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

Rapid Purification and Formulation of Radiopharmaceuticals via Thin-Layer Chromatography

Before formulating radiopharmaceuticals for injection, it is necessary to remove various impurities via purification. Conventional synthesis methods involve relatively large quantities of reagents, requiring high-resolution and high-capacity chromatographic methods (e.g., semi-preparative radio-HPLC) to ensure adequate purity of the radiopharmaceutical. Due to the use of organic solvents during purification, additional processing is needed to reformulate the radiopharmaceutical into an injectable buffer. Recent developments in microscale radiosynthesis have made it possible to synthesize radiopharmaceuticals with vastly reduced reagent masses, minimizing impurities. This enables purification with lower-capacity methods, such as analytical HPLC, with a reduction of purification time and volume (that shortens downstream re-formulation). Still, the need for a bulky and expensive HPLC system undermines many of the advantages of microfluidics. This study demonstrates the feasibility of using radio-TLC for the purification of radiopharmaceuticals. This technique combines high-performance (high-resolution, high-speed separation) with the advantages of a compact and low-cost setup. A further advantage is that no downstream re-formulation step is needed. Production and purification of clinical scale batches of [¹⁸F]PBR-06 and [¹⁸F]Fallypride are demonstrated with high yield, purity, and specific activity. Automating this radio-TLC method could provide an attractive solution for the purification step in microscale radiochemistry systems.

Factors Controlling Degradation of Biologically Relevant Synthetic Polymers in Solution and Solid State

The field of biodegradable synthetic polymers, which is central for regenerative engineering and drug delivery applications, encompasses a multitude of hydrolytically sensitive macromolecular structures and diverse processing approaches. The ideal degradation behavior for a specific life science application must comply with a set of requirements, which include a clinically relevant kinetic profile, adequate biocompatibility, benign degradation products, and controlled structural evolution. Although significant advances have been made in tailoring materials characteristics to satisfy these requirements, the impacts of autocatalytic reactions and microenvironments are often overlooked resulting in uncontrollable and unpredictable outcomes. Therefore, roles of surface versus bulk erosion, in situ microenvironment, and autocatalytic mechanisms should be understood to enable rational design of degradable systems. In an attempt to individually evaluate the physical state and form factors influencing autocatalytic hydrolysis of degradable polymers, this Review follows a hierarchical analysis that starts with hydrolytic degradation of water-soluble polymers before building up to 2D-like materials, such as ultrathin coatings and capsules, and then to solid-state degradation. We argue that chemical reactivity largely governs solution degradation while diffusivity and geometry control the degradation of bulk materials, with thin "2D" materials remaining largely unexplored. Following this classification, this Review explores techniques to analyze degradation in vitro and in vivo and summarizes recent advances toward understanding degradation behavior for traditional and innovative polymer systems. Finally, we highlight challenges encountered in analytical methodology and standardization of results and provide perspective on the future trends in the development of biodegradable polymers.

PET Molecular Imaging in Drug Development: The Imaging and Chemistry Perspective

Positron emission tomography with selective radioligands advances the drug discovery and development process by revealing information about target engagement, proof of mechanism, pharmacokinetic and pharmacodynamic profiles. Positron emission tomography (PET) is an essential and highly significant tool to study therapeutic drug development, dose regimen, and the drug plasma concentrations of new drug candidates. Selective radioligands bring up target-specific information in several disease states including cancer, cardiovascular, and neurological conditions by quantifying various rates of biological processes with PET, which are associated with its physiological changes in living subjects, thus it reveals disease progression and also advances the clinical investigation. This study explores the major roles, applications, and advances of PET molecular imaging in drug discovery and development process with a wide range of radiochemistry as well as clinical outcomes of positron-emitting carbon-11 and fluorine-18 radiotracers.

Economical Production of Radiopharmaceuticals for Preclinical Imaging Using Microdroplet Radiochemistry

The short-lived radiolabeled "tracers" needed for performing whole body imaging in animals or patients with positron-emission tomography (PET) are generally produced via automated "radiosynthesizers". Most current radiosynthesizers are designed for routine production of relatively large clinical batches and are very wasteful when only a small batch of a tracer is needed, such as is the case for preclinical in vivo PET imaging studies. To overcome the prohibitively high cost of producing small batches of PET tracers, we developed a droplet microreactor system that performs radiochemistry at the 1-10μL scale instead of the milliliter scale of conventional technologies. The overall yield for the droplet-based production of many PET tracers is comparable to conventional approaches, but 10-100× less reagents are consumed, the synthesis can be completed in much less time (<30 min), and only a small laboratory footprint and minimal radiation shielding are needed. By combining these advantages, droplet microreactors enable the economical production of small batches PET tracers on demand. Here, we describe the fabrication method of the droplet microreactor and the droplet-based synthesis of an example radiotracer ([¹⁸F]fallypride).

Nuclear Medicine in Times of COVID-19: How Radiopharmaceuticals Could Help to Fight the Current and Future Pandemics

The emergence and global spread of COVID-19, an infectious disease caused by the novel coronavirus SARS-CoV-2, has resulted in a continuing pandemic threat to global health. Nuclear medicine techniques can be used for functional imaging of (patho)physiological processes at the cellular or molecular level and for treatment approaches based on targeted delivery of therapeutic radionuclides. Ongoing development of radiolabeling methods has significantly improved the accessibility of radiopharmaceuticals for in vivo molecular imaging or targeted radionuclide therapy, but their use for biosafety threats such as SARS-CoV-2 is restricted by the contagious nature of these agents. Here, we highlight several potential uses of nuclear medicine in the context of SARS-CoV-2 and COVID-19, many of which could also be performed in laboratories without dedicated containment measures. In addition, we provide a broad overview of experimental or repurposed SARS-CoV-2-targeting drugs and describe how radiolabeled analogs of these compounds could facilitate antiviral drug development and translation to the clinic, reduce the incidence of late-stage failures and possibly provide the basis for radionuclide-based treatment strategies. Based on the continuing threat by emerging coronaviruses and other pathogens, it is anticipated that these applications of nuclear medicine will become a more important part of future antiviral drug development and treatment.

An original radio-biomimetic approach to synthesize radiometabolites for PET imaging

To fully exploit the potential of positron emission tomography (PET) imaging to assess drug distribution and pharmacokinetics in the central nervous system, the contribution of radiometabolites to the PET signal has to be determined for correct interpretation of data. However, radiosynthesis and extensive study of radiometabolites are rarely investigated and very challenging for complex drugs. Therefore, an original radio-biomimetic (RBM) approach was developed to rapidly synthesize radiometabolites and non-invasively investigate their kinetics with PET imaging. This method enabled the challenging radiosynthesis of [11C]nor-buprenorphine ([11C]nor-BUP), the main metabolite of buprenorphine (BUP) which has been identified as a substrate of the P-glycoprotein (P-gp) transport function at the blood-brain barrier (BBB). Biomimetic conditions using cytochromes P450 3A4 to convert BUP into nor-BUP were optimized taking into account the short half-life of carbon-11 (t1/2 = 20.4 min). Those conditions afforded 32% of conversion within 20 min and were applied to the biomimetic radiosynthesis of [11C]nor-BUP from [11C]BUP. Automated radiosynthesis of [11C]BUP according to a procedure described in the literature followed by optimized RBM conditions afforded [11C]nor-BUP in 1.5% decay-corrected radiochemical yield within 90 min and 90 ± 15 GBq/μmol molar activity. HPLC quality control showed chemical and radiochemical purities above 98%. To demonstrate the applicability of the RBM approach to preclinical studies, brain PET images in rats showed a drastic lower uptake of [11C]nor-BUP (0.067 ± 0.023%ID/cm-3) compared to [11C]BUP (0.436 ± 0.054%ID/cm-3). P-gp inhibition using Tariquidar increased the brain uptake of [11C]nor-BUP (0.557 ± 0.077%ID/cm-3).

Rapid, efficient, and economical synthesis of PET tracers in a droplet microreactor: application to O-(2-[18F]fluoroethyl)-L-tyrosine ([18F]FET)

BACKGROUND

Conventional scale production of small batches of PET tracers (e.g. for preclinical imaging) is an inefficient use of resources. Using O-(2-[18F]fluoroethyl)-L-tyrosine ([18F]FET), we demonstrate that simple microvolume radiosynthesis techniques can improve the efficiency of production by consuming tiny amounts of precursor, and maintaining high molar activity of the tracers even with low starting activity.

PROCEDURES

The synthesis was carried out in microvolume droplets manipulated on a disposable patterned silicon "chip" affixed to a heater. A droplet of [18F]fluoride containing TBAHCO3 was first deposited onto a chip and dried at 100 °C. Subsequently, a droplet containing 60 nmol of precursor was added to the chip and the fluorination reaction was performed at 90 °C for 5 min. Removal of protecting groups was accomplished with a droplet of HCl heated at 90 °C for 3 min. Finally, the crude product was collected in a methanol-water mixture, purified via analytical-scale radio-HPLC and formulated in saline. As a demonstration, using [18F]FET produced on the chip, we prepared aliquots with different molar activities to explore the impact on preclinical PET imaging of tumor-bearing mice.

RESULTS

The microdroplet synthesis exhibited an overall decay-corrected radiochemical yield of 55 ± 7% (n = 4) after purification and formulation. When automated, the synthesis could be completed in 35 min. Starting with < 370 MBq of activity, ~ 150 MBq of [18F]FET could be produced, sufficient for multiple in vivo experiments, with high molar activities (48-119 GBq/μmol). The demonstration imaging study revealed the uptake of [18F]FET in subcutaneous tumors, but no significant differences in tumor uptake as a result of molar activity differences (ranging 0.37-48 GBq/μmol) were observed.

CONCLUSIONS

A microdroplet synthesis of [18F]FET was developed demonstrating low reagent consumption, high yield, and high molar activity. The approach can be expanded to tracers other than [18F]FET, and adapted to produce higher quantities of the tracer sufficient for clinical PET imaging.

Radiosynthesis and PET Bioimaging of 76Br-Bedaquiline in a Murine Model of Tuberculosis

Bedaquiline is a promising drug against tuberculosis (TB), but limited data are available on its intralesional pharmacokinetics. Moreover, current techniques rely on invasive tissue resection, which is difficult in humans and generally limited even in animals. In this study, we developed a novel radiosynthesis for 76Br-bedaquiline and performed noninvasive, longitudinal whole-body positron emission tomography (PET) in live, Mycobacterium tuberculosis-infected mice over 48 h. After the intravenous injection, 76Br-bedaquiline distributed to all organs and selectively localized to adipose tissue and liver, with excellent penetration into infected lung lesions (86%) and measurable penetration into the brain parenchyma (15%). Ex vivo high resolution, two-dimensional autoradiography, and same section hematoxylin/eosin and immunofluorescence provided detailed intralesional drug biodistribution. PET bioimaging and high-resolution autoradiography are novel techniques that can provide detailed, multicompartment, and intralesional pharmacokinetics of new and existing TB drugs. These technologies can significantly advance efforts to optimize drug dosing.

Use of Molecular Imaging in Clinical Drug Development: a Systematic Review

Background

Molecular imaging such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) can provide the crucial pharmacokinetic-pharmacodynamic information of a drug non-invasively at an early stage of clinical drug development. Nevertheless, not much has been known how molecular imaging has been actually used in drug development studies.

Methods

We searched PubMed using such keywords as molecular imaging, PET, SPECT, drug development, and new drug, or any combination of those to select papers in English, published from January 1, 1990, to December 31, 2015. The information about the publication year, therapeutic area of a drug candidate, drug development phase, and imaging modality and utility of imaging were extracted.

Results

Of 10,264 papers initially screened, 208 papers met the eligibility criteria. The more recent the publication year, the bigger the number of papers, particularly since 2010. The two major therapeutic areas using molecular imaging to develop drugs were oncology (47.6%) and the central nervous system (CNS, 36.5%), in which efficacy (63.5%) and proof-of-concept through either receptor occupancy (RO) or other than RO (29.7%), respectively, were the primary utility of molecular imaging. PET was used 4.7 times more frequently than SPECT. Molecular imaging was most frequently used in phase I clinical trials (40.8%), whereas it was employed rarely in phase 0 or exploratory IND studies (1.4%).

Conclusions

The present study confirmed the trend that molecular imaging has been more actively employed in recent clinical drug development studies although its adoption was rather slow and rare in phase 0 studies.

Direct Incorporation of [11C]CO2 into Asymmetric [11C]Carbonates

A novel carbon-11 radiolabelling methodology for the synthesis of the dialkylcarbonate functional group has been developed. The method uses cyclotron-produced short-lived [¹¹C]CO₂ (half-life 20.4 min) directly from the cyclotron target in a one-pot synthesis. Alcohol in the presence of base trapped [¹¹C]CO₂ efficiently forming an [¹¹C]alkylcarbonate intermediate that subsequently reacted with an alkylchloride producing the di-substituted [¹¹C]carbonate (34% radiochemical yield, determined by radio-HPLC) in 5 minutes from the end of [¹¹C]CO₂ cyclotron delivery.

Technologies for Measuring Pharmacokinetic Profiles

The creation of a pharmacokinetic (PK) curve, which follows the plasma concentration of an administered drug as a function of time, is a critical aspect of the drug development process and includes such information as the drug's bioavailability, clearance, and elimination half-life. Prior to a drug of interest gaining clearance for use in human clinical trials, research is performed during the preclinical stages to establish drug safety and dosing metrics from data obtained from the PK studies. Both in vivo animal models and in vitro platforms have limitations in predicting human reaction to a drug due to differences in species and associated simplifications, respectively. As a result, in silico experiments using computer simulation have been implemented to accurately predict PK parameters in human studies. This review assesses these three approaches (in vitro, in vivo, and in silico) when establishing PK parameters and evaluates the potential for in silico studies to be the future gold standard of PK preclinical studies.

Early Detection of Aβ Deposition in the 5xFAD Mouse by Amyloid PET

Purpose. ¹⁸F-FC119S is a positron emission tomography (PET) tracer for imaging β-amyloid (Aβ) plaques in Alzheimer's disease (AD). The aim of this study is to evaluate the efficacy of ¹⁸F-FC119S in quantitating Aβ deposition in a mouse model of early amyloid deposition (5xFAD) by PET. Method. Dynamic ¹⁸F-FC119S PET images were obtained in 5xFAD (n = 5) and wild-type (WT) mice (n = 7). The brain PET images were spatially normalized to the M. Mirrione T2-weighted mouse brain MR template, and the volumes of interest were then automatically drawn on the cortex, hippocampus, thalamus, and cerebellum. The specific binding of ¹⁸F-FC119S to Aβ was quantified as the distribution volume ratio using Logan graphical analysis with the cerebellum as a reference tissue. The Aβ levels in the brain were also confirmed by immunohistochemical analysis. Result. For the 5xFAD group, radioactivity levels in the cortex, the hippocampus, and the thalamus were higher than those for the WT group. In these regions, specific binding was approximately 1.2-fold higher in 5xFAD mice than in WT. Immunohistochemistry supported these findings; the 5xFAD showed severe Aβ deposition in the cortex and hippocampus in contrast to the WT group. Conclusion. These results demonstrated that ¹⁸F-FC119S PET can successfully distinguish Aβ depositions in 5xFAD mice from WT.

Automatic concentration and reformulation of PET tracers via microfluidic membrane distillation

Short-lived radiolabeled tracers for positron emission tomography (PET) must be rapidly synthesized, purified, and formulated into injectable solution just prior to imaging. Current radiosynthesizers are generally designed for clinical use, and the HPLC purification and SPE formulation processes often result in a final volume that is too large for preclinical and emerging in vitro applications. Conventional technologies and techniques for reducing this volume tend to be slow, resulting in radioactive decay of the product, and often require manual handling of the radioactive materials. We present a fully-automated microfluidic system based on sweeping gas membrane distillation to rapidly perform the concentration and formulation process. After detailed characterization of the system, we demonstrate fast and efficient concentration and formulation of several PET tracers, evaluate residual solvent content to establish the safety of the formulated tracers for injection, and show that the formulated tracer can be used for in vivo imaging.

Use of Accelerator Mass Spectrometry in Human Health and Molecular Toxicology

Accelerator mass spectrometry (AMS) has been adopted as a powerful bioanalytical method for human studies in the areas of pharmacology and toxicology. The exquisite sensitivity (10-18 mol) of AMS has facilitated studies of toxins and drugs at environmentally and physiologically relevant concentrations in humans. Such studies include risk assessment of environmental toxicants, drug candidate selection, absolute bioavailability determination, and more recently, assessment of drug-target binding as a biomarker of response to chemotherapy. Combining AMS with complementary capabilities such as high performance liquid chromatography (HPLC) can maximize data within a single experiment and provide additional insight when assessing drugs and toxins, such as metabolic profiling. Recent advances in the AMS technology at Lawrence Livermore National Laboratory have allowed for direct coupling of AMS with complementary capabilities such as HPLC via a liquid sample moving wire interface, offering greater sensitivity compared to that of graphite-based analysis, therefore enabling the use of lower 14C and chemical doses, which are imperative for clinical testing. The aim of this review is to highlight the recent efforts in human studies using AMS, including technological advancements and discussion of the continued promise of AMS for innovative clinical based research.

Quantitating drug-target engagement in single cells in vitro and in vivo

Quantitation of drug target engagement in single cells has proven to be difficult, often leaving unanswered questions in the drug development process. We found that intracellular target engagement of unlabeled new therapeutics can be quantitated using polarized microscopy combined with competitive binding of matched fluorescent companion imaging probes. We quantitated the dynamics of target engagement of covalent BTK inhibitors, as well as reversible PARP inhibitors, in populations of single cells using a single companion imaging probe for each target. We then determined average in vivo tumor concentrations and found marked population heterogeneity following systemic delivery, revealing single cells with low target occupancy at high average target engagement in vivo.

Pharmacokinetics of gene recombined angiogenesis inhibitor Kringle 5 in vivo using 131I specific markers and SPECT/CT

The previous pharmacokinetic methods can be only limited to drug analysis in vitro, which provide less information on the distribution and metabolismof drugs, and limit the interpretation and assessment of pharmacokinetics, the determination of metabolic principles, and evaluation of treatment effect. The objective of the study was to investigate the pharmacokinetic characteristics of gene recombination angiogenesis inhibitor Kringle 5 in vivo. The SPECT/CT and specific ¹³¹I-Kringle 5 marked by Iodogen method were both applied to explore the pharmacokinetic characteristics of ¹³¹I-Kringle 5 in vivo, and to investigate the dynamic distributions of ¹³¹I-Kringle 5 in target organs. Labeling recombinant angiogenesis inhibitor Kringle 5 using ¹³¹I with longer half-life and imaging in vivo using SPECT instead of PET, could overcome the limitations of previous methods. When the doses of ¹³¹I-Kringle 5 were 10.0, 7.5 and 5.0 g/kg, respectively, the two-compartment open models can be determined within all the metabolic process in vivo. There were no significant differences in t_1/2α, t_1/2β, apparent volume of distribution and CL between those three levels. The ratio of AUC_(0~∞) among three different groups of 10.0, 7.5 and 5.0 g/kg was 2.56:1.44:1.0, which was close to the ratio (2:1.5:1.0). It could be clear that in the range of 5.0-10.0 g/kg, Kringle 5 was characterized by the first-order pharmacokinetics. Approximately 30 min after ¹³¹I-Kringle 5 was injected, ¹³¹I-Kringle 5 could be observed to concentrate in the heart, kidneys, liver and other organs by means of planar imaging and tomography. After 1 h of being injected, more radionuclide retained in the bladder, but not in intestinal. It could be concluded that ¹³¹I-Kringle 5 is mainly excreted through the kidneys. About 2 h after the injection of ¹³¹I-Kringle 5, the radionuclide in the heart, kidneys, liver and other organs was gradually reduced, while more radionuclide was concentrated in the bladder. The radionuclide was completely metabolized within 24 h, and the distribution of radioactivity in rats was similar to normal levels. In our study, the specific marker ¹³¹I-Kringle 5 and SPECT/CT were successfully used to explore pharmacokinetic characteristics of Kringle 5 in rats. The study could provide a new evaluation platform of the specific, in vivo and real-time functional imaging and pharmacokinetics for the clinical application of ¹³¹I-Kringle 5.

Whole-body direct 4D parametric PET imaging employing nested generalized Patlak expectation-maximization reconstruction

Whole-body (WB) dynamic PET has recently demonstrated its potential in translating the quantitative benefits of parametric imaging to the clinic. Post-reconstruction standard Patlak (sPatlak) WB graphical analysis utilizes multi-bed multi-pass PET acquisition to produce quantitative WB images of the tracer influx rate K i as a complimentary metric to the semi-quantitative standardized uptake value (SUV). The resulting K i images may suffer from high noise due to the need for short acquisition frames. Meanwhile, a generalized Patlak (gPatlak) WB post-reconstruction method had been suggested to limit K i bias of sPatlak analysis at regions with non-negligible (18)F-FDG uptake reversibility; however, gPatlak analysis is non-linear and thus can further amplify noise. In the present study, we implemented, within the open-source software for tomographic image reconstruction platform, a clinically adoptable 4D WB reconstruction framework enabling efficient estimation of sPatlak and gPatlak images directly from dynamic multi-bed PET raw data with substantial noise reduction. Furthermore, we employed the optimization transfer methodology to accelerate 4D expectation-maximization (EM) convergence by nesting the fast image-based estimation of Patlak parameters within each iteration cycle of the slower projection-based estimation of dynamic PET images. The novel gPatlak 4D method was initialized from an optimized set of sPatlak ML-EM iterations to facilitate EM convergence. Initially, realistic simulations were conducted utilizing published (18)F-FDG kinetic parameters coupled with the XCAT phantom. Quantitative analyses illustrated enhanced K i target-to-background ratio (TBR) and especially contrast-to-noise ratio (CNR) performance for the 4D versus the indirect methods and static SUV. Furthermore, considerable convergence acceleration was observed for the nested algorithms involving 10-20 sub-iterations. Moreover, systematic reduction in K i % bias and improved TBR were observed for gPatlak versus sPatlak. Finally, validation on clinical WB dynamic data demonstrated the clinical feasibility and superior K i CNR performance for the proposed 4D framework compared to indirect Patlak and SUV imaging.

Fluorine-18 Radiolabeled PET Tracers for Imaging Monoamine Transporters: Dopamine, Serotonin, and Norepinephrine

This review focuses on the development of fluorine-18 radiolabeled PET tracers for imaging the dopamine transporter (DAT), serotonin transporter (SERT), and norepinephrine transporter (NET). All successful DAT PET tracers reported to date are members of the 3β-phenyl tropane class and are synthesized from cocaine. Currently available carbon-11 SERT PET tracers come from both the diphenylsulfide and 3β-phenyl nortropane class, but so far only the nortropanes have found success with fluorine-18 derivatives. NET imaging has so far employed carbon-11 and fluorine-18 derivatives of reboxetine but due to defluorination of the fluorine-18 derivatives further research is still necessary.

(18)F-Labeling of Arenes and Heteroarenes for Applications in Positron Emission Tomography

Diverse radiochemistry is an essential component of nuclear medicine; this includes imaging techniques such as positron emission tomography (PET). As such, PET can track diseases at an early stage of development, help patient care planning through personalized medicine and support drug discovery programs. Fluorine-18 is the most frequently used radioisotope in PET radiopharmaceuticals for both clinical and preclinical research. Its physical and nuclear characteristics (97% β(+) decay, 109.8 min half-life, 635 keV positron energy) and high specific activity make it an attractive nuclide for labeling and molecular imaging. Arenes and heteroarenes are privileged candidates for (18)F-incorporation as they are metabolically robust and therefore widely used by medicinal chemists and radiochemists alike. For many years, the range of (hetero)arenes amenable to (18)F-fluorination was limited by the lack of chemically diverse precursors, and of radiochemical methods allowing (18)F-incorporation in high selectivity and efficiency (radiochemical yield and purity, specific activity, and radio-scalability). The appearance of late-stage fluorination reactions catalyzed by transition metal or small organic molecules (organocatalysis) has encouraged much research on the use of these activation manifolds for (18)F-fluorination. In this piece, we review all of the reactions known to date to install the (18)F substituent and other key (18)F-motifs (e.g., CF3, CHF2, OCF3, SCF3, OCHF2) of medicinal relevance onto (hetero)arenes. The field has changed significantly in the past five years, and the current trend suggests that the radiochemical space available for PET applications will expand rapidly in the near future.

Synthesis, F-18 radiolabeling, and microPET evaluation of 3-(2,4-dichlorophenyl)-N-alkyl-N-fluoroalkyl-2,5-dimethylpyrazolo[1,5-a]pyrimidin-7-amines as ligands of the corticotropin-releasing factor type-1 (CRF1) receptor

A series of 3-(2,4-dichlorophenyl)-N-alkyl-N-fluoroalkyl-2,5-dimethylpyrazolo[1,5-a]pyrimidin-7-amines were synthesized and evaluated as potential positron emission tomography (PET) tracers for the corticotropin-releasing factor type-1 (CRF1) receptor. Compounds 27, 28, 29, and 30 all displayed high binding affinity (⩽1.2 nM) to the CRF1 receptor when assessed by in vitro competition binding assays at 23 °C, whereas a decrease in affinity (⩾10-fold) was observed with compound 26. The logP7.4 values of [(18)F]26-[(18)F]29 were in the range of ∼2.2-2.8 and microPET evaluation of [(18)F]26-[(18)F]29 in an anesthetized male cynomolgus monkey demonstrated brain penetrance, but specific binding was not sufficient enough to differentiate regions of high CRF1 receptor density from regions of low CRF1 receptor density. Radioactivity uptake in the skull, and sphenoid bone and/or sphenoid sinus during studies with [(18)F]28, [(18)F]28-d8, and [(18)F]29 was attributed to a combination of [(18)F]fluoride generated by metabolic defluorination of the radiotracer and binding of intact radiotracer to CRF1 receptors expressed on mast cells in the bone marrow. Uptake of [(18)F]26 and [(18)F]27 in the skull and sphenoid region was rapid but then steadily washed out which suggests that this behavior was the result of binding to CRF1 receptors expressed on mast cells in the bone marrow with no contribution from [(18)F]fluoride.

In Vitro Testing for Orally Inhaled Products: Developments in Science-Based Regulatory Approaches

This article is part of a series of reports from the "Orlando Inhalation Conference-Approaches in International Regulation" which was held in March 2014, and coorganized by the University of Florida and the International Pharmaceutical Aerosol Consortium on Regulation and Science (IPAC-RS). The goal of the conference was to foster the exchange of ideas and knowledge across the global scientific and regulatory community in order to identify and help move towards strategies for internationally harmonized, science-based regulatory approaches for the development and marketing approval of inhalation medicines, including innovator and second entry products. This article provides an integrated perspective of case studies and discussion related to in vitro testing of orally inhaled products, including in vitro-in vivo correlations and requirements for in vitro data and statistical analysis that support quality or bioequivalence for regulatory applications.

Non linear mixed effects analysis in PET PK-receptor occupancy studies

The characterisation of a pharmacokinetic-receptor occupancy (PK-RO) relationship derived from a PET study is typically modelled in a conventional non-linear least squares (NLLS) framework. In the present work, we explore the application of a non-linear mixed effects approach (NLME) and compare this with NLLS estimation (using both naive pooled data and two-stage approaches) in the context of a direct PK-RO relationship described by an Emax model, using simulated data sets. Target and reference tissue time-activity curves were simulated using a two-tissue compartmental model and an arterial plasma input function for a typical PET study (12 subjects in 3 dose groups with 3 scans each). A range of different PET scenarios was considered to evaluate the impact of between-subject variability and reference region availability. The PET outcome measures derived from the simulations were then used to estimate the parameters of the PK-RO model. The performance of the two approaches was compared in terms of parameters estimates (square mean error SME, root mean square error RMSE) and prediction of the exposure-occupancy relationship. In general, both NLME and NLLS estimation methods provided unbiassed and precise population estimates for the Emax model parameters, although a slight bias was observed for the individual-NLLS method due to a few outliers. The increased value of NLME over NLLS was most notable in the estimation of the between-subject variability (BSV), especially in the case of a more complex PK-RO model when no reference region was available (maximum SME and RMSE values related to BSV of EC₅₀ of 27.6% and 86.5% from NLME versus 264.6% and 689.5% from NLLS). Overall, the NLME approach provided a more robust estimation and produced less-biassed estimates of the population means and variances than either the NLLS approach for the simulations considered.

Molecular medicine: a path towards a personalized medicine

Psychiatric disorders are among the most common human illnesses; still, the molecular and cellular mechanisms underlying their complex pathophysiology remain to be fully elucidated. Over the past 10 years, our group has been investigating the molecular abnormalities in major signaling pathways involved in psychiatric disorders. Recent evidences obtained by the Instituto Nacional de Ciência e Tecnologia de Medicina Molecular (National Institute of Science and Technology - Molecular Medicine, INCT-MM) and others using behavioral analysis of animal models provided valuable insights into the underlying molecular alterations responsible for many complex neuropsychiatric disorders, suggesting that "defects" in critical intracellular signaling pathways have an important role in regulating neurodevelopment, as well as in pathophysiology and treatment efficacy. Resources from the INCT have allowed us to start doing research in the field of molecular imaging. Molecular imaging is a research discipline that visualizes, characterizes, and quantifies the biologic processes taking place at cellular and molecular levels in humans and other living systems through the results of image within the reality of the physiological environment. In order to recognize targets, molecular imaging applies specific instruments (e.g., PET) that enable visualization and quantification in space and in real-time of signals from molecular imaging agents. The objective of molecular medicine is to individualize treatment and improve patient care. Thus, molecular imaging is an additional tool to achieve our ultimate goal.

Liposome imaging agents in personalized medicine

In recent years the importance of molecular and diagnostic imaging has increased dramatically in the treatment planning of many diseases and in particular in cancer therapy. Within nanomedicine there are particularly interesting possibilities for combining imaging and therapy. Engineered liposomes that selectively localize in tumor tissue can transport both drugs and imaging agents, which allows for a theranostic approach with great potential in personalized medicine. Radiolabeling of liposomes have for many years been used in preclinical studies for evaluating liposome in vivo performance and has been an important tool in the development of liposomal drugs. However, advanced imaging systems now provide new possibilities for non-invasive monitoring of liposome biodistribution in humans. Thus, advances in imaging and developments in liposome radiolabeling techniques allow us to enter a new arena where we start to consider how to use imaging for patient selection and treatment monitoring in connection to nanocarrier based medicines. Nanocarrier imaging agents could furthermore have interesting properties for disease diagnostics and staging. Here, we review the major advances in the development of radiolabeled liposomes for imaging as a tool in personalized medicine.

Dynamic in vivo imaging of receptors in small animals using positron emission tomography

Positron emission tomography (PET) is a functional imaging technique with the potential to image and quantify receptors in vivo with high sensitivity. PET has been used extensively to study major neurotransmitters such as dopamine, serotonin, and benzodiazepine in humans as well as proving to be a very powerful tool to accelerate development and assessment of existing and novel drugs. With the recent development of dedicated PET scanners for small animals, such as the microPET, it is now possible to perform functional imaging in small animals such as rodents at high resolution. This will allow the study of animal models of disease and longitudinal studies in these models to monitor disease progression or effect of treatment in the same animal. Furthermore, the complete pharmacokinetics of a drug as well as pharmacodynamic information can be obtained in a single animal. Thus, small animal imaging will significantly reduce the number of animals needed for this type of experiment as well as reducing the effect of inter-animal variation. Experimental protocols in small animal imaging potentially can be very labor intensive. In this chapter, we discuss methods and practical aspects related to this type of experiment using the microPET system.

Phase 0 - Microdosing strategy in clinical trials

Drug development is an activity that is long, complex and expensive. In 2004, attrition in the drug development paradigm prompted the US Food and Drug Administration (FDA) to introduce its ‘Critical Path’ document, which highlighted the serious discordance between major scientific advances and limited drug development process. One issue addressed was that of microdosing. The concept of microdosing involves the use of extremely low, nonpharmacologically active doses of a drug to define the pharmacokinetic profile of the medication in human subjects. Microdosing, thus, appears as a new viable concept in the ‘toolbox’ of the drug development activity. It appears that microdosing strategy could complement standard animal-to-human scaling, redefining the existing concept of phase I clinical research. In future, when research methods and technology involved in Phase 0 studies become more sophisticated, human microdosing may be applied to a number of drugs developed subsequently.

Challenges in assessing regional distribution of inhaled drug in the human lungs

INTRODUCTION

Both the total amount of drug deposited in the lungs (whole lung deposition) and the amount deposited in different lung regions (regional lung deposition) are potentially important factors that determine the safety and efficacy of inhaled drugs. Radionuclide imaging is well established for quantifying the whole lung deposition of inhaled drugs, but the assessment of regional lung deposition is less straightforward, because of the complex nature of the lung anatomy.

AREAS COVERED

This review describes the challenges and problems associated with quantifying regional lung deposition by the two-dimensional (2D) radionuclide imaging method of gamma scintigraphy, and by the three-dimensional (3D) radionuclide imaging methods of single-photon-emission computed tomography (SPECT) and positron-emission tomography (PET). The advantages and disadvantages of each method for assessing regional lung deposition are discussed.

EXPERT OPINION

Owing to its 2D nature, gamma scintigraphy provides limited information about regional lung deposition. SPECT provides regional lung deposition data in three dimensions, but usually involves a (99m)Tc radiolabel. PET enables the regional lung deposition of radiolabeled drug molecules to be quantified in three dimensions, but poses the greatest logistical and technical difficulties. Despite their more challenging nature, 3D imaging methods should be considered as an alternative to gamma scintigraphy whenever the determination of regional lung deposition of pharmaceutical aerosols is a major study objective.

Translational pharmacokinetics: challenges of an emerging approach to drug development in stroke

INTRODUCTION

There is increasing recognition of the importance of translational pharmacokinetics in stroke research, lack of which has been cited as one of the main contributing factors to failure of Phase III trials.

AREAS COVERED

The article reviews the translational issues in administration, distribution and sampling in the pharmacokinetics of putative therapeutic drugs in stroke. In addition, the role of translational pharmacometrics in drug development is discussed. The review uses the anti-inflammatory agent, IL-1 receptor antagonist, as an example. The reader will gain an insight into the pitfalls that are commonplace in translating pharmacokinetics from the preclinical to the clinical scenario. The reader will also gain an understanding of the complexities of blood-central nervous system (CNS) barriers in relation to brain pharmacokinetics and the increasing use of translational pharmacometrics in stroke research.

EXPERT OPINION

The translation of preclinical to clinical pharmacokinetics is a discipline that is traditionally overlooked and is likely to be a key factor responsible for failure of clinical trials. With a clear comprehensive insight into the benefits and limitations of translational pharmacokinetics in stroke, translational pharmacokinetics can be safely used to enhance the efficacy of clinical trials in stroke and their likelihood of success.

Food and drug administration's critical path initiative and innovations in drug development paradigm: Challenges, progress, and controversies

During the last decade, despite increased investment in drug research and development related activity, stagnation in new drug discovery has been documented. Despite a 70% increase in investment in research and development-related activities, a 40% fall in launch of new chemical entities was seen during 1994-2004. A steep rise in the attrition rate of drug development has complicated the matter. Rising cost and increased attrition rates proved major barriers to investment in higher risk drugs or in therapies for uncommon diseases or diseases that predominantly afflict the poor. This prompted Food and Drug Administration (FDA) to highlight this problem in a 2004 white paper classified as "Critical Path Initiative" (CPI) and to initiate steps to target stagnation and rise in attrition rates. Many new drug development projects have started worldwide taking cue from CPI; adopting microdosing, adaptive designs and taking advantage of newly developed biomarkers under the CPI. This review discusses the various strategies adopted under CPI to decrease attrition rate and stagnation of new drug development, and the challenges and controversies associated with CPI.

Exploring new routes for neuroprotective drug development in traumatic brain injury

Worldwide, traumatic brain injury (TBI) is a major cause of mortality and morbidity with a substantial predicted increase in incidence. Despite an obvious need, there are no pharmacological treatment options for TBI because translation of neuroprotection from preclinical studies to clinical practice has so far failed. Here, we identify potential causes for this failure. We suggest that the monitoring and investigation tools that are commonly used in patients with TBI may provide an experimental medicine route to facilitate a more rational approach to translational research. This suggestion is underpinned by existing research data on disease biology, drug delivery, and treatment response obtained with these methods.

Synthesis, radiosynthesis, and biological evaluation of fluorine-18-labeled 2beta-carbo(fluoroalkoxy)-3beta-(3'-((Z)-2-haloethenyl)phenyl)nortropanes: candidate radioligands for in vivo imaging of the serotonin transporter with positron emission tomography

The meta-vinylhalide fluoroalkyl ester nortropanes 1-4 were synthesized as ligands of the serotonin transporter (SERT) for use as positron emission tomography (PET) imaging agents. In vitro competition binding assays demonstrated that 1-4 have a high affinity for the SERT (K(i) values = 0.3-0.4 nM) and are selective for the SERT over the dopamine and norepinephrine transporters (DAT and NET). MicroPET imaging in anesthetized cynomolgus monkeys with [(18)F]1-[(18)F]4 demonstrated that all four tracers behave similarly with peak uptake in the SERT-rich brain regions achieved after 45-55 min, followed by a steady washout. An awake monkey study was performed with [(18)F]1, which demonstrated that the uptake of [(18)F]1 was not influenced by anesthesia. Chase studies with the SERT ligand 15 displaced [(18)F]1-[(18)F]4, but chase studies with the DAT ligand 16 did not displace [(18)F]1-[(18)F]4 thus indicating that the tracers were binding specifically to the SERT.

Synthesis and Bioimaging of Positron-Emitting15O-Labeled 2-Deoxy-D-glucose of Two-Minute Half-Life

In positron emission tomography (PET), which exploits the affinity of a radiopharmaceutical for the target organ, a systematic repertoire of oxygen-15-labeled PET tracers is expected to be useful for bioimaging owing to the ubiquity of oxygen atoms in organic compounds. However, because of the 2-min half-life of 15O, the synthesis of complex biologically active 15O-labeled organic molecules has not yet been achieved. A state-of-the-art synthesis now makes available an 15O-labeled complex organic molecule, 6-[15O]-2-deoxy-D-glucose. Ultrarapid radical hydroxylation of 2,6-dideoxy-6-iodo-D-glucose with molecular oxygen labeled with 15O of two-minute half-life provided the target 15O-labeled molecule. The labeling reaction with 15O was complete in 1.3 min, and the entire operation time starting from the generation of 15O-containing dioxygen by a cyclotron to the purification of the labeled sugar was 7 min. The labeled sugar accumulated in the metabolically active organs as well as in the bladder of mice and rats. 15O-labeling offers the possibility of repetitive scanning and the use of multiple PET tracers in the same body within a short time, and hence should significantly expand the scope of PET studies of small animals.

Elevation of radiolabelled thymidine uptake in RIF-1 fibrosarcoma and HT29 colon adenocarcinoma cells after treatment with thymidylate synthase inhibitors

PURPOSE

We recently showed an increase in tumour uptake of 2-[(11)C]thymidine in patients with gastrointestinal malignancies after thymidylate synthase (TS) inhibition. To understand the phenomenon in more detail, we investigated whether TS inhibition by different TS inhibitors leads to a dose- and time-dependent change in the uptake of radiolabelled thymidine, and whether radiotracer uptake is related to changes in cell viability resulting from treatment.

METHODS

RIF-1 and HT29 cells were treated with the TS inhibitors 5-fluorouracil (5-FU) and AG337 (nolatrexed dihydrochloride), as well as cisplatin as control. The cell viability and net accumulation of [(3)H]thymidine after a 1-h pulse was determined at different times after drug treatment.

RESULTS

In both cell lines, [(3)H]thymidine uptake increased after a 2-h treatment with 5-FU, in a dose- and time-dependent manner. [(3)H]thymidine uptake decreased at 24 and 48 h post treatment. AG337 also produced a similar effect. In contrast to the TS inhibitors, cisplatin decreased [(3)H]thymidine uptake in RIF-1 and HT29 cells at all time points. Cell viability was compromised only after 24 h.

CONCLUSION

Using two types of TS inhibitor, we have shown an increase in [(3)H]thymidine uptake, in a dose-dependent manner, a few hours after TS inhibition when the cell viability was not compromised. This effect was not seen with a non-TS inhibitor. These findings suggest that 2-[(11)C]thymidine positron emission tomography can be used to study TS inhibition in vivo at early time points when cell viability is not compromised and may therefore be helpful in the development of new TS inhibitors and in differentiating between patients with tumours sensitive to TS inhibitors and those unlikely to respond.