Effects of vascular endothelial and platelet-derived growth factor receptor inhibitors on long-term cultures from normal human bone marrow

Endothelial cells and fibroblasts are important constituents of the haemopo ietic microenvironment. Growth and function of these cells are controlled b y a variety of cytokines, including vascular endothelial growth factor (VEG F) and platelet-derived growth factor (PDGF). We analysed the effects of no vel tyrosine kinase inhibitors targeting the VEGF and PDGF receptors (compo unds SU5614 and SU5768) on the performance of long-term cultures from norma l human bone marrow. In developing cultures, the inhibitors induced a dose-dependent reduction i n stromal. fibroblasts, macrophages and endothelial cells with a concomitan t decrease in blood cell production and an increase in fat cells. For SU561 4, the concentration Whiting stroma formation by 50% (IC50) was 123nM, and the IC50 for haemopoietic. colony forming cell output was 186 nM. For SU576 8, the respective values were 871 nM and 331 nM. Changes in stroma composit ion and inhibition of haemopoietic cell production were also demonstrable a fter delayed addition of the inhibitors to established cultures. By contras t, haemopoietic colony formation in clonogenic agar cultures was unimpaired (IC50 not reached at 100 muM). Immunofluorescence studies and time course analyses suggested that the primary effect of the inhibitors was interferen ce with the proliferation and function of fibroblasts and endothelial cells which in turn resulted in decreased haemopoiesis and increased adipogenesi s. This was associated with decreased levels in conditioned media of granul ocyte-macrophage colony-stimulating factor, interleukin-6 and leptin. VEGF and PDGF may play a hitherto underestimated role in the control of blo od cell formation. VEGF/PDGF receptor inhibitors may have therapeutic poten tial in stroma diseases such as myelofibrosis. Since they weaken the stimul atory signals provided by the microenvironment, they may also be of value i n the treatment of leukaemia and other neoplastic bone marrow diseases.