Efficacy of rituximab in neuromyelitis optica: A meta-analysis

Background: Neuromyelitis optica (NMO) is a severe autoimmune disorder of inflammatory central nervous system, which often resulting in paralysis or blindness. Rituximab (RTX) is a mouse-human chimeric monoclonal antibody specific for the CD20 antigen on B lymphocytes used to treat many autoimmune diseases. This review performed a meta-analysis of the efficacy of rituximab use in NMO. Methods: We searched through the databases of PubMed, Embase, and Cochrane Library. We compiled 28 studies in this meta-analysis: 19 used annualized relapse rate (ARR) ratio, 24 used Expanded Disability Status Scale (EDSS) score. Differences in the ARR ratio and EDSS score before and after rituximab therapy were the main efficacy measures. After a consistency test, the publication bias was evaluated and a sensitivity analysis was performed with mean difference (MD) of the efficacy of rituximab. Results: A meta-analysis of 28 studies with 613 participants total was conducted. NMO patients have antibodies against aquaporin 4 autoantibody (AQP4-Ab) were recorded in 440 of 613 (71.78%). The mean difference of ARR ratio after rituximab therapy was 1.59 (95% CI, 1.33 to 1.85), and a mean difference 1.14 (95%CI, 0.95 to 1.33) reduction in the mean EDSS score. 345 of 563 patients (61.28%) reached a relapse-free state. 94 of 613 (15.33%) patients had adverse reactions. Conclusions: RTX has acceptable tolerance, reduces the frequency of relapse, and improves disability in most patients. However, the potential impact of early diagnosis of NMO and treatment with RTX in reducing health-care costs and improving functional outcome should be carefully addressed in future studies.

3 4 autoantibody (AQP4-Ab) [1][2][3]. Serum autoantibodies targeting AQP4-Ab have become sensitive and specific biomarker that enable early diagnosis of NMO and are found in most patients. Because the progression of NMO disability is related to the severity of the attacks, prophylactic treatment of NMO recurrence must be performed as soon as NMO is identified. Since studies have found that NMO patients have antibodies against AQP4-Ab, several studies have proposed treatment for B cells in NMO [4].
Rituximab (RTX) is a chimeric monoclonal antibody directed against CD20 epitope expressed on pre-B and mature B cells, and is used to treat B-cell-derived lymphoid neoplasms and antibody-mediated autoimmune diseases [5,6]. The depletion of CD20 provides a theoretical basis for the treatment of autoimmune diseases in which B cells and autoantibodies play a key role, such as NMO, in which AQP4-Ab have been associated with the disease [7]. Here, we perform a meta-analysis to test rituximab efficacy is both well safe and tolerated, and examined the treatment efficacies using relapse rates and disability in NMO.

Literature search
This search was restricted only to articles published in English language. We searched for publications on the PubMed, Embase, Cochrane Library, without any temporal restriction.
We did keyword and Medical Subject Heading (MeSH) searches for our theme, and MeSH terms, key words and their synonyms related to "rituximab" and "neuromyelitis optica".
After excluding duplicates (n = 85) and inappropriate articles (n =962), we retained for analysis 28 relevant studies published between 2008 and 2018. A flowchart of the search strategy is shown in Fig. 1. One of us used a standardized form of data extraction to extract data, another person checks it, and revisits the data that does not match, and resolves the 4 differences through discussion and consensus.
Inclusion and exclusion criteria for the literature Studies were included if they fulfilled the following criteria: 1) Published in various journals on RTX in patients with NMO; 2) Patients with NMO do not limit their age, gender, ethnicity, and whether they have received treatment before; 3) Main variables include ARR and/or EDSS; Exclusion criterion: 1) Case reports and studies that included fewer than 2 patients, review, meta-analysis; 2) studies with incomplete data.

Main variables
Among the 28 articles selected, we extracted the values (means and standard deviations) of DESS and ARR directly available. Disability was measured by the EDSS. The ARR were calculated using the total number of relapses per patient-year. A comprehensive search of the literature was undertaken in order to identify research the efficacy of rituximab in NMO.

Statistical Analysis
Data analysis was performed using statistical software provided by the Cochrane Efficacy on the ARR Ratio Fig. 2 shows a forest plot of the mean difference in the ARR ratio before and after rituximab therapy. This finding suggested that the mean difference of ARR ratio after rituximab therapy was 1.59 (95% CI, 1.33 to 1.85). A random-effect model was used with I2 of 81%.
In the subgroup analysis based on different rituximab doses, the 1000 mg infused twice during a 2-week interval has a greater degree of heterogeneity (I2=0%, P=0.74). Similar degrees of heterogeneity were demonstrated in the subgroup analyses by the 375 mg/m2 infused once weekly for 4 weeks (I2=93%, P<0.001). The heterogeneity of 100 mg once per week for 3 consecutive weeks is low (I2=0%, P=0.55), the 375 mg/m2 infused once weekly 6 for 4 weeks and/or 1000mg infused twice during a 2-week interval was moderate (I2=45%, P=0.09), and the not mention a clear dose was at least moderate (I2=57%, P=0.1) (Fig. 3).
Sensitivity analyses were performed by removing each study in turn and re-analyzed. We performed a sensitivity analysis on the 375 mg/m2 infused once weekly for 4 weeks group and found that Cabre 2018 significantly affected heterogeneity(I2=0%,P=0.78).
Efficacy on the EDSS Score

Publication bias
The funnel plot for studies on the incidence of ARR and EDSS were symmetrical. The funnel plots indicated an absence of publication bias. (Fig. 5) Discussion 7 NMO is a relapsing disease with a high early mortality rate. Without employing appropriate immunosuppressant treatment, more than 50% of patients with NMO will be functionally blind, or will progress to wheelchair dependence, within 5 years [8,9]. Its treatment options are based on case series and expert opinion, and immunosuppressive therapy is the main method to prevent recurrence and prevent disability. The successful use of RTX has been widely reported in NMO. However, there are no randomized controlled trials in NMO and there are currently no established guidelines for the treatment of RTX. Although rituximab is expensive, it can offset the cost of recurrence and plasma exchange with its good therapeutic effect [10,11].
We performed a subgroup analysis of ARR and found that its heterogeneity was significantly correlated with drug dose. Sensitivity analysis of the subgroup revealed that Cabre 2018 [12]had a greater impact on heterogeneity. A careful reading of Cabre 2018 found that the baseline ARR of this document was lower than the baseline ARR observed in other literatures. The therapeutic effect of RTX varies among patients with different outcomes.
One obvious factor is the duration of RTX treatment. Differences in follow-up time, ethnic differences, and other immunosuppressive treatments in some patients before and after RTX treatment were important reasons for heterogeneity.
With the increasing number of treatment options, the choice of the right therapeutic agent for the right patient becomes more important. Monoclonal antibody therapies are perspective options for NMO patients to prevent relapses compared with traditional immunosuppressants, especially in refractory patients. Some studies have used RTX as a first-line treatment for prophylactic treatment, which can reduce the severity of disease recurrence [13,14]. Quan et al. has reported that rituximab treatment is well tolerated and resulted in a gradual recovery of neurological function. Rituximab mainly exerted its 8 function through two stages. It not only eliminates or inhibits the pathogenic effector B cells to achieve early effects, but also regulates the proportion of functional B cells to make a more lasting improvement in efficacy [15].
Although AQP4-Ab is critical in the diagnosis of NMO, its involvement in the pathogenesis of the disease remains controversial [9]. Some studies suggest that AQP4-Ab is generally used as a marker of disease activity in an individual patient, and AQP4-IgG titers increase significantly during NMO recurrence period, but these indices decline during remission period [16,17]. Therefore, there is a correlation between rises in antibody levels and clinical attacks, and treatment with RTX resulted in the most pronounced decline in AQP4-Ab levels. However, many studies have shown that AQP4-Ab is only a diagnostic marker for NMO, which can be detectable in serum during relapse as well as during remission. RTX can effectively reduce CD20 B cells in blood and cerebrospinal fluid, but it does not eliminate plasma cells in the bone marrow. This may be other mechanisms beyond AQP4-Ab to maintain disease stabilization, including inhibition of B cell/T cell interactions, increased regulatory T cells, and decrease of CD20 cells with reduction of proinflammatory cytokines, as well as modulation of the T cell compartment [18,19]. Therefore, there is a lack of correlation between AQP4-Ab titers and clinical disease activity. Partial AQP4-Ab negative NMO patients still have a good response to the treatment of RTX [20].
Maintaining the consumption of memory B cells by repeated treatment may be the pivotal to the clinical effects of RTX in NMO patients [21]. Some studies have developed a personalized maintenance treatment program for CD19 + CD27 + memory B cell detection, and achieved good results. However, the increase in the relapses rate of NMO also occurs in the case of low levels of CD19+ B cells, which makes it difficult to control the disease [12,14,[22][23][24]. Therefore, CD27+ memory B cells can better detect the therapeutic effect of 9 RTX relative to CD19 + B cells.
Yang et al. found that in Asian NMO, lower doses of RTX can achieve better reactivity with cost and availability advantages [25]. Low doses of RTX can effectively reduce the recurrence rate and improve the prognosis in most NMOs cases. Repeated RTX treatments are superior to single RTX doses for NMO therapy. Some patients required an increased frequency of RTX infusion to maintain low levels of CD19+ cells, and long-term use of lower dose RTX may lead to cost savings [19,26].
Due to ethical issues, the current study of rituximab lacks a placebo control group [12].
However, Zhang et al. has investigated the NMO patients treated with RTX and acetazolamide (AZA), study has found that RTX is more effective in preventing NMO recurrence than patients treated with AZA and can improve symptoms [27]. The most common reported adverse drug reactions are infections. A decrease in immunoglobulin levels was observed in some patients following rituximab treatment, which may increase the probability of infection in patients [28,29]. Relatively few reports of serious adverse reactions leading to death. In our meta-analysis, most of the 6 patients died due to serious illness and related complications. Not related to the treatment of RTX.
Our meta-analysis was not only to present efficacy data, but also to expand the knowledge about the safety of RTX treatments. Although RTX has significant benefits for the treatment of NMO, long-term benefits and risks remain to be determined. Moreover, most patients receive other immunotherapies before and after RTX treatment, so the benefits and risks of treatment are inaccurate due to a single drug [5,20,30,31]. And it is unclear whether patients have the appropriate time to discontinue RTX treatment without the risk of further relapse.

Conclusions
RTX has acceptable tolerance, reduces the frequency of relapse, and improves disability in most patients. However, the potential impact of early diagnosis of NMO and treatment with RTX in reducing health-care costs and improving functional outcome should be carefully addressed in future studies.

Declarations
Acknowledgements None Funding This work did not receive any specific grant from funding agencies in the public, commercial, or non-profit sectors.

Availability of data and materials
All data analyzed during this study are included in this article.   Figure 1 Flow chart presenting the process of the study selection for NMO meta-analysis.

Figure 2
Forest plot of the mean difference in the ARR ratio before and after rituximab therapy.  Forest Plot Showing the EDSS score of Patients with NMO after Rituximab Therapy.

Figure 5
Funnel plot showing the incidence of ARR and EDSS of Patients with NMO after Rituximab Therapy.