Summary
In this study, the fibrin binding properties of liposomes containing a number of plasminogen
(Pig) molecules on the outside were compared to those of free (non-liposomal) Pig
in an in vitro model system. Fibrin monolayer coated 96-wells plates were used, containing
fibrin monomer at a density of around 3.4 to 3.9 × 10-4 nmol/cm2. These densities are similar to liposomal Plg-densities, thus allowing multivalent
interactions to occur.
In the panel of experimental conditions that was chosen, binding of free Pig and liposomes
with Pig showed three main differences in characteristics. Firstly, in the fibrin
binding of Plg-liposomes not all Pig may be involved, but on the average 40% of the
total amount of liposomal Pig. This was shown by lysing the liposomes after binding
to the fibrin and estimation of truly bound Pig. With Plg-densities on the liposomes
below the fibrin binding sites density, the maximal number of bound Pig molecules
remains below the amount of available fibrin binding sites. Secondly, a higher binding
rate by at least one order of magnitude was observed for liposomes with Pig compared
to free Pig. Thirdly, liposomes with Pig exhibit a fibrin binding affinity which increases
with Plg-density, because of the multivalent character of interaction. Liposomal Pig
can successfully compete for fibrin binding sites with a 100 fold higher concentration
of free Pig.
These in vitro findings indicate that in view of avid and rapid fibrin binding, liposomes
with attached plasminogen may be suitable for in vivo targeting to fibrin based thrombi.
