MEK inhibitor

PI3K and MEK inhibitor combinations: examining the evidence in selected tumor types

Carolyn D. Britten

Received: 14 January 2013 / Accepted: 8 February 2013 / Published online: 27 February 2013 ti Springer-Verlag Berlin Heidelberg 2013

Abstract The PI3K/AKT/mTOR and RAS/RAF/MEK/
ERK pathways are two of the most frequently dysregulated kinase cascades in human cancer. Molecular alterations in these pathways are implicated in tumorigenesis and resis- tance to anticancer therapies. The PI3K/AKT/mTOR and RAS/RAF/MEK/ERK pathways are known to interact with each other at several nodes, and mounting evidence sug- gests that dual blockade of both pathways may be required to achieve anticancer effects in certain contexts. This may include tumor types with a high frequency of RAS/RAF/
MEK/ERK pathway activation, or situations in which dual pathway strategies may be required to overcome resistance to current targeted therapies. Several clinical studies are currently evaluating the combination of PI3K and MEK inhibitors in a variety of different cancers with certain types of molecular alterations. This review will summarize existing knowledge of the PI3K/AKT/mTOR and RAS/
RAF/MEK/ERK pathways, the cross-talk between them, and the current generation of PI3K and MEK inhibitors that target them. The preclinical rationale for dual pathway inhibition will be discussed within the context of the major tumor types currently being explored in ongoing clinical trials, namely malignant melanoma with BRAF or NRAS mutations, and colorectal, ovarian, pancreatic, and basal- like breast cancers. The emerging clinical profile of PI3K and MEK inhibitor combinations, as reported in Phase I trials, will also be discussed.
Keywords BRAF mutation ti MEK inhibitor ti NRAS mutation ti PI3K inhibitor ti PI3K/AKT/mTOR pathway RAS/RAF/MEK/ERK pathway ti

Introduction

The PI3K/AKT/mTOR and RAS/RAF/MEK/ERK path- ways are two of the most frequently dysregulated kinase cascades in human cancer [1, 2]. Both pathways represent important signal transduction mechanisms that facilitate the proliferation and survival of cancers driven by growth factor receptors, such as human epidermal growth factor receptor 2 (HER2) or epidermal growth factor receptor (EGFR). The individual downstream components of these signaling cascades are also known to be frequently altered in cancer, either through somatic mutation or epigenetic modification, thus contributing to tumorigenesis and resistance to anticancer therapies [3]. Our understanding of both pathways and their role in cancer continues to grow, and agents that target specific components of these path- ways have recently been approved for use in certain tumor types, with many more agents in clinical development.
The PI3K/AKT/mTOR and RAS/RAF/MEK/ERK pathways are known to interact with each other at several nodes, and mounting evidence suggests that dual blockade of both pathways may be required to achieve anticancer effects in certain contexts. Several clinical studies are currently evaluating the particular combination of PI3K and MEK inhibitors in a variety of different cancers with

C. D. Britten (&)
Division of Hematology/Oncology, Department of Medicine, Hollings Cancer Center, Medical University of South Carolina, 96 Jonathan Lucas Street, Clinical Science Building, Suite 903, Charleston, SC 29425, USA
e-mail: [email protected]
certain types of molecular alterations. This review will summarize existing knowledge on the PI3K/AKT/mTOR and RAS/RAF/MEK/ERK pathways, the cross-talk between them, and the current generation of investigational agents that target them. The preclinical rationale for dual

pathway inhibition will then be discussed within the con- text of the major tumor types currently being explored in ongoing clinical trials.

The PI3K/AKT/mTOR pathway

The PI3K protein family comprises three different classes of lipid kinase, of which Class IA has been most frequently implicated in human cancer [2, 4, 5]. Class IA PI3Ks are heterodimers that consist of a regulatory and a catalytic subunit. Several different isoforms of the catalytic (p110a, p110b, and p110d) and the regulatory [p85a, p55a, p50a (all splice variants of the same gene), p85b, and p55c] subunits have been described. In particular, amplification or mutation of PIK3CA (which encodes the p110a catalytic subunit) has frequently been described in a broad range of human cancers.
The PI3K/AKT/mTOR kinase cascade is triggered by receptor tyrosine kinases (RTKs) and G-protein-coupled receptors (GPCRs) located at the cell surface (Fig. 1). When these receptors bind their extracellular ligands and become phosphorylated, they sequester the regulatory subunit of PI3K, leaving the catalytic subunit free to catalyze the phos- phorylation of phosphatidylinositol 4,5-bisphosphate (PIP2) to phosphatidylinositol 3,4,5-trisphosphate (PIP3). PIP3 is an important propagator of intracellular signaling, and its pro- duction is antagonized by the tumor suppressor phosphatase and tensin homolog (PTEN), which has been found to be frequently silenced or inactivated in a range of cancers. PIP3 directly binds the pleckstrin homology domains of various proteins, including phosphoinositide-dependent kinase 1 (PDK1) and AKT. PDK1 binds PIP3 at the plasma membrane and phosphorylates AKT, which is then free to phosphorylate a multitude of targets in the nucleus and cytoplasm.
AKT promotes cell survival by phosphorylating MDM2 (a negative regulator of the p53 tumor suppressor) and by neg- atively regulating the pro-apoptotic Bcl-2 family members BAD and BAX, and forkhead transcription factors, such as FOXO. Activated AKT also negatively regulates tuberous sclerosis protein complex 1 (TSC1) and TSC2, which leads to activation of mTOR complex 1 (mTORC1), a key regulator of cellular growth and protein synthesis. mTORC1 stimulates S6 kinase activity, which has multiple substrates, including the ribosomal protein S6. mTORC1 also phosphorylates factor 4E-binding protein 1 (4E-BP1), releasing eukaryotic initiation factor 4E (eIF4E) to permit assembly of the cap- binding complex, initiating translation of specific mRNAs.

The RAS/RAF/MEK/ERK pathway

The RAS/RAF/MEK/ERK pathway is triggered by RTKs, GPCRs, and cytokine receptors [1, 6]. RAS, the key

controller of the pathway, comprises a family of four small GTPases: HRAS, NRAS, and two forms of KRAS (KRAS4A and KRAS4B), which arise through alternative splicing. RAS proteins are recruited to ligand-bound receptors at the plasma membrane via activation of the Grb2/SOS coupling complex; however, this process is dependent on RAS undergoing post-translational modifi- cation, including farnesylation, geranylgeranylation, or palmitoylation. Activation of RAS, via the exchange of GDP for GTP, allows the recruitment of the serine/threo- nine RAF to the plasma membrane.
The RAF family of proteins comprises three members: ARAF, BRAF (which is frequently mutated in certain cancers), and CRAF (RAF-1). The regulatory control of these kinases is highly complex, involving dimerization, phosphorylation at multiple sites, and further interactions with other proteins. Activated RAF phosphorylates MEK, a dual-specificity tyrosine and serine/threonine kinase, which has the predominant role of activating the serine/
threonine kinases ERK1 and ERK2. ERKs have over 160 different targets and directly phosphorylate cytoplasmic proteins, such as p90 ribosomal S6 kinase (RSK), and many transcription factors, including ETS-1, c-JUN, and c-MYC. Moreover, ERKs are known to have key roles in the control of cell cycle regulation, proliferation, and survival.

PI3K/AKT/mTOR and RAS/RAF/MEK/ERK pathway cross-talk

The PI3K/AKT/mTOR and RAS/RAF/MEK/ERK path- ways are known to interact at multiple points, resulting in cross-activation, cross-inhibition, and pathway conver- gence (Fig. 1) [7]. First, and most importantly, RAS is capable of directly activating PI3K through a mechanism that is independent of the p85 regulatory subunit [8]. Cross- activation between the two pathways also occurs through ERK, which phosphorylates TSC2 at sites distinct from those phosphorylated by AKT [9]. Phosphorylation by either ERK or AKT suppresses TSC2, thus promoting activation of mTORC1.
Under certain circumstances, cross-inhibition between the PI3K/AKT/mTOR and RAS/RAF/MEK/ERK pathways can occur; for example, AKT can phosphorylate the reg- ulatory domain of RAF in the setting of strong insulin-like growth factor 1 (IGF1) stimulation, thus inhibiting RAF activity [10]. Similarities between certain pathway com- ponents can also result in pathway convergence. RSK, AKT, and S6K are all AGC kinases (named after the protein kinase A, G, and C families), and as such, they are sometimes capable of phosphorylating the same motif within the same protein [7]. This, and many other

Fig. 1 PI3K/AKT/mTOR and RAS/RAF/MEK/ERK pathways. 4E- BP1 factor 4E-binding protein 1, AKT (PKB) protein kinase B, BAD Bcl-2-associated death promoter, BAX Bcl-2-associated X protein, COT cancer Osaka thyroid, eIF4E eukaryotic initiation factor 4E, ERK extracellular signal-regulated kinase, FOXO forkhead box O, GRB2 growth factor receptor-bound 2, IGF insulin-like growth factor, IRS-1 insulin receptor substrate 1, MEK mitogen-activated protein

kinase/ERK kinase, mTORC mammalian target of rapamycin com- plex, PI3K phosphatidylinositol 3-kinase, PIP2 phosphatidylinositol 4,5-bisphosphate, PIP3 phosphatidylinositol 3,4,5-trisphosphate, PTEN phosphatase and tensin homolog, RAF rapidly accelerated fibrosarcoma, RHEB Ras homolog enriched in brain, RSK ribosomal S6K, RAS rat sarcoma, SOS son of sevenless, S6K S6 kinase, TSC tuberous sclerosis protein complex

mechanisms of cooperation between the two pathways, is summarized elsewhere [7, 11].
The network of cross-talk between the PI3K/AKT/
mTOR and RAS/RAF/MEK/ERK pathways is complex, and the relative importance of each interaction under physiologic conditions may vary from observations made in vitro. Pathway signaling and cross-talk may also vary across different tissue or tumor types, and can also depend on the relative strengths of different growth factors, and/or other pathway inputs. Data from preclinical experiments and retrospective studies suggest that tumors can utilize redundancy in signaling as a means of developing resis- tance to anticancer agents that target upstream RTKs or components of the pathways themselves.

Current-generation pathway inhibitors

The observation that the PI3K/AKT/mTOR and RAS/RAF/
MEK/ERKpathwaysarefrequentlyactivatedincancer,andthat componentsofthesepathwaysarefrequentlymutatedoraltered, has driven the development of an expanding range of small- molecule inhibitors that target various nodes of both pathways.

PI3K/AKT/mTOR pathway inhibitors

Inhibitors of the PI3K/AKT/mTOR pathway include the first-generation agent rapamycin and second-generation agents everolimus and temsirolimus, all of which allos- terically inhibit the mTORC1 complex by interacting with FK-binding protein 12 (FKBP12) [2, 12]. Although these second-generation agents have been proven effective in certain tumor types, it has been speculated that their broader efficacy may be limited by their inability to inhibit mTORC2 (an activator of AKT) and the resulting abro- gation of negative feedback loops that occur when mTORC1 activity is inhibited [12, 13]. The current gen- eration of PI3K inhibitors was designed with the hope of avoiding the issues of negative feedback seen with mTORC1 inhibitors, by targeting the pathway further upstream.
In contrast to mTORC1 inhibitors, which allosterically inhibit their target, current-generation PI3K inhibitors bind to the ATP-binding pocket of the catalytic domain of PI3K [13]. mTOR and PI3K share structural similarities, and as a consequence, many of the first PI3K inhibitors to be inves- tigated have been dual inhibitors of Class IA PI3K and both

mTOR complexes [13]. It has been speculated that dual PI3K/mTOR inhibition could offer advantages over selec- tive inhibition of PI3K or mTOR, by virtue of more complete blockade of the pathway [12]. However, the identification of more specific inhibitors has continued to be pursued due to the possibility of improved toxicity profiles, which could enable higher therapeutic doses to be given, especially when combined with other therapies [12, 13].
Advances in drug design led to the development of ‘‘pure’’ PI3K inhibitors, which selectively target the cata- lytic sites of Class I PI3K only [12, 13]. Isoform-specific PI3K inhibitors have also recently emerged, including agents that target p110a, the catalytic subunit isoform encoded by PIK3CA, which is most commonly implicated in solid cancers, and p110d, an isoform of importance in chronic lymphocytic leukemia and indolent non-Hodgkin’s lymphoma. PI3K inhibitors in current clinical development are outlined in Table 1A. While the clinical data are in evolution, common toxicities noted to date include gas- trointestinal effects (diarrhea, nausea, vomiting, anorexia), rash, hyperglycemia, transaminase elevation, fatigue, and pneumonia [12, 13]. Reversible mood alteration has been observed with at least one agent (BKM120) and may be related to the effects of PI3K on neurocognitive dysfunc- tion [14]. Properties of individual agents are reviewed in detail elsewhere [12, 13].

RAS/RAF/MEK/ERK pathway inhibitors

Early attempts at targeting the RAS/RAF/MEK/ERK pathway focused on inhibiting RAS by preventing its activation via farnesylation. Unfortunately, these farnesyl- transferase inhibitors proved disappointing in the clinic, due to the realization that RAS could still be activated through geranylgeranylation [15]. The observation that BRAF is frequently mutated in several human cancers (namely melanoma, colorectal cancer, thyroid cancer, and low-grade serous ovarian cancer) led to the development of BRAF-specific inhibitors, and one such inhibitor, vemu- rafenib, is approved for use in late-stage melanoma with BRAF V600E mutation [16]. Inhibition of MEK provides another approach, with potential applications for malig- nancies with alterations in BRAF and NRAS.
MEK inhibitors in clinical development are outlined in Table 1B. Common toxicities include rash and/or dermatitis acneiform, diarrhea, peripheral edema, and fatigue [17, 18]. Ocular events, particularly blurred vision, central serous retinopathy, and retinal vein occlusion, are considered class effects [18]. Asymptomatic reversible ventricular dysfunc- tion has also been observed in a small percentage of patients [17]. Properties of individual agents are available in clinical trial results cited throughout this review.

Rationale for dual pathway inhibition

Interaction between the parallel PI3K/AKT/mTOR and RAS/RAF/MEK/ERK pathways may explain the modest single-agent activity of agents that target these pathways. Aside from the encouraging results with the PI3Kd-specific inhibitor CAL-101 (now GS-1101) in indolent non-Hodg- kin’s disease [19], early-phase single-agent trials of PI3K inhibitors have frequently demonstrated cytostatic rather than cytotoxic clinical activity, with stable disease but few partial responses in patients with advanced solid tumors [2, 12–14, 20–24]. Regarding MEK inhibitors, although BRAF-mutant disease has demonstrated sensitivity [17], studies in more heterogeneous patient populations have only shown modest clinical efficacy [18, 25–28].
In tumors driven by activated or amplified RTKs (such as EGFR-mutant lung cancer or HER2-amplified breast cancer), oncogenic signaling can potentially take place via either or both pathways [29, 30]. Single-agent inhibition of PI3K or MEK in these tumor types has been shown to lead to compensatory increases in ERK or AKT activity, respectively [29–31]. Many Phase II/III clinical trials with PI3K, mTOR, or MEK inhibitors are currently examining inhibitors as combination therapy with receptor-targeted therapies [32]. However, different tumor types exhibit different mechanisms of resistance to receptor-targeted therapies, suggesting that the addition of PI3K or MEK inhibitors will not always be capable of reversing resis- tance to receptor-targeted therapies [33]. In these circum- stances, combined PI3K and MEK inhibition may ensure the blockade of downstream proliferation, growth, and survival signals in tumors that develop resistance to RTK- directed therapy.
Not all cancers demonstrate expression or activation of a known receptor that can be therapeutically targeted. RAS is capable of signaling through both the PI3K/AKT/mTOR and RAS/RAF/MEK/ERK pathways, and activating muta- tions in RAS are generally recognized as one of the most frequently observed alterations in cancer, being found in up to approximately 30 % of all cases [34]. Preclinical studies regarding the use of KRAS mutations as a predictive marker for resistance to single-agent PI3K inhibitors have yielded mixed results [35–38], whereas in the clinic, at least one response with single-agent PI3K inhibitor therapy has been observed in a patient with KRAS-mutant triple-negative breast cancer [14]. Although MEK inhibitors have dem- onstrated preclinical activity in some RAS-mutant cancers, sensitivity to MEK inhibitors is likely to be dependent on additional factors, such as tumor type, or the presence of coexisting molecular alterations, such as PIK3CA muta- tion, PTEN loss, or the activation of compensatory path- ways other than PI3K/AKT/mTOR [37, 39–42].

Table 1 PI3K and MEK inhibitors currently in clinical development

Agent

A.PI3K inhibitors

Company

ROA Target

Phase of development

BKM120 Novartis Oral Class I PI3K Phase III

GS-1101
Gilead Sciences Oral Isoform specific
(PI3Kd)
Phase III

BAY 80-6946 Bayer IV Class I PI3K Phase II
BEZ235 Novartis Oral PI3K/mTORC1/2 Phase II
GDC-0941 Genentech Oral Class I PI3K Phase II
PF-04691502 Pfizer Oral PI3K/mTORC1/2 Phase II
PF-05212384 Pfizer IV PI3K/mTORC1/2 Phase II
PX-866 Oncothyreon Oral Class I PI3K Phase II

BYL719
Novartis
Oral Isoform specific
(PI3Ka)
Phase II

GDC-0980 Genentech Oral PI3K/mTORC1/2 Phase II
SAR245408 (XL147) Sanofi Oral Class I PI3K Phase I/II
SAR245409 (XL765) Sanofi Oral PI3K/mTORC1/2 Phase I/II

GSK2126458
B.MEK inhibitors
GlaxoSmithKline Oral PI3K/mTORC1/2 Phase I

Trametinib (GSK1120212) GlaxoSmithKline Oral MEK1/2 Phase III
Selumetinib (AZD6244) AstraZeneca Oral MEK1/2 Phase II
GDC-0973 (XL518) Genentech Oral MEK1/2 Phase I/II
BAY 86-9766 Bayer Oral MEK 1/2 Phase I/II

Pimasertib (AS703026/
MSC1936369B)
Merck Oral MEK 1/2
Phase I

PD325901 Pfizer Oral MEK1/2 Discontinued as a single
agent but under

IV intravenous, ROA route of administration
Source: ClinicalTrials.gov

CI-1040 (PD184352)

Pfizer

Oral MEK1/2
evaluation in combination with PI3K inhibitors
Discontinued

Since RAS can potentially signal through both the PI3K/
AKT/mTOR and RAF/MEK/ERK pathways, combined inhibition of both pathways may be necessary to affect a clinical response in certain tumor types or in tumors with alterations in both pathways. The following sections dis- cuss preclinical evidence and the rationale for PI3K plus MEK inhibitor combinations in selected cancers with a high frequency of RAS/RAF/MEK pathway activation, or in which dual pathway strategies may be required to overcome resistance to current targeted therapies.

Rationale for PI3K plus MEK inhibitor combinations in selected cancers

Melanoma

BRAF-mutant melanoma

A total of 40–60 % of cutaneous melanomas harbor a BRAF mutation, and 90 % of these are BRAF V600E [16, 43, 44].

Vemurafenib demonstrates an impressive 80 % response rate in patients with BRAF V600E-positive melanoma [16, 43], and its introduction represented an important break- through in this disease. Although the activity of vemurafenib in patients with sensitizing mutations has been extremely encouraging, the duration of response is limited and tumors quickly develop resistance via additional molecular altera- tions in other pathway components [45].

Resistance to BRAF inhibitors Preclinical data and tumor genomic profiling suggest that resistance to BRAF inhibi- tors may arise through several mechanisms, including increased expression or aberrant splicing of BRAF [46, 47], increased expression of CRAF [48], increased expression of COT [49], activating mutations in NRAS [50, 51], activating mutations in MEK1 [52], and activation or altered expression of RTKs, such as platelet-derived growth factor receptor b (PDGFRb) [51].
Current-generation MEK inhibitors have shown some promise in the treatment of BRAF-mutant melanoma [17, 53, 54]. When compared with chemotherapy, the MEK

inhibitor trametinib improved progression-free survival and overall survival in patients with V600E or V600K BRAF- mutant metastatic melanoma [17]. Furthermore, preclinical experiments with dabrafenib and trametinib suggest that BRAF and MEK inhibitor combinations may be capable of overcoming resistance to BRAF inhibitors caused by cer- tain mutations in NRAS or MEK [55]. In addition, the combination of a MEK inhibitor with a BRAF inhibitor may prevent BRAF inhibitor-induced tumorigenesis in latent keratinocytes [56, 57]. A randomized Phase II trial in metastatic BRAF V600-mutant melanoma compared the BRAF inhibitor dabrafenib in combination with trametinib versus single-agent dabrafenib [58]. Combination therapy resulted in a significant improvement in progression-free survival (median 9.4 vs. 5.8 months for dabrafenib mono- therapy) and a trend toward decreased incidence of cuta- neous squamous cell carcinoma [58].
The heterogeneous range of resistance mechanisms observed in BRAF inhibitor-refractory cancer cells implies that additional inhibitor combinations beyond those of BRAF plus MEK inhibitors may be required to fully address the problem of BRAF inhibitor-refractory disease [50]. Activation of the PI3K/AKT/mTOR pathway, through RTK activation, alterations in PI3K pathway components, or alterations in RAS, represents one poten- tially important route of resistance and could limit the effectiveness of therapies solely targeting the RAS/RAF/
MEK/ERK pathway.

Preclinical evidence with PI3K and MEK inhibitor com- binations Evidence supporting the use of PI3K and MEK inhibitor combinations in vemurafenib-resistant BRAF- mutant melanoma comes from several preclinical experi- ments. In vitro experiments suggest that resistance to BRAF inhibitors may confer cross-resistance to MEK inhibitors [55, 59]. BRAF-mutant melanoma cell lines resistant to vemurafenib and the MEK inhibitor, selumet- inib, were shown to be dependent upon the induction or persistence of AKT signaling for survival [59]. In these cell lines, the addition of an AKT inhibitor was able to over- come the resistance to single-agent MEK inhibition [59]. Similarly, in vitro generation of dabrafenib-resistant BRAF V600E cell lines, arising through MEK or NRAS muta- tions, was resistant to single-agent trametinib but showed enhanced cell growth inhibition when combined with the PI3K inhibitor GSK2126458 [55].
Vemurafenib-resistant BRAF V600E melanoma cell lines with PDGFRb upregulation showed dual activation of ERK and AKT in vitro [60]. Although the combination of vemurafenib and the PI3K/mTOR inhibitor BEZ235 was capable of synergistically inhibiting growth, antiapoptotic effects were not observed. However, the combination of selumetinib and BEZ235 was found to consistently trigger

apoptosis in all cell lines investigated [60]. Lastly, in BRAF-mutant melanoma cells chronically treated with the BRAF inhibitor SB-590885, resistance was found to occur through the upregulation of alternative RAF isoforms [61]. Single-agent MEK inhibition in these cells was found to have cytostatic rather than cytotoxic effects, and resistance was associated with increased IGF-1R and pAKT levels, which were also identified in a post-relapse tumor sample [61]. Combined treatment with selumetinib and an IGF-1R or PI3K inhibitor was able to overcome this resistance mechanism in vitro, leading to apoptosis [61].

NRAS-mutant melanoma

Around 15–20 % of cutaneous melanomas harbor a mutation in NRAS [62, 63], and these alterations are generally not found to coexist with BRAF mutations [62, 63]. There are currently no approved drugs that directly target altered NRAS, and the treatment of melanomas with these alterations represents an unmet medical need.
NRAS-mutant melanoma cell lines have been shown to be partially sensitive to MEK inhibition [64]. However, single-agent MEK inhibitors have shown disappointing activity against NRAS-mutant melanoma in the clinic. In a Phase I trial of the MEK inhibitor selumetinib in patients with advanced malignancies, one patient with uveal NRAS-mutant melanoma plus renal cancer had stabiliza- tion of disease for 88 weeks [65]. A randomized Phase II trial of single-agent selumetinib versus temozolomide showed no difference in progression-free survival in 200 unselected melanoma patients [66]. In this trial, 10 patients with NRAS-positive melanoma were treated with selu- metinib, and none responded. Lastly, a Phase II study with the MEK inhibitor MEK162 demonstrated a response rate of 8 % among 24 patients with NRAS-mutant melanoma [53].
Since RAF has multiple downstream pathways, combi- nation therapy with PI3K and MEK inhibitors may be more effective than single-agent strategies in treating RAS- mutant melanoma. As an example, genetically engineered mouse models of RAS-mutant melanoma demonstrated resistance to both selumetinib and the BEZ235 mono- therapy; however, the combination of both agents was able to induce tumor regression and increase survival [67].

Colorectal cancer

The anti-EGFR antibodies cetuximab and panitumumab are active as single agents in chemorefractory metastatic colorectal cancer; however, these therapies are only effective in patients with KRAS wild-type tumors [68, 69]. KRAS mutations are observed in 35–40 % of colorectal cancers, while NRAS and HRAS mutations are less

common (1–3 %) [70]. In addition to causing primary resistance to anti-EGFR therapy, the emergence of KRAS mutations has also been associated with acquired resistance to these agents [71]. BRAF mutations occur in 5–15 % of colorectal cancers, but are mutually exclusive with KRAS mutations in this disease [70, 72, 73]. Although BRAF mutations do not appear to predict for resistance to ce- tuximab, they do predict for poor prognosis [72, 74].
Since perturbations in the RAS/RAF/MEK/ERK path- way are frequent in colorectal cancer, therapies targeted against this axis are urgently required. Unfortunately, efforts to treat advanced colorectal cancer with MEK inhibitors have thus far been disappointing. A Phase II trial with the first-generation MEK inhibitor CI-1040 identified an amenable safety profile, but very little antitumor activity in a heterogeneous patient population that included 20 patients with advanced colorectal cancer [26]. While CI- 1040 may be an inferior drug compared with the next generation of MEK inhibitors, selumetinib (second-gener- ation MEK inhibitor) has not performed well in unselected metastatic colorectal cancers either. A Phase II trial with selumetinib in patients with advanced colorectal cancer demonstrated similar efficacy to capecitabine in patients who had progressed after one or two prior lines of che- motherapy [27].
Experiments suggest that RAS/RAF wild-type cell lines are resistant to MEK inhibitors, whereas RAF-mutant, and to a lesser extent RAS-mutant, cell lines are sensitive [64]. In KRAS-mutant colorectal cancer cells, resistance to MEK inhibitors may arise through a variety of mecha- nisms, including amplification of the oncogene [75, 76], upregulation of the Wnt signaling pathway [77], or through coexisting mutations in PIK3CA, and/or activation of the PI3K pathway [39, 40, 78, 79]. Importantly, PIK3CA mutations occur in about 15 % of colorectal cancers [70, 80], and two large series with a combined number of over 1,200 patients demonstrated that 8–9 % of colorectal can- cers harbor mutations in both PIK3CA and KRAS [70, 80].
Preclinical data indicate that combined MEK and PI3K inhibition will be required to instigate or maintain a pro- apoptotic effect in KRAS-mutant colon cancer with alter- ations in the PI3K pathway. The HCT116 cell line, which harbors mutations in both KRAS and PIK3CA, was resis- tant to the PI3K inhibitors WAY-266176 and WAY- 266175; however, combined therapy with the MEK inhibitors CI-1040 or UO126 led to growth suppression and apoptosis [81]. In other investigations with the KRAS/
PIK3CA-double-mutant HCT116 and DLD-1 cell lines, selective knockout of mutant PIK3CA conferred sensitivity to the MEK inhibitor PD0325901 [40, 78], whereas exog- enous re-expression of mutant PIK3CA restored resistance [78]. In addition, murine xenografts of the HCT116 cell line were resistant to single-agent MEK inhibition

(PD0325901); however, xenografts engineered with knockout of mutant PIK3CA demonstrated sensitivity [40, 78]. Similar increased antitumor activity has been observed with combined MEK and AKT inhibition in KRAS/
PIK3CA-double-mutant HCT15 and BRAF/PIK3CA-dou- ble-mutant RKO colon cancer models [39, 78].
Recent evidence suggests that the mechanism of synergy between PI3K and MEK inhibitors in colorectal cancer cells can be modulated by mTOR inhibition [82]. The pan- PI3K inhibitor GDC-0941 demonstrated greater synergy than the dual PI3K/mTOR inhibitor BEZ235 when com- bined with the MEK inhibitors selumetinib and PD0325901 in BRAF/PIK3CA-double-mutant HT29 and KRAS/
PIK3CA-double-mutant HCT116 colorectal cancer cell lines [82]. The addition of the mTORC1/2 inhibitor KU0063794 impaired the synergistic effect of GDC-0941 with MEK inhibitors. This suggests that different types of PI3K inhibitors should be evaluated as part of combination therapy with MEK inhibitors [82].
Despite the strong preclinical rationale for PI3K and MEK inhibitor combinations in colorectal cancer, some studies suggest that combined blockade of both pathways may not consistently lead to durable responses or regressions in this tumor type [83]. Migliardi et al. [83] recently reported on an analysis of 40 different patient-derived murine xeno- grafts of metastatic colorectal cancer. In these models, the combination of BEZ235 and selumetinib demonstrated greater rates of disease stabilization than monotherapy (disease control rates were 70 % for combined therapy, and 27.5 and 42.5 % for selumetinib and BEZ235, respectively); however, substantial tumor regression was not observed [83]. These findings could suggest that PI3K and MEK inhibitor combinations may not be capable of durable responses in patients with KRAS-mutant colorectal cancer and that additional biomarkers may be required to identify those patients who will respond best [83].

Ovarian cancer

Ovarian cancer is a heterogeneous disease comprising different histologic subtypes that are characterized by fre- quent loco-regional dissemination to the ovary and related pelvic organs; as such, a considerable proportion of tumors classified as ‘‘ovarian cancer’’ may not arise from ovarian tissue [84]. The current model for epithelial ovarian car- cinogenesis defines two broad categories, designated type I and type II [85]. Type I tumors are typically low-grade neoplasms that develop from borderline tumors. Type I tumors include low-grade serous carcinoma, mucinous carcinomas, endometrioid carcinomas, malignant Brenner tumors, and clear-cell carcinomas [86]. The type I molec- ular signature is characterized by frequent BRAF and KRAS mutations, and wild-type p53 and BRCA1/2 [85].

Type II tumors are typically high-grade tumors that have no recognizable precursor lesion. Type II tumors include high-grade serous carcinoma (the most prevalent epithelial ovarian cancer), malignant mixed mesodermal tumors (carcinosarcoma), and undifferentiated carcinomas [86]. The type II molecular signature is characterized by mutated p53, dysregulated BRCA1/2, and wild-type BRAF and KRAS [85].
Low-grade serous ovarian cancers are slow growing but very resistant to standard-of-care platinum therapies [85]. As the prototypical type I epithelial ovarian cancer, they are associated with a high rate of KRAS and BRAF mutation, implying that MEK inhibitors could be a relevant strategy for this specific subtype of tumor. In a Phase II trial of single-agent selumetinib in 52 patients with low- grade serous ovarian carcinoma, eight patients (15.4 %) had complete or partial responses, and 34 patients (65.4 %) had stable disease [87]. Although 41 % of evaluable patients had KRAS mutation, this was not found to predict response to MEK inhibition [87]. Due to the significant cross-talk between the RAS/RAF/MEK/ERK and PI3K/
AKT/mTOR pathways, the addition of a PI3K inhibitor could potentiate single-agent MEK inhibition in low-grade serous ovarian cancer.
Combined therapy with PI3K and MEK inhibitors could also play a role in the treatment of ovarian cancer subtypes that harbor abnormalities in the PI3K/AKT/mTOR path- way. Clear-cell carcinomas have a PIK3CA mutation rate of approximately 50 % and a PTEN deletion rate of about 20 %, whereas low-grade endometrioid carcinomas have a PIK3CA mutation rate of 20 % and PTEN mutation rate of 20 % [85]. PI3K and MEK inhibitors have shown synergy in preclinical experiments with ovarian cancer models harboring abnormalities in the PI3K/AKT/mTOR pathway. For example, in a KRAS-mutant/PTEN deletion mouse model of epithelial ovarian cancer, the PI3K/mTOR inhibitor, PF-04691502, did not induce tumor regression or long-term growth inhibition. However, the combination of PF-04691502 and PD0325901 instigated striking tumor regression and observable pro-apoptotic effects [88].
Type II tumors are not generally associated with base- line alterations in the PI3K/AKT/mTOR or RAS/RAF/
MEK/ERK pathways, and for these high-grade tumors, other strategies are more likely to be successful.

Pancreatic cancer

Pancreatic cancer is characterized by an extremely high frequency of KRAS mutation (50–80 % of cases) [89, 90], and preclinical experiments have confirmed that KRAS expression is required for oncogenesis and tumor mainte- nance [37, 91]. In KRAS-driven mouse models of pan- creatic cancer, signaling appears to predominantly occur

via the RAS/RAF/MEK/ERK pathway, with pERK expression readily detectable and pAKT expression com- paratively low [37]. Pancreatic cancer xenografts were sensitive to MEK inhibition with selumetinib and relatively resistant to PI3K inhibition with GDC-0941, whereas the combination of selumetinib and GDC-0941 demonstrated synergistic antitumor effects [37]. In vitro models of pan- creatic cancer have demonstrated that dual inhibition of the PI3K/AKT/mTOR and RAS/RAF/MEK/ERK pathways leads to synergistic apoptotic effects through the activation of FOXO transcription factors [92].
The pivotal role of RAS in pancreatic cancer has led to clinical trials with MEK inhibitors in this disease. A single- agent Phase I trial of the MEK inhibitor CI-1040 enrolled 77 subjects with solid tumors and demonstrated one partial response in a subject with pancreatic cancer [93]. A sub- sequent Phase II trial with CI-1040 in subjects with colo- rectal, non-small cell lung, breast, or pancreatic cancer, without pre-defined molecular abnormalities, failed to demonstrate any responses [26]. Most recently, a Phase II trial of selumetinib versus capecitabine in patients with advanced pancreatic cancer identified only a marginal improvement in median survival (5.4 vs. 5.0 months, respectively) [28]. It has been hypothesized that the low efficacy observed in the Phase II trial of selumetinib could be due to the advanced stage of the patients enrolled, who would have received several rounds of prior therapies [28, 37]. Nevertheless, the findings of these trials suggest that single-agent MEK inhibitors will not be sufficient to improve current standards of pancreatic cancer care.

Basal-like breast cancer

Basal-like breast cancer is a molecular intrinsic subtype that exhibits a gene expression profile similar to the basal/
myoepithelial layer of normal breast cells, as defined by microarray analysis [94]. A substantial proportion of basal- like breast cancers correspond with the ‘‘triple-negative’’ subtype and therefore do not express estrogen receptor, progesterone receptor, or HER2 [95]. These tumors are aggressive, highly proliferative, and not sensitive to treat- ment with conventional anti-HER2 or hormonal therapies [94, 95]. In the absence of targeted therapy, the identifi- cation of new therapeutic targets that can be exploited in basal-like or triple-negative breast cancer is an important research goal.
KRAS mutations are almost never observed in triple- negative breast cancer [96]; however, triple-negative breast cancer cell lines have been shown to exhibit gene expres- sion profiles similar to those of KRAS-mutant cancers [97, 98]. Breast cancer cell lines with these gene expression profiles demonstrate upregulation of pMEK and pERK and are sensitive to MEK inhibition [97, 98]. In contrast, cell

Table 2 Ongoing clinical trials with PI3K and MEK inhibitor combinations

Trial Sponsor PI3K inhibitor
MEK inhibitor
Phase Patient population

NCT01363232 Novartis BKM120 MEK162 Phase I Advanced solid cancers, including triple-negative breast cancer,
pancreatic cancer, colorectal cancer, malignant melanoma, non- small cell lung cancer, and other cancers with KRAS, BRAF, and NRAS mutation
NCT01337765 Novartis BEZ235 MEK162 Phase I Advanced solid cancers, including triple-negative breast cancer, pancreatic cancer, colorectal cancer, malignant melanoma, non- small cell lung cancer, and other cancers with KRAS, BRAF, and NRAS mutation

NCT01390818 EMD Serono Sanofi
SAR245409 Pimasertib
(MSC1936369B)
Phase I Advanced solid tumors, including either of the following: (1) cancer with diagnosed alteration in one or more of the following genes: PTEN, BRAF, KRAS, NRAS, PI3KCA, ErbB1, ErbB2, MET, RET, c-KIT, GNAQ, GNA11; or (2) any of the following cancers: pancreatic, thyroid, colorectal, non-small cell lung, endometrial, renal, breast, ovarian carcinoma, or melanoma

NCT01347866 Pfizer
PF-04691502 or PF- 05212384
PD0325901
Phase I Advanced cancers demonstrating KRAS or BRAF mutation and patients with advanced colorectal cancer with evidence of KRAS mutation and no more than 1 prior regimen of systemic therapy

NCT01155453 Novartis BKM120
Trametinib
(GSK1120212)
Phase I Advanced solid tumors, including expansion arms consisting of RAS- or BRAF-mutant advanced non-small cell lung cancer, ovarian cancer, or pancreatic cancer

NCT01392521 Bayer BAY80-6946 BAY86-9766
Phase
Ib
Advanced solid cancers

NCT01449058 Novartis BYL719
MEK162
Phase
II
Advanced colorectal cancer, esophageal cancer, pancreatic cancer, non-small cell lung cancer, or other advanced solid tumors with documented RAS or BRAF mutations

NCT00996892 Genentech GDC-0941 GDC-0973 Phase I Advanced solid cancers

NCT01248858 GSK GSK2126458 Trametinib
(GSK1120212)
Phase I Advanced solid tumors

Source: ClinicalTrials.gov

lines with PTEN loss, or other mechanisms of PI3K acti- vation, are resistant to single-agent MEK inhibition [97, 98]. In basal-type breast cancer cells, MEK inhibition has been shown to lead to activation of the PI3K pathway, thus limiting the efficacy of MEK inhibitors [98]. The combi- nation of PI3K and MEK inhibition has been shown to have synergistic antitumor effects in these tumor cells, both in vitro and in vivo, regardless of PI3K pathway alterations [97–99]. Given the lack of therapeutic targets in triple- negative or basal-like breast cancer, PI3K plus MEK inhibitor combinations could be a rational therapeutic strategy for further investigation.

The emerging clinical profile of PI3K and MEK inhibitor combination therapy

Several trials evaluating PI3K plus MEK inhibitors are underway in patients with advanced solid tumors (Table 2), and preliminary data are now available for three Phase I trials [100–102]. The combination of GDC-0973 and GDC-
0941 was evaluated in 78 patients with advanced solid tumors [100]. This combination was well tolerated with toxicities similar to those observed in single-agent Phase I trials. Dose-limiting toxicities (DLTs) were grade 3 lipase and grade 4 creatine phosphokinase (CPK) elevation. Common adverse events were rash, nausea, fatigue, vom- iting, decreased appetite, dysgeusia, and elevated CPK. Partial responses were observed in three patients: one patient BRAF-mutant melanoma, one patient with BRAF- mutant pancreatic cancer, and one patient with KRAS- mutant endometrial cancer. Five patients had stable disease lasting more than 5 months.
The combination of trametinib and BKM120 was eval- uated in 49 patients with advanced RAS- or BRAF-mutant cancers [101]. Combination treatment with these agents was also well tolerated. DLTs included gastrointestinal events, left ventricular ejection fraction decrease, and CPK increase. Common adverse events were dermatitis acnei- form/rash, diarrhea, nausea, vomiting, CPK increase, decreased appetite, stomatitis, and hyperglycemia. Partial responses were observed in three patients with KRAS-

mutant ovarian cancer, two of whom maintained their responses for more than 9 months. Two patients with BRAF-mutant melanoma had stable disease.
In another combination study, the dual oral PI3K/mTOR inhibitor PF-04691502 was combined with either the MEK inhibitor PD-0325901 or irinotecan, while in a third arm, the intravenous PI3K/mTOR inhibitor PF-05212384 was combined with irinotecan [102]. In the PF-04691502/PD- 0325901 combination arm, the maximum tolerated dose was exceeded in the first dosing cohort, with four of six evaluable patients developing DLTs including one patient with grade 3 diarrhea, nausea, and vomiting; one patient with grade 4 mucositis; one patient with grade 3 increased alkaline phosphatase; and one patient with grade 2 uveitis. Among the three treatment arms, PF-05212384 plus iri- notecan showed the most promise because it was well tolerated, and one patient with KRAS wild-type metastatic colorectal cancer, previously treated with irinotecan, responded. A fourth arm combining PF-05212384 with PD- 0325901 is now underway.
In the first two trials, PI3K and MEK inhibitor combi- nations were generally well tolerated, and therapeutically active dose levels were achievable in patients with advanced solid cancers. Regarding the third trial, data from the PF-05212384 plus PD-0325901 arm will be required before an assessment of the viability of PI3K inhibitor combinations with PD-0325901 can be made. Although hyperglycemia (an ‘‘on-target’’ side effect of PI3K inhibi- tion) was reported in the BKM120/trametinib study, serous- like retinopathies (a class effect of MEK inhibition [103, 104]) were not reported in any of the three trials described above. This may have been due to improved patient selec- tion and adverse event management, informed by previous clinical experience with MEK inhibitors. Data from these trials suggest that some tumors may be more likely to respond to therapy than others; however, these empirical observations should be treated with caution, due to the small sample sizes and lack of statistical analysis [100, 101].
Despite the common inclusion of colorectal cancer patients into early-phase clinical trials with PI3K inhibitors, MEK inhibitors, or PI3K/MEK inhibitor combinations, tumor shrinkage was rarely observed. A recent Phase I study attempted to use molecular profiling to optimize current targeted therapies for advanced chemorefractory colorectal cancer [105]. Patients with PIK3CA mutation or PTEN loss (n = 32) received a PI3K inhibitor, patients with KRAS or BRAF mutation (n = 11) received PI3 K and MEK inhibitors in combination, patients with wild-type KRAS received second-generation anti-EGFR monoclonal antibodies (n = 11), and patients with MET amplification (n = 10) received an mTOR inhibitor plus anti-IGFR1 monoclonal antibody (if low PTEN) or BRAF inhibitor (for BRAF-mutant tumors) [105]. Only one partial response was

observed (PI3K inhibitor therapy; PIK3CA mutation) sug- gesting that current targeted agents may not be capable of effectively treating chemorefractory colorectal cancer, even if patients are stratified by tumor molecular status [105]. Additional targets or therapeutic combinations, potentially including chemotherapy, may therefore be required to advance the treatment of colorectal cancer.
Based on the preliminary reports of tumor shrinkage in trials of PI3K and MEK inhibitor combination therapy, advanced melanoma and low-grade serous ovarian cancer appear to be more sensitive. Patients with low-grade serous ovarian cancer have previously demonstrated an excellent rate of response to single-agent MEK inhibitor therapy [87]. Further studies are required to determine if the anti- tumor activity demonstrated with MEK inhibitors can be potentiated by the addition of PI3K inhibitors. Similarly, single-agent MEK inhibitors have also shown good responses in BRAF inhibitor-naı¨ve malignant melanoma [17, 54]; however, the addition of PI3K inhibitors could still be required for patients who develop resistance. Additional studies with PI3K and MEK inhibitor combi- nations may be warranted in both of these tumor types.

Conclusion

The PI3K/AKT/mTOR and RAS/RAF/MEK/ERK path- ways are two of the most important signaling cascades in cancer, and both are frequently involved in resistance to current target therapies. Dual inhibition of both pathways may be needed to overcome certain resistance mechanisms or to treat cancers with driver mutations that cannot be directly targeted, such as mutational activation of RAS. At least two PI3K and MEK inhibitor combinations are well tolerated and can be administered at therapeutic doses; however, additional work is required to establish the tumor types that will respond best to therapy.

Acknowledgments Financial support for medical editorial assis- tance was provided by Novartis Pharmaceuticals. Ben Holtom DPhil provided medical editorial assistance with this manuscript. The author retained full editorial control over the content of the manuscript and received no compensation from any party for this work. Carolyn Britten is appointed as the SmartState Endowed Chair in GI Malig- nancy Diagnostic & Therapeutic Trials in the South Carolina SmartStateti Center for Gastrointestinal Cancer Diagnostics. Carolyn Britten has received research funding from Bayer HealthCare Phar- maceuticals, Genentech, Novartis Pharmaceuticals, and Pfizer. She has received honoraria from Bayer HealthCare Pharmaceuticals.

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