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Lymphoma |
aFred Hutchinson Cancer Research Center, Seattle, Washington, USA; bService of Nuclear Medicine, University Hospital of Lausanne, Lausanne, Switzerland; cService of Nuclear Medicine, University Hospital of Geneva, Geneva, Switzerland; dDivision of Oncology, Department of Medicine, University of Washington, Seattle, Washington, USA; e Multidisciplinary Oncology Center, University Hospital of Lausanne, Lausanne, Switzerland
Key Words. Follicular lymphoma • Radioimmunotherapy • Antitumor antibodies • Chemotherapy • Cytokines • Combination therapy
Correspondence: Franz Buchegger, M.D., Service of Nuclear Medicine, University Hospital of Lausanne, CH-1011 Lausanne, Switzerland. Telephone: 41-21-31-44-373; Fax: 41-21-31-44-349; e-mail: Franz.Buchegger{at}CHUV.CH
Received January 25, 2008; accepted for publication April 23, 2008.
Disclosure: A.B.D. has acted as a consultant for Bayer Schering (Zevalin®). No other potential conflicts of interest were reported by the authors, planners, reviewers, or staff managers of this article.
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Learning Objectives
Top
Learning Objectives
Abstract
Introduction
Antibody-Based Biological...
Cytokines
Perspective
Conclusion
Author Contributions
Acknowledgments
References
After completing this course, the reader will be able to:
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ABSTRACT |
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TopLearning ObjectivesAbstractIntroductionAntibody-Based Biological...CytokinesPerspectiveConclusionAuthor ContributionsAcknowledgmentsReferences |
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INTRODUCTION |
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TopLearning ObjectivesAbstractIntroductionAntibody-Based Biological...CytokinesPerspectiveConclusionAuthor ContributionsAcknowledgmentsReferences |
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Unlabeled and radiolabeled antibodies appear to exemplify fundamentally different therapeutic concepts in the management of NHL and other hematologic malignancies. Antibodies such as rituximab represent a biological treatment [5] exploiting the potential activation of various immunological mechanisms including antibody-dependent cellular cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), and direct induction of apoptosis [6]. The induced expression of the chemokines CCL3 and CCL4 and their potential for attracting neutrophils, macrophages, natural killer (NK) cells, and T cells into lymphoma may represent yet another mechanism of action of rituximab [7]. In addition to rituximab, multiple other antibodies and antibody derivatives directed against either CD20 or other lymphoma-associated surface antigens are being investigated in preclinical and clinical trials for therapy of lymphoma and leukemia. On the other hand, RIT with either 131I-tositumomab (Bexxar®; GlaxoSmithKline, Philadelphia) or 90Y-ibritumomab (Zevalin®; Biogen Idec Inc., Cambridge, MA) acts primarily by killing lymphoma cells via emission of beta particles [8–10]. Because involved-field irradiation is the only potentially curative treatment for localized follicular lymphoma [11], an exquisitely radiosensitive tumor [12], targeted irradiation offered by RIT appears particularly promising as an adjunct to chemo- and immunotherapy.
Autologous stem cell transplantation (ASCT) has a major impact on remission duration when applied in first remission to patients with follicular lymphoma, as demonstrated in several phase III studies [13–15]. However, while an advantage in survival was shown for ASCT given in relapse [16], current data from follicular lymphoma patients treated in first remission with ASCT have not convincingly demonstrated a longer overall survival time [14, 15]. The authors of one of these trials therefore concluded that ASCT should be reserved for patients in relapse [15]. It should be noted, however, that these studies were performed before the introduction of rituximab. The value of ASCT in first remission in patients induced with chemotherapy integrated with either unlabeled or radiolabeled antibodies is thus currently not defined. Based primarily on an excess of secondary malignancies in the ASCT arm, the authors of the Groupe Ouest Est d'Etude des Leucémies et Autres Maladies du Sang (GOELAMS) study concluded that ASCT, as practiced in their trial, should not be recommended as first-line consolidation of follicular lymphoma. They suggested instead that new approaches intended to decrease the risk for relapse following transplant should be investigated, such as improving the conditioning regimen with radioimmunoconjugates or using rituximab as an in vivo purging agent [14].
The intention of this review is to discuss the potential of antibodies as biological treatments, possibly combined with cytokines, and review radioimmunotherapy and its delivery of various forms of radiation. During this discourse, we explore the potential of RIT for upfront treatment as compared with therapy of late-stage disease. Finally, it is our conviction that optimized biological treatment combined with radiolabeled antibodies and chemotherapy will afford the greatest potential for durable complete responses and possibly cure for a currently incurable disease with conventional methods.
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ANTIBODY-BASED BIOLOGICAL TREATMENTS |
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TopLearning ObjectivesAbstractIntroductionAntibody-Based Biological...CytokinesPerspectiveConclusionAuthor ContributionsAcknowledgmentsReferences |
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In the treatment of follicular lymphoma, the remarkable efficacy of rituximab is particularly evident when employed in combination with chemotherapy. Cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP) plus rituximab (R-CHOP), as a representative combination, has shown significantly better therapeutic efficacy in terms of response rates and duration, without adding major adverse effects, compared with CHOP treatment alone [17–19]. Similar improvements were observed when adding rituximab to cyclophosphamide, vincristine, and prednisone (R-CVP) [20, 21] or with fludarabine as either single-agent chemotherapy or in combination with other drugs such as cyclophosphamide and mitoxantrone (FCM) [22–24]. An impact on long-term treatment outcome by combining rituximab treatment with chemotherapy has been demonstrated convincingly by multiple studies [1, 2, 25–29]. Finally, greater efficacy has been observed after maintenance treatment with rituximab [30–33], and such maintenance might also merit stronger consideration in upfront combination therapies.
Other antibodies directed against different target proteins in lymphoma and leukemia are under investigation in preclinical and clinical trials. Further improved biological efficacy in NHL might be achieved by combining anti-CD20 rituximab treatment with other antibodies directed against different antigens, such as anti-CD22 [34] or anti-CD40 [35], using humanized antibodies, or using novel anti-CD20 antibodies with modified Fc domains that provide greater affinity for Fc receptors and superior effector functions [36, 37].
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CYTOKINES |
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[38–42]. Cytokines might act by enhancing antibody efficacy, chemotherapy, or RIT or be effective by themselves. However, IL-2 and other cytokines may also exert a direct impact on cellular immunity by stimulating T and NK cells. Allogeneic stem cell transplantation affords the most impressive demonstration of the efficacy of cellular immunotherapy of cancer and affords an opportunity for cure of a variety of leukemias and lymphomas even after they become resistant to other treatments [43]. The importance of NK cells in these transplants is becoming increasingly apparent [44, 45]. Cytokines have a significant impact after allogeneic stem cell transplantation and ASCT by promoting more rapid hematologic and immunologic recovery and possibly by fostering the development of independent antilymphoma and antileukemia effects [19].
Toxicity has been a major obstacle impeding the wider clinical application of various cytokines. Efforts to improve the efficacy/toxicity profile of IL-2 and other cytokines have included coupling them to carrier proteins or to antibodies targeting tumor antigens [46–48]. Several groups have also shown that the efficacy of IL-2 is significantly enhanced when delivered as antibody–IL-2 complexes [49–51]. Efficacy was similarly observed when IL-15 was administered as a complex with the IL-15/R receptor [52, 53]. Pronounced stimulation of CD4 and CD8 T cells, NK cells, and NKT cells has been observed following the administration of such preformed complexes. The amounts of IL-2 evaluated in mice as antibody-bound complexes [50, 51] corresponded, on a per kg basis, to IL-2 concentrations considered as low or intermediate in human applications. Considering that B- and T-cell functions are frequently impaired in lymphoma and leukemia patients and can be further attenuated by chemotherapy, RIT, or rituximab treatment, the potential of a prompt recovery after administration of cytokines might be especially important for such patients.
RIT
Encouraging results for RIT with 131I-labeled anti-CD20 monoclonal antibodies (131I-tositumomab) were first reported in 1993 when both nonmyeloablative and myeloablative regimens were reported for patients with recurrent B-cell NHL [54, 55]. Subsequent long-term observations following RIT using 90Y-ibritumomab and 131I-tositumomab have confirmed very promising overall survival and progression-free survival (PFS) times at both myeloablative and nonmyeloablative dosages when applied to patients with advanced and heavily pretreated B-cell NHL [56–62]. Upfront treatment with RIT alone at conventional, nonmyeloablative doses has also shown a very high complete response rate and a median PFS duration of 6.1 years [63].
Very encouraging long-term results have been described employing CHOP or fludarabine followed by 131I-tositumomab or 90Y-ibritumomab in the upfront treatment of follicular lymphoma [64–66]. RIT administered after chemotherapy converts many partial remissions into complete remissions [64, 67]. 90Y-ibritumomab was also given for consolidation after first-line chemotherapy and was able to induce a high number of complete remissions and produce a significantly longer PFS time both in patients having reached partial remission after chemotherapy as well as in patients already in complete remission [67]. Subsequent to the phase III study results [67], the European Commission extended the marketing authorization for 90Y-ibritumomab in Europe. 90Y-ibritumomab can now be used in the course of first-line therapy after remission induction in previously untreated patients with follicular lymphoma. Otherwise, 90Y-ibritumomab is accessible for relapsed patients both in Europe and the U.S. while 131I-tositumomab is commercialized only in the U.S.
RIT might be further improved by several additional strategies. RIT is generally combined with the injection of significant amounts of the respective nonradiolabeled anti-CD20 antibodies (predosing), a concept developed in patients with significant tumor burden [54, 55]. Predosing with nonradiolabeled antibodies has been shown to be favorable for obtaining better tumor-to–normal tissue ratios of radiolabeled antibodies in patients with measurable disease [54, 55]. Furthermore, as mentioned previously, nonradiolabeled antibodies are able, based on their different biological effector functions, to reduce tumor burden and the amount of B cells expressing CD20, and consequently improve the efficiency of subsequent RIT. It is not clear, however, if predosing is as appropriate in patients with minimal residual disease or in partial remission after debulking, because competition for tumor targeting between nonradiolabeled and radiolabeled antibodies in the latter situation might be significant. Reducing this competition by decreasing the amount of preinjected nonradiolabeled antibody could allow the delivery of larger quantities of radiolabeled antibody into the residual tumor. Alternatively, predosing with lower amounts of unlabeled anti-CD20 antibody could be favorably employed in conjunction with a noncompeting antibody directed against a different antigen [34, 35, 68]. The reduction of normal precursor B cells in bone marrow by the combined action of different antibodies against B-cell antigens could have a beneficial effect in RIT by favoring tumor uptake of the radiolabeled antibody and simultaneously reducing bone marrow targeting and subsequent irradiation of stem cells by the β-radiation crossfire effect.
A repeat RIT cycle could be introduced in patients with minimal disease or clinical complete remission aimed to eradicate residual tumor cells that might be more vulnerable and better accessible to antibodies after debulking [69, 70]. Antibodies coupled with radioisotopes emitting low energy β-radiation such as 131I or 177Lu, or possibly an -emitter or an Auger electron-emitter in the case of internalizing antibodies [71], could be most efficient in this situation (Fig. 1). The capacity to eradicate minimal residual disease with 131I-labeled antibodies has been shown experimentally in lung and liver metastasis models of colorectal cancer in mice [72, 73]. Antibodies labeled with 90Y that emits high-energy β-radiation appear better suited to treat larger nodular disease [74], even though the capacity of 90Y-ibritumomab to produce prolonged PFS times in patients in complete remission [67] may partially contradict the theoretical extrapolations. Whether RIT with a low-energy β-radiation emitter would be more efficient in this situation or could further prolong the PFS duration when given as a second RIT remains to be shown.
Enhancing tumor-to–normal tissue radioactivity ratios with two- and three-step targeting is another approach aimed at gaining RIT efficacy. Different strategies in this regard include the development of bispecific antibodies combined with the injection of small radiolabeled haptens [75] and antibodies coupled with streptavidin combined with the subsequent injection of a blood-clearing agent and the small radiolabeled biotin (Fig. 2) [76]. These concepts have been explored in preclinical and clinical studies [77–79].
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RIT in Special Situations
Though RIT had been shown to be an efficient treatment for lymphoma several years before the description of rituximab's efficacy in patients, the ease of application and minimal toxicity of rituximab allowed its rapid and widespread implementation in just a few years. The number of patients treated with RIT remains small in comparison with those treated with rituximab. However, RIT has also been shown to exhibit clinical utility in aggressive lymphoma [10]. RIT has shown its efficacy in patients with tumor infiltration of the bone marrow exceeding 25%, a situation that was an exclusion criterion in previous RIT studies [84]. Several reports have also shown that RIT after stem cell grafting is possible [10, 58, 85, 86]. However, the numbers of patients in the latter studies were generally small and a consensus on the dose to be used following stem cell transplantation still needs to be defined. Conflicting reports have variably indicated that the maximum-tolerated dose was observed at either a reduced dose or at conventional dosing [10, 58, 85, 86].
High-dose RIT or RIT at the conventional, nonmyeloablative dose has been integrated, with most promising results, in conditioning regimens before ASCT. RIT was used either alone or in combination with chemotherapy as a substitute for total body irradiation [57, 87–89]. Such an approach may significantly reduce the risk for relapse compared with chemotherapy alone.
RIT Side Effects and the Risk for Myelodysplastic Syndrome
The major side effect of RIT is myelosuppression that is grade 4 in severity in 10%–30% of patients using conventional nonmyeloablative dosing. Most patients recover spontaneously from RIT-induced myelodepression after about 2–3 months. Nonhematological toxicities are generally mild. Hypersensitivity reactions may be observed but they generally occur after infusion of nonradiolabeled antibody previous to the radiolabeled antibody. 131I-labeled antibodies induce hypothyroidism in 10%–20% of patients despite thyroid protection with nonradiolabeled iodine. Human anti-mouse IgG antibodies (HAMAs) are found in a varying percentage of patients after treatment with tositumomab, depending on the number and intensity of preceding treatments. HAMA occurrence was highest after RIT of previously untreated patients (63% of patients) [63]. Other side effects such as flu-like syndromes early after injection are observed after RIT as well as in rituximab treatment.
Two studies have evaluated the risk for myelodysplastic syndrome (MDS) and acute myeloid leukemia following RIT with either 90Y-ibritumomab or 131I-tositumomab. These retrospective analyses of large numbers of patients both showed that RIT did not produce a higher risk for MDS in comparison with similar patient populations treated with multiple chemotherapies alone [90, 91]. MDS has rarely been observed in patients treated upfront with RIT using 131I-tositumomab alone [63].
RIT and Subsequent Treatments
Standard-dose RIT may produce a prolonged myelosuppressive effect in some patients who may therefore not qualify for salvage chemotherapy regimens for about 2–3 months. This limitation should be remembered when giving RIT alone to patients with a modest chance of responding. Transformed lymphomas or high-grade lymphoma generally respond less frequently than untransformed follicular lymphoma. RIT rarely produces myelosuppression lasting >3 months, and salvage treatment after recovery can generally be administered without particular precautions [92, 93].
Antibody Forms for RIT
The current antibodies approved for RIT of NHL are of murine origin, 131I-tositumomab is a mouse IgG2a capable of inducing ADCC and CDC, while 90Y-ibritumomab is a mouse IgG1 and therefore devoid of the aforementioned functions. While 131I-tositumomab is combined with the injection of unlabeled tositumomab antibody, injection of 90Y-ibritumomab is combined with rituximab, which is a chimeric antibody bearing a human IgG1 constant domain that can elicit ADCC and CDC. Many other chimeric and humanized IgG1 antibodies against other CD antigens expressed by lymphoma and leukemia are currently being tested in clinical trials. Chimeric human–mouse IgGs are obtained from mouse antibodies by genetically engineering the mouse variable heavy and light chain domains onto the corresponding human IgG constant domains. Humanized antibodies are obtained by selectively engineering only the three hypervariable regions of the heavy and light chains from a given mouse or rat antibody into an adequate framework of a human IgG. Chimeric IgGs may still induce human antichimeric IgG antibodies in a significant number of patients, but this is rarely observed in lymphoma and leukemia patients because of their impaired immunocompetence. Humanized antibodies could theoretically also induce anti-idiotypic antibodies in patients (human anti-humanized IgG antibodies), but their frequency is extremely low, especially in leukemia and lymphoma patients. Some observations with patients developing HAMAs after treatment with mouse antibodies tend to indicate that these patients have a better chance of tumor response than patients without HAMAs [61, 94]. While most observers believe that the higher response rates observed in patients who develop HAMAs reflects their better immune status, it cannot be excluded that HAMAs themselves could positively influence antitumor responses, possibly via an anti-anti-idiotype mechanism of the immune network [95–97].
Induction of an immune response might, however, also become of concern when using antibody fusion proteins or conventional couplings of antibodies with agents such as bacterial or fungal toxins or streptavidin, or when using radiolabeled haptens in two- or three-step targeting. Only a single treatment course is generally possible when using highly immunogenic reagents.
Choice of the Adequate Radioisotope for RIT
The optimal radioisotope for RIT with intact murine or humanized antibodies should have a long physical half-life similar to that of the antibody itself in order to maximize tumor irradiation because tumor localization becomes optimal 1–2 days after injection. On the other hand, when using small radiolabeled haptens or radiolabeled biotin in two- or three-step targeting approaches, short-lived radioisotopes such as many -radiation emitters may be preferred because these small radiolabeled molecules rapidly localize into the tumor or are rapidly excreted if unbound (Fig. 2).
In radioimmunotherapy it is desirable to document the tissue distribution of the radioconjugate and rely on a minimal dosimetry in order to prevent major side effects. Unfortunately, reliable bone marrow dosimetry that correlates well with RIT-induced myelosuppression in individual patients is currently not available. Therapy with 131I-labeled antibodies can be followed by scintigraphy based on the 
-radiation that is emitted simultaneously with the therapeutic β-radiation. However, the high energy -radiation of 131I unfortunately also nonspecifically irradiates the patient himself and mandates that patients, family members, and health care workers are monitored carefully during therapy. 177Lu emits -radiation of lower energy and at a lower percentage than 131I, making it an ideal radioisotope to be monitored during therapy. Furthermore, treatment with 177Lu radioconjugates is much more conducive to therapy in an ambulatory setting than 131I because of lesser environmental irradiation. 90Y, on the other hand, has no significant or positron emission. It can be ideally followed by positron emission tomography–based imaging and dosimetry using 86Y as a surrogate tracer that has nearly identical chemical and biological properties as 90Y. However, 86Y is not readily available and has the disadvantage of high-energy -radiation emissions that require strict radiation protection. In practice, 111In is more frequently used as a surrogate isotope for imaging prior to 90Y therapy, though 111In itself does not display identical chemical and biological behavior to 90Y. These differences are minimal as long as the isotopes remain bound to the antibody, but become more concerning if the radioisotope is released in free form into the patient's body.
The currently approved RIT reagents provide therapeutic radiation effects via β-radiation emissions, that is, electrons emitted by 131I and 90Y. A peculiarity of β-radiation is the fact that a large spectrum of different energies is emitted. It is therefore not adequate to consider only the maximal electron energy that is emitted by a given radioisotope, though this maximal energy is physically characteristic. The maximal electron energy emitted by 90Y and 131I is high and intermediate, respectively, but both radioisotopes also emit electrons of lower energy.
The influence of β-radiation energy on tissue absorption may be minimal in large, clinically measurable tumors, as long as all cells are accessible to antibodies. This can be assumed to be the case in lymphomas and even more so in leukemias. However, for treatment of small tumor nodules, the energy range of the selected radioisotope can be more critical. Small tumor nodules of 10 mg in mass would absorb only 23% of medium-energy electrons emitted by 90Y but 77% of those emitted by 131I [98, 99]. Absorbed energies in even smaller nodules would be less for 90Y and differences between the β-emitters even more marked. Thus, in small cell clusters of 0.1 mm diameter that might still represent hundreds of cells, absorbed fractions of β-radiation emitted by 90Y and 177Lu would be as low as 1% and 9%, respectively. For such small nodules, the short tissue path length of particles (30–80 µm) would be much more desirable (Fig. 1). This would translate, in a cell-rich tissue, to an -radiation tissue range of two to four cells. The absence of vascularization in very small tumor nodules might, however, still be another parameter modulating accessibility for antibodies and treatment efficacy.
In conclusion, 131I and 90Y each have their particular advantages and disadvantages as radiolabels for RIT. Whether very small residual lymphoma cell burdens require RIT or could be efficiently treated by native antibodies alone or together with cytokines remains to be demonstrated.
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PERSPECTIVE |
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We hypothesize that the combination of these three treatments is most likely to optimize clinical benefit, because rituximab and RIT are both clearly distinct treatments with additive efficacy in combination with chemotherapy. However, RIT might benefit from substitution of an optimal low-energy β-radiation emitter, such as 177Lu, possibly in repetitive cycles of RIT. New developments with two- and three-step targeting might further augment the tumor radiation dose while limiting the radiation exposure of normal tissues. Unlabeled antibody treatment with rituximab could possibly be combined favorably with other antibodies directed against other tumor antigens or combined with cytokines to further improve cellular immune defenses. Such an optimized triple-therapy approach in the upfront setting based on optimal chemotherapy, full-dose biological treatment, and targeted internal radiation therapy with radiolabeled antibodies would currently appear to offer the most promising and potentially curative approach for advanced-stage follicular lymphoma.
Similar observations also seem to apply in mantle cell lymphoma (MCL). While an aggressive combination chemotherapy (HyperCVAD) with rituximab in the upfront setting resulted in a most promising failure-free survival time reported for MCL [101], no apparent survival plateau was observed in this trial. On the other hand, upfront treatment of MCL with aggressive induction chemotherapy plus rituximab followed by intensification with ASCT in responsive MCL patients [102–104] appears to show a plateau, suggesting that survival may be improved with such an approach.
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CONCLUSION |
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Multiple studies in NHL indicate that chemotherapy combined with rituximab or RIT yields superior results compared with chemotherapy alone. We argue that chemotherapy combined with both RIT and full-dose biological treatment has an even higher efficacy potential. The triple-therapy approach employing upfront chemotherapy combined with optimized RIT and extended biologic treatment with antibodies may represent the best chance for prolonged disease-free survival, and potential cure, keeping in reserve the possibility of intensification with ASCT or allografting for relapsed patients.
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AUTHOR CONTRIBUTIONS |
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Data analysis and interpretation: Franz Buchegger, Oliver W. Press, Angelika Bischof Delaloye, Nicolas Ketterer
Manuscript writing: Franz Buchegger, Oliver W. Press, Angelika Bischof Delaloye, Nicolas Ketterer
Final approval of manuscript: Franz Buchegger, Oliver W. Press, Angelika Bischof Delaloye, Nicolas Ketterer
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ACKNOWLEDGMENTS |
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TopLearning ObjectivesAbstractIntroductionAntibody-Based Biological...CytokinesPerspectiveConclusionAuthor ContributionsAcknowledgmentsReferences |
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REFERENCES |
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) with a bold line, that of blood activity is shown with a thin line. The second step involves injection of a biotinylated clearing agent (
) 20 hours after the antibody–streptavidin conjugate is injected, which binds to excess antibody–streptavidin in the circulation, resulting in first-pass hepatic clearance. A high tumor-to-blood ratio of the antibody–streptavidin conjugate is thus established (still without administration of any radioactive agent). On the right, the evolution of tumor uptake of antibody–streptavidin conjugate as a function of time (
, bold line) and the evolution of uptake in the blood (thin line) are shown in red. Note that the small radiolabeled biotin molecule rapidly localizes into the tumor while unbound radio-biotin is rapidly cleared through the kidneys. High tumor-to-blood and tumor-to-tissue radioactivity ratios are rapidly established and allow effective irradiation of tumor, while blood, bone marrow, and normal tissue irradiation remains low to moderate.