Radiolabeled constructs for evaluation of the asialoglycoprotein receptor status and hepatic functional reserves

ASGR1 and Its Enigmatic Relative, CLEC10A

The large family of C-type lectin (CLEC) receptors comprises carbohydrate-binding proteins that require Ca²⁺ to bind a ligand. The prototypic receptor is the asialoglycoprotein receptor-1 (ASGR1, CLEC4H1) that is expressed primarily by hepatocytes. The early work on ASGR1, which is highly specific for N-acetylgalactosamine (GalNAc), established the foundation for understanding the overall function of CLEC receptors. Cells of the immune system generally express more than one CLEC receptor that serve diverse functions such as pathogen-recognition, initiation of cellular signaling, cellular adhesion, glycoprotein turnover, inflammation and immune responses. The receptor CLEC10A (C-type lectin domain family 10 member A, CD301; also called the macrophage galactose-type lectin, MGL) contains a carbohydrate-recognition domain (CRD) that is homologous to the CRD of ASGR1, and thus, is also specific for GalNAc. CLEC10A is most highly expressed on immature DCs, monocyte-derived DCs, and alternatively activated macrophages (subtype M2a) as well as oocytes and progenitor cells at several stages of embryonic development. This receptor is involved in initiation of T_H1, T_H2, and T_H17 immune responses and induction of tolerance in naïve T cells. Ligand-mediated endocytosis of CLEC receptors initiates a Ca²⁺ signal that interestingly has different outcomes depending on ligand properties, concentration, and frequency of administration. This review summarizes studies that have been carried out on these receptors.

Cell therapy for liver disease: From liver transplantation to cell factory

Work over several decades has laid solid foundations for the advancement of liver cell therapy. To date liver cell therapy in people has taken the form of hepatocyte transplantation for metabolic disorders with a hepatic basis, and for acute or chronic liver failure. Although clinical trials using various types of autologous cells have been implemented to promote liver regeneration or reduce liver fibrosis, clear evidence of therapeutic benefits have so far been lacking. Cell types that have shown efficacy in preclinical models include hepatocytes, liver sinusoidal endothelial cells, mesenchymal stem cells, endothelial progenitor cells, and macrophages. However, positive results in animal models have not always translated through to successful clinical therapies and more realistic preclinical models need to be developed. Studies defining the optimal repopulation by transplanted cells, including routes of cell transplantation, superior engraftment and proliferation of transplanted cells, as well as optimal immunosuppression regimens are required. Tissue engineering approaches to transplant cells in extrahepatic locations have also been proposed. The derivation of hepatocytes from pluripotent or reprogrammed cells raises hope that donor organ and cell shortages could be overcome in the future. Critical hurdles to be overcome include the production of hepatocytes from pluripotent cells with equal functional capacity to primary hepatocytes and long-term phenotypic stability in vivo.

A nonhuman primate model of human radiation-induced venocclusive liver disease and hepatocyte injury

BACKGROUND

Human liver has an unusual sensitivity to radiation that limits its use in cancer therapy or in preconditioning for hepatocyte transplantation. Because the characteristic veno-occlusive lesions of radiation-induced liver disease do not occur in rodents, there has been no experimental model to investigate the limits of safe radiation therapy or explore the pathogenesis of hepatic veno-occlusive disease.

METHODS AND MATERIALS

We performed a dose-escalation study in a primate, the cynomolgus monkey, using hypofractionated stereotactic body radiotherapy in 13 animals.

RESULTS

At doses ≥40 Gy, animals developed a systemic syndrome resembling human radiation-induced liver disease, consisting of decreased albumin, elevated alkaline phosphatase, loss of appetite, ascites, and normal bilirubin. Higher radiation doses were lethal, causing severe disease that required euthanasia approximately 10 weeks after radiation. Even at lower doses in which radiation-induced liver disease was mild or nonexistent, latent and significant injury to hepatocytes was demonstrated by asialoglycoprotein-mediated functional imaging. These monkeys developed hepatic failure with encephalopathy when they received parenteral nutrition containing high concentrations of glucose. Histologically, livers showed central obstruction via an unusual intimal swelling that progressed to central fibrosis.

CONCLUSIONS

The cynomolgus monkey, as the first animal model of human veno-occlusive radiation-induced liver disease, provides a resource for characterizing the early changes and pathogenesis of venocclusion, for establishing nonlethal therapeutic dosages, and for examining experimental therapies to minimize radiation injury.

Liver-targeting of interferon-alpha with tissue-specific domain antibodies

Interferon alpha (IFNα) is used for the treatment of hepatitis C infection and whilst efficacious it is associated with multiple adverse events including reduced leukocyte, erythrocyte, and platelet counts, fatigue, and depression. These events are most likely caused by systemic exposure to interferon. We therefore hypothesise that targeting the therapeutic directly to the intended site of action in the liver would reduce exposure in blood and peripheral tissue and hence improve the safety and tolerability of IFNα therapy. We genetically fused IFN to a domain antibody (dAb) specific to a hepatocyte restricted antigen, asialoglycoprotein receptor (ASGPR). Our results show that the murine IFNα2 homolog (mIFNα2) fused to an ASGPR specific dAb, termed DOM26h-196-61, could be expressed in mammalian tissue culture systems and retains the desirable biophysical properties and activity of both fusion partners when measured in vitro. Furthermore a clear increase in in vivo targeting of the liver by mIFNα2-ASGPR dAb fusion protein, compared to that observed with either unfused mIFNα2 or mIFNα2 fused to an isotype control dAb V_HD2 (which does not bind ASGPR) was demonstrated using microSPECT imaging. We suggest that these findings may be applicable in the development of a liver-targeted human IFN molecule with improved safety and patient compliance in comparison to the current standard of care, which could ultimately be used as a treatment for human hepatitis virus infections.

A single-species approach considering additional physiological information for prediction of hepatic clearance of glycoprotein derivate therapeutics

BACKGROUND AND OBJECTIVES

Existing methods for the prediction of human clearance of therapeutic proteins involve the use of allometry approaches. In general, these approaches have concentrated on the role of body weight, with only occasional attention given to more specific physiological parameters. The objective of this study was to develop a mechanism-based model of hepatic clearance (CL(H)), which combines a single-species scaling approach with liver physiology, for predicting CL(H) of selected glycoprotein derivate therapeutics, and to compare the outcome of this novel method with those of two empirical methods obtained from the literature - namely, the single-exponent theory and multiple-species allometry. Thus, this study was designed as an explanatory study to verify if the addition of physiological information is of benefit for extrapolating clearance of selected therapeutic proteins from one species to another.

METHODS

Five glycoprotein derivate therapeutics that are known to be principally eliminated by asialoglycoprotein receptors (ASGPRs) under in vivo conditions were selected. It was assumed that the interspecies differences in CL(H) reported for these compounds are reflected by the interspecies differences in the abundance of these receptors. Therefore, key scaling factors related to these differences were integrated into one model. Fourteen extrapolation (prediction) scenarios across species were used in this study while comparing the single-species model, based on physiology, with the single-exponent theory. In addition, the physiological model was compared with multiple-species allometry for three proteins.

RESULTS

In general, the novel physiological model is superior to the derived allometric methods. Overall, the physiological model produced a predicted CL(H) value with levels of accuracy of 100% within 3-fold, 100% within 2-fold and about 82% within 1.5-fold, compared with the observed values, whereas the levels of accuracy decreased to 93%, 77% and 53%, respectively, for allometry. The proposed physiological model is also superior to allometry on the basis of the root mean square error and absolute average fold error values.

CONCLUSIONS

It has been demonstrated that interspecies differences in the abundance of ASGPRs principally govern interspecies variations in CL(H) of compounds that are principally eliminated by ASGPRs. Overall, the proposed physiological model is an additional tool, which should facilitate investigation and prediction of human CL(H) of specific glycoproteins solely on the basis of clearance data determined in a single preclinical species.

Carborane-carbohydrate derivatives — Versatile platforms for developing targeted radiopharmaceuticals

A series of carborane-carbohydrate derivatives were prepared as model platforms for developing targeted radiopharmaceuticals in which a single ligand can be tagged with two different classes of radionuclides. A nido-carborane linked to glucose was synthesized and the cage labelled with a radiometal (99mTc) and a radiohalogen (125I) in good radiochemical yield. The 99mTc-metallocarborane derivative was prepared via microwave-assisted reactions, while the iodinated derivative was prepared using oxidative labelling methods in mere minutes. Fully characterized reference standards were prepared using nonradioactive rhenium and iodine and used to demonstrate that the isolated radioactive compounds were in fact the desired targets. The work reported, which is one of the rare examples of a single ligand system that can be labelled with both radiohalogens and radiometals, forms the basis of a new paradigm for developing targeted radiopharmaceuticals.Key words: carboranes, medical isotopes, technetium-99m, iodine-125, glucose.

Prolonged survival of porcine hepatocytes in cynomolgus monkeys

BACKGROUND & AIMS

Management of patients with liver failure can be a significant medical challenge, and transplantation of the liver is the only definitive therapy. Whole liver allotransplantation is limited by a shortage of human donors and the risks of the surgery in those most ill. Transplants consisting of xenogeneic hepatocytes might overcome these problems, and work in rodents indicates that such transplants can correct some metabolic deficiencies and can prevent the complications and mortality associated with hepatic failure. As a prelude to clinical application, we tested the feasibility of hepatocyte xenotransplantation in nonhuman primates.

METHODS

One to 2 billion hepatocytes from outbred swine were transplanted into the spleens of cynomolgus monkeys using conventional immunosuppression to control rejection. Duration of graft function was determined based on assay for porcine albumin.

RESULTS

Following a single infusion, xenogeneic hepatocytes functioned for more than 80 days and, following re-transplantation, for more than 253 days. Engraftment in the spleen was confirmed 40 days after transplantation by asialoglycoprotein receptor-directed nuclear scanning. The humoral immune response to the transplanted porcine cells had no discernible impact on the survival of the grafts.

CONCLUSIONS

Xenotransplantation of hepatocytes should be explored as a readily available, minimally invasive form of therapy for hepatic failure.

Green upconversion nanocrystals for DNA detection

By combining magnetic-field-assisted bioseparation and concentration technology with magnetite nanoparticles, novel green upconversion (UC) fluorescence nanocrystals (NaYF4:Yb3+/Er3+) have been applied to the sensitive detection of DNA.