Ionizing radiation induces CD20 surface expression on human B cells

Prospects of Synergy: Local Interventions and CAR T Cell Therapy in Solid Tumors

Chimeric antigen receptor T cell therapy has been established in the treatment of various B cell malignancies. However, translating this therapeutic effect to treat solid tumors has been challenging because of their inter-tumoral as well as intratumoral heterogeneity and immunosuppressive microenvironment. Local interventions, such as surgery, radiotherapy, local ablation, and locoregional drug delivery, can enhance chimeric antigen receptor T cell therapy in solid tumors by improving tumor infiltration and reducing systemic toxicities. Additionally, ablation and radiotherapy have proven to (re-)activate systemic immune responses via abscopal effects and reprogram the tumor microenvironment on a physical, cellular, and chemical level. This review highlights the potential synergy of the combined approaches to overcome barriers of chimeric antigen receptor T cell therapy and summarizes recent studies that may pave the way for new treatment regimens.

CAR T cell-based immunotherapy and radiation therapy: potential, promises and risks

CAR T cell-based therapies have revolutionized the treatment of hematological malignancies such as leukemia and lymphoma within the last years. In contrast to the success in hematological cancers, the treatment of solid tumors with CAR T cells is still a major challenge in the field and attempts to overcome these hurdles have not been successful yet. Radiation therapy is used for management of various malignancies for decades and its therapeutic role ranges from local therapy to a priming agent in cancer immunotherapy. Combinations of radiation with immune checkpoint inhibitors have already proven successful in clinical trials. Therefore, a combination of radiation therapy may have the potential to overcome the current limitations of CAR T cell therapy in solid tumor entities. So far, only limited research was conducted in the area of CAR T cells and radiation. In this review we will discuss the potential and risks of such a combination in the treatment of cancer patients.

Infiltrating immune cells in prostate cancer tissue after androgen deprivation and radiotherapy

OBJECTIVES

Androgen deprivation therapy (ADT) has long been a cornerstone in treatment of advanced prostate cancer (PCa), and is known to improve the results of radiotherapy (RT) for high-risk disease. The purpose of our study was to use a multiplexed immunohistochemical (mIHC) approach to investigate the infiltration of immune cells in PCa tissue after eight weeks of ADT and/or RT with 10 Gy.

METHODS

From a cohort of 48 patients divided into two treatment arms, we obtained biopsies before and after treatment and used a mIHC method with multispectral imaging to analyze the infiltration of immune cells in tumor stroma and tumor epithelium, focusing on areas with high infiltration.

RESULTS

Tumor stroma showed a significantly higher infiltration of immune cells compared to tumor epithelium. The most prominent immune cells were CD20⁺ B-lymphocytes, followed by CD68⁺ macrophages, CD8⁺ cytotoxic T-cells, FOXP3⁺ regulatory T-cells (Tregs), and T-bet⁺ Th1-cells. Neoadjuvant ADT followed by RT significantly increased the infiltration of all five immune cells. Numbers of Th1-cells and Tregs significantly increased after single treatment with ADT or RT. In addition, ADT alone increased the number of cytotoxic T-cells and RT increased the number of B-cells.

CONCLUSIONS

Neoadjuvant ADT in combination with RT results in a higher inflammatory response compared to RT or ADT alone. The mIHC method may be a useful tool for investigating infiltrating immune cells in PCa biopsies to understand how immunotherapeutic approaches can be combined with current PCa therapies.

Radiotherapy induced immunogenic cell death by remodeling tumor immune microenvironment

Emerging evidence indicates that the induction of radiotherapy(RT) on the immunogenic cell death (ICD) is not only dependent on its direct cytotoxic effect, changes in the tumor immune microenvironment also play an important role in it. Tumor immune microenvironment (TIME) refers to the immune microenvironment that tumor cells exist, including tumor cells, inflammatory cells, immune cells, various signaling molecules and extracellular matrix. TIME has a barrier effect on the anti-tumor function of immune cells, which can inhibit all stages of anti-tumor immune response. The remodeling of TIME caused by RT may affect the degree of immunogenicity, and make it change from immunosuppressive phenotype to immunostimulatory phenotype. It is of great significance to reveal the causes of immune escape of tumor cells, especially for the treatment of drug-resistant tumor. In this review, we focus on the effect of RT on the TIME, the mechanism of RT in reversing the TIME to suppress intrinsic immunity, and the sensitization effect of the remodeling of TIME caused by RT on the effectiveness of immunotherapy.

Interaction between Fibroblasts and Immune Cells Following DNA Damage Induced by Ionizing Radiation

Cancer-associated fibroblasts (CAF) form the basis of tumor microenvironment and possess immunomodulatory functions by interacting with other cells surrounding tumor, including T lymphocytes, macrophages, dendritic cells and natural killer cells. Ionizing radiation is a broadly-used method in radiotherapy to target tumors. In mammalian cells, ionizing radiation induces various types of DNA damages and DNA damage response. Being unspecific, radiotherapy affects all the cells in tumor microenvironment, including the tumor itself, CAFs and immune cells. CAFs are extremely radio-resistant and do not initiate apoptosis even at high doses of radiation. However, following radiation, CAFs become senescent and produce a distinct combination of immunoregulatory molecules. Radiosensitivity of immune cells varies depending on the cell type due to inefficient DNA repair in, for example, monocytes and granulocytes. In this minireview, we are summarizing recent findings on the interaction between CAF, ionizing radiation and immune cells in the tumor microenvironment.

177Lu-Lilotomab Satetraxetan Has the Potential to Counteract Resistance to Rituximab in Non-Hodgkin Lymphoma

Patients with non-Hodgkin lymphoma (NHL) who are treated with rituximab may develop resistant disease, often associated with changes in expression of CD20. The next-generation β-particle–emitting radioimmunoconjugate ¹⁷⁷Lu-lilotomab-satetraxetan (Betalutin) was shown to up-regulate CD20 expression in different rituximab-sensitive NHL cell lines and to act synergistically with rituximab in a rituximab-sensitive NHL animal model. We hypothesized that ¹⁷⁷Lu-lilotomab-satetraxetan may be used to reverse rituximab resistance in NHL. Methods: The rituximab-resistant Raji2R and the parental Raji cell lines were used. CD20 expression was measured by flow cytometry. Antibody-dependent cellular cytotoxicity (ADCC) was measured by a bioluminescence reporter assay. The efficacies of combined treatments with ¹⁷⁷Lu-lilotomab-satetraxetan (150 or 350 MBq/kg) and rituximab (4 × 10 mg/kg) were compared with those of single agents or phosphate-buffered saline in a Raji2R-xenograft model. Cox regression and the Bliss independence model were used to assess synergism. Results: Rituximab binding in Raji2R cells was 36% ± 5% of that in the rituximab-sensitive Raji cells. ¹⁷⁷Lu-lilotomab-satetraxetan treatment of Raji2R cells increased the binding to 53% ± 3% of the parental cell line. Rituximab ADCC induction in Raji2R cells was 20% ± 2% of that induced in Raji cells, whereas treatment with ¹⁷⁷Lu-lilotomab-satetraxetan increased the ADCC induction to 30% ± 3% of that in Raji cells, representing a 50% increase (P < 0.05). The combination of rituximab with 350 MBq/kg ¹⁷⁷Lu-lilotomab-satetraxetan synergistically suppressed Raji2R tumor growth in athymic Foxn1^nu mice. Conclusion:¹⁷⁷Lu-lilotomab-satetraxetan has the potential to reverse rituximab resistance; it can increase rituximab binding and ADCC activity in vitro and can synergistically improve antitumor efficacy in vivo.

Radiation, inflammation and the immune response in cancer

Radiation is an important component of cancer treatment with more than half of all patients receive radiotherapy during their cancer experience. While the impact of radiation on tumour morphology is routinely examined in the pre-clinical and clinical setting, the impact of radiation on the tumour microenvironment and more specifically the inflammatory/immune response is less well characterised. Inflammation is a key contributor to short- and long-term cancer eradication, with significant tumour and normal tissue consequences. Therefore, the role of radiation in modulating the inflammatory response is highly topical given the current wave of targeted and immuno-therapeutic treatments for cancer. This review provides a general overview of how radiation modulates the inflammatory and immune response—(i) how radiation induces the inflammatory/immune system, (ii) the cellular changes that take place, (iii) how radiation dose delivery affects the immune response, and (iv) a discussion on research directions to improve patient survival, reduce side effects, improve quality of life, and reduce financial costs in the immediate future. Harnessing the benefits of radiation on the immune response will enhance its maximal therapeutic benefit and reduce radiation-induced toxicity.

Abscopal effects of radiotherapy and combined mRNA-based immunotherapy in a syngeneic, OVA-expressing thymoma mouse model

BACKGROUND

Tumor metastasis and immune evasion present major challenges of cancer treatment. Radiotherapy can overcome immunosuppressive tumor microenvironments. Anecdotal reports suggest abscopal anti-tumor immune responses. This study assesses abscopal effects of radiotherapy in combination with mRNA-based cancer vaccination (RNActive®).

METHODS

C57BL/6 mice were injected with ovalbumin-expressing thymoma cells into the right hind leg (primary tumor) and left flank (secondary tumor) with a delay of 4 days. Primary tumors were irradiated with 3 × 2 Gy, while secondary tumors were shielded. RNA and combined treatment groups received mRNA-based RNActive® vaccination.

RESULTS

Radiotherapy and combined radioimmunotherapy significantly delayed primary tumor growth with a tumor control in 15 and 53% of mice, respectively. In small secondary tumors, radioimmunotherapy significantly slowed growth rate compared to vaccination (p = 0.002) and control groups (p = 0.01). Cytokine microarray analysis of secondary tumors showed changes in the cytokine microenvironment, even in the non-irradiated contralateral tumors after combination treatment.

CONCLUSION

Combined irradiation and immunotherapy is able to induce abscopal responses, even with low, normofractionated radiation doses. Thus, the combination of mRNA-based vaccination with irradiation might be an effective regimen to induce systemic anti-tumor immunity.

Development of Novel Anti-Cd20 Monoclonal Antibodies and Modulation in Cd20 Levels on Cell Surface: Looking to Improve Immunotherapy Response

Rituximab has been revolutionized and validated CD20 targeting monoclonal antibody. Although, it is widely used for lymphoma therapy and many patients have been benefited. However significant numbers of patients are refractory or developed resistance to current therapies due to low level of CD20 expression and/or availability on cells surface. Thus development of novel anti-CD20 mAbs with great cell killing ability and enhance CD20 levels on cell surface can potentially exploit lymphoma therapy. In this scenario, we are summarizing the recently developed mAbs against CD20 and compounds that have ability to induce CD20 expression at significant level. We also are providing information regarding combination strategy for use of radiation and anti-CD20 mAbs in vitro. However, it will need to be determined by rigorous at pre-clinical and clinic testing. We hope this review will be beneficial for current research in the area of immunotherapy or radio-immunotherapy.

Surface levels of CD20 determine anti-CD20 antibodies mediated cell death in vitro

Background

The sensitivity of human Burkitt's lymphoma cells to rituximab (Rtx) and tositumomab (Tst) was assessed on cells expressing different levels of CD20 on surface. Cells that harbor low CD20 levels may resists against therapeutics response to CD20-specific antibodies. We postulated that, radiation-induced modulation of CD20 surface levels may play a crucial and central role in determining the relative efficacy of rituximab and tositumomab in treating Burkitt's lymphoma disease. Here, we examined the γ-radiation-induced CD20 expression in the Burkitt lymphoma cell line ‘Daudi’ and the relation of differential levels of CD20 with anti-CD20 mAbs mediated cell death.

Methodology

In this study we examined kinetics of CD20 expression following sub lethal doses ofγ-radiation to Daudi cells and thereafter anti-CD20 mAbs (rituximab and tositumomab) were added in cell suspensions. The correlation of kinetics of CD20 expression and cells treated with anti-CD20 mAbs/or corresponding isotype Abs with special reference to changes in mitochondrial membrane potential and reactive oxygen species generation was also examined. Further, we also investigated the efficacy of anti-CD20 mAbs and possible induction of cell death in relation to levels of CD20 cell surface expression.

Conclusion

This report provides evidence that CD20 expression can be induced by exposure of cells to γ-radiation. In addition, these findings demonstrated that the efficacy of anti-CD20 mAbs is dependent on the surface levels of CD20. Based on these findings, we hypothesized (i) irradiation just prior to immunotherapy may provide new treatment options even in aggressive B cell tumors, which are resistant to current therapies in vivo (ii) The efficacy of induction of apoptosis varies with type of monoclonal antibodies in vitro.

Combination strategies to enhance antitumor ADCC

The clinical efficacy of monoclonal antibodies as cancer therapeutics is largely dependent upon their ability to target the tumor and induce a functional antitumor immune response. This two-step process of ADCC utilizes the response of innate immune cells to provide antitumor cytotoxicity triggered by the interaction of the Fc portion of the antibody with the Fc receptor on the immune cell. Immunotherapeutics that target NK cells, γδ T cells, macrophages and dendritic cells can, by augmenting the function of the immune response, enhance the antitumor activity of the antibodies. Advantages of such combination strategies include: the application to multiple existing antibodies (even across multiple diseases), the feasibility (from a regulatory perspective) of combining with previously approved agents and the assurance (to physicians and trial participants) that one of the ingredients - the antitumor antibody - has proven efficacy on its own. Here we discuss current strategies, including biologic rationale and clinical results, which enhance ADCC in the following ways: strategies that increase total target-monoclonal antibody-effector binding, strategies that trigger effector cell 'activating' signals and strategies that block effector cell 'inhibitory' signals.

Treatment strategies in limited stage follicular NHL

Limited stage (I-II) follicular lymphoma is an uncommon entity, since most patients with this disease have generalized adenopathy (stage III) or bone marrow involvement (stage IV). Although patients who present with stage III-IV disease often are considered to be incurable, ~50% of patients with limited disease will enjoy long-term freedom-from progression, usually following treatment with radiation therapy. Relapse among these patients is uncommon after 10 years and exceedingly rare after 15 years. Radiation treatment is generally restricted to the involved nodal region(s) with modest (~5 cm.) extension proximally and distally. Radiation dose is generally 30 Gy, but may be boosted slightly (36 Gy total) in the presence of bulky disease. Randomized clinical trials have been insufficiently powered to define the value of any additional treatment beyond radiation therapy, although single arm studies suggest a benefit to the addition of chemotherapy. There have been no reported experiences with chemo-immunotherapy or radioimmunotherapy. Patients should be monitored during follow up to identify transformation to a more aggressive lymphoma.

Bortezomib modulates surface CD20 in B-cell malignancies and affects rituximab-mediated complement-dependent cytotoxicity

Unresponsiveness to rituximab treatment develops in many patients prompting elucidation of underlying molecular pathways. It was recently observed that rituximab-resistant lymphoma cells exhibit up-regulation of components of the ubiquitin-proteasome system (UPS). Therefore, we investigated in more detail the role of this system in the regulation of CD20 levels and the influence of proteasome inhibitors on rituximab-mediated complement-dependent cytotoxicity (R-CDC). We observed that incubation of Raji cells with rituximab leads to increased levels of ubiquitinated CD20. However, inhibition of the UPS was not associated with up-regulation of surface CD20 levels, although it significantly increased its ubiquitination. Short-term (24 hours) incubation of Raji cells with 10 or 20nM bortezomib did not change surface CD20 levels, but sensitized CD20+ lymphoma cells to R-CDC. Prolonged (48 hours) incubation with 20nM bortezomib, or incubation with 50nM bortezomib for 24 hours led to a significant decrease in surface CD20 levels as well as R-CDC. These effects were partly reversed by bafilomycin A1, an inhibitor of lysosomal/autophagosomal pathway of protein degradation. These studies indicate that CD20 levels are regulated by 2 proteolytic systems and that the use of proteasome inhibitors may be associated with unexpected negative influence on R-CDC.

Report from the Radiation Therapy Committee of the Southwest Oncology Group (SWOG): Research Objectives Workshop 2008

Strategic planning for the Radiation Therapy Committee of the Southwest Oncology Group (SWOG) is comprehensively evaluated every six years in an effort to maintain a current and relevant scientific focus, and to provide a standard platform for future development of protocol concepts. Participants in the 2008 Strategic Planning Workshop included clinical trial experts from multiple specialties, industry representatives from both pharmaceuticals and equipment manufacturers, and basic scientists. High-priority research areas such as image-guided radiation therapy for control of limited metastatic disease, analysis of biomarkers for treatment response and late toxicity, assessment of novel agents in combination with radiation, standardization of radiation target delineation, and the assessment of new imaging techniques to individualize cancer therapy, were discussed. Research priorities included clinical study designs featuring translational end points that identify patients most likely to benefit from combined modality therapy; intervention including combination radiation with standard chemotherapy; radiation with radiosensitizing molecular-targeted therapies; and stereotactic radiation for treatment of patients with regard to asymptomatic metastasis and radiation-induced tumor autoimmunity. The Committee concluded that the future research opportunities are among the most exciting to have developed in the last decade, and work is in progress to embark on these plans.

Yttrium-90-ibritumomab tiuxetan in combination with intravenous busulfan, cyclophosphamide, and etoposide followed by autologous stem cell transplantation in patients with relapsed or refractory B-cell non-Hodgkin's lymphoma

BACKGROUND

Radiolabelled immunotherapy agents have an increasingly significant role in autologous stem cell transplantation (ASCT) by improving the tolerability and increasing the efficacy of the conditioning regimen, thereby reducing the relapse risk. We evaluated the efficacy and safety of yttrium-90-ibritumomab tiuxetan ((90)Y-ibritumomab) combined with intravenous busulfan, cyclophosphamide, and etoposide (Bu/Cy/E) followed by ASCT in patients with relapsed or refractory B-cell non-Hodgkin's lymphoma (NHL).

METHODS

Each patient received a single dose of (90)Y-ibritumomab (0.4 mCi/kg on day -14) with Bu/Cy/E as a conditioning regimen.

RESULTS

The patient cohort consisted of 19 individuals (ten males), of median age 51 years (range, 25-63 years). Sixteen patients had received two or more chemotherapy regimens before transplantation. Histologies were diffuse large B-cell (n = 14), follicular (n = 2), mantle cell (n = 2), and Burkitt lymphoma (n = 1). All patients engrafted. The median time to neutrophil engraftment was 10 days and time to platelet engraftment was 10 days. Nineteen patients were evaluable for response. The objective overall response rate was 84.2% (16/19): continued CR, 36.8% (7/19); induced CR, 36.8% (7/19); and PR, 10.5% (2/19). With a median follow-up of 29.4 months (13.4-36.6), the estimated 3-year overall survival and event-free survival rates were 52.6% (95% confidence interval [CI] 45.8-59.4) and 26.3% (95% CI 19.8-32.8), respectively. Adverse events were similar to those seen historically with Bu/Cy/E alone, and there were no treatment related deaths.

CONCLUSION

In conclusion, (90)Y-ibritumomab with Bu/Cy/E and ASCT is feasible in patients with relapsed or refractory B-cell NHL, without increased toxicity.

Mechanisms involved in radiation enhancement of intratumoral dendritic cell therapy

We have previously reported that local tumor irradiation, without inducing cell death, can augment the therapeutic efficacy of intratumoral (IT) dendritic cell (DC) vaccination. This study examined potential mechanisms underlying radiation enhancement of IT DC therapy in this setting. Even though ionizing radiation did not mediate tumor cell killing, bone marrow-derived DCs acquired in vitro tumor antigens from irradiated D5 murine melanoma cells more efficiently than from untreated cells. This radiation-enhanced loading of DCs did not induce DC maturation, but was associated with improved cross-priming of T cells both in vitro and in vivo. Furthermore, in vivo pulsing of DCs with irradiated versus untreated tumor cells resulted in superior presentation of tumor antigens to T cells. In addition, tumor irradiation facilitated homing of IT administered DCs to the draining lymph node, possibly by down-regulating CCL21 expression within the tumor mass. Studies of the tumor microenvironment in irradiated versus untreated tumors did not reveal significant inflammatory changes. Moreover, radiation did not promote accumulation of CD4 or CD8 effector T cells within solid tumors. Our results indicate that, without inducing cytotoxicity, tumor irradiation can enhance the ability of DCs to capture tumor antigens, migrate to the draining lymph node, and present processed antigens to T cells. These findings may prove useful in designing future strategies for human cancer immunotherapy.

Regulation of CD20 expression by radiation-induced changes in intracellular redox status

Increasing the levels of CD20 expression in cells that harbor low CD20 levels may enhance their responsiveness to CD20-specific antibody therapies. Here, we examined the regulation of CD20 expression after treatment with 0.5-2.0 Gy X-irradiation and hydrogen peroxide (H(2)O(2)), in the presence or absence of known antioxidants, in the Burkitt lymphoma cell lines Daudi and Raji. Irradiation of cells enhanced cell-surface CD20 expression; the kinetics and extent of this change were cell-type specific and time-dependent. The kinetics of reactive oxygen species generation and changes in mitochondrial membrane potential after irradiation were also correlated with changes in CD20 expression. Raji and Daudi cells treated with H(2)O(2) showed a 2-to 2.5-fold increase in CD20 expression at 12 and 20 h, respectively. Buthionine sulfoximine, which depletes glutathione, also increased surface CD20, whereas antioxidants, such as PEG-catalase, PEG-SOD, vitamin C, and amifostine, decreased CD20 expression induced by radiation or H(2)O(2). The antioxidant-mediated decrease in CD20 expression induced by radiation or H(2)O(2) suggests a mechanism involving redox regulation. These results demonstrate the critical role of radiation-induced oxidative stress in CD20 expression and may have implications for defining and improving the efficacy of CD20-targeted antibody therapy and radioimmunotherapy.

Systemic targeted radionuclide therapy: potential new areas

Radiation oncology is entering an exciting new era with therapies being delivered in a targeted fashion through an increasing number of novel approaches. External beam radiotherapy now integrates functional and anatomic tumor imaging to guide delivery of conformal radiation to the tumor target. Systemic targeted radionuclide therapy (STaRT) adds an important new dimension by making available to the radiation oncologist biologically targeted radiation therapy. Impressive clinical results with antibody-targeted radiotherapy, leading to the Food and Drug Administration's approval of two anti-CD20 radiolabeled antibodies, highlight the potential of STaRT. Optimization strategies will further improve the efficacy of STaRT by improving delivery systems, modifying the tumor microenvironment to increase targeted dose, and maximizing dose effect. Ultimately, the greatest potential for STaRT will not be as monotherapy, but as therapy integrated into established multimodality regimens and used as adjuvant or consolidative therapy in patients with minimal or micrometastatic disease.

Rituximab enhances radiation-triggered apoptosis in non-Hodgkin's lymphoma cells via caspase-dependent and - independent mechanisms

Rituximab (RTX), a chimeric human anti-CD20 monoclonal antibody, is currently employed in the treatment of malignant non-Hodgkin's lymphoma (NHL) either alone or in combination with other cytotoxic approaches. The present study examines the effects of ionizing radiation in combination with RTX on proliferation and apoptosis development in B-lymphoma RL and Raji cells. RTX was used at a concentration of 10 microg/mL 24 hours prior to irradiation at a single dose of 9 Gy. CD20 expression, cell viability, apoptosis, mitochondrial membrane potential and apoptosis-related proteins were evaluated in the treated B cells. The constitutive level of CD20 expression in RL and Raji lymphoma cells did not play an essential role in RTX-induced cell growth delay. Both lymphoma cells showed similar inhibition of cell proliferation without apoptosis development in response to RTX treatment. Exposure to ionizing radiation induced cell growth delay and apoptosis in RL cells, whereas Raji cells showed moderate radio-resistance and activation of cell growth at 24 hours after irradiation, which was accompanied by increased radiation-triggered CD20 expression. The simultaneous exposure of lymphoma cells to ionizing radiation and RTX abrogated radioresistance of Raji cells and significantly enhanced cell growth delay and apoptosis in RL cells. X-linked inhibitor of apoptosis protein (XIAP) and the inducible form of heat shock protein 70 (Hsp70) were positively modulated by RTX in combination with ionizing radiation in order to induce apoptosis. Furthermore, it was demonstrated that mitochondrial membrane potential dissipation is not an essential component to induce apoptosis-inducing factor (AIF) maturation and apoptosis. Our results show that RTX-triggered enhancement of radiation-induced apoptosis and cell growth delay is achieved by modulation of proteins involved in programmed cell death.

Pretreatment with rituximab enhances radiosensitivity of non-Hodgkin's lymphoma cells

The present study examines the effects of ionizing radiation in combination with rituximab (RTX), a chimeric human anti-CD20 monoclonal antibody, on proliferation, cell cycle distribution and apoptosis in B-lymphoma RL and Raji cells. Exposure to ionizing radiation (9 Gy) induced cell growth delay and apoptosis in RL cells, whereas Raji cells showed moderate radio-resistance. The simultaneous exposure of lymphoma cells to ionizing radiation and RTX (10 microg/mL) markedly enhanced apoptosis and cell growth delay in RL and Raji cells. Cooperative antiproliferative and apoptotic effects of RTX and radiation were achieved through the inhibition of c-myc and bcl-XL expression. Furthermore, RTX-modulated expression of cell cycle regulating proteins, such as p53, p21/WAF1, p27/KIP1, contributed to the development of radiation-induced cell killing and growth arrest. Each NHL cell line that underwent apoptosis induced by combination treatment revealed enhanced caspase-3 and poly (ADP-ribose) polymerase (PARP) cleavage as compared to only irradiated cells. These findings show that rituximab synergistically enhances radiation-induced apoptosis and cell growth delay through the expression of proteins involved in the programmed cell death and cell cycle regulation pathways.

Cellular and molecular signal transduction pathways modulated by rituximab (rituxan, anti-CD20 mAb) in non-Hodgkin's lymphoma: implications in chemosensitization and therapeutic intervention

The clinical application of rituximab (chimeric mouse anti-human CD20 mAb, Rituxan, IDEC-C2B8), alone and/or combined with chemotherapy, has significantly ameliorated the treatment outcome of patients with relapsed and refractory low-grade or follicular non-Hodgkin's lymphoma (NHL). The exact in vivo mechanisms of action of rituximab are not fully understood, although antibody-dependent cell-mediated cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), and apoptosis have been suggested. We have proposed that modifications of the cellular signaling pathways by rituximab may be crucial for its clinical response. The B-cell restricted cell surface phosphoprotein CD20 is involved in many cellular signaling events including proliferation, activation, differentiation, and apoptosis upon crosslinking. Monomeric rituximab chemosensitizes drug-resistant NHL cells via selective downregulation of antiapoptotic factors through the type II mitochondrial apoptotic pathway. Several signaling pathways are affected by rituximab which are implicated in the underlying molecular mechanisms of chemosensitization. ARL (acquired immunodeficiency syndrome (AIDS)-related lymphoma) and non-ARL cell lines have been examined as in vitro model systems. In ARL, rituximab diminishes the activity of the p38MAPK signaling pathway resulting in inhibition of the interleukin (IL)-10/IL-10R autocrine/paracrine cytokine autoregulatory loop leading to the inhibition of constitutive STAT-3 activity and subsequent downregulation of Bcl-2 expression leading to chemosensitization. Rituximab upregulates Raf-1 kinase inhibitor protein (RKIP) expression in non-ARL cells. Through physical association with Raf-1 and nuclear factor kappaB (NF-kappa B)-inducing kinase (NIK), RKIP negatively regulates two major survival pathways, namely, the extracellular signal-regulated kinase1/2 (ERK1/2) and the NF-kappa B pathways, respectively. Downmodulation of the ERK1/2 and NF-kappa B pathways inhibits the transcriptional activity of AP-1 and NF-kappa B transcription factors, respectively, both of which lead to the downregulation of Bcl-(xL) (Bcl-2 related gene (long alternatively spliced variant of Bcl-x gene)) transcription and expression and sensitization to drug-induced apoptosis. Bcl-(xL)-overexpressing cells corroborated the pivotal role of Bcl-(xL) in chemosensitization. The specificity of rituximab-mediated signaling and functional effects were corroborated by the use of specific pharmacological inhibitors. Many patients do not respond and/or relapse and the mechanisms of unresponsiveness are unknown. Rituximab-resistant B-NHL clones were generated to investigate the acquired resistance to rituximab-mediated signaling, and chemosensitization. Resistant clones display different phenotypic, genetic and functional properties compared to wild-type cells. This review summarizes the data highlighting a novel role of rituximab as a signal-inducing antibody and as a chemosensitizing agent through negative regulation of major survival pathways. Studies presented herein also reveal several intracellular targets modified by rituximab, which can be exploited for therapeutic and prognostic purposes in the treatment of patients with rituximab- and drug-refractory NHL.

Efficient elimination of B-lineage lymphomas by anti-CD20-auristatin conjugates

The anti-CD20 antibody rituximab is useful in the treatment of certain B-cell malignancies, most notably non-Hodgkin's lymphoma. Its efficacy has been increased when used in combination with chemotherapy, yet anti-CD20 monoclonal antibodies (mAbs) directly conjugated with drugs such as doxorubicin (Dox) have failed to deliver drug or to demonstrate antitumor activity. We have produced anti-CD20 antibody-drug conjugates that possess potent antitumor activity by using the anti-mitotic agent, monomethyl auristatin E (MMAE), linked via the lysosomally cleavable dipeptide, valine-citrulline (vc). Two anti-CD20 conjugates, rituximab-vcMMAE and 1F5-vcMMAE, were selectively cytotoxic against CD20(+) B-lymphoma cell lines, with IC(50) values ranging from 50 ng/mL to 1 microg/mL. Unlike rituximab, which showed diffuse surface localization, rituximab-vcMMAE capped and was internalized within 4 hours after binding to CD20(+) B cells. Internalization of rituximab-vcMMAE was followed by rapid G(2)-M phase arrest and onset of apoptosis. Anti-CD20 antibody-drug conjugates prepared with Dox were internalized and localized as with rituximab-vcMMAE, yet these were not effective for drug delivery (IC(50) > 50 microg/mL). Consistent with in vitro activity, rituximab-vcMMAE showed antitumor efficacy in xenograft models of CD20-positive lymphoma at doses where rituximab or rituximab-Dox conjugates were ineffective. These data indicate that anti-CD20-based antibody-drug conjugates are effective antitumor agents when prepared with a stable, enzyme-cleavable peptide linkage to highly potent cytotoxic agents such as MMAE.

Radiobiology of radioimmunotherapy: targeting CD20 B-cell antigen in non-Hodgkin's lymphoma

The radiobiology of radioimmunotherapy is an important determinant of both the toxicity and the efficacy associated with the treatment of B-cell non-Hodgkin's lymphoma with radiolabeled anti-CD20 monoclonal antibodies. The properties of the target, CD20, and the mechanisms of action of both the monoclonal antibodies and the associated exponentially decreasing low-dose-rate radiotherapy are described. The radiation dose and dose-rate effects are discussed and related to both the tumor responses and normal organ toxicity. Finally, the use of either unlabeled or radiolabeled anti-CD20 monoclonal antibodies as a component of combined modality therapy (including the sequential or concurrent use of sensitizers) and future directions of the field are discussed.

Radiobiology of radioimmunotherapy with 90Y ibritumomab tiuxetan (zevalin)

Radioimmunotherapy represents a significant advance over unlabeled immunotherapy for the treatment of patients with B-cell non-Hodgkin's lymphoma. The efficacy of radioimmunotherapeutic agents depends in large part on the basic biological effects associated with their components, monoclonal antibodies and radionuclides, separately and in combination. The radiobiological effects associated with yttrium 90-labeled ibritumomab tiuxetan (Zevalin; Biogen Idec Inc, Cambridge, MA) include the induction of apoptosis and cell-cycle redistribution (eg, arrest of cells in the G(2)/M phase of the cell cycle). Because of dose-rate effects, tumor cells may, in some cases, be more susceptible to the low-dose-rate radiation used in radioimmunotherapy than to the high-dose-rate radiation used in external beam radiotherapy. The efficacy of radioimmunotherapy may potentially be optimized through a variety of approaches, including the use of agents that increase the expression of certain tumor antigens (thus facilitating improved biodistribution of radiolabeled monoclonal antibodies) or that sensitize tumor cells to radiation.