99mTc-neoglycoalbumin (NGA)-binding to human hepatic binding protein (HBP) in vitro

Research progress on asialoglycoprotein receptor-targeted radiotracers designed for hepatic nuclear medicine imaging

Asialoglycoprotein receptor (ASGPR) specifically recognizes glycans terminated with β-d-galactose or N-acetylgalactosamine. Its exclusive expression in mammalian hepatocytes renders it an ideal hepatic-targeted biomarker. To date, ASGPR-targeted ligands have been actively developed for drug delivery and hepatic imaging. This review provides a comprehensive summary of the progress achieved to-date in the field of developing ASGPR-targeted nuclear medicine imaging (NMI) radiotracers, highlighting the recent advancements over the last decade in terms of structure, radionuclides and labeling strategies. The biodistribution patterns, imaging characteristics, challenges and future prospective are discussed.

Evaluation of GalNAc-siRNA Conjugate Activity in Pre-clinical Animal Models with Reduced Asialoglycoprotein Receptor Expression

The hepatocyte-specific asialoglycoprotein receptor (ASGPR) is an ideal candidate for targeted drug delivery to the liver due to its high capacity for substrate clearance from circulation together with its well-conserved expression and function across species. The development of GalNAc-siRNA conjugates, in which a synthetic triantennary N-acetylgalactosamine-based ligand is conjugated to chemically modified siRNA, has enabled efficient, ASGPR-mediated delivery to hepatocytes. To investigate the potential impact of variations in receptor expression on the efficiency of GalNAc-siRNA conjugate delivery, we evaluated the pharmacokinetics and pharmacodynamics of GalNAc-siRNA conjugates in multiple pre-clinical models with reduced receptor expression. Despite greater than 50% reduction in ASGPR levels, GalNAc conjugate activity was retained, suggesting that the remaining receptor capacity was sufficient to mediate efficient uptake of potent GalNAc-siRNAs at pharmacologically relevant dose levels. Collectively, our data support a broad application of the GalNAc-siRNA technology for hepatic targeting, including disease states where ASGPR expression may be reduced.

Development of ⁶⁸Ga-labelled DTPA galactosyl human serum albumin for liver function imaging

PURPOSE

The hepatic asialoglycoprotein receptor is responsible for degradation of desialylated glycoproteins through receptor-mediated endocytosis. It has been shown that imaging of the receptor density using [(99m)Tc]diethylenetriamine pentaacetic acid (DTPA) galactosyl human serum albumin ([(99m)Tc]GSA) allows non-invasive determination of functional hepatocellular mass. Here we present the synthesis and evaluation of [(68)Ga]GSA for the potential use with positron emission tomography (PET).

METHODS

Labelling of GSA with (68)Ga was carried out using a fractionated elution protocol. For quality control thin-layer chromatography (TLC), high-performance liquid chromatography (HPLC) and size exclusion chromatography (SEC) techniques were evaluated. Stability of [(68)Ga]GSA was studied in phosphate-buffered saline (PBS) and human serum. For in vivo evaluation [(68)Ga]GSA distribution in Lewis rats was compared with [(99m)Tc]GSA by using a dual isotope protocol. PET and planar imaging studies were performed using the same scaled molar dose of [(68)Ga]GSA and [(99m)Tc]GSA. Time-activity curves (TAC) for heart and liver were generated and corresponding parameters calculated (t50, t90).

RESULTS

[(68)Ga]GSA can be produced with high radiochemical purity. The best TLC methods for determining potential free (68)Ga include 0.1 M sodium citrate as eluent. None of the TLC methods tested were able to determine potential colloids. This can be achieved by SEC. HPLC confirmed high radiochemical purity (>98%). Stability after 120 min incubation at 37 °C was high in PBS (>95% intact tracer) and low in human serum (∼27% intact tracer). Biodistribution studies simultaneously injecting both tracers showed comparable liver uptake, whereas activity concentration in blood was higher for [(68)Ga]GSA compared to [(99m)Tc]GSA. The [(99m)Tc]GSA TACs exhibited a small degree of hepatic metabolism compared to the [(68)Ga]GSA curves. The mean [(68)Ga]GSA t90 was higher than the mean t90 for [(99m)Tc]GSA. The mean [(68)Ga]GSA t50 was not significantly different from the mean t50 for [(99m)Tc]GSA.

CONCLUSION

This study provides a promising new (68)Ga-labelled compound based on a commercially used kit for imaging the functional hepatocellular mass.

Scintigraphic evaluation of functional hepatic mass in patients with advanced breast cancer

Recent studies suggest a high specificity of 99mTc-galactosyl neoglycoalbumin (99mTc-NGA) receptor scanning in vivo by providing both morphological and functional diagnosis of liver disease. In 22 patients with advanced breast cancer 99mTc-NGA (150 MBq; 50 nmol) was exclusively trapped by the liver, the images showing 'cold spots' in areas of liver metastases formation. A two-tailed analysis was performed: the time activity curves recorded for the liver and precordial area were subjected to a kinetic receptor-calculating model allowing an estimation of the NGA-receptor concentration of the liver (i.e. hepatic binding protein, HBP) as well as calculation of the residual functional liver volume (RFLV) via the S.P.E.C.T.-study. In breast cancer patients with liver metastases a significantly (P < 0.01) lower HBP-concentration was estimated (0.65 +/- 0.16 vs 0.82 +/- 0.17 mumol l-1) as evidenced by a lower 99mTc-NGA-accumulation in the liver resulting also in a significantly (P < 0.001) lower RFLV (739 +/- 348 vs 1336 +/- 184 ml). In four amonafide-treated patients (800 mg m-2 intravenous infusion over 3 h) approximately one week after one chemotherapy cycle a significant (P < 0.05) increase in HBP-concentration (0.56 +/- 0.10 vs 0.72 +/- 0.06 mumol l-1) of the liver was found corresponding with an increase in RVLF (546 +/- 297 vs 670 +/- 265 ml). These regulatory mechanisms at the HBP level measured in vivo provide further evidence that 99mTc-NGA should have promise as a clinically useful receptor radiopharmaceutical for both quantification of liver function and assessment of liver morphology.

Liver function in acute viral hepatitis as determined by a hepatocyte-specific ligand: 99mTc-galactosyl-neoglycoalbumin

Twelve patients with recently diagnosed acute viral hepatitis underwent serial 99mTc-galactosyl neoglycoalbumin scanning of the liver (for up to 8 mo). Injection of 99mTc-galactosyl neoglycoalbumin (150 mBq) at a rate of 3.5 mg (50 nmol; 1 ml) revealed that the liver is the exclusive site of tracer uptake. Simulation of 99mTc-galactosyl neoglycoalbumin kinetics allowed quantification of galactosyl neoglycoalbumin binding to human hepatic binding protein. Return of liver function test scores to normal values was associated in two patients with hepatitis A, in four patients with hepatitis B and in two patients with non-A, non-B hepatitis virus infection, with increases in hepatic binding protein concentration (up to three times the initial concentration), binding rate constant and hepatic blood flow. In the other four patients (three patients with hepatitis B and one patient with cytomegalovirus infection) a prolonged course of disease was monitored. In the mean, hepatic binding protein increased from 0.41 +/- 0.11 mumol/L after onset of acute hepatitis (n = 12) to 0.78 +/- 0.21 mumol/L after 6 mo of follow-up (n = 10) (p less than 0.001). During this period, binding rate constant (72.4 +/- 12.6 vs. 82 +/- 11.5 mumol/L/sec; p less than 0.05) and hepatic blood flow (0.027 +/- 0.0051 vs. 0.031 +/- 0.0083 L/sec; p less than 0.05) increased. Hepatic binding protein concentration correlated highly with actual laboratory test results for liver function (r = 0.98; p = 0.0001). We conclude that scintigraphic evaluation of functional liver cell mass using the new receptor-tracer 99mTc-galactosyl neoglycoalbumin could provide an in vivo diagnostic means of quantifying liver function and assessing liver morphology. In addition, our findings suggest that changes in hepatic binding protein-receptor concentration are likely to occur in vivo.

Decreased hepatic function in patients with hepatoma or liver metastasis monitored by a hepatocyte specific galactosylated radioligand

99mTc-galactosylated neoglycoalbumin (99mTc-NGA) is a hepatocyte-specific tracer that, after injection into the blood stream, delivers radioactivity selectively to the liver. This is based upon chemical recognition and binding by the hepatic binding protein (HBP), a receptor specific for galactosylated glycoproteins. Liver tissue samples were obtained intraoperatively from patients undergoing surgery for various cancers. The concentration of specific HBP receptors in the liver (normal liver, hepatoma, liver metastasis) was calculated from the in vitro binding of 99mTc-NGA. One week after surgery, the in vivo HBP density was also measured in some of these patients after injection of 3.5 mg (50 nmol per patient) 99mTc-NGA (150-200 MBq) for simulation of 99mTc-NGA kinetics. Comparison of in vitro and in vivo HBP concentration in the liver showed values in the same concentration range. In patients with hepatoma or liver metastasis a significantly (P less than 0.01) decreased global HBP density was found in vivo compared to controls. The values obtained for in vivo HBP concentration in the liver amounted to 0.38 +/- 0.05 mumol l-1 liver for patients with hepatoma, to 0.4 +/- 0.1 mumol l-1 in patients with liver metastasis and to 94 +/- 0.05 mumol l-1 liver in cancer patients without liver malignancy. In vitro investigation of HBP density revealed the malignant liver tissue to have a significantly (P less than 0.0001) decreased or almost (completely) absent HBP receptor density compared to the normal tissue apart from the cancer area. It is concluded that determination of HBP density in vivo via a specific tracer is a new, simple and reliable approach for the determination of remaining hepatic function in patients with primary or secondary liver cancer.