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Targeted Therapies in Melanoma: Translational Research at Its Finest

  • Allen W. Ho
    Affiliations
    Department of Dermatology, Massachusetts General Hospital, Boston, Massachusetts, USA
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  • Hensin Tsao
    Correspondence
    Wellman Center for Photomedicine, Massachusetts General Hospital, Edwards 211, 55 Fruit Street, Boston, Massachusetts 02114, USA
    Affiliations
    Department of Dermatology, Massachusetts General Hospital, Boston, Massachusetts, USA

    Wellman Center for Photomedicine, Massachusetts General Hospital, Boston, Massachusetts, USA
    Search for articles by this author
      Recent progress in melanoma drug development highlights the critical impact that translational research plays in advancing patient care. Prior to 2011, dacarbazine, IL-2, and IFNα-2b were the only US Food and Drug Administration (FDA)-approved treatment options for metastatic melanoma. These early therapies resulted in poor and inconsistent overall response rates (~10–15%;
      • Eggermont A.M.
      • Kirkwood J.M.
      Re-evaluating the role of dacarbazine in metastatic melanoma: what have we learned in 30 years?.
      ). A renaissance in melanoma therapeutics occurred with the recognition that molecular aberrations in the mitogen-activated protein kinase (MAPK) pathway (Figure 1) were present in a majority fraction of melanomas (
      • Davies H.
      • Bignell G.R.
      • Cox C.
      • et al.
      Mutations of the BRAF gene in human cancer.
      ). These investigations resulted in the vigorous pursuit of small-molecule kinase inhibitors and the eventual FDA approval of three novel MAPK pathway inhibitors for the treatment of advanced melanoma (Figure 1). Although key insights into immune checkpoint blockade have generated a similar number of recent immunotherapeutic breakthroughs (
      • Brahmer J.R.
      • Tykodi S.S.
      • Chow L.Q.
      • et al.
      Safety and activity of anti-PD-L1 antibody in patients with advanced cancer.
      ;
      • Hamid O.
      • Robert C.
      • Daud A.
      • et al.
      Safety and tumor responses with lambrolizumab (anti-PD-1) in melanoma.
      ;
      • Hodi F.S.
      • O'Day S.J.
      • McDermott D.F.
      • et al.
      Improved survival with ipilimumab in patients with metastatic melanoma.
      ), this Editorial focuses on the development of molecular targeted therapies.
      Figure thumbnail gr1
      Figure 1Molecular targeting of the mitogen activated protein kinase (MAPK), PI3K, and CDK4 pathways and the associated mutation rates of potential molecular targets in advanced melanoma (
      • Hodis E.
      • Watson I.R.
      • Kryukov G.V.
      • et al.
      A landscape of driver mutations in melanoma.
      ). Activation of a receptor tyrosinekinase such as c-KIT results in the propagation of signal via the MAPK pathway leading to activation of RAS, RAF, MEK, and ERK. This ultimately results in gene expression and promotes cellular proliferation and survival. Mutated BRAF bypasses this ordered pathway and stimulates constitutive signaling, making it a prime target for vemurafenib and dabrafenib. Trametinib targets the downstream effector molecule MEK. Dysregulation of the PI3K pathway promotes melanoma progression. Smallmolecule inhibitors of the PI3K pathway are being clinically tested in combination with MAPK pathway inhibition. Similarly, CDK4 is an attractive candidate molecule to target in melanoma and is the focus of multiple clinical trials. Red, activated; gray, inactivated; green, normal function. Drugs (shown in boxes) that have been approved (boldface) or are in trial (italics) are indicated. Please note that trials and drug approvals are subject to change.
      Melanoma arises from the activation, or inactivation, of genes that regulate critical cellular functions including proliferation, cell-cycle regulation, survival, angiogenesis, and cell migration. Efforts to systematically codify these changes have uncovered molecularly discrete subsets of melanoma (
      • Curtin J.A.
      • Fridlyand J.
      • Kageshita T.
      • et al.
      Distinct sets of genetic alterations in melanoma.
      ). For instance, melanomas arising in the context of non–chronic sun damaged skin are associated with BRAF and NRAS mutations, whereas lesions that arise from mucosal and acral surfaces are linked to KIT alterations (
      • Curtin J.A.
      • Fridlyand J.
      • Kageshita T.
      • et al.
      Distinct sets of genetic alterations in melanoma.
      ). Moreover, some melanomas that lack NRAS or BRAF mutations harbor deleterious lesions in NF1 or amplifications of CCND1 and CDK4 (
      • Lin W.M.
      • Baker A.C.
      • Beroukhim R.
      • et al.
      Modeling genomic diversity and tumor dependency in malignant melanoma.
      ), which are downstream mediators of cell-cycle progression in the MAPK pathway (Figure 1). These data suggest that inasmuch as melanomas should be classified histologically, molecular subtyping may provide a more pragmatic approach as specific therapies targeting receptor tyrosine kinases, downstream kinases, and other signaling molecules become available. Table 1 highlights key clinical trials that have expanded the treatment landscape of melanoma with targeted therapies.
      Table 1Key clinical trials in targeted melanoma therapy
      Table thumbnail fx1

      BRAF inhibition

      The rationale for targeting BRAF is evident because the Val-600-Glu mutation in the BRAF kinase domain is the single most common mutation in cutaneous melanoma (
      • Davies H.
      • Bignell G.R.
      • Cox C.
      • et al.
      Mutations of the BRAF gene in human cancer.
      ). This substitution constitutively activates BRAF and its attendant downstream MAPK pathway effectors such as MEK (Figure 1). Although 50% of melanomas harbor the BRAF V600E mutation, nearly 80% of benign nevi also contain the identical mutation (
      • Pollock P.M.
      • Harper U.L.
      • Hansen K.S.
      • et al.
      High frequency of BRAF mutations in nevi.
      ). Thus, it is an early genetic lesion that is neither necessary nor sufficient to fully induce melanoma. Preclinical work demonstrated that BRAF knockdown reduced tumor formation in murine xenograft models, and selective small-molecule inhibitors of RAF suppressed BRAF-mutant melanoma cell lines (
      • Hoeflich K.P.
      • Gray D.C.
      • Eby M.T.
      • et al.
      Oncogenic BRAF is required for tumor growth and maintenance in melanoma models.
      ;
      • Joseph E.W.
      • Pratilas C.A.
      • Poulikakos P.I.
      • et al.
      The RAF inhibitor PLX4032 inhibits ERK signaling and tumor cell proliferation in a V600E BRAF-selective manner.
      ). These translational studies show that BRAF is a well-validated target and set the stage for the development of BRAF inhibitors for clinical use.
      Early studies targeting RAF employed sorafenib, a multikinase inhibitor that has activity against BRAF and CRAF, but resulted in little to no clinical activity as monotherapy (
      • Eisen T.
      • Ahmad T.
      • Flaherty K.T.
      • et al.
      Sorafenib in advanced melanoma: a Phase II randomised discontinuation trial analysis.
      ;
      • Ott P.A.
      • Hamilton A.
      • Min C.
      • et al.
      A phase II trial of sorafenib in metastatic melanoma with tissue correlates.
      ). Selective BRAF inhibitors (i.e., those agents that specifically target mutant BRAF over wild-type BRAF), however, demonstrated impressive results in melanoma. The small-molecule inhibitors vemurafenib and dabrafenib selectively bind the active conformation of BRAF and inhibit signal transduction between BRAF and MEK. A phase III trial, BRIM-3, of vemurafenib versus dacarbazine as first-line therapy for BRAF V600E–mutated metastatic melanoma demonstrated improved median progression-free survival (PFS; 5.3 vs. 1.6 months) and better overall survival (OS; 84% vs. 64%) at 6 months in the vemurafenib versus dacarbazine groups, respectively (
      • Chapman P.B.
      • Hauschild A.
      • Robert C.
      • et al.
      Improved survival with vemurafenib in melanoma with BRAF V600E mutation.
      ). The most commonly detected toxicities of vemurafenib included cutaneous eruptions, arthralgias, photosensitivity reactions, and cutaneous squamous-cell carcinomas that were observed in 26% of patients. These results led to the FDA approval of vemurafenib (Zelboraf) in August 2011 for the treatment of unresectable BRAF V600E mutant melanoma.
      Another phase III trial, BREAK-3, compared dabrafenib to dacarbazine in the treatment of patients with unresectable, metastatic, BRAF V600E mutation–positive melanoma. BREAK-3 demonstrated similarly impressive results as BRIM-3. Patients in the dabrafenib arm had improved median PFS when compared to those in the dacarbazine arm, 5.1 versus 2.7 months, respectively, with a hazard ratio (HR) for progression of 0.30 (95% confidence interval (95% CI) 0.18–0.51; P < 0.0001) (
      • Hauschild A.
      • Grob J.J.
      • Demidov L.V.
      • et al.
      Dabrafenib in BRAF-mutated metastatic melanoma: a multicentre, open-label, phase 3 randomised controlled trial.
      ). However, one important distinction between the two trials is that the primary endpoint for BREAK-3 was PFS, whereas the co-primary endpoint for BRIM-3 was PFS and OS. Dabrafenib also demonstrated remarkable efficacy in the treatment of intracranial metastases (
      • Long G.V.
      • Trefzer U.
      • Davies M.A.
      • et al.
      Dabrafenib in patients with Val600Glu or Val600Lys BRAF-mutant melanoma metastatic to the brain (BREAK-MB): a multicentre, open-label, phase 2 trial.
      ). Although vemurafenib and dabrafenib appear to have similar efficacy with respect to overall response rates, patients in the vemurafenib trials had higher rates of cutaneous squamous-cell carcinomas, 18–25%, when compared to those in the dabrafenib trials, 6–11% (
      • Chapman P.B.
      • Hauschild A.
      • Robert C.
      • et al.
      Improved survival with vemurafenib in melanoma with BRAF V600E mutation.
      ;
      • Hauschild A.
      • Grob J.J.
      • Demidov L.V.
      • et al.
      Dabrafenib in BRAF-mutated metastatic melanoma: a multicentre, open-label, phase 3 randomised controlled trial.
      ). BREAK-3 led to the FDA approval of dabrafenib (Tafinlar) in May 2013 for the treatment of unresectable melanoma harboring BRAF V600E.

      MEK inhibition

      • Solit D.B.
      • Garraway L.A.
      • Pratilas C.A.
      • et al.
      BRAF mutation predicts sensitivity to MEK inhibition.
      reported early preclinical results that melanoma sensitivity to MEK inhibition was also correlated with the presence of the BRAF V600E mutation. Thus, pharmacologic attenuation of MEK signaling represents another possible approach for BRAF-mutated tumors. Exome sequencing of metastatic melanoma specimens identified somatic mutations in MEK1 and MEK2 as potential clinically significant aberrations, characterizing MEK1 and MEK2 mutations in 8% of melanomas (
      • Nikolaev S.I.
      • Rimoldi D.
      • Iseli C.
      • et al.
      Exome sequencing identifies recurrent somatic MAP2K1 and MAP2K2 mutations in melanoma.
      ). Moreover, pharmacological MEK blockade completely abrogated tumor growth in BRAF mutant xenografts (
      • Solit D.B.
      • Garraway L.A.
      • Pratilas C.A.
      • et al.
      BRAF mutation predicts sensitivity to MEK inhibition.
      ). These data provided the rationale for a phase III trial, METRIC, which compared trametinib, a small-molecule selective MEK1/2 inhibitor, to chemotherapy (dacarbazine or paclitaxel) in the treatment of patients with BRAF V600E/K mutant—positive metastatic melanoma. Compared with patients receiving chemotherapy, patients treated with trametinib demonstrated significant improvement in median PFS (1.5 vs. 4.8 months; HR 0.45; 95% CI 0.33–0.63; P < 0.001) and 6-month OS (67% vs. 81%; HR 0.54; 95% CI 0.32–0.92; P = 0.01), despite being permitted to cross over to trametinib. Although cutaneous eruptions were observed as an adverse effect in 87% of patients, trametinib treatment was minimally associated with the development of cutaneous squamous-cell carcinomas. Other toxic effects such as diarrhea and peripheral edema occurred in 35% and 27% of patients, respectively (
      • Flaherty K.T.
      • Robert C.
      • Hersey P.
      • et al.
      Improved survival with MEK inhibition in BRAF-mutated melanoma.
      ). Trametinib (Mekinist) gained FDA approval in May 2013 for the first-line treatment of patients with unresectable BRAF V600E/K mutant—positive melanoma.

      Combination BRAF and MEK inhibition

      Despite the impressive levels of tumor shrinkage observed in BRAF mutant melanoma patients treated with small-molecule BRAF inhibitors, responses are typically short lived, with a PFS of approximately 7 months (
      • Chapman P.B.
      • Hauschild A.
      • Robert C.
      • et al.
      Improved survival with vemurafenib in melanoma with BRAF V600E mutation.
      ;
      • Hauschild A.
      • Grob J.J.
      • Demidov L.V.
      • et al.
      Dabrafenib in BRAF-mutated metastatic melanoma: a multicentre, open-label, phase 3 randomised controlled trial.
      ). Importantly, molecular studies characterized a number of potential mechanisms of resistance and demonstrated that combined BRAF and MEK inhibition effectively abrogated resistance mediated by MEK1 mutations, BRAF truncation, and acquired NRAS mutations (
      • Paraiso K.H.
      • Fedorenko I.V.
      • Cantini L.P.
      • et al.
      Recovery of phospho-ERK activity allows melanoma cells to escape from BRAF inhibitor therapy.
      ;
      • Poulikakos P.I.
      • Persaud Y.
      • Janakiraman M.
      • et al.
      RAF inhibitor resistance is mediated by dimerization of aberrantly spliced BRAF(V600E).
      ;
      • Wagle N.
      • Emery C.
      • Berger M.F.
      • et al.
      Dissecting therapeutic resistance to RAF inhibition in melanoma by tumor genomic profiling.
      ). To “oversuppress” MAPK signaling, a phase I/II trial demonstrated that combination dabrafenib and trametinib at full monotherapy doses improved response rate (76% vs. 54%, P = 0.03) and PFS (10.5 vs. 5.6 months; HR 0.39; 95% CI 0.25–0.52; P < 0.001) when compared to dabrafenib alone, respectively (
      • Flaherty K.T.
      • Infante J.R.
      • Daud A.
      • et al.
      Combined BRAF and MEK inhibition in melanoma with BRAF V600 mutations.
      ). Based on these results, both dabrafenib and trametinib received accelerated FDA approval in January 2014 for use in the treatment of patients with metastatic melanoma with a BRAF V600E/K mutation.
      Three recent phase III trials corroborated these early results. The combination of vemurafenib and the MEK inhibitor cobimetinib improved PFS (9.9 vs. 6.2 months; HR 0.51; 95% CI 0.39–0.68; P < 0.001) and OS (68% vs. 45%; P < 0.001) when compared to vemurafenib alone (
      • Larkin J.
      • Ascierto P.A.
      • Dreno B.
      • et al.
      Combined vemurafenib and cobimetinib in BRAF-mutated melanoma.
      ). In another expanded study, the combination of dabrafenib and trametinib improved PFS (9.3 vs. 8.8 months, HR 0.75; 95% CI 0.57–0.99; P = 0.03) and OS (67% vs. 51%, P = 0.002) when compared to dabrafenib alone (
      • Long G.V.
      • Stroyakovskiy D.
      • Gogas H.
      • et al.
      Combined BRAF and MEK inhibition versus BRAF inhibition alone in melanoma.
      ). The much anticipated trial of the combination of dabrafenib and trametinib demonstrated improved PFS (11.4 vs. 7.3 months, HR 0.56, 95% CI 0.46–0.69; P < 0.001) and OS (72% vs. 65%, HR 0.69; 95% CI 0.53–0.89, P = 0.005) when compared to vemurafenib as monotherapy (
      • Robert C.
      • Karaszewska B.
      • Schachter J.
      • et al.
      Improved Overall Survival in Melanoma with Combined Dabrafenib and Trametinib.
      ). Consistent with earlier studies, combination treatment led to lower rates of cutaneous squamous-cell carcinoma formation compared to BRAF monotherapy. Thus, at this juncture, dual BRAF and MEK inhibition is quickly emerging as a standard of care for BRAF V600E—mutated melanomas.

      Looking forward

      Indeed, molecular targeted therapy has proven successful in the treatment of melanoma. Future directions include optimizing the currently available drugs for maximal clinical benefit and also the identification of novel therapeutic targets in melanoma. The previous discussion highlights the significant clinical benefit of selective BRAF inhibitors and the eventual relapse that occurs in patients treated with this modality. Combination therapy targeting BRAF and MEK is one potential avenue to abrogate this resistance that has been explored clinically. Work in human melanoma xenograft models suggests another approach to delay BRAF inhibitor resistance. These studies demonstrate that vemurafenib-resistant melanoma cells maintain dependency on BRAF V600E signaling via BRAF V600E overexpression. Intriguingly, the vemurafenib-resistant tumors in this model demonstrate dependence on vemurafenib for continued proliferation, such that cessation of the drug leads to tumor regression (
      • Das Thakur M.
      • Salangsang F.
      • Landman A.S.
      • et al.
      Modelling vemurafenib resistance in melanoma reveals a strategy to forestall drug resistance.
      ). This suggests that a pulsed dosing strategy may forestall eventual vemurafenib-resistant tumors. As novel targeted therapies for melanoma emerge, one key clinical challenge will be developing optimal therapeutic regimens or combinations of therapies that will provide the most durable clinical response.
      Inasmuch as the ideal treatment regimens of the existing melanoma drugs are being investigated, therapies targeting novel melanoma targets are in development. Given the genetic diversity of melanoma cells, there exist other attractive therapeutic targets including PI3K, CDK4, ERK, NF1, PPP6C, BCL-2, HSP90, mTOR, PDGF, Notch, MITF, and RAC1. Preclinical studies have begun to lay a foundation for further efforts to confirm clinical relevance.

      PTEN/PI3K

      Molecular aberrations in the PI3K pathway play an important role in the pathogenesis of melanoma (Figure 1). The tumor suppressor PTEN negatively regulates PI3K signaling and has emerged as the dominant genetic target in the PI3K pathway for melanoma therapies. PTEN mutations were found in 40% of melanoma cell lines and 10% of primary melanomas (
      • Guldberg P.
      • thor Straten P.
      • Birck A.
      • et al.
      Disruption of the MMAC1/PTEN gene by deletion or mutation is a frequent event in malignant melanoma.
      ;
      • Tsao H.
      • Benoit E.
      • Sober A.J.
      • et al.
      Novel mutations in the p16/CDKN2A binding region of the cyclin-dependent kinase-4 gene.
      ). Moreover, forced expression of PTEN in PTEN-deficient melanoma tumor cells abrogates activation of the downstream PI3K effector molecule AKT and cell growth (
      • Robertson G.P.
      Functional and therapeutic significance of Akt deregulation in malignant melanoma.
      ). Mouse models have suggested that PTEN dysregulation acts in concert with BRAF V600E to promote melanoma tumorigenesis and that targeting both MEK and the PI3K pathway in these mice inhibits tumor growth (
      • Dankort D.
      • Curley D.P.
      • Cartlidge R.A.
      • et al.
      Braf(V600E) cooperates with Pten loss to induce metastatic melanoma.
      ). These data strongly implicate PTEN and the PI3K pathway in the development of melanoma. Clinical trials targeting the PI3K pathway in combination with inhibiting the MAPK pathway in melanoma are currently underway (Table 2).
      Table 2Combined melanoma targeted therapies in progress (trials subject to change)
      Table thumbnail fx2

      CDK4/6

      Inappropriate CDK4/6 activity can result from rare activating mutations of CDK4 (
      • Tsao H.
      • Zhang X.
      • Benoit E.
      • et al.
      Identification of PTEN/MMAC1 alterations in uncultured melanomas and melanoma cell lines.
      ;
      • Zuo L.
      • Weger J.
      • Yang Q.
      • et al.
      Germline mutations in the p16INK4a binding domain of CDK4 in familial melanoma.
      ) or, more commonly, loss of p16 (Figure 1). Knock-in mice expressing CDK4 mutations display enhanced melanoma in response to carcinogen exposure (
      • Sotillo R.
      • Garcia J.F.
      • Ortega S.
      • et al.
      Invasive melanoma in Cdk4-targeted mice.
      ). Thus, small-molecule inhibitors targeting CDK4/6 may also be an effective strategy to target the cell cycle, and several agents are currently in clinical trials (Table 2).
      The development of novel targeted therapeutic interventions in the treatment of advanced melanoma demonstrates the importance of translational studies. Characterization of the molecular aberrations present in different subsets of melanoma cells has driven the development of therapies targeted specifically at mutations proven to contribute to melanomagenesis. Preclinical and clinical efforts have led to the recent approval of a number of therapeutic agents that have improved the PFS and OS of advanced melanoma patients. Despite these successes, there remains much work to be done to characterize and exploit other molecular targets, to overcome resistance and adverse effects associated with the recently approved agents, and to evaluate how best to combine these new interventions to maximal benefit. The bench-to-bedside model provides an effective and efficient means to achieve these goals.
      Mentorship during the writing of this work was supported in part by the National Institutes of Health (K24 CA149202 to HT).

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