B-Cell Targeted Therapies

Targeting B Cells

Over the past few years, there has been considerable progress defining the potential role of B cells in normal immunity and also in the pathogenesis of autoimmune conditions including rheumatoid arthritis (RA). Among the activities of B cells that may contribute to the initiation and sustenance of rheumatoid inflammation are their ability to:

• Act as antigen-presenting cells
• Secrete various inflammatory molecules, including cytokines
• Produce autoantibodies, including rheumatoid factor (RF) and anti- cyclic citrullinated peptide
• (CCP) antibodies
• Modulate lymphoid organization within inflammatory sites (Figure 15-1).

Greater understanding of the potential role played by B cells in autoimmune disease has spurred an interest in targeting B cells therapeutically.

Enlarge 

Rituximab (Rituxan)

Rituximab is a chimeric anti-CD20 monoclonal antibody originally developed for the treatment of B-cell non-Hodgkin’s lymphoma (approved since 1997 in the USA) that is now used in a number of autoimmune diseases. CD20 is a B-cell surface molecule that is expressed only on pre-B cells and mature B cells; it is not present on stem cells or pro-B cells and is lost before differentiation of B cells into plasma cells. Rituximab causes a rapid selective depletion of CD20+ B cells. B-cell depletion by rituximab may be mediated by various mechanisms, including antibody-dependent cellular cytotoxicity, complement-mediated B-cell lysis and induction of apoptosis.

Indication

Rituximab is approved in the United States in combination with methotrexate (MTX) for the treatment of adult patients with moderately to severely active RA who have had an inadequate response to one or more anti- tumor necrosis factor (TNF) therapies, as well as for other autoimmune conditions (granulomatosis with polyangiitis, microscopic polyangiitis, pemphigus vulgaris). Rituximab is also commonly used for autoimmune diseases where it has not received Food and Drug Administration (FDA) approval, especially systemic lupus erythematosus (SLE).

Clinical Efficacy

Initially, evidence of the efficacy of rituximab in patients with RA came from small open-label trials. In these early studies, rituximab was given in conjunction with the antimetabolite cyclophosphamide as well as relatively large doses of glucocorticosteroids, thereby raising questions as to the specific efficacy of rituximab itself. Other early trials, also in small numbers of patients with active seropositive RA, suggested rituximab could be efficacious in patients who had experienced an inadequate response to treatment with anti-TNF agents and/or traditional disease-modifying antirheumatic drug (DMARDs).

Subsequently, a randomized controlled trial compared the effect of rituximab and various concomitant therapies in 161 patients with active RA despite concomitant MTX. Patients were randomly assigned to receive one of four treatments: MTX only, rituximab only, rituximab plus cyclophosphamide and rituximab plus MTX. Rituximab was administered as two IV infusions of 1,000 mg 2 weeks apart. All patients received corticosteroid therapy over the initial 2 weeks, with a total dose of 930 mg. At the end of 6 months, the rituximab + MTX and rituximab + cyclophosphamide groups had significantly greater ACR50 rates (the primary end point) compared with the MTX-alone group (Figure 15-2). In all groups treated with rituximab, a significantly higher proportion of patients reached ACR20. Clinical responses for a number of patients were maintained through 48 weeks, particularly in the rituximab–MTX group (Figure 15-2).

Rituximab therapy induced profound peripheral blood B-cell depletion for >6 months and it was also associated with a decrease in RF levels that was maintained after 24 weeks. However, there was no correlation between changes in B cell numbers, or RF or other antibody or autoantibody levels and clinical responses. B-cell numbers began to rise after about 6 months, mostly returning to pretreatment levels thereafter. Treatment was generally well tolerated. Despite the decrease in numbers of circulating B cells, serum immunoglobulin levels decreased only slightly, largely remaining within normal ranges.

Some patients who participated in the above trial were followed for 2 years after their single course of rituximab therapy. Substantially greater numbers of patients in the rituximab + MTX group experienced sustained clinical responses, including maintenance of improvements in functional status, as measured by the Health Assessment Questionnaire Disability (HAQ) Index. The main significance of this study is that it proved the utility of rituximab in combination with MTX, thereby obviating the need for cyclophosphamide in patients with RA. Although cyclophosphamide has been used in RA and other autoimmune disorders, its side effect profile is severe and many clinicians are reluctant to use it if there are less-toxic alternatives.

A phase 2b, randomized, double-blind, placebo-controlled trial known as DANCER assessed the efficacy and safety of two different dosages of rituximab (500 mg or 1000 mg, each given twice) with various corticosteroid regimens (placebo, IV steroids only, or IV plus oral steroids). The study population included 465 patients with active RA despite ongoing treatment with MTX and previous lack or loss of response to treatment with at least one other DMARD or biologic agent; of note, 31% of patients had previously received therapy with TNF inhibitors. Rituximab (0, 500, or 1000 mg) was administered on days 1 and 15. Patients were also randomized to one of three corticosteroid premedication regimens: placebo, 100-mg methylprednisolone IV pretreatment on days on days 1 and 15 and methylprednisolone 100 mg IV on days 1 and 15, plus 60-mg oral prednisone on days 2 to 7 and 30 mg on days 8 to 14. All patients continued on stable MTX doses.

At week 24, significantly more patients who received either 2 × 500 mg or 2 × 1,000 mg of rituximab improved clinically, as assessed by ACR20 responses (ACR20: 55% and 54%, respectively) compared with placebo (28%; P <0.0001) (Figure 15-3). In addition, the ACR50 and ACR70 response rates in rituximab-treated patients were significantly higher than in placebo-treated patients. Analysis of the results according to concomitant steroid usage showed that although patients receiving steroids may have responded more quickly, there was no difference in clinical outcome as assessed by the primary study end point (ACR20 response at 24 weeks), regardless of glucocorticoid use. Pretreatment with IV glucocorticoids did however improve the tolerability of the first rituximab infusion.

The Randomized Evaluation of Long-Term Efficacy of Rituximab in RA (REFLEX) Trial was a multicenter, randomized, double-blind, placebo-controlled, phase 3 study of rituximab therapy in 520 patients with active RA and an inadequate response to ≥1 anti-TNF agents. Patients were randomized to receive IV rituximab (one course, consisting of 2 infusions of 1000 mg each) or placebo, both with background MTX. At week 24, significantly more (P <0.0001) rituximab-treated patients than placebo-treated patients demonstrated ACR20 (51% vs 18%), ACR50 (27% vs 5%) and ACR70 (12% vs 1%) responses (Figure 15-4). Treatment with rituximab was associated with a rapid and complete depletion of peripheral B cells and a decrease in serum RF levels (55%). However, these did not correlate with clinical outcome.

The Study Evaluating Rituximab’s Efficacy in MTX Inadequate Responders (SERENE) was another multicenter, randomized, double-blind, placebo-controlled, phase 3 trial in 512 patients with active RA who had an inadequate response to MTX and who were naïve to prior biological treatment. After a ≥2-week washout period for all DMARDs, patients were randomized (1:1:1) to rituximab 2 × 500 mg, rituximab 2 × 1,000 mg, or placebo. All treatments were administered by IV infusions on days 1 and 15. All infusions (including placebo) were premedicated with IV methylprednisolone 100 mg. All patients remained on stable doses of MTX (10 to 25 mg/week). The primary end point was ACR20 response at week 24. Patients not in remission at week 24 (disease activity score (DAS)28-ESR ≥2.6) who also met predefined safety criteria (neutrophil count >1,500 cells/μl) were retreated with their randomized dose of rituximab or, if initially assigned to placebo, switched to receive rituximab (2 × 500 mg). All patients were followed until week 48.

At week 24, a significantly greater proportion of patients receiving rituximab 2 × 500 mg or 2 × 1,000 mg + MTX achieved an ACR20 response compared with patients receiving placebo (54.5%, 50.6% and 23.3%, respectively; P <0.0001) (Figure 15-5). In addition, significantly greater proportions of patients receiving either rituximab dose also achieved an ACR50 response compared with those receiving placebo + MTX (26.3% and 25.9% vs 9.3%, respectively; P <0.0001) (Figure 15-5). Although ACR70 responses were achieved by a greater proportion of patients receiving either dose of rituximab vs those receiving placebo + MTX, the differences did not achieve statistical significance (Figure 15-5). Also at week 24, there were significantly greater mean changes in DAS28-ESR (P <0.0001) in both rituximab dose groups compared with the placebo group. In addition, both rituximab regimens produced statistically significant improvements in patient-reported outcomes (e.g., changes in HAQ-DI, FACIT-F and SF-36 scores).

By week 48, ~90% of patients in all treatment groups had received a second course of treatment. Mean DAS28-ESR scores for all treatment arms were maintained or improved throughout the 48-week period with both doses of rituximab + MTX. Mean changes in DAS at week 48 were greater than at week 24 for both rituximab dose groups. At week 48, clinical responses (ACR and EULAR responses) were maintained in both rituximab groups, although there were no significant differences between the rituximab doses for any clinical end point.

Studies that used highly sensitive flow cytometry (HSFC) have shown that most patients who do not exhibit an adequate response to rituximab treatment have persistent circulating B cell levels at week 2 after initial treatment. Vital and associates treated patients with RA with a first cycle of rituximab (2 infusions of 1000 mg). Patients had been previously treated with two nonbiologic DMARDs (including MTX) and one anti-TNF agent. Patients were categorized as either first-cycle responders (n = 65) or first-cycle nonresponders (n = 38) using EULAR criteria for clinical response. Six months later, patients were retreated with their previous rituximab regimen in order to determine whether an additional cycle of rituximab would improve B cell depletion and clinical response in patients whose disease did not respond to the initial cycle.

HSFC was performed at baseline, immediately prior to the second infusion (week 2), 1 month after the second infusion (week 6), then every 3 months for each cycle of rituximab. Complete B cell depletion was defined as being <0.0001 × 109 cells/liter. At baseline, the number of preplasma cells was significantly higher in first-cycle nonresponders than in first-cycle responders (P = 0.003). Following the first infusion of the first cycle of rituximab, only 9% of first-cycle nonresponders exhibited complete depletion of B-lineage cells compared with 37% of first-cycle responders (P = 0.007). Following the first infusion of the second cycle of rituximab, 38% of first-cycle nonresponders exhibited complete depletion. Twenty-six weeks after the second cycle, there was a significant improvement in the DAS28 joints, with 72% of patients exhibiting a EULAR response. The authors conclude that these findings indicate that an additional cycle of rituximab administered prior to total B cell repopulation enhances B cell depletion and clinical responses.

The MIRROR study assessed three treatment/retreatment regimens comprising two courses of rituximab given 24 weeks apart: 2 × 500 mg then 2 × 500 mg; 2 × 500 mg then 2 × 1000 mg (dose escalation); and 2 × 1000, then 2 × 1000 mg in 378 patients with active disease despite stable MTX and no more than one biologic agent. A total of 378 patients were randomized, of whom 314 (83%) completed the 48 week of the study. The primary end point was the proportion of patients achieving ACR20 at week 48. At week 48, ACR and EULAR outcomes with the rituximab 1000/1000 mg regimen were consistently higher compared with the rituximab 500/500 mg and 500/1000 mg regimens. However, the differences between groups were not significantly different. At week 48, the rituximab 1000/1000 mg regimen was associated with a higher proportion of patients who, following retreatment, maintained or improved their week 24 responses.

As discussed in Pharmacotherapy and Management Guidelines, there has been a shift to a treat-to-target (TT) treatment strategy for patients with RA since tight control of disease activity has been shown to halt joint damage and improve long-term outcomes irrespective of the type of DMARD, synthetic, or biological treatment. A retrospective analysis of data from 493 RA patient recruited into rituximab phase 2/3 studies who received further courses of open-label rituximab assessed the efficacy and safety profiles of two different rituximab retreatment strategies: (1) TT in which patients were assessed at 24 weeks after each course and retreated if not in remission (DAS28-ESR ≥2.6); (2) treatment as needed (PRN) in which patients were retreated at the physician’s discretion ≥24 weeks following the first course and ≥16 weeks following further courses, if both swollen and tender joint counts were ≥8. All courses consisted of rituximab 2 × 1000 mg 2 weeks apart plus MTX.

As shown in Figure 15-6, the TT regimen was associated with significantly greater mean improvement in disease activity and lower HAQ-DI scores than the PRN regimen. From week 40, the difference in disease activity resulting from the two approaches was statistically significant and potentially clinically relevant. Furthermore, significantly fewer patients treated using the TT regimen experienced disease flares, with tighter control of disease activity throughout each rituximab treatment course. Despite more frequent retreatment with TT compared with PRN (median number of courses 4 and 3, respectively), the rates of serious adverse events and serious infections were comparable between regimens.

Enlarge 

Enlarge 

Enlarge 

Enlarge 

Enlarge 

Safety and Tolerability

In general, rituximab therapy in RA was well tolerated. In the clinical trials, acute infusion reactions (manifested by fever, chills, rigors, etc.) were the most common adverse events, experienced by approximately 30% of rituximab-treated patients following the first infusion compared with about 20% of placebo-treated patients. The incidence of infusion reactions following the second infusion was less, approximately 10% for both rituximab and placebo and serious infusion reactions were uncommon. Infusion reactions to rituximab appeared to be less common and less severe among RA patients as compared with patients with lymphoma.

As is seen with virtually all immune-modulating drugs in RA patients, infections are observed and tend to be slightly higher among patients receiving the active agent. The most common infections observed in trials of rituximab in RA were nasopharyngitis, upper respiratory tract infections, urinary tract infections, bronchitis and sinusitis. Two cases of progressive multifocal leukoencephalopathy (PML) have been reported from open-label use of rituximab in patients with systemic lupus erythematosus (SLE). While no cases have been reported to data among RA patients, and while other opportunistic infections have typically not been observed in clinical trials of rituximab in RA, clinicians must always be vigilant for potential signs and symptoms of uncommon infections. For patients achieving clinical benefit to rituximab who develop allergic reactions, alternate molecules targeting CD20 might be considered.

Dosage and Administration

Rituximab is generally given as two 1,000-mg IV infusions separated by 2 weeks. Glucocorticoids administered as methylprednisolone 100 mg IV or its equivalent 30 minutes prior to each infusion have traditionally been utilized to reduce infusion reactions. Rituximab is currently approved for use in combination with MTX in RA.

Despite the efficacy of rituximab in some patients with RA, which in some cases can last many months after a single treatment course, disease activity almost always recurs. This highlights the need for delineation of the efficacy and safety of repeated courses of rituximab therapy in RA. While open-label experience to date suggests that repeat courses of rituximab appear to be as effective and as well tolerated as initial courses, the optimal treatment paradigms have not yet been fully delineated in controlled trials. A number of RA patients, including many patients responding to therapy in clinical trials, have received two to five repeat courses, most commonly two infusions of 1,000 mg of rituximab per course. Most repeat courses were administered no sooner than 24 weeks after the previous course. Importantly, decreases in circulating B cell numbers may become more prolonged after repeated courses of treatment. Also, levels of serum immunoglobulin appear to decrease in some patients with repeated courses of treatment. Therefore, close attention will be paid to the safety of repeated courses of rituximab.

In addition to questions concerning the optimal treatment paradigm and long-term safety of B-cell targeted therapy with rituximab in RA, a number of other questions remain (Figure 15-1). For example, what effect if any will genetic variations in Fc receptors have on therapeutic safety or efficacy of rituximab in RA? Also, what are the consequences, if any, of additional immune-modulating therapy in patients who have received rituximab?

Enlarge 

Use in Other Autoimmune Diseases

In addition to RA, B cells may play a pivotal role in the pathogenesis of a number of other autoimmune conditions. Targeting of B cells with rituximab has been investigated in several conditions, most notably ANCA-associated vasculitis (AAV). Notable positive results led to the approval of rituximab for the treatment of patients with granulomatosis with polyangiitis (previously called Wegener’s granulomatosis) and microscopic polyangiitis, two types of AAV that heretofore generally required cyclophosphamide treatment. Rituximab has also shown positive results as a therapy for relapsing-remitting multiple sclerosis and pemphigus vulgaris.

Interestingly, data in SLE, a prototypical autoantibody mediated autoimmune disease, have been mixed. Early positive results in small, open-label trials could not be replicated in controlled trials, although the failure may relate in part to factors such as study design. Other autoimmune conditions in which rituximab has been assessed include Sjogren’s syndrome, inflammatory myositis and others. Trials currently under way should provide answers to questions regarding optimal dosing, efficacy and tolerability in these various diseases.