Blood Coagulation Factor Binding and Hepatocyte Tropism
Modification of Ad5 with pHPMA (Green et al. 2004) and with a dense shield of 20 kDa PEG (Wortmann et al. 2008; Doronin et al. 2009) was shown to ablate the liver tropism of intravenously injected Ad vectors. Although physical measurements to determine the binding of blood coagulation factor X to PEGylated adenoviral capsids is still lacking, it may be assumed that dense shields with large PEG moieties at least significantly reduce FX binding and thus directly influence the hepatocyte tropism of the vector particles.
However, it has to be mentioned that a polymer shield which is dense enough to prevent unwanted interactions can render the vector particles non-infectious and inert and thus useless for gene transfer purposes. Strategies have been developed to attach ligands at random positions on top of the vector shields (for HPMA see Fisher et al. (2001), for PEG Romanczuk et al. (1999); Lanciotti et al. (2003), for a review see Kreppel and Kochanek (2008)) for retargeting purposes. However, only very few ligands (and in particular those with relatively broad specificity) were successfully used in such approaches. This may be attributed to the fact that it is very difficult to attach a ligand at a defined position of the polymer shield. Campos and Barry (2006) published data on the effect of ligand positioning on the capsid for targeting purposes . They noticed that while a ligand can be functional when presented by the Ad fiber, the same ligand induced aberrant intracellular trafficking when presented on a hexon or IX. Later similar observations and refinements were made by Corjon et al. (2008). It appears that both the position of the ligand on the capsid and the intracellular fate of the ligand determine the success of targeting approaches to a large degree.
From a scientific point of view it may be noted that Ad vectors shielded by amine- directed attachment of synthetic polymers represent some kind of “black box” and are of limited scientific use to describe and understand biological barriers for Ad vectors. The history of hepatocyte detargeting nicely illustrates this fact. Long circulating, liver-detargeted Ad vector particles had been generated years before the discovery and molecular description of the role of FX for Ad hepatocyte tropism (Green et al. 2004). However, for successful vector development it should be kept in mind that only a comprehensive understanding of biological mechanisms is a robust basis for the development of truly improved Ad gene transfer vectors.