Buparlisib

Buparlisib in breast cancer

Bhawna Sirohi*,1, Sameer Rastogi2 & Shaheenah Dawood3

ABSTRACT Buparlisib (formerly BKM 120), an oral 2,6-dimorpholino pyrimidine derivative is a potent pan-PI3K inhibitor causing inhibition of PI3K downstream signaling including downregulation of p-Akt and p-S6R and apoptosis of cancer cells. Buparlisib is rapidly absorbed, has more than 90% bioavailability, good blood–brain barrier penetration and half-life of 40 h. Phase I trials have shown good disease control rate with tolerable toxicity profile at the recommended doses of 100 mg. The most common adverse events noted with buparlisib are rash, hyperglycemia, derangement of liver functions and psychiatric events. Several clinical trials with buparlisib alone or in combination with chemotherapy and targeted therapies are underway. Buparlisib has not yet been approved for regular use. Further randomized trials are required before buparlisib is approved for treatment of breast cancer.

Metastatic breast cancer, either diagnosed at initial presentation or diagnosed after treatment of loco-regionally confined disease, occurs in 40–50% of breast cancer patients with median survival ranging from 2 to 3 years [1,2]. Among women, breast cancer is the second leading cause of mortal- ity after lung cancer in the USA while it is the most common cause of mortality equaling cervical cancer in India [3]. Despite significant advances, metastatic breast cancer remains incurable with a median survival of 2–3 years [4]. Conventionally, metastatic breast cancer has been treated with chemotherapy, hormonal therapy and anti Her2-targeted therapies. In this postgenomic era there is a constant search for newer agents that can target various molecular pathways, hoping to achieve good clinical outcomes at the cost of minimal toxicity. The PI3K/AKT/mTOR pathway is among the most commonly activated molecular signaling pathways in cancer [5]. Buparlisib, an oral revers- ible inhibitor of all class I PI3Ks has been shown to be active in both preclinical and clinical stud- ies in advanced breast cancer. In this review, we will further elaborate on the PI3K/AKT/mTOR pathway, its role in breast cancer and drugs acting on this pathway with emphasis on buparlisib.

PI3K/AKT/mTOR pathway
PI3Ks belong to a family of lipid kinases that phosphorylate the 3-hydroxyl group of phospho- inositides. This causes formation of products such as phosphatidylinositol-3, 4, 5-trisphosphate or PIP3, a critical second messenger that recruits AKT resulting in activation of growth, proliferation and survival signaling, There are three classes of PI3Ks (I–III) with different structure, cellular distribution, mechanism of action and substrate preference. So far only class I has been implicated in oncogenesis [6]. The class I PI3Ks are further subdivided into class IA, which are activated by
KEYWORDS
⦁ breast cancer • buparlisib
⦁ mTOR • PI3K

1Department of Medical Oncology, Mazumdar Shaw Cancer Centre, Narayana Health, Bangalore, India
2Department of Medical Oncology, Tata Memorial Centre, Mumbai, India
3Department of Medical Oncology, Dubai Hospital, UAE
*Author for correspondence: Tel.: +91 9538145698; [email protected]

growth factor signaling through receptor tyrosine kinases (RTK), and class IB, which are activated by G-protein-coupled receptors (GPCR). From an oncologists viewpoint, the class IA PI3Ks are very important. These consist of a p85 regulatory subunit and a catalytic subunit at p110 [7,8].
Regulatory subunits of class IA PI3Ks com- prises of three genes, PIK3R1, PIK3R2 and PIK3R3, encoding p85 (and its splicing vari- ants p55 and p50), p85 and p55 regulatory subunits, respectively, collectively called p85 [7,8]. Similarly catalytic unit consists of three class IA p110 isoforms (,  and ) encoded by three genes (PIK3CA, PIK3CB and PIK3CD, respec- tively) [7]. Of these, the  and  isoforms are thought to be expressed ubiquitously, whereas the
 isoform is expressed only in hematopoietic cells.
Deeper understanding of widespread role of this pathway in cancer has given the way to develop- ment of numerous drugs targeting various com- ponents of these pathways [9].
Mutations in PI3K pathway are frequent in breast cancer and also play a pivotal role in resistance to hormonal therapy and Her-2 tar- geted therapy [10]. Activation of the PI3K/AKT/ mTOR pathway – defined as mutation or ampli- fication of the PIK3CA gene, which encodes the p110a catalytic subunit of PI3K; loss of PTEN protein expression; or overexpression of AKT – was identified in around 70% of breast cancers in one cohort [11].

PFS of 4 months in patients with hormone-posi- tive, HER2-negative advanced breast cancer [12]. Similarly TAMRAD, a randomized Phase II study that compared everolimus and tamox- ifen versus tamoxifen alone in a population of metastatic breast cancer patients who had pro- gressed after prior aromatase inhibitor therapy showed improvement of PFS as well as overall survival [13]. Everolimus and exemestane combi- nation is approved in the USA and Europe for the treatment of postmenopausal women with hor- mone receptor positive HER2-negative advanced breast cancer recurring or progressing after prior nonsteroidal aromatase inhibitors [12].
Novel agents undergoing clinical trials target- ing this pathway include the pan-PI3K inhibi- tor buparlisib, the dual pan-PI3K/mTORC1/2 inhibitor BEZ235 and the selective (isoform specific) p110 inhibitor BYL719. These oral compound agents are structurally imidazo quinolone derivatives. Buparlisib inhibits PI3K and mTOR kinase activity by binding to the ATP-binding cleft of these enzymes. Studies in human cell lines and in vivo human tumor stud- ies showed NVP-BEZ235 to be potent inhibitor of this pathway with favorable pharmacokinetic profile [14]. This is undergoing Phase I/II trials for further evaluation. BYL719 is an oral small-
molecule inhibitor of the p110 catalytic subu-
nit of PI3K, which is encoded by the PIK3CA
gene. Results of first-in-human study pre-

sented last year at American Society of Clinical

Overview of market of various PI3K pathway inhibitors
Drugs targeting this pathway can be classi- fied into allosteric mTORC1 inhibitors, Akt inhibitors, inhibitors of all four class I PI3K isoforms, namely , ,  and  (so-called pan- class I PI3K inhibitors), dual pan-class I PI3K and mTORC1/2 inhibitors, and, most recently, isoform-specific PI3K inhibitors. Inhibition of
Oncology meeting in patients with advanced solid tumors with PI3KCA gene somatic muta- tion suggest favorable pharmacokinetics and safety profile and early indication of clinical efficacy [15].
The focus of rest of the review is on novel agent buparlisib that is undergoing Phase II/III trials in various solid tumors and is upcoming targeted therapy for advanced breast cancer.

this pathway decreases tumor cell proliferation,

causes cell death and also impacts angiogenesis, metastasis and metabolism [9]. It is not yet clear which of isoform-specific or pan-PI3K inhibitors would be clinically more effective.
As PI3K pathway is implicated in breast cancer progression and resistance to hormonal therapy, everolimus, a rapamycin derivative that inhibits mTOR was studied. In the Phase III randomized trial BOLERO-2, everolimus and exemestaneas compared with placebo and exemestane more than doubled the median progression-free sur- vival (PFS) with an absolute difference in median
Chemistry
Buparlisib (NovartisPharma AG, Basel, Switzerland) is an oral 2,6-dimorpholino pyrimidine derivative that is a potent pan-PI3K inhibitor of all isoforms of class I PI3K (, ,
 and ) [16]. It is also active against p110
somatic mutations frequently described in vari- ous human cancers. Buparlisib is minimally effective in biochemical assays against the PI3K class III family member Vps34, class IV PIKK protein kinases mTOR, ATR and DNA-PK, and the distinct lipid kinase PI4Kb [16].

Preclinical pharmacodynamics
The mechanism of action of buparlisibas seen from its cocrystal structure in the PI3K p110 isoform and by comparative modelling in the PI3K p110 isoform is binding to the ATP binding cleft of PI3K enzyme in a competi- tive manner [16]. Buparlisib causes inhibition of wild-type and mutant PI3K isoforms and PI3K ,  and  isoforms at nanomolar concen- trations (e.g., IC50 <100 nM, p110) and it does not significantly inhibit other protein or lipid kinases (e.g., IC50 = 2.8 ± 1.6 M, mTOR) [16].
It acts in a concentration dependent manner in
decreasing level of AKT in mechanistic models and different cell lines. Further, in a panel of 353 cell lines, the lines that had the presence of an oncogenic mutation in the PIK3CA had statistically significant sensitivity to buparlisib than cells bearing a nonmutated wild-type form of the gene [16]. Maira et al. showed buparlisib to be highly specific in cellular models by test- ing the compound in presence of various ago- nists of other signaling pathways such as EGFR, PDGFR and IL-STAT pathway. When cells were exposed to mitogenic stimuli such as PDGF or EGF, buparlisib blocked S473P-Akt induction by the respective growth factors but did not affect receptor activation (Y751P-PDGFR and Y845P-EGFR, respectively) and activation of the MAPK pathways [16].
Buparlisib has demonstrated agonistic prop-
erties when used with both targeted therapies and cytotoxic drugs. In HER2-positive breast cell lines, synergy in form of significant tumor regression was observed when both trastuzumab and buparlisib were combined as compared with both agents alone [16]. Similar to other PI3K inhibitors buparlisib has strong anti-angiogenic properties [16,17]. There is evidence that there is correlation between trastuzumab resistance and PI3K/AKT/mTOR pathway activation through either PTEN loss or activation of PIK3CA muta- tion [18,19]. O’Brien et al. assessed efficacy of buparlisib (alone or in combination with tras- tuzumab) in a model of in vivo trastuzumab resistance generated through long-term treat-
receptor (ER)-positive breast cancer, Miller et al. established human breast cancer cell lines after long-term estrogen deprivation (LTED) [21]. On proteomic profiling, LTED cells showed increased phosphorylation of mTOR substrates p70S6 kinase and p85S6 kinase and PI3K sub- strate AKT suggesting activation of PI3K path- way. Furthermore, inhibition of PI3K pathway and mTOR caused apoptosis of LTED cells. This implies that in presence of hormone deprivation, breast cancer cells depend on the PI3K pathway to a large extent. Sanchez et al. [22], looked at var- ious PI3K pathway inhibitors (including BKM 120) and endocrine therapy combinations in ER-positive breast cancer cell lines. The authors made two important observations. First the presence of estrogen deprivation increased the apoptotic effects of PI3K inhibitors providing a rational for the first line combination of PI3K inhibitors and an aromatase inhibitor. Second in LTED cells the persistent expression of ERs was associated with resistance to PI3K inhibitors that was reversed by fulvestrant suggesting that the combination of these two agents could be explored aromatase inhibitor resistant hormone receptor positive breast cancer. Miller et al. [23] then explored whether the PI3K pathway acti- vates ER in the LTED cells essentially trying to prove whether cross talk exists between the two pathways. The investigators were unable to detect such interactions between the two path- ways in the LTED cells indicated that signaling from both pathways were important and required for hormone independent growth. The authors then went on to demonstrate that the combined downregulation of ER with fulvestrant and inhibition of the PI3K pathway with BKM 120 in ER-positive breast cancer xenografts in mice devoid of estrogen supplementation resulted in near complete tumor regression which was sig- nificantly more effective than either agent (i.e, fulvestrant or BKM 120) alone. Collectively all the preclinical data indicated that a syngergistic activity was present between endocrine therapy and PI3K inhibitors such as BKM 120 which needed to further explored in the clinical setting.

ment of the trastuzumab sensitive BT474 cells

(BT-TR) [20]. Though single agent buparlisib induced complete inhibition of tumor prolifera- tion as monotherapy but it was only in combi- nation with trastuzumab that it induced tumor regression in the trastuzumab resistant tumors. In an effort to understand the mechanism to escape from hormone dependence in estrogen
Preclinical pharmacokinetics & metabolism
Buparlisib has good oral bioavailability in mice. It has been used in mice with doses ranging from 30 mg/kg to 60 mg/kg once a day [16,24]. In p110  mutation tumor-bearing animals treated orally with once daily dose of either 30

or 60 mg/kg, maximum levels in tumor tissues and plasma compartment were achieved in 1 h which coincided with maximum phosphorylated Akt inhibition [16]. Also it has shown excellent ability to cross blood–brain barrier and is being explored in glioblastomamultiforme [16,24].
In preclinical animal studies, buparlisib was well tolerated. Maira et al. used female harlan nude mice for in vivo experiments and after induction of skin tumors in mice, buparlisib was given orally once per day, using an applica- tion volume of 10 ml/kg. Similarly Koul et al. tested the antitumor efficacy in intracra- nial xenografts in mice using oral buparlisib. Though Maira et al. didn’t mention the exact nature of the safety profile but Koul et al. dem- onstrated that after treatment with buparlisib there were no adverse events with respect to body weight, water intake and general activ- ity [16,24]. Preclinical efficacy, pharmacokinetics and side effects were acceptable to proceed to human trials.

22 (63%) of the patients. Grade 3/4 events that were observed in two or more patients were – rash, hyperglycemia, decline in perfor- mance status, mood alteration and pruritus. Hyperglycemia that is also seen with other PI3K/AKT/mTOR pathway inhibitors is a likely on-target class effect of PI3K inhibition, since this signaling axis also mediates the actions of insulin, including glucose transport and gly- cogen synthesis [29,30]. Mood alterations with buparlisib is probably due to its ability to cross the blood–brain barrier and inhibition of PI3K signaling in the brain parenchyma. Alteration of PI3K pathway in brain has been linked to anxiety and schizophrenia [31,32].
Pharmacokinetics of buparlisib in this Phase I trial revealed rapid absorption rate after oral administration with a median time to maximum serum concentration of 0.5 and 4 h post oral dose. On achieving peak drug concentration its concentration declines in a biexponential man- ner, thus exhibiting apparent long terminal elim-

ination half-life (t1/2). Buparlisib has a mean

Phase I trial of buparlisib
Table 1 summarizes the Phase I studies with buparlisib. In the first in human Phase I trial of buparlisib, Bendell et al. enrolled 35 patients with various types of advanced solid tumor and used doses ranging from 12.5 to 150 mg per day [25]. The primary objective of this first- in-human, Phase I, dose escalation study was the determination of maximum-tolerated dose (MTD) of oral buparlisib in adult patients and secondary objectives included assessment of safety and tolerability, preliminary antitumor activity and characterization of the pharmacoki- netic and pharmacodynamic profiles. Patients with colorectal (n = 15; 43%) or breast (n = 9; 26%) cancer constituted majority of patients in this trial. Patients received oral buparlisib capsules once a day every 28 days. Out of 30 patients evaluable for dose limiting toxicities (DLT), one patient (one of six) at 80 mg had grade 2 mood alteration, four patients (four of 16) at 100 mg – one each had grade 3 epigastral- gia, grade 3 rash, grade 2 mood alteration and grade 3 mood alteration and two patients (two of three) at 150 mg – both had grade 4 hyper- glycemia. The MTD in this trial was defined as 100 mg on the basis of the largest probability of DLT rate in the target toxicity interval and with less than 25% risk of overdose [25].
With regard to safety, grade 3/4 adverse events
regardless of causality in this trial, occurred in
half-life of 40 h which is also supported by the fact that it accumulates threefold during steady state concentration. Buparlisib has oral bioavail- ability of >90% and first pass hepatic extraction is less than 10% [25]. Of the 31 evaluable patients for response assessment, 16 patients (52%) had stable disease for more than 6 weeks, includ- ing five patients of colorectal and breast cancer each, giving a preliminary evidence of efficacy. Buparlisib showed dose-dependent pharma- codynamic effects on [18F] FDG-PET, fast- ing C-peptide, fasting blood glucose and pS6. Tumor molecular alterations did not correlate well with clinical efficacy.
Ando et al. did another Phase I drug escalat- ing study of buparlisib in Japanese patients with advanced solid tumors. Of the 15 patients, three received 25 mg/day, three received 50 mg/day and nine patients received 100 mg/day [26]. Only one DLT, grade 4 abnormal liver dysfunction occurred in a patient treated with 100 mg/day dose in first cycle. The non-Japanese Phase II recommended dose is 100 mg/day but given the safety information other than DLT, even though the Bayesian logistic regression model (BLRM) allowed the dose to be increased to 150 mg/day, it was decided to go with 100 mg/day as the dose for buparlisib for use in Japanese patients.
The most common treatment related toxicities were rash, abnormal hepatic function, increased blood insulin levels and increased eosinophil

Table 1. Trials of buparlisib reported till date.
Author Year Phase Patients (n) Patient population Treatment Response Ref.
Bendell et al. 2012 I 35 Advanced solid tumors progressed on standard treatment Buparlisib PR: 1/31
SD ≥6 weeks: 16/31 (52%) [25]
Ando et al. 2014 I 15 Advanced solid tumors progressed on standard treatment Buparlisib SD: 6/13 (46%) [26]
Saura et al. 2014 Ib 18 Her-2+ breast cancer resistant to trastuzumab Buparlisib PR: 2/12 (17%) [27]
Trastuzumab SD ≥6 weeks: 7/12 (58%)
Mayer et al. 2014 Ib 51 ER+ breast cancer refractory to at least one line of endocrine therapy Buparlisib Letrozole SD ≥6 months: 16 /51 SD ≥6 months: 16 /51 PR: 1
CR: 1 [28]
CR: Complete response; PR: Partial response; SD: Stable disease.

count. The pharmacokinetics of buparlisib was similar to the previous Phase I study with rapid absorption in dose dependent manner, achieving Cmax 1–1.5 h after test dose and half-life con- sistent with approximately 40 h [26]. Buparlisib showed preliminary activity in this study with disease control rate of 40%. Preliminary data on this agent are promising, but further robust results from large randomized trials are needed.
patients who had progressed on one or more tras- tuzumab-based therapy for advanced/metastatic disease or those who developed metastatic breast cancer within 12 months of adjuvant or neoad- juvant trastuzumab) [27]. Buparlisib was given at the dose of 50 mg/day with weekly trastu- zumab. Dose escalation in this study was guided by an adaptive BLRM with overdose control. Of the 17 patients who received at least one dose

of buparlisib, five patients received 50 mg/day

Phase Ib studies
With the background, that in murine model, buparlisib has been shown to restore sensitiv- ity to trastuzumab in HER2-positive breast cancer resistant to trastuzumab, Saura et al. conducted a Phase Ib/II study to determine the clinical activity of the buparlisib in patients with HER2-positive advanced breast cancer resistant to trastuzumab based therapy (defined as those
while 12 received 100 mg/day. Based on previ- ous studies, dose escalation beyond 100 mg was not permitted in this study [25]. Pharmacokinetic and toxicity profile of buparlisib was similar to previous studies [25]. Since only one DLT was observed at 100 mg/day of buparlisib, and dose escalation beyond that was not permitted, MTD dose of buparlisib could not be reached in this trial. Other than this, they also evaluated skin

Table 2. Current ongoing Phase I/II/ III trials of buparlisib.
Trial Phase Description of the trial
NCT01248494 I BKM120 or BEZ235 plus endocrine treatment in postmenopausal patients with hormone receptor-positive metastatic breast cancer
NCT01339442 I BKM120 and fulvestrant for treating postmenopausal patients with estrogen receptor-positive stage IV breast cancer
NCT01300962 I BKM120 or BEZ235 and capecitabine in patients with metastatic breast cancer
NCT01285466 I Dose-escalation study of Oral BEZ235 and BKM120 in combination with weekly paclitaxel (WP) in patients with advanced solid tumors and WP/trastuzumab in patients with HER2+ metastatic breast cancer
NCT01589861 I/II BKM120 and lapatinib in HER2+/PI3K-activated, trastuzumab-resistant advanced breast cancer
NCT01816594 II Neoadjuvant Trastuzumab + BKM120 in combination with weekly paclitaxel in HER2-positive primary breast cancer
NCT01923168 II Study of letrozole with or without BYL719 or buparlisib, for the neoadjuvant treatment of postmenopausal women
NCT01572727 BELLE 4 II Phase II study of buparlisib plus paclitaxel in HER2– advanced breast cancer, with or without PI3K pathway activation
NCT01629615 II BKM120 in patients with triple-negative metastatic breast cancer
NCT02000882 II BKM120 plus capecitabine in triple-negative (ER-, PgR-, HER2-) breast cancer (TNBC) patients with measurable brain metastases
NCT01610284 (BELLE 2) III Phase III study of buparlisib plus fulvestrant in HR+/HER2- advanced breast cancer that has progressed on or after aromatase inhibitor therapy
NCT01633060 BELLE 3 III Phase III study of buparlisib plus fulvestrant in HR+/HER2- advanced breast cancer previously treated with aromatase inhibitors and refractory to endocrine and mTOR inhibitor combination therapy

biopsies for pharmacodynamics for the level of pS6 and p4E-BP1. As seen in previous reports, the maximum inhibition of pS6 was seen at highest doses of buparlisib. Greatest reduction in pS6 correlated with best clinical response further mandating the use of pS6 as a surrogate marker for response. As compared with pS6, the levels of p4E-BP1 in skin biopsy did not change with buparlisib. Of patients who received buparlisib 100 mg/day (n = 12), there were two (17%) partial responses, 7 (58%) patients had stable disease (6 weeks) and the disease control rate was 75%.
Mayer et al. recently reported results of a mul- ticenter, open label Phase Ib trial of buparlisib in combination with oral aromatase inhibitor letro- zole in patients with metastatic ER-positive breast cancer refractory to endocrine therapies [28]. The primary objective was to assess safety and effi- cacy of the combination and secondary objective was to evaluate antitumor activity and tumor metabolic response. Hormone refractoriness was defined as tumor refractory to at least one line of endocrine therapy in the metastatic setting, or diagnosed with metastatic breast cancer during or within 1 year of adjuvant endocrine therapy. Patients were allocated sequentially to continu- ous or intermittent (5 days on and 2 days off) buparlisib administration on a 4-week schedule.

Of the 51 patients enrolled, 31 were enrolled in the intermittent arm and 20 patients were allo- cated to the continuous arm. Buparlisib MTD in both arms was 100 mg/day, with less than 25% of patients on either arm requiring buparlisib interruption or dose reduction during the first 8 weeks of treatment. Combination of buparl- isib and letrozole was well tolerated with grade 3 side effects occurring in 14 (27%) patients and no grade 4 side effects. Of the 51 patients, 16 had lack of progression 6 months includ- ing two objective responses, suggesting a clini- cal benefit rate of 31%. The most common side effects were GI disorders (80%), transaminitis and hyperglycemia (60% each) and mood disor- ders (45%). Furthermore, from molecular point of view, the efficacy of buparlisib and letrozole did not correlate with PIK3CA hot-spot muta- tions. Around half of the patients who had no disease progression 12 months had a PIK3CA- mutated cancer. Since buparlisib was effective in even wild-type PIK3CA patients so it seems that mutations other than PIK3CA might be operating to activate PI3K pathway.
Buparlisib has shown good safety profile and response rates in Phase I/ II trials, hence quite a few clinical trials are running with either single-agent buparlisib or in combination with chemotherapy or targeted therapy (Table 2).

EXECUTIVE SUMMARY
Mechanism of action
⦁ Buparlisib is a pan-PI3K inhibitor causing inhibition of PI3K downstream signaling.
⦁ Buparlisib downregulates p-Akt and p-S6R and causes apoptosis of cancer cells.
Pharmacokinetics, dosage & administration
⦁ Buparlisib has a rapid absorption after oral administration in dose-dependent manner, achieving Cmax 1–1.5 h after the dose.
⦁ Buparlisib has a mean half-life of 40 h.
⦁ Buparlisib has oral bioavailability of >90% with a first pass hepatic extraction of less than 10%.
⦁ Buparlisb is given orally 100 mg /day.
Clinical efficacy
⦁ Clinical activity of letrozole and buparlisib is seen independent of the PIK3CA mutation status.
⦁ The combination of buparlisib and trastuzumab shows promising clinical activity.
Safety & tolerability
⦁ Buparlisib is a safe drug and has been tested in combination with other drugs in breast cancer.
⦁ The most common side effects with buparlisib are gastrointestinal disorders, transaminitis, hyperglycemia, mood disorders and rash.

Conclusion
There is great enthusiasm in the development of new targeted therapies in the treatment of breast cancer, buparlisib being one of them. Data from Phase I/ II trials suggest good tolerability and
(NCT01816594 and NCT01923168) and in the setting of brain metastasis (NCT02000882, in view of good brain penetration) are also under- way and will hopefully define the place of bupar- lisib in the treatment paradigm.

disease control in breast cancer patients refrac-

tory to multiple lines of therapy. However, a great deal needs to be learnt to delineate the sub- group of patients, it will benefit most. Till now the role of PIK3CA mutations as predictive bio- markers has not been successful. Furthermore, in the presence of multiple drugs targeting PI3K/ AKT/mTOR pathway, the exact role of bupar- lisib needs to be defined to know comparison of its efficacy, toxicity profile with other drugs. Trials addressing its role in neoadjuvant setting
Financial & competing interests disclosure
The authors have no relevant affiliations or financial involvement with any organization or entity with a finan- cial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.
No writing assistance was utilized in the production of this manuscript.

References
Papers of special note have been highlighted as:
⦁ of interest; •• of considerable interest
⦁ Cardoso F, Costa A, Norton L et al.
ESO-ESMO 2nd international consensus guidelines for advanced breast cancer (ABC2). Breast 23(5), 489–502 (2014).
•• This is the most recent guideline update on the treatment of advanced breast cancer and includes the risk versus benefit versus burden in the evidence grading system.
⦁ Castano Z, Tracy K, McAllister SS. The tumor macroenvironment and systemic regulation of breast cancer progression. Int. J. Dev. Biol. 55(7–9), 889–897 (2011).
⦁ Ferlay JSI, Ervik M, Dikshit R et al. GLOBOCAN 2012 v1.0, Cancer Incidence and Mortality Worldwide: IARC CancerBase
No. 11. International Agency for Research on Cancer, Lyon, France (2013).
⦁ Morris PG, McArthur HL, Hudis CA. Therapeutic options for metastatic breast cancer. Expert Opin. Pharmacother. 10(6), 967–981 (2009).
⦁ Liu P, Cheng H, Roberts TM, Zhao JJ. Targeting the phosphoinositide 3-kinase pathway in cancer. Nat. Rev. Drug Discov. 8(8), 627–644 (2009).
⦁ Yuan TL, Cantley LC. P.I3K pathway alterations in cancer: variations on a theme. Oncogene 27(41), 5497–5510 (2008).
⦁ Engelman JA, Luo J, Cantley LC. T.he evolution of phosphatidylinositol 3-kinases as regulators of growth and metabolism. Nat. Rev. Genet. 7(8), 606–619 (2006).
⦁ Katso R, Okkenhaug K, Ahmadi K, White S, Timms J, Waterfield MD. Cellular function of phosphoinositide 3-kinases: implications
for development, homeostasis, and cancer.
Annu. Rev. Cell Dev. Biol. 17, 615–675
(2001).
⦁ Courtney KD, Corcoran RB, Engelman JA. The PI3K pathway as drug target in human cancer. J. Clin. Oncol. 28(6), 1075–1083 (2010).
⦁ Baselga J. Targeting the phosphoinositide-3 (PI3) kinase pathway in breast cancer. Oncologist 16(Suppl. 1), 12–19 (2011).
⦁ Lopez-Knowles E, O’Toole SA, McNeil CM et al. PI3K pathway activation in breast cancer is associated with the basal-like phenotype and cancer-specific mortality. Int. J.
Cancer 126(5), 1121–1131 (2010).
⦁ Yardley DA, Noguchi S, Pritchard KI et al. Everolimus plus exemestane in postmenopausal patients with HR+ breast cancer: BOLERO-2 final progression-free survival analysis. Adv. Ther. 30(10), 870–884 (2013).
⦁ Bachelot T, Bourgier C, Cropet C et al. Randomized Phase II trial of everolimus in combination with tamoxifen in patients with hormone receptor-positive, human epidermal growth factor receptor 2-negative metastatic breast cancer with prior exposure to aromatase inhibitors: a GINECO study.
J. Clin. Oncol. 30(22), 2718–2724 (2012).
⦁ Maira SM, Stauffer F, Brueggen J et al. Identification and characterization of NVP-BEZ235, a new orally available dual phosphatidylinositol 3-kinase/mammalian
target of rapamycin inhibitor with potent in vivo antitumor activity. Mol. Cancer
Ther. 7(7), 1851–1863 (2008).
⦁ Gonzalez-Angulo AM, Juric D, Argilés G et al. Safety, pharmacokinetics, and preliminary activity of the -specific PI3K
inhibitor BYL719: Results from the first-in-human study. J. Clin. Oncol. 31(Suppl.), Abstract 2531 (2013).
⦁ Maira SM, Pecchi S, Huang A et al.
Identification and characterization of
NVP-BKM120, an orally available pan-class I PI3-kinase inhibitor. Mol. Cancer Ther. 11(2), 317–328 (2012).
⦁ Schnell CR, Stauffer F, Allegrini PR C et al.
Effects of the dual phosphatidylinositol
3-kinase/mammalian target of rapamycin inhibitor NVP-BEZ235 on the tumor vasculature: implications for clinical imaging. Cancer Res. 68(16), 6598–6607 (2008).
⦁ Nagata Y, Lan KH, Zhou X et al. PTEN activation contributes to tumor inhibition by trastuzumab, and loss of PTEN predicts trastuzumab resistance in patients. Cancer Cell 6(2), 117–127 (2004).
⦁ This paper highlighted one of the crucial pathways for trastuzumab resistance.
⦁ Berns K, Horlings HM, Hennessy BT et al. A functional genetic approach identifies the PI3K pathway as a major determinant of trastuzumab resistance in breast cancer. Cancer Cell 12(4), 395–402 (2007).
⦁ This paper highlighted PI3K pathway as one of the critical pathways for trastuzumab resistance.
⦁ O’Brien NA, Tong L, Von Euw E, Conklin DO. Kalous: PI3K/mTOR inhibition overcomes in vitro and in vivo trastuzumab resistance independent of feedback activation of pAKT. Cancer Res. 72(24 Suppl.), Abstract nr P4-08-01 (2012).
⦁ Miller TW, Hennessy BT, Gonzalez-Angulo AM et al. Hyperactivation of phosphatidylinositol-3 kinase promotes escape

from hormone dependence in estrogen receptor-positive human breast cancer. J. Clin. Invest. 120(7), 2406–2413 (2010).
⦁ Sanchez CG, Ma CX, Crowder RJ et al. Preclinical modeling of combined phosphatidylinositol-3-kinase inhibition with endocrine therapy for estrogen receptor- positive breast cancer. Breast Cancer
Res. 13(2), R21 (2011).
⦁ Miller TW, Balko JM, Fox EM et al.
ER-dependent E2F transcription can mediate resistance to estrogen deprivation in human breast cancer. Cancer Discov. 1(4), 338–351 (2011).
⦁ Koul D, Fu J, Shen R et al. Antitumor activity of NVP-BKM120 – a selective pan class I PI3 kinase inhibitor showed differential forms of cell death based on p53 status of glioma cells. Clin. Cancer Res. 18(1), 184–195 (2012).
⦁ This study shows the ability of buparlisb to cross the blood–brain barrier with a potential for use in brain tumors.
⦁ Bendell JC, Rodon J, Burris HA et al. Phase I, dose-escalation study of BKM120, an oral
pan-class I PI3K inhibitor, in patients with advanced solid tumors. J. Clin. Oncol. 30(3), 282–290 (2012).
•• This key study has established 100 mg per day as the maximum tolerated dose of buparlisib.
⦁ Ando Y, Inada-Inoue M, Mitsuma A et al. Phase I dose-escalation study of buparlisib (BKM120), an oral pan-class I PI3K inhibitor, in Japanese patients with advanced solid tumors. Cancer Sci. 105(3), 347–353 (2014).
⦁ Saura C, Bendell J, Jerusalem G et al.
Phase Ib study of buparlisib plus trastuzumab in patients with HER2-positive advanced or metastatic breast cancer that has progressed on trastuzumab-based therapy. Clin. Cancer Res. 20(7), 1935–1945 (2014).
⦁ Mayer IA, Abramson VG, Isakoff SJ et al.
Stand up to cancer Phase Ib study of pan-phosphoinositide-3-kinase inhibitor buparlisib with letrozole in estrogen
receptor-positive/human epidermal growth factor receptor 2-negative metastatic breast cancer. J. Clin. Oncol. 32(12), 1202–1209
(2014).
⦁ This study has shown that the use of letrozole and buparlisib is safe and feasible. The clinical activity in the study was independent of the PIK3CA mutation status.
⦁ Foukas LC, Claret M, Pearce W et al. Critical role for the p110alpha phosphoinositide-3- OH kinase in growth and metabolic regulation. Nature 441(7091), 366–370 (2006).
⦁ Taniguchi CM, Emanuelli B, Kahn CR. C. ritical nodes in signalling pathways: insights into insulin action. Nat. Rev. Mol. Cell. Biol. 7(2), 85–96 (2006).
⦁ Ackermann TF, Hortnagl H, Wolfer DP et al. Phosphatidylinositide dependent kinase deficiency increases anxiety and decreases GABA and serotonin abundance in the amygdala. Cell. Physiol. Biochem. 22(5–6), 735–744 (2008).
⦁ Kalkman HO. The role of the phosphatidylinositide 3-kinase-protein kinase B pathway in schizophrenia. Pharmacol. Ther. 110(1), 117–134 (2006).