The treatment of B-cell malignancies has been revolutionized by the availability of safe and effective monoclonal antibodies. The addition of rituximab to standard chemotherapy regimens prolongs the survival of patients with diffuse large B-cell lymphoma (DLBCL) and follicular non-Hodgkin lymphoma. Nevertheless, indolent and mantle cell lymphomas remain incurable, and 30% to 40% of patients with DLBCL still die from their disease. Much ongoing research has focused on optimizing monoclonal antibody use, integrating them into multiagent regimens, and developing newer antibodies. Attempts to improve on the efficacy of monoclonal antibody–based therapy have included altering the dosing schedule, optimizing patient selection, maintenance therapy, improving upon the rituximab molecule, radioimmunotherapy, as well as combinations with cytotoxic molecules and other novel agents. Preliminary data with a number of treatment regimens are promising in indolent and aggressive lymphomas. The eventual goal of targeted therapies is to individualize treatment to increase response and survival, while reducing treatment-related toxicity.
B-cell malignancies include the non-Hodgkin lymphomas (NHLs), with an estimated incidence of 65,980 in the United States for 2009, and chronic lymphocytic leukemia (CLL), with about 15,000 newly diagnosed patients per year. The NHLs include a wide spectrum of diseases that range from those that are indolent to others that are highly aggressive. In the US, 85% are of B-cell origin and only 15% are T-cell NHLs. Traditionally, advanced B-cell malignancies have been treated with either single-agent or combination chemotherapy. However, over the past decade the availability of safe and effective monoclonal antibodies has altered treatment strategies resulting in improved outcomes. Whereas, as single agents, these antibodies have demonstrated clinically meaningful activity, much of the ongoing research has focused on optimizing their use, finding how best to integrate them into multiagent regimens, and developing newer monoclonal antibodies.
Mechanisms of Action and Resistance
For an antibody to be effective, it needs to target an appropriate antigen, preferably one that is expressed abundantly and exclusively on the malignant cells. Once the antibody binds to its target antigen, it induces cell death through a variety of mechanisms, including complement-dependent cytotoxicity (CDC), antibody-dependent cellular cytotoxicity (ADCC), and apoptosis. Other hypothesized mechanisms of action include cytokine inhibition, cytotoxic T-lymphocyte generation, and phagocytosis.
The mechanisms by which malignant lymphocytes become resistant to antibody therapy are not fully elucidated, but include impairment to the above-mentioned mechanisms of action. For example, upregulation of complement inhibitory proteins, such as CD55 and CD59, can lead to decreased CDC.[2,3] ADCC, which occurs as a result of the Fc portion of the monoclonal antibody binding to the Fcγ receptor on the effector cell, can vary with certain Fcγ-receptor polymorphisms. Some are associated with rituximab (Rituxan) resistance, while others are associated with hypersensitivity in NHL.[4-6] Similarly, impaired apoptosis can result in resistance to a variety of therapeutics.[7,8] Retrospective data suggest that certain gene-expression patterns may be associated with rituximab resistance.
The role of genes such as Bcl-2 as prognostic factors has yet to be established. Mounier et al published data from a phase III trial in which diffuse large B-cell lymphoma (DLBCL) patients were randomized to cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP) alone or with rituximab (R-CHOP). In patients whose tumors were bcl-2–positive (greater than 50% expression of the bcl-2 protein), those who received R-CHOP had a higher overall response rate (78% vs 60%, P = .01), longer 2-year overall survival rate (67% vs 48%, P = .004), and longer event-free survival rate (58% vs 32%, P < .001) than those who were treated with CHOP alone. The investigators found no statistically significant difference in overall response rate or overall survival in the bcl-2–negative population. These data suggest that rituximab overcomes bcl-2 resistance.
Wilson et al demonstrated that the addition of rituximab to dose-adjusted etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (DA-EPOCH) was also beneficial for DLBCL patients with bcl-2 overexpression, improving the 5-year progression-free survival from 50% to 80%. However, in a study by Winter et al, the addition of rituximab did not improve overall survival or failure-free survival in patients with bcl-2 or bcl-6 overexpression. Discrepancies among the studies may be explained by differences in distribution of activated B-cell vs germinal center B-cell lymphomas, assay techniques, sample size, or other unknown factors.
Rituximab, a chimeric anti-CD20 monoclonal antibody, was initially approved by the US Food and Drug Administration (FDA) in 1997 as the first monoclonal antibody for the treatment of a human malignancy—relapsed or refractory, follicular or low-grade NHL. This approval was based largely on a study involving 166 patients who received 375 mg/m2 of rituximab weekly for 4 weeks. The overall response rate was 48%, with 6% complete responses (CRs) and a median duration of response of about 1 year. Subsequent trials have confirmed its efficacy as a single agent in both the upfront and refractory disease settings (Table 1).[11-16] Moreover, addition of the antibody to standard chemotherapy regimens has prolonged the survival of patients with DLBCL and follicular NHL (Table 2).[17-27]
Nevertheless, indolent and mantle cell lymphomas remain incurable, and 30% to 40% of patients with DLBCL still die from their disease. Attempts to improve on the efficacy of rituximab have included strategies such as altering the dosing schedule, rituximab as maintenance therapy, improving on the molecule, radioimmunotherapy, and combinations with novel agents including immunomodulators, apoptosis targeting drugs, cytotoxic molecules, or other monoclonal antibodies. Other antibodies and related molecules are also in development.
Dose and Schedule
Intensification of dose and schedule of rituximab has been evaluated in a phase I/II trial by Poeschel et al, in which elderly patients (aged 61–80 years, stages I–IV) received six cycles of CHOP every 2 weeks along with 12 infusions of rituximab at 375 mg/m2 on days 0, 1, 4, 8, 15, 22, 29, 43, 57, 71, 85, and 99. Of the first 20 patients, 3 died from therapy-associated complications (1 sepsis, 2 interstitial pneumonitis). Another 4 patients developed interstitial pneumonia. Prophylaxis for Pneumocystis carinii (cotrimoxazole) and cytomegalovirus (acyclovir) was initiated after the first 20 patients, with no additional episodes of interstitial pneumonia. Disease progression was not seen in the 46 patients who completed this regimen during the 9 months of follow-up.
The RICOVER trial (Rituximab With CHOP Over Age 60 Years) was a phase III comparison of six or eight cycles of CHOP or R-CHOP in a 14-day dosing regimen in 1,222 elderly patients with aggressive B-cell lymphoma. Patients who received rituximab had a longer progression-free and overall survival. Fifty patients in the eight-cycle group and 42 patients in the six-cycle group experienced treatment-related deaths, the majority due to infection. However, whether R-CHOP should be administered every 2 (R-CHOP-14) or 3 (R-CHOP-21) weeks remains controversial.
Studies evaluating this question have been conducted by the Groupe d’Etude des Lymphomes de l’Adulte (GELA) and the National Cancer Research Institute (NCRI). Preliminary results from the NCRI trial show similar complete response rates between R-CHOP-21 and R-CHOP-14 (47% in both arms), but no final data regarding overall survival are yet available. Nevertheless, CR rates tend to correlate with time-dependent endpoints. The most common grade 3 and 4 toxicities include neutropenia in 57% of patients given R-CHOP-21 and 31% of R-CHOP-14 recipients. Of note, patients in the latter group received granulocyte colony-stimulating factor (Neupogen) with each cycle, which likely contributed to the decreased frequency of neutropenia. Other adverse effects include thrombocytopenia occurring in 5% and 9% and infection in 22% and 17% of patients, respectively.
Another method of enhancing the benefit of rituximab is to use it as maintenance following induction therapy (Table 3).[20,25,30-38] Hainsworth et al first published a phase II trial of 62 previously untreated patients with follicular lymphoma or small lymphocytic lymphoma (SLL) who received rituximab as four weekly infusions followed by an additional four weekly doses every 6 months until disease progression, or to a maximum of 2 years. The overall and complete response rates increased from 47% and 7% following the initial infusions to 73% and 37%, respectively, at the end of maintenance. With a median follow-up of 30 months, the progression-free survival was an encouraging 34 months.
Nevertheless, randomized studies have failed to show a survival benefit with maintenance rituximab. Ghielmini et al reported 202 patients with untreated or previously treated follicular lymphoma treated with four weekly infusions of rituximab followed by observation or maintenance. At a median follow-up of 8.9 years, the median event-free survival was 13 months for the observation arm and 24 months for the consolidation arm (P = .0012). Overall survival revealed a trend favoring prolonged treatment (P = .09). Van Oers et al randomized 474 minimally pretreated follicular lymphoma patients to CHOP or R-CHOP with a second randomization to observation or maintenance rituximab. Progression-free survival was longer with maintenance therapy (3.7 vs 1.3 years, P < .0001). An initial suggestion of a survival benefit disappeared with additional follow-up (5-year overall survival rates were 74% with maintenance and 64% with observation, P = .07).
Forstpointner et al randomized 195 patients with relapsed follicular or mantle cell lymphoma to fludarabine, cyclosphosphamide, and mitoxantrone (FCM) alone or with rituximab maintenance. They reported a significant prolongation of the duration of response in both follicular lymphoma (P = .035) and mantle cell lymphoma (P = .049), but the estimated 3-year overall survival was not different (77% with maintenance and 57% with observation, P = .1). In the mantle cell cohort, a higher rate of ongoing remissions was noted beyond 2 years with maintenance rituximab—45% vs 9% with observation. Whether a survival benefit will emerge requires longer follow-up. The European Mantle Cell Lymphoma Network is currently studying the efficacy of maintenance rituximab vs maintenance interferon following R-CHOP or the combination of rituximab, fludarabine, and cyclophosphamide in mantle cell lymphoma (NCT00209209).
Hochster et al conducted a phase III trial in which 311 patients with untreated advanced indolent NHL underwent induction therapy with cyclophosphamide, vincristine, and prednisone (CVP) and subsequently were stratified to either observation or maintenance rituximab. Progression-free survival was significantly longer with maintenance therapy (68% vs 33%), but there was no improvement in overall survival.
In a meta-analysis of five trials of approximately 1,000 follicular lymphoma patients randomized to rituximab maintenance or observation after induction, patients who received maintenance appeared to have a significantly longer survival. However, this analysis was problematic: Whereas the Eastern Cooperative Oncology Group (ECOG) 1496 study cohorts were included individually, the final analysis was not.[34,36,37] Second, only the earlier data from both van Oers et al suggesting survival prolongation were used, not the longer follow-up showing no difference.[20,38] Similarly, only the initial data from the ECOG 1496 trial were included. Preliminary results of patients who received CVP induction showed an improvement in overall survival with maintenance therapy, but with long-term follow-up this disappeared as well. The meta-analysis may have produced different results with longer follow-up of this indolent patient population.
It is also not clear if maintenance rituximab is more efficacious than re-treatment at time of progression. Hainsworth et al randomized 114 patients with relapsed indolent NHL to 4 weeks of rituximab followed by 4 weeks of maintenance rituximab every 6 months or rituximab retreatment at time of progression. A longer progression-free survival was seen with maintenance therapy, but there was no difference in survival. The ECOG 4402 Rituximab Extended Schedule or Re-treatment Trial (RESORT) investigated this question in untreated indolent NHL, and results are pending.
A number of factors should enter into the decision regarding the use of maintenance rituximab. First is the expense without an associated survival benefit. Second, there is no standard maintenance regimen, as schedules vary in frequency and duration (Table 3). Moreover, toxicities associated with maintenance rituximab include an increase in grade 3/4 infections as well as less common consequences such as progressive multifocal leukoencephalopathy. In addition, maintenance may compromise response to subsequent therapies. Finally, none of the published studies in follicular lymphoma involve maintenance that follows the current standard of chemoimmunotherapy as the initial approach to the disease.
In the Primary Rituximab and Maintenance (PRIMA) study, patients with untreated stage III/IV follicular lymphoma were treated with rituximab in combination with CHOP, CVP, or FCM, selected on an institutional basis. Patients who responded to induction were randomized to maintenance rituximab or observation. The study was closed early due to a planned interim analysis in which the primary endpoint of progression-free survival was prolonged with maintenance rituximab. Response rates, toxicity, and overall survival data are pending. Since no studies to date demonstrate a survival advantage for maintenance rituximab in follicular NHL, yet there is an increase in toxicities, maintenance is hard to justify on the basis of the available data.
The lack of a role for maintenance in DLBCL is demonstrated by Habermann et al, who conducted a phase III trial in which elderly patients with untreated DLBCL were randomized to CHOP or R-CHOP followed by four weekly doses of rituximab every 6 months or observation. Although the primary endpoint of the study was reached with an improvement in failure-free survival (53% vs 46%), no survival benefit was seen from maintenance rituximab with R-CHOP.
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