New horizons in pulmonary arterial hypertension therapies

Macitentan

Macitentan is a novel dual ERA with sustained receptor binding [21], and optimised physicochemical properties leading to enhanced tissue penetration [22]. Studies have shown that macitentan has a limited drug–drug interaction profile [23, 24]. It also has no significant inhibitory effects on hepatic bile salt transport [25] and, therefore, has the potential for a favourable liver safety profile [26].

Data from the large, multicentre, double-blind phase III SERAPHIN (Study with an Endothelin Receptor Antagonist in Pulmonary Arterial Hypertension to Improve Clinical Outcome) trial have recently been published [27]. SERAPHIN was an event-driven trial with a primary end-point of time to first morbidity or mortality event. Morbidity and mortality were chosen as a primary end-point to reflect recommendations from the 4th World Symposium on Pulmonary Hypertension [28], with the aim of providing robust, clinically meaningful data that is relevant to the long-term nature of PAH. In the SERAPHIN study, 10 mg of macitentan significantly reduced the risk of morbidity and mortality by 45% (p<0.001). Macitentan was well tolerated by the patients in this trial and, notably, adverse events commonly associated with the ERA drug class (elevated liver aminotransferases and peripheral oedema) occurred at a similar rate to patients who received placebo across all groups [27]. Macitentan has recently been approved by the US Food and Drug Administration for the treatment of PAH patients.

Selexipag

Selexipag is a highly selective oral IP receptor agonist that is rapidly hydrolysed to a long-acting metabolite [29]. Both selexipag and its active metabolite have a high binding affinity for the IP receptor and, unlike prostacyclin analogues, do not activate other prostanoid receptors. The selective nature of selexipag may translate into an improved safety profile, particularly with regards to gastrointestinal effects as, unlike iloprost and beraprost, selexipag and its metabolite do not stimulate gastric smooth muscle via the prostaglandin E (EP) receptors EP3 and EP1 [30].

In a phase II proof-of-concept study in 43 patients with symptomatic PAH receiving either ERA or PDE-5i therapy, selexipag significantly improved PVR compared with placebo (treatment effect, per protocol analysis, 30.3%; p=0.0045) (fig. 3) [31]. Selexipag-treated patients also experienced an increase in 6-min walk distance (6MWD) (mean change from baseline 24.7 m) although this did not reach significance when compared with placebo. Selexipag was well tolerated at doses up to 800 mg twice daily [31]. The encouraging preliminary results from this pilot study provided the rationale for investigating the effect of selexipag on morbidity and mortality in the event-driven phase III GRIPHON (Prostaglandin Receptor Agonist in Pulmonary Arterial Hypertension) study [32].

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Figure 3.

Change in pulmonary vascular resistance (PVR) from baseline to week 17 (per protocol analysis). Data are presented as geometric means and error bars represent 95% confidence limits (CL). Baseline PVR values (mean±sd) for per protocol population for selexipag were 951.9±434.5 dyn·s·cm⁻⁵ and for placebo were 826.8±195.8 dyn·s·cm⁻⁵. TE: treatment effect. Reproduced from [31].

Oral drugs acting on the prostacyclin pathway, such as selexipag, are an attractive therapeutic proposition. Intravenous prostacyclin analogues are potent drugs but their invasive mode of administration is a major drawback to their utility and also acceptance by the patient. Although selexipag is the first oral IP receptor agonist investigated for PAH, it is not the first oral drug to be developed for this pathway. Beraprost is an oral prostacyclin analogue that is already in clinical use. In clinical trials, beraprost was shown to have initial effects in improving exercise capacity [17] and reducing disease progression [16]; however, these effects were not sustainable over the longer term [33]. This lack of sustainability coupled with an adverse event profile typical of prostacyclin therapy means its risk–benefit profile is not ideal [16].

The efficacy of another oral prostacyclin analogue, treprostinil, was assessed in the FREEDOM-C trial. The addition of oral treprostinil therapy to background ERA or PDE-5i therapy failed to produce significant improvements in either 6MWD or Borg dyspnoea score [34]. It seems unlikely that oral treprostinil will be approved by the regulatory authorities.

Riociguat

Riociguat is a stimulator of soluble guanylate cyclase (sGC) that targets the NO pathway. Riociguat also sensitises sGC to NO and triggers downstream increases in cyclic guanosine monophosphate that will promote vasorelaxation [35].

In a phase II study in patients with functional class II/III PAH or CTEPH, treatment with riociguat for 12 weeks resulted in a significant increase in 6MWD and reductions in PVR [36]. Subsequently, riociguat has been evaluated in two large phase III trials: PATENT-1 (Pulmonary Arterial Hypertension sGC-stimulator Trial) in 445 treatment-experienced and -naïve patients with PAH [37] and CHEST-1 (Chronic Thromboembolic Pulmonary Hypertension sGC-Stimulator Trial) in 263 patients with inoperable CTEPH [38].

In PATENT-1, 12 weeks of treatment with riociguat was well tolerated and resulted in significant improvements in 6MWD (placebo-corrected improvement 35.8 m; p<0.001), PVR (p<0.001), functional class (p=0.003), time to clinical worsening (p=0.005) and Borg dyspnoea score (p=0.002) [37]. Interim data from the long-term extension study PATENT-2 have recently been reported: after 1 year of treatment, 6MWD continued to improve with a mean increase in the overall cohort (n=214) of 48±72 m compared with baseline [39]. Functional class also continued to improve and 68% of the overall cohort were in functional class I/II after 1 year of treatment (versus 38–49% at baseline); 36% and 57% of patients showed improved or stable functional class, respectively. Riociguat has recently been approved by the US Food and Drug Administration for the treatment of PAH patients.

In CHEST-1, patients with CTEPH treated for 16 weeks showed a significant increase in 6MWD (46 m; p<0.001) and secondary end-points, including PVR (p<0.001), reductions in N-terminal pro-brain natriuretic peptide (NT-proBNP; p=0.001) and functional class (p=0.003), compared with placebo [38]. Previously, there was little evidence that PAH-specific therapies were useful in the treatment of patients with CTEPH. The BENEFIT (Bosentan Effects in Inoperable Forms Of Chronic Thromboembolic Pulmonary Hypertension) trial explored the efficacy of bosentan in the treatment of 157 patients with inoperable CTEPH and showed significant improvement in PVR versus placebo, but no significant improvement in 6MWD [40]. Riociguat has recently been approved by the US Food and Drug Administration for the treatment of inoperable CTEPH patients or operable CTEPH with recurrent PH.

Imatinib

Imatinib is an anti-proliferative agent that was originally designed to inhibit the oncogenic Bcr-abl tyrosine kinase in patients with chronic myeloid leukaemia [41]. Imatinib also has an inhibitory effect on platelet-derived growth factor and c-KIT signalling. These compounds are both important in vascular smooth muscle cell proliferation and hyperplasia and have been implicated in the development of PH [42–44].

Imatinib has been shown to reverse pulmonary vascular disease in animal models of PAH [45], and has been shown to have in vitro anti-proliferative and pro-apoptotic effects on pulmonary artery smooth muscle cells taken from patients with PAH [46].

An initial case study reported the effects of imatinib in a patient with PAH classified as New York Heart Association (NYHA) functional class IV, whose condition was deteriorating despite receiving combination therapy with oral bosentan, inhaled iloprost and sildenafil (but who refused intravenous prostacyclin therapy) [47]. Treatment with imatinib produced improvements in the patient’s 6MWD and haemodynamics and the patient improved from NYHA functional class IV to II. This effect was sustained after 6 months of treatment [47].

In a 24-week, phase II study in 59 patients with PAH (functional class II–IV) and who had an inadequate response to previous therapy, patients were additionally treated with imatinib or placebo. Patients treated with imatinib showed a mean improvement of 22 m in 6MWD compared with a decline of 1 m in the placebo group, although this difference was not significant [48]. However, significant improvements were seen in PVR (imatinib -300 dyn·s·cm⁻⁵ versus placebo -78 dyn·s·cm⁻⁵; p<0.01) and cardiac output (imatinib +0.6 L·min⁻¹ versus placebo -0.1 L·min⁻¹; p=0.02).

In the recently reported phase III IMPRES (Imatinib in Pulmonary Arterial Hypertension, a Randomised, Efficacy Study) trial, imatinib used as add-on therapy significantly improved exercise capacity and haemodynamics in patients with advanced PAH. These advanced patients were symptomatic despite treatment with two or more PAH-specific therapies. However, imatinib did not provide a benefit in terms of functional class, time to clinical worsening or mortality [41].

There are concerns regarding the side-effect profile of imatinib in PAH. In the phase II trial, serious adverse events were reported in 39% of patients treated with imatinib versus 23% in the placebo group [48]. Both serious adverse events and discontinuations were more frequent with imatinib than placebo in the phase III trial (44% versus 30% and 33% versus 18%, respectively) [41]. Adverse events reported in the phase III trial were similar to those reported with the use of imatinib in other indications; however there was an unexpectedly high incidence of subdural haematoma patients receiving imatinib in addition to oral anticoagulants [41].

Other kinase inhibitors have been considered for the treatment of PAH. Of these, the most promising is nilotinib, a follow-on compound from imatinib. Rat models suggest that this may be a more efficacious treatment compared with imatinib [49], and the efficacy and safety of this treatment in PAH has recently been investigated [50]. The results have not yet been published. The more general kinase inhibitors sorafenib and dastinib have also been explored for the treatment of PAH; however, due to safety concerns they are unlikely to undergo further development [51].

Vasoactive intestinal peptide

Vasoactive intestinal peptide (VIP) is a molecule that acts to relax the vasculature and reduce the effects of vasoconstrictors [53]. The importance of VIP in PAH was demonstrated in mutant mice models, where deletion of the VIP gene led to significant increases in right ventricular (RV) systolic pressures, vascular remodelling and inflammation [54]. In these VIP knockout mice, both the vascular and right ventricular remodelling were attenuated by treatment with VIP. Further experiments in rat models with monocrotaline-induced PAH suggested that treatment with VIP may be more effective than treatment with bosentan and that combination therapy with bosentan could potentially give a greater efficacy than either VIP or bosentan therapy alone [53].

In humans, a small study observing the effects of VIP in 20 patients with idiopathic PAH during right heart catheterisation demonstrated that a single dose of VIP temporarily improved haemodynamics [55]. The single-centre, open-label study of eight patients found that haemodynamics and 6MWD were improved following 3 months of treatment with VIP [56]. The multicentre phase II study in 56 patients suggested that there was no reduction in PVR or increase in exercise capacity over 12 weeks compared with placebo [57, 58]. It is not clear to date if there will be additional trials with VIP.

Endothelial NO synthase couplers

Endothelial NO is a well-known and established contributory factor to PAH and it is important in the regulation of vascular tone. The production of endothelial NO is dependent on endothelial NO synthase (eNOS). An increased understanding of this enzyme has found that eNOS must be dimerised, or coupled, in order to produce NO and to cease production of vasoconstrictors such as superoxide [59]. Because eNOS can produce both vasorelaxant and vasoconstrictive molecules, the coupling of eNOS has a two-fold impact on the balance of vasoactive factors released in the endothelium.

The efficacy of the eNOS coupling molecule, the eNOS co-factor sapropterin dihydrochloride, with PH has been assessed in the treatment of patients [60]. The small study of 18 patients monitored markers of NO synthesis, inflammation, exercise capacity and cardiac function [60]. Although NO synthesis did not appear to increase following treatment with sapropterin dihydrochloride, there were significant (p=0.002) improvements in exercise capacity and the treatment was well tolerated, indicating this is a treatment that may warrant further study.

Rho kinases

Rho kinases mediate vasoconstriction and endothelial dysfunction; two key components of PAH [61]. The rho-kinase pathway causes downstream changes to intracellular calcium and causes calcium sensitisation. Changes in response to and levels of intracellular calcium are important as intracellular calcium signalling is involved in vasoconstriction and pulmonary arterial smooth muscle cell proliferation [62].

The rho-kinase inhibitor fasudil has shown promising results in the treatment of PAH in monocrotaline-induced rat models, even when compared to the licensed treatments bosentan and sildenafil [63].

In a small cohort of 15 patients with PAH, exposure to fasudil during right heart catheterisation produced significant (p<0.05) decreases in mean pulmonary arterial pressures and tended to decrease PVR, although it did not affect cardiac output [64]. This study tested the same parameters following exposure to NO and found similar results, suggesting that fasudil might produce similar results to treatment with NO.

Serotonin

Serotonin is an important vasoactive and mitogenic compound that is synthesised by the endothelial cells and acts on pulmonary arterial smooth muscle cells [65]. The serotonin transporter is required for serotonin to elicit its mitogenic effects and transgenic mice overexpressing the serotonin transporter have been shown to spontaneously develop PH [66].

Chronic administration of the serotonin receptor antagonist terguride has been shown to dose-dependently prevent the development and progression of monocrotaline-induced PAH in rats [67]. A 16-week study investigated the effect of terguride on PVR, 6MWD, haemodynamics, NT-proBNP and time to clinical worsening in 78 patients with PAH [68]. Although there were no overall significant improvements in PVR or the other end-points investigated, subgroup analysis found that PVR significantly improved in patients who were also treated with an ERA [68]. Terguride is already marketed in Japan for the treatment of hyperprolactinaemia and, as such, has already passed some of the stages and testing that are required for a treatment to be approved.

Apelin

Apelin is a peptide that is thought to play a role in angiogenesis, and regulate endothelial and smooth muscle cell apoptosis and proliferation, and has wider effects by altering eNOS expression [69]. Patients with PAH typically have lower levels of apelin.

Reduced or altered expression of BMPR2 has been linked to PAH [70]. Disruption of BMPR2 signalling via the peroxisome proliferator-activated receptor (PPAR)γ/β-catenin complex was found to reduce levels of apelin promoting pulmonary arterial smooth muscle cell proliferation and reduce endothelial cell apoptosis [71].

Results from animal models show that apelin elicits a vasodilatory effect [72, 73] and can reduce pulmonary arterial pressures [73]. Chronic administration in animal models also shows a pressure-reducing effect that may be due to a stabilising effect of apelin on the endothelial cells [71, 74].

As apelin is a peptide, some of the key challenges to the use of apelin in the treatment of PAH are its short half-life, developing an appropriate mode of administration, and managing the systemic vasodilatory effect of apelin [69].

Immunosuppressants

The prevalence of PAH in patients with autoimmune diseases such as systemic sclerosis and HIV have long indicated a link between immunity and PAH. Preclinical studies have shown that levels of pleiotropic cytokines such as interleukin (IL)-6 and IL-1β, which have roles in immune regulation, haematopoiesis and inflammation, are increased in patients with PAH [65, 75]. Further to this, a case report of a patient with Castleman’s-related PAH showed that treatment of the patient with the anti-IL-6 monoclonal antibody tocilizumab was effective [75]. Tocilizumab is currently licensed for the treatment of rheumatoid arthritis and is also licensed for the treatment of Castleman’s disease in Japan. As tocilizumab is already in clinical use, its safety profile in patients without PAH is well established, and the preclinical and case study data suggest this may be an interesting therapeutic option for patients with some forms of PAH, perhaps related to autoimmunity.

Another immunosuppressant that has been investigated in animal models of PAH is rapamycin. Rapamycin is a potent anti-proliferative agent that has been shown to prevent the RV wall thickening and increased proliferation of pulmonary vasculature that leads to the development of PAH in hypoxic mice [76]. In addition, rapamycin also reversed pulmonary vascular remodelling in mice that were allowed to develop hypoxia-induced PAH. Therefore, both these immunosuppressants, rapamycin and tocilizumab, may provide a novel therapeutic strategy for PAH [76].

Epidermal growth factor receptor blockers

Epidermal growth factor (EGF) has been shown to increase pulmonary arterial smooth muscle cell proliferation that can contribute to the vascular remodelling, which is, in part, causative of PAH [77]. Serine elastases act to break down extracellular matrix and this can lead to the release of EGF. In mouse models of monocrotaline-induced PAH, the use of serine elastase inhibitors has been shown to reverse the monocrotaline-induced PAH [78].

In rats with monocrotaline-induced PAH, the EGF receptor inhibitors gefitinib, erlotinib and lapatinib were shown to inhibit EGF-induced smooth muscle cell proliferation of the pulmonary vasculature [77]. However, at their highest tolerated dose, there was no significant improvement in RV systolic pressure or hypertrophy observed. Furthermore, no upregulation of EGF receptors was observed in lung tissue of patients with idiopathic PAH [77], suggesting that EGF receptor antagonists may not be as promising for the treatment of PAH as was originally thought.

Nevertheless, the results from the experiments with the serine elastase inhibitors show that targeting the EGF pathway might still be an effective treatment strategy for PAH and that this is an area that requires further investigation.

PPARγ agonists

PPARγ is a transcription factor that appears to be important in pulmonary arterial smooth muscle cell proliferation. Mutant mice with the PPARγ gene knocked out of either the endothelial or smooth muscle cells have been shown to develop PAH [79, 80].

As previously mentioned, BMPR2 mutations can be causative of PAH and are responsible for many cases of seemingly idiopathic PAH [70]. PPARγ is a downstream target of BMPR2 signalling and on formation of a complex with β-catenin appears to trigger the transcription on apelin (the importance of which was described previously) [71].

Treatment of rats with experimental PAH using the PPARγ agonist rosiglitazone attenuated the development of hypoxia-induced PAH [81]. When compared with hypoxic rats not treated with rosiglitazone or normal rats, the expression of the markers endothelin and vascular endothelial growth factor was similar in the rosiglitazone-treated and normal mice; they had deteriorated in the hypoxic, non-treated rats [81].

A recent study explored the vasodilatory effects of PPARγ antagonists in human pulmonary arteries taken from patients without PAH during resection of lung carcinoma. This study found that the PPARγ antagonists pioglitazone and rosiglitazone act to relax human pulmonary arteries, and that rosiglitazone was the most potent vasodilator [82]. These results further suggest that PPARγ antagonists may also be a novel therapeutic route for the treatment of PAH.

Dichloroacetate

In healthy people, adenosine triphosphate (ATP) is produced by the Krebs cycle; however, evidence has shown that when a patient with PAH experiences RV hypertrophy, ATP production is increasingly driven by the less efficient process of glycolysis [83, 84]. It has been hypothesised that the shift to increased glycolysis could, in part, be caused by activation of pyruvate dehydrogenase kinase (PDK). This prevents pyruvate from entering the Krebs cycle so that energy must then be generated via glycolysis [83]. The PDK inhibitor dichloroacetate has been shown to decrease proliferation and increase apoptosis of pulmonary arterial smooth muscle cells from monocrotaline-induced mice models of PAH, as well as reducing growth in pulmonary arterial smooth muscle cells from patients with idiopathic PAH [85]. Further studies in rat models of PAH found that dichloroacetate increased glucose uptake and, with chronic therapy, improved cardiac output [86].

As stimulation of serotonin receptors in pulmonary arterial smooth muscle cells has been shown to decrease apoptosis and activate the PDK pathway, it is possible that the serotonin inhibitor terguride, described earlier in this review, may also act, at least in part, via a PDK pathway [87].