Targeting phosphoinositide 3-kinase (PI3K) in head and neck squamous cell carcinoma (HNSCC)

Targeting phosphoinositide 3-kinase (PI3K) in head and neck squamous cell carcinoma (HNSCC) Jung et al. Cancers of the Head & Neck (2018) 3:3 Cancers of the https://doi.org/10.1186/s41199-018-0030-z Head & Neck REVIEW Open Access Targeting phosphoinositide 3-kinase (PI3K) in head and neck squamous cell carcinoma (HNSCC) 1* 2 2 Kyungsuk Jung , Hyunseok Kang and Ranee Mehra Abstract: The landscape of head and neck squamous cell carcinoma (HNSCC) has been changing rapidly due to growing proportion of HPV-related disease and development of new therapeutic agents. At the same time, there has been a constant need for individually tailored treatment based on genetic biomarkers in order to optimize patient survival and alleviate treatment-related toxicities. In this regard, aberrations of PI3K pathway have important clinical implications in the treatment of HNSCC. They frequently constitute ‘gain of function’ mutations which trigger oncogenesis, and PI3K mutations can also lead to emergence of drug resistance after treatment with EGFR inhibitors. In this article, we review PI3K pathway as a target of treatment for HNSCC and summarize PI3K/mTOR inhibitors that are currently under clinical trials. In light of recent advancement of immune checkpoint inhibitors, consideration of PI3K inhibitors as potential immune modulators is also suggested. Keywords: HNSCC, PI3K, mTOR, Akt, EGFR, PIK3CA, HPV, Drug resistance, Precision medicine Background related HNSCC frequently harbors mutations in the hel- Head and neck squamous cell carcinoma (HNSCC) ical domain of PIK3CA, yet its biological significance arises from mucosal epithelium of oral cavity, pharynx has not been fully elucidated. In the era of precision and larynx. An estimate of 61,000 new cases of HNSCC medicine, it is becoming more important to understand were diagnosed in the US in 2016, with 13,190 deaths at- key genomic alterations and their therapeutic implica- tributable to the disease [1]. Traditional risk factors in- tions [9]. This review will focus on the role of PI3K-Akt- clude tobacco smoking, alcohol consumption, betel nut mTOR pathway in relation to epidermal growth factor chewing and genetic predisposition such as Fanconi receptor (EGFR) and their clinical applications in anemia [2–4]. Human papillomavirus (HPV) has re- HNSCC. cently emerged as a major and distinct risk factor for HNSCC. HPV-related HNSCC most commonly arises in Phosphoinositide 3-kinase (PI3K) and PI3K-Akt- oropharynx and has been associated with younger age of mTOR pathway disease onset, less smoking history, better performance PI3K is a family of phospholipid kinase that is divided status and favorable prognosis [5]. The proportion of into three classes based on structure, function and sub- HPV-positive oropharyngeal squamous cell cancer has strate specificity. Class I PI3K is a heterodimer that con- significantly increased for the past decade regardless of sists of a regulatory and a catalytic subunit. It is further sex and race [6], raising the need for a separate thera- divided into class IA and IB. For class IA PI3K, there are peutic strategy. three variants of catalytic subunit, p110α, p110β and Comprehensive genomic analysis of HNSCC revealed p110δ (encoded by PIK3CA, PIK3CB and PIK3CD), and frequent alterations in genes encoding molecules in five variants of regulatory subunit, p85α, p55α, p50α phosphoinositide 3-kinase (PI3K) pathway including (encoded by PIK3R1 and splice variants), p85β and p55δ PIK3CA, PTEN and PIK3R1 [7, 8]. In particular, HPV- (encoded by PIK3R2 and PIK3R3). p85 regulatory sub- unit contains Src homology 2 (SH2) domain which binds * Correspondence: kyungsuk.jung@fccc.edu to phosphorylated Y-X-X-M motif in receptor tyrosine Department of Medicine, Fox Chase Cancer Center, 333 Cottman Ave, kinase [10]. It was found that five isoforms of regulatory Philadelphia, PA, USA Full list of author information is available at the end of the article subunit express different affinities to tyrosine kinases © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Jung et al. Cancers of the Head & Neck (2018) 3:3 Page 2 of 13 [11], and each p110 subunit is selectively recruited to re- Class II PI3K is a monomer of catalytic isoforms, C2α, ceptor activation [12, 13]. These findings are consistent C2β and C2γ (encoded by PIK3C2A, PIK3C2B and with selective mutation of p110 in various types of can- PIK3C2G), and lacks regulatory subunit. Class II lipid cer and provides important prospect for targeted ther- kinase produces PI-3,4-P2 from PIP and PI-3-P from PI. apy. PIK3CA is one of the most commonly mutated and C2α isoform found in endosomes was suggested to play extensively studied oncogenes in various types of human a role in angiogenesis and vascular barrier formation cancer. An analysis of The Cancer Genome Atlas [18]. Class III PI3K is a heterodimer of a regulatory (TCGA) data showed that PIK3CA was the most fre- (Vps15, encoded by PIK3R4) subunit and a catalytic sub- quently mutated gene in breast cancer samples, second unit (Vps34, encoded by PIK3C3), which converts PI to most frequently mutated gene in uterine corpus endo- PI-3-P. Little is known about physiologic role of class III metrial cancer and third most commonly mutated gene PI3K, but it was implicated in induction of autophagy in in HNSCC [14]. PIK3CA is also heavily mutated in lung the state of nutrient deficiency [19]. squamous cell carcinoma, urothelial carcinoma of blad- The family of PI3K proteins mainly regulates cellular der and colorectal adenocarcinoma [14]. Molecular com- growth and cycle. Its activation is triggered by upstream position of p110α, the product of PIK3CA, and p85α are receptor tyrosine kinase such as ErbB family receptor illustrated in Fig. 1. (including EGFR), platelet-derived growth factor re- Class IB PI3K consists of p110γ catalytic subunit ceptor (PDGFR), insulin-like growth factor 1 receptor (encoded by PIK3CG) and p101 or p87 regulatory sub- (IGF-1R) or G protein-coupled receptor (GPCR). PI3K unit (encoded by PIK3R5, PIK3R6). Class IA and IB attaches a phosphate group to the 3′ hydroxyl of the in- PI3K phosphorylate 3-hydroxyl group of phos- ositol head of PIP2, converting it to PIP3 [20]. Inositol phatidylinositol (PI), phosphatidylinositol 4-phosphate phospholipids constitute a minor part of the cellular (PIP) and phosphatidylinositol 4,5-bisphosphate (PIP2), membrane and phosphorylation of inositol head has little producing phosphatidylinositol 3-phosphate (PI-3-P), effect on membrane structure. However, phosphorylated phosphatidylinositol 3,4-bisphosphate (PI-3,4-P2) and inositol head protruding from the membrane provides an phosphatidylinositol 3,4,5-triphosphate (PIP3), respect- anchoring site for secondary signaling molecules that are ively [15]. Expressions of p110δ and p110γ are found floating in the cytosol. Once PIP3 is formed by PI3K, cyto- exclusively in lymphocytic immune system whereas p110α solic molecules such as Akt/Protein kinase B localize to and p110β are expressed ubiquitously [16]. Idelalisib, a plasma membrane and become tethered to the head of drug used for treatment of lymphoma, is a selective inhibi- PIP3 via Pleckstrin homology (PH) domain in N terminal tor of p110δ which is abundantly expressed in malignant [21]. Activated Akt, in turn, phosphorylates a series of B cells [17]. molecules including mechanistic target of rapamycin Fig. 1 Linear composition of p110α and p85α molecules. Red arrowheads in p110α indicate ‘hotspot’ mutations. C2 in p110α is a putative membrane-binding domain. Breakpoint cluster region-homology (BH) domain in p85α has shown GTPase activating protein (GAP) activity toward Rab family. Rab GTPase induces degradation and deregulation of activated growth factor receptors, and mutated Rab GAP induces cell transformation [148]. However, it is unclear if this function is still active in complex with p110α [149]. BH domain in p85α is flanked by proline-rich domain, implying an auto-regulatory mechanism in interaction with its SH3 domain [150] Jung et al. Cancers of the Head & Neck (2018) 3:3 Page 3 of 13 (mTOR) that promotes cell survival, proliferation and mo- combination with radiotherapy [27]. Cetuximab is cur- tility. The action of PI3K, conversion of PIP2 to PIP3, is rently used with platinum-based chemotherapy as the first negatively regulated by reverse phosphatases, such as line treatment for HNSCC or for recurrent or metastatic phosphatase and tensin homolog (PTEN). Other cytoplas- (R/M) disease [28, 29]. However, efforts to develop a pre- mic molecules that contain PH domain and interact with dictive biomarker for EGFR-targeting treatment have not PIP3 include Rho-guanine nucleotide exchange factor been successful. In particular, overexpression of EGFR (GEF). Rho family proteins, when activated by GEF, re- assessed by immunohistochemistry (IHC) could not be model cytoskeleton, decrease contact inhibition and in- correlated with the level of treatment response to cetuxi- crease cell motility, all of which elevate invasiveness in mab [30–32]. Additionally, resistance to cetuximab has cancer cells [22]. been widely observed in various types of cancer including HNSCC. Several evasive mechanisms may serve to restore Implications of PI3K pathway alteration for EGFR original oncogene dependence, circumventing the initial pathway in HNSCC targeting treatment. Receptors can potentially abrogate in- EGFR is a cell surface receptor tyrosine kinase in ErbB hibitory action of therapeutic agents as they obtain second family and has been an attractive therapeutic target for mutations that result in pharmacokinetic changes [33]. A various human cancers including HNSCC. The receptor well-known mutation of EGFR, T790M, enhances affinity becomes activated by ligand binding which transitions of the kinase pocket for ATP, which competitively blocks EGFR monomers to the allosteric homodimer. Receptor binding of tyrosine kinase inhibitors [34]. Copy number dimerization stimulates tyrosine kinase activity in C ter- gains of target genes also reactivate dependent pathway minal domain and initiates downstream phosphorylation and counteract the treatment effect. For example, amplifi- cascade through PI3K-Akt-mTOR, Raf-MEK-MAP kin- cation of BRAF via copy number gains was found in 8% of ase or JAK/STAT pathways (Fig. 2). the tumor samples from metastatic melanoma treated It has been well known that EGFR overexpression is in- with BRAF inhibitors [35]. Studies with HNSCC demon- volved in carcinogenesis of HNSCC [23, 24], and associ- strated as well that copy number alteration by amplifica- ated with poor prognosis [25, 26]. EGFR-targeting strategy tion of 7p11.2 accounts for a number of cases of EGFR with a monoclonal antibody, cetuximab, has prolonged activation [36–38]. It was also hypothesized that ligand survival of patients with locally advanced HNSCC in overexpression or receptor cross phosphorylation triggers Fig. 2 Interactive signaling pathway of EGFR-PI3K-mTOR. PI3K binds to cytoplasmic tail of receptor tyrosine kinase via SH domains within p85 regulatory subunit. Activation signal can also be transferred through Ras-binding domain in p110 catalytic subunit which tethers PI3K molecule to Ras protein in growth receptors. p110 activation by Ras binding is inhibited by p85 subunit which can be released by co-stimulation of SH do- main by tyrosine kinase [151] Jung et al. Cancers of the Head & Neck (2018) 3:3 Page 4 of 13 uncontrolled EGFR hyperactivity. A genetic profiling of [7]. Frequency of these ‘hotspot’ mutations is also higher HNSCC samples with EGFR activation revealed that in HPV-positive oropharyngeal cancers [50]. EGFR ligands (including TGFα)were highlyexpressed in Targeting PIK3CA alteration in human squamous cell a subset, suggesting an establishment of an autocrine loop xenografts has demonstrated susceptibility to treatment [39]. in vitro and in vivo, leading a path for its clinical impli- Alternatively, the function of target gene can be cation. Inhibition of PI3K by competitive blockage of bypassed by activating downstream molecules of the sig- ATP binding site led to decreased phosphorylation of naling cascade or switching dependence to an alternative Akt in several studies [51–54]. In a number of the pathway for cell growth and proliferation [40]. As the patient-derived xenografts harboring E545K and tumor progresses and develops genomic heterogeneity, H1047R mutations, PI3K inhibitors were effective in cells with genetic survival benefit outgrow through evo- achieving control of tumor growth [43, 55, 56]. Add- lutionary selection pressure. In consistent with this the- itionally, activation of PI3K/mTOR pathway from either ory, whole-exome sequencing of melanoma cells that are mutation or gene amplification was positively correlated resistant to BRAF inhibitor revealed diverse genetic al- with tumor susceptibility to PI3K inhibitors in xenograft terations in the downstream MAPK pathway [41]. Simi- models [52, 57–59]. However, preclinical data also sug- larly, KRAS amplification or mutation was found in gested that additional molecular change should interact tumor samples from colorectal cancer patients who de- with PIK3CA alteration for tumorigenesis. Cell lines veloped resistance to EGFR inhibitors [42]. Relevant to engineered to harbor PIK3CA mutations in the ‘hot- our review, compensatory activation of downstream spots’ responded more favorably to PI3K/mTOR dual in- pathway, mainly PI3K, has been proposed as one of the hibition than PI3K inhibition only, indicating that tumor major resistance mechanisms to EGFR inhibitors in survival is not strictly dependent on the activated PI3K HNSCC. Gene expression of the molecules in PI3K [60]. In a similar sense, PI3K inhibition demonstrated pathway was elevated in cetuximab-resistant strains markedly synergistic effect when combined with EGFR compared to cetuximab-susceptible cells [43], and or MEK inhibition [61]. Interestingly, PIK3CA activation addition of mTOR/PI3K inhibitor effectively achieved in HPV-positive HNSCC did not necessarily lead to in- control of cell growth in HNSCC that acquired resist- creased Akt target phosphorylation, but instead, led to ance to EGFR inhibitors [44, 45]. increased mTOR activity and showed more sensitivity to PI3K/mTOR dual inhibition than Akt inhibition [62]. This finding can be extended to more favorable efficacy PI3K-mTOR alteration in HNSCC of PI3K/mTOR inhibitors over Akt inhibitors in clinical 66% of HNSCC harbor genomic alterations in one of the settings [63]. major components of PI3K pathway [46]. An analysis of Locations of mutations affect PI3K structure and func- whole-exome sequencing of 151 HNSCC tumors re- tion, resulting in different responsiveness to inhibition and vealed that PI3K is the most commonly mutated mito- clinical outcome. Regulatory subunit p85 normally sup- genic pathway among PI3K, JAK/STAT and MAPK and presses catalytic function of p110 at resting stage. Conse- that presence of multiple mutations in PI3K signaling quently, C terminal truncation or internal deletion of p85 pathway is correlated with more advanced disease [8]. releases p110 from negative regulation and constitutively Physiologic data confirms that an aberrant PI3K-mTOR activates the PI3K pathway [64, 65]. Additionally, as fre- pathway is associated with cell motility, invasion and quently mutated E542 and E545 in p110 are located at a metastasis. PI3K-PTEN balance has a direct effect on distance from the kinase domain, it is plausible that muta- chemotaxis and cell motility as it controls actin cytoskel- tions at these spots alter regulatory control of p85. Indeed, eton via Rho family proteins, such as Rho, Rac and E545K mutation in the helical domain of p110 changes CDC42 [22, 47]. PIP3 and PIP2 determine epithelial po- acid-base charge and disrupts inhibitory interaction be- larity in individual cells, thus dysfunctional PI3K results tween p85 and p110 [66]. H1047R mutation in the kinase in epithelial-mesenchymal transition, a critical event in domain, on the other hand, shifts orientation of the resi- tumor invasion [48]. due and changes conformation of the two loops of kinase PI3KCA is among the most frequently mutated genes that contact cell membrane. This allows for kinase access in HNSCC, affected both in HPV-positive and negative to phospholipid that is less regulated by p85 [67]. diseases (56 and 34%, respectively) [7]. PIK3CA muta- Independently from p110, p85 as a monomer also tions in HPV-positive HNSCCs are concentrated in hel- down-regulates PI3K activation: p85 is naturally more ical domain, whereas mutations are more spread out in abundant than p110 and excess p85 monomers can se- HPV-negative diseases [9, 49]. TCGA data presents that questrate insulin receptor substrate 1 (IRS-1), an adaptor 73% of PIK3CA mutations are located at E542, E545 in molecule that mediates signal transduction between the helical domain and in H1047 in the kinase domain IGF-1R and downstream PI3K [68]. Thus, in wild-type Jung et al. Cancers of the Head & Neck (2018) 3:3 Page 5 of 13 cells, the p85 monomer competes with the p85-p110 4–6 months compared to 3–5 months in the placebo dimer for IRS binding and signal transduction. In het- plus paclitaxel group (p = 0.011) [77]. In this trial, com- erozygous knock out cells, the amount of p85 monomers parable proportions of the patients had a mutation in decreases more than p85-p110 dimers which up- PIK3CA, 11% and 13% in the buparlisib and control regulates the PI3K pathway [69]. However, in null cells, arm, respectively. Patients taking buparlisib also main- complete absence of regulatory subunit to stabilize p110 tained stable quality of life and demonstrated good toler- leads to significantly decreased signal transduction caus- ance to the treatment compared to the placebo group, as ing cell apoptosis [69]. Although not as frequent as in similar proportions of patients discontinued the treat- PIK3CA, mutations in PIK3R1 (encoding p85α) can be ment due to adverse effects [77]. However, this study found in 3% of HPV-positive HNSCC and 1% of HPV- failed to demonstrate significant improvement in overall negative HNSCC according to TCGA data [7]. survival (OS) with buparlisib partly because of insuffi- Alteration of PTEN tumor suppressor gene is among cient power. There are several ongoing clinical trials to the frequently found somatic mutations in human can- evaluate the efficacy and safety of buparlisib with or cers as well as germline mutations causing hereditary without additional therapy (Table 1). cancer syndromes. PTEN dephosphorylates PIP3 to PIP2, inhibiting mitogenic signal transduction in the PX-866 PI3K pathway. PTEN also interacts with PI3K, which PX-866 is an analog of wortmannin that irreversibly in- plays a key role in chemotaxis and tumor metastasis hibits class I PI3K by binding to Lys in ATP catalytic site [47, 48]. Clinical data has shown that loss of PTEN ex- [78]. Potent and irreversible binding of PX-866 enables pression is a poor prognostic marker in oral squamous sub-nanomolar IC values of 0.1, 1.0 and 2.9 nmol/L for cell cancer [70]. However, PTEN loss was found in only PI3Kα PI3Kγ and PI3Kδ, respectively, in contrast to a small number of HNSCC (8.16%), implying that it is a much higher IC of > 300 nmol/L for PI3Kβ [79]. In relatively minor component in PI3K pathway activation vivo studies revealed antitumor activities of PX-866 [8]. against human colon cancer, ovarian cancer and lung cancer xenografts [80]. It enhanced antitumor activities Targeting PI3K-Akt-mTOR pathway in clinic of cisplatin and radiation treatment in colon cancer and PI3K inhibitor ovarian cancer cells, respectively [80]. PX-866 also ef- Buparlisib (BKM120) fectively overcame resistance to EGFR inhibitor in hu- Buparlisib is an orally bioavailable pan-PI3K inhibitor, man lung cancer cells lacking expression of ErbB-3 [79]. targeting the ATP binding site of p110 kinase domain. PX-866 induced cessation of tumor growth in xenograft Its inhibitory potency is equitable on class IA isoforms models of human HNSCC which included one case of of p110α, β and δ, but slightly less against class IB p110γ PIK3CA gene amplification and another case of E545K [51]. An in vitro study demonstrated IC values for Akt [43]. However, clinical trials of PX-866 failed to show inhibition of 104 ± 18, 234 ± 47 and 463 ± 87 nmol/L promising results. In phase II clinical trials, combined for PI3Kα, β and δ, respectively [51]. Buparlisib is rapidly use of PX-866 with either cetuximab or docetaxel failed absorbed orally and its serum concentration increases to achieve improved PFS or OS compared to each treat- proportionately to dosage [71]. The molecule also pene- ment alone [81, 82]. trates blood brain barrier and administration of buparli- sib by gavage effectively controlled metastatic growth of Alpelisib (BYL719) human breast cancer in mouse brain [72]. Based on pre- Theoretically, a selective inhibitor of PI3Kα can achieve clinical data, its antitumor activity was also attributed to antitumor activity without affecting other isoforms of suppression of microtubular dynamics [73], and antian- PI3K, allowing for a more favorable side effect profile. giongenic effect [51]. A combination of buparlisib, Alpelisib was designed as a specific inhibitor of PI3Kα, cetuximab and radiation exerted a synergistic antiprolif- the product of frequently mutated PIK3CA [83]. The erative effect on human head and neck cancer cell lines molecule inhibits wild-type PI3Kα (IC = 4.6 nmol/L) as [74, 75]. In vivo, buparlisib inhibited PI3K activity in cell well as PI3Kα with common PI3KCA mutations, such as lines with wild-type PIK3CA as well as mutant form har- E545K or H1047R (IC = 4 nmol/L), more potently than boring any hotspot mutation of E542K, E545K or PI3Kδ (IC = 290 nmol/L) or PI3Kγ (IC = 250 nmol/L) 50 50 H1047R [76]. In a phase I dose-escalation study for ad- [52]. Preclinical data also suggested that PIK3CA mutation vanced solid tumors, most common side effects included makes cancer cells more vulnerable to PI3K inhibition by rash, abnormal hepatic function, alteration in glucose alpelisib. In vitro pharmacologic sensitivity screen among metabolism and fatigue [71]. In a recent randomized a broad panel of cancer cell lines revealed that sensitivity phase II trial with R/M HNSCC, adding buparlisib to to alpelisib was positively associated with the presence of paclitaxel improved progression-free survival (PFS) to PIK3CA mutation, amplification or copy number gain Jung et al. Cancers of the Head & Neck (2018) 3:3 Page 6 of 13 Table 1 Clinical trials evaluating PI3K or mTOR inhibitor in patients with HNSCC Agent Clinical Trial Other Targeted Additional Conditions Phase Status Identifier Agent Therapy PI3K inhibitor Alpelisib (BYL719) NCT02145312–– R/M HNSCC, failed to respond to II Not yet recruiting platinum-based therapy NCT02537223 – Cisplatin, Locoregionally advanced HNSCC, not I Active, recruiting radiation previously treated NCT01602315 Cetuximab – R/M HNSCC I/II Terminated (sponsor withdrawal) NCT02298595 Cetuximab Cisplatin HPV-associated oropharyngeal SCC I/II Not yet recruiting Buparlisib NCT01816984 Cetuximab – R/M HNC I/II Active, not recruiting (BKM120) NCT01737450–– Recurrent or progressive HNC II Active, recruiting NCT02113878 – Cisplatin, Locally advanced HNSCC I Active, recruiting radiation PX-866 NCT01252628 Cetuximab R/M HNSCC II Completed NCT01204099 Docetaxel Locally advanced or R/M HNSCC II Completed Copanlisib NCT02822482 Cetuximab – HNSCC with PI3KCA mutation/ I/II Active, recruiting amplification or PTEN loss INCB050465 NCT02646748 Itacitinib Pembrolizumab Advanced solid tumors I Active, recruiting mTOR inhibitor Sirolimus NCT01195922–– Advanced HNSCC, not previously I/II Completed treated Temsirolimus NCT01172769–– R/M HNSCC II Completed NCT01009203 Erlotinib – Advanced HNSCC, refractory to II Terminated (high patient platinum withdrawal rate) NCT01016769 – Paclitaxel, R/M HNSCC I/II Active, not recruiting carboplatin NCT02215720 Cetuximab – Advanced or metastatic solid tumors I Active, recruiting NCT00703625 – Docetaxel Resistant solid malignancies I Completed Everolimus NCT01332279 Erlotinib Radiation Recurrent HNC, previously treated with I Withdrawn (sponsor (RAD001) radiation withdrawal) NCT01313390 – Docetaxel R/M HNSCC I/II Terminated (lack of recruitment) NCT01009346 Cetuximab Cisplatin, R/M HNSCC I/II Terminated (toxicity) carboplatin NCT01051791–– R/M HNSCC II Active, not recruiting PI3K/mTOR dual inhibitor SF1126 NCT02644122–– R/M HNSCC II Active, recruiting Gedatolisib NCT03065062 Palbociclib – Advanced HNSCC I Active, recruiting Dactolisib NCT00620594–– Advanced solid tumors I Completed (BEZ235) PI3K/HDAC dual inhibitor CUDC-907 NCT02307240–– Advanced or relapsed solid tumors I Active, recruiting [84], which was confirmed by an in vivo study using sensitization to radiation [55]. Compared to HNSCC cell mouse models [52]. In a HNSCC cell line (Cal-33) and a lines with wild-type PIK3CA, cell lines with PIK3CA patient-derived xenograft model, both harboring H1047R H1047R mutation were more susceptible to antiprolifera- mutation in PIK3CA, administration of alpelisib using tive effect of alpelisib [56]. In another in vivo study, nanoparticles induced inhibition of tumor growth and PIK3CA mutation, regardless of its location, was the Jung et al. Cancers of the Head & Neck (2018) 3:3 Page 7 of 13 strongest predictive feature that correlated with favorable adjacent lymph nodes [95]. In a phase I trial of sirolimus response to alpelisib [52]. Compensatory hyperactivation and bevacizumab for patients with advanced malignan- of PIK3CA is one of the major mechanisms of treatment cies, no objective response was observed among the par- resistance, thus PI3K inhibitors are being tested with other ticipants with HNSCC [96]. However, among the targeted therapies, such as EGFR inhibitors. Inhibition of patients with stage II-IVA, untreated HNSCC, neoadju- PI3K with alpelisib enhanced tumor sensitivity to vant trial of sirolimus followed by definitive therapy (sur- cetuximab in HNSCC xenograft models [85]. A phase I gery or chemoradiation) demonstrated significant clinical trial of alpelisib combined with cetuximab in R/M responses (one CR, one PR and 14 SDs among 16 pa- HNSCC resulted in one partial response (PR), three tients) with good patient tolerance [97]. Sirolimus is unconfirmed PRs and five stable diseases (SDs) among known for poor bioavailability and low predictability of 32 cases with relatively good patient tolerance [86]. serum concentration after intestinal absorption, thus its PI3K activation statuswas unknowninthistrial.Ina narrow therapeutic window and a long half-life require more recent phase I trial of alpelisib, any of complete regular drug concentration monitoring [98]. Based on response (CR), PR or SD was achieved in 13 out of 19 these concerns, analogs of sirolimus have been developed study participants with PIK3CA-mutant HNSCC to improve pharmacokinetic properties. (NCT01219699) [87]. Copanlisib Temsirolimus Copanlisib is a potent inhibitor of class I PI3K with sub- Temsirolimus is a water-soluble analog of sirolimus and nanomolar IC . The molecule exhibits preferential ac- can be administered parenterally [99]. It undergoes hy- tivity against PI3Kα and PI3Kδ over PI3Kβ and PI3Kγ drolysis after administration to form sirolimus, but the (IC values of 0.5 and 0.7 nmol/L over 3.7 and 6. medication itself is also capable of inhibiting mTOR. 4 nmol/L, respectively) [57, 88]. It demonstrated super- Temsirolimus is currently FDA approved for the treat- ior inhibitory effect in cells with PIK3CA activating ment of advanced renal cell carcinoma [100]. Several pre- mutations over wild-type in breast cancer and non- clinical studies proved that a combination of temsirolimus small cell lung cancer xenografts [57]. Phase I trials in and cetuximab induces synergistic antitumor effect, as it patients with advanced or refractory solid tumors mitigates or prevents compensatory downstream mTOR presented good patient tolerance and evidence of dis- over-activation induced by EGFR inhibitor [101–105]. ease control [89, 90]. Efficacy and safety of combined There have been a number of phase I/II trials using tem- copanlisib and cetuximab for HNSCC is under study sirolimus in patients with HNSCC. In a phase I study of (NCT02822482). temsirolimus used with carboplatin and paclitaxel in R/M HNSCC, 22% of the patients exhibited objective PRs mTOR inhibitor [106]. The information regarding PI3K activation status Sirolimus (rapamycin) was lacking in this study. In TEMHEAD trial, a phase II Sirolimus was initially developed as an antifungal metab- study of temsirolimus in R/M HNSCC refractory to plat- olite, extracted from the bacterium Streptomyces hygro- inum and cetuximab, tumor shrinkage occurred in 39.4% scopicus [91]. However, since its immunosuppressive and of the patients mostly within the first six weeks of the antiproliferative properties were revealed, this macrolide treatment. However, no objective response was achieved, molecule has been more widely used for oncologic treat- nor did PI3KCA mutational status (H1048Y and G1050S) ment and for prevention of graft rejection or coronary predict treatment success [107]. In another trial including stent blockage. Sirolimus binds with FKBP12 (12 kDa a broad range of advanced malignancies, the combination FK506-binding protein) to form a gain-of-function com- of bevacizumab, cetuximab and temsirolimus was effective plex that function as an inhibitor of mTOR complex 1 in achieving PRs in 25% of the patients with HNSCC, but (mTORC1) [92]. This compound, as a result, inhibits a few patients were withdrawn from the trial because of metabolic alteration and cell proliferation which is trig- toxicities [108]. In this study, treatment-responders did gered by upstream gain-of-function mutations, such as not carry PIK3CA mutation in HNSCC cells. A trial com- PI3K and Akt. Sirolimus demonstrated antiproliferative bining temsirolimus with erlotinib for R/M HNSCC was activity in HNSCC cell lines inducing synergistic effect closed early due to toxicity and patient death [109]. In a with chemotherapeutic agents or radiation [93, 94]. In phase I pharmacokinetic study of temsirolimus, dose- HNSCC xenograft models with activated PI3K-Akt path- limiting toxicities occurred such as thrombocytopenia, way, administration of sirolimus induced marked inhib- stomatitis or mucositis, asthenia, manic-depressive syn- ition of tumor growth and cell apoptosis [58, 59]. It also drome and rash [110]. Thus, the treatment effect of tem- suppressed lymphangiogenesis in HNSCC xenograft sirolimus should be evaluated against potential toxicities models and prevented spread of the cancer cells to and more clinical trials are ongoing. Jung et al. Cancers of the Head & Neck (2018) 3:3 Page 8 of 13 Everolimus (RAD001) E545K or H1047R in PIK3CA as well as wild-type [124, Everolimus is a hydroxyethyl derivative of rapamycin, of- 125]. Gedatolisib also inhibited cell proliferation and in- fering improved oral bioavailability. The medication has creased radiosensitivity of human nasopharyngeal cancer a short half-life, allowing for quick establishment of cells with PI3K/mTOR hyperactivation [126]. Addition- stable status and improved drug safety [111]. After intes- ally, use of gedatolisib in EGFR inhibitor-resistant tinal absorption, everolimus is not converted to rapamy- HNSCC suppressed cell survival and induced apoptosis cin, instead forms a complex with FKBP12 and inhibits [45]. Phase I trials with gedatolisib for patients with ad- mTOR [112]. It is currently approved by the FDA for vanced cancer demonstrated potential antitumor activ- treatment of multiple malignancies including advanced ities with PRs and acceptable tolerance [127, 128]. breast cancer, kidney cancer, neuroendocrine tumor However, no apparent relationship between PIK3CA al- (NET) of pancreas, progressive NET of GI and lung, tu- teration and treatment response was observed in these berous sclerosis-associated renal angiomyolipoma and trials. There is an ongoing phase I trial of gedatolisib subependymal giant cell astrocytoma [113]. Although combined with palbociclib (CDK4/CDK6 inhibitor) for everolimus was effective in arresting tumor growth in advanced solid tumors including HNSCC (NCT0306 HNSCC xenograft models [114, 115], clinical data was 5062). not as encouraging. Several phase I studies demon- strated PRs among patients with HNSCC [116–119], but Dactolisib (BEZ235) the doses of everolimus used were different depending Dactolisib is an ATP-competitive dual inhibitor of PI3K on other treatments combined, such as platinum, doce- and mTOR, exerts more potency on PI3Kα,PI3Kδ,PI3Kγ taxel, cetuximab or radiation. Phase II trials with everoli- and mTOR (IC values of 4, 7, 5 and 21 nmol/L, respect- mus also failed to demonstrate clinical benefit for ively) than PI3Kβ (IC =75 nmol/L) [54, 129]. Dactolisib HNSCC. Either as monotherapy or combination with er- exhibited potent antiproliferative activity, halting cell cy- lotinib, treatment with everolimus was not successful in cles at G1 [54] and attenuating VEGF expression [129]. achieving objective response in patients with previously HNSCC cell lines with H1047R mutation were more sus- treated R/M HNSCC [120, 121]. There is a currently ac- ceptible to inhibition with lower IC whereas E545K con- 50, tive clinical trial testing everolimus monotherapy in pa- ferred only slightly increased sensitivity [60]. In clinical tients with R/M HNSCC (NCT01051791). settings, however, there has been little evidence to support drug efficacy and safety. When dactolisib was used for PI3K/mTOR dual inhibitor patients with castration-resistant prostate cancer or SF1126 everolimus-resistant pancreatic NET, the trials were dis- SF1126 is a peptide-conjugated prodrug of LY294002, with continued due to dose-limiting toxicities, such as stoma- improved water solubility and pharmacokinetics. RGDS titis, vomiting, diarrhea or hyperglycemia [130, 131]. conjugation enables the molecule to bind to specific integ- Combination of dactolisib and everolimus tested in pa- rins within the tumor, enhancing drug permeability [53]. tients with various advanced solid tumors, including one LY294002 is a pan-PI3K inhibitor, with IC values of case of HNSCC, failed to demonstrate objective response 720 nmol/L, 306 nmol/L, 1.33 μmol/L and 1.6 μmol/L for [132]. Another phase I trial of dactolisib treatment for PI3Kα,PI3Kβ, PI3Kδ and PI3Kγ respectively, and similar various, advanced solid tumors is now complete and the IC for mTOR (1.5 μmol/L) [53, 122]. In a phase I trial, result is being awaited (NCT00620594). SF1126 as a single agent was effective in maintaining stable diseases in patients with GIST and clear cell renal PI3K/HDAC dual inhibitor cancer, and in combination with rituximab decreased ab- CUDC-907 solute lymphocyte count and lymph node/spleen size in CUDC-907 is an orally administered inhibitor of class I CLL [123]. SF1126 monotherapy is now being evaluated PI3K isoforms and histone deacetylase (HDAC). IC values for treatment of R/M HNSCC (NCT02644122). for PI3Kα,PI3Kβ,PI3Kδ and PI3Kγ are 19, 54, 38 and 311 nmol/L, respectively [133]. Simultaneous inhibition of Gedatolisib PI3K and HDAC has demonstrated synergistic effect com- Gedatolisib is a potent and reversible inhibitor of class I pared to the combined level of growth suppression PI3K and mTOR. IC values for PI3Kα, PI3Kβ,PI3Kδ, achieved by single compound of HDAC inhibitor, vorino- PI3Kγ and mTOR are 0.4 nmol/L, 6 nmol/L, 8 nmol/L, stat, and PI3K inhibitor, GDC-0941 [133]. CUDC907 has 6 nmol/L and 10 nmol/L, respectively [124]. The inhibi- proved to be therapeutic against B cell lymphoma by de- tory activity against PI3Kα with hotspot mutations, such creasing MYC protein levels [134]. The effect of dual inhib- as E545K and H1047R, are comparatively low (0.6 nmol/ ition synergistically induced apoptosis of MYC-altered cells L and 0.8 nmol/L) [124]. Its antitumor activity was dem- in diffuse large B-cell lymphoma (DLBCL) [135]. For cancer onstrated in in vitro studies using mutant cells harboring cells that developed resistance to PI3K inhibition through Jung et al. Cancers of the Head & Neck (2018) 3:3 Page 9 of 13 alternative pathway activation, concurrent inhibition of in macrophages plays a key role in inducing immune sup- HDAC can down-regulate other signaling proteins and cir- pression by inhibiting NFκB pathway. Inhibition of PI3Kγ cumvent treatment resistance. This potential benefit of dual in macrophages, therefore, stimulated NFκB activation and inhibition was supported by an in vitro finding which dem- promoted an immunostimulatory transcriptional program, onstrated that administration of HDAC inhibitor success- restoring T cell activation. Another report suggests that fully overcame resistance to mTOR inhibitor in lymphoma PI3K-Akt pathway activation may mediate Tim-3 expres- cells [136]. An in vivo study has also revealed that dual in- sion in HNSCC, which is associated with more exhausted hibition of PI3K and HDAC can defeat cancer resistance to phenotype of tumor infiltrating lymphocytes, and cause re- platinum-based treatment by suppressing multidrug resist- sistance to immune checkpoint blockade [147]. However, ance transporters and DNA repairs [137]. The first phase I the role of PI3K pathway in cancer immunology needs to trial of CUDC-907 for the treatment of relapsed/refractory be clinically investigated further. Thereare phaseItrials of lymphoma achieved two CRs and three PRs in patients combining PI3Kδ inhibitor (INCB050465) with pembroli- with DLBCL [138]. There is an actively ongoing phase I zumab in advanced solid tumors (NCT02646748), and trial of CUDC-907 for the patients with advanced or re- combining PI3Kβ inhibitor (GSK2636771) with pembroli- lapsed solid tumors (NCT02307240), and another phase I zumab in advanced melanoma (NCT03131908). With re- trial for the patients with metastatic or locally advanced cent approvals of immune checkpoint inhibitors for the thyroid cancer (NCT03002623). treatment of R/M HNSCC, effects of adding PI3K inhibi- tors to immune checkpoint inhibitors will be further Inhibition of PI3K pathway and immune system explored. It has been well known that inhibitors of mTOR, such as sirolimus, modulate immune system. Clinically, they Conclusions have been used as immune suppressive agents to prevent PI3K plays a key role in the progression of HNSCC and rejection for patients who had undergone organ trans- development of resistance against cetuximab. Genomic plant. In fact, PI3K family controls many aspects of cell alterations affecting PI3K are common among both development, differentiation and function in both innate HPV-positive and HPV-negative diseases and serve as an and adaptive immune system [139]. Especially, PI3Kγ attractive target for the treatment of HNSCC. Early clin- and PI3Kδ are highly expressed in all subtypes of ical trials evaluating PI3K inhibitors have shown disap- leukocyte, and inhibition of PI3Kγ suppressed progres- pointing results, but further evaluation with more potent sion of breast cancer in an animal model by inhibiting agents and careful patient selection might lead to devel- tumor inflammation and myeloid cell-mediated angio- opment of effective PI3K inhibitors in HNSCC. In light genesis [140]. Furthermore, it has been revealed that of recent success of immune checkpoint inhibitors, po- PI3Kγ in macrophage has a critical role in the interplay tential impacts of PI3K inhibition on immune system between immune stimulation and suppression during in- should be considered in the future development of flammation or cancer development [141]. Class I PI3K PI3K-targeted therapy. signaling becomes activated by antigen receptors Abbreviations expressed by T and B cells, altering adaptive immune anti-PD1: anti-programmed death 1; BH: Breakpoint cluster region-homology; system. Therefore, inhibition of PI3Kδ dampens regula- CR: Complete response; DLBCL: Diffuse large B-cell lymphoma; tory T cells, enhances activity of cytotoxic T cells and in- EGFR: Epidermal growth factor receptor; Erk: Extracellular signal-regulated kin- ase; FKBP12: 12 kDa FK506-binding protein; GAP: GTPase activating protein; duces tumor regression as shown in animal models of GEF: Guanine nucleotide exchange factor; GPCR: G protein-coupled receptor; melanoma, lung cancer, thymoma and breast cancer HDAC: Histone deacetylase; HNSCC: Head and neck squamous cell [142]. Various mutations in genes encoding PI3Kδ may carcinoma; HPV: Human papillomavirus; IGF-1R: Insulin-like growth factor 1 receptor; IHC: Immunohistochemistry; IRS-1: Insulin receptor substrate 1; as well lead to immunodeficiency syndromes [143]. MEK: MAPK(mitogen-activated protein kinase)/Erk kinase; mTOR: mechanistic Immune checkpoint inhibitors such as anti-programmed target of rapamycin; mTORC1: mTOR complex1; NET: Neuroendocrine tumor; death 1 (anti-PD1) antibodies have demonstrated remark- OS: Overall survival; PDGFR: Platelet-derived growth factor receptor; PDK1: Phosphoinositide-dependent kinase 1; PD-L1: Programmed death- able activities in HNSCC [144, 145]. Interestingly, the level ligand 1; PFS: Progression-free survival; PH: Pleckstrin homology; of immune checkpoint ligands such as programmed death PI: Phosphatidylinositol; PI-3,4-P2: Phosphatidylinositol 3,4-bisphosphate; ligand 1 (PD-L1) appears to be regulated by the PI3K-Akt- PI3K: Phosphoinositide 3-kinase; PI-3-P: Phosphatidylinositol 3-phosphate; PIP: Phosphatidylinositol 4-phosphate; PIP2: Phosphatidylinositol 4,5- mTOR pathway: inhibition of PI3K, Akt or mTOR de- bisphosphate; PIP3: Phosphatidylinositol 3,4,5-triphosphate; PKB: Protein creased expression of PD-L1 in a non-small cell lung cancer kinase B; PR: Partial response; PTEN: Phosphatase and tensin homolog; model in vitro and in vivo [146]. Furthermore, combination Ral: Ras-like protein; Rheb: Ras homolog enriched in brain; SD: Stable disease; SH: Src homology; TCGA: The cancer genome atlas; TSC: Tuberous sclerosis of PI3Kγ blockade and immune checkpoint blockade with complex anti-PD1 therapy induced a synergistic growth inhibitory effect in animal models of both HPV-positive and negative Funding HNSCC [141]. In this study, the authors showed that PI3Kγ There was no financial support or funding for this review article. Jung et al. Cancers of the Head & Neck (2018) 3:3 Page 10 of 13 Authors’ contributions 17. Furman RR, Sharman JP, Coutre SE, Cheson BD, Pagel JM, Hillmen P, et al. KJ searched literature and RM compiled current clinical trials. KJ and HK Idelalisib and rituximab in relapsed chronic lymphocytic leukemia. N Engl J wrote the initial manuscript. RM and HK provided revisions. All authors read Med. 2014;370(11):997–1007. and approved the final manuscript. 18. 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Targeting phosphoinositide 3-kinase (PI3K) in head and neck squamous cell carcinoma (HNSCC)

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Abstract

Jung et al. Cancers of the Head & Neck (2018) 3:3 Cancers of the https://doi.org/10.1186/s41199-018-0030-z Head & Neck REVIEW Open Access Targeting phosphoinositide 3-kinase (PI3K) in head and neck squamous cell carcinoma (HNSCC) 1* 2 2 Kyungsuk Jung , Hyunseok Kang and Ranee Mehra Abstract: The landscape of head and neck squamous cell carcinoma (HNSCC) has been changing rapidly due to growing proportion of HPV-related disease and development of new therapeutic agents. At the same time, there has been a constant need for individually tailored treatment based on genetic biomarkers in order to optimize patient survival and alleviate treatment-related toxicities. In this regard, aberrations of PI3K pathway have important clinical implications in the treatment of HNSCC. They frequently constitute ‘gain of function’ mutations which trigger oncogenesis, and PI3K mutations can also lead to emergence of drug resistance after treatment with EGFR inhibitors. In this article, we review PI3K pathway as a target of treatment for HNSCC and summarize PI3K/mTOR inhibitors that are currently under clinical trials. In light of recent advancement of immune checkpoint inhibitors, consideration of PI3K inhibitors as potential immune modulators is also suggested. Keywords: HNSCC, PI3K, mTOR, Akt, EGFR, PIK3CA, HPV, Drug resistance, Precision medicine Background related HNSCC frequently harbors mutations in the hel- Head and neck squamous cell carcinoma (HNSCC) ical domain of PIK3CA, yet its biological significance arises from mucosal epithelium of oral cavity, pharynx has not been fully elucidated. In the era of precision and larynx. An estimate of 61,000 new cases of HNSCC medicine, it is becoming more important to understand were diagnosed in the US in 2016, with 13,190 deaths at- key genomic alterations and their therapeutic implica- tributable to the disease [1]. Traditional risk factors in- tions [9]. This review will focus on the role of PI3K-Akt- clude tobacco smoking, alcohol consumption, betel nut mTOR pathway in relation to epidermal growth factor chewing and genetic predisposition such as Fanconi receptor (EGFR) and their clinical applications in anemia [2–4]. Human papillomavirus (HPV) has re- HNSCC. cently emerged as a major and distinct risk factor for HNSCC. HPV-related HNSCC most commonly arises in Phosphoinositide 3-kinase (PI3K) and PI3K-Akt- oropharynx and has been associated with younger age of mTOR pathway disease onset, less smoking history, better performance PI3K is a family of phospholipid kinase that is divided status and favorable prognosis [5]. The proportion of into three classes based on structure, function and sub- HPV-positive oropharyngeal squamous cell cancer has strate specificity. Class I PI3K is a heterodimer that con- significantly increased for the past decade regardless of sists of a regulatory and a catalytic subunit. It is further sex and race [6], raising the need for a separate thera- divided into class IA and IB. For class IA PI3K, there are peutic strategy. three variants of catalytic subunit, p110α, p110β and Comprehensive genomic analysis of HNSCC revealed p110δ (encoded by PIK3CA, PIK3CB and PIK3CD), and frequent alterations in genes encoding molecules in five variants of regulatory subunit, p85α, p55α, p50α phosphoinositide 3-kinase (PI3K) pathway including (encoded by PIK3R1 and splice variants), p85β and p55δ PIK3CA, PTEN and PIK3R1 [7, 8]. In particular, HPV- (encoded by PIK3R2 and PIK3R3). p85 regulatory sub- unit contains Src homology 2 (SH2) domain which binds * Correspondence: kyungsuk.jung@fccc.edu to phosphorylated Y-X-X-M motif in receptor tyrosine Department of Medicine, Fox Chase Cancer Center, 333 Cottman Ave, kinase [10]. It was found that five isoforms of regulatory Philadelphia, PA, USA Full list of author information is available at the end of the article subunit express different affinities to tyrosine kinases © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Jung et al. Cancers of the Head & Neck (2018) 3:3 Page 2 of 13 [11], and each p110 subunit is selectively recruited to re- Class II PI3K is a monomer of catalytic isoforms, C2α, ceptor activation [12, 13]. These findings are consistent C2β and C2γ (encoded by PIK3C2A, PIK3C2B and with selective mutation of p110 in various types of can- PIK3C2G), and lacks regulatory subunit. Class II lipid cer and provides important prospect for targeted ther- kinase produces PI-3,4-P2 from PIP and PI-3-P from PI. apy. PIK3CA is one of the most commonly mutated and C2α isoform found in endosomes was suggested to play extensively studied oncogenes in various types of human a role in angiogenesis and vascular barrier formation cancer. An analysis of The Cancer Genome Atlas [18]. Class III PI3K is a heterodimer of a regulatory (TCGA) data showed that PIK3CA was the most fre- (Vps15, encoded by PIK3R4) subunit and a catalytic sub- quently mutated gene in breast cancer samples, second unit (Vps34, encoded by PIK3C3), which converts PI to most frequently mutated gene in uterine corpus endo- PI-3-P. Little is known about physiologic role of class III metrial cancer and third most commonly mutated gene PI3K, but it was implicated in induction of autophagy in in HNSCC [14]. PIK3CA is also heavily mutated in lung the state of nutrient deficiency [19]. squamous cell carcinoma, urothelial carcinoma of blad- The family of PI3K proteins mainly regulates cellular der and colorectal adenocarcinoma [14]. Molecular com- growth and cycle. Its activation is triggered by upstream position of p110α, the product of PIK3CA, and p85α are receptor tyrosine kinase such as ErbB family receptor illustrated in Fig. 1. (including EGFR), platelet-derived growth factor re- Class IB PI3K consists of p110γ catalytic subunit ceptor (PDGFR), insulin-like growth factor 1 receptor (encoded by PIK3CG) and p101 or p87 regulatory sub- (IGF-1R) or G protein-coupled receptor (GPCR). PI3K unit (encoded by PIK3R5, PIK3R6). Class IA and IB attaches a phosphate group to the 3′ hydroxyl of the in- PI3K phosphorylate 3-hydroxyl group of phos- ositol head of PIP2, converting it to PIP3 [20]. Inositol phatidylinositol (PI), phosphatidylinositol 4-phosphate phospholipids constitute a minor part of the cellular (PIP) and phosphatidylinositol 4,5-bisphosphate (PIP2), membrane and phosphorylation of inositol head has little producing phosphatidylinositol 3-phosphate (PI-3-P), effect on membrane structure. However, phosphorylated phosphatidylinositol 3,4-bisphosphate (PI-3,4-P2) and inositol head protruding from the membrane provides an phosphatidylinositol 3,4,5-triphosphate (PIP3), respect- anchoring site for secondary signaling molecules that are ively [15]. Expressions of p110δ and p110γ are found floating in the cytosol. Once PIP3 is formed by PI3K, cyto- exclusively in lymphocytic immune system whereas p110α solic molecules such as Akt/Protein kinase B localize to and p110β are expressed ubiquitously [16]. Idelalisib, a plasma membrane and become tethered to the head of drug used for treatment of lymphoma, is a selective inhibi- PIP3 via Pleckstrin homology (PH) domain in N terminal tor of p110δ which is abundantly expressed in malignant [21]. Activated Akt, in turn, phosphorylates a series of B cells [17]. molecules including mechanistic target of rapamycin Fig. 1 Linear composition of p110α and p85α molecules. Red arrowheads in p110α indicate ‘hotspot’ mutations. C2 in p110α is a putative membrane-binding domain. Breakpoint cluster region-homology (BH) domain in p85α has shown GTPase activating protein (GAP) activity toward Rab family. Rab GTPase induces degradation and deregulation of activated growth factor receptors, and mutated Rab GAP induces cell transformation [148]. However, it is unclear if this function is still active in complex with p110α [149]. BH domain in p85α is flanked by proline-rich domain, implying an auto-regulatory mechanism in interaction with its SH3 domain [150] Jung et al. Cancers of the Head & Neck (2018) 3:3 Page 3 of 13 (mTOR) that promotes cell survival, proliferation and mo- combination with radiotherapy [27]. Cetuximab is cur- tility. The action of PI3K, conversion of PIP2 to PIP3, is rently used with platinum-based chemotherapy as the first negatively regulated by reverse phosphatases, such as line treatment for HNSCC or for recurrent or metastatic phosphatase and tensin homolog (PTEN). Other cytoplas- (R/M) disease [28, 29]. However, efforts to develop a pre- mic molecules that contain PH domain and interact with dictive biomarker for EGFR-targeting treatment have not PIP3 include Rho-guanine nucleotide exchange factor been successful. In particular, overexpression of EGFR (GEF). Rho family proteins, when activated by GEF, re- assessed by immunohistochemistry (IHC) could not be model cytoskeleton, decrease contact inhibition and in- correlated with the level of treatment response to cetuxi- crease cell motility, all of which elevate invasiveness in mab [30–32]. Additionally, resistance to cetuximab has cancer cells [22]. been widely observed in various types of cancer including HNSCC. Several evasive mechanisms may serve to restore Implications of PI3K pathway alteration for EGFR original oncogene dependence, circumventing the initial pathway in HNSCC targeting treatment. Receptors can potentially abrogate in- EGFR is a cell surface receptor tyrosine kinase in ErbB hibitory action of therapeutic agents as they obtain second family and has been an attractive therapeutic target for mutations that result in pharmacokinetic changes [33]. A various human cancers including HNSCC. The receptor well-known mutation of EGFR, T790M, enhances affinity becomes activated by ligand binding which transitions of the kinase pocket for ATP, which competitively blocks EGFR monomers to the allosteric homodimer. Receptor binding of tyrosine kinase inhibitors [34]. Copy number dimerization stimulates tyrosine kinase activity in C ter- gains of target genes also reactivate dependent pathway minal domain and initiates downstream phosphorylation and counteract the treatment effect. For example, amplifi- cascade through PI3K-Akt-mTOR, Raf-MEK-MAP kin- cation of BRAF via copy number gains was found in 8% of ase or JAK/STAT pathways (Fig. 2). the tumor samples from metastatic melanoma treated It has been well known that EGFR overexpression is in- with BRAF inhibitors [35]. Studies with HNSCC demon- volved in carcinogenesis of HNSCC [23, 24], and associ- strated as well that copy number alteration by amplifica- ated with poor prognosis [25, 26]. EGFR-targeting strategy tion of 7p11.2 accounts for a number of cases of EGFR with a monoclonal antibody, cetuximab, has prolonged activation [36–38]. It was also hypothesized that ligand survival of patients with locally advanced HNSCC in overexpression or receptor cross phosphorylation triggers Fig. 2 Interactive signaling pathway of EGFR-PI3K-mTOR. PI3K binds to cytoplasmic tail of receptor tyrosine kinase via SH domains within p85 regulatory subunit. Activation signal can also be transferred through Ras-binding domain in p110 catalytic subunit which tethers PI3K molecule to Ras protein in growth receptors. p110 activation by Ras binding is inhibited by p85 subunit which can be released by co-stimulation of SH do- main by tyrosine kinase [151] Jung et al. Cancers of the Head & Neck (2018) 3:3 Page 4 of 13 uncontrolled EGFR hyperactivity. A genetic profiling of [7]. Frequency of these ‘hotspot’ mutations is also higher HNSCC samples with EGFR activation revealed that in HPV-positive oropharyngeal cancers [50]. EGFR ligands (including TGFα)were highlyexpressed in Targeting PIK3CA alteration in human squamous cell a subset, suggesting an establishment of an autocrine loop xenografts has demonstrated susceptibility to treatment [39]. in vitro and in vivo, leading a path for its clinical impli- Alternatively, the function of target gene can be cation. Inhibition of PI3K by competitive blockage of bypassed by activating downstream molecules of the sig- ATP binding site led to decreased phosphorylation of naling cascade or switching dependence to an alternative Akt in several studies [51–54]. In a number of the pathway for cell growth and proliferation [40]. As the patient-derived xenografts harboring E545K and tumor progresses and develops genomic heterogeneity, H1047R mutations, PI3K inhibitors were effective in cells with genetic survival benefit outgrow through evo- achieving control of tumor growth [43, 55, 56]. Add- lutionary selection pressure. In consistent with this the- itionally, activation of PI3K/mTOR pathway from either ory, whole-exome sequencing of melanoma cells that are mutation or gene amplification was positively correlated resistant to BRAF inhibitor revealed diverse genetic al- with tumor susceptibility to PI3K inhibitors in xenograft terations in the downstream MAPK pathway [41]. Simi- models [52, 57–59]. However, preclinical data also sug- larly, KRAS amplification or mutation was found in gested that additional molecular change should interact tumor samples from colorectal cancer patients who de- with PIK3CA alteration for tumorigenesis. Cell lines veloped resistance to EGFR inhibitors [42]. Relevant to engineered to harbor PIK3CA mutations in the ‘hot- our review, compensatory activation of downstream spots’ responded more favorably to PI3K/mTOR dual in- pathway, mainly PI3K, has been proposed as one of the hibition than PI3K inhibition only, indicating that tumor major resistance mechanisms to EGFR inhibitors in survival is not strictly dependent on the activated PI3K HNSCC. Gene expression of the molecules in PI3K [60]. In a similar sense, PI3K inhibition demonstrated pathway was elevated in cetuximab-resistant strains markedly synergistic effect when combined with EGFR compared to cetuximab-susceptible cells [43], and or MEK inhibition [61]. Interestingly, PIK3CA activation addition of mTOR/PI3K inhibitor effectively achieved in HPV-positive HNSCC did not necessarily lead to in- control of cell growth in HNSCC that acquired resist- creased Akt target phosphorylation, but instead, led to ance to EGFR inhibitors [44, 45]. increased mTOR activity and showed more sensitivity to PI3K/mTOR dual inhibition than Akt inhibition [62]. This finding can be extended to more favorable efficacy PI3K-mTOR alteration in HNSCC of PI3K/mTOR inhibitors over Akt inhibitors in clinical 66% of HNSCC harbor genomic alterations in one of the settings [63]. major components of PI3K pathway [46]. An analysis of Locations of mutations affect PI3K structure and func- whole-exome sequencing of 151 HNSCC tumors re- tion, resulting in different responsiveness to inhibition and vealed that PI3K is the most commonly mutated mito- clinical outcome. Regulatory subunit p85 normally sup- genic pathway among PI3K, JAK/STAT and MAPK and presses catalytic function of p110 at resting stage. Conse- that presence of multiple mutations in PI3K signaling quently, C terminal truncation or internal deletion of p85 pathway is correlated with more advanced disease [8]. releases p110 from negative regulation and constitutively Physiologic data confirms that an aberrant PI3K-mTOR activates the PI3K pathway [64, 65]. Additionally, as fre- pathway is associated with cell motility, invasion and quently mutated E542 and E545 in p110 are located at a metastasis. PI3K-PTEN balance has a direct effect on distance from the kinase domain, it is plausible that muta- chemotaxis and cell motility as it controls actin cytoskel- tions at these spots alter regulatory control of p85. Indeed, eton via Rho family proteins, such as Rho, Rac and E545K mutation in the helical domain of p110 changes CDC42 [22, 47]. PIP3 and PIP2 determine epithelial po- acid-base charge and disrupts inhibitory interaction be- larity in individual cells, thus dysfunctional PI3K results tween p85 and p110 [66]. H1047R mutation in the kinase in epithelial-mesenchymal transition, a critical event in domain, on the other hand, shifts orientation of the resi- tumor invasion [48]. due and changes conformation of the two loops of kinase PI3KCA is among the most frequently mutated genes that contact cell membrane. This allows for kinase access in HNSCC, affected both in HPV-positive and negative to phospholipid that is less regulated by p85 [67]. diseases (56 and 34%, respectively) [7]. PIK3CA muta- Independently from p110, p85 as a monomer also tions in HPV-positive HNSCCs are concentrated in hel- down-regulates PI3K activation: p85 is naturally more ical domain, whereas mutations are more spread out in abundant than p110 and excess p85 monomers can se- HPV-negative diseases [9, 49]. TCGA data presents that questrate insulin receptor substrate 1 (IRS-1), an adaptor 73% of PIK3CA mutations are located at E542, E545 in molecule that mediates signal transduction between the helical domain and in H1047 in the kinase domain IGF-1R and downstream PI3K [68]. Thus, in wild-type Jung et al. Cancers of the Head & Neck (2018) 3:3 Page 5 of 13 cells, the p85 monomer competes with the p85-p110 4–6 months compared to 3–5 months in the placebo dimer for IRS binding and signal transduction. In het- plus paclitaxel group (p = 0.011) [77]. In this trial, com- erozygous knock out cells, the amount of p85 monomers parable proportions of the patients had a mutation in decreases more than p85-p110 dimers which up- PIK3CA, 11% and 13% in the buparlisib and control regulates the PI3K pathway [69]. However, in null cells, arm, respectively. Patients taking buparlisib also main- complete absence of regulatory subunit to stabilize p110 tained stable quality of life and demonstrated good toler- leads to significantly decreased signal transduction caus- ance to the treatment compared to the placebo group, as ing cell apoptosis [69]. Although not as frequent as in similar proportions of patients discontinued the treat- PIK3CA, mutations in PIK3R1 (encoding p85α) can be ment due to adverse effects [77]. However, this study found in 3% of HPV-positive HNSCC and 1% of HPV- failed to demonstrate significant improvement in overall negative HNSCC according to TCGA data [7]. survival (OS) with buparlisib partly because of insuffi- Alteration of PTEN tumor suppressor gene is among cient power. There are several ongoing clinical trials to the frequently found somatic mutations in human can- evaluate the efficacy and safety of buparlisib with or cers as well as germline mutations causing hereditary without additional therapy (Table 1). cancer syndromes. PTEN dephosphorylates PIP3 to PIP2, inhibiting mitogenic signal transduction in the PX-866 PI3K pathway. PTEN also interacts with PI3K, which PX-866 is an analog of wortmannin that irreversibly in- plays a key role in chemotaxis and tumor metastasis hibits class I PI3K by binding to Lys in ATP catalytic site [47, 48]. Clinical data has shown that loss of PTEN ex- [78]. Potent and irreversible binding of PX-866 enables pression is a poor prognostic marker in oral squamous sub-nanomolar IC values of 0.1, 1.0 and 2.9 nmol/L for cell cancer [70]. However, PTEN loss was found in only PI3Kα PI3Kγ and PI3Kδ, respectively, in contrast to a small number of HNSCC (8.16%), implying that it is a much higher IC of > 300 nmol/L for PI3Kβ [79]. In relatively minor component in PI3K pathway activation vivo studies revealed antitumor activities of PX-866 [8]. against human colon cancer, ovarian cancer and lung cancer xenografts [80]. It enhanced antitumor activities Targeting PI3K-Akt-mTOR pathway in clinic of cisplatin and radiation treatment in colon cancer and PI3K inhibitor ovarian cancer cells, respectively [80]. PX-866 also ef- Buparlisib (BKM120) fectively overcame resistance to EGFR inhibitor in hu- Buparlisib is an orally bioavailable pan-PI3K inhibitor, man lung cancer cells lacking expression of ErbB-3 [79]. targeting the ATP binding site of p110 kinase domain. PX-866 induced cessation of tumor growth in xenograft Its inhibitory potency is equitable on class IA isoforms models of human HNSCC which included one case of of p110α, β and δ, but slightly less against class IB p110γ PIK3CA gene amplification and another case of E545K [51]. An in vitro study demonstrated IC values for Akt [43]. However, clinical trials of PX-866 failed to show inhibition of 104 ± 18, 234 ± 47 and 463 ± 87 nmol/L promising results. In phase II clinical trials, combined for PI3Kα, β and δ, respectively [51]. Buparlisib is rapidly use of PX-866 with either cetuximab or docetaxel failed absorbed orally and its serum concentration increases to achieve improved PFS or OS compared to each treat- proportionately to dosage [71]. The molecule also pene- ment alone [81, 82]. trates blood brain barrier and administration of buparli- sib by gavage effectively controlled metastatic growth of Alpelisib (BYL719) human breast cancer in mouse brain [72]. Based on pre- Theoretically, a selective inhibitor of PI3Kα can achieve clinical data, its antitumor activity was also attributed to antitumor activity without affecting other isoforms of suppression of microtubular dynamics [73], and antian- PI3K, allowing for a more favorable side effect profile. giongenic effect [51]. A combination of buparlisib, Alpelisib was designed as a specific inhibitor of PI3Kα, cetuximab and radiation exerted a synergistic antiprolif- the product of frequently mutated PIK3CA [83]. The erative effect on human head and neck cancer cell lines molecule inhibits wild-type PI3Kα (IC = 4.6 nmol/L) as [74, 75]. In vivo, buparlisib inhibited PI3K activity in cell well as PI3Kα with common PI3KCA mutations, such as lines with wild-type PIK3CA as well as mutant form har- E545K or H1047R (IC = 4 nmol/L), more potently than boring any hotspot mutation of E542K, E545K or PI3Kδ (IC = 290 nmol/L) or PI3Kγ (IC = 250 nmol/L) 50 50 H1047R [76]. In a phase I dose-escalation study for ad- [52]. Preclinical data also suggested that PIK3CA mutation vanced solid tumors, most common side effects included makes cancer cells more vulnerable to PI3K inhibition by rash, abnormal hepatic function, alteration in glucose alpelisib. In vitro pharmacologic sensitivity screen among metabolism and fatigue [71]. In a recent randomized a broad panel of cancer cell lines revealed that sensitivity phase II trial with R/M HNSCC, adding buparlisib to to alpelisib was positively associated with the presence of paclitaxel improved progression-free survival (PFS) to PIK3CA mutation, amplification or copy number gain Jung et al. Cancers of the Head & Neck (2018) 3:3 Page 6 of 13 Table 1 Clinical trials evaluating PI3K or mTOR inhibitor in patients with HNSCC Agent Clinical Trial Other Targeted Additional Conditions Phase Status Identifier Agent Therapy PI3K inhibitor Alpelisib (BYL719) NCT02145312–– R/M HNSCC, failed to respond to II Not yet recruiting platinum-based therapy NCT02537223 – Cisplatin, Locoregionally advanced HNSCC, not I Active, recruiting radiation previously treated NCT01602315 Cetuximab – R/M HNSCC I/II Terminated (sponsor withdrawal) NCT02298595 Cetuximab Cisplatin HPV-associated oropharyngeal SCC I/II Not yet recruiting Buparlisib NCT01816984 Cetuximab – R/M HNC I/II Active, not recruiting (BKM120) NCT01737450–– Recurrent or progressive HNC II Active, recruiting NCT02113878 – Cisplatin, Locally advanced HNSCC I Active, recruiting radiation PX-866 NCT01252628 Cetuximab R/M HNSCC II Completed NCT01204099 Docetaxel Locally advanced or R/M HNSCC II Completed Copanlisib NCT02822482 Cetuximab – HNSCC with PI3KCA mutation/ I/II Active, recruiting amplification or PTEN loss INCB050465 NCT02646748 Itacitinib Pembrolizumab Advanced solid tumors I Active, recruiting mTOR inhibitor Sirolimus NCT01195922–– Advanced HNSCC, not previously I/II Completed treated Temsirolimus NCT01172769–– R/M HNSCC II Completed NCT01009203 Erlotinib – Advanced HNSCC, refractory to II Terminated (high patient platinum withdrawal rate) NCT01016769 – Paclitaxel, R/M HNSCC I/II Active, not recruiting carboplatin NCT02215720 Cetuximab – Advanced or metastatic solid tumors I Active, recruiting NCT00703625 – Docetaxel Resistant solid malignancies I Completed Everolimus NCT01332279 Erlotinib Radiation Recurrent HNC, previously treated with I Withdrawn (sponsor (RAD001) radiation withdrawal) NCT01313390 – Docetaxel R/M HNSCC I/II Terminated (lack of recruitment) NCT01009346 Cetuximab Cisplatin, R/M HNSCC I/II Terminated (toxicity) carboplatin NCT01051791–– R/M HNSCC II Active, not recruiting PI3K/mTOR dual inhibitor SF1126 NCT02644122–– R/M HNSCC II Active, recruiting Gedatolisib NCT03065062 Palbociclib – Advanced HNSCC I Active, recruiting Dactolisib NCT00620594–– Advanced solid tumors I Completed (BEZ235) PI3K/HDAC dual inhibitor CUDC-907 NCT02307240–– Advanced or relapsed solid tumors I Active, recruiting [84], which was confirmed by an in vivo study using sensitization to radiation [55]. Compared to HNSCC cell mouse models [52]. In a HNSCC cell line (Cal-33) and a lines with wild-type PIK3CA, cell lines with PIK3CA patient-derived xenograft model, both harboring H1047R H1047R mutation were more susceptible to antiprolifera- mutation in PIK3CA, administration of alpelisib using tive effect of alpelisib [56]. In another in vivo study, nanoparticles induced inhibition of tumor growth and PIK3CA mutation, regardless of its location, was the Jung et al. Cancers of the Head & Neck (2018) 3:3 Page 7 of 13 strongest predictive feature that correlated with favorable adjacent lymph nodes [95]. In a phase I trial of sirolimus response to alpelisib [52]. Compensatory hyperactivation and bevacizumab for patients with advanced malignan- of PIK3CA is one of the major mechanisms of treatment cies, no objective response was observed among the par- resistance, thus PI3K inhibitors are being tested with other ticipants with HNSCC [96]. However, among the targeted therapies, such as EGFR inhibitors. Inhibition of patients with stage II-IVA, untreated HNSCC, neoadju- PI3K with alpelisib enhanced tumor sensitivity to vant trial of sirolimus followed by definitive therapy (sur- cetuximab in HNSCC xenograft models [85]. A phase I gery or chemoradiation) demonstrated significant clinical trial of alpelisib combined with cetuximab in R/M responses (one CR, one PR and 14 SDs among 16 pa- HNSCC resulted in one partial response (PR), three tients) with good patient tolerance [97]. Sirolimus is unconfirmed PRs and five stable diseases (SDs) among known for poor bioavailability and low predictability of 32 cases with relatively good patient tolerance [86]. serum concentration after intestinal absorption, thus its PI3K activation statuswas unknowninthistrial.Ina narrow therapeutic window and a long half-life require more recent phase I trial of alpelisib, any of complete regular drug concentration monitoring [98]. Based on response (CR), PR or SD was achieved in 13 out of 19 these concerns, analogs of sirolimus have been developed study participants with PIK3CA-mutant HNSCC to improve pharmacokinetic properties. (NCT01219699) [87]. Copanlisib Temsirolimus Copanlisib is a potent inhibitor of class I PI3K with sub- Temsirolimus is a water-soluble analog of sirolimus and nanomolar IC . The molecule exhibits preferential ac- can be administered parenterally [99]. It undergoes hy- tivity against PI3Kα and PI3Kδ over PI3Kβ and PI3Kγ drolysis after administration to form sirolimus, but the (IC values of 0.5 and 0.7 nmol/L over 3.7 and 6. medication itself is also capable of inhibiting mTOR. 4 nmol/L, respectively) [57, 88]. It demonstrated super- Temsirolimus is currently FDA approved for the treat- ior inhibitory effect in cells with PIK3CA activating ment of advanced renal cell carcinoma [100]. Several pre- mutations over wild-type in breast cancer and non- clinical studies proved that a combination of temsirolimus small cell lung cancer xenografts [57]. Phase I trials in and cetuximab induces synergistic antitumor effect, as it patients with advanced or refractory solid tumors mitigates or prevents compensatory downstream mTOR presented good patient tolerance and evidence of dis- over-activation induced by EGFR inhibitor [101–105]. ease control [89, 90]. Efficacy and safety of combined There have been a number of phase I/II trials using tem- copanlisib and cetuximab for HNSCC is under study sirolimus in patients with HNSCC. In a phase I study of (NCT02822482). temsirolimus used with carboplatin and paclitaxel in R/M HNSCC, 22% of the patients exhibited objective PRs mTOR inhibitor [106]. The information regarding PI3K activation status Sirolimus (rapamycin) was lacking in this study. In TEMHEAD trial, a phase II Sirolimus was initially developed as an antifungal metab- study of temsirolimus in R/M HNSCC refractory to plat- olite, extracted from the bacterium Streptomyces hygro- inum and cetuximab, tumor shrinkage occurred in 39.4% scopicus [91]. However, since its immunosuppressive and of the patients mostly within the first six weeks of the antiproliferative properties were revealed, this macrolide treatment. However, no objective response was achieved, molecule has been more widely used for oncologic treat- nor did PI3KCA mutational status (H1048Y and G1050S) ment and for prevention of graft rejection or coronary predict treatment success [107]. In another trial including stent blockage. Sirolimus binds with FKBP12 (12 kDa a broad range of advanced malignancies, the combination FK506-binding protein) to form a gain-of-function com- of bevacizumab, cetuximab and temsirolimus was effective plex that function as an inhibitor of mTOR complex 1 in achieving PRs in 25% of the patients with HNSCC, but (mTORC1) [92]. This compound, as a result, inhibits a few patients were withdrawn from the trial because of metabolic alteration and cell proliferation which is trig- toxicities [108]. In this study, treatment-responders did gered by upstream gain-of-function mutations, such as not carry PIK3CA mutation in HNSCC cells. A trial com- PI3K and Akt. Sirolimus demonstrated antiproliferative bining temsirolimus with erlotinib for R/M HNSCC was activity in HNSCC cell lines inducing synergistic effect closed early due to toxicity and patient death [109]. In a with chemotherapeutic agents or radiation [93, 94]. In phase I pharmacokinetic study of temsirolimus, dose- HNSCC xenograft models with activated PI3K-Akt path- limiting toxicities occurred such as thrombocytopenia, way, administration of sirolimus induced marked inhib- stomatitis or mucositis, asthenia, manic-depressive syn- ition of tumor growth and cell apoptosis [58, 59]. It also drome and rash [110]. Thus, the treatment effect of tem- suppressed lymphangiogenesis in HNSCC xenograft sirolimus should be evaluated against potential toxicities models and prevented spread of the cancer cells to and more clinical trials are ongoing. Jung et al. Cancers of the Head & Neck (2018) 3:3 Page 8 of 13 Everolimus (RAD001) E545K or H1047R in PIK3CA as well as wild-type [124, Everolimus is a hydroxyethyl derivative of rapamycin, of- 125]. Gedatolisib also inhibited cell proliferation and in- fering improved oral bioavailability. The medication has creased radiosensitivity of human nasopharyngeal cancer a short half-life, allowing for quick establishment of cells with PI3K/mTOR hyperactivation [126]. Addition- stable status and improved drug safety [111]. After intes- ally, use of gedatolisib in EGFR inhibitor-resistant tinal absorption, everolimus is not converted to rapamy- HNSCC suppressed cell survival and induced apoptosis cin, instead forms a complex with FKBP12 and inhibits [45]. Phase I trials with gedatolisib for patients with ad- mTOR [112]. It is currently approved by the FDA for vanced cancer demonstrated potential antitumor activ- treatment of multiple malignancies including advanced ities with PRs and acceptable tolerance [127, 128]. breast cancer, kidney cancer, neuroendocrine tumor However, no apparent relationship between PIK3CA al- (NET) of pancreas, progressive NET of GI and lung, tu- teration and treatment response was observed in these berous sclerosis-associated renal angiomyolipoma and trials. There is an ongoing phase I trial of gedatolisib subependymal giant cell astrocytoma [113]. Although combined with palbociclib (CDK4/CDK6 inhibitor) for everolimus was effective in arresting tumor growth in advanced solid tumors including HNSCC (NCT0306 HNSCC xenograft models [114, 115], clinical data was 5062). not as encouraging. Several phase I studies demon- strated PRs among patients with HNSCC [116–119], but Dactolisib (BEZ235) the doses of everolimus used were different depending Dactolisib is an ATP-competitive dual inhibitor of PI3K on other treatments combined, such as platinum, doce- and mTOR, exerts more potency on PI3Kα,PI3Kδ,PI3Kγ taxel, cetuximab or radiation. Phase II trials with everoli- and mTOR (IC values of 4, 7, 5 and 21 nmol/L, respect- mus also failed to demonstrate clinical benefit for ively) than PI3Kβ (IC =75 nmol/L) [54, 129]. Dactolisib HNSCC. Either as monotherapy or combination with er- exhibited potent antiproliferative activity, halting cell cy- lotinib, treatment with everolimus was not successful in cles at G1 [54] and attenuating VEGF expression [129]. achieving objective response in patients with previously HNSCC cell lines with H1047R mutation were more sus- treated R/M HNSCC [120, 121]. There is a currently ac- ceptible to inhibition with lower IC whereas E545K con- 50, tive clinical trial testing everolimus monotherapy in pa- ferred only slightly increased sensitivity [60]. In clinical tients with R/M HNSCC (NCT01051791). settings, however, there has been little evidence to support drug efficacy and safety. When dactolisib was used for PI3K/mTOR dual inhibitor patients with castration-resistant prostate cancer or SF1126 everolimus-resistant pancreatic NET, the trials were dis- SF1126 is a peptide-conjugated prodrug of LY294002, with continued due to dose-limiting toxicities, such as stoma- improved water solubility and pharmacokinetics. RGDS titis, vomiting, diarrhea or hyperglycemia [130, 131]. conjugation enables the molecule to bind to specific integ- Combination of dactolisib and everolimus tested in pa- rins within the tumor, enhancing drug permeability [53]. tients with various advanced solid tumors, including one LY294002 is a pan-PI3K inhibitor, with IC values of case of HNSCC, failed to demonstrate objective response 720 nmol/L, 306 nmol/L, 1.33 μmol/L and 1.6 μmol/L for [132]. Another phase I trial of dactolisib treatment for PI3Kα,PI3Kβ, PI3Kδ and PI3Kγ respectively, and similar various, advanced solid tumors is now complete and the IC for mTOR (1.5 μmol/L) [53, 122]. In a phase I trial, result is being awaited (NCT00620594). SF1126 as a single agent was effective in maintaining stable diseases in patients with GIST and clear cell renal PI3K/HDAC dual inhibitor cancer, and in combination with rituximab decreased ab- CUDC-907 solute lymphocyte count and lymph node/spleen size in CUDC-907 is an orally administered inhibitor of class I CLL [123]. SF1126 monotherapy is now being evaluated PI3K isoforms and histone deacetylase (HDAC). IC values for treatment of R/M HNSCC (NCT02644122). for PI3Kα,PI3Kβ,PI3Kδ and PI3Kγ are 19, 54, 38 and 311 nmol/L, respectively [133]. Simultaneous inhibition of Gedatolisib PI3K and HDAC has demonstrated synergistic effect com- Gedatolisib is a potent and reversible inhibitor of class I pared to the combined level of growth suppression PI3K and mTOR. IC values for PI3Kα, PI3Kβ,PI3Kδ, achieved by single compound of HDAC inhibitor, vorino- PI3Kγ and mTOR are 0.4 nmol/L, 6 nmol/L, 8 nmol/L, stat, and PI3K inhibitor, GDC-0941 [133]. CUDC907 has 6 nmol/L and 10 nmol/L, respectively [124]. The inhibi- proved to be therapeutic against B cell lymphoma by de- tory activity against PI3Kα with hotspot mutations, such creasing MYC protein levels [134]. The effect of dual inhib- as E545K and H1047R, are comparatively low (0.6 nmol/ ition synergistically induced apoptosis of MYC-altered cells L and 0.8 nmol/L) [124]. Its antitumor activity was dem- in diffuse large B-cell lymphoma (DLBCL) [135]. For cancer onstrated in in vitro studies using mutant cells harboring cells that developed resistance to PI3K inhibition through Jung et al. Cancers of the Head & Neck (2018) 3:3 Page 9 of 13 alternative pathway activation, concurrent inhibition of in macrophages plays a key role in inducing immune sup- HDAC can down-regulate other signaling proteins and cir- pression by inhibiting NFκB pathway. Inhibition of PI3Kγ cumvent treatment resistance. This potential benefit of dual in macrophages, therefore, stimulated NFκB activation and inhibition was supported by an in vitro finding which dem- promoted an immunostimulatory transcriptional program, onstrated that administration of HDAC inhibitor success- restoring T cell activation. Another report suggests that fully overcame resistance to mTOR inhibitor in lymphoma PI3K-Akt pathway activation may mediate Tim-3 expres- cells [136]. An in vivo study has also revealed that dual in- sion in HNSCC, which is associated with more exhausted hibition of PI3K and HDAC can defeat cancer resistance to phenotype of tumor infiltrating lymphocytes, and cause re- platinum-based treatment by suppressing multidrug resist- sistance to immune checkpoint blockade [147]. However, ance transporters and DNA repairs [137]. The first phase I the role of PI3K pathway in cancer immunology needs to trial of CUDC-907 for the treatment of relapsed/refractory be clinically investigated further. Thereare phaseItrials of lymphoma achieved two CRs and three PRs in patients combining PI3Kδ inhibitor (INCB050465) with pembroli- with DLBCL [138]. There is an actively ongoing phase I zumab in advanced solid tumors (NCT02646748), and trial of CUDC-907 for the patients with advanced or re- combining PI3Kβ inhibitor (GSK2636771) with pembroli- lapsed solid tumors (NCT02307240), and another phase I zumab in advanced melanoma (NCT03131908). With re- trial for the patients with metastatic or locally advanced cent approvals of immune checkpoint inhibitors for the thyroid cancer (NCT03002623). treatment of R/M HNSCC, effects of adding PI3K inhibi- tors to immune checkpoint inhibitors will be further Inhibition of PI3K pathway and immune system explored. It has been well known that inhibitors of mTOR, such as sirolimus, modulate immune system. Clinically, they Conclusions have been used as immune suppressive agents to prevent PI3K plays a key role in the progression of HNSCC and rejection for patients who had undergone organ trans- development of resistance against cetuximab. Genomic plant. In fact, PI3K family controls many aspects of cell alterations affecting PI3K are common among both development, differentiation and function in both innate HPV-positive and HPV-negative diseases and serve as an and adaptive immune system [139]. Especially, PI3Kγ attractive target for the treatment of HNSCC. Early clin- and PI3Kδ are highly expressed in all subtypes of ical trials evaluating PI3K inhibitors have shown disap- leukocyte, and inhibition of PI3Kγ suppressed progres- pointing results, but further evaluation with more potent sion of breast cancer in an animal model by inhibiting agents and careful patient selection might lead to devel- tumor inflammation and myeloid cell-mediated angio- opment of effective PI3K inhibitors in HNSCC. In light genesis [140]. Furthermore, it has been revealed that of recent success of immune checkpoint inhibitors, po- PI3Kγ in macrophage has a critical role in the interplay tential impacts of PI3K inhibition on immune system between immune stimulation and suppression during in- should be considered in the future development of flammation or cancer development [141]. Class I PI3K PI3K-targeted therapy. signaling becomes activated by antigen receptors Abbreviations expressed by T and B cells, altering adaptive immune anti-PD1: anti-programmed death 1; BH: Breakpoint cluster region-homology; system. Therefore, inhibition of PI3Kδ dampens regula- CR: Complete response; DLBCL: Diffuse large B-cell lymphoma; tory T cells, enhances activity of cytotoxic T cells and in- EGFR: Epidermal growth factor receptor; Erk: Extracellular signal-regulated kin- ase; FKBP12: 12 kDa FK506-binding protein; GAP: GTPase activating protein; duces tumor regression as shown in animal models of GEF: Guanine nucleotide exchange factor; GPCR: G protein-coupled receptor; melanoma, lung cancer, thymoma and breast cancer HDAC: Histone deacetylase; HNSCC: Head and neck squamous cell [142]. Various mutations in genes encoding PI3Kδ may carcinoma; HPV: Human papillomavirus; IGF-1R: Insulin-like growth factor 1 receptor; IHC: Immunohistochemistry; IRS-1: Insulin receptor substrate 1; as well lead to immunodeficiency syndromes [143]. MEK: MAPK(mitogen-activated protein kinase)/Erk kinase; mTOR: mechanistic Immune checkpoint inhibitors such as anti-programmed target of rapamycin; mTORC1: mTOR complex1; NET: Neuroendocrine tumor; death 1 (anti-PD1) antibodies have demonstrated remark- OS: Overall survival; PDGFR: Platelet-derived growth factor receptor; PDK1: Phosphoinositide-dependent kinase 1; PD-L1: Programmed death- able activities in HNSCC [144, 145]. Interestingly, the level ligand 1; PFS: Progression-free survival; PH: Pleckstrin homology; of immune checkpoint ligands such as programmed death PI: Phosphatidylinositol; PI-3,4-P2: Phosphatidylinositol 3,4-bisphosphate; ligand 1 (PD-L1) appears to be regulated by the PI3K-Akt- PI3K: Phosphoinositide 3-kinase; PI-3-P: Phosphatidylinositol 3-phosphate; PIP: Phosphatidylinositol 4-phosphate; PIP2: Phosphatidylinositol 4,5- mTOR pathway: inhibition of PI3K, Akt or mTOR de- bisphosphate; PIP3: Phosphatidylinositol 3,4,5-triphosphate; PKB: Protein creased expression of PD-L1 in a non-small cell lung cancer kinase B; PR: Partial response; PTEN: Phosphatase and tensin homolog; model in vitro and in vivo [146]. Furthermore, combination Ral: Ras-like protein; Rheb: Ras homolog enriched in brain; SD: Stable disease; SH: Src homology; TCGA: The cancer genome atlas; TSC: Tuberous sclerosis of PI3Kγ blockade and immune checkpoint blockade with complex anti-PD1 therapy induced a synergistic growth inhibitory effect in animal models of both HPV-positive and negative Funding HNSCC [141]. In this study, the authors showed that PI3Kγ There was no financial support or funding for this review article. Jung et al. Cancers of the Head & Neck (2018) 3:3 Page 10 of 13 Authors’ contributions 17. Furman RR, Sharman JP, Coutre SE, Cheson BD, Pagel JM, Hillmen P, et al. KJ searched literature and RM compiled current clinical trials. KJ and HK Idelalisib and rituximab in relapsed chronic lymphocytic leukemia. N Engl J wrote the initial manuscript. RM and HK provided revisions. All authors read Med. 2014;370(11):997–1007. and approved the final manuscript. 18. 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Cancers of the Head & NeckSpringer Journals

Published: Jun 4, 2018

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