: A series of potent dual PDGFR/cKIT inhibitors has been identified. The series was optimized for duration of action in the lung. A novel kinase occupancy assay was used to directly measure target occupancy after it dosing. Compound 25 shows 24 hour occupancy of the PDGFR kinase domain, after a single it dose and has efficacy at 0.03 mg/kg, in the rat moncrotaline model of pulmonary arterial hypertension. Examination of PK/PD data from the optimization effort has revealed in vitro: in vivo correlations which link duration of action in vivo with low permeability and high basicity, and demonstrate that nonspecific binding to lung tissue increases with lipophilicity.