Positron emission tomography microdosing: a new concept with application in tracer and early clinical drug development

Contribution of Clinical Neuroimaging to the Understanding of the Pharmacology of Methylphenidate

Methylphenidate (MPH) is currently the most widely used molecule in the pharmacologic treatment of attention-deficit hyperactivity disorder (ADHD). Although experience of its application now extends over several decades, its psychotropic nature, prolonged use in children, and chemical relation to amphetamines still raise doubts in the minds of prescribers and the families of the patients. Brain imaging has shed considerable light on the neuropharmacology of MPH. The two main in vivo neuroimaging techniques are positron-emission tomography (PET) and magnetic resonance imaging (MRI), and these can be applied in both animal models and humans. The present review seeks to show how human molecular and functional imaging has contributed to determining not only the molecular targets of MPH, and the action kinetics of the various pharmaceutical forms available, but also the connectivity and brain networks activated by treatment. We also discuss the perspectives opened up by new hybrid PET-MRI techniques that enable multimodal tracking of the impact of methylphenidate on neurotransmission.

High-efficiency γ-ray flash generation via multiple-laser scattering in ponderomotive potential well

γ-ray flash generation in near-critical-density target irradiated by four symmetrical colliding laser pulses is numerically investigated. With peak intensities about 10^{23} W/cm^{2}, the laser pulses boost electron energy through direct laser acceleration, while pushing them inward with the ponderomotive force. After backscattering with counterpropagating laser, the accelerated electron is trapped in the electromagnetic standing waves or the ponderomotive potential well created by the coherent overlapping of the laser pulses, and emits γ-ray photons in a multiple-laser-scattering regime, where electrons act as a medium transferring energy from the laser to γ rays in the ponderomotive potential valley.

Tactics for preclinical validation of receptor-binding radiotracers

INTRODUCTION

Aspects of radiopharmaceutical development are illustrated through preclinical studies of [125I]-(E)-1-(2-(2,3-dihydrobenzofuran-5-yl)ethyl)-4-(iodoallyl)piperazine ([125I]-E-IA-BF-PE-PIPZE), a radioligand for sigma-1 (σ1) receptors, coupled with examples from the recent literature. Findings are compared to those previously observed for [125I]-(E)-1-(2-(2,3-dimethoxy-5-yl)ethyl)-4-(iodoallyl)piperazine ([125I]-E-IA-DM-PE-PIPZE).

METHODS

Syntheses of E-IA-BF-PE-PIPZE and [125I]-E-IA-BF-PE-PIPZE were accomplished by standard methods. In vitro receptor binding studies and autoradiography were performed, and binding potential was predicted. Measurements of lipophilicity and protein binding were obtained. In vivo studies were conducted in mice to evaluate radioligand stability, as well as specific binding to σ1 sites in brain, brain regions and peripheral organs in the presence and absence of potential blockers.

RESULTS

E-IA-BF-PE-PIPZE exhibited high affinity and selectivity for σ1 receptors (Ki = 0.43 ± 0.03 nM, σ2/σ1 = 173). [125I]-E-IA-BF-PE-PIPZE was prepared in good yield and purity, with high specific activity. Radioligand binding provided dissociation (koff) and association (kon) rate constants, along with a measured Kd of 0.24 ± 0.01 nM and Bmax of 472 ± 13 fmol/mg protein. The radioligand proved suitable for quantitative autoradiography in vitro using brain sections. Moderate lipophilicity, Log D7.4 2.69 ± 0.28, was determined, and protein binding was 71 ± 0.3%. In vivo, high initial whole brain uptake, >6% injected dose/g, cleared slowly over 24 h. Specific binding represented 75% to 93% of total binding from 15 min to 24 h. Findings were confirmed and extended by regional brain biodistribution. Radiometabolites were not observed in brain (1%).

CONCLUSIONS

Substitution of dihydrobenzofuranylethyl for dimethoxyphenethyl increased radioligand affinity for σ1 receptors by 16-fold. While high specific binding to σ1 receptors was observed for both radioligands in vivo, [125I]-E-IA-BF-PE-PIPZE displayed much slower clearance kinetics than [125I]-E-IA-DM-PE-PIPZE. Thus, minor structural modifications of σ1 receptor radioligands lead to major differences in binding properties in vitro and in vivo.

Palladium-mediated11C-carbonylations using aryl halides and cyanamide

A rapid, efficient and high-yielding synthesis of¹¹C-cyanobenzamides, including novel analogs of various drug molecules, is described.

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.

Imaging Mass Spectrometry Revealed the Accumulation Characteristics of the 2-Nitroimidazole-Based Agent "Pimonidazole" in Hypoxia

Hypoxia, or low oxygen concentration, is a key factor promoting tumor progression and angiogenesis and resistance of cancer to radiotherapy and chemotherapy. 2-Nitroimidazole-based agents have been widely used in pathological and nuclear medicine examinations to detect hypoxic regions in tumors; in particular, pimonidazole is used for histochemical staining of hypoxic regions. It is considered to accumulate in hypoxic cells via covalent binding with macromolecules or by forming reductive metabolites after reduction of its nitro group. However, the detailed mechanism of its accumulation remains unknown. In this study, we investigated the accumulation mechanism of pimonidazole in hypoxic tumor tissues in a mouse model by mass spectrometric analyses including imaging mass spectrometry (IMS). Pimonidazole and its reductive metabolites were observed in the tumor tissues. However, their locations in the tumor sections were not similar to the positively stained areas in pimonidazole-immunohistochemistry, an area considered hypoxic. The glutathione conjugate of reduced pimonidazole, a low-molecular-weight metabolite of pimonidazole, was found in tumor tissues by LC-MS analysis, and our IMS study determined that the intratumor localization of the glutathione conjugate was consistent with the area positively immunostained for pimonidazole. We also found complementary localization of the glutathione conjugate and reduced glutathione (GSH), implying that formation of the glutathione conjugate occurred in the tumor tissue. These results suggest that in hypoxic tumor cells, pimonidazole is reduced at its nitro group, followed by conjugation with GSH.

Strategies towards in vivo imaging of active transglutaminase type 2 using positron emission tomography

Transglutaminase type 2 (TG2) is increasingly linked to the pathogenesis of several diseases, such as celiac disease, cancer, and fibrotic and neurodegenerative diseases. In parallel with becoming an attractive target for therapy, interest in the development of compounds for in vivo imaging of TG2 is rising. Such imaging biomarkers might assist in clarifying the role of TG2 in pathology and in monitoring TG2 inhibition in vivo and thus assist in drug development. In this review, the latest results together with various strategies in TG2 PET tracer development are discussed, including radiolabelling of irreversible and reversible active-site inhibitors, as well as allosteric inhibitors, acyl-donor and acyl-acceptor substrates, and anti-TG2 monoclonal antibodies.

Asymmetric 18F-fluorination for applications in positron emission tomography

Positron emission tomography (PET) is becoming more frequently used by medicinal chemists to facilitate the selection of the most promising lead compounds for further evaluation. For PET, this entails the preparation of 11C- or 18F-labeled drugs or radioligands. With the importance of chirality and fluorine substitution in drug development, chemists can be faced with the challenge of preparing enantiopure molecules featuring the 18F-tag on a stereogenic carbon. Asymmetric 18F-fluorination is an emerging field of research that provides an alternative to resolution or conventional SN2-based radiochemistry. To date, both transition metal complexes and organomediators have been successfully employed for 18F-incorporation at a stereogenic carbon.

A New Highly Reactive and Low Lipophilicity Fluorine-18 Labeled Tetrazine Derivative for Pretargeted PET Imaging

A new (18)F-labeled tetrazine derivative was developed aiming at optimal radiochemistry, fast reaction kinetics in inverse electron-demand Diels-Alder cycloaddition (IEDDA), and favorable pharmacokinetics for in vivo bioorthogonal chemistry. The radiolabeling of the tetrazine was achieved in high yield, purity, and specific activity under mild reaction conditions via conjugation with 5-[(18)F]fluoro-5-deoxyribose, providing a glycosylated tetrazine derivative with low lipophilicity. The (18)F-tetrazine showed fast reaction kinetics toward the most commonly used dienophiles in IEDDA reactions. It exhibited excellent chemical and enzymatic stability in mouse plasma and in phosphate-buffered saline (pH 7.41). Biodistribution in mice revealed favorable pharmacokinetics with major elimination via urinary excretion. The results indicate that the glycosylated (18)F-labeled tetrazine is an excellent candidate for in vivo bioorthogonal chemistry applications in pretargeted PET imaging approaches.

Development of a complementary PET/MR dual-modal imaging probe for targeting prostate-specific membrane antigen (PSMA)

We tried to develop a dual-modal PET/MR imaging probe using a straightforward one-pot method by encapsulation with specific amphiphiles. In this study, iron oxide (IO) nanoparticles were encapsulated with three amphiphiles containing PEG, DOTA and the prostate-specific membrane antigen (PSMA)-targeting ligand in aqueous medium. The diameter of the prepared nanoparticle DOTA-IO-GUL was 11.01±1.54nm. DOTA-IO-GUL was labeled with (68)Ga in high efficiency. The DOTA-IO-GUL showed a dose-dependent binding to LNCaP (PSMA positive) cells via a competitive binding study against (125)I-labeled MIP-1072 (PSMA-targeting agent). Additionally, PET and MR imaging results showed PSMA selective uptake by only 22Rv1 (PSMA positive) but not PC-3 (PSMA negative) in dual-tumor xenograft mouse model study. MR imaging showed high resolution, and PET imaging enabled quantification and confirmation of the specificity. In conclusion, we have successfully developed the specific PSMA-targeting IO nanoparticle, DOTA-IO-GUL, as a dual-modality probe for complementary PET/MR imaging.

FROM THE CLINICAL EDITOR

The combination of using Positron Emission Tomography (PET) and computed tomography (CT) in clinical practice is now the norm. With advances in technology, the next step would be to develop combined PET and Magnetic Resonance (MR) dual-imaging. In this article, the authors described their positive study on the development of a dual-modal PET/MR imaging probe using a prostate cancer model.

Intraarterial Microdosing: A Novel Drug Development Approach, Proof-of-Concept PET Study in Rats

Intraarterial microdosing (IAM) is a novel drug development approach combining intraarterial drug delivery and microdosing. We aimed to demonstrate that IAM leads to target exposure similar to that of systemic full-dose administration but with minimal systemic exposure. IAM could enable the safe, inexpensive, and early study of novel drugs at the first-in-human stage and the study of established drugs in vulnerable populations.

METHODS

Insulin was administered intraarterially (ipsilateral femoral artery) or systemically to 8 CD IGS rats just before blood sampling or 60-min (18)F-FDG uptake PET imaging of ipsilateral and contralateral leg muscles (lateral gastrocnemius) and systemic muscles (spinotrapezius). The (18)F-FDG uptake slope analysis was used to compare the interventions. Plasma levels of insulin and glucose were compared using area under the curve calculated by the linear trapezoidal method. A physiologically based computational pharmacokinetics/pharmacodynamics model was constructed to simulate the relationship between the administered dose and response over time.

RESULTS

(18)F-FDG slope analysis found no difference between IAM and systemic full-dose slopes (0.0066 and 0.0061, respectively; 95% confidence interval [CI], -0.024 to 0.029; P = 0.7895), but IAM slope was statistically significantly greater than systemic microdose (0.0018; 95% CI, -0.045 to -0.007; P = 0.0147) and sham intervention (-0.0015; 95% CI, 0.023-0.058; P = 0.0052). The pharmacokinetics/pharmacodynamics data were used to identify model parameters that describe membrane insulin binding and glucose-insulin dynamics.

CONCLUSION

Target exposure after IAM was similar to systemic full dose administration but with minimal systemic effects. The computational pharmacokinetics/pharmacodynamics model can be generalized to predict whole-body response. Findings should be validated in larger, controlled studies in animals and humans using a range of targets and classes of drugs.

A Systemic Review on Aloe arborescens Pharmacological Profile: Biological Activities and Pilot Clinical Trials

Since ancient times, plants and herbal preparations have been used as medicine. Research carried out in the last few decades has verified several such claims. Aloe arborescens Miller, belonging to the Aloe genus (Family Asphodelaceae), is one of the main varieties of Aloe used worldwide. The popularity of the plant in traditional medicine for several ailments (antitumor, immunomodulatory, antiinflammatory, antiulcer, antimicrobial and antifungal activity) focused the investigator's interest on this plant. Most importantly, the reported studies have shown the plant effectiveness on various cancer types such as liver, colon, duodenal, skin, pancreatic, intestinal, lung and kidney types. These multiple biological actions make Aloe an important resource for developing new natural therapies. However, the biological activities of isolated compounds such as glycoprotein, polysaccharides, enzyme and phenolics were insufficient. Considering all these, this contribution provides a systematic review outlining the evidence on the biological efficacy of the plant including the pharmacology and the related mechanisms of action, with specific attention to the various safety precautions, and preclinical and clinical studies, indicating the future research prospects of this plant.

PET tracers for serotonin receptors and their applications

Serotonin receptors (5-HTRs) are implicated in the pathophysiology of a variety of neuropsychiatric and neurodegenerative disorders and are also targets for drug therapy. In the CNS, most of these receptors are expressed in high abundance in specific brain regions reflecting their role in brain functions. Quantifying binding to 5-HTRs in vivo may permit assessment of physiologic and pathologic conditions, and monitoring disease progression, evaluating treatment response, and for investigating new treatment modalities. Positron emission tomography (PET) molecular imaging has the sensitivity to quantify binding of 5-HTRs in CNS disorders and to measure drug occupancy as part of a process of new drug development. Although research on PET imaging of 5-HTRs have been performed more than two decades, the successful radiotracers so far developed for human studies are limited to 5-HT₁AR, 5-HT₁BR, 5-HT₂AR, 5-HT₄R and 5-HT₆R. Herein we review the development and application of radioligands for PET imaging of 5-HTRs in living brain.

68Ga-Based radiopharmaceuticals: production and application relationship

The contribution of ⁶⁸Ga to the promotion and expansion of clinical research and routine positron emission tomography (PET) for earlier better diagnostics and individualized medicine is considerable. The potential applications of ⁶⁸Ga-comprising imaging agents include targeted, pre-targeted and non-targeted imaging. This review discusses the key aspects of the production of ⁶⁸Ga and ⁶⁸Ga-based radiopharmaceuticals in the light of the impact of regulatory requirements and endpoint pre-clinical and clinical applications.

Synthesis of ([(11)C]carbonyl)raclopride and a comparison with ([(11)C]methyl)raclopride in a monkey PET study

INTRODUCTION

The selective dopamine D2 receptor antagonist raclopride is usually labeled with carbon-11 using [(11)C]methyl iodide or [(11)C]methyl triflate for use in the quantification of dopamine D2 receptors in human brain. The aim of this work was to label raclopride at the carbonyl position using [(11)C]carbon monoxide chemistry and to compare ([(11)C]carbonyl)raclopride with ([(11)C]methyl)raclopride in non-human primate (NHP) using PET with regard to quantitative outcome measurement, metabolism of the labeled tracers and protein binding.

METHODS

Palladium-mediated carbonylation using [(11)C]carbon monoxide, 4,6-dichloro-2-iodo-3-methoxyphenol and (S)-(-)-2-aminomethyl-1-ethylpyrrolidine was applied in the synthesis of ([(11)C]carbonyl)raclopride. The reaction was performed at atmospheric pressure using xantphos as supporting phosphine ligand and palladium (π-cinnamyl) chloride dimer as the palladium source. ([(11)C]Methyl)raclopride was prepared by a previously published method. In the PET study, two female cynomolgus monkeys were used under gas anesthesia of sevoflurane. A dynamic PET measurement was performed for 63 min with an HRRT PET camera after intravenous injection of ([(11)C]carbonyl)raclopride and ([(11)C]methyl)raclopride, respectively, during the same day. The order of injection of the two PET radioligands was changed between the two monkeys. The venous blood sample for measurement of protein binding was taken 3 min prior to the PET scan. Binding potential (BPND) of the putamen and caudate was calculated with SRTM using the cerebellum as a reference region.

RESULTS

The target compound ([(11)C]carbonyl)raclopride was obtained with 50 ± 5% decay corrected radiochemical yield and 95% radiochemical purity. The trapping efficiency (TE) of [(11)C]carbon monoxide was 65 ± 5% and the specific radioactivity of the final product was 34 ± 1 GBq/μmol after a 50 min of synthesis time. The radiochemical yield of ([(11)C]methyl)raclopride was in the same range as published previously i. e. 50-60% and specific radioactivity of those two batches which were used in the present PET study were 192 GBq/μmol and 638 GBq/μmol respectively after a synthesis time of 32 min. In monkey PET studies, the percentage difference of BPND in putamen was <3% and that in caudate was <9% for the two radioligands. The plasma protein binding was 86.2 ± 0.3% and 85.7 ± 0.6% for ([(11)C]carbonyl)raclopride and ([(11)C]methyl)raclopride, respectively. The radiometabolite pattern was similar for both radioligands.

CONCLUSION

Raclopride was (11)C-labeled at the carbonyl position using a palladium-mediated [(11)C]carbonylation reaction. A comparison between ([(11)C]carbonyl)raclopride and ([(11)C]methyl)raclopride with regard to quantitative PET outcome measurements, metabolism of radioligands and protein binding in monkey was performed. The monkey PET study with ([(11)C]carbonyl)raclopride showed similar results as for ([(11)C]methyl)raclopride. The PET studies were performed on 2 subjects.

Improved synthesis and application of [(11) C]benzyl iodide in positron emission tomography radiotracer production

Positron emission tomography has increased the demand for new carbon-11 radiolabeled tracers and building blocks. A promising radiolabeling synthon is [(11) C]benzyl iodide ([(11) C]BnI), because the benzyl group is a widely present functionality in biologically active compounds. Unfortunately, synthesis of [(11) C]BnI has received little attention, resulting in limited application. Therefore, we investigated the synthesis in order to significantly improve, automate, and apply it for labeling of the dopamine D2 antagonist [(11) C]clebopride as a proof of concept. [(11) C]BnI was synthesized from [(11) C]CO2 via a Grignard reaction and purified prior the reaction with desbenzyl clebopride. According to a one-pot procedure, [(11) C]BnI was synthesized in 11 min from [(11) C]CO2 with high yield, purity, and specific activity, 52 ± 3% (end of the cyclotron bombardment), 95 ± 3%, and 123 ± 17 GBq/µmol (end of the synthesis), respectively. Changes in the [(11) C]BnI synthesis are reduced amounts of reagents, a lower temperature in the Grignard reaction, and the introduction of a solid-phase intermediate purification. [(11) C]Clebopride was synthesized within 28 min from [(11) C]CO2 in an isolated decay-corrected yield of 11 ± 3% (end of the cyclotron bombardment) with a purity of >98% and specific activity (SA) of 54 ± 4 GBq/µmol (n = 3) at the end of the synthesis. Conversion of [(11) C]BnI to product was 82 ± 11%. The reliable synthesis of [(11) C]BnI allows the broad application of this synthon in positron emission tomography radiopharmaceutical development.

Dihydroquinoline Carbamate Derivatives as "Bio-oxidizable" Prodrugs for Brain Delivery of Acetylcholinesterase Inhibitors: [¹¹C] Radiosynthesis and Biological Evaluation

With the aim of improving the efficiency of marketed acetylcholinesterase (AChE) inhibitors in the symptomatic treatment of Alzheimer's disease, plagued by adverse effects arising from peripheral cholinergic activation, this work reports a biological evaluation of new central AChE inhibitors based on an original "bio-oxidizable" prodrug strategy. After peripheral injection of the prodrug 1a [IC50 > 1 mM (hAChE)] in mice, monitoring markers of central and peripheral cholinergic activation provided in vivo proof-of-concept for brain delivery of the drug 2a [IC50 = 20 nM (hAChE)] through central redox activation of 1a. Interestingly, peripheral cholinergic activation has been shown to be limited in time, likely due to the presence of a permanent positive charge in 2a promoting rapid elimination of the AChE inhibitor from the circulation of mice. To support these assumptions, the radiosynthesis with carbon-11 of prodrug 1a was developed for additional ex vivo studies in rats. Whole-body biodistribution of radioactivity revealed high accumulation in excretory organs along with moderate but rapid brain uptake. Radio-HPLC analyses of brain samples confirm rapid CNS penetration of [(11)C]1a, while identification of [(11)C]2a and [(11)C]3a both accounts for central redox activation of 1a and pseudoirreversible inhibition of AChE, respectively. Finally, Caco-2 permeability assays predicted metabolite 3a as a substrate for efflux transporters (P-gp inter alia), suggesting that metabolite 3a might possibly be actively transported out of the brain. Overall, a large body of evidence from in vivo and ex vivo studies on small animals has been collected to validate this "bio-oxidizable" prodrug approach, emerging as a very promising strategy in the rational design of selective central AChE inhibitors.

In Vivo Tissue Pharmacokinetics of Carbon-11-Labeled Clozapine in Healthy Volunteers: A Positron Emission Tomography Study

We investigated clozapine (CLZ) tissue pharmacokinetics in vivo by using carbon-11-labeled CLZ (¹¹C-CLZ) and positron emission tomography (PET). Eight healthy volunteers underwent ¹¹C-CLZ studies wherein computed tomography image acquisition was followed by PET scans (whole-body, four; brain, four). After bolus intravenous ¹¹C-CLZ injection, PET images were acquired at various timepoints for 2–3 hours. Tissue ¹¹C-CLZ signals were plotted over time, and pharmacokinetic parameters were determined. High ¹¹C-CLZ radioactivity was detected in the liver and brain, implying CLZ hepatic metabolism and efficient blood–brain barrier penetration. The urinary and hepatobiliary tracts were involved in ¹¹C-CLZ excretion. Moderate to high radioactivity was observed in the dopaminergic and serotonergic receptor-rich brain regions, indicating CLZ binding to multiple receptor types. To our knowledge, this is the first study to report the determination of ¹¹C-CLZ tissue pharmacokinetics in humans. PET using radiolabeled drugs can provide valuable information that could complement plasma pharmacokinetic data.

Noninvasive detection of tumor-infiltrating T cells by PET reporter imaging

Adoptive transfer of tumor-reactive T cells can successfully reduce tumor burden; however, in rare cases, lethal on-target/off-tumor effects have been reported. A noninvasive method to track engineered cells with high sensitivity and resolution would allow observation of correct cell homing and/or identification of dangerous off-target locations in preclinical and clinical applications. Human deoxycytidine kinase triple mutant (hdCK3mut) is a nonimmunogenic PET reporter that was previously shown to be an effective tool to monitor whole-body hematopoiesis. Here, we engineered a construct in which hdCK3mut is coexpressed with the anti-melanoma T cell receptor F5, introduced this construct into human CD34 cells or PBMCs, and evaluated this approach in multiple immunotherapy models. Expression of hdCK3mut allowed engrafted cells to be visualized within recipient bone marrow, while accumulation of [18F]-L-FMAU in hdCK3mut-expressing T cells permitted detection of intratumoral homing. Animals that received T cells coexpressing hdCK3mut and the anti-melanoma T cell receptor had demonstrably higher signals in HLA-matched tumors compared with those in animals that received cells solely expressing hdCK3mut. Engineered T cells caused cytotoxicity in HLA/antigen-matched tumors and induced IFN-γ production and activation. Moreover, hdCK3mut permitted simultaneous monitoring of engraftment and tumor infiltration, without affecting T cell function. Our findings suggest that hdCK3mut reporter imaging can be applied in clinical immunotherapies for whole-body detection of engineered cell locations.

Large Variation in Brain Exposure of Reference CNS Drugs: a PET Study in Nonhuman Primates

BACKGROUND

Positron emission tomography microdosing of radiolabeled drugs allows for noninvasive studies of organ exposure in vivo. The aim of the present study was to examine and compare the brain exposure of 12 commercially available CNS drugs and one non-CNS drug.

METHODS

The drugs were radiolabeled with (11)C (t 1/2 = 20.4 minutes) and examined using a high resolution research tomograph. In cynomolgus monkeys, each drug was examined twice. In rhesus monkeys, a first positron emission tomography microdosing measurement was repeated after preadministration with unlabeled drug to examine potential dose-dependent effects on brain exposure. Partition coefficients between brain and plasma (KP) were calculated by dividing the AUC0-90 min for brain with that for plasma or by a compartmental analysis (VT). Unbound KP (KP u,u) was obtained by correction for the free fraction in brain and plasma.

RESULTS

After intravenous injection, the maximum radioactivity concentration (C max, %ID) in brain ranged from 0.01% to 6.2%. For 10 of the 12 CNS drugs, C max, %ID was >2%, indicating a preferential distribution to brain. A lower C max, %ID was observed for morphine, sulpiride, and verapamil. K P ranged from 0.002 (sulpiride) to 68 (sertraline) and 7 of 13 drugs had KP u,u close to unity. For morphine, sulpiride, and verapamil, K P u,u was <0.3, indicating impaired diffusion and/or active efflux. Brain exposure at microdosing agreed with pharmacological dosing conditions for the investigated drugs.

CONCLUSIONS

This study represents the largest positron emission tomography study on brain exposure of commercially available CNS drugs in nonhuman primates and may guide interpretation of positron emission tomography microdosing data for novel drug candidates.

Positron emission tomography to elucidate pharmacokinetic differences of regioisomeric retinoid x receptor agonists

RXR partial agonist NEt-4IB (2a, 6-[ethyl-(4-isobutoxy-3-isopropylphenyl)amino]pyridine-3-carboxylic acid: EC50 = 169 nM, E max = 55%) showed a blood concentration higher than its E max after single oral administration at 30 mg/kg to mice, and repeated oral administration at 10 mg/kg/day to KK-A(y) mice afforded antitype 2 diabetes activity without the side effects caused by RXR full agonists. However, RXR full agonist NEt-3IB (1a), in which the isobutoxy and isopropyl groups of 2a are interchanged, gave a much lower blood concentration than 2a. Here we used positron emission tomography (PET) with tracers [(11)C]1a, [(11)C]2a and fluorinated derivatives [(18)F]1b, [(18)F]2b, which have longer half-lives, to examine the reason why 1a and 2a exhibited significantly different blood concentrations. As a result, the reason for the high blood concentration of 2a after oral administration was found to be linked to higher intestinal absorbability together with lower biliary excretion, compared with 1a.

Development of rapid multistep carbon-11 radiosynthesis of the myeloperoxidase inhibitor AZD3241 to assess brain exposure by PET microdosing

INTRODUCTION

The myeloperoxidase inhibitor AZD3241 has been selected as a candidate drug currently being developed to delay progression in patients with neurodegenerative brain disorders. Part of the decision tree for translation of AZD3241 into clinical studies included the need for assessment of brain exposure in non-human primates by PET microdosing. For that purpose a rapid multistep method for (11)C-labeling of AZD3241 was developed.

METHODS

AZD3241 was labeled in the thio-carbonyl position starting from [(11)C]potassium cyanide in a 4-step procedure using microwave assisted heating. In the first step [(11)C]potassium cyanide was converted to [(11)C]potassium thiocyanate followed by reaction with benzoyl chloride to yield benzoyl [(11)C]isothiocyanate. The benzoyl [(11)C]isothiocyanate was subsequently reacted with the precursor ethyl 3-(2-isopropoxyethylamino)-1H-pyrrole-2-carboxylate and the formed intermediate underwent a base catalyzed cyclization to obtain [(11)C]AZD3241 in the final step. To assess [(11)C]AZD3241 brain exposure PET measurements were performed in three cynomolgus monkeys.

RESULTS

[(11)C]AZD3241 was produced in good and reproducible radiochemical yield 710 ± 294 MBq (mean ± SD, n = 7). Total time of synthesis was 60 min from end of bombardment. The specific radioactivity was 9 ± 4GBq/μmol and the radiochemical purity was >98%. Following iv administration of [(11)C]AZD3241 there was a rapid presence of radioactivity in brain in each of the three monkeys. The distribution of [(11)C]AZD3241 to brain was fast and a Cmax of 1.9 to 2.6% of the injected radioactivity was observed within 1.5 min. [(11)C]AZD3241 was homogeneously distributed in brain.

CONCLUSION

The MPO inhibitor AZD3241 was successfully labeled with carbon-11 in a challenging 4-step procedure in good radiochemical yield allowing PET microdosing studies in cynomolgus monkey. [(11)C]AZD3241 rapidly entered brain and confirmed adequate brain exposure to support translation of AZD3241 to phase 2a studies in patients.

Application of cross-species PET imaging to assess neurotransmitter release in brain

RATIONALE

This review attempts to summarize the current status in relation to the use of positron emission tomography (PET) imaging in the assessment of synaptic concentrations of endogenous mediators in the living brain.

OBJECTIVES

Although PET radioligands are now available for more than 40 CNS targets, at the initiation of the Innovative Medicines Initiative (IMI) "Novel Methods leading to New Medications in Depression and Schizophrenia" (NEWMEDS) in 2009, PET radioligands sensitive to an endogenous neurotransmitter were only validated for dopamine. NEWMEDS work-package 5, "Cross-species and neurochemical imaging (PET) methods for drug discovery", commenced with a focus on developing methods enabling assessment of changes in extracellular concentrations of serotonin and noradrenaline in the brain.

RESULTS

Sharing the workload across institutions, we utilized in vitro techniques with cells and tissues, in vivo receptor binding and microdialysis techniques in rodents, and in vivo PET imaging in non-human primates and humans. Here, we discuss these efforts and review other recently published reports on the use of radioligands to assess changes in endogenous levels of dopamine, serotonin, noradrenaline, γ-aminobutyric acid, glutamate, acetylcholine, and opioid peptides. The emphasis is on assessment of the availability of appropriate translational tools (PET radioligands, pharmacological challenge agents) and on studies in non-human primates and human subjects, as well as current challenges and future directions.

CONCLUSIONS

PET imaging directed at investigating changes in endogenous neurochemicals, including the work done in NEWMEDS, have highlighted an opportunity to further extend the capability and application of this technology in drug development.

Molecular imaging of targeted therapies with positron emission tomography: the visualization of personalized cancer care

INTRODUCTION

Molecular imaging has been defined as the visualization, characterization and measurement of biological processes at the molecular and cellular level in humans and other living systems. In oncology it enables to visualize (part of) the functional behaviour of tumour cells, in contrast to anatomical imaging that focuses on the size and location of malignant lesions. Available molecular imaging techniques include single photon emission computed tomography (SPECT), positron emission tomography (PET) and optical imaging. In PET, a radiotracer consisting of a positron emitting radionuclide attached to the biologically active molecule of interest is administrated to the patient. Several approaches have been undertaken to use PET for the improvement of personalized cancer care. For example, a variety of radiolabelled ligands have been investigated for intratumoural target identification and radiolabelled drugs have been developed for direct visualization of the biodistibution in vivo, including intratumoural therapy uptake. First indications of the clinical value of PET for target identification and response prediction in oncology have been reported. This new imaging approach is rapidly developing, but uniformity of scanning processes, standardized methods for outcome evaluation and implementation in daily clinical practice are still in progress. In this review we discuss the available literature on molecular imaging with PET for personalized targeted treatment strategies.

CONCLUSION

Molecular imaging with radiolabelled targeted anticancer drugs has great potential for the improvement of personalized cancer care. The non-invasive quantification of drug accumulation in tumours and normal tissues provides understanding of the biodistribution in relation to therapeutic and toxic effects.

Distribution of intravenously administered acetylcholinesterase inhibitor and acetylcholinesterase activity in the adrenal gland: 11C-donepezil PET study in the normal rat

Purpose

Acetylcholinesterase (AChE) inhibitors have been used for patients with Alzheimer's disease. However, its pharmacokinetics in non-target organs other than the brain has not been clarified yet. The purpose of this study was to evaluate the relationship between the whole-body distribution of intravenously administered ¹¹C-Donepezil (DNP) and the AChE activity in the normal rat, with special focus on the adrenal glands.

Methods

The distribution of ¹¹C-DNP was investigated by PET/CT in 6 normal male Wistar rats (8 weeks old, body weight  = 220±8.9 g). A 30-min dynamic scan was started simultaneously with an intravenous bolus injection of ¹¹C-DNP (45.0±10.7 MBq). The whole-body distribution of the ¹¹C-DNP PET was evaluated based on the Vt (total distribution volume) by Logan-plot analysis. A fluorometric assay was performed to quantify the AChE activity in homogenized tissue solutions of the major organs.

Results

The PET analysis using Vt showed that the adrenal glands had the 2nd highest level of ¹¹C-DNP in the body (following the liver) (13.33±1.08 and 19.43±1.29 ml/cm³, respectively), indicating that the distribution of ¹¹C-DNP was the highest in the adrenal glands, except for that in the excretory organs. The AChE activity was the third highest in the adrenal glands (following the small intestine and the stomach) (24.9±1.6, 83.1±3.0, and 38.5±8.1 mU/mg, respectively), indicating high activity of AChE in the adrenal glands.

Conclusions

We demonstrated the whole-body distribution of ¹¹C-DNP by PET and the AChE activity in the major organs by fluorometric assay in the normal rat. High accumulation of ¹¹C-DNP was observed in the adrenal glands, which suggested the risk of enhanced cholinergic synaptic transmission by the use of AChE inhibitors.

Why are we failing to implement imaging studies with radiolabelled new molecular entities in early oncology drug development?

In early drug development advanced imaging techniques can help with progressing new molecular entities (NME) to subsequent phases of drug development and thus reduce attrition. However, several organizational, operational, and regulatory hurdles pose a significant barrier, potentially limiting the impact these techniques can have on modern drug development. Positron emission tomography (PET) of radiolabelled NME is arguably the best example of a complex technique with a potential to deliver unique decision-making data in small cohorts of subjects. However, to realise this potential the impediments to timely inclusion of PET into the drug development process must be overcome. In the present paper, we discuss the value of PET imaging with radiolabelled NME during early anticancer drug development, as exemplified with one such NME. We outline the multiple hurdles and propose options on how to streamline the organizational steps for future studies.

Considerations for the clinical use of contrast agents for cellular MRI in regenerative medicine

Advances in regenerative medicine are rapidly transforming healthcare. A cornerstone of regenerative medicine is the introduction of cells that were grown or manipulated in vitro. Key questions that arise after these cells are re-introduced are: whether these cells are localized in the appropriate site; whether cells survive; and whether these cells migrate. These questions predominantly relate to the safety of the therapeutic approach (i.e. tumorigenesis), but certain aspects can also influence the efficacy of the therapeutic approach (e.g. site of injection). The European Medicines Agency has indicated that suitable methods for stem cell tracking should be applied where these methods are available. We here discuss the European regulatory framework, as well as the scientific evidence, that should be considered to facilitate the potential clinical implementation of magnetic resonance imaging contrast media to track implanted/injected cells in human studies.

Compact microfluidic device for rapid concentration of PET tracers

HPLC purification and reformulation of positron emission tomography (PET) tracers can lead to significant dilution of the final product, making it difficult to produce a sufficiently high radioactivity concentration for some applications (e.g. small animal imaging, in vitro assays, and labelling of proteins with prosthetic groups). This is especially true for molecules with lengthy or low-yield syntheses. Starting the synthesis with more radioactivity increases the final radioactivity concentration but increases hazards and complexity of handling. An alternative is to concentrate the final product by a process such as rotary evaporation prior to downstream use. Because a rotovap requires significant space within a hot cell that could be put to more productive use, we developed a compact microfluidic system for concentration of PET tracers. This system also provides advantages in terms of repeatability, interfacing and potential for automation. We present here the design and performance characterization of the system, and demonstrate the concentration of several tracers in aqueous-based HPLC mobile phases.

Using positron emission tomography to study transporter-mediated drug-drug interactions in tissues

Drug disposition is highly regulated by membrane transporters. Some transporter-mediated drug–drug interactions (DDIs) may not manifest themselves in changes in systemic exposure but rather in changes in tissue exposure of drugs. To better assess the impact of transporter-mediated DDIs in tissues, positron emission tomography (PET)—a noninvasive imaging method—plays an increasingly important role. In this article, we provide examples of how PET can be used to assess transporter-mediated DDIs in different organs.

Quantitative evaluation of the improvement in the pharmacokinetics of a nucleic acid drug delivery system by dynamic PET imaging with (18)F-incorporated oligodeoxynucleotides

Recently, we demonstrated the utility of positron emission tomography (PET) imaging-based pharmacokinetic evaluation studies for preclinical experiments and microdose clinical trials, mainly focused on low molecular weight compounds. In order to investigate the pharmacokinetics of nucleic acid drugs and their drug delivery systems (DDSs) in vivo by using PET imaging, we developed a novel and efficient method for radiolabeling oligodeoxynucleotides with the positron-emitting radionuclide (18)F (stoichiometry-focused Huisgen-type (18)F labeling). By using this method, we succeeded in synthesizing a variety of (18)F-labeled oligodeoxynucleotides with not only phosphodiesters (PO) in natural forms, but also phosphorothioate (PS) and bridged nucleic acid (BNA) in artificial forms, and then performed PET studies and radioactive metabolite analyses of these (18)F-labeled oligodeoxynucleotides. The tissue-distribution and dynamic changes in radioactivity showed significantly different profiles between these antisense oligodeoxynucleotides. The radioactivity of (18)F-labeled PO-DNA and PO-BNA rapidly accumulated in the kidneys and liver and then moved to the renal medulla, ureter, bladder, and intestine. However, the radioactivity of (18)F-labeled PS-DNA and PS-BNA, possessing PS backbone structures, was retained in the blood for relatively long periods and then gradually accumulated in the liver and kidneys. The metabolite analysis showed that (18)F-labeled PO-DNA rapidly degraded by 5min and (18)F-labeled PO-BNA gradually degraded over time by 60min. Conversely, (18)F-labeled PS-DNA and PS-BNA were shown to be much more stable. To demonstrate the usefulness of the PET imaging technique for evaluating the improved targeting potential of the DDS, we designed and synthesized a cholesterol-modified oligodeoxynucleotide, that we developed as an antisense nucleic acid drug against proprotein convertase subtilisin/kexin type 9 (PCSK9) for hypercholesterolemia therapy, and evaluated its pharmacokinetics using PET imaging. As expected, the (18)F-labeled cholesterol-modified PS-BNA-type oligodeoxynucleotide showed much higher and more rapid accumulation in the delivery target organ, that is, the liver, which encourages us to develop this drug. These results suggest that dynamic PET studies using (18)F-incorporated oligodeoxynucleotide synthesized by stoichiometry-focused Huisgen-type labeling is useful for quantitative pharmacokinetic evaluation of nucleic acid drugs and their delivery systems.

Prospective of ⁶⁸Ga-radiopharmaceutical development

Positron Emission Tomography (PET) experienced accelerated development and has become an established method for medical research and clinical routine diagnostics on patient individualized basis. Development and availability of new radiopharmaceuticals specific for particular diseases is one of the driving forces of the expansion of clinical PET. The future development of the ⁶⁸Ga-radiopharmaceuticals must be put in the context of several aspects such as role of PET in nuclear medicine, unmet medical needs, identification of new biomarkers, targets and corresponding ligands, production and availability of ⁶⁸Ga, automation of the radiopharmaceutical production, progress of positron emission tomography technologies and image analysis methodologies for improved quantitation accuracy, PET radiopharmaceutical regulations as well as advances in radiopharmaceutical chemistry. The review presents the prospects of the ⁶⁸Ga-based radiopharmaceutical development on the basis of the current status of these aspects as well as wide range and variety of imaging agents.

Translational PET imaging research

The goal of any early central nervous system (CNS) drug development program is always to test the mechanism and not the molecule in order to support additional research investments in late phase clinical trials. Confirmation that drugs reach their targets using translational positron emission tomography (PET) imaging markers of engagement is central to successful clinical proof-of-concept testing and has become an important feature of most neuropsychiatric drug development programs. CNS PET imaging can also play an important role in the clinical investigation of the neuropharmacological basis of psychiatric disease and the optimization of drug therapy.

Effects of anesthesia and species on the uptake or binding of radioligands in vivo in the Göttingen minipig

Progress in neuroscience research often involves animals, as no adequate alternatives exist to animal models of living systems. However, both the physiological characteristics of the species used and the effects of anesthesia raise questions of common concern. Here, we demonstrate the confounding influences of these effects on tracer binding in positron emission tomography (PET). We determined the effects of two routinely used anesthetics (isoflurane and propofol) on the binding of two tracers of monoamine function, [¹¹C]SCH23390, a tracer of the dopamine D1 and D5 receptors, and the alpha2-adrenoceptor antagonist, [¹¹C]yohimbine, in Göttingen minipigs. The kinetics of SCH23390 in the pigs differed from those of our earlier studies in primates. With two different graphical analyses of uptake of SCH23390, the initial clearance values of this tracer were higher with isoflurane than with propofol anesthesia, indicative of differences in blood flow, whereas no significant differences were observed for the volumes of distribution of yohimbine. The study underscores the importance of differences of anesthesia and species when the properties of radioligands are evaluated under different circumstances that may affect blood flow and tracer uptake. These differences must be considered in the choice of a particular animal species and mode of anesthesia for a particular application.

Performance evaluation of the small-animal nanoScan PET/MRI system

nanoScan is a high-resolution integrated system for consecutive PET and MR imaging of small laboratory animals. We evaluated the performance of the system, using the NEMA NU 4-2008 protocol for the PET component and the NEMA MS 1-2007, MS 2-2008, and MS 3-2007 standards for the MR imaging component.

METHODS

The imaging system uses magnetically shielded position-sensitive photomultiplier tubes and a compact 1-T permanent-magnet MR imaging platform. Spatial resolution, sensitivity, counting rate capabilities, and image quality parameters were evaluated in accordance with the aforementioned NEMA standards. Further in vivo evaluation experiments complement the physical validation results.

RESULTS

The spatial resolution of the PET system enabled the 0.8-mm rods of a Derenzo phantom to be resolved. With point source and 2-dimensional filtered backprojection reconstruction, the resolution varied from 1.50 to 2.01 mm in full width at half maximum in the radial direction and from 1.32 to 1.65 mm in the tangential direction within the radius of 25 mm. Peak absolute sensitivity was 8.41%. Scatter fraction was 17.3% and 34.0%, and maximum noise-equivalent counting rate was 406 and 119 kcps in the mouselike and ratlike phantom, respectively. The image quality test found a nonuniformity of 3.52% and a spillover ratio of 6.2% and 5.8% in water and air, respectively. In testing of the MR imaging component, artifact-free images with high signal-to-noise ratio were recorded. Geometric distortion was below 5%, and image uniformity was at least 94.5% and 96.6% for the 60- and 35-mm radiofrequency coils, respectively.

CONCLUSION

The nanoScan integrated small-animal PET/MR imaging system has excellent spatial resolution and sensitivity. The performance characteristics of the PET and the MR imaging components are not compromised as a result of their integration onto a single platform. Because of its combination of features and performance parameters, the system provides crucial advantages for preclinical imaging studies over existing PET/CT systems, especially in neurologic and oncologic research.

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.

Modeling of PET data in CNS drug discovery and development

Positron emission tomography (PET) is increasingly used in drug discovery and development for evaluation of CNS drug disposition and for studies of disease biomarkers to monitor drug effects on brain pathology. The quantitative analysis of PET data is based on kinetic modeling of radioactivity concentrations in plasma and brain tissue compartments. A number of quantitative methods of analysis have been developed that allow the determination of parameters describing drug pharmacokinetics and interaction with target binding sites in the brain. The optimal method of quantification depends on the properties of the radiolabeled drug or radioligand and the binding site studied. We here review the most frequently used methods for quantification of PET data in relation to CNS drug discovery and development. The utility of PET kinetic modeling in the development of novel CNS drugs is illustrated by examples from studies of the brain kinetic properties of radiolabeled drug molecules.

Toward prediction of efficacy of chemotherapy: a proof of concept study in lung cancer patients using [¹¹C]docetaxel and positron emission tomography

PURPOSE

Pharmacokinetics of docetaxel can be measured in vivo using positron emission tomography (PET) and a microdose of radiolabeled docetaxel ([(11)C]docetaxel). The objective of this study was to investigate whether a [(11)C]docetaxel PET microdosing study could predict tumor uptake of therapeutic doses of docetaxel.

EXPERIMENTAL DESIGN

Docetaxel-naïve lung cancer patients underwent 2 [(11)C]docetaxel PET scans; one after bolus injection of [(11)C]docetaxel and another during combined infusion of [(11)C]docetaxel and a therapeutic dose of docetaxel (75 mg·m(-2)). Compartmental and spectral analyses were used to quantify [(11)C]docetaxel tumor kinetics. [(11)C]docetaxel PET measurements were used to estimate the area under the curve (AUC) of docetaxel in tumors. Tumor response was evaluated using computed tomography scans.

RESULTS

Net rates of influx (Ki) of [(11)C]docetaxel in tumors were comparable during microdosing and therapeutic scans. [(11)C]docetaxel AUCTumor during the therapeutic scan could be predicted reliably using an impulse response function derived from the microdosing scan together with the plasma curve of [(11)C]docetaxel during the therapeutic scan. At 90 minutes, the accumulated amount of docetaxel in tumors was less than 1% of the total infused dose of docetaxel. [(11)C]docetaxel Ki derived from the microdosing scan correlated with AUCTumor of docetaxel (Spearman ρ = 0.715; P = 0.004) during the therapeutic scan and with tumor response to docetaxel therapy (Spearman ρ = -0.800; P = 0.010).

CONCLUSIONS

Microdosing data of [(11)C]docetaxel PET can be used to predict tumor uptake of docetaxel during chemotherapy. The present study provides a framework for investigating the PET microdosing concept for radiolabeled anticancer drugs in patients.