Ambrisentan (BSF 208075) is an endothelin receptor antagonist used in the therapy of pulmonary arterial hypertension (PAH). Ambrisentan has been associated with a low rate of serum enzyme elevations during therapy but has yet to be implicated in cases of clinically apparent acute liver injury.

Hepatic hydroxyproline content in the Ambrisentan group is significantly lower compared to the control group (18.0 μg/g±6.1 μg/g vs 33.9 μg/g±13.5 μg/g liver, respectively, P=0.014). Hepatic fibrosis estimated by Sirius red staining and areas positive for α-smooth muscle actin, indicative of activated hepatic stellate cells, are also significantly lower in the Ambrisentan group (0.46%±0.18% vs 1.11%±0.28%, respectively, P=0.0003; and 0.12%±0.08% vs 0.25%±0.11%, respectively, P=0.047). In addition, hepatic RNA expression levels of procollagen-1 and tissue inhibitor of metalloproteinase-1 (TIMP-1) are significantly lower by 60% and 45%, respectively, in the Ambrisentan group. Inflammation, steatosis, and endothelin-related mRNA expression in the liver are not significantly different between the groups. Ambrisentan attenuates the progression of hepatic fibrosis by inhibiting hepatic stellate cell activation and reducing procollagen-1 and TIMP-1 gene expression. Ambrisentan did not affect inflammation or steatosis[6].

In vitro PTPase assays: Individual PTPases (0.01 μg/reaction) in 50 μL of PTPase buffer [50 mM Tris (pH 7.4)] are incubated at 22 ℃ for 10 min or as indicated in the absence or presence of inhibitory compounds. Substrates (0.2 mM phosphotyrosine peptide) are then added and allows to react at 22 ℃ for 18 hr. PTPase activity of individual reactions is measured by adding 100 μL of malachite green solution (UBI) and then quantifying the amounts of free phosphate cleaved by the PTPase from the peptide substrate by spectrometry (A660 nm). Relative PTPase activities are calculated based on the formula [(PTPase activity in the presence of an inhibitory compound)/(PTPase activity in the absence of the compound) × 100%]. Reactions performed under comparable conditions in the absence of recombinant PTPases only are used as controls and shows no detectable PTPase activity.

Ambrisentan is dissolved in DMSO and stored, and then diluted with appropriate medium before use. Unless otherwise stated, for each BMEC experiment cells are randomly divided into 4 groups: (1) normoxia vehicle control (Nx-CTRL); (2) normoxia-treated; (3) hypoxia (24 h) control (Hx-CTRL) and (4) hypoxia (24 h) treated. As previously described, Nrf2 activators are added 24 h prior to any hypoxic exposures. Cell treatments are; Protandim (100 μg/mL), methazolamide (125 μg/mL, nifedipine (7 μg/mL) or Ambrisentan (40 μg/mL). In addition, some cells are treated with Nrf2 siRNA. In these experiments, siRNA is added 24 h prior to drug treatments. The rationale for 24 h hypoxia exposure for BMEC is to ensure that cells remained transfected with siRNA for the pre-treatment of drugs (24 h in normoxia) and during the 24 h hypoxia exposure. Data is collected from at least three separate cell culture preparations on three separate days (n=9)[2].