Selumetinib in the Treatment of NSCLC

Selumetinib in the treatment of NSCLC

Reyes Bernabé1,2, Ana Patrao1, Louise Carter1, Fiona Blackhall3,1, Emma Dean3,1

Affiliations:
1 The Christie NHS Foundation Trust, Manchester, UK
2 Hospital Valme, Seville, Spain
3 The University of Manchester, Manchester, UK

Conflicts of Interest
Dr. Emma Dean has been Chief Investigator on commercial trials of selumetinib.

Introduction

Lung cancer is the commonest cancer worldwide with an estimated 1.8 million new cases per year and high mortality rates representing 1 in 5 deaths from cancer [1]. NSCLC (non-small cell lung cancer) accounts for approximately 85% of all cases of lung cancer and about 70% present with locally advanced or metastatic disease at diagnosis [2]. Chemotherapy is the backbone of treatment for many patients, but increased knowledge about cancer biology garnered interest in the development of targeted therapies against molecular drivers within the tumour. Epidermal growth factor receptor (EGFR) and anaplastic lymphoma kinase (ALK) inhibitors have proven efficacy with response rates of around 45-65%, and are standard treatment strategies in patients with EGFR mutant and ALK mutant disease, respectively [3, 4].

The RAS-RAF-MEK-ERK pathway has been extensively researched in the past 25 years and it is estimated that one third of human cancers contain mutations in this pathway [5]. KRAS mutations are the most common mutations seen in NSCLC with a prevalence of approximately 30% in patients with adenocarcinoma and about 6% in squamous histology [6]. RAS proteins are anchored to the cytoplasmic side of the plasma membrane and are responsible for the communication of external cellular signals to the nucleus [7]. In normal cells, when RAS signalling is activated, it interacts with RAF (A-RAF, B-RAF, C-RAF and RAF-1), a serine/threonine kinase, leading to its activation. The activated RAF phosphorylates and activates MEK 1 and MEK 2 kinases, leading to downstream phosphorylation and activation of extracellular signal-regulated kinases, ERK 1 and 2. This activation triggers downstream activation of nuclear and cytoplasmic targets associated with transcription, cell proliferation, differentiation and metabolism, [6,7], Figure 1.

Mutations in the KRAS pathway lead to constitutive activation of the RAF-MEK-ERK pathway. Originally, KRAS mutations were considered to have similar clinical and biological activity but recent data suggests that different KRAS mutations might produce different downstream signalling effects. KRAS G12D, the dominant mutation found in the tumours of non-smokers seems to transduce signals via the PI3K-AKT, INK, p38 and FAK pathways [8]. There is also some evidence that MEK signalling can be activated independent of RAS signal by MAP3k1 and MAP3k8 which are mutated in some tumour types [6].

Overview of RAS-RAF-MEK-ERK targeted therapies

Recognition of the RAS-RAF-MEK-ERK pathway as an oncogenic driver in tumours has led to the development of targeted agents that can inhibit its activity. Efforts to inhibit mutant RAS or its membrane association using farnesyl transferase inhibitors failed to show clinical benefit [9]. Two BRAF inhibitors are currently approved for the treatment of BRAF mutated malignant melanoma, vemurafenib and dabrafenib. Although impressive initial responses are observed, unfortunately the majority of patients relapse in less than one year. Second generation B-RAF inhibitors are being developed [5]. The most advanced in development is encorafenib (LGX-818, Novartis, Array), that is currently being tested in the phase III trial COLUMBUS versus vemurafenib either alone and in combination with a MEK-inhibitor [10].

MEK is a central component of the signalling cascade. MEK 1/2 inhibitors have their greatest anti-tumour effects in patients harbouring RAS or BRAF mutations but there are examples of activity in non-mutant patients. MEK1/2 inhibitors have also been implicated in modifying the response to cytotoxic chemotherapy and other targeted agents [11]. MEK inhibitors have been shown to induce apoptosis by reducing cyclin D1 levels and inducing p27KIP1 expression, as well as the dephosphorylation of Retinoblastoma protein (Rb) causing the arrest of cells in G1 phase. Furthermore, MEK inhibitors alter the balance between proapoptotic / prosurvival proteins from the Bcl2 family in favour of apoptosis [7].

Even though MEK seemed a promising target, the first generation MEK inhibitors, PD098059 and U0126, did not have in vivo activity [12,13]. CI-1040 and subsequently PD0325901 were the first to be tested in clinical trials but research was suspended due to lack of clinical activity and toxicity profile [14,15]. Second generation MEK inhibitors, believed to be more potent and less toxic, have shown more promise pre-clinically and have demonstrated efficacy in clinical trials. The only approved MEK inhibitor to date is trametinib, which is used in combination with dabrafenib for the treatment of malignant melanoma patients, but several others are in development [5]. In NSCLC, the MEK inhibitor selumetinib has shown the most promising results to date. Table 1 summarizes the MEK inhibitors that are currently in clinical trial or have been tested in NSCLC patients. Several other MEK inhibitors such as Pimasertinib, Refametinib, E06201 and cobimetinib are in development in other cancer disease types [5].

ERK is the only substrate to MEK kinase and has been considered a druggable target although most efforts have focussed on upstream blockade. Several ERK inhibitors have been studied in phase I trials; Ulixertinib (BVD-523), GDC-0994 (RG-7842), CC-90003 and SCH900353. The results of the Phase I trial of Ulixertinib were presented at ASCO 2015 where it was reported that the toxicity profile was manageable and the maximum tolerated dose was 600 mg twice a day. Other ERK inhibitor trials are still ongoing [16].

Selumetinib

Selumetinib (AZD6244: ARRY-142866) is an orally available, potent, selective inhibitor of MEK 1 and 2 [6]. This drug has been extensively researched in several tumour types with mixed results, Table 2. In this review we will analyse its pharmacokinetic and pharmacodynamic properties as well as discuss trials results considering its clinical efficacy, toxicity and potential developments in the future in NSCLC.

Chemistry

Selumetinib, (6-(4-bromo-2-chloroanilino)-7-fluoro-N-(2-hydroxyethoxy)-3-methylbenzimidazole-5-carboxamide) has the molecular formula C17H15BrClFN4O3 and the molecular weight of 457.681403 g/mol. It is a second generation, orally active small molecule that acts as a selective and ATP-uncompetitive inhibitor of MEK 1 and 2, binding to the allosteric binding site.

Pharmacodynamics

The activity of selumetinib has been investigated in a number of preclinical studies. Selumetinib inhibits the enzymatic activity of purified constitutively active MEK 1 with a half maximal inhibitory concentration (IC50) of 14 nmol/L. It is highly selective for MEK 1 and 2 compared to 40 other serine/threonine and tyrosine kinases at concentrations of up to 10 µmol/L [17]. Selumetinib causes inhibition of growth in a range of cell lines including NSCLC, melanoma, pancreatic and colorectal cell lines. Analysis of the IC50 in cell lines showed a tendency for cell lines with BRAF and RAS mutations to be more sensitive to selumetinib than those that were wild type for the genes, though this trend was not absolute, particularly amongst KRAS mutated cell lines [18]. Selumetinib had little effect on the growth of Malme-3, the control cell line to the melanoma cell line Malme-3M, suggesting its effects are not due to general cytotoxicity [17].

Analysis of a range of cell lines showed that selumetinib effectively inhibits the phosphorylation of ERK 1 and 2, which are substrates of MEK 1 and 2 in the MAP kinase pathway [17,18]. The inhibition of ERK phosphorylation by selumetinib has also been confirmed in clinical trials through the analysis of both circulating lymphocytes and tumour samples before and after dosing. In circulating lymphocytes up to 100% inhibition of ERK phosphorylation was seen 1 hour after the first dose of selumetinib and with continued dosing up to 90% inhibition was seen in trough samples at day 15 and 22, confirming target inhibition [19]. In the analysis of paired tumour biopsies, inhibition of ERK phosphorylation by on average 79% was also demonstrated by IHC, though Ki67 was not as consistently reduced in these samples [19].

Preclinical animal studies have confirmed that selumetinib causes tumour growth inhibition in mouse models bearing xenografts containing both KRAS and BRAF wild type and mutated genes [17,18]. Analysis of the effect of chronic twice daily dosing of 25mg/kg of selumetinib showed stasis in Colo-205 tumours, moderate inhibition of growth in SW-620 tumours (colon) and strong inhibition of growth in Calu-6 tumours (head and neck), suggesting that in vitro sensitivity generally predicts for in vivo sensitivity [18]. Inhibition of ERK phosphorylation was also demonstrated, with an inverse correlation between the plasma drug concentration and the levels of phosphorylated ERK seen in Calu-6 xenografts [18]. Increased markers of apoptosis such as cleaved caspase 3 and decreased cell proliferation were seen in response to treatment with selumetinib in the xenograft models [18].

Selumetinib has been shown in preclinical studies to be effective when combined with both standard cytotoxics such as docetaxel and irinotecan, and targeted agents such as mTOR inhibitors and AKT inhibitors [11,18,20,21,22,23,24]. The combination of docetaxel and selumetinib has been shown to be synergistic in studies utilising SW-620 xenograft mouse models [18]. The response of genetically engineered KRAS NSCLC mouse models to docetaxel was reduced if there was concomitant loss of p53 or Lkb1 [25]. However, the addition of selumetinib to docetaxel showed significant augmentation of the response to docetaxel in the KRAS mutant models and the KRAS/p53 mutant mouse models. The KRAS/Lkb1 mutant mouse models were resistant to docetaxel and selumetinib. This suggests that response to selumetinib may be influenced by a combination of mutations rather than driven by a single mutation.

Pharmacokinetics and Metabolism

Selumetinib was originally formulated as a free-base suspension which was also referred to as selumetinib “mix and drink” formulation. After a single dose, the free-base suspension had a median half-life (t1/2) of 8.3 hours in a study of 57 patients [19]. The mean area under the plasma concentration-time curve (AUC) after both single doses and at steady state increased with increasing doses but in a less than dose-proportional manner. A capsule formulation incorporating a hydrogen sulfate salt (Hyd-Sulfate) was latterly formulated for ease of dosing. A phase I trial looking at the Hyd-Sulfate capsule showed it was rapidly absorbed with a median time to maximum plasma concentration (tmax) of 1 to 1.6 hours and median t1/2 of 5 to 8 hours [26]. The total body clearance (CL/F) and volume of distribution in the steady state (Vss/F) were consistent across the dose range studied, with mean values of 12 to 23L/h and 87 to 126 L respectively.

Comparison of the pharmacokinetics of the two selumetinib formulations was undertaken in a phase I trial in which single doses of each formula at maximum tolerated dose (MTD) were given sequentially to patients with a wash-out between the two formulations [26]. Analysis of results from 27 patients showed similar tmax and mean t1/2 obtained for both formulations as were CL/F and Vss/F. However, both the maximum serum concentration and AUC over 24 hours (AUC0-24) were higher at the MTD dose for the Hyd-Sulfate formulation than the free base solution at 1,316 ng/ml and 4,545 ng x h/ml respectively in contrast to the 523 ng/mland 2,260 ng x h/ml. The estimated oral bioavailability, calculated using the AUC0-24 of the Hyd-Sulfate formulation relative to the free-base suspension was 263% (90% CI 241-322%).

A randomised phase I study was performed to assess the effect of food on the absorption of selumetinib Hyd-Sulfate capsules [27]. It demonstrated that both the Cmax and AUC of selumetinib were decreased by 62% and 19% respectively in the fed versus the fasted state. The rate of absorption of selumetinib was delayed by approximately 2.5 hours in the presence of food. These results have led to the recommendation that selumetinib should be taken on an empty stomach (no food or drink for two hours prior and one hour after dosing).

Selumetinib is metabolised in the liver by the cytochrome P450 enzymes 1A2, 2C19 and 3A4 with CYP1A2 being responsible for the metabolism of Selumetinib to the active metabolite N-desmethyl-selumetinib [27]. In comparison to selumetinib, N-desmethyl-selumetinib showed 3 to 5 fold greater potency for the inhibition of MEK 1 but lower exposure (AUC) was noted. [26] Elimination of selumetinib is likely to be predominantly through glucuronidation as the majority of selumetinib metabolites are detected as glucuronide conjugates [27]. The selumetinib metabolites are then excreted in faeces. Trametinib and selumetinib have similar tmax values of approximately 1.5 hours but the t1/2 of trametinib is much longer than selumetinib at 4 days in contrast to less than 8 hours, highlighting one of the differences of Selumetinib compared to other MEK inhibitors [8].

CLINICAL TRIALS

PHASE I TRIALS

Initial phase I trials of selumetinib in patients with solid tumours established the MTD of the two formulations: free-base suspension and capsule. Subsequent phase I trials were performed to assess novel combinations of selumetinib with chemotherapies or targeted treatments.

Adjei et al. reported [19] that AZD6244 formulated as free based suspension, is well tolerated up to 100 mg bid. The maximum tolerated dose (MTD) was 200 mg bid, but due to a dose dependent increase in the frequency and severity of rash, a lower dose level (100 mg) was the recommended dose for phase II trials. Among the 57 patients analyzed, the most frequent toxicity was rash occurring in 74% of all patients. Nine of the 43 episodes of rash were grade 3 or 4 (20%). Other grade 3 or 4 toxicities reported were fatigue, oedema and ALT elevation. The best overall response was stable disease (SD) in 17 patients with solid tumours.

Barneji et al. reported the MTD of the hydrogen sulfate oral capsule formulation of selumetinib at 75 mg bid [26]. The overall evaluation of the safety and tolerability of Hyd-sulfate capsules showed a toxicity profile similar to that observed with the free-base suspension formulation. The most frequent grade 3/4 toxicity was fatigue (17%). Other grade 3/4 toxicities were acneiform dermatitis (5.7%), vomiting (5.7%), peripheral edema (2.9%), and exertional dyspnea (2.9%). Out of 55 patients with solid tumours evaluable for response, the best overall response seen was a complete response in one patient and a further 26 patients had SD.

Selumetinib and chemotherapy trials

The combination of docetaxel and selumetinib has been shown to be synergistic in vivo [18]. Two schedules were compared: a single dose of docetaxel (15 mg/kg) followed 24 hours later by selumetinib (25 mg/kg bid) for 7 days or selumetinib administered for 7 days followed 24 hours later by docetaxel. The tumor growth inhibition was 110% when docetaxel was administered before selumetinib compared with 61% when docetaxel was administered after selumetinib. These results suggested that the best sequence of treatment in order to enhance the efficacy was docetaxel followed by selumetinib [11].