Soluble GC stimulators and activators: Past, present and future

The discovery of soluble GC (sGC) stimulators and sGC activators provided valuable tools to elucidate NO-sGC signalling and opened novel pharmacological opportunities for cardiovascular indications and beyond. The first-in-class sGC stimulator riociguat was approved for pulmonary hypertension in 2013 and vericiguat very recently for heart failure. sGC stimulators enhance sGC activity independent of NO and also act synergistically with endogenous NO. The sGC activators specifically bind to, and activate, the oxidised haem-free form of sGC. Substantial research efforts improved on the first-generation sGC activators such as cinaciguat, culminating in the discovery of runcaciguat, currently in clinical Phase II trials for chronic kidney disease and diabetic retinopathy. Here, we highlight the discovery and development of sGC stimulators and sGC activators, their unique modes of action, their preclinical characteristics and the clinical studies. In the future, we expect to see more sGC agonists in new indications, reflecting their unique therapeutic potential.

Soluble Guanylate Cyclase Stimulators and Activators

When Furchgott, Murad, and Ignarro were honored with the Nobel prize for the identification of nitric oxide (NO) in 1998, the therapeutic implications of this discovery could not be fully anticipated. This was due to the fact that available therapeutics like NO donors did not allow a constant and long-lasting cyclic guanylyl monophosphate (cGMP) stimulation and had a narrow therapeutic window. Now, 20 years later, the stimulator of soluble guanylate cyclase (sGC), riociguat, is on the market and is the only drug approved for the treatment of two forms of pulmonary hypertension (PAH/CTEPH), and a variety of other sGC stimulators and sGC activators are in preclinical and clinical development for additional indications. The discovery of sGC stimulators and sGC activators is a milestone in the field of NO/sGC/cGMP pharmacology. The sGC stimulators and sGC activators bind directly to reduced, heme-containing and oxidized, heme-free sGC, respectively, which results in an increase in cGMP production. The action of sGC stimulators at the heme-containing enzyme is independent of NO but is enhanced in the presence of NO whereas the sGC activators interact with the heme-free form of sGC. These highly innovative pharmacological principles of sGC stimulation and activation seem to have a very broad therapeutic potential. Therefore, in both academia and industry, intensive research and development efforts have been undertaken to fully exploit the therapeutic benefit of these new compound classes. Here we summarize the discovery of sGC stimulators and sGC activators and the current developments in both compound classes, including the mode of action, the chemical structures, and the genesis of the terminology and nomenclature. In addition, preclinical studies exploring multiple aspects of their in vitro, ex vivo, and in vivo pharmacology are reviewed, providing an overview of multiple potential applications. Finally, the clinical developments, investigating the treatment potential of these compounds in various diseases like heart failure, diabetic kidney disease, fibrotic diseases, and hypertension, are reported. In summary, sGC stimulators and sGC activators have a unique mode of action with a broad treatment potential in cardiovascular diseases and beyond.

Clinical trial design in phase 2 and 3 trials for pulmonary hypertension

This article on clinical trial design incorporates the broad experience of members of the Pulmonary Vascular Research Institute's (PVRI) Innovative Drug Development Initiative (IDDI) as an open debate platform for academia, the pharmaceutical industry and regulatory experts surrounding the future design of clinical trials in pulmonary hypertension. It is increasingly clear that the design of phase 2 and 3 trials in pulmonary hypertension will have to diversify from the traditional randomised double-blind design, given the anticipated need to trial novel therapeutic approaches in the immediate future. This article reviews a wide range of differing approaches and places these into context within the field of pulmonary hypertension.

Biomarkers in Pulmonary Vascular Disease: Gauging Response to Therapy

Biomarkers are increasingly being investigated in the treatment of pulmonary vascular disease. In particular, the signaling pathways targeted by therapies for pulmonary arterial hypertension provide biomarkers that potentially can be used to guide therapy and to assess clinical response as an alternative to invasive procedures such as right-sided cardiac catheterization. Moreover, the growing use of combination therapy for both the initial and subsequent treatment of pulmonary arterial hypertension highlights the need for biomarkers in this treatment approach. Currently approved therapies for pulmonary arterial hypertension target 3 major signaling pathways: the nitric oxide-soluble guanylate cyclase-cyclic guanosine monophosphate pathway, the endothelin pathway, and the prostacyclin pathway. Although the main biomarker used in practice and evaluated in clinical trials is N-terminal pro-brain natriuretic peptide, other putative biomarkers include the endogenous nitric oxide (NO) synthase inhibitor asymmetric dimethylarginine, NO metabolites including S-nitrosothiols and nitrite, exhaled NO, endothelins, cyclic guanosine monophosphate, cyclic adenosine monophosphate, and atrial natriuretic peptide. This review describes accessible biomarkers, related to the actual molecules targeted by current therapies, for measuring and predicting response to the individual pulmonary arterial hypertension treatment classes as well as combination therapy.

Rationale and study design of RESPITE: An open-label, phase 3b study of riociguat in patients with pulmonary arterial hypertension who demonstrate an insufficient response to treatment with phosphodiesterase-5 inhibitors

Patients with pulmonary arterial hypertension (PAH) who do not have an adequate response to therapy with phosphodiesterase-5 inhibitors (PDE-5i) may have insufficient synthesis of cyclic guanosine monophosphate (cGMP). These patients may respond to a direct soluble guanylate cyclase (sGC) stimulator such as riociguat. RESPITE (NCT02007629) was an open-label, multicenter, uncontrolled, single-arm phase 3b study of riociguat in patients with PAH who demonstrated an insufficient response to treatment with PDE-5i. Insufficient response was defined as World Health Organization functional class (WHO FC) III despite PDE-5i therapy for at least 90 days; 6-min walk distance (6MWD) of 165-440 m, and right-heart catheterization showing mean pulmonary artery pressure >30 mmHg, cardiac index <3.0 L/min/m², and pulmonary vascular resistance >400 dyn s cm^-5. PAH patients with an insufficient response to stable doses of sildenafil or tadalafil-either as monotherapy or in combination with an endothelin receptor antagonist-for at least 90 days were switched to riociguat for 24 weeks. Starting at 1.0 mg TID, the dose of riociguat was increased during the 8-week titration phase in 0.5-mg increments in 2-week intervals if the patient had no signs or symptoms of hypotension. In the ensuing 16-week maintenance phase, riociguat was continued at the optimal individual dose. All efficacy outcomes were exploratory and include change from baseline to 24 weeks in 6MWD, cardiac index, N-terminal pro-brain natriuretic peptide, WHO FC, and quality of life and the proportion of patients with clinical worsening. The following biomarkers were to be measured: cGMP, asymmetric dimethyl arginine, growth-differentiation factor-15, and ST2. Results from RESPITE will help to determine if PAH patients who do not respond to PDE-5i are likely to benefit from treatment with an sGC stimulator. The study may also identify biomarkers that are able to suggest which patients are more likely to respond to sGC stimulators.

Ambrisentan for the treatment of adults with pulmonary arterial hypertension: a review

OBJECTIVE

To provide an overview of the drug profile of the orally active, selective endothelin A receptor antagonist ambrisentan, and its efficacy and safety in the treatment of patients with pulmonary arterial hypertension (PAH).

RESEARCH DESIGN AND METHODS

Medical literature on the use of ambrisentan in PAH was identified using MEDLINE and EMBASE. Additional references were identified from the reference lists of published articles and from the authors' own bibliographies.

RESULTS

Significant improvements in exercise capacity were observed with approved dosages of ambrisentan (5 or 10 mg once daily) in the AMB-220 dose-ranging study and the pivotal ARIES-1 and ARIES-2 trials, with sustained effects up to 2 years observed in ARIES-E. Improvements in cardiopulmonary hemodynamic variables were reported in AMB-220 and ARIES-E (subset analysis). Ambrisentan had little or no effect on hepatic transporters in in vitro studies and displayed a low risk of potential drug-drug interactions, including those with other PAH therapies. Results from the VOLT post-marketing program confirmed the safety profile of ambrisentan observed in the ARIES studies, including the low incidence of liver function test abnormalities. Peripheral edema and anemia were common side effects of endothelin receptor antagonist therapies, including ambrisentan. In the recently completed AMBITION study (ClinicalTrials.gov Identifier: NCT01178073), upfront initial combination therapy with ambrisentan and tadalafil significantly reduced the risk of clinical failure (primary endpoint) by 50% compared with the pooled monotherapy groups.

CONCLUSIONS

The long-term efficacy and safety profile of ambrisentan in patients with PAH is supported by data from a comprehensive clinical trial program and real-life, post-marketing observations.

An update on the use of ambrisentan in pulmonary arterial hypertension

The development of effective oral treatments that are capable of modulating the activity of endothelin receptor 1 (ET-1) represents a significant milestone in the field of pulmonary arterial hypertension (PAH). Randomized clinical trials confirm that endothelin receptor antagonist (ERA) treatments confer significant improvements on important clinical endpoints, such as exercise capacity, functional class, quality of life and pulmonary hemodynamics. Moreover, ERAs may prevent or delay clinical worsening and retard disease progression.

Ambrisentan is a propanoic acid-based ERA, showing preferential affinity for the type A ET-1 over the type B receptor. It provides another valuable, effective treatment option in PAH. Two large, randomized-placebo controlled trials demonstrated the efficacy of ambrisentan in PAH at improving exercise tolerance as measured by the 6 min walk distance. Additional secondary measures of improvement including time to clinical worsening, survival, functional class, quality of life and hemodynamic variables have been reported in clinical trials. A favorably low incidence of aminotransferase elevation indicating lower hepatic toxicity than other ERAs has been observed. Ambrisentan can be safely administered with warfarin or sildenafil without the need for dose adjustment of either therapy. A once daily oral medication with relatively few side effects is an attractive option, especially as the use of therapies in combination continues to increase. Long-term data and hemodynamic data confirm the benefits can be compared with other ERAs with fewer drug–drug interactions and a better liver safety profile.

The pharmacological treatment of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a life-threatening and progressive disease of various origins characterized by pulmonary vascular remodeling that leads to increased pulmonary vascular resistance and pulmonary arterial pressure, most often resulting in right-sided heart failure. The most common symptom at presentation is breathlessness, with impaired exercise capacity as a hallmark of the disease. Advances in understanding the pathobiology over the last 2 decades have led to therapies (endothelin receptor antagonists, phosphodiesterase type 5 inhibitors, and prostacyclins or analogs) initially directed at reversing the pulmonary vasoconstriction and more recently directed toward reversing endothelial cell dysfunction and smooth muscle cell proliferation. Despite these advances, disease progression is common even with use of combination regimens targeting multiple mechanistic pathways. Overall 5-year survival for PAH has increased significantly from approximately 30% in the 1980s to approximately 60% at present, yet remains abysmal. This review summarizes the mechanisms of action, clinical data, and regulatory histories of approved PAH therapies and describes the latest agents in late-stage clinical development.