N-(4-[18F]fluorobenzyl)cholylglycine, a novel tracer for PET of enterohepatic circulation of bile acids: Radiosynthesis and proof-of-concept studies in rats

N-(4-[18F]fluorobenzyl)cholylglycine, a potential tracer for positron emission tomography of enterohepatic circulation and drug-induced inhibition of ileal bile acid transport. A proof-of-concept PET/CT study in pigs

INTRODUCTION

Enterohepatic circulation (EHC) of conjugated bile acids is an important physiological process crucial for bile acids to function as detergents and signal carriers. Perturbation of the EHC by disease or drugs may lead to serious and life-threatening liver and gastrointestinal disorders. In this proof-of-concept study in pigs, we investigate the potential of N-(4-[¹⁸F]fluorobenzyl)cholylglycine ([¹⁸F]FBCGly) as tracer for quantitative positron emission tomography (PET) of the EHC of conjugated bile acids.

METHODS

The biodistribution of [¹⁸F]FBCGly was investigated by PET/CT in domestic pigs following intravenous and intraileal administration of the tracer. Hepatic kinetics were estimated from PET and blood data using a 2-tissue compartmental model and dual-input of [¹⁸F]FBCGly. The ileal uptake of [¹⁸F]FBCGly was investigated with co-injection of nifedipine and endogenous cholyltaurine. Dosimetry was estimated from the PET data using the Olinda 2.0 software. Blood, bile and urine samples were analyzed for possible fluorine-18 labelled metabolites of [¹⁸F]FBCGly.

RESULTS

[¹⁸F]FBCGly was rapidly taken up by the liver and excreted into bile, and underwent EHC without being metabolized. Both nifedipine and endogenous cholyltaurine inhibited the ileal uptake of [¹⁸F]FBCGly. The flow-dependent hepatic uptake clearance was estimated to median 1.2 mL blood/min/mL liver tissue. The mean residence time of [¹⁸F]FBCGly in hepatocytes was 4.0 ± 1.1 min. Critical organs for [¹⁸F]FBCGly were the gallbladder wall (0.94 mGy/MBq) and the small intestine (0.50 mGy/MBq). The effective dose for [¹⁸F]FBCGly was 36 μSv/MBq.

CONCLUSION

We have shown that [¹⁸F]FBCGly undergoes EHC in pigs without being metabolized and that its ileal uptake is inhibited by nifedipine and endogenous bile acids. Combined with our previous findings in rats, we believe that [¹⁸F]FBCGly has potential as PET tracer for assessment of EHC of conjugated bile acids under physiological conditions as well as conditions with perturbed hepatic and ileal bile acid transport.

Clinical importance of laboratory biomarkers in liver fibrosis

Hepatic cirrhosis is a major health problem across the world, causing high morbidity and mortality. This disease has many etiologies, yet the result of chronic hepatic injury is hepatic fibrosis causing cirrhosis and hepatocellular carcinoma, as the liver’s architecture is progressively destroyed. While liver biopsy is currently the gold standard for fibrosis staging, it has significant disadvantages, leading to a growing interest in non-invasive markers. Direct biomarkers – hyaluronic acid, laminin, collagen type III N-peptide, type IV collagen and cholylglycine – are new and rarely applied in routine clinical practice. This is the case primarily because there is no general consensus regarding the clinical application and effectiveness of the individual biomarkers. The usage of these markers in routine clinical practice could be advantageous for patients with liver fibrosis, requiring a simple blood test instead of a biopsy. The former option would be especially attractive for patients who are contraindicated for the latter. This review summarizes recent findings on direct biomarkers of liver fibrosis and highlights their possible applications and potential benefit for liver fibrosis diagnostics and/or staging.

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.

Human biodistribution, dosimetry, radiosynthesis and quality control of the bile acid PET tracer [N-methyl-11C]cholylsarcosine

INTRODUCTION

[N-methyl-¹¹C]cholylsarcosine ([¹¹C]CSar) is a tracer for imaging and quantitative assessment of intrahepatic cholestatic liver diseases and drug-induced cholestasis by positron emission tomography (PET). The purpose of this study is to determine whole-body biodistribution and dosimetry of [¹¹C]CSar in healthy humans. The results are compared with findings in a patient with primary sclerosing cholangitis (PSC) and a patient with primary biliary cholangitis (PBC) as well as with preclinical findings in pigs. Radiosynthesis and quality control for preparation of [¹¹C]CSar for clinical use are also presented.

METHODS

Radiosynthesis and quality control of [¹¹C]CSar were set up in compliance with Danish/European regulations. Both healthy participants (3 females, 3 males) and patients underwent whole-body PET/CT to determine the biodistribution of [¹¹C]CSar. The two patients were under treatment with ursodeoxycholic acid at the time of the study. Dosimetry was estimated from the PET data using the Olinda 2.0 software.

RESULTS

The radiosynthesis provided [¹¹C]CSar in a solution ready for injection. The biodistribution studies revealed that gallbladder wall, small intestine, and liver were critical organs in both healthy participants and patients with the gallbladder wall receiving the highest dose (up to 0.5 mGy/MBq). The gender-averaged (±SD) effective dose for the healthy participants was 6.2 ± 1.4 μSv/MBq. The effective dose for the PSC and the PBC patient was 5.2 and 7.0 μSv/MBq, respectively.

CONCLUSION

A radiosynthesis for preparation of [¹¹C]CSar for clinical use was developed and approved by the Danish Medicines Agency. The most critical organ was the gallbladder wall although the amount of [¹¹C]CSar in the gallbladder was found to vary significantly between individuals. The estimated effective dose for humans was comparable to that estimated in anesthetized pigs although the absorbed dose estimates to some organs, such as the stomach, was different. ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE: [¹¹C]CSar PET/CT enables detailed quantitative assessment of patients with cholestatic liver disease by tracing the separate hepatobiliary transport steps of endogenous bile acids. The present work offers a radiosynthetic method and dosimetry data suitable for clinical implementation of [¹¹C]CSar.

Validation of hepatobiliary transport PET imaging in liver function assessment: Evaluation of 3β-[18F]FCA in mouse models of liver disease

Recently, our research group reported on the development of 3β-[¹⁸F]Fluorocholic acid (3β-[¹⁸F]FCA), a ¹⁸F labeled bile acid to detect drug interference with the bile acid transporters (drug-induced cholestasis). It was hypothesized that 3β-[¹⁸F]FCA could also be used as a non-invasive tool to monitor (regional) liver function in vivo in different liver diseases through altered expression of bile acid transporters.

METHODS

Hepatobiliary transport of 3β-[¹⁸F]FCA was evaluated in four murine liver disease models. Acute liver injury was induced by oral gavage of an acetaminophen (APAP) overdose (300 mg/kg). Chronic cholangiopathy and non-alcoholic steatohepatitis (NASH) were induced by feeding mice 3,5-diethoxycarbonyl- 1,4-dihydrocollidine (DDC) diet or methionine and choline deficient (MCD) diet, respectively. Hepatocellular carcinoma (HCC) was evoked by intraperitoneal injection of 35 mg/kg diethylnitrosamine (DEN) once a week for 23 weeks. Gene expression of the murine bile acid transporters was determined by RT-qPCR.

RESULTS

Hepatobiliary transport of 3β-[¹⁸F]FCA was not significantly altered after an APAP overdose. Mice fed the DDC or MCD diet showed impaired transport of 3β-[¹⁸F]FCA compared to baseline, which was associated with altered expression of the bile acid transporters ntcp, oatp4 and mrp2. After recovery from DDC- and MCD-induced liver injury, 3β-[¹⁸F]FCA parameters returned to baseline. Global hepatobiliary transport of 3β-[¹⁸F]FCA in HCC bearing mice was not significantly different compared to control mice. However, HCC lesions showed reduced hepatic uptake of the tracer (tumor-to-background: 0.45 ± 0.13), which was in line with decreased in expression of basolateral bile acid uptake transporters nctp and oatp4 in tumor tissue.

CONCLUSION

3β-[¹⁸F]FCA is a useful tool to assess and longitudinally follow-up liver function in several mouse models for liver diseases that are associated with altered expression of the bile acid transporters. These results point towards the (pre)clinical utility of 3β-[¹⁸F]FCA as a PET tracer to monitor altered liver functionality in patients with chronic liver diseases.