Microvascular effects of the low molecular weight heparins in a colorectal xenograft model: an intravital microscopy study

BackgroundLow molecular weight heparins (LMWHs) are the cornerstone of the prevention of thromboembolic events in surgical patients. Recent clinical data suggest that LMWHs might improve survival of cancer patients, independent of their anticoagulant effect. The anti-cancer mechanism of LMWHs is incompletely understood, but may include effects on tumor angiogenesis. We assessed the effects of LMWHs on tumor angiogenesis and microcirculation in a mouse colorectal xenograft model using in vivo microscopy in window chambers.MethodsHT29 human colorectal cancers were implanted in dorsal skinfold window chambers in athymic mice. Animals (n = 8 per group) were treated with 200 IU of nadroparin, enoxaparin, or saline for 8 d. Repeated intravital fluorescence microscopy was performed to calculate functional microcirculatory parameters as follows: number (N) and length (L) of microvessels, vascular area fraction (AF), and red blood cell velocity. Microvessel density, microvessel fractal dimension, and pericyte coverage were assessed histologically.ResultsActive angiogenesis was observed in control animals, resulting in a significant increase in N, L, and AF. In nadroparin- and enoxaparin-treated animals, however, AF did not change significantly over time and N and L remained significantly lower than untreated animals on day 7. Compared with control animals, nadroparin- and enoxaparin-treated animals showed a significantly lower microvessel density, but a higher pericyte coverage index, indicating a more mature microvessel network.ConclusionsThe LMWHs nadroparin and enoxaparin inhibit tumor angiogenesis and result in microvessel normalization in this in vivo observed colorectal xenograft model.