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Annu. Rev. Med. 2003. 54:34369doi: 10.1146/annurev.med.54.101601.152442

Copyright c 2003 by Annual Reviews. All rights reserved

MONOCLONALANTIBODYTHERAPY FORCANCER

Margaret von Mehren, Gregory P. Adams,and Louis M. Weiner

Department of Medical Oncology, Fox Chase Cancer Center, 7701 Burholme Avenue,

Philadelphia, Pennsylvania 19111; e-mail: m [email protected],

gp [email protected], lm [email protected]

Key Words immunoconjugate, immunotoxin, radioimmunotherapy,antibody-dependent cellular cytotoxicity

Abstract Monoclonal antibody therapy has emerged as an important therapeuticmodality for cancer. Unconjugated antibodies show significant efficacy in the treat-ment of breast cancer, non-Hodgkins lymphoma, and chronic lymphocytic leukemia.Promising new targets for unconjugated antibody therapy include cellular growth fac-tor receptors, receptors or mediators of tumor-driven angiogenesis, and B cell surfaceantigens other than CD20. Immunoconjugates composed of antibodies conjugated toradionuclides or toxins show efficacy in non-Hodgkins lymphoma. One immunocon-

jugate containing an antibody and a chemotherapy agent exhibits clinically meaningfulantitumor activity in acute myeloid leukemia. Numerous efforts to exploit the abilityof antibodies to focus the activities of toxic payloads at tumor sites are under way andshow early promise. The ability to create essentially human antibody structures hasreduced the likelihood of host-protective immune responses that otherwise limit theduration of therapy. Antibody structures now can be readily manipulated to facilitateselective interaction with host immune effectors. Other structural manipulations thatimprove the selective targeting properties and rapid systemic clearance of immuno-conjugates should lead to the design of effective new treatments, particularly for solid

tumors.

INTRODUCTION

The description of techniques for the production of monoclonal antibodies (MAb)

by Kohler & Milstein 25 years ago led to the development of multiple reagents

employing these structures. Antibodies, which initially were viewed as target-

ing missiles, have proved much more complex in their targeting and biologic

properties than the fields pioneers envisioned them. The ability to manipulate thegenes of antibodies with microbiologic techniques has allowed significant mod-

ifications of these structures. Murine proteins can be readily transformed into

human or humanized formats that are not readily recognized as foreign by the

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344 VON MEHREN ADAMS WEINER

human immune system. In addition, novel antibody-based structures with multi-

ple antigen recognition sites, altered size, or effector domains have been shown

to influence the targeting ability of antibodies. Coupled with the identification

of appropriate cancer targets, antibody-based therapeutics are finding increasingapplications in cancer treatment, and they can be effective alone, in conjunction

with chemotherapy or radiation therapy, or when conjugated to toxic moieties

such as toxins, chemotherapy agents, or radionuclides. This review focuses on

antibody-based agents with significant efficacy and novel approaches with clinical

promise.

IMMUNOGLOBULIN STRUCTURE

IgG

IgG molecules are the most common antibodies employed in cancer therapy. They

are comprised of two identical light chains and two identical heavy chains, each

composed of variable domains and constant domains (Figure 1). The variable re-

gion contains the hypervariable or complementarity-determining regions (CDRs).

These short, highly variable amino acid sequences are the major site of interaction

with antigen. There are three CDRs (CDR1, CDR2, and CDR3) on each variable

chain. Both the light chains and the heavy chains are held together by disulfide

bonds and noncovalent interactions between several of the domains. The complexis oriented with bilateral symmetry: light chainheavy chainheavy chainlight

chain. The IgG molecule is divided into three functional domains, two antigen-

binding (Fab) domains with identical antigen specificity connected by a highly

flexible hinge domain to the Fc (effector) domain that interacts with complement,

Figure 1 Schematic diagram depicting the structures of an IgGmolecule and a variety

of engineered antibody molecules derived from various IgG domains.

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CANCER ANTIBODIES 345

immune effector cells, and receptors involved in maintaining constant concentra-

tions of IgG in the circulation.

Antibody Fragments

For many years, IgG molecules have been enzymatically digested into smaller

functional fragments [Fab and F(ab)2]. More recently, the genes encoding the

variable and constant domains have been used to construct a variety of recombi-

nant antibody-based fragments. These range in size from peptides comprised of an

individual CDR (1) to multidomained molecules with four antigen binding sites

(2). The smallest antibody-based fragments, individual CDRs and single-variable

domains, often exhibit lower affinity and less antigen specificity than correspond-

ing intact antibody molecules (1, 3). Single-chain Fv (scFv) molecules, composedof peptide-linked VH and VL domains, are rapidly emerging as key players in

the engineered antibody field. With a size of25 kDa, these molecules are both

effective targeting vehicles in their own right and versatile components for the con-

struction of larger antibody-based regents. ScFv molecules can be produced from

genes isolated from hybridomas expressing MAb of a desired specificity (4) or can

be isolated by panning (selection) from a combinatorial phage display library (5).

The small size of scFv molecules mediates their rapid renal elimination, leading

to highly specific tumor localization. Because of this property, radiolabeled scFv

molecules are highly attractive agents for use in tumor detection (6).ScFv can be dimerized to yield larger molecules that exhibit a slower systemic

clearance and a greater avidity for cells that overexpress their target antigen, making

them potentially useful in tumor therapy. Various strategies can be employed to

dimerize scFv molecules. These include the creation of disulfide-linked (scFv)2 (7)

or gene-fused (scFv)2, with a peptide spacer joining two scFv, and the shortening of

the linker between the light and heavy chain, forcing the production of diabodies by

noncovalent associations between two molecules (8). Larger divalent molecules,

termed minibodies, can be produced by fusing the gene for an scFv to that for a

CH3 domain (9). An additional advantage of the minibody format is the greater

stability of the complex conferred by the affinitybetween the CH3 domains. Tandem

diabodies, with four antigen-binding sites, can be produced by joining two diabody

components with peptide spacer (2).

The ideal antibody-based molecule depends on the application. If a naked

antibody can directly mediate an antitumor effect, the greatest possible bioavail-

ability is desired. This is best achieved by leaving the antibody in the native IgG

format, which will exhibit a prolonged residence in the circulation. However, the

native IgG format could lead to unacceptable bone marrow toxicity if the antibody

is conjugated to a therapeutic radioisotope for radioimmunotherapy (RAIT) appli-

cations. For RAIT, smaller, more rapidly cleared molecules such as minibodies or

diabodies can reduce nontargeted marrow toxicity. Furthermore, because physio-

logic barriers such as heterogeneous blood supply and elevated interstitial pressure

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(10) impede the penetration of antibodies into tumors, these smaller molecules may

have other advantages over intact IgG. Jain & Baxter (11) determined that an intact

IgG molecule would take 54 h to move 1 mm into a solid tumor, whereas a Fab

fragment would travel the same distance in only 16 h.

DESIRABLE CHARACTERISTICS OF ANTIBODY TARGETS

A variety of protein and carbohydrate molecules on the surface of cancer cells are

potential targets for antibody-directed therapies. Clinically useful targets may be

uniquely expressed by cancer cells or expressed at higher levels by cancer cells than

by normal cells. As a result, antibodies will selectively bind to cancer cells. Recep-

tors on the surface of cancer cells are particularly attractive targets for therapeutic

antibodies if binding of the receptor perturbs a downstream signaling event, or

inhibits the binding of a natural ligand to its receptor and therefore interferes with

a normal signaling pathway. If an antibody is linked to a radioactive particle, im-

munotoxin, or chemotherapeutic agent, it may be beneficial to target a receptor that

internalizes upon antibody binding, leading to internalization of the toxic agent.

MECHANISMS OF ACTION FOR ANTIBODIESAS THERAPEUTIC AGENTS

Antibodies may exert antitumor effects by inducing apoptosis (12), interfering with

ligand-receptor interactions (13), or preventing the expression of proteins that are

critical to the neoplastic phenotype. In addition, antibodies have been developed

that target components of the tumor microenvironment, perturbing vital structures

such as the formation of tumor-associated vasculature. Some antibodies target

receptors whose ligands are growth factors, such as the epidermal growth fac-

tor receptor. The antibody thus inhibits natural ligands that stimulate cell growth

from binding to targeted tumor cells. Alternatively, antibodies may induce an

anti-idiotype network (14), complement-mediated cytotoxicity (15), or antibody-

dependent cellular cytotoxicity (ADCC) (16). An anti-idiotypic network results

from the recognition of the antibody, called Ab1, by the host immune system. The

antigenic binding site within the variable regions of the antibody is seen as foreign.

When an antibody, termed Ab2, is formed against this binding site, it recapitulates

the three-dimensional structure of the antigen targeted by Ab1. This chain of events

can keep occurring, thus perpetuating the immune response against the primary

antigen (17, 18). Some classes of immunoglobulins can activate complement or

natural killer (NK) cells. The first component of the complement cascade, C1, is

capable of binding the Fc portion of IgM and IgG molecules. C1 activation triggers

the classical complement cascade, leading to the recruitment of phagocytic cells

and death of the antibody-bound cell. This process occurs more efficiently with

IgM molecules, but it can also occur if IgG molecules are clustered on the cell sur-

face. Cells coated by IgG1 or IgG3 isotype antibodies can also activate effector cells

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via the binding of the terminal Fc portion of the antibody to Fc receptors, found

on NK cells, neutrophils, mononuclear phagocytes, some T cells, and eosinophils.

ADCC occurs with the release of cytoplasmic granules containing perforins and

granzymes from these effector cells. This phenomenon can be amplified by con-structing bispecific antibodies that contain two binding domains, one that targets

a tumor antigen and one that targets the immunoglobulin Fc receptor on immune

effector cells, thereby activating the effector cells for tumor lysis.

ANTIBODY LIMITATIONS

Initial clinical trials with MAb produced some striking antitumor effects (19),

but most of the early experiences illustrated the obstacles to successful therapy

(Table 1). Most of the MAb employed in early clinical trials were derived from

mice, and patients exposed to them developed human antimouse antibody (HAMA)

responses, which limited the number of treatments patients could safely receive

(20). Molecular engineering techniques have overcome this obstacle by grafting

critical sequences in the human heavy-chain backbone onto the xenogeneic murine

antibody structure, paring the interspecies differences and reducing the immuno-

genicity of the resulting antibodies. Numerous techniques have further reduced

immunogenicity, and it is now possible to create fully human immunoglobulins

with limited capacity to induce HAMA. However, these approaches do not inhibit

the production of anti-idiotype antibodies (see previous section).

Some tumor antigens are shed or secreted. Antibodies that target these antigens

will bind their targets in the circulation, limiting the amount of unbound antibody

available to bind to tumor (21). Barriers that impede antibody distribution within

tumors include (a) disordered tumor vasculature, (b) increased hydrostatic pressure

within tumors and (c) heterogeneity of antigen distribution within tumors (11). It

has been estimated that, due to these barriers, an IgG molecule will take 2 days to

travel 1 mm and 78 months to travel 1 cm within a tumor. Estimates for a smaller

antibody-based molecule, such as a Fab fragment, are 1 day to travel 1 mm and

2 months to travel 1 cm. If antibodies do reach their targets, there is little evidence

TABLE 1 Obstacles to effective antibody

therapy

1. Immunogenicity of xenogeneic antibodies

2. Shedding of antigen into circulation

3. Disordered vasculature in tumors

4. Increased hydrostatic pressure in tumors

5. Heterogeneity of antigen on tumor surface

6. Limited numbers of effector cells at tumor

7. Immunosuppressive tumor microenvironment

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that they efficiently mediate ADCC in vivo. For this to occur, sufficient numbers

of effector cells, such as macrophages, NK cells, or cytotoxic T cells, must be

present in the tumor (22). Finally, many tumors are known to secrete compounds

that downregulate the immune response (23, 24), or to have decreased effectorcells as a consequence of hypoxia (25). Despite these impediments, preclinical and

clinical data with improved antibody-based molecules continue to demonstrate a

role for antibody-based therapy as a component of the oncologic armamentarium.

ANTIBODY THERAPEUTICS

The ability to identify therapeutic targets and produce antibodies with limited

immunogenicity has led to the production and testing of a host of agents, several ofwhich have demonstrated clinically important antitumor activity and have received

FDA approval for treatment of cancer. We focus here on antibodies with proven

efficacy, classified according to the tumor or antigen system that is targeted.

Hematologic Malignancy Antigens

B CELL ANTIGENS Initial studies employing antibodies directed against B cell

determinants showed that the passive administration of these antibodies led to

clearance of circulating tumor cells and rare objective clinical responses (26). In

a series of landmark studies, Levy and colleagues prepared customized antibodiesreactive with a given lymphoma patients idiotype that was uniquely expressed

on the surface of the malignant B cell clone. Each patients idiotype served as a

tumor-specific signature that could be targeted by a customized MAb. The pro-

cedures for preparing such antibodies for each patient were laborious, but 50%

of treated patients experienced significant clinical responses, with some patients

achieving durable complete remissions (19). The addition of chemotherapy agents,

interferon, or other cytokines did not appreciably improve treatment outcomes.

Effective therapy had to overcome circulating lymphoma idiotype proteins that

diverted antibodies from their cellular targets, and resistance to therapy resulted inpart from the emergence of idiotype-negative variants. The mechanisms underlying

responses to these antibodies have not been completely elucidated but may include

mechanisms such as ADCC (see below) and perturbation of signal transduction

through idiotype engagement (27). Due to the immunosuppression of lymphoma

patients, relatively few patients developed HAMA that interfered with repeated

therapeutic antibody administration. Thisexciting approach was very cumbersome,

as it required the generation of patient-customized reagents. These results could

not be replicated using antibodies that recognize shared idiotypes expressed by a

large proportion of lymphoma patients (28). Despite this set of impediments, theseimportant observations have informed much of the subsequent work in this field.

CD52 The CAMPATH-1 antibody has specificity for CD52, a glycopeptide that

is highly expressed on T and B lymphocytes. It has been tested as a therapeutic

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agent for chronic lymphocytic and promyelocytic leukemias, as well as other

non-Hodgkins lymphomas, and as a means to deplete T cells from allogeneic trans-

plant grafts. Half of the patients with fludarabine-resistant chronic lymphocytic

leukemia or B-prolymphocytic leukemia exhibited clinical responses toCAMPATH-1 (29). A larger phase II study reported a 42% response rate in pa-

tients with relapsed or refractory chronic lymphocytic leukemia, but at the cost of

an increase in opportunistic infections and septicemia. CAMPATH-1 has also been

evaluated as first-line therapy for patients with chronic lymphocytic leukemia. All

patients responded with loss of peripheral blood malignant lymphocytes. How-

ever, patients with involvement of lymph nodes and/or spleen were less likely to

respond completely. There was evidence of reactivation of cytomegalovirus in-

fections. Subcutaneous administration of the antibody was found to be safe and

effective. A humanized version of the antibody (CAMPATH-1H) has been exten-sively tested.

In a phase II multicenter study of CAMPATH-1H in previously treated patients

with low-grade non-Hodgkins lymphomas, 50 patients with relapsed or refrac-

tory disease were treated with 30 mg of CAMPATH-1H 3 times weekly for up

to 12 weeks (30). Infection, anemia, and thrombocytopenia were common, and

myocardial infarction occurred in one patient with a prior history of angina and

congestive heart failure. The overall response rate was 20% (16% partial response,

4% complete reponse). Responses were short in duration, with a median time to

progression of 4 months. Patients with mycosis fungoides responded more fre-quently and had a longer time to progression (10 months) than did patients with

low-grade non-Hodgkins lymphoma (4 months). Treatment was associated with

reactivation of herpes simplex, oral candidiasis,pneumocystis cariniipneumonia,

cytomegalovirus pneumonitis, pulmonary aspergillosis, disseminated tuberculo-

sis, and seven cases of pneumonia and septicemia.

CAMPATH-1 has been used to deplete T cells from allogeneic transplant grafts

in patients with hematologic malignancies (31, 32). The initial study suggested

that the addition of CAMPATH-1 significantly decreased graft rejection and graft-

versus-host disease compared with conventional therapy. However, the frequencyof graft rejection and graft failure was higher. A retrospective review of patients

with acute lymphocytic or acute myelogenous leukemia who underwent allogeneic

transplantation with CAMPATH-1purged marrow suggested no impact on the

graft-versus-leukemia effect, as there was no increase in leukemia relapse (33). Ad-

ditionally, the incidence of Epstein-Barr virus (EBV)related lymphoproliferative

disorders was decreased in allogeneic transplant patients who had T cell depletion

with CAMPATH-1 therapy compared with other methods of T cell depletion (e.g.,

E-rosettes or other MAb) (34). CAMPATH-1 eliminated B cells within the graft,

thus eliminating a potential reservoir of EBV or targets for subsequent infection.

CD20 The testing and evaluation of the chimeric anti-CD20 antibody, IDEC-

C2B8, also known as rituximab, demonstrated impressive clinical responses.

Rituximab is the first MAb to be approved by the FDA for use in human malignancy

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(35, 36). Rituximab is humanized and multiple doses can be safely administered.

In vitro studies have demonstrated multiple mechanisms by which anti-CD20 an-

tibodies lead to cell death: ADCC, complement-mediated lysis, and apoptosis that

is diminished by inhibitors of Lck and Fyn tyrosine kinases, calcium chelators,and caspase inhibitors (37).

In the phase I study to determine the maximum tolerated dose, patients with re-

lapsed low-grade and intermediate/high-grade non-Hodgkins lymphoma received

four weekly infusions of rituximab (37). Thrombocytopenia and B cell lympho-

cytopenia were observed. The lymphocytopenia persisted for 36 months. Six of

18 patients, all of whom had low-grade lymphomas, demonstrated partial responses

(33%). Phase II studies using the maximum tolerated dose, 375 mg/m2, confirmed

the efficacy of this therapy, demonstrating 46% and 48% response rates in two

separate studies (38, 39). Although the numbers of circulating B cells were re-duced by therapy, there were no documented changes in serum immunoglobulin

levels. Bacterial and viral infections were seen in patients with relapsed indo-

lent lymphomas, but in contrast to the CAMPATH-1 experience, treatment with

rituximab did not result in significant morbidity due to infections. Patients with

small-lymphocytic-B-cell lymphoma had lower response rates, probably related

to the lesser expression of CD20 on these tumor cells.

Although phase I testing had not suggested significant activity in intermediate/

high-grade lymphomas, a phase II trial evaluated rituximab in relapsing or refrac-

tory diffuse large B cell lymphoma, mantle cell lymphoma, or other intermediate-or high-grade B cell non-Hodgkins lymphomas (40). The study randomized

54 patients to either 8 weekly treatments of 375 mg/m2 intravenous rituximab or

375 mg/m2 in week one followed by 7 weekly intravenous infusions of 500 mg/m2.

Five complete responses and 12 partial responses were observed, for an overall

response rate of 31%; there was no evidence of superiority of either treatment reg-

imen. Patients with refractory disease and those with histologies other than diffuse

large B cell lymphoma appeared to have lower response rates. A novel use of rit-

uximab has been reported for cutaneous B cell lymphoma; patients who received

intralesional injections of the antibody showed partial clearing of nodules (41).Rituximab has been tested in conjunction with chemotherapy (42). Preclinical

data show that this antibody can sensitize chemotherapy-resistant cell lines to the

cytotoxic effects of chemotherapy (43). Forty patients with low-grade or follicular

B cell non-Hodgkins lymphoma were enrolled in the study. Thirty-five patients

received all six planned cycles of CHOP every 21 days, with six infusions of

rituximab at a dose of 375 mg/m2 given before, during, and after chemotherapy.

Three patients did not complete treatment due to intercurrent infections (n = 2)

and patient choice (n = 1), and two patients were withdrawn from the study prior to

therapy. The overall response rate was 95% (38/40), with 55% complete responsesand 40% partial responses. Fewer complete responses were noted in patients with

bulky disease. Median response duration and time to progression had not been

reached after 29+ months of follow-up. Seven of 8 patients who had initially

been positive for the bcl-2 translocation became negative for the translocation

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by PCR assay after therapy; this has not been seen with CHOP chemotherapy

alone (44). A preliminary analysis of a randomized study in elderly patients with

diffuse large-cell lymphoma comparing standard CHOP chemotherapy to CHOP

with rituximab demonstrated a 76% complete response rate in the combinationarm compared with 60% complete response rate in the chemotherapy arm without

significant differences in toxicity between the two groups (40). Furthermore, the

addition of rituximab prolonged event-free survival and overall survival.

Rituximab also has been evaluated in conjunction with fludarabine in low-grade

lymphomas. Significant leukopenias were associated with thiscombination, requir-

ing a dose reduction in the fludarabine. An interim analysis after accruing 30 of the

40 planned patients revealed a response rate of 93% with median response dura-

tion of over 14 months. Cytogenetic responses were also observed in some patients

(45). The significance of such responses is unclear. In a study of rituximab withCHOP chemotherapy in patients with mantle cell lymphoma, loss of a cytogenetic

marker was not associated with improved progression-free survival (46).

Another setting in which rituximab has been tested is following high-dose

chemotherapy with autologous transplantation (47). Patients with at least a 75%

reduction in tumor volume following CHOP chemotherapy underwent high-dose

chemotherapy with stem cell transplantation. Two and six months following trans-

plant, patients received four weekly treatments with rituximab. A preliminary

report of this study describes four patients who have received one post-transplant

cycle and eight who have received two post-transplant cycles. Two patients are re-ported with a partial response and another two patients with unconfirmed complete

response.

Circulating CD20-positive cells, including lymphoma cells, may affect the ef-

ficacy of rituximab. Peak levels of circulating antibody inversely correlate with

pretreatment B cell counts as well as the bulk of tumor (38, 39). Greater numbers

of peripheral lymphocytes and/or tumor bulk serve as an antigen sink, removing

antibody from the circulation. For patients with bulky disease, a higher antibody

dose or a greater number of cycles may be warranted, since patients with lower

serum rituximab concentrations have had statistically significant lower responserates. A dosage of eight cycles of weekly rituximab has been used safely in low-

grade and follicular non-Hodgkins lymphoma (48). The efficacy of rituximab in

chronic lymphocytic leukemia is clearly affected by lower circulative levels of

the antibody. Initial studies revealed a 20% response rate. Chronic lymphocytic

leukemia has lower antigen density than many lymphomas that express CD20, and

the cells circulate, acting as an antigen sink. A dose-escalation study demonstrated

a clear dose-response relationship (49).

Rituximab therapy rarely selects for the emergence of an antigen-negative pop-

ulation of tumor cells. At the present time, little is known about mechanisms ofresistance (49a). This phenomenon has been documented in a patient with a follic-

ular mixed small- and large-cell lymphoma, who was treated with rituximab after

multiple chemotherapy regimens (50). Approaches to overcome antigen-negative

populations include combining rituximab with chemotherapy or with antibodies

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targeting other antigens expressed by the lymphoma or leukemia being treated.

For example, chronic lymphocytic leukemia expresses both CD20 and CD52.

Studies are evaluating combination therapy with both rituxan and CAMPATH-1H

(51). Other antigens that could target B cell lymphomas and leukemias includeCD22, which is expressed on pro-B and pre-B cell lymphomas, and the ma-

jor histocompatability class II antigen, HLA-DR, found on B lymphocytes as

well as macrophages and dendritic cells. Epratuzumab, which targets CD22, and

apolizumab, which targets a subset of HLA-DR molecules, are currently undergo-

ing testing (52). Clearly these agents are candidates for combination therapy with

rituximab.

In some patients with circulating blood tumor cells, rituximab therapy has

induced an infusion-related syndrome characterized by fever, rigors, thrombocy-

topenia, tumor lysis, bronchospasm, and hypoxemia, requiring discontinuationof the antibody infusion. Symptoms typically resolve with supportive care and

patients may continue further therapy without sequelae (53). Another case report

documents rapid tumor lysis in a patient with B cell chronic lymphocytic leukemia

with a pretreatment lymphocytosis of>100 109/liter (54). Occasional severe

mucocutaneous reactions have been reported, some of which have been fatal.

These present as paraneoplastic pemphigus, Stevens-Johnson syndrome, and toxic

epidermal necrolysis (55).

Solid Tumor Antigens

HER-2/NEU HER-2/neu(c-erbB-2), a member of the epidermal growth factor re-

ceptor (EGFR) family, has been targeted for antibody therapy because it is over-

expressed on 25% of breast cancers, as well as other adenocarcinomas of the

ovary, prostate, lung, and gastrointestinal tract. Trastuzumab (56, 57), also known

as Herceptin and rhuMAb HER2, is a humanized antibody derived from 4D5, a

murineMAb, which recognizes an epitope on the extracellular domain of HER-

2/neu. A phase II trial in women with metastatic breast cancer reported an objec-

tive response rate of 11.6%, with responses seen in the liver, mediastinum, lymph

nodes, and chest wall. Patients received ten or more treatments with the antibody,

and none developed an antibody response against trastuzumab. In a second phase

II study, 222 women with metastatic breast cancer were treated with 2 mg/kg of

trastuzumab weekly, with an objective response rate of 16% (56). The median

response duration was 9.1 months, with a median overall survival of 13 months,

both of which are superior to outcomes reported for second-line chemotherapy in

metastatic disease. In each of these trials, about 30% of the patients had stable dis-

ease, lasting more than 5 months. Overexpression of HER-2/neuthat is associated

with gene amplification correlates with a clinical response to trastuzumab. Earlier

studies utilized variable criteria to define HER2 positivity that may account for

the differences in the response rates. The standard assay for determining HER2

expression is the Hercept test. Breast tumors with 3+ expression by this assay

correlate with gene overexpression; those with 2+ expression require testing by

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fluorescence in situ hybridization to confirm gene amplification. Intriguingly, pre-

clinical studies have demonstrated decreased expression of vascular endothelial

cell growth factor (VEGF) and vascular permeability factor with 4D5 therapy, sug-

gesting that an antiangiogenesis mechanism may account for some of the clinicalimpact of this antibody (58). Trastuzumab continues to be evaluated clinically in

diverse adenocarcinoma types.

A large, randomized phase III trial evaluating cytotoxic chemotherapy alone

and with trastuzumab has shown the efficacy of combination therapy (59). Patients

receiving initial therapy for metastatic breast cancer were treated with cyclophos-

phamide plus doxorubicin or epirubicin, or with paclitaxel if they had received

an anthracycline in the adjuvant setting. Patients were randomized to receive this

chemotherapy alone or in combination with weekly antibody therapy. Response

rates for combination therapy with an anthracycline regimen increased from 43%to 52% with the addition of trastuzumab. Using a taxane regimen, response rates

increased from 16% to 42% with the addition of trastuzumab. In addition, there

was evidence that the addition of trastuzumab to chemotherapy improved sur-

vival at one year by 16% (60) and improved survival at 29 months by 25% (61).

Myocardial dysfunction was observed more frequently in patients receiving dox-

orubicin or epirubicin when trastuzumab was added. Therefore, trastuzumab is not

recommended in combination with anthracyclines.

Based on these clinical trial results, trastuzumab was approved by the FDA

to treat women with metastatic breast cancer with HER-2/neu overexpression,given either alone or in combination with paclitaxel. Herceptin also has activity in

combination with vinorelbine (62), docetaxel, cisplatin (63), and the combination

of gemcitabine and paclitaxel (64). In addition, breast cancer patients with lymph

node involvement whose cancers overexpress HER-2/neu are candidates for partic-

ipation in a randomized phase III trial evaluating standard adjuvant chemotherapy

with or without trastuzumab.

The use of Herceptin for breast cancer has been a therapeutic success. Al-

though other adenocarcinomas may overexpress HER-2/neu, clinical trials have

not documented consistent therapeutic successes in other cancers. The Gyneco-logic Oncology Group evaluated Herceptin in patients with recurrent or refractory

ovarian or primary peritoneal carcinoma (65). An overall response rate of 7.3% was

found with a median treatment of 8 weeks and median progression-free interval of

2 months. HER-2/neuhas also been found on prostate cancer and nonsmall-cell

lung cancer.

Epidermal Growth Factor Receptor

EGFR is overexpressed on many cancers. The receptor and its ligands, epidermal

growth factor (EGF) and transforming growth factoralpha (TGF), act in an au-

tocrine loop to stimulate the growth of breast cancer cells. In vitro, some anti-EGFR

antibodies have been shown to inhibit the binding of the receptor ligands (66, 67).

Antibodies that block the binding of EGFR ligands limit receptor activation by

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tyrosine kinases and inhibit growth of normal fibroblasts (68) as well as tumor cells

in culture (69). Also, combining anti-EGFR antibodies with cisplatin significantly

decreases the IC50of cisplatin (70), and cures of established tumors have occurred

when anti-EGFR antibodies are combined with cisplatin (71) or doxorubicin (72).Sequential treatment with topotecan followed by C225 (a chimerized version of the

anti-EGFR antibody MAb225) in vitro leads to supra-additive growth inhibition in

breast, ovary, and colon cancer cell lines (73). C225 delivered simultaneously with

radiation reduces phosphorylation of the EGFR and STAT-3, enhancing apoptosis

(74). In vitro and in vivo studies suggest that anti-EGFR antibodies may lead to

terminal differentiation of squamous cell carcinoma cells, with accumulation of

cells in G0G1 phases of the cell cycle and expression of cell surface markers such

as involucrin and cytokeratin-10 (75).

MAb225 blocks in vitro phosphorylation of the EGFR and induces receptor in-ternalization, as occurs with binding of the natural ligand (76). However, receptor

processing is slower with antibody engagement than with natural ligand engage-

ment (77). Smaller bivalent F(ab)2and univalent Fab forms of this antibody also

inhibit growth and decrease receptor phosphorylation, although the bivalent form

is superior to the monovalent form (78). Since the smaller fragments lead to tumor

regressions, the efficacy of antibody therapy is not dependent on ADCC, as these

smaller fragments lack the Fc portion of the antibody required for ADCC. Rather,

the efficacy of MAb225 is due to its ability to inhibit binding of the natural ligand,

limit receptor phosphorylation and thus downstream signals, and induce receptorinternalization.

The chimeric form of MAb225, C225, has been evaluated in vitro and in vivo in

hormone-sensitive and hormone-refractory prostate cancer (79). EGF is a strong

chemoattractant for prostate cancer cells. Blocking the EGFR receptor with C225

in vitro decreases migration of prostate cancer cells in a dose-dependent manner by

decreasing phosphorylation of the EGFR (80). C225 has also been shown to cause

cell cycle arrest and decrease proliferation of prostate cancer cells (81, 82). The

binding of C225 to the EGFR results in multiple events that decrease proliferation

and possibly metastatic potential in prostate cancer. It has also been shown toinhibit the expression of VEGF and vascular permeability factor, both of which

are involved in the induction of angiogenesis (59).

Phase I clinical trials of C225 alone or in combination with cisplatin have been

conducted in patients with cancers overexpressing EGFR (83). Skin toxicity in

the form of flushing, seborrheic dermatitis, and acneiform rash were observed at

doses higher than 100 mg/m2. The rash commonly presents on the head, neck,

and trunk and resolves without scarring once the treatment has been discontinued

(84). Epiglottitis resulting in severe shortness of breath was noted when C225

was combined with cisplatin at 100 mg/m2, but not at 60 mg/m2 (83). Anotherphase I study, in which patients with recurrent head and neck cancer received

cisplatin every three weeks and C225 weekly, found no such toxicity (85). Of the

9 evaluable patients in that study, 2 patients previously treated with cisplatin had

complete responses and an additional 4 had partial responses. In addition, tumor

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biopsies demonstrated a dose-dependent saturation of the EGFR and loss of EGFR

tyrosine kinase activity following repeated doses of the antibody. Phase III trials

are now evaluating the impact of adding C225 to cisplatin in patients with recurrent

or metastatic head and neck cancer and the benefit of adding C225 to radiationtherapy in patients with locally advanced head and neck cancer. Studies are also

ongoing in patients with nonsmall-cell lung cancer, who receive C225 along with

various chemotherapy regimens including carboplatin and paclitaxel, gemcitabine

and carboplatin, and docetaxel (84).

ABX-EGF, another antibody that targets EGFR, is also under clinical develop-

ment. This antibody was produced in a transgenic mouse in which murine antibody

genes were inactivated and replaced by genes that encode the human sequences

(86). The resulting fully human antibody has been shown to cause inhibition of

the tyrosine phosphorylation of the receptor and internalization of the receptor.The inhibitory concentration of the antibody is lower than that of MAb225, the

parent antibody of C225. Preclinical studies in animals have shown eradication

of EGFR tumors with ABX-EGF antibody therapy alone, whereas similar studies

with C225 have required the addition of chemotherapy to produce similar results

(87). ABX-EGF is currently in clinical trials.

Ep-CAM (EGP-2/GA 733-2)

The 17-1A antibody, which recognizes Ep-CAM, has undergone extensive clinicaltesting, with some studies suggesting efficacy in colorectal carcinomas. The murine

antibody has been replaced by a human chimeric construct that offers increased

mononuclear cell-mediated ADCC (88), a longer half-life, no development of hu-

man antimouse antibody (HAMA), and radiolocalization to known sites of disease

(89). Clinical trials have also incorporated cytokines because of in vitro data that

demonstrate improved target cell killing by apoptosis with interferon- (90) and

increased ADCC with interferon- (91), GM-CSF (92, 93), the combination of

GM-CSF and interleukin (IL)-2 (94), IL-4 (95), and IL-8 (96). These studies have

formed the basis for clinical trials incorporating the antibody and cytokines suchas interferon- (97) and GM-CSF (98). Although these studies have shown ev-

idence for cytokine immunomodulation, no consistent pattern of clinical benefit

has emerged from therapy with these combinations.

The therapeutic use of 17-1A has also been shown to induce potentially effective

anti-idiotypic antibodies (99101), an effect that was increased by concomitant

therapy with GM-CSF (102), and to increase infiltration of macrophages and of

CD4+ and CD8+ T cells within tumors (103). Induction of T cells against anti-

idiotypic epitopes has also been evaluated (104). Five of ten patients with anti-

idiotypic antibodies were also found to have induction of EP-CAM antigen-specificT cells. Four of these patients showed a clinical response and their T cells demon-

strated a proliferative response in vitro when stimulated with an anti-idiotypic

antibody, in contrast to the six patients without evidence of T cells against anti-

idiotypic epitopes.

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Initial phase I studies with 17-1A yielded promising results. Several studies

demonstrated responses in patients with metastatic cancers of the gastrointesti-

nal tract with only one intravenous dose of antibody. One patient received an

intrahepatic infusion of autologous mononuclear cells mixed with 17-1A, whichresulted in regression of hepatic metastases. Antibody therapy was well tolerated

with mild nausea, vomiting, or diarrhea. Phase II studies in colon and pancreatic

cancers were less encouraging (105, 106). Repeat-dose injections and combina-

tions with cytokines to enhance effector cell number and activity did not result in

significant response rates, although in vitro assays of patient effector cells revealed

increased activity with cytokine therapy. Repeat-dose schedules with higher doses

were theorized to induce tolerance to the murine antibody, but this maneuver had

no significant impact on clinical response (106). Some trials reported evidence

of induction of anti-idiotypic antibodies, correlating with clinical response in onetrial. The overall lack of efficacy seen in these studies may have resulted from the

large tumor burden or the associated immunosuppression in these patients with

metastatic disease.

The initial study evaluating 17-1A in the adjuvant setting suggested possible

efficacy. A phase III clinical trial of patients with lymph-nodepositive colorectal

cancer randomized patients to observation or therapy with 17-1A. The surgical

approach was standardized and agreed to by all participating surgeons. All pa-

tients were followed post-operatively in a similar manner, irrespective of treat-

ment. Of 189 patients, 166 were evaluable for overall survival and disease-freesurvival. Therapy with 17-1A was well tolerated except for malaise, low-grade

fevers and chills, and mild gastrointestinal discomfort. Four anaphylactic reactions

were treated without sequelae. At five years of follow-up, the death rate was 36%

in the 17-1A group in contrast to 51% in the observation group, and the calculated

recurrence rate was 48.7% versus 66.5% (107). At seven years the death rates were

43% (17-1A) and 63% (observation), and the calculated recurrence rates were 52%

and 68%, respectively, demonstrating a continued benefit in patients who received

17-1A (108). Treatment with 17-1A was associated with a decreased incidence

of metastatic disease but did not alter the incidence of local failure. This shiftin failure pattern was felt to represent the ability of 17-1A to eradicate isolated

metastatic cancer cells but not bulkier disease. Alternatively, altered vasculature

due to surgery and scar tissue may have limited antibody diffusion to tumor cells.

Another factor potentially accounting for the apparent lack of efficacy of 17-1A

on local control is that 11 patients in the observation group received pre- or post-

operative radiation therapy alone or in combination with chemotherapy. This trial

has been criticized because of higher rates of recurrence and death in the control

arm than would be anticipated. However, it is intriguing because it demonstrates

an effect of antibody-based therapy in the adjuvant setting of colorectal cancer.Recent clinical trials were designed to confirm these results in stage II colon

cancers and to test the value of adding 17-1A to standard chemotherapy in patients

with stage III disease. Therapy with antibody alone is unlikely to be effective in all

patients owing to the antigenic heterogeneity of cancer cells. Combining standard

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adjuvant therapy with 17-1A would introduce therapies that have different mech-

anisms of action, are cell-cycledependent and -independent, and allow death of

cancer cells irrespective of antigen expression patterns. A preliminary report of

the phase III trial of 17-1A versus no therapy in the adjuvant setting revealed nodifference in overall survival in patients with stage II colorectal at a median follow

up of 13.4 months (109); a randomized study of chemotherapy with or without the

antibody in patients with stage III disease has shown a minimal benefit of ques-

tionable significance in a preliminary analysis. Overall survival was 81.6% for

combination therapy versus 78.9% for chemotherapy alone; this was not a statisti-

cally significant difference. Irrespective of the final analyses of these large studies,

the value of treating colorectal cancer patients with this antibody is unproven and

likely to be low.

Vascular Endothelial Growth Factor

Molecules that are selectively expressed in the tissue matrix surrounding tumor

are potentially important targets for antibody therapy. The first of these targets

to be exploited resides in the vasculature that feeds tumor cells. Bevacizumab is

a humanized MAb that blocks binding of the vascular endothelial growth factor

(VEGF) to its receptor on vascular endothelium. VEGF is produced by many

cancers to stimulate the growth of new blood vessels, and in some studies its

expression at tumor sites is correlated with risk for metastases. A phase I clinicalstudy of bevacizumab tested doses of 0.110 mg/kg infused on days 1, 28, 35, and

42 (110). No severe toxicities were found. At doses of 310 mg/kg, increases in

systolic and diastolic blood pressure of>10 mm Hg were noted during therapy

(110), and subsequent studies have confirmed that hypertension is a toxicity of this

agent (111). Two patients receiving3 mg/kg experiencedbleeding intotheir tumors.

One patient with hepatocellular carcinoma bled into a previously undiagnosed

brain metastasis. Another patient with an extremity sarcoma and lung metastases

bled into the extremity mass and experienced hemoptysis.

In a phase II study of bevacizumab combined with carboplatin and paclitaxelin patients with nonsmall-cell lung cancer, 6 of 66 patients developed pulmonary

hemorrhage, 4 of whom died (112). Patients with squamous cell lung cancers and

tumors with evidence of cavitation or squamous cell histology appeared to be at

higher risk of hemoptysis. Interestingly, a study of bevacizumab and chemotherapy

in metastatic colorectal cancer found an increase in thrombotic episodes as well

as hemorrhage. The incidence of thrombotic events was higher at the 5 mg/kg

dose level than at the 10 mg/kg dose level. The majority of cases were deep venous

thromboses, but one cerebral vascular accident and one pulmonary embolism were

reported.Clinical trials have suggested a therapeutic benefit from bevacizumab. Although

the phase I study demonstrated no objective responses, 12 of 23 patients had

stable disease during the trial (70 days) and an additional patient with renal cell

carcinoma experienced a minor response (110). A phase II study was conducted

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in 25 women whose metastatic breast cancer had progressed following at least one

anthracycline or taxane-based regimen. Clinical responses were noted at doses of

3 mg/kg (n = 1/18) and 10 mg/kg (n = 2/17) (111). All responses were in lymph

nodes or subcutaneous skin nodules. Bevacizumab has also been combined withchemotherapy in patients with nonsmall-cell lung cancer (113) and metastatic

colorectal cancer (114). The response rate and time to progression in patients

with lung cancer improved when carboplatin and paclitaxel were combined with

bevacizumab at 10 mg/kg, but not at 5 mg/kg. A similar result was found in patients

with colorectal cancer receiving 5-fluorouracil and folinic acid plus bevacizumab,

but the improved response rate was seen at the 5 mg/kg dose. Time to progression

improved at the both 5 mg/kg and 10 mg/kg, but to a greater extent at 5 mg/kg.

RADIOIMMUNOTHERAPY

Radioimmunoconjugates

A tumor-specific antibody that does not alter tumor growth properties can be

armed with a radioisotope to create a cytotoxic agent. Even antibodies that are

effective antitumor agents in an unconjugated state can have enhanced efficacy

with the addition of a cytotoxic radioisotope. For example, although therapy with

the anti-CD20 MAb rituximab is associated with a significant antitumor effect

in patients with non-Hodgkins lymphoma, significantly more patients exhibitcomplete responses when the 90Y parent MAb Y2B8 is added to the treatment

protocol (115). The recent availability of a panel of antibody-based constructs with

a range of in vivo half-lives now makes it possible to select antibody/radioisotope

combinations with matched biologic and physical half-lives. This type of pairing

enables the majority of the decay, and therefore cytotoxic emissions, to occur

while the greatest quantity of labeled antibody is localized in the tumor. Williams

et al. developed formulas, termed the imaging figure of merit (IFOM) and therapy

figure of merit (TFOM), to determine which isotopes best pair with a series of

antibodies and fragments based on the rate of MAb clearance, isotope half-life, and

emission track length (116). This approach can be used to optimize the efficiency

and specificity of an imaging or treatment system.

Until recently, most RAIT studies employed 131I, but improvements in labeling

strategies and the availability of large quantities of radiopharmaceutical-grade 90Y

have enabled the study of 90Y conjugates. Now, a wide variety of therapeutic

radioisotopes are available for RAIT. They fall into two general categories, those

that emit shorttrack-length (e.g., one cell length) alpha particles with a high

linear energy transfer (LET) and those that emit longertrack-length (e.g., 50 cell

lengths) beta particles with a low LET. Because the LET is a measure of the energy

deposited along the track of the emitted particle, a high LET is associated with a

greater probability of tumor cell death. This is in part dictated by the difference

in the size of the particles. An alpha particle is equivalent to a helium nucleus

whereas a beta particle is an electron. Alpha particles tend to kill cells by causing

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irreparable double-strand DNA breaks; beta particles lead to single-strand DNA

breaks.

Of the available beta-emitting radioisotopes, 90Y (t1/2 = 64 h) is rapidly gaining

dominance in most RAIT protocols owing to its ease of labeling and absence of agamma emission. The other commonly used radionuclide, 131I (t1/2 = 8.02 days),

requires a trained radiopharmacist and appropriately shielded/vented radiophar-

macy because of its volatile nature. Other beta-emitting radioisotopes that have

shown promise for radioimmunotherapy include 67Cu (t1/2 = 61.8 h) and 177Lu

(t1/2 = 6.73 days). There are also a number of alpha-emitting radioisotopes that are

potential partners with antibody-based molecules for RAIT. These include 213Bi

(t1/2 = 45 min), 212Bi (t1/2 = 61 min), and

211At (t1/2 = 7.2 h). Whereas the

half-life of 211At is compatible with the pharmacokinetics of several engineered

antibody-based molecules, the very short half-lives of the bismuth radioisotopeslimit their utility to the setting of disseminated disease or to pretargeted RAIT

(described below).

Some clinical trials have demonstrated partial, short-lived clinical responses in

some patients with advanced, solid tumors, but most of the significant advances in

RAIT have occurred in the treatment of hematologic neoplasms. Lymphomas and

leukemias remain the most sensitive tumor targets for RAIT, presumably because

of their intrinsic sensitivity to radiation and the relatively good access of radioim-

munoconjugates to the malignant cells that comprise these neoplasms. Patients

with hematologic neoplasms are also less likely to mount HAMA responses whenforeign MAbs are employed.

Radioimmunotherapy for Hematologic Malignancies

B-1 The CD20 antigen has been a very effective target for RAIT. Press and col-

leagues used high, marrow-ablative doses of 131I-B1 (tositumomab or Bexxar)

anti-CD20 RAIT in the setting of autologous bone marrow transplantation. In a

total of 42 patients with chemotherapy-refractory lymphomas, 24 exhibited MAb

distributions that were predictive of a favorable response and 19 received high-

dose RAIT. Of the RAIT group, 84% exhibited a complete remission, and 11%

had a partial remission (117). Many of the complete remissions observed with this

approach have proven to be durable, with 62% exhibiting progression-free survival

at 2 years. Low-dose, non-myeloablative RAIT with 131I-B1 has also proven effi-

cacious, showing 50% response rates, with a median duration of 16.5 months, in

patients with chemotherapy-refractory B cell lymphoma (118, 119). Preliminary

studies have also shown that RAIT may be useful as a conditioning regimen prior to

bone marrow transplantation in patients with acute myelogenous leukemia (120).

IBRITUMOMAB TIUXETAN Ibritumomab tiuxetan (Zevalin) is another MAb that

targets an epitope on the CD20 antigen that is distinct from the B1 epitope. Another

distinction between the ibritumomab trials and the tositumomab trials described

above is that Ibritumomab employs 90Y in place of131I. This antibody is the first,

and to date only, MAb licensed for use in RAIT in the United States.

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A number of clinical trials have evaluated ibritumomab in over 230 patients

with relapsed or refractory low- or intermediate-grade non-Hodgkins lymphoma

(reviewed in Reference 121). The most comprehensive study was a randomized

phase III trial that compared the safety and efficacy of a single dose of ibritumomabwith a standard treatment course of rituximab in 143 patients with low-grade, fol-

licular or transformed non-Hodgkins lymphoma. In this trial, an overall response

rate of 80% and a complete response rate of 30% was observed in the group treated

with ibritumomab versus an overall response rate of 56% and a complete response

rate of 16% in the group treated with rituximab (122).

LYM-1 Lym-1 is specific for a human leukocyte antigen (HLA-DR) expressed

in >95% of B cell tumors. This murine MAb has not been humanized. In phase

I/II clinical trials,131

I-Lym-1 has shown significant antitumor activity. Low-dose,fractionated treatment of 30 patients (25 with non-Hodgkins lymphoma and 5

with chronic lymphocytic leukemia) resulted in 3 complete responses and 14 partial

responses (123). When treated at the non-myeloablative maxiumum tolerated dose

of 100 mCi/m2, 11 of 21 patients responded favorably (7 complete responses, 4

partial responses) (124).

M195 The cell surface antigen CD33 is expressed on most myeloid leukemic blasts

and leukemic progenitor cells. Its normal tissue expression is limited to committed

normal myelomonocytic and erythroid progenitor cells and (at low levels) earlyhematopoietic stem cells. M195, a murine anti-CD33 MAb, has been used to de-

liver therapeutic doses of131I in combination with busulfan or cyclophosphamide

to eliminate disease before bone marrow transplantation (125). HuM195, a hu-

manized version of M195, has been employed as a vehicle for the RAIT of acute

and chronic myelogenous leukemia. HuM195 RAIT resulted in minor responses

in 8 of 12 patients treated with 90Y-conjugated Mab and 13 of 18 patients treated

with 213Bi-conjugated Mab (126, 127). The 213Bi studies were the first use of alpha

particles in RAIT.

Radioimmunotherapy of Solid Malignancies

Significantly less progress has been made in the treatment of solid malignancies

with RAIT. In most reported trials in patients with solid tumors (e.g., breast car-

cinoma, colon carcinoma, ovarian carcinoma), RAIT is associated with, at best,

stable disease. Although reports of a greater response do appear, their rarity gives

them the aura of an Elvis sighting. A phase II trial of 75 mCi/m2 of 131I-labeled

CC49 MAb with interferon (to enhance target antigen expression) in patients with

metastatic prostate cancer reported minor radiographic responses and relief of pain

(128). Similar results were reported for a phase I study of 90Y-2IT-BAD-M170 in

patients with androgen-independent metastatic prostate cancer (129). In a recent

phase I trial in patients with recurrent or persistent ovarian cancer, intraperitoneal177Lu-labeled CC49 MAb, plus interferon and Taxolled to partial responses in 4 of

17 treated patients and progression-free intervals in patients without measurable

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disease (130). Transient clinical responses were also observed in a phase I trial in

metastatic breast cancer patients treated with 90Y-BrE-3 (131). Still, several new

agents are currently undergoing clinical evaluation, many of which have novel char-

acteristics (e.g., domain deletions or significant antitumor properties as a nakedMAb) that may translate into improved results.

Pretargeted Radioimmunotherapy

One new RAIT strategy recently examined in the clinic is pretargeted radioim-

munotherapy (PRIT). PRIT differs from traditional RAIT in that the antibody and

the radioisotope are delivered to the tumor in separate steps. In the PRIT protocols

employed to date, MAb-streptavidin (MAb-SA) or MAb-biotin (MAb-B) conju-

gates were administered to the patient. After the MAb conjugate localized in the tu-

mor, a clearing agent [e.g., a biotingalactosehuman serum albumin conjugate

or streptavidin (SA)] is administered that binds to the circulating MAb conjugate

and directs its clearance by the liver. After an additional 24 h, a biotin-chelate-90Y

complex is administered that binds specifically to the MAb-SA or MAB-B-SA

prelocalized in the tumor.

PRIT clinical trials have been performed in patients with glioma (132), colon

cancer (133), prostate cancer, and non-Hodgkins lymphoma (134). Of 48 glioma

patients treated by PRIT at doses of 6080 mCi/m2, 25% (12 patients) exhibited

a reduction in tumor mass ranging from>25% to 100%, with 8 responses lastinglonger than one year (132). In the PRIT treatment of patients with colon can-

cer, 8% (2 of 25) of the patients treated with pretargeted NR-LU-10 MAb and

110 mCi/m2 of90Y-DOTA-biotin had a partial response, and 16% (4 of 25) exhib-

ited stable disease. However, significant hematologic and nonhematologic toxic-

ities (diarrhea) were reported (133). A companion trial directed against prostate

cancer revealed similar toxicities (I. Horak, personal communication). Signifi-

cantly more success was observed in the PRIT of non-Hodgkins lymphoma. In

this trial, rituximab, which has antitumor properties in an unconjugated form, was

conjugated to SA as the first-step reagent. Six of seven patients treated with dosesof 30 or 50 mCi/m2 of90Y-DOTA-biotin exhibited objective regressions, with one

partial response and three complete responses (134). The toxicities reported in this

trial were also less severe and were limited to grade I/II nonhematologic toxicity

and transient grade III hematologic toxicity.

CONCLUSIONS

Therapeutic antibodies have made the transition from concept to clinical real-

ity over the past two decades. Many are now being tested as adjuvant or first-line

therapies to assess their efficacy in improving or prolonging survival. Modified an-

tibody structures, such as bispecific antibodies or smaller antibody fragments, have

yet to claim defined therapeutic roles, whereas radioimmunotherapy has clearly

demonstrated efficacy in some disease settings.

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TheAnnual Review of Medicineis online at http://med.annualreviews.org

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