Endothelial dysfunction seems to be a key factor in the development of several complications observed early after hematopoietic stem cell transplantation (HSCT). The conditioning regimen and many other factors associated with the procedure are responsible for this endothelial damage. The effects of immunosuppressive agents on endothelial function have not been explored in detail. We evaluated the effects of 3 drugs commonly used in HSCT: 2 calcineurin inhibitors, cyclosporine A (CSA) and tacrolimus (TAC), and an inhibitor of mTOR, sirolimus (SIR). We also evaluated the effect of the combination of TAC and SIR (TAC+SIR), which is used increasingly in clinical practice. Microvascular endothelial cells (HMEC-1) were exposed to these drugs to evaluate changes in (1) intercellular adhesion molecule (ICAM)-1 expression on the cell surface, assessed by immunofluorescence labeling and expressed as the mean gray value (MGV); (2) reactivity of the extracellular matrix (ECM) toward platelets, upon exposure of the ECM to circulating blood; and (3) whole-blood clot formation, assessed by thromboelastometry. Studies were conducted in the absence and presence of defibrotide (DF) to assess its possible protective effect. The exposure of HMEC-1 to CSA and TAC+SIR significantly increased the expression of ICAM-1 (157.5 ± 11.6 and 153.4 ± 9.5 MGV, respectively, versus 105.7 ± 6.5 MGV in controls [both P < .05]). TAC applied alone increased ICAM-1 slightly (120.3 ± 8.2 MGV), and SIR had no effect (108.9 ± 7.4 MGV). ECM reactivity increased significantly only in response to CSA (surface covered by platelets of 41.2% ± 5.4% versus 30.1% ± 2.0%, P < .05). DF attenuated all these changes. No significant changes in the viscoelastic properties of clot formation were observed in any condition with blood samples incubated in vitro. In conclusion, CSA and TAC+SIR had a proinflammatory effect, but only CSA exhibited an additional prothrombotic effect. Interestingly, DF exerted clear protective anti-inflammatory and antithrombotic effects on the endothelium.

5319 Endothelial dysfunction is a key factor in the development of hematopoietic stem cell transplantation (HSCT) complications due largely to the conditioning treatments. The effect of immunosuppressive therapy on endothelial function has not been explored in detail in this setting. We evaluated the effect of three immunosuppressive drugs: two calcineurin inhibitors (cyclosporine and tacrolimus) and an inhibitor of mTOR (rapamycin). Microvascular endothelial cells (HMEC) were exposed to the three drugs to assess changes in ICAM-1 expression on the cell surface, the reactivity of the extracellular matrix (ECM) generated, and in signaling proteins. Studies were conducted in the absence and presence of defibrotide (DF) to assess its protective effect. Only the exposure of HMEC to cyclosporine (CyA) increased significantly the expression of ICAM-1 (4-fold, p<0.01) and the reactivity of the ECM (surface covered by platelets of 32.1±2.6% vs. 26.5±2.5%, p<0.01). While tacrolimus itself only increased slightly ICAM-1 expression, rapamycin moderately decreased the ECM reactivity. Those changes due to CyA were prevented by DF. CyA induced the activation of AKT and Raf, among others, whose expression was inversely related (4- and 0.5-fold, respectively, with respect to control cells). DF reversed this trend (0.4- and 2.7-fold, respectively, with respect to CyA treated cells). In conclusion, both calcineurin inhibitors showed a proinflammatory effect, though only CyA exhibited significant proinflammatory and prothrombotic effects on the endothelium that correlated with AKT/Raf pathway activation. Interestingly, the protective anti-inflammatory and anti-thrombotic effect of defibrotide seems to occur through the same signaling pathway. In contrast, rapamycin did not exert any significant action on endothelium. Disclosures: No relevant conflicts of interest to declare.