Review: Clinical Trial Outcomes
`
`Strategies to overcome endocrine therapy resistance in
`hormone receptor-positive advanced breast cancer
`
`Rowan T Chlebowski
`Chief of Medical Oncology & Hematology,
`Harbor-UCLA Medical Center, Torrance,
`CA 90502, USA
`Tel.: +1 310 222 2219
`Fax: +1 310 320 2564
`E-mail: rowanchlebowski@gmail.com
`
`Clin. Invest. (2014) 4(1), 19–33
`
`Use of endocrine therapies has made hormone receptor-positive breast
`cancer a manageable disease if diagnosed at an early stage. However,
`endocrine therapy resistance is a persistent problem in patients with
`advanced breast cancer. Ongoing research has identified a number
`of mechanisms that may mediate resistance, including estrogen-
`independent activation of the estrogen receptor; increased signaling
`through the RAS/MAPK, NF-kB, or PI3K/Akt/mTOR pathway; and
`maintenance of cyclin D1 expression. Based on such findings, various
`strategies to overcome endocrine resistance have been developed.
`Although some therapies are in early development, others are available
`in the clinic. The novel treatment strategies under evaluation in clinical
`trials for managing patients with endocrine-resistant advanced breast
`cancer will be reviewed in combination with endocrine therapy.
`
`Keywords: endocrine resistance • growth factor receptor
`• hormone receptor-positive • PI3K pathway • RAS/MAPK pathway • tyrosine kinases
`
`In 2013, approximately 232,000 women will be diagnosed with invasive breast
`cancer (BC) in the USA and 40,000 will die of the disease [1]. Approximately
`70% of BCs are hormone (estrogen and/or progesterone) receptor-positive (HR+)
`[2], and patients with HR+ BC have a better prognosis, in part, because of their
`responsiveness to endocrine therapies [3,4]. Tamoxifen, a selective estrogen recep-
`tor (ER) modulator, is a partial ER antagonist, initially used as first-line therapy
`in patients with HR+ advanced BC (ABC) [201]. Aromatase inhibitors (AIs) lower
`endogenous estrogen levels by inhibiting androgen to estrogen conversion [5]. The
`three third-generation AIs – anastrozole, letrozole and exemestane – were shown
`to be superior to tamoxifen in terms of efficacy and safety in postmenopausal
`women with ABC [6–12]. In addition, approximately 40% of patients treated with
`adjuvant endocrine therapy and almost all patients with metastatic BC (MBC)
`die of the disease [13].
`
`Overview of endocrine resistance & strategies to overcome resistance
`Endocrine resistance is a significant problem in treating BC. Patients can pres-
`ent with primary/acquired resistance (no response to initial endocrine therapy),
`or disease progression or recurrence can develop while the patient is receiving
`therapy (secondary/acquired resistance). In fact, approximately 30% of patients
`with MBC regress with initial endocrine therapy, whereas another 20% have pro-
`longed stable disease [14]; and the duration of response to second and subsequent
`lines of therapy is substantially lower [15,16]. Since current endocrine therapy is
`effective, it is important to identify those patients who respond and those who
`do not respond to therapy in order to improve treatment decisions. However,
`predicting response can be challenging. Molecular profiling of snap-frozen tumor
`
`10.4155/CLI.13.123 © 2014 Future Science Ltd
`
`ISSN 2041-6792
`
`19
`
`Ex. 1085-0001
`
`

`
`Review: Clinical Trial Outcomes Chlebowski
`
`biopsies has demonstrated that patients who recur early
`while on adjuvant tamoxifen therapy have a different
`molecular profile than those patients who experience
`a later recurrence while receiving tamoxifen [17]. In
`addition, molecular profiling analyses of patients who
`became resistant to aromatase inhibitors have found
`that patients who develop resistance to endocrine
`therapy have different gene expression patterns, sug-
`gesting that endocrine resistance is not a homogeneous
`phenomenon [18]. In addition, preclinical models have
`suggested that endocrine resistance results in upregu-
`lation of alternate signaling pathways that might be
`suitable targets for targeted therapies [19]. These data
`suggest that endocrine resistance could be a hetero-
`geneous phenomenon that might necessitate molecu-
`lar profiling to determine the appropriate course of
`action when resistance occurs. Additionally, because
`resistance might result in the upregulation of multiple
`pathways, the use of more than one therapy to target
`these pathways might be necessary.
`Ongoing research has provided insight into the
`causes of endocrine resistance and a number of mecha-
`nisms have been proposed, such as loss of ERa (one
`of the two types of ER) expression through methyla-
`tion; alterations in the expression of ER coactivators;
`and mutations in ERa [13,20]. However, no large-scale
`clinical data are available for most of these endocrine
`resistance mechanisms and they will not be discussed
`further.
`Some of the mechanisms of endocrine resistance
`that have been studied in clinical trials (Figure 1) [21]
`include ligand (estrogen)-independent activation of
`ER, increased signaling through the RAS/MAPK
`pathway, NF-kB pathway, or PI3K/Akt/mTOR path-
`way, increased growth factor (GF) expression and sig-
`naling, and loss of ERa-mediated signaling through
`chromatin remodeling [13,20].
`
` ■ Targeting the ER
`Unlike AIs and tamoxifen, fulvestrant, a selective
`ER down regulator, induces rapid degradation of ER
`[22]. In a Phase III trial in patients with previously
`untreated ABC comparing tamoxifen with fulves-
`trant at the initially evaluated dose of 250 mg every
`28 days, now considered low-dose, fulvestrant was as
`effective, but not superior to, tamoxifen [23]. Although
`steady-state drug concentrations are reached only after
`3–6 months with low-dose fulvestrant therapy, steady
`state levels were reached in less time using a loading
`dose consisting of 500 mg on day 1, 250 mg on days
`14 and 28, and 250 mg every 28 days thereafter, and a
`high-dose, consisting of 500 mg on days 1, 14, and 28
`of the first month and 500 mg every 28 days thereafter
`[24]. A Phase II, open-label trial (FIRST) in patients
`
`with ABC who may have had prior adjuvant endo-
`crine therapy for early disease more than 12 months
`before randomization but with no previous endocrine
`therapy exposure for advanced disease, evaluated the
`efficacy of high-dose fulvestrant versus anastrozole
`[25]. Although there was no difference in the primary
`end point of clinical benefit rate (CBR) in the FIRST
`trial, the median time to progression (TTP) was sub-
`stantially greater with high-dose fulvestrant (23.4 vs
`13.1 months; hazard ratio: 0.66; 95% CI: 0.47–0.92;
`p = 0.01) [25,26]. An ongoing Phase III randomized
`trial is attempting to provide definitive evaluation of
`those results.
`As AIs lower estrogen levels, endocrine resistance
`may result from development of estrogen-indepen-
`dent ER-mediated signaling [5]. Since fulvestrant
`promotes degradation of ERa through the ubiqui-
`tin–proteasome pathway in a preclinical setting [22],
`it was hypothesized that fulvestrant could overcome
`endocrine resistance. A combined ana lysis of two
`Phase III trials [27,28] in the second-line setting in
`postmenopausal women with locally ABC or MBC
`and disease progression during previous endocrine
`therapies, found low-dose fulvestrant as effective as
`anastrozole [29], leading to US FDA approval for use
`in postmenopausal women with HR+ ABC after pro-
`gression on previous anti-estrogen therapy. In another
`Phase III trial, low-dose fulvestrant and exemestane
`were equally effective in patients with HR+ ABC after
`previous nonsteroidal AI therapy [30]. Recently, the
`effectiveness of high- and low-dose fulvestrant in post-
`menopausal women with ABC or MBC progressing
`during previous endocrine therapy has been directly
`compared in the Phase III (CONFIRM) trial. In that
`setting, high-dose fulvestrant, compared with low-
`dose, significantly improved median progression-free
`survival (PFS; 6.5 vs 5.5 months; hazard ratio = 0.80;
`95% CI: 0.68–0.94; p = 0.006) [31] and overall sur-
`vival (OS; 26.4 vs 22.3 months; hazard ratio = 0.81;
`95% CI: 0.69–0.96; p = 0.016) [32]. Consequently, the
`FDA approved the high-dose fulvestrant schedule as a
`second-line therapy for postmenopausal women with
`HR+ metastatic disease [33].
`Although preclinical models suggested that combin-
`ing AI and fulvestrant may be an effective treatment
`option [34,35], clinical trial results have been conflicting.
`Results from the SWOG S0226 trial in post menopausal
`women with previously untreated MBC, showed that
`low-dose fulvestrant plus anastrozole significantly
`improved median PFS (15.0 vs 13.5 months; p = 0.007)
`and median OS (47.7 vs 41.3 months; p = 0.049) than
`anastrozole alone [36]. In contrast, results from another
`Phase III trial (FACT) in postmenopausal women with
`HR+ ABC in the same setting, comparing low-dose
`
`20
`
`www.future-science.com
`
`future science group
`
`Ex. 1085-0002
`
`

`
`Endocrine resistance in advanced breast cancer Review: Clinical Trial Outcomes
`
`Estrogen
`
`Growth factors
`
`A
`
`Plasma
`membrane
`
`PI3K
`
`B
`
`RTKs:
`EGFR,
`ERB2
`and IGFR
`
`C
`
`ER
`
`Ras
`
`Erk
`
`ER
`
`D
`
`PTEN
`
`Akt
`
`PI3K
`
`mTORC1
`
`ER
`
`Src
`
`M
`
`FAK
`
`E
`
`PELP1
`
`ER
`
`Src
`
`Src
`
`S6K1
`
`4E-BP1
`
`Akt
`
`S6K1
`
`elF4E
`
`Translation
`
`Erk
`
`PP
`
`ER
`
`P P
`
`Cytoplasm
`
`ER
`
`ER
`
`Nucleus
`
`CoA HAT
`
`ER ER
`
`CoA
`
`ER
`
`Ap1 Sp1
`
`TFs
`
`TFs
`
`TF
`
`TF
`
`ERE
`
`SRE
`
`RE
`
`mRNA
`
`AAAA
`
`MYC, cyclin D1,
`cyclin E1 and cyclin E2
`
`Growth proliferation and survival
`
`Clinical Investigation © Future Science Group (2014)
`
`Figure 1. Mechanisms that may mediate endocrine resistance. (A) Ligand-bound ER activates gene expression either directly or
`through protein–protein interactions. (B) RTKs such as the EGFR, ERBB2 (also known as HER2) and the IGFR, can activate downstream
`signaling events, thereby regulating translation and transcription. (C) ER localized at the cell membrane or in the cytoplasm can mediate
`nongenomic signaling. (D) Formation of an ER–PI3K–Src–FAK complex can activate Akt, resulting in activation of downstream signaling
`cascade. (E) Activation of Erk by ER–Src–PELP1 complexes can activate downstream signaling events. All together these complex
`signaling cascades regulate growth, proliferation and survival, resulting in endocrine therapy resistance.
`CoA: Coactivator; EGFR: EGF receptor; ER: Estrogen receptor; ERE: Estrogen response element; IGFR: IGF receptor; RE: Response element;
`RTK: Receptor tyrosine kinase; SRE: Serum response element; TF: Transcription factor.
`Reprinted with permission from [21].
`
`Clin. Invest. (2014) 4(1)
`
`21
`
`future science group
`
`Ex. 1085-0003
`
`

`
`Review: Clinical Trial Outcomes Chlebowski
`
`fulvestrant plus anastrozole with anastrozole alone,
`did not find statistically significant differences in TTP,
`CBR, objective response rate (ORR) or OS between the
`two treatments [37]. Results from another Phase III trial
`(SoFEA), comparing low-dose fulvestrant plus anas-
`trozole with anastrozole alone, also found no differ-
`ences in PFS, ORR, CBR or OS [38]. Therefore, current
`data are inconclusive for combining AI with low-dose
`fulvestrant to overcome endocrine resistance.
`
` ■ Inhibition of intracellular signaling cascade
`In preclinical models, activation of intra cellular signal-
`ing pathways (Figure 1) [21] – such as the PI3K/Akt/
`mTOR pathway, RAS/MAPK pathway, Src kinase sig-
`naling cascade, or NF-kB pathway – has been shown
`to mediate endocrine resistance. Strategies inhibiting
`one or more of these pathways in combination with
`endocrine therapies are under clinical evaluation.
`
`Inhibition of the PI3K/Akt/mTOR pathway
`Preclinical models of BC cells resistant to estrogen
`deprivation were found to have amplified PI3K/Akt/
`mTOR-mediated signaling [39], suggesting that acti-
`vation of the PI3K pathway may facilitate survival of
`these cells under hormone deprivation. Additionally,
`by using cell lysates from hormone receptor-positive
`primary breast tumors, PI3K pathway activation was
`associated with poor disease outcome after adjuvant
`therapy [39]. Activation of PI3K results in activation
`of its downstream target Akt and in cells expressing
`activated Akt, the efficacy of tamoxifen to induce
`growth inhibition was dramatically reduced [40]. Since
`treatment with mTOR inhibitors reversed tamoxifen
`resistance in cells over expressing activated Akt [40,41]
`and reduced tumor growth in xenograft models [40],
`mTOR inhibitors (sirolimus, temsirolimus and evero-
`limus) have been tested in clinical trials to overcome
`endocrine resistance.
`
`Sirolimus & temsirolimus
`Sirolimus, in combination with tamoxifen, has recently
`been shown to significantly improve median TTP and
`response rates compared with tamoxifen alone in a
`Phase II trial in postmenopausal women with HR +
`ABC in whom previous tamoxifen and/or AI therapy
`was ineffective [42]. Although temsirolimus was effec-
`tive in combination with letrozole in patients with
`ABC with disease progression during or after tamoxi-
`fen therapy in a Phase II trial [43], results from HORI-
`ZON, a Phase III trial in postmenopausal women with
`AI-naive, locally advanced or MBC, did not show ben-
`efit for the temsirolimus combination compared with
`letrozole alone in the first-line setting, with results
`perhaps limited by the substantial toxicity seen [44].
`
`Everolimus
`In a Phase II neoadjuvant trial in patients with oper-
`able ER+ BC, everolimus in combination with letro-
`zole improved the clinical response rate compared with
`letrozole alone [45]. In another Phase II trial, everolimus
`plus tamoxifen was compared with tamoxifen alone in
`postmenopausal patients with HR+, HER2– MBC with
`previous exposure to AIs, and the combination signifi-
`cantly improved CBR (61 vs 42%; p < 0.045), TTP
`(8.6 vs 4.5 months; p < 0.002), and OS (hazard ratio:
`0.45; 95% CI: 0.24–0.81; exploratory p = 0.007) [46].
`In the BOLERO-2 Phase III trial evaluating everoli-
`mus plus exemestane versus exemestane alone in post-
`menopausal women with HR+, HER2– ABC after previ-
`ous letrozole or anastrozole, median PFS at 7.1 months
`was significantly improved with the everolimus combi-
`nation (6.9 vs 2.8 months by local assessment; hazard
`ratio 0.43; 95% CI: 0.35–0.54; p < 0.001) [47]. The
`Kaplan–Meier plot shows substantial visual separa-
`tion of the PFS curves at the 6-week reimaging period.
`Analy ses from the 12.5- and 18-month follow-up data
`from BOLERO-2 produced similar results [48,49]. Based
`on the BOLERO-2 trial, everolimus (10 mg/day) in
`combination with exemestane (25 mg/day) was FDA
`approved for managing postmenopausal women with
`HR+ ABC and is currently the only approved mTOR
`inhibitor for managing patients with ABC after failure
`with letrozole or anastrozole [50]. In additional analy-
`ses from BOLERO-2, the everolimus combination was
`associated with a longer time to definitive deterioration
`of health-related quality of life (8.3 vs 5.8 months; haz-
`ard ratio: 0.74; p = 0.0084) [51] and was effective regard-
`less of whether patients had visceral metastases [52].
`Finally, exploratory analyses in the study found favorable
`effects on bone turnover, and less progression in bone
`metastases was seen for the everolimus combination [53].
`Of potential clinical relevance are findings from
`the Phase III BOLERO-3 trial, in which everolimus
`(5 mg/day) addition was compared with trastuzumab
`and weekly vinorelbine alone in patients with HER2+
`ABC [54]. Everolimus addition improved median PFS
`(hazard ratio: 0.78; p = 0.0067), but subgroup analyses
`suggested an effect limited to women with ER– dis-
`ease (hazard ratio: 0.65; 95% CI: 0.49–0.86). These
`findings raise the hypothesis that combination therapy
`targeting both ER and HER2 pathways may be needed
`to optimize outcome in ER+, HER2+ ABC.
`
`Ongoing trials
`A Phase III adjuvant trial evaluating 1-year therapy
`with everolimus in addition to adjuvant endocrine
`therapy in high-risk patients with HR+ HER2– invasive
`BC is ongoing (Table 1) [202]. Other ongoing BC stud-
`ies evaluating everolimus are also outlined in Table 1.
`
`22
`
`www.future-science.com
`
`future science group
`
`Ex. 1085-0004
`
`

`
`Table 1. Ongoing clinical trials using PI3K/Akt/mTOR inhibitors to overcome endocrine resistance in patients with
`advanced breast cancer.
`Clinical trial
`Intervention
`identifier
`NCT01231659
`
`Expected
`enrollment (n)
`70
`
`Select primary and secondary
`outcomes
`Primary: ORR
`Secondary: PFS, OS, DCR, safety
`
`Patient population
`
`Phase
`
`II
`
`Postmenopausal women
`with locally advanced BC
`or MBC after recurrence
`or progression on
`tamoxifen, anastrozole
`or exemestane
`Postmenopausal women
`with endocrine-resistant
`advanced BC
`Postmenopausal women
`with AI-resistant MBC
`
`Letrozole +
`everolimus
`
`NCT01499160
`
`NCT01797120
`
`NCT01698918
`
`NCT01437566
`
`NCT01783444
`
`Letrozole +
`lapatinib +
`everolimus
`Fulvestrant +
`everolimus
`
`Everolimus +
`letrozole
`
`Fulvestrant ±
`GDC-0941
`fulvestrant ±
`GDC-0980
`Everolimus
`Capecitabine
`Everolimus +
`exemestane
`
`NCT01626222
`
`Exemestane
`+ everolimus
`
`NCT01633060
`
`Fulvestrant ±
`BKM120
`
`NCT01610284
`
`Fulvestrant ±
`BKM120
`
`NCT01674140
`
`Adjuvant
`Primary: IDFS assessed up to
`hormone
`10 years
`therapy ±
`Secondary: OS, DRFS assessed
`everolimus
`up to 10 years, toxicity
`AI: Aromatase inhibitor; BC: Breast cancer; CBR: Clinical-benefit rate; DCR: Disease-control rate; DOR: Duration of response; DRFS: Distant recurrence-free survival;
`ECOG: Eastern Cooperative Oncology Group; ER: Estrogren receptor; HR: Hormone receptor; HR-QoL: Health-related QoL; IDFS: Invasive disease-free survival;
`MBC: Metastatic breast cancer; ORR: Overall-response rate; OS: Overall survival; PFS: Progression-free survival; PK: Pharmacokinetic; QoL: Quality of life; TTP: Time to
`progression.
`
`Endocrine resistance in advanced breast cancer Review: Clinical Trial Outcomes
`
`Ref.
`
`[214]
`
`[215]
`
`[216]
`
`[217]
`
`[218]
`
`[219]
`
`[220]
`
`[221]
`
`[222]
`
`[202]
`
`II
`
`II
`
`II
`
`II
`
`II
`
`76
`
`130
`
`200
`
`270
`
`300
`
`Primary: CBR
`Secondary: PFS
`
`Primary: PFS
`Secondary: ORR, TTP, OS
`toxicity
`Primary: PFS
`Secondary: ORR, OS, reduction
`in severity and duration of
`stomatitis, safety
`Primary: PFS, safety
`Secondary: ORR, DOR,
`PK parameters
`
`Primary: PFS
`Secondary: OS, ORR, CBR,
`change in ECOG status, QoL,
`safety
`
`IIIb
`
`300
`
`Primary: ORR
`Secondary: PFS, OS, safety, HR-
`QoL, resource use
`
`III
`
`III
`
`615
`
`842
`
`Primary: PFS
`Secondary: OS, ORR, CBR,
`safety, PK, QoL
`
`Primary: PFS
`Secondary: OS, ORR, CBR,
`safety, PK, QoL
`
`III
`
`3500
`
`Postmenopausal women
`with ER+ MBC as first-
`line therapy
`
`AI-resistant patients
`with advanced BC or
`MBC
`
`Postmenopausal women
`with locally advanced
`BC, recurrent BC or
`MBC after recurrence
`or progression on prior
`letrozole or anastrozole
`Postmenopausal
`women with ER+, locally
`advanced BC or MBC
`progressing on prior
`therapies
`Postmenopausal women
`with HR+ HER2– AI
`treated with locally
`advanced BC or MBC
`progressing on or after
`mTOR inhibitor therapy
`Postemenopausal
`women with HR+ HER2–
`AI treated with locally
`advanced BC or MBC
`refractory to AI therapy
`High-risk patients with
`HR+, HER2– BC
`
`Clin. Invest. (2014) 4(1)
`
`23
`
`future science group
`
`Ex. 1085-0005
`
`

`
`Review: Clinical Trial Outcomes Chlebowski
`
` ■ Identifying biomarkers of response to everolimus
`A key aspect of administering everolimus therapy is deter-
`mining which patients would benefit most from treat-
`ment. However, identification of appropriate biomark-
`ers has been challenging. Although PIC3AK-activating
`mutations are common in BC [55], data have suggested
`that activation of PIK3CA mutations is not predictive
`of clinical benefit to mTOR inhibitors. An ana lysis of
`primary ER+ BC tumor samples found that high levels
`of the PIK3CA-GS gene signature expression (indicative
`of a PIK3CA mutant phenotype) is indicative of low
`mTOR-pathway activation [56]. Additionally, an explor-
`atory ana lysis of the BOLERO-2 study that evaluated
`the mutational status of key genes (PIK3CA, CCND1
`or FGFR1/2) found that, when examining these genes
`individually, patients whose genes were altered derived
`similar benefit to everolimus, compared with the overall
`trial population [57]. Also, patients who had wild-type
`or a single genetic alteration in the PIK3CA, CCND1,
`or FGFR1/2 genes seemed to derive greater benefit from
`everolimus than the overall BOLERO-2 population
`[57]. Another method to evaluate the potential benefit
`of mTOR inhibitors is the evaluation of the pattern of
`protein expression with mTOR pathway activation. In
`an exploratory translational ana lysis of the TAMRAD
`data, high p4EBP, low LKB1 and low PI3K seemed to be
`associated with everolimus efficacy [58]. However, these
`data must be validated in larger studies.
`
` ■ Investigational strategies to inhibit PI3K/Akt/
`mTOR pathway
`Additional PI3K/Akt/mTOR pathway inhibitors are being
`evaluated in clinical trials. For example, NVP-BEZ235 (a
`dual pan-PI3K/mTOR inhibitor) and BKM120 (a pan-
`PI3K inhibitor) are being evaluated in combination with
`letrozole in a Phase I trial in patients with HR+ MBC [203].
`
`Inhibition of the RAS/MAPK pathway
`The proto-oncoprotein RAS is a central mediator of many
`GF receptor-mediated signals, and activation of GF-medi-
`ated signaling results in ligand-independent activation of
`ER-mediated signaling through phosphorylation of ER
`or its coactivators [59]. Farnesyltransferase inhibitors that
`inhibit RAS have been shown to synergize with endocrine
`therapies to inhibit cell growth and induce apoptosis in a
`preclinical setting [60]. Despite promising preclinical stud-
`ies, results from Phase II clinical trials evaluating tipifa-
`rinib, a farnesyltransferase inhibitor, with tamoxifen [61]
`or fulvestrant [62] have been disappointing.
`
`Inhibition of Src-Kinase signaling cascade
`In preclinical studies, Src-kinase activity was increased in
`tamoxifen-resistant cells, resulting in increased migration,
`and inhibition of Src was found to reverse this aggressive
`
`phenotype [63] and restore sensitivity to tamoxifen [64].
`Dasatinib, an Src-kinase inhibitor, blocked the Src-
`induced proliferation of tamoxifen-resistant cells [65].
`However, in the clinic, dasatinib addition to exemestane
`in a Phase II trial did not increase PFS, compared with
`exemestane alone [204]. Nonetheless, several additional
`Phase II combination studies are ongoing (Table 2).
`
`NF-kB pathway & proteasome inhibition
`Active NF-kB in BC tissue identified a high-risk subset of
`ER+ BC patients [66]. In preclinical studies, inhibition of
`NF-kB activation with proteasome inhibitors stimulated
`the growth inhibitory effect of tamoxifen [67,68]. Bortezo-
`mib, a proteasome inhibitor that blocks NF-kB activation,
`was studied in combination with endocrine therapy in a
`small Phase II trial [69]. Although no clinical response was
`observed, a Phase II trial in combination with fulvestrant
`is ongoing (Table 2) [205].
`
` ■ Inhibition of aberrant GF receptor activation
`Endocrine resistance has been linked to aberrant expres-
`sion, activation or signaling through GF RTKs. Aber-
`rant activation of GF RTKs have been shown to activate
`a number of intracellular signal transduction cascades
`(Figure 1) [21], including the PI3K/Akt/mTOR, Src-kinase,
`and RAS/MAPK pathways [70,71]. Hence, a number of GF
`receptor inhibitors are being studied in clinical trials to
`overcome endocrine resistance.
`
`HER2 inhibition
`Approximately 10% of HR+ BC are also HER2+ [72].
`Using archival tumor blocks from the ATAC trial, time
`to recurrence was shorter in patients with HER2+ BC
`who were treated with either anastrozole or tamoxifen,
`suggesting that the effectiveness of tamoxifen might
`be impeded by HER2 positivity [73]. Additionally, pre-
`clinical evidence has suggested that crosstalk between
`HER2 and ER might lead to endocrine resistance [74,75].
`In a Phase III trial comparing letrozole plus lapatinib, a
`dual HER2 and EGF receptor (EGFR) inhibitor, with
`letrozole alone as first-line therapy, the combination sig-
`nificantly improved CBR and PFS in postmenopausal
`women with HER2+ HR+ MBC but not in patients with
`HR+ HER2– MBC [76]. Currently, lapatinib in combi-
`nation with letrozole is FDA approved for patients with
`HR+, HER2+ MBC [77]. Two trials exploring the effi-
`cacy and safety of fulvestrant with lapatinib in patients
`with previous exposure to endocrine therapy are ongoing
`(Table 3) [206,207].
`Trastuzumab, a HER2 inhibitor, in combination
`with anastrozole, was studied in a Phase III trial in
`postmenopausal patients with HER2+ HR+ MBC,
`some of whom had previous exposure to endocrine
`therapy [78]. Although the combination significantly
`
`24
`
`www.future-science.com
`
`future science group
`
`Ex. 1085-0006
`
`

`
`Endocrine resistance in advanced breast cancer Review: Clinical Trial Outcomes
`
`improved PFS by 2.4 months, the PFS for patients
`receiving the combination was only 4.8 months [78].
`A Phase III trial comparing letrozole plus trastuzumab
`with letrozole alone in patients with HER2+ HR+ MBC,
`when approximately 50% of patients had received pre-
`vious tamoxifen therapy, showed that the median TTP
`with the combination therapy was 14.1 months, com-
`pared with 3.3 months with letrozole alone [79]. All
`together, these observations indicate that combining
`a HER2 inhibitor with an AI may be clinically effec-
`tive. A Phase II trial in patients with HR+ HER2– ABC
`that progressed during previous endocrine therapy is
`currently evaluating the efficacy of MM-21, an inhibi-
`tor of HER3 ligand-stimulated dimerization between
`HER2 and HER3, in combination with exemestane
`(Table 3) [208].
`
`gefitinib in addition to anastrozole significantly improved
`median PFS by 6.3 months, compared with anastro-
`zole alone [84]; however, another Phase II trial failed
`to find significant benefit for tamoxifen plus gefitinib
`[85]. Gefitinib is currently being studied in a Phase II
`trial in patients with MBC who progressed during first-
`line endocrine therapy in combination with fulvestrant
`(Table 3) [209]. Overall, the results for gefitinib addition to
`endocrine therapy are inconclusive. In addition to gefi-
`tinib, vandetanib, a novel tyrosine kinase inhibitor with
`activity against a number of RTKs, including EGFR
`and the VEGF receptor (VEGFR) [86], is currently being
`studied in a Phase II trial in combination with fulves-
`trant in patients with predominantly bone metastasis
`HR+ MBC who progressed during previous endocrine
`therapy (Table 3) [210].
`
`EGF-receptor inhibition
`Approximately 30% of HR+ BC is EGFR+ [80], and pre-
`clinical studies have suggested that EGFR pathway acti-
`vation may mediate endocrine resistance [81]. In a Phase II
`trial, CBR was improved with gefitinib, an EGFR inhibi-
`tor, in patients with HR+ tamoxifen-resistant tumors,
`compared with HR– tamoxifen-resistant tumors [82].
`However, results from another Phase II trial failed to
`support those findings and reported higher toxicity with
`the combination [83]. In another Phase II trial in patients
`with HR+ MBC with no previous endocrine therapy for
`MBC or who progressed on adjuvant tamoxifen therapy,
`
`VEGFR inhibition
`Preclinical studies have shown that estradiol stimu-
`lates proliferation of human endothelial cells and that
`anti-estrogen inhibits these effects [87]. ER was shown
`to bind to the promoter of VEGF and activate its
`transcription, resulting in increased angiogenesis [88].
`Retrospective studies of tumor samples have associ-
`ated higher levels of VEGF with decreased response
`to endocrine therapy [89]. Bevacizumab, a monoclonal
`antibody that prevents VEGF and VEGFR interaction,
`in combination with letrozole, resulted in a median PFS
`of 17.1 months [90]. The combination of fulvestrant and
`
`Table 2. Ongoing clinical trials using other signal transduction inhibitors to overcome endocrine resistance in patients
`with advanced breast cancer.
`Clinical trial
`Intervention
`identifier
`Src-kinase inhibitors
`NCT00754325
`Fulvestrant ± dasatinib
`
`Patient population
`
`Phase
`
`Enrollment
`expected (n)
`
`Select primary and
`secondary outcomes
`
`II
`
`100
`
`Primary: PFS
`Secondary: ORR, TTF,
`bone markers, toxicity,
`bone pain, BMD
`
`Men and
`postmenopausal
`women with advanced
`BC previously treated
`with AI
`Postmenopausal
`women with
`unresectable, locally
`recurrent, or MBC – as
`first- and second-line
`therapy
`
`NCT00696072
`
`Letrozole ± dasatinib
`
`II
`
`120
`
`Primary: CBR
`Secondary: ORR, PFS,
`TTF, bone markers,
`toxicity, bone pain,
`BMD
`
`NF-kB pathway inhibitors
`NCT01142401
`Fulvestrant ± bortezomib Postmenopausal
`Primary: CBR
`women with locally
`Secondary: OS, PFS,
`advanced BC or MBC
`CBR at 12 and 24
`resistant to AI therapy
`weeks
`AI: Aromatase inhibitor; BC: Breast cancer; BMD: Bone mineral density; CBR: Clinical-benefit rate; MBC: Metastatic breast cancer; ORR: Overall-response rate;
`OS: Overall survival; PFS: Progression-free survival; TTF: Time to failure.
`
`118
`
`II
`
`Clin. Invest. (2014) 4(1)
`
`Ref.
`
`[223]
`
`[224]
`
`[225]
`
`25
`
`future science group
`
`Ex. 1085-0007
`
`

`
`Table 3. Ongoing clinical trials using growth factor-receptor inhibitors to overcome endocrine resistance in patients with
`advanced breast cancer.
`Clinical trial
`Intervention
`identifier
`HER2 inhibitors
`NCT01151046
`
`Exemestane ±
`MM-121
`
`NCT00390455
`
`NCT00688194
`
`Fulvestrant ±
`lapatinib
`
`Fulvestrant ±
`lapatinib ± AI
`
`Postmenopausal women with
`HR+ HER2– locally advanced
`BC or MBC
`Postmenopausal women with
`HR+ advanced BC
`
`Postmenopausal women with
`MBC who progressed after
`prior AI therapy
`
`EGFR inhibitors
`NCT00811369
`
`Fulvestrant ±
`vandetanib
`
`Postmenopausal women with
`bone-predominant HR+ MBC
`
`NCT00570258
`
`Fulvestrant ±
`erlotinib
`
`Patients with HR+ MBC who
`progressed on first-line
`hormonal therapy
`
`VEGFR inhibitors
`NCT01466972
`
`NCT00545077
`
`Anastrozole
`or letrozole +
`pazopanib
`Fulvestrant
`or letrozole ±
`bevacizumab
`
`Patients with HR+ advanced
`BC progressing on NSAI
`therapy
`Postmenopausal women with
`advanced BC or MBC; as first-
`line therapy
`
`II
`
`131
`
`Primary: PFS
`Secondary: NR
`
`III
`
`III
`
`II
`
`II
`
`II
`
`324
`
`396
`
`126
`
`130
`
`30
`
`III
`
`338
`
`Primary: PFS
`Secondary: OS, DOR, ORR,
`safety, QoL
`Primary: PFS
`Secondary: OS, TTP, RR, CBR
`
`Primary: Decrease in bone
`marker (NTx)
`Secondary: PFS, response,
`improvement in pain
`Primary: TTP
`Secondary: CBR, RR
`
`Primary: CBR
`Secondary: TTP, safety
`
`Primary: PFS
`Secondary: OS, TTF, RR, DOR,
`clinical benefit proportion,
`safety
`
`Ref.
`
`[208]
`
`[206]
`
`[207]
`
`[210]
`
`[209
`
`[226]
`
`[211]
`
`[212]
`
`Review: Clinical Trial Outcomes Chlebowski
`
`Patient population
`
`Phase
`
`Enrollment
`expected (n)
`
`Select primary and
`secondary outcomes
`
`FGFR inhibitors
`NCT01528345
`
`Fulvestrant ±
`dovitinib
`
`Postmenopausal women with
`locally advanced BC or MBC
`not amenable to curative
`surgery or radiotherapy and
`progressing on or after prior
`endocrine therapy
`AI: Aromatase inhibitor; BC: Breast cancer; CBR: Clinical benefit rate; DOR: Duration of response; EGFR: EGF receptor; FGFR: FGF receptor; HR: Hormone receptor;
`MBC: Metastatic breast cancer; NR: Not reported; NSAI: Nonsteroidal aromatase inhibitor; ORR: Overall-response rate; OS: Overall survival; PFS: Progression-free
`survival; QoL: Quality of life; RR: Response rate; TTF: Time to failure; TTP: Time to progression; VEGFR: VEGF receptor.
`
`II
`
`150
`
`Primary: PFS
`Secondary: OS, ORR, DOR,
`safety
`
`bevacizumab was tested in a Phase II trial in postmeno-
`pausal women with newly diagnosed MBC who were
`intolerant to an AI or who progressed while receiving
`an AI [91]. In this same trial, anastrozole plus bevaci-
`zumab was studied in patients with earlier stage disease.
`The median TTP was 21 months with anastrozole plus
`bevacizumab, essentially as first-line therapy, whereas
`it was 9 months with fulvestrant and bevacizumab as
`second-line therapy [91]. Although these results are dif-
`ficult to interpret, bevacizumab, in combination with
`endocrine therapy, is currently being tested as a first-line
`therapy in a Phase III trial (Table 3) [211].
`
`FGF receptor inhibition
`Aberrant expression of FGF receptor (FGFR)-1, -2,
`-3, and -4 have all been shown to result in BC devel-
`opment and to mediate endocrine resistance [92,93].
`Brivanib, a dual FGF and VEGF RTK inhibitor, was
`shown in preclinical studies to inhibit FGF-stimulated
`growth in FGFR-1-amplified BC cells [94]. Dovitinib,
`an inhibitor of FGFR, VEGFR, and the PDGF recep-
`tor, was shown in a Phase II trial to have some activity
`in patients who were heavily pretreated [95]. Dovitinib
`is currently undergoing another Phase II trial in com-
`bination with fulvestrant (Table 3) in postmenopausal
`
`26
`
`www.future-science.com
`
`future science group
`
`Ex. 1085-0008
`
`

`
`Endocrine