Introduction
Monoclonal antibodies are the first example of successful therapy for lymphoma using our knowledge of the immune system. In 1975, Kohler and Milstein[1] discovered that the antibody-producing cells from a mouse spleen could be immortalized by fusion with myeloma cells. These fused cells (hybridoma) were able to live indefinitely and produce antibodies that provided the reagents and systems for further experiments. The first attempt at using monoclonal antibodies in lymphoma therapy was reported by Nadler et al[2] in 1980. In that study, a transient decrease in the white blood cell count was observed following each treatment; however, no significant antitumor effect was noted—perhaps due to the presence of a soluble antigen in the serum that blocked the antibody.
Each lymphoma is a clone of identical cells, all of which possess the same immunoglobulin on their surface. Each B cell’s immunoglobulin is unique. The idiotype—the unique portion of the immunoglobulin—is an ideal target for therapy. Early studies targeted the idiotype with individual monoclonal antibodies designed for that patient’s lymphoma. In many patients, such therapy produced direct antilymphoma responses that lasted for several years, with very limited toxicity.[3] In an attempt to create a more generic antilymphoma antibody, molecules found on many B-cell lymphoma cells became targets for antibodies. Clinical trials with antibodies have mostly targeted CD20, which is present on 95% of all B-cell lymphomas, as well as CD19 and CD22.
This concept of using the idiotype to broaden the antilymphoma effect and to fashion a vaccine model has recently been evaluated. This approach would theoretically produce an active immunization, inducing humoral and cellular responses that would be longer acting than passive antibodies alone. The response is heterogeneous and polyclonal, which may be an advantage. Individual studies utilizing both of these approaches will be outlined in this article.
Unconjugated Antibodies
Rituximab for Indolent Non-Hodgkin’s Lymphoma
The most extensively studied and clinically used unconjugated monoclonal antibody is the chimeric anti-CD20 monoclonal antibody rituximab(Drug information on rituximab) (Rituxan). This antibody consists of the murine variable regions of the parent 2B8 murine anti-CD20 grafted onto a human immunoglobulin G1-constant region.[4] The CD20 antigen is an excellent target for immunotherapy because it is found only on mature B cells and not on precursor B cells (which would lead to long-term depletion), and the antigen is not shed, internalized, or modulated to any great extent once antigen-antibody binding has occurred.[5] Apoptosis also appears to be triggered by this antibody-antigen binding motif.[6]
Pivotal Trial: Initial phase I studies of the rituximab anti-CD20 antibody used a bolus of the antibody that was repeated weekly in patients with relapsed CD20-positive non-Hodgkin’s lymphoma (NHL).[7] Multiple phase II studies enrolled many patients with relapsed, indolent CD20-positive NHL. Patients received a dose of 375 mg/m2 of rituximab weekly for 4 weeks. The pivotal trial showed a 6% complete response rate and a 42% partial response rate, for an overall response rate of 48% in this patient population with relapsed, indolent NHL.[8]
In this trial, patients had received a median of two prior chemotherapy regimens before being treated with rituximab. Patients who were tested before and after therapy with rituximab could clear a detectable bcl-2 rearrangement (by polymerase chain reaction [PCR] analysis for the t[14;18] translocation) in the blood and marrow compartments. However, some of these patients still had bcl-2-positive lymph nodes.[9] Median time to progression for the responders in the pivotal trial was 13 months (Figure 1). Relapses with a CD20-negative lymphoma have now been reported, although these represent a minority of such cases.[10]
Due to concerns about penetration of the antibody in large masses, a separate study was also performed in patients with bulky disease. In this trial, patients with masses > 10 cm with relapsed or refractory indolent NHL received rituximab for four doses as per previous trials. The overall response rate in this population was 43%, with a median time to progression of 8 months.[11]
More recently, studies of rituximab in previously untreated patients have been evaluated. Gutheil et al[12] reported on 20 patients with previously untreated indolent lymphoma who received rituximab in a phase II trial. An overall response rate of 50% was reported in these patients, with the follow-up currently too short to obtain a median time to relapse.
Combination Therapy: In addition to the single-agent use of rituximab for indolent lymphoma, it has also been combined with conventional chemotherapy. This combination therapy is based on the in vitro study that demonstrated synergism with the anti-CD20 antibody and chemotherapy in sensitizing previously chemotherapy-resistant cell lines.[13]
In the first combination study, reported by Czuczman et al,[14] 38 patients with indolent lymphoma (31 previously untreated) were treated with rituximab and CHOP (cyclophosphamide [Cytoxan, Neosar]/doxorubicin HCl/vincristine [Oncovin]/prednisone). The response rate was 100%, including a 66% complete response rate. With no comparison other than a historical one, it is difficult to know whether an improvement in event-free survival will be shown over CHOP alone. A prospective, randomized trial will be needed for this documentation.
Ongoing trials are studying combinations of rituximab with fludarabine (Fludara), CVP (cyclophosphamide/vincristine/prednisone), and FND (fludarabine/mitoxantrone [Novantrone]/dexamethasone), among other regimens.
Rituximab for Aggressive NHL
With respect to aggressive NHL, only a few trials have been published. Coiffier et al[15] evaluated 54 patients with relapsed diffuse, large cell or mantle cell lymphoma who received rituximab for eight weekly doses. A total of 5 complete responses and 12 partial responses were observed, for an overall response rate of 31%. Another study, by Foran et al,[16] evaluated 120 patients with newly diagnosed or recurrent mantle cell lymphoma, immunocytoma, or small lymphocytic lymphoma, who received rituximab for four weekly infusions. The overall response rate in these patients was 30% (36 out of 120 patients). A total of 10 patients (all with mantle cell lymphoma) achieved a complete response with this therapy.
Combination Therapy: With respect to combination therapy for aggressive NHL, the most mature study is a phase II trial, in which 33 patients with newly diagnosed diffuse, large B-cell NHL received rituximab at 375 mg/m2 on day 1 of each cycle, followed by CHOP on day 3 of each cycle. The complete response rate in this trial was 61%, and the partial response rate was 35%, for an overall response rate of 97%.[17] All patients were able to receive all cycles of therapy, with a 27% rate of neutropenia and < 1% incidence of thrombocytopenia. With a minimum follow-up of 24 months, 29 of 33 patients (88%) are alive and disease free following this therapy.
Recently, the preliminary results of a large prospective randomized trial evaluating the addition of rituximab to CHOP as initial therapy for aggressive NHL have been presented.[18] In this trial, patients with diffuse large B-cell NHL who were ³ 60 years of age were randomized to receive either 6 cycles of CHOP alone or rituximab combined with CHOP. There were 159 patients in the CHOP alone arm and 169 patients in the combination arm. The combination regimen had a CR rate of 76% compared with 60% for the CHOP alone arm (P = .004). In addition, the event-free survival rate was 69% in the combination arm compared to 49% in the CHOP alone arm (P < .0005). The overall survival was also improved in the combination arm—83% at 1 year, compared to 68% at 1 year in the CHOP alone arm (P < .01). Another confirmatory trial is currently close to finishing accrual in the ECOG study group.
The combination of CHOP plus rituximab has also been evaluated in patients with newly diagnosed mantle cell lymphoma. A report from the Dana-Farber Cancer Institute evaluated 40 patients who received this regimen.[19] The complete response rate was 33%, the unconfirmed complete response rate was 15%, and the partial response rate was 49%, for an overall response rate of 98%. The median progression-free survival rate was 16 months—no different from similar historical control patients receiving CHOP alone. Although 11 of the 23 patients with detectable bcl-1 rearrangements became PCR-negative, this did not correlate with the overall response to therapy.
Rituximab Plus High-Dose Therapy/Stem Cell Transplant
Because rituximab has been shown to "clear" bcl-2-positive cells from the blood and marrow, it has recently been tested in the transplant setting as an additive therapy. A few preliminary studies have now been presented in abstract form.
Salles et al[20] evaluated 26 patients—with follicular lymphoma (n = 17), mantle cell lymphoma (n = 4), or marginal zone or small lymphocytic lymphoma (n = 5)—who were treated with rituximab prior to hematopoietic stem cell harvest. Patients received four doses of rituximab at 375 mg/m2. Mobilization was attempted with high-dose cyclophosphamide(Drug information on cyclophosphamide), etoposide(Drug information on etoposide), and granulocyte colony-stimulating factor (G-CSF [Neupogen]) at 4 weeks after the administration of rituximab. Of the 26 patients, 3 failed to mobilize and the remainder received stem cell transplants. Among patients with adequate follow-up (minimum 1 year) 10 of 11 were alive and in complete remission. The PCR analysis in the stem cell product demonstrated PCR negativity in 6 of 7 patients with follicular lymphoma, while 2 patients with mantle cell lymphoma and 1 patient with small lymphocytic lymphoma showed residual PCR-positive cells.
Another trial, by Flinn et al,[21] evaluated 40 patients with follicular lymphoma (n = 19), mantle cell lymphoma (n = 9), or small lymphocytic, marginal zone, or lymphoplasmacytic disease (n = 12), who were receiving autologous transplantation. Patients received rituximab, 375 mg/m2, on day 1 of mobilization, followed by cyclophosphamide and G-CSF or granulocyte-macrophage colony-stimulating factor (GM-CSF [Leukine]). The last 15 patients were also selected for CD34 positivity. Preliminary results demonstrated that 7 of the patients with unmanipulated grafts had a PCR-positive product, whereas none of the CD34-positive-selected grafts did. Final results from both of these trials are pending.
At the University of Nebraska Medical Center, we recently completed a trial of 30 chemotherapy-sensitive patients with follicular or mantle cell lymphoma. In this study design, patients received rituximab with the BEAM transplant regimen (carmustine [BiCNU]/etoposide/cytarabine [Ara-C]/melphalan [Alkeran]). Patients were given one dose of rituximab prior to initiation of the BEAM regimen and at 30 and 60 days posttransplant. With very early follow-up, 29 of 30 patients are alive following the transplant, with a complete response rate of 88%. No additional toxicity was seen with the supplemental rituximab, and engraftment results were identical to those seen in patients receiving BEAM alone.
Our newest study uses the principles outlined above to take advantage of rituximab premobilization to clear the blood: Rituximab is administered during the regimen to sensitize the lymphoma cells, and another four doses are given at 6 months posttransplant when the B cells begin to recover (Figure 2). Quantitative bcl-2 assays will also be performed on these patients.
Rituximab for Posttransplant Lymphoproliferative Disorder
In patients who have received solid organ or bone marrow transplants, posttransplant lymphoproliferative disorder (PTLD) can be a serious complication. The problem is believed to result from the rapid proliferation of Epstein-Barr virus-transformed B cells in the setting of T-cell deficiency, a phenomenon that occurs in immunosuppressed patients. The first line of therapy for PTLD is typically a reduction of the immunosuppression. However, this does not succeed in all patients, and further therapy is often necessary.
Several pilot studies have evaluated the use of rituximab for the treatment of PTLD. In a study by Milpied et al,[22] 32 patients with PTLD received rituximab at 375 mg/m2 for four weekly doses. Of the 32 patients, 26 had solid organ transplants and 6 had bone marrow transplants. The overall response rate was 69%, with 63% of patients having a complete response and 6% experiencing a partial response. Median time to response was 54 days (range: 11 to 148 days). Of the responding patients, 59% were still in remission at a median follow-up of 10 months posttreatment.
Several other small investigations of rituximab for the treatment of PTLD have been published in abstract form.[23,24] Since many of these patients are quite ill at the time that PTLD is diagnosed, a therapeutic intervention with a less toxic agent is ideal in this clinical situation.
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