Animal Models Related to Congenital Heart Disease and Clinical Research in Pulmonary Hypertension

There are several animal models for studying human pulmonary hypertension (PH). An increased flow model in pigs was developed at the University Hospital in Heidelberg in order to simulate congenital heart disease. The high pulmonary blood flow is achieved by installation of a Blalock-Taussig anastomosis. In order to further improve this model by adding a pressure component, the left pulmonary artery is ligated. An acute model, which is used at the Innsbruck Medical University, addresses another disease entity. Human meconium is placed deeply into the trachea of the pigs in order to induce an acute respiratory distress syndrome-like response in the lungs. Animals were randomly assigned to four treatment groups. Inhaled iloprost, due to its pulmonary and intrapulmonary selectivity, was the only substance that significantly reduced intrapulmonary shunt volumes. In humans, PH encompasses multiple disease subtypes. Pulmonary arterial hypertension (PAH) accounts for only 6% of PH cases, however, all existing treatments are indicated only for PAH. This means that for 94% of patients with PH, no specific medication is available. Therefore, huge efforts have been made to better understand the pathophysiology of PH and to detect new signalling pathways that may allow new compounds to be developed that will ultimately improve the prognosis of PAH and non-PAH PH patients. Promising new substances include riociguat, a stimulator of the soluble guanylate cyclase (sGC), as well as cinaciguat, a sGC activator, and an elastase inhibitor. Riociguat (BAY 63-2521) is an oral agent that targets the intact/native form of sGC. It enhances the sensitivity of sGC to low levels of bioavailable nitric oxide (NO) and is also capable of stimulating native sGC independently of NO. Thus, unlike phosphodiesterase-5 inhibitors, the effect of riociguat is not limited by low NO levels. In a multicentre open-label phase II study, riociguat exerted strong and significant effects on pulmonary haemodynamics and exercise capacity in patients with PAH and in patients with chronic thromboembolic pulmonary hypertension (CTEPH). Riociguat is currently being evaluated in phase III clinical trials both in PAH and in CTEPH patients. Clinical studies with riociguat in earlier development stages have addressed PH in lung diseases and systolic left ventricular failure. An elastase inhibitor is currently being investigated in phase I clinical trials in patients with PH owing to chronic obstructive pulmonary disease.