GSK1120212 | Gsk3 Signal

Trametinib (GSK1120212) in the treatment of melanoma
April KS Salama & Kevin B Kim†
†The University of Texas MD Anderson Cancer Center, Department of Melanoma Medical Oncology, Houston, TX, USA
Introduction: The discovery of somatic mutations in melanoma has advanced our knowledge of the biology of the disease. The mutations, such as those in NRAS, BRAF, GNAQ and GNA11, promote the growth of melanoma cells in most part through the mitogen-activated protein kinase (MAPK) pathway. Understanding the molecular pathways of some of these mutations has resulted in the successful development of selective BRAF inhibitors. Yet, a cure for advanced melanoma is far from reality. Targeting MAPK/ERK kinase (MEK), an essential intermediary kinase protein within the MAPK pathway, may be a promising way to treat patients with BRAF or other genomic mutation.
Areas covered: The authors discuss the MAPK pathway in melanoma and review the preclinical and clinical studies of the MEK inhibitor, trametinib, in melanoma. They also discuss the potential of using trametinib in the targeted therapy of advanced melanoma.
Expert opinion: Studies have demonstrated the activity of trametinib in BRAF- mutant melanoma, suggesting that it could be a very reasonable alternative to BRAF inhibitors for these patients. Current clinical investigations have shown great promise with the combination of trametinib and dabrafenib in patients with BRAF-mutant melanoma; a number of clinical trials of trametinib in combination with other targeted drugs are underway.

Keywords: BRAF mutation, MAP kinase, MEK inhibitor, metastatic melanoma, trametinib

Expert Opin. Pharmacother. (2013) 14(5):619-627

⦁ Introduction

Melanoma has garnered a great deal of attention in recent years, as landmark advan- ces in both targeted treatment and immunotherapy have been made. Prior to 2011, only two systemic agents, dacarbazine and interleukin-2, had received approval by the US Food and Drug Administration (FDA) for the treatment of metastatic mela- noma. The fact that neither agent had demonstrated a survival benefit in Phase III studies underscores the significance of these advances. Used as a single agent or with other cytotoxic chemotherapeutic agents, dacarbazine had been the historical mainstay of treatment. However, response rates were 7 — 19% at best and the response rarely lasted for more than a few months [1-6]. High-dose interleukin-2 had been shown to induce durable complete remissions in ~ 5% of patients, but
most patients were not candidates for this therapy due to the significant toxicity
of the treatment [7,8]. More recently, an increased understanding of mediators of immune regulation led to the development of ipilimumab, an anti-cytotoxic T-lymphocyte-associated antigen 4 antibody, which has demonstrated a survival benefit in two Phase III studies [4,9]. In addition, early data suggest that ipilimumab can induce durable responses in ~ 20% of patients [4,9].
The discovery of BRAFV600 mutations as an oncogenic driver in ~ 50% of cutaneous
melanomas represents a paradigm shift in the understanding of the pathophysiology of
this disease [10-13]. The clinical efficacy of BRAF-targeted therapy with the selective BRAF inhibitors, vemurafenib and dabrafenib, has been validated in two Phase III

Box 1. Drug summary.
Drug name Trametinib (GSK1120212)
Phase Phases I — III
Indication No FDA approval at this time. Trametinib is being investigated for use in patients with cancer, including melanoma
Pharmacology description Reversible, selective allosteric MEK1/MEK2 inhibitor Route of administration Alimentary
Chemical structure F l

O HN

N N

O N O

N H
Pivotal trials An Open-Label, Multiple-Dose, Dose-Escalation Study to Investigate the Safety, Pharmacokinetics, and Pharmacodynamics of the MEK Inhibitor GSK1120212 in Subjects with Solid Tumors or Lymphoma (NCT00687622)

An Open-Label, Multicenter Study to Investigate the Objective Response Rate, Safety and Pharmacokinetics of GSK1120212,
a MEK Inhibitor, in BRAF Mutation-positive Melanoma Subjects Previously Treated with or without a BRAF Inhibitor (NCT01037127)

A Phase III Randomized, Open-label Study Comparing GSK1120212 to Chemotherapy in Subjects with Advanced or Metastatic BRAF V600E/K Mutation-positive Melanoma (NCT01245062)

An Open-Label, Dose-Escalation, Phase I/II Study to Investigate the Safety, Pharmacokinetics, Pharmacodynamics and Clinical Activity of the BRAF Inhibitor GSK2118436 in Combination with the MEK Inhibitor GSK1120212 in Subjects with BRAF Mutant Metastatic Melanoma (NCT01072175)

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clinical trials [5,14]. Despite the promise of these two agents, met- astatic melanoma remains an incurable disease for the majority of patients and novel approaches are still desperately needed. Vemurafenib and dabrafenib can dramatically shrink tumors in most patients; yet, most cases relapse within a year [10,11].
Thus, while landmark translational discoveries have now been brought to clinical fruition, numerous questions remain to be answered in order to optimize treatment and continue improving outcomes for patients diagnosed with this devastating disease.

⦁ Mitogen-activated protein kinase pathway in melanoma
The mitogen-activated protein kinase (MAPK) pathway trans-
mits signals from activated cell surface receptors to a number of intracellular effectors [15,16]. The RAF-MEK-ERK cascade is one of the most well-defined examples of this pathway and is a key driver of oncogenesis in a number of malignancies [17]. Under normal conditions, activation of RAS induces membrane localization and dimerization of RAF. Three isoforms of RAF have been identified in humans: ARAF, BRAF and CRAF. All three RAF family members can phosphorylate MAPK/ERK kinase (MEK), although BRAF may be the most critical
activator in melanoma [15]. MEK1 and MEK2 are dual-specific- ity protein kinases and are the only known substrates of BRAF. De novo mutations of MEK are rare in cancer, but MEK activity appears to be critical for mutant BRAF signaling [18,19]. Activated MEK is capable of phosphorylating ERK, its primary downstream target. ERK subsequently activates a wide array of targets, notably including the nuclear transcrip- tion factors Elk1, Fos, Jun and Myc. Additionally, ERK controls a number of cytoplasmic kinases and phosphatases across multiple pathways, as well as cytoskeletal proteins. Several proapoptotic proteins and proteinases, among them Bim-EL and Caspase 9, are also inhibited by ERK [15,16,20].
Through this diverse signaling network, the ERK cascade is a key regulator of cell proliferation and growth.
Alterations in the MAPK pathway are common in mela- noma. First identified in 2002, somatic BRAF mutations are now known to be present in ~ 50% of cutaneous melanomas, while 15 — 25% of cutaneous melanomas have a mutation in NRAS [10,21]. An exception is uveal melanoma, for which there
are no known BRAF mutations [22,23]. The MAPK pathway does, however, appear to be critical in the development of metastatic uveal melanoma: the majority of patients have a mutation in one of the heterotrimeric G proteins, Galphaq

Figure 1. Diagram of MAPK pathway and site of trametinib action.

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(GNAQ) or Galpha11 (GNA11), both of which are capable of inducing MAPK activation [22,24].

⦁ MEK inhibition in melanoma
The knowledge that the MAPK pathway is dysregulated in a large proportion of melanomas and other malignancies makes MEK an attractive potential therapeutic target. Preclinical data suggested that BRAF-mutated cell lines are extremely sensitive to MEK inhibition, with half-maximal inhibitory
concentration values of 0.024 — 0.11 µM in response to
MEK inhibitor, CI-1040 [18]. Furthermore, MEK inhibition suppresses tumor growth in melanoma BRAFV600E xenograft models [18]. Despite the promising preclinical evidence early on, MEK inhibition has only recently demonstrated a marked clinical benefit for patients with melanoma [25]. A number of MEK inhibitors have entered early-phase testing, but response rates have been low and the potential results have been often limited by toxicity. These shortcomings may be due to the narrow therapeutic window of some MEK inhibitors and to patient selection, as some studies on melanoma have included patients irrespective of their BRAF mutation status [26-29].

⦁ Trametinib development

Trametinib (GSK1120212) is an orally available allosteric inhibitor of MEK1 and MEK2 activity that has promising clinical activity in melanoma (see Box 1). Normal activation of MEK requires dual phosphorylation. It is believed that in addition to inhibiting the catalytic activity of MEK1 and MEK2 (Figure 1), trametinib inhibits activation of MEK by
preferentially inhibiting phosphorylation at serine 217, result- ing in a predominantly monophosphorylated protein at serine 221 [30]. The resultant kinase has a fraction of the activity seen with dual phosphorylation.
Among patients with BRAF-mutated melanoma, trameti- nib has been shown to lengthen survival duration compared with patients treated with standard chemotherapy [25]. Based on these results, a new drug application was submitted to the FDA in August 2012 for the treatment of unresectable metastatic melanoma with a BRAFV600 mutation.

⦁ Preclinical studies
In vitro data have demonstrated that sustained level of trameti- nib induces sustained inhibition of phosphorylated ERK1/2. Notably, this suppression of ERK1/2 phosphorylation is observed in spite of the fact that in some cell lines MEK inhibition induces increased RAF-mediated phosphorylation of MEK through inhibition of a negative feedback loop [30,31]. In xenograft tumor models, activating mutations in BRAF or RAS seem to be predictive of sensitivity to MEK inhibi- tion [18,30], consistent with previous data. In the xenograft stud- ies, trametinib was administered orally at 3 mg/kg for 14 days and the tumors assessed for tumor growth inhibition (TGI), as defined by the percentage volume differential between treated and untreated tumors. Consistent with other analyses, the BRAF-mutant tumors treated with trametinib demonstrated the most marked TGI (80 — 87%) and clear evidence of tumor shrinkage [30]. KRAS-mutant tumors exhibited high levels of TGI, at 75 and 83%, in two KRAS-mutant cell lines, but had no significant tumor regression. Models with both

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wild-type BRAF and wild-type RAS demonstrated more modest levels of TGI (44 — 46%), also with no tumor regression.

⦁ Pharmacokinetics
The pharmacokinetic profile of trametinib is unique among MEK inhibitors in that it has a long half-life and a low peak-to-trough ratio. Preclinical pharmacokinetic data have demonstrated that trametinib inhibits MEK1/2-dependent phosphorylation of ERK1/2 at residues threonine 202 and tyrosine 204 for up to 72 h [30]. A decrease in MEK1/2 phosphorylation was also noted in a number of tumor cell lines. This inhibition was sustained in BRAFV600E SK- MEL-28 cells, yet in BRAFV600E A375-P(F11) cells and the KRAS-mutant cell line HCT-116, this effect appeared to be temporary and there was a resultant gradual increase in phos- phorylated MEK levels [30]. This phenomenon is thought to be due to inhibition of a negative feedback loop, resulting in increased CRAF activity, which has been observed with other MEK inhibitors [31-33].
In Phase I clinical pharmacokinetic analyses, trametinib demonstrated rapid absorption, with a median time to maxi- mum concentration (Cmax) of 1.5 h for the 2 mg/day dose, and a half-life of ~ 4 days [34]. The effect of loading doses did not appear to significantly alter the Cmax or area under the
curve at day 15. The 2 mg/day dose demonstrated a Cmax of
22.2 ng/ml (between-subject coefficient of variation = 28% [Cmax range: 14 -- 32.9]), resulting in a favorable mean peak-to-trough ratio of 1.81. At this dose level, the mean concentration of trametinib exceeded the established preclinical antiproliferation target of 10.4 ng/ml at 24 h for BRAF-mutant cell lines. There appeared to be low interpatient variability [34].

⦁ Pharmacodynamics
In vivo analyses with an A375P (BRAFV600E melanoma cell line) xenograft model demonstrated that a single dose of tra- metinib markedly reduced ERK phosphorylation for more than 8 h [27]. Evidence of antiproliferative effects was also seen, with a decrease in Ki67 expression and an increase in p27 expression after 4 days of administration. In that study, trametinib did not appear to cross the blood–brain barrier, as very low levels were detected in brain tissue [30]. Similar results were found in a clinical study in which pharmacodynamic bio- markers were assessed in patients with paired biopsy samples. At the recommended Phase II dose of 2 mg/day, inhibition of phosphorylated ERK and reduction of Ki67 expression were observed [34]. These changes were most marked in the cohort of patients with either BRAF- or NRAS-mutated mela- noma, with a median decrease in ERK phosphorylation of 62% and a decrease in Ki67 expression of 83% at day 15 compared with baseline.

⦁ Clinical efficacy of single-agent studies
⦁ Phase I and Phase II studies
In a Phase I study of 206 patients with advanced tumors (including melanoma), three dosing regimens of trametinib
were assessed: once daily administration for 21 days with a 7-day break, a 1- to 2-day loading dose followed by continu- ous daily dosing, or continuous once daily dosing with no prior loading dose [34]. This trial established the maximum tolerated dose of trametinib to be 3 mg/day, and the recom- mended Phase II dose of 2 mg/day, as there was increased toxicity with 3 mg dosing beyond cycle 1. Responses were observed in 21 patients (10%), and patients with BRAF- mutated melanoma appeared to be the most sensitive to treat- ment. A total of 97 patients with metastatic melanoma were enrolled in the study, including 36 patients with BRAFV600 mel- anoma, 39 with wild-type BRAF non-uveal melanoma, 6 whose BRAF status was unknown, and 16 with uveal melanoma [35]. Nearly all of these patients received at least 2 mg/day of trame- tinib; only four patients received a dose lower than 2 mg/day. For the 30 patients with BRAF-mutated melanoma who had not received prior therapy with a BRAF inhibitor, two com- plete responses and eight partial responses were observed, for a total confirmed response rate of 33% (unconfirmed response rate of 40%), and the median progression-free survival (PFS) duration was 5.7 months (95% confidence interval [CI], 4 — 7.4 months). In this BRAF inhibitor-na¨ıve group, 19 patients had some degree of tumor shrinkage, and 4 patients were on therapy for more than 1 year. Among the six patients who had previously received BRAF-targeted therapy, one had an unconfirmed partial response and four had stable disease.
Among 20 melanoma patients with both wild-type BRAF and wild-type NRAS who were participating in a Phase I study, four (20%) achieved a confirmed partial response and six patients continued on therapy for more than 1 year, sug- gesting that the clinical activity of trametinib in this genomic subset is promising [32]. Among the trial’s seven patients with an NRAS mutation, two had stable disease, one of whom had received treatment for 48 weeks [35].
In a Phase II study that included 57 BRAF inhibitor- na¨ıve patients with advanced BRAF-mutant melanoma, 14 (25%) patients had a confirmed response and 29 (51%) experienced stable disease [36]. The median PFS was 4 months (95% CI: 3.6 — 5.6 months). For the 40 trial participants with prior BRAF inhibitor therapy, no responses were observed but 11 patients had stable disease. In this cohort, the median PFS was only 1.8 months (95% CI: 1.8 — 2 months). Interestingly, three of the four patients with a non-BRAFV600E/K mutation (one each with V600R/V600K, K601E and L597V) had a con- firmed clinical response across the Phase I and Phase II studies of trametinib [35,36].

⦁ Phase III study
A recent Phase III randomized study confirmed the efficacy of MEK inhibition in treating melanoma. In this study, 322 patients with BRAFV600E/K melanoma were assigned to receive, at a 2:1 ratio, trametinib 2 mg/day or a standard cyto- toxic chemotherapy (dacarbazine or paclitaxel) [25]. Patients who had received prior therapy with a BRAF inhibitor, MEK

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inhibitor or ipilimumab were excluded from the trial. The haz- ard ratio for PFS, the primary endpoint of the study, was 0.45 (95% CI: 0.33 — 0.63, p < 0.001) and favored the trame- tinib arm. Median PFS duration was 4.8 months in the trame- tinib arm and 1.5 months in the chemotherapy arm. The 6-month overall survival (OS) rate was significantly higher for patients who received trametinib rather than chemotherapy (81 vs 67%). The hazard ratio for death was 0.54 (95% CI:
0.32 -- 0.92, p = 0.01), although 47% of the patients in the chemotherapy arm were crossed over at the time of disease progression to receive trametinib treatment. At the time of the initial publication of the clinical data for this study, median OS had not been reached for both arms.

⦁ Safety and tolerability
Safety data have demonstrated that trametinib overall appears to be well tolerated. Cutaneous adverse events have been the most frequently reported side effect, with rash (primarily acneiform dermatitis) occurring in 75 -- 90% of patients treated with the 2 mg dose [34-36]. Cutaneous squamous cell carcinomas and other proliferative skin lesions have not been reported. Diarrhea, the most common gastrointestinal toxicity, has been reported for ~ 45% of patients, although
most cases were grade 1 and 10% or less was grade 2. Other
gastrointestinal events have included nausea and vomiting, which occurred in < 20% of patients, but these cases were generally responsive to standard supportive care measures. Additional adverse events with trametinib include peripheral and periorbital edema, which is consistent with effects seen with this class of agents. Ocular effects can include dry eye and blurred or impaired vision, which appear to resolve with discontinuation of the drug. Central serous retinopathy and, rarely, retinal vein occlusion have also been reported. There were three events of central serous retinopathy at the various dosing regimens, which were resolved upon the treatment dis- continuation. One case of retinal vein occlusion that occurred at the 2 mg/day dosing regimen was improved with intraocu- lar treatment of antibodies against vascular endothelial growth factors. A drug-related decrease in left ventricular ejection fraction has been reported for a minority of patients. Hemato- logic toxicity is rare with trametinib. The overall incidence of thrombocytopenia has been low, occurring in 5% of patients, and no grade 4 thrombocytopenia was observed at the 2 mg/day dose [34,35].

⦁ Role of MEK inhibition in combination therapy Although treatment with a BRAF inhibitor can relatively rapidly provide clinical benefit and dramatically shrink tumors, for most patients the responses do not appear to be durable: the median PFS ranges from 6 to 7 months [5,14,37,38]. The lack of durable response has led to questions regarding the potential mecha- nisms of disease resistance, and data suggest that it is a heteroge- neous process. It is interesting to note that resistance to BRAF inhibitor therapy does not appear to be due to novel mutations in BRAF itself and that tumors seem to retain the original BRAF
mutation at the time of disease progression [39-42]. A potential mechanism is reactivation of the MAPK pathway via develop- ment of activating mutations in NRAS or MEK, dimerization of RAF kinases, increased expression of COT-1 serine/ threonine kinase protein, or alternate splicing of the BRAF gene. Dual MEK/BRAF inhibition has evolved as a strategy for more effective target blockade [41-46]. Reactivation of the MAPK kinase pathway was recognized early as a key mediator of resistance to BRAF inhibitor therapy, and early data have indicated the promise of dual BRAF/MEK inhibition.
With preclinical data implying a synergistic effect of dual inhibition as well as a potential decrease in hyperproliferative skin lesions in a BRAFV600E melanoma model, a three-part Phase I/II combination trial was conducted [47]. An initial cohort included a drug--drug interaction study to determine the effect of trametinib on the BRAF inhibitor dabrafenib, which was followed by dose escalation and ultimately multiple expansion cohorts stratified by disease and prior therapy. Patients with a BRAFV600 malignancy and stable brain metas- tases for 12 weeks were initially enrolled. It was determined that full monotherapy doses of both drugs could be safely administered in combination, dabrafenib at 150 mg b.i.d. and trametinib at 2 mg/day. Initial results from 109 patients reported one dose-limiting toxicity, grade 2 neutrophilic pan- niculitis. Among the 71 BRAF inhibitor-na¨ıve melanoma patients, 5 experienced a complete response (7%) and 42 a partial response (59%); a number of these patients experi- enced durable responses [47]. A final analysis of the randomiza- tion cohorts of 162 patients (dabrafenib 150 mg b.i.d. alone; dabrafenib 150 mg b.i.d. + trametinib 1 mg/day; or dabrafe- nib 150 mg b.i.d. + trametinib 2 mg/day) revealed that the confirmed clinical response rate and the median PFS were sig- nificantly greater with the combination regimen (dabrafenib 150 mg b.i.d. + trametinib 2 mg/day) than with single-agent dabrafenib (response rate, 76% vs 54%; PFS = 9.4 months [95% CI: 8.6 -- 16.7 months] vs 5.8 months [95% CI,
4.6 -- 7.4 months]) [48].
The most common adverse events of dabrafenib plus trametinib were pyrexia, chills, fatigue, nausea, vomiting, diarrhea, headache, peripheral edema, cough, arthralgia and rash [48]. Overall, the combination appeared to be well toler- ated, and there appeared to be a lower incidence of rash with this combination than with the above-described single- agent trametinib therapies. Furthermore, the combination appeared to prevent the development of hyperproliferative skin lesions (including cutaneous squamous cell carcinoma, which is commonly seen with single-agent BRAF inhibitor therapy) [48], with lower rates of cutaneous events than those seen with either agent alone. Phase III studies comparing the combination of BRAF and MEK inhibition with BRAF-targeted therapy alone are currently underway (NCT01584648 and NCT01597908).

⦁ Role of MEK inhibition in uveal melanoma Metastatic uveal melanoma continues to be an exceedingly difficult disease to treat. Because of its relative rarity compared

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with other subtypes of melanoma, robust clinical data are often lacking, but advanced disease has been resistant to most types of systemic therapy [49]. BRAF mutations have yet to be identified in uveal melanoma, but mutations in GNAQ or GNA11 have been shown to alter MAPK pathway activation and are likely key mediators of oncogenesis in this disease [22,24]. In a Phase I study of trametinib, which included a cohort of 16 patients with uveal melanoma, two patients, one of whom had GNAQ mutation, experienced 24% tumor reduction and four patients had stable disease lasting for 16 weeks or longer [35]. Mutations in GNAQ or GNA11 were identified in six patients, but there was no clear correlation with response.

⦁ Conclusion

The development of selectively targeted therapy represents a paradigm shift in the treatment of melanoma. Trametinib is an orally bioavailable, highly selective inhibitor of MEK1 and MEK2 with a favorable pharmacokinetic profile. Preclin- ical data suggest that MAPK dysregulation is an important driver of melanomagenesis and that MEK inhibition, either alone or in combination with other therapy, is an important strategy for targeting this pathway. Trametinib has shown promising clinical efficacy in early-phase trials, particularly in patients with BRAFV600 melanoma, and longer PFS and OS has been observed with this agent over cytotoxic chemo- therapeutic drugs in a recent Phase III study. Future strategies include optimizing patient selection and using combination strategies to investigate synergistic effects and overcome resistance. Further evaluation of trametinib in patients with non-BRAFV600 mutations and other distinct genomic subsets is warranted.

⦁ Expert opinion

Selective BRAF inhibitors, such as vemurafenib and dabrafe- nib, significantly dephosphorylate MEK and ERK1/2 proteins in melanomas harboring a BRAF mutation [37,50] and in the Phase III studies, vemurafenib and dabrafenib also resulted in longer OS or PFS than did dacarbazine [10,11]. As a result, vemurafenib was approved by the FDA in 2011 for the treatment of advanced melanoma harboring a BRAF muta- tion, and dabrafenib is now under review for possible approval. However, resistance to BRAF inhibitors develops quickly, frequently within 6 months of treatment, via a number of escape mechanisms, many of which are related to the reactiva- tion of the MAPK pathway [42,45,46,51]. Although NRAS and BRAF mutations are mutually exclusive within the same mel- anoma, secondary NRAS mutations may develop in BRAF- mutant melanomas at the time of disease progression during treatment with a BRAF inhibitor [41] In addition, develop- ment of new mutations in MEK gene, dimerization of RAF kinases, increased expression of COT-1 serine/threonine kinase protein, or development of alternate splicing of BRAF
gene could render cells resistant to the BRAF inhibitors [41,44]. These genetic and epigenetic aberrations are responsible for reactivation of the MAPK pathway despite continual inhibition of mutated BRAF kinase.
Accordingly, targeting MEK protein could be an attractive strategy for inhibiting the MAPK pathway. A number of MEK inhibitors have been investigated in clinical studies. The early-generation MEK inhibitors CI-1040 and PD0325901 (both from Pfizer, New York, NY, USA) had limited clinical efficacy and notable adverse events, including retinal vein occlusion [27,52], so further development in advanced mela- noma was halted. Selumetinib (AZD6244, AstraZeneca, London, UK), a selective, allosteric inhibitor of MEK1/2, was tested in a randomized Phase II clinical study against temozolomide with chemotherapy-na¨ıve patients with meta- static melanoma [26]. Unfortunately, there appeared to be no clinical benefit over temozolomide with regard to response rate and median PFS, and the response rate of selumetinib among patients with BRAF-mutant melanoma was only 11%.
In preclinical studies, trametinib had antiproliferative activity in cell lines of various cancer types, but especially toward BRAF- and RAS-mutant cancer cells [30,31]. Likewise, in vivo studies showed that trametinib caused the greatest TGI in BRAF- mutant tumors [30,31]. In a Phase I study, the recommended Phase II dose was determined to be 2 mg/day based on the safety profile, pharmacokinetic and pharmacodynamic data, and clinical activity, although the maximum tolerated dose was 3 mg/day [34]. The most common adverse events at 2 mg/day were rash/dermatitis acneiform, peripheral edema, fatigue and nausea; life-threatening adverse events associated with typical cytotoxic drugs (such as severe myelosuppression) were not a major concern. The Phase II study demonstrated that the response rate of trametinib was 25% and the median PFS was
4 months among BRAF inhibitor-na¨ıve patients previously treated with systemic therapy [36]. The promising clinical activity observed in the Phase II study was confirmed in a Phase III trial in which both PFS and OS were superior with trametinib com- pared with cytotoxic chemotherapy (dacarbazine or paclitaxel) in patients with BRAFV600E/K melanoma [25].
Thus far, there has been no head-to-head study to compare BRAF inhibitors and trametinib for clinical outcome in this patient population; therefore, it is premature to conclude whether one class of drug is better than the other. However, among patients with advanced BRAF-mutant melanoma, the response rate is ~ 50% with selective BRAF inhibitors and
22 -- 25% with trametinib [5,14,25,36]. Phase III studies of
the combination of dabrafenib and trametinib is currently underway, and the results will be available shortly.
Based on the available data, it is likely that BRAF inhibitors will be favored over trametinib for treating BRAFV600 mela- noma even if the latter receives approval from the FDA for treatment of melanoma harboring a BRAF mutation. How- ever, trametinib can be a good alternative treatment option for patients who do not tolerate BRAF inhibitors. In addition, rare non-BRAFV600E/K melanomas might be more sensitive to

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MEK inhibitors than BRAF inhibitors. In the Phase I and Phase II studies of trametinib, three of the four patients with a non-BRAFV600E/K mutation had a confirmed clinical response [36]. It is interesting to note that melanoma cells with activated MAPK pathway induced by either a BRAFL597 or a BRAFK601E mutation had a more dramatic decrease of phospho-ERK expression with trametinib than with vemurafenib in in vitro experiments [53], suggesting that trametinib may have a better clinical activity than BRAF inhibitors in patients with a non-BRAFV600 mutation. However, the clin- ical benefit of trametinib and BRAF inhibitors needs to be evaluated with larger cohorts of patients before conclusions can be drawn.
BRAF-mutant melanomas that acquire resistance to a BRAF inhibitor also appear to be resistant to subsequent treatment with trametinib alone. In the Phase II study of tra- metinib, there was no case of confirmed response among 40 patients who had previously been treated with a selective BRAF inhibitor [36]. In contrast, 5 of 26 patients previously treated with a BRAF inhibitor had a clinical response to the combination of dabrafenib and trametinib in a Phase I/II study [54]. These findings suggest that the mechanisms of resis- tance to BRAF inhibitors and MEK inhibitors overlap to a certain degree. The findings also suggest the potential clinical benefit of continually inhibiting mutant BRAF in subsequent treatment even after disease progression with a previous BRAF-inhibiting treatment.
The best use of trametinib or other selective MEK inhibitors may be in combination with other targeted drugs rather than as a single agent. It appears to be the case, especially in the combi- nation with a BRAF inhibitor. In the recent Phase I/II study of the combination of trametinib and dabrafenib in patients with BRAFV600E/K melanoma, the response rate was greater and the median PFS time was longer in the higher-dose combination regimen (150 mg b.i.d. dabrafenib/2 mg/day trametinib) than
with dabrafenib monotherapy [48]. Moreover, the combination regimen was well tolerated, and parts of the toxicity profile of the combination regimen were more favorable, especially with regard to skin rash, cutaneous keratoacanthoma and squamous cell carcinoma [48]. These observations indicated inhibition of the paradoxical MAPK activation induced by a BRAF inhibitor in wild-type BRAF normal tissues, such as keratinocytes, as pre- dicted by the preclinical data. Two large randomized Phase III studies currently underway will more definitely clarify the clinical superiority of the combination regimen over the single-agent BRAF inhibitor.
Trametinib or other MEK inhibitors might be able inhibit the MAPK pathway in melanomas harboring a NRAS, GNAQ or GNA11 mutation because these mutations induce melanoma cell proliferation, at least in part through the activation of MEK. In a Phase II study of the selective MEK inhibitor MEK162 (Novartis, Basel, Switzerland), three confirmed and three unconfirmed responses were observed among 28 evaluable patients with NRAS-mutated advanced melanoma, suggesting the potential benefit of MEK inhibition for this patient population [55]. Strong clinical evidence for trametinib’s activity in patients with NRAS-, GNAQ- or GNA11-mutant melanoma is lacking at this time. A number of trials of trametinib-based combinations are currently being investigated with patients who have advanced melanoma with these genomic mutations. Further clinical investigation of tremetinib in combination with another targeted drug or drugs in patients with melanoma with wild-type BRAF and NRAS, a less well-defined genomic subset, is certainly warranted.

Declaration of interest

KB Kim has received research funding from GlaxoSmithKline (no personal compensation). AKS Salama declares no conflict of interest.

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Affiliation
April KS Salama1 MD & Kevin B Kim†2 MD
†Author for correspondence
1Instructor of Medicine,
Duke University Medical Center, Division of Medical Oncology, Durham, NC, USA
2Associate Professor,
The University of Texas MD Anderson Cancer Center, Department of Melanoma Medical Oncology,
Unit 430, 1515 Holcombe Blvd, Houston, TX 77030, USA
Tel: +1 713 792 2921; Fax: +1 713 745 1046;
E-mail: [email protected]