Primary analysis
Rituximab, a genetically engineered chimeric anti-CD20 monoclonal antibody, is currently approved for the treatment of relapsed or refractory, low grade, or follicular CD20+ B cell non-Hodgkin's lymphoma. Rituximab selectively depletes B cells that bear the CD20 surface marker via multiple mechanisms that include antibody-dependent cellular cytotoxicity and complement-dependent cytotoxicity, and via the induction of apoptosis.
Edwards [4] hypothesized that depletion of B lymphocytes could represent a new treatment for RA. In order to assess better the efficacy of this B cell targeted therapy, a randomized, double-blind, controlled study was carried out to examine the effect of selective depletion of B cells with rituximab in patients with RA [5].
A total of 161 patients with active RA who had failed to respond to treatment with methotrexate (at least 10 mg/week for at least 16 weeks) were randomly assigned to one of four treatment regimens: oral methotrexate as a control arm; intravenous rituximab alone (1000 mg on days 1 and 15); intravenous rituximab plus cyclophosphamide (750 mg on days 3 and 17); or rituximab plus methotrexate. All patients received 100 mg methylprednisolone just before each treatment (or intravenous placebo), as well as prednisone 60 mg/day on day 2 and days 4–7, and 30 mg/day on days 8–14.
Clinical assessments were performed at baseline (day 1) and at weeks 12, 16, 20 and 24, using the American College of Rheumatology (ACR) core set of disease activity measures: swollen and tender joint counts (66 joints evaluated), patients' evaluation of pain based on a scale of 0 (no pain) to 100 (unbearable pain), patients' global assessment of disease activity on a scale from 0 (disease inactivity) to 100 (maximal disease activity), physicians' assessment of disease activity, assessment of physical function reported by patients utilizing a health assessment questionnaire, and laboratory evaluation of serum C-reactive protein levels and erythrocyte sedimentation rate.
Patient responses were assessed at week 24 for the primary analyses and at week 48 for the exploratory analyses. The primary end-point was the proportion of patients with an ACR50 response at week 24. Secondary end-points were ACR20 (a 20% improvement according to ACR criteria) and ACR70 (a 70% improvement according to ACR criteria) responses, as well as a response based on the European League Against Rheumatism criteria.
At week 24, the proportion of patients with an ACR50 response was significantly greater for the group of patients taking the rituximab–methotrexate combination (43%; P = 0.005) and the rituximab–cyclophosphamide group (41%; P = 0.005) than for those receiving methotrexate alone (13%). Investigators also noted that the number of patients achieving an ACR50 response in the group receiving rituximab alone was greater than that in the control group, but this failed to reach statistical significance (P = 0.059).
Notably, the mean change from baseline in Disease Activity Score at week 24 reflected significant improvement over methotrexate alone in all rituximab groups (P = 0.002). The clinical parameters employed in the calculation of the Disease Activity Score include number of tender joints, number of swollen joints, erythrocyte sedimentation rate, and the patient's subjective assessment of disease activity.
Exploratory analysis: a 48 week study
An exploratory analysis of ACR responses at 48 weeks found that, of patients in the rituximab–methotrexate group, 35% (P = 0.002) and 15% (P = 0.03) had ACR50 and ACR70 responses, respectively, as compared with 5% and 0% in the methotrexate control group [5, 6]. In addition, 27% of patients in the rituximab–cyclophosphamide arm achieved an ACR50 response (P = 0.01; Fig. 1).
In terms of pharmacodynamic outcomes, treatment with rituximab was associated with nearly complete depletion of peripheral blood B cells throughout the entire 24-week period. Although such a profound reduction in B cells may suggest greater susceptibility to infection, the overall incidence of infection was similar in the control and rituximab groups at 24 and 48 weeks. By week 24, one patient in the control group and four in the rituximab groups had suffered a serious infection. An additional two serious infections – including a fatal bronchopneumonia – were reported during the extended 48-week period in the rituximab group. There was no accumulation of any particular type of infection in rituximab treated groups.
Despite B cell depletion, immunoglobulin levels did not change substantially. Patients in the rituximab groups experienced substantial and rapid reductions in rheumatoid factor levels, whereas those in the methotrexate alone group experienced only a moderate reduction in rheumatoid factor levels [5] (Fig. 2).
Adverse events
Overall, 73–85% of patients in all treatment groups reported at least one adverse event, with hypertension, hypotension, nasopharyngitis, arthralgia, back pain, hyperglycaemia, cough, flushing and headache reported most often. Of patients in each group, 30–45% experienced events associated with initial infusion, although 85–95% of adverse events related to rituximab infusions were characterized as mild or moderate.
Infusion reactions occurred during the first infusion in approximately one-third of patients in the groups receiving rituximab and the placebo group (36% and 30%, respectively). The rate of first infusion reaction in patients with RA was considerably lower than rates seen in patients with non-Hodgkin's lymphoma (36% versus 70–80%). According to the 48-week data, the most common infusion related reactions reported by patients were hypotension (14% versus 10% in placebo group), hypertension (9% versus 5%), flushing (6%), pruritus (6%), and rash (6%). Six patients during the primary 24-week trial period and an additional three at 48 weeks withdrew from the trial because of adverse events, including exacerbation of ongoing RA, hypotension and bronchopneumonia, staphylococcal septicaemia, renal impairment and rash.
Rituximab in systemic lupus erythematosus
Albert and coworkers [7] conducted a phase I study to determine the safety and efficacy of B cell depletion with the anti-CD20 monoclonal antibody rituximab as treatment for SLE. Seven patients with active and persistent SLE who had failed at least one immunosuppressive agent enrolled in this pilot study. Each patient received 4-weekly infusions of rituximab at 375 mg/m2.
Six out of seven patients exhibited a clinical response, defined as improvement in SLEDAI. All but one patient had greater than 99% B cell depletion lasting more than 3 months. The patient who exhibited only 95% B cell depletion had no clinical response. Interestingly, this patient also showed responses to immunizations, whereas two other patients with complete B cell depletion (and clinical response) failed to achieve adequate immune responses. Among clinical responders, 50% had a brief remission (6 weeks to 6 months), and the other 50% had a more prolonged remission (6–9 months).
Among longer term responders, the steroid dose was either stabilized or lowered; one patient had been in remission for 14 months without the need for steroid therapy. All brief responders returned to an immunosuppressive regimen. There was a trend toward a decrease in the percentage of B cells expressing CD86 among all patients. There was no distinctive pattern of other cell surface marker expression among responders and nonresponders. No consistent changes were found in serum chemistry, serum complement levels, autoantibody titres and immunoglobulin levels.
A study conducted by Anolik and coworkers [8] found that, compared with normal control individuals, lupus patients exhibited several abnormalities in peripheral B cell homeo-stasis at baseline, including naïve lymphopenia, and expansion of circulating plasmablasts. Remarkably, these abnormalities partially resolved after effective B cell depletion with rituximab and immune reconstitution [8].
The frequency of autoreactive VH4.34 memory B cells also decreased 1 year following treatment, despite the presence of low levels of residual memory B cells at the point of maximal B cell depletion and persistently elevated serum autoantibody titres in most patients. This study demonstrates that, in SLE patients, specific B cell depletion therapy with rituximab dramatically improves the abnormalities in B cell homeostasis that are characteristic of this disease.
Looney and coworkers [9], in a phase I/II study, examined the potential use of rituximab in B cell depletion for patients with SLE. In order to establish the safety and efficacy of rituximab in this patient population, those investigators performed a dose escalation trial of rituximab added to ongoing therapy in SLE. They administered rituximab as a single infusion of 100 mg/m2 (low dose), a single infusion of 375 mg/m2 (intermediate dose), or as four infusions (1 week apart) of 375 mg/m2 (high dose). CD19+ lymphocytes were measured to determine the effectiveness of B cell depletion. The Systemic Lupus Activity Measure (SLAM) score was used as the primary outcome for clinical efficacy.
Looney and coworkers demonstrated that rituximab was well tolerated in this patient population, with most experiencing no significant adverse effects. Three patients had serious adverse events requiring hospitalization (one Staphylococcus aureus abscess of the thigh, one localized case of herpes zoster and a transient ischaemic attack), although none was deemed related to rituximab administration. The majority of patients (11 out of 17) had profound B cell depletion (CD19+ B cell count < 5/μl). In these patients the SLAM score was significantly improved at 2 and 3 months compared with baseline (P = 0.0016 and P = 0.0022, respectively). This improvement persisted for 12 months, despite the absence of a significant change in anti-dsDNA antibody and complement levels. Six patients developed human antichimeric antibodies at a level of 100 ng/ml or greater (range of peak levels 631–9930 ng/ml). These human antichimeric antibody titres were associated with African American ancestry, higher baseline SLAM scores, reduced B cell depletion and lower levels of rituximab at 2 months after initial infusion.