PI3K/AKT/mTOR-Targeted Therapy for Breast Cancer

Abstract

Phosphatidylinositol 3-kinase (PI3K), protein kinase B (PKB/AKT) and mechanistic target of rapamycin (mTOR) (PAM) pathways play important roles in breast tumorigenesis and confer worse prognosis in breast cancer patients. The inhibitors targeting three key nodes of these pathways, PI3K, AKT and mTOR, are continuously developed. For breast cancer patients to truly benefit from PAM pathway inhibitors, it is necessary to clarify the frequency and mechanism of abnormal alterations in the PAM pathway in different breast cancer subtypes, and further explore reliable biomarkers to identify the appropriate population for precision therapy. Some PI3K and mTOR inhibitors have been approved by regulatory authorities for the treatment of specific breast cancer patient populations, and many new-generation PI3K/mTOR inhibitors and AKT isoform inhibitors have also been shown to have good prospects for cancer therapy. This review summarizes the changes in the PAM signaling pathway in different subtypes of breast cancer, and the latest research progress about the biomarkers and clinical application of PAM-targeted inhibitors.

Full text

Citation: Zhu, K.; Wu, Y.; He, P.; Fan,
Y.; Zhong, X.; Zheng, H.; Luo, T.
PI3K/AKT/mTOR-Targeted Therapy
for Breast Cancer. Cells 2022,11, 2508.
https://doi.org/10.3390/cells11162508
Academic Editor: Anand Singh
Received: 9 July 2022
Accepted: 9 August 2022
Published: 12 August 2022
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cells
Review
PI3K/AKT/mTOR-Targeted Therapy for Breast Cancer
Kunrui Zhu 1, 2, †, Yanqi Wu 1,† , Ping He 1,2, Yu Fan 2, Xiaorong Zhong 1,2, Hong Zheng 1,2,* and Ting Luo 1,2 ,*
1Breast Disease Center, Cancer Center, West China Hospital, Sichuan University, Chengdu 610000, China
2Multi-Omics Laboratory of Breast Diseases, State Key Laboratory of Biotherapy, National Collaborative,
Innovation Center for Biotherapy, West China Hospital, Sichuan University, Chengdu 610000, China
*Correspondence: hzheng@scu.edu.cn (H.Z.); luoting@wchscu.cn (T.L.)
† These authors contributed equally to this work.
Abstract:
Phosphatidylinositol 3-kinase (PI3K), protein kinase B (PKB/AKT) and mechanistic target
of rapamycin (mTOR) (PAM) pathways play important roles in breast tumorigenesis and confer worse
prognosis in breast cancer patients. The inhibitors targeting three key nodes of these pathways, PI3K,
AKT and mTOR, are continuously developed. For breast cancer patients to truly benefit from PAM
pathway inhibitors, it is necessary to clarify the frequency and mechanism of abnormal alterations
in the PAM pathway in different breast cancer subtypes, and further explore reliable biomarkers to
identify the appropriate population for precision therapy. Some PI3K and mTOR inhibitors have been
approved by regulatory authorities for the treatment of specific breast cancer patient populations,
and many new-generation PI3K/mTOR inhibitors and AKT isoform inhibitors have also been shown
to have good prospects for cancer therapy. This review summarizes the changes in the PAM signaling
pathway in different subtypes of breast cancer, and the latest research progress about the biomarkers
and clinical application of PAM-targeted inhibitors.
Keywords: AKT; biomarker; breast cancer; cancer therapy; mTOR; PI3K
1. Introduction
Phosphatidylinositol 3-kinases (PI3Ks) are lipid kinases that can fall into three classes
(I, II and III) in mammals [
1
]. It has been extensively studied for its important functions
in physiology and diseases. In particular, class I PI3K is a well-studied subtype and has
been confirmed to be associated with the occurrence and development of cancer [
2
]. Class
I PI3Ks consist of a catalytic subunit p110 (p110
α
, p110
β
, p110
γ
or p110
δ
) encoded by
PIK3CA, PIK3CB and PIK3CD, respectively, and a regulatory subunit p85 (p85
α
, p85
β
and
p85
γ
) encoded by PIK3R1, PIK3R2 and PIK3R3, respectively (Figure 1). The class I PI3K,
protein kinase B (PKB/AKT) and mechanistic target of rapamycin (mTOR) pathway (PAM
pathway) display abnormal activation frequently in human cancer and plays a vital part in
cell survival, proliferation, motility and metabolism (Figure 2) [3–5].
Cells 2022,11, 2508. https://doi.org/10.3390/cells11162508 https://www.mdpi.com/journal/cells

Cells 2022,11, 2508 2 of 20
Cells 2022, 11, x FOR PEER REVIEW 2 of 21
Figure 1. The genes and proteins of class I PI3K. The catalytic subunits p110α/p110β, the regulatory
subunits p85α/p85β and the genes that encoded these subunits (PIK3CA, PIK3CB, PIK3R1 and
PIK3R2) are shown. ABD, albumin binding domain; RBD, RAS-binding domain; C2, protein kinase
C conserved region 2; Rho-GAP, Rho GTPase-activating protein; SH2, Src homology 2 domain; iSH2,
inter-SH2; cSH2, C-terminal SH2.
Figure 2. The PAM signaling pathways. PAM pathway can be activated by G protein-coupled re-
ceptor (GPCR) and receptor tyrosine kinases (RTKs), including human epidermal growth factor re-
ceptor 2 (HER2/ERBB2), fibroblast growth factor receptor (FGFR), insulin and insulin-like growth
factor-1 receptor (InsR/IGF-1R), which PtdIns (4,5) P2 (PIP2) to generate the second messenger
PtdIns (3,4,5)
P3 (PIP3) [6]. PTEN (phosphatase and tensin homolog) dephosphorylates PIP3 to gen-
erate PIP2, while INPP4B (inositol polyphosphate-4-phosphatase type II B) dephosphorylates
PtdIns(3,4)P2 to generate PtdIns(3)P (PI3P). Proteins, containing a pleckstrin homology (PH) do-
main, are recruited to the cytomembrane by PIP3, including AKT, 3-phosphoinositide-dependent
kinase 1 (PDK1), and serum and glucocorticoid-induced kinase (SGK). The main downstream target
of PI3K is AKT. It is activated by PDK1 and mTOR complex 2 (mTORC2), and phosphorylates a
large number of downstream effector proteins, including mTOR complex 1 (mTORC1), forkhead
box protein O1 (FoxO1), glycogen synthesis kinase (GSK3) and murine double minute 2 (MDM2).
The AKT-mediated phosphorylation of GSK3β, FOXO1 and MDM2 directly or indirectly controls
cellular growth and survival. The activated mTORC1 ultimately regulates cellular processes, such
as the initiation of mRNA transcription, cell growth, autophagy and protein synthesis, via phos-
phorylation of 4EBP1 and S6K1 [1,7]. In addition, mTORC2 can phosphorylate both IGF-1R and
AKT [8]. S6K-mediated phosphorylation of PDK1 negatively feed back to inhibit PDK1 [7]. PKC and
SGK are also involved in PI3K signaling independent of AKT.
Figure 1.
The genes and proteins of class I PI3K. The catalytic subunits p110
α
/p110
β
, the regulatory
subunits p85
α
/p85
β
and the genes that encoded these subunits (PIK3CA, PIK3CB, PIK3R1 and
PIK3R2) are shown. ABD, albumin binding domain; RBD, RAS-binding domain; C2, protein kinase C
conserved region 2; Rho-GAP, Rho GTPase-activating protein; SH2, Src homology 2 domain; iSH2,
inter-SH2; cSH2, C-terminal SH2.
Cells 2022, 11, x FOR PEER REVIEW 2 of 21
Figure 1. The genes and proteins of class I PI3K. The catalytic subunits p110α/p110β, the regulatory
subunits p85α/p85β and the genes that encoded these subunits (PIK3CA, PIK3CB, PIK3R1 and
PIK3R2) are shown. ABD, albumin binding domain; RBD, RAS-binding domain; C2, protein kinase
C conserved region 2; Rho-GAP, Rho GTPase-activating protein; SH2, Src homology 2 domain; iSH2,
inter-SH2; cSH2, C-terminal SH2.
Figure 2. The PAM signaling pathways. PAM pathway can be activated by G protein-coupled re-
ceptor (GPCR) and receptor tyrosine kinases (RTKs), including human epidermal growth factor re-
ceptor 2 (HER2/ERBB2), fibroblast growth factor receptor (FGFR), insulin and insulin-like growth
factor-1 receptor (InsR/IGF-1R), which PtdIns (4,5) P2 (PIP2) to generate the second messenger
PtdIns (3,4,5)
P3 (PIP3) [6]. PTEN (phosphatase and tensin homolog) dephosphorylates PIP3 to gen-
erate PIP2, while INPP4B (inositol polyphosphate-4-phosphatase type II B) dephosphorylates
PtdIns(3,4)P2 to generate PtdIns(3)P (PI3P). Proteins, containing a pleckstrin homology (PH) do-
main, are recruited to the cytomembrane by PIP3, including AKT, 3-phosphoinositide-dependent
kinase 1 (PDK1), and serum and glucocorticoid-induced kinase (SGK). The main downstream target
of PI3K is AKT. It is activated by PDK1 and mTOR complex 2 (mTORC2), and phosphorylates a
large number of downstream effector proteins, including mTOR complex 1 (mTORC1), forkhead
box protein O1 (FoxO1), glycogen synthesis kinase (GSK3) and murine double minute 2 (MDM2).
The AKT-mediated phosphorylation of GSK3β, FOXO1 and MDM2 directly or indirectly controls
cellular growth and survival. The activated mTORC1 ultimately regulates cellular processes, such
as the initiation of mRNA transcription, cell growth, autophagy and protein synthesis, via phos-
phorylation of 4EBP1 and S6K1 [1,7]. In addition, mTORC2 can phosphorylate both IGF-1R and
AKT [8]. S6K-mediated phosphorylation of PDK1 negatively feed back to inhibit PDK1 [7]. PKC and
SGK are also involved in PI3K signaling independent of AKT.
Figure 2.
The PAM signaling pathways. PAM pathway can be activated by G protein-coupled
receptor (GPCR) and receptor tyrosine kinases (RTKs), including human epidermal growth factor
receptor 2 (HER2/ERBB2), fibroblast growth factor receptor (FGFR), insulin and insulin-like growth
factor-1 receptor (InsR/IGF-1R), which PtdIns (4,5) P2 (PIP2) to generate the second messenger
PtdIns (3,4,5) P3 (PIP3) [
6
]. PTEN (phosphatase and tensin homolog) dephosphorylates PIP3 to
generate PIP2, while INPP4B (inositol polyphosphate-4-phosphatase type II B) dephosphorylates
PtdIns(3,4)P2 to generate PtdIns(3)P (PI3P). Proteins, containing a pleckstrin homology (PH) domain,
are recruited to the cytomembrane by PIP3, including AKT, 3-phosphoinositide-dependent kinase 1
(PDK1), and serum and glucocorticoid-induced kinase (SGK). The main downstream target of PI3K is
AKT. It is activated by PDK1 and mTOR complex 2 (mTORC2), and phosphorylates a large number
of downstream effector proteins, including mTOR complex 1 (mTORC1), forkhead box protein O1
(FoxO1), glycogen synthesis kinase (GSK3) and murine double minute 2 (MDM2). The AKT-mediated
phosphorylation of GSK3
β
, FOXO1 and MDM2 directly or indirectly controls cellular growth and
survival. The activated mTORC1 ultimately regulates cellular processes, such as the initiation of
mRNA transcription, cell growth, autophagy and protein synthesis, via phosphorylation of 4EBP1
and S6K1 [
1
,
7
]. In addition, mTORC2 can phosphorylate both IGF-1R and AKT [
8
]. S6K-mediated
phosphorylation of PDK1 negatively feed back to inhibit PDK1 [7]. PKC and SGK are also involved
in PI3K signaling independent of AKT.

Cells 2022,11, 2508 3 of 20
Different from class I PI3K(PIK3C1), class II/III PI3K (PIK3C2/3) mainly phosphory-
lates phosphatidylinositol to phosphatidylinositol 3-phosphate (PI3P), thereby regulating
autophagy and vesicular trafficking [
9
,
10
]. Class II PI3K is also involved in angiogenesis
and cell migration. In breast cancer, low expression of PIK3C2 seems to increase sensitiv-
ity to chemotherapeutic drugs [
11
]. However, PIK3C2 is less sensitive to classical PI3K
inhibitors. It warrants further studies to develop selective PIK3C2 inhibitors [
12
,
13
]. In
addition, PIK3C3 has been confirmed to regulate tumor cell proliferation by inducing au-
tophagy and regulating iron metabolism [
14
]. Preclinical studies have shown that targeting
PIK3C3 has anti-tumor activity in breast, colorectal and prostate cancer [
15
,
16
]. This review
focuses on the well-studied class I PI3K and the related PAM pathway in breast cancer.
Abnormal enhancement of the PI3K/AKT/mTOR pathway often promotes excessive
cell multiplication and resistance to apoptosis, and participates in the development and
progression of various tumors [
17
]. Disturbance of the PAM pathway is particularly usual
in breast cancer, with approximately 70% of breast cancer patients having alterations in
this pathway [
18
,
19
]. A huge number of experiments
in vitro
and
in vivo
have shown
that inhibiting key components of the PAM pathway can inhibit cancer cell proliferation
and survival, and affect tumor microenvironment, angiogenesis, cancer metastasis and
metabolism, thereby exerting anti-tumor effects and overcoming endocrine therapy resis-
tance [
20
–
24
]. Many of small-molecule inhibitors targeting the PAM pathway have been
tested in preclinical and clinical studies. Thus far, only a few PI3K and mTOR inhibitors
have been approved for the treatment of breast cancer [25].
2. Changes of the PAM Pathway in Different Breast Cancer Subtypes
The mechanism of abnormal activation of the PAM pathways in breast cancer in-
cludes variations in key molecules, such as amplification or overexpression of RTKs (e.g.,
HER2/ERBB2) and KRAS (kirsten rat sarcoma viral oncogene) mutation [
26
]. PIK3CA,
PIK3CB and PIK3R1 mutations are frequently detected in breast cancer [
22
,
27
–
29
]. There
are common p110
α
(PIK3CA) variants in the acidic cluster of the helical domain (E542, E545,
and Q546) and the histidine residue (H1047) in the kinase domain (Figure 1). In addition,
AKT1 mutations and inactivation of tumor suppressor gene PTEN, TSC1/2 (Tuberous
Sclerosis Complex 1/2) or INPP4B [
18
,
30
–
32
] are involved in the abnormal activation of
the PAM pathways. Some of these changes can be prognostic factors or biomarkers for
targeted therapy (Table 1).
Table 1. Frequencies of changes of PAM pathway in different molecular subtypes of breast cancer.
Gene
(Protein) Alteration Effect on
Signaling
Correlation with
Prognosis
Frequency
Reference
Luminal (ER+) HER2+ TNBC (ER−,
PR−, HER2−)
A B
PTEN
Inactivation
and mutation/
reduced
expression
over activation of
PI3K signaling Negative in TNBC 29–44% 22% 67% [33–35]
PIK3CA
(p110α/PI3K) Activating
mutation Hyperactivation of
PI3K signaling
* Positive in
luminal,
negative in
metastatic/HER2+
breast cancer
47% 33% 23–39% 8–25% [33,36–38]
PIK3CB
(p110β/PI3K) Amplification/
Mutation PIP3 accumulates
and activates AKT Irrelevant 5% [29,39]
PIK3R1
(p85α/PI3K) Inactivating
mutation
Derepression of
catalytic activity
of p110α
-2% of Early breast cancer
11% of Metastatic breast cancer [40,41]

Cells 2022,11, 2508 4 of 20
Table 1. Cont.
Gene
(Protein) Alteration Effect on
Signaling
Correlation with
Prognosis
Frequency
Reference
Luminal (ER+) HER2+ TNBC (ER−,
PR−, HER2−)
A B
AKT1 Activating
mutation
Hyperactivation
of AKT
Irrelevant 2.6–7.4% [33,42–44]
AKT2 Amplification Irrelevant 2.8–4% [18]
AKT3 Amplification Positive in luminal
A breast cancer 15% [40]
PDK1 Amplification Hyperactivation
of AKT - 20–38% [45]
(mTOR) p-mTOR
expression Hyperactivation
of mTOR Negative in TNBC 39% 37.5–72.1% [46]
* Positive: Associated with a better prognosis; Negative: Associated with a worse prognosis; Irrelevant: No
significant correlation with prognosis; p-mTOR: phosphorylated mTOR.
The frequency of changes in the above-mentioned genes may vary among different
subtypes of breast cancer (Table 1) [
47
–
53
]. Breast cancer can be roughly divided into
four subtypes: luminal A (60–70%), luminal B (10–20%), HER2-enriched (13–15%) and
triple-negative (10–15%) [
52
]. In estrogen receptor (ER)+ and HER2+ breast cancer, the most
common mechanism of abnormal activation of the PAM pathway is PIK3CA mutation, ac-
counting for 47% of cases of the ER+/HER2
−
(luminal A) subtype, 33% of the ER+/HER2+
(luminal B) subtype, 23%–39% of the ER-/HER2+ subtype and 8–25% of the triple-negative
breast cancer (TNBC) subtype [
33
,
34
,
36
]. In advanced and metastatic breast cancer, PI3KCA
mutations may lead to chemotherapy resistance and a poor prognosis. For HER2 positive
breast cancer, PIK3CA mutations are associated with worse prognosis [
37
,
54
]. Among pa-
tients receiving neoadjuvant regimens containing docetaxel, carboplatin, trastuzumab and
lapatinib, those with tumors with PIK3CA/ERBB family mutations seem to develop patho-
logic complete response (pCR) more than those with wild-type tumors [
55
]. For TNBC,
the most common abnormal mechanism of PAM is PTEN inactivation or downregulation,
accounting for 67% of cases [
56
,
57
]. In addition, mTOR hyperphosphorylation is associated
with poor outcomes of patients with stage I/II TNBC [
46
]. Metaplastic breast cancer is a
type of TNBC. Several studies found strong enrichment in mutations of PIK3CA/PIK3R1,
P53 and PTEN, and aberrations of RAS-MAPK pathways in metaplastic breast cancer [
38
].
In addition, PDK1 amplification is present in 20–38% of all breast cancer subtypes and is
involved in aberrant activation of the PAM pathway [
35
,
45
]. AKT1 activating mutations
(E17K) are also observed in 7% of ER+ metastatic breast cancer patients [
42
]. Therefore, it is
necessary to select corresponding biomarkers according to different breast cancer subtypes
to guide targeted therapy and evaluate prognosis.
Inhibiting the key nodes in PAM pathway can exert anti-tumor effects [
43
,
44
]. In
HR+ breast cancer, the activation of PI3K pathway by PIK3CA mutation promotes ligand-
independent ER activation, which is one of the important mechanisms of endocrine therapy
resistance [
53
,
58
–
60
]. Inhibition of PI3K can delay or reverse endocrine therapy resistance
and improve patient prognosis. In addition, AKT activation may confer resistance to
anticancer agents such as the ER antagonists tamoxifen and fulvestrant. Combination of
AKT inhibitors with tamoxifen and fulvestrant may improve their effectiveness [
61
,
62
].
Pan-mTORC1/2 inhibitors can also reverse endocrine resistance, chemoresistance and
radiation resistance [
63
]. For patients with HER2+ breast cancer, inhibition of PI3K or
mTOR can become a new therapeutic regimen after secondary resistance to anti-HER2
therapy [
64
–
66
]. For TNBC, PAM inhibitors may have clinical benefits in PTEN-deficient
population
[67,68]
. Owing to the complexity and interactions among the diverse compo-
nents in PAM pathway, inhibition of a single target may result in compensatory changes in
other targets. After inhibition of PI3K, the mTOR pathway can be abnormally activated to
counteract this effect [
69
–
71
]. The mTOR inhibitor may promote the expression of insulin
receptor substrates, which may upregulate the AKT pathway [72].

Cells 2022,11, 2508 5 of 20
Multiple changes in PAM pathways may co-exist in breast cancer. For example, the
co-existence of PIK3CA mutation, PTEN deletion and HER2 amplification is detected in
breast cancer [
2
,
73
]. Hence, inhibition of a single target may not achieve anti-tumor effects
in these circumstances. This not only suggests the molecular mechanism of drug resistance
in breast cancer, but also the feasibility of combined therapy. Yang et al. have shown
that temsirolimus (mTORC1 inhibitor) in combination with dactolisib (dual PI3K-mTOR
inhibitor) or ZSTK474 (pan-PI3K inhibitor) can collectively inhibit cancer cell growth and
overcome cellular resistance to temsirolimus [
74
]. Tang et al. also proved that using
different PAM inhibitors at the same time can reach better anti-cancer effects [75].
3. Preclinical and Clinical Development of PAM Pathway Inhibitors in Different
Subtypes of Breast Cancer
3.1. PAM Inhibitors for Treating ER+/HER2−Breast Cancer
About 70% of breast cancer is ER+ and HER2
−
[
37
]. Endocrine therapy is the standard
regimen for these patients [
25
]. Abnormal activation of the PAM pathway is one of the
important reasons of endocrine resistance [
59
], which can be overcome or reversed by
targeting pathways’ components which activated during acquired drug resistance [
76
,
77
].
Therefore, PAM pathway inhibitors have been extensively studied in this population
(Table 2). PAM pathway inhibitors are classified into PI3K, AKT, mTOR, and dual PI3K-
mTOR inhibitors. Alpelisib (PI3K inhibitor) and everolimus (mTOR inhibitor) have been
approved by the U.S. Food and Drug Administration (FDA) for the clinical treatment of
breast cancer [
25
]. Many AKT and mTOR inhibitors have initially shown preclinical activity
or are currently undergoing clinical trials.
Table 2. Clinical trials of PI3K inhibitors in ER+/HER2−breast cancer.
Target Drug Study (Phase) Patient Population Regimen and Outcome FDA/EMA
Approval Reference
Pan-PI3K Buparisib
(BKMI20)
BELLE-2
(III)
HR (+), HER2 (−),
ABC/MBC
(second line)
buparlisib + fulvestrant
vs. placebo + fulvestrant
(mPFS: 6.9 vs. 5.0 months;
HR: 0.78; p= 0.00021)
N [78]
BELLE-3
(II)
HR (+), HER2 (−),
ABC/MBC relapsed
on or after endocrine
therapy and mTOR
inhibitors
buparlisib vs. Placebo
(mPFS: 3.9 vs. 1.8 months; HR:
0.67; p= 0.0003)
[79]
Pictilisib
(GDC-0941)
FERGI
(II)
HR (+), HER2 (−),
ABC/MBC
Al-resistant
pictilisib + fulvestrant
vs. placebo + fulvestrant
(mPFS:6.6 vs. 5.1 months;
HR: 0.74; p= 0.096)
N [80]
PEGGY
(II)
HR (+), HER2 (−)
metastatic breast
cancer
Pictilisib + paclitaxel vs.
placebo + paclitaxel (mPFS:8.2
vs. 7.8 months; HR: 0.95)
[81]
PI3K
(p110α)
Alpelisib
(BYL719)
SOLAR-1
(III)
HR (+), HER2 (−),
ABC
Received endocrine
therapy previously
PIK3CA-mutated:
alpelisib vs. placebo
(mPFS 11.0 vs. 5.7 months;
HR: 0.65; p< 0.001);
(mOS: 39.3 vs. 31.4 months;
HR: 0.86; p= 0.15)
Y [82]
BYLieve
(II)
HR (+), HER2 (−),
PIK3CA-mutant ABC
progressed on/after
prior therapy,
including CDK
inhibitors
proportion of without disease
progression at 6 month was
50.4% (95% CI: 41.2–59.6).
[83]

Cells 2022,11, 2508 6 of 20
Table 2. Cont.
Target Drug Study (Phase) Patient Population Regimen and Outcome FDA/EMA
Approval Reference
NEO-ORB
(II)
HR (+), HER2 (−)
Postmenopausal
women
Tlc-T3 breast cancer
Alpelisib + letrozole
vs. placebo + letrozde,
ORR: 43% vs. 45%,
PIK3CA-wild-type vs. mutant
ORR: 63% vs. 61%
[84]
Taselisib
(GDC0032)
SANDPIPER
(III)
Postmenopausal
women, disease
recurrence/progression
during/after AI
Taselisib vs. placebo
(PFS: 7.4 vs. 5.4 months;
HR: 0.70: p= 0.0037)
N [85]
PI3K-mTOR Gedatolisib NCT02684032
(I)
metastatic
breast cancer NA N [86]
Apitolisib NCT01254526
(Ib)
locally recurrent
breast cancer
or metastatic
breast cancer
NA N [87]
Samotolisib NCT02057133
(I)
In combination with:
letrozole, anastrozole,
tamoxifen, exemestane
NA N [88]
Al, aromatase inhibitor; mPFS, median progression-free survival; HR, hazard ratio: MBC, metastatic breast
cancer. ABC, advanced breast cancer; ORR, objective response rate; NA, not applicable or discontinued owing
to drug toxicity; EMA, European Medicines Agency; N, not yet approved; Y, approved. FDA, Food and Drug
Administration.
3.1.1. PI3K Inhibitors for Treating ER+/HER2−Breast Cancer
PI3K inhibitors can fall into three categories. The first category includes pan-PI3K
inhibitors, which non-selectively act on the ATP-binding pockets of all class I PI3K isoforms
(p110
α
, p110
β
, p110
γ
, and p110
δ
) [
89
]. Both buparisib and pictilisib are pan-PI3K in-
hibitors. Phase III clinical trials show that, as the second-line treatment of ER+ and HER2
−
advanced or metastatic breast cancer patients, buparisib combined with fulvestrant can
significantly improve PFS, while the most frequent grade 3–4 adverse events are elevated
alanine aminotransferase, hyperglycemia, hypertension and fatigue [
78
,
79
]. These severe
adverse reactions may result in discontinuation of the drug. Due to the high blood–brain
barrier-penetrating properties of buparlisib, depression and anxiety are also common psy-
chiatric side-effects. Pictilisib, another pan-PI3K inhibitor, did not result in a significant
survival benefit [
80
,
81
]. However, these studies consider that combining PI3K inhibition
with endocrine therapy is reasonable in patients with ER +/HER2
−
breast cancer [
36
,
90
]
(Table 2).
The second class of specific PI3K inhibitors are represented by the drugs alpelisib
(BYL719) and taselisib (GDC-0032). The ATP binding sites of type I PI3K are highly homol-
ogous. The different residues next to the ATP binding sites can be divided into the adjacent
hinge region (four residues) and the variable region located at the p-loop. Non-conserved
residues in these two regions are key to the subtype selectivity of pan-PI3K inhibitors.
Alpelisib can form a dihydrogen bond with Q859 in the hinge region of PI3K
α
, while other
subunits at this site are too short to form the same structure, so it can selectively inhibit
p110
α
[
91
]. So far, alpelisib is the only PI3K inhibitor approved by the FDA for breast
cancer treatment. In the phase III SOLAR-1 trial, ER+/HER2
−
advanced breast cancer
patients with relapsed or progressed disease after endocrine therapy received alpelisib and
fulvestrant treatment. This trial showed that alpelisib plus fulvestrant demonstrated better
overall efficacy compared with placebo (26.6% vs. 12.8%). Furthermore, in patients with
PIK3CA mutation, combination of alpelisib and fulvestrant greatly prolonged the mPFS
(
p= 0.001
, Table 2). In contrast, fulvestrant plus alpelisib had no PFS benefit (
7.4 months
vs.
5.6 months,
HR: 0.85; 95% CI: 0.58–1.25) in the PIK3CA wild-type group [
81
,
82
]. The
positive results of SOLAR-1 trial led to the approvement of alpelisib plus fulvestrant
for treating advanced or metastatic breast cancer with ER expression and PIK3CA muta-

Cells 2022,11, 2508 7 of 20
tion [
25
]. The recently updated American Society of Clinical Oncology (ASCO) guideline
recommended that patients with locally recurrent unresectable or metastatic hormone
receptor positive and HER2-negative breast cancer should be subject to testing of PIK3CA
mutations to determine their eligibility for treatment with the alpelisib plus fulvestrant [
92
].
The BYLieve phase II trial assessment of the effectiveness and security of alpelisib plus
fulvestrant for patients treated with CDK4/6 inhibitors [
83
]. Other studies have attempted
to use alpelisib for neoadjuvant treatment of breast cancer, but the results of the NEO-
ORB phase II study showed that alpelisib combined with letrozole for ER+/HER2
−
and
early-stage breast cancer had no additional clinical benefit [
84
]. Taselisib inhibits p110
α
,
δ
and
γ
, but is 30-fold less potent against p110
β
. The SANDPIPER phase III clinical trial
evaluated the safety of taselisib plus fulvestrant for postmenopausal breast cancer with
disease recurrence/progression during or after an aromatase inhibitor. This trial indicated
that taselisib increased the frequency of grade 3–5 adverse events (16.4% in placebo arm
vs. 49.5% in taselisib arm). Based on this trail, taselisib combined with fulvestrant was
discontinued, suggesting that follow-up PI3K inhibitors should improve selectivity for key
isoforms, not just selectivity [
85
]. Other compounds, such as GDC0077 [
93
], eganelisib and
samotolisib are also in preclinical studies.
Dual PI3K-mTOR inhibitors can target the catalytic pockets of mTOR and PI3K en-
zymes based on structural similarity. Since mTOR inhibitors may enhance the PI3K/PDK1
axis, an inhibitor targeting both PI3K and mTOR may have better anti-cancer activity [
94
].
Gedatolisib (PF-05212384) is the represent drug of dual PI3K-mTOR inhibitors. Preclinical
studies have demonstrated that gedatolisib combined with letrozole/palbociclib or ful-
vestrant/palbociclib has antitumor activity with manageable toxicity [
86
,
95
]. In addition,
a phase I trial of samotolisib (dual PI3K-mTOR inhibitors) in combination with cyclin
dependent kinase (CDK) inhibitors in ER+ breast cancer patients is ongoing [
88
]. Recent
studies indicate that PI3K/mTOR inhibitors combined with paclitaxel can enhance tumor
response to immunosuppressants and may provide a viable treatment for metastatic breast
cancer. Therefore, dual target inhibitors in combination with immunotherapy will also be
the focus of future research [87,88,96].
3.1.2. AKT Inhibitors for Treating ER+/HER2−Breast Cancer
There are different kinds of AKT inhibitors. Pan-AKT inhibitors bind to the ATP pocket
of AKT1/2/3 and suppress their activity [
97
] (Table 3). Another kind of AKT inhibitor is
allosteric inhibitor [
98
]. Allosteric Akt inhibitor such as MK-2206 is a kind of PH-domain
dependent inhibitor [
99
]. A phase II trial (NCT01776008) showed that MK-2206 did not
increase the efficacy of anastrozole monotherapy in patients with PIK3CA-mutated ER+
breast cancer [100].
Table 3. Clinical trials of AKT and mTOR inhibitors in ER+/HER2−breast cancer.
Target Drug Study (Phase) Patient Population Regimen and Outcome FDA/EMA
Approved Reference
AKT1-3 Capivasertib
(AZD5363)
BEECH
(II)
ER (+), HER2 (−)
ABC/MBC (first-line)
capivasertib + paclitaxel
vs. placebo + paclitaxel (mPFS:
10.9 vs. 8.4 months;
HR: 0.80; p= 0.308)
PIK3CA+ sub-population
(mPFS: 10.9 vs. 10.8 months;
HR: 1.11; p= 0.760)
N [101]
FAKTION
(II)
ER (+)/HER (−)
ABC/MBC;
Postmenopausal
relapsed or
progressed on AI
capivasertib + fulvestrant
vs. placebo + fulvestrant
(mPFS: 10.3 vs. 4.8 months;
HR: 0.58; p= 0.0044;
mOS: 29.3 vs. 23.4 months;
HR: 0.66; p= 0.035)
[102,103]

Cells 2022,11, 2508 8 of 20
Table 3. Cont.
Target Drug Study (Phase) Patient Population Regimen and Outcome FDA/EMA
Approved Reference
Pan-AKT MK-2206 NCT01776008
(II)
Endocrine resistant,
ER+ breast cancer 0% pCR N -
mTOR
(mTORC1)
Everolimus
(RAD001)
BOLERO-2
(III)
ER (+)/HER2−,
AI-resistant and
postmenopausal
ABC
Everolimus + exemestrane vs.
placebo + exemestrane (final
PFS:11.0 vs. 4.1 months;
HR: 0.38; p< 0.0001)
[104]
MANTA
(II)
HR+,
postmenopausal and
AI-resistant locally
ABC or MBC
Everolimus +fulvestrant vs.
fulvestrant (mPFS: 12.3 vs.
5.4 months; HR: 0.63; p= 0.01)
A [105]
PrE0102
(II)
ER (+)/HER2−,
AI-resistant and
postmenopausal MBC
Everolimus + fulvestrant vs.
placebo + fulvestrant
(mPFS:10.3 vs.5.1 months; HR:
0.61; p= 0.02)
ORR: 18.2 vs. 12.3%; p= 0.47
[106]
BOLERO-6
(III) ER (+)/HER2−ABC
Everolimus + exemestrane vs.
Exemestrane vs. capecitabine
mOS: 23.1 vs. 29.3 months vs.
25.6 months
[107]
NCT02123823
(I-II)
ER (+)/HER2−
ABC and MBC
xentuzumab + everolimus +
exemestane, vs. exemestane +
everolimus (mPFS: 7.3 vs.
5.6 months; p= 0.9057)
[108]
mTOR
(mTORC1/2) MLN0128 NCT02049957
(I-II)
HR+/HER2−and
AI-resistant MBC
everolimus-sensitive vs.
everolimus-resistant cohorts,
1CBR-16: 45% vs. 23%,
2ORR: 8% vs. 2%
N [109]
Apanisertib NCT02756364
(II)
HR+/HER2−and
AI-resistant MBC NA N -
Pan-mTOR Temsirolimus HORIZON
(III)
HR+,
postmenopausal and
AI-naïve ABC
Temsirolimus
vs. placebo + letrozole
(mPFS: 9.0 vs. 5.6 months;
HR: 0.7, p< 0.009)
N [110]
1CBR-16, clinical benefit rate at 16 weeks; 2ORR, overall response rate.
Capivasertib (AZD5363) is another inhibitor of all three AKT isoforms [
101
]. A ran-
domized study assessed the effects of capivasertib plus fulvestrant in ER+, HER2
−
ad-
vanced breast cancer patients resistant to endocrine therapy (FAKTION). This trial and
its updated analysis showed that the addition of capivasertib to fulvestrant resulted in a
significant improvement of progression-free survival, objective response rate (ORR) [
102
]
and overall survival [
103
] in participants with aromatase inhibitor-resistant ER-positive,
HER2
−
negative advanced breast cancer. Additionally, the expanded biomarker testing
suggested that capivasertib was predominantly effective in patients with PI3K/AKT/PTEN
pathway-altered tumors (38.9 vs. 20.0 months, p= 0.0047) [
103
]. The grade 3–4 adverse
events were hypertension (capivasertib arm vs. placebo: 32% vs. 24%), diarrhea (14% vs.
4%) and rash (20% vs. 0) [
102
]. Further study and stricter monitoring and management
of adverse reactions are needed. Moreover, a basket trial of capivasertib treatment of
patients with AKT1 (E17K)-mutated tumors demonstrated an objective response rate of
33%, with clinical benefit in ER+/HER2
−
breast cancer [
111
]. Multiple preclinical studies
have confirmed that the combination of PARP and PI3K/AKT pathway inhibitors has
synergistic antitumor activity in breast cancer susceptibility gene (BRCA)-deficient cancer
models [
112
]. Phase I trials of olaparib, a PARP inhibitor, and capivasertib in BRCA1/2 and
non-BRCA1/2 mutated breast cancer patients are ongoing [113].

Cells 2022,11, 2508 9 of 20
3.1.3. mTOR Inhibitors for Treating ER+/HER2−Breast Cancer
It is known that mTOR can phosphorylate ER
α
at ser118, making it insensitive to ta-
moxifen [
71
,
114
]. The BOLERO-2 study showed that the median progression-free survival
in postmenopausal ER+/HER2
−
breast cancer patients resistant to aromatase inhibitor was
improved by everolimus in combination with exemestane compared to placebo and exemes-
tane [
104
]. These findings prompted the FDA to approve everolimus for the patients with
ER+, HER2
−
advanced disease or its combination with exemestane for the treatment of
relapse or progression after the use of nonsteroidal aromatase inhibitors in postmenopausal
ER+/HER2
−
advanced breast cancer patients without visceral disease [
25
,
105
,
106
,
108
].
The European Medicines Agency (EMA) also approved everolimus for ER+/HER2
−
ad-
vanced breast cancer patients after failure of non-steroidal aromatase inhibitors treatment.
Exploration of everolimus in the neoadjuvant offsetting for breast cancer has yielded initial
results. A study of everolimus plus letrozole for preoperative neoadjuvant treatment of
breast cancer was conducted. Everolimus combined with letrozole resulted in a superior
response and inhibition of tumor proliferation than letrozole alone [
110
,
115
]. However,
further studies are required to confirm the effectiveness and safety of the drug.
Sapanisertib (MLN0128) has dual specificity for the mTOR complex (mTORC1 and
mTORC2), and is a new generation of ATP-competitive mTOR kinase inhibitors. A phase II
study of sapanisertib in combination with exemestane or fulvestrant in postmenopausal
women with previously treated everolimus-sensitive or-resistant breast cancer was con-
ducted. It was well tolerated and showed significant clinical benefit [
109
]. However,
follow-up research is necessary. Some studies have found that compensatory IGF signaling
could reduce the effectiveness of mTOR inhibitor in combination with endocrine ther-
apy [
108
]. Further studies may focus on the effects of combining IGF axis inhibitors and
mTOR inhibitors.
3.2. PAM Inhibitors for Treating HER2+ Breast Cancer
HER2 is overexpressed in about 20–25% of breast cancers [
52
]. HER2
−
targeted therapy
is the standard of care for these patients [
26
]. PAM pathway is one of the major signaling
pathway downstream of HER2. Its abnormal activation, such as PIK3CA mutation and
constitutively active AKT, is involved in the development of primary and secondary
resistance to HER2
−
targeted therapy [
116
,
117
]. PAM pathway inhibitors may help restore
tumor sensitivity to anti-HER2 therapy (Table 4), but the efficacy and safety need to be
further explored.
Table 4. Clinical trials of PAM inhibitors in HER2+ breast cancer.
Target Drug Study (Phase) Patient Population Regimen and Outcome Reference
Pan-PI3K Buparlisib
(BKM120) BKM120 (II) Trastuzumab-resistant
HER2+ breast cancer buparlisib + trastuzumab: ORR:10%
(ORR ≥25%) [118]
PIKHER
(II) Trastuzumab-resistant
HER2+ ABC DCR: 79%; 95% CI: 57–92%,
CBR: 29%; 95% CI: 12–51%. [119]
Alpelisib BYL-719
(I)
Trastuzumab- and
taxane-resistant
HER2+ MBC
Alpelisib + T-DM1: ORR: 43%.
T-DM1-resistant (n = 10): ORR 30%.
mPFS 8.1 months [120]
Pan-Akt MK-2206 SPY2
(II)
High-risk, early-stage
Breast cancer with
neoadjuvant therapy
MK-2206 vs. control: pCR 61.8% vs. 35% (control:
standard taxane- and anthracycline-based
neoadjuvant therapy) [121]
AKT-1 Ipatasertib
(IPAT) SOLTI-1507
(Ib) HER2+ ABC or MBC with
PIK3CA mutation NA -
mTOR Everolimus BOLERO-1
(III) HER2+, HR-
primary ABC Everolimus vs. placebo + trastuzumab
mPFS: 20.3 vs.13.1 months; HR: 0.66; p= 0.0049 [122]
BOLERO-3
(III)
Taxane-pretreated and
trastuzumab-resistant
HER2+ ABC
Everolimus vs. Placebo
+ trastuzumab, vinorelbine
mPFS: 7.0 vs.5.8 months; HR: 0.78; p= 0.0067 [123]
Sirolimus M124188
(II) HER2(+) MBC Trastuzumab + sirolimus
ORR: 1/9 (11%) CBR: 4/9 (44%) [124]

Cells 2022,11, 2508 10 of 20
Buparlisib is a pan-PI3K inhibitor [
118
,
119
]. A phase II trial of buparlisib plus
trastuzumab in trastuzumab-resistant, HER2
−
positive advanced breast cancer patients
was conducted. Similar to many pan-PI3K inhibitors, the common adverse reactions of
buparlisib were diarrhea (54%) and nausea (48%). Unfortunately, this trial failed to demon-
strate a benefit of addling buparlisib to trastuzumab [
119
]. Copanlisib (BAY80-6946), a
PI3K
α
/
δ
inhibitor, was shown to be synergistic with anti-HER2 therapy in trastuzumab-
resistant breast cancer cells [
125
]. A phase Ib trial indicated that copanlisib in combination
with trastuzumab was well-tolerated in HER2+ metastatic breast cancer patients [
126
].
In a phase I trial (BYL-719) for alpelisib in combination with T-DM1(Ado-trastuzumab
emtansine) in trastuzumab-resistant and/or T-DM1-resistant, HER2+ and metastatic breast
cancer patients, the CBR (CR + PR) in the entire patient population and patients with
prior T-DM1 treatment was 71% and 60%, respectively [
120
]. To further improve the
efficacy and safety, specific PI3K inhibitors are still in development. Of note, GDC0941
and XL-147(pan-PI3K inhibitors) or BEZ235 (dual PI3K/mTOR inhibitor) can increase
HER2/3 expression thereby promoting RAS/MAPK (mitogen-activated protein kinases)
signaling
[
127
–
129
]. Hence, PAM inhibitors combined with MEK inhibitors may be useful
to overcome drug resistance [130].
The combination of MK-2206 with standard neoadjuvant therapy given rise to pCR
rates in breast cancer patients with HER2 over expression [
121
]. However, MK-2206 has not
yet been developed further. AKT/mTOR is highly activated in lapatinib-resistant HER2+
breast cancer cells [
131
]. The mTOR inhibitor INK-128 can restore the sensitivity of lapatinib-
resistant HER2+ breast cancer cells to TKIs [
127
]. Recent study indicates that everolimus
combined with T-DM1 has a strong
in vivo
and
in vitro
antitumor effect on HER2 positive
breast cancer [
132
]. In a phase II clinical trial, the mTOR inhibitor sirolimus combined with
trastuzumab was well tolerated in patients with trastuzumab-resistant, HER2-positive and
advanced breast cancer [
124
]. The phase III BOLERO-3 study of everolimus combined
with vinorelbine and trastuzumab in HER2-positive, trastuzumab-resistant advanced
breast cancer was conducted. Compared with the placebo arm, everolimus combined
with vinorelbine and trastuzumab significantly prolonged the mPFS (5.78 vs. 7.0 month;
p= 0.0067)
[
123
]. The exploratory analysis found that patients with PIK3CA mutations,
PTEN deletions or tumors with an overactive PI3K pathway could gain PFS benefit from
everolimus [122]. However, further studies are still in progress [124].
3.3. PAM Inhibitors for Treating Triple-Negative Breast Cancer (TNBC)
TNBC has a high degree of malignancy and rapid metastasis [
52
]. Abnormal activation
of the PAM pathway is also common in TNBC, especially the type of luminal androgen
receptor (LAR) in the Fudan classification [
133
]. This population may benefit from PI3K-
targeted therapy (Table 5). A trial of the pan-PI3K inhibitor buparlisib in metastatic
TNBC patients was conducted. Although the downregulation of vital components in the
PI3K pathway was observed in patients with stable disease, the trial failed to observe
a clear objective response. Inhibition of PI3K alone may not be sufficient for treating
TNBC [
134
]. A phase II study of the sequential treatment of metastatic TNBC with PI3K-
α
inhibitor serabelisib, cisplatin and nab-paclitaxel is underway. In addition, a phase I study
of eganelisib (dual PI3K-mTOR inhibitor) for treating advanced or metastatic TNBC is
under recruitment.
Table 5. Clinical trials of PAM inhibitors in triple negative breast cancer (TNBC).
Target Drug Study (Phase) Patient Population Regimen and Outcome Reference
Pan-PI3K Buparlisib
(BKM120) NCT01790932
(II) Metastatic TNBC CBR:12% (6 patients, all SD ≥4 months) mPFS:
1.8 months (95% CI: 1.6–2.3)
mOS: 11.2 months (95% CI: 6.2–25) [134]
AKT1-3 Capivasertib (II) Metastatic TNBC Capivasertib vs. Placebo + paclitaxel
(mPFS: 5.9 vs. 4.2 months; p= 0.06)
(mOS 19.1 vs. 12.6 months; p= 0.04) [135]

Cells 2022,11, 2508 11 of 20
Table 5. Cont.
Target Drug Study (Phase) Patient Population Regimen and Outcome Reference
Pan-AKT GDC-0068
(Ipatasertib)
LOTU
(II) Metastatic TNBC Ipatasertib vs. placebo + paclitaxel
(mPFS 6.2 vs. 4.9 months; HR: 0.60; p= 0.037) [136]
FAIRLANE
(II) Early TNBC Ipatasertib + paclitaxel vs. placebo + paclitaxel
pCR rates: 17% vs. 13% [137]
LY2780301 TAKTIC
(Ib/II) HER2-ABC 6-month ORR:63.9% [48.8–76.8] [138]
mTOR Everolimus
(DAE) NCT00930930
(II) II/III TNBC
(Neoadjuvant therapy) Everolimus vs. placebo
(5pCR: 36% vs. 49%) [139]
Temsirolimus
(DAT) NCT00761644
(II) Metaplastic TNBC
Doxorubicin + bevacizumab + DAT or DAE
ORR: 21%; CBR: 40%
PI3K pathway alteration: ORR: (31% vs. 0%;
p= 0.04); CBR (44% vs. 45%; p> 0.99).
[140]
PI3K-mTOR Eganelisib NCT03719326
(I/Ib) Advanced or metastatic
TNBC
In combination with pegylated liposomal
doxorubicin (PLD) or A2aR/A2bR
antagonist-1(AB928)
NA
-
CI, confident interval; SD, stable disease; mOS, median overall survival; mPFS, median Progression-Free Survival;
pCR, pathological complete response; NA: not available.
AKT inhibitors combined with paclitaxel have shown remarkable antitumor activity
as first-line drugs in the treatment of metastatic breast cancer. In the phase II PAKT study of
the AKT1-3 isoform inhibitor capivasertib combined with paclitaxel as a first-line treatment
of TNBC, combination of capivasertib and paclitaxel significantly prolonged PFS and OS
compared with paclitaxel alone (p= 0.04, Table 5). The difference in survival benefit was
more obvious in patients with PIK3CA/AKT1/PTEN-altered tumors (9.3 vs. 3.7 months;
p= 0.01) [
135
]. In the LOTU phase II study for first-line treatment of TNBC, the pan-AKT
inhibitor GDC-0068 (Ipatasertib) also showed an advantage in prolonging PFS compared
with paclitaxel alone (p= 0.037) [
136
]. Another ongoing phase II trial, FAIRLANE, is
also exploring the efficacy and safety of GDC-0068 in combination with paclitaxel in
patients with grade IA-IIIA TNBC [
137
]. Some studies have confirmed that suppressing
AKT and/or p70S6K (p70 ribosomal protein S6 kinase) activation might synergize with
paclitaxel [
141
]. LY2780301 is a dual inhibitor of p70S6K and AKT. The phase Ib/II TAKTIC
trial aims to evaluate LY2780301 in combination with weekly paclitaxel for treating HER2-
negative advanced breast cancer patients. The combination of LY2780301 and paclitaxel
demonstrated ORR benefit, while the main grade 3–4 drug-related adverse events included
neuropathy (8%) and ungual toxicity (25%) [138].
The mTOR inhibitor everolimus was also used in the neoadjuvant treatment of TNBC.
However, compared with the placebo group, combination of everolimus with cisplatin and
paclitaxel did not demonstrate CR benefit (pCR: 36% vs. 49%) [
139
]. A study of tamirolimus
or everolimus in combination with doxorubicin and bevacizumab for the treatment of
metastatic TNBC showed initial results. mTOR inhibitors prolonged the ORR to 21%, and
improved the 6-month clinical benefit rate to 40% [
140
]. The objective response benefit was
associated with PI3K pathway aberration (p= 0.04) [
140
]. The status of PTEN may be a
biomarker of PAM inhibitors for TNBC [142].
4. Conclusions and Perspectives
The PI3K/AKT/mTOR pathway adjusts cell proliferation and metabolism. Abnor-
malities in key targets lead to over-activation of this pathway, which is involved in tumor
development, progression and drug resistance. PAM pathway inhibitors may be reliable
antitumor agents (Table 6). At present, the PI3K inhibitor alpelisib and mTOR inhibitor
everolimus have been approved by the FDA and EMA for the treatment of advanced ER+
breast cancer. The new generation of AKT inhibitors capivasertib and the highly selective
ATP-competitive mTOR kinase inhibitors sapanisertib and sirolimus remain to be exten-
sively evaluated. Most PAM inhibitors have limited effectiveness due to weak isoform
selection inhibition, feedback regulation of downstream pathways, and tandem interfer-

Cells 2022,11, 2508 12 of 20
ence with other signaling pathways. Perhaps stronger PI3K subtype-specific inhibitors
such as GDC-0077(p110
α
inhibitor) may show a better antitumor efficacy. In addition, the
novel AKT inhibitor INY-03-041, which is composed of Ipatasertib-NH2, a ten-hydrocarbon
linker and a cereblon ligand lenalidomide, can target all three AKT protein for proteasomal
degradation [
143
]. More and more AKT degraders have been developed [
144
]. Preclinical
studies demonstrate that these AKT degraders can effectively suppress tumor growth [
144
].
It remains to know whether these compounds are tolerable and have superior efficacy in
clinical setting.
Table 6. Lists of clinical trials of some PAM inhibitors in different subtypes of breast cancer.
Target Drug HR+, HER2−HER2+ Triple Negative Breast Cancer
Class I PI3K Buparisib
NCT01633060 - NCT01790932
NCT01339442
NCT01610284
Pictilisib NCT01437566 NCT00928330 NCT01918306
NCT01740336
Alpelisib
NCT02379247 NCT05230810 NCT02038010
NCT03386162
NCT04208178
Taselisib NCT02340221 NCT02390427 NCT02457910
NCT02273973
PI3K-mTOR Gedatolisib NCT02684032 NCT03698383 NCT03243331
NCT01920061
Apitolisib NCT01254526 - -
Samotolisib NCT02057133 - -
Eganelisib - - NCT03961698
AKT Capivasertib NCT01277757 - NCT03997123
NCT01992952 NCT03742102
MK-2206 NCT01776008 - -
Ipatasertib -NCT03840200 NCT02301988
NCT03800836
LY2780301 - - NCT01980277
mTOR
Everolimus
NCT02216786 NCT00912340 NCT01931163
NCT01797120 NCT00876395
NCT01783444
NCT02123823
MLN0128 NCT02049957 - NCT02719691
NCT02988986
Apanisertib NCT02756364 - -
Temsirolimus NCT02152943 NCT00411788 NCT02723877
NCT01248494
Sirolimus NCT00411788 NCT01783444 -
Ridaforolimus - NCT00736970 -

Cells 2022,11, 2508 13 of 20
In conclusion, different combination treatments may be considered for different breast
cancer subtypes. In ER+ breast cancer patients, PAM inhibitors combined with CDK in-
hibitors or other anti-estrogen therapies are expected to overcome drug resistance and
prolong survival. For breast cancer with HER2-overexpressed breast cancer, the combina-
tion of mTOR inhibitor with anti-HER2 drugs (trastuzumab, TDM-1) may be an alternative
after the progression of second-line therapy. For TNBC, concurrent targeting PAM and
other key pathway nodes (EGFR, MEK) may be of benefit. Moreover, it remains to evaluate
the efficacy of combined treatment of breast cancer patients with immune checkpoint
inhibitors and PAM inhibitors. Currently, the combination of PARP and AKT inhibitors has
also shown prospective results in breast cancer patients with BRCA1/2 mutation [
145
,
146
].
Future studies should focus on improving subtype selectivity and reducing toxic side
effects. The combination of different PAM pathway inhibitors, or a combination with im-
munotherapy drugs, may also be a strategy to further improve efficacy [
147
]. Screening for
reliable biomarkers that predict the effectiveness of combination regiments is also critical.
Author Contributions:
Conceptualization, T.L. and X.Z.; Supervision, H.Z. and P.H.;
writing—original
draft preparation, K.Z. and Y.W.; visualization, Y.F.; Writing—review and editing, T.L. and H.Z. All
authors have read and agreed to the published version of the manuscript.
Funding:
This research was funded by the Key Research and Development Project of Science and
Technology Department of Sichuan Province, grant number 00402053A2231 and the 135 project for
disciplines of excellence, West China Hospital, Sichuan University, grant number: ZYGD18012.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments:
Thanks to all authors for their contributions to this manuscript. Figure 2was cre-
ated with BioRender, thanks to BioRender (https://app.biorender.com/ (accessed on 26 June 2022).
Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design
of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript; or
in the decision to publish the results.
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