Tumor-selective vessel occlusions by platelets after vascular targeting chemotherapy using paclitaxel encapsulated in cationic liposomes.

Paclitaxel encapsulated in cationic liposomes (EndoTAG-1) significantly impairs tumor growth by a significant reduction of functional tumor microcirculation and induction of endothelial cell apoptosis. The aim of the study was to analyze whether platelet activation within the tumor microcirculation contributes to the antivascular effects of vascular targeting chemotherapy using EndoTAG-1. In vitro, FACS analysis revealed a significant activation of platelets upon treatment with EndoTAG-1. In vivo, using A-Mel-3 tumors in Syrian Golden hamsters equipped with dorsal skinfold chamber preparations, the contribution of platelets to the antivascular effects of EndoTAG-1 was evaluated by fluorescence and laser-scanning microscopy. Immediately after a single treatment with EndoTAG-1 or cationic liposomes devoid of paclitaxel, an increase of platelet adherence in tumor microvessels was observed. This was accompanied by an acute impairment of the microcirculation within the treated tumors leading to reduced tumor perfusion. After repetitive therapy, an increase of platelet adherence and subsequent tumor microvessel occlusions occurred only after treatment with EndoTAG-1. Comparing to "tumor free" normal tissue controls these microthromboses were tumor selective. Significantly disbalancing the coagulation system within tumors by targeted induction of microthromboses within the tumor microcirculation appears to be an important mechanism of EndoTAG-1 therapy.

Extracellular MRP8/14 is a regulator of ß2 integrin-dependent neutrophil slow rolling and adhesion.

Myeloid-related proteins (MRPs) 8 and 14 are cytosolic proteins secreted from myeloid cells as proinflammatory mediators. Currently, the functional role of circulating extracellular MRP8/14 is unclear. Our present study identifies extracellular MRP8/14 as an autocrine player in the leukocyte adhesion cascade. We show that E-selectin-PSGL-1 interaction during neutrophil rolling triggers Mrp8/14 secretion. Released MRP8/14 in turn activates a TLR4-mediated, Rap1-GTPase-dependent pathway of rapid ß2 integrin activation in neutrophils. This extracellular activation loop reduces leukocyte rolling velocity and stimulates adhesion. Thus, we identify Mrp8/14 and TLR4 as important modulators of the leukocyte recruitment cascade during inflammation in vivo.