In vitro inhibition by defibrotide of monocyte superoxide anion generation: A possible mechanism for the antithrombotic effect of a polydeoxyribonucleotide-derived drug

In an attempt to elucidate the antithrombotic potential of defibrotide (D) we have evaluated several functions of monocytes from 7 healthy subjects before and after in vitro incubation of the cells with increasing concentrations of this drug. At concentrations as high as 40 μg/ml, D hardly affected the expression of both the procoagulant activity of monocytes and the formation of superoxide anion in response to l mg/ml zymosan (STZ). In contrast, at concentrations that may be achieved in vivo following the administration of the drug (5-20 μg/ml), D impaired in a 2^-dose-dependent manner (p <0.05) the generation of O₂ in response to N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP, l μM) or calcium ionophore A23l87 (l0 μM). Regardless of the agonist employed, at concentrations between l and 5 mM, extracellular Ca²⁺ had little effect on hbe impairment of superoxide anion generation by D. In contrast, the inhibitory effect was time-dependent, the maximum impairment (> 30%) being observed when the cells were preincubated with the drug for 20 h. These data support the concept that the antithrombotic potential of D involves the ability of the drug to affect the generation of free radicals by leukocytes and suggest that future in vivo studies for the evaluation of the activity of D should take into account the role of monocytes in hemostasis and thrombosis.