Methods to assess tissue-specific distribution and metabolism of drugs

Most drugs exert their effects not within the plasma compartment, but in defined target tissues into which drugs have to distribute from the central compartment. Unfortunately, a complete and lasting equilibration between blood and tissue cannot always be taken for granted. Drug distribution processes may be characterized by a high intertissue- and intersubject variability and target site drug levels may substantially differ from corresponding plasma levels. Suboptimal target site concentrations may have important clinical implications, as it is a potential explanation for therapeutic failures. Therefore, determination of drug tissue penetration plays an important role in clinical drug development. In recent years, the assessment of tissue concentrations after administration of very low, sub-pharmacological drug amounts (microdosing) has attracted major interest in early clinical drug development, which calls for the availability of highly sensitive analytical methods. The present article will review the most important techniques that are currently available for studying drug disposition in humans. These can be classified as semi-invasive (microdialysis, MD) and non-invasive (positron emission tomography, PET, and magnetic resonance spectroscopy, MRS). We will discuss individual strengths and shortcomings of each method and provide some recent examples with particular focus on antiinfective and anticancer drugs. Whereas MD and MRS also lend themselves to the assessment of tissue-specific drug metabolism, PET usually does not provide metabolic information. For some drugs, such as the anticancer agents 5-fluorouracil and capecitabine, measurement of drug metabolites is particularly important as these represent the therapeutically active species.     

Considerations in the use of cerebrospinal fluid pharmacokinetics to predict brain target concentrations in the clinical setting: implications of the barriers between blood and brain

In the clinical setting, drug concentrations in cerebrospinal fluid (CSF) are sometimes used as a surrogate for drug concentrations at the target site within the brain. However, the brain consists of multiple compartments and many factors are involved in the transport of drugs from plasma into the brain and the distribution within the brain. In particular, active transport processes at the level of the blood-brain barrier and blood-CSF barrier, such as those mediated by P-glycoprotein, may lead to complex relationships between concentrations in plasma, ventricular and lumbar CSF, and other brain compartments. Therefore, CSF concentrations may be difficult to interpret and may have limited value. Pharmacokinetic data obtained by intracerebral microdialysis monitoring may be used instead, providing more valuable information. As non-invasive alternative techniques, positron emission tomography or magnetic resonance spectroscopy may be of added value.     

Ocular microdialysis: a continuous sampling technique to study pharmacokinetics and pharmacodynamics in the eye

The unique anatomy and physiology of the eye present many challenges to the successful development and delivery of ophthalmic drugs. Any therapeutic strategy developed to control the progression of anterior and posterior segment diseases requires continuous monitoring of effective drug concentrations in the relevant ocular tissues and fluids. Ocular microdialysis has gained popularity in recent years due to its ability to continuously monitor drug concentrations and substantially reduce the number of animals needed. The intrusive nature of ocular microdialysis experimentation has restricted these studies to animal models. This review article intends to highlight various aspects of ocular microdialysis and its relevance in examining the disposition of drugs in the anterior and posterior segments.     

Application of an in vivo brain microdialysis technique to studies of drug transport across the blood-brain barrier

There is a wide range of methods available for studying the transport of drugs across the blood-brain barrier (BBB) which is equipped with several systems to transport drugs as well as endogenous nutrients and waste products. The in vivo brain microdialysis technique, which allows direct sampling of the brain interstitial fluid (ISF), is a powerful means of characterizing influx and efflux transport across the BBB. In this paper, we review our results from the successful application of this technique to BBB drug transport studies. The drugs investigated include novel and CNS-active peptides, some agents that are actively removed from the brain ISF across the BBB, and a brain-directed prodrug.     

Skin and soft tissue concentrations of tedizolid (formerly torezolid), a novel oxazolidinone, following a single oral dose in healthy volunteers

Plasma concentrations of antimicrobial drugs have long been used to correlate exposure with effect, yet one cannot always assume that unbound plasma and tissue concentrations are similar. Knowledge about unbound tissue concentrations is important in the development of antimicrobial drugs, since most infections are localised in tissues. Therefore, a clinical microdialysis study was conducted to evaluate the distribution of tedizolid (TR-700), the active moiety of the antimicrobial prodrug tedizolid phosphate (TR-701), into interstitial fluid (ISF) of subcutaneous adipose and skeletal muscle tissues following a single oral 600 mg dose of tedizolid phosphate in fasting conditions. Twelve healthy adult subjects were enrolled. Two microdialysis probes were implanted into the thigh of each subject, one into the vastus medialis muscle and one into subcutaneous adipose tissue. Probes were calibrated using retrodialysis. Dialysate samples were collected every 20 min for 12h following a single oral dose of 600 mg tedizolid phosphate, and blood samples were drawn over 24h. Unbound tedizolid levels in plasma were similar to those in muscle and adipose tissue. The ratios of unbound (free) AUC in tissues over unbound AUC in plasma (fAUC(tissue)/fAUC(plasma)) were 1.1 ± 0.2 and 1.2 ± 0.2 for adipose and muscle tissue, respectively. The median half-life was 8.1, 9.2 and 9.6h for plasma, adipose tissue and muscle tissue, respectively. Mean protein binding was 87.2 ± 1.8%. The study drug was very well tolerated. The results of this study show that tedizolid distributes well into ISF of adipose and muscle tissues. Unbound levels of tedizolid in plasma, adipose tissue and muscle tissue were well correlated. Free plasma levels are indicative of unbound levels in the ISF of muscle and adipose tissues.     

Application of in vivo brain microdialysis to the study of blood-brain barrier transport of drugs

Recent advances in blood-brain barrier (BBB) research have led to a new understanding of drug transport processes at the BBB. The BBB acts as a dynamic regulatory interface at which nutrients necessary for neural activity are actively taken up into the brain from the blood circulation, and actively excludes metabolites that might interfere with the maintenance of brain homeostasis. Such influx and efflux transport functions at the BBB would also control the concentrations of various drugs in the brain interstitial fluid (ISF), which are an important determinant of the central nervous system (CNS) effects. Thus, direct measurement of the brain ISF concentration of drugs can provide significant information for clarifying the influx and efflux transport functions of drugs across the BBB. Although several experimental techniques have been developed to investigate transport functions across the BBB, in vivo brain microdialysis seems to be one of the most suitable techniques for characterizing the influx and efflux transport functions across the BBB under physiological and pathological conditions. This review covers studies during the past decade, in which the influx and efflux transport of drugs across the BBB was kinetically and mechanistically evaluated by means of the brain microdialysis technique. Some applications of brain microdialysis to studies on neuronal function and neurotherapeutics are also included.     

Therapeutic Drug Monitoring in Interstitial Fluid: A Feasibility Study Using a Comprehensive Panel of Drugs

This study compared drug concentration–time profiles in interstitial fluid (ISF) and blood, using an established animal model and a comprehensive panel of drugs, to examine the feasibility of therapeutic drug monitoring (TDM) in ISF. An intravenous bolus of vancomycin, gentamicin, tacrolimus, cyclosporine, mycophenolate, valproic acid, phenobarbital, phenytoin, carboplatin, cisplatin, methotrexate, theophylline, or digoxin was administered into the ear vein (n = 4–6) of rabbits. Serial (0–72 h after dose) blood and ISF concentrations (collected via an ultrafiltration probe) were determined by validated analytical assays. Pharmacokinetic parameters were generated by noncompartmental analysis. Vancomycin, gentamicin, and carboplatin showed no significant difference in area under the curve (AUC) values in ISF and blood, respectively. Other AUCs were lower (mycophenolic acid, valproic acid, phenobarbital, cisplatin, methotrexate, theophylline, and digoxin) or not measurable (tacrolimus, cyclosporine, and phenytoin) in ISF with our extraction technique. Similar concentration–time profiles in the two matrices were evident for a selection of drugs tested. Using a comprehensive panel of drugs in a single experimental setting, we have identified agents that can be quantified in ISF. Our newly developed scoring algorithm can help determine the feasibility of conducting TDM in ISF.     

α1-Adrenoceptors and muscarinic receptors in voiding function - binding characteristics of therapeutic agents in relation to the pharmacokinetics

In vivo and ex vivo binding of α(1)-adrenoceptor and muscarinic receptors involved in voiding function is reviewed with therapeutic agents (α(1)-adrenoceptor antagonists: prazosin, tamsulosin and silodosin; and muscarinic receptor antagonists: oxybutynin, tolterodine, solifenacin, propiverine, imiafenacin and darifenacin) in lower urinary tract symptoms. This approach allows estimation of the inhibition of a well-characterized selective (standard) radioligand by unlabelled potential drugs or direct measurement of the distribution and receptor binding of a standard radioligand or radiolabelled form of a novel drug. In fact, these studies could be conducted in various tissues from animals pretreated with radioligands and/or unlabelled novel drugs, by conventional radioligand binding assay, radioactivity measurement, autoradiography and positron emission tomography. In vivo and ex vivo receptor binding with α(1)-adrenoceptor antagonists and muscarinic receptor antagonists have been proved to be useful in predicting the potency, organ selectivity and duration of action of drugs in relation to their pharmacokinetics. Such evaluations of drug-receptor binding reveal that adverse effects could be avoided by the use of new α(1)-adrenoceptor antagonists and muscarinic receptor antagonists for the treatment of lower urinary tract symptoms. Thus, the comparative analysis of α(1)-adrenoceptor and muscarinic receptor binding characteristics in the lower urinary tract and other tissues after systemic administration of therapeutic agents allows the rationale for their pharmacological characteristics from the integrated viewpoint of pharmacokinetics and pharmacodynamics. The current review emphasizes the usefulness of in vivo and ex vivo receptor binding in the discovery and development of novel drugs for the treatment of not only urinary dysfunction but also other disorders.     

Molecular imaging with SPECT as a tool for drug development

Molecular imaging techniques are increasingly being used as valuable tools in the drug development process. Radionuclide-based imaging modalities such as single-photon emission computed tomography (SPECT) and positron emission tomography (PET) have proven to be useful in phases ranging from preclinical development to the initial stages of clinical testing. The high sensitivity of these imaging modalities makes them particularly suited for exploratory investigational new drug (IND) studies as they have the potential to characterize in vivo pharmacokinetics and biodistribution of the compounds using only a fraction of the intended therapeutic dose (microdosing). This information obtained at an early stage of clinical testing results in a better selection among promising drug candidates, thereby increasing the success rate of agents entering clinical trials and the overall efficiency of the process. In this article, we will review the potential applications of SPECT imaging in the drug development process with an emphasis on its applications in exploratory IND studies.     

Important drug interactions in dermatology

Drug interactions can occur at any step from absorption to elimination of a drug, and can induce adverse as well as beneficial effects. Since systemic drugs are increasingly available and important in the treatment of dermatological diseases, a variety of possible interactions between concomitantly administered drugs have to be considered by dermatologists. The xenobiotic-metabolising enzyme system cytochrome P450 (CYP) is involved in the metabolism of many drugs, regulating their plasma concentrations and activities. Furthermore, the adverse effects of many drugs depend on the basal activity and inducibility of particular CYP isoenzymes in an individual patient. Since drug therapy in dermatological practice is of increasing complexity, and an increasing number of potent systemic drugs have become commonly used therapeutic agents, this review focuses on the following topics with the aim of optimising dermatological drug therapy. In the first section, all the different types of drug interactions that can occur through pharmacokinetic and pharmacodynamic mechanisms are introduced briefly, and then discussed systematically with special reference to drugs important for dermatologists. Then, the network of drug interactions that may occur from absorption to elimination is presented. The most important drug interactions mediated by CYP isoenzymes are listed. Finally, the importance of pharmacogenetics for the development of new drugs and its potential impact on the optimisation of individual therapy regimens is discussed.     

Microdialysis in peripheral tissues

The objective of this review is to survey the recent literature regarding the applications of microdialysis in pharmacokinetic studies and facilitating many other studies in peripheral tissues such as muscle, subcutaneous adipose tissue, heart, lung, etc. It has been reported extensively that microdialysis is a useful technique for monitoring free concentrations of compounds in extracellular fluid (ECF), and it is gaining popularity in pharmacokinetic and pharmacodynamic studies, both in experimental animals and humans. The first part of this review discusses the use of microdialysis technique for ECF sampling in peripheral tissues in animal studies. The second part of the review describes the use of microdialysis for ECF sampling in peripheral tissues in human studies. Microdialysis has been applied extensively to measure both endogenous and exogenous compounds in ECF. Of particular benefit is the fact that microdialysis measures the unbound concentrations in the peripheral tissue fluid which have been shown to be responsible for the pharmacological effects. With the increasing number of applications of microdialysis, it is obvious that this method will have an important place in studying drug pharmacokinetics and pharmacodynamics.     

Microdialysis. A novel tool for clinical studies of anti-infective agents

In vivo microdialysis (MD) is an innovative clinical technique that has been employed in preclinical research and metabolic studies in patients for more than a decade. Recently, MD has been adopted for human drug studies and has opened up the opportunity to quantify tissue drug distribution in vivo. The particular advantage of MD for the anti-infective field relates to the fact that MD allows for online measurement of the unbound, pharmacologically active drug fraction in the interstitial space fluid (ISF), the anatomically defined target site for most bacterial infections. The aim of this review is to provide an overview of the current literature about MD in anti-microbial drug studies. It will be shown that MD has become feasible in most human tissues including brain and lung. So far, several MD studies have demonstrated that anti-microbial concentrations at the effect site may be subinhibitory, although effective concentrations are attained in serum, a finding that has significant impact on clinical decision making. In addition to its property as a pharmacokinetic sampling technique, MD offers unique opportunities in pharmacokinetic-pharmacodynamic (PK-PD) research and has the potential to streamline the decision process on proper drug dosing in drug development.     

Clinical microdialysis in skin and soft tissues: an update

Traditionally, plasma or serum drug concentrations have been used for the assessment of bioavailability and bioequivalence. Since in the majority of cases the site of drug action is in the tissue rather than the blood, the use of corresponding free, unbound concentrations in the tissue is a much more meaningful approach. This can become especially important for topical drug administrations, where locally active drug concentrations can significantly exceed free concentrations in plasma. The ability to measure these free concentrations at the site of drug action over time makes microdialysis a very valuable tool for the assessment of bioavailability and bioequivalence. This has been recognized by industry and regulatory authorities, resulting in a recommendation of the microdialysis technique as a tool for bioequivalence determination of topical dermatologic products. The aim of this article is to provide an updated review of the microdialysis technique, its applications in skin and soft tissues, and the resulting impact on clinical drug development.     

Microdialysis assessment of drug delivery systems for vitreoretinal targets

Posterior segment drug delivery challenges inherent in the treatment of many sight-threatening diseases have become increasingly apparent. Therapeutic interventions for ocular diseases such as neovascular retinopathies, inflammatory and/or infectious diseases may involve drug delivery to vitreoretinal targets. An important part of successful therapeutic strategies for such diseases involves verification that efficacious concentrations of the pharmacological agent are achieved within relevant intraocular regions. Microdialysis has been effectively employed for characterizing intraocular disposition in both anterior and posterior segments, providing important documentation of successful drug delivery to desired targets. Recent papers that showcase the maturation in the model development of microdialysis approaches for estimating posterior segment pharmacokinetics and further validation of the methodology are described in this review. Special problems examined include anterior and posterior ocular clearance mechanisms, intraocular metabolism and active transport of drugs.     

Antimicrobial drug delivery in food animals and microbial food safety concerns: an overview of in vitro and in vivo factors potentially affecting the animal gut microflora

This review provides an overview of considerations particular to the delivery of antimicrobial agents to food animals. Antimicrobial drugs are used in food animals for a variety of purposes. These drugs may have therapeutic effects against disease agents, or may cause changes in the structure and/or function of systems within the target animal. Routes of administration, quantity, duration, and potency of an antimicrobial drug are all important factors affecting their action(s) and success. Not only might targeted pathogens be affected, but also bacteria residing in (or on) the treated food animals, especially in the intestines (gastrointestinal tract microflora). Resistance to antimicrobial agents can occur through a number of mechanisms. The extent to which resistance develops is greatly affected by the amount of drug [or its metabolite(s)] a bacterium is exposed to, the duration of exposure, and the interaction between an individual antimicrobial agent and a particular bacterium. The impact of antimicrobial agents on the emergence of resistance in vitro and in vivo may not readily correlate.     

PET imaging in clinical drug abuse research

Over the last two decades, SPECT (single photon emission computed tomography) and especially PET (positron emission tomography) have proven increasingly effective imaging modalities in the study of human psychopharmacology. Abusing populations can be studied at multiple times after abstinence begins, to give information about neurochemical and physiological adaptations of the brain during recovery from addiction. Individual human subjects can be studied using multiple positron labeled radiotracers, so as to probe more than one facet of brain function. PET and SPECT have been used to help our understanding of many aspects of the pharmacokinetics and pharmacodynamics of abused drugs, and have made valuable contributions in terms of drug mechanisms, drug interactions (e.g. cocaine and alcohol) and drug toxicities. They have also been employed to study the acute effects of drugs on populations of active drug abusers and of normal controls, and to evaluate the neurochemical consequences of candidate therapies for drug abuse. A particularly productive strategy has been the use of PET in conjunction with neuropsychological testing of subjects, to allow correlation of imaging data with uniquely human aspects of the effects of drugs, such as euphoria and craving.     

Noninvasive brain imaging for experimental medicine in drug discovery

There is a widely shared perception that it is becoming increasingly urgent to find reliable measures related to therapeutic efficacy for use as early as possible in drug development. The inadequate understanding of diseases, limitations of animal models and difficulties in using their responses to anticipate drug effects in humans highlights the need to develop tools in experimental medicine to characterise human disease directly. Noninvasive imaging, particularly positron emission tomography and magnetic resonance imaging, provide a powerful range of methods for serial observation of drug distribution and interactions, and for assessing potential therapeutic mechanisms. Using imaging technology to establish biological proof-of-principle and as a pharmacodynamic marker for dose ranging would contribute greatly to the speed and efficiency of early decision making in new drug development. Imaging methods offer the ultimate promise for the development of clinically predictive surrogate markers of disease responses to new drugs.     

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.     

Microdialysis in the study of drug transporters in the CNS

Quantitative microdialysis in the central nervous system (CNS) has recently provided evidence for the existence of transporters as they relate to the brain distribution of a variety of drugs. Support for the existence of drug transporters in the blood-brain barrier (or in the blood-CSF barrier) comes from investigations that have found: unbound drug concentrations in brain fluids that are lower than corresponding levels in plasma; saturability of transport clearances across the blood-brain barrier and; the regulation of transport by putative inhibitors. Additional confirmatory evidence for the existence of active transport or carrier-mediated processes has also been derived from models that relate observed drug levels in the CNS with those in plasma or blood. The conclusion that reduced drug levels in brain fluids generally indicate the existence of active efflux transport is questioned. In the case of relatively polar compounds with modest blood-brain barrier permeability, lower unbound concentrations in brain may be a consequence of dilution by turnover of brain fluids. This review summarizes recent reports (grouped by class of compounds) where investigators have used microdialysis to examine the distribution of therapeutic agents to the CNS, and have reached conclusions regarding the functional presence of drug transporters in the brain.