Pharmacokinetics of 18F-labeled fluconazole in healthy human subjects by positron emission tomography

Preclinical Models for Cryptococcosis of the CNS and Their Characterization Using In Vivo Imaging Techniques

Infections caused by Cryptococcus neoformans and Cryptococcus gattii remain a challenge to our healthcare systems as they are still difficult to treat. In order to improve treatment success, in particular for infections that have disseminated to the central nervous system, a better understanding of the disease is needed, addressing questions like how it evolves from a pulmonary to a brain disease and how novel treatment approaches can be developed and validated. This requires not only clinical research and research on the microorganisms in a laboratory environment but also preclinical models in order to study cryptococci in the host. We provide an overview of available preclinical models, with particular emphasis on models of cryptococcosis in rodents. In order to further improve the characterization of rodent models, in particular the dynamic aspects of disease manifestation, development, and ultimate treatment, preclinical in vivo imaging methods are increasingly used, mainly in research for oncological, neurological, and cardiac diseases. In vivo imaging applications for fungal infections are rather sparse. A second aspect of this review is how research on models of cryptococcosis can benefit from in vivo imaging methods that not only provide information on morphology and tissue structure but also on function, metabolism, and cellular properties in a non-invasive way.

Antifungal Resistance in Cryptococcal Infections

Antifungal therapy, especially with the azoles, could promote the incidence of less susceptible isolates of Cryptococcus neoformans and C. gattii species complexes (SC), mostly in developing countries. Given that these species affect mostly the immunocompromised host, the infections are severe and difficult to treat. This review encompasses the following topics: 1. infecting species and their virulence, 2. treatment, 3. antifungal susceptibility methods and available categorical endpoints, 4. genetic mechanisms of resistance, 5. clinical resistance, 6. fluconazole minimal inhibitory concentrations (MICs), clinical outcome, 7. environmental influences, and 8. the relevance of host factors, including pharmacokinetic/pharmacodynamic (PK/PD) parameters, in predicting the clinical outcome to therapy. As of now, epidemiologic cutoff endpoints (ECVs/ECOFFs) are the most reliable antifungal resistance detectors for these species, as only one clinical breakpoint (amphotericin B and C. neoformans VNI) is available.

Antifungal Drug Concentration Impacts the Spectrum of Adaptive Mutations in Candida albicans

Invasive fungal infections are a leading global cause of human mortality. Only three major classes of antifungal drugs are widely used, and resistance to all three classes can arise rapidly. The most widely prescribed antifungal drug, fluconazole, disseminates rapidly and reaches a wide range of concentrations throughout the body. The impact of drug concentration on the spectrum and effect of mutations acquired during adaptation is not known for any fungal pathogen, and how the specific level of a given stress influences the distribution of beneficial mutations has been poorly explored in general. We evolved 144 lineages from three genetically distinct clinical isolates of Candida albicans to four concentrations of fluconazole (0, 1, 8, and 64 μg/ml) and performed comprehensive phenotypic and genomic comparisons of ancestral and evolved populations. Adaptation to different fluconazole concentrations resulted in distinct adaptive trajectories. In general, lineages evolved to drug concentrations close to their MIC50 (the level of drug that reduces growth by 50% in the ancestor) tended to rapidly evolve an increased MIC50 and acquired distinct segmental aneuploidies and copy number variations. By contrast, lineages evolved to drug concentrations above their ancestral MIC50 tended to acquire a different suite of mutational changes and increased in drug tolerance (the ability of a subpopulation of cells to grow slowly above their MIC50). This is the first evidence that different concentrations of drug can select for different genotypic and phenotypic outcomes in vitro and may explain observed in vivo drug response variation.

Imaging of Invasive Fungal Infections- The Role of PET/CT

Over the last decades, the population at risk for invasive fungal disease (IFD) has increased because of medical therapy advances and diseases compromising patients' immune systems. The high morbidity and mortality associated with invasive fungal disease in the immunocompromised present the challenge of early diagnosis of the IFD and the need to closely monitor the infection during treatment. The definitive diagnosis of invasive fungal disease based on culture or histopathological methods often has reduced diagnostic accuracy in the immunocompromised and may be very invasive. Less invasive and indirect evidence of the fungal infection by serology and imaging has been used for the early diagnosis of fungal infection before definitive results are available or when the definitive methods of diagnosis are suboptimal. Imaging in invasive fungal disease is a non-invasive biomarker that helps in the early diagnosis of invasive fungal disease but helps follow-up the infection during treatment. Different imaging modalities are used in the workup to evaluate fungal disease. The different imaging modalities have advantages and disadvantages at different sites in the body and may complement each other in the management of IFD. Positron emission tomography integrated with computed tomography with [18F]Fluorodeoxyglucose (FDG PET/CT) has helped manage IFD. The combined functional data from PET and anatomical data from the CT from almost the whole body allows noninvasive evaluation of IFD and provides a semiquantitative means of assessing therapy. FDG PET/CT adds value to anatomic-based only imaging modalities. The nonspecificity of FDG uptake has led to the evaluation of other tracers in the assessment of IFD. However, these are mainly still at the preclinical level and are yet to be translated to humans. FDG PET/CT remains the most widely evaluated radionuclide-based imaging modality in IFD management. The limitations of FDG PET/CT must be well understood, and more extensive prospective studies in uniform populations are needed to validate its role in the management of IFD that can be international guidelines.

Consensus guidelines for optimising antifungal drug delivery and monitoring to avoid toxicity and improve outcomes in patients with haematological malignancy and haemopoietic stem cell transplant recipients, 2021

Antifungal agents can have complex dosing and the potential for drug interaction, both of which can lead to subtherapeutic antifungal drug concentrations and poorer clinical outcomes for patients with haematological malignancy and haemopoietic stem cell transplant recipients. Antifungal agents can also be associated with significant toxicities when drug concentrations are too high. Suboptimal dosing can be minimised by clinical assessment, laboratory monitoring, avoidance of interacting drugs, and dose modification. Therapeutic drug monitoring (TDM) plays an increasingly important role in antifungal therapy, particularly for antifungal agents that have an established exposure-response relationship with either a narrow therapeutic window, large dose-exposure variability, cytochrome P450 gene polymorphism affecting drug metabolism, the presence of antifungal drug interactions or unexpected toxicity, and/or concerns for non-compliance or inadequate absorption of oral antifungals. These guidelines provide recommendations on antifungal drug monitoring and TDM-guided dosing adjustment for selected antifungal agents, and include suggested resources for identifying and analysing antifungal drug interactions. Recommended competencies for optimal interpretation of antifungal TDM and dose recommendations are also provided.

Radionuclide Imaging of Invasive Fungal Disease in Immunocompromised Hosts

Invasive fungal disease (IFD) leads to increased mortality, morbidity, and costs of treatment in patients with immunosuppressive conditions. The definitive diagnosis of IFD relies on the isolation of the causative fungal agents through microscopy, culture, or nucleic acid testing in tissue samples obtained from the sites of the disease. Biopsy is not always feasible or safe to be undertaken in immunocompromised hosts at risk of IFD. Noninvasive diagnostic techniques are, therefore, needed for the diagnosis and treatment response assessment of IFD. The available techniques that identify fungal-specific antigens in biological samples for diagnosing IFD have variable sensitivity and specificity. They also have limited utility in response assessment. Imaging has, therefore, been applied for the noninvasive detection of IFD. Morphologic imaging with computed tomography (CT) and magnetic resonance imaging (MRI) is the most applied technique. These techniques are neither sufficiently sensitive nor specific for the early diagnosis of IFD. Morphologic changes evaluated by CT and MRI occur later in the disease course and during recovery after successful treatment. These modalities may, therefore, not be ideal for early diagnosis and early response to therapy determination. Radionuclide imaging allows for targeting the host response to pathogenic fungi or specific structures of the pathogen itself. This makes radionuclide imaging techniques suitable for the early diagnosis and treatment response assessment of IFD. In this review, we aimed to discuss the interplay of host immunity, immunosuppression, and the occurrence of IFD. We also discuss the currently available radionuclide probes that have been evaluated in preclinical and clinical studies for their ability to detect IFD.

Visualizing and quantifying antimicrobial drug distribution in tissue

The biodistribution and pharmacokinetics of drugs are vital to the mechanistic understanding of their efficacy. Measuring antimicrobial drug efficacy has been challenging as plasma drug concentration is used as a surrogate for tissue drug concentration, yet typically does not reflect that at the intended site(s) of action. Utilizing an image-guided approach, it is feasible to accurately quantify the biodistribution and pharmacokinetics within the desired site(s) of action. We outline imaging modalities used in visualizing drug distribution with examples ranging from in vitro cellular drug uptake to clinical treatment of microbial infections. The imaging modalities of interest are: radio-labeling, magnetic resonance, mass spectrometry imaging, computed tomography, fluorescence, and Raman spectroscopy. We outline the progress, limitations, and future outlook for each methodology. Further advances in these optical approaches would benefit patients and researchers alike, as non-invasive imaging could yield more profound insights with a lower clinical burden than invasive measurement approaches used today.

Successful management of post-COVID-19 acanthamoebic encephalitis

Acanthamoebic encephalitis is a rare and highly fatal disease that has no standard management protocol. Coronavirus disease 2019 (COVID-19) causes immune dysfunction and may predispose patients to this infection.

The present study describes successful management of acanthamoebic encephalitis in a young male who recently recovered from COVID-19 using a combination of medical and surgical approaches.

A combination of miltefosine with other agents with trophicidal and cysticidal activities should be used in the regimen. Surgical excision of the abscess should be undertaken whenever feasible.

Advances in the In Vivo Molecular Imaging of Invasive Aspergillosis

Invasive pulmonary aspergillosis (IPA) is a life-threatening infection of immunocompromised patients with Aspergillus fumigatus, a ubiquitous environmental mould. While there are numerous functioning antifungal therapies, their high cost, substantial side effects and fear of overt resistance development preclude permanent prophylactic medication of risk-patients. Hence, a fast and definitive diagnosis of IPA is desirable, to quickly identify those patients that really require aggressive antimycotic treatment and to follow the course of the therapeutic intervention. However, despite decades of research into this issue, such a diagnostic procedure is still not available. Here, we discuss the array of currently available methods for IPA detection and their limits. We then show that molecular imaging using positron emission tomography (PET) combined with morphological computed tomography or magnetic imaging is highly promising to become a future non-invasive approach for IPA diagnosis and therapy monitoring, albeit still requiring thorough validation and relying on further acceptance and dissemination of the approach. Thereby, our approach using the A. fumigatus-specific humanized monoclonal antibody hJF5 labelled with ⁶⁴Cu as PET-tracer has proven highly effective in pre-clinical models and hence bears high potential for human application.

Antifungal Drugs: Special Problems Treating Central Nervous System Infections

Treating fungal infections in the central nervous system (CNS) remains a challenge despite the availability of new antifungal agents. Therapy is limited by poor understanding of the kinetic properties of antifungal drugs in the CNS compounded by lack of data for many agents. In some cases, clinical response rates do not correspond to data on drug concentrations in the cerebral spinal fluid and/or brain parenchyma. In order to better characterize the use of antifungal agents in treating CNS infections, a review of the essential principles of neuroPK are reviewed. Specific data regarding antifungal drug concentrations in the cerebral spinal fluid and brain tissue are described from human data where available. Alternative dosing regimens and the role of antifungal drug concentration monitoring in treating fungal infections in the CNS are also discussed. Having a better understanding of these key concepts will help guide clinicians in determining the best treatment courses for patients with these devastating infections.

Beyond tissue concentrations: antifungal penetration at the site of infection

Despite advances in antifungal therapy, invasive fungal infections remain a significant cause of morbidity and mortality worldwide. One important factor contributing to the relative ineffectiveness of existing antifungal drugs is insufficient drug exposure at the site of infection. Despite the importance of this aspect of antifungal therapy, we generally lack a full appreciation of how antifungal drugs distribute, penetrate, and interact with their target organisms in different tissue subcompartments. A better understanding of drug distribution will be critical to guide appropriate use of currently available antifungal drugs, as well as to aid development of new agents. Herein we briefly review current perspectives of antifungal drug exposure at the site of infection and describe a new technique, matrix-assisted laser desorption ionization (MALDI) mass spectrometry imaging, which has the potential to greatly expand our understanding of drug penetration.

Imaging Techniques in the Diagnosis and Management of Ocular Tumors: Prospects and Challenges

Different types of imaging modalities are used in the diagnosis of ocular cancer. Selection of an imaging modality is based on the features of a tumor as well as the inherent characteristics of the imaging technique. It is vital to select an appropriate imaging modality in diagnosis of ocular tumor with confidence. This review focuses on five most commonly used imaging modalities, i.e., positron emission tomography-computed tomography (PET/CT), single photon emission computed tomography (SPECT), optical coherence tomography (OCT), ultrasound (US), and magnetic resonance imaging (MRI). The principal of imaging modalities is briefly explained, along with their role in the diagnosis and management of the most common ocular tumors such as retinoblastoma and uveal melanoma. Further, the diagnostic features of ocular tumors corresponding to each imaging modality and possibilities of utilizing imaging techniques in the process of ocular drug development are included in this review.

Population Pharmacokinetics and Cerebrospinal Fluid Penetration of Fluconazole in Adults with Cryptococcal Meningitis

Robust population pharmacokinetic (PK) data for fluconazole are scarce. The variability of fluconazole penetration into the central nervous system (CNS) is not known.

Robust population pharmacokinetic (PK) data for fluconazole are scarce. The variability of fluconazole penetration into the central nervous system (CNS) is not known. A fluconazole PK study was conducted in 43 patients receiving oral fluconazole (usually 800 mg every 24 h [q24h]) in combination with amphotericin B deoxycholate (1 mg/kg q24h) for cryptococcal meningitis (CM). A four-compartment PK model was developed, and Monte Carlo simulations were performed for a range of fluconazole dosages. A meta-analysis of trials reporting outcomes of CM patients treated with fluconazole monotherapy was performed. Adjusted for bioavailability, the PK parameter means (standard deviation) were the following: clearance, 0.72 (0.24) liters/h; volume of the central compartment, 18.07 (6.31) liters; volume of the CNS compartment, 32.07 (17.60) liters; first-order rate constant from the central to peripheral compartment, 12.20 (11.17) h⁻¹, from the peripheral to central compartment, 18.10 (8.25) h⁻¹, from the central to CNS compartment, 35.43 (13.74) h⁻¹, and from the CNS to central the compartment, 28.63 (10.03) h⁻¹. Simulations of the area under concentration-time curve resulted in median (interquartile range) values of 1,143.2 (range, 988.4 to 1,378.0) mg · h/liter in plasma (AUC_plasma) and 982.9 (range, 781.0 to 1,185.9) mg · h/liter in cerebrospinal fluid (AUC_CSF) after a dosage of 1,200 mg q24h. The mean simulated ratio of AUC_CSF/AUC_plasma was 0.89 (standard deviation [SD], 0.44). The recommended dosage of fluconazole for CM induction therapy fails to attain the pharmacodynamic (PD) target in respect to the wild-type MIC distribution for C. neoformans. The meta-analysis suggested modest improvements in both CSF sterility and mortality outcomes with escalating dosage. This study provides the pharmacodynamic rationale for the long-recognized fact that fluconazole monotherapy is an inadequate induction regimen for CM.

Identification of Fungal Species in Brain Tissue from Alzheimer's Disease by Next-Generation Sequencing

The possibility that patients diagnosed with Alzheimer's disease (AD) have disseminated fungal infection has been recently advanced by the demonstration of fungal proteins and DNA in nervous tissue from AD patients. In the present study, next-generation sequencing (NGS) was used to identify fungal species present in the central nervous system (CNS) of AD patients. Initially, DNA was extracted from frozen tissue from four different CNS regions of one AD patient and the fungi in each region were identified by NGS. Notably, whereas a great variety of species were identified using the Illumina platform, Botrytis cinerea and Cryptococcus curvatus were common to all four CNS regions analyzed. Further analysis of entorhinal/cortex hippocampus samples from an additional eight AD patients revealed a variety of fungal species, although some were more prominent than others. Five genera were common to all nine patients: Alternaria, Botrytis, Candida, Cladosporium, and Malassezia. These observations could be used to guide targeted antifungal therapy for AD patients. Moreover, the differences found between the fungal species in each patient may constitute a basis to understand the evolution and severity of clinical symptoms in AD.

Plasma vs heart tissue concentration in humans - literature data analysis of drugs distribution

Little is known about the uptake of drugs into the human heart, although it is of great importance nowadays, when science desires to predict tissue level behavior rather than to measure it. Although the drug concentration in cardiac tissue seems a better predictor for physiological and electrophysiological changes than its level in plasma, knowledge of this value is very limited. Tissue to plasma partition coefficients (Kp) come to rescue since they characterize the distribution of a drug among tissues as being one of the input parameters in physiologically based pharmacokinetic (PBPK) models. The article reviews cardiac surgery and forensic medical studies to provide a reference for drug concentrations in human cardiac tissue. Firstly, the focus is on whether a drug penetrates into heart tissue at a therapeutic level; the provided values refer to antibiotics, antifungals and anticancer drugs. Drugs that directly affect cardiomyocyte electrophysiology are another group of interest. Measured levels of amiodarone, digoxin, perhexiline and verapamil in different sites in human cardiac tissue where the compounds might meet ion channels, gives an insight into how these more lipophilic drugs penetrate the heart. Much data are derived from postmortem studies and they provide insight to the cardiac distribution of more than 200 drugs. The analysis depicts potential problems in defining the active concentration location, what may indirectly suggest multiple mechanisms involved in the drug distribution within the heart. Copyright © 2015 John Wiley & Sons, Ltd.

Tissue penetration of antifungal agents

Understanding the tissue penetration of systemically administered antifungal agents is critical for a proper appreciation of their antifungal efficacy in animals and humans. Both the time course of an antifungal drug and its absolute concentrations within tissues may differ significantly from those observed in the bloodstream. In addition, tissue concentrations must also be interpreted within the context of the pathogenesis of the various invasive fungal infections, which differ significantly. There are major technical obstacles to the estimation of concentrations of antifungal agents in various tissue subcompartments, yet these agents, even those within the same class, may exhibit markedly different tissue distributions. This review explores these issues and provides a summary of tissue concentrations of 11 currently licensed systemic antifungal agents. It also explores the therapeutic implications of their distribution at various sites of infection.

Quantification of brain voriconazole levels in healthy adults using fluorine magnetic resonance spectroscopy

Voriconazole is more effective for aspergillosis infections with central nervous system involvement than other antifungal agents. The clinical efficacy of voriconazole for central nervous system infections has been attributed to its ability to cross the blood-brain barrier. However, pharmacokinetic studies are limited to plasma and cerebrospinal fluid, so it remains unclear how much of the drug enters the brain. Fluorinated compounds such as voriconazole can be quantified in the brain using fluorine-19 magnetic resonance spectroscopy (MRS). Twelve healthy adult males participated in a pharmacokinetic analysis of voriconazole levels in the brain and plasma. Open-label voriconazole was dosed per clinical protocol with a loading dose of 400 mg every 12 h on day 1, followed by 200 mg every 12 h administered orally over a 3-day period. MRS was performed before and after dosing on the third day. Voriconazole levels in the brain exceeded the MIC for Aspergillus. The brain/plasma ratios were 3.0 at steady state on day 3 (predose) and 1.9 postdose. We found that voriconazole is able to penetrate the brain tissue, which can be quantified using a noninvasive MRS technique. (This study has been registered at ClinicalTrials.gov under registration no. NCT00300677.).

Medical imaging in new drug clinical development

Medical imaging can help answer key questions that arise during the drug development process. The role of medical imaging in new drug clinical trials includes identification of likely responders; detection and diagnosis of lesions and evaluation of their severity; and therapy monitoring and follow-up. Nuclear imaging techniques such as PET can be used to monitor drug pharmacokinetics and distribution and study specific molecular endpoints. In assessing drug efficacy, imaging biomarkers and imaging surrogate endpoints can be more objective and faster to measure than clinical outcomes, and allow small group sizes, quick results and good statistical power. Imaging also has important role in drug safety monitoring, particularly when there is no other suitable biomarkers available. Despite the long history of radiological sciences, its application to the drug development process is relatively recent. This review highlights the processes, opportunities, and challenges of medical imaging in new drug development.

Osteoarticular infection by Candida albicans in an infant with cystic fibrosis

Invasive candidiasis is rare in children after the neonatal period, but can occur in children with (secondary) immunodeficiency with a damaged gastrointestinal or skin barrier, or when receiving antibiotics. A 10-month-old girl was diagnosed as suffering from cystic fibrosis (CF) when showing failure to thrive, pulmonary symptoms and hypoproteinaemia. At that time, Candida albicans was identified from blood culture and treated intravenously with liposomal amphotericin B for 13 days. Six weeks later, the girl presented with osteoarticular infection of the left knee caused by C. albicans. The infection showed insufficient response to therapy with liposomal amphotericin B, but the patient recovered after therapy with fluconazole and flucytosine. Follow-up over 4 years revealed no sequelae. In conclusion, invasive Candida infections may occur in patients with CF, and preventive measures might be considered in patients at risk. In the case of an invasive infection, prolonged treatment with a combination of antifungal drugs may be required.

Fluconazole dosing for the prevention or treatment of invasive candidiasis in young infants

BACKGROUND

Young infants are susceptible to developmental factors influencing the pharmacokinetics of drugs. Fluconazole is increasingly used to prevent and treat invasive candidiasis in infants. Dosing guidance remains empiric and variable because limited pharmacokinetic data exist.

METHODS

Our population pharmacokinetic model derived from 357 fluconazole plasma concentrations from 55 infants (23-40 week gestation) illustrates expected changes in fluconazole clearance based upon gestational age, postnatal age, weight, and creatinine. We used a Monte Carlo simulation approach based on parametric description of a patient population's pharmacokinetic response to fluconazole to predict fluconazole exposure (median: 10th and 90th percentile population variability range) after 3, 6, and 12 mg/kg dosing.

RESULTS

For the treatment of invasive candidiasis, a dose of at least 12 mg/kg/d in the first 90 days after birth is needed to achieve an area under the concentration curve (AUC) of >400 mg*h/L and an AUC/minimum inhibitory concentration (MIC) >50 for Candida species with MIC <8 microg/mL in > or =90% of <30 week gestation infants and 80% of 30 to 40 week gestation infants. The more preterm infants achieve a higher median AUC (682 mg*hr/L) compared with more mature infants (520 mg*hr/L). For early prevention of candidiasis in 23 to 29 week infants, a dose of 3 or 6 mg/kg twice weekly during the first 42 days of life is equivalent to an AUC of 50 and 100 mg*hr/L, respectively, and maintains fluconazole concentrations > or =2 or 4 microg/mL, respectively, for half of the dosing interval. For late prevention, the 6 mg/kg dose every 72 hours provides similar exposure to 3 mg/kg daily dose. Infants with serum creatinine > or =1.3 mg/dL have delayed drug clearance and dose adjustment is indicated if creatinine does not improve within 96 hours.

CONCLUSIONS

A therapeutic concentration of fluconazole in premature infants with invasive candidiasis requires dosing substantially greater than commonly recommended in most reference texts. To prevent invasive candidiasis, twice weekly prophylaxis regimens can provide adequate exposure when unit specific MICs are taken into account.

Microdosing studies in humans: the role of positron emission tomography

Positron emission tomography (PET)-microdosing comprises the administration of a carbon-11- or fluorine-18-labelled drug candidate to human subjects in order to describe the drug's concentration-time profile in body tissues targeted for treatment. As PET microdosing involves the administration of only microgram amounts of unlabelled drug, the potential toxicological risk to human subjects is very limited. Consequently, regulatory authorities require reduced preclinical safety testing as compared with conventional phase 1 studies. Microdose studies are gaining increasing importance in clinical drug research as they have the potential to shorten time-lines and cut costs along the critical path of drug development. Current applications of PET in anticancer, anti-infective and CNS system drug research are reviewed.

Microdialysis versus other techniques for the clinical assessment of in vivo tissue drug distribution

Quantification of target site pharmacokinetics (PK) is crucial for drug discovery and development. Clinical microdialysis (MD) has increasingly been employed for the description of drug distribution and receptor phase PK of the unbound fraction of various analytes. Costs for MD experiments are comparably low and given suitable analytics, target tissue PK of virtually any drug molecule can be quantified. The major limitation of MD stems from the fact that organs such as brain, lung or liver are not readily accessible without surgery. Recently, non-invasive imaging techniques, i.e. positron emission tomography (PET) or magnetic resonance spectroscopy (MRS), have become available for in vivo drug distribution assessment and allow for drug concentration measurements in practically every human organ. Spatial resolution of MRS imaging, however, is low and although PET enables monitoring of regional drug concentration differences with a spatial resolution of a few millimetres, discrimination between bound and unbound drug or parent compound and metabolite is difficult. Radiotracer development is furthermore time and labour intensive and requires special expertise and radiation exposure and costs originating from running a PET facility cannot be neglected. The recent complementary use of MD and imaging has permitted to exploit individual strengths of these diverse techniques. In conclusion, MD and imaging techniques have provided drug distribution data that have so far not been available. Used alone or in combination, these methods may potentially play an important role in future drug research and development with the potential to serve as translational tools for clinical decision making.

Candida spondylodiscitis and epidural abscess: management with shorter courses of anti-fungal therapy in combination with surgical debridement

Epidural abscess associated with candidal spondylodiscitis is rarely seen, particularly when it involves the cervical and thoracic spine. We report two such cases that were successfully managed with early surgical debridement, as well as medical therapy with intravenous amphotericin followed by oral fluconazole. The literature related to candidal spinal infection is reviewed, and a rational approach to the management of this uncommon condition is proposed. A good outcome may generally be expected with early diagnosis as well as appropriate surgical and pharmacological treatment. Oral fluconazole appears to be useful in the management of candida spondylodiscitis complicated by epidural abscess formation. Treatment until a normal ESR is attained is ideal, and this may be as short as 3 months when surgical drainage has been adequately performed.

Using imaging biomarkers to accelerate drug development and clinical trials

There is increasing evidence that human medical imaging can help answer key questions that arise during the drug development process. Imaging modalities such as magnetic resonance imaging, computed tomography and positron emission tomography can offer significant insights into the bioactivity, pharmacokinetics and dosing of drugs, in addition to supporting registration applications. In this review, examples from oncology, neurology, psychiatry, infectious diseases and inflammatory diseases are used to illustrate the role imaging can play. We conclude with some remarks concerning new developments that will be required to significantly advance the field of pharmaco-imaging.

Microdialysis

Microdialysis is a probe-based sampling method, which, if linked to analytical devices, allows for the measurement of drug concentration profiles in selected tissues. During the last two decades, microdialysis has become increasingly popular for preclinical and clinical pharmacokinetic studies. The advantage of in vivo microdialysis over traditional methods relates to its ability to continuously sample the unbound drug fraction in the interstitial space fluid (ISF). This is of particular importance because the ISF may be regarded as the actual target compartment for many drugs, e.g. antimicrobial agents or other drugs mediating their action through surface receptors. In contrast, plasma concentrations are increasingly recognised as inadequately predicting tissue drug concentrations and therapeutic success in many patient populations. Thus, the minimally invasive microdialysis technique has evolved into an important tool for the direct assessment of drug concentrations at the site of drug delivery in virtually all tissues. In particular, concentrations of transdermally applied drugs, neurotransmitters, antibacterials, cytotoxic agents, hormones, large molecules such as cytokines and proteins, and many other compounds were described by means of microdialysis. The combined use of microdialysis with non-invasive imaging methods such as positron emission tomography and single photon emission tomography opened the window to exactly explore and describe the fate and pharmacokinetics of a drug in the body. Linking pharmacokinetic data from the ISF to pharmacodynamic information appears to be a straightforward approach to predicting drug action and therapeutic success, and may be used for decision making for adequate drug administration and dosing regimens. Hence, microdialysis is nowadays used in clinical studies to test new drug candidates that are in the pharmaceutical industry drug development pipeline.

Preparation and pharmacokinetics of 11C labeled stavudine (d4T)

Stavudine, a potent antiviral agent for treating human immunodeficiency virus (HIV) infections, was radiolabeled with (11)C by methylation of a specifically designed precursor, 5'-O-(2-tetrahydropyranyl)-5-bromo-2',3'-didehydro-3'-deoxythymidine, with (11)C H(3)I. The radiolabeled drug was isolated by reverse phase HPLC. A total time of approximately 45 minutes was required for synthesis, purification and isolation of (11)C stavudine with chemical and radiochemical purities of greater than 98%. (11)C stavudine was combined with unlabeled drug (2.0 mg/kg) and used to study its pharmacokinetics in rats by measurement of radioactivity in excised tissues. In this species, there was rapid accumulation of drug in all tissue. In all tissues, with the exceptions of testis and brain, highest concentrations of drug were detected at 5 minutes after injection and decreased monotonically thereafter. The peak concentration (microg/g) of stavudine in blood was 1.78 +/- 0.16 and similar levels were achieved in most other tissues; heart 1.66 +/- 0.11, lung 1.60 +/- 0.15, liver 2.13 +/- 0.17, spleen 1.61 +/- 0.15, adrenal 1.47 +/- 0.20, stomach 1.40 +/- 0.11, GI tract 1.44 +/- 0.14, skeletal muscle 1.38 +/- 0.15 and bone 1.30 +/- 0.16. Much higher peak concentrations were achieved in kidney; 7.23 +/- 0.57 microg/g. Concentrations in testis were lower and remained relatively constant over 1 hour; peak 0.62 +/- 0.14 microg/g at 15 min Brain concentrations were low but increased monotonically over time; peak 0.26 +/- 0.02 microg/g at 60 min. Future PET studies with this radiopharmaceutical will allow in vivo measurements of the pharmacokinetics of stavudine in both animal models and human subjects.

Human subcutaneous tissue distribution of fluconazole: comparison of microdialysis and suction blister techniques

AIMS

To investigate uptake of fluconazole into the interstitial fluid of human subcutaneous tissue using the microdialysis and suction blister techniques.

METHODS

A sterile microdialysis probe (CMA/60) was inserted subcutaneously into the upper arm of five healthy volunteers following an overnight fast. Blisters were induced on the lower arm using gentle suction prior to ingestion of a single oral dose of fluconazole (200 mg). Microdialysate, blister fluid and blood were sampled over 8 h. Fluconazole concentrations were determined in each sample using a validated HPLC assay. In vivo recovery of fluconazole from the microdialysis probe was determined in each subject by perfusing the probe with fluconazole solution at the end of the 8 h sampling period. Individual in vivo recovery was used to calculate fluconazole concentrations in subcutaneous interstitial fluid. A physiologically based pharmacokinetic (PBPK) model was used to predict fluconazole concentrations in human subcutaneous interstitial fluid.

RESULTS

There was a lag-time (approximately 0.5 h) between detection of fluconazole in microdialysate compared with plasma in each subject. The in vivo recovery of fluconazole from the microdialysis probe ranged from 57.0 to 67.2%. The subcutaneous interstitial fluid concentrations obtained by microdialysis were very similar to the unbound concentrations of fluconazole in plasma with maximum concentration of 4.29 +/- 1.19 microg ml(-1) in subcutaneous interstitial fluid and 3.58 +/- 0.14 microg ml(-1) in plasma. Subcutaneous interstitial fluid-to-plasma partition coefficient (Kp) of fluconazole was 1.16 +/- 0.22 (95% CI 0.96, 1.35). By contrast, fluconazole concentrations in blister fluid were significantly lower (P < 0.05, paired t-test) than unbound plasma concentrations over the first 3 h and maximum concentrations in blister fluid had not been achieved at the end of the sampling period. There was good agreement between fluconazole concentrations derived from microdialysis sampling and those estimated using a blood flow-limited PBPK model.

CONCLUSIONS

Microdialysis and suction blister techniques did not yield comparable results. It appears that microdialysis is a more appropriate technique for studying the rate of uptake of fluconazole into subcutaneous tissue. PBPK model simulation suggested that the distribution of fluconazole into subcutaneous interstitial fluid is dependent on tissue blood flow.

Pharmacokinetics and the most suitable dosing regimen of fluconazole in critically ill patients receiving continuous hemodiafiltration

OBJECTIVE

To evaluate fluconazole pharmacokinetics and the dosage best suited to maintain effective plasma concentration in patients with continuous hemodiafiltration.

DESIGN AND SETTING

Prospective study in the general intensive care unit of a university hospital.

PATIENTS

Four critically ill patients being treated with fluconazole and receiving continuous hemodiafiltration.

INTERVENTIONS

Fluconazole was administered at three dosing regimens: 200 and 400 mg every 24 h, 400 mg every 12 h, and 800 mg every 24 h.

MEASUREMENTS AND RESULTS

The following pharmacokinetic variables for fluconazole were obtained: The mean volume distribution of steady state dosed at 400 mg every 12 h and 800 mg every 24 h were 0.55+/-0.23 and 0.71+/-0.16 l/kg, half-life of the elimination phase 8.08+/-0.83 and 9.12+/-0.75 h, total body clearance of fluconazole 1.14+/-0.44 and 0.98+/-0.20 ml/kg per minute, respectively. None of the dosing regimens reached the effective plasma trough concentration of fluconazole; however, simulation study found the recommended dose.

CONCLUSIONS

Continuous hemodiafiltration is highly effective in removing fluconazole from circulation. We recommend fluconazole to be dosed at 500-600 mg intravenously every 12 h in patients receiving hemodiafiltration. This dosing regimen resulted in adequate trough plasma levels for systemic fungal infection.

Pharmacokinetic imaging: a noninvasive method for determining drug distribution and action

Advances in positron emission tomography (PET), single photon emission computed tomography (SPECT) and magnetic resonance spectroscopy (MRS), and the ability to label a wide variety of compounds for in vivo use in humans, have created a new technology for making precise physiological and pharmacological measurements. Due to the noninvasive nature of these approaches, repetitive and/or continuous measurements have become possible. Thus far, these techniques have been primarily used for one-time assessments of individuals. However, experience suggests that a major use of this technology will be in the evaluation of new drug therapies. Already, these techniques have been used to measure precisely and noninvasively the pharmacokinetics of a variety of antimicrobial, antineoplastic and CNS agents. In the case of CNS drugs, imaging techniques (particularly PET) have been used to define the classes of neuroreceptors with which the drug interacts. The physiological, pharmacological and biochemical measurements that can be performed noninvasively using modern imaging techniques can greatly facilitate the evaluation of new therapies. These measurements are most likely to be useful during drug development in preclinical studies and in phase I/II human studies. Preclinically, new drugs can be precisely compared with standard therapies, or a series of analogues can be screened for further development on the basis of performance in animal models. In Phase I/II, imaging measurements can be combined with classical pharmacokinetic data to establish optimal administration schedules, evaluate the utility of interventions in specific clinical situations, and aid in the design of Phase III trials.

Expression of ubiquitin gene in Microsporum canis and Trichophyton mentagrophytes cultured with fluconazole

The expression of the ubiquitin (Ub) gene in dermatophytes was examined for its relation to resistance against the antifungal drug fluconazole. The nucleotide sequences and the deduced amino acid sequences of the Ub gene in Microsporum canis were proven to be 99% similar to those of the Ub gene in Trichophyton mentagrophytes. Expression of mRNA of Ub in M. canis and T. mentagrophytes was enhanced when the fungi were cultured with fluconazole. The antifungal activity of fluconazole against these dermatophytes was increased in the presence of Ub proteasome inhibitor.

In vivo pharmacokinetics and pharmacodynamics in drug development using positron-emission tomography

Positron-emission tomography (PET) is a sensitive technique that can be used to measure drug pharmacokinetics and pharmacodynamics non-invasively in target tissues of patients. Here we focus on the application of this technology to address some of the bottlenecks in drug development, including: elucidation of pathophysiology, evaluation of pharmacokinetics, proof of principle of mechanism, and assessment of efficacy and/or response to therapy.

Serum levels of fluconazole in patients after cytotoxic chemotherapy for hematological malignancy

We performed a prospective evaluation of pharmacokinetics of fluconazole administered for prophylactic purposes to 19 patients after cytotoxic chemotherapy for hematological malignancies. On days 7 and 15, we obtained 5 ml of blood from each patient. If fluconazole was administered orally, blood samples were drawn 2, 8, and 24 hr after ingestion of the drug. If it was administered intravenously, blood samples were drawn 1, 8, and 24 hr post-injection. Serum fluconazole levels were analyzed by HPLC with ultraviolet light detection. In patients receiving 200 or 400 mg of fluconazole per day, maximal serum levels were 7.9 and 15.6 mg/l and minimum levels were 5.0 and 10.3 mg/l, respectively. There was no significant difference in serum fluconazole levels comparing the levels after oral and intravenous administration, and pharmacokinetic parameters of fluconazole were comparable at each time point within one dose level. However, considerable variation in serum fluconazole levels was noted in this study, as the maximal serum levels ranged from 4.0 to 13.3 mg/l and from 8.7 to 26.9 mg/l in patients receiving 200 and 400 mg of fluconazole orally, respectively. These variations may be associated with prophylactic failures for patients with insufficient fluconazole concentrations. Multiple regression analysis showed significant correlation between serum fluconazole levels and some variables including dose of fluconazole, age, serum aspartate aminotransferase levels and blood urea nitrogen levels. These variations may be associated with disturbance of body water balance, such as massive hemorrhage and dehydration.

Fluconazole penetration into the pancreas

Because of antibiotic prophylaxis for necrotizing pancreatitis, the frequency of fungal superinfection in patients with pancreatic necrosis is increasing. In this study we analyzed the penetration of fluconazole into the human pancreas and in experimental acute pancreatitis. In human pancreatic tissues, the mean fluconazole concentration was 8.19 +/- 3.38 microg/g (96% of the corresponding concentration in serum). In experimental edematous and necrotizing pancreatitis, 88 and 91% of the serum fluconazole concentration was found in the pancreas. These data show that fluconazole penetration into the pancreas is sufficient to prevent and/or treat fungal contamination in patients with pancreatic necrosis.

In vivo monitoring of drugs using radiotracer techniques

There is an increasing realization of the role of non-invasive monitoring of drug pharmacology. In this review, we discuss the role of positron emission tomography in such monitoring of tumour and normal tissue drug pharmacokinetics as well as assessment of tumour response, drug-receptor interactions and mechanisms of drug action and resistance. These studies represent a multidisciplinary research effort involving radiochemists, imaging scientists, clinicians, pharmacologists and mathematical modellers. This review evaluates achievements in the field from assessment of commonly used therapeutic agents such as 5-fluorouracil to target specific molecules such as markers for gene expression. It is envisaged that application of this technology will facilitate rational drug design and rapid translation of new ideas to the bedside.

The penetration of anti-infectives into the central nervous system

The blood-brain barrier, blood-cerebrospinal fluid (CSF) barrier, and meninges are a complex and difficult-to-study system charged with protecting the central nervous system (CNS) from toxins, including drugs. Current estimates of CNS drug exposure are limited to CSF to blood ratios, of which area-under-the curve (AUC) estimates provide the most robust measure of drug exposure. Different classes of drugs and individual drugs within classes have different CNS penetration potential that is dependent upon a variety of biologic and pharmacologic factors. Clinical data (AUC and point ratios) regarding the penetration of several anti-infective agents used for the treatment of CNS infections are provided in this article.

Fluconazole, with or without dexamethasone for experimental cryptococcosis: impact of treatment timing

The time of initiation of fluconazole treatment with or without dexamethasone, and the impact on mycological outcome and drug pharmacokinetics were assessed in a murine model of disseminated cryptococcosis. Non-infected mice and mice with disseminated cryptococcosis were given saline, dexamethasone, or fluconazole +/- dexamethasone, 1 or 8 days after infection. Cfus were counted in tissues, and fluconazole concentrations were determined in plasma and tissues by HPLC and a bioassay. Despite fluconazole tissue and plasma concentrations which were above the minimal inhibitory concentration, the numbers of cfus in brain and lung tissues were reduced after early (P = 0.002 and 0.04, respectively), but not after late fluconazole treatment. The administration of dexamethasone did not have a deleterious effect on the number of cfus, fluconazole pharmacokinetics or antifungal activity. In conclusion, the size of the fungal burden influences the effective level of fluconazole activity in lung and brain. These results strongly suggest that potential antifungal agents should be studied following both early and late administration in experimental cryptococcosis.

Pharmacokinetics of [18F]trovafloxacin in healthy human subjects studied with positron emission tomography

Tissue pharmacokinetics of trovafloxacin, a new broad-spectrum fluoroquinolone antimicrobial agent, were measured by positron emission tomography (PET) with [18F]trovafloxacin in 16 healthy volunteers (12 men and 4 women). Each subject received a single oral dose of trovafloxacin (200 mg) daily beginning 5 to 8 days before the PET measurements. Approximately 2 h after the final oral dose, the subject was positioned in the gantry of the PET camera, and 1 h later 10 to 20 mCi of [18F]trovafloxacin was infused intravenously over 1 to 2 min. Serial PET images and blood samples were collected for 6 to 8 h, starting at the initiation of the infusion. Drug concentrations were expressed as the percentage of injected dose per gram, and absolute concentrations were estimated by assuming complete absorption of the final oral dose. In most tissues, there was rapid accumulation of the radiolabeled drug, with high levels achieved within 10 min after tracer infusion. Peak concentrations of more than five times the MIC at which 90% of the isolates are inhibited (MIC90) for most members of Enterobacteriaceae and anaerobes (>10-fold for most organisms) were achieved in virtually all tissues, and the concentrations remained above this level for more than 6 to 8 h. Particularly high peak concentrations (micrograms per gram; mean +/- standard error of the mean [SEM]) were achieved in the liver (35.06 +/- 5.89), pancreas (32.36 +/- 20. 18), kidney (27.20 +/- 10.68), lung (22.51 +/- 7.11), and spleen (21. 77 +/- 11.33). Plateau concentrations (measured at 2 to 8 h; micrograms per gram; mean +/- SEM) were 3.25 +/- 0.43 in the myocardium, 7.23 +/- 0.95 in the lung, 11.29 +/- 0.75 in the liver, 9.50 +/- 2.72 in the pancreas, 4.74 +/- 0.54 in the spleen, 1.32 +/- 0.09 in the bowel, 4.42 +/- 0.32 in the kidney, 1.51 +/- 0.15 in the bone, 2.46 +/- 0.17 in the muscle, 4.94 +/- 1.17 in the prostate, and 3.27 +/- 0.49 in the uterus. In the brain, the concentrations (peak, approximately 2.63 +/- 1.49 microg/g; plateau, approximately 0.91 +/- 0.15 microg/g) exceeded the MIC90s for such common causes of central nervous system infections as Streptococcus pneumoniae (MIC90, <0.2 microg/ml), Neisseria meningitidis (MIC90, <0.008 microg/ml), and Haemophilus influenzae (MIC90, <0.03 microg/ml). These PET results suggest that trovafloxacin will be useful in the treatment of a broad range of infections at diverse anatomic sites.