Trypanocidal effect of Ir‐(COD)‐pentamidine tetraphenylborate on Trypanosoma brucei and T. b. gambiense rodent models and serum kinetics in sheep

Pentamidine di‐(iridium cyclo‐octadiene)tetraphenylborate, called Ir‐(COD)‐pentamidine tetraphenylborate, was selected from a primary screening as a promising trypanocidal compound. The compound was evaluated against three isolates: Trypanosoma brucei brucei CMP, T. b. brucei GVR 35 and T. b. gambiense Feo. On the T. b. brucei GVR 35 murine CNS model, no mouse was cured when the treatment was commenced 21 days post‐infection whatever the treatment regimen. Nevertheless, in vitro the compound killed the trypomastigote forms of T. b. gambiense Feo at 0.6 μm. In vivo, the compound cured all mice infected 1 hour previously with T. b. gambiense Feo after a 10 mg/kg (6.3 μmol/kg) treatment subcutaneously administered in a single dose. Moreover, the compound was active at 1 mg/kg (0.6 μmol/kg) in a single dose against the early stage of the T. b. brucei Antat 1‐9 sheep model. Serum kinetics data showed that pentamidine di‐(iridium cyclo‐octadiene) tetraphenylborate was distributed within deep compartment according to a monocompartmental model. The maximum iridium serum concentration was 198 μg/l corresponding to 1 μmol/kg of iridium derivative and this value remained stable for 30–50 hours post‐treatment. Iridium was completely eliminated from the serum 700 hours post‐treatment. All data obtained from these models are in favour of an activity in the early stage of the disease but indicate that the compound could not cross the blood–brain barrier despite its lipophilicity. Although iterative treatments with the compound rapidly induced the selection of iridium derivative refractory populations, the compound could be studied on pentamidine refractory strains.