Microscopic polyangiitis (MPA) and granulomatosis with polyangiitis (GPA) are systemic small-vessel vasculitides that are part of the group of diseases referred to as anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV). Until the introduction of cyclophosphamide (CY) combined with glucocorticoid (GC), the prognosis of AAV, and particularly of GPA, was very poor [1,2]. Recent guidelines for MPA and GPA from the Japan Research Committee of the Ministry of Health, Labour, and Welfare for Intractable Vasculitis (JPVAS), as well as those from the British Society of Rheumatology (BSR)/British Health Professionals in Rheumatology (BHPR), the European League Against Rheumatism (EULAR)/European Renal Association-European Dialysis and Transplant Association (ERA-EDTA), and the Kidney Disease Improving Global Outcomes (KDIGO) all recommend the use of CY or rituximab plus GC as first-line therapy [3-6]. These recommendations are based on initial cohort studies of CY combined with GC [1,2] and the results of several clinical trials in which CY or rituximab combined with GC was tested.

Despite the established efficacies of CY plus GC and rituximab plus GC for the treatment of MPA and GPA, a substantial proportion of patients fail to show an appropriate therapeutic response, as some patients experience disease relapse, even those under remission-maintenance treatment with GC plus azathioprine or other immunosuppressive agents. The use of CY or rituximab plus GC is associated with several serious adverse drug reactions that decrease the quality of life of patients. A possible solution to this clinical challenge is the development of molecular targeted drugs with novel mechanisms of action, a strategy that has markedly improved the outcomes of patients with rheumatoid arthritis [7]. Recent advances in basic and clinical research have accumulated evidence of the efficacy of targeting molecules in patients with MPA and GPA. In this article, we review the efficacy and safety of the standard regimens for MPA and GPA and discuss the molecular targeted drugs under clinical development. Demonstration of similar or improved efficacy of these drugs and their greater safety may enable a decrease in the concomitant dose or even the exclusion of GC for the treatment of MPA and GPA.

In a systematic review recently published by JPVAS [8], a randomized controlled trial (RCT) and two cohort studies were evaluated using the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) system. Neither the RCT [9] nor one of the cohort studies [10] revealed a significant difference between GC monotherapy and GC plus CY regarding death, end-stage renal disease, regarding death, endstage renal disease, and remission of MPA or GPA. However, in both studies, the certainty of evidence of each evaluated outcome was downgraded to very low because of the risk of bias and imprecision. In the other cohort study [2], the relative risk (95% confidence interval [CI]) of GC plus CY for remission versus GC monotherapy was 1.51 (95% CI, 1.05 to 2.17), which was statistically significant. However, the risk of bias and imprecision again resulted in a very low certainty of evidence of the outcome. Despite the lack of sufficient evidence for the superior efficacy of CY plus GC versus GC monotherapy, a spate of clinical trials has demonstrated good clinical outcomes in patients with MPA and GPA treated with CY plus GC [11-15].

Safety concerns regarding the use of CY plus GC pose a challenge to physicians treating patients with MPA and GPA. GC use is associated with serious infection, osteoporosis, osteonecrosis, glaucoma, cardiovascular diseases, cerebrovascular diseases, and glucose intolerance, and CY is associated with increased risk of serious infection, cytopenia, malignancy, liver dysfunction, and premature gonadal failure. In an observational study of 535 patients who participated in four RCTs conducted by the European Vasculitis Study Group (EUVAS), adverse events including osteoporosis, malignancy, and diabetes were reported at rates exceeding 10% each. Lower glomerular filtration rates, four or more flares, and longer use of GC were independent risk factors for high-level damage (i.e., Vascular Damage Index ≥ 5) [16]. Consequently, novel agents that produce a better benefit-risk balance than CY plus GC in patients with MPA and GPA are urgently needed.

The anti-human CD20 chimeric antibody rituximab has been approved in many countries for use in treating patients with MPA and GPA, and in clinical practice guidelines it is recommended as a first-line agent [3-6]. Two RCTs have established the efficacy and safety of rituximab in patients with MPA and GPA (Table 1). In the Rituximab in ANCA-associated Vasculitis (RAVE) trial [15], patients with newly diagnosed or relapsing MPA or GPA were randomly assigned to the rituximab (375 mg/m² of body surface area, once weekly for 4 weeks) or the control group (daily oral dose of 2 mg CY/kg followed by a daily azathioprine dose of 2 mg /kg). At month 6 of treatment, rituximab was not inferior to daily oral CY for the primary endpoint of remission without the use of prednisone (64% vs. 53%, respectively; p < 0.001). In addition, rituximab was more efficacious than CY among patients with relapsing disease in achieving the primary endpoint (67% vs. 42%, respectively; p = 0.01). The Rituximab versus CY in ANCA-associated Renal Vasculitis (RITUXVAS) trial enrolled patients with newly diagnosed MPA or GPA with renal involvement [14] and then randomized them to the rituximab group, which received rituximab with two doses of intravenous (IV) CY, or the control group, which received only IVCY for 3 to 6 months followed by treatment with azathioprine. Sustained-remission rates at month 12 were similar between the rituximab and control groups (76% vs. 82%, respectively; p = 0.68). In both the RAVE and the RITUXVAS trials, the two groups of patients did not significantly differ in the number of severe adverse events. The long-term follow-up of patients enrolled in these trials showed that the relapse rates in patients treated with a single course of rituximab without maintenance therapy were similar to those in patients treated with CY followed by azathioprine maintenance therapy: 32% versus 29%, respectively, at month 18 in the RAVE trial and 21% versus 18%, respectively, at month 24 in the RITUXVAS trial [17,18].

The Maintenance of Remission using Rituximab in Systemic ANCA-Associated Vasculitis (MAINRITSAN) trial evaluated the efficacy and safety of rituximab vs. azathioprine for remission maintenance in patients with MPA and GPA (Table 1) [19]. Patients with newly diagnosed or relapsing GPA, MPA, or renal-limited AAV who had achieved remission with IVCY plus GC were enrolled and then randomly assigned to receive either 500 mg rituximab on days 0 and 14 and at months 6, 12, and 18 or daily azathioprine at 2 mg/kg until month 22. Fewer patients had a major relapse at month 28 in the rituximab group than in the azathioprine group (5% vs. 29%, respectively; p = 0.002). A longterm analysis of this trial demonstrated that rituximab remained superior to azathioprine for sustaining remission at month 60 (57.9% vs. 37.2%, respectively; p = 0.012) [20]. The subsequent trial, MAINRITSAN2, compared tailored versus fixed-schedule rituximab reinfusion for remission maintenance in patients with MPA and GPA (Table 1) [21]. Patients in the tailored-infusion group received a 500 mg rituximab infusion at randomization, with additional rituximab infusions until month 18 based on ANCA and CD19+ lymphocyte counts. Fixed-schedule patients received 500 mg rituximab infusions on days 0 and 14 and then at months 6, 12, and 18. The two groups did not significantly differ in terms of the proportion of patients with relapse at month 28: 17.3% in the tailored-infusion group vs. 9.9% in the fixed-schedule patients (p = 0.22). Patients in the former group received fewer rituximab infusions.

The results from these clinical trials established the similar efficacy and safety of rituximab plus GC compared to CY plus GC in patients with MPA and GPA. Their findings also strongly suggest the involvement of CD20+ B cells in the immunopathogenesis of these vasculitides.

Accumulating evidence suggests problems in B-cell depletion therapy when rituximab is used for patients with MPA and GPA, such as a requirement for concomitant high-dose GC in the induction of remission and long-term low-dose GC to prevent relapses. Several studies have reported increases in the levels of B-cell activating factor (BAF) in patients with MPA and GPA [22-24]. Blockade of BAF in combination with rituximab may deplete B-cells more broadly and to a greater extent and may inhibit the recovery of autoreactive B-cells, thereby contributing to the maintenance of remission. A clinical trial to evaluate the efficacy and safety of this combined anti-B-cell therapy approach for AAV is under preparation [25].

The approximately 30 molecules that make up the complement system play essential roles in both innate and adaptive immunity [26]. There are three pathways that lead to complement activation: the classic, alternative, and lectin pathways. All three include the formation of C3a, C3b, C5a, and the terminal complement complex C5b6789. C5a is a potent pro-inflammatory mediator that, after binding to the C5a receptor (C5aR, CD88), stimulates leukocyte migration, activation, degranulation, vascular permeability, and the release of proteinases and oxidative free radicals. CD88 is expressed by neutrophils, mast cells, basophils, eosinophils, monocytes, and vascular endothelial cells, among others. Although complement deposition is not observed in AAV-related pauci-immune glomerulonephritis, growing evidence strongly suggests the involvement of the complement system in the pathogenesis of AAV [27].

Avacopan is an orally administered antagonist of C5aR that is under clinical development as a treatment for AAV [28-30]. Xiao et al. [30] established a mouse model of ANCA-associated renal vasculitis, in which mouse anti-myeloperoxidase (MPO) immunoglobulin G (IgG)-induced necrotizing and crescentic glomerulonephritis (NCGN) develops in mice with knockedin human C5aR/CD88 and knocked-out mouse C5aR/CD88. Oral administration of avacopan almost completely ameliorates anti-MPO IgG-induced NCGN in this model. Along with this dose-dependent response are significant improvements in hematuria, proteinuria, and leukocyturia. In the human myeloid cell line U937, avacopan significantly inhibits the binding of [125I]-C5a to C5aR as well as C5a-induced migration and the intracellular release of calcium [28]. Avacopan also strongly suppresses the binding of [125I]-C5a to C5aR, C5a-induced intracellular calcium release, chemotaxis, the upregulation of CD11b, and the oxidative burst in human neutrophils in vitro. Oral administration of avacopan 1 hour prior to blood sampling significantly inhibits C5a-induced upregulation of CD11b in neutrophils of mice with knocked-in human C5aR/CD88 [30].

In a phase I study of avacopan that enrolled 48 healthy volunteers to evaluate the safety and tolerability of the drug in humans, avacopan was well tolerated and dose-limiting or serious adverse events were not reported [28]. Commonly observed adverse events included diarrhea, dizziness, lower abdominal pain, nausea, and oropharyngeal pain. The average maximal plasma level of avacopan after the administration of 30 mg twice a day was 191 ng/mL, and the terminal halflife was 129 hours. A single 30 mg dose of avacopan or a 7-day regimen of 30 mg twice a day dose-dependently inhibited C5a-induced CD11b expression in peripheral blood neutrophils [28].

In the clinical trial, a randomized, double-blind, placebo-controlled, phase 2 study to evaluate the safety and efficacy of CCX168 in subjects With anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis on background of cyclophosphamide or rituximab treatment (CLEAR), patients with newly diagnosed or relapsed AAV received placebo plus prednisone starting at 60 mg daily (control group, n = 23), 30 mg avacopan twice a day plus reduced prednisone starting at 20 mg daily (n = 22), or 30 mg avacopan twice a day without prednisone (n = 22) [29]. All patients received IVCY at a dose of 15 mg/kg at weeks 0, 2, 4, 8, and 12 followed by azathioprine at a target dose of 2 mg/kg per day up to week 24 or IV rituximab at 375 mg/m² per week for 4 weeks. The aim of the study was to determine the feasibility of reducing or replacing GC with avacopan without compromising efficacy in treating AAV. The primary endpoint was the proportion of patients with a decrease in their Birmingham Vasculitis Activity Scores (BVAS) ≥ 50% from baseline and no disease worsening in any body system. Secondary endpoints were the proportion of patients with a renal response, BVAS-confirmed remission, and changes in the BVAS and main laboratory parameters compared to the baseline. Patients in the three groups ranged in age from 57 to 59.1 years old. Of the 67 patients enrolled, 33 (49%) each had GPA and MPA and one had unknown disease type; 35 (52%) and 29 (43%) patients were positive for MPO-ANCA and proteinase 3 (PR3)-ANCA, respectively. Almost all patients had renal involvement (97%) and the mean BVAS of the three groups ranged from 13.2 to 14.3. The primary endpoint was met in 14 of 20 patients (70.0%) in the control group, 19 of 22 (86.4%) in the avacopan plus reduced-dose prednisone group, and 17 of 21 (81.0%) in the avacopan without prednisone group. The differences compared to the control group were 16.4% for the avacopan plus reduced prednisone group (90% CI, –4.3 to 37.1; p = 0.002) and 11.0% for the avacopan without prednisone group (90% CI, –11.0 to 32.9; p = 0.01), thus demonstrating the non-inferiority of these two groups compared to the control group. Adverse events were reported in 21 of the 23 patients (91%) in the control group, 19 of the 22 patients (86%) in the avacopan plus reduced prednisone group, and 21 of the 22 patients (96%) in the avacopan without prednisone group. Changes in the BVAS tended to be larger in the avacopan-treated groups than in the control group but the difference was not statistically significant, while the changes in the EuroQoL-5D-5L visual analog scale at week 12 were significantly better in the two avacopan groups than in the control group. Short form-36 physical functioning scores, role emotional scores, urinary albumin/creatinine ratios, and urinary monocyte chemoattractant protein-1/creatinine ratios improved significantly at week 12 in the avacopan plus reduced-dose prednisone group versus the control group [29].

Based on the promising results from the phase II study, a multi-national phase III study (ADVOCATE) (ClinicalTrials.gov Identifier: NCT02994927) is currently in progress. Eligible patients are those with incident or relapsed MPA or GPA with ANCA positivity for which treatment with CY or rituximab is needed; with at least one major item, three non-major items, or two renal items of proteinuria and hematuria according to BVAS; and with an estimated glomerular filtration rate > 15 mL/min/1.73 m2 at the time of screening. Patients are allocated to either the avacopan plus rituximab or CY (IV or oral) group or to the prednisone plus rituximab or CY group. Following initial treatment, both groups receive azathioprine. The primary endpoints are the proportion of patients achieving disease remission as determined by BVAS at week 26 and the proportion of patients achieving sustained remission assessed by BVAS at week 52 [31]. The demonstrated success of avacopan may open the door to a new era of GC-free AAV treatment.

Although the results of ongoing or scheduled clinical trials are currently pending, the clinical development of molecular targeted therapy for MPA and GPA is promising and is expected to rapidly transform treatment so that it will be GC-free or at least less GC-dependent. Decreasing or eliminating the use of GC will reduce the burden of accrued damage and improve the longterm outcomes for patients. However, several pertinent clinical questions about molecular targeted therapy for MPA and GPA still need to be addressed: Are there subgroups of patients who respond better to one molecular targeted therapy than to others? Which molecular targeted therapy should be used first? Are good biomarkers available for each form of molecular targeted therapy? Can treatment with molecular targeted therapy be discontinued or switched to conventional immunosuppressants without increasing the risk for major flares? Can patients be treated with molecular targeted therapies at a reasonable cost? Analyses of multi-national, national, or regional registries, claims databases, and the results of well-designed clinical trials should provide answers to these questions.