We describe here the effects of Delta(24(25)) sterol methenyl transferase inhibitors (SMTI) on promastigote and axenic amastigote forms of Leishmania amazonensis. When these cells were exposed to 20-piperidin-2-yl-5 alpha-pregnan-3 beta-20-diol (22,26-azasterol; AZA), hydrazone-imidazol-2-yl-5 alpha-pregnan3 beta-ol (IMI), 20-hydrazone-pyridin-2-yl-5 alpha-pregnan-3 beta-ol (PYR) or 24(R, S), 25-epiiminolanosterol (EIL), a concentration- and time-dependent inhibition of growth was observed, with IC50 values in the submicromolar range. Ultrastructural alterations in treated cells were mainly observed in the mitochondrion, which displayed an intense swelling and a reduction of the electron density of the matrix with remarkable changes in the inner mitochondrial membranes. Mitochondrial transmembrane electric potential (Delta psi) was measured using spectrophotometric methods in control and treated promastigotes permeabilized with digitonin. After energization with the substrates for complexes I, II or IV of the respiratory chain, it was possible to detect marked changes of Delta psi in promastigotes treated with 1 mu M of the SMTI for 48 or 72 h when compared with normal cells, indicating that these compounds led to the loss of the energy-transducing properties of the mitochondrial inner membrane, probably related to the alteration of its lipid composition. The present study confirms these findings, showing that in Leishmania amazonensis the mitochondrial complex appears to be the first organelle affected after treatment with different SMTI. (c) 2007 Elsevier GmbH. All rights reserved.1584447456