Selective functional activity measurement of a PEGylated protein with a modification-dependent activity assay.

BAX 855 (ADYNOVATE) is a PEGylated recombinant factor VIII (rFVIII) that showed prolonged circulatory half-life compared to unmodified rFVIII in hemophilic patients. Here, the development and validation of a novel assay is described that selectively measures the activity of BAX 855 as cofactor for the serine protease factor IX, which actives factor X. This method type, termed modification-dependent activity assay, is based on PEG-specific capture of BAX 855 by an anti-PEG IgG preparation, followed by a chromogenic FVIII activity assay. The assay principle enabled sensitive measurement of the FVIII cofactor activity of BAX 855 down to the pM-range without interference by non-PEGylated FVIII. The selectivity of the capture step, shown by competition studies to primarily target the terminal methoxy group of PEG, also allowed assessment of the intactness of the attached PEG chains. Altogether, the modification-dependent activity not only enriches, but complements the group of methods to selectively, accurately, and precisely measure a PEGylated drug in complex biological matrices. In contrast to all other methods described so far, it allows measurement of the biological activity of the PEGylated protein. Data obtained demonstrate that this new method principle can be extended to protein modifications other than PEGylation and to a variety of functional activity assays.

Modulation of ADAMTS13 secretion and specific activity by a combination of common amino acid polymorphisms and a missense mutation.

Sequence analysis of the ADAMTS13 locus of 2 patients with hereditary thrombotic thrombocytopenic purpura (TTP) revealed the homozygous presence of 4 single nucleotide polymorphisms (SNPs) (R7W, Q448E, P618A, A732V) and a rare missense mutation (R1336W). Analysis of the individual effect of any amino acid exchanges showed that several sequence variations can interact with each other, thereby altering the phenotype of ADAMTS13 deficiency. Introduction of polymorphisms R7W, Q448E, and A732V had no or only minor effects on ADAMTS13 secretion. In contrast, P618A, R1336W, and the A732V-P618A combination strongly reduced ADAMTS13-specific activity and antigen levels. Surprisingly, R7W and Q448E were positive modifiers of ADAMTS13 secretion in the context of P618A and A732V but neither could rescue the severely reduced specific activity conferred by P618A. However, in the context of R1336W, polymorphisms R7W and Q448E enhanced the detrimental effect of the missense mutation and led to undetectable enzyme activity. We show that dependent on the sequence context, the same polymorphisms might be either positive or negative modifiers of gene expression. Our results might therefore be widely relevant to understanding the influence of polymorphisms on the phenotypic expression of complex diseases.

Nonneutralizing IgM and IgG antibodies to von Willebrand factor-cleaving protease (ADAMTS-13) in a patient with thrombotic thrombocytopenic purpura.

Acquired thrombotic thrombocytopenic purpura (TTP) has been linked to severe deficiency of ADAMTS-13 activity caused by autoantibodies inhibitory to ADAMTS-13. We report data on a patient with confirmed TTP who had severely reduced ADAMTS-13 activity but showed no ADAMTS-13 inhibition in a widely used fluid phase activity assay. With a newly developed enzyme-linked immunosorbent assay, using immobilized recombinant ADAMTS-13, we found high titers of IgM and IgG antibodies that bound to ADAMTS-13, but did not neutralize protease activity. These autoantibodies probably influenced the half-life of ADAMTS-13 or its binding to the endothelial cell surface, thereby compromising ADAMTS-13 activity in vivo. Given that ADAMTS-13 may interact physiologically with various receptors or ligands, the occurrence, distribution, and the epitope mapping of nonneutralizing antibodies will be an important area for future research.