Development of quantitative PET/MR imaging for measurements of hepatic portal vein input function: a phantom study

BACKGROUND

Accurate pharmacokinetic modelling in PET necessitates measurements of an input function, which ideally is acquired non-invasively from image data. For hepatic pharmacokinetic modelling two input functions need to be considered, to account for the blood supply from the hepatic artery and portal vein. Image-derived measurements at the portal vein are challenging due to its small size and image artifacts caused by respiratory motion. In this work we seek to demonstrate, using phantom experiments, how a dedicated PET/MR protocol can tackle these challenges and potentially provide input function measurements of the portal vein in a clinical setup.

METHODS

A custom 3D printed PET/MR phantom was constructed to mimic the liver and portal vein. PET/MR acquisitions were made with emulated respiratory motion. The PET/MR imaging protocol consisted of high-resolution anatomical MR imaging of the portal vein, followed by a PET acquisition in parallel to a dedicated motion-tracking MR sequence. Motion tracking and deformation information were extracted from PET data and subsequently used in PET reconstruction to produce dynamic series of motion-free PET images. Anatomical MR images were used post PET reconstruction for partial volume correction of the input function measurements.

RESULTS

Reconstruction of dynamic PET data with motion-compensation provided nearly motion-free series of PET frame data, suitable for image derived input function measurements of the portal vein. After partial volume correction, the individual input function measurements were within a 16.1% error range from the true activity in the portal vein compartment at the time of PET acquisition.

CONCLUSION

The proposed protocol demonstrates clinically feasible PET/MR imaging of the liver for pharmacokinetic studies with accurate quantification of the portal vein input function, including correction for respiratory motion and partial volume effects.

First-in-human evaluation of 6-bromo-7-[11C]methylpurine, a PET tracer for assessing the function of multidrug resistance-associated proteins in different tissues

PURPOSE

Multidrug resistance-associated protein 1 (MRP1) is a transport protein with a widespread tissue distribution, which has been implicated in the pathophysiology of Alzheimer's and chronic respiratory disease. PET with 6-bromo-7-[11C]methylpurine ([11C]BMP) has been used to measure MRP1 function in rodents. In this study, [11C]BMP was for the first time characterised in humans to assess the function of MRP1 and other MRP subtypes in different tissues.

METHODS

Thirteen healthy volunteers (7 men, 6 women) underwent dynamic whole-body PET scans on a long axial field-of-view (LAFOV) PET/CT system after intravenous injection of [11C]BMP. Three subjects of each sex were scanned a second time to assess reproducibility. Volumes of interest were outlined for MRP-expressing tissues (cerebral cortex, cerebellum, choroid plexus, retina, lungs, myocardium, kidneys, and liver). From the time-activity curves, the elimination rate constant (kE, h- 1) was derived as a parameter for tissue MRP function and its test-retest variability (TRTV, %) was calculated. Radiation dosimetry was calculated using the Medical Internal Radiation Dose (MIRD) methodology.

RESULTS

Mean kE and corresponding TRTV values were: cerebral cortex: 0.055 ± 0.010 h- 1 (- 4 ± 24%), cerebellum: 0.033 ± 0.009 h- 1 (1 ± 39%), choroid plexus: 0.292 ± 0.059 h- 1 (0.1 ± 16%), retina: 0.234 ± 0.045 h- 1 (30 ± 38%), lungs: 0.875 ± 0.095 h- 1 (- 3 ± 11%), myocardium: 0.641 ± 0.105 h- 1 (11 ± 25%), kidneys: 1.378 ± 0.266 h- 1 (14 ± 16%), and liver: 0.685 ± 0.072 h- 1 (7 ± 9%). Significant sex differences were found for kE in the cerebellum, lungs and kidneys. Effective dose was 4.67 ± 0.18 µSv/MBq for men and 4.55 ± 0.18 µSv/MBq for women.

CONCLUSION

LAFOV PET/CT with [11C]BMP potentially allows for simultaneous assessment of MRP function in multiple human tissues. Mean TRTV of kE in different tissues was in an acceptable range, except for the retina. The radiation dosimetry of [11C]BMP was in the typical range of 11C-tracers. LAFOV PET/CT holds great potential to assess at a whole-body, multi-tissue level molecular targets relevant for drug disposition in humans.

TRIAL REGISTRATION

EudraCT 2021-006348-29. Registered 15 December 2021.

[11C]Metoclopramide PET can detect a seizure-induced up-regulation of cerebral P-glycoprotein in epilepsy patients

BACKGROUND

P-glycoprotein (P-gp) is an efflux transporter which is abundantly expressed at the blood-brain barrier (BBB) and which has been implicated in the pathophysiology of various brain diseases. The radiolabelled antiemetic drug [11C]metoclopramide is a P-gp substrate for positron emission tomography (PET) imaging of P-gp function at the BBB. To assess whether [11C]metoclopramide can detect increased P-gp function in the human brain, we employed drug-resistant temporal lobe epilepsy (TLE) as a model disease with a well characterised, regional P-gp up-regulation at the BBB.

METHODS

Eight patients with drug-resistant (DRE) TLE, 5 seizure-free patients with drug-sensitive (DSE) focal epilepsy, and 15 healthy subjects underwent brain PET imaging with [11C]metoclopramide on a fully-integrated PET/MRI system. Concurrent with PET, arterial blood sampling was performed to generate a metabolite-corrected arterial plasma input function for kinetic modelling. The choroid plexus was outmasked on the PET images to remove signal contamination from the neighbouring hippocampus. Using a brain atlas, 10 temporal lobe sub-regions were defined and analysed with a 1-tissue-2-rate constant compartmental model to estimate the rate constants for radiotracer transfer from plasma to brain (K1) and from brain to plasma (k2), and the total volume of distribution (VT = K1/k2).

RESULTS

DRE patients but not DSE patients showed significantly higher k2 values and a trend towards lower VT values in several temporal lobe sub-regions located ipsilateral to the epileptic focus as compared to healthy subjects (k2: hippocampus: +34%, anterior temporal lobe, medial part: +28%, superior temporal gyrus, posterior part: +21%).

CONCLUSIONS

[11C]Metoclopramide PET can detect a seizure-induced P-gp up-regulation in the epileptic brain. The efflux rate constant k2 seems to be the most sensitive parameter to measure increased P-gp function with [11C]metoclopramide. Our study provides evidence that disease-induced alterations in P-gp expression at the BBB can lead to changes in the distribution of a central nervous system-active drug to the human brain, which could affect the efficacy and/or safety of drugs. [11C]Metoclopramide PET may be used to assess or predict the contribution of increased P-gp function to drug resistance and disease pathophysiology in various brain diseases.

TRIAL REGISTRATION

EudraCT 2019-003137-42. Registered 28 February 2020.

Tolerability of tariquidar - A third generation P-gp inhibitor as add-on medication to antiseizure medications in drug-resistant epilepsy

PURPOSE

P-glycoprotein (P-gp) has been hypothesized to be involved in drug-resistance of epilepsy by actively extruding antiseizure medications (ASMs) from the brain. The P-gp inhibitor tariquidar (TQD) has been shown to effectively inhibit P-gp at the human blood-brain barrier, improving brain entry of several ASMs. A potential strategy to overcome drug-resistance is the co-administration of P-gp inhibitors such as TQD to ASMs. Here we present data on the tolerability of single-dose TQD as a potential add-on medication to ASMs.

METHODS

We performed a multi-centre cohort study including drug-resistant epilepsy patients and healthy controls from the United Kingdom and Austria. TQD was administered intravenously at five different doses (2 mg/kg or 3 mg/kg of TQD were given to drug-resistant epilepsy patients and healthy controls, higher doses of TQD at 4 mg/kg, 6 mg/kg and 8 mg/kg as well as a prolonged infusion aiming at a dose of 6 mg/kg were only given to healthy controls). Adverse events were recorded and graded using the Common Terminology Criteria (CTCAE) scale. Additionally, TQD plasma concentration levels were measured and compared between drug-resistant patients and healthy controls.

RESULTS

In total, 108 participants received TQD once at variable doses and it was overall well tolerated. At doses of 2 or 3 mg/kg TQD, only two of the 19 drug-resistant epilepsy patients and a third of the healthy controls (n = 14/42) reported adverse events probably related to TQD. The majority of those adverse events (96 %) were reported as mild. One drug-resistant epilepsy patient reported adverse events 24-hours after TQD administration possibly related to TQD-induced increased ASMs levels in the brain.

CONCLUSIONS

TQD is an effective and well tolerated P-gp inhibitor as a single dose and could potentially be used intermittently in conjunction with ASMs to improve efficacy. This promising strategy to overcome drug-resistance in epilepsy should be investigated further in clinical randomised controlled trials.

The impact of ATP-binding cassette transporters in the diseased brain: Context matters

ATP-binding cassette (ABC) transporters facilitate the movement of diverse molecules across cellular membranes, including those within the CNS. While most extensively studied in microvascular endothelial cells forming the blood-brain barrier (BBB), other CNS cell types also express these transporters. Importantly, disruptions in the CNS microenvironment during disease can alter transporter expression and function. Through this comprehensive review, we explore the modulation of ABC transporters in various brain pathologies and the context-dependent consequences of these changes. For instance, downregulation of ABCB1 may exacerbate amyloid beta plaque deposition in Alzheimer's disease and facilitate neurotoxic compound entry in Parkinson's disease. Upregulation may worsen neuroinflammation by aiding chemokine-mediated CD8 T cell influx into multiple sclerosis lesions. Overall, ABC transporters at the BBB hinder drug entry, presenting challenges for effective pharmacotherapy. Understanding the context-dependent changes in ABC transporter expression and function is crucial for elucidating the etiology and developing treatments for brain diseases.

St. John's wort extract with a high hyperforin content does not induce P-glycoprotein activity at the human blood-brain barrier

St. John's wort (SJW) extract, a herbal medicine with antidepressant effects, is a potent inducer of intestinal and/or hepatic cytochrome P450 (CYP) enzymes and P-glycoprotein (P-gp), which can cause clinically relevant drug interactions. It is currently not known whether SJW can also induce P-gp activity at the human blood-brain barrier (BBB), which may potentially lead to decreased brain exposure and efficacy of certain central nervous system (CNS)-targeted P-gp substrate drugs. In this study, we used a combination of positron emission tomography (PET) imaging and cocktail phenotyping to gain a comprehensive picture on the effect of SJW on central and peripheral P-gp and CYP activities. Before and after treatment of healthy volunteers (n = 10) with SJW extract with a high hyperforin content (3-6%) for 12-19 days (1800 mg/day), the activity of P-gp at the BBB was assessed by means of PET imaging with the P-gp substrate [11C]metoclopramide and the activity of peripheral P-gp and CYPs was assessed by administering a low-dose phenotyping cocktail (caffeine, omeprazole, dextromethorphan, and midazolam or fexofenadine). SJW significantly increased peripheral P-gp, CYP3A, and CYP2C19 activity. Conversely, no significant changes in the peripheral metabolism, brain distribution, and P-gp-mediated efflux of [11C]metoclopramide across the BBB were observed following the treatment with SJW extract. Our data suggest that SJW does not lead to significant P-gp induction at the human BBB despite its ability to induce peripheral P-gp and CYPs. Simultaneous intake of SJW with CNS-targeted P-gp substrate drugs is not expected to lead to P-gp-mediated drug interactions at the BBB.

Advancing 6-bromo-7-[11C]methylpurine to clinical use: improved regioselective radiosynthesis, non-clinical toxicity data and human dosimetry estimates

BACKGROUND

6-Bromo-7-[11C]methylpurine ([11C]BMP) is a radiotracer for positron emission tomography (PET) to measure multidrug resistance-associated protein 1 (MRP1) transport activity in different tissues. Previously reported radiosyntheses of [11C]BMP afforded a mixture of 7- and 9-[11C]methyl regioisomers. To prepare for clinical use, we here report an improved regioselective radiosynthesis of [11C]BMP, the results of a non-clinical toxicity study as well as human dosimetry estimates based on mouse PET data.

RESULTS

[11C]BMP was synthesised by regioselective N7-methylation of 6-bromo-7H-purine (prepared under good manufacturing practice) with [11C]methyl triflate in presence of 2,2,6,6-tetramethylpiperidine magnesium chloride in a TRACERlab™ FX2 C synthesis module. [11C]BMP was obtained within a total synthesis time of approximately 43 min in a decay-corrected radiochemical yield of 20.5 ± 5.2%, based on starting [11C]methyl iodide, with a radiochemical purity > 99% and a molar activity at end of synthesis of 197 ± 130 GBq/μmol (n = 28). An extended single-dose toxicity study conducted in male and female Wistar rats under good laboratory practice after single intravenous (i.v.) administration of unlabelled BMP (2 mg/kg body weight) revealed no test item related adverse effects. Human dosimetry estimates, based on dynamic whole-body PET data in female C57BL/6J mice, suggested that an i.v. injected activity amount of 400 MBq of [11C]BMP will deliver an effective dose in the typical range of 11C-labelled radiotracers.

CONCLUSIONS

[11C]BMP can be produced in sufficient amounts and acceptable quality for clinical use. Data from the non-clinical safety evaluation showed no adverse effects and suggested that the administration of [11C]BMP will be safe and well tolerated in humans.

Performance and Sensitivity of [99mTc]Tc-sestamibi Compared with Positron Emission Tomography Radiotracers to Measure P-glycoprotein Function in the Kidneys and Liver

P-glycoprotein (P-gp, encoded in humans by the ABCB1 gene and in rodents by the Abcb1a/b genes) is a membrane transporter that can restrict the intestinal absorption and tissue distribution of many drugs and may also contribute to renal and hepatobiliary drug excretion. The aim of this study was to compare the performance and sensitivity of currently available radiolabeled P-gp substrates for positron emission tomography (PET) with the single-photon emission computed tomography (SPECT) radiotracer [99mTc]Tc-sestamibi for measuring the P-gp function in the kidneys and liver. Wild-type, heterozygous (Abcb1a/b(+/-)), and homozygous (Abcb1a/b(-/-)) Abcb1a/b knockout mice were used as models of different P-gp abundance in excretory organs. Animals underwent either dynamic PET scans after intravenous injection of [11C]N-desmethyl-loperamide, (R)-[11C]verapamil, or [11C]metoclopramide or consecutive static SPECT scans after intravenous injection of [99mTc]Tc-sestamibi. P-gp in the kidneys and liver of the mouse models was analyzed with immunofluorescence labeling and Western blotting. In the kidneys, Abcb1a/b() mice had intermediate P-gp abundance compared with wild-type and Abcb1a/b(-/-) mice. Among the four tested radiotracers, renal clearance of radioactivity (CLurine,kidney) was significantly reduced (-83%) in Abcb1a/b(-/-) mice only for [99mTc]Tc-sestamibi. Biliary clearance of radioactivity (CLbile,liver) was significantly reduced in Abcb1a/b(-/-) mice for [11C]N-desmethyl-loperamide (-47%), [11C]metoclopramide (-25%), and [99mTc]Tc-sestamibi (-79%). However, in Abcb1a/b(+/-) mice, CLbile,liver was significantly reduced (-47%) only for [99mTc]Tc-sestamibi. Among the tested radiotracers, [99mTc]Tc-sestamibi performed best in measuring the P-gp function in the kidneys and liver. Owing to its widespread clinical availability, [99mTc]Tc-sestamibi represents a promising probe substrate to assess systemic P-gp-mediated drug-drug interactions and to measure renal and hepatic P-gp function under different (patho-)physiological conditions.

[11C]metoclopramide is a sensitive radiotracer to measure moderate decreases in P-glycoprotein function at the blood-brain barrier

The efflux transporter P-glycoprotein (P-gp) at the blood-brain barrier limits the cerebral uptake of various xenobiotics. To assess the sensitivity of [11C]metoclopramide to measure decreased cerebral P-gp function, we performed [11C]metoclopramide PET scans without (baseline) and with partial P-gp inhibition by tariquidar in wild-type, heterozygous Abcb1a/b(+/-) and homozygous Abcb1a/b(-/-) mice as models with controlled levels of cerebral P-gp expression. Brains were collected to quantify P-gp expression with immunohistochemistry. Brain uptake of [11C]metoclopramide was expressed as the area under the brain time-activity curve (AUCbrain) and compared with data previously obtained with (R)-[11C]verapamil and [11C]N-desmethyl-loperamide. Abcb1a/b(+/-) mice had intermediate P-gp expression compared to wild-type and Abcb1a/b(-/-) mice. In baseline scans, all three radiotracers were able to discriminate Abcb1a/b(-/-) from wild-type mice (2.5- to 4.6-fold increased AUCbrain, p ≤ 0.0001). However, only [11C]metoclopramide could discriminate Abcb1a/b(+/-) from wild-type mice (1.46-fold increased AUCbrain, p ≤ 0.001). After partial P-gp inhibition, differences in [11C]metoclopramide AUCbrain between Abcb1a/b(+/-) and wild-type mice (1.39-fold, p ≤ 0.001) remained comparable to baseline. There was a negative correlation between baseline [11C]metoclopramide AUCbrain and ex-vivo-measured P-gp immunofluorescence (r = -0.9875, p ≤ 0.0001). Our data suggest that [11C]metoclopramide is a sensitive radiotracer to measure moderate, but (patho-)physiologically relevant decreases in cerebral P-gp function without the need to co-administer a P-gp inhibitor.

Parametric Imaging of P-Glycoprotein Function at the Blood-Brain Barrier Using kE,brain-maps Generated from [11C]Metoclopramide PET Data in Rats, Nonhuman Primates and Humans

PURPOSE

PET imaging using [11C]metoclopramide revealed the importance of P-glycoprotein (P-gp, ABCB1) in mediating the brain-to-blood efflux of substrates across the blood-brain barrier (BBB). In this work, the elimination rate constant from the brain (kE,brain), calculated from dynamic PET images without the need for arterial blood sampling, was evaluated as an outcome parameter for the interpretation of [11C]metoclopramide PET data.

PROCEDURES

kE,brain parameter was obtained by linear regression of log-transformed brain time-activity curves (TACs). kE,brain values (h-1) obtained under baseline conditions were compared with values obtained after complete P-gp inhibition using tariquidar in rats (n = 4) and baboons (n = 4) or after partial inhibition using cyclosporine A in humans (n = 10). In baboons, the sensitivity of kE,brain to measure complete P-gp inhibition was compared with outcome parameters derived from kinetic modeling using a 1-tissue compartment model (1-TCM). Finally, kE,brain-maps were generated in each species using PMOD software.

RESULTS

The linear part of the log-transformed brain TACs occurred from 10 to 30 min after radiotracer injection in rats, from 15 to 60 min in baboons, and from 20 to 60 min in humans. P-gp inhibition significantly decreased kE,brain values by 39 ± 12% in rats (p < 0.01), by 32 ± 6% in baboons (p < 0.001), and by 37 ± 22% in humans (p < 0.001). In baboons, P-gp inhibition consistently decreased the brain-to-plasma efflux rate constant k2 (36 ± 9%, p < 0.01) leading to an increase in the total brain volume of distribution (VT, 101 ± 12%, p < 0.001). In all studied species, brain kE,brain-maps displayed decreased P-gp-mediated efflux across the BBB.

CONCLUSIONS

kE,brain of [11C]metoclopramide provides a simple outcome parameter to describe P-gp function in the living brain when arterial input function data are unavailable, although less sensitive than VT. kE,brain-maps represent easy to compute parametric images reflecting the effect of P-gp on [11C]metoclopramide elimination from the brain.

Dissimilar Effect of P-Glycoprotein and Breast Cancer Resistance Protein Inhibition on the Distribution of Erlotinib to the Retina and Brain in Humans and Mice

P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) are two ATP-binding cassette efflux transporters that are coexpressed at the human blood-brain barrier (BBB) and blood-retina barrier (BRB). While pharmacological inhibition of P-gp and/or BCRP results in increased brain distribution of dual P-gp/BCRP substrate drugs, such as the tyrosine kinase inhibitor erlotinib, the effect of P-gp and/or BCRP inhibition on the retinal distribution of such drugs has hardly been investigated. In this study, we used positron emission tomography (PET) imaging to assess the effect of transporter inhibition on the distribution of [11C]erlotinib to the human retina and brain. Twenty two healthy volunteers underwent two PET scans after intravenous (i.v.) injection of a microdose (<5 μg) of [11C]erlotinib, a baseline scan, and a second scan either with concurrent i.v. infusion of tariquidar to inhibit P-gp (n = 5) or after oral intake of single ascending doses of erlotinib (300 mg, 650 mg, or 1000 mg, n = 17) to saturate erlotinib transport. In addition, transport of [3H]erlotinib to the retina and brain was assessed in mice by in situ carotid perfusion under various drug transporter inhibition settings. In comparison to the baseline PET scan, coadministration of tariquidar or erlotinib led to a significant decrease of [11C]erlotinib total volume of distribution (VT) in the human retina by -25 ± 8% (p ≤ 0.05) and -41 ± 16% (p ≤ 0.001), respectively. In contrast, erlotinib intake led to a significant increase in [11C]erlotinib VT in the human brain (+20 ± 16%, p ≤ 0.001), while administration of tariquidar did not result in any significant changes. In situ carotid perfusion experiments showed that both P-gp and BCRP significantly limit the distribution of erlotinib to the mouse retina and brain but revealed a similar discordant effect at the mouse BRB and BBB following co-perfusion with tariquidar and erlotinib as in humans. Co-perfusion with prototypical inhibitors of solute carrier transporters did not reveal a significant contribution of organic cation transporters (e.g., OCTs and OCTNs) and organic anion-transporting polypeptides (e.g., OATP2B1) to the retinal and cerebral distribution of erlotinib. In conclusion, we observed a dissimilar effect after P-gp and/or BCRP inhibition on the retinal and cerebral distribution of [11C]erlotinib. The exact mechanism for this discrepancy remains unclear but may be related to the function of an unidentified erlotinib uptake carrier sensitive to tariquidar inhibition at the BRB. Our study highlights the great potential of PET to study drug distribution to the human retina and to assess the functional impact of membrane transporters on ocular drug distribution.

Imaging the impact of blood-brain barrier disruption induced by focused ultrasound on P-glycoprotein function

The P-glycoprotein (P-gp/ABCB1) is a major efflux transporter which impedes the brain delivery of many drugs across the blood-brain barrier (BBB). Focused ultrasound with microbubbles (FUS) enables BBB disruption, which immediate and delayed impact on P-gp function remains unclear. Positron emission tomography (PET) imaging using the radiolabeled substrate [11C]metoclopramide provides a sensitive and translational method to study P-gp function at the living BBB. A FUS protocol was devised in rats to induce a substantial and targeted disruption of the BBB in the left hemisphere. BBB disruption was confirmed by the Evan's Blue extravasation test or the minimally-invasive contrast-enhanced MRI. The expression of P-gp was measured 24 h or 48 h after FUS using immunostaining and fluorescence microscopy. The brain kinetics of [11C]metoclopramide was studied by PET at baseline, and both immediately or 24 h after FUS, with or without half-maximum P-gp inhibition (tariquidar 1 mg/kg). In each condition (n = 4-5 rats per group), brain exposure of [11C]metoclopramide was estimated as the area-under-the-curve (AUC) in regions corresponding to the sonicated volume in the left hemisphere, and the contralateral volume. Kinetic modeling was performed to estimate the uptake clearance ratio (R1) of [11C]metoclopramide in the sonicated volume relative to the contralateral volume. In the absence of FUS, half-maximum P-gp inhibition increased brain exposure (+135.0 ± 12.9%, p < 0.05) but did not impact R1 (p > 0.05). Immediately after FUS, BBB integrity was selectively disrupted in the left hemisphere without any detectable impact on the brain kinetics of [11C]metoclopramide compared with the baseline group (p > 0.05) or the contralateral volume (p > 0.05). 24 h after FUS, BBB integrity was fully restored while P-gp expression was maximally down-regulated (-45.0 ± 4.5%, p < 0.001) in the sonicated volume. This neither impacted AUC nor R1 in the FUS + 24 h group (p > 0.05). Only when P-gp was inhibited with tariquidar were the brain exposure (+130 ± 70%) and R1(+29.1 ± 15.4%) significantly increased in the FUS + 24 h/tariquidar group, relative to the baseline group (p < 0.001). We conclude that the brain kinetics of [11C]metoclopramide specifically depends on P-gp function rather than BBB integrity. Delayed FUS-induced down-regulation of P-gp function can be detected. Our results suggest that almost complete down-regulation is required to substantially enhance the brain delivery of P-gp substrates.

Effect of budesonide on pulmonary activity of multidrug resistance-associated protein 1 assessed with PET imaging in rats

Multidrug resistance-associated protein 1 (MRP1/ABCC1) is a highly abundant efflux transporter in the lungs, which protects cells from toxins and oxidative stress and has been implicated in the pathophysiology of chronic obstructive pulmonary disease and cystic fibrosis. There is evidence from in vitro studies that the inhaled glucocorticoid budesonide can inhibit MRP1 activity. We used positron emission tomography (PET) imaging with 6-bromo-7-[¹¹C]methylpurine ([¹¹C]BMP), which is transformed in vivo into a radiolabeled MRP1 substrate, to assess whether intratracheally (i.t.) aerosolized budesonide affects pulmonary MRP1 activity in rats. Three groups of rats (n = 5-6 each) underwent dynamic PET scans of the lungs after i.t. aerosolization of either [¹¹C]BMP alone, or [¹¹C]BMP mixed with either budesonide (0.04 mg, corresponding to the maximum soluble dose) or the model MRP1 inhibitor MK571 (2 mg). From PET-measured radioactivity concentration-time curves, the rate constant describing radioactivity elimination from the right lung (k_E,lung) and the area under the curve (AUC_lung) were calculated from 0 to 5 min after start of the PET scan as measures of pulmonary MRP1 activity. Co-administration of MK571 resulted in a pronounced decrease in k_E,lung (25-fold, p < 0.0001) and an increase in AUC_lung (5.3-fold, p < 0.0001) when compared with vehicle-treated animals. In contrast, in budesonide-treated animals k_E,lung and AUC_lung were not significantly different from the vehicle group. Our results show that i.t. aerosolized budesonide at an approximately 5 times higher dose than the maximum clinical dose leads to no change in pulmonary MRP1 activity, suggesting a lack of an effect of inhaled budesonide treatment on the MRP1-mediated cellular detoxifying capacity of the lungs. However, the strong effect observed for MK571 raises the possibility for the occurrence of transporter-mediated drug-drug interactions at the pulmonary epithelium with inhaled medicines.

Influence of P-glycoprotein on pulmonary disposition of the model substrate [11C]metoclopramide assessed by PET imaging in rats

In the lungs, the membrane transporter P-glycoprotein (P-gp) is expressed in the apical (i.e. lumen-facing) membrane of airway epithelial cells and in the luminal (blood-facing) membrane of pulmonary capillary endothelial cells. To better understand the influence of P-gp on the pulmonary disposition of inhaled P-gp substrate drugs, we measured the intrapulmonary pharmacokinetics of the intratracheally (i.t.) aerosolized model P-gp substrate [¹¹C]metoclopramide in presence and absence of P-gp activity by means of positron emission tomography (PET) imaging in rats. Data were compared to data previously acquired with the model P-gp substrates (R)-[¹¹C]verapamil and [¹¹C]N-desmethyl-loperamide, using the same experimental set-up. Groups of wild-type rats, either untreated or treated with the P-gp inhibitor tariquidar, and Abcb1a/b^(-/-) rats underwent 90-min dynamic PET scans after i.t. aerosolization of [¹¹C]metoclopramide. Lung exposure to [¹¹C]metoclopramide was expressed as the area under the right lung concentration-time curve (AUC_lung). AUC_lung values were significantly higher in Abcb1a/b^(-/-) rats (1.8-fold, p ≤ 0.0001) and in tariquidar-treated wild-type rats (1.6-fold, p ≤ 0.01) than in untreated wild-type rats. This differed from previously obtained results with (R)-[¹¹C]verapamil and [¹¹C]N-desmethyl-loperamide, which showed decreased exposure in the rat lung in absence of P-gp activity. Our results suggest that transepithelial transfer of [¹¹C]metoclopramide was not or only to a small extent affected by P-gp activity, presumably due to the compound's high passive permeability. The increased lung retention of [¹¹C]metoclopramide may be due to decreased P-gp-mediated clearance into the blood in absence of P-gp activity in capillary endothelial cells. The overall effect of P-gp on the lung exposure to inhaled P-gp substrate drugs may, thus, be determined by a balance of opposing effects at the pulmonary epithelium and endothelium.

Positron Emission Tomography-Based Pharmacokinetic Analysis To Assess Renal Transporter-Mediated Drug-Drug Interactions of Antimicrobial Drugs

Transporter-mediated drug-drug interactions (DDIs) are of concern in antimicrobial drug development, as they can have serious safety consequences. We used positron emission tomography (PET) imaging-based pharmacokinetic (PK) analysis to assess the effect of different drugs, which may cause transporter-mediated DDIs, on the tissue distribution and excretion of [18F]ciprofloxacin as a radiolabeled model antimicrobial drug. Mice underwent PET scans after intravenous injection of [18F]ciprofloxacin, without and with pretreatment with either probenecid (150 mg/kg), cimetidine (50 mg/kg), or pyrimethamine (5 mg/kg). A 3-compartment kidney PK model was used to assess the involvement of renal transporters in the examined DDIs. Pretreatment with probenecid and cimetidine significantly decreased the renal clearance (CLrenal) of [18F]ciprofloxacin. The effect of cimetidine (-86%) was greater than that of probenecid (-63%), which contrasted with previously published clinical data. The kidney PK model revealed that the decrease in CLrenal was caused by inhibition of basal uptake transporters and apical efflux transporters in kidney proximal tubule cells. Changes in the urinary excretion of [18F]ciprofloxacin after pretreatment with probenecid and cimetidine resulted in increased blood and organ exposure to [18F]ciprofloxacin. Our results suggest that multiple membrane transporters mediate the tubular secretion of ciprofloxacin, with possible species differences between mice and humans. Concomitant medication inhibiting renal transporters may precipitate DDIs, leading to decreased urinary excretion and increased blood and organ exposure to ciprofloxacin, potentially exacerbating adverse effects. Our study highlights the strength of PET imaging-based PK analysis to assess transporter-mediated DDIs at a whole-body level.

Knockout of ABCC1 in NCI-H441 cells reveals CF to be a suboptimal substrate to study MRP1 activity in organotypic in vitro models

Multidrug resistance-associated protein 1 (MRP1/ABCC1) is an efflux transporter responsible for the extrusion of endogenous substances as well as xenobiotics and their respective metabolites. Its high expression levels in lung tissue imply a key role in pulmonary drug disposition. Moreover, its association with inflammatory lung diseases underline MRP1's relevance in drug development and precision-medicine. With the aim to develop a tool to better understand MRP1's role in drug disposition and lung disease, we generated an ABCC1^-/- clone based on the human distal lung epithelial cell line NCI-H441 via a targeted CRISPR/Cas9 approach. Successful knockout (KO) of MRP1 was confirmed by qPCR, immunoblot and Sanger sequencing. To assess potential compensatory upregulation of transporters with a similar substrate recognition pattern as MRP1, expression levels of MRP2-9 as well as OAT1-4, 6, 7 and 10 were measured. Functional transporter activity was determined via release studies with two prodrug/substrate pairs, i.e. 5(6)-carboxyfluorescein (CF; formed from its diacetate prodrug) and S-(6-(7-methylpurinyl))glutathione (MPG; formed from its prodrug 6-bromo-7-methylpurine, BMP), respectively. Lastly, transepithelial electrical resistance (TEER) of monolayers of the KO clone were compared with wildtype (WT) NCI-H441 cells. Of eight initially generated clones, the M2 titled clone showed complete absence of mRNA and protein in accordance with the designed genome edit. In transport studies using the substrate CF, however, no differences between the KO clone and WT NCI-H441 cells were observed, whilst no differences in expression of potential compensatory transporters was noted. On the other hand, when using BMP/MPG, the release of MPG was reduced to 11.5% in the KO clone. Based on these results, CF appears to be a suboptimal probe for the study of MRP1 function, particularly in organotypic in vitro and ex vivo models. Our ABCC1^-/- NCI-H441 clone further retained the ability to form electrically tight barriers, making it a useful model to study MRP1 function in vitro.

Nuclear medicine imaging methods as novel tools in the assessment of pulmonary drug disposition

INTRODUCTION

Drugs for the treatment of respiratory diseases are commonly administered by oral inhalation. Yet surprisingly little is known about the pulmonary pharmacokinetics of inhaled molecules. Nuclear medicine imaging techniques (i.e. planar gamma scintigraphy, single-photon emission computed tomography [SPECT] and positron emission tomography [PET]) enable the noninvasive dynamic measurement of the lung concentrations of radiolabeled drugs or drug formulations. This review discusses the potential of nuclear medicine imaging techniques in inhalation biopharmaceutical research.

AREAS COVERED

(i) Planar gamma scintigraphy studies with radiolabeled inhalation formulations to assess initial pulmonary drug deposition; (ii) imaging studies with radiolabeled drugs to assess their intrapulmonary pharmacokinetics; (iii) receptor occupancy studies to quantify the pharmacodynamic effect of inhaled drugs.

EXPERT OPINION

Imaging techniques hold potential to bridge the knowledge gap between animal models and humans with respect to the pulmonary disposition of inhaled drugs. However, beyond the mere assessment of the initial lung deposition of inhaled formulations with planar gamma scintigraphy, imaging techniques have rarely been employed in pulmonary drug development. This may be related to several technical challenges encountered with such studies. Considering the wealth of information that can be obtained with imaging studies their use in inhalation biopharmaceutics should be further investigated.

Impact of P-gp and BCRP on pulmonary drug disposition assessed by PET imaging in rats

P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) are two efflux transporters which are expressed in the apical (i.e. airway lumen-facing) membranes of lung epithelial cells. To assess the influence of P-gp and BCRP on the pulmonary disposition of inhaled drugs, we performed positron emission tomography (PET) imaging in rats after intratracheal aerosolization of two model P-gp/BCRP substrate radiotracers (i.e. [¹¹C]erlotinib and [¹¹C]tariquidar). We studied rat groups in which both transporters were active (i.e. wild-type rats), either of the two transporters was inactive (Abcb1a/b^(-/-) and Abcg2^(-/-) rats) or both transporters were inactive (Abcg2^(-/-) rats in which pulmonary P-gp activity was inhibited by treatment with unlabeled tariquidar). PET-measured lung distribution data were compared with brain-to-plasma radioactivity concentration ratios measured in a gamma counter at the end of the PET scan. For [¹¹C]erlotinib, lung exposure (AUC_lungs) was moderately but not significantly increased in Abcb1a/b^(-/-) rats (1.6-fold) and Abcg2^(-/-) rats (1.5-fold), and markedly (3.6-fold, p < 0.0001) increased in tariquidar-treated Abcg2^(-/-) rats, compared to wild-type rats. Similarly, the brain uptake of [¹¹C]erlotinib was substantially (4.5-fold, p < 0.0001) increased when both P-gp and BCRP activities were impaired. For [¹¹C]tariquidar, differences in AUC_lungs between groups pointed into a similar direction as for [¹¹C]erlotinib, but were less pronounced and lacked statistical significance. Our study demonstrates functional P-gp and BCRP activity in vivo in the lungs and further suggests functional redundancy between P-gp and BCRP in limiting the pulmonary uptake of a model P-gp/BCRP substrate, analogous to the blood-brain barrier. Our results suggest that pulmonary efflux transporters are important for the efficacy and safety of inhaled drugs and that their modulation may be exploited in order to improve the pharmacokinetic and pharmacodynamic performance of pulmonary delivered drugs.

Use of PET Imaging to Assess the Efficacy of Thiethylperazine to Stimulate Cerebral MRP1 Transport Activity in Wild-Type and APP/PS1-21 Mice

Multidrug resistance-associated protein 1 (MRP1, encoded by the ABCC1 gene) may contribute to the clearance of amyloid-beta (Aβ) peptides from the brain into the blood and stimulation of MRP1 transport activity may be a therapeutic approach to enhance brain Aβ clearance. In this study, we assessed the effect of thiethylperazine, an antiemetic drug which was shown to stimulate MRP1 activity in vitro and to decrease Aβ load in a rapid β-amyloidosis mouse model (APP/PS1-21), on MRP1 transport activity by means of positron emission tomography (PET) imaging with the MRP1 tracer 6-bromo-7-[11C]methylpurine. Groups of wild-type, APP/PS1-21 and Abcc1(-/-) mice underwent PET scans before and after a 5-day oral treatment period with thiethylperazine (15 mg/kg, once daily). The elimination rate constant of radioactivity (kelim) was calculated from time-activity curves in the brain and the lungs as a measure of tissue MRP1 activity. Treatment with thiethylperazine had no significant effect on MRP1 activity in the brain and the lungs of wild-type and APP/PS1-21 mice. This may either be related to a lack of an MRP1-stimulating effect of thiethylperazine in vivo or to other factors, such as substrate-dependent MRP1 stimulation, insufficient target tissue exposure to thiethylperazine or limited sensitivity of the PET tracer to measure MRP1 stimulation.

Strategic, feasibility, economic, and cultural aspects of phase 0 approaches: Is it time to change the drug development process in order to increase productivity?

Research conducted over the past 2 decades has enhanced the validity and expanded the applications of microdosing and other phase 0 approaches in drug development. Phase 0 approaches can accelerate drug development timelines and reduce attrition in clinical development by increasing the quality of candidates entering clinical development and by reducing the time to "go-no-go" decisions. This can be done by adding clinical trial data (both healthy volunteers and patients) to preclinical candidate selection, and by applying methodological and operational advantages that phase 0 have over traditional approaches. The main feature of phase 0 approaches is the limited, subtherapeutic exposure to the test article. This means a reduced risk to research volunteers, and reduced regulatory requirements, timelines, and costs of first-in-human (FIH) testing. Whereas many operational aspects of phase 0 approaches are similar to those of other early phase clinical development programs, they have some unique strategic, regulatory, ethical, feasibility, economic, and cultural aspects. Here, we provide a guidance to these operational aspects and include case studies to highlight their potential impact in a range of clinical development scenarios.

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

BACKGROUND AND OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

CLINICAL TRIAL REGISTRATION

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

Pharmacokinetic Imaging Using 99mTc-Mebrofenin to Untangle the Pattern of Hepatocyte Transporter Disruptions Induced by Endotoxemia in Rats

Endotoxemia-induced inflammation may impact the activity of hepatocyte transporters, which control the hepatobiliary elimination of drugs and bile acids. ^99mTc-mebrofenin is a non-metabolized substrate of transporters expressed at the different poles of hepatocytes. ^99mTc-mebrofenin imaging was performed in rats after the injection of lipopolysaccharide (LPS). Changes in transporter expression were assessed using quantitative polymerase chain reaction of resected liver samples. Moreover, the particular impact of pharmacokinetic drug–drug interactions in the context of endotoxemia was investigated using rifampicin (40 mg/kg), a potent inhibitor of hepatocyte transporters. LPS increased ^99mTc-mebrofenin exposure in the liver (1.7 ± 0.4-fold). Kinetic modeling revealed that endotoxemia did not impact the blood-to-liver uptake of ^99mTc-mebrofenin, which is mediated by organic anion-transporting polypeptide (Oatp) transporters. However, liver-to-bile and liver-to-blood efflux rates were dramatically decreased, leading to liver accumulation. The transcriptomic profile of hepatocyte transporters consistently showed a downregulation of multidrug resistance-associated proteins 2 and 3 (Mrp2 and Mrp3), which mediate the canalicular and sinusoidal efflux of ^99mTc-mebrofenin in hepatocytes, respectively. Rifampicin effectively blocked both the Oatp-mediated influx and the Mrp2/3-related efflux of ^99mTc-mebrofenin. The additive impact of endotoxemia and rifampicin led to a 3.0 ± 1.3-fold increase in blood exposure compared with healthy non-treated animals. ^99mTc-mebrofenin imaging is useful to investigate disease-associated change in hepatocyte transporter function.

Comparative vulnerability of PET radioligands to partial inhibition of P-glycoprotein at the blood-brain barrier: A criterion of choice?

Only partial deficiency/inhibition of P-glycoprotein (P-gp, ABCB1) function at the blood-brain barrier (BBB) is likely to occur in pathophysiological situations or drug-drug interactions. This raises questions regarding the sensitivity of available PET imaging probes to detect moderate changes in P-gp function at the living BBB. In vitro, the half-maximum inhibitory concentration (IC₅₀) of the potent P-gp inhibitor tariquidar in P-gp-overexpressing cells was significantly different using either [¹¹C]verapamil (44 nM), [¹¹C]N-desmethyl-loperamide (19 nM) or [¹¹C]metoclopramide (4 nM) as substrate probes. In vivo PET imaging in rats showed that the half-maximum inhibition of P-gp-mediated efflux of [¹¹C]metoclopramide, achieved using 1 mg/kg tariquidar (in vivo IC₅₀ = 82 nM in plasma), increased brain exposure by 2.1-fold for [¹¹C]metoclopramide (p < 0.05, n = 4) and 2.4-fold for [¹¹C]verapamil (p < 0.05, n = 4), whereby cerebral uptake of the "avid" substrate [¹¹C]N-desmethyl-loperamide was unaffected (p > 0.05, n = 4). This comparative study points to differences in the "vulnerability" to P-gp inhibition among radiolabeled substrates, which were apparently unrelated to their "avidity" (maximal response to P-gp inhibition). Herein, we advocate that partial inhibition of transporter function, in addition to complete inhibition, should be a primary criterion of evaluation regarding the sensitivity of radiolabeled substrates to detect moderate but physiologically-relevant changes in transporter function in vivo.

PET imaging to assess the impact of P-glycoprotein on pulmonary drug delivery in rats

Several drugs approved for inhalation for the treatment of pulmonary diseases are substrates of the adenosine triphosphate-binding cassette (ABC) transporter P-glycoprotein (P-gp). P-gp is expressed in the apical membrane of pulmonary epithelial cells and could play a role in modulating the pulmonary absorption and distribution of inhaled drugs, thereby potentially contributing to variability in therapeutic response and/or systemic side effects. We developed a new in vivo experimental approach to assess the functional impact of P-gp on the pulmonary delivery of inhaled drugs in rats. By using positron emission tomography (PET) imaging, we measured the intrapulmonary pharmacokinetics of the model P-gp substrates (R)-[¹¹C]verapamil ([¹¹C]VPM) and [¹¹C]-N-desmethyl-loperamide ([¹¹C]dLOP) administered by intratracheal aerosolization in three rat groups: wild-type, Abcb1a/b^(-/-) and wild-type treated with the P-gp inhibitor tariquidar. Lung exposure (AUC_{lung_right}) to [¹¹C]VPM was 64% and 50% lower (p < 0.05) in tariquidar-treated and in Abcb1a/b^(-/-) rats, respectively, compared to untreated wild-type rats. For [¹¹C]dLOP, AUC_{lung_right} was 59% and 34% lower (p < 0.05) in tariquidar-treated and in Abcb1a/b^(-/-) rats, respectively. Our results show that P-gp can affect the pulmonary disposition of inhaled P-gp substrates, whereby a decrease in P-gp activity may lead to lower lung exposure and potentially to a decrease in therapeutic efficacy. Our study highlights the potential of PET imaging with intratracheally aerosolized radiotracers to assess the impact of membrane transporters on pulmonary drug delivery, in rodents and potentially also in humans.

Characterization of an APP/tau rat model of Alzheimer's disease by positron emission tomography and immunofluorescent labeling

BACKGROUND

To better understand the etiology and pathomechanisms of Alzheimer's disease, several transgenic animal models that overexpress human tau or human amyloid-beta (Aβ) have been developed. In the present study, we generated a novel transgenic rat model by cross-breeding amyloid precursor protein (APP) rats with tau rats. We characterized this model by performing positron emission tomography scans combined with immunofluorescent labeling and cerebrospinal fluid analyses.

METHODS

APP/Tau rats were generated by cross-breeding male McGill-R-Thy1-APP transgenic rats with female hTau-40/P301L transgenic rats. APP/Tau double transgenic rats and non-transgenic (ntg) littermates aged 7, 13, and 21 months were subjected to dynamic [11C] PiB scan and dynamic [18F]THK-5317 scans. For regional brain analysis, a template was generated from anatomical MR images of selected animals, which was co-registered with the PET images. Regional analysis was performed by application of the simplified reference tissue model ([11C]PiB data), whereas [18F]THK-5317 data were analyzed using a 2-tissue compartment model and Logan graphical analysis. In addition, immunofluorescent labeling (tau, amyloid) and cerebrospinal fluid analyses were performed.

RESULTS

[11C]PiB binding potential (BPND) and [18F]THK-5317 volume of distribution (VT) showed an increase with age in several brain regions in the APP/Tau group but not in the ntg control group. Immunohistochemical analysis of brain slices of PET-scanned animals revealed a positive correlation between Aβ labeling and [11C]PiB regional BPND. Tau staining yielded a trend towards higher levels in the cortex and hippocampus of APP/Tau rats compared with ntg littermates, but without reaching statistical significance. No correlation was found between tau immunofluorescence labeling results and the respective [18F]THK-5317 VT values.

CONCLUSIONS

We thoroughly characterized a novel APP/Tau rat model using combined PET imaging and immunofluorescence analysis. We observed an age-related increase in [11C]PiB and [18F]THK-5317 binding in several brain regions in the APP/Tau group but not in the ntg group. Although we were able to reveal a positive correlation between amyloid labeling and [11C]PiB regional brain uptake, we observed relatively low human tau and amyloid fibril expression levels and a somewhat unstable brain pathology which questions the utility of this animal model for further studies.