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Novel Antibody-Based Therapeutic Agents Targeting CD70: A Potential Approach for Treating Waldenström's Macroglobulinemia
Novel Antibody-Based Therapeutic Agents Targeting CD70: A Potential Approach for Treating Waldenström’s Macroglobulinemia Che-Leung Law, Julie A. McEarchern, Iqbal S. Grewal
Abstract Targeting leukocyte differentiation antigens is a validated approach to develop therapeutic agents for the treatment of cancer, autoimmunity, and inflammatory diseases. A subset of activation antigens transiently induced on leukocytes is particularly interesting because many of them are absent from normal tissues, including those of most vital organs, and therapeutic agents’ targeting of such antigens is expected to impart limited toxicity. One such antigen, CD70, has recently emerged as an attractive potential drug target for the treatment of cancers. Whereas CD70 is only transiently expressed on activation T and B cells and mature dendritic cells, it is found to be aberrantly expressed on a variety of tumor cells, including Waldenström's macroglobulinemia. In this report, we discuss potential antibody-based therapeutic approaches targeting CD70 for tumor elimination where various mechanisms such as antibody effector functions, immune enhancement, blockade of paracrine growth loop, and delivery of cytotoxic payloads can be exploited to achieve efficacy. Indeed, early clinical trials with therapeutic anti-CD70 antibodies are currently in progress, and those for anti-CD70 drug conjugates will soon follow. Clinical Lymphoma & Myeloma, Vol. 9, No. 1, 90-93, 2009; DOI: 10.3816/CLM.2009.n.024 Keywords: Antibody-dependent cell-mediated cytotoxicity, Drug conjugate, Monoclonal antibodies, Paracrine growth loop, T cells, Waldenström
Introduction More than 20 monoclonal antibody (MoAb)–based therapeutic agents have been approved. About half of them are directed against cell surface receptors restricted to hematopoietic cells.1 The MoAbs rituximab and alemtuzumab and the immunoconjugates gemtuzumab ozogamicin, ibritumomab tiuxetan, and iodine 131 tositumomab targeting the leukocyte differentiation antigens (LDAs) CD20, CD52, or CD33, respectively, provide significant clinical benefits in patients with non-Hodgkin lymphoma (NHL), chronic lymphocytic leukemia, or acute myeloid leukemia. The success in targeting LDAs can be partly explained by the fact that most of them are not expressed to any detectable level in normal nonhematopoietic cells of vital organs. As such, minimal on-target toxic effects to nonhematopoietic tissues are expected from MoAbbased therapeutic agents targeting LDAs. It is therefore likely that efforts to further evaluate and interrogate other cell surface receptors restricted to leukocytes might yield additional drug targets. In this respect, we have evaluated the potential of targeting CD70.2 We have developed a humanized anti-CD70 antibody, Department of Preclinical Therapeutics, Seattle Genetics, Inc, Bothell, WA Meeting date: October 15-19, 2008; Published date: March 18, 2009 Address for correspondence: Iqbal S. Grewal, PhD, FRCPath, Seattle Genetics, Inc, 21823 30th Dr SE, Bothell, WA 98021 Fax: 425-527-4119; e-mail: [email protected]
SGN-70, capable of mediating antibody effector functions and eliciting significant antitumor activity in preclinical cancer xenograft models.3,4 Additionally, we have conjugated this antibody to an auristatin-based cytotoxic drug MMAF, SGN-75, for delivering MMAF to CD70+ malignant cells.5,6 SGN-75 exhibits enhanced antitumor activity in solid tumor xenograft models compared to SGN-70, confirming increased therapeutic efficacy through cytotoxic drug delivery. Thus, preclinical animal models have provided strong evidence that targeting CD70 either with unconjugated antibodies or with antibody-drug conjugates represents a promising approach to treat human malignancies. Optimized unconjugated anti-CD70 antibodies as well as antibodydrug conjugates to target CD70-expressing cancer cells are heading toward their clinical application.
CD70: Expression and Functions CD70 (TNFSF7) is a member of the TNF superfamily. Its expression is tightly regulated, and it has not been reported on any normal nonhematopoietic cells so far. Within the hematopoietic lineages, CD70 is only transiently induced on activated B and T cells upon antigen receptor stimulation as well as on mature dendritic cells (DCs).7,8 The only receptor that has been defined for CD70 is CD27 (TNFRSF7), a type I integral membrane protein of the TNF receptor superfamily. Whereas CD70 is a typical activation marker, CD27 is expressed on resting mature T and memory B cells, germinal center B cells, and natural killer (NK) cells.7,8
This article might include the discussion of investigational and/or unlabeled uses of drugs and/or devices that might not be approved by the FDA. Electronic forwarding or copying is a violation of US and international copyright laws. Authorization to photocopy items for internal or personal use, or the internal or personal use of specific clients, is granted by CIG Media Group, LP, ISSN #1557-9190, provided the appropriate fee is paid directly to Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923 USA. www.copyright.com 978-750-8400.
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Figure 1 CD70 Pathway in Normal Immune Response and Targeting CD70 for Cancer Therapy Ag-primed T cell
A
Effector and memory cell differentiation
Activated T, B, NK cells, Mature DC
Memory B cell Plasma cell differentiation
CD27
CD70
CD70 as a Tumor-Associated Antigen Besides being an activation marker, CD70 is also expressed by B- and T-lineage tumors, including diffuse large B-cell lymphoma,14 follicular lymphoma, B-cell lymphocytic leukemia, Burkitt lymphoma, mantle cell lymphomas, adult T lymphoma,15 Waldenström’s macroglobulinemia (WM),16 Hodgkin lymphoma Reed-Sternberg cells,17 and multiple myeloma (MM).3,4 More interestingly, aberrant CD70 expression is observed on transformed cells of epithelial and neural origins, including renal cell,5 nasopharynx,18 ovary, colon and thymus carcniomas19, as well as glioblastoma20,21 and astrocytoma.21 The presence of CD70 on lymphoid cancers might reflect the transformation of lymphocytes arrested at specific maturational stages during which CD70 is transiently induced. Transforming viruses like the human T leukemia virus-1 (HTLV-1)21 and the Epstein-Barr virus18 might regulate CD70 expression on ATL, HL, and nasopharyngeal carcinoma, respectively. However, the mechanistic basis for aberrant CD70 expression on transformed nonhematopoietic cells in the absence of underlying viral infection remains to be elucidated. Defective extracellular signal-regulated kinase (ERK) signaling and DNA methylation have been correlated to CD70 overexpression on T cells in lupus-prone mice and in lupus patients.22 It would therefore be interesting to examine the methylation status of the CD70 gene locus in transformed cells. CD70 expression on transformed cells might constitute an escape strategy against immune surveillance. Thus, clear-cell renal cell carcinoma (RCC)23 and glioblastoma20,21 mediate T-cell apoptosis in a CD70-dependent manner. In RCC, activation of SIVA, a CD27binding signaling intermediate, correlates with RCC-mediated T-cell apoptosis.23 The apoptosis-inducing activity of glioblastoma cells has also been correlated to their ability to release gangliosides GM2 and GD1.20 Induction of regulatory T cell (Treg) differentiation by CD70+ NHL B cells24 and a paracrine growth loop involving CD70 in WM16 are additional mechanisms by which CD70 can contribute to cancer development. It is, however, important to realize that CD70 can also stimulate long-lasting antitumor immunity.25 Because of these contrasting functions, more investigations are clearly needed to determine how immune surveillance and evasion are regulated by CD70 expressed on neoplastic cells.
Targeting CD70 as a Therapeutic Approach for the Treatment of Waldenström’s Macroglobulinemia: Mechanistic Possibilities Current treatment options for WM include alkylating agents, nucleoside analogues, or rituximab. Strategies combining alkylating
NK cell NK cell– mediated cytotoxicity
B
Cytotoxic Drug Delivery
Antibody Effector Functions
CD70 + cancer cell WM cell
X
+
CD70 + NHL B cell
X
Interaction between CD70 and CD27 constitutes an important co-stimulatory pathway in adaptive and innate immunity (Figure 1A), as genetic manipulation of CD70 and CD27 expression in mice results in profound alterations in immune functions.9-11 In the T lineage, CD70-CD27 interaction is required for efficient T-cell priming, survival, and expansion of antigen-stimulated T cells, and the differentiation of effector and memory T cells.7,8 In Blineage cells, CD70-CD27 co-stimulation promotes B-cell expansion, germinal center formation, immunoglobulin (Ig) secretion, and plasma cell differentiation.7,8 In NK cells, ligation of CD27 enhances proliferation and cytolytic activity, as well as IFNG production.12,13
Mast cell
CD4+ Foxp3+ Treg
Treg Differentiation Blockade
Paracrine Loop Blockade KEY CD70
CD27
Soluble CD27
CD154
CD40
Anti-CD70
APRIL
BCMA/TACI
Anti-CD70 Drug Conjugate
(A) CD70-CD27 interaction promotes innate and adaptive immune response. (B) Multiple mechanisms might contribute to therapeutic efficacy of targeting CD70 for cancer indications. Abbreviations: APRIL = A proliferation-inducing ligand; BCMA = B-cell maturation protein A; NHL = non-Hodgkin lymphoma; NK = natural killer; TACI = transmembrane activator and calcium modulator and cyclophilin ligand interactor
agents with vinca alkaloids or nucleoside analogues, rituximab with CHOP (cyclophosphamide/doxorubicin/vincristine/prednisone), or high-dose therapy with stem cell transplantation are also applied.26,27 Unfortunately, WM remains an incurable disease with a median survival of 5 years.26 Several experimental therapeutic approaches based on the understanding of WM biology are being pursued.26,28 The discovery of CD70 expression on WM cell might provide another avenue for targeted therapies.16 The current literature supports at least 4 potential mechanisms for MoAb-based therapeutic agents targeting CD70 for the treatment of WM and other cancers (Figure 1B).
Antibody Effector Functions Certain mouse anti-human CD70 MoAbs including LD6 (IgG2b) and Ki-24 (IgG3) elicit CDC against CD70+ Burkitt lymphoma cell lines and demonstrate antitumor activity in Burkitt lymphoma xenograft models.29 We3,4 and others30 have generated chimeric and humanized anti-CD70 MoAbs of human IgG1
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Novel Antibody-Based Therapeutic Agents Targeting CD70 isotype. In addition to CDC, these MoAbs mediate antibodydependent cell-mediated cytotoxicity (ADCC) and antibodydependent cellular phagocytosis (ADCP), block CD70-CD27 interaction,4 and prolong survival of mice harboring lymphoma xenografts.3,4 Testing of one such humanized anti-CD70 (SGN-70) in early phase clinical trials has recently been initiated. Although SGN-70 does not directly induce apoptosis in CD70+ target cells, it is effective in the elimination of CD70+ tumor cells by virtue of its effector functions (ADCC, ADCP, and CDC) against multiple CD70+ tumor lines representing NHL, HL, RCC, and MM, suggesting that this MoAb will be broadly effective. When tested in vivo, SGN-70 is efficacious in disseminated lymphoma and MM xenograft models established with CD70+ tumor cell lines.3,4 In a SCID-hu WM xenograft model SGN-70 also delays tumor progression.16 Although paracrine blockade (see below) has been proposed to be a mechanism in this report, it is also possible that antibody effector functions of SGN-70 have contributed directly to tumor cell elimination.
Immune Enhancement As SGN-70 mediates ADCP of CD70+ tumor cells by macrophages,4 it might potentially enhance presentation of tumor-associated antigens, resulting in the induction of adaptive antitumor immune response. Furthermore, CD4+CD25–Foxp3+ tumor-infiltrating lymphocytes with suppressive activity are present in NHL patient biopsies. CD70+ lymphoma B cells augment activation-induced Foxp3 expression in these lymphocytes, leading to the hypothesis that CD70 expression on malignant cells might contribute to immune suppression by inducing Treg.24 Interestingly, anti-CD70–mediated blockade abrogates Foxp3 upregulation. Together with the observation that CD70 blockade suppresses the apoptosis-inducing activity of CD70+ RCC23 and glioblastoma cells,20 therapeutic anti-CD70 MoAbs might act not only to deplete CD70+ malignant cells, but could also potentially impede acquisition of T-cell immune regulatory functions within the tumor microenvironment (Figure 1B).
Blockade of a Paracrine Growth Loop CD70 and CD27 are co-expressed in a subset of NHL cell lines. Apart from malignant cells, CD27+ mononuclear infiltrates are often present in the tumor microenvironments of patients with HL, MM, and NHL.4 These findings are consistent with the potential of autocrine CD70-induced proliferation or survival of transformed lymphoma cells via CD27 signaling (Figure 1B). Consistent with this idea, Ho et al have recently provided evidence for a CD70-mediated paracrine stimulatory loop in WM.16 Mast cells isolated from WM tumors express CD70 and respond to WM-derived soluble CD27 (sCD27) by upregulating CD40L and APRIL (A proliferation-inducing ligand), which in turn support the growth and survival of lymphoplasmacytic cells through interactions with CD40 and BCMA/TACI (B-cell maturation protein A/transmembrane activator and calcium modulator and cyclophilin ligand interactor), respectively. The humanized antiCD70, SGN-70, prevents upregulation of CD40L and APRIL on mast cells and blocks disease progression in SCID-hu mice bearing established WM.
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Delivery of Cytotoxic Payloads CD70 can be readily modulated off the cell surface through receptor-mediated endocytosis. This property has been exploited for cytotoxic payload delivery. Anti-CD70 drug conjugates carrying a tubulin-binding agent, an auristatin class,5 or DNA binding drugs31 specifically induce cytotoxicity in CD70+ transformed cell lines. The antibody drug conjugate approach is particularly attractive for the treatment of solid tumor and bulky lymphoma in which access of immune effector cells to the tumor interior might be restricted, leading to reduced therapeutic effect of unconjugated MoAbs. We have demonstrated significant antitumor activity with auristatinbased anti-CD70 drug conjugates in xenografts modeling RCC and glioblastoma.5,6 SGN-75 induces potent cytotoxic activity via internalization and subcellular trafficking through the endosomallysosomal pathway and by disrupting the microtubule network leading to G2-M phase cell-cycle arrest.5 The favorable toxicity and safety profiles in animals at dose levels in excess of the efficacious dose validate CD70 as an attractive target for antibody-based therapeutic agents. Thus, SGN-75 offers a new treatment option for patients with cancer and is currently under development to enter clinical trials in the near future.
Conclusion The unique expression profile and properties of CD70 have clearly made it a very attractive target for MoAb-based therapeutic agents. Not only can these novel therapeutic agents take advantage of multiple mechanisms of action to deliver their therapeutic effects against WM, but they also have broad implications in the therapies against other liquid and solid tumors. As CD70 plays critical role in the immune system, targeting CD70 might also be beneficial to patients with autoimmune and inflammatory diseases.
References 1. Grewal IS. Overview of antibody-based therapeutics: present and future promise. In: Grewal IS, ed. Emerging Biotherapeutics. Boca Raton:CRC Press, FL; 2009:3-35. 2. Grewal IS. CD70 as a therapeutic target in human malignancies. Expert Opin Ther Targets 2008; 12:341-51. 3. McEarchern JA, Oflazoglu E, Francisco L, et al. Engineered anti-CD70 antibody with multiple effector functions exhibits in vitro and in vivo antitumor activities. Blood 2007; 109:1185-92. 4. McEarchern JA, Smith LM, McDonagh CF, et al. Preclinical characterization of SGN-70, a humanized antibody directed against CD70. Clin Cancer Res 2008; 14:7763-72. 5. Law CL, Gordon KA, Toki BE, et al. Lymphocyte activation antigen CD70 expressed by renal cell carcinoma is a potential therapeutic target for anti-CD70 antibody-drug conjugates. Cancer Res 2006; 66:2328-37. 6. Oflazoglu E, Stone IJ, Gordon K, et al. Potent anticarcinoma activity of the humanized anti-CD70 antibody h1F6 conjugated to the tubulin inhibitor auristatin via an uncleavable linker. Clin Cancer Res 2008; 14:6171-80. 7. Croft M. Costimulation of T cells by OX40, 4-1BB, and CD27. Cytokine Growth Factor Rev 2003; 14:265-73. 8. Borst J, Hendriks J, Xiao Y. CD27 and CD70 in T cell and B cell activation. Curr Opin Immunol 2005; 17:275-81. 9. Hendriks J, Gravestein LA, Tesselaar K, et al. CD27 is required for generation and long-term maintenance of T cell immunity. Nat Immunol 2000; 1:433-40. 10. Arens R, Tesselaar K, Baars PA, et al. Constitutive CD27/CD70 interaction induces expansion of effector-type T cells and results in IFNgamma-mediated B cell depletion. Immunity 2001; 15:801-12. 11. Tesselaar K, Arens R, van Schijndel GM, et al. Lethal T cell immunodeficiency induced by chronic costimulation via CD27-CD70 interactions. Nat Immunol 2003; 4:49-54. 12. Takeda K, Oshima H, Hayakawa Y, et al. CD27-mediated activation of murine NK cells. J Immunol 2000; 164:1741-5. 13. Yang FC, Agematsu K, Nakazawa T, et al. CD27/CD70 interaction directly induces natural killer cell killing activity. Immunology 1996; 88:289-93. 14. Lens SM, Drillenburg P, den Drijver BF, et al. Aberrant expression and reverse signalling of CD70 on malignant B cells. Br J Haematol 1999; 106:491-503.
Che-Leung Law et al 15. Baba M, Okamoto M, Hamasaki T, et al. Highly enhanced expression of CD70 on human T-lymphotropic virus type 1-carrying T-cell lines and adult T-cell leukemia cells. J Virol 2008; 82:3843-52. 16. Ho AW, Hatjiharissi E, Ciccarelli BT, et al. CD27-CD70 interactions in the pathogenesis of Waldenstrom macroglobulinemia. Blood 2008; 112:4683-9. 17. Gruss HJ, Kadin ME. Pathophysiology of Hodgkin’s disease: functional and molecular aspects. Baillieres Clin Haematol 1996; 9:417-46. 18. Agathanggelou A, Niedobitek G, Chen R, et al. Expression of immune regulatory molecules in Epstein-Barr virus-associated nasopharyngeal carcinomas with prominent lymphoid stroma. Evidence for a functional interaction between epithelial tumor cells and infiltrating lymphoid cells. Am J Pathol 1995; 147:1152-60. 19. Hishima T, Fukayama M, Hayashi Y, et al. CD70 expression in thymic carcinoma. Am J Surg Pathol 2000; 24:742-6. 20. Chahlavi A, Rayman P, Richmond AL, et al. Glioblastomas induce T-lymphocyte death by two distinct pathways involving gangliosides and CD70. Cancer Res 2005; 65:5428-38. 21. Wischhusen J, Jung G, Radovanovic I, et al. Identification of CD70-mediated apoptosis of immune effector cells as a novel immune escape pathway of human glioblastoma. Cancer Res 2002; 62:2592-9. 22. Sawalha AH, Jeffries M, Webb R, et al. Defective T-cell ERK signaling induces interferon-regulated gene expression and overexpression of methylation-sensitive genes similar to lupus patients. Genes Immun 2008; 9:368-78.
23. Diegmann J, Junker K, Loncarevic IF, et al. Immune escape for renal cell carcinoma: CD70 mediates apoptosis in lymphocytes. Neoplasia 2006; 8:933-8. 24. Yang ZZ, Novak AJ, Ziesmer SC, et al. CD70+ non-Hodgkin lymphoma B cells induce Foxp3 expression and regulatory function in intratumoral CD4+CD25 T cells. Blood 2007; 110:2537-44. 25. Aulwurm S, Wischhusen J, Friese M, et al. Immune stimulatory effects of CD70 override CD70-mediated immune cell apoptosis in rodent glioma models and confer long-lasting antiglioma immunity in vivo. Int J Cancer 2006; 118:1728-35. 26. Vijay A, Gertz MA. Waldenstrom macroglobulinemia. Blood 2007; 109:5096-103. 27. Dimopoulos MA, Anagnostopoulos A. Treatment of Waldenstrom's Macroglobulinemia. Curr Treat Options Oncol 2007; 8:144-53. 28. Treon SP, Hatjiharissi E, Leleu X, et al. Novel agents in the treatment of Waldenstrom's macroglobulinemia. Clin Lymphoma Myeloma 2007; 7(suppl 5): S199-206. 29. Israel BF, Gulley M, Elmore S, et al. Anti-CD70 antibodies: a potential treatment for EBV+ CD70-expressing lymphomas. Mol Cancer Ther 2005; 4:2037-44. 30. Terrett JA, Pan C, Gangwar S, et al. CD70 antibody based drugs: two different mechanisms of action for the treatment of multiple cancer types. Proc Amer Assoc Cancer Res 2006; 47:(Abstract 1995). 31. Jeffrey SC, Nguyen MT, Moser RF, et al. Minor groove binder antibody conjugates employing a water soluble beta-glucuronide linker. Bioorg Med Chem Lett 2007; 17:2278-80.
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Abstract Targeting leukocyte differentiation antigens is a validated approach to develop therapeutic agents for the treatment of cancer, autoimmunity, and inflammatory diseases. A subset of activation antigens transiently induced on leukocytes is particularly interesting because many of them are absent from normal tissues, including those of most vital organs, and therapeutic agents’ targeting of such antigens is expected to impart limited toxicity. One such antigen, CD70, has recently emerged as an attractive potential drug target for the treatment of cancers. Whereas CD70 is only transiently expressed on activation T and B cells and mature dendritic cells, it is found to be aberrantly expressed on a variety of tumor cells, including Waldenström's macroglobulinemia. In this report, we discuss potential antibody-based therapeutic approaches targeting CD70 for tumor elimination where various mechanisms such as antibody effector functions, immune enhancement, blockade of paracrine growth loop, and delivery of cytotoxic payloads can be exploited to achieve efficacy. Indeed, early clinical trials with therapeutic anti-CD70 antibodies are currently in progress, and those for anti-CD70 drug conjugates will soon follow. Clinical Lymphoma & Myeloma, Vol. 9, No. 1, 90-93, 2009; DOI: 10.3816/CLM.2009.n.024 Keywords: Antibody-dependent cell-mediated cytotoxicity, Drug conjugate, Monoclonal antibodies, Paracrine growth loop, T cells, Waldenström
Introduction More than 20 monoclonal antibody (MoAb)–based therapeutic agents have been approved. About half of them are directed against cell surface receptors restricted to hematopoietic cells.1 The MoAbs rituximab and alemtuzumab and the immunoconjugates gemtuzumab ozogamicin, ibritumomab tiuxetan, and iodine 131 tositumomab targeting the leukocyte differentiation antigens (LDAs) CD20, CD52, or CD33, respectively, provide significant clinical benefits in patients with non-Hodgkin lymphoma (NHL), chronic lymphocytic leukemia, or acute myeloid leukemia. The success in targeting LDAs can be partly explained by the fact that most of them are not expressed to any detectable level in normal nonhematopoietic cells of vital organs. As such, minimal on-target toxic effects to nonhematopoietic tissues are expected from MoAbbased therapeutic agents targeting LDAs. It is therefore likely that efforts to further evaluate and interrogate other cell surface receptors restricted to leukocytes might yield additional drug targets. In this respect, we have evaluated the potential of targeting CD70.2 We have developed a humanized anti-CD70 antibody, Department of Preclinical Therapeutics, Seattle Genetics, Inc, Bothell, WA Meeting date: October 15-19, 2008; Published date: March 18, 2009 Address for correspondence: Iqbal S. Grewal, PhD, FRCPath, Seattle Genetics, Inc, 21823 30th Dr SE, Bothell, WA 98021 Fax: 425-527-4119; e-mail: [email protected]
SGN-70, capable of mediating antibody effector functions and eliciting significant antitumor activity in preclinical cancer xenograft models.3,4 Additionally, we have conjugated this antibody to an auristatin-based cytotoxic drug MMAF, SGN-75, for delivering MMAF to CD70+ malignant cells.5,6 SGN-75 exhibits enhanced antitumor activity in solid tumor xenograft models compared to SGN-70, confirming increased therapeutic efficacy through cytotoxic drug delivery. Thus, preclinical animal models have provided strong evidence that targeting CD70 either with unconjugated antibodies or with antibody-drug conjugates represents a promising approach to treat human malignancies. Optimized unconjugated anti-CD70 antibodies as well as antibodydrug conjugates to target CD70-expressing cancer cells are heading toward their clinical application.
CD70: Expression and Functions CD70 (TNFSF7) is a member of the TNF superfamily. Its expression is tightly regulated, and it has not been reported on any normal nonhematopoietic cells so far. Within the hematopoietic lineages, CD70 is only transiently induced on activated B and T cells upon antigen receptor stimulation as well as on mature dendritic cells (DCs).7,8 The only receptor that has been defined for CD70 is CD27 (TNFRSF7), a type I integral membrane protein of the TNF receptor superfamily. Whereas CD70 is a typical activation marker, CD27 is expressed on resting mature T and memory B cells, germinal center B cells, and natural killer (NK) cells.7,8
This article might include the discussion of investigational and/or unlabeled uses of drugs and/or devices that might not be approved by the FDA. Electronic forwarding or copying is a violation of US and international copyright laws. Authorization to photocopy items for internal or personal use, or the internal or personal use of specific clients, is granted by CIG Media Group, LP, ISSN #1557-9190, provided the appropriate fee is paid directly to Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923 USA. www.copyright.com 978-750-8400.
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Figure 1 CD70 Pathway in Normal Immune Response and Targeting CD70 for Cancer Therapy Ag-primed T cell
A
Effector and memory cell differentiation
Activated T, B, NK cells, Mature DC
Memory B cell Plasma cell differentiation
CD27
CD70
CD70 as a Tumor-Associated Antigen Besides being an activation marker, CD70 is also expressed by B- and T-lineage tumors, including diffuse large B-cell lymphoma,14 follicular lymphoma, B-cell lymphocytic leukemia, Burkitt lymphoma, mantle cell lymphomas, adult T lymphoma,15 Waldenström’s macroglobulinemia (WM),16 Hodgkin lymphoma Reed-Sternberg cells,17 and multiple myeloma (MM).3,4 More interestingly, aberrant CD70 expression is observed on transformed cells of epithelial and neural origins, including renal cell,5 nasopharynx,18 ovary, colon and thymus carcniomas19, as well as glioblastoma20,21 and astrocytoma.21 The presence of CD70 on lymphoid cancers might reflect the transformation of lymphocytes arrested at specific maturational stages during which CD70 is transiently induced. Transforming viruses like the human T leukemia virus-1 (HTLV-1)21 and the Epstein-Barr virus18 might regulate CD70 expression on ATL, HL, and nasopharyngeal carcinoma, respectively. However, the mechanistic basis for aberrant CD70 expression on transformed nonhematopoietic cells in the absence of underlying viral infection remains to be elucidated. Defective extracellular signal-regulated kinase (ERK) signaling and DNA methylation have been correlated to CD70 overexpression on T cells in lupus-prone mice and in lupus patients.22 It would therefore be interesting to examine the methylation status of the CD70 gene locus in transformed cells. CD70 expression on transformed cells might constitute an escape strategy against immune surveillance. Thus, clear-cell renal cell carcinoma (RCC)23 and glioblastoma20,21 mediate T-cell apoptosis in a CD70-dependent manner. In RCC, activation of SIVA, a CD27binding signaling intermediate, correlates with RCC-mediated T-cell apoptosis.23 The apoptosis-inducing activity of glioblastoma cells has also been correlated to their ability to release gangliosides GM2 and GD1.20 Induction of regulatory T cell (Treg) differentiation by CD70+ NHL B cells24 and a paracrine growth loop involving CD70 in WM16 are additional mechanisms by which CD70 can contribute to cancer development. It is, however, important to realize that CD70 can also stimulate long-lasting antitumor immunity.25 Because of these contrasting functions, more investigations are clearly needed to determine how immune surveillance and evasion are regulated by CD70 expressed on neoplastic cells.
Targeting CD70 as a Therapeutic Approach for the Treatment of Waldenström’s Macroglobulinemia: Mechanistic Possibilities Current treatment options for WM include alkylating agents, nucleoside analogues, or rituximab. Strategies combining alkylating
NK cell NK cell– mediated cytotoxicity
B
Cytotoxic Drug Delivery
Antibody Effector Functions
CD70 + cancer cell WM cell
X
+
CD70 + NHL B cell
X
Interaction between CD70 and CD27 constitutes an important co-stimulatory pathway in adaptive and innate immunity (Figure 1A), as genetic manipulation of CD70 and CD27 expression in mice results in profound alterations in immune functions.9-11 In the T lineage, CD70-CD27 interaction is required for efficient T-cell priming, survival, and expansion of antigen-stimulated T cells, and the differentiation of effector and memory T cells.7,8 In Blineage cells, CD70-CD27 co-stimulation promotes B-cell expansion, germinal center formation, immunoglobulin (Ig) secretion, and plasma cell differentiation.7,8 In NK cells, ligation of CD27 enhances proliferation and cytolytic activity, as well as IFNG production.12,13
Mast cell
CD4+ Foxp3+ Treg
Treg Differentiation Blockade
Paracrine Loop Blockade KEY CD70
CD27
Soluble CD27
CD154
CD40
Anti-CD70
APRIL
BCMA/TACI
Anti-CD70 Drug Conjugate
(A) CD70-CD27 interaction promotes innate and adaptive immune response. (B) Multiple mechanisms might contribute to therapeutic efficacy of targeting CD70 for cancer indications. Abbreviations: APRIL = A proliferation-inducing ligand; BCMA = B-cell maturation protein A; NHL = non-Hodgkin lymphoma; NK = natural killer; TACI = transmembrane activator and calcium modulator and cyclophilin ligand interactor
agents with vinca alkaloids or nucleoside analogues, rituximab with CHOP (cyclophosphamide/doxorubicin/vincristine/prednisone), or high-dose therapy with stem cell transplantation are also applied.26,27 Unfortunately, WM remains an incurable disease with a median survival of 5 years.26 Several experimental therapeutic approaches based on the understanding of WM biology are being pursued.26,28 The discovery of CD70 expression on WM cell might provide another avenue for targeted therapies.16 The current literature supports at least 4 potential mechanisms for MoAb-based therapeutic agents targeting CD70 for the treatment of WM and other cancers (Figure 1B).
Antibody Effector Functions Certain mouse anti-human CD70 MoAbs including LD6 (IgG2b) and Ki-24 (IgG3) elicit CDC against CD70+ Burkitt lymphoma cell lines and demonstrate antitumor activity in Burkitt lymphoma xenograft models.29 We3,4 and others30 have generated chimeric and humanized anti-CD70 MoAbs of human IgG1
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Novel Antibody-Based Therapeutic Agents Targeting CD70 isotype. In addition to CDC, these MoAbs mediate antibodydependent cell-mediated cytotoxicity (ADCC) and antibodydependent cellular phagocytosis (ADCP), block CD70-CD27 interaction,4 and prolong survival of mice harboring lymphoma xenografts.3,4 Testing of one such humanized anti-CD70 (SGN-70) in early phase clinical trials has recently been initiated. Although SGN-70 does not directly induce apoptosis in CD70+ target cells, it is effective in the elimination of CD70+ tumor cells by virtue of its effector functions (ADCC, ADCP, and CDC) against multiple CD70+ tumor lines representing NHL, HL, RCC, and MM, suggesting that this MoAb will be broadly effective. When tested in vivo, SGN-70 is efficacious in disseminated lymphoma and MM xenograft models established with CD70+ tumor cell lines.3,4 In a SCID-hu WM xenograft model SGN-70 also delays tumor progression.16 Although paracrine blockade (see below) has been proposed to be a mechanism in this report, it is also possible that antibody effector functions of SGN-70 have contributed directly to tumor cell elimination.
Immune Enhancement As SGN-70 mediates ADCP of CD70+ tumor cells by macrophages,4 it might potentially enhance presentation of tumor-associated antigens, resulting in the induction of adaptive antitumor immune response. Furthermore, CD4+CD25–Foxp3+ tumor-infiltrating lymphocytes with suppressive activity are present in NHL patient biopsies. CD70+ lymphoma B cells augment activation-induced Foxp3 expression in these lymphocytes, leading to the hypothesis that CD70 expression on malignant cells might contribute to immune suppression by inducing Treg.24 Interestingly, anti-CD70–mediated blockade abrogates Foxp3 upregulation. Together with the observation that CD70 blockade suppresses the apoptosis-inducing activity of CD70+ RCC23 and glioblastoma cells,20 therapeutic anti-CD70 MoAbs might act not only to deplete CD70+ malignant cells, but could also potentially impede acquisition of T-cell immune regulatory functions within the tumor microenvironment (Figure 1B).
Blockade of a Paracrine Growth Loop CD70 and CD27 are co-expressed in a subset of NHL cell lines. Apart from malignant cells, CD27+ mononuclear infiltrates are often present in the tumor microenvironments of patients with HL, MM, and NHL.4 These findings are consistent with the potential of autocrine CD70-induced proliferation or survival of transformed lymphoma cells via CD27 signaling (Figure 1B). Consistent with this idea, Ho et al have recently provided evidence for a CD70-mediated paracrine stimulatory loop in WM.16 Mast cells isolated from WM tumors express CD70 and respond to WM-derived soluble CD27 (sCD27) by upregulating CD40L and APRIL (A proliferation-inducing ligand), which in turn support the growth and survival of lymphoplasmacytic cells through interactions with CD40 and BCMA/TACI (B-cell maturation protein A/transmembrane activator and calcium modulator and cyclophilin ligand interactor), respectively. The humanized antiCD70, SGN-70, prevents upregulation of CD40L and APRIL on mast cells and blocks disease progression in SCID-hu mice bearing established WM.
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Delivery of Cytotoxic Payloads CD70 can be readily modulated off the cell surface through receptor-mediated endocytosis. This property has been exploited for cytotoxic payload delivery. Anti-CD70 drug conjugates carrying a tubulin-binding agent, an auristatin class,5 or DNA binding drugs31 specifically induce cytotoxicity in CD70+ transformed cell lines. The antibody drug conjugate approach is particularly attractive for the treatment of solid tumor and bulky lymphoma in which access of immune effector cells to the tumor interior might be restricted, leading to reduced therapeutic effect of unconjugated MoAbs. We have demonstrated significant antitumor activity with auristatinbased anti-CD70 drug conjugates in xenografts modeling RCC and glioblastoma.5,6 SGN-75 induces potent cytotoxic activity via internalization and subcellular trafficking through the endosomallysosomal pathway and by disrupting the microtubule network leading to G2-M phase cell-cycle arrest.5 The favorable toxicity and safety profiles in animals at dose levels in excess of the efficacious dose validate CD70 as an attractive target for antibody-based therapeutic agents. Thus, SGN-75 offers a new treatment option for patients with cancer and is currently under development to enter clinical trials in the near future.
Conclusion The unique expression profile and properties of CD70 have clearly made it a very attractive target for MoAb-based therapeutic agents. Not only can these novel therapeutic agents take advantage of multiple mechanisms of action to deliver their therapeutic effects against WM, but they also have broad implications in the therapies against other liquid and solid tumors. As CD70 plays critical role in the immune system, targeting CD70 might also be beneficial to patients with autoimmune and inflammatory diseases.
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