Summary
Metastasis is still the most serious reason for the high mortality of cancer patients.
It is a complex process in which platelets play a crucial role. Several attempts have
been performed to inhibit the metastatic process, some of these using modified liposomes.
The aggregation behaviour of human platelets and HT29 colon carcinoma cells in the
presence of liposomes with a modified surface has been investigated in the present
study. Liposomes (PC/CH/DMPE) were unmodified, sterically stabilized by polyethylene
glycol (PEG-DSPE), or equipped with the carbohydrate ligand sialyl LewisX (conjugated to PEG-DMPE or DMPE as anchor) intended to specifically compete with
ligands expressed by HT29 cells. We found in vitro that an addition of surface modified liposomes to human platelets in plasma caused
an up to 2.9-fold increase in platelet aggregation. In addition, when HT29 tumor cells
were mixed with platelets and surface modified lipo somes, the number of tumor cells
found in aggregates increased significantly from 8.3 % (only tumor cells) to 30.2
%. This result was supported by fluorescence micrographs demonstrating a strong association
of platelets and liposomes around the tumor cells. In addition, a clear decrease in
number and a change in the distribution of metastases after intravenous injection
of HT29 cells in combination with liposomes was observed in vivo. While in control mice metastases in lung, liver and in intestine were prevailing,
liposomal treatment resulted in a new localization of metastases in muscles. Taking
together, the ability of surface modified liposomes to enhance aggregate formation
of platelets and tumor cells has been demonstrated for the first time. The capability
of these vesicles to interfere with the metastatic process might have implications
for the use of such liposomes for therapeutic applications.
Keywords
Liposomes - sialyl Lewis X - platelets - metastasis