Understanding the efficacy of individual Janus kinase inhibitors in the treatment of ulcerative colitis for future positioning in inflammatory bowel disease treatment

Abstract Recent studies have gradually elucidated the pathogenesis of inflammatory bowel disease; thus, the Janus kinase (JAK)-signal transducers and activators of transcription pathway are strongly involved in the pathophysiology of inflammatory bowel disease. Generally, Janus kinase inhibitors are being used for the treatment of rheumatoid arthritis and other immunological diseases, with the therapeutic promising effects. Currently, in Japan, three Janus kinase inhibitors, namely tofacitinib, filgotinib, and upadacitinib, are available for the treatment of patients with active ulcerative colitis. Therefore, evaluating the efficacy and safety of each JAK inhibitor is essential for determining the role of JAK inhibitors in future therapeutic strategies for inflammatory bowel disease (IBD).


Introduction
Steroids and 5-aminosalicylic acid have been the mainstays of treatment for inflammatory bowel disease (IBD). However, based on the elucidation of the pathophysiology of IBD, numerous other drugs have been developed [1]. Among them, anti-tumor necrosis factor (TNF)-a antibody drugs have revolutionized IBD treatment. Since their emergence, anti-interleukin (IL)-12/23p40 and anti-integrin a4b7 antibodies, which markedly improved the quality of life (QoL) of patients with IBD, have been developed [2]. Recently, small-molecule compounds, such as Janus kinase (JAK) inhibitors, which target the JAK-signal transducers and activators of transcription (STAT) pathway, have become available for IBD patients [3]. In Japan, three JAK inhibitors, namely tofacitinib, filgotinib, and upadacitinib, are now clinically available for the management of patients with active ulcerative colitis (UC), but not Crohn's disease. In this review, we describe the significance of JAK-STAT pathway in the pathophysiology of IBD and focus on the efficacy and safety of three JAK inhibitors for patients with UC to determine their future potential.

What is the JAK-STAT pathway?
JAKs are cytoplasmic tyrosine kinases, that mediate cytokine signaling from membrane receptors to STAT transcription factors. In 1989, the first two JAKs (JAK1 and JAK2) were discovered; they are named as 'just another kinase' because of their unknown function [4]. Subsequently, their function was elucidated. They were found to include both a catalytic domain and a kinase-like domain and were, therefore, renamed 'Janus kinases' after the Roman two-faced god. The discovery of the JAK-STAT pathway two decades ago was a breakthrough in the understanding of cytokine biology. Since then, numerous studies have shown that this pathway plays important roles in various biological processes, including cell development, growth, survival, and various diseases [5,6]. In addition, the JAK-STAT pathway is intrinsically involved in the mucosal immune responses to bacteria, viruses, and other antigens. Four JAK family members are known: JAK1, JAK2, JAK3, and tyrosine kinase (TYK) 2 [7] ( Table 1). The significance of each JAK in the immune system has been elucidated by studying specific JAK-knockout mice. In mice, complete loss of JAK1 leads to perinatal lethality, and newborn mice display a strong reduction in thymocytes and B cells [8]. The-in vivo-importance of Jak 2 in cytokine-dependent signaling was demonstrated by the fact that germline deletion of Jak 2 in mice resulted in embryonic lethality by day 12.5 owing to a lack of hematopoiesis [9]. Mice and humans lacking Jak 3 have severe immune deficiencies, including defects in B and T lymphocyte development and function [10,11].
The JAK-STAT pathway mediates signaling in various types of cells in the human intestinal tract, including immune, epithelial, and stromal cells, and has been suggested to be involved in the pathogenesis of IBD [3]. Genetic studies have reported that gain-of-function variants of JAK2, TYK2, STAT1, STAT3, and STAT4 are associated with both UC and Crohn's disease [13][14][15][16][17][18]. In addition, the expression of all JAKs reportedly is increased in the intestinal mucosa of patients with IBD [19][20][21][22][23][24][25] but decreases in those who respond to anti-inflammatory treatments.

Efficacy of JAK inhibitors in active UC
In Japan, three JAK inhibitors are currently available for the treatment of patients with UC: tofacitinib (TOF), filgotinib (FIL), and upadacitinib (UPA) ( Table 2). Table 2 shows the IC50 concentrations for JAK1, 2, 3, and Tyk 2 pathways for each drug, indicating that FIL and UPA have JAK1-preferential suppressive effects, while TOF is a non-selective inhibitor. However, we cannot rule out the possibility that increasing the concentrations of all JAK inhibitors may affect each pathway, and we should consider that selectivity is only relative. In the subsequent sub-section, the therapeutic efficacy and safety of these JAK inhibitors based on clinical trial data are presented to determine the role of each JAK in future treatment strategies for UC.

Tofacitinib
TOF was the first JAK inhibitor to become clinically available for IBD patients in May 2018. TOF inhibits all JAKs but preferentially inhibits JAK1 and JAK3 [12,26]. The results of three phase 3, randomized, double-blind, placebo-controlled trials of TOF therapy in adult patients with UC have been reported [27]. In the OCTAVE Induction 1 and 2 trials, 598 and 541 patients, respectively, who had moderately to severely active UC despite previous conventional therapy or therapy with a TNF-a antagonist were randomly assigned to receive induction therapy with TOF (10 mg twice daily [BID]) or placebo for 8 weeks. The primary end point was remission at 8 weeks. The clinical remission rate was significantly higher in the TOF 10 mg twice daily group than in the placebo group (18.5 vs. 8.2%; p ¼ .007 in the OCTAVE Induction 1 trial and 16.6 vs. 3.6%; p < .001 in the OCTAVE Induction 2 trial). In the OCTAVE Sustain trial, 593 patients who had a clinical response to induction therapy were randomly assigned to receive maintenance therapy with TOF (5 or 10 mg BID) or placebo for 52 weeks. The primary end point was remission at 52 weeks. Clinical remission rates at 52 weeks in the patients in the 5-mg tofacitinib group, 10-mg tofacitinib group, and placebo group were 34.3, 40.6, and 11.1%, respectively (p < .001 for both compared with the placebo).
We experienced the relatively rapid onset of the effects of JAK inhibitors in clinical practice. Hanauer et al. performed a post-hoc analysis of the impact of TOF on early symptom improvement in the OCTAVE 1 and 2 induction studies, using data within 15 days of treatment initiation [28]. By day 3, patients receiving TOF had substantially higher reductions from baseline in stool frequency subscore, total number of daily bowel movements, and rectal bleeding subscore than those receiving placebo. These data indicated that the rapid onset of effect of TOF in patients with UC.
Considering the side effects of TOF, it would be ideal if remission could be maintained at 5 mg TOF. The RIVETING trial was designed to provide clinically relevant information regarding the use of TOF 5 and 10 mg BID during maintenance to assess the efficacy and safety of dose reduction to 5 mg in patients with UC in stable remission on TOF 10 mg BID maintenance therapy [29]. Patients who had received TOF 10 mg BID for !2 consecutive years and been in stable remission for !6 months were enrolled. The primary endpoint was modified Mayo score remission at month 6. One hundred and forty patients were randomized to receive TOF 5 or 10 mg BID [1:1]. The result showed that 77.1 and 90.0% of patients in the 5 and 10 mg BID groups, respectively, were in modified Mayo score remission at month 6. Most patients with UC in stable remission on 10 mg BID maintenance therapy have kept remission following dose de-escalation. In patients who dose de-escalated, those in deep endoscopic remission and those without prior anti-TNF a treatment failure were more likely to maintain remission. One limitation of this study was that the efficacy data were limited to the first 6 months after randomization.
Some patients with UC have to remain on TOF 10 mg BID. However, the long-term efficacy and safety of TOF 10 mg BID in these patients have remained unclear. Therefore, the OCTAVE Open trial investigated the short-and long-term (month 36) efficacy and safety of TOF 10 mg BID in patients achieving a clinical response following extended induction therapy (16 weeks) who failed to respond to initial induction treatment (8 weeks) [30]. The results of this study showed that 52.2% of patients who did not achieve a clinical response after 8 weeks of induction treatment with TOF 10 mg BID achieved a clinical response following extended induction (delayed responders). At month 12, 70.3, 56.8, and 44.6% of delayed responders maintained a clinical response and achieved endoscopic improvement and remission, respectively. Corresponding values at month 36 were 56.1, 52.0, and 44.6%. The safety profile of TOF 10 mg BID as induction therapy for up to 16 weeks was a comparable to that of 8 weeks of induction therapy. These data supported the recommended dose of TOF 10 mg BID for induction (8 weeks) or extended induction (16 weeks). Most patients who responded to extended induction maintained a clinical response for up to 36 months.
Data on the therapeutic effect of TOF on acute severe UC (ASUC) are limited. Berinstein et al. reported that three of four patients (75%) with ASUC responded to TOF 30 mg daily [31]. A retrospective case-control study evaluating the effectiveness of TOF in biologic-experienced patients with ASUC requiring intravenous corticosteroids indicated that 30 mg TOF daily acted protective for colectomy [hazard ratio (HR), 0.11; 95%CI, 0.02-0.56; p ¼ .008], whereas 20 mg TOF daily was not effective (HR, 0.66; 95%CI, 0.21-2.09; p ¼ .5) [32]. In case series reported by Kotwani et al. and Gilmore et al. eight patients with ASUC refractory to biologics achieved clinical remission without colectomy at 90 days, whereas one patient did not [33,34]. These results indicate a potential therapeutic effect of high dose TOF on ASUC. However, prospective studies are required to determine the optimal dose and evaluate safety at high doses.

Filgotinib
Given the importance of the JAK-STAT pathway in the hematopoietic cells, it has been presumed that a selective JAK1 inhibitor with an improved safety profile may reduce the rate of AEs. This rationale had led to the development of FIL, a selective inhibitor of JAK1. Compared to TOF, FIL has a longer half-life of $6 h for the parent compound and $23 h for the active metabolite [35]. In Japan, FIL has become available for UC treatment since March 2022. The efficacy and safety of FIL in patients with moderately to severely active UC have been tested in a phase 2b/3, randomized, double-blind SELECTION study [36]. This clinical trial consisted of two induction studies and one maintenance study. In the induction studies, both biologic-naive patients (induction study A) and those who had previously failed biologic therapy including any TNF-a inhibitors or vedolizumab (VED) (induction study B) were eligible. This clinical trial is unique in that the bio-naive and bio-failure groups were separated before the start of FIL. Therefore, patients with UC in the induction B cohort have higher disease activity and endoscopic activity. In total, 659 biologic-naive and 689 biologic-experienced patients (of whom 43.1% had experienced failure of both a TNF-a inhibitor and VED) were included and randomized 2:2:1 into three groups: FIL 200 mg daily, FIL 100 mg daily or placebo. The primary endpoint was clinical remission at week 10 as Mayo endoscopic subscore 1, rectal bleeding subscore ¼ 0, and a ! 1-point decrease in stool frequency from induction baseline to achieve a subscore 1. In biologic-naive patients, the clinical remission was significantly higher in the FIL 200 mg group than in the placebo group (26.1 vs. 15.3%, p ¼ .0157). In biologic-experienced patients, the clinical remission rate was significantly higher in the FIL 200 mg group than in the placebo group (11.5 vs. 4.2%, p ¼ .0103), although the efficacy rate was lower than that in biologic-naive group.
The 58-week maintenance study included patients who achieved clinical remission or a clinical response after 10 weeks. Patients treated with FIL during the induction phase were rerandomized 2:1 to induction FIL dose or placebo, and patients previously randomized to placebo continued placebo. The primary endpoint was clinical remission at week 58, defined the same as that at week 10. The clinical remission rate in the FIL 200 mg and 100 mg group was significantly higher than that in the placebo group (37.2 vs. 11.2 and 23.8 vs. 13.5%, p < .025 for both comparisons). There was no significant difference in the rate of serious AEs between the FIL 200 and FIL 100 mg groups and the placebo groups (4.5 and 4.5 vs. 4.3%). Venous thromboses or pulmonary embolisms were not diagnosed in FIL-treated patients and the incidence of herpes zoster was very low (<1%).
A recent study conducted post-hoc analyses of the SELECTION trial to evaluate the association between early symptomatic improvements and health-related QoL outcomes [37]. The data showed the significant improvement in rectal bleeding and stool frequency within 7 days and achievement of partial Mayo clinical score (pMCS) remission by week 2 in FIL 200 mg-treated patients with UC compared to the placebo group, and a similar treatment effect was observed at week 58. Day 7 rectal bleeding and stool frequency subscores contributed to the MCS response at weeks 10 and 58. Patients in pMCS remission at weeks 10 and 58 had greater improvements in the IBD questionnaire score than those who did not achieve pMCS remission.
Another study conducting post-hoc analyses of the SELECTION trial assessed the corticosteroidsparing effects of FIL [38]. The results showed 30 A post-hoc analysis of the phase 2b/3 SELECTION trial to evaluate the efficacy and safety of FIL 100 and 200 mg for induction and maintenance in Japanese patients has been reported [39]. In this study, 37 biologic-naive and 72 biologic-experienced Japanese patients were enrolled in two induction studies and the maintenance study. Numerically higher proportions of FIL 200 mg-treated than placebo-treated achieved clinical remission in induction (26.7 vs. 0%) and the maintenance studies (25.0 vs. 0%) in biologic-naive patients, but not in biologicexperienced patients (3.4 vs. 7.1%).

Upadacitinib
UPA, a selective JAK1 inhibitor that also partly acts on JAK2, became available for clinical use in Japan in September 2022. To assess the efficacy and safety of UPA in patients with moderate to severe UC, a phase 3, multicenter, randomized, double-blind, placebo-controlled clinical program that consisted of two replicate induction studies (U-ACHIEVE induction [UC1] and U-ACCOMPLISH [UC2]) and a single maintenance study (U-ACHIEVE maintenance [UC3]) was performed [40].
Recently, post-hoc analyses of UC1 and UC2 were performed to assess how rapidly UPA 45 mg daily improve UC symptom by measuring inflammatory markers, high-sensitivity C-reactive protein (hs-CRP) and fecal calprotectin (FCP) and QoL within the first 14 days of treatment [41]. Patients treated with UPA showed significant improvements in all UC symptoms between days 1 and 3 and up to day 14 compared to those given placebo. Furthermore, 75.7 and 48.2% of patients treated with UPA had a more than 50% reduction from baseline in hs-CRP and FCP levels, respectively (p < .001 vs. placebo), and QoL at weeks 2 and 8 was significantly improved. Interestingly, early improvement in stool frequency and bowel urgency by day 3 and reductions in hs-CRP and FCP by week 2 significantly contributed to clinical remission/response at week 8.

Safety of JAK inhibitors in active UC
When using JAK inhibitors in daily clinical practice, physicians are concerned about infections, major adverse cardiovascular events (MACE), venous thromboembolic events (VTEs; including deep vein thrombosis [DVT] and pulmonary embolisms [PE]), etc. Herpes zoster (HZ) infection, is of particular concern because of its high incidence among Asians and elderly patients treated with JAK inhibitors. This chapter discusses the frequency of adverse effects for each JAK inhibitor based on clinical trial data.
Tables 3-5 summarize the HZ, major MACE, and VTEs obtained from the clinical trial data to date [27,29,36,37,42,43]. These data suggested that the frequency of HZ increases in a dose-dependent manner for all drugs. A recent network metaanalysis by Din et al. also showed that among advanced therapies, including biologics, JAK inhibitors were associated with the highest incidence of HZ [44]. On the other hand, the incidence of cardiovascular events is lower, which tends to be different from the frequency observed in studies of patients with rheumatoid arthritis (RA) [45]. Regarding VTEs, it is well-known that the risk of thrombosis is inherently higher in patients with IBD. However, the risk of thrombosis has not increased after therapy with any JAK inhibitors. Next, we will discuss the reported side effects of each drug in a detail.   a Patients who responded placebo in the induction studies and continued to receive placebo in the maintenance study. b Patients who responded FIL 100 mg in the induction studies and were randomly assigned to placebo in the maintenance study. c Patients who responded FIL 200 mg in the induction studies and were randomly assigned to placebo in the maintenance study.

Safety of TOF
TOF is the longest-used JAK inhibitor for patients with UC patients; therefore, data on its adverse effects are reliable.  [46]. The most frequently reported malignancy was colorectal cancer. NMSC was reported in 22 patients in the Overall plus P3b/4 Cohort. Of the 22 patients, five and 17 were receiving PD TOF 5 and 10 mg BID, respectively. In multivariable analysis, disease duration and history of NMSC were significant risk factors for malignancies excluding NMSC, while older age and history of NMSC were significant risk factors for NMSC [46]. In clinical practice, the particularly high incidence of herpes zoster in the elderly (>65 years old) and in patients who received corticosteroids during TOF treatment is concerning. Furthermore, venous thromboembolism has been reported at high doses of TOF, and in patients with risk factors for cardiovascular events (smoking, hypertension, diabetes mellitus, and a history of coronary artery disease), TOF should be administered with caution. Finally, TOF can cause hypercholesteremia, which should be monitored regularly.

Safety of FIL
In the SELECTION study, no differences were found between the FIL 200 mg and FIL 100 mg groups and the placebo groups in the rates of serious AEs (4.3 and 5.0 vs. 4.7%) and serious infections (0. 6 and 1.1 vs. 1.1%). HZ infections occurred in four patients (one in the FIL 100 mg group and three in the FIL 200 mg group). Only one patient in the FIL 200 mg group had pulmonary embolism. Both doses were well-tolerated, and no new safety signals were noted. Only one FIL 200 mg-treated patient had HZ in each of the induction and the maintenance studies. Data from the FIL UC program including 1069 patients exposed to FIL in induction, maintenance, and long-term extension studies and 279 participants treated with placebo in the induction phase was reported [47]. The rates of HZ infections (0. Among JAK inhibitors, there is concern about the impact of FIL on the testes because preclinical studies of FIL in rats and dogs demonstrated that germ cell depletion/degeneration and/or tubular vacuolation in testes with correlating findings in epididymides. Therefore, Hellstrom et al. designed the MANTA and MANTA-Ray trials with global health agencies to find out if FIL decreases the quality of semen in men with active inflammatory bowel disease (UC or Crohn's disease) (MANTA) or rheumatic disease (rheumatoid arthritis, psoriatic arthritis, ankylosing spondylitis or non-radiographic axial spondylitis) (MANTA-RAy) [48]. This trial will find patients with at least a 50% reduction in sperm cells while taking 200 mg of FIL and will follow them for recovery or up to 52 weeks. The results of the trial are awaited.

Safety of UPA
In UC 1 and UC 2, the proportion of serious AEs and AEs leading to discontinuation was lower in both UPA groups than in the placebo group. As shown in Table 3, events of adjudicated MACE, VTEs, and cancers were infrequent. However, the incidence of HZ in patients with UC who received UPA 15 or 30 mg as remission maintenance therapy appears to be higher than with other JAK inhibitors. Hence, the frequency of HZ during the long-term course of patients receiving UPA, similar to TOF, must be followed carefully.
UPA has been used to treat various immune disorders, including rheumatoid arthritis (RA), psoriatic arthritis (PsA), moderate-to-severe atopic dermatitis (AD), and ankylosing spondylitis. It is important to assess the differences in type and frequency of AEs related to UPA in each immune disease. Colombel et al. assessed AE data on UPA therapy in patients with UC in comparison with AEs reported in clinical trials of various immune diseases to evaluate the long-term safety profile of UPA [49]. Safety data were collated from UC1, UC2, and UC3, and from two AD (15 and 30 mg doses), six RA (15 mg dose in all, 30 mg dose in four), and two PsA (15 and 30 mg doses) phase 3 trials. The rates of any AEs, serious AEs, severe AEs, and AEs leading to treatment discontinuation in patients with UC who received maintenance treatment with UPA 15 or 30 mg were within the ranges previously reported in AD, RA, and PsA, except for neutropenia. Moreover, apart from a higher rate of venous thromboembolism, the rates of AEs of special interest in the UC maintenance study were generally similar to those reported in patients receiving long-term treatment with UPA 15 or 30 mg daily in AD, RA, or PsA.

JAK inhibitors during the COVID-19 pandemic
During the COVID-19 pandemic, we were initially concerned about using JAK inhibitors in patients with autoimmune diseases. However, several reports indicate that anti-TNFa antibodies and JAK inhibitors could regulate the cytokine storm caused by COVID-19 infection [50][51][52][53]. In addition, clinical trials supported the suppression of COVID-19 severity by JAK inhibitors [54]. Registry data on Japanese patients with IBD and COVID-19 (J-COSMOS) showed no severe COVID-19 in IBD patients receiving JAK inhibitors (personal communication). Thus, these results suggest that JAK inhibitors may have minimal impact on the clinical course of patients with IBD and COVID-19.

How to use the three JAK inhibitors
To decide which of the three JAK inhibitors should be used, the selectivity of each JAK, their metabolic pathways, and concomitant use with immunomodulators should be considered. The differences among the JAK inhibitors have been presented in Table 2. First, regarding their selectivity, as mentioned above, TOF acts on all JAKs, FIL specifically on JAK1, and UPA on JAK1 and partly JAK2. Theoretically, the more JAKs inhibited, the stronger the immunosuppressive effect and the greater the risk of infection and shingles. In vitro and ex vivo pharmacological studies of the three JAK inhibitors have shown that FIL is the most selective for JAK1 among the three drugs and that UPA was the most potent in suppressing signaling pathways mediated by JAK1 and JAK2 [3,36,37]. Next, considering their major metabolic pathways-hepatic metabolism for TOF and UPA and renal excretion for FIL-it is essential to check hepatic and renal function when using these drugs. FIL may be used in patients receiving thiopurines, therefore it may be advantageous to provide FIL to patients who relapse while in remission or to transition from an anti-TNF-a antibody to FIL in patients who relapse while receiving a biologic agent and a thiopurine. However, the efficacy of IM alone vs. IM plus FIL is not currently known. In the field of rheumatology, the efficacy of methotrexate (MTX) þ FIL (200 and 100 mg) against MTX alone in active RA with limited or no prior MTX exposure was assessed [55]. The primary endpoint was achieved by 81% of patients receiving FIL200 þ MTX vs. 71% receiving MTX (p < .001). A more significant proportion treated with FIL100 þ MTX than MTX achieved an ACR20 response (80%, p ¼ .017) at week 24. Based on these results, a combination effect may be predicted in patients with UC with limited or no prior IM exposure.
Despite the lack of a head-to-head comparison study of JAK inhibitors, Lasa et al. conducted a systematic review and network meta-analysis to compare the relative efficacy and safety of biologics and small-molecule drugs for treating patients with moderate-to-severe UC [56]. Their results demonstrated that UPA was significantly superior to all other JAKs in inducing clinical remission (TOF, OR 2Á84; 95% CI, 1.28-6.31; FIL 100 mg, OR, 6.15; 95% CI, 2.98-12.72; FIL 200 mg, OR, 4.49; 95% CI, 2.18-9.24), but also ranked highest in terms of AEs. There are few direct comparisons of the efficacy and safety of JAK inhibitors, therefore, further clinical data should be accumulated to aid physicians in positioning drugs in treatment algorithms.
In conclusion, JAK inhibitors have undoubtedly changed the management aspects of patients with IBD, including treatment concepts, disease concepts, and therapeutic goals. The era has come in which we can treat patients based on the mechanism of action of these drugs and IBD pathophysiology. The use of JAK inhibitors in IBD has just begun.

Funding
This work was supported by the Health and Labour Sciences Research Grants for research on intractable diseases from the Ministry of Health, Labour and Welfare of Japan (Investigation and Research for intractable Inflammatory Bowel Disease).