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Introduction
A large number of genes in eukaryotic cells encode for
proteins that function as trans-membrane cell surface
receptors, of which many are endowed with intrinsic protein
tyrosine kinase activity. These receptor tyrosine kinases
(RTK) play important roles in the control of some of the
most fundamental cellular processes, including cell cycling,
proliferation and differentiation[1,2]. Abnormal cell growth
resulting from aberrant signal transduction has been
implicated in the initiation and progression of a variety of human
cancers[1,2], suggesting that various RTK pathways may
represent excellent targets for effective cancer
intervention[3,4]. In the past few years, many efforts have been
made by both academic laboratories and pharmaceutical/biotechnology
companies to identify and develop effective RTK
antagonists for cancer therapy[3_6]. Our understanding of how each
RTK signal transduction pathway participates in abnormal
cell growth and the molecules responsible for these events
has led to a variety of novel and increasingly
mechanism-based approaches to the development of RTK antagonists
for cancer therapy. As a result, a number of selective RTK
antagonists, including monoclonal antibodies (mAb) and
small molecular compounds that selectively inhibit growth
factor receptor activation and signal transduction, have been
successfully developed for the treatment of patients with a
variety of cancers[5_9].
Epidermal growth factor receptor (EGFR) is a
trans-membrane receptor encoded by the
c-erbB1 proto-oncogene with a molecular weight of approximately 170
kDa[10_13]. EGFR belongs to the subclass I family of RTK and is the receptor
to at least six distinct ligands, including EGF, transforming
growth factor-α (TGF-α), heparin-binding EGF, amphiregulin,
betacellulin and epiregulin[10_13]. The subclass I family of
RTK consists of EGFR (also known as HER1), HER2/neu
(erbB-2), HER3 (erbB-3) and HER-4
(erbB-4)[14_16]. Much evidence suggests that these receptors function in various
homodi-meric and heterodimeric pairs, depending on their
density on the cell surface, the concentrations of a particular
ligand and intrinsic dimerization preference between the
receptors[17]. EGFR is normally expressed in a wide variety of
epithelial tissues as well as in the central nervous system.
Binding of a ligand to the extracellular domain of EGFR leads to
receptor dimerization, followed by activation of the intrinsic
RTK activity and autophosphorylation of specific residues
within the receptor's cytoplasmic domain. These
phosphorylated residues serve as docking sites for other molecules
involved in the regulation of intracellular signaling cascades.
The major signaling cascades activated by EGFR include the
Ras/MAP kinase, PLC-gamma, PI-3 kinase/Akt and STAT3
pathways. The integrated biological responses to EGFR
signaling are pleiotropic and include enhanced cell mitogenesis,
cell motility, protein secretion, cell adhesion, invasion,
differentiation or dedifferentiation, and increased
neovascula-rization[11_16].
A large body of experimental evidence supports a role
for EGFR activation and signaling in the pathogenesis of
human cancers. In 1984, analyses of the EGFR gene and
protein revealed that its sequence was homologous to the
v-erbB proto-oncogene[10,18]. Direct preclinical evidence for
a role of EGFR in malignant transformation emerged from
studies in which the transfection of EGFR or TGF-α cDNA
was associated with cellular
transformation[19]. EGFR is
expressed in a variety of human solid tumors, including
squamous cell cancer of the head and neck (SCCHN) and
carcinomas of the colon and rectum, pancreas, lung, cervix, renal
cell, prostate, bladder and breast, as well as melanoma,
glioblastoma and meningioma[20,21]. Accumulating evidence
suggests that the level of EGFR overexpression is an important
factor that directly correlates with active proliferation of
malignant cells and poor prognosis of
patients[20,21]. In addition, several tumor types have been shown to coexpress EGFR and its ligands, leading to an autocrine
activation of the receptor and poor outcome in the
clinic[20,21]. Finally, mutants of EGFR, due to gene rearrangements that result in
in-frame deletion of portions of the extracellular domain of
the receptor, have been found in a significant fraction of
EGFR-expressing tumors. For example, the most common
mutation, EGFR variant III (EGFRvIII), with a deletion of
amino acids 6_273, which is frequently found in brain
tumors such as glioblastoma, results in a protein with
defective ligand binding capacity, but is constitutively activated
and its tumorigenicity in vivo is
enhanced[22,23]. Taken together, these data indicate that expression of EGFR in
human cancers has a significant effect on their biological
behavior; thus, providing the rationale for the development
of EGFR antagonists as potentially useful therapeutic
strategies for the treatment of EGFR-expressing
cancers[3_8,24].
Monoclonal antibodies as cancer therapeutics
In recent years, mAb, owing to their high specificity
towards a given target, have rapidly evolved from the ideal
of a "magic bullet" to a new class of practical and efficacious
cancer therapeutics. Traditional obstacles in antibody
therapy, such as immunogenicity of rodent-derived
antibodies and difficulty in producing antibodies in sufficient
quantity and quality for commercial application, are being rapidly
superseded by advancement in antibody engineering technologies, including antibody chimerization,
humanization and direct generation of fully human antibodies from
either phage display libraries or human transgenic
mice[25_28], as well as the development of efficient manufacturing
processes for high level production of mAb at costs that are
more economical than ever[29]. Since 1994, the United States
Food and Drug Administration (FDA) has approved 21
therapeutic mAb for clinical use, including 9 for oncology
indica-tions. In addition, there are several hundred mAb currently
being tested in clinical trials worldwide for various indications.
The majority of antibodies currently being used for
targeted therapy belong to the IgG class of immunoglobulins.
The effectiveness of an antibody-based therapeutic depends
on its ability to induce one of several biological mechanisms.
These include:
1. Blocking growth factor/receptor interaction and/or
downregulating expression of oncogenic proteins (or
receptors) on the cell surface. By interfering with important
growth factor/receptor signaling pathways, the mAb can
influence the growth and survival of tumor cells, and may
also potentiate the cytotoxic effects of chemotherapeutic
drugs and radiation. Several antibodies are believed to exert
their therapeutic efficacy mainly via this mechanism,
including bevacizumab (Avastin, an IgG1 antibody to vascular
endothelial growth factor [VEGF]), cetuximab (Erbitux, an IgG1
antibody to EGFR) and trastuzumab (Herceptin, an IgG1
antibody to HER2/neu)[30_32].
2. Recruiting effector mechanisms of the immune system,
such as antibody-dependent cellular cytotoxicity (ADCC)
and complement-mediated cytotoxicity (CMC). The ability
to mediate both ADCC and CMC is dependent on the isotype
of the mAb. For example, human IgG1 and IgG3 bind with
significantly higher affinity to human Fc receptors and are
much more effective mediators of ADCC than IgG2 and IgG4.
There is evidence to suggest that effector mechanisms play
an important role in the clinical antitumor efficacy of rituximab
(Rituxan, an IgG1 antibody to CD20), trastuzumab and
cetuximab[33_36]; all 3 mAb are of the human IgG1 isotype.
3. As a conjugate, the antibody acts as a carrier molecule
to deliver an attached chemotherapeutic agent or toxin or
radioisotope to cells displaying a specific antigen. A
number of antibody conjugates have been approved by the FDA
for oncology indications, including gemtuzumab ozogamicin
(Mylotarg, an anti-CD30 IgG4 antibody-calicheamicin
conjugate) for acute myeloid
leukemia[37], ibritumomab tiuxetan (Zevalin, a
90Y-labeled anti-CD20
antibody)[38] and tositumomab (Bexxar, an
131I-labeled anti-CD20
antibody)[39] for non-Hodgkin's lymphoma.
4. Other mechanisms; for example, antibodies to
stimulate the anti-idiotype network to generate antitumor
anti-anti-idiotypic antibody
response[40], and antibodies to enhance a patient's immune response to tumors by stimulating
cytotoxic T lymphocytes via CD40[41] or by antagonizing
endogenous immune inhibitory factors such as
CTLA-4[42]. Recently, functional antibodies have been expressed
intracellularly as "intrabodies". These intrabodies exert their
biological effects through interfering with the function of the
targeted molecules via a variety of mechanisms, including
altering their intracellular trafficking, localization and/or
surface expression, blocking their interaction with other
molecules, or directly neutralizing their enzymatic activity
(for kinase targets)[43,44].
Cetuximab (Erbitux, IMC-C225)
Cetuximab in preclinical studies Cetuximab is an IgG1
anti-EGFR antibody that is being developed for the
treatment of EGFR-expressing human cancers (ImClone Systems
Incorporated, New York, NY, USA). Cetuximab is a chimeric
version of the murine anti-EGFR mAb
M225[45,46]. It efficiently competes with EGF, and
TGF-α for binding to EGFR, inhibits ligand-stimulated activation of the receptor and
downstream signaling molecules, and inhibits tumor cell
mitogenesis in vitro. Binding of cetuximab to EGFR also induces
receptor internalization; hence, effectively stripping the
receptor from the tumor cell surface. Cetuximab also induces
apoptosis in some EGFR-overexpressing tumor cell lines.
Furthermore, as a human IgG1 isotype, cetuximab mediates
effective ADCC to a variety of human tumor cell lines in
vitro, and the extent of cell lysis correlates directly with the
expression level of EGFR by the tumor
cells[47,48]. The tumor growth inhibitory activities of cetuximab were confirmed
in vivo in a variety of human tumor xenograft models that
include carcinomas of colon, epidermoid, lung, pancreas,
bladder, breast, prostate and renal
cells[49].
It has been shown that tumor cells often upregulate the
expression of growth factors and their receptors; for example,
EGF and EGFR, in response to cellular stress or cytotoxic
insult in order to activate the survival
mechanisms[50_53]. Several in
vivo studies have demonstrated a direct
correlation between increased EGFR expression and decreased
response (ie the development of resistance) to chemotherapy
and radiotherapy regimens for several different tumor
types[54_56]. To this end, a number of preclinical studies have
shown that cetuximab could augment the antitumor activity
of various anticancer agents, including cisplatin,
doxo-rubicin, fluorouracil, gemcitabine, paclitaxel and topotecan,
both in vitro and in vivo in human tumor xenograft models in
mice[57_62]. In these studies, combination treatment with both
cetuximab and cytotoxic drugs resulted in markedly enhanced
tumor inhibition over treatment with either agent alone, and
in some models led to tumor regression and the eradication
of established tumors. It is pertinent to note that the tumor
cells used in some of these studies were poorly responsive
to the cytotoxic agents alone, but were sensitized to these
agents by concurrent cetuximab
treatment[57,58]. For example, cetuximab has been shown to significantly potentiate, in an
additive or synergistic manner, the antitumor activity of
irinotecan (CPT-11) in a variety of preclinical animal models,
in which irinotecan and/or cetuximab exhibit poor efficacy as
monotherapies[63]. Similar enhancement of antitumor
activity has been observed in studies in which cetuximab is used
in combination with radiation[64_67]. These observations
suggest that simultaneous inhibition of EGFR-mediated
biological activities may improve (or sensitize) tumor response to
conventional cytotoxic therapies.
Mechanisms of action of cetuximab Cetuximab, once
bound to EGFR, blocks ligands (EGF and TGF-α) from
association with the receptor, resulting in inhibition of receptor
activation and downstream signal
transduction[49,68,69]. In addition, binding of cetuximab to cell surfaces induces
internalization of the receptor, leading to surface receptor
down-regulation[70]. As a result, the cellular process necessary for
cell survival and proliferation does not ensue properly.
Several molecular mechanisms of action may play a role in the
antitumor activity of cetuximab. These mechanisms include
inhibiting cell cycle progression, inducing cell apoptosis,
inhibiting angiogenesis, inhibiting invasion and metastasis,
inhibiting DNA repair and recovery after chemotherapy
and/or radiation, and inducing immunological effector responses
(including both ADCC and CMC)[49,68,69]. In the case of
combinational therapy, blockade of the EGFR pathway by
cetuximab inhibits DNA synthesis and repairing processes
following the cytotoxic insults, thereby enhancing the
antitumor activity of chemotherapeutic agents and
radiation[49,68,69].
Cetuximab in clinical studies in patients with metastatic
colorectal cancer EGFR is expressed in a significant
percentage (from 25% to 80%) of human colorectal tumors and
its overexpression is usually associated with advanced
diseases. Clinical trials have been conducted using both
cetuximab alone and cetuximab in combination with
conventional chemotherapeutic regimens in the treatment of
metastatic colorectal cancer (mCRC) patients in various stages
and settings. These trials aimed to demonstrate that
treatment with cetuximab, either as a single agent or in
combination with various chemotherapies, would not only be safe,
but would also lead to significant antitumor activity, such as
the activity observed in the preclinical studies.
EMR-007 (BOND), a randomized Phase II study, is
designed to evaluate the activity of cetuximab alone or in
combination with irinotecan in a prospective
manner[71]. In this design, patients with EGFR-positive tumors who had
recently failed irinotecan therapy were randomized in a 2:1
fashion to receive the combination of cetuximab and
irinotecan or cetuximab alone. A total of 329 patients were
randomized, 218 to receive the combination of irinotecan and
cetuximab and 111 to receive cetuximab alone. Partial
response (PR) (ie tumor shrinkage by more than 50% after
treatment) was achieved in 22.9% of the patients who
received both cetuximab and irinotecan, and in 10.8% of the
patients treated with cetuximab alone (P=0.007). Time-to-progression (TTP) was also significantly different between
the two groups in favor of the combination regimen; the
median TTP for the combination arm is 4.1 months versus
1.5 months for the mono-therapy group (P<0.001). The most
common toxicity associated with cetuximab treatment was
an acne-like skin rash, which occurred in 80% of treated
patients (grade 3/4 in 9.4% patients in the combination group
and 5.2% patients in the monotherapy group). There seems
to be a direct correlation between the patients' response to
antibody therapy and the severity of the skin rash, but not
to the EGFR staining intensity as determined by standard
immunohistochemistry (IHC) methods (see Sections 4.1 and
4.2 for a detailed discussion). Severe anaphylactic reactions
to cetuximab developed in 4 (1.2%) patients requiring the
discontinuation of the treatment. Other grade 3 or 4
toxicities in the combination group included diarrhea (21.2%),
asthenia (13.7%), neutropenia (9.4%), nausea and vomiting
(7.1%) and dyspnea (1.4%), compared to rates of 1.7%,
10.4%, 0%, 4.3%, and 13%, respectively, in the antibody
monotherapy group. The tumor response rate in this trial
was consistent with that observed in an earlier trial in a
similar patient population reported by Saltz et
al (study CP02-9923). In this trial, a single group of 120 mCRC patients who
had failed irinotecan was treated with a combination of
cetuximab and irinotecan, and PR was observed in 22.5% of
the treated patients[72]. Based on these observations,
cetuximab was approved by the FDA in February 2004, both
in combination with irinotecan for the treatment of
EGFR-expressing mCRC who are refractory to irinotecan-based
chemotherapy, and as a single agent for the treatment of
EGFR-expressing mCRC patients who are intolerant to
irinotecan-based chemotherapy.
Two non-randomized, single-arm studies provided
additional data on the antitumor activity of cetuximab as a
monotherapy. In a small single arm study (CP02-0141), 57
patients with documented progression on irinotecan or an
irinotecan-based regimen were treated with
cetuximab[73]. Five patients (9%) achieved a PR. Twenty-one additional
patients had stable disease (SD) or minor responses. The
median survival time was 6.4 months. Eighty-three percent
of patients developed a skin rash (18% with grade 3), two
patients (3.5%) had grade 3 allergic reactions, and 56% of
patients experienced asthenia, fatigue, malaise or lethargy
(9% grade 3). In another trial (IMCL-0144) reported by Lenz
et al[74], cetuximab as a single agent yielded an 11.6% PR rate in
346 patients refractory to both irinotecan and oxaliplatin,
with another 31.8% of patients experiencing SD for at least 6
weeks. Median overall survival was 6.7 months. The most
common adverse events were very similar to those observed
in the CP02-0141 trial, including an acne-like skin rash (90%,
with 6% of grade 3/4) and fatigue/malaise (48%, grade 3/4
10%). Detailed analysis of the response rate among patient
subgroups, who had received 2 to 9 regimens (median, 4) of
prior chemotherapy (including 259 patients who had received
oxaliplatin after irinotecan failure and 87 patients who had
received irinotecan after or with oxaliplatin), revealed that
cetuximab was equally active in all patient subgroups
regardless of the numbers of prior therapy or the sequence
of prior agents[75].
Most recently, a randomized, multi-center, Phase III trial
(NCIC CTG CO.17, also known as BMS-025) compared cetuximab plus best supportive care (BSC) to BSC alone in
572 patients with mCRC whose disease was refractory to all
available chemotherapy, including irinotecan, oxaliplatin and
fluoropyrimidines. The study met its primary end-point of
overall survival:patients who received cetuximab lived an
average of 6.1 months compared to 4.6 months for patients
who received BSC alone, representing a 23% increase in
overall survival (P=0.005). Cetuximab treatment also resulted
in PR in 23 patients (8%) compared to 0% in patients who
received BSC alone (P<0.0001), and a 32% reduction in the
risk of disease progression (P<0.0001). Further, SD was seen
in an additional 31.4% of patients receiving cetuximab, but
only in 10.9% of patients on BSC. The antibody was
generally well tolerated with a rash as the most common toxicity.
These are the first data of an anticancer therapy to
demonstrate overall survival in refractory mCRC patients.
Another Phase III randomized study, known as EPIC
(Erbitux Plus Irinotecan in Colorectal Cancer), compared
irinotecan to irinotecan plus cetuximab in second-line
settings in patients whose disease was not responding to
first-line oxaliplatin and fluoropyrimidine chemotherapy. A total
of 1298 patients were randomized (1 to 1) into two groups
receiving irinotecan plus cetuximab (Arm A) or irinotecan
alone (Arm B) until disease progression, when the study
treatment was stopped and further treatment was at the
discretion of the physician. As a result, 47% of patients in Arm
B received post-study cetuximab (87% of them in
combination with irinotecan). The most common grade 3/4 adverse
events included (Arm A vs Arm B): neutropenia (31.8%
vs 25.4%), rash (8.2% vs 0.5%), infusion reaction (1.4%
vs 0.8%) and hypomagnesemia (3.3% vs 0.4%), diarrhea
(28.8% vs16.2%) and fatigue (9.2% vs
4.9%). Secondary efficacy end-points strongly favored the combination of
cetuximab plus irinotecan over irinotecan alone; the
response rate and progression-free survivaI (PFS) were
16.4% and 4.0 months in Arm A, respectively, compared to
4.2% (P<0.0001) and 2.6 months (P<0.0001) in Arm B,
respectively. However, the primary end-point, overall
survival was not significantly different between the two arms
(10.7 months in Arm A vs 10 months in Arm B). Post-hoc
analysis suggests this may result from the substantial
post-study use of cetuximab, which potentially confounded the
interpretation of this end-point. As observed in the two
third-line studies (EMR-007 and NCIC CTG CO.17) discussed
above, cetuximab, alone or in combination with irinotecan,
significantly increased patients' response rates and/or
overall survival after their failure with irinotecan therapy.
Cetuximab has also been evaluated for safety and
efficacy in mCRC patients in first-line settings in combination
with various chemotherapy regimens. In one Phase II
study, cetuximab was given in combination with FOLFOX-4
(oxaliplatin/5-fluorouracil/leucovorin) to patients with
non-resectable mCRC[76]. In a preliminary analysis of 42
patients, 10% had a complete response (CR), 71% had a PR
and 17% had SD. Median PFS was 12.3 months. Nine
patients (21%) subsequently underwent surgery of their
metastases. The major grade 3/4 toxicities were acne-like
rash (30%), neurotoxicity (30%), diarrhea (26%),
neutropenia (21%) and stomatitis/mucositis (16%). Recently,
ImClone Systems announced that the results of a Phase III
study (CRYSTAL study) of cetuximab plus FOLFIRI (irinotecan/5-fluorouracil/leucovorin) met the primary
end-point of increasing median duration of PFS over FOLFIRI
alone in patients with previously untreated mCRC. This
study enrolled more than 1000 patients around the world.
Final results have been presented at the annual meeting of
the American Society of Clinical Oncology (ASCO) in June
2007.
Currently, several additional Phase III randomized
clinical trials with cetuximab in first-line settings in mCRC
patients are being carried out. These trials include a trial
comparing FOLFOX±cetuximab (OPUS trial, 300 patients,
enrollment completed), a trial comparing continuous FOLFOX
versus intermittent FOLFOX with or without cetuximab (COIN
trial, 2400 patients, ongoing), a trial comparing XELOX
(capecitabine plus oxaliplatin) plus bevacizumab with or
without cetuximab (CAIRO II trial, 750 patients, ongoing), and an
intergroup trial comparing chemotherapy (FOLFOX or
FOLFIRI) plus bevacizumab versus chemotherapy plus
cetuximab versus chemotherapy plus bevacizumab and
cetuximab (CALGB 80405 trial, 2289 patients, ongoing). In
mCRC adjuvant settings, cetuximab is being tested in two
Phase III trials: NCCTG 147 intergroup trial (2300 patients,
ongoing) and PETACC European trial (2000 patients,
ongoing), both comparing FOLFOX±cetuximab in mCRC
patients with a high likelihood of recurrence.
Cetuximab in squamous cell carcinoma of the head and
neck The safety and efficacy of cetuximab was studied both
as a single agent and in combination with radiation in
patients with SCCHN. In a Phase III trial (IMCL-9815
study)[77], 424 patients with locally or regionally advanced
SCCHN were randomized (1:1) to receive radiation alone
or cetuximab plus radiation. The median duration of
local-regional disease control, the primary end-point, was
24.2 months in the combination group compared with 14.9
months in patients who received only radiation
(p=0.005). The medium overall survival in the combination group was
49 months compared with 29.3 months in the radiation alone
group (p= 0.03). It is important to note that, except for an
infusion reaction (3% in the combination group versus 0%
in the radiation alone group) and skin rash, the use of
cetuximab did not significantly increase the incidence of
the major toxicities associated with radiation, particularly
mucositis/stomatitis (56% in the combination group
vs 52% in the radiation alone group). In another single-arm trial,
cetuximab alone was given to 103 patients with recurrent or
metastatic SCCHN with documented progression within 30
days after 2_6 cycles of a platinum-based chemotherapy.
The PR rate was 13% and the median duration of response
was 5.8 months. Based on these results, cetuximab was
approved by the FDA in March 2006, in combination with
radiation for the treatment of locally or regionally advanced
SCCHN, and as a single agent for the treatment of patients
with recurrent or metastatic SCCHN who had failed prior
platinum-based therapy.
Cetuximab has also been studied in combination with
chemotherapeutic agents in patients with SCCHN. In a
randomized Phase III trial, patients with recurrent or metastatic
SCCHN were treated with cisplatin alone or in combination
with cetuximab[78]. In 117 analyzable patients, response rates
were 26% in the combination group and 10% in the cisplatin
alone group (p=0.03). Median PFS and medium overall
survival were 4.2 months and 9.2 months, respectively, in the
combination group, and 2.7 months and 8.0 months, respectively, in patients who received cisplatin alone.
Although there was a survival advantage for patients who
received cetuximab, the difference was not significant mainly
because of the fact that the trial was not sufficiently powered.
In several other trials in SCCHN patients with advanced
disease refractory to platinum-based regimen, cetuximab, either
alone or in combination with the same dose and schedule of
platinum (that patients had failed), yielded a response rate of
10% to 13%[79,80], which compares favorably to the expected
response rate in a similar patient population treated with
more toxic second-line chemotherapeutic agents. In April
2007, ImClone Systems announced that a first-line Phase III
study of cetuximab combined with platinum-based
chemotherapy met the primary end-point of increasing overall
survival in patients with recurrent and/or metastatic SCCHN.
The randomized, multi-center study, known as EXTREME,
studied more than 400 patients treated with cetuximab in
combination with 5-fluorouracil plus either cisplatin or
carboplatin and compared the results to patients treated with
5-fluorouracil plus either cisplatin or carboplatin alone.
Final results have been presented at the annual meeting of
ASCO in June 2007.
Cetuximab in other cancers In addition to mCRC and
SCCHN, cetuximab is also being tested in other cancer
indications, including patients with pancreatic
carcinoma[81], non-small cell lung carcinoma
(NSCLC)[82, 83] and ovarian
carcinoma[84; see 85,86 for reviews]. In April 2007, ImClone Systems
announced the preliminary results of an open-label,
randomized study comparing cetuximab plus gemcitabine to
gemcitabine alone in more than 700 patients with pancreatic
cancer in a first-line treatment setting. Conducted by the
Southwest Oncology Group (SWOG) in centers throughout
the United States and Canada, the study failed to meet its
primary end-point of statistically improving overall survival.
Detailed analysis of the data is currently being performed.
Another large Phase III trial, the FLEX study, examining in
first-line settings cisplatin/vinorelbine alone or in
combination with cetuximab in patients with NSCLC, has been fully
enrolled (1124 patients). The primary end-point for this study
is overall survival and results are expected in late 2007.
Panitumumab (ABGX-EGF)
Panitumumab is a fully human anti-EGFR antibody
generated using the transgenic XenoMouse technology being
developed by Amgen (Thousand Oaks, CA, USA)
[87,88]. The antibody binds to EGFR with a high affinity (~50 pmol/L)
and is able to completely regress certain human xenografts
in animal models as a single agent
therapy[89]. As an IgG2 subclass antibody, panitumumab does not mediate a
significant level of ADCC on EGFR-expressing tumor cells. In
clinical studies panitumumab has been well tolerated at doses
ranging up to 10 mg/kg, causing low infusion reaction (<1%),
and has not elicited any human antibody response. In a
dose-escalating trial, 88 patients with metastatic renal cell
carcinoma were treated with panitumumab at weekly doses
of 1.0, 1.5, 2.0, or 2.5 mg/kg with no loading dose. PR was
achieved in three patients, and two patients had minor
responses. Forty-four patients (50%) also had SD at their
first 8-week assessment, and the median PFS was 100 days.
The principal toxicity, skin rash, occurred in 68%, 95%, 87%,
and 100% of patients who received at least three doses of
ABGX-EGF at 1.0, 1.5, 2.0, and 2.5 mg/kg per week,
respectively[90].
In a multi-center, open-label, single-arm study, a total of
148 CRC patients were grouped into two cohorts: cohort A
with higher EGFR staining intensity (2+ or 3+ in >10%
evaluated tumor cells [104 patients]) and cohort B with
lower EGFR staining intensity (1+ or 2+ or 3+ in <10%
evaluated tumor cells [44 patients]). The patients were first
treated with panitumumab at 2.5 mg/kg every week for a total
of 8 doses, followed by radiographic evaluation of the
tumor response and repeated 8-week treatment cycles until
disease progression or unacceptable toxicity. The antibody
was well tolerated, with the major toxicity, skin rash (including
3% grade 3), occurring in 95% of patients. Interim analysis
revealed PR in 11% of cohort A patients and 9% of cohort B
patients. Of the 15 patients with a response, 13 patients
responded at the time of the first 8-week evaluation. The
median overall survival time was 7.9 months and the median
TTP was 2.0 months[91]. As seen in the cetuximab trials, there
seems to be a direct correlation between tumor response and
skin rash, but not between tumor response and EGFR
staining intensity in the tumors, in panitumumab-treated patients.
In a recently completed open-label Phase III trial, 463
patients with mCRC who had previously failed standard
chemotherapy, including oxaliplatin and irinotecan, were
randomized to receive panitumumab at 6 mg/kg every 2 weeks
plus BSC (n=231) or BSC alone (n=232). The PR rate was 8%
with panitumumab versus zero with BSC alone, and the
median duration of response was 17 weeks. The SD rate was
28% with panitumumab versus 10% with BSC alone. Furthermore, patients who received panitumumab showed a
46% decrease in tumor progression rate versus those who
received BSC alone. Approximately 75% of the BSC patients
entered a crossover arm to receive panitumumab after their
disease had progressed (n=174). Panitumumab treatment
also showed a clinical benefit in patients who crossed over;
in these patients, panitumumab treatment resulted in a 9%
PR, 32% SD and one CR. Despite these findings, an interim
analysis revealed that the overall survival between the two
groups was similar. The investigators believed that rate (75%)
and timing (median 7.0 weeks) of crossover from the BSC
alone arm to receiving panitumumab, and the antitumor
activity observed after crossover, are likely to have
confounded the ability to demonstrate a treatment effect on
overall survival[92]. Based on these observations, the FDA
approved panitumumab in September 2006 for use as a single
agent in patients with refractory mCRC.
In March 2007, Amgen announced that it had
discontinued the Panitumumab Advanced Colorectal Cancer
Evaluation (PACCE) trial, a Phase IIIb randomized, open-label
clinical trial evaluating oxaliplatin-based and irinotecan-based
chemotherapy and bevacizumab with and without panitumumab in the first-line treatment of patients with
mCRC. The trial enrolled 1054 patients (824 patients were
randomized to receive oxaliplatin-based chemotherapy and
230 patients were randomized to receive irinotecan-based
chemotherapy) in the United States between the first quarter
of 2005 and the third quarter of 2006. A pre-planned interim
efficacy analysis scheduled after the first 231 events (death
or disease progression) revealed a statistically significant
difference in PFS in favor of the control arm (bevacizumab
plus chemotherapy). An unplanned analysis of overall
survival also demonstrated a statistically significant difference
favoring the control arm. Furthermore, a review of the
interim analysis showed an increased incidence of grade 3
severe events of diarrhea, dehydration and infections in the
panitumumab-treated patients. In addition, increased
incidence of pulmonary embolism was observed in patients who
received panitumumab compared with those who did not
(4% and 2%, respectively). One fatal event of pulmonary
embolism occurred in a patient receiving panitumumab.
Final results of this trial are due to be presented in late 2007.
Nimotuzumab (h-R3)
Nimotuzumab (YM Biosciences, Mississauga, ON, Canada, and Center of Molecular Immunology, Havana,
Cuba) is a humanized IgG1 form of the murine IgG2a
antibody R3 specific for EGFR[93,94]. Nimotuzumab binds to the
EGFR extracellular domain with a moderate affinity (about 1
nmol/L), blocks EGF binding to its receptor and
ligand-dependent receptor autophosphorylation, and inhibits cell
growth in EGFR-expressing cells[94]. Studies have shown
that the antitumor effect of nimotuzumab may result from its
combined effects on tumor cell proliferation, survival and
angiogenesis[95]. Multiple clinical trials are currently being
conducted to examine the therapeutic efficacy of the
antibody in a number of cancers, including a Phase III trial in
pediatric pontine glioma (first-line in combination with
radiation, data expected in 2007) and Phase II studies in
patients with carcinomas of the pancreas, esophagus and
cervix, and NSCLC and hormone-refractory prostate cancer.
In a Phase II trial, pediatric patients (n=34) with relapsed or
resistant high-grade gliomas received weekly nimotuzumab
(150 mg/m2) for 6 weeks, followed by a consolidation arm of
four infusions in a 3-week interval if no disease progression
were observed. One PR and 11 SD were observed and a
median of 7.5 months of PFS was achieved in 8 patients in
the consolidation phase of the trial. No severe side effects
were observed. In a Phase I/II trial, 29 patients (16
gliobla-stoma, 12 anaplastic astrocytoma and 1 anaplastic
oligo-dendroglioma) received 6-weekly infusions of nimotuzumab
at a dose of 200 mg in combination with external beam
radiotherapy. The antibody was very well tolerated without
grade 3/4 adverse events. None of the patients developed
acneiform rash or allergic reactions. One patient developed
a positive anti-idiotypic response. The objective response
rate was 37.9% (17.2% CR, 20.7% PR) and SD occurred in
41.4% of the patients. With a median follow up time of 29
months, the median survival was 22.17 months for all
subjects[96].
Nimotuzumab is also being tested in combination with
radiotherapy in the treatment of locally advanced SCCHN
patients. In one trial, 24 patients received 6-weekly
infusions of nimotuzumab at 4 dose levels in combination with
radiation. The combination therapy was well tolerated. Aside
from infusion reactions, no skin or allergic toxicities were
observed. Overall survival was significantly increased after
the use of the higher antibody
doses[97]. In another Phase II trial, nimotuzumab (100 mg, iv once weekly for 8 weeks) in
combination with radiotherapy demonstrated greater efficacy
against nasopharyngeal carcinoma than radiation alone. Of
the 130 patients in the intent-to-treat analysis, 90.6% in the
combination arm had a CR, compared with 51.5% in the
radiation-alone group. Nimotuzumab in combination with
radiation therapy has been approved for the treatment of
locally advanced inoperable head and neck carcinomas in
several countries, including Cuba, Argentina, Columbia, China
and India.
To date about 600 patients have been treated with
nimotuzumab. Early clinical experience with nimotuzumab
suggests that it may lack some of the toxicities commonly
associated with other EGFR-targeting agents, including
cetuximab, panitumumab and small molecule tyrosine kinase
inhibitors (TKI). In particular, grade 1/2 skin rash (no grade
3/4) has only been reported in about 6% patients, compared
to the high frequency often seen with both cetuximab and
panitumumab. Furthermore, no significant hypomagnesemia
occurred in patients treated with nimotuzumab. Whether
these observations have positive or negative clinical
implications for nimotuzumab remains to be seen. A distinct
toxicity profile may result from nimotuzumab binding to EGFR
with lower affinity than other EGFR-specific antibodies.
Alternatively, nimotuzumab may bind to a different epitope
on the receptor, thereby inducing an alternative intracellular
signal. Other toxicities of nimotuzumab observed to date
include mild or moderate (grade 1/2) fever,
hypotension/hypertension, vomiting, diarrhea and nausea, dry mouth, nail
inflammation, and tremors; these were controlled with
standard medications.
Matuzumab (EMD 72000)
Matuzumab is a humanized anti-EGFR antibody (Merck
KGA, Germany) that has demonstrated antitumor activity in
preclinical tumor models both as a single agent and in
combination with chemotherapy and
radiation[98]. To date, over 320 patients have been treated with the antibody and have
tolerated it well, with the most commonly reported toxicities
being skin rash, fever and headache. The maximum tolerated
dose of matuzumab is 1600 mg, iv weekly, and the
dose-limiting toxicities are grade 3 fever and headache. In a
clinical trial (EMD 72000-018), 22 patients with mixed solid
tumors were given the antibody at 400 mg, 800 mg, 1200 mg,
1600 mg or 2000 mg, iv per week. Grade 3 fever and headache
were observed in patients at greater than 1600 mg dose levels.
PR was achieved in 5 patients (2/5 of RCC patients, 3/5
SCCHN patients), with SD in an additional 6
patients[99]. In another single arm Phase II trial, 37 patients with recurrent,
EGFR-positive ovarian or primary peritoneal cancers were
treated with matuzumab. The antibody was well tolerated,
and the most common toxicities included rash, acne, dry skin
and paronychia, as well as headache, fatigue and diarrhea.
Seven patients (21%) achieved SD and remained on therapy
for more than 3 months[100].
Clinical trials are also being carried out to investigate the
antitumor activity of matuzumab in combination with
chemotherapy. In a Phase I combination study (EMD
72000-020), matuzumab, at a weekly dose ranging from 100 to 800
mg, was given in combination with paclitaxel to advanced
NSCLC patients. One patient at an antibody dose of 800 mg
showed a grade 4 neutropenia. Grade 1/2 acneiform skin
rash in 14 patients was the most frequent side effect. Grade
2 toxicities included pruritus (n=2), bronchospasm
(n=1), fissures (n=1), abdominal pain
(n=1) and hot flushes (n=1). Out of 18 patients evaluated, 1 CR (previously untreated), 3 PR (2
previously untreated) and 6 SD were
achieved[101]. In
another Phase I study, three groups of chemotherapy-naive
advanced pancreatic adenocarcinoma patients
(n=17)
received escalating weekly doses of matuzumab (400 mg,
800 mg weekly, or 800 mg biweekly) and gemcitabine. Severe
treatment-related toxicities were limited to grade 3
neutropenia (n=3), leucopenia (n=1) and decreased white blood cell
count (n=1). Common study drug-related adverse events
were skin toxicities (6 grade 2 and 7 grade 1) and fever (grade
1). Matuzumab inhibited EGFR phosphorylation and affected
receptor-dependent signaling and transduction in tumor
biopsies from all dose groups. PR or SD were achieved in 8
of 12 evaluated patients (66.7%), with 3 PR among 6
evaluated patients in the group receiving 800 mg
weekly[102]. Matuzumab is currently also in Phase II trials in NSCLC,
gastric and colorectal cancer patients.
Zalutumumab (HuMax-EGFr, SF8, 2F8)
Zalutuzumab (Genmab A/S, Copenhagen, Denmark) is a
human IgG1 anti-EGFR mAb generated using Medarex's
(Princeton, NJ, USA) transgenic mouse technology. The
antibody blocks the binding of growth factors to tumor cells,
inhibits phosphorylation of EGFR and cell proliferation with
an approximate EC50 value of 1 µg/mL, causes tumor cell
killing by ADCC, and directly slows the rate of tumor
growth[103]. In animal studies, complete eradication of tumors was
observed between 9 and 14 days after three injections of the
antibody. When administered 1 day after tumor inoculation,
zalutumumab was capable of completely preventing tumor
formation at a dose of 50 µg; the antibody was also capable
of eradicating well-established tumors in mice at a total dose
as low as 125 µg. A recent mechanistic study showed that
the antibody locks the EGFR into an inactive configuration,
preventing the growth factor from binding and the
subsequent receptor dimerization and activation.
Results from a Phase I/II trial in patients with head and
neck cancers were reported at the 2005 ASCO annual meeting.
In this trial, 27 patients who had previously failed standard
therapies were given a single dose of zalutumumab, ranging
from 0.15, 0.5, 1.0, 2.0, 4.0 to 8.0 mg/kg, and were followed for
4 weeks before receiving 4 further doses at weekly intervals.
Twenty patients received all five infusions. None of the
patients receiving doses of up to 8 mg/kg experienced
dose-limiting toxicity and preliminary pharmacokinetic data
suggested that saturation of the EGFR was provided by doses
close to 2 mg/kg. Assessed by 18-fluoro-2-deoxyglucose
positron emission tomography (FDG-PET), 7 of 18 patients
that could be evaluated achieved partial metabolic response
and 4 had stable metabolic disease 1 week after their last
treatment. These results were achieved in the 4 higher dose
groups, including 9 out of 11 patients in the 2 highest dose
groups. Computed tomography (CT) scans showed that 2
of 19 patients that could be evaluated achieved PR and 9
patients had SD according to Response Evaluation
Criteria in Solid Tumors (RECIST) criteria. The PR was maintained at
week 12 in 1 of the 2 patients. In the 2 highest dose groups,
7 out of 10 patients obtained PR or SD. The most frequent
adverse event was acneiform rashes in 56% of the patients;
this event was antibody dose dependent with 10 of 11
patients in the 4 and 8 mg/kg dose groups reporting the rash.
Other adverse events included rigors, fatigue, pyrexia,
nausea, flushing and increased sweating. One patient
reported a serious adverse event, a grade 2 pyrexia, which
developed during the first infusion. The patient recovered
and completed the study.
In September 2006, Genmab started a randomized pivotal
Phase III trial in 273 SCCHN patients who were refractory to
or intolerant of platinum chemotherapy. In this study,
patients have been randomized into 2 treatment groups:
zalutumumab in combination with BSC or BSC alone.
Patients in the antibody group will be given an initial dose of
8 mg/kg of zalutumumab, followed by weekly infusions of a
maintenance dose until disease progression. The
maintenance dose will be adjusted as necessary until the patient
develops a dose-limiting skin rash, up to a maximum dose of
16 mg/kg. Disease status will be assessed every 8 weeks by
CT scan or magnetic resonance imaging (MRI) according to
RECIST criteria until disease progression, and patients will
be followed for survival. In addition, a Phase I/II trial of
zalutumumab in combination with chemoradiation in SCCHN
patients was initiated in October 2006.
mAb 806
mAb 806 (Ludwig Institute for Cancer Research, New
York, NY, USA) is an antibody raised against a truncated
form of EGFR, delta2-7 EGFR (the variant III or
EGFRvIII)[104]. The binding epitope(s) of mAb 806 is not exposed on
inactive wild-type EGFR, but is exposed on a transitional form of
the receptor[105,106]. This is supported by IHC staining
demonstrating that the antibody binds to a broad range of
epithelial cancers and to gliomas, but not to normal human
tissues expressing wild-type receptor in the absence of gene
amplification[104,107]. The antibody has significant
anticancer activity against human tumor xenografts expressing
amplified EGFR (such as A431 tumor model) or mutant EGFRvIII
(such as U87MG delta2-7 model) as a single
agent[108,109], and was synergistic with small molecular EGFR
inhibitors[110]. Clinical trials using a chimeric version of mAb 806 (ch-806),
which has similar binding affinity to the unique EGFR epitope
as the parent antibody, were initiated in late 2003, and
preliminary results were reported at the ASCO annual meeting
in June 2006. The trials confirmed the excellent targeting of
ch-806 to cancers including squamous cell carcinomas of
the lung, head and neck, and skin, and colorectal cancer,
mesothelioma and glioma. Importantly, there was no
evidence of localization of ch-806 to normal tissue. No
significant toxicities were
observed[111].
IMC-11F8
IMC-11F8 is a fully human IgG1 antibody derived from a
Fab fragment originally isolated from an antibody phage
display library (ImClone Systems). The antibody binds to EGFR
with high affinity, blocks EGF-stimulated receptor activation,
signal transduction and cell proliferation, and mediates
efficient ADCC on tumor cells expressing the
receptor[112]. The antibody also showed significant tumor inhibitory activity
in vivo in a variety of xenograft models as a single
agent[113_115] and demonstrated additive or synergistic antitumor effect
when combined with conventional therapeutic agents. A
Phase I clinical trial in patients of various advanced solid
malignancies has been completed. The trial had 6 cohorts,
testing weekly or biweekly doses of 100, 200, 400, 600, 800, or
1000 mg. The most frequent adverse events were grade 1/2
skin rashes, nausea, vomiting, fatigue and headache. No
infusion reactions were observed. Interim analysis of the
first 30 patients demonstrated 2 PR (1 with mCRC and the
other with metastatic melanoma) and 9 SD of between 14 and
57 weeks[116]. Phase II/III trials are expected to begin in the
latter half of 2007.
Perspectives
A number of anti-EGFR antibodies, exemplified by
cetuximab, acting either as a single agent or in combination
with other cytotoxic regimens including chemotherapy and
radiation, have demonstrated good safety
profiles[117,118] and significant antitumor activity both in preclinical studies and
in the clinic in patients with various
malignancies[85,86]. Cetuximab has been approved by the FDA for use, both as a
single agent and in combination with chemotherapy or
radiation, in patients with mCRC and SCCHN, and panitumumab has been approved as a single agent in
patients with refractory mCRC. Despite the overall clinical
success, there are a number of biological and clinical
questions associated with anti-EGFR therapy that still need to be
addressed. Answers to these questions will, undoubtedly,
greatly facilitate further clinical development of these
antibodies in a more rational and efficient manner.
EGFR expression and patient selection Unlike the
case of trastuzumab in the treatment of HER2-expressing
breast cancer, where overexpression of HER2 in tumors is
positively associated with the patients' response, based on
all the clinical trials carried out to date it seems apparent
that the levels of EGFR expression in tumors, either as the
percentage of EGFR-positive tumor cells or as the
staining intensity determined by standard IHC, do not correlate
with tumor response to anti-EGFR therapy. For example,
in the EMR-007 study, patients with tumors that stained
faint, weak, moderate, or strong for EGFR expression
demonstrated response rates to cetuximab treatment of 20.8%,
24.7%, and 22.7%, respectively (p=0.64). The response rates
were also similar among patients with tumors that had either
£10% EGFR-positive cells or >35% EGFR-positive
cells[71]. It is even more intriguing that in a recent study in which 16
refractory EGFR-negative (by IHC) mCRC patients were
treated with either cetuximab alone (2 patients) or in
combination with irinotecan (14 patients), 4 PR (including 1 patient
received cetuximab alone) and 2 minor responses were
achieved[119]. Furthermore, it has been shown that
expression of EGFR in primary tumors may not necessarily
correlate with receptor status in metastatic sites. In a
retrospective study, primary tumors and related metastatic sites from
99 mCRC patients were examined for EGFR expression using
IHC. EGFR expression was seen in the primary tumors in 53
(53%) patients, but the corresponding metastatic sites from
19 (36%) of these patients were found to be EGFR negative.
In contrast, 7 patients (15%) showed positive EGFR staining
in the metastatic sites, but not in the primary
tumors[120]. These results suggest that screening of EGFR expression
using current IHC methods in primary CRC tumors may not
be adequate for patient selection for anti-EGFR-based
therapy.
As an alternative approach for assessing EGFR status in
tumors, a recent study examined the gene copy numbers of
the receptor using fluorescence in-situ hybridization (FISH)
in 31 patients (10 responders and 21 non-responders) treated
with cetuximab or panitumumab. Eight of 9 patients with an
objective response had increased EGFR gene copy numbers
compared to 1 of 20 non-responders, suggesting that gene
amplification of EGFR may serve as a better criterion for
patient selection and as an indicator of patient response to
therapy[121]. Another recent report examining the correlation
between EGFR gene amplification and protein expression by
IHC revealed that only a small percentage of EGFR-positive
(by IHC) tumors also harbor gene amplification (>5
copies/nucleus) _ in 158 primary or metastatic CRC tumors studied,
positive IHC staining was detected in 85% of primary and
79% of metastatic tumors, whereas gene amplification was
only seen in 12% of primary and 8% of metastatic
tumors[122]. Taken together, all these findings suggest that large
perspective clinical trials are clearly needed to further delineate
the true relevance of the levels of EGFR expression (or gene
amplification) in tumors and their response to anti-EGFR
therapy.
Biomarkers for efficacy In addition to EGFR
expres-sion, significant efforts are aiming to identify other
biomarkers, or surrogate markers, for patient stratification
and for predicting patients' response to anti-EGFR therapies.
One obvious phenomenon that is positively associated with
patients' response to anti-EGFR therapy is the development
of a skin rash or other types of skin reactions. As noted,
patients who experienced skin rash were more likely to
respond to antibody treatment than those who did not, and
the severity of the skin rash seems to correlate well with
patients' response and overall
survival[71_74]. For example, in the EMR-007 study, patients with any degree of skin
reaction to cetuximab therapy yielded response rates of 25.8%
(the combination group) and 13% (the antibody alone group)
compared with 6.3% and 0% in patients without any skin
reactions, respectively. Furthermore, patients who
developed grade 3/4 skin reactions also showed higher response
rates than those with grade 1/2 reactions; 55.2% versus
20.4% (in the combination group) and 33.3% versus 11.6%
(in the antibody alone group), respectively. The median
survival time among patients with skin reactions and those
without skin reactions were 9.1 and 3.0 months, respectively,
in the combination group, and 8.1 and 2.5 months,
respec-tively, in the antibody alone group. These observations
suggest that skin rash may be used as a pharmacodynamic
marker; that is, an indication of EGFR inhibition, for
biological activity of anti-EGFR antibody therapy. In contrast, one
should keep in mind that activity seen in normal skin is not
likely to be an accurate indication of tumor inhibition
because the downstream consequences of EGFR blockade
are clearly different in the skin and the
tumors[123]. Neverthe-less, clinical studies are currently being conducted where
the patients are given escalating doses of cetuximab until
the development of a skin rash occurs, in the hope of further
enhancing the biological activity of the antibody by
achieving maximum receptor saturation/blockade.
On a molecular level, several preclinical and clinical
studies have been carried out in an attempt to identify molecular
markers as predictive indicators for the outcome of anti-EGFR
therapy. One preclinical study examined the proteome
profile of two CRC cell lines with high expression of EGFR, but a
different response to cetuximab. Using two-dimensional
electrophoresis and subsequent mass spectrometry, 14 proteins
were identified that expressed differentially among the two
cell lines, the responder Caco-2 and the non-responder
HRT-18. While all the proteins identified are involved in the
metabolic pathways and malignant growth, expression of certain
proteins, such as fatty acid binding protein and heat shock
protein 27, were implicated in the anti-apoptotic activity
responsible for the non-responsiveness to cetuximab
treatment by the HRT-18 cells[124]. In a retrospective clinical study,
tumor specimens from 39 patients enrolled in the IMCL-0144
trial were examined for intratumoral mRNA levels of EGFR,
VEGF, cyclin D1, cyclooxygenase 2 and interleukin 8, using
real-time RT-PCR following laser-capture microdissection.
High levels of VEGF were associated with resistance to
cetuximab, whereas the combination of low levels of EGFR,
cyclooxygenase 2 and interleukin 8 were significantly
associated with longer overall
survival[125]. Both findings were independent of skin rash, which in itself is correlated to
survival. In several early clinical studies using EMD72000
or small molecule EGFR TKI it was noted that while there
was complete inhibition of tumor phosphorylated EGFR and
MAP kinases in treated patients, phosphorylated Akt and
Ki67 (a cell proliferation marker) were only inhibited in
patients who responded to the
therapy[79,126,127]. Recently, K-ras mutation was found to be associated with rapid
disease progression and significantly decreased TTP in patients
treated with cetuximab. K-ras mutation was detected in 22
out of 59 chemotherapy refractory patients treated with
cetuximab and, remarkably, none was detected in the 12
patients who had a clinical response to the antibody,
suggesting that K-ras mutation may represent a highly
predictive factor to cetuximab
efficacy[125]. Taken together, these observations suggest that in addition to the target EGFR
itself, careful and detailed analysis of downstream signaling
pathways may yield molecular biomarkers that can be used
as useful indicators for the prediction of the efficacy of
anti-EGFR-based therapies.
EGFR mutation and efficacy of anti-EGFR
therapy It has been shown that in a number of clinical trials in NSCLC
patients treated with two small molecular TKI to EGFR,
gefitinib and erlotinib, a subset of patients, including those
with bronchioalveolar carcinoma, women, never-smokers and
Asian patients, had higher response rates and better clinical
outcomes. In addition, it was later discovered that NSCLC
patients with somatic mutations in the EGFR kinase
domain had a much better response to gefitinib and erlotinib[129_134,for review see 135]. Subsequent sequencing studies of
a large panel of lung cancer specimens revealed that the
overall rate of EGFR kinase mutations was approximately
19.6% (149 mutations out of 759 tumors studies), and the
mutations were more frequent in tumors from women
(37.5%) than men (13%), never-smokers (50.8%) than
smokers (9%), adenocarcinomas (31.3%) than tumors of other
histology (2.3%), and from patients of Asian origin (29.1%) than
those of non-Asian origin
(7.9%)[126_128,133]. Overall, the incidence of EGFR kinase mutations in patients from the United
States is 9.5%. This observation is rather intriguing in that
the subgroups of patients harboring higher EGFR kinase
mutation rates represent the same population of patients who
had a better response to gefitinib and erlotinib. Detailed
analysis of the mutation sites (192 in total identified to date)
demonstrated that the majority of the mutations occur in two
hot spots _ one (55.8%) is an in-frame deletion of 4 highly
conserved amino acids (LREA) encoded by exon 19 and the
other (44.2%) is a point mutation in exon 21 that lead to an
amino acid substitution at position 858
(L858R)[129_131,136,137]. Amid all these findings, perspective clinical trials to
correlate these mutations with actual patient responses to either
gefitinib or erlotinib are, however, yet to be conducted. In
contrast, it is very encouraging to note that two recent
preclinical studies showed that cetuximab was very effective in
inhibiting both in vitro and in vivo growth of tumor cells
expressing either wild-type EGFR or mutant EGFR (HCC-827
cell line with delE746-A750 mutation, and NCI-1940 cell line
with L858R and T790M
mutations)[138,139]. Furthermore, cetuximab also demonstrated good inhibitory activity to
tumor cells that expressed the EGFRvIII
variant[140]. Recently, Wong et
al reported that transgenic mice with inducible
expression of either L858R or LREA deletion mutant in type
II pneumocytes developed lung adenocarcinoma after
sustained EGFR mutant expression, which is also essential
for tumor maintenance. Treatment with small molecular TKI
(erlotinib) or cetuximab led to dramatic tumor regression in
these mice[141]. Taken together, these observations strongly
suggest that cetuximab may represent an effective
therapeutic agent against both wild-type and/or mutant EGFR-expressing tumors.
In contrast to NSCLC patients, earlier studies failed to
positively identify any EGFR TK domain mutations in other
tumors[136,142,143]. Two recent reports have, however, revealed
that EGFR kinase mutations may also be present in other
human malignancies, including CRC and SCCHN. In a study
by Nagahara et al[144], although none of the 11 CRC cell lines
examined exhibited somatic mutations, 4 of 33 clinical tumors
(12%) exhibited mutations in the EGFR kinase domain.
Similarly, Lee et al[145] observed 3 mutations (7.3%), all the
same in-frame deletion mutation in exon 19 (del746-750), in
41 SCCHN patients analyzed. In another recent report, EGFR
kinase domain in tumor specimens from 38 NSCLC and 39
mCRC patients participating in two separate cetuximab
monotherapy studies were sequenced. Three mutations were
identified in the 38 NSCLC patients _ two del746-750
mutations in 13 patients experiencing SD and one L861Q
mutation in 21 patients with progressive disease. No mutations
were found in the one patient who achieved a PR and 3
patients whose response data were unavailable. In the 39
mCRC patients, including 20 experiencing PR and one CR,
no mutations were identified[146]. Further sequencing
analysis of 160 biopsy samples of previously untreated CRC
tumors from patients outside of cetuximab trials did not reveal
any mutations in exons 18, 19 and 21 in the EGFR kinase
domain. Taken together, these results suggest that, in
contrast to the case of small molecular EGFR TKI, the presence
of EGFR kinase mutations may not represent a major
predictive and prognostic factor for the efficacy of cetuximab
therapy in CRC patients.
Cetuximab in combination with other targeted
agents One of the hallmarks of effective cancer treatment is the use
of combinational therapeutic regimens comprising several
cytotoxic or cytostatic agents that target cancer cells via
different mechanisms. To this end, cetuximab has
demonstrated significant enhanced antitumor activity in
combination with either chemotherapeutics or radiation in the clinic;
for example, with irinotecan in mCRC and with radiation in
SCCHN. The side-effects of these combination therapies
are usually associated with the cytotoxic components in the
regimens. Based on these observations, it is plausible that a
combination of anti-EGFR antibodies with other targeted
therapeutic agents, including small molecular TKI and mAb
directed against different tumor-associated targets, may yield
enhanced therapeutic activity without adding severe
unwanted systemic toxicities. A number of preclinical
studies have shown that combination of cetuximab with a small
molecular TKI, gefitinib or erlotinib, resulted in enhanced
tumor growth inhibition both in vitro and
in vivo of a number of different tumor cell
lines[147,148]. There was, however, at
least one study that showed that the combination of
cetuximab and gefitinib was rather
antagonistic[149]. Taken together, these observations suggest that the concept of
combining an anti-EGFR antibody with a small molecular TKI
for double-hitting the same target in tumor cells, while
encouraging, needs more preclinical validation and should
proceed with caution in clinical trials. Similarly, additive or
synergistic antitumor activity has also been observed when
cetuximab was used in combination with antisense
oligonucleotides targeting other molecules, such as protein A
kinase and VEGF[150,151].
Cetuximab has also demonstrated additive or synergistic
antitumor activities in various xenograft models when used
in combination with mAb targeting other growth factor
receptors, including those directed against
HER2[152], VEGF receptor 2[153, 154]
and insulin-like growth factor
receptor[115]. In a recent Phase II clinical trial (NCI-6444), patients with
mCRC refractory to irinotecan were given both cetuximab
and bevacizumab, with or without irinotecan. Of 41 patients
who received cetuximab/bevacizumab plus irinotecan, 37%
had a PR and the median TTP was 7.9 months. Of 40 patients
who received cetuximab/beva-cizumab alone, 20% had a PR
and the median TTP was 5.6 months. The most commonly
reported adverse events in the
cetuximab/bevacizumab/irino-tecan arm were skin rash (grade 2, 60%; grade 3, 17%),
diarrhea (grade 2, 29%; grade 3/4, 24%), fatigue (grade 2, 32%;
grade 3, 10%) and neutropenia (grade 3/4, 22%), and the
most commonly reported adverse event in the
cetuximab/bevacizumab alone arm was skin rash (grade 2, 65%; grade 3,
20%)[155]. As a historical control, in the EMR-007 trial,
patients who received cetuximab alone or cetuximab plus
irinotecan had PR rates of 10.8% and 22.9%,
respectively[71]. Furthermore, it is also very intriguing to note that
bevaci-zumab has failed to demonstrate significant clinical benefits
in refractory mCRC patients who have exhausted standard
chemotherapeutic options, including both irinotecan-based
and oxaliplatin-based regimens. In a single-arm, multi-center
trial, 350 refractory patients were treated with bevacizumab
plus bolus or infusional fluorouracil (FU) and leucovorin
(LV). Of the first 100 patients evaluated, PR was confirmed in
only one patient and medium PFS was 3.5
months[156]. Based on these results, a randomized Phase III trial is being
conducted by CALGB/NCI in which chemotherapy naïve
patients (about 2300 patients) are randomized into three groups
to receive cetuximab plus chemotherapy (FOLFOX or FOLFIRI depending on the choice of the individual
investigators), bevacizumab plus chemotherapy or cetuximab
and bevacizumab plus chemotherapy in first-line settings.
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