Targeted Therapy of Cancer
`
`Targeted Therapy of Cancer: New
`Prospects for Antibodies and
`Immunoconjugates1
`
`Robert M. Sharkey, PhD; David M. Goldenberg, ScD, MD
`
`Dr. Sharkey is Member and Direc-
`tor of Clinical Research, Garden
`State Cancer Center at the Center for
`Molecular Medicine and Immunol-
`ogy, Belleville, NJ.
`
`Dr. Goldenberg is President, Gar-
`den State Cancer Center at the Cen-
`ter
`for Molecular Medicine and
`Immunology, Belleville, NJ.
`
`This article is available online at
`http://CAonline.AmCancerSoc.org
`
`ABSTRACT Immunotherapy of cancer has been explored for over a century, but it is only in the
`last decade that various antibody-based products have been introduced into the management of
`patients with diverse cancers. At present, this is one of the most active areas of clinical research, with
`eight therapeutic products already approved in oncology. Antibodies against tumor-associated
`markers have been a part of medical practice in immunohistology and in vitro immunoassays for
`several decades, have even been used as radioconjugates in diagnostic imaging, and are now
`becoming increasingly recognized as important biological agents for the detection and treatment of
`cancer. Molecular engineering has improved the prospects for such antibody-based therapeutics,
`resulting in different constructs and humanized/human antibodies that can be administered fre-
`quently. Consequently, a renewed interest in the development of antibodies conjugated with radio-
`nuclides, drugs, and toxins has emerged. We review how antibodies and immunoconjugates have influenced cancer detection and therapy,
`and also describe promising new developments and challenges for broader applications. (CA Cancer J Clin 2006;56:226–243.) © American
`Cancer Society, Inc., 2006.
`
`INTRODUCTION
`
`The search for a mechanism to target diseases selectively was first realized when resistance to infectious disease
`could be transferred from one animal to another through their serum, a process known as passive serotherapy.1 Five
`years later, in 1895, Hericourt and Richet immunized dogs with a human sarcoma and then transferred the serum
`to patients.2 This anticipated the “magic bullet” concept of Paul Ehrlich in 1908, that “toxins” could be targeted to
`cancer and other diseases.3 Another half-century passed before antibodies were identified as the substance in serum
`responsible for these effects.
`Despite being potent immune system instigators for killing infectious agents, clinical research initially focused on
`immunoconjugates prepared with radionuclides, drugs, or toxins, since unconjugated or “naked” antibodies had little
`therapeutic benefit in oncology compared with the immunoconjugates. Early immunotherapy trials failed to show
`substantial responses,4 – 6 but antibodies against carcinoembryonic antigen (CEA) could selectively target and disclose
`sites of CEA-expressing cancers in patients, and also deliver cytotoxic radioactivity in human colonic cancer
`xenografts having CEA.7,8 Thereafter, DeNardo, et al.9 reported responses in lymphoma patients to radiolabeled
`antibodies, and soon others confirmed that radiolabeled antibodies had antitumor activity in non-Hodgkin lym-
`phoma (NHL), but there was also early evidence that the naked antibodies themselves might be effective.10 –12 It was
`during this same period that rituximab (Rituxan, Genentech, and biogen idec), an anti-CD20 IgG, became of interest
`as a therapeutic for NHL without being radiolabeled.13 The experience and subsequent introduction of rituximab
`into the treatment of NHL can be credited for the expanded interest in unconjugated antibodies for cancer therapy.
`
`1This work was supported in part by USPHS grant P01-CA103985 from the National Cancer Institute, NIH, and grant 06-1853-FS-N0 from the New Jersey
`Department of Health and Senior Services.
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`Antibodies (eg, IgG, which is the most com-
`monly used immunoglobulin form, Figure 1)
`are unique proteins with dual
`functionality.
`All naturally occurring antibodies are multi-
`valent, with IgG having two binding ‘arms.’
`Antigen-binding specificity is encoded by
`three complementarity-determining regions (CDRs),
`while the Fc-region is responsible for binding to
`serum proteins (eg, complement) or cells. An
`antibody itself usually is not responsible for
`killing target cells, but instead marks the cells
`that other components or effector cells of the
`body’s immune system should attack, or it can
`initiate signaling mechanisms in the targeted
`cell that leads to the cell’s self-destruction (Fig-
`ure 2). The former two attack mechanisms are
`referred to as antibody-dependent complement-
`mediated
`cytotoxicity
`and antibody-
`(CMC)
`dependent cellular cytotoxicity (ADCC). ADCC
`involves the recognition of the antibody by
`immune cells that engage the antibody-marked
`cells and either through their direct action, or
`through the recruitment of other cell types,
`lead to the tagged-cell’s death. CMC is a pro-
`cess where a cascade of different complement
`proteins become activated, usually when sev-
`eral IgGs are in close proximity to each other,
`either with one direct outcome being cell lysis,
`or one indirect outcome being attracting other
`immune cells to this location for effector cell
`function.
`Antibodies, when bound to key substances
`found on the cell surface, also can induce cells
`to undergo programmed cell death, orapoptosis
`(Figure 2). For example, if rituximab binds to
`two CD20 molecules, this triggers signals in-
`side the cell that can induce apoptosis.14 If
`rituximab is cross-linked by other antiantibod-
`ies, the apoptotic signal is intensified.15 This
`cross-linking could also occur when the anti-
`body is bound by another immune cell through
`its Fc-gamma receptors (Fc␥R). Other anti-
`bodies, such as trastuzumab (anti-HER2/neu;
`Herceptin, Genentech) and cetuximab (anti-
`epidermal growth factor receptor, EGFR; Er-
`bitux,
`ImClone Systems and Bristol-Myers
`Squibb) also have the ability to inhibit cell
`proliferation.16 –18 Because cells
`frequently
`have alternative pathways for critical functions,
`interrupting a single signaling pathway alone
`
`might not be sufficient to ensure cell death.
`From this perspective, it is not surprising that
`antibodies are often best used in combination
`with chemotherapy and radiation therapy to
`augment their antitumor effects.19 –21
`Bevacizumab (Avastin, Genentech) is yet
`another example of how antibodies can be
`used therapeutically. This antibody binds to
`vascular endothelial growth factor (VEGF)
`that is made by tumor cells to promote vessel
`formation, thereby preventing it from inter-
`acting with endothelial cells to form new
`blood vessels (Figure 2).22 Antibodies can
`also be used to modulate immune response.
`Antibodies to the cytotoxic T-lymphocyte
`associated antigen-4 (CTLA-4)
`stimulate
`T-cell
`immune responses by blocking the
`inhibitory effects of CTLA-4, which can en-
`hance tumor rejection.23 However, release of
`this innate inhibitory mechanism can also
`increase the risk of autoimmunity.24 Two
`human anti-CTLA-4 antibodies are currently
`in early clinical trials (MDX-010, Medarex,
`and CP-675,206, Pfizer), with evidence that
`they may have activity in melanoma.24 There are
`already a number of antibodies used or be-
`ing studied as therapeutic agents in cancer as
`well as autoimmune diseases (eg, alemtuzumab,
`daclizumab,
`infliximab,
`rituximab,
`epratu-
`zumab).25–31 Antibodies also can block molecules
`associated with cell adhesion, thereby inhibiting
`tumor metastasis.32,33 With such diverse mecha-
`nisms of action, there are a number of opportu-
`nities for building antibody-based therapeutics.
`Antibodies naturally have long serum half-
`lives. For immunotherapy,
`this property is
`helpful because the antibody is maintained in
`the body fluids, where it can continually inter-
`act with its target. For other targeting strategies,
`most notably with radioconjugates, it can be
`harmful because the highly radiosensitive bone
`marrow is continually exposed to radiation,
`resulting in dose-limiting myelosuppression.
`The large size of an antibody impacts its ability
`to move through a tumor mass. A high inter-
`stitial pressure inhibits the diffusion of larger
`molecules within the tumor.34 Migration
`within the tumor is also inhibited by a binding-
`site barrier, a process where the antibody as it is
`leaving the tumor’s blood vessels binds to the
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`FIGURE 1 Schematic representation of an IgG molecule, its chemically produced fragments, and several recombinant
`antibody fragments with their nominal molecular weights. At the bottom, a schematic representation of the process in-
`volved in engineering murine MAbs to reduce their immunogenicity is provided. A chimeric antibody splices the VL and
`VH portions of the murine IgG to a human IgG. A humanized antibody splices only the CDR portions from the murine
`MAb, along with some of the adjacent “framework” regions to help maintain the conformational structure of the CDRs. A
`fully human IgG can be isolated from specialized transgenic mice bred to produce human IgG after immunizing with tu-
`mor antigen or by a specialized phage display method.
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`FIGURE 2 Mechanisms of action associated with unconjugated antibodies. In this example, the antigen is shown to be
`floating in lipid rafts within the tumor cell membrane. (A) Antibodies can activate apoptotic signals by cross-linking anti-
`gen, particularly across different lipid rafts. Additional cross-linking of antibody by immune cells can also enhance cellu-
`lar signaling. (B) Immune cells themselves can attack the antibody-coated cell (eg, phagocytosis), and/or they can
`liberate additional factors, such as cytokines that attract other cytotoxic cells. (C) If antibodies are positioned closely to-
`gether, they can initiate the complement cascade that can disrupt the membrane, but some of the complement compo-
`nents also are chemo-attractants for immune effector cells and stimulate blood flow. (D) Tumors also can produce
`angiogenic factors that initiate neovascularization. Antibodies can neutralize these substances by binding to them, or
`they can bind directly to unique antigens presented in the new blood vessels, where they could exert similar activities.
`
`first available antigen, concentrating the anti-
`body in the perivascular space.35 High-affinity
`antibodies are less likely to migrate into the
`tumor bed.36 Administering higher doses of the
`antibody can reduce the effect of the binding
`site barrier and allow the antibody to diffuse
`more deeply into the tumor bed.37 For cyto-
`toxic agents that must be internalized to kill the
`cell (eg, toxins, cytotoxic drugs), the ability to
`distribute throughout the tumor is important.
`Radioconjugates are less affected by this be-
`cause some radioactive particles can traverse as
`much as 1.0 cm from where they are deposited
`(bystander or crossfire effect).
`
`THERAPY WITH UNCONJUGATED ANTIBODIES
`
`A renewed interest in the effects of un-
`conjugated antibodies in cancer began in the
`early 1980s, after murine monoclonal anti-
`(MAbs) became available.38 These
`bodies
`initial trials were performed in hematological
`malignancies, as well as in colorectal cancer
`and melanoma.4 – 6,39 – 41 As with many inno-
`
`vative treatment approaches that are some-
`times introduced before the technology has
`matured sufficiently to extract maximum
`benefit, only occasional clinical
`responses
`were observed. With insufficient efficacy and
`the immunogenicity of the foreign murine
`MAb, most of these studies were terminated.
`Fortunately, some investigators persevered.
`An excellent lesson on the tribulations of the
`development of an antibody product be-
`tween an academic group and industry is that
`of alemtuzumab (Campath, Berlex, and Gen-
`zyme).42 Alemtuzumab (anti-CD52) had one
`of the earliest and protracted developments
`of an antibody ultimately commercialized. It
`took over 20 years from the development of
`the first rat immunoglobulin against CD52,
`changing the immunoglobulin type, and fi-
`nally developing a humanized, recombinant
`form, and involved several commercial firms
`during this
`time. Chemotherapy-refractive
`chronic lymphocytic leukemia was the indi-
`cation finally approved in 2001.
`Due in part to the contributions made by the
`groups led by Morrison (Columbia and Stan-
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`ford Universities) and Winter (Cambridge),
`MAbs now are engineered to remove a signif-
`icant portion of the murine component of the
`IgG, substituting human IgG components be-
`fore entering clinical studies.43– 45 Chimeric
`antibodies essentially splice VL and VH regions
`on the murine antibody to the human IgG,
`making a molecule that is 75% human and 25%
`murine IgG, whereas a humanized antibody
`grafts the CDR regions from a murine MAb,
`along with some of the surrounding “frame-
`work” regions to maintain CDR conforma-
`tion, onto a human IgG, essentially making a
`molecule with 5% of its sequence from the
`parental MAb (Figure 1).45 More recent ad-
`vances have made available, either by genetic
`or phage-display methods, the development of
`fully human MAbs that have now entered clin-
`ical trials.46 Such engineered MAbs are postu-
`lated to greatly reduce the immunogenicity of
`antibodies, allowing multiple injections to be
`given, and the human Fc enhances the inter-
`action with other immune system elements.
`Rituximab is perhaps the most prominent
`example of a highly successful paradigm of an-
`tibody therapy. As a chimeric antibody, not
`only did it have reduced immunogenicity, but
`its effector function (associated with the Fc-
`portion) was improved. For example, when
`testing ADCC activity against follicular lym-
`phoma isolated from 43 patients, Weng, et al.
`reported that only rituximab, not its parent
`murine anti-CD20 IgG (2B8), had activity in
`vitro.47 Rituximab was initially approved as a
`single agent therapy for relapsed or refractory
`low-grade, follicular B-cell NHL, having an
`overall response rate of 48% (10% were com-
`plete responses, CR) with a median duration of
`11.8 months.48,49 Since CD20 is not expressed
`on precursors B-cells, rituximab induces a de-
`pletion of only mature B-cells. Rituximab’s
`major side effects, which are thought to be
`associated with the activation of complement
`pathways, occur during or shortly after its in-
`fusion. Other less common side effects include
`symptoms associated with tumor lysis
`syn-
`drome, severe mucotaneous reactions, renal
`toxicity, cardiac arrhythmias, hypersensitivity
`reactions, and reactivation of hepatitis B (pri-
`
`marily when used in combination with chemo-
`therapy).49
`Rituximab’s activity is unique among cancer
`treatments because 40% of
`the patients re-
`treated with rituximab could again respond
`with a similar duration.50 Extending the dura-
`tion of rituximab therapy can improve the re-
`sponse
`rate, particularly
`the number of
`complete responses, and its duration. However,
`whether given as a maintenance regimen or
`retreating at the first sign of progression, the
`time to chemotherapy was the same.51 With
`both approaches having equal benefit, retreat-
`ment is generally favored because of the higher
`expense of a maintenance regimen. Despite the
`success of rituximab as a monotherapy, there
`are still a number of patients who do not re-
`spond to the initial treatment, and over time,
`many of those who do will relapse. In an at-
`tempt to improve outcome, rituximab has been
`combined with chemotherapy regimens,
`in-
`cluding CHOP, CVP, and MCP, as front-line
`treatments, with very promising results in not
`only follicular B-cell lymphomas, but also in
`lymphomas.52,53 Indeed,
`diffuse large B-cell
`trials examining front-line combinations of rit-
`uximab and chemotherapy have already dem-
`onstrated improvements in response rates, time
`to progression, and event-free survival, and
`while the overall response rates are promising
`based on current 2- to 3-year follow-up data,
`more time will be required to fully appre-
`ciate its impact.52 Even in chronic lymphocytic
`leukemia (CLL), where initial testing of ritux-
`imab was disappointing, dose intensifica-
`tion and combinations with chemotherapy
`have provided significant
`improvements
`in
`response.54,55 Early clinical studies combining
`rituximab with a humanized anti-CD22,
`epratuzumab (Immunomedics, Inc.) suggested
`the potential for additional benefit, particularly
`in patients with diffuse large B-cell lympho-
`mas.56,57 Studies have also assessed the possible
`role of an anti-CD80 MAb (galiximab, biogen
`idec) as a monotherapy in NHL,58,59 and clin-
`ical trials are in progress testing its combination
`with rituximab.
`Considerable attention has been devoted to
`understanding the mechanism of action of rit-
`uximab, particularly why some B-cell lympho-
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`mas are affected and why not all patients with
`follicular lymphomas respond. As mentioned
`earlier, rituximab has been shown to have
`CMC, ADCC, and apoptotic activity, with the
`former two mechanisms believed to have the
`greatest impact, although there are conflicting
`views of which of these two pathways contrib-
`utes the most to the response.14,60 – 66 Studies in
`transgenic and other mouse models have sup-
`ported the importance of
`the Fc-receptor-
`mediated mechanism of
`action
`for
`rituximab.67,68 These efforts have contributed
`in part to a better understanding of the role of
`various Fc receptors found on a variety of im-
`mune effector cells (eg, B-cells, neutrophils,
`natural killer cells, and monocytes) on (in the
`case of rituximab) the clearance of B-cells, as
`well as the plasma half-life of antibodies.69 Not
`only do the various Fc-receptors
`influence
`binding, but the absence of certain carbohy-
`drates on the Fc portion of the IgG can affect
`both ADCC and CMC activities.70,71 Cartron,
`et al. found that the expression of the homozy-
`gous Fc-gamma RIIIa receptor (CD16) 158V
`genotype correlated with a higher response rate
`to rituximab, but it did not have an impact on
`the progression-free survival.72 Weng, et al.
`found a similar correlation and also noted that
`the homozygous expression of the Fc-gamma
`RIIa histidine/histidine genotype correlated
`independently with a higher response rate, par-
`ticularly when assessing the response status ⱖ6
`months from treatment.47 By unraveling the
`molecular basis for antibody cytotoxicity, not
`only can more effective antibodies be designed,
`but it could lead to a more rational approach
`for combinations to enhance activity, such as
`the finding that G-CSF up-regulates CD64
`(Fc-gamma receptor I), which can enhance the
`binding of neutrophils and monocytes
`to
`B-cells coated with rituximab.73 IL-12 has a
`similar stimulatory effect in mouse models and
`more recently has been applied clinically with
`promising results.74,75 These discoveries are
`also having an impact on the development of
`antibodies for treating other cancers.76 – 80
`The approved antibodies listed in Table 1 in-
`dicate that immunotherapy is not restricted to
`hematological malignancies, but includes diverse
`target antigens and receptors having different
`
`functions. Trastuzumab is an anti-
`biological
`HER2/neu antibody that has had a major impact
`on the therapy of breast cancer and is used alone
`and in combination with drugs.81– 83 HER2/neu
`is overexpressed on a proportion of breast and
`other cancers, and trastuzumab binds with an
`extracellular epitope of
`this
`target molecule.
`About 15% of women whose tumors overexpress
`HER2/neu respond to trastuzumab, but its effi-
`cacy is clearly best when used in combination
`with chemotherapy, where a 25% increase in the
`median survival (to 29 months) has been report-
`ed.81 Further, the addition of this antibody to
`adjuvant chemotherapy for breast cancer has im-
`proved survival markedly.83 Since only a portion
`of breast cancer patients overexpress HER2/neu
`and respond to trastuzumab, selection of suitable
`patients is important. New data are emerging that
`suggest trastuzumab treatment after adjuvant che-
`motherapy can have a significant benefit com-
`pared with observation, particularly in reducing
`the rate of distant recurrence.82
`As a member of a family of receptor tyrosine
`kinases, the binding of HER2 by trastuzumab
`can interrupt intracellular signaling and affect
`tumor cell growth. Izumi, et al. showed that
`trastuzumab also has antiangiogenic proper-
`ties.84 While this may be an important under-
`lying mechanism of action, other evidence
`suggests that trastuzumab’s activity is princi-
`pally governed by ADCC.85 However, trastu-
`zumab combined with chemotherapy improves
`response rates, despite the immunosuppressive
`activity of the chemotherapy, and trastuzum-
`ab’s activity is enhanced when combined with
`other, nonantibody, Erb inhibitors, such as ge-
`fitinib and erlotinib, all of which suggest that its
`ability to interfere with signaling is impor-
`tant.86 Since HER2 is a member of a family of
`growth factors known as
`the neuregulins/
`heregulin and is expressed in multiple neuronal
`and non-neuronal tissues in embryos and adult
`animals, including the heart, it is not surprising
`that cardiomyopathy has been associated with
`trastuzumab, particularly when combined with
`paclitaxel and anthracyclines.87–90
`EGFR is also overexpressed in many solid
`cancers, and when bound by its ligand, cell
`growth is stimulated. However, when engaged
`by an EGFR-specific antibody, receptor phos-
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`TABLE 1 FDA-approved Antibodies for the Parenteral Use in Detection and Treatment of Cancer
`
`Generic Name
`
`Trade name
`
`Agent/Target
`
`Cancer Indication
`
`Approval
`
`Unconjugated
`Rituximab
`Trastuzumab
`Alemtuzumab
`Cetuximab
`
`Bevacizumab
`Radioconjugates
`Satumomab pendetide
`Nofetumomab merpentan
`Arcitumomab
`Capromab pendetide
`Ibritumomab tiuxetan
`Tositumomab
`Drug conjugates
`Gemtuzumab ozogamicin
`
`Rituxan
`Herceptin
`Campath-1H
`Erbitux
`
`Avastin
`
`OncoScint*
`Verluma*
`CEA-Scan*
`ProstaScint
`Zevalin
`Bexxar
`
`Mylotarg
`
`*No longer commercially available.
`†CLL ⫽ chronic lymphocytic leukemia.
`‡SCLC ⫽ small cell lung cancer.
`§AML ⫽ acute myelogenous leukemia.
`
`Chimeric anti-CD20 IgG1
`Humanized anti-HER2 IgG1
`Humanized anti-CD52
`Chimeric anti-EGFR
`
`Chimeric anti-VEGF
`
`111In-murine anti-TAG-72 IgG
`99mTc-murine anti-EGP-1 Fab’
`99mTc-murine anti-CEA Fab’
`111In-murine anti-PSMA
`90Y-murine anti-CD20 IgG ⫹ rituximab
`131I-murine anti-CD20 IgG ⫹ unlabeled tositumomab
`
`B-cell lymphoma
`Breast
`CLL†
`Colorectal
`Head/neck
`Colorectal
`
`Colorectal, ovarian
`SCLC‡
`Colorectal
`Prostate
`B-cell lymphoma
`B-cell lymphoma
`
`Humanized anti-CD33 IgG4 conjugated to colicheamicin
`
`AML§
`
`1997
`1998
`2001
`2004
`2006
`2004
`
`1992
`1996
`1996
`1996
`2002
`2003
`
`2000
`
`phorylation is decreased and cell growth is inhib-
`ited. The chimeric antibody against EGFR,
`cetuximab, also has an effect on neovasculariza-
`tion.91,92 Cetuximab works best in combination
`with chemotherapy in colorectal cancer,
`for
`which it was initially approved, and with external
`irradiation in head and neck cancers, which was
`recently FDA-approved.17,93 Beside the usual
`risks associated with antibody infusions, cetux-
`imab causes an acneform rash and other skin
`reactions in most patients, with 10% of these
`being severe. There is evidence suggesting that
`the intensity of the skin rash is associated with its
`antitumor response and even survival.94 Other
`EGFR antibodies, particularly humanized and
`fully human forms, also are in development, as
`indicated in Table 2, and may in fact be better
`tolerated and show evidence of activity without
`being combined with cytotoxic chemotherapy,
`which is currently being evaluated in Phase III
`trials. It is too early to speculate whether they
`will, in fact, provide any therapeutic advantages
`over cetuximab.
`Bevacizumab targets and blocks vascular en-
`dothelial growth factor (VEGF) and VEGF’s
`binding to its receptor on the vascular endo-
`thelium. Since VEGF is released by many
`cancers to stimulate proliferation of new blood
`vessels, the combination of bevacizumab and
`chemotherapy was found to increase objective
`
`responses, median time to progression, and sur-
`vival
`in patients with metastatic colorectal
`cancer, compared with chemotherapy alone,
`but earlier preclinical
`studies
`indicated that
`anti-VEGF antibodies were active alone, as
`well as in combination with radiation.22,95,96 It
`is currently being studied clinically in renal cell,
`breast, and lung cancers, as well as in a number
`of other nonhematological and hematological
`malignancies.97–99 As might be expected, bev-
`acizumab may cause gastrointestinal perfora-
`tions and delayed wound healing, as well as
`hemorrhagic events (primarily seen in small cell
`lung cancer trials, where bevacizumab is not
`approved). Arterial
`thromboembolic events
`(eg, cerebral infarction, transient ischemic at-
`tacks, myocardial infarction, angina) and pro-
`teinurea also have been reported.100
`
`IMMUNOCONJUGATES
`
`Antibodies also function as carriers of cyto-
`toxic substances, such as radioisotopes, drugs, and
`toxins (Figure 3). In NHL, anti-CD20 radiocon-
`jugates have superior antitumor activity com-
`pared with their unconjugated antibody coun-
`terparts, but there is increased, albeit manageable,
`hematological toxicity.101,102 These findings are
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`TABLE 2 Selected Unconjugated Antibody Therapeutics in Advanced Clinical Testing
`
`Generic Name
`
`Agent/Target
`
`Apolizumab
`Chimeric 14.18
`Epratuzumab
`Galiximab
`HuMax-CD4
`Lumiliximab
`MDX-010
`Matuzumab
`Orgegovomab
`Panitumumab
`Pertuzumab
`Rencarex
`Vitaxin
`
`Human anti-HLA-DR
`Chimeric anti-ganglioside (GD2)
`Humanized anti-CD22
`Humanized anti-CD80
`Fully human anti-CD4
`Humanized anti-CD23 (Fc-epsilon RII)
`Anti-CTLA-4
`Humanized anti-EGFR
`Murine anti-CA-125
`Human anti-EGFR
`Humanized anti-HER-2
`Chimeric anti-G250
`Humanized anti-␣v␤3 integrin
`
`*CLL ⫽ chronic lymphocytic leukemia.
`†CRC ⫽ colorectal cancer.
`‡CTCL ⫽ cutaneous T-cell lymphoma.
`§NHL ⫽ non-Hodgkin lymphoma.
`¶NSCLC ⫽ non-small cell lung cancer.
`**SLL ⫽ small lymphocytic lymphomas.
`
`CA Cancer J Clin 2006;56:226–243
`
`Cancer
`
`CLL*, SLL**
`Neuroblastoma
`NHL§
`NHL§
`CTCL‡
`CLL*
`Melanoma
`CRC†
`Ovarian
`NSCLC¶, CRC†, renal
`Breast, prostate, ovarian
`Kidney
`Melanoma, prostate
`
`FIGURE 3 Immunoconjugates are formed primarily by chemical reactions. Radioconjugates can be formed by coupling
`radioiodine to tyrosine residues, or by binding chelates to lysine residues, which are then used to bind a variety of radio-
`metals, such as 90Y. Cysteine residues are also useful for coupling radionuclides, particularly 99mTc and rhenium, but
`cysteine is also used for conjugation of drugs and toxins, which can also be coupled to lysine residues. In addition, the
`carbohydrates found on IgG can be modified to allow the coupling of chelates or drugs. Drugs have also been coupled
`to an intermediate carrier that allows for a higher number of drugs to be bound to the antibody. Toxin conjugates usually
`need to be modified to remove their innate cell binding properties, with the biologically active portion then coupled to the
`antibody or used as a portion of a recombinant fusion protein.
`
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`Targeted Therapy of Cancer
`
`strong incentives to continue the pursuit of im-
`munoconjugates for cancer therapy.
`
`Radionuclides
`
`Radiolabeled antibodies were the first group
`of immunoconjugates to be examined.103 Ta-
`ble 3 lists some of the more commonly used
`radionuclides conjugated to antibodies
`for
`cancer treatment. Because the radioactivity can
`be detected easily by external scintigraphy, it is
`also noteworthy to mention the additional ap-
`plication of radiolabeled antibodies for imag-
`ing. The demonstration of cancer targeting
`with a radiolabeled antibody fragment to CEA
`resulted in the development of radiolabeled
`antibodies for cancer imaging.7 Since then,
`99mTc- and 111In-radioconjugates have been
`commonly used for this application, but with
`the advent of positron-emission tomography
`(PET), investigators are now beginning to take
`advantage of this technologically superior im-
`aging system by radiolabeling tumor-associated
`antibodies with positron-emitters.104 –107
`Whereas external beam radiation delivers a
`focused beam of high dose rate radiation for
`short bursts that are divided over several weeks
`and is designed to treat local disease, radioim-
`munotherapy (RAIT) is typically given as an
`intravenous injection, thereby allowing radio-
`activity to be delivered to tumors throughout
`the body. Tumor uptake of a radiolabeled IgG
`occurs gradually, taking 1 to 2 days before peak
`uptake occurs. Peak uptake is typically ⬍0.01%
`of the total injected dose per gram tumor, but
`the radioactivity deposited in the tumor can be
`detected several weeks later.108 Because of its
`kinetics, the radiation-absorbed dose delivered
`by RAIT occurs at a much lower dose rate than
`external beam irradiation, but is continually
`present for a period of time defined by the
`physical half-life of the radionuclide and the
`biological half-life of the antibody residing in
`the tumor. This continuous, low dose rate ra-
`diation exposure can be as effective as intermit-
`tent, high dose rate radiation.109,110
`When it comes to choices of radionuclides for
`therapy, tumor size is the primary consideration.
`Medium-energy beta-emitters, such as 131I (0.5
`MeV) and 177Lu (0.8 MeV), can traverse 1.0 mm,
`
`while high-energy beta-emitters, such as 90Y or
`188Re (2.1 MeV), can penetrate up to 11 mm,
`making it possible for beta-emitters to kill across
`several hundred cells, referred to earlier as a by-
`stander or crossfire effect.111 This is considered a
`significant attribute for radioconjugates compared
`with other immunoconjugates, since they can be
`therapeutically active even if heterogeneous anti-
`gen expression, tumor architecture, or other fac-
`tors impede targeting of every cell. Although
`higher energy beta-emitters have the potential of
`killing cells across a longer path-length, the ab-
`sorbed fraction is higher for the lower energy
`beta-emitters (ie, probability of hitting the nu-
`clear DNA), making them efficient killers. Alpha-
`emitters, such as 213Bi and 211At, traverse only a
`few cell diameters, but an alpha particle is also a
`far more efficient (energetic) killer than even a
`low-energy beta particle, requiring fewer “hits”
`to damage cellular processes.111 Low-energy
`electrons, such as are produced by Auger emitters
`(125I, 67Ga, or 111In, for example) have to be in
`close contact, preferably inside a cell or in the
`nucleus, to exert a cytotoxic effect. As one might
`expect, beta-emitters are most likely best applied
`in situations where the tumors are ⱖ0.5 cm in
`diameter; otherwise a substantial portion of the
`energy from the radioactive decay will be ab-
`sorbed in the surrounding normal tissue. The
`alpha and low-energy electron emitters are best
`applied when the disease burden is smaller, more
`localized, or where there may be single or small
`clusters of cells (eg,
`leukemia, malignant as-
`cites).112,113
`The primary concern for using radionuclide-
`labeled IgG is that it remains in the blood for an
`extended period of time, which continually exposes
`the highly sensitive red marrow to radiation, result-
`ing in dose-limiting myelosuppression. Smaller
`forms of the antibodies, such as a F(ab=)2 or Fab’, and
`more recently, molecularly engineered antibody
`subfragments (Figure 1) with more favorable phar-
`macokinetic properties, are removed more rapidly
`from the blood, thereby improving tumor/blood
`ratios.114,115 There have been reports of improved
`therapeutic responses using smaller-sized antibodies,
`but these smaller entities frequently are cleared from
`the blood by renal filtration, and as a result, many
`radionuclides (eg, radiometals) become trapped in a
`higher concentration in the kidneys than in the
`
`234
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`CA Cancer J Clin 2006;56:226–243
`
`TABLE 3 Physical Properties of Several Examples of Radionuclides Used for Radioconjugate Therapy
`
`Radionuclide
`
`Emission
`
`Half