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RP5063, a multimodal dopamine (DA) and serotonin (5-HT) modulator with high affinity for DA2/3/4 and 5-HT2A/2B/7 receptors and moderate affinity for SERT, is a novel therapeutic of special interest in the treatment of pulmonary arterial hypertension (PAH). Evidence indicates that therapeutics targeting the 5-HT2A/2B receptors can influence the pathogenesis of PAH. However, the therapeutic effect of RP5063 in humans has yet to be investigated. A Sugen 5416-hypoxia (SuHx)–induced PAH model was used to evaluate twice-daily (b.i.d.) RP5063 at 10 mg/kg (RP-10) and 20 mg/kg (RP-20), as compared with positive (sildenafil 50 mg/kg b.i.d.; Sil-50) and negative controls (SuHx+vehicle; SuHx+veh), in 24 adult male Wistar-Kyoto rats. RP5063 showed significantly lower systolic pulmonary arterial (both doses) and systolic right ventricular (RP-10) pressures, and improvement in oxygen saturation (RP-20). It significantly reduced small-vessel wall thickness (RP-20), lowered the percentage of muscular vessels (both doses). Both doses limited arterial obliteration due to endothelial cell proliferation, prevented plexiform lesion formation, and stemmed the release of leukotriene B4. Sildenafil showed statistically greater effects on vessel structure than that seen in both RP5063 groups and improved oxygen saturation. Additionally, Sildenafil did not demonstrate any significant effect on arterial obliteration, plexiform lesion development, or pulmonary arterial or right ventricular pressure. As PAH gains in severity, the impact of RP5063 inhibition of 5HT2B increases, preventing arterial constriction and improving pulmonary hemodynamics. Due to its functional, structural, and chemokine effects, RP5063 represents a promising candidate for investigation in late-phase PAH.