MEK Inhibitors

The Journal of Targeted Therapies in Cancer, April 2014, Volume 3, Issue 2

The RAS-RAF-MEK-extracellular signal–regulated kinase 1 and 2 (ERK1/2) pathway is the most mutated pathway in human cancer. Thus, components of this pathway have been seen as promising targets for cancer therapy.

Alex A. Adjei, MD, PhD

Professor and Chair, Department of Medicine

Senior Vice President of Clinical Research

The Katherine Anne Gioia Chair in Cancer Medicine

Roswell Park Cancer Institute


The RAS-RAF-MEK-extracellular signal—regulated kinase 1 and 2 (ERK1/2) pathway is the most mutated pathway in human cancer. Thus, components of this pathway have been seen as promising targets for cancer therapy. MEK inhibitors have been in clinical evaluation for more than a decade. While low-level clinical activity has been documented with virtually all MEK inhibitors tested in clinical trials, dramatic responses have been few. Several inhibitors are at various stages of clinical evaluation. Singleagent activity has been detected in tumors harboring RAS/RAF mutations such as melanoma, and one inhibitor, trametinib, has been approved by the US Food and Drug Administration (FDA) for the treatment of melanoma. Combinations of MEK inhibitors with cytotoxic chemotherapy or other targeted agents are being investigated.


Three well-characterized subfamilies of mitogenactivated protein kinases (MAPKs) control many physiologic processes in humans. Because of its role in cell proliferation and carcinogenesis, the most characterized MAP kinase pathway is the RAS-RAF- MEK-ERK pathway. This is the most frequently dysregulated signal transduction pathway in human cancer. Common aberrations are gain-of-function mutations ofRASandRAFfamily members.

Mutations inKRAShave been found in 90% of pancreatic, in 20% of non-small cell lung cancer (NSCLC), and in up to 50% of colorectal and thyroid cancers,1while mutations ofBRAFhave been identified in more than 50% of melanoma and in 40% to 60% of papillary thyroid cancers.2-4Although mitogen- activated protein kinase 1/2 (MEK1/2) is rarely mutated, constitutively active MEK has been found in more than 30% of primary tumor cell lines tested.5

The RAS-RAF-MEK-ERK pathway is activated by a wide array of growth factors and cytokines such as epidermal growth factor (EGF), insulin-like growth factor (IGF), and transforming growth factor (TGF), which initially bind to, and activate, transmembrane receptors located on the cell surface. Through sequential interactions with adapter proteins and nucleotide exchange proteins, these growth factors activate RAS through a conversion from the inactive guanosine diphosphate (GDP)-bound form to the active guanosine triphosphate (GTP)-bound form.

Activated RAS recruits RAF kinase to the membrane, where it is activated by multiple phosphorylation events. Activated RAF phosphorylates and activates MEK kinase. MEK kinase in turn phosphorylates and activates ERK kinase. Phosphorylated ERK can translocate to the nucleus, where it phosphorylates and activates various transcription factors.6-8This leads to altered gene transcription and cellular proliferation.9-12

Clinical Pearls

  • Most MEK inhibitors in clinical trials are allosteric inhibitors.
  • Single-agent activity has been seen consistently only in tumors withBRAFmutations andNRASmutations, such as melanoma.
  • Mechanism-based toxicities with these agents include ocular toxicities, skin rash, peripheral edema, left ventricular dysfunction, creatine phosphokinase elevation, and central nervous system abnormalities such as confusion and hallucinations.
  • Combinations of MEK inhibitors with BRAF inhibitors have demonstrated increased efficacy accompanied with reduced toxicity.
  • Trametinib is the first MEK inhibitor to be approved by the FDA for the treatment of melanoma, alone and in combination with the BRAF inhibitor, dabrafenib.
  • MEK inhibitors, when used as single agents, show no activity inBRAF-mutant melanoma that has become refractory to BRAF inhibitors and should not be used in this setting.

History of MEK Inhibitors in Clinical Studies

Notably, the RAF family consists of A-RAF, B-RAF, and RAF-1, all sharing RAS as a common upstream activator, and MEK1/2 as principal kinase effectors.13MEK1/2 are dual-specificity kinases, catalyzing the phosphorylation of both tyrosine and threonine on ERK1 and ERK2, their only known physiologic substrates.14In contrast to RAF and MEK1/2, which have narrow substrate specificity, activated ERK1/2 phosphorylates numerous cytoplasmic and nuclear substrates. This process regulates many cellular responses, such as angiogenesis, cell differentiation, embryogenesis, metastasis, metabolism, and apoptosis (Figure 1).15-22The structure of the MEK1/2 protein makes it ideal for targeting. It possesses a pocket structure adjacent to the adenoside triphosphate (ATP)-binding site that is only conserved in MEK proteins. Upon inhibitor binding, several conformational changes occur and lock the unphosphorylated MEK1/2 into a catalytically inactive state. Since this ATP-noncompetitive mechanism does not exert inhibitory effects through the highly conserved ATP pocket, it potentially avoids undesired, off-target side effects associated with inadvertent inhibition of other protein kinases, and the challenge of competing with millimolar intracellular concentrations of ATP.23,24Several compounds with highly potent, specific and allosteric inhibitory activity of MEK1/2 have been developed and tested in clinical studies.

Figure 1. Simplified Schematic of MEK Signaling

The first MEK inhibitor, CI-1040, entered phase I clinical trials in 2000. Since then, only one MEK inhibitor, trametinib, has been approved for clinical use. This is because these agents have failed to demonstrate significant clinical activity in most tumor types. One potential mechanism underlying this modest activity has been elucidated preclinically. An autoregulatory negative feedback loop between ERK and RAF that mediates sensitivity to MEK inhibitors has been described. Activated ERK releases tonic inhibition of RAF kinases, thereby leading to activated RAF, which activates anti-apoptotic downstream RAF targets, thus abrogating the cytotoxic activity of MEK inhibitors (Figure 2). Tumors harboringBRAFV600Emutations lack this negative feedback loop and are sensitive to MEK inhibitors.25These data predicted that a combination of MEK inhibitors and RAF inhibitors would be synergistic. This prediction has been confirmed in the clinic, as described in the trametinib section later on in this article.

Class Toxicities of MEK Inhibitors

Since 2000, more than 10 MEK inhibitors have entered clinical trials, providing a large body of data on mechanism-based toxicities of these compounds. Some toxicities, such as fatigue, diarrhea, and skin rash, are common to many small-molecule kinase inhibitors. Others are relatively unique to MEK inhibitors, and include ocular toxicities manifest as blurred vision and loss of visual acuity; retinal vein occlusions26; and the most common toxicity, central serous retinopathy27(Figure 3). This potentially serious toxicity involves subretinal fluid accumulation, which generally subsides after drug interruption followed by dose reduction. Peripheral edema, particularly periorbital edema, occurs as dose elevation of creatine phosphokinase without any associated troponin abnormalities, evidence of rhabdomyolysis, or any underlying pathology.

Selumetinib (AZD6244, ARRY-142886)

Rare cases of left ventricular dysfunction have been reported. Also, central nervous system (CNS) effects have occurred, such as hallucinations and confusion (presumably with the subset of agents with good CNS penetration).Table 1illustrates selected MEK inhibitors that are in clinical trials. Selumetinib is the MEK inhibitor with the largest amount of published data. Trametinib has received FDA approval alone and in combination with dabrafenib. These two compounds are discussed in the sections that follow. A brief description of other selected MEK inhibitors is also provided.Selumetinib, a second-generation MEK inhibitor, was introduced in the clinic together with PD- 0325901 after the development of CI-1040 was discontinued because of limited potency and poor pharmacology.24In its initial phase I study, skin rash was the most frequent and dose-limiting toxicity (DLT). Inhibition of ERK phosphorylation was established as a pharmacodynamic biomarker. Prolonged disease stabilization was achieved in one medullary thyroid cancer patient and in one patient with uveal melanoma (out of 34 evaluable patients).28A solid oral capsule formulation of this compound was subsequently developed with improved pharmacologic properties. A prolonged complete response (CR) in a patient with melanoma bearing aBRAFV600Emutation was observed with this formulation in a phase I study.29

Figure 2. Feedback inhibition of ERK on RAF is abrogated by MEK inhibitors, leading to activation of MEK independent downstream RAF targets and resistance to apoptosis. This may explain the lack of activity of single-agent MEK inhibitors in a number of tumors. In addition, this figure may explain the synergy between MEK inhibitors and BRAF inhibitors.

Single-agent selumetinib has been evaluated in multiple phase II studies in a variety of solid tumors as well as in hematologic malignancies.30-34Treatment with single-agent selumetinib in 28 patients with metastatic biliary cancers yielded 3 objective responses (ORs) (12%) and 14 stable diseases (SDs) lasting for more than 4 months, whereas no significant antitumor activity was observed in either papillary thyroid carcinoma or hepatocellular carcinoma. In aBRAFV600E/K-mutated melanoma study, no antitumor activity was observed in the 10 patients with high phosphorylated AKT (pAKT) levels, while 3 of 5 patients with low pAKT levels achieved tumor regression, suggesting a potential role of PI3/AKT activation in MEK inhibitor resistance. In a phase II study, 47 patients with relapsed/refractory acute myeloid leukemia (AML) or who were 60 years or older with untreated AML were stratified byFLT3-internal tandem duplication (ITD) mutation status and were treated with selumetinib. In theFLT3wildtype cohort, 6 of 36 patients (17%) had a response (1 partial response, 3 minor responses, 2 unconfirmed minor responses [uMR]). No patient withFLT3-ITD responded.NRASandKRASmutations were detected in 7% and 2% of patients, respectively. The sole patient withKRASmutation had uMR with hematologic improvement in platelets. Baseline p-ERK activation was observed in 85% of patients analyzed, but this activation did not correlate with a response. A single-nucleotide polymorphism (SNP), rs3733542 in exon 18 of theKITgene, was detected in a significantly higher number of patients with response/ stable disease compared with nonresponders (60% vs 23%;P= .027).34

Selumetinib has also been evaluated in combination with other anticancer agents. In a phase I study combining selumetinib with the AKT inhibitor MK- 2206, DLTs were rash, stomatitis, grade 2 detached retinal pigment epithelium, diarrhea, grade 4 lipase elevation, bilateral posterior, grade 1 subcapsular cataracts, and fatigue. In this 51-patient trial, 1 patient withKRAS-mutated NSCLC and 1 patient withKRAS-mutated ovarian cancer achieved durable confirmed partial responses (PR), and 1 unconfirmed PR was seen in a patient with pancreatic cancer.35In a phase I study assessing selumetinib and cetuximab in solid tumors andKRAS-mutated colorectal cancer (CRC), the most common toxicities reported were acneiform rash, fatigue, nausea/vomiting, and diarrhea, while the DLT was grade 4 hypomagnesemia.36Among 13 evaluable patients treated in the dose-escalation cohort, 2 PRs were observed in patients with CRC, and SD was achieved in 1 patient with tonsillar squamous cell carcinoma, 1 patient with NSCLC, and 2 patients with CRC. Results in theKRAS-mutated CRC expansion cohort have not been reported.

Figure 3. Central serous retinopathy from a MEK inhibitor. Retinal picture and OCT stain pretreatment (A, B). Fluid collection behind retina after 2 weeks of treatment on OCT stain (C, D).

Randomized phase II studies comparing selumetinib versus temozolomide in chemotherapynaïve melanoma, selumetinib versus pemetrexed in NSCLC beyond first/second-line therapies, selumetinib versus capecitabine in pancreatic cancer after failing gemcitabine, and selumetinib versus capecitabine in CRC beyond first/second-line therapies did not demonstrate superiority, even though antitumor activity as single agent was observed in each study.37-40

In previously treated NSCLC withKRASmutation, the addition of selumetinib to docetaxel did not yield a statistically significantly improved median overall survival (OS) (9.4 months vs 5.2 months; hazard ratio [HR] 0.80; 80% confidence interval [CI]: 0.56- 1.14; 1-sidedP= .21), although improvement in median progression-free survival (PFS) (5.3 months vs 2.1 months; HR = 0.58; 80% CI: 0.42-0.79; 1-sidedP= .014) and OR rate (37% vs 0%; P <.0001) were seen in this study. A higher incidence of myelosuppression and fatigue was found in the selumetinib group.40 Based on the high OR rate of this combination, a phase III study has been started. Selumetinib plus dacarbazine versus placebo plus dacarbazine was compared in patients withBRAF-mutant melanoma as first-line treatment in a phase II study. Although significantly improved PFS was observed with the addition of selumetinib, no OS benefit was demonstrated.41

Finally, the effect of selumetinib on radioiodine sensitivity in radioiodine-resistant differentiated thyroid cancers was tested in a 24-patient pilot study. A 4-week treatment of selumetinib restored sensitivity to radioiodine that exceeded the threshold for radioiodine treatment in 8 patients, allowing administration of therapeutic radioiodine; PR and SD were achieved in 5 and 3 patients, respectively.

Trametinib (GSK1120212,JTP-74057)

In this small study, selumetinib appeared to convert a higher percentage ofNRAS-mutant tumors to radioiodine sensitivity thanBRAF-mutated tumors.42Phase I and II trials combining selumetinib with vandetanib, cixutumumab, gemcitabine, and irinotecan are under way or have been completed.40,43Trametinib, a third-generation MEK inhibitor, was evaluated in a 206-patient, phase I study.27The most common adverse events were rash and diarrhea, and DLTs were rash, diarrhea, and central serous retinopathy. The effective half-life of trametinib was found to be about 4 days. Although a dose level of 3 mg/day was determined to be the maximum tolerated dose (MTD), because of poor tolerance beyond the first cycle of treatment, the recommended phase II dose was 2 mg/day. Twenty-one (10%) ORs were noted at all dose levels, with the most sensitive population beingBRAF-mutant melanoma.27Subanalysis of participants withBRAF-mutant melanoma revealed a 33% response rate among 30 patients who were BRAF inhibitor-naïve. This result was further confirmed in a phase II study, in which a 25% OR rate was achieved in patients who were BRAF inhibitor- naïve, while only minimal clinical activity was observed in patients with BRAF inhibitor-resistant disease.44

Table 1. MEK Inhibitors in Clinical Development



Stage of Clinical Development

AZD8330 (ARRY- 424704)

AstraZeneca, Array Biopharma

Phase II

Rafametinib (BAY 86- 9766 RDEA119)

Bayer, Ardea Bioscience

Phase II

CI-1040 (PD 184352)



Cobimetinib (GDC-0973; XL-518, RG7421)


Phase III


Eisai Inc.

Phase II

MEK162 (ARRY-438162)

Array Biopharma, Norvartis

Phase III



Phase II

Pimasertib (AS703026, MSC1936369B)

EMD Serono

Phase II

RO4987655 (CH4987655)

Hoffmann-La Roche

Phase II


Hoffmann-La Roche

Phase II

Selumetinib (AZD6244, ARRY-142886)


Phase III



Phase I

Trametinib (GSK1120212)


FDA approved



Phase I

Trametinib was subsequently advanced to a phase III study. Patients withBRAFV600E- orBRAFV600K-mutant melanoma who were not previously treated with a BRAF or MEK inhibitor or with ipilimumab were randomized to receive either trametinib or chemotherapy (dacarbazine or paclitaxel). The trametinib arm had a median PFS of 4.8 months, compared with 1.5 months in the chemotherapy group. Median OS has not been reached.45However, based on these results, the FDA granted approval of trametinib as a single agent for the treatment ofBRAFV600EorBRAFV600Kmutation—positive unresectable or metastatic melanoma. The preclinical data suggest that the efficacy of MEK inhibitors may be enhanced by combination with a RAF inhibitor.25In addition, activation of the MAPK pathway has been postulated as a potential resistance mechanism to RAF inhibitors in melanoma. Simultaneous inhibition of RAF and MEK is therefore a rational approach to overcoming resistance to BRAF inhibitors. Trametinib was combined with dabrafenib, a BRAF inhibitor, in a phase I/II trial to determine tolerability, and to compare single-agent dabrafenib to the combination. Cutaneous squamous cell carcinoma, a BRAF inhibitor-associated adverse event presumed to be due to paradoxical MAPK pathway activation, was lower in the combination arm. A significant improvement in PFS was observed in the combination arm (HR, 0.39; 95% CI: 0.25-0.62;P<.001), indicating the potential of MAPK inhibition to delay resistance to BRAF inhibition. This study was further extended into a phase III trial (COMBI-D) in 162 patients with histologically confirmed stage IIIC or IV melanoma with aBRAFV600E(85%) orBRAFV600K(15%) mutation. Only one prior chemotherapy regimen and/or interleukin-2 therapy was permitted. Patients with prior exposure to BRAF inhibitors or MEK inhibitors were ineligible. Fifty-four patients with performance status (PS) 0 to 1 each were assigned to 3 arms; trametinib (1 mg in one arm, 2 mg in the other arm) orally, once daily in combination with dabrafenib 150 mg orally, twice daily, with single-agent dabrafenib as the control arm. In this study, 67% of patients had M1c disease, and 81% had not received prior anticancer therapy for unresectable or metastatic disease. Objective response rates and response durations were 76% (95% CI: 62-87) and 10.5 months (95% CI: 7-15), respectively, in the trametinib 2-mgplus- dabrafenib combination arm and 54% (95% CI: 40-67) and 5.6 months (95% CI: 5-7), respectively, in the single-agent dabrafenib arm. Objective response rates were similar in subgroups defined byBRAFV600mutation subtype,BRAFV600EandBRAFV600K.The trial’s primary safety endpoint, the incidence of cutaneous squamous cell carcinoma (including squamous cell carcinomas of the skin and keratoacanthomas), was 7% (95% CI: 2-18) in the trametinib 2 mg plus dabrafenib combination arm compared with 19% (95% CI: 9-32) in the single-agent dabrafenib arm. The most frequent (≥20% incidence) adverse reactions from trametinib in combination with dabrafenib were fever, chills, fatigue, rash, nausea, vomiting, diarrhea, abdominal pain, peripheral edema, cough, headache, arthralgia, night sweats, decreased appetite, constipation, and myalgia. The most frequent grades 3 and 4 adverse events (at least 5% incidence) were acute renal failure, fever, hemorrhage, and back pain. Serious but less common adverse drug reactions occurring in patients taking trametinib in combination with dabrafenib were hemorrhage, venous thromboembolism, new primary malignancy, febrile reactions, cardiomyopathy, skin toxicity, and eye disorders such as retinal pigmented epithelial detachments. Based on these data, the FDA granted accelerated approval to trametinib in January 2014 for use in combination with dabrafenib in the treatment of patients with unresectable or metastatic melanoma with aBRAFV600Eor aBRAFV600Kmutation.46The trametinib/dabrafenib combination is also being evaluated in the adjuvant setting after surgical resection (COMBI-AD), and in a phase III study comparing this combination with another RAF inhibitor, vemurafenib (COMBI-v).

It must be noted, however, that trametinib and other MEK inhibitors when used as a single agent have no activity againstBRAF-mutant melanoma that has progressed on a BRAF inhibitor such as vemurafenib. In fact, anecdotal data suggest that such treatment may lead to a disease flare and a rapid demise of patients, and should be avoided.

Cobimetinib (GDC-0973, XL-518, RG7421)

Trametinib combinations have also been evaluated in several other tumor types. In a randomized, placebo-controlled study, the addition of trametinib to gemcitabine failed to improve efficacy parameters in 160 patients with metastatic pancreatic cancer irrespective of theirKRASmutation status.47Other phase I/Ib studies evaluating combinations of trametinib with various targeted therapeutic and conventional chemotherapy agents are under study, such as everolimus, pazopanib, dabrafenib, the AKT inhibitor GSK2141795, the PI3K inhibitor BKM120, erlotinib, 5-fluorouracil, and radiation therapy.48-51A second third-generation MEK inhibitor, cobimetinib, is also undergoing phase III clinical trials. Cobimetinib was well tolerated in phase I trials, but there were no ORs.52Cobimetinib was further evaluated in a phase Ib study in combination with the PI3-kinase inhibitor, GDC-0941, in advanced solid tumors. The most common adverse events of this combination were diarrhea, fatigue, nausea, and rash. Among the 46 evaluable patients, PRs were observed in 1 patient withBRAF-mutated melanoma, 1 withBRAF-mutated pancreatic cancer, and 1 withKRAS-mutated endometrial cancer.53,54Cobimetinib was also combined with vemurafenib inBRAFV600E-mutated melanoma in another phase Ib study. Tumor shrinkage was observed in all 8 evaluable patients who were vemurafenib naïve. Notably, of the 44 subjects, only 1 developed cutaneous squamous cell carcinoma.54On the basis of these results, a phase III study comparing this combination with vemurafenib has been initiated. Phase II studies combining cobimetinib with other targeted agents, including GDC-0941 and the AKT inhibitor GDC-0068, are ongoing.

Rafametinib (BAY 86-9766, RDEA119)

Pimasertib (AS703026, MSC1936369B)

Other MEK Inhibitors


Other MEK inhibitors are in phase Ib, phase I/II, and phase II clinical trials and are briefly described in the sections that follow.In a phase I study of rafametinib, 1 patient with CRC of a total of 53 evaluable patients achieved a PR.55Based on preclinical data56and results of a phase I study, rafametinib was combined with sorafenib as first-line systemic treatment for patients with hepatocellular carcinoma in a phase II study. Among 70 subjects, 3 had confirmed PR and 25 had prolonged (>4 months) SD. However, there were 4 grade 5—related adverse events reported, and almost all patients required dose modifications because of adverse events.57A phase I/II study evaluating rafametinib in combination with gemcitabine in patients with metastatic pancreatic cancer is ongoing.58Phase I studies of pimasertib found no ORs in a total of 180 patients. However, 5 patients with eitherBRAForNRASmutations had confirmed tumor shrinkage, but not enough to meet OR criteria.59,60A combination study with 5-fluorouracil, leucovorin, and irinotecan (FOLFIRI) as second-line treatment inKRAS-mutated metastatic CRC was discontinued because of toxicity.61Studies combining pimasertib with a number of agents are ongoing. These include the PI3K/mTOR inhibitor SAR245409 (phase Ib) and temsirolimus (phase I). There are also phase II studies comparing pimasertib and dacarbazine in NRASmutated melanoma, pimasertib combined with gemcitabine versus gemcitabine alone in pancreatic adenocarcinoma, and a phase II study of pimasertib in patients with advanced hematologic malignancies.Phase I studies evaluating AZD8330, WX-554, E6201, RO4987655, TAK-733, and RO5126766, either alone or in multiple combinations are ongoing or have been recently completed.62-67A randomized, phase III study with MEK162 compared with dacarbazine in patients with advanced unresectable or metastaticNRASmutation-positive melanoma is currently enrolling patients.68A phase III, randomized 3-arm open-label study of MEK162 plus LGX818 and LGX818 monotherapy compared with vemurafenib in patients with unresectable or metastaticBRAFV600-mutant melaoma is also currently enrolling patients.69Despite the prevalence of aberrant MEK signaling in human cancers, highly potent and specific MEK inhibitors have failed to demonstrate striking singleagent activity in most solid tumors and hematologic malignancies. Apart fromBRAF-mutant and NRASmutant melanoma, clinical activity has been reported in biliary cancers, serous ovarian cancer, and uveal melanoma. While these agents are not active inBRAF-mutant tumors that develop resistance to BRAF inhibitors, activity has been demonstrated in combination with BRAF inhibitors. This combination is superior to single-agent BRAF inhibitors in terms of response rate and duration of response. Combinations with other targeted agents may also improve efficacy and overcome resistance. However, toxicity of a number of these combinations (AKT inhibitors, PI3-kinase inhibitors) has caused problems. Combinations with classical cytotoxic agents may also be efficacious, and results of the ongoing phase III trial of the combination of selumetinib with docetaxel are awaited with interest.


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