Influence of Multidrug Resistance-Associated Proteins on the Excretion of the ABCC1 Imaging Probe 6-Bromo-7-[11C]Methylpurine in Mice

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.

Improved synthesis of 6-bromo-7-[11C]methylpurine for clinical use

BACKGROUND

Multidrug resistance-associated protein 1 (MRP1), an energy-dependent efflux pump, is expressed widely in various tissues and contributes to many physiological and pathophysiological processes. 6-Bromo-7-[11C]methylpurine ([11C]7m6BP) is expected to be useful for the assessment of MRP1 activity in the human brain and lungs. However, the radiochemical yield (RCY) in the synthesis of [11C]7m6BP was low, limiting its clinical application, because the methylation of the precursor with [11C]CH3I provided primarily the undesired isomer, 6-bromo-9-[11C]methylpurine ([11C]9m6BP). To increase the RCY of [11C]7m6BP, we investigated conditions for improving the [11C]7m6BP/[11C]9m6BP selectivity of the methylation reaction.

RESULTS

[11C]7m6BP was manually synthesized via the methylation of 6-bromopurine with [11C]CH3I in various solvents and at different temperatures in the presence of potassium carbonate for 5 min. Several less polar solvents, including tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), and ethyl acetate (AcOEt) improved the [11C]7m6BP/[11C]9m6BP selectivity from 1:1 to 2:1, compared with the conventionally used solvents for the alkylation of 6-halopurines, acetone, acetonitrile, and N,N-dimethylformamide. However, a higher temperature (140 °C or 180 °C) was needed to progress the 11C-methylation in the less polar solvents, and the manual conditions could not be directly translated to an automated synthesis. [11C]Methyl triflate ([11C]CH3OTf) was thus used as a methylating agent to increase the conversion at a lower temperature. The 11C-methylation using [11C]CH3OTf at 100 °C proceeded efficiently in THF, 2-MeTHF, and AcOEt with maintenance of the improved selectivity. Starting from 28 to 34 GBq [11C]CO2, [11C]7m6BP was produced with 2.3-2.6 GBq for THF, 2.7-3.3 GBq for AcOEt, and 2.8-3.9 GBq for 2-MeTHF at approximately 30 min after the end of bombardment (n = 3 per solvent). The isolated RCYs (decay corrected) for THF, 2-MeTHF, and AcOEt were 24-28%, 29-35%, and 22-31% (n = 3), respectively.

CONCLUSIONS

The use of THF, 2-MeTHF, and AcOEt improved the [11C]7m6BP/[11C]9m6BP selectivity in the methylation reaction, and the improved method provided [11C]7m6BP with sufficient radioactivity for clinical use.

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.

Cytotoxicity and reversal effect of sertraline, fluoxetine, and citalopram on MRP1- and MRP7-mediated MDR

Cancer is one of the leading causes of death worldwide, and the development of resistance to chemotherapy drugs is a major challenge in treating malignancies. In recent years, researchers have focused on understanding the mechanisms of multidrug resistance (MDR) in cancer cells and have identified the overexpression of ATP-binding cassette (ABC) transporters, including ABCC1/MRP1 and ABCC10/MRP7, as a key factor in the development of MDR. In this study, we aimed to investigate whether three drugs (sertraline, fluoxetine, and citalopram) from the selective serotonin reuptake inhibitor (SSRI) family, commonly used as antidepressants, could be repurposed as inhibitors of MRP1 and MRP7 transporters and reverse MDR in cancer cells. Using a combination of in silico predictions and in vitro validations, we analyzed the interaction of MRP1 and MRP7 with the drugs and evaluated their ability to hinder cell resistance. We used computational tools to identify and analyze the binding site of these three molecules and determine their binding energy. Subsequently, we conducted experimental assays to assess cell viability when treated with various standard chemotherapies, both with and without the presence of SSRI inhibitors. Our results show that all three SSRI drugs exhibited inhibitory/reversal effects in the presence of chemotherapies on both MRP1-overexpressed cells and MRP7-overexpressed cells, suggesting that these medications have the potential to be repurposed to target MDR in cancer cells. These findings may open the door to using FDA-approved medications in combination therapy protocols to treat highly resistant malignancies and improve the efficacy of chemotherapy treatment. Our research highlights the importance of investigating and repurposing existing drugs to overcome MDR in cancer treatment.

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.

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.

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.

Influence of ABC transporters on the excretion of ciprofloxacin assessed with PET imaging in mice

Ciprofloxacin is a commonly prescribed fluoroquinolone antibiotic which is cleared by active tubular secretion and intestinal excretion. Ciprofloxacin is a known substrate of the ATP-binding cassette (ABC) transporters breast cancer resistance protein (BCRP) and multidrug resistance-associated protein 4 (MRP4). In this work, we used positron emission tomography (PET) imaging to investigate the influence of BCRP, MRP4, MRP2 and P-glycoprotein (P-gp) on the excretion of [¹⁸F]ciprofloxacin in mice. Dynamic 90-min PET scans were performed after intravenous injection of [¹⁸F]ciprofloxacin in wild-type mice without and with pre-treatment with the broad-spectrum MRP inhibitor MK571. Moreover, [¹⁸F]ciprofloxacin PET scans were performed in Abcc4^(-/-), Abcc2^(-/-), Abcc4^(-/-)Abcg2^(-/-) and Abcb1a/b^(-/-)Abcg2^(-/-) mice. In addition to non-compartmental pharmacokinetic (PK) analysis, a novel three-compartment PK model was developed for a detailed assessment of the renal disposition of [¹⁸F]ciprofloxacin. In MK571 pre-treated mice, a significant increase in the blood exposure to [¹⁸F]ciprofloxacin was observed along with a significant reduction in the renal and intestinal clearances. PK modelling revealed a significant reduction in renal radioactivity uptake (CL₁) and in the rate constants for transfer of radioactivity from the corticomedullary renal region into blood (k₂) and urine (k₃), respectively, after MK571 administration. No changes in the renal clearance or in the estimated kidney PK model parameters were observed in any of the studied knockout models, while a significant reduction in the intestinal clearance was observed in Abcc2^(-/-) and Abcc4^(-/-)Abcg2^(-/-) mice. Our data failed to reveal a role of any of the studied ABC transporters in the tubular secretion of ciprofloxacin. This may indicate that ciprofloxacin is handled in the kidneys by more than one transporter family, most likely with a great degree of mutual functional redundancy. Our study highlights the potential of PET imaging for an assessment of transporter-mediated renal excretion of radiolabelled drugs.

Measurement of cerebral ABCC1 transport activity in wild-type and APP/PS1-21 mice with positron emission tomography

Previous data suggest a possible link between multidrug resistance-associated protein 1 (ABCC1) and brain clearance of beta-amyloid (Aβ). We used PET with 6-bromo-7-[11C]methylpurine ([11C]BMP) to measure cerebral ABCC1 transport activity in a beta-amyloidosis mouse model (APP/PS1-21) and in wild-type mice aged 50 and 170 days, without and with pretreatment with the ABCC1 inhibitor MK571. One hundred seventy days-old-animals additionally underwent [11C]PiB PET scans to measure Aβ load. While baseline [11C]BMP PET scans detected no differences in the elimination slope of radioactivity washout from the brain (kelim) between APP/PS1-21 and wild-type mice of both age groups, PET scans after MK571 pretreatment revealed significantly higher kelim values in APP/PS1-21 mice than in wild-type mice aged 170 days, suggesting increased ABCC1 activity. The observed increase in kelim occurred across all investigated brain regions and was independent of the presence of Aβ plaques measured with [11C]PiB. Western blot analysis revealed a trend towards increased whole brain ABCC1 levels in 170 days-old-APP/PS1-21 mice versus wild-type mice and a significant positive correlation between ABCC1 levels and kelim. Our data point to an upregulation of ABCC1 in APP/PS1-21 mice, which may be related to an induction of ABCC1 in astrocytes as a protective mechanism against oxidative stress.

Mechanisms of glutathione-conjugate efflux from the brain into blood: Involvement of multiple transporters in the course

Accumulation of detrimental glutathione-conjugated metabolites in the brain potentially causes neurological disorders, and must therefore be exported from the brain. However, in vivo mechanisms of glutathione-conjugates efflux from the brain remain unknown. We investigated the involvement of transporters in glutathione-conjugates efflux using 6-bromo-7-[¹¹C]methylpurine ([¹¹C]1), which enters the brain and is converted into its glutathione conjugate, S-(7-[¹¹C]methylpurin-6-yl)glutathione ([¹¹C]2). In mice of control and knockout of P-glycoprotein/breast cancer resistance protein and multidrug resistance-associated protein 2 ([Mrp2]^-/-), [¹¹C]2 formed in the brain was rapidly cleared, with no significant difference in efflux rate. In contrast, [¹¹C]2 formed in the brain of Mrp1^-/- mice was slowly cleared, whereas [¹¹C]2 microinjected into the brain of control and Mrp1^-/- mice was 75% cleared within 60 min, with no significant difference in efflux rate. These suggest that Mrp1 contributes to [¹¹C]2 efflux across cell membranes, but not BBB. Efflux rate of [¹¹C]2 formed in the brain was significantly lower in Mrp4^-/- and organic anion transporter 3 (Oat3)^-/- mice compared with control mice. In conclusion, Mrp1, Oat3, and Mrp4 mediate [¹¹C]2 efflux from the brain. Mrp1 may contribute to [¹¹C]2 efflux from brain parenchymal cells, while extracellular [¹¹C]2 is likely cleared across the BBB, partly by Oat3 and Mrp4.

Generation and Characterization of an Abcc1 Humanized Mouse Model (hABCC1flx/flx ) with Knockout Capability

ATP-binding cassette (ABC) transporters such as ABCB1 (P-glycoprotein), ABCC1 (MRP1), and ABCG2 (BCRP) are well known for their role in rendering cancer cells resistant to chemotherapy. Additionally, recent research provided evidence that, along with other ABC transporters (ABCA1 and ABCA7), they might be cornerstones to tackle neurodegenerative diseases. Overcoming chemoresistance in cancer, understanding drug-drug interactions, and developing efficient and specific drugs that alter ABC transporter function are hindered by a lack of in vivo research models, which are fully predictive for humans. Hence, the humanization of ABC transporters in mice has become a major focus in pharmaceutical and neurodegenerative research. Here, we present a characterization of the first Abcc1 humanized mouse line. To preserve endogenous expression profiles, we chose to generate a knockin mouse model that leads to the expression of a chimeric protein that is fully human except for one amino acid. We found robust mRNA and protein expression within all major organs analyzed (brain, lung, spleen, and kidney). Furthermore, we demonstrate the functionality of the expressed human ABCC1 protein in brain and lungs using functional positron emission tomography imaging in vivo. Through the introduction of loxP sites, we additionally enabled this humanized mouse model for highly sophisticated studies involving cell type-specific transporter ablation. Based on our data, the presented mouse model appears to be a promising tool for the investigation of cell-specific ABCC1 function. It can provide a new basis for better translation of preclinical research.