdose studies focused primarily on plasma pharmacokinetics, we set out to evaluate the feasibility of microdosing for a pharmacokinetic assessment in subcutaneous tissue and epithelial lining fluid.

Methods

Healthy subjects received a single intravenous bolus injection of a microdose of [¹⁴C]ciprofloxacin (1.1 µg, 7 kBq) with (cohort A, n = 9) or without (cohort B, n = 9) a prior intravenous infusion of a therapeutic dose of unlabeled ciprofloxacin (400 mg). Microdialysis and bronchoalveolar lavage were applied for determination of subcutaneous and intrapulmonary drug concentrations. Microdose [¹⁴C]ciprofloxacin was quantified by accelerator mass spectrometry and therapeutic-dose ciprofloxacin by liquid chromatography–tandem mass spectrometry.

Results

The pharmacokinetics of therapeutic-dose ciprofloxacin (cohort A) in plasma, subcutaneous tissue, and epithelial lining fluid was in accordance with previous data. In plasma and subcutaneous tissue, the dose-adjusted area under the concentration–time curve of microdose ciprofloxacin was similar in cohorts A and B and within an 0.8-fold to 1.1-fold range of the area under the concentration–time curve of therapeutic-dose ciprofloxacin. Penetration of microdose ciprofloxacin into subcutaneous tissue was similar in cohorts A and B and comparable to that of therapeutic-dose ciprofloxacin with subcutaneous tissue-to-plasma area under the concentration–time curve ratios of 0.44, 0.44, and 0.38, respectively. Penetration of microdose ciprofloxacin into epithelial lining fluid was highly variable and failed to predict the epithelial lining fluid penetration of therapeutic-dose ciprofloxacin.

Conclusions

Our study confirms the feasibility of microdosing for pharmacokinetic measurements in plasma and subcutaneous tissue. Microdosing combined with microdialysis is a potentially useful tool in clinical antimicrobial drug development, but its applicability for the assessment of pulmonary pharmacokinetics with bronchoalveolar lavage requires further studies.

Clinical Trial Registration

ClinicalTrials.gov NCT03177720 (registered 6 June, 2017).

Supplementary Information

The online version contains supplementary material available at 10.1007/s40262-021-01091-1.

Moxifloxacin Concentrations in the Knee Joint, Tibial Bone, and Soft Tissue When Combined with Rifampicin: A Randomized Porcine Microdialysis Study

BACKGROUND

Peri and postoperative antibiotics are key adjuvant treatment tools in the management of periprosthetic joint infection (PJI). The aim of this study was to evaluate the effect of rifampicin on the area under the moxifloxacin concentration-time curve from 0 to 24 hours (AUC0-24) in the synovial fluid of the knee joint, tibial bone, and adjacent subcutaneous tissue under steady-state conditions using microdialysis in a porcine model.

METHODS

Twenty female pigs were randomized to receive oral treatment with moxifloxacin monotherapy (Group A, n = 10) of 400 mg once daily for 3 days or a combination therapy (Group B, n = 10) of 400 mg of moxifloxacin once daily for 3 days and 450 mg of rifampicin twice daily for 7 days. Microdialysis was used for sampling the synovial fluid of the knee joint, tibial cancellous and cortical bone, and adjacent subcutaneous tissues. Plasma samples were taken as a reference. Measurements were obtained for 24 hours.

RESULTS

Coadministration of moxifloxacin and rifampicin resulted in reductions of the moxifloxacin AUC0-24 in all targeted tissue compartments by 67% to 85% (p < 0.05). The corresponding change in plasma was 20% (p = 0.49). For both groups, the tissue penetration (the ratio of tissue free fraction AUC0-24 to plasma free fraction AUC0-24 [fAUCtissue/fAUCplasma]) was incomplete in all investigated compartments. The highest moxifloxacin tissue penetration was in the knee joint synovial fluid: 0.59 (Group A) and 0.24 (Group B). The lowest tissue penetration was in the cortical bone: 0.17 (Group A) and 0.03 (Group B).

CONCLUSIONS

We found a significant reduction of the moxifloxacin concentration, expressed as the AUC0-24, in tissues relevant to acute PJI treatment when coadministered with rifampicin.

CLINICAL RELEVANCE

The concentrations within the targeted tissue compartments were reduced significantly more than the concentrations in plasma, which may be particularly important as plasma concentrations are used in clinical practice to assess moxifloxacin treatment sufficiency.

Weight-based cefuroxime dosing provides comparable orthopedic target tissue concentrations between weight groups - a microdialysis porcine study

Antibiotic prophylaxis is a key element in prevention of surgical site infections. For the majority of orthopedic procedures, antibiotic administration follows fixed dosing regimens irrespective of weight. However, this may result in insufficient antibiotic target tissue concentrations and higher risk of surgical site infections in obese individuals. The aim of this study was to investigate the effect of weight-based cefuroxime dosing on plasma and target tissue concentrations. Eighteen female pigs were allocated into three groups differentiated by weight: 53-57 kg, 73-77 kg, and 93-97 kg. Microdialysis catheters were placed for continuous sampling in bone, muscle, and subcutaneous tissue during an 8h sampling interval. Blood samples were collected as reference. Cefuroxime was administered intravenously as a bolus according to weight (20 mg/kg). The primary endpoint was the time above the cefuroxime minimal inhibitory concentration for Staphylococcus aureus (T > MIC (4 μg/mL)). Comparable target tissue T > MICs (4 μg/mL) were found between weight groups. Mean T > MIC ranged between 116-137 min for plasma, 118-154 min for bone, 109-146 min for the skeletal muscle, and 117-165 min for subcutaneous tissue across the groups. Weight-based cefuroxime (20 mg/kg) dosing approach provides comparable perioperative plasma and target tissue T > MIC (4 μg/mL) in animals between 50-100 kg body weight, and thus a comparable prophylaxis of surgical site infections.

Plasma and Lung Tissue Pharmacokinetics of Ceftaroline Fosamil in Patients Undergoing Cardiac Surgery with Cardiopulmonary Bypass: an In Vivo Microdialysis Study

Ceftaroline fosamil, a fifth-generation cephalosporin antibiotic with activity against methicillin-resistant Staphylococcus aureus (MRSA), is currently approved for the treatment of pneumonia and complicated skin and soft tissue infections. However, pharmacokinetics data on free lung tissue concentrations in critical patient populations are lacking. The aim of this study was to evaluate the pharmacokinetics of the high-dose regimen of ceftaroline in plasma and lung tissue in cardiac surgery patients during intermittent and continuous administration. Nine patients undergoing elective cardiac surgery on cardiopulmonary bypass were included in this study and randomly assigned to intermittent or continuous administration. Eighteen hundred milligrams of ceftaroline fosamil was administered intravenously as either 600 mg over 2 h every 8 h (q8h) (intermittent group) or 600 mg over 2 h (loading dose) plus 1,200 mg over 22 h (continuous group). Interstitial lung tissue concentrations were measured by in vivo microdialysis. Relevant pharmacokinetics parameters were calculated for each group. Plasma exposure levels during intermittent and continuous administration were comparable to those of previously published studies and did not differ significantly between the two groups. In vivo microdialysis demonstrated reliable and adequate penetration of ceftaroline into lung tissue during intermittent and continuous administration. The steady-state area under the concentration-time curve from 0 to 8 h (AUCss 0-8) and the ratio of AUCSS 0-8 in lung tissue and AUC in plasma (AUClung/plasma) were descriptively higher in the continuous group. Continuous administration of ceftaroline fosamil achieved a significantly higher proportion of time for which the free drug concentration remained above 4 times the minimal inhibitory concentration (MIC) during the dosing interval (% fT>4xMIC) than intermittent administration for pathogens with a MIC of 1 mg/liter. Ceftaroline showed adequate penetration into interstitial lung tissue of critically ill patients undergoing major cardiothoracic surgery, supporting its use for pneumonia caused by susceptible pathogens.

Single-dose pharmacokinetics of temocillin in plasma and soft tissues of healthy volunteers after intravenous and subcutaneous administration: a randomized crossover microdialysis trial

BACKGROUND

The antibiotic temocillin has recently been rediscovered as a promising therapeutic option against MDR Gram-negative bacteria. However, some aspects of the pharmacokinetic (PK) profile of the drug are still to be elucidated: subcutaneous administration of temocillin might be of interest as an alternative to the intravenous route in selected patients. Similarly, information on the penetration of temocillin into human soft tissues is lacking.

OBJECTIVES

To investigate the feasibility and plasma PK of subcutaneous dosing as well as soft tissue PK of temocillin after intravenous administration to healthy volunteers.

METHODS

Eight healthy volunteers received 2 g of temocillin both as intravenous and subcutaneous infusion in a randomized two-period crossover study. Concentration-time profiles of total temocillin in plasma (after both routes) and of unbound temocillin in plasma, muscle and subcutis (only after intravenous dosing) were determined up to 12 h post-dose.

RESULTS

Subcutaneous dosing caused some infusion site discomfort but resulted in sustained drug concentrations over time with only slightly decreased overall exposure compared with intravenous dosing. Plasma protein binding of temocillin showed concentration-dependent behaviour and was higher than previously reported. Still, unbound drug concentrations in muscle and subcutis determined by microdialysis markedly exceeded those in plasma, suggesting good tissue penetration of temocillin.

CONCLUSIONS

The subcutaneous administration of temocillin is a valid and feasible alternative to intravenous dosing. With the description of plasma protein binding and soft tissue PK of temocillin in healthy volunteers, this study provides important information that adds to the ongoing characterization of the PK profile of temocillin and might serve as input for PK/PD considerations.

Evaluation of Benzylpenicillin as an Internal Standard for Measurement of Piperacillin Bone Concentrations Via Microdialysis

Microdialysis is a pharmacokinetic tool that can be advantageous when obtaining tissues' pharmacokinetic information. Since absolute extracellular tissue concentrations are needed in pharmacokinetic studies, calibrating the microdialysis system is necessary. The internal standard method is superior when compared to other calibration methods. However, thorough evaluation of the internal standard is required before it can be used. In vitro experiments and an in vivo study on pigs (n = 8) were conducted to assess the relative recoveries by gain and by loss for piperacillin, both with and without a benzylpenicillin concentration of 5 µg/mL. Furthermore, the in vivo setup allowed for an evaluation of piperacillin cancellous bone and subcutaneous tissue concentrations in a single 8 h dosing interval. Ultra-high performance liquid chromatography (UHPLC) was used to determine piperacillin and benzylpenicillin concentrations. Relative recovery by loss for benzylpenicillin and relative recovery by gain for piperacillin were similar in in vitro and in vivo. Presence of benzylpenicillin did not affect the relative recovery for piperacillin. Relative recovery, pharmacokinetic parameters and fT>MIC were similar when comparing the retrodialysis by drug and the internal standard calibration methods (p > 0.31). Mean fT>MIC (16 µg/mL) for plasma, cancellous bone and subcutaneous tissue were 232 min, 255 min and 295 min, respectively. Our findings suggest that benzylpenicillin is suitable as an internal standard for piperacillin in microdialysis studies. Mean fT>MIC (16 µg/mL) for plasma, cancellous bone, and subcutaneous tissue reached a target of 50% fT>MIC under the investigated conditions (mean range: 52%-66%); however, the target was not obtained in all pigs in all compartments. Moreover, 100% fT>MIC was not obtained in any case, suggesting that different strategies must be taken into consideration if higher targets are employed.

Plasma and soft tissue pharmacokinetics of ceftolozane/tazobactam in healthy volunteers after single and multiple intravenous infusion: a microdialysis study

OBJECTIVES

To investigate ceftolozane/tazobactam pharmacokinetics (PK) in plasma and interstitial space fluid (ISF) of muscle and subcutaneous tissue and establish a population PK model.

METHODS

Eight healthy volunteers received four IV doses of 1000/500 mg ceftolozane/tazobactam q8h in a prospective, open-labelled PK study. ISF concentration-time profiles were determined via in vivo microdialysis up to 8 h post-dose and efficacy of unbound ceftolozane and tazobactam was estimated using the time above MIC (%ƒT>MIC) and time above threshold concentration (%T>CT), respectively. A population PK model was established by merging derived plasma and soft tissue PK data.

RESULTS

Ceftolozane reached %ƒT>MIC values of 100% in plasma, muscle and subcutaneous ISF for Enterobacteriaceae and 87%, 89% and 87%, respectively, for Pseudomonas aeruginosa. Tazobactam %T>CT was 21%, 22% and 21% in plasma, muscle and subcutaneous ISF, respectively. Plasma protein binding was 6.3% for ceftolozane and 8.0% for tazobactam. Multiple-dose ceftolozane AUC0-8 ISF/plasma ratios were 0.92 ± 0.17 in muscle and 0.88 ± 0.18 in subcutis, and tazobactam ratios were 0.89 ± 0.25 in muscle and 0.87 ± 0.21 in subcutis, suggesting substantial soft tissue penetration.

CONCLUSIONS

Tazobactam %T>CT values were distinctly below proposed target values, indicating that tazobactam might be underdosed in the investigated drug combination. However, ISF/unbound plasma ratios of ceftolozane and tazobactam support their use in soft tissue infections. A plasma and soft tissue PK model adds important information on the PK profile of ceftolozane/tazobactam. Further investigations in patients suffering from wound infections are needed to confirm these findings.

Analysis of Pharmacokinetics in the Cochlea of the Inner Ear

Hearing loss affects >5% of the global population and therefore, has a great social and clinical impact. Sensorineural hearing loss, which can be caused by different factors, such as acoustic trauma, aging, and administration of certain classes of drugs, stems primarily from a dysfunction of the cochlea in the inner ear. Few therapeutic strategies against sensorineural hearing loss are available. To develop effective treatments for this disease, it is crucial to precisely determine the behavior of ototoxic and therapeutic agents in the microenvironment of the cochlea in live animals. Since the 1980s, a number of studies have addressed this issue by different methodologies. However, there is much less information on pharmacokinetics in the cochlea than that in other organs; the delay in ontological pharmacology is likely due to technical difficulties with accessing the cochlea, a tiny organ that is encased with a bony wall and has a fine and complicated internal structure. In this review, we not only summarize the observations and insights obtained in classic and recent studies on pharmacokinetics in the cochlea but also describe relevant analytical techniques, with their strengths, limitations, and prospects.

Double-dose cefuroxime concentrations in bone, synovial fluid of the knee joint and subcutaneous adipose tissue-A randomised porcine microdialysis study

This study evaluated target tissue concentrations of double dose cefuroxime administered intravenously as either one 15 min infusion of 3000 mg (Group 1) or two single 15 min infusions of 1500 mg administered 4 h apart (Group 2). Sixteen pigs were randomised into two groups of eight. Cortical and cancellous bone, synovial fluid of the knee joint and subcutaneous adipose tissue concentrations were measured based on sampling via microdialysis. Plasma samples were collected as a reference. Comparison of the groups was based on time with concentrations above relevant minimal inhibitory concentrations (fT>MIC) of 4 μg/mL. The mean time fT>MIC (4 μg/mL) across compartments was longer for Group 2 (280-394 min) than for Group 1 (207-253 min) (p<0.01). Cortical bone showed a tendency towards longer fT>MIC (4 μg/mL) in Group 2 (280 min) than in Group 1 (207 min) (p = 0.053). Within 50 min after administration, the mean concentration of 4 μg/mL was reached in all compartments for both groups. The mean concentrations decreased below 4 μg/mL after approximately 4 h (Group 1) and 3 h (Group 2) from initiation of administration (time zero). During an 8 h interval, double-dose cefuroxime administered as 2 × 1500 mg with a 4 h interval provides longer time above MIC breakpoint for Staphylococcus aureus (4 μg/mL) than a single bolus of 3000 mg cefuroxime. To maintain sufficient tissue concentrations during longer surgeries, re-administration of cefuroxime (1500 mg) should be considered 3 h after the first administration.

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.

Influence of anastomoses on intestine ischemia and cefuroxime concentrations: Evaluated in the ileum and colon in a porcine model

BACKGROUND

Anastomotic leakage is a serious complication following gastrointestinal surgery and is associated with increased morbidity and mortality. The incidence of anastomotic leakage is determined by anatomy and is reported to be between 4%-33% for colon anastomosis and 1%-3% for small intestine anastomosis. The etiology of anastomotic leakage of the intestine has been divided into three main factors: healing disturbances, communication between intra- and extra-luminal compartments, and infection. All three factors interact, and one factor will inevitably lead to the other two factors resulting in tissue ischemia, tissue necrosis, and anastomotic leakage.

AIM

To evaluate ischemic metabolites and cefuroxime concentrations in both anastomosis and non-anastomosis ileum and colon in a porcine model.

METHODS

Eight healthy female pigs (Danish Landrace breed, weight 58-62 kg) were included in this study. Microdialysis catheters were placed for sampling of ischemic metabolites (glucose, lactate, glycerol, and pyruvate) and cefuroxime concentrations in both anastomosis and non-anastomosis ileum and colon. Cefuroxime 1.5 g was administered as an intravenous infusion over 15 min. Subsequently, dialysates and blood samples were collected over 8 h and the ischemic metabolites and cefuroxime concentrations were quantified in all samples. The concentrations of glucose, lactate, glycerol and pyruvate were determined using the CMA 600 Microdialysis Analyzer with Reagent Set A (M Dialysis AB, Sweden), and the concentrations of cefuroxime and meropenem were quantified using a validated ultra-high-performance liquid chromatography assay.

RESULTS

Only the colon anastomosis induced mean ischemic lactate/pyruvate ratios above 25 (ischemic cut-off) throughout the entire sampling interval, and simultaneously decreased glucose concentrations. The mean time for which cefuroxime concentrations were maintained above the clinical breakpoint minimal inhibitory concentration for Escherichia coli (8 µg/mL) ranged between 116-128 min across all the investigated compartments, and was similar between the anastomosis and non-anastomosis ileum and colon. For all pigs and in all the investigated compartments, a cefuroxime concentration of 8 µg/mL was reached within 10 min after administration. When comparing the pharmacokinetic parameters between the anastomosis and non-anastomosis sites for both ileum and colon, only colon T_max and half-life differed between anastomosis and non-anastomosis (P < 0.03). Incomplete tissue penetrations were found in all tissues except for the non-anastomosis colon.

CONCLUSION

Administering 1.5 g cefuroxime 10 min prior to intestine surgery seems sufficient, and effective concentrations are sustained for approximately 2 h. Only colon anastomosis was locally vulnerable to ischemia.

Flucloxacillin bone and soft tissue concentrations assessed by microdialysis in pigs after intravenous and oral administration

Aims

Flucloxacillin is commonly administered intravenously for perioperative antimicrobial prophylaxis, while oral administration is typical for prophylaxis following smaller traumatic wounds. We assessed the time, for which the free flucloxacillin concentration was maintained above the minimum inhibitory concentration (fT > MIC) for methicillin-susceptible Staphylococcus aureus in soft and bone tissue, after intravenous and oral administration, using microdialysis in a porcine model.

Methods

A total of 16 pigs were randomly allocated to either intravenous (Group IV) or oral (Group PO) flucloxacillin 1 g every six hours during a 24-hour period. Microdialysis was used for sampling in cancellous and cortical bone, subcutaneous tissue, and the knee joint. In addition, plasma was sampled. The flucloxacillin fT > MIC was evaluated using a low MIC target (0.5 μg/ml) and a high MIC target (2.0 μg/ml).

Results

Intravenous administration resulted in longer fT > MIC (0.5 μg/ml) compared to oral administration, except for cortical bone. In Group IV, all pigs reached a concentration of 0.5 μg/ml in all compartments. The mean fT > MIC (0.5 μg/ml) was 149 minutes (95% confidence interval (CI) 119 to 179; range 68 to 323) in subcutaneous tissue and 61 minutes (95% CI 29 to 94; range 0 to 121) to 106 minutes (95% CI 76 to 136; range 71 to 154) in bone tissue. In Group PO, 0/8 pigs reached a concentration of 0.5 μg/ml in all compartments. For the high MIC target (2.0 μg/ml), fT > MIC was close to zero minutes in both groups across compartments.

Conclusion

Although intravenous administration of flucloxacillin 1 g provided higher fT > MIC for the low MIC target compared to oral administration, concentrations were surprisingly low, particularly for bone tissue. Achievement of sufficient bone and soft tissue flucloxacillin concentrations may require a dose increase or continuous administration.

Cite this article: Bone Joint Res 2021;10(1):60–67.

Dexamethasone-Enhanced Microdialysis and Penetration Injury

Microdialysis probes, electrochemical microsensors, and neural prosthetics are often used for in vivo monitoring, but these are invasive devices that are implanted directly into brain tissue. Although the selectivity, sensitivity, and temporal resolution of these devices have been characterized in detail, less attention has been paid to the impact of the trauma they inflict on the tissue or the effect of any such trauma on the outcome of the measurements they are used to perform. Factors affecting brain tissue reaction to the implanted devices include: the mechanical trauma during insertion, the foreign body response, implantation method, and physical properties of the device (size, shape, and surface characteristics. Modulation of the immune response is an important step toward making these devices with reliable long-term performance. Local release of anti-inflammatory agents such as dexamethasone (DEX) are often used to mitigate the foreign body response. In this article microdialysis is used to locally deliver DEX to the surrounding brain tissue. This work discusses the immune response resulting from microdialysis probe implantation. We briefly review the principles of microdialysis and the applications of DEX with microdialysis in (i) neuronal devices, (ii) dopamine and fast scan cyclic voltammetry, (iii) the attenuation of microglial cells, (iv) macrophage polarization states, and (v) spreading depolarizations. The difficulties and complexities in these applications are herein discussed.

Timing of Antimicrobial Prophylaxis and Tourniquet Inflation: A Randomized Controlled Microdialysis Study

BACKGROUND

Tourniquets are widely used during extremity surgery. In order to prevent surgical site infection, correct timing of antimicrobial prophylaxis and tourniquet inflation is important. We aimed to evaluate the time for which the free drug concentration of cefuroxime is maintained above the minimum inhibitory concentration (t > MIC) in porcine subcutaneous adipose tissue and calcaneal cancellous bone during 3 clinically relevant tourniquet application scenarios.

METHODS

Twenty-four female Danish Landrace pigs were included. Microdialysis catheters were placed bilaterally for sampling of cefuroxime concentrations in calcaneal cancellous bone and subcutaneous adipose tissue, and a tourniquet was applied to a randomly picked leg of each pig. Subsequently, the pigs were randomized into 3 groups to receive 1.5 g of cefuroxime by intravenous injection 15 minutes prior to tourniquet inflation (Group A), 45 minutes prior to tourniquet inflation (Group B), and at the time of tourniquet release (Group C). The tourniquet duration was 90 minutes in all groups. Dialysates and venous blood samples were collected for 8 hours after cefuroxime administration. Cefuroxime and various ischemic marker concentrations were quantified.

RESULTS

Cefuroxime concentrations were maintained above the clinical breakpoint MIC for Staphylococcus aureus (4 µg/mL) in calcaneal cancellous bone and subcutaneous adipose tissue throughout the 90-minute tourniquet duration in Groups A and B. Cefuroxime administration at the time of tourniquet release (Group C) resulted in concentrations of >4 µg/mL for approximately of 3.5 hours in the tissues on the tourniquet side. Furthermore, tourniquet application induced ischemia (increased lactate:pyruvate ratio) and cell damage (increased glycerol) in subcutaneous adipose tissue and calcaneal cancellous bone. Tissue ischemia was sustained for 2.5 hours after tourniquet release in calcaneal cancellous bone.

CONCLUSIONS

Administration of cefuroxime (1.5 g) in the 15 to 45-minute window prior to tourniquet inflation resulted in sufficient concentrations in calcaneal cancellous bone and subcutaneous adipose tissue throughout the 90-minute tourniquet application. Furthermore, tourniquet-induced tissue ischemia fully resolved 2.5 hours after tourniquet release.

CLINICAL RELEVANCE

Cefuroxime administration 15 to 45 minutes prior to tourniquet inflation seems to be a safe window. If the goal is to maintain postoperative cefuroxime concentrations above relevant MIC values, our results suggest that a second dose of cefuroxime should be administered at the time of tourniquet release.

Methods for evaluating penetration of drug into the skin: A review

BACKGROUND

Skin being the largest organ of the human body plays a very important role in the permeation and penetration of the drug. In addition, the transdermal drug delivery system (TDDS) plays a major role in managing dermal infections and attaining sustained plasma drug concentration. Thus, evaluation of percutaneous penetration of the drug through the skin is important in developing TDDS for human use.

MATERIAL AND METHODS

Various techniques are used for getting the desired drug penetration, permeation, and absorption through the skin in managing these dermal disorders. The development of novel pharmaceutical dosage forms for dermal use is much explored in the current era. However, it is very important to evaluate these methods to determine the bioequivalence and risk of these topically applied drugs, which ultimately penetrate and are absorbed through the skin.

RESULTS

Currently, numerous skin permeation models are being developed and persuasively used in studying dermatopharmacokinetic (DPK) profile and various models have been developed, to evaluate the TDD which include ex vivo human skin, ex vivo animal skin, and artificial or reconstructed skin models.

CONCLUSION

This review discusses the general physiology of the skin, the physiochemical characteristics affecting particle penetration, understand the models used for human skin permeation studies and understanding their advantages, and disadvantages.

Perspectives on Nanodelivery to the Brain: Prerequisites for Successful Brain Treatment

Nanocarriers (NCs) are promising tools to improve drug delivery across the blood–brain barrier (BBB) for more effective treatment of brain disorders, although there is a scarcity of clinical translation of brain-directed NCs. In order to drive the development of brain-oriented NCs toward clinical success, it is essential to understand the prerequisites for nanodelivery to be successful in brain treatment. In this Perspective, we present how pharmacokinetic/pharmacodynamic (PK/PD), formulation and nanotoxicity factors impact the therapeutic success of brain-specific nanodelivery. Properties including high loading efficiency, slow in vivo drug release, long systemic circulation, an increase in unbound brain-to-plasma concentration/exposure ratio (K_p,uu,brain), high drug potency, and minimal nanotoxicity are prerequisites that should preferably be combined to maximize the therapeutic potential of a brain-targeted NC. The PK of brain-directed NCs needs to be evaluated in a more therapeutically relevant manner, focusing on the released, unbound drug. It is more crucial to increase the K_p,uu,brain than to improve the ability of the NC to cross the BBB in its intact form. Brain-targeted NCs, which are mostly developed for treating brain tumors, including metastases, should aim to enhance drug delivery not just to tumor regions with disrupted BBB, but equally important to regions with intact BBB where the drugs themselves have problems reaching. This article provides critical insights into how a brain-targeted nanoformulation needs to be designed and optimized to achieve therapeutic success in the brain.

Local concentrations of gentamicin obtained by microdialysis after a controlled application of a GentaColl sponge in a porcine model

Local treatment with gentamicin may be an important tool in the prevention and treatment of surgical site infections in high-risk procedures and patients. The aim of this study was to evaluate the pharmacokinetic profile of gentamicin in bone and surrounding tissue, released from a controlled application of a GentaColl sponge in a porcine model. In eight female pigs, a GentaColl sponge of 10 × 10 cm (1.3 mg gentamicin/cm² ) was placed in a cancellous bone cavity in the proximal tibia. Microdialysis was used for sampling of gentamicin concentrations over 48 hours from the cavity with the implanted GentaColl sponge, cancellous bone parallel to the cavity over and under the epiphyseal plate, cortical bone, the intramedullary canal, subcutaneous tissue, and the joint cavity of the knee. Venous blood samples were obtained as reference. The main finding was a mean peak drug concentration (95% CI) of gentamicin in the cancellous bone cavity containing the implanted GentaColl sponge of 11 315 (9049-13 581) μg/mL, persisting above 1000 μg/mL until approximately 40 hours after application. Moreover, the concentrations were low (<1 μg/mL) in the surrounding tissues as well as in plasma. The mean peak gentamicin concentration from the cancellous bone cavity after a controlled application of a GentaColl sponge was high and may be adequate for the prevention of biofilm formation. However, high MIC strains and uncontrolled application of the GentaColl sponge may jeopardize this conclusion.

Pharmacokinetics of double-dose cefuroxime in porcine intervertebral disc and vertebral cancellous bone-a randomized microdialysis study

BACKGROUND CONTEXT

Postoperative pyogenic spondylodiscitis is associated with prolonged antimicrobial therapy and high relapse rates. Cefuroxime is a time-dependent antimicrobial widely used for intravenous perioperative prophylaxis in spine surgery. A previous study has indicated that a single dose of cefuroxime (1.5 g) provides insufficient spine tissue concentrations for spine procedures lasting more than 2 to 3 hours.

PURPOSE

To evaluate the time with concentrations above relevant minimal inhibitory concentrations (T>MIC) in plasma, subcutaneous adipose tissue, vertebral cancellous bone, and intervertebral disc after a twofold increase of the standard dosage of 1.5 g cefuroxime given as one double dose (1×3 g) or two single doses (2×1.5 g) with a four-hour interval.

METHODS

Sixteen pigs were randomized into two groups: Group 1 received one double dose of cefuroxime (1×3 g) as an intravenous bolus and Group 2 received two single doses of cefuroxime (2×1.5 g) as an intravenous bolus with a four-hour interval. Cefuroxime measurements were obtained from plasma, subcutaneous adipose tissue, vertebral cancellous bone, and intervertebral disc for eight hours thereafter. Microdialysis was applied for sampling in solid tissues. The cefuroxime concentrations were determined using ultra-high performance liquid chromatography. This work was supported by grants from the Health Research Foundation of Central Denmark Region (Level E). The funding source did not play any role in the investigation.

RESULTS

The time with concentrations above the Staphylococcus aureus clinical breakpoint minimal inhibitory concentration of 4 μg/mL was higher in all compartments for Group 2 compared to Group 1. The mean T>MIC (4 μg/mL) in all compartments ranged between 47% and 67% for Group 1 and 72% and 92% for Group 2. Furthermore, a delayed tissue penetration into all tissues for both groups was demonstrated.

CONCLUSIONS

This study suggests that cefuroxime should be given at least 45 minutes prior to spine procedures and as two single doses at a maximum interval of four hours for extended spine procedures. Clinical studies verifying these results are warranted.

CLINICAL SIGNIFICANCE

Administering cefuroxime as two single doses (2×1.5 g) with a four-hour interval compared to one double dose (1×3 g) resulted in higher T>MIC. Furthermore, we found delayed and incomplete cefuroxime tissue penetration.

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.

Evaluating the Pharmacokinetics and Systemic Effects of a Permeability Enhancer Sodium N-[8-(2-hydroxybenzoyl)amino] Caprylate in Rats

Oral administration is the preferred route for drug delivery and its success is highly dependent on a compound's ADME properties, of which, permeability plays a major role. Therefore, permeability enhancers are an attractive area of research in the pharmaceutical industry. Recent data suggest that sodium N-[8-(2-hydroxybenzoyl) amino] caprylate (SNAC) is an effective permeability enhancer, yet the pharmacokinetic (PK) and systemic effects of SNAC are poorly understood, specifically its oral bioavailability and systemic effects on distribution, which could influence the safety of certain drugs. To answer these questions, both in vitro and in vivo studies were conducted. Of 3 permeability enhancers (SNAC, 4-CNAB, and 5-CNAC), SNAC was found to have the greatest effect on the absorption of cyanocobalamin in rats. It was also found that SNAC is orally bioavailable (almost 40%) when dosed to rats. Based on these findings, tool compounds were co-dosed in rats to further evaluate the systemic effects of SNAC. Oral co-dosing of SNAC with an intravenous infusion of 2 poorly brain penetrant compounds, quinidine, and gabapentin, did not increase brain ISF: plasma ratio or total brain:plasma ratio for either of these compounds, implying that SNAC is effective only in the intestine at pharmacologically relevant concentrations.

Exclusion of unsuitable CNS drug candidates based on their physicochemical properties and unbound fractions in biomatrices for brain microdialysis investigations

Microdialysis has been the only direct method of continuously measuring the unbound drug concentrations in extracellular fluid at a specific brain region with respect to time in the same animal. However, not every compound is suitable for microdialysis system as demonstrated by their inconsistent "by gain" and "by loss" in-vitro microdialysis probe recoveries leading to over- or under- estimated in-vivo concentrations. Therefore, our current study was proposed aiming to develop simple exclusion criteria for drug candidates that are not suitable for microdialysis system investigation. Through literature research, the properties ((LogP, pKa, water solubility and unbound fraction in plasma and brain) of drugs that have been reported for microdialysis studies were summarized. The exclusion criteria were developed by evaluating the impact of such properties on the consistency of in-vitro "by gain" and "by loss" recoveries of microdialysis probe. As a result, forty-five compounds were identified from literatures, among which doxorubicin, docetaxel, omeprazole, donepezil and phenytoin were found to have inconsistent in-vitro "by gain" and "by loss" microdialysis probe recoveries and subsequently selected for the exclusion criteria analysis. It was found that compounds with limited water solubility (less than 1 g/L) and unbound fraction in plasma (f_u,plasma less than 30%) and brain homogenate (f_u,brain less than 10%) were more likely to have inconsistent "by gain" and "by loss" microdialysis probe recoveries. Our proposed exclusion criteria were further validated using carbamazepine (limited water solubility only), DB213 (limited f_u,brain only) and piperine (both limited water solubility and limited f_u,plasma, f_u,brain). Our current proposed exclusion criteria will help excluding the CNS drug candidates that are highly unlikely suitable for brain microdialysis approach leading to a better success rate in brain microdialysis approach development.

The Brain Exposure Efficiency (BEE) Score

The blood-brain barrier (BBB), composed of microvascular tight junctions and glial cell sheathing, selectively controls drug permeation into the central nervous system (CNS) by either passive diffusion or active transport. Computational techniques capable of predicting molecular brain penetration are important to neurological drug design. A novel prediction algorithm, termed the Brain Exposure Efficiency Score (BEE), is presented. BEE addresses the need to incorporate the role of trans-BBB influx and efflux active transporters by considering key brain penetrance parameters, namely, steady state unbound brain to plasma ratio of drug (K_p,uu) and dose normalized unbound concentration of drug in brain (C_u,b). BEE was devised using quantitative structure-activity relationships (QSARs) and molecular modeling studies on known transporter proteins and their ligands. The developed algorithms are provided as a user-friendly open source calculator to assist in optimizing a brain penetrance strategy during the early phases of small molecule molecular therapeutic design.

Simultaneous Retrodialysis by Drug for Cefuroxime Using Meropenem as an Internal Standard-A Microdialysis Validation Study

Microdialysis is a valuable pharmacokinetic tool for obtaining samples of drug concentrations from tissues of interest. When an absolute tissue concentration is needed, a calibration of the microdialysis catheter is required. The use of an internal standard offers a number of advantages compared to standard calibration methods. However, meticulous validation both in vitro and in vivo is needed, as this method requires an internal standard with physiochemical similarities to the analyte of interest with no interference. A series of in vitro and in vivo setups were conducted to determine the relative recovery by gain and by loss for cefuroxime, with and without a constant meropenem concentration. The cefuroxime and meropenem concentrations were determined using ultra-HPLC. The main finding was that cefuroxime and meropenem relative recovery behaved similarly both in vitro and in vivo, signifying that meropenem is a representative internal standard for cefuroxime. Furthermore, cefuroxime relative recovery in vitro was not affected by either the cefuroxime concentration or the presence of meropenem, and the in vivo meropenem relative recovery was constant over 6 h.

Lack of dermal penetration of topically applied gentamicin as pharmacokinetic evidence indicating insufficient efficacy

Background

Treatment of skin and superficial soft tissue infections with topically applied antibiotics is a controversial topic, because only few clinical studies exist and target site concentrations after topical treatment are widely unknown.

Objectives

This study aimed to investigate the target site concentration of topically applied gentamicin as a potential cause of therapeutic failure and to explore if microporation by laser might be used to improve penetration of gentamicin through the skin barrier.

Methods

Six healthy volunteers were included in this cross-over Phase 1 study. On two study days, separated by a washout period, microdialysate and plasma sampling was performed for 6 h after administration of 500 mg of gentamicin cream on a predefined area. On one of the study days the skin was microporated before drug application using the P.L.E.A.S.E. Professional laser system.

Results

In intact skin, Cmax and AUC values were 3.3 ± 5.64 ng/mL and 5.4 ± 10.4 ng·h/mL, respectively; thereby far under the threshold needed to treat common pathogens. With a Cmax of 474.2 ± 555.3 ng/mL laser application showed a significant increase in tissue penetration and decrease in pharmacokinetic variability; however, even after microporation no therapeutically active concentrations were achieved as indicated by Cmax/epidemiological cut-off ratios of 0.237 and 0.059 for Staphylococcus aureus and Pseudomonas aeruginosa, respectively. Solely after administration on microporated skin, plasma concentrations of gentamicin were quantifiable (lower limit of quantification 10 pg/mL).

Conclusions

This study confirmed that after topical administration gentamicin penetration through the dermal barrier is insufficient, providing pharmacokinetic evidence that topical gentamicin in its current form might be inappropriate to treat skin infections.

Practical approaches to evaluating and optimizing brain exposure in early drug discovery

Developing drugs for CNS related diseases continues to be one of the most challenging endeavors in drug discovery. This is at least in part related to the existence of the Blood Brain Barrier (BBB), a complex multicellular organization that provides selective access to required nutrients and hormones, while removing waste and restricting exposure to potential harmful toxins, pathogens, and xenobiotics. Consequently, designing and selecting molecules that can overcame this protection system are unique and critical aspects of the CNS drug discovery. Here we review modern CNS pharmacokinetic concepts and methods suitable for early drug discovery, and medicinal chemistry strategies towards molecules with optimal CNS exposure.

[Development of in vivo drug sensing system with needle-type diamond microelectrode]

Continuous and real-time measurement of local concentrations of systemically administered drugs in vivo must be crucial for pharmacological studies. Nevertheless, conventional methods require considerable samples quantity and have poor sampling rates. Additionally, they cannot determine how drug kinetics correlates with target function over time. Here, we describe a system with two different sensors. One is a needle-type microsensor composed of boron-doped diamond with a tip of ~40 μm in diameter, and the other is a glass microelectrode. We first tested bumetanide. This diuretic can induce deafness. In the guinea-pig cochlea injected intravenously with bumetanide, the changes of the drug concentration and the extracellular potential underlying hearing were simultaneously measured in real time. We further examined an antiepileptic drug lamotrigine in the rat brain, and tracked its kinetics and at the same time the local field potentials representing neuronal activity. The action of the anticancer reagent doxorubicin was also monitored in the cochlea. This microsensing system may be applied to analyze pharmacokinetics and pharmacodynamics of various drugs at local sites in vivo, and contribute to promoting the pharmacological researches.

Determination of Dermal Pharmacokinetics by Microdialysis Sampling in Rats

The evaluation of absorption and availability at the site of action of a drug candidate is an important element of drug discovery and development, as clinical response is a function of the bioavailability of the active agent and its continued presence at the site of action. Evaluation of dermal pharmacokinetics facilitates the selection of new compounds or chemical structures for advancement as possible clinical candidates. An advantage of microdialysis is that it allows the measurement of compound concentrations at the site of action without disturbing the tissue milieu, making it possible to determine the relationship between this important variable and plasma concentrations of the agent. Described in this unit are laboratory protocols for performing dermal microdialysis experiments in rat for the purpose of defining the pharmacokinetics parameters of test agents. © 2019 by John Wiley & Sons, Inc.

Skin microdialysis: methods, applications and future opportunities-an EAACI position paper

Skin microdialysis (SMD) is a versatile sampling technique that can be used to recover soluble endogenous and exogenous molecules from the extracellular compartment of human skin. Due to its minimally invasive character, SMD can be applied in both clinical and preclinical settings. Despite being available since the 1990s, the technique has still not reached its full potential use as a tool to explore pathophysiological mechanisms of allergic and inflammatory reactions in the skin. Therefore, an EAACI Task Force on SMD was formed to disseminate knowledge about the technique and its many applications. This position paper from the task force provides an overview of the current use of SMD in the investigation of the pathogenesis of chronic inflammatory skin diseases, such as atopic dermatitis, chronic urticaria, psoriasis, and in studies of cutaneous events during type 1 hypersensitivity reactions. Furthermore, this paper covers drug hypersensitivity, UVB-induced- and neurogenic inflammation, and drug penetration investigated by SMD. The aim of this paper is to encourage the use of SMD and to make the technique easily accessible by providing an overview of methodology and applications, supported by standardized operating procedures for SMD in vivo and ex vivo.

Use of microdialysis for the assessment of fluoroquinolone pharmacokinetics in the clinical practice

Antibacterial drugs, including fluoroquinolones, can exert their therapeutic action only with adequate penetration at the infection site. Multiple factors, such as rate of protein binding, drug liposolubility and organ blood-flow all influence ability of antibiotics to penetrate target tissues. Microdialysis is an in vivo sampling technique that has been successfully applied to measure the distribution of fluoroquinolones in the interstitial fluid of different tissues both in animal studies and clinical setting. Tissue concentrations need to be interpreted within the context of the pathogenesis and causative agents implicated in infections. Integration of microdialysis -derived tissue pharmacokinetics with pharmacodynamic data offers crucial information for correlating exposure with antibacterial effect. This review explores these concepts and provides an overview of tissue concentrations of fluoroquinolones derived from microdialysis studies and explores the therapeutic implications of fluoroquinolone distribution at various target tissues.