Endothelin Receptor Antagonists for the Treatment of Raynaud's Phenomenon and Digital Ulcers in Systemic Sclerosis

Systemic sclerosis is a connective tissue disease characterized by fibrosis of the skin, internal organs, and widespread vasculopathy. Raynaud's phenomenon and digital ulcers are vascular manifestations of this disease and cause significant morbidity. Current treatments are only moderately effective in reducing the severity of Raynaud's in a portion of patients and typically do not lead to substantial benefit in terms of the healing or prevention of digital ulcers. Several studies have evaluated the efficacy of targeting the vasoconstrictor endothelin-1 for the treatment of systemic sclerosis-associated vascular disease. The purpose of this paper is to summarize the published studies and case reports evaluating the efficacy of endothelin receptor antagonists in the treatment of Raynaud's phenomenon and digital ulcers associated with systemic sclerosis.


Introduction
Systemic sclerosis (SSc) is a connective tissue disease characterized by fibrosis of the skin and internal organs and widespread vasculopathy. Raynaud's phenomenon (RP) is often the first manifestation of SSc, frequently preceding the onset of cutaneous sclerosis by several years particularly in patients with limited disease, and eventually occurs in 95% of patients with SSc [1]. Vasospasm of the digital arteries leads to the three characteristic phases of pallor, cyanosis, then erythema correlating with reduced blood flow, total loss of oxygen supply, and reperfusion. Episodes of RP are usually triggered by cold exposure or stress and can be associated with numbness and pain, resulting in significant disability [2]. Recurrent episodes of ischemia-reperfusion injury and the subsequent generation of reactive oxygen species can result in ischemic damage to distal tissue sites. Digital ulcers (DUs) are necrotic lesions that occur either at distal aspects of digits (fingers or toes) or over bony prominences and occur in up to 50% of patients with limited or diffuse cutaneous SSc [3]. These lesions are exquisitely painful, heal slowly, and interfere with activities of daily living often leading to substantial functional disability. Other complications associated with DU include scarring with loss of distal tissue, infection that can lead to osteomyelitis, and progression to gangrene requiring amputation [4,5]. DUs that develop at distal aspects of digits are thought to be related to recurrent ischemia from various processes, including vasospasm from RP, thrombosis of digital arteries, calcinosis, and structural microvascular changes related to the underlying SSc [4,[6][7][8]. Recurrent trauma, particularly in patients with joint contractures, also contributes to the development of DU in patients with SSc. Ulcerations on the lower extremities proximal to the feet can occur in patients with SSc who likely have macrovascular disease as well. Current treatments for both RP and DU consist of vasodilators including calcium channel blockers (CCBs), alpha-adrenergic inhibitors, angiotensin converting enzyme (ACE) inhibitors, angiotensin receptor blockers, and nitroglycerin analogues. These medications are moderately effective in reducing the severity of RP in a portion of SSc patients [9], but typically do not lead to substantial benefit in terms of the healing and prevention of DU.
With the availability of powerful vasodilator therapies for the treatment of pulmonary arterial hypertension (PAH), options for the treatment of severe RP, DU, and progressive digital ischemia have increased. Prostacyclin analogues have been shown to accelerate the healing of DU, however, those agents found to be effective thus far require intravenous or 2 International Journal of Rheumatology subcutaneous delivery [10][11][12]. Small studies have indicated that oral phosphodiesterase-5 inhibitors (PDE-5-I) are effective in reducing the severity of RP and promoting the healing of DU [13][14][15]. Large multicenter randomized controlled studies are underway to further evaluate the efficacy of PDE-5-I in the treatment of RP and DU. Several studies have evaluated the efficacy of targeting the vasoconstrictor endothelin-1 (ET-1) for the treatment of RP and/or DU. The purpose of this paper is to summarize the published studies evaluating endothelin receptor antagonists (ETRA) in the treatment of RP and/or ischemic DU associated with SSc.

The Role of Endothelin in the Pathogenesis of SSc-Associated RP and DU
The initial events leading to SSc vasculopathy are thought to involve endothelial cell injury [16] with subsequent loss of normal vasodilatory mediators such as prostacyclin and nitric oxide [17][18][19][20]. In addition, endothelial injury results in increased release of the vasoconstrictor endothelin-1 (ET-1) [21,22]. ET-1 is a 21-amino acid polypeptide expressed primarily by endothelial cells, but has also been found to be expressed by epithelial cells, macrophages, fibroblasts, and cardiomyocytes among others [23,24]. It acts locally, binding to the surface of smooth muscle cells and acts on the vascular endothelium itself in an autocrine manner. Levels of ET-1 have been found to be increased in the serum of patients with RP and SSc [25][26][27]. In addition to its role as a biomarker of vascular disease, ET-1 itself may be contributing to the fibrotic and vasculopathic aspects of SSc as it has been shown to stimulate fibroblast and smooth muscle proliferation [28,29]. ET-1 signaling is mediated by two transmembrane G-protein-coupled receptors (ET A and ET B ) with different binding affinity and physiologic effects [23,30]. ET A receptors are expressed on vascular smooth muscle cells and primarily mediate vasoconstriction whereas ET B receptors are expressed on both endothelial cells, mediating vasodilatation, and on smooth muscle cells, mediating vasoconstriction [31].

Endothelin Receptor Antagonists
Endothelin receptor antagonists are a class of PAH-specific drugs that block the interaction of ET-1 with its receptors (  [37]. After 6 months of standard bosentan therapy (62.5 mg BID × 4 weeks then 125 mg BID), she experienced resolution of her DU correlating with a decrease in plasma ET-1 concentration. The second case reported a 4-yearold girl with pansclerotic morphea unresponsive to corticosteroids, methotrexate, CCB, ACE-inhibitors, and Dpenicillamine [38]. Within the first months of bosentan therapy, both her widespread sclerotic skin lesions and her limb ulcers improved. Another case report published in 2007 described a 39-year-old female with limited cutaneous SSc and worsening DU despite IV prostacyclin therapy [39]. After 6 weeks of the standard approved dose of bosentan, her DU completely healed. In 2008, a report was published describing a 62-year-old female with long-standing SSc who experienced healing of a large pretibial ulceration after 6 months of standard bosentan therapy [40]. Finally, in 2009, a case report described a 39-year-old female with diffuse cutaneous SSc and recalcitrant DU treated with sitaxsentan 100 mg daily. After 6 months of therapy, her DU significantly improved and no new DU developed [41]. Table 3 outlines the prospective studies investigating the utility of ETRAs in the treatment of RP and/or DU. The first prospective study published in 2006 described 3 patients with RP in the setting of prescleroderma (defined as RP associated with sclerodermatous nailfold capillaroscopic changes and SSc-specific autoantibodies) or limited cutaneous SSc independent of a history of DU [43]. The participants received the standard dosing of bosentan and at the end of the 16-week treatment course pain, RP disease activity and severity were noted to be reduced. A larger prospective observational study published in 2008 evaluated the long-term efficacy and tolerability of bosentan in 15 patients with SSc with current or a prior history of DU [44]. The patient population in the study was particularly heterogeneous with a wide range in age (11-72 years), 0 to 26 DU at baseline, and included 6 patients with interstitial lung   was noted in the DU of 4 additional patients. Overall, the mean number of DU per patient was reduced at 6, 12, and 36 months (P < 0.001). Additionally, healing of the DU was evidenced by complete reepithelialization on skin biopsy which was performed on 5 of the 26 participants. In a 24-week prospective open-label trial, 10 patients with SSc were treated with bosentan; skin fibrosis as assessed by the modified Rodnan skin score (MRSS) was the primary endpoint, but evaluation of DU was a secondary outcome assessment [48]. At each visit, examination of DU was performed by the same evaluator and categorized as present, indeterminate (>50% reduction in their surface area), or healed (total reepithelialization). 88% of DU were categorized as healed at the end of the 24-week treatment compared with 42% at baseline (P = 0.0019). Finally, in 2010, a 48week observational study was published evaluating the effectiveness of bosentan for RP without DU in patients with SSc-associated PAH [49]. 14 patients who were on stable doses of other PAH-specific therapies (excluding patients treated with parenteral prostanoids within the previous 6 months) were treated with bosentan as add-on therapy. For patients with limited or diffuse cutaneous SSc, the number of RP attacks from baseline to 48 weeks decreased from 3.4 ± 1.8 to 2.1 ± 1.4 and 3.8 ± 1.7 to 2.1 ± 1.8, respectively (P < 0.05). Likewise, the duration of RP attacks decreased for patients with limited or diffuse cutaneous SSc from baseline to 48 weeks from 62.5 ± 43.7 to 22.1 ± 13.8 minutes and 61.0 ± 38.9 to 29.2 ± 13.7 minutes, respectively (P < 0.01).

Double-Blind, Randomized, Placebo-Controlled Studies.
The first randomized placebo-controlled double-blind clinical trial evaluating an ETRA for the prevention of DU in patients with SSc was published in 2004 and is commonly known as the RAPIDS-1 (Randomized Placebo-controlled Investigation of Digital ulcers in Scleroderma) trial [42]. 122 patients with SSc and current DU or a history of at least 1 in the prior 12 months, enrolled across 17 centers in Europe and North America. 79 were randomized to receive bosentan and 43 to receive matching placebo during the 16-week treatment phase. The mean number of new ulcers during the treatment period was 1.4 for patients on bosentan versus 2.7 for patients on placebo (P = 0.0083) representing a 48% reduction in the number of new DU. However, there were no differences in the reduction of preexisting DU in the 63% of patients with active DUs at baseline. The most notable adverse event occurring in more patients on bosentan than placebo was elevated transaminase levels (14% versus 0%). 3 patients developed a marked transaminitis (>8x ULN), and 5 patients (6%) discontinued the study due to these laboratory abnormalities, but in all cases the transaminase values returned to normal when bosentan was discontinued. The results of a second randomized placebo-controlled doubleblind clinical trial investigating bosentan for the treatment of SSc-related DU (RAPIDS-2) were recently published in 2011 [50]. This trial involved a longer 24-week treatment phase and all 188 patients enrolled across 41 sites in Europe and North America were required to have at least 1 active DU, the largest called the "cardinal ulcer," at baseline. The mean number of new DU over 24 weeks was 1.9 ± 0.2 for patients on bosentan versus 2.7 ± 0.3 for patients on placebo (P = 0.035) representing a 30% reduction in the number of new lesions. As with the RAPIDS-1 study, the RAPIDS-2 study failed to show a benefit in terms of healing of existing ulcers. Adverse events occurring in more patients on bosentan than placebo included peripheral edema (18.8% versus 4.4%) and elevated aminotransferases (12.5% versus 2.2%). Markedly increased aminotransferases (>3x upper limit of normal (ULN) and 1 case of >8x ULN) occurred in 10.5% of patients in the bosentan group, but these abnormalities resolved during continued treatment, after a decrease in dose, or following temporary or permanent treatment discontinuation. Only one placebo-controlled double-blind clinical trial evaluating an ETRA for the treatment of RP in patients with SSc has been published to date [47]. 17 patients without preexisting DU were randomized to either standard bosentan therapy or matching placebo during the 16-week treatment phase. Severity of RP was assessed via the Raynaud's Condition Score (RCS), a validated composite self-assessment of the severity of RP encompassing the number and duration of episodes, the associated symptoms, such as pain and numbness, and the degree of hand disability. The RCS is measured on a scale of 0-10 with 0 indicating no disability related to RP and 10 indicating extremely severe disability from RP. Patients recorded the frequency, duration, and severity of RP attacks in daily symptom diaries. The mean RCS score was reduced for both the bosentan and placebo groups (−31% and −36%) at week 16, but the improvements were not statistically significant compared with baseline nor were they different between the groups. Frequency of RP attacks significantly decreased in the bosentan and placebo groups, however, at week 16 only patients in the placebo group maintained a statistically significant decrease in RP frequency (bosentan: −30%; placebo: −57%, P = 0.017). In 16 weeks, a significant reduction in mean duration of RP attacks was observed for both bosentan (−26%; P = 0.012) and placebo (−60%; P = 0.028) compared with baseline, however, there was no significant difference between the groups. Interestingly, despite the lack of improvement on measures of RP activity and severity with bosentan, patients in the treatment group demonstrated statistically significant improvements in functional status as assessed by the scleroderma Health Assessment Questionnaire (P = 0.03 and P = 0.01 at weeks 12 and 20, respectively) and the UK functional score (P = 0.04 at weeks 8 and 16) compared with those treated with placebo. No serious adverse events were noted, and only 1 participant withdrew due to treatmentrelated peripheral edema. The authors did mention that DU developed in 1 patient on placebo and 2 on bosentan, but these resolved with similar healing times.

Conclusion
The findings from the literature reviewed here indicate that ETRAs may play a role in the treatment of RP and DU in addition to their indication for the treatment of PAH. The two large, randomized, placebo-controlled studies using bosentan have shown that this agent is useful in the prevention of new DU in patients with SSc (RAPIDS 1 and 2) confirming findings in uncontrolled observational studies. However, both of these studies failed to show a benefit in terms of healing of existing ulcers [42,50]. Although observational studies demonstrated an improvement in RP with ETRA treatment [43,45,49], the one randomized, placebocontrolled study using bosentan for the treatment of RP [47] did not show a statistically significant difference between bosentan and placebo. Although this was a relatively small study, the results highlight the importance of performing RCT in the assessment of potential treatments for RP and DU. In addition to the benefit of ETRA for DU, 3 of the case reports suggested benefit in nondigital ischemic ulcers in patients with SSc, but larger studies are necessary to verify these results [36,38,40]. This is important as nondigital ulcers are seen in up to 4% of patients with SSc [51]. The majority of studies published thus far have described the effects of the ETRA bosentan on RP and/or DU. It is unknown whether ETRAs with greater selectivity for the ET A receptor, would show better tolerability and efficacy than bosentan in the treatment of SSc-associated RP and DU. Ambrisentan may be preferable to bosentan given the lower incidence of liver function test abnormalities, once daily dosing and lack of interaction with warfarin. Our center has recently completed the first open-label prospective study of ambrisentan for the treatment of SSc-associated DU and the results will soon be available. ETRAs may be preferable to prostacyclins given their oral bioavailability, but physicians must be cognizant of their teratogenicity and potential side effects including liver toxicity, edema, and anemia. Additional RCTs are necessary to better assess the role of ETRAs for the treatment of RP and DU in patients with SSc.