BRAF Inhibition: Halting Cancer Cell Growth

Lisa Raedler, PhD, RPh

Cancers develop when mutations in critical genes alter cells’ ability to proliferate, differentiate, and die. One of these critical genes is the BRAF gene. Mutations in BRAF result in overactive, or oncogenic, BRAF protein, which ultimately enhances cell proliferation and survival.¹ Because BRAF mutations are present in more than half of melanomas—the deadliest form of skin cancer—and to a lesser degree in other solid tumors,² oncogenic BRAF has become a crucial target of small-molecule drug discovery efforts.³ Since 2011, 3 kinase inhibitors have been approved for use in patients with BRAF mutation–positive unresectable or metastatic melanoma as detected by a US Food and Drug Administration (FDA)-approved test.^4-6 At least 4 BRAF inhibitors are in clinical development for melanoma, papillary thyroid cancer, colorectal cancer, ovarian cancer, non-small cell lung cancer, and other solid tumors.⁷ What does BRAF do in normal cells? RAF kinases are a component of the RAS-RAF pathway,⁸ serving as intermediaries in extracellular signaling from cell surface receptors.¹ BRAF, one member of the RAF family of serine/threonine protein kinases, is encoded by the BRAF gene.^1,2 BRAF plays an important role as an intermediary in RAS-RAF signaling and, ultimately, in cell proliferation and survival.^8-10 In normal cells, extracellular signaling through the RAS-RAF pathway starts with growth factors that bind to and activate receptor tyrosine kinases on the cell surface. This, in turn, activates a signaling cascade that includes mitogen-activated protein kinase kinase (MAPKK, also known as MEK) and extracellular signalregulated kinase (ERK), as seen in the Figure. Specific steps are involved in this cell signaling cascade^8-10:

• Growth factors in the extracellular space activate receptor tyrosine kinases
• Receptor tyrosine kinases initiate RAS-RAF signaling
• Activation of RAS activates RAF proteins, including BRAF
• Activation of RAF leads to MEK phosphorylation
• MEK then phosphorylates ERK, which directly and indirectly activates various gene transcription factors
• Gene transcription factors direct the expression of specific genes that govern cell proliferation and survival.

Why target BRAF in cancer? More than 25 years ago, RAF kinases were discovered to be involved in oncogenesis. Ten years later, researchers specifically linked RAF to RAS, the oncogene most frequently mutated in human cancers, and to the MEK-ERK signaling pathway. The subsequent discovery of activating BRAF mutations in various cancers, including melanoma, thyroid cancer, ovarian cancer, colorectal cancer, and other solid tumors, conclusively established RAF (specifically the BRAF isotype) as a major player in tumor development.^11,12 Most of the known BRAF mutations in melanoma are BRAF V600E mutations, which involve substitution of glutamic acid for valine at position V600 of the protein chain.^12,13 These mutated BRAF genes lead to overactive BRAF signaling and uncontrolled downstream signaling via the MEK-ERK pathway.⁹ Ultimately, overactive BRAF results in cell proliferation and resistance to apoptosis.¹¹ For the organism, these genetic mutations and their downstream effects facilitate the development and progression of cancer.^11,12 What is the current clinical role of BRAF (and MEK) inhibitors? Based on knowledge of oncogenic BRAF and its role in MEK-ERK cell signaling, several inhibitors of BRAF as well as MEK have been developed and tested in patients with BRAF-mutated solid tumors.^8,11,14 As of June 2013, 2 BRAF inhibitors, vemurafenib and dabrafenib, and 1 MEK inhibitor, trametinib, are currently approved for use in the United States. Table 1 summarizes these approved kinase inhibitors, and provides examples of additional areas of clinical development for these drugs.

Vemurafenib

Vemurafenib (Zelboraf), the first commercialized BRAF inhibitor, was approved by the FDA in August 2011 for the treatment of patients with unresectable or metastatic melanoma with BRAF V600E mutation as detected by an FDA-approved test. The cobas 4800 BRAF V600 Mutation Test, a DNA-based companion diagnostic used to identify patients whose tumors carry a BRAF mutation, was simultaneously approved in the United States.¹⁵ Vemurafenib’s approval was based on positive results from an interim analysis of BRIM3, a global, randomized, open-label, controlled phase 3 study. This trial compared vemurafenib given orally twice daily with dacarbazine given intravenously (IV) in 675 patients with previously untreated BRAF V600E mutation-positive, unresectable or metastatic melanoma.¹⁶ Interim analysis showed that 6-month overall survival (OS) was improved for patients who received vemurafenib relative to those who received dacarbazine: 84% versus 64%, respectively. Median progression-free survival (PFS) was 5.3 months for patients who received vemurafenib compared with 1.6 months for those who received dacarbazine (hazard ratio [HR] 0.26; p<.0001).¹⁶ Adverse events (AEs) in the vemurafenib group included cutaneous events, arthralgia, and fatigue. Photosensitivity skin reactions of grade 2 or 3 were seen in 12% of the patients. Grade 3 photosensitivity reactions, which were characterized by blistering, were successfully prevented with sunblock. AEs led to dose modification or interruption in 38% of patients in the vemurafenib group and in 16% of patients in the dacarbazine group. Cutaneous squamous cell carcinoma (SCC), keratoacanthoma, or both developed in 18% of patients receiving vemurafenib. All lesions were treated by simple excision without the need for dose modification of vemurafenib. No other secondary neoplasias were observed in vemurafenib recipients.¹⁶ In June 2012, updated results from the BRIM3 study were reported at the 48th Annual American Society of Clinical Oncology (ASCO) Meeting.¹⁷ In this analysis, which included patient crossover from the dacarbazine arm to the vemurafenib arm, vemurafenib improved OS. In patients censored at crossover, OS was 13.6 months for vemurafenib and 9.7 months for dacarbazine (HR 0.76; P<.001 post hoc). Vemurafenib also improved PFS; median PFS was 6.9 months in the vemurafenib arm versus 1.6 months in the dacarbazine arm (HR 0.38; P<.001 log-rank post hoc).¹⁷ Selected AEs that occurred more frequently in the vemurafenib group included cutaneous SCC, keratoacanthoma, and skin papilloma. Eight patients in the vemurafenib group reported new primary melanomas.¹⁷

Dabrafenib

The second BRAF inhibitor authorized for marketing in the United States, dabrafenib (Tafinlar), is a reversible kinase inhibitor of V600E-mutant BRAF. Approved by the FDA in May 2013, dabrafenib is indicated for patients with advanced melanoma that contains the V600E mutation of BRAF.⁵ Concurrent with this approval, the FDA approved the THxID BRAF assay for detection of BRAF V600E mutations. In the drug’s pivotal phase 3 trial, known as BREAK-3, 250 patients with unresectable stage III or stage IV melanoma and a documented BRAF V600E mutation were randomly assigned to dabrafenib given orally twice daily or dacarbazine given intravenously in a 3:1 ratio, respectively.¹⁸ The primary end point of the trial was PFS as determined by investigators. (PFS was independently reviewed as a secondary trial end point.) Patients in BREAK-3 were allowed to cross over to the alternative treatment upon development of progressive disease. The study demonstrated that dabrafenib significantly increased median PFS compared with dacarbazine: 5.1 months versus 2.7 months, respectively (HR 0.33; 95% confidence interval [CI], 0.20-0.54). Independent review of the data found that median PFS findings were similarly improved with dabrafenib relative to dacarbazine: 6.7 months versus 2.9 months, respectively (HR 0.35; 95% CI, 0.20-0.61).¹⁸ Treatment-related AEs (grade 2 or higher) occurred in 53% of the patients who received dabrafenib and 44% of the patients receiving dacarbazine. The most common AEs with dabrafenib were skin-related toxic effects, fever, fatigue, arthralgia, and headache. Grade 3 or 4 AEs were uncommon in both groups.¹⁸ Updated OS data from the BREAK-3 study were presented during the June 2013 ASCO meeting.¹⁹ With a median follow-up of 15.2 months and 12.7 months for the 2 groups, OS favored patients treated with dabrafenib (18.2 months vs 15.6 months; HR 0.76; 95% CI, 0.48-1.21), but the difference was not statistically significant. Because more than half (57%) of the patients originally treated with dacarbazine had crossed over to dabrafenib, the OS benefit from initial dabrafenib therapy was likely obscured.¹⁹ The updated safety profile of dabrafenib did not change significantly relative to the initial data analysis. The most common AEs in the dabrafenib arm were hyperkeratosis, headache, arthralgia, and pyrexia. Serious AEs occurring in 5% or more of patients on the dabrafenib arm included cutaneous SCC/keratoacanthoma (10%) and pyrexia (5%).¹⁸

Trametinib

The first MEK inhibitor approved in the United States, trametinib (Mekinist), is indicated for the treatment of patients with unresectable or metastatic melanoma with BRAF V600E or V600K mutations as detected by an FDA-approved test. Concurrent with this May 2013 approval, the FDA approved the THxID BRAF assay for detection of BRAF V600E and V600K mutations. Trametinib is not indicated for the treatment of patients who have received prior BRAF inhibitor therapy.⁶ Trametinib’s approval was based on the demonstration of an improved PFS in METRIC, a phase 3, international, open-label, randomized, active-controlled trial that enrolled 322 patients with histologically confirmed stage IIIc or IV melanoma.6,23 Patients were determined to be BRAF V600E or V600K mutation–positive based on centralized testing. No more than 1 prior chemotherapy regimen for advanced or metastatic disease was permitted, and patients with prior exposure to BRAF inhibitors or MEK inhibitors were ineligible for the trial. Patients in the METRIC trial were randomized to receive trametinib given orally once daily or chemotherapy, either dacarbazine or paclitaxel given intravenously every 3 weeks. Upon disease progression, patients who were randomized to chemotherapy could cross over to trametinib.²³ Both PFS, the study’s primary end point, and OS, a secondary end point, were significantly improved with trametinib. Investigators reported a statistically significant increase in PFS in patients treated with trametinib compared with chemotherapy (HR 0.45; 95% CI, 0.33-0.63; P<.001). The median PFS was 4.8 months for patients taking trametinib (95% CI, 4.3-4.9) compared with 1.5 months for chemotherapy (95% CI, 1.4-2.7). In the trametinib group, the 6-month OS rate was 81%, compared with 67% in the chemotherapy group, a significant difference despite crossover (HR 0.54; 95% CI, 0.32-0.92; P=.01). (At the time of disease progression, almost half [47%] of patients receiving chemotherapy crossed over to receive trametinib.²³) Rash, diarrhea, peripheral edema, and fatigue were the most common AEs in the trametinib group. These were managed with dose interruption and dose reduction. Asymptomatic and reversible reductions in cardiac ejection fraction, as well as ocular events, occurred infrequently in the METRIC study. No secondary skin neoplasms were observed.²³ Both dabrafenib and trametinib are currently FDA approved as monotherapy, not in combination with each other or with other melanoma therapies.^5,6 However, trametinib and dabrafenib have been combined in clinical trials in an effort to delay development of treatment resistance, and to minimize toxicities associated with BRAF inhibition.²³ Several phase 3 trials of dabrafenib and trametinib (DT) are currently under way. One of these trials, known as COMBI-AD, is evaluating the efficacy of DT relative to placebo in the adjuvant treatment of high-risk patients with BRAF V600 melanoma.²⁴ A second phase 3 trial compares DT with single-agent vemurafenib in unresectable or metastatic melanoma with BRAF V600E or V600K mutations (COMBI-v).²⁵ What are the next steps in BRAF science and drug development? Use of BRAF inhibitors is now a standard option in BRAF-mutated metastatic melanoma. The successful development of vemurafenib and dabrafenib has been described as a milestone in the care of patients with this deadly disease.²⁶ However, treatment resistance to BRAF inhibition as well as unique toxicities (ie, cutaneous SCCs) and secondary premalignancies and malignancies, have emerged.^27,28 Despite impressive initial responses to BRAF inhibitors, it is now known that BRAF-mutated tumor cells eventually develop resistance. This is likely due to the activation of various alternative signaling pathways that can perpetuate cell growth and survival.⁷ In 2013, researchers reported that BRAF-positive metastatic melanomas develop resistance to drugs that target BRAF and MEK pathways by altering their metabolism.²⁷ By switching to a process known as oxidative phosphorylation to supply energy, tumor cells enhance their ability to survive despite BRAF inhibitor treatment.²⁷ Combination use of BRAF inhibitors and mitochondrial inhibitors is being explored as an effective strategy to improve the long-term efficacy of BRAF inhibitors.²⁷ The mechanisms underlying development of secondary tumors in patients receiving BRAF inhibitors are yet unclear. Some have hypothesized a paradoxical activation of the MEK pathway.²⁸ Although it appears that combination use of BRAF and MEK inhibitors delays the onset of treatment-induced SCCs, it is not yet known whether combination treatment will mitigate emergence of all BRAF-inhibitor–driven pathologies.²⁸ Research is ongoing to better understand and address these limiting features of BRAF inhibitors. To overcome shortcomings associated with current BRAF inhibitors, researchers are also identifying and developing novel BRAF inhibitors. At least 4 other BRAF inhibitors, including BMS-908662, RAF265, LGX818, and PLX3603 (RO5212054), are in early stages development for melanoma and other solid tumors. Examples of ongoing clinical trials that include these agents are provided in Table 2.^7,29-33 Since their introduction, BRAF inhibitors have transformed oncologists’ management of metastatic melanoma in patients who have a BRAF mutation. Only a handful of years ago, such patients were expected to live approximately 6 to 9 months from the time of diagnosis.³⁴ Building upon knowledge of RAF kinases, BRAF mutations, and the effects of BRAF on MEKERK signaling, scientists are now exploring novel combinations such as BRAF inhibitors plus MEK inhibitors, immunotherapies, or mitochondrial inhibitors, in the hope that the number and variety of safe and effective treatments for patients with metastatic melanoma and other solid tumors will continue to expand.

References

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