Pitfalls and solutions of the fully-automated radiosynthesis of [11C]metoclopramide

Intramolecular Friedel-Crafts Acylation of [11C]Isocyanates Enabling the Radiolabeling of [carbonyl-11C]DPQ

α,β-aromatic lactams are highly abundant in biologically active molecules, yet so far they cannot be radiolabeled with short-lived (t1/2=20.3 min), β+-decaying carbon-11, which has prevented their application as positron emission tomography tracers. Herein, we developed, optimized, and applied a widely applicable, one-pot, quick, robust and automatable radiolabeling method for α,β-aromatic lactams starting from [11C]CO2 using the reagent POCl3⋅AlCl3. This method proceeds via intramolecular Friedel-Crafts acylation of in situ formed [11C]isocyanates and shows a broad substrate scope for the formation of five- and six-membered rings. We implemented our developed labeling method for the radiosynthesis of the potential PARP1 PET tracer [carbonyl-11C]DPQ in a clinical radiotracer production facility following the standards of the European Pharmacopoeia.

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.

Development and In Vivo Evaluation of Small-Molecule Ligands for Positron Emission Tomography of Immune Checkpoint Modulation Targeting Programmed Cell Death 1 Ligand 1

A substantial portion of patients do not benefit from programmed cell death protein 1/programmed cell death 1 ligand 1 (PD-1/PD-L1) checkpoint inhibition therapies, necessitating a deeper understanding of predictive biomarkers. Immunohistochemistry (IHC) has played a pivotal role in assessing PD-L1 expression, but small-molecule positron emission tomography (PET) tracers could offer a promising avenue to address IHC-associated limitations, i.e., invasiveness and PD-L1 expression heterogeneity. PET tracers would allow for improved quantification of PD-L1 through noninvasive whole-body imaging, thereby enhancing patient stratification. Here, a large series of PD-L1 targeting small molecules were synthesized, leveraging advantageous substructures to achieve exceptionally low nanomolar affinities. Compound 5c emerged as a promising candidate (IC50 = 10.2 nM) and underwent successful carbon-11 radiolabeling. However, a lack of in vivo tracer uptake in xenografts and notable accumulation in excretory organs was observed, underscoring the challenges encountered in small-molecule PD-L1 PET tracer development. The findings, including structure-activity relationships and in vivo biodistribution data, stand to illuminate the path forward for refining small-molecule PD-L1 PET tracers.

Cardiac PET Imaging of ATP Binding Cassette (ABC) Transporters: Opportunities and Challenges

Adenosine triphosphate binding cassette (ABC) transporters are a broad family of membrane protein complexes that use energy to transport molecules across cells and/or intracellular organelle lipid membranes. Many drugs used to treat cardiac diseases have an affinity for these transporters. Among others, P-glycoprotein (P-gp) plays an essential role in regulating drug concentrations that reach cardiac tissue and therefore contribute to cardiotoxicity. As a molecular imaging modality, positron emission tomography (PET) has emerged as a viable technique to investigate the function of P-gp in organs and tissues. Using PET imaging to evaluate cardiac P-gp function provides new insights for drug development and improves the precise use of medications. Nevertheless, information in this field is limited. In this review, we aim to examine the current applications of ABC transporter PET imaging and its tracers in the heart, with a specific emphasis on P-gp. Furthermore, the opportunities and challenges in this novel field will be discussed.

On the Road towards Small-Molecule Programmed Cell Death 1 Ligand 1 Positron Emission Tomography Tracers: A Ligand-Based Drug Design Approach

PD-1/PD-L1 immune checkpoint blockade for cancer therapy showed promising results in clinical studies. Further endeavors are required to enhance patient stratification, as, at present, only a small portion of patients with PD-L1-positive tumors (as determined by PD-L1 targeted immunohistochemistry; IHC) benefit from anti-PD-1/PD-L1 immunotherapy. This can be explained by the heterogeneity of tumor lesions and the intrinsic limitation of multiple biopsies. Consequently, non-invasive in vivo quantification of PD-L1 on tumors and metastases throughout the entire body using positron emission tomography (PET) imaging holds the potential to augment patient stratification. Within the scope of this work, six new small molecules were synthesized by following a ligand-based drug design approach supported by computational docking utilizing lead structures based on the (2-methyl-[1,1'-biphenyl]-3-yl)methanol scaffold and evaluated in vitro for potential future use as PD-L1 PET tracers. The results demonstrated binding affinities in the nanomolar to micromolar range for lead structures and newly prepared molecules, respectively. Carbon-11 labeling was successfully and selectively established and optimized with very good radiochemical conversions of up to 57%. The obtained insights into the significance of polar intermolecular interactions, along with the successful radiosyntheses, could contribute substantially to the future development of small-molecule PD-L1 PET tracers.

Comparison of the Blood-Brain Barrier Transport and Vulnerability to P-Glycoprotein-Mediated Drug-Drug Interaction of Domperidone versus Metoclopramide Assessed Using In Vitro Assay and PET Imaging

Domperidone and metoclopramide are widely prescribed antiemetic drugs with distinct neurological side effects. The impact of P-glycoprotein (P-gp)-mediated efflux at the blood−brain barrier (BBB) on brain exposure and BBB permeation was compared in vitro and in vivo using positron emission tomography (PET) imaging in rats with the radiolabeled analogs [11C]domperidone and [11C]metoclopramide. In P-gp-overexpressing cells, the IC50 of tariquidar, a potent P-gp inhibitor, was drastically different using [11C]domperidone (221 nM [198−248 nM]) or [11C]metoclopramide (4 nM [2−8 nM]) as the substrate. Complete P-gp inhibition led to a 1.8-fold higher increase in the cellular uptake of [11C]domperidone compared with [11C]metoclopramide (p < 0.0001). Brain PET imaging revealed that the baseline brain exposure (AUCbrain) of [11C]metoclopramide was 2.4-fold higher compared with [11C]domperidone (p < 0.001), consistent with a 1.8-fold higher BBB penetration (AUCbrain/AUCplasma). The maximal increase in the brain exposure (2.9-fold, p < 0.0001) and BBB penetration (2.9-fold, p < 0.0001) of [11C]metoclopramide was achieved using 8 mg/kg of tariquidar. In comparison, neither 8 nor 15 mg/kg of tariquidar increased the brain exposure of [11C]domperidone (p > 0.05). Domperidone is an avid P-gp substrate that was in vitro compared with metoclopramide. Domperidone benefits from a lower brain exposure and a limited risk for P-gp-mediated drug−drug interaction involving P-gp inhibition at the BBB.

Human Biodistribution and Radiation Dosimetry of the P-Glycoprotein Radiotracer [11C]Metoclopramide

Purpose

To assess in healthy volunteers the whole-body distribution and dosimetry of [¹¹C]metoclopramide, a new positron emission tomography (PET) tracer to measure P-glycoprotein activity at the blood-brain barrier.

Procedures

Ten healthy volunteers (five women, five men) were intravenously injected with 387 ± 49 MBq of [¹¹C]metoclopramide after low dose CT scans and were then imaged by whole-body PET scans from head to upper thigh over approximately 70 min. Ten source organs (brain, thyroid gland, right lung, myocardium, liver, gall bladder, left kidney, red bone marrow, muscle and the contents of the urinary bladder) were manually delineated on whole-body images. Absorbed doses were calculated with QDOSE (ABX-CRO) using the integrated IDAC-Dose 2.1 module.

Results

The majority of the administered dose of [¹¹C]metoclopramide was taken up into the liver followed by urinary excretion and, to a smaller extent, biliary excretion of radioactivity. The mean effective dose of [¹¹C]metoclopramide was 1.69 ± 0.26 μSv/MBq for female subjects and 1.55 ± 0.07 μSv/MBq for male subjects. The two organs receiving the highest radiation doses were the urinary bladder (10.81 ± 0.23 μGy/MBq and 8.78 ± 0.89 μGy/MBq) and the liver (6.80 ± 0.78 μGy/MBq and 4.91 ± 0.74 μGy/MBq) for female and male subjects, respectively.

Conclusions

[¹¹C]Metoclopramide showed predominantly renal excretion, and is safe and well tolerated in healthy adults. The effective dose of [¹¹C]metoclopramide was comparable to other ¹¹C-labeled PET tracers.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11307-021-01582-4.

Impaired Clearance From the Brain Increases the Brain Exposure to Metoclopramide in Elderly Subjects

The antiemetic and gastroprokinetic drug metoclopramide is a weak substrate of the blood‐brain barrier (BBB) efflux transporter P‐gp and displays central nervous system (CNS) side effects (i.e., extrapyramidal symptoms and tardive dyskinesia) caused by dopamine D₂ receptor blockade in the basal ganglia. These side effects occur with a higher incidence in elderly people. We used positron emission tomography to assess the brain distribution of [¹¹C]metoclopramide in young (n = 11, 26 ± 3 years) and elderly (n = 7, 68 ± 9 years) healthy men both after administration of a microdose (9 ± 7 µg) and a microdose co‐injected with a therapeutic dose of unlabeled metoclopramide (10 mg). For both doses, elderly subjects had a significantly higher total volume of distribution (V_T) of [¹¹C]metoclopramide in the basal ganglia than young subjects (microdose: +26%, therapeutic dose: +41%). Increases in V_T (= K₁/k₂) were caused by significant decreases in the transfer rate constant of [¹¹C]metoclopramide from brain into plasma (k₂, microdose: −18%, therapeutic dose: −30%), whereas the distributional clearance from plasma into brain (K₁) remained unaltered. This reduction in the clearance of [¹¹C]metoclopramide (k₂) from the brains of elderly subjects may be caused by an age‐related decrease in the activity of P‐gp at the BBB and may contribute to the higher incidence of CNS side effects of metoclopramide in the aged population. Our data suggest that an age‐associated decrease in the clearance properties of the BBB may modulate the CNS effects or side effects of clinically used P‐gp substrates.

Carbon-11 carboxylation of trialkoxysilane and trimethylsilane derivatives using [11C]CO2

A simple and rapid carbon-11 carboxylation radiosynthesis method.

A novel carboxylation radiosynthesis methodology is described starting from cyclotron-produced [¹¹C]CO₂ and fluoride-activated silane derivatives. Six carbon-11 labelled carboxylic acids were obtained from their corresponding trimethylsilyl and trialkoxysilyl precursors in a one-pot labelling methodology. The radiochemical yields ranged from 19% to 93% within 12 minutes post [¹¹C]CO₂ delivery with a trapping efficiency of 21–89%.