Inhalation of the Novel Tryptophan Hydroxylase 1 Inhibitor TPT-004 Alleviates Pulmonary Arterial Hypertension in Rats

Inhaled pharmacotherapies are promising treatment options for patients with pulmonary arterial hypertension (PAH) as they minimize extrapulmonary adverse effects. Recently, we developed a highly specific inhibitor (TPHi) of the serotonin synthesizing enzyme tryptophan hydroxylase 1, TPT-004. We hypothesized that repetitive nose-only inhalation of TPT-004 alleviates PAH and pulmonary vascular remodeling in the Sugen-hypoxia (SuHx) rat model. Male Sprague-Dawley rats were divided into 3 groups: (i) ConNx, control animals kept in room air during the study; (ii) SuHx [rats subcutaneously injected with the VEGFR2-inhibitor SU5416, then exposed to chronic hypoxia (3 weeks), followed by 5.5 weeks of room air]; (iii) SuHx + TPHi [SuHx animals treated with TPHi by inhalation for 4 weeks]. Closed-chest right-left heart catheterization and cardiac MRI were performed in spontaneously breathing rats. Lungs underwent histological and mRNA-seq analysis. SuHx-exposed rats had severe PAH, RV hypertrophy, and RV dilation. In comparison with SuHx-exposed rats, TPHi-treated SuHx rats had significantly lower RV systolic pressure (67.25 vs.51.47mmHg; p<0.0001), normalized RV end-systolic volume (182.6 vs. 105.1µL; p<0.0001) and improved RV ejection fraction by cardiac MRI (47.9 vs. 66.8%; p<0.0001). Inhaled TPT-004 did not affect LV end-diastolic or systemic blood pressure. TPT-004 therapy reversed pulmonary vascular remodeling and alveolar macrophage infiltration. RNA-sequencing unraveled TPHi-induced changes in pulmonary gene expression: 1) increased cell adhesion and reduced cell motility/migration; 2) suppressed extracellular matrix remodeling; 3) modulated immune response; 4) suppressed pulmonary vascular remodeling via modulating proliferation, apoptosis, and homeostasis. Taken together, TPT-004 is an effective therapeutic PAH agent, does not cause any hemodynamic adverse effects in rodents, and thus, should be tested further towards a clinical phase 1b/phase 2 study in PAH patients.