Pentamidine is an antiprotozoal drug with additional antiinflammatory activities that are not well understood. We now report that pentamidine inhibited the human whole blood production of the chemotactic cytokines (chemokines) interleukin (IL)-8, growth related gene alpha (GRO alpha) and monocyte chemotactic protein-1 (MCP-1). The title compound dose-dependently suppressed the lipopolysaccharide (LPS)- and phytohemagglutinin (PHA)-stimulated whole blood generation of these chemokines with IC50-values of 2.1 and 2.2 microM (IL-8), 2.4 and 1.8 microM (GRO alpha) and 2.8 and 2.4 microM (MCP-1). The inhibition was specific: when tested at 10 microM, pentamidine had no significant inhibitory effect on the PHA-induced generation of the non-chemotactic cytokines tumor necrosis factor-alpha (TNF-alpha), IL-1 beta, IL-2, IL-4, IL-5, IL-10 and interferon-gamma (IFN-gamma), except for a partial inhibition on IL-6. Time course experiments indicated that pentamidine (10 microM) retained its ability to inhibit PHA-stimulated IL-8 production even when its addition was delayed for up to 24h after mitogen stimulation. Furthermore, reverse transcription PCR studies showed that pentamidine had no effect on IL-8 mRNA expression. These findings indicate that pentamidine is a post-transcription acting inhibitor of human chemokine production. This activity may contribute to the anti-inflammatory action ascribed to the title compound.