Pharmacokinetics and mechanisms of plasma removal of hemoglobin-based oxygen carriers

Recognition mechanisms of hemoglobin particles by monocytes - CD163 may just be one

Hemoglobin-based oxygen carriers (HBOCs) as blood substitutes are one of the great hopes of modern transfusion and emergency medicine. After the major safety-relevant challenges of the last decades seem to be largely overcome, current developments have in common that they are affected by degradation and excretion at an early stage in test organisms. Several possible mechanisms that may be responsible for this are discussed in the literature. One of them is CD163, the receptor of the complex of haptoglobin (Hp) and hemoglobin (Hb). The receptor has been shown in various studies to have a direct affinity for Hb in the absence of Hp. Thus, it seems reasonable that CD163 could possibly also bind Hb within HBOCs and cause phagocytosis of the particles. In this work we investigated the role of CD163 in the uptake of our hemoglobin sub-micron particles (HbMPs) in monocytes and additionally screened for alternative ways of particle recognition by monocytes. In our experiments, blockade of CD163 by specific monoclonal antibodies proved to partly inhibit HbMP uptake by monocytes. It appears, however, that several other phagocytosis pathways for HbMPs might exist, independent of CD163 and also Hb.

The Use of Hemoglobin-Based Oxygen Carriers in Ex Vivo Machine Perfusion of Donor Organs for Transplantation

There has been significant progress in the development of ex vivo machine perfusion for the nonischemic preservation of donor organs. However, several complications remain, including the logistics of using human blood for graft oxygenation and hemolysis occurring as a result of mechanical technology. Recently, hemoglobin-based oxygen carriers, originally developed for use as blood substitutes, have been studied as an alternative to red blood cell-based perfusates. Although research in this field is somewhat limited, the findings are promising. We offer a brief review of the use of hemoglobin-based oxygen carriers in ex vivo machine perfusion and discuss future directions that will likely have a major impact in progressing oxygen carrier use in clinical practice.

Towards understanding the cell surface phenotype, metabolic properties and immune functions of resident macrophages of the peritoneal cavity and splenic red pulp using high resolution quantitative proteomics

Background:Resident macrophages (Mϕs) are distributed throughout the body and are important for maintaining tissue homeostasis and for defence against infections. Tissue Mϕs are highly adapted to their microenvironment and thought to mediate tissue-specific functions involving metabolism and immune defence that are not fully elucidated. Methods:We have used high resolution quantitative proteomics to gain insights into the functions of two types of resident tissue Mϕs: peritoneal cavity Mϕs and splenic red pulp Mϕs.  The cellular expression levels of many proteins were validated by flow cytometry and were consistently in agreement with the proteomics data.Results:Peritoneal and splenic red pulp macrophages displayed major differences in cell surface phenotype reflecting their adaptation to different tissue microenvironments and tissue-specific functions. Peritoneal Mϕs were shown to be enriched in a number of key enzymes and metabolic pathways normally associated with the liver, such as metabolism of fructose, detoxification, nitrogen homeostasis and the urea cycle. Supporting these observations, we show that peritoneal Mϕs are able to utilise glutamine and glutamate which are rich in peritoneum for urea generation.  In comparison, splenic red pulp Mϕs were enriched in proteins important for adaptive immunity such as antigen presenting MHC molecules, in addition to proteins required for erythrocyte homeostasis and iron turnover. We also show that these tissue Mϕs may utilise carbon and nitrogen substrates for different metabolic fates to support distinct tissue-specific roles.Conclusions:This study provides new insights into the functions of tissue Mϕs in immunity and homeostasis.  The comprehensive proteomics data sets are a valuable resource for biologists and immunologists.

Haemoglobin-based oxygen carriers and myocardial infarction

The incidence of investigator diagnosed myocardial infarction (MI) is greater in patients treated with haemoglobin-based oxygen carriers (HBOCs) than controls. Clinical trials and literature pertaining to possible HBOC toxicity mechanisms have been analyzed in order to identify possible reasons for this imbalance. MI diagnosis is hampered by potential interference of troponin assays by haemoglobin, haemolysis and bilirubin. Nevertheless, insofar as the reported incidence correlates with actual occurrence, there is a positive relationship between MI and HBOC dose and size. Preclinical and clinical data suggest that direct cardiac toxicity and coronary vasoconstriction are unlikely. More probable are detrimental intravascular interactions between HBOCs and components of the coagulation cascade, particularly dysfunctional endothelium. Elucidation of mechanisms is impeded by a lack of clinical data. Measurement of relevant biomarkers would be extremely useful in this regard and in improving patient selection criteria. Conduct of clinical trials in carefully selected patient populations after the development of improved protocols for MI diagnosis, along with concomitant biomarker data collection, is recommended.

Comparison of the Pharmacokinetic Properties of Hemoglobin-Based Oxygen Carriers

Hemoglobin (Hb) is an ideal material for use in the development of an oxygen carrier in view of its innate biological properties. However, the vascular retention of free Hb is too short to permit a full therapeutic effect because Hb is rapidly cleared from the kidney via glomerular filtration or from the liver via the haptogloblin-CD 163 pathway when free Hb is administered in the blood circulation. Attempts have been made to develop alternate acellular and cellular types of Hb based oxygen carriers (HBOCs), in which Hb is processed via various routes in order to regulate its pharmacokinetic properties. These HBOCs have been demonstrated to have superior pharmacokinetic properties including a longer half-life than the Hb molecule in preclinical and clinical trials. The present review summarizes and compares the pharmacokinetic properties of acellular and cellular type HBOCs that have been developed through different approaches, such as polymerization, PEGylation, cross-linking, and encapsulation.