Rationale: Whether idiopathic, familial, or secondary to another disease, pulmonary arterial hypertension (PAH) is characterized by increased vascular tone, neointimal hyperplasia, medial hypertrophy, and adventitial fibrosis. Imatinib, a potent receptor tyrosine kinase inhibitor, reverses pulmonary remodeling in animal models of PAH and improves hemodynamics and exercise capacity in selected patients with PAH.

Objectives: Here we use both imatinib and knockout animals to determine the relationship between platelet-derived growth factor receptor (PDGFR) and serotonin signaling and investigate the PAH pathologies each mediates.

Methods: We investigated the effects of imatinib (100 mg/kg) on hemodynamics, vascular remodeling, and downstream molecular signatures in the chronic hypoxia/SU5416 murine model of PAH.

Measurements and Main Results: Treatment with imatinib reduced all measures of PAH pathology observed in hypoxia/SU5416 mice. In addition, 5-hydroxytryptamine (5-HT) and tryptophan hydroxylase 1 (Tph1) expression were reduced compared with the normoxia/SU5416 control group. Imatinib attenuated hypoxia-induced increases in Tph1 expression in pulmonary endothelial cells in vitro via inhibition of the PDGFR-β pathway. To better understand the consequences of this novel mode of action for imatinib, we examined the development of PAH after hypoxic/SU5416 exposure in Tph1-deficient mice (Tph1^−/−). The extensive changes in pulmonary vascular remodeling and hemodynamics in response to hypoxia/SU5416 were attenuated in Tph1^−/− mice and further decreased after imatinib treatment. However, imatinib did not significantly further impact collagen deposition and collagen 3a1 expression in hypoxic Tph1^−/− mice. Post hoc subgroup analysis suggests that patients with PAH with greater hemodynamic impairment showed significantly reduced 5-HT plasma levels after imatinib treatment compared with placebo.

Conclusions: We report a novel mode of action for imatinib, demonstrating TPH1 down-regulation via inhibition of PDGFR-β signaling. Our data reveal interplay between PDGF and 5-HT pathways within PAH, demonstrating TPH1-dependent imatinib efficacy in collagen-mediated mechanisms of fibrosis.