Half-life-extended recombinant coagulation factor IX-albumin fusion protein is recycled via the FcRn-mediated pathway.

The neonatal Fc receptor (FcRn) has a pivotal role in albumin and IgG homeostasis. Internalized IgG captured by FcRn under acidic endosomal conditions is recycled to the cell surface where exocytosis and a shift to neutral pH promote extracellular IgG release. Although a similar mechanism is proposed for FcRn-mediated albumin intracellular trafficking and recycling, this pathway is less well defined but is relevant to the development of therapeutics exploiting FcRn to extend the half-life of short-lived plasma proteins. Recently, a long-acting recombinant coagulation factor IX-albumin fusion protein (rIX-FP) has been approved for the management of hemophilia B. Fusion to albumin potentially enables internalized proteins to engage FcRn and escape lysosomal degradation. In this study, we present for the first time a detailed investigation of the FcRn-mediated recycling of albumin and the albumin fusion protein rIX-FP. We demonstrate that following internalization via FcRn at low pH, rIX-FP, like albumin, is detectable within the early endosome and rapidly (within 10-15 min) traffics into the Rab11+ recycling endosomes, from where it is exported from the cell. Similarly, rIX-FP and albumin taken up by fluid-phase endocytosis at physiological pH traffics into the Rab11+ recycling compartment in FcRn-positive cells but into the lysosomal compartment in FcRn-negative cells. As expected, recombinant factor IX (without albumin fusion) and an FcRn interaction-defective albumin variant localized to the lysosomal compartments of both FcRn-expressing and nonexpressing cells. These results indicate that FcRn-mediated recycling via the albumin moiety is a mechanism for the half-life extension of rIX-FP observed in clinical studies.

Isoagglutinin reduction by a dedicated immunoaffinity chromatography step in the manufacturing process of human immunoglobulin products.

BACKGROUND: The passive transfer of antibodies specific to blood groups A and B (also called isoagglutinins) contained in immunoglobulin (Ig)G products for intravenous administration (IVIG) is believed to be largely responsible for rare but sometimes serious IVIG-related hemolytic events. We present in this work a modification of the manufacturing process of Privigen-a 10% l-proline-stabilized IVIG product-that allows extensive reduction of isoagglutinin concentrations in the final product. STUDY DESIGN AND METHODS: An additional immunoaffinity chromatography (IAC) step was introduced toward the end of the manufacturing process of Privigen. Isoagglutinin titers were measured using the indirect agglutination method and a published flow cytometry-based binding assay. Quality attributes, such as microorganism counts and concentration of endotoxins, IgG, IgA, IgM, aggregates, and so forth were measured using standardized procedures. RESULTS: The introduction of an IAC step in the manufacturing process of Privigen resulted in an 88% to 90% reduction in isoagglutinins between the feed of the chromatography column and the flow-through fraction. All other product quality attributes measured were nearly identical before and after IAC. This process modification resulted in a three-titer-step reduction in isoagglutinin levels in the final IgG product compared to Privigen lots produced by the unmodified process. CONCLUSION: Introducing an isoagglutinin-specific IAC step in the manufacturing process of Privigen is an efficient strategy for reduction of anti-A and anti-B titers. Such reductions might help minimize the risk of hemolytic events in patients receiving IVIG therapy.

Increased potency of recombinant VWF D'D3 albumin fusion proteins engineered for enhanced affinity for coagulation factor VIII.

BACKGROUND: We have recently reported on a recombinant von Willebrand factor (VWF) D'D3 albumin fusion protein (rD'D3-FP) developed to extend the half-life of coagulation factor VIII (FVIII) for the treatment of hemophilia A. Based on predictive modelling presented in this study, we hypothesized that modifying rD'D3-FP to improve FVIII interaction would reduce exchange with endogenous VWF and provide additional FVIII half-life benefit. OBJECTIVES: The aim of this study was to identify novel rD'D3-FP variants with enhanced therapeutic efficacy in extending FVIII half-life. METHODS: Through both directed mutagenesis and random mutagenesis using a novel mammalian display platform, we identified novel rD'D3-FP variants with increased affinity for FVIII (rVIII-SingleChain) under both neutral and acidic conditions and assessed their ability to extend FVIII half-life in vitro and in vivo. RESULTS: In rat preclinical studies, rD'D3-FP variants with increased affinity for FVIII displayed enhanced potency, with reduced dose levels required to achieve equivalent rVIII-SingleChain half-life extension. In cell-based imaging studies in vitro, we also demonstrated reduced dissociation of rVIII-SingleChain from the rD'D3-FP variants within acidic endosomes and more efficient co-recycling of the rD'D3-FP/rVIII-SingleChain complex via the FcRn recycling system. CONCLUSIONS: In summary, at potential clinical doses, the rD'D3-FP variants provide marked benefits with respect to dose levels and half-life extension of co-administered FVIII, supporting their development for use in the treatment of hemophilia A.

The pseudo-dimeric tyrosyl-tRNA synthetase of T. brucei aminoacylates cytosolic and mitochondrial tRNATyr and requires both monomeric units for activity.

Aminoacyl-tRNA synthetases are essential for protein synthesis. The single-copy tyrosyl-tRNA synthetase (Tb-TyrRS) of T. brucei has an unusual structure and forms a pseudo-dimer. It is therefore twice the size than tyrosyl-tRNA synthetases of most other organisms. Here we show by inducible RNAi that Tb-TyrRS is essential for normal growth of procyclic T. brucei. Furthermore we demonstrate that Tb-TyrRS aminoacylates cytosolic as well as mitochondrial tRNATyr indicating that it is dually localized. Finally we show that individual deletion of the 36 N- or C-terminal amino acids abolishes the function of Tb-TyrRS. This indicates that both monomeric units of the enzyme, the C-terminal one of which is predicted to lack enzymatic activity, are essential for Tb-TyrRS function. In summary our results together with previous studies support the notion that Tb-TyrRS might be a suitable drug target.