Targeting the PI3K/AKT/mTOR pathway in triple‑negative breast cancer: a review

Ricardo L. B. Costa1 · Hyo Sook Han1 · William J. Gradishar2

Received: 7 December 2017 / Accepted: 29 January 2018
© Springer Science+Business Media, LLC, part of Springer Nature 2018

Abstract
Purpose Triple-negative breast cancer (TNBC) accounts for approximately 20% of breast cancer cases. Although there have been advances in the treatment of hormone receptor-positive and human epidermal growth factor receptor 2-positive breast cancers, targeted therapies for TNBC remain unavailable. In this narrative review, we summarize recent discoveries related to the underlying biology of the phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT)/mechanistic target of rapamycin (mTOR) pathway in TNBC, examine clinical progress to date, and suggest rational future approaches for investigational therapies in TNBC.
Results As with other subtypes of breast cancer, aberrations in the PI3K/AKT/mTOR pathway are common in TNBC. Pre- clinical data support the notion that these aberrations predict TNBC inhibition by targeted agents. In a recently published phase 2 clinical trial, an AKT inhibitor (ipatasertib) improved outcomes in a subset of patients with metastatic TNBC when combined with paclitaxel in the first-line setting. In addition, new compounds with distinct specificity and potency targeting different PI3K/AKT/mTOR components and cognate molecules (e.g., mitogen-activated protein kinase) are being devel- oped. These agents present a wide range of toxicity profiles and early efficacy signals, which must be considered prior to the advancement of new agents in later-phase clinical trials.
Conclusions The development of drugs targeting the PI3K/AKT/mTOR pathway for the treatment of TNBC is an evolving field that should take into account the efficacies and toxicities of new agents in addition to their interactions with different cancer pathways.

Keywords Triple-negative breast cancer · Targeted therapy · PI3K · mTOR

Purpose

Triple-negative breast cancer (TNBC) is defined by the absence of targetable aberrations, such as hormone recep- tor (HR) and human epidermal growth factor receptor 2 (HER2), represents up to 20% of all breast cancers, and is associated with poor clinical outcomes [1]. Despite multi- modal treatments, patients with TNBC have a 3-year free- dom-from-recurrence rate, which compares unfavorably
with non-TNBC tumors (63 vs. 76%, p < .0001) [2, 3]. Chemotherapy continues to be the standard-of-care treat- ment for TNBC, with disappointing results; for example, chemotherapy regimens studied in the metastatic TNBC set- ting have shown overall response rates (ORRs) ranging from 10 to 35% and median progression-free survival (PFS) of only 3–4 months [4–6]. We performed a literature review to identify research that could have an impact on TNBC treat- ment. We describe recent discoveries related to the underly- ing biology of the phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT)/mechanistic target of rapamycin (mTOR)

*
[email protected]
pathway in TNBC, examine clinical progress to date, and suggest rational future approaches for investigational thera-

1Department of Breast Oncology, Moffitt McKinley Outpatient Center, H. Lee Moffitt Cancer Center
and Research Institute, 10920 North McKinley Drive, BR-Program, Tampa, FL 33612, USA
2Robert H. Lurie Comprehensive Cancer Center of Northwestern University, Chicago, IL, USA
pies in TNBC.

Vol.:(0123456789)

Results

PI3K/AKT/mTOR pathway in TNBC

Aberrations in the PI3K/AKT/mTOR pathway are among the most common genomic abnormalities in breast cancer across various subtypes [7]. PI3Ks are pivotal molecules in this cascade that ultimately lead to tumor-cell growth (Fig. 1). PI3Ks are heterodimers composed of regulatory (p85) and catalytic (p110) subunits and exist in 4 isoforms (α, β, δ, and γ) [8]. The signaling pathway is activated by stimulation of receptor tyrosine kinases, which in turn trig- ger PI3K activation, followed by phosphorylation of AKT and mTOR complex 1 (mTORC1). In TNBC, oncogenic activation of the PI3K/AKT/mTOR pathway can happen as a function of overexpression of upstream regulators (e.g., epidermal growth factor receptor [EGFR]), activating mutations of PI3K catalytic subunit α (PIK3CA), loss of function or expression of phosphatase and tensin homolog (PTEN), and the proline-rich inositol polyphosphatase, which are downregulators of PI3K [9–11]. Conversely, activating mutations of downstream molecules to PI3K (i.e., AKT and mTOR) and cognate pathways (MAPK and RAS) are rare in TNBC [7]. Furthermore, several onco- genic pathways (i.e., FGFR, cMET, RAF) unleashed by

P53 inactivation converge to activate the PI3K pathway [12].
When compared to placebo, mTOR inhibitor everoli- mus in combination with aromatase inhibitor exemestane has improved the median PFS of patients with HR-positive (HR+) HER2-negative (HER2-) metastatic breast cancer (MBC, 10.6 vs. 4.1 months, p < .001) and is now a com- monly used treatment option [13]. PI3K/AKT/mTOR-tar- geted therapies have not yet been successfully developed for TNBC, although an early-phase clinical trial has suggested the efficacy of agents targeting this pathway. Ipatasertib (an AKT inhibitor) combined with paclitaxel improved the PFS of patients with TNBC when compared to paclitaxel alone (6.2 vs. 4.9 months, p = .037), indicating that PI3K pathway- targeted therapies may lead to promising results in a subset of patients [14].

Genomic aberrations in TNBC

TNBC samples can be grouped as basal-like breast tumors, which present gene expression cluster characteristics of basal epidermal cells, including keratin 5, keratin 17, integrin-β4, and laminin [15]. In addition, the mutational landscape of TNBC was analyzed through whole-exome sequencing [7]. Basal-like breast cancers showed stagger- ing genomic heterogeneity epitomized by low mutational

HER

EGFR

Cytoplasm
TKI inhibitor

PI3K inhibitor
PI3K
PIPP

RAS

AKT inhibitor

AKT
PTEN

MEK
MEK inhibitor
mTOR inhibitor mTORC1/2 mTORC2

Nucleus

P Phosphorylated site
↑cell proliferation, invasion, survival

Fig. 1 Potential targets in the PI3K/AKT/mTOR pathway for drug development for triple-negative breast cancer. PI3K/AKT/mTOR signal transduction: cytoplasmatic activation of PI3K is secondary to activating mutations in the α catalytic unit and/or PTEN and/or PIPP loss. Alternatively, activation of the receptor tyrosine kinase occurs by homodimerization or heterodimerization with other HER fam- ily members, such as HER2. Activated EGFR initiates downstream signaling through the PI3K/AKT/mTOR and RAS-MEK pathways,

promoting cell proliferation and survival. Lines with flat ends repre- sent the inhibitory activity of targeted therapies, and lines with arrow- heads represent the stimulatory activity of upstream molecules. ADC Antibody drug conjugate, EGFR epidermal growth factor receptor, HER human epidermal receptor, mTORC mechanistic target of rapa- mycin complex, PTEN phosphatase and tensin homolog, PIPP prolin- rich inositol polyphosphatase, TKI tyrosine kinase inhibitor

frequencies (i.e., < 5%) in all genes except tumor suppressor genes P53 (80% of basal-like tumors) and PIK3CA (9%).
In a seminal work, Lehmann et al. showed that TNBC could be clustered based on gene ontologies, with the expression profiling of 587 tumor samples (386 in the train- ing set and 201 in the validation set) [16]. Despite significant heterogeneity among TNBC samples, 6 TNBC types were proposed: basal-like 1, basal-like 2, immune modulatory, mesenchymal, mesenchymal stem-like (MSL), and lumi- nal androgen receptor. The basal-like 2 subtype displayed cognate genes involving growth factors (epidermal growth factor, MET, Wnt/β-catenin, and insulin growth factor recep- tor 1) and growth factor receptors (EGFR, MET receptor, and EPAH2). These receptors and cognate downstream molecules have been implicated in the PI3K/AKT/mTOR- signaling pathway. Most importantly, TNBC cell lines clas- sified as luminal androgen receptor, and MSL were found to frequently harbor PIK3CA and PTEN mutations and exhibited significant sensitivity to PI3K and mTOR inhibi- tion by NVP-BEZ235 (a dual PI3K and mTOR inhibitor) both in vivo and in vitro, indicating that the PI3K/AKT/
mTOR pathway drives the biology of different subtypes of TNBC. Smaller studies also support the presence of PI3K/
AKT/mTOR aberrations in a subset of patients with TNBC, as 40–70% of cases present overexpression of phosphoryl- ated mTOR protein and approximately 22% present with PIK3CA mutations [17–19]. Furthermore, as many as 48% of patients with metastatic TNBC have tumors harboring low PTEN expression [14].

Targeting PI3K/AKT/mTOR in TNBC

PI3K inhibitors in TNBC

PI3K inhibitors exhibit different potencies against dis- tinct PI3K types (i.e., α, β, δ, and γ subunits). PIK3CA is the most commonly activated isoform in breast cancer, including TNBC. Thus far, class IA pan-PI3K inhibitors have been more extensively studied than isoform-specific inhibitors. For example, the clinical efficacy of buparl- isib (BKM120), a PI3K antagonist with activity against all PI3K isoforms, has already been assessed in a phase 3 trial [20]. In the BELLE-2 trial (Baselga et al.), a total of 1147 patients with progressive HR+ HER2- MBC after treatment with antiestrogen therapy were randomized to treatment with fulvestrant at standard dosage, with or without buparlisib (100 mg/day). The median PFS was 6.9 months in the buparlisib group versus 5 months in the placebo group (p = .00021) [21]. Interestingly, among the 372 patients with tumors harboring PI3K activation (i.e., mutations on PIK3CA exons 1, 7, 9, or 20 and/or immunohistochemistry-based PTEN loss), buparlisib improved the median PFS from 4 to 6.8 months (p = .014).

The most common adverse events (AEs) in the buparlisib cohort were grade ≥ 3 alanine aminotransferase (25%) and aspartate aminotransferase elevations (18%), hyperglyce- mia (15%), and rash (8%). Buparlisib is currently under investigation for the treatment of TNBC in a phase 2 trial (NCT01790932).
Pictilisib is another pan-PI3K inhibitor with greater subu- nit α-inhibitor activity than buparlisib (IC50 = 3 nM versus IC50 = 52 nM, respectively), which showed a favorable tox- icity profile in a multiple histology, phase 1 trial. The most common AEs were grades 1–2 nausea, rash, and fatigue [22, 23]. In a randomized phase 2 trial, however, pictilisib was given at 2 different dose levels (340 mg and 260 mg) in combination with fulvestrant for the treatment of progres- sive HR+ HER2- MBC after antiestrogen therapy [24]. At the 340 mg dose, grade ≥ 3 AEs occurred in 54 out of 89 (61%) patients, and at the 260 mg dose level, grade ≥ 3 AEs occurred in 15 out of 42 (36%) patients. Diarrhea and skin rash were the most common AEs in both groups. These results suggest that agents with greater PI3K specificity may lead to higher frequencies of AEs when compared to less potent agents. As preclinical studies continue to explore the efficacy of pictilisib alone or in combination with other targeted agents (such as the insulin growth factor receptor 1 pathway) in TNBC models, translational trials will need to account for the higher absolute risk of AEs associated with this compound [25]. Finally, although other pan-PI3K inhibitors (e.g., SAR245408, taselisib, and PX-866) are under development for the treatment of HER2+ and HR+ breast cancer subtypes, studies in the realm of TNBC remain lacking [26–28].
Isoform-specific PI3K inhibitors are being tested in early- phase clinical trials. Alpelisib is an oral class I α-specific PI3K inhibitor compound, which has shown preliminary evidence of antitumor activity for the treatment of HR+ HER2- MBC [29]. In a phase 1b trial, 26 patients with anti- estrogen refractory MBC were treated with letrozole and alpelisib [30]. At the maximum-tolerated dose of 300 mg daily, 2 patients experienced grade 3 AEs of diarrhea, 2 of hyperglycemia, and 1 of transaminase elevation. Among patients with PIK3CA-mutated tumors, the 6-month clini- cal benefit rate was 44%, while it was 20% among patients with PIK3CA wild-type tumors. Grade 3 toxicities asso- ciated with alpelisib were uncommon, and transaminitis and gastrointestinal side effects were less frequent than for pan-PI3K inhibitors. As alpelisib continues to be devel- oped for the treatment of HR+ HER2- and HER2+ MBC (NCT03056755, NCT02167854), preclinical data support the notion that alpelisib induces decreased AKT phospho- rylation across different TNBC subtypes (i.e., basal-like 1, mesenchymal, and MSL) [31]. Furthermore, a phase 1b trial of enzalutamide combined with alpelisib for the treatment of TNBC is ongoing (NCT03207529).

Serabelisib is a potent and selective PIK3CA inhibitor (IC50 21 nM) that is under development for the treatment of solid tumors [32]. In a phase 1 trial, a total of 71 patients received at least 1 dose of serabelisib, and 54% of patients reported at least 1 ≥ grade 3 AE, indicating that intermittent dosing was required [32]. Sixteen patients had MBC (type not reported), 3 of whom achieved a partial response; all 3 had tumors harboring PIK3CA mutations.

AKT inhibitors in TNBC

AKT is downstream to PI3K (Fig. 1) and is inhibited by ipatasertib [33]. In the recently published LOTUS (LOng- Term follow-Up Study) trial, a total of 124 patients with untreated metastatic TNBC were randomized to 80 mg/m2 paclitaxel (on days 1, 8, and 15) with 400 mg oral ipatasertib or placebo (days 1 through 21 on 28-day cycles) [14]. The median PFS was significantly improved among molecularly unselected patients treated with ipatasertib compared to pla- cebo (6.2 vs. 4.9 months, hazard ratio 0.6, p = .037). Grades 3–4 diarrhea (23%), neutropenia (10%), and stomatitis (2%) were observed. No treatment-related deaths, pneumonitis, or significant weight loss were reported, which compares favorably with the performance of the FDA-approved mTOR inhibitor everolimus [13]. In addition, the absolute risk of grade ≥ 3 AEs was only modestly greater among patients treated with ipatasertib than among those treated with pacli- taxel alone (54 vs. 42%, respectively), and diarrhea was the most common grade 3–4 AE in the ipatasertib arm (23%). PFS showed significant correlation with PI3K/AKT/PTEN alterations (hazard ratio 0.44, p = .041) indicating that the PI3K/AKT/mTOR pathway indeed drives the biology of a subset of patients with TNBC. Other AKT inhibitors are already under clinical development as monotherapy or in combination with chemotherapy or targeted agents for the treatment of MBC, including TNBC (e.g., AZD5363, MK-2206) [34–36].

Mechanistic target of rapamycin inhibitors in TNBC

Data from 177 patients with TNBC showed that 72% of tumors harbored phosphorylated mTOR [19]. After of a median follow-up period of 69 months, univariate analysis showed significant correlation of phosphorylated mTOR with shorter overall- and recurrence-free rates of survival for patients with stages I and II TNBC (p = .001 and .004, respectively). A patient-derived xerograph TNBC model testing the mTOR inhibitor rapamycin showed 77–99% tumor-growth inhibition, which is significantly more than that has been seen with doxorubicin; protein phosphoryla- tion studies indicated that constitutive activation of the mTOR pathway decreased with treatment [37].

More recently, 52 women with metaplastic TNBC were treated with liposomal doxorubicin, bevacizumab, and tem- sirolimus or everolimus in a phase 1 trial. Among unselected patients, the ORR was 21% and the clinical benefit rate at 6 months was 40%. Tissue samples from 43 patients were available for testing, and 32 (74%) were found to have a PI3K pathway aberration. The presence of activating PI3K pathway aberrations was associated with a significant improvement in ORR (31 vs. 0%, p = .04) but not the clini- cal benefit rate [38]. As most patients enrolled in this trial had received prior anthracycline-based (75%) and/or taxane- based therapies (81%), the results compare favorably with historical controls.
Targeting both mTORC1 and mTORC2 has the theo- retical advantage of blocking the feedback loop leading to AKT activation by mTORC2 upon inhibition of mTORC1 [39]. Preclinical data support in vivo activity and induc- tion of decreased levels of p-AKT by mTORC1/2 inhibi- tor AZD2014, when compared with everolimus in TNBC patient-derived xerograph models [39]. Other mTORC1/2 inhibitors have shown antitumor activity in vitro across different breast cancer cell lines, including TNBC (e.g., MLN0128) [40].

Dual PI3K/AKT/mTOR inhibitors

As the blockade of mTOR gives rise to a mechanism of negative feedback that leads to the activation of upstream molecules, it is hypothetically possible that blockade of the PI3K/AKT/mTOR pathway can lead to increased antitumor activity in TNBC [41]. Preclinical data have shown that the combination of compounds targeting different cognate mol- ecules in the PI3K/AKT/mTOR pathway leads to synergis- tic activity. Xu et al. reported results of a PTEN-knocked basal-like breast cancer model in which the combination of MK-8669 (mTOR inhibitor) and MK-2206 (AKT inhibitor) improved the mean percentage of tumor-growth inhibition compared to either one alone in vivo (p < .001) [42].
On the basis of these findings, new compounds target- ing different components of the PI3K/AKT/mTOR path- way simultaneously continue to be developed. For example, gedatolisib inhibits mutant forms of PI3K-α with elevated kinase activity at concentrations equivalent to the IC50 for wild-type PI3K-α. PI3K-β, -δ, and -γ isoforms were inhib- ited by gedatolisib at concentrations approximately 10-fold higher than those observed for PI3K-α. Mechanistic target of rapamycin kinase inhibition (IC50 = 1 nmol/L) indicated that gedatolisib was an equipotent PI3Kα/mTOR inhibitor and is more potent than currently approved mTOR inhibi- tor everolimus [43]. Another advantage of simultaneously targeting PI3K and mTOR is the ensuing more robust inhi- bition of receptor tyrosine kinase-positive feedback loops seen with isolated PI3K inhibition [44]. TNBC cell line

MDA-MB-231 showed significant inhibition by gedatolisib in vitro (IC50 0.042 μmol/L). The safety and antitumor activity of gedatolisib have been assessed in early-phase clinical trials among patients with endometrial cancer, and the drug appears to have a favorable toxicity profile [45, 46]. Gedatolisib is currently under development for the treatment of TNBC, in combination with PTK7 antibody–drug conju- gate (NCT03243331).
Apitolisib (GDC-0980) is a PI3K inhibitor (subunits α, δ, and γ) that also targets mTORC [47]. In a cell-derived TNBC model (MDA-MB-231), mice treated with GDC- 0980 achieved tumor-growth inhibition of over 50% [48]. A phase 1 study is ongoing (NCT00854126). Omipalisib is a potent pan-PI3K and mTORC inhibitor that showed in vivo activity in a HER2+ model (ABT414 cells) [49].
Preclinical data continue to propel other dual PI3K/
AKT/mTOR inhibitors into early-phase clinical trials, but few have moved specifically to clinical trials for TNBC. For instance, omipalisib (GSK2126458) is still in its initial stages of clinical development in a phase 1 multiple-histol- ogy trial (NCT00972686). Voxtalisib (SAR245409, XL765) is a PI3K (subunits α, β, δ, and γ) and mTOR inhibitor with clinical trials underway for its use in the treatment of HR+ tumors only [50]. Other PI3K inhibitors with high affini- ties to isoforms that are not found in TNBC (e.g., isoform δ) have been developed for the treatment of hematologic malignancies [51].

Targeting PI3K/AKT/mTOR pathway interactions in TNBC

Epidermal Growth Factor Receptor

EGFR is upstream to PI3K and activates this pathway [52]. Results of in vitro studies of MDA-MB-231 and MDA-MB-468 TNBC cell models treated with rapamycin (0.78–100 nmol/L) and/or lapatinib (0.156–20 nmol/L) showed that in both cell lines, lapatinib alone produced a gradual dose-dependent growth inhibition, and the lapatinib- rapamycin combination produced significant dose-dependent cell growth inhibition. In CDX models (MDA-MB-468 and MDA-MB-231) the combination of lapatinib and rapamycin also showed synergistic antitumor effects [9].

Mitogen‑activated protein kinase kinase

Analysis of TNBC tumor specimens showed high levels of activation of the RAS/MEK/extracellular signal-regulated kinase (ERK) pathway, supporting MEK as a suitable target for therapeutic intervention in a subset of TNBC patients [53]. Mirzoeva et al. demonstrated that in the presence of epidermal growth factor, MEK inhibitor-induced activation of AKT (by feedback activation of PI3K induced by MEK

blockade) is abrogated by the treatment of MDA-MB-231 cells with gefitinib, indicating cross talk between PI3K and MEK pathways [54]. Dactolisib (BEZ235) is a PI3K (subu- nits α, β, δ, and γ) and mTOR inhibitor that is currently under development in combination with MEK162 (MEK inhibitor) for the treatment of TNBC (NCT01337765) [55].

Poly (ADP‑ribose) polymerase

Deregulation of BRCA1, a protein with critical roles in the homologous recombination-dependent DNA repair path- way, has been attributed to several mechanisms, including BRCA1-promoter methylation and overexpression of the negative regulators ID4 and HMG [56–58]. The premise for the synthetic lethality approach is tumor sensitivity to blockade of DNA single-strand breaks by poly (ADP-ribose) polymerase (PARP) inhibition [59]. Under normal condi- tions, the PI3K/AKT/mTOR pathway stabilizes and pre- serves DNA double-strand break repair by interacting with the homologous recombination complex, and it is necessary for DNA repair during ionizing radiation [60]. In a TNBC cell model, PI3K inhibition with buparlisib impairs homolo- gous recombinant DNA repair, inducing further genomic aberrations [61]. De et al. showed that the combination of GDC-0980 (pan-PI3K and mTOR inhibitor) with ABT888 (inhibitor of PARP) led to significant tumor-growth inhibi- tion in a PTEN-null in vivo TNBC model (MDA-MB468) [62]. Mechanistic target of rapamycin complex 1/2 inhibitor AZD2014 in combination with PARP inhibitor olaparib is currently under the initial stages of clinical development for the treatment of TNBC (NCT02208375).

PI3K/AKT/mTOR and immunotherapy in TNBC

Immune checkpoint inhibitor antibodies selectively block the interaction between programed cell death 1 (PD-1) and PD-1 ligand 1 (PD-L1) that are expressed on cytotoxic CD8+ T cells and tumor cells, respectively, leading to the activa- tion of cellular immunity. Pembrolizumab, a monoclonal anti-PD-1 antibody, was tested in a phase 1b trial involving 32 heavily pretreated women with PD-L1 IHC+ recurrent metastatic TNBC; the ORR was 18.5% [63]. Avelumab, a PDL-1 inhibitor, also showed signs of preliminary activity in patients with MBC in a phase 1b trial [64]. Forty-four percent of patients (4 of 9) who had PD-L1+ immune cells within the tumor had partial responses, whereas 2.6% of TNBC patients (1 of 39) with PD-L1- immune cells had the same outcome. The mutational burden of tumors cor- relates with clinical benefit from immune checkpoint inhibi- tors, with the tumors displaying the highest rates of muta- tions having remarkable antitumor effects [65]. In TNBC cells, PI3K inhibition impairs homologous recombinant DNA repair, inducing further genomic aberrations, which

could lead to increases in sensitivity to immune checkpoint antibodies [61]. Evidence also supports regulation of the adaptive cytotoxic immune system through upregulation of PD-L1 via activation of PI3K/AKT/mTOR in TNBC. Mit- tendorf et al. showed that PTEN loss (induced by PTEN antiviral PTEN small hairpin RNA vector) led to increased PD-L1 cell surface expression in a MDA-MB-157 cell line (p < .001) [66]. PTEN knockdown in the MDA-MB-231 cell line resulted in a greater increase in PD-L1 expression than the addition of IFN-γ, which is known to enhance PD-L1 expression. Most importantly, treatment of those TNBC cells with AKT inhibitor or mTOR (MK2206 or rapamy- cin, respectively) reduced expression of PD-L1 (p < .01). An early-phase clinical trial is currently assessing the safety and efficacy of an immune checkpoint inhibitor (PDR001) combined with an mTOR inhibitor (everolimus) in patients with TNBC (NCT02890069).

Conclusions

The PI3K/AKT/mTOR pathway drives the biology of a sub- set of TNBC, with the obvious caveat that this represents a minority of cases [16]. Preclinical and multiple-histology early-phase clinical trials suggest that careful selection of compounds based on their specificity and potency to signal- ing pathways of interest is mandatory when designing early- phase clinical trials assessing the safety and efficacy of new agents. This is illustrated by the example of a phase 2 trial comparing gedatolisib (pan-PI3K and mTORC inhibitor) and PF-04691502 (pan-PI3K and AKT inhibitor) for the treatment of recurrent endometrial tumors [46]. Enrollment was stopped due to unacceptable toxicities that included pneumonitis and pneumonia and the deaths of 9 study par- ticipants. The 9 deaths occurred during the course of the study while patients were receiving PF-04691502, and thus far, gedatolisib has demonstrated favorable toxicity.
Results of the LOTUS trial are worth careful considera- tion. Notwithstanding the modest absolute improvement in clinical outcomes seen in patients treated with ipatasertib (median PFS of 6.2 months), important lessons were learned

Table 1 Ongoing clinical trials assessing safety and efficacy of PI3K/AKT/mTOR-targeted agents in TNBC
Compound Additional agent Compound inhibitory selectivity Study phase Trial ID
PI3K-α PI3K-β PI3K-δ PI3K-γ AKT mTOR
BYL719 [29] NA √ 2 NCT02506556

Taselisib [67] Enzalutamide (non-steroidal antiandro-
gen)
√ √ √ √ 1b/2 NCT02457910

Gedatolisib [43] Paclitaxel, docetaxel, dacomitinib (pan-
HER inhibitor)
√
√
1b
NCT01920061

AZD8186 [68] Docetaxel √ √ √ 1 NCT03218826
AZD8186 [68] AZD2014 (AKT and mTOR inhibitor) √ √ √ 1 NCT01884285
Gedatolisib [43] PTK7-ADC √ √ 1 NCT03243331
Everolimus [69] PDR001 (PD-1 monoclonal antibody) √ 1 NCT02890069
Everolimus [69] Vinorelbine √ √ 2 NCT01520103
Everolimus [69] Erlotinib (EGFR inhibitor) √ 1/2 NCT00574366
Everolimus [69] Eribulin √ 1 NCT02616848
AZD2014 [39] Selumetinib (MEK inhibitor) √ √ 1/2 NCT02583542
SAR245409 [50] MSC1936369B (MEK inhibitor) √ √ √ √ 1b NCT01390818
MK2206 [70] NA √ 2 NCT01277757
AZD5363 [71] Paclitaxel √ 2 NCT02423603
GSK2141795 [72] Trametinib (MEK inhibitor) √ NCT01964924
Ipatasertib [33] Paclitaxel 2 NCT02301988
AZD2014 [39] Olaparib (PARP inhibitor) √ √ 1/2 NCT02208375
Temsirolimus [73] Neratinib (EGFR and HER2 inhibitor) √ 1/2 NCT01111825
Data extracted from www.clinicaltrials.gov, accessed on November 2, 2017. Includes multiple histology, all breast cancers, and exclusively TNBC trials
ADC antibody drug conjugate, AKT protein kinase B, EGFR epidermal growth factor receptor, HER human epidermal growth factor receptor, ID identification, mTOR mechanistic target of rapamycin, NA not applicable, PARP poly (ADP-ribose) polymerase, PD-1 programed cell death 1, PI3K phosphoinositide 3-kinase, PTK7 protein-tyrosine kinase-like 7, TNBC triple-negative breast cancer

from this study: (i) the magnitude of benefit was greater among patients with PI3K/AKT/mTOR-aberrant tumors, indicating the need for more personalized strategies and (ii) 48% of patients had low expression of PTEN that did not overlap with genomic aberrations for many, suggesting that proteomic aberrations should also be explored as predictive markers in future trials.
Finally, moving forward, the field of developmental therapeutics needs to take into account preclinical evidence of significant interactions between PI3K/AKT/mTOR and other oncogenic signaling pathways and cognate molecules, such as transmembrane receptors and DNA repair pathways (Table 1). Also, a subset of TNBCs has prominent expres- sion of immune-modulatory genes; indeed, early-phase trials already support activity of immune checkpoint inhibitors in a small subset of patients with TNBC [16, 63, 64]. Data on the intersection of the PI3K/AKT/mTOR-signaling pathway and the adaptive immune system in TNBC are beginning to emerge.
In conclusion, the development of drugs targeting the PI3K/AKT/mTOR pathway for the treatment of TNBC is an evolving field that should take into account the efficacies and toxicities of new agents in addition to their interactions with different cancer pathways.

Acknowledgements We thank Sonya Smyk (Moffitt Cancer Center) for editorial assistance. She received no compensation beyond her regular salary.

Funding This research did not receive any specific grant funding from agencies in the public, commercial, or not-for-profit sectors.

Compliance with ethical standards

Disclosure The authors declare no potential conflicts of interest.

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