Dynamic Contrast-Enhanced MRI of OATP Dysfunction in Diabetes

Species-Specific Hepatic Uptake of [64Cu]Cu-EOB-NOTA, A Newly Designed Hepatospecific PET Agent

PURPOSE

Measuring hepatic flux rates of transportable substrates has the potential for assessing liver function. PET imaging of a PET-enabled substrate may provide a more straightforward measurement of time-dependent substrate concentration through the liver than MRI using an MRI contrast agent. Here we synthesized and evaluated the hepatobiliary transport of a new hepatospecific PET agent designed for stable Cu2+ chelation and transport by hepatic OATPs, [64Cu]Cu-EOB-NOTA.

PROCEDURES

EOB-NOTA was synthesized, its two enantiomers separated by chiral HPLC, and individually radiolabeled with [64Cu]Cu2+. Cocktails of each enantiomer of [64Cu]Cu-EOB-NOTA and Gd-EOB-DTPA were formulated for simultaneous PET/MRI imaging of hepatic flux by PET and MRI. Two mouse models were evaluated: wild-type mice and mice expressing only human hepatic OATPs.

RESULTS

In wild-type mice, [64Cu]Cu-EOB-NOTA hepatic influx and efflux was high, but slower compared to Gd-EOB-DTPA. Neither enantiomer of [64Cu]Cu-EOB-NOTA exhibited detectable transport into the liver in mice expressing human OATPs. This was validated by waste clearance studies and in vitro uptake assays in cells engineered to express rodent and human OATPs.

CONCLUSION

[64Cu]Cu-EOB-NOTA exhibited no detectable hepatic uptake by transgenic mice expressing human hepatic transporters. This finding was surprising given the efficient transport of the structurally similar metal chelate Gd-EOB-DTPA, and underscores challenges in the design of imaging molecular probes, including poor predictability for hepatic transport, and the value of validating new agents in mice expressing human hepatic transporters.

DCE-MRI Detects OATP-expressing Transplanted Cells Using Clinical Doses of Gadolinium Contrast Agent

PURPOSE

Hepatic organic anion transporting polypeptides (OATPs) transport off-the-shelf, FDA-approved, hepatospecific Gd-based MRI contrast agents into cells that express the transporters enhancing signal on T1-weighted MRI. Studies have used MRI to identify OATP-overexpressing tumors and metastases transplanted in mice following the delivery of Gd-EOB-DTPA at 27-67-fold higher than clinical doses. With safety and regulatory concerns over Gd-based contrast agents, translating OATPs as an MRI reporter protein to humans for regenerative medicine will require substantially lower doses of agent.

PROCEDURES

We engineered the MyC-CaP mouse tumor cell line to express rat OATP1B2, which influxes both Gd-EOB-DTPA and Gd-BOPTA, resulting in signal enhancement on T1-weighted MRI. We then inoculated mice with rat OATP1B2 and non-expressing cells bilaterally to generate tumors. 3-4 weeks after inoculation, when tumors had formed, in-vivo MRI imaging was performed with delivery of 0.025 mmol/kg or 0.25 mmol/kg of the Gd-based contrast agents. We complemented static T1-weighted MRI and T1-mapping with dynamic contrast enhanced (DCE)-MRI and performed area under the curve (AUC) analysis to discriminate the two tumor types.

RESULTS

While all OATP1B2-expressing tumors were easily visible at the high dose of 0.25 mmol/kg on T1-weighted MRI and easy to distinguish from control tumors, OATP1B2-expressing tumors were hard to identify and distinguish from non-expressing tumors at the lower, clinical dose of 0.025 mmol/kg with standard T1-weighted MRI or T1-mapping. However, AUC analyses of the DCE-MRI curves could identify and distinguish these tumors, needing 30 (Gd-EOB-DTPA) or 45 (Gd-BOPTA) minutes acquisition time.

CONCLUSIONS

By performing AUC analyses of DCE-MRI curves following delivery of clinical concentration of MRI contrast agents, OATP1B2-expressing tumors could be identified and distinguished from control tumors, suggesting this imaging approach as a path to substantially reducing the amount of contrast agent needed to use OATPs as a clinically viable reporter protein for imaging regenerative medicine.

MRI-Based Cell Tracking of OATP-Expressing Cell Transplants by Pre-Labeling with Gd-EOB-DTPA

PURPOSE

A critical step in cell-based therapies is determining the exact position of transplanted cells immediately post-transplant. Here, we devised a method to detect cell transplants immediately post-transplant, using a clinical gadolinium-based contrast agent. These cells were detected as hyperintense signals using a clinically familiar T1-weighted MRI protocol.

PROCEDURES

HEK293 cells were stably transduced to express human OATP1B3, a hepatic organic anion transporting polypeptide that transports Gd-EOB-DTPA into cells that express the transporters, the intracellular accumulation of which cells causes signal enhancement on T1-weighted MRI. Cells were pre-labeled prior to injection in media containing Gd-EOB-DTPA for MRI evaluation and indocyanine green for cryofluorescence tomography validation. Labeled cells were injected into chicken hearts, in vitro, after which MRI and cryofluorescence tomography were performed in sequence.

RESULTS

OATP1B3-expressing cells had substantially reduced T1 following labeling with Gd-EOB-DTPA in culture. Following their implantation into chicken heart, these cells were robustly identified in T1-weighted MRI, with image-derived injection volumes of cells commensurate with intended injection volumes. Cryofluorescence tomography showed that the areas of signal enhancement in MRI overlapped with areas of indocyanine green signal, indicating that MRI signal enhancement was due to the transplanted cells.

CONCLUSIONS

OATP1B3-expressing cells can be pre-labeled with Gd-EOB-DTPA prior to injection into tissue, affording the use of clinically familiar T1-weighted MRI to robustly detect cell transplants immediately after transplant. This procedure is easily generalizable and has potential advantages over the use of iron oxide based cell labeling agents and imaging procedures.

MRI-based cell tracking of OATP-expressing cell transplants by pre-labeling with Gd-EOB-DTPA

Purpose

A critical step in cell-based therapies is determining the exact position of transplanted cells immediately post-transplant. Here, we devised a method to detect cell transplants immediately post-transplant, using a clinical gadolinium-based contrast agent. These cells were detected as hyperintense signals using a clinically familiar T1-weighted MRI protocol.

Procedures

HEK293 cells were stably transduced to express human OATP1B3, a hepatic organic anion transporting polypeptide that transports Gd-EOB-DTPA into cells that express the transporters, the intracellular accumulation of which cells causes signal enhancement on T1-weighted MRI. Cells were pre-labeled prior to injection in media containing Gd-EOB-DTPA for MRI evaluation and indocyanine green for cryofluorescence tomography validation. Labeled cells were injected into chicken hearts, in vitro, after which MRI and cryofluorescence tomography were performed in sequence.

Results

OATP1B3-expressing cells had substantially reduced T1 following labeling with Gd-EOB-DTPA in culture. Following their implantation into chicken heart, these cells were robustly identified in T1-weighted MRI, with image-derived injection volumes of cells commensurate with intended injection volumes. Cryofluorescence tomography showed that the areas of signal enhancement in MRI overlapped with areas of indocyanine green signal, indicating that MRI signal enhancement was due to the transplanted cells.

Conclusions

OATP1B3-expressing cells can be pre-labeled with Gd-EOB-DTPA prior to injection into tissue, affording the use of clinically familiar T1-weighted MRI to robustly detect cell transplants immediately after transplant. This procedure is easily generalizable and has potential advantages over the use of iron oxide based cell labeling agents and imaging procedures.

Application of Pharmacokinetic Modeling to Characterize Hepatobiliary Disposition of Imaging Agents and Alterations due to Liver Injury in Isolated Perfused Rat Livers

BACKGROUND

Understanding the impact of altered hepatic uptake and/or efflux on the hepatobiliary disposition of the imaging agents [99mTc]Mebrofenin (MEB) and [153Gd]Gadobenate dimeglumine (BOPTA) is important for proper estimation of liver function.

METHODS

A multi-compartmental pharmacokinetic (PK) model describing MEB and BOPTA disposition in isolated perfused rat livers (IPRLs) was developed. The PK model was simultaneously fit to MEB and BOPTA concentration-time data in the extracellular space, hepatocytes, bile canaliculi, and sinusoidal efflux in livers from healthy rats, and to BOPTA concentration-time data in rats pretreated with monocrotaline (MCT).

RESULTS

The model adequately described MEB and BOPTA disposition in each compartment. The hepatocyte uptake clearance was much higher for MEB (55.3 mL/min) than BOPTA (6.67 mL/min), whereas the sinusoidal efflux clearance for MEB (0.000831 mL/min) was lower than BOPTA (0.0127 mL/min). The clearance from hepatocytes to bile (CLbc) for MEB (0.658 mL/min) was similar to BOPTA (0.642 mL/min) in healthy rat livers. The BOPTA CLbc was reduced in livers from MCT-pretreated rats (0.496 mL/min), while the sinusoidal efflux clearance was increased (0.0644 mL/min).

CONCLUSION

A PK model developed to characterize MEB and BOPTA disposition in IPRLs was used to quantify changes in the hepatobiliary disposition of BOPTA caused by MCT pretreatment of rats to induce liver toxicity. This PK model could be applied to simulate changes in the hepatobiliary disposition of these imaging agents in rats in response to altered hepatocyte uptake or efflux associated with disease, toxicity, or drug-drug interactions.

Effect of type 2 diabetes on liver images of GD-EOB-DTPA-enhanced MRI during the hepatobiliary phase

To analyze alterations of the liver appearance during the hepatobiliary phase of individuals with type 2 diabetes who are receiving gadolinium ethoxybenzyl diethylenetriamine pentaacetic acid (Gd-EOB-DTPA) enhanced magnetic resonance imaging (MRI). Fifty-seven individuals who received Gd-EOB-DTPA-enhanced MRI and had normal liver and renal function but did not have (control group) or have type 2 diabetes (observation group) were retrospectively included in this study. The liver enhancement ratio (LER) and contrast between liver parenchyma and portal vein (LPC) were calculated from hepatobiliary phase images. Utilizing liver to kidney signal intensity, signs of the biliary system, and signs of the portal vein, a functional liver imaging score (FLIS) was calculated. Wilcoxon rank-sum test was used to assess the between-group differences in LER, LPC, and FLIS. FLIS constituent ratios between the two groups were tested using the χ² test. The effectiveness of LER, LPC, and FLIS for identifying type 2 diabetes was assessed by receiver operating characteristic curves (ROCs). The interobserver consistency of FLIS was evaluated using the intraclass correlation coefficients. The observation group's LER and LPC were lower than the control group. The constituent ratio of the FLIS score (liver to kidney signal intensity, p = 0.011) showed a significant between-group difference. According to ROCs, LER and LPC were associated with the identification of type 2 diabetes. LER = 0.54 and LPC = 1.46 were the optimal cutoff for identifying type 2 diabetes, respectively. FLIS demonstrated excellent inter-reader agreement. The relative signal intensity of the liver during the hepatobiliary phase is decreased in patients with type 2 diabetes. This should be considered when individuals with type 2 diabetes undergo Gd-EOB-DTPA-enhanced MRI to avoid misdiagnoses, such as small hepatocellular carcinoma or abnormal liver function.

New and Emerging Research on Solute Carrier and ATP Binding Cassette Transporters in Drug Discovery and Development: Outlook From the International Transporter Consortium

Enabled by a plethora of new technologies, research in membrane transporters has exploded in the past decade. The goal of this state-of-the-art article is to describe recent advances in research on membrane transporters that are particularly relevant to drug discovery and development. This review covers advances in basic, translational, and clinical research that has led to an increased understanding of membrane transporters at all levels. At the basic level, we describe the available crystal structures of membrane transporters in both the solute carrier (SLC) and ATP binding cassette superfamilies, which has been enabled by the development of cryogenic electron microscopy methods. Next, we describe new research on lysosomal and mitochondrial transporters as well as recently deorphaned transporters in the SLC superfamily. The translational section includes a summary of proteomic research, which has led to a quantitative understanding of transporter levels in various cell types and tissues and new methods to modulate transporter function, such as allosteric modulators and targeted protein degraders of transporters. The section ends with a review of the effect of the gut microbiome on modulation of transporter function followed by a presentation of 3D cell cultures, which may enable in vivo predictions of transporter function. In the clinical section, we describe new genomic and pharmacogenomic research, highlighting important polymorphisms in transporters that are clinically relevant to many drugs. Finally, we describe new clinical tools, which are becoming increasingly available to enable precision medicine, with the application of tissue-derived small extracellular vesicles and real-world biomarkers.

Gadolinium-free Magnetic Resonance Imaging of the Liver via an Oatp1-Targeted Manganese(III) Porphyrin

Controversy surrounding gadolinium-based contrast agents (GBCAs) has rendered their continued utility highly contentious, but the liver-specific GBCA Gd(III) ethoxybenzyl-diethylene triamine pentaacetic acid (Gd(III)-EOB-DTPA) remains in use because it provides unique diagnostic information that could not be obtained by any other means. To address the need for an alternate liver-specific MRI probe, we synthesized Mn(III) 20-(4-ethoxyphenyl) porphyrin-5,10,15-tricarboxylate (Mn(III)TriCP-PhOEt), which exhibited significantly higher r1 relaxivity than Gd(III)-EOB-DTPA in vitro, while also targeting hepatocyte-specific organic anion-transporting polypeptide 1 (Oatp1) channels as a marker of viability. In mice, Mn(III)TriCP-PhOEt resulted in significant and specific increases in liver signal intensity on T1-weighted images and significant decreases in liver T1 time relative to pre-contrast measurements. Our findings suggest that Mn(III)TriCP-PhOEt operates as a specific and sensitive MR probe for Oatp1-targeted imaging in vivo.

Empagliflozin Treatment Attenuates Hepatic Steatosis by Promoting White Adipose Expansion in Obese TallyHo Mice

Sodium-glucose co-transporters (SGLTs) serve to reabsorb glucose in the kidney. Recently, these transporters, mainly SGLT2, have emerged as new therapeutic targets for patients with diabetes and kidney disease; by inhibiting glucose reabsorption, they promote glycosuria, weight loss, and improve glucose tolerance. They have also been linked to cardiac protection and mitigation of liver injury. However, to date, the mechanism(s) by which SGLT2 inhibition promotes systemic improvements is not fully appreciated. Using an obese TallyHo mouse model which recapitulates the human condition of diabetes and nonalcoholic fatty liver disease (NAFLD), we sought to determine how modulation of renal glucose handling impacts liver structure and function. Apart from an attenuation of hyperglycemia, Empagliflozin was found to decrease circulating triglycerides and lipid accumulation in the liver in male TallyHo mice. This correlated with lowered hepatic cholesterol esters. Using in vivo MRI analysis, we further determined that the reduction in hepatic steatosis in male TallyHo mice was associated with an increase in nuchal white fat indicative of "healthy adipose expansion". Notably, this whitening of the adipose came at the expense of brown adipose tissue. Collectively, these data indicate that the modulation of renal glucose handling has systemic effects and may be useful as a treatment option for NAFLD and steatohepatitis.

Imaging in experimental models of diabetes

Translational medicine, experimental medicine and experimental animal models, in particular mice and rats, represent a multidisciplinary field that has made it possible to achieve, in the last decades, important scientific progress. In this review, we have summarized the most frequently used imaging animal models, such as ultrasound (US), micro-CT, MRI and the optical imaging methods, and their main implications in diagnostic and therapeutic fields, with a particular focus on diabetes mellitus, a multifactorial disease extremely widespread among the general population.

Effect of type 2 diabetes on Gd-EOB-DTPA uptake into liver parenchyma: replication study in human subjects

PURPOSE

Gadolinium ethoxybenzyl diethylenetriamine pentaacetic acid (Gd-EOB-DTPA) is a contrast agent for magnetic resonance imaging (MRI), which specifically taken up by hepatocytes through organic anion-transporting polypeptides (OATPs). Previous research in mice has shown that type 2 diabetes is associated with reduced uptake of Gd-EOB-DTPA into the liver parenchyma, reflecting reduced expression of OATP. Since considerable differences in OATP expression exist between mice and humans, human studies are necessary to clarify the effect of diabetes to Gd-EOB-DTPA uptake. The purpose of this study was to validate the effect of diabetes to Gd-EOB-DTPA liver uptake by a confirmatory study in humans.

METHODS

Patients who underwent Gd-EOB-DTPA-enhanced MRI were retrospectively reviewed and divided into two groups: severe or uncontrolled diabetic group (patients with insulin therapy and/or HbA1c ≥ 8.4%) and the control group. Liver-to-spleen ratio (LSR) and relative enhancement of the liver (REL) were calculated to represent Gd-EOB-DTPA liver uptake.

RESULTS

A total of 94 patients fulfilled the criteria. The severe or uncontrolled diabetic group (n = 15) showed significantly lower LSR (1.74 ± 0.26 vs. 1.98 ± 0.31, p = 0.007) and REL (0.69 ± 0.23 vs. 0.87 ± 0.31, p = 0.005), compared to the control group (n = 79).

CONCLUSION

Our study revealed decreased uptake of Gd-EOB-DTPA into liver parenchyma in the severe or uncontrolled diabetic patients. Further studies to determine the impact of the reduced liver enhancement on clinical diagnostic practice will be needed.

Organic Anion Transporting Polypeptide 1B1 Is a Potential Reporter for Dual MR and Optical Imaging

Membrane proteins responsible for transporting magnetic resonance (MR) and fluorescent contrast agents are of particular importance because they are potential reporter proteins in noninvasive molecular imaging. Gadobenate dimeglumine (Gd-BOPTA), a liver-specific MR contrast agent, has been used globally for more than 10 years. However, the corresponding molecular transportation mechanism has not been validated. We previously reported that the organic anion transporting polypeptide (OATP) 1B3 has an uptake capability for both MR agents (Gd-EOB-DTPA) and indocyanine green (ICG), a clinically available near-infrared (NIR) fluorescent dye. This study further evaluated OATP1B1, another polypeptide of the OATP family, to determine its reporter capability. In the OATP1B1 transfected 293T transient expression model, both Gd-BOPTA and Gd-EOB-DTPA uptake were confirmed through 1.5 T MR imaging. In the constant OAPT1B1 and OATP1B3 expression model in the HT-1080 cell line, both HT-1080-OAPT1B1 and HT-1080-OATP1B3 were observed to ingest Gd-BOPTA and Gd-EOB-DTPA. Lastly, we validated the ICG uptake capability of both OATP1B1 and OATP1B3. OAPT1B3 exhibited a superior ICG uptake capability to that of OAPT1B1. We conclude that OATP1B1 is a potential reporter for dual MR and NIR fluorescent molecular imaging, especially in conjunction with Gd-BOPTA.

Use of imaging to assess the activity of hepatic transporters

Introduction: Membrane transporters of the SLC and ABC families are abundantly expressed in the liver, where they control the transfer of drugs/drug metabolites across the sinusoidal and canalicular hepatocyte membranes and play a pivotal role in hepatic drug clearance. Noninvasive imaging methods, such as PET, SPECT or MRI, allow for measuring the activity of hepatic transporters in vivo, provided that suitable transporter imaging probes are available.Areas covered: We give an overview of the working principles of imaging-based assessment of hepatic transporter activity. We discuss different currently available PET/SPECT radiotracers and MRI contrast agents and their applications to measure hepatic transporter activity in health and disease. We cover mathematical modeling approaches to obtain quantitative parameters of transporter activity and provide a critical assessment of methodological limitations and challenges associated with this approach.Expert opinion: PET in combination with pharmacokinetic modeling can be potentially applied in drug development to study the distribution of new drug candidates to the liver and their clearance mechanisms. This approach bears potential to mechanistically assess transporter-mediated drug-drug interactions, to assess the influence of disease on hepatic drug disposition and to validate and refine currently available in vitro-in vivo extrapolation methods to predict hepatic clearance of drugs.

Chimeric mouse model for MRI contrast agent evaluation

PURPOSE

While rodents are the primary animal models for contrast agent evaluation, rodents can potentially misrepresent human organ clearance of newly developed contrast agents. For example, gadolinium (Gd)-BOPTA has ~50% hepatic clearance in rodents, but ~5% in humans. This study demonstrates the benefit of chimeric mice expressing human hepatic OATPs (organic anion-transporting polypeptides) to improve evaluation of novel contrast agents for clinical use.

METHODS

FVB (wild-type) and OATP1B1/1B3 knock-in mice were injected with hepatospecific MRI contrast agents (Gd-EOB-DTPA, Gd-BOPTA) and nonspecific Gd-DTPA. T₁ -weighted dynamic contrast-enhanced MRI was performed on mice injected intravenously. Hepatic MRI signal enhancement was calculated per time point. Mass of gadolinium cleared per time point and percentage elimination by means of feces and urine were also measured.

RESULTS

Following intravenous injection of Gd-BOPTA in chimeric OATP1B1/1B3 knock-in mice, hepatic MRI signal enhancement and elimination by liver was more reflective of human hepatic clearance than that measured in wild-type mice. Gd-BOPTA hepatic MRI signal enhancement was reduced to 22% relative to wild-type mice. Gd-BOPTA elimination in wild-type mice was 83% fecal compared with 32% fecal in chimeric mice. Hepatic MRI signal enhancement and elimination for Gd-EOB-DTPA and Gd-DTPA were similar between wild-type and chimeric cohorts.

CONCLUSION

Hepatic MRI signal enhancement and elimination of Gd-EOB-DTPA, Gd-BOPTA, and Gd-DTPA in chimeric OATP1B1/1B3 knock-in mice closely mimics that seen in humans. This study provides evidence that the chimeric knock-in mouse is a more useful screening tool for novel MRI contrast agents destined for clinical use as compared to the traditionally used wild-type models.