Substantial evidence from animal models supports the concept of inhibiting peripheral serotonin synthesis for the treatment of pulmonary arterial hypertension (PAH), as demonstrated by pharmacological blockade or genetic deletion of tryptophan hydroxylase 1 (TPH1) [1-5]. Most recently, we have shown that daily inhalation of TPT-004, a novel class TPH1 inhibitor, can alleviate PAH in the Sugen-hypoxia (SuHx) rat model [1].