ABSTRACT: The phosphatidylinositol 3-kinase/mammalian target
of rapamycin (PI3K/mTOR) pathway is commonly dysregulated in breast
cancer. In preclinical studies, hyperactivation of the PI3K pathway has
been linked to resistance to both endocrine therapy and trastuzumab(Drug information on trastuzumab)
(Herceptin). Rapalogs, agents that primarily inhibit mTOR-raptor
complex 1, have been studied in combination with endocrine therapy to
overcome endocrine resistance. Trials of combination endocrine therapy
and rapalogs in metastatic hormone receptor–positive breast cancer have
demonstrated variable results. However, two independent trials have
recently shown that combination everolimus (Afinitor) and tamoxifen(Drug information on tamoxifen) or combination everolimus and exemestane(Drug information on exemestane)
(Aromasin) is more effective than either endocrine agent alone. These
trials selected patients with cancer refractory to endocrine therapy,
which may be important in sensitizing tumors to inhibition of this
pathway. In human epidermal growth factor receptor 2 (HER2)-positive
breast cancer, the early clinical data with combinations of PI3K/mTOR
inhibitors and anti-HER2 therapies are encouraging. Efforts to identify
clinical biomarkers of response or resistance to mTOR inhibitors are
ongoing. This review will summarize results of preclinical and clinical
studies as well as ongoing clinical trials with mTOR or dual PI3K/mTOR
inhibitors.
The
phosphatidylinositol 3-kinase (PI3K)/Akt pathway, of which mammalian
target of rapamycin (mTOR) protein is an important component, is
commonly dysregulated in cancer. TOR protein, a highly conserved serine/
threonine protein kinase, was first identified in 1991 through yeast
studies examining the mechanism of rapamycin.[1] Complex regulatory
mechanisms of the mTOR signaling pathway have been elucidated. These
mechanisms have been important in the development of mTOR inhibitors for
treatment of cancer and also in identifying predictors of response or
resistance.
mTOR controls various cellular processes,
including growth, survival, and autophagy. It receives input from
upstream growth factor receptors, such as the PI3K pathway, and senses
nutrient availability; its central role is in integrating these signals
and altering cellular processes. Based on nutrient availability, the
mTOR pathway can either promote cell growth or it can inhibit growth
during the nutrient deprivation state.[2] Autophagy, a catabolic process
involved in eradicating damaged cellular material, is under the
delicate control of the mTOR pathway. It is utilized as an adaptive
rescue mechanism for starving cells to conserve energy and is highly
dependent on nutrient availability.[3]
The mTOR pathway also receives input from the
adenosine(Drug information on adenosine)
monophosphate–activated protein kinase (AMPK) pathway. The AMPK pathway
senses cellular energy and negatively regulates the mTOR pathway
through the tuberin (TSC1)/hamartin (TSC2) complex. When energy stores
are low, AMPK and TSC2 are activated, thereby inhibiting the mTOR
pathway. An additional negative regulator of mTOR is phosphatase and
tensin homolog (
PTEN), which also tightly regulates the PI3K
pathway.[4] There are two distinct complexes of mTOR—mTORC1 and
mTORC2—which have independent regulatory mechanisms and exert their
cellular growth effects through different downstream targets. Activated
mTOR-raptor complex 1 (mTORC1) results in enhanced protein synthesis and
also inhibits PI3K signaling. Activated mTOR-rictor complex 2 (mTORC2)
promotes cell survival.[4]
Activating
PI3K mutations are frequent in human cancers and have been identified
as oncogenic, making this pathway an attractive therapeutic target in
cancer.[5] These mutations can occur in any component of the PI3K
pathway, resulting in its dysregulation; a number of mechanisms,
including mutations, methylation, and loss of heterozygosity, may be
involved.
PIK3CA (p110 catalytic subunit alpha) mutations have
been identified as a common occurrence in breast cancer,[6] with a
higher frequency in the estrogen receptor (ER)-positive and human
epidermal growth factor receptor 2 (HER2)-positive subtypes than in
triple-negative breast cancer (TNBC).[7] Studies have confirmed that the
PIK3CA
gene is among the most highly mutated genes in breast cancer: mutations
occur at a frequency of 27% to 36%.[8,9] One such study evaluated the
mutational spectrum of
PIK3CA by breast cancer subtype,[8] determined by gene expression profiling.[10,11] The
PIK3CA somatic mutation spectrum differed both by the frequency of mutation and by the type of
PIK3CA mutation seen in each subtype. The luminal A subtype of breast cancer had the highest frequency of
PIK3CA
mutation (45%), and the basal subtype had the lowest (9%). These data
are consistent with the results of prior studies, as luminal A and
basal-like subtypes roughly correspond to ER-positive and
triple-negative breast cancer by immunohistochemistry (IHC),
respectively. Even though
PIK3CA mutations are oncogenic, they
are a good prognostic factor and are associated with improved
survival.[12] This is important to consider when assessing patient
survival in trials in patients with
PIK3CA mutations.
Additional PI3K pathway alterations in breast cancer include
Akt and
PTEN
mutations, or loss of PTEN protein.[7,13] Activation of the PI3K
pathway in breast cancer can occur via a PI3K pathway component
aberration or through activation of another crosstalk pathway. Beyond
identifying PI3K pathway mutations for understanding breast cancer
biology, there are important considerations when this information is
used for patient selection for treatment. The specific
PI3K mutations and the altered components of the PI3K pathway may both impact treatment response.
Rapamycin,
a macrolide, was first isolated from a soil sample on Easter Island
(Rapa Nui) in 1975, and was shown to have antifungal properties.[14] It
was initially used clinically as an immune suppressant to prevent
allograft rejection in renal transplant patients.
Sirolimus(Drug information on sirolimus)
(Rapamune), a rapamycin analog (rapalog), has been shown to inhibit the
growth of cancer cell lines and xenografts from different tumor
subtypes.[15,16] The first generation of mTOR inhibitors target mTORC1,
but they do not bind to mTORC2, which is mostly considered to be
rapamycin-insensitive.[2] However, there are limited data that rapamycin
reduces mTORC2 levels and inhibits Akt activation.[17] Targeting only
mTORC1 with rapalogs leads to increased signaling through upstream
receptor tyrosine kinases and increased Akt activation, which promotes
cell survival. It has been speculated that rapalogs have had limited
clinical activity in cancer due to this mechanism, as well as activation
of parallel signaling pathways. This limitation of rapalogs has fueled
development of alternate methods of targeting the PI3K signaling
pathway, with either adenosine triphosphate (ATP)-competitive mTOR
inhibitors that target both mTORC1 and mTORC2, or by using dual
PI3K/mTOR inhibitors. Several mTORC1 inhibitors are in clinical trials
for various tumor subtypes, including everolimus (Afinitor),
temsirolimus (Torisel), and ridaforolimus (AP23573). Temsirolimus was
the first rapalog approved by the US Food and Drug Administration (FDA);
it was approved in 2007 for the treatment of advanced renal cell
cancer.
In breast cancer, the majority of studies have exploited
the use of mTORC1 inhibitors in ER-positive or HER2-positive breast
cancers, primarily to reverse treatment resistance. The focus of this
review will be these preclinical and clinical studies by breast cancer
subtype. We will also discuss ongoing breast cancer clinical studies
using ATP-competitive mTOR inhibitors, which target mTORC1/mTORC2, and
dual PI3K/mTOR inhibitors.
Preclinical
studies, using hormone receptor (HR)-positive cell lines, have
demonstrated activation of the PI3K/mTOR pathway after long-term
estrogen deprivation.[18,19] Based on these studies, it appeared that
estrogen-deprived cells relied heavily on the PI3K signaling pathway,
making this an important mechanism of acquired endocrine resistance.
This suggested that priming of the PI3K pathway with anti-hormonal
treatment might be important in sensitizing these cells to PI3K/mTOR
inhibitors. A natural next step was to use combination therapy,
simultaneously targeting both the ER and PI3K pathways. Early
combination studies showed that rapalogs were synergistic with
anti-estrogens, including tamoxifen and
letrozole(Drug information on letrozole)
(Femara); blocking both pathways not only enhanced antitumor activity
but also reversed endocrine therapy resistance related to PI3K
signaling.[20-22] Moreover, high Akt activity has also been shown to
contribute to resistance to endocrine therapy,[23] and this also can be
reversed by rapalogs.[20,22]
Metastatic setting.
Almost all patients with HR-positive breast cancer treated with
endocrine therapy develop tumor resistance to treatment. Preclinical
data, as described earlier, implicate the PI3K/mTOR pathway in acquired
resistance to endocrine therapy, and synergistic preclinical anti-tumor
activity has been seen with the combination of rapalogs and
anti-estrogens. Based on this biological rationale, clinical trials have
combined mTORC1 inhibitors and endocrine therapy in HR-positive breast
cancer. Initial studies with temsirolimus and everolimus as single
agents in the metastatic setting demonstrated response rates of 9% to
12%.[24,25] Another study with temsirolimus alone was limited to
HR-positive or HER2-positive metastatic breast cancer, to enrich it for
PIK3CA mutations.[26] Clinical activity was again limited. Primary tumors from this study were analyzed for
PIK3CA
mutations and PTEN expression by IHC, but no association was seen with
clinical response.[26] A limitation of this study is that the
PIK3CA mutation status of primary tumors was analyzed, as opposed to the metastatic site, which can be discordant.[27]
The
next approach was to combine anti-estrogens and mTORC1 inhibitors in
clinical trials. A randomized phase II study of HR-positive metastatic
breast cancer tested combination letrozole and temsirolimus vs letrozole
alone and found that patients who received combination therapy had
superior median progression-free survival (PFS) (13.2 vs 11.6 months).
However, the clinical benefit rate (CBR) and the objective response rate
(ORR) for patients in the combination arm were not significantly
different from the rates in patients who received letrozole alone.[28]
Given these somewhat encouraging results, a large randomized phase III
trial (N = 1112) was conducted in postmenopausal women with metastatic
disease, with letrozole either alone or in combination with temsirolimus
as first-line endocrine therapy. The trial was terminated early due to
lack of benefit.[29] It has been speculated that this trial failed since
it limited the use of mTOR inhibition in combination with endocrine
therapy to the first-line metastatic setting. Given lack of prior
hormonal therapy exposure, the tumors might not have been dependent on
the PI3K/mTOR pathway, thereby remaining insensitive to mTOR pathway
inhibition. This highlights the need for identification and selection of
patients, whose tumors are dependent on PI3K pathway activation.
The
Tamoxifen Plus Everolimus (TAMRAD) study (N = 111) randomized patients
with prior exposure to an aromatase inhibitor (AI) in the metastatic
setting, to tamoxifen alone versus combination tamoxifen and
everolimus.[30] This study demonstrated improvement in CBR (42% vs 61%;
P
= .045), the primary endpoint, and in time to progression (TTP) (4.5 vs
8.6 months; hazard ratio [HR] = 0.54; 95% confidence interval [CI],
0.36–0.81;
P = .002) favoring the combination treatment. This
supports that prior endocrine therapy resulting in priming of the
PI3K/mTOR pathway may allow for meaningful synergy through attempts to
overcome acquired endocrine resistance. In an exploratory analysis,
patients were stratified based on primary hormone resistance, defined as
relapse during adjuvant AI therapy or progression within 6 months of AI
treatment in the metastatic setting, or secondary hormone resistance,
defined as late relapse or progression on an AI in the metastatic
setting more than 6 months after treatment. A higher CBR (48% vs 74%
[secondary]; 36% vs 46% [primary]) and increased TTP (5.5 vs 14.8
months; HR = 0.46; 95% CI, 0.26–0.83;
P = .009 [secondary]; 3.8 vs 5.4 months; HR = 0.70; 95% CI, 0.40–1.21;
P = nonsignificant [primary]) was predominantly observed in patients with secondary hormone resistance in the everolimus arm.[30]
A
phase III trial, Breast Cancer Trials of Oral Everolimus-2 (BOLERO-2),
enrolled 724 patients with HR-positive advanced breast cancer to assess
the efficacy of everolimus (at a dose of 10 mg per day) and exemestane
(Aromasin), in patients with disease refractory to nonsteroidal AIs,
including letrozole or
anastrozole(Drug information on anastrozole)
(Arimidex).[31] An improved median PFS was observed by both local and
central assessment with the combination of exemestane and everolimus
(2.8 vs 6.9 months; HR = 0.43; 95% CI, 0.35–0.54;
P < .001 [local]; 4.1 vs 10.6 months; HR = 0.36; 95% CI, 0.27–0.47;
P
< .001 [central]). Overall survival results have not been reported. A
total of 23% of patients receiving everolimus had serious adverse
events compared with 12% receiving placebo, resulting in everolimus
discontinuation in 19% of the combination group vs 4% in the placebo
group. The most common grade 3 or 4 events with combination therapy were
stomatitis, anemia, hyperglycemia, dyspnea, fatigue, and pneumonitis.
Notably, there were seven deaths attributed to adverse events (1%) in
the everolimus arm; these were due to sepsis, pneumonia, tumor
hemorrhage, cerebrovascular incident, renal failure, and suicide.[31]
Forthcoming
trials are assessing the role of mTORC1 and dual PI3K/mTOR inhibitors
in combination with other anti-hormonal therapies and even
chemotherapies, in various lines of metastatic disease (
Table).
In
summary, trials of combination endocrine therapy and mTORC1 inhibitors
in metastatic HR-positive breast cancer have demonstrated variable
results. Single-agent temsirolimus or everolimus has limited clinical
activity in metastastic breast cancer. A large study that combined
temsirolimus with letrozole vs letrozole alone in first-line hormonal
therapy for metastatic disease found no benefit from the combination.
Two trials have found that combination everolimus and tamoxifen (TAMRAD
study) or combination everolimus and exemestane (BOLERO-2) is more
effective than either endocrine agent alone. The variability among the
reported studies may be related to patient selection, prior endocrine
therapy exposure, and the specific drug combination being tested. It is
noteworthy that both of the positive studies selected patients who were
previously exposed to endocrine therapy in the metastatic setting. Thus,
prior endocrine therapy exposure may be an important factor in priming
the PI3K/mTOR pathway and thereby sensitizing the tumors to inhibition
of this pathway.
Adjuvant/neoadjuvant setting. Everolimus
was studied as a single agent in a neoadjuvant trial and was associated
with a significant reduction in Ki67 after 14 days of therapy.[32] A
neoadjuvant, randomized study in postmenopausal women (N = 270) with
ER-positive breast cancer compared letrozole and everolimus vs letrozole
and placebo.[33] There was an improved clinical response rate and
decreased proliferation in the everolimus-plus-letrozole arm compared
with letrozole alone. Response was seen in both wild-type and mutant
PI3K tumors. In addition, a reduction in phospho-S6, a pharmacodynamic
marker, was noted in post-treatment biopsies in the
everolimus-containing arm, signifying that mTOR was being inhibited at
the dose used.[33]
In the adjuvant setting, a phase III
randomized trial is evaluating the role of combining everolimus with
standard adjuvant endocrine therapy, for women with high-risk breast
cancer (T
able).
Based
upon the available data, there is no standard role for adjuvant or
neoadjuvant use of mTOR or dual PI3K/mTOR inhibitors in combination with
endocrine therapy or chemotherapy.