Expert consensus on the clinical application of PI3K/AKT/mTOR inhibitors in the treatment of advanced breast cancer

Abstract Phosphoinositide 3‐kinase (PI3K)/protein kinase B (PKB or AKT)/mammalian target of rapamycin (mTOR) signaling pathway (PAM pathway) plays an important role in the development of breast cancer and are closely associated with the resistance to endocrine therapy in advanced breast cancer. Therefore, anticancer treatment targeting key molecules in this signaling pathway has become a research hotspot in recent years. Randomized clinical trials have demonstrated that PI3K/AKT/mTOR inhibitors bring significant clinical benefit to patients with advanced breast cancer, especially to those with hormone receptor (HR)‐positive, human epidermal growth factor receptor (HER) 2‐negative advanced breast cancer. Alpelisib, a PI3K inhibitor, and everolimus, an mTOR inhibitor, have been approved by FDA. Based on their high efficacy and relatively good safety profile, an expanded indication of everolimus in breast cancer has been approved by National Medical Products Administration (NMPA). Alpelisib is expected to be approved in China in the near future. The members of the consensus expert panel reached this consensus to comprehensively define the role of PI3K/AKT/mTOR signaling pathway in breast cancer, efficacy and clinical applications of PI3K/AKT/mTOR inhibitors, management of adverse reactions, and PIK3CA mutation detection, to promote the understanding of PI3K/AKT/mTOR inhibitors for Chinese oncologists, improve clinical decision‐making, and prolong the survival of target patient population.

and clinical applications of PI3K/AKT/mTOR inhibitors, management of adverse reactions, and PIK3CA mutation detection, to promote the understanding of PI3K/AKT/mTOR inhibitors for Chinese oncologists, improve clinical decision-making, and prolong the survival of target patient population.
breast cancer, PI3K/AKT/mTOR inhibitors 1 | INTRODUCTION Among all malignancies, breast cancer has the highest incidence and fifth highest mortality rate worldwide in the female population [1].According to GLOBOCAN 2020, the global cancer data, there were 2.261 million new cases and 685 thousand deaths in 2020 [1], while in China, 416 thousand new cases and 117 thousand deaths were reported in 2020 [2].Breast cancer is considered a heterogeneous group of diseases at the molecular level, whose comprehensive care regimen depends on the molecular subtype of each individual patient, and approximately 70% of all breast cancer cases are hormone receptor (HR)-positive [3,4].Although endocrine therapy, chemotherapy, and targeted therapy (such as CDK4/6 inhibitors and anti-HER2 therapy) have significantly decreased the risk of recurrence, most patients still end up with drug resistance, resulting in disease progression.Consequently, there is an urgent need to explore the mechanisms of resistance and develop an effective strategy to overcome drug resistance [5,6].
Phosphoinositide 3-kinase (PI3K)/protein kinase B (PKB or AKT)/mammalian target of rapamycin (mTOR) signaling pathway (PAM pathway) is involved in the regulation of tumor cell growth, proliferation, survival, angiogenesis, and so forth, which is important in the pathogenesis and progression of breast cancer [6][7][8][9][10].Meanwhile, normalizing the overactivation of the PAM pathway is also one of the major mechanisms of action in endocrine therapy, chemotherapy, and targeted therapy.Over the course of treatment, not only do the breast cancer cells adapt to estrogen deprivation and develop resistance to chemotherapy via multidrug resistance-associated proteins as well as the antiapoptotic mechanisms, but proteins associated with the PAM pathway also contribute to the resistance to CDK4/6 inhibitors and anti-HER2 treatment [5-7, 11, 12].
Several PAM-specific-targeted therapies are either already available or under research, and PAM pathway inhibitors can be categorized into three major classes: (1) PI3K inhibitors, such as alpelisib, taselisib, and inavolisib; (2) AKT inhibitors, such as capivasertib and ipatasertib; and (3) mTOR inhibitors, such as everolimus [5,13].Alpelisib is the first PI3K inhibitor that has been approved for breast cancer, and the combination therapy of alpelisib and fulvestrant has been approved by the United States Food and Drug Administration (FDA) for male and postmenopausal female patients with PIK3CA (the gene encoding p110α, the PI3K catalytic subunit)-mutated, HRpositive/HER2-negative advanced breast cancer who suffer from disease progression during or after endocrine therapy [5,11,14].In HR-positive/HER2negative breast cancer, PIK3CA is one of the most commonly mutated genes, and is also more prevalent among patients in China than in western countries [15].Everolimus in combination with exemestane has also been approved by the FDA and China's National Medical Products Administration, in postmenopausal patients with HR-positive/HER2-negative advanced breast cancer who failed letrozole or anastrozole therapy [16].Multiple phase II/III trials have confirmed that in terms of progression-free survival, patients with endocrine-resistant, HR-positive/HER2negative advanced breast cancer can benefit from combination therapy of everolimus plus endocrine therapy (such as letrozole, exemestane, or fulvestrant) [17][18][19][20][21].According to a multicenter, retrospective study in China, in patients with HR-positive/HER2negative advanced breast cancer who exhibited disease progression after palbociclib therapy, a CDK4/6 inhibitor, an increase of up to 5.1 months in median PFS was observed after receiving everolimus in combination with endocrine therapy [22].In addition, AKT inhibitors are still under research, but none of them has been approved as of now.
To further promote clinical studies on PAMtargeted drugs and provide standardized management of their clinical application and safe practice, a consensus on the clinical application of PAM-specific therapy has been reached, with collaborated efforts of experts brought together by the Society of Clinical Research of Oncology Medications of China Anticancer Association, Breast Cancer Expert Committee of National Cancer Quality Control Center, the Society of Onco-Pathology of China Anticancer Association, and Boao Cancer Innovation Institute.This consensus will focus on currently available PAM inhibitors both in China and abroad, summarizing their mechanisms of action, profiles of clinical benefit and adverse reactions, as well as key points on the detection of PIK3CA mutation, to guide their clinical application, efficacy monitoring, and management of adverse effects of these medications.

| PAM PATHWAY MEDIATES THE PATHOGENESIS AND PROGRESSION OF BREAST CANCER AND DRUG RESISTANCE 2.1 | The role of PAM pathway in the pathogenesis and progression of breast cancer
Activation of receptor tyrosine kinase (RTK) stimulates the PI3K complex (p85 and p110), which converts phosphatidylinositol 4,5-bisphosphate to phosphatidylinositol 3,4,5-trisphosphate (PIP3).Then, PIP3 phosphorylates and activates AKT, resulting in the activation of the mTOR complex after a series of phosphorylations [8,10,23].In breast cancer, PAM is the most commonly activated pathway, which is closely related to tumor pathogenesis, progression, and metastasis [7][8][9][10].The PI3K complex functions the upstream of PAM pathway and there are three classes of PI3K.Class I is the major class of PI3K that drives tumor pathogenesis [24].Studies have demonstrated that the gain-of-function mutation in PIK3CA (the gene encoding p110α, the catalytic subunit) is associated with the overactivation of PI3K, which is an effective mechanism of oncogenesis, especially in breast cancer [5,24].Meanwhile, AKT is one of the major effector molecules downstream, and once activated, phosphorylates many target molecules in the nucleus and cytosol, which then activates a large number of downstream substrates that regulates cell proliferation, contributing to tumor cell growth, proliferation, and angiogenesis [7,23,25].Furthermore, AKT facilitates survival and inhibits apoptosis of tumor cells by inhibiting BAD and BAX, members of the Bcl-2 family, which is a group of proapoptotic genes, and downregulating forkhead transcription factors like FOXO.A downstream target of PI3K/AKT is mTOR, activation of which results in phosphorylation of ribosomal protein S6 kinase (S6K) and eukaryotic translation initiation factor 4E (eIF4E)binding protein 1 (4EBP-1), permitting the transcription, F I G U R E 1 PAM signaling pathway and mechanisms of action of PAM pathway inhibitors [10].

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The role of PAM pathway in the treatment of resistant breast cancer

| Resistance to endocrine therapy
A potential mechanism of resistance to endocrine therapy is the overactivation of PAM pathway.Preclinical data have shown that PI3K and AKT can phosphorylate the Ser167 locus of ERα, independently activating ERα in the absence of estrogen.Therefore, the interaction between estrogen receptor (ER) and the overactivation of PAM pathway enables breast cancer cells to adapt to estrogen deprivation, resulting in desensitization of breast cancer cells to endocrine therapy [5,7].During the development of resistance to endocrine therapy, after prolonged estrogen deprivation, the main activation targets for PI3K include MCF-7 and p70S6 kinase, the substrate of mTOR in MDA-MB-361 cells.Meanwhile, PIK3CA mutation and PTEN loss trigger estrogenindependent growth of breast cancer cells.In PAM pathway, PIK3CA is the most common mutated gene, detected in almost 50% of ER-positive breast cancer patients [5,24].In addition, upregulation of RTK, which causes enhanced activation of the downstream PAM pathway, is also involved in the mechanisms of resistance [5].

| Resistance to CDK4/6 inhibitors
Additionally, PAM pathway is also involved in the acquired resistance to CDK4/6 inhibitors.In a model of acquired resistance to palbociclib monotherapy, compared to parental cell lines, resistant cells underwent significant changes in PAM pathway-associated proteins (such as upregulation of pAKT S473 , pAKT th308 , and P70S6K, as well as downregulation of PTEN), including 25 upregulated and 18 downregulated proteins.By means of pathway enrichment analysis, several changes in cancer-associated signaling pathways have been identified, including PI3K, mTOR, AMPK, and apoptosis induction pathways [12].Among the various mechanisms mediating drug resistance to CDK4/6 inhibitors found in HR-positive/HER2negative breast cancer cells, upregulation of the PI3K/mTOR pathway is a very common one, making it a potential target for reversal of drug resistance [12].Furthermore, in the case of acquired resistance, palbociclib is unable to inhibit the Rb signal, and thus loses control of tumor cell growth, while targeted PI3K therapy can block PI3K/AKT/mTOR pathway, thus effectively inhibiting the proliferation of resistant cells [12].

| Resistance to anti-HER2 therapies
As a member of the receptor tyrosine kinase ErbB family, HER2 forms heterodimers with other members of this family, triggering self-phosphorylation of the receptor tyrosine kinase domain and activating downstream pathways, including PAM pathway.Aberrant activation of PAM pathway is closely associated with resistance to anti-HER2 therapies [5,28].The most common variation of HER2 receptor molecule is a truncated form of HER2 (p95-HER2), and overexpression of p95-HER2 can induce resistance to anti-HER2 therapies via the activation of the PI3K/AKT signaling pathway [5].Plus, trastuzumab, an anti-HER2 agent, facilitates the expression of HER3 in breast cancer cells, and its overexpression can also activate PI3K/AKT pathway, resulting in resistance to anti-HER2 therapies [5].Other activating factors of PAM pathway, including PIK3CA mutation, AKT1 mutation, AKT2 amplification, and loss of cancer suppressor gene PTEN, can also cause resistance to anti-HER2 agents, such as trastuzumab and lapatinib [5,28].
Extensive preclinical evidence has shown that PAM inhibitors can overcome the resistance to anti-HER2 therapies.Copanlisib, a PI3K inhibitor has been proved capable of sensitizing anti-HER2 therapy-resistant cells, potentiating the effects of trastuzumab and lapatinib.The growth of breast tumor can also be inhibited, to a great extent, by the combination of alpelisib and anti-HER2 monoclonal antibodies [5].

| Resistance to chemotherapy
Transcription of breast cancer resistance protein (BCRP) and expression of P-glycoprotein (P-gp) are facilitated by PAM pathway via the KEAP1-Nrf2 axis and NF-κB pathway, with the exact mechanisms still unclear.The efflux of chemotherapy agents is increased by BCRP and P-gp, resulting in chemoresistance [5].The phosphorylation of AKT is also proved to be positively correlated to the activity, migration, and apoptosis of breast cancer cells, potentially leading to chemoresistance [29].
The efficacy of PAM pathway inhibitors in combination with chemotherapy in patients with chemoresistant breast cancer is currently under research in many clinical studies.For example, the efficacy of paclitaxel in combination with PI3K inhibitors (pictilisib and buparlisib) has been assessed in two randomized controlled trials, PEGGY (NCT01740336) and BELLE-4 (NCT01572727).
Nonetheless, the median PFS is only slightly prolonged in both studies without significant benefit.Other PI3K inhibitors are also under investigation, such as the application of alpelisib in combination with albuminbound paclitaxel in advanced triple-negative breast cancer with PIK3CA mutation or PTEN loss [5].Patients with inoperable, locally triple-negative advanced or metastatic breast cancer were included in another phase II LOTUS study (NCT02162719) and were divided into two groups: one treated with paclitaxel and ipatasertib, an AKT inhibitor, and the other treated with paclitaxel and placebo.Improved PFS was observed in the group treated with paclitaxel combined with iparasertib (6.2 vs. 4.9 months, p = 0.037), providing evidence for targeted treatment with AKT inhibitors in patients with triplenegative breast cancer [5].

| The role of PAM pathway inhibitors in the treatment of breast cancer
Up till now, a variety of PAM pathway inhibitors are either already available or under research.Targeted PI3K inhibitors include alpelisib (BYL719), taselisib (GDC-0032), and inavolisib (GDC-0077), and alpelisib is the first oral PI3K inhibitor specifically targeting the p110α subtype that has been approved for the treatment of breast cancer [5].Iparasertib (GDC-0068) and capivasertib (AZD-5363) are both AKT inhibitors currently under clinical research, but no AKT inhibitor has yet been approved either in China or abroad [5].Everolimus is the first mTOR inhibitor approved for the treatment of HR-positive/HER2negative breast cancer [5].

| Alpelisib
Patients with HR-positive/HER2-negative advanced breast cancer who had received endocrine therapy were enrolled in the SOLAR-1 trial [30], and the efficacy and safety of fulvestrant and alpelisib combination therapy were assessed in comparison with fulvestrant and placebo.The results showed that in patients with PIK3CA mutation, PFS was significantly prolonged (11.0 vs. 5.7 months) and the risk of disease progression or death was decreased by 35% in patients treated with combined alpelisib and fulvestrant treatment, compared to fulvestrant monotherapy.Objective response rate (ORR), clinical benefit rate (CBR), and tumor shrinkage rate were all higher in the group treated with alpelisib and fulvestrant than in the group treated with fulvestrant and placebo.Meanwhile, regardless of whether the patient had received CDK4/6 inhibitors before, they could benefit from combination therapy of alpelisib and fulvestrant either as a first-or second-line therapy [31].According to an update from ESMO in 2020, [32] the median overall survival in the combination therapy group was prolonged by 7.9 months (39.3 vs. 31.4months) compared to the other group, further confirming the efficacy of this regimen.For HR-positive/HER2negative advanced breast cancer patients with PIK3CA mutation who had received CDK4/6 inhibitor treatment, it is proved by the BYLieve study [33] for the first time that they can benefit from alpelisib treatment in combination with aromatiase inhibitors (AI), with the efficacy and safety profile unaffected by alpelisib.Thus, alpelisib has been confirmed as an important backup option for patients with CDK4/6 inhibitor-resistant breast cancer [33][34][35][36].For premenopausal female patients with HR-positive/HER2-negative advanced breast cancer, alpelisib in combination with endocrine therapy has been shown in the B-YOND study [37] to be a potentially effective regimen (with a median PFS of 25.2 months and an ORR of 50.0%).Compared to patients without PIK3CA mutation, more tumor shrinkage was observed in those with PIK3CA mutation after treatment with alpelisib.
Based on these results, it is recommended in the NCCN guidelines [38] that for postmenopausal HRpositive/HER2-negative breast cancer patients with PIK3CA mutation, the second-line treatment of choice should be a combination therapy of alpelisib and fulvestrant (category 1).The ABC6 guidelines [39] also recommend that for patients with PIK3CA (exon 9 or 20) mutation and appropriate HbA1c level who has been exposed to AI, alpelisib combined with fulvestrant could be a treatment option.For ER-positive/HER2-negative metastatic breast cancer, alpelisib should only be administered in patients with PIK3CA mutation.Similarly, according to the Guidelines and Standard of Diagnosis and Treatment of Breast Cancer by China Anticancer Association [40], in the second-line treatment of advanced breast cancer, there is evidence supporting the combination therapy of alpelisib, the PI3Kα inhibitor, and endocrine therapy in patients with PI3Kα mutation (detected by ctDNA testing of tumor tissue or peripheral blood) (Table 1).It was reported at the 2020 SABCS conference that a phase I/Ib clinical study [41] explored the efficacy and safety of inavolisib combined with palbociclib and fulvestrant in HR-positive/HER2-negative metastatic breast cancer patients with PIK3CA mutation.This study found that inavolisib at 9 mg, which was the recommended dose in phase II clinical study, in combination with palbociclib and fulvestrant at standard doses, exhibited controllable safety profiles and some antitumor activity.In addition, a pilot study of correlation analysis between clinical outcomes and PIK3CA mutation status in patients with HR-positive/HER2-negative metastatic breast cancer treated with inavolisib was also presented at this conference [42], suggesting that a higher ORR was observed in patients with multiple PIK3CA mutations detected in baseline ctDNA testing, compared to those with single PIK3CA mutation.Results from Cohort D were reported at the 2021 SABCS conference [43], revealing a median PFS of 7.1 months and a CBR of 48%.Currently, a phase III clinical study on the efficacy of inavolisib in combination with palbociclib and fulvestrant (NCT04191499) is still underway.

| Ipatasertib
The IPATunity130 study is an international, multicenter, randomized, placebo-controlled, double-blind phase III clinical trial, designed to assess the efficacy and safety of ipatasertib in combination with paclitaxel as first-line treatment in triple-negative or HR-positive/HER2negative breast cancer patients with PIK3CA/AKT1/ PTEN mutation.The main findings of its Cohort A were presented at the 2020 SABCS conference [44], and no significant difference in PFS was found between the combination therapy group and the paclitaxel monotherapy group in patients with PIK3CA/AKT1/PTEN mutation (median PFS: 7.4 vs. 6.1 months), unable to achieve its goal in terms of the primary endpoint.

| Capivasertib
The FAKTION trial [45] [46].An ORR of 20% was achieved in the monotherapy group, while in the combination therapy group, for patients who had previously received fulvestrant therapy, the ORR was 36%, but for those who had never received fulvestrant before, the ORR was 20%.
Therefore, capivasertib is a potential option for the treatment of ER-positive metastatic breast cancer.2).The BOLERO-5 trial [19] was designed to assess the efficacy of everolimus and exemestane combination therapy in postmenopausal ER-positive/ HER2-negative advanced breast cancer in China.Similar to the BOLERO-2 trial, an international multicenter trial, a significantly prolonged PFS (median PFS: 7.4 months) was observed in the BOLERO-5 trial, further supporting the application of everolimus and exemestane combination therapy in postmenopausal patients with ER-positive/ HER2-negative advanced breast cancer in China (Table 2).The BOLERO-4 trial [20] demonstrated that a median PFS of 22 months was achieved by combination therapy of everolimus and letrozole as the first-line treatment in postmenopausal patients with HR-positive/ HER2-negative advanced breast cancer (Table 2).The MIRACLE trial [21] also demonstrated that PFS was significantly prolonged by combination therapy of everolimus and endocrine therapy, compared to endocrine therapy alone (19.4 vs. 12.9 months) in premenopausal patients with HR-positive/HER2-negative advanced breast cancer who had experienced treatment failure with endocrine therapy (Table 2).Furthermore, the TRINITI-1 trial, a single-center, retrospective study in the United States, and a multicenter, retrospective study in China all confirmed that in terms of PFS, benefits could be also achieved with combination therapy of everolimus and endocrine therapy with or without CDK4/6 inhibitors in patients with HR-positive/HER2-negative advanced or metastatic breast cancer who had experienced disease progression with CDK4/6 inhibitors [22,49,50] (Table 3).
The multicenter, retrospective study in China found that in terms of PFS, combination therapy of everolimus and endocrine therapy provided benefits for patients with HRpositive/HER2-negative metastatic breast cancer who had experienced disease progression with CDK4/6 inhibitors [22] (Table 3).Subgroup analyses in both the BOLERO-2 trial [17] and the MIRACLE trial [21] reported that the PFS benefit brought on by combination therapy of everolimus and endocrine therapy was not affected by factors, such as age (<65 vs. ≥65 years), presence or absence of visceral metastasis, presence or absence of bone metastasis, primary or secondary resistance to endocrine therapy, presence or absence of recurrence during adjuvant therapy, and so forth.

| Sirolimus
A retrospective study [51] has shown that for patients with HR-positive advanced breast cancer, similar median PFS can be achieved with either combination therapy of sirolimus and endocrine therapy or combination therapy of everolimus and endocrine therapy (4.9 and 5.5 months, respectively; HR = 1.56; p = 0.142), indicating that sirolimus could be an alternative to everolimus.However, till now, there has been no large-scale, prospective clinical evidence on this, and sirolimus has not yet been approved for the treatment of breast cancer 3.2 | Indicated population

| PI3K inhibitors (alpelisib)
According to the FDA label, a combination of alpelisib and fulvestrant can be added to an endocrine therapybased regimen in the treatment of male and postmenopausal female patients with PIK3CA-mutated, HR-positive/HER2-negative advanced or metastatic breast cancer who exhibited disease progression during or after treatment [14].According to the drug label in Hong Kong, combination therapy of alpelisib and fulvestant can be administered in postmenopausal female patients with PIK3CA-mutated, HR-positive/HER2-negative locally advanced or metastatic breast cancer who exhibited disease progression after endocrine monotherapy [52].As for now, no indication has been approved in mainland China for alpelisib.
Expert panel recommendations: Based on the findings of SOLAR-1 and BYLieve studies, combination therapy of alpelisib and fulvestrant may be used in patients with PIK3CA-mutated, HR-positive/HER2negative breast cancer who experienced disease progression during or after endocrine therapy-based regimens.For those with PIK3CA mutation who experienced disease progression with CDK4/6 inhibitors, a combination of alpelisib and endocrine therapy (AI or fulvestrant) is also a preferred option.
T A B L E 3 Clinical trials and real-world studies on combination therapy of everolimus plus endocrine therapy with or without CDK4/6 inhibitors in CDK4/6 inhibitor-resistant, HR-positive/HER2-negative advanced or metastatic breast cancer.Abbreviations: CBR, clinical benefit rate; HER2, human epidermal growth factor receptor 2; HR, hormone receptor; PFS, progression-free survival.

| mTOR inhibitors (everolimus)
In light of information on the drug label and new indications approved by NMPA, everolimus and exemestane combination therapy can be used in postmenopausal patients with HR-positive/HER2-negative advanced breast cancer after treatment failure with letrozole or anastrozole therapy [16].
Expert panel recommendations: With combination therapy of CDK4/6 inhibitors plus endocrine therapy becoming the first-line treatment of HR-positive/ HER2-negative advanced breast cancer now, in general, CDK4/6 inhibitor plus fulvestrant is also administered in patients who failed AI therapy.Based on the findings of BOLERO-2 and BOLERO-5, as well as real-world studies, everolimus and exemestane combination therapy may be used in patients with HRpositive/HER2-negative advanced breast cancer after treatment failure with endocrine monotherapy or combined therapy.Thus, everolimus and exemestane combination therapy is a very valuable option for cases resistant to CDK4/6 inhibitors but without PIK3CA mutation.Moreover, combination therapies of everolimus plus fulvestrant or tamoxifen have also been proved feasible by PrE0102 and GINECO trials in postmenopausal patients with HR-positive/HER2negative advanced breast cancer after treatment failure with endocrine therapy.

| AKT inhibitors
As of now, no drug in this class has been approved for breast cancer.

| Drug information
Based on the FDA drug label, drug information about alpelisib [14] and everolimus [16] is summarized in Table 4.

| PI3K inhibitors (alpelisib)
When taken with strong CYP3A4 inducers, the concentration of alpelisib will be decreased, and thus the effects diminished.Such coadministration should be avoided [15].Coadministration of alpelisib with a breast cancer resistance protein inhibitor may increase alpelisib concentration, heightening the risk of toxicities.Such If a dose of alpelisib is missed, it can be taken with food within 9 h after the time it is usually taken.After more than 9 h, skip the dose for that day.The next day, take alpelisib at the usual time.If the patient vomits after taking the dose, advise the patient not to take an additional dose on that day, and to resume the dosing schedule the next day at the usual time.
c When combined with fulvestrant, the recommended dose of fulvestrant is 500 mg administered on Days 1, 15, and 29, and once monthly thereafter.
d Swallow tablets as is.Tablets should not be chewed or crushed before swallowing, and should be administered orally at the same time each day.Take everolimus once a day, with or without food.Swallow the tablets whole with a glass of water.
e Combined with exemestane.
coadministration should also be avoided [14].Coadministration of alpelisib with CYP2C9 substrates can decrease the plasma concentration of these substrates, diminishing their effects.Thus, the plasma concentration of these drugs should be monitored in the case of coadministration [14].See Table 5 for details about the classification of coadministered drugs and precautions for alpelisib.

| mTOR inhibitors (everolimus)
Everolimus is the substrate of CYP3A4 and Pglycoprotein (PgP), a multidrug efflux pump, which might interact with CYP3A4 and/or PgP inhibitors and inducers [16,53].The metabolism of everolimus is slowed down by CYP3A4 and/or PgP inhibitors, raising its plasma concentration, and thus increasing its adverse effects.Therefore, coadministration of everolimus with moderate and strong inhibitors of CYP3A4 and/or PgP inhibitors should be avoided, and under the circumstances where they must be coadministered, the dose of everolimus should be lowered [16,53].On the contrary, CYP3A4 and/or PgP inducers accelerate the metabolism of everolism, lowering its plasma concentration, thus diminishing the effects of everolimus.Therefore, coadministration of everolimus and strong CYP3A4 and/or PgP inducers should be avoided [16,53].See Table 6 for details about the classification of coadministered drugs and precautions for everolimus.

| PI3K inhibitors (alpelisib)
Based on the findings in animal experiments and studies on its mechanisms of action, alpelisib can cause fetal harm when administered to pregnant women, so pregnant patients should be informed of the potential risks to the fetus.Serious adverse effects can also be brought onto the breastfeeding infants, so for lactating women, breastfeeding should be avoided during alpelisib treatment and a week after the last dose [14].For women of childbearing age, effective contraceptive measures should be taken during alpelisib treatment and a week after the last dose [14].For the elderly population (≥65 years old), the incidence of Grades 3 and 4 hyperglycemia is higher than in those younger than 65 years old (44% vs. 32%), so close monitoring for adverse effects is recommended and dosage should be modified accordingly [14].As for patients with mild to moderate renal impairment, no dose modification is required, while for those with severe renal impairment, no empirical data are available, and alpelisib should be administered with caution.
For patients with liver impairment, no dose modification is needed [14].After taking the adverse effects of alpelisib into consideration, its use in specific populations is presented in Table 7.

| mTOR inhibitors (everolimus)
For pregnant patients, the embryotoxicity of everolimus has been found in mechanism studies.Thus, everolimus should not be used in pregnant patients unless its benefits outweigh the potential risks to the fetus.Serious adverse effects can also be brought onto the breastfeeding infants, so for lactating women, breastfeeding should be avoided during everolimus treatment and 2 weeks after the last dose [16].For women of childbearing age, effective contraceptive measures should be taken during everolimus treatment and 8 weeks after the last dose [16].When used in the elderly Precautions for coadministration of alpelisib with other drugs [14] Classification Drug names Precautions for alpelisib

CYP3A4 inducers Including but not limited to rifampin
Coadministration with a strong CYP3A4 inducer may decrease the concentration of alpelisib, diminishing its activity.Such coadministration should be avoided.

BCRP inhibitors Including but not limited to cyclosporine
Coadministration with BCRP inhibitors may increase the concentration of alpelisib, increasing the risk of toxicities.For patients on alpelisib, BCRP inhibitors should be avoided.If no alternative regimen is available and alpelisib must be coadministered with BCRP inhibitors, adverse effects should be closely monitored due to the risk of increased toxicity.

CYP2C9 substrates Including but not limited to warfarin
Coadministration with CYP2C9 substrates may decrease the plasma concentration of these substrates, diminishing their activity.Therefore, when coadministered, the concentration of the CYP2C9 substrates should be closely monitored.
population (≥65 years old), the incidence of adverse effects that lead to drug discontinuation is higher than in the younger population (33% vs. 17%), so close monitoring for adverse effects is recommended and dosage should be modified accordingly [16].For patients with mild (Child-Pugh class A) or moderate (Child-Pugh class B) liver impairment, a decreased dose is recommended [16].After taking the adverse effects of everolimus into consideration, its use in specific populations is presented in Table 8.Monitoring parameters, frequencies, and related recommendations on clinical management are summarized in Table 9, which is in accordance with the FDA drug label [14].
Fasting blood glucose and HbA1c should be monitored before and after alpelisib treatment.Including but not limited to amprenavir, fosamprenavir, aprepitant, erythromycin, fluconazole, verapamil, and diltiazem.
Moderate CYP3A4 inhibitor increases the plasma concentration of everolimus.These drugs should be coadministered with caution.For those who must coadminister moderate CYP3A4 and/or PgP inhibitors, the dose of everolimus should be decreased to 2.5 mg/day.If well tolerated, consider increasing the dose from 2.5 mg/day to 5 mg/day.When discontinuing the moderate CYP3A4 and/or PgP inhibitors, a wash-out period of 2-3 days should be allowed before increasing the dose of everolimus.
After the discontinuation, the dose of everolimus should be increased to the same dose before the moderate CYP3A4 and/or PgP inhibitors were added.
Strong CYP3A4 inducers decrease the plasma concentration of everolimus.Coadministration should be avoided.When strong CYP3A4 inducers must be coadministered, consider doubling the daily dose of everolimus using increments of 5 mg or less.If the strong inducer is discontinued, consider a washout period of 3 to 5 days, before the everolimus dose is returned to the dose used before initiation of the strong CYP3A4/PgP inducer Abbreviation: PgP, P-glycoprotein.a When on everolimus, foods with known cytochrome P450 and PgP inhibiting effects should be avoided, such as grapefruit, grapefruit juice, and so forth.
b St. John's wort (Hypericum perforatum) can decrease the exposing dose of everolimus unexpectedly, which should also be avoided.T A B L E 9 Recommendations on monitoring parameters and frequencies in patients receiving alpelisib [14] Parameters Alpelisib

FPG a
• Blood glucose should be monitored before the initiation of alpelisib treatment.
• After initiation, blood glucose and/or FPG should be monitored at least once a week in the first 2 weeks, and once every four weeks after that.Additional tests are also recommended if considered necessary.

HbA1c
• In the beginning, HbA1c should be monitored before the initiation of alpelisib treatment.
• Over the course of alpelisib treatment, HbA1c should be monitored at least once every 3 months.Additional tests are also recommended if considered necessary.
Abbreviation: HbA1c, hemoglobin A1c, glycated hemoglobin.a FPG, fasting plasma glucose.Blood glucose status should be monitored over the course of alpelisib treatment.If the blood glucose level is examined after the initiation of alpelisib treatment, instead of FPG, it should be fasting blood glucose.
T A B L E 10 Recommendations on monitoring parameters and frequencies in patients receiving everolimus [16] Parameters Everolimus

Renal function
Elevated blood creatinine levels and proteinuria have been reported in patients receiving everolimus.
Monitoring of renal function before the initiation of everolimus treatment is recommended, including blood urea nitrogen (BUN), urine protein, or blood creatinine tests, as well as regular tests afterward.For those with risk factors for further renal impairment, special attention should be paid to their renal function.

Blood glucose and lipid panel
For patients who already have hyperglycemia, hypercholesterolemia, and hypertriglyceridemia, fasting blood glucose and lipid panel should be examined before the initiation of everolimus treatment.Regular tests are also recommended after initiation, and appropriate management should be provided.If everolimus is coadministered with other medications that might cause hyperglycemia, more frequent tests are recommended.If possible, ideal blood glucose control should be obtained before initiation.
Hematologic parameters Decreased hemoglobin, lymphocytes, and platelets have been reported in patients receiving everolimus.It is recommended that a complete blood count should be examined before the initiation of everolimus treatment, and it should also be tested regularly afterward.
T A B L E 11 Recommendations on dose modification and management of adverse reactions during alpelisib therapy [14,59] Adverse reactions Severity a Recommendations on dose modification and management Discontinue alpelisib.Initiate or intensify topical/systemic corticosteroid and oral anti-histaminic therapies.Once the skin reaction is alleviated to grade 1 or lower, reinstitute alpelisib at the same dose as when the rash first appears, and then lower the dose when the rash appears again.

Hyperglycemia
creatinine, diarrhea, skin rash, decreased lymphocyte count, and so forth.Most common Grades 3 or 4 adverse reactions (with an incidence of ≥2%) include hyperglycemia, skin rash, diarrhea, and so forth [14].The management of alpelisib-associated adverse reactions over the course of treatment should be consistent with the grading [14], with details in Table 11.

| Hyperglycemia
The incidence of alpelisib-associated hyperglycemia is 65%, and the incidences of Grades 3 and 4 hyperglycemia are 33% and 3.9%, respectively [14], with a higher incidence during the first 1-2 months of alpelisib administration, which can be managed with anti-hyperglycemic agents.When the dose of alpelisib is as low as 200 mg, the incidence of hyperglycemia is greatly reduced.This hyperglycemia is also reversible after discontinuation of alpelisib [14].For patients with a history of diabetes, blood glucose should be closely monitored and anti-hyperglycemic treatment should be intensified [14].Alpelisib can be an option for patients with type 2 diabetes and good blood glucose control, while its safety profile is still unclear in patients with type 1 diabetes or patients with type 2 diabetes and suboptimal blood glucose control [14].The PI3K pathway activates AKT and downstream pathways, participating in sugar and lipid metabolism and inhibiting gluconeogenesis, which explains the mechanism of alpelisib-induced hyperglycemia [54,55].Preclinical data have shown that inhibition of PI3Kα stimulates liver glycogenolysis and decreases signal transduction of insulin receptors, resulting in insulin resistance and hyperglycemia [56][57][58].Expert panel recommendations: Before the initiation of alpelisib treatment, FPG and HbA1c should be tested, and blood glucose control should be optimized in advance if necessary.After initiation, regular monitoring is recommended, and initiation or optimization of antihyperglycemic treatment should be considered according to the clinical parameters (Tables 9 and 11).Currently, metformin is the drug of choice for alpelisib-associated hyperglycemia, as it somewhat increases the sensitivity to insulin.Moreover, due to the desensitization of insulin caused by alpelisib, there is usually an increase in the dose of insulin.There is also evidence supporting the effectiveness of SGLT-2 inhibitors, thiazolidinediones, and glucagon-like peptide-1 (GLP-1) analogs in controlling alpelisib-associated hyperglycemia.If a patient has suboptimal blood glucose control, endocrinology consultation should be arranged as soon as possible, to obtain a plan for blood glucose management.

| Skin rash
Among patients who experienced Grades 2 or 3 skin rash, the median time to the first onset is 12 days [14].In the SOLAR-1 trial, preventive medication can lower the incidence and mitigate the severity of skin rash, compared to those who did not receive anti-rash medication (rash of any grade: 26.7% vs. 64.1%;rash of Grade 3: 11.6% vs. 22.7%) [14].Therefore, preventive management has a positive impact on modifying the development and severity of skin rash.
Expert panel recommendations: Preventive antihistaminic medication before the initiation of alpelisib is recommended, such as cetirizine (10 mg q.d.), loratadine (10 mg q.d.), and so forth, to lower the risk and severity of skin rash.Patients should be advised to use gentle, unscented soap and laundry detergent, as well as noncomedogenic skin moisturizer and sunscreen, to prevent dry skin.Unprotected sun exposure and skincare products containing irritative components, such as alcohol, salicylic acid, ammonium lactate, and urea should also be avoided.Most of the rashes are reversible after appropriate treatment and discontinuation of alpelisib.Symptomatic treatment and severity-appropriate management should be undertaken according to the type and severity of skin rash (Tables 11 and 12).Anti-infective medications can also be administered if the infection is present.For the skin rash that cannot be relieved with conventional treatment, a dermatology consultation should be arranged in time, to obtain a plan of management.

| Diarrhea
The incidence of alpelisib-associated diarrhea is 58%, and the median time to onset of Grades 2 or 3 diarrhea is 46 days.Severe diarrhea might occur in patients on alpelisib, resulting in dehydration and acute kidney injury [14].For diarrhea of different levels of severity, management should be provided according to Table 11, such as skipping a dose, lowering the dose, or discontinuation of alpelisib.If diarrhea occurs during alpelisib treatment, patients should be advised to start antidiarrheal treatment and oral rehydration, and their healthcare decision-makers should also be informed [14].

| Noninfectious pneumonitis
Severe noninfectious pneumonitis has been reported in patients receiving alpelisib treatment, including acute interstitial pneumonitis and interstitial lung disease [14].About noninfectious pneumonitis was found in 1.8% of the patients receiving alpelisib [14].Thus, for patients with nonspecific respiratory signs and symptoms (such as hypoxia, cough, dyspnea, and interstitial lung infiltration on radiologic examinations, after excluding infection, tumor, and other etiologies), noninfectious pneumonitis should be considered [14].

| Other adverse reactions
Other adverse reactions, such as nausea (45%), fatigue (42%), loss of appetite (36%), stomatitis (30%), and vomiting (27%) have also been reported in patients receiving alpelisib [14].In general, for adverse reactions of Grades 1 or 2, no dose modification is needed, but medication and monitoring can be undertaken if necessary.For adverse reactions of Grade 3, alpelisib should be discontinued until the reactions are alleviated to Grade 1 or lower.Then, alpelisib should be reinstituted at a lower dose.However, when adverse reactions of Grade 4 occur, alpelisib should be permanently discontinued [14].

| Severe hypersensitivity
The incidence of alpelisib-associated Grades 3 or 4 hypersensitivity is 0.7% [14].Patients should be informed of the signs and symptoms of severe hypersensitivity (including but not limited to dyspnea, flushing, skin rash, fever, and tachycardia).Alpelisib should be permanently discontinued when severe hypersensitivity occurs [14].

| Severe skin reaction
Stevens-Johnson syndrome (SJS) and erythema multiforme (EM) (both are severe skin reactions) have been reported in about 0.4% and 1.1% of patients receiving alpelisib, respectively [14].Thus, alpelisib treatment should not be instituted in patients with a history of SJS, EM, or toxic epidermal necrolysis (TEN).Patients should be informed of the signs and symptoms of severe skin reactions (such as prodromal fever, flu-like symptoms, mucosal lesions, or progressively worsening skin rash) [14].When signs or symptoms of severe skin reactions occur, alpelisib should be discontinued until the etiology of the skin reaction is determined and dermatology consultation is also advisable.If SJS, TEN, or EM is confirmed, alpelisib should be permanently discontinued.For patients who have ever experienced severe skin reactions during alpelisib treatment, alpelisib should never be reinstituted.If the skin reactions are not severe enough to be diagnosed as SJS, TEN, or EM, the dose of alpelisib may be modified and topical corticosteroids or oral anti-histamines are indicated [14].

| mTOR inhibitors (everolimus)
Most common adverse reactions (of all grades, with an incidence of ≥30%) of everolimus in patients with HRpositive/HER2-negative advanced breast cancer include stomatitis, infection, skin rash, fatigue, diarrhea, and loss of appetite.Most common Grades 3 or 4 adverse reactions (with an incidence of ≥2%) include stomatitis, infection, hyperglycemia, fatigue, dyspnea, pneumonia, and diarrhea [16, 18-21, 48, 49, 60].Most of the everolimus-associated adverse reactions are of Grades 1 or 2, occur at the early stage of everolimus therapy, and can be controlled by decreasing the dose or by discontinuing everolimus [18-21, 48, 49, 60-62].Good management of the adverse reactions can improve compliance and thus maintain its effectiveness.The management of alpelisib-associated adverse reactions over the course of treatment should be consistent with the grading, as shown in Table 13.

| Stomatitis
Special attention should be paid to oral mucositis during everolimus therapy, which usually manifests as clearly demarcated, gray, elliptical, cold sore-like ulcers surrounded by a red halo [63].Due to its destructive effects on the mucosal barrier, the risk of systemic infection is increased.Clinical data shows that the incidence of everolimus-associated stomatitis is 33.8%-69% and the incidence of Grades 3 or 4 stomatitis is 2.9%-11% [16, 18-21, 48, 49, 60].Stomatitis is associated with the immunosuppressive effects of everolimus, and its incidence and severity can be reduced by gargling with alcohol-free water or saline water and applying topical analgesics or steroids so that dose reduction or even discontinuation can be avoided [16].The SWISH trial, a Phase II clinical trial [64] confirms that in a postmenopausal patient with HR-positive/HER2-negative advanced breast cancer receiving everolimus plus endocrine therapy, preventive use of glucocorticoidcontaining mouthwash could decrease the risk of T A B L E 13 Recommendations on dose modification and management of adverse reactions during everolimus therapy [16,69] Adverse reactions Severity a

Recommendations on dose modification b and management
Stomatitis [16] Grade 1 (Asymptomatic or mildly symptomatic, no intervention required) • No need for dose modification.Gargle with alcohol-free water or normal saline (0.9%) several times a day.
Grade 2 (Moderate pain, swallowing unaffected, texture-modified food required) • Temporarily discontinue everolimus until the symptoms are alleviated to Grade 1 or lower.Reinstitute everolimus at the same dose.
• If Grade 2 stomatitis recurs, discontinue everolimus until symptoms are alleviated to Grade 1 or lower.Reinstitute everolimus at a lower dose.Treat with topical analgesics (such as benzocaine, butamben, tetracaine hydrochloride, menthol, or phenol).Add topical corticosteroids as appropriate (such as triamcinolone oral patch) c .
Grade 3 (Severe pain, swallowing affected) • Temporarily discontinue everolimus until the symptoms are alleviated to Grade 1 or lower.Reinstitute at a lower dose.Treat with topical analgesics (such as benzocaine, butamben, tetracaine hydrochloride, menthol, or phenol).Add topical corticosteroids as appropriate (such as triamcinolone oral patch) c .
Grade 4 (With life-threatening outcomes, emergent intervention is required) • Terminate everolimus therapy.Provide appropriate treatment.
Grade 2 (Symptomatic, medication required, activities of daily life unaffected) • Consider discontinuation.Obtain chest CT, exclude infectious etiologies, and consider corticosteroid treatment until symptoms are alleviated to Grade 1 or lower.Reinstitute everolimus at a lower dose.
• Terminate everolimus therapy if no relief is observed after 4 weeks.
Grade 3 (Severe symptoms, daily life affected, oxygen supplementation required) • Discontinue everolimus.Obtain chest CT, exclude infectious etiologies, and consider corticosteroid treatment until symptoms are alleviated to grade 1 or lower.Reinstitute everolimus at a lower dose.
• Obtain pulmonary function test and bronchoscopic examination with bronchoalveolar lavage and/or biopsy.
Grade 4 (Life-threatening impairment of respiratory function, emergent intervention required [such as tracheostomy or intubation]) • Terminate everolimus therapy.Obtain chest CT, exclude infectious etiologies, and treat with corticosteroids.• Obtain pulmonary function test and bronchoscopic examination with bronchoalveolar lavage and/or biopsy.
Other nonhematologic toxicities (Except metabolic events) [16] Grade 1 • If tolerated, no need for dose modification.Treat and monitor as appropriate.
Grade 2 • If tolerated, no need for dose modification.Treat and monitor as appropriate.
stomatitis.Compared to the BOLERO-2 trial, the incidences of stomatitis in all grades (21.2% vs. 67%) and of Grade 2 or higher (2.4% vs. 33%) are both remarkably decreased at Week 8. Expert panel recommendations: For mild stomatitis, topical supportive treatment is recommended in general, including gargling with alcohol-free water or saline water and a cold compress.For moderate to severe stomatitis, topical medications may be added, such as local anesthetics (e.g., lidocaine), glucocorticoids, growth factors, Kangfuxin solution, and so forth.Meanwhile, when deemed appropriate, systemic medications (antibiotics, antifungals, antivirals, analgesics, etc.) can also be added.If necessary, everolimus therapy should be discontinued or terminated (Table 13).Prevention of stomatitis is just as important as treatment, which requires gargling with steroid-containing mouthwash before everolimus therapy, as well as complete dental examination, regular oral care and cleansing, healthy oral hygiene habits, and avoidance of hot, acidic, spicy, hard, crispy, or other irritative food during everolimus therapy.

Adverse reactions Severity a
Recommendations on dose modification b and management • If not tolerated, temporarily discontinue everolimus until symptoms are alleviated to Grade 1 or lower.Reinstitute everolimus at the same dose.
• If Grade 2 event recurs, discontinue everolimus until symptoms are alleviated to Grade 1 or lower.Reinstitute everolimus at a lower dose.
Grade 3 • Temporarily discontinue everolimus until symptoms are alleviated to Grade 1 or lower.Treat and monitor as appropriate.Consider reinstitution at a lower dose.
• If Grade 2 event recurs, consider termination of everolimus therapy.
Grade 2 (e.g., for hyperglycemia, fasting blood glucose is between 160 and 250 mg/dl, and for hypercholesterolemia, cholesterol is between 2.5× ULN and 5× ULN) • No need for dose modification.Treat and monitor as appropriate.
Grade 3 (e.g., for hyperglycemia, fasting blood glucose is between 250 and 500 mg/dl, and for hypercholesterolemia, cholesterol is between 5× ULN and 10× ULN) • Temporarily discontinue everolimus and reinstitute it at a lower level.Treat and monitor as appropriate.
Abbreviation: ULN, upper limit of normal.
b If a lower dose is needed, it is recommended to administer everolimus at about 50% of the previous dose.If the daily dose is smaller than a tablet, consider administering everolimus every other day.
When treating stomatitis, avoid using products containing hydrogen peroxide, iodine, and thyme derivatives because these may worsen oral ulcers.

.2.2 | Infection
The risk of infection is increased by the immunosuppressive effects of everolimus, and the possible infectious agents include bacteria, fungi, viruses, and parasites, including opportunistic infections [16].Both local and systemic infections have been reported in patients receiving everolimus, including pneumonia, mycobacterial infections, other bacterial infections, invasive fungal infections (such as aspergillosis, candidiasis, and Pneumocystis jiroveci pneumonia [PJP]), and viral infections (such as reactivation of hepatitis B virus).Some of the infections are severe or life-threatening (such as those leading to sepsis, respiratory failure, or liver failure) [16].Before the initiation of everolimus therapy, the pre-existing invasive fungal infections should be thoroughly treated, and signs of symptoms of infection should be monitored during everolimus therapy.If diagnosed as infection, appropriate treatment should be initiated rapidly and discontinuation or termination of everolimus therapy should be considered [16].Cases of PJP have been reported in patients receiving everolimus, with death being the outcome in some of them.Concomitant administration of corticosteroids or other immunosuppressants might be associated with PJP.Thus, preventive treatment for PJP should be considered when concomitant corticosteroids or other immunosuppressants are required [16].

| Skin rash
Skin rash is a common everolimus-associated cutaneous toxicity, with an incidence of 27%-44%, and the incidence of Grades 3 or 4 skin rash is less than 1%-4%, which usually occurs in the early stage of everolimus therapy [16, 18-21, 48, 49, 60].Most everolimus-associated skin rash resolves without intervention [65].Symptoms can also be relieved by topical application of hydrocortisone ointment or moisturizer.However, for patients susceptible to infection and/or hyperglycemic patients, steroids should be used with caution in the treatment of skin rash [65].

| Noninfectious pneumonitis
pneumonitis is a common adverse reaction associated with everolimus.According to the international trials of BOLERO-2 [60] and BOLERO-4 [20], the incidences of everolimus-associated pneumonitis are 16% and 18%, respectively, while in the BOLERO-5 trial [19] conducted in China, the incidence of everolimus-associated noninfectious pneumonitis is 23.8%, slightly higher than the aforementioned international trials.Nonetheless, everolimus-associated lung toxicity is not lethal and is correlated with its efficacy.The YOUNGBC-5 trial [61], a smaller-scale, single-center, retrospective study in China suggests that everolimus-associated pneumonitis was found in over half of the patients with advanced breast cancer receiving everolimus therapy, more than 80% of which occurred during the early stage of everolimus therapy.However, only less than one-third of the patients with everolimus-associated pneumonitis shown on radiologic images were actually symptomatic, and the symptoms resolved after discontinuation of everolimus in most of these patients.Compared to patients who did not experience everolimus-associated pneumonitis, those who did benefited more from everolimus in terms of survival.
Expert panel recommendations: The following precautions should be taken before the initiation of everolimus therapy [66].(1) Obtain a thorough history of respiratory diseases.(2) Obtain a recent chest X-ray image or baseline high-resolution CT scan (if available).
(3) Instruct the patient to report any new or worsening respiratory symptoms (such as cough, tachypnea, and fever) in a timely manner.( 4) Advise the patient to take a pulmonary function test if there are respiratory symptoms.( 5) Use everolimus with caution and follow up closely if there is a pre-existing diffuse interstitial lung disease.Pathologic diagnosis of interstitial pneumonitis should only be made with a multidisciplinary approach after taking clinical symptoms, imaging evidence, and pathologic findings into consideration.High-resolution CT provides reliable evidence for clinical diagnosis and differential diagnosis.Multidisciplinary consultations are recommended, if necessary [67].
If asymptomatic noninfectious pneumonitis is found but there are changes in the disease course, additional attention should be paid.If there is newonset, symptomatic noninfectious pneumonitis, it is recommended that corticosteroid be used until the symptoms are alleviated to Grade 1 or lower with close monitoring.Everolimus should be discontinued temporarily if there is severe interstitial lung disease or pneumonitis.Reinstitution from a lower dose can be considered after the symptoms resolve.For adverse reactions not responsive to conventional treatment, timely consultation with specialists and multidisciplinary management is recommended.

| Hyperglycemia
The incidence of everolimus-associated hyperglycemia is 14%-42.5% and the incidence of Grades 3 or 4 hyperglycemia is 3%-10% [16-21, 49, 60].For those with carbohydrate metabolism disorders, fasting blood glucose levels should be monitored before receiving everolimus treatment and blood glucose control should be optimized according to the standardized management in the guidelines [65].The glucose-lowering medication of choice is metformin and other options include SGLT-2 inhibitors (dapagliflozin, empagliflozin, etc.), pioglitazone, and so forth.Insulin should be added in time if ideal blood glucose control cannot be achieved [68].An endocrinology consultation should be arranged if necessary.

| General principles of managing adverse reactions
Management of adverse reactions is essential a patient's quality of life.As each targeted agent usually has a distinctive spectrum of adverse reactions, a comprehensive understanding of the potential adverse reactions for different PAM pathway inhibitors is important and close attention should be paid.Early prevention, close monitoring, and timely, appropriate, and effective interventions should be implemented.As for severe adverse reactions, PAM pathway inhibitors should be temporarily or permanently discontinued.Another critical aspect of managing adverse reactions involves multidisciplinary teams.The collaboration between endocrinologists, dermatologists, gastroenterologists, and cardiologists should be strengthened, which is of great significance in the management of adverse reactions during treatment with PAM pathway inhibitors.

| Overview of the PIK3CA gene
As the initiating molecule of the PAM pathway, PI3K molecules can be categorized into Classes I, II, and III, of which Class I PI3K is the driving factor for cancer.Class I PI3K can be further divided into Class IA and Class IB [24].Of these two classes, Class IA is a heterodimer formed by a catalytic subunit p110 and the regulatory subunit p85 [26].For PI3K Class IA, the isozymes p110α, p110β, and p110δ are encoded by PIK3CA, PIK3CB, and PIK3CD, respectively [26].In contrast, PI3K class IB is formed by p110γ only, encoded by PIK3CG.The PIK3CA gene encodes p110α protein, one of the catalytic subunits of the PI3K enzyme, playing a vital role in the pathogenesis of cancer driven by PIK3CA mutations, as well as in the development of resistance in breast cancer [70][71][72].Most of the PIK3CA mutations occur in the "hot-spots" of the gene sequence, including E542K ad E545K in exon 9 (helical domain) and H1047R in exon 20 (kinase domain) [70,73].Multiple clinical studies revealed that the prevalence of PIK3CA mutations is 18%-46.5% in breast cancer [15,[74][75][76][77][78][79][80][81].It is also reported by clinical studies on mutation analysis in patients with breast cancer in China that PIK3CA is also common in Chinese patients with breast cancer, with a prevalence of 32%-46.5% [15,80,81].According to a clinical study published on Breast Cancer in 2021, compared to the TCGA datasets, which mainly consist of data from patients with breast cancer in western countries, the prevalence of PIK3CA mutation in patients with breast cancer in China is higher (45.6% vs. 34.7%,p < 0.001).Among the three common mutation loci (p.E545K, p.E542K, and p.H1047R), p.H1047R is the most common one (66.1%).Among the four molecular subtypes of breast cancer, ER-positive/HER2-positive breast cancer has the highest PIK3CA mutation rate (51.6%), followed by ER-positive/HER2-negative breast cancer (48.7%), and ER-negative/HER2-negative breast cancer has the lowest mutation rate (30.0%) but the highest PIK3CA amplification rate (19.05%) [15].Guidelines for Advanced Breast Cancer (ABC 5), for patients with HR-positive/HER2-negative advanced or metastatic breast cancer, tissue biopsy for PIK3CA mutation testing is indicated when there is disease progression after endocrine-based therapy [38,82,83].
The PIK3CA mutation testing can be performed with either cancer tissue or plasma ctDNA specimens.Tissue biopsy specimens are preferred for patients with advanced breast cancer.If PIK3CA is not detected in the plasma ctDNA specimen, it is recommended obtain cancer tissue specimens to clarify the PIK3CA mutation status [38,[82][83][84].Due to the similar distribution of PIK3CA mutation in primary and metastatic sites, either fresh biopsy tissue samples or archived tissue samples can be used for mutation testing [85].As reported in the SOLAR-1 trial, whether PIK3CA mutation is detected in plasma ctDNA or in cancer tissue, patients can benefit remarkably from alpelisib and fulvestrant combination therapy [31].

| Commonly used methods for PIK3CA mutation testing
Currently, commonly used methods for PIK3CA mutation testing include fluorescent dye-based real-time PCR (RT-PCR) and NGS.Compared to RT-PCR, more mutation sites can be detected with NGS.Other methods include droplet digital PCR (ddPCR), BEAMing, and Sanger sequencing [79,89,90].With alpelisib approved by FDA, as an accompanying product for diagnosis, the QIAGEN therascreen®PIK3CA RGQ PCR kit also became available for PIK3CA mutation testing with tissue and/or peripheral blood ctDNA (liquid biopsy) specimens [14].Then, F1LCDx, an NGS-based accompanying product for liquid biopsy, is also approved by FDA for the detection of PIK3CA mutations [87].Each testing method has its own advantages and disadvantages, with test results affected by detection limit, type of specimen, quantity, quality of the specimen, and so forth.
Expert panel recommendations: PIK3CA mutation testing is recommended for patients with HR-positive/ HER2-negative advanced or metastatic breast cancer who exhibited disease progression after endocrinebased therapy.As there is not yet a national standard for mutation testing, either medical centers with the capability of genetic testing or fully certified genetic testing laboratories should be selected for PIK3CA mutation testing.All testing procedures in the laboratories should be approved by authorities, to ensure the accuracy of test results.Either tumor tissue or plasma ctDNA can be collected for PIK3CA mutation testing, but fresh, paraffin-embedded tumor tissue is preferred.If unavailable, previously harvested paraffin-embedded tumor tissue or slices, as well as plasma, can also be considered.If PIK3CA mutation is not detected in plasma ctDNA specimen, it is recommended to harvest tumor tissue specimen to further clarify the PIK3CA mutation status.Due to the similar distribution of PIK3CA mutation in primary and metastatic sites, either fresh biopsy tissue samples or archived tissue samples can be used for mutation testing.It is also recommended that relevant mutation sites of PIK3CA reported by the SOLAR-1 trial and real-world studies be tested, such as mutations in exons 7, 9, and 20 (C420R, E542K, E545A/D/G/K, Q546E/R, and H1047L/R/Y), and so forth.For other mutation sites, there is still no clear evidence supporting their association with clinical efficacy, and further studies are needed.

| FUTURE PERSPECTIVES
In previous studies, PAM pathway inhibitors were mainly examined as a first-line or late-line treatment for HR-positive/HER2-negative advanced or metastatic breast cancer.However, aberrant activation of PAM pathway is not only a potential mechanism for resistance to endocrine therapy and CDK4/6 inhibitors but is also closely associated with resistance to anti-HER2 therapy and chemotherapy.Therefore, the application of PAM pathway inhibitors in triple-negative breast cancer and HER2-positive breast cancer is currently being explored.

| PI3K inhibitors
Outstanding efficacy and safety profiles have been reported on alpelisib in patients with PIK3CA-mutated, HR-positive/ HER2-negative advanced breast cancer, and some efficacy has also been shown in triple-negative breast cancer and HER2-positive breast cancer.The NCT02038010 study [91] found that good tolerance and preliminary efficacy were achieved by combination therapy of alpelisib T-DM1 in trastuzumab-refractory, HER2-positive metastatic breast cancer, laying the foundation for further studies.Moreover, a phase III trial (CBYL719C2201/NCT04544189) on alpelisib plus fulvestrant in Chinese patients with PIK3CAmutated, HR-positive/HER2-negative breast cancer who previously received AI therapy, a phase III trial (CBYL719H12301/NCT04251533) on alpelisib plus nabpaclitaxel in triple-negative breast cancer patients with PIK3CA mutation or with PTEN loss but without PIK3CA mutation, and a phase III trial (CBYL719G12301/ NCT04208178) on alpelisib plus trastuzumab and pertuzumab as maintenance therapy in patients with PIK3CAmutated, HER2-positive advanced breast cancer are still underway.
Other PI3K inhibitors, such as taselisib, eganelisib, and MEN1611, are still being explored in the treatment of breast cancer.The LORELEI study has proved for the first time that compared to letrozole monotherapy, taselisib plus letrozole can significantly improve the ORR in patients with ER-positive/HER2-negative earlystage breast cancer, supporting its role in overcoming the resistance to neoadjuvant endocrine therapy [92].The MARIO-3 study provided preliminary evidence supporting the efficacy and safety of the combination therapy of eganelisib, tezolizumab, and chemotherapy in unresectable, triple-negative locally advanced, or metastatic breast cancer.Good tolerance was also observed from the combination therapy of MEN1611 and trastuzumab with or without fulvestrant in PIK3CA-mutated, HER2positive advanced or metastatic breast cancer [93].

| AKT inhibitors
Besides ipatasertib and capivasertib, other AKT inhibitors under research include MK-2206, vevorisertib, and so forth, but no positive clinical data have been obtained.

| mTOR inhibitors
Combination therapy of everolimus plus endocrine therapy in patients with HR-positive/HER2-negative advanced or metastatic breast cancer after treatment failure with endocrine therapy has been established in clinical practice and included in many guidelines.Multiple real-world studies have observed the advantages this combination therapy over other endocrine therapies in patients with advanced breast cancer who had experienced disease progression with CDK4/6 inhibitors.This combination therapy is also recommended by international guidelines as well as Chinese guidelines in this population.
The anti-HER2 effects of PAM pathway inhibitors have also been predicted by early-phase studies, including BOLERO-1 and BOLERO-3 trials.In the BOLERO-3 trial, PFS in patients with HER2-positive breast cancer was significantly improved in the group treated with everolimus plus trastuzumab plus vinorelbine, in comparison to placebo plus trastuzumab plus vinorelbine.Although the effects of everolimus on PFS in BOLERO-1 did not reach the preset significant level in the overall analysis, median PFS in ER-negative patients turned out to be 7.2 months longer than in the control group (Table 14).
Other mTOR inhibitors under investigation include sapanisertib and vistusertib.The Phase II trial of sapanisertib and fulvestrant combination therapy failed to show any actual improvement in prognosis in postmenopausal female patients with ER-positive/HER-negative advanced breast cancer after disease progression with AI therapy [94].In the MANTA study, no significant difference in PFS was found between vistusertib plus fulvestrant group and the fulvestrant monotherapy group [95].

| CONCLUSION
Currently, although combination therapy of endocrine therapy plus CDK4/6 inhibitors remains the standard first-line treatment for patients with HR-positive/HER2negative advanced breast cancer, drug resistance still needs to be addressed [3,6].The PAM pathway plays an essential role in cell growth, survival, proliferation, angiogenesis, as well as resistance to endocrine therapy and CDK4/6 inhibitors in breast cancer, making it an important treatment target.As of now, many PAM pathway inhibitors are still under development, which might potentially improve the management of breast cancer [6,12,13].Those that have already been approved, including a PI3K inhibitor (alpelisib) and an mTOR inhibitor (everolimus), provide more options for patients with HR-positive/HER2-negative advanced breast cancer after disease progression with endocrine therapy.In breast cancer of other molecular subtypes, PAM pathway inhibitors also showed some preliminary efficacy, indicating that more patients might benefit from PAM pathway inhibitors in the future.
[16]B L E 6 Precautions for coadministration of everolimus with other drugs[16] Liver impairment can increase the exposure to everolimus.Thus, recommended dose modification is listed as follows.(1) Mild liver impairment (Child-Pugh class A): recommended dose is 7.5 mg/day; if not well-tolerated, consider lowering the dose to 5 mg/day.(2) Moderate liver impairment (Child-Pugh class B): recommended dose is 5 mg/day; if not well-tolerated, consider lowering the dose to 2.5 mg/day.(3) Severe liver impairment (Child-Pugh class C): if the expected benefit outweighs the risks, everolimus can be administered at 2.5 mg/day, but the dose should not exceed this level.Over the course of everolimus treatment, if there is any change in the liver function status (Child-Pugh score), the dose should be modified accordingly.
[16]B L E 7 Contraindications of alpelisib in specific populations[14]Monitoring parameters, frequencies, and related recommendations on clinical management are summarized in Table10, which is in accordance with the FDA drug label[16].Renal function, blood glucose, lipid panel, and hematologic parameters should be monitored before and after everolimus treatment.3.7 | Management of adverse reactions3.7.1 | PI3K inhibitors (alpelisib)Most common adverse reactions (of all grades, with an incidence of ≥20%) of alpelisib in patients with PIK3CAmutated, HR-positive/HER2-negative advanced or metastatic breast cancer include elevated blood glucose, elevated T A B L E 8 Contraindications of everolimus in specific populations[16]a T A B L E 14 Clinical studies related to PAM pathway inhibitors