Targeted Therapy in Systemic Sclerosis

Targeted therapies use an understanding of the pathophysiology of a disease in an individual patient. Although targeted therapy for systemic sclerosis (SSc, scleroderma) has not yet reached the level of patient-specific treatments, recent developments in the understanding of the global pathophysiology of the disease have led to new treatments based on the cells and pathways that have been shown to be involved in the disease pathogenesis. The presence of a B cell signature in skin biopsies has led to the trial of rituximab, an anti-CD20 antibody, in SSc. The well-known properties of transforming growth factor (TGF)-β in promoting collagen synthesis and secretion has led to a small trial of fresolimumab, a human IgG4 monoclonal antibody capable of neutralizing TGF-β. Evidence supporting important roles for interleukin-6 in the pathogenesis of SSc have led to a large trial of tocilizumab in SSc. Soluble guanylate cyclase (sGC) is an enzyme that catalyzes the production of cyclic guanosine monophosphate (cGMP) upon binding of nitric oxide (NO) to the sGC molecule. Processes such as cell growth and proliferation are regulated by cGMP. Evidence that sGC may play a role in SSc has led to a trial of riociguat, a molecule that sensitizes sGC to endogenous NO. Tyrosine kinases (TKs) are involved in a wide variety of physiologic and pathological processes including vascular remodeling and fibrogenesis such as occurs in SSc. This has led to a trial of nintedanib, a next-generation tyrosine-kinase (TK) inhibitor which targets multiple TKs, in SSc.


INT RODUCTION
Sy stemic sclerosis (SSc, or scleroderma) is a serious multi-sy stem disorder of connective tissue disease characterized clinically by thickening and fibrosis of the skin and inv olvement of internal organs, most commonly the lungs, gastrointestinal tract , and heart. Sy stemic sclerosis affects predo minantly women in the prime of their life and is associated with increased morbidity and mortality . Sy stemic sclerosis is a rare disease with prevalence estimates v ary ing from 30 to 443/million population. 1 ,2 Because the disease is so heterogeneous and rare, it has been difficult to perform high-quality randomized controlled clinical trials. Unfortunately, the few such trials performed, especially early ones, hav e not been uniformly successful. Therapies studied in randomized controlled trials that have failed include ketanserin, 3 5-fluorouracil, 4 ketotifen, 5 interferongamma, 6 interferon-alpha, 7 penicillamine, 8 relaxin, 9,1 0 methotrexate, 11 oral collagen, 12 imatinib, 1 3,1 4 IV IG, 15 macitentan, 16 and bosentan. 17,18 More recently there have been some hopeful positive trials with statins, 19 cyclophosphamide, 20 and autologous stem cell transplantation. 21 ,22 Could we be more successful with a more targeted approach to therapy in SSc, similar to that taken in other diseases such as cancer? For ex ample, in melanoma, BRAF kinase-activating mutations can turn BRAF into an oncogene, and the presence of such mutations has led to specific therapy. 23 Similar analy ses of non-small cell lung cancer has found that mutations in the epidermal growth factor receptor (EGFR), translocations inv olving the anaplastic ly mphoma kinase (ALK) tyrosine kinase, oncogenic RAS mutations, and other driver mutations, such as BRAF, can be treated with therapy targeted at these abnormalities. 24 In SSc we are entering a v ery ex citing and hopeful era with 31 6 trials for SSc listed on ClinicalTrials.gov. 25 Some of these trials have used, or are using, new biologic agents or kinase inhibitors such as abatacept, 26 rituximab, 25 imatinib, dasatinib, fasudil, 27 nintedanib, 28 and fresolimumab, an antitransforming growth factor (TGF) antibody . 29 A phase III trial of tocilizumab is underway. Some of these trials are based on the concept of targeting specific pathophysiologic abnormalities that hav e been found in SSc . We hav e thus hopefully mov ed away from general immunosuppressiv es such as cy clophosphamide or methotrex ate to drugs that inhibit specific pathways or cells thought to be active in SSc. These represent the first steps toward personalized targeted medicine in SSc.

RIT UXIMAB
One promising approach to possible biomarkers that may indicate that a certain subset may respond to specific targeted therapy has come from the study of the gene ex pression found in skin biopsies. [29][30][31][32][33][34][35][36][37] Early work demonstrated that some patients with SSc had a skin gene ex pression signature consistent with the presence of active B cells. 31 This observ ation has led directly to two trials of rituximab (RTX), an anti-CD20 monoclonal antibody , in SSc. 38, 39 In the first publication 15 patients with diffuse cutaneous sy stemic sclerosis (dcSSc) received two doses of ritux imab 1 g each, 2 weeks apart. 39 The mean change in the modified Rodnan skin score (MRSS) between baseline and 6 months was not significant. Results of pulmonary function tests and other measures of major organ inv olvement were stable. The modest B cell infiltrates that were present in most skin biopsy specimens at baseline were completely depleted at 6 months in most patients. In the other trial, 1 4 patients with SSc were ev aluated. 38 Eight patients were randomized to receiv e two cy cles of RTX at baseline and 24 weeks, whereas six patients (control group) receiv ed standard treatment alone. There was a significant increase of forced v ital capacity (FV C) in the RTX group compared with baseline. The median percentage of improv ement of FV C in the RTX group was 10.25%, whereas that of deterioration in the controls was 5.04% (P=0.002). Skin thickening, assessed with the MRSS, improv ed significantly in the RTX group compared with the baseline score (mean±SD 1 3.5±6.84 v ersus 8.37 ±6.45 at baseline v ersus 1 y ear, respectively, P<0.001 ). Ritux imab depleted both circulating B cells and dermal B cells. In the RTX-treated group, there was a significant reduction of collagen deposition in the papillary dermis at 24 weeks compared with baseline which was not seen in the control group. The EUSTAR group also published an observational study assessing the effects of RTX on skin and lung fibrosis in patients with SSc. 40 Comparison between RTX-treated patients and matched controls revealed a significant difference in fav or of RTX. Howev er, to this author's knowledge, ritux imab has not strictly been employed as targeted therapy such as treating only those patients with a skin biopsy showing B cell activation or analy zing the results in patients with skin biopsies that show B cell activation versus those with no such pattern.

FRESOLIMUMAB
It is well-known that TGF-β promotes collagen sy nthesis, secretion, processing, and cross-linking as well as secretion of other matrix molecules, such as fibronectin and thrombospondin. 41 This has led to an interest in using anti-TGF-β in SSc. The first trial of CAT-1 92, a recombinant human antibody that neutralizes TGF-β1, was not successful. 42 However, a more recent study tested fresolimumab, a first-inclass human IgG4 κ monoclonal antibody capable of neutralizing all mammalian isoforms of TGF-β. 29 There is a four-gene, pharmacodynamic biomarker of SSc skin disease, based on gene ex pression in a mid-forearm skin biopsy . 29 Two of the four genes making up the biomarker, thrombospondin-1 (THBS1 ) and cartilage oligomeric protein (COMP), are highly regulated by TGF-β. In this open uncontrolled trial, the predefined primary efficacy outcome was change in COMP and THBS1 mRNA ex pression in skin after treatment compared with that at baseline. Subjects showed rapid declines in THBS1 and COMP gene ex pression in skin biopsies after treatment with fresolimumab. THBS1 and COMP gene ex pression was strikingly higher in SSc patient cohorts than in healthy control skin, and changes in gene ex pression in study patients generally correlated with changes in MRSS.

T OCILIZUMAB
Another promising therapy consists of inhibition of interleukin (IL)-6. Ev idence supports important roles for IL-6 in the pathogenesis of SSc, e.g. dermal fibroblasts from SSc patients constitutively express higher lev els of IL-6 than found in healthy controls; serum and skin lev els of IL-6 are elev ated in SSc patients with early disease and in patients with SSc or SSc-interstitial lung disease (ILD), and increased IL-6 lev els have been associated with higher mortality , more severe skin inv olv ement, and increased incidence of progressiv e pulmonary decline. [43][44][45][46][47][48] Strategies to block the IL-6 response resulted in a significant reduction of procollagen ty pe I in cultured SSc fibroblasts and myofibroblastic differentiation in dermal fibroblasts in a bleomy cin-induced model of dermal sclerosis. 49,50 Some indirect ev idence of increased effect of IL-6 in SSc derives from the fact that CRP is elev ated in SSc although not to lev els associated with diseases such as rheumatoid arthritis. 51 C-reactive protein (CRP) lev els in dcSSc are higher than in limited cutaneous SSc (lcSSc). 51,52 These findings hav e led to a phase II trial of tocilizumab in early dcSSc. 53 The primary end-point showed a treatment difference of -2.7 0 MRSS units in fav or of tocilizumab at week 24 but did not quite reach statistical significance. Ex ploratory analysis of lung function showed that fewer patients in the tocilizumab arm had a decline in percentagepredicted forced v ital capacity than in the placebo arm by comparison of the cumulative distribution by week 48. Tocilizumab specifically downregulated the ex pression of my eloid-associated genes in the skin and decreased circulating lev els of CCL1 8, a chemokine associated with fibrosis and progression of SSc-associated lung disease. A phase III trial is underway .

RIOCIGUAT
Soluble guany late cyclase (sGC) is an enzy me that catalyzes the production of cyclic guanosine monophosphate (cGMP) upon binding of nitric ox ide (NO) to the sGC molecule. Once released by the sGC, cGMP can act as a second messenger to activate further downstream targets, such as cGMP-regulated ion channels, protein kinases (G-kinases), and phosphodiesterases (PDEs). Through those effectors, cGMP regulates a v ariety of physiological processes, including cell growth and proliferation, v ascular tone and remodeling, immune responses, and neuronal transmission. Riociguat is a molecule that sensitizes sGC to endogenous NO by stabilizing NO-sGC binding. 54,55 Riociguat also directly stimulates sGC, independent of NO, resulting in increased generation of cGMP. Riociguat has been shown in large randomized controlled clinical trials to be effectiv e in patients with different forms of pulmonary hypertension including patients with SScrelated pulmonary arterial hy pertension (PAH).
There is ev idence that sGC may play a role in SSc. Soluble guany late cyclase activators inhibited the release of TGF-β-induced extracellular matrix proteins from primary dermal fibroblasts obtained from both normal volunteers and SSc subjects, and dermal fibrosis was reduced in the bleomycin skin fibrosis model of SSc. 56,57 Riociguat has been shown in large randomized controlled clinical trials to be effectiv e in patients with different forms of pulmonary hy pertension, including patients with SScrelated PAH, 58 and is now in a trial for the skin thickening of SSc.
Nintedanib reduced differentiation of myofibroblasts and the release of collagen of dermal fibroblasts from patients with SSc and healthy individuals. Nintedanib also showed anti-fibrotic effects in a dose-dependent manner in different animal models of SSc , including the bleomycin skin fibrosis model both in preventive and therapeutic applications, the chronic graft-versus-host disease model, and the Tsk-1 model. Interestingly , in the Fra-2 tg mouse model, nintedanib did not only inhibit skin and lung fibrosis but also improved the pulmonary vascular lesions resembling PAH. 61 Based on these results, a large, randomized, placebo -controlled trial with nintedanib is currently initiated in patients with SSc for pulmonary fibrosis.

CONCLUSION
Although we are in an exciting new era of drug trials for SSc, and although the drugs discussed here are being used because of their abilities to interfere with specific cells or pathways that have been found to be inv olved in SSc or SSc models, the field of targeted therapy has not y et progressed quite as far as it has in cancer. The nex t big step will be to understand if the new drugs are effectiv e specifically in patients who demonstrate abnormalities of the cells or pathway s that are inhibited or activ ated by these new medications. Then we will be closer to developing a truly personalized approach to the treatment of this serious disease.