- Email: [email protected]
BRAF inhibitors: the current and the future
Available online at www.sciencedirect.com
ScienceDirect BRAF inhibitors: the current and the future Weijiang Zhang The introduction of BRAF inhibitors (BRAFi), vemurafenib and dabrafenib, revolutionized BRAFV600-mutated metastatic melanoma treatment with improved response rate and overall survival compared to standard chemotherapy. However, the mechanism related cutaneous toxicity remains a concern. In addition, intrinsic and acquired resistance remain the key challenges in BRAFi therapy. Extensive efforts to elucidate the mechanisms have led to an improved understanding of disease biology and resulted in exploration of multiple new therapeutic options. While the future looks bright with multiple new therapeutic strategies in exploration and possible new generations of BRAFi, there are questions remaining to be answered to enable more efficient use of BRAFi in cancer therapy. Address Roche Innovation Center New York, 430 E. 29th Street, New York, NY 10016, United States Corresponding author: Zhang, Weijiang ([email protected])
Current Opinion in Pharmacology 2015, 23:68–73 This review comes from a themed issue on Cancer Edited by Alex Phipps
mutation in approximately 5–10% of human cancers, including in 66% of melanoma patients [1]. The majority (>90%) of the BRAF mutations happen at codon 600 with a change from valine to glutamic acid (V600E). The substitution of valine with other amino acids at codon 600 (V600K, V600D, or V600R) is less frequent. Before the development of vemurafenib to treat BRAFV600 mutated advanced melanoma, sorafenib was approved for the treatment of advanced renal cell carcinoma and unresectable hepatocellular carcinoma. Sorafenib was discovered as a RAF kinase inhibitor and was later found to be a multikinase inhibitor [2]. In addition to CRAF (IC50 value of 6 nM) and BRAF (IC50 values of 22 nM and 38 nM for wild type and V600 mutant, respectively), sorafenib also inhibits a number of other kinases, including vascular endothelial growth factor receptor (VEGFR) (IC50 values of 90 nM and 20 nM for VEGFR-2 and VEGFR-3, respectively) [2]. Clinical studies suggest that the clinical efficacy of sorafenib probably results from inhibition of VEGFR rather than inhibition of BRAF as no efficacy was observed in melanoma patients with either wild-type or mutated BRAF [3]. Compared to the non-selective inhibition of BRAF kinases of sorafenib, vemurafenib and the more latterly approved dabrafenib, selectively inhibit the oncogenic V600 mutated BRAF kinase.
http://dx.doi.org/10.1016/j.coph.2015.05.015 1471-4892/# 2015 Elsevier Ltd. All rights reserved.
Introduction The approval of ipilimumab and vemurafenib by FDA in 2011 started a new era for advanced melanoma treatment. Vemurafenib is an orally available small molecule, it is a potent and highly selective inhibitor to oncogenic BRAFV600 kinase. BRAF is a member of RAF serine/ threonine kinases (including CRAF and ARAF), which have important roles in mitogen-activated protein kinase (MAPK) signaling pathways (RAS–RAF–MEK–ERK cascade). Once active, RAF kinases phosphorylate and activate downstream MEK kinases, which in turn phosphorylate and activate ERK kinases, subsequently regulating a number of important cellular functions, such as cell survival, proliferation and apoptosis resistance. MAPK pathway is often found to be hyperactivated due to activation of upstream growth factors or mutations in RAS or BRAF, resulting in constitutive signaling leading to the oncogenic effects of cell proliferation and survival. In 2002, Davies et al. discovered BRAF Current Opinion in Pharmacology 2015, 23:68–73
The introduction of selective BRAFV600 inhibitors revolutionized advanced melanoma therapy with substantially improved therapeutic benefit to the patients, the discovery of various mechanisms of resistance to this class of compounds also inspired numerous research interests in the area, which will potentially nourish new therapeutic strategies and further improve clinical outcome. This review summarizes the clinical activity of BRAF inhibitors (BRAFi), the key challenges when using these drugs and potential strategies to overcome the challenges based on recent research progress in this area.
Current BRAFi Clinical activity
Vemurafenib and dabrafenib were approved for the treatment of BRAFV600 mutated unresectable or metastatic melanoma. Compared to the previously available chemotherapy dacarbazine (DTIC), vemurafenib and dabrafenib substantially improved response rates and overall survival (OS) in patients with BRAFV600 mutated metastatic melanoma. Vemurafenib achieved 50–80% objective response rate in clinical trials in patients with BRAF mutated unresectable or metastatic melanoma [4–6]. Compared to dacarbazine, vemurafenib prolonged the median progression-free survival (PFS) from two to seven www.sciencedirect.com
BRAF inhibitors: the current and the future Zhang 69
months, and the median OS was prolonged from nine to 14 months [6]. Similar results were also observed for dabrafenib in clinical trials [7]. The key features and properties of vemurafenib and dabrafenib, including pharmacokinetics/efficacy/safety information, are listed in Table 1 [8,9]. Safety and tolerability
Vemurafenib and dabrafenib are generally well tolerated in advanced melanoma patients with manageable adverse effects as shown in Table 1. Approximately one-third of patients require dose modification or interruption to manage the adverse events for either drug. Despite many similar adverse effects of these two drugs, there are some differences in abnormal laboratory values. Hyperglycemia (6% grades 3 and 4), hypophosphatemia (6% grades 3 and 4) and hyponatremia (2% grades 3 and 4) were only observed with dabrafenib; while elevated liver enzymes (gamma-glutamyl transpeptidase (GGT), alanine transaminase (ALT), aspartate transaminase (AST), bilirubin) were only reported for vemurafenib [8,9]. In addition, QT prolongation was observed with vemurafenib but not dabrafenib [8,9]. Cutaneous toxicities, most notably squamous cell carcinomas (SCC), are considered a mechanism-related class effect of BRAFi. The development of cutaneous toxicities of BRAFi may be explained by paradoxical activation of the MAPK pathway in wild-type BRAF cells [10,11]. The formation of homo or hetero RAF dimers in wild-type BRAF cells in the presence of oncogenic
RAS mutation and subsequently activation of MEK is considered the major cause for the observed cutaneous adverse effects of BRAFi [10,11]. The mechanism is not only suggested by preclinical studies, it is also demonstrated by the high prevalence of oncogenic RAS mutation in clinical samples for patients who developed SCC with BRAF inhibitor treatment [12–14]. The reported prevalence (30–70%) of RAS mutation in patients who developed SCC with BRAF inhibitor treatment is much higher than that in patients who developed SCC without the presence of BRAF inhibitor (3.2%) [12–14]. The mechanism of SCC in the absence of RAS mutation is still undergoing further research; it was suggested that AKT activation or amplification of epidermal growth factor receptor (EGFR) may play a role [15]. Given the mechanism of paradoxical activation of the MAPK pathway for the cutaneous toxicity, dual inhibition of mutated BRAF and MEK or ERK has been evaluated to block the pathway. Concomitant MEK inhibitor administration has improved the skin toxicity profile of BRAF inhibitor by multiple clinical studies [16–18]. Recently, Sanlorenzo et al. directly compared skin toxicity in BRAFi/MEKi combination versus BRAFi monotherapy in melanoma; in the analysis, 44 patients either received vemurafenib alone, dabrafenib alone, vemurafenib with cobimetinib, or dabrafenib with trametinib [19]. As expected, cutaneous adverse events were less frequent in patients with BRAFi/MEKi combination compared with those in patients with BRAFi alone; despite each inhibitor and combination having a particular cutaneous safety profile. In addition to combination with MEKi or ERKi, efforts to
Table 1 Key features and properties of vemurafenib and dabrafenib [8,9]
Dose proportionality Recommended dose Dosing instruction Absolute bioavailability Absorption affected by pH Single dose food effect Metabolism/transporter
Metabolites Estimated half-life Excretion (oral) Induction/inhibition of enzymes/transporters Phase III efficacy: Median OS Phase III efficacy: Median PFS Most common adverse reactions in package insert
www.sciencedirect.com
Vemurafenib
Dabrafenib
240–960 mg 960 mg BID With or without a meal NA No "Cmax 2.5 fold "AUC 5 fold CYP3A4 substrate (unlikely the major pathway) P-gp and BCRP substrate <5% in plasma Median (57 h) by population PK analysis 94% feces, 1% urine Induce CYP3A4; inhibit CYP1A2, Pgp, and BCRP
12–300 mg 150 mg BID At least 1 h before or at least 2 h after a meal 95% Yes #Cmax 51% #AUC 31% Primarily metabolized by CYP2C8 and CYP3A4 P-gp and BCRP substrate
Vemurafenib: 13.6 months Dacarbazine: 9.7 months Vemurafenib: 5.3 months Dacarbazine: 1.6 months Arthralgia, rash, alopecia, fatigue, photosensitivity reaction, nausea, pruritus and skin papilloma
>10% and probably active Mean 8 h 71% feces 23% urine Induce CYP3A4, CYP2C8, CYP2C9, CYP2C19, CYP2B6, UGT; inhibit BCRP, OATP1B1, OATP1B3, OAT1, and OAT3 Dabrafenib: 18.2 months Dacarbazine: 15.6 months Dabrafenib: 6.7 months Dacarbazine: 2.9 months Hyperkeratosis, headache, pyrexia, arthralgia, papilloma, alopecia, and palmar-plantar erythrodysesthesia syndrome
Current Opinion in Pharmacology 2015, 23:68–73
70 Cancer
develop a new generation of BRAFi without the paradoxical effects in wild-type cells is ongoing to potentially provide therapeutic benefit to patients with an improved skin toxicity profile [20,21]. Resistance
Intrinsic and acquired resistance remain the key challenges in BRAF inhibitor therapy. Approximately 10–15% of patients with mutant BRAF never respond to BRAF inhibitor therapy due to intrinsic resistance. For those patients who respond, acquired resistance to BRAF inhibitor usually develops within 6–8 months, resulting in disease progression. Various mechanisms of resistance have been proposed and tested in vitro and in vivo; some of them have been confirmed by clinical samples. One common observation is that the resistance does not involve a second mutation in BRAF. The vigorous research on the mechanism of resistance is leading to more insights into the biology of disease, bringing the hope for more effective treatment strategies. Intrinsic resistance
Intrinsic resistance has been identified to be associated with a number of pre-existing molecular characteristics. The detection of heterogeneity of the BRAFV600E mutation within individual melanoma tumor specimens, and among multiple specimens from individuals by Yancovitz et al. provided a possible explanation for the intrinsic resistance [22]. A number of other factors were also reported to be associated with BRAF inhibitor intrinsic resistance and were recently reviewed [23,24]. Over expression of cyclin D1, which regulates cyclin dependent kinases 4 (CDK4) and CDK6, was demonstrated to confer resistance to BRAFi. The loss of phosphatase and tensin homolog (PTEN), a tumor suppressor in PI3K-AKTmTOR pathway, was also reported to be associated with BRAFi resistance. Other reported mechanisms include RACIp298 mutations, COT overexpression, loss of NF1, hepatocyte growth factor (HGF) secreted by stromal cells, alternative receptor tyrosine kinase (RTK) signaling, HOXD8 mutations and increased IGF-I signaling.
one member of the dimer. Increased NRAS expression without active mutation in NRAS gene was also reported from recent in vitro studies by Lidsky et al. as a mechanism for acquired vemurafenib resistance [25]. Secondary mutations in downstream MEK1 and MEK2 were also linked to MAPK reactivation. The RAF level signals include dimerization of aberrantly spliced BRAFV600E and BRAFV600E amplification, elevated CRAF level leading to increased ERK phosphorylation bypassing BRAF. Most recently, Goetz et al. identified multiple point mutations in ERK1 (MAPK3) and ERK2 (MAPK1) that could confer resistance to ERK or RAF/MEK inhibitors [26]. The PI3K-AKT-mTOR pathway regulates important cellular functions including cell proliferation, growth, and survival. The cross reaction between the PI3K pathway and the MAPK pathway is well recognized. The PI3K pathway can be activated by multiple factors, such as RTKs, active mutations, gene amplification and functional loss of the pathway (e.g. loss of PTEN). The active mutations in the pathway, such as AKT1/3 mutations and pathway positive-regulator and negative-regulator mutations may activate PI3K pathway, leading to the resistance. Using transposon mutagenesis in a melanoma model, Perna et al. recently reported that AKT-mediated inactivation of BAD (BCL2-associated agonist of cell death) constitutes a pathway that may contribute to HGF-mediated BRAFi resistance [27]. Numerous other factors have been reported to be activated in resistance to BRAF inhibition, including platelet-derived growth factor b (PDGFRb), PDGFRa, insulin-like growth factor 1 receptor (IGF1R), HGF–MET, fibroblast growth factor receptor 3 (FGFR3), and EGFR [23,24]. The signaling of RTKs is complicated, and often affects both MAPK and PI3K pathways, thus making it an attractive target option for efficient treatment. Most recently, Miao et al. reported induction of EPHA2, a member of Eph receptor family, as a mechanism for BRAF inhibition resistance, also suggesting EPHA2 as a target in BRAF inhibitor-resistant melanoma [28].
Acquired resistance
The reactivation of MAPK signaling pathway is the major mechanism for acquired resistance. Less frequently observed resistance mechanisms involve activation of other proliferation or pro-survival pathways. PI3K-AKT-mTOR pathway up-regulation represents another crucial mechanism for BRAFi acquired resistance. The reactivation of MAPK pathway, mostly bypassing mutated BRAF, can be triggered by upstream, downstream or RAF level signals [23,24]. Upstream upregulation of RTKs and activating NRAS mutation were reported to be associated with MAPK reactivation. NRAS mutation increases RAF dimer formation and results in paradoxical activation of MEK with BRAFi binding to Current Opinion in Pharmacology 2015, 23:68–73
The role of autophagy in BRAF mutated tumor has been recently recognized [29–31]. Autophagy inhibition, together with MEK inhibition, has been demonstrated to restore cell death in vemurafenib-resistant cells [31]. In addition, the possible involvement of two trans-membrane proteins, P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP), has been suggested as a resistance mechanism based on in vitro experiments since both vemurafenib and dabrafenib are P-gp and BCRP substrates [32,33]. However, considering that vemurafenib also inhibits both P-gp and BCRP and dabrafenib inhibits BCRP [8–9], it is unlikely that these transporters play crucial role in the resistance to BRAF inhibition. Smit et al. recently published results using (phospho) www.sciencedirect.com
BRAF inhibitors: the current and the future Zhang 71
proteomic and functional genomic platforms to identify potential drug target for BRAF mutant melanoma. ROCK1 silencing was shown to increase melanoma cell elimination when combined with BRAF or ERK inhibitor treatment, representing a new resistance mechanism and drug target [34].
preclinical models and altered dosing may prevent the emergence of lethal drug resistance [38]. Cessation of dose is often used in clinical cases to manage side effects [39]. Considering tumor heterogeneity [22] and preclinical and clinical differences in pharmacokinetic/pharmacodynamic properties of BRAFi, it will be interesting to see how these preclinical findings may translate into clinical outcome.
Future of BRAFi The BRAFi, as standard therapy for BRAF mutated advanced melanoma, improved clinical benefit compared to chemotherapy. However, intrinsic and acquired resistance are key challenges when using these drugs. The active research to understand the mechanisms of resistance has made remarkable progress in understanding the disease biology, continuously bringing new therapeutic strategies into scope. There are many questions to be answered while optimizing the therapeutic strategy for BRAFi: can BRAFi benefit patients with BRAF mutation in tumor types other than melanoma; will intermittent dosing reduce toxicity and prevent resistance; what is the best strategy for combination: what drugs to combine with and how to combine (simultaneous or sequential treatment schedules); will next-generation of BRAFi be devoid of paradoxical effects in wild-type cells or with dual inhibition activities; will the target therapy be more ‘personalized’ in the future based on individual tumor micro environment. The answers to these questions will probably shape the future of BRAFi and are discussed further.
Combinations
Drug combination is the most recognized strategy to improve cutaneous safety profile and delay resistance for BRARi; new combination strategies constantly emerge as the knowledge of drug resistance grows [40,41]. There are more than 100 ongoing clinical trials for combination therapy with BRAFi, including combinations with MEK inhibitor, immunotherapy, PI3K pathway inhibitor, chemotherapy, RTK inhibitor, etc. [42]. The combination of BRAF inhibitor with MEK inhibitor improved clinical response and skin toxicity profile, as demonstrated in recent clinical trials [16–18]. The combination of BRAF inhibitor and EGFR inhibitor in BRAF mutant metastatic CRC patients is reported to be well tolerated, with less cutaneous toxicity and results in modest clinical activity (partial responses in two patients of 12 evaluable patients, and stable disease lasting over six months in two patients) [43]. The optimized combination strategy for BRAF inhibitor will continue to be refined with ongoing and future clinical trials. The next-generation BRAF inhibitor
BRAF inhibitor in other tumor types
In addition to its high prevalence in melanoma, BRAF mutation is also found in many other cancers, such as colorectal cancer (CRC), thyroid cancer, ovarian cancer, non-small cell lung cancer (NSCLC), gastric cancer, cholangiocarcinoma (CLC), hairy cell leukemia (HCL) [35]. The effectiveness of BRAFi in these tumor types remain to be validated by clinical trials although results from clinical case reports and clinical trials to evaluate BRAFi in different tumor types are emerging. Del Bufalo et al. recently observed some clinical benefit in a case of BRAFV600E mutated cervicomedullary ganglioglioma treated with vemurafenib as single agent [36]. The preliminary results from the vemurafenib BASKET study with multiple BRAFV600 mutated tumor types are being presented at various meetings with promising activity in patients with NSCLC, CLC and Erdheim-Chester disease (ECD)/Langerhans cell histiocytosis (LCH) (ASCO 2014, ASH 2014). Impressive clinical results have been observed in refractory HCL patients treated with vemurafenib in multiple clinical trials, summarized by a recent review [37]. Intermittent dosing
Das Thakur et al. reported that cessation of drug administration (4-week on drug and 2-week off drug schedule) leads to regression of vemurafenib resistant tumors in www.sciencedirect.com
There are multiple BRAFi in clinical development (e.g. LGX818, XL281, RO51267566, CEP-32496). With the experience from current approved BRAFi, most clinical trials are evaluating candidates in combination with immunotherapy or inhibitors to MEK, CDK4/6, PI3K, etc. Among them, RO51267566 is a dual MEK/RAF inhibitor with activity in RAS-mutated malignant tumor cells. New generation BRAFi (e.g. PLX7904, PLX8394) are being tested in preclinical animal models and human xenograft models. Recently, promising in vitro results have been reported for a new class of RAF inhibitors, paradox-breaker-04 (PB04, PLX7904) and its clinical analog PB03 (PLX8394) [20,21]. These compounds are selective, non-paradox-inducing RAF inhibitors, that demonstrated activities in mutant NRAS-mediated vemurafenib-resistant cells or vemurafenib-resistant cells with distinct BRAFV600E splice variants, suggesting potential for further investigation of these paradox-breaker RAF inhibitors to overcome BRAFi resistance [20,21]. More ‘personalized’ medicine?
Considering the complicated multiple mechanisms involved in cancer and tumor heterogeneity [22], with the advances in technology and vigorous research on understanding disease biology [27,34], will the target therapy be more ‘personalized’ based on genomic or proteomic screening of individual tumor micro environment? The Current Opinion in Pharmacology 2015, 23:68–73
72 Cancer
discovery of BRAF inhibitor has been revolutionary for personalized medicine in advanced melanoma. Whether further research in this field will bring more breakthroughs in personalized medicine remains to be answered.
Conflict of interest statement None declared.
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest 1.
Davies H, Bignell GR, Cox C et al.: Mutations of the BRAF gene in human cancer. Nature 2002, 417:949-954.
2.
Wilhelm SM, Carter C, Tang L, Wilkie D, McNabola A et al.: BAY 43-9006 exhibits broad spectrum oral antitumor activity and targets the RAF/MEK/ERK pathway and receptor tyrosine kinases involved in tumor progression and angiogenesis. Cancer Res 2004, 64:7099-7109.
3.
Hauschild A, Agarwala SS, Trefzer U, Hogg D, Robert C, Hersey P et al.: Results of a phase III, randomized, placebo-controlled study of sorafenib in combination with carboplatin and paclitaxel as second-line treatment in patients with unresectable stage III or stage IV melanoma. J Clin Oncol 2009, 27:2823-2830.
4.
Flaherty KT, Puzanov I, Kim KB et al.: Inhibition of mutated, activated BRAF in metastatic melanoma. N Engl J Med 2010, 363:809-819.
5.
Sosman JA, Kim KB, Schuchter L et al.: Survival in BRAF V600-mutant advanced melanoma treated with vemurafenib. N Engl J Med 2012, 366:707-714.
6.
McArthur GA, Chapman PB, Robert C, Larkin J, Haanen JB, Dummer R et al.: Safety and efficacy of vemurafenib in BRAFV600E and BRAFV600K mutation-positive melanoma (BRIM-3): extended follow-up of a phase 3, randomised, openlabel study. Lancet Oncol 2014, 15:323-332.
7.
Trinh VA, Davis JE, Anderson JE, Kim KB: Dabrafenib therapy for advanced melanoma. Ann Pharmacother 2014, 48:519-529.
8.
Tafinlar1 (Dabrafenib) US prescribing information. Research Triangle Park, NC: GlaxoSmithKline. http://us.gsk.com/products/ assets/us_tafinlar.pdf.
9.
Zelboraf1 (Vemurafenib) US prescribing information. South San Francisco, CA: Genentech USA, Inc. http://www. gene.com/ download/pdf/zelboraf_prescribing.pdf.
10. Heidorn SJ, Milagre C, Whittaker S et al.: Kinase-dead BRAF and oncogenic RAS cooperate to drive tumor progression through CRAF. Cell 2010, 140:209-221. 11. Hatzivassiliou G, Song K, Yen I et al.: RAF inhibitors prime wildtype RAF to activate the MAPK pathway and enhance growth. Nature 2010, 464:431-435. 12. Su F, Viros A, Milagre C et al.: RAS mutations in cutaneous squamous-cell carcinomas in patients treated with BRAF inhibitors. N Engl J Med 2012, 36:207-215. 13. Oberholzer PA, Kee D, Dziunycz P et al.: RAS mutations are associated with the development of cutaneous squamous cell tumors in patients treated with RAF inhibitors. J Clin Oncol 2012, 30:316-321. 14. Anforth R, Tembe V, Blumetti T, Fernandez-Pen˜as P: Mutational analysis of cutaneous squamous cell carcinomas and verrucal keratosis in patients taking BRAF inhibitors. Pigment Cell Melanoma Res 2012, 25:569-572. Current Opinion in Pharmacology 2015, 23:68–73
15. Uribe P, Gonzalez S: Epidermal growth factor receptor (EGFR) and squamous cell carcinoma of the skin: molecular bases for EGFR-targeted therapy. Pathol Res Pract 2011, 207:337-344. 16. Robert C, Karaszewska B, Schachter J, Rutkowski P, Mackiewicz A, Stroiakovski D, Lichinitser M, Dummer R, Grange F, Mortier L et al.: Improved overall survival in melanoma with combined dabrafenib and trametinib. N Engl J Med 2015, 372:30-39. 17. Long GV, Stroyakovskiy D, Gogas H, Levchenko E, de Braud F, Larkin J, Garbe C, Jouary T, Hauschild A, Grob JJ et al.: Combined BRAF and MEK inhibition versus BRAF inhibition alone in melanoma. N Engl J Med 2014, 371:1877-1888. 18. Ribas A, Gonzalez R, Pavlick A, Hamid O, Gajewski TF, Daud A, Flaherty L, Logan T, Chmielowski B, Lewis K et al.: Combination of vemurafenib and cobimetinib in patients with advanced BRAF(V600)-mutated melanoma: a phase 1b study. Lancet Oncol 2014, 15:954-965. 19. Sanlorenzo M, Choudhry A et al.: Comparative profile of cutaneous adverse events: BRAF/MEK inhibitor combination therapy versus BRAF monotherapy in melanoma. J Am Acad Dermatol 2014, 71:1102-1109. The analysis directly compared skin toxicity in BRAFi/MEKi combination versus BRAFi monotherapy in melanoma; in the analysis, 44 patients either received vemurafenib alone, dabrafenib alone, or vemurafenib with cobimetinib, dabrafenib with trametinib. As expected, cutaneous adverse events were less frequent in patients with BRAFi/MEKi combination compared with those in patients with BRAFi alone; despite each inhibitor and combination had a particular cutaneous safety profile. 20. Le K, Blomain ES, Rodeck U, Aplin AE: Selective RAF inhibitor impairs ERK1/2 phosphorylation and growth in mutant NRAS, vemurafenibresistant melanoma cells. Pigment Cell Melanoma Res 2013, 26:509-517. 21. Basile KJ, Le K, Hartsough EJ, Aplin AE: Inhibition of mutant BRAF splice variant signaling by next-generation, selective RAF inhibitors. Pigment Cell Melanoma Res 2014, 27:479-484. 22. Yancovitz M, Litterman A, Yoon J, Ng E, Shapiro RL, Berman RS, Pavlick AC, Darvishian F, Christos P, Mazumdar M, Osman I, Polsky D: Intra- and inter-tumor heterogeneity of BRAF(V600E) mutations in primary and metastatic melanoma. PLoS One 2012, 7:e29336. 23. Spagnolo F, Ghiorzo P et al.: BRAF-mutant melanoma: treatment approaches, resistance mechanisms, and diagnostic strategies. Onco Targets Ther 2015, 8:157-168. 24. Spagnolo F, Ghiorzo P, Queirolo P: Overcoming resistance to BRAF inhibition in BRAF-mutated metastatic melanoma. Oncotarget 2014, 5:10206-10221. 25. Lidsky M, Antoun G, Speicher P, Adams B, Turley R et al.: Mitogen-activated protein kinase (MAPK) hyperactivation and enhanced NRAS expression drive acquired vemurafenib resistance in V600E BRAF melanoma cells. J Biol Chem 2014, 289:27714-27726. In vitro study results indicate a mechanism of acquired vemurafenib resistance in V600E BRAF+ve melanoma cells that involves increased activation of all three human MAPKs and the PI3K pathway, as well as increased NRAS expression, which, contrary to previous reports, was not associated with mutations in the NRAS gene. 26. Goetz EM, Ghandi M, Treacy DJ, Wagle N, Garraway LA: ERK mutations confer resistance to mitogen-activated protein kinase pathway inhibitors. Cancer Res 2014, 74:7079-7089. A new mechanism of BRAFi resistance was identified using a random mutagenesis screen. Multiple point mutations in ERK1 (MAPK3) and ERK2 (MAPK1) that could confer resistance to ERK or RAF/MEK inhibitors. 27. Perna D, Karreth FA et al.: BRAF inhibitor resistance mediated by the AKT pathway in an oncogenic BRAF mouse melanoma model. Proc Natl Acad Sci U S A 2015, 112:E536-E545. Use of transposon mutagenesis in a melanoma model to identify that AKT-mediated inactivation of BAD (BCL2-associated agonist of cell death) constitutes a pathway that may contribute to HGF-mediated BRAFi resistance. 28. Miao B, Ji Z, Tan L et al.: EPHA2 is a mediator of vemurafenib resistance and a novel therapeutic target in melanoma. Cancer Discov 2015, 5:274-287. www.sciencedirect.com
BRAF inhibitors: the current and the future Zhang 73
A new mechanism of resistance is reported, the induction of EPHA2, a member of Eph receptor family was identified as a mechanism for BRAF inhibition resistance, it also demonstrated EPHA2 as a target in BRAF inhibitor-resistant melanoma. 29. Strohecker AM, White E: Autophagy promotes BrafV600Edriven lung tumorigenesis by preserving mitochondrial metabolism. Autophagy 2014, 10:384-385. 30. Levy JM, Thompson JC et al.: Autophagy inhibition improves chemosensitivity in BRAF(V600E) brain tumors. Cancer Discov 2014, 4:773-780. 31. Martin S, Dudek-Peric´ AM et al.: Concurrent MEK and autophagy inhibition is required to restore cell death associated danger-signalling in vemurafenib-resistant melanoma cells. Biochem Pharmacol 2015, 93:290-304.
35. Bollag G, Tsai J, Zhang J, Zhang C et al.: Vemurafenib: the first drug approved for BRAF-mutant cancer. Nat Rev Drug Discov 2012, 11:873-886. 36. Del Bufalo F, Carai A et al.: Response of recurrent BRAFV600E mutated ganglioglioma to vemurafenib as single agent. J Transl Med 2014, 12:356-362. 37. Jain P, Polliack A, Ravandi F: Novel therapeutic options for relapsed hairy cell leukemia. Leuk Lymphoma 2015, 4:1-9. 38. Das Thakur M, Salangsang F et al.: Modelling vemurafenib resistance in melanoma reveals a strategy to forestall drug resistance. Nature 2013, 494:251-255. 39. Dooley AJ, Gupta A, Bhattacharyya M, Middleton MR: Intermittent dosing with vemurafenib in BRAF V600E-mutant melanoma: review of a case series. Ther Adv Med Oncol 2014, 6:262-266.
32. Michaelis M, Rothweiler F et al.: Association between acquired resistance to PLX4032 (vemurafenib) and ATP-binding cassette transporter expression. BMC Res Notes 2014, 7:710-716.
40. Hu-Lieskovan S, Robert L, Homet Moreno B, Ribas A: Combining targeted therapy with immunotherapy in BRAF-mutant melanoma: promise and challenges. J Clin Oncol 2014, 32:2248-2254.
33. Wu CP, Sim HM, Huang YH, Liu YC, Hsiao SH, Cheng HW, Li YQ, Ambudkar SV, Hsu SC: Overexpression of ATP-binding cassette transporter ABCG2 as a potential mechanism of acquired resistance to vemurafenib in BRAF(V600E) mutant cancer cells. Biochem Pharmacol 2013, 85:325-334.
41. Kwong LN, Davies MA: Targeted therapy for melanoma: rational combinatorial approaches. Oncogene 2014, 33:1-9.
34. Smit MA, Maddalo G et al.: ROCK1 is a potential combinatorial drug target for BRAF mutant melanoma. Mol Syst Biol 2014, 10:772-786. The study used (phospho) proteomic and functional genomic platforms to identify potential drug target for BRAF mutant melanoma, ROCK1 silencing was shown to increase melanoma cell elimination when combined with BRAF or ERK inhibitor treatment, representing a new resistance mechanism and drug target.
www.sciencedirect.com
42. https://clinicaltrials.gov. 43. Yaeger R, Cercek A, O’Reilly EM, Reidy DL et al.: Pilot trial of combined BRAF and EGFR inhibition in BRAF-mutant metastatic colorectal cancer patients. Clin Cancer Res 2015, 21:1313-1320. A pilot study with dual inhibition of BRAF and EGFR, indicating that the combination of BRAF inhibitor and EGFR inhibitor in BRAF mutant metastatic CRC patients is well tolerated, with less cutaneous toxicity and results in partial responses in two patients (of 12 evaluable patients) and stable disease lasting over six months in two patients.
Current Opinion in Pharmacology 2015, 23:68–73
ScienceDirect BRAF inhibitors: the current and the future Weijiang Zhang The introduction of BRAF inhibitors (BRAFi), vemurafenib and dabrafenib, revolutionized BRAFV600-mutated metastatic melanoma treatment with improved response rate and overall survival compared to standard chemotherapy. However, the mechanism related cutaneous toxicity remains a concern. In addition, intrinsic and acquired resistance remain the key challenges in BRAFi therapy. Extensive efforts to elucidate the mechanisms have led to an improved understanding of disease biology and resulted in exploration of multiple new therapeutic options. While the future looks bright with multiple new therapeutic strategies in exploration and possible new generations of BRAFi, there are questions remaining to be answered to enable more efficient use of BRAFi in cancer therapy. Address Roche Innovation Center New York, 430 E. 29th Street, New York, NY 10016, United States Corresponding author: Zhang, Weijiang ([email protected])
Current Opinion in Pharmacology 2015, 23:68–73 This review comes from a themed issue on Cancer Edited by Alex Phipps
mutation in approximately 5–10% of human cancers, including in 66% of melanoma patients [1]. The majority (>90%) of the BRAF mutations happen at codon 600 with a change from valine to glutamic acid (V600E). The substitution of valine with other amino acids at codon 600 (V600K, V600D, or V600R) is less frequent. Before the development of vemurafenib to treat BRAFV600 mutated advanced melanoma, sorafenib was approved for the treatment of advanced renal cell carcinoma and unresectable hepatocellular carcinoma. Sorafenib was discovered as a RAF kinase inhibitor and was later found to be a multikinase inhibitor [2]. In addition to CRAF (IC50 value of 6 nM) and BRAF (IC50 values of 22 nM and 38 nM for wild type and V600 mutant, respectively), sorafenib also inhibits a number of other kinases, including vascular endothelial growth factor receptor (VEGFR) (IC50 values of 90 nM and 20 nM for VEGFR-2 and VEGFR-3, respectively) [2]. Clinical studies suggest that the clinical efficacy of sorafenib probably results from inhibition of VEGFR rather than inhibition of BRAF as no efficacy was observed in melanoma patients with either wild-type or mutated BRAF [3]. Compared to the non-selective inhibition of BRAF kinases of sorafenib, vemurafenib and the more latterly approved dabrafenib, selectively inhibit the oncogenic V600 mutated BRAF kinase.
http://dx.doi.org/10.1016/j.coph.2015.05.015 1471-4892/# 2015 Elsevier Ltd. All rights reserved.
Introduction The approval of ipilimumab and vemurafenib by FDA in 2011 started a new era for advanced melanoma treatment. Vemurafenib is an orally available small molecule, it is a potent and highly selective inhibitor to oncogenic BRAFV600 kinase. BRAF is a member of RAF serine/ threonine kinases (including CRAF and ARAF), which have important roles in mitogen-activated protein kinase (MAPK) signaling pathways (RAS–RAF–MEK–ERK cascade). Once active, RAF kinases phosphorylate and activate downstream MEK kinases, which in turn phosphorylate and activate ERK kinases, subsequently regulating a number of important cellular functions, such as cell survival, proliferation and apoptosis resistance. MAPK pathway is often found to be hyperactivated due to activation of upstream growth factors or mutations in RAS or BRAF, resulting in constitutive signaling leading to the oncogenic effects of cell proliferation and survival. In 2002, Davies et al. discovered BRAF Current Opinion in Pharmacology 2015, 23:68–73
The introduction of selective BRAFV600 inhibitors revolutionized advanced melanoma therapy with substantially improved therapeutic benefit to the patients, the discovery of various mechanisms of resistance to this class of compounds also inspired numerous research interests in the area, which will potentially nourish new therapeutic strategies and further improve clinical outcome. This review summarizes the clinical activity of BRAF inhibitors (BRAFi), the key challenges when using these drugs and potential strategies to overcome the challenges based on recent research progress in this area.
Current BRAFi Clinical activity
Vemurafenib and dabrafenib were approved for the treatment of BRAFV600 mutated unresectable or metastatic melanoma. Compared to the previously available chemotherapy dacarbazine (DTIC), vemurafenib and dabrafenib substantially improved response rates and overall survival (OS) in patients with BRAFV600 mutated metastatic melanoma. Vemurafenib achieved 50–80% objective response rate in clinical trials in patients with BRAF mutated unresectable or metastatic melanoma [4–6]. Compared to dacarbazine, vemurafenib prolonged the median progression-free survival (PFS) from two to seven www.sciencedirect.com
BRAF inhibitors: the current and the future Zhang 69
months, and the median OS was prolonged from nine to 14 months [6]. Similar results were also observed for dabrafenib in clinical trials [7]. The key features and properties of vemurafenib and dabrafenib, including pharmacokinetics/efficacy/safety information, are listed in Table 1 [8,9]. Safety and tolerability
Vemurafenib and dabrafenib are generally well tolerated in advanced melanoma patients with manageable adverse effects as shown in Table 1. Approximately one-third of patients require dose modification or interruption to manage the adverse events for either drug. Despite many similar adverse effects of these two drugs, there are some differences in abnormal laboratory values. Hyperglycemia (6% grades 3 and 4), hypophosphatemia (6% grades 3 and 4) and hyponatremia (2% grades 3 and 4) were only observed with dabrafenib; while elevated liver enzymes (gamma-glutamyl transpeptidase (GGT), alanine transaminase (ALT), aspartate transaminase (AST), bilirubin) were only reported for vemurafenib [8,9]. In addition, QT prolongation was observed with vemurafenib but not dabrafenib [8,9]. Cutaneous toxicities, most notably squamous cell carcinomas (SCC), are considered a mechanism-related class effect of BRAFi. The development of cutaneous toxicities of BRAFi may be explained by paradoxical activation of the MAPK pathway in wild-type BRAF cells [10,11]. The formation of homo or hetero RAF dimers in wild-type BRAF cells in the presence of oncogenic
RAS mutation and subsequently activation of MEK is considered the major cause for the observed cutaneous adverse effects of BRAFi [10,11]. The mechanism is not only suggested by preclinical studies, it is also demonstrated by the high prevalence of oncogenic RAS mutation in clinical samples for patients who developed SCC with BRAF inhibitor treatment [12–14]. The reported prevalence (30–70%) of RAS mutation in patients who developed SCC with BRAF inhibitor treatment is much higher than that in patients who developed SCC without the presence of BRAF inhibitor (3.2%) [12–14]. The mechanism of SCC in the absence of RAS mutation is still undergoing further research; it was suggested that AKT activation or amplification of epidermal growth factor receptor (EGFR) may play a role [15]. Given the mechanism of paradoxical activation of the MAPK pathway for the cutaneous toxicity, dual inhibition of mutated BRAF and MEK or ERK has been evaluated to block the pathway. Concomitant MEK inhibitor administration has improved the skin toxicity profile of BRAF inhibitor by multiple clinical studies [16–18]. Recently, Sanlorenzo et al. directly compared skin toxicity in BRAFi/MEKi combination versus BRAFi monotherapy in melanoma; in the analysis, 44 patients either received vemurafenib alone, dabrafenib alone, vemurafenib with cobimetinib, or dabrafenib with trametinib [19]. As expected, cutaneous adverse events were less frequent in patients with BRAFi/MEKi combination compared with those in patients with BRAFi alone; despite each inhibitor and combination having a particular cutaneous safety profile. In addition to combination with MEKi or ERKi, efforts to
Table 1 Key features and properties of vemurafenib and dabrafenib [8,9]
Dose proportionality Recommended dose Dosing instruction Absolute bioavailability Absorption affected by pH Single dose food effect Metabolism/transporter
Metabolites Estimated half-life Excretion (oral) Induction/inhibition of enzymes/transporters Phase III efficacy: Median OS Phase III efficacy: Median PFS Most common adverse reactions in package insert
www.sciencedirect.com
Vemurafenib
Dabrafenib
240–960 mg 960 mg BID With or without a meal NA No "Cmax 2.5 fold "AUC 5 fold CYP3A4 substrate (unlikely the major pathway) P-gp and BCRP substrate <5% in plasma Median (57 h) by population PK analysis 94% feces, 1% urine Induce CYP3A4; inhibit CYP1A2, Pgp, and BCRP
12–300 mg 150 mg BID At least 1 h before or at least 2 h after a meal 95% Yes #Cmax 51% #AUC 31% Primarily metabolized by CYP2C8 and CYP3A4 P-gp and BCRP substrate
Vemurafenib: 13.6 months Dacarbazine: 9.7 months Vemurafenib: 5.3 months Dacarbazine: 1.6 months Arthralgia, rash, alopecia, fatigue, photosensitivity reaction, nausea, pruritus and skin papilloma
>10% and probably active Mean 8 h 71% feces 23% urine Induce CYP3A4, CYP2C8, CYP2C9, CYP2C19, CYP2B6, UGT; inhibit BCRP, OATP1B1, OATP1B3, OAT1, and OAT3 Dabrafenib: 18.2 months Dacarbazine: 15.6 months Dabrafenib: 6.7 months Dacarbazine: 2.9 months Hyperkeratosis, headache, pyrexia, arthralgia, papilloma, alopecia, and palmar-plantar erythrodysesthesia syndrome
Current Opinion in Pharmacology 2015, 23:68–73
70 Cancer
develop a new generation of BRAFi without the paradoxical effects in wild-type cells is ongoing to potentially provide therapeutic benefit to patients with an improved skin toxicity profile [20,21]. Resistance
Intrinsic and acquired resistance remain the key challenges in BRAF inhibitor therapy. Approximately 10–15% of patients with mutant BRAF never respond to BRAF inhibitor therapy due to intrinsic resistance. For those patients who respond, acquired resistance to BRAF inhibitor usually develops within 6–8 months, resulting in disease progression. Various mechanisms of resistance have been proposed and tested in vitro and in vivo; some of them have been confirmed by clinical samples. One common observation is that the resistance does not involve a second mutation in BRAF. The vigorous research on the mechanism of resistance is leading to more insights into the biology of disease, bringing the hope for more effective treatment strategies. Intrinsic resistance
Intrinsic resistance has been identified to be associated with a number of pre-existing molecular characteristics. The detection of heterogeneity of the BRAFV600E mutation within individual melanoma tumor specimens, and among multiple specimens from individuals by Yancovitz et al. provided a possible explanation for the intrinsic resistance [22]. A number of other factors were also reported to be associated with BRAF inhibitor intrinsic resistance and were recently reviewed [23,24]. Over expression of cyclin D1, which regulates cyclin dependent kinases 4 (CDK4) and CDK6, was demonstrated to confer resistance to BRAFi. The loss of phosphatase and tensin homolog (PTEN), a tumor suppressor in PI3K-AKTmTOR pathway, was also reported to be associated with BRAFi resistance. Other reported mechanisms include RACIp298 mutations, COT overexpression, loss of NF1, hepatocyte growth factor (HGF) secreted by stromal cells, alternative receptor tyrosine kinase (RTK) signaling, HOXD8 mutations and increased IGF-I signaling.
one member of the dimer. Increased NRAS expression without active mutation in NRAS gene was also reported from recent in vitro studies by Lidsky et al. as a mechanism for acquired vemurafenib resistance [25]. Secondary mutations in downstream MEK1 and MEK2 were also linked to MAPK reactivation. The RAF level signals include dimerization of aberrantly spliced BRAFV600E and BRAFV600E amplification, elevated CRAF level leading to increased ERK phosphorylation bypassing BRAF. Most recently, Goetz et al. identified multiple point mutations in ERK1 (MAPK3) and ERK2 (MAPK1) that could confer resistance to ERK or RAF/MEK inhibitors [26]. The PI3K-AKT-mTOR pathway regulates important cellular functions including cell proliferation, growth, and survival. The cross reaction between the PI3K pathway and the MAPK pathway is well recognized. The PI3K pathway can be activated by multiple factors, such as RTKs, active mutations, gene amplification and functional loss of the pathway (e.g. loss of PTEN). The active mutations in the pathway, such as AKT1/3 mutations and pathway positive-regulator and negative-regulator mutations may activate PI3K pathway, leading to the resistance. Using transposon mutagenesis in a melanoma model, Perna et al. recently reported that AKT-mediated inactivation of BAD (BCL2-associated agonist of cell death) constitutes a pathway that may contribute to HGF-mediated BRAFi resistance [27]. Numerous other factors have been reported to be activated in resistance to BRAF inhibition, including platelet-derived growth factor b (PDGFRb), PDGFRa, insulin-like growth factor 1 receptor (IGF1R), HGF–MET, fibroblast growth factor receptor 3 (FGFR3), and EGFR [23,24]. The signaling of RTKs is complicated, and often affects both MAPK and PI3K pathways, thus making it an attractive target option for efficient treatment. Most recently, Miao et al. reported induction of EPHA2, a member of Eph receptor family, as a mechanism for BRAF inhibition resistance, also suggesting EPHA2 as a target in BRAF inhibitor-resistant melanoma [28].
Acquired resistance
The reactivation of MAPK signaling pathway is the major mechanism for acquired resistance. Less frequently observed resistance mechanisms involve activation of other proliferation or pro-survival pathways. PI3K-AKT-mTOR pathway up-regulation represents another crucial mechanism for BRAFi acquired resistance. The reactivation of MAPK pathway, mostly bypassing mutated BRAF, can be triggered by upstream, downstream or RAF level signals [23,24]. Upstream upregulation of RTKs and activating NRAS mutation were reported to be associated with MAPK reactivation. NRAS mutation increases RAF dimer formation and results in paradoxical activation of MEK with BRAFi binding to Current Opinion in Pharmacology 2015, 23:68–73
The role of autophagy in BRAF mutated tumor has been recently recognized [29–31]. Autophagy inhibition, together with MEK inhibition, has been demonstrated to restore cell death in vemurafenib-resistant cells [31]. In addition, the possible involvement of two trans-membrane proteins, P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP), has been suggested as a resistance mechanism based on in vitro experiments since both vemurafenib and dabrafenib are P-gp and BCRP substrates [32,33]. However, considering that vemurafenib also inhibits both P-gp and BCRP and dabrafenib inhibits BCRP [8–9], it is unlikely that these transporters play crucial role in the resistance to BRAF inhibition. Smit et al. recently published results using (phospho) www.sciencedirect.com
BRAF inhibitors: the current and the future Zhang 71
proteomic and functional genomic platforms to identify potential drug target for BRAF mutant melanoma. ROCK1 silencing was shown to increase melanoma cell elimination when combined with BRAF or ERK inhibitor treatment, representing a new resistance mechanism and drug target [34].
preclinical models and altered dosing may prevent the emergence of lethal drug resistance [38]. Cessation of dose is often used in clinical cases to manage side effects [39]. Considering tumor heterogeneity [22] and preclinical and clinical differences in pharmacokinetic/pharmacodynamic properties of BRAFi, it will be interesting to see how these preclinical findings may translate into clinical outcome.
Future of BRAFi The BRAFi, as standard therapy for BRAF mutated advanced melanoma, improved clinical benefit compared to chemotherapy. However, intrinsic and acquired resistance are key challenges when using these drugs. The active research to understand the mechanisms of resistance has made remarkable progress in understanding the disease biology, continuously bringing new therapeutic strategies into scope. There are many questions to be answered while optimizing the therapeutic strategy for BRAFi: can BRAFi benefit patients with BRAF mutation in tumor types other than melanoma; will intermittent dosing reduce toxicity and prevent resistance; what is the best strategy for combination: what drugs to combine with and how to combine (simultaneous or sequential treatment schedules); will next-generation of BRAFi be devoid of paradoxical effects in wild-type cells or with dual inhibition activities; will the target therapy be more ‘personalized’ in the future based on individual tumor micro environment. The answers to these questions will probably shape the future of BRAFi and are discussed further.
Combinations
Drug combination is the most recognized strategy to improve cutaneous safety profile and delay resistance for BRARi; new combination strategies constantly emerge as the knowledge of drug resistance grows [40,41]. There are more than 100 ongoing clinical trials for combination therapy with BRAFi, including combinations with MEK inhibitor, immunotherapy, PI3K pathway inhibitor, chemotherapy, RTK inhibitor, etc. [42]. The combination of BRAF inhibitor with MEK inhibitor improved clinical response and skin toxicity profile, as demonstrated in recent clinical trials [16–18]. The combination of BRAF inhibitor and EGFR inhibitor in BRAF mutant metastatic CRC patients is reported to be well tolerated, with less cutaneous toxicity and results in modest clinical activity (partial responses in two patients of 12 evaluable patients, and stable disease lasting over six months in two patients) [43]. The optimized combination strategy for BRAF inhibitor will continue to be refined with ongoing and future clinical trials. The next-generation BRAF inhibitor
BRAF inhibitor in other tumor types
In addition to its high prevalence in melanoma, BRAF mutation is also found in many other cancers, such as colorectal cancer (CRC), thyroid cancer, ovarian cancer, non-small cell lung cancer (NSCLC), gastric cancer, cholangiocarcinoma (CLC), hairy cell leukemia (HCL) [35]. The effectiveness of BRAFi in these tumor types remain to be validated by clinical trials although results from clinical case reports and clinical trials to evaluate BRAFi in different tumor types are emerging. Del Bufalo et al. recently observed some clinical benefit in a case of BRAFV600E mutated cervicomedullary ganglioglioma treated with vemurafenib as single agent [36]. The preliminary results from the vemurafenib BASKET study with multiple BRAFV600 mutated tumor types are being presented at various meetings with promising activity in patients with NSCLC, CLC and Erdheim-Chester disease (ECD)/Langerhans cell histiocytosis (LCH) (ASCO 2014, ASH 2014). Impressive clinical results have been observed in refractory HCL patients treated with vemurafenib in multiple clinical trials, summarized by a recent review [37]. Intermittent dosing
Das Thakur et al. reported that cessation of drug administration (4-week on drug and 2-week off drug schedule) leads to regression of vemurafenib resistant tumors in www.sciencedirect.com
There are multiple BRAFi in clinical development (e.g. LGX818, XL281, RO51267566, CEP-32496). With the experience from current approved BRAFi, most clinical trials are evaluating candidates in combination with immunotherapy or inhibitors to MEK, CDK4/6, PI3K, etc. Among them, RO51267566 is a dual MEK/RAF inhibitor with activity in RAS-mutated malignant tumor cells. New generation BRAFi (e.g. PLX7904, PLX8394) are being tested in preclinical animal models and human xenograft models. Recently, promising in vitro results have been reported for a new class of RAF inhibitors, paradox-breaker-04 (PB04, PLX7904) and its clinical analog PB03 (PLX8394) [20,21]. These compounds are selective, non-paradox-inducing RAF inhibitors, that demonstrated activities in mutant NRAS-mediated vemurafenib-resistant cells or vemurafenib-resistant cells with distinct BRAFV600E splice variants, suggesting potential for further investigation of these paradox-breaker RAF inhibitors to overcome BRAFi resistance [20,21]. More ‘personalized’ medicine?
Considering the complicated multiple mechanisms involved in cancer and tumor heterogeneity [22], with the advances in technology and vigorous research on understanding disease biology [27,34], will the target therapy be more ‘personalized’ based on genomic or proteomic screening of individual tumor micro environment? The Current Opinion in Pharmacology 2015, 23:68–73
72 Cancer
discovery of BRAF inhibitor has been revolutionary for personalized medicine in advanced melanoma. Whether further research in this field will bring more breakthroughs in personalized medicine remains to be answered.
Conflict of interest statement None declared.
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest 1.
Davies H, Bignell GR, Cox C et al.: Mutations of the BRAF gene in human cancer. Nature 2002, 417:949-954.
2.
Wilhelm SM, Carter C, Tang L, Wilkie D, McNabola A et al.: BAY 43-9006 exhibits broad spectrum oral antitumor activity and targets the RAF/MEK/ERK pathway and receptor tyrosine kinases involved in tumor progression and angiogenesis. Cancer Res 2004, 64:7099-7109.
3.
Hauschild A, Agarwala SS, Trefzer U, Hogg D, Robert C, Hersey P et al.: Results of a phase III, randomized, placebo-controlled study of sorafenib in combination with carboplatin and paclitaxel as second-line treatment in patients with unresectable stage III or stage IV melanoma. J Clin Oncol 2009, 27:2823-2830.
4.
Flaherty KT, Puzanov I, Kim KB et al.: Inhibition of mutated, activated BRAF in metastatic melanoma. N Engl J Med 2010, 363:809-819.
5.
Sosman JA, Kim KB, Schuchter L et al.: Survival in BRAF V600-mutant advanced melanoma treated with vemurafenib. N Engl J Med 2012, 366:707-714.
6.
McArthur GA, Chapman PB, Robert C, Larkin J, Haanen JB, Dummer R et al.: Safety and efficacy of vemurafenib in BRAFV600E and BRAFV600K mutation-positive melanoma (BRIM-3): extended follow-up of a phase 3, randomised, openlabel study. Lancet Oncol 2014, 15:323-332.
7.
Trinh VA, Davis JE, Anderson JE, Kim KB: Dabrafenib therapy for advanced melanoma. Ann Pharmacother 2014, 48:519-529.
8.
Tafinlar1 (Dabrafenib) US prescribing information. Research Triangle Park, NC: GlaxoSmithKline. http://us.gsk.com/products/ assets/us_tafinlar.pdf.
9.
Zelboraf1 (Vemurafenib) US prescribing information. South San Francisco, CA: Genentech USA, Inc. http://www. gene.com/ download/pdf/zelboraf_prescribing.pdf.
10. Heidorn SJ, Milagre C, Whittaker S et al.: Kinase-dead BRAF and oncogenic RAS cooperate to drive tumor progression through CRAF. Cell 2010, 140:209-221. 11. Hatzivassiliou G, Song K, Yen I et al.: RAF inhibitors prime wildtype RAF to activate the MAPK pathway and enhance growth. Nature 2010, 464:431-435. 12. Su F, Viros A, Milagre C et al.: RAS mutations in cutaneous squamous-cell carcinomas in patients treated with BRAF inhibitors. N Engl J Med 2012, 36:207-215. 13. Oberholzer PA, Kee D, Dziunycz P et al.: RAS mutations are associated with the development of cutaneous squamous cell tumors in patients treated with RAF inhibitors. J Clin Oncol 2012, 30:316-321. 14. Anforth R, Tembe V, Blumetti T, Fernandez-Pen˜as P: Mutational analysis of cutaneous squamous cell carcinomas and verrucal keratosis in patients taking BRAF inhibitors. Pigment Cell Melanoma Res 2012, 25:569-572. Current Opinion in Pharmacology 2015, 23:68–73
15. Uribe P, Gonzalez S: Epidermal growth factor receptor (EGFR) and squamous cell carcinoma of the skin: molecular bases for EGFR-targeted therapy. Pathol Res Pract 2011, 207:337-344. 16. Robert C, Karaszewska B, Schachter J, Rutkowski P, Mackiewicz A, Stroiakovski D, Lichinitser M, Dummer R, Grange F, Mortier L et al.: Improved overall survival in melanoma with combined dabrafenib and trametinib. N Engl J Med 2015, 372:30-39. 17. Long GV, Stroyakovskiy D, Gogas H, Levchenko E, de Braud F, Larkin J, Garbe C, Jouary T, Hauschild A, Grob JJ et al.: Combined BRAF and MEK inhibition versus BRAF inhibition alone in melanoma. N Engl J Med 2014, 371:1877-1888. 18. Ribas A, Gonzalez R, Pavlick A, Hamid O, Gajewski TF, Daud A, Flaherty L, Logan T, Chmielowski B, Lewis K et al.: Combination of vemurafenib and cobimetinib in patients with advanced BRAF(V600)-mutated melanoma: a phase 1b study. Lancet Oncol 2014, 15:954-965. 19. Sanlorenzo M, Choudhry A et al.: Comparative profile of cutaneous adverse events: BRAF/MEK inhibitor combination therapy versus BRAF monotherapy in melanoma. J Am Acad Dermatol 2014, 71:1102-1109. The analysis directly compared skin toxicity in BRAFi/MEKi combination versus BRAFi monotherapy in melanoma; in the analysis, 44 patients either received vemurafenib alone, dabrafenib alone, or vemurafenib with cobimetinib, dabrafenib with trametinib. As expected, cutaneous adverse events were less frequent in patients with BRAFi/MEKi combination compared with those in patients with BRAFi alone; despite each inhibitor and combination had a particular cutaneous safety profile. 20. Le K, Blomain ES, Rodeck U, Aplin AE: Selective RAF inhibitor impairs ERK1/2 phosphorylation and growth in mutant NRAS, vemurafenibresistant melanoma cells. Pigment Cell Melanoma Res 2013, 26:509-517. 21. Basile KJ, Le K, Hartsough EJ, Aplin AE: Inhibition of mutant BRAF splice variant signaling by next-generation, selective RAF inhibitors. Pigment Cell Melanoma Res 2014, 27:479-484. 22. Yancovitz M, Litterman A, Yoon J, Ng E, Shapiro RL, Berman RS, Pavlick AC, Darvishian F, Christos P, Mazumdar M, Osman I, Polsky D: Intra- and inter-tumor heterogeneity of BRAF(V600E) mutations in primary and metastatic melanoma. PLoS One 2012, 7:e29336. 23. Spagnolo F, Ghiorzo P et al.: BRAF-mutant melanoma: treatment approaches, resistance mechanisms, and diagnostic strategies. Onco Targets Ther 2015, 8:157-168. 24. Spagnolo F, Ghiorzo P, Queirolo P: Overcoming resistance to BRAF inhibition in BRAF-mutated metastatic melanoma. Oncotarget 2014, 5:10206-10221. 25. Lidsky M, Antoun G, Speicher P, Adams B, Turley R et al.: Mitogen-activated protein kinase (MAPK) hyperactivation and enhanced NRAS expression drive acquired vemurafenib resistance in V600E BRAF melanoma cells. J Biol Chem 2014, 289:27714-27726. In vitro study results indicate a mechanism of acquired vemurafenib resistance in V600E BRAF+ve melanoma cells that involves increased activation of all three human MAPKs and the PI3K pathway, as well as increased NRAS expression, which, contrary to previous reports, was not associated with mutations in the NRAS gene. 26. Goetz EM, Ghandi M, Treacy DJ, Wagle N, Garraway LA: ERK mutations confer resistance to mitogen-activated protein kinase pathway inhibitors. Cancer Res 2014, 74:7079-7089. A new mechanism of BRAFi resistance was identified using a random mutagenesis screen. Multiple point mutations in ERK1 (MAPK3) and ERK2 (MAPK1) that could confer resistance to ERK or RAF/MEK inhibitors. 27. Perna D, Karreth FA et al.: BRAF inhibitor resistance mediated by the AKT pathway in an oncogenic BRAF mouse melanoma model. Proc Natl Acad Sci U S A 2015, 112:E536-E545. Use of transposon mutagenesis in a melanoma model to identify that AKT-mediated inactivation of BAD (BCL2-associated agonist of cell death) constitutes a pathway that may contribute to HGF-mediated BRAFi resistance. 28. Miao B, Ji Z, Tan L et al.: EPHA2 is a mediator of vemurafenib resistance and a novel therapeutic target in melanoma. Cancer Discov 2015, 5:274-287. www.sciencedirect.com
BRAF inhibitors: the current and the future Zhang 73
A new mechanism of resistance is reported, the induction of EPHA2, a member of Eph receptor family was identified as a mechanism for BRAF inhibition resistance, it also demonstrated EPHA2 as a target in BRAF inhibitor-resistant melanoma. 29. Strohecker AM, White E: Autophagy promotes BrafV600Edriven lung tumorigenesis by preserving mitochondrial metabolism. Autophagy 2014, 10:384-385. 30. Levy JM, Thompson JC et al.: Autophagy inhibition improves chemosensitivity in BRAF(V600E) brain tumors. Cancer Discov 2014, 4:773-780. 31. Martin S, Dudek-Peric´ AM et al.: Concurrent MEK and autophagy inhibition is required to restore cell death associated danger-signalling in vemurafenib-resistant melanoma cells. Biochem Pharmacol 2015, 93:290-304.
35. Bollag G, Tsai J, Zhang J, Zhang C et al.: Vemurafenib: the first drug approved for BRAF-mutant cancer. Nat Rev Drug Discov 2012, 11:873-886. 36. Del Bufalo F, Carai A et al.: Response of recurrent BRAFV600E mutated ganglioglioma to vemurafenib as single agent. J Transl Med 2014, 12:356-362. 37. Jain P, Polliack A, Ravandi F: Novel therapeutic options for relapsed hairy cell leukemia. Leuk Lymphoma 2015, 4:1-9. 38. Das Thakur M, Salangsang F et al.: Modelling vemurafenib resistance in melanoma reveals a strategy to forestall drug resistance. Nature 2013, 494:251-255. 39. Dooley AJ, Gupta A, Bhattacharyya M, Middleton MR: Intermittent dosing with vemurafenib in BRAF V600E-mutant melanoma: review of a case series. Ther Adv Med Oncol 2014, 6:262-266.
32. Michaelis M, Rothweiler F et al.: Association between acquired resistance to PLX4032 (vemurafenib) and ATP-binding cassette transporter expression. BMC Res Notes 2014, 7:710-716.
40. Hu-Lieskovan S, Robert L, Homet Moreno B, Ribas A: Combining targeted therapy with immunotherapy in BRAF-mutant melanoma: promise and challenges. J Clin Oncol 2014, 32:2248-2254.
33. Wu CP, Sim HM, Huang YH, Liu YC, Hsiao SH, Cheng HW, Li YQ, Ambudkar SV, Hsu SC: Overexpression of ATP-binding cassette transporter ABCG2 as a potential mechanism of acquired resistance to vemurafenib in BRAF(V600E) mutant cancer cells. Biochem Pharmacol 2013, 85:325-334.
41. Kwong LN, Davies MA: Targeted therapy for melanoma: rational combinatorial approaches. Oncogene 2014, 33:1-9.
34. Smit MA, Maddalo G et al.: ROCK1 is a potential combinatorial drug target for BRAF mutant melanoma. Mol Syst Biol 2014, 10:772-786. The study used (phospho) proteomic and functional genomic platforms to identify potential drug target for BRAF mutant melanoma, ROCK1 silencing was shown to increase melanoma cell elimination when combined with BRAF or ERK inhibitor treatment, representing a new resistance mechanism and drug target.
www.sciencedirect.com
42. https://clinicaltrials.gov. 43. Yaeger R, Cercek A, O’Reilly EM, Reidy DL et al.: Pilot trial of combined BRAF and EGFR inhibition in BRAF-mutant metastatic colorectal cancer patients. Clin Cancer Res 2015, 21:1313-1320. A pilot study with dual inhibition of BRAF and EGFR, indicating that the combination of BRAF inhibitor and EGFR inhibitor in BRAF mutant metastatic CRC patients is well tolerated, with less cutaneous toxicity and results in partial responses in two patients (of 12 evaluable patients) and stable disease lasting over six months in two patients.
Current Opinion in Pharmacology 2015, 23:68–73