SAN FRANCISCOPotentially therapeutic concentrations
of ZD1839 (Iressa) inhibit phosphorylation in HER2-overexpressing breast tumor
cell lines that coexpress HER1, Stacy Moulder, MD, Vanderbilt University
Medical Center, said at the 37th Annual Meeting of the American Society of
Clinical Oncology (ASCO).
Iressa is an investigational oral epidermal growth factor
receptor (EGFR, also known as HER1) tyrosine kinase inhibitor, made by
AstraZeneca. Preclinical evidence suggests that Iressa inhibits signaling by
blocking the putative HER1/HER2 heterodimer and that inhibition can be
augmented by the addition of the anti-HER2 monoclonal antibody trastuzumab
In her presentation at an ASCO integrated symposium, Dr.
Moulder said that HER2 is overexpressed in 20% to 30% of breast carcinomas.
HER1 may not have as high an expression rate by immunohistochemistry (IHC);
however, because HER1 and HER2 act together, the HER1 kinase may be responsible
for activating the HER2 receptor.
She explained that HER2 lacks an activating ligand and is
usually turned on by partnering with another receptor. The fact that HER2 is
constantly activated in some breast cancers hints that transactivation of its
kinase may occur via HER1.
An agent such as Iressa that blocks the HER1 receptor,
therefore, potentially could block HER2 phosphorylation as well. "Since it
has been established clinically that antibodies against HER2 are a therapeutic
strategy in breast cancer patients, we wanted to test this preclinically," Dr. Moulder said in an interview.
The investigators first examined the effect of Iressa against
isolated HER1 and HER2 receptors. They found that the concentration of Iressa
needed to block HER1 (an IC50 of 0.02 µM) was much lower than that needed to
block HER2 (IC50 of 3.7 µM). "This is important," Dr. Moulder
stated, "because steady state levels of 0.9 µM have been achieved in
phase I/II clinical trials among patients receiving 250- to 500-mg doses of
In BT-474 and SKBR-3 breast cancer cell lines, which coexpress
HER1, much lower concentrations of Iressa (1 µM) were able to block HER2. The
MDA-453 breast cancer cell line, which does not coexpress HER1, however, was
not affected by 1 µM levels of Iressa.
These findings suggests that Iressa-mediated inhibition of the
HER1 kinase results in in vivo inhibition of HER2 phosphorylation, she said.
Dr. Moulder underscored that HER1 coexpression is difficult to
measure and may not be detected in some tumors using standard tumor-staining
techniques. "We have lots of HER2 on the cell surface of these cells, and
we have coexpression of HER1, meaning that it is merely also there," she
said. In the Vanderbilt studies, the researchers used flow cytometry to detect
the receptor, which is much more sensitive than IHC.
"It may be that clinically we can’t detect HER1, but
that doesn’t mean that there is not a functioning HER1 molecule there that
can be used to block HER2," Dr. Moulder said.
The investigators next looked at possible synergy with therapy
combining Iressa and trastuzumab to block the family of receptors from
interacting with one another. Trastuzumab, Dr. Moulder noted, sequesters HER2,
preventing it from interacting with any of the other EGFR family members.
"Looking at the SKBR-3s, a cell line that overexpresses
HER2 and is a HER1 coexpressor, we found that Iressa inhibited growth better
than trastuzumab, and that putting the two together did not augment the
effect," she said. Based on that experiment alone, there is no rationale
for combining the drugs, she said. However, investigators also used flow
cytometry to look at the percentage of apoptotic cells induced by Iressa,
trastuzumab, or the combination. The percentage was 0.56% in untreated cells,
1.06% with Iressa, 1.52% with trastuzumab, and 5.26% with the combination of
Iressa/trastuzumab. "We saw a fivefold increase with the combination,
compared with either agent alone," she said.
The researchers also looked at the combination’s effect on
murine xenograft breast cancer models. Tumors were allowed to grow to 200 mm3
before initiating Iressa, trastuzumab, or both. The results showed that all
three inhibited cell growth, compared with controls, with substantial tumor
regression seen with the combination, compared with trastuzumab alone (P =
Finally, analysis of the agents’ effect on harvested breast
cancer tumors showed 29% of controls to be in S phase, compared with 14.0% for
Iressa, 22% for trastuzumab, and 10.4% for the combination (P = .001, compared
with trastuzumab alone or controls).
Dr. Moulder noted that trastuzumab has demonstrated efficacy in
combination with breast cancer chemotherapy regimens and has been tolerated as
a single agent for prolonged periods (more than 5 years). "Iressa is
targeted therapy that will possibly offer lower toxicity. We hope that patients
will be able to tolerate this biologic therapy for longer periods, compared
with combination chemotherapy/Herceptin regimens," she said.
After the meeting, Dr. Moulder told ONI that a phase II ECOG
trial of trastuzumab plus Iressa in metastatic breast cancer has received NIH
approval and should begin enrolling patients shortly. Criteria for patients
will include HER2 positive (+3 by IHC) and/or HER2 amplified by FISH; two or
fewer prior chemotherapy or hormonal therapies for metastatic disease; and no
prior therapy with trastuzumab. Treatment will be trastuzumab (2 mg/kg/wk) plus
Iressa (500 mg/d) until disease progression. Endpoints will be response rate
and time to disease progression.