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Treatment of Advanced Melanoma in 2020 and Beyond

  • Russell W. Jenkins
    Affiliations
    Center for Cancer Research, Department of Medicine, MGH Cancer Center, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts, USA

    Laboratory for Systems Pharmacology, Harvard Program in Therapeutic Sciences, Harvard Medical School, Boston, Massachusetts, USA
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  • David E. Fisher
    Correspondence
    Corresponding author: David E. Fisher, Cutaneous Biology Research Center, Department of Dermatology and MGH Cancer Center, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts 02114, USA.
    Affiliations
    Cutaneous Biology Research Center, Department of Dermatology and MGH Cancer Center, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts, USA
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Published:April 05, 2020DOI:https://doi.org/10.1016/j.jid.2020.03.943
      The melanoma field has seen an unprecedented set of clinical advances over the past decade. Therapeutic efficacy for advanced or metastatic melanoma went from being one of the most poorly responsive to one of the more responsive. Perhaps most strikingly, the advances that transformed management of the disease are based upon modern mechanism-based therapeutic strategies. The targeted approaches that primarily suppress the BRAF oncoprotein pathway have a high predictability of efficacy although less optimal depth or durability of response. Immunotherapy is primarily based on blockade of one or two immune checkpoints and has a lower predictability of response but higher fractions of durable remissions. This article reviews the clinical progress in management of advanced melanoma and also discusses the impact of the same therapies on earlier stage disease, where the agents have shown significant promise in treating resectable but high-risk clinical scenarios. Collectively, the progress in melanoma therapeutics has transformed the standard of care for patients, informed new approaches that are increasingly utilized for treatment of other malignancies, and suggest novel strategies to further boost efficacy for the many patients not yet receiving optimal benefit from these approaches.

      Abbreviations:

      BRAFi (BRAF inhibitor), D (dabrafenib), ICB (immune checkpoint blockade), irAE (immune-related adverse event), MEK (MAPK kinase), MEKi (MAPK kinase inhibitor), OS (overall survival), RFS (recurrence-free survival), T (trametinib), T-VEC (talimogene laherparepvec), V (vemurafenib)
      Progress lies not in enhancing what is, but in advancing toward what will be.”- Khalil Gibran

      Introduction

      The treatment of metastatic melanoma has undergone a dramatic transformation over the past decade with the advent of molecular targeted therapy and immunotherapy. Today, one in two patients with metastatic melanoma are alive five years after diagnosis when treated with combination immunotherapy (
      • Larkin J.
      • Chiarion-Sileni V.
      • Gonzalez R.
      • Grob J.J.
      • Rutkowski P.
      • Lao C.D.
      • et al.
      Five-year survival with combined nivolumab and ipilimumab in advanced melanoma.
      ), and over one in three patients are alive following years of combination BRAF/MAPK kinase (MEK) targeted therapy (
      • Robert C.
      • Grob J.J.
      • Stroyakovskiy D.
      • Karaszewska B.
      • Hauschild A.
      • Levchenko E.
      • et al.
      Five-year outcomes with dabrafenib plus trametinib in metastatic melanoma.
      ) or single-agent PD-1 blockade (
      • Hamid O.
      • Robert C.
      • Daud A.
      • Hodi F.S.
      • Hwu W.J.
      • Kefford R.
      • et al.
      Five-year survival outcomes for patients with advanced melanoma treated with pembrolizumab in KEYNOTE-001.
      ). This is in contrast to 10 years ago, when metastatic melanoma was considered uniformly fatal with an overall survival <5% (
      • Dickson P.V.
      • Gershenwald J.E.
      Staging and prognosis of cutaneous melanoma.
      ). Despite these advances, additional therapeutic approaches are needed for patients resistant to available targeted and immune treatments. Here, we review the currently approved systemic and local therapies for advanced melanoma and emphasize areas of uncertainty and unmet needs.

       Melanoma in 2020: The scope of the problem

      Melanoma arises from a malignant transformation of melanocytes, the cells throughout the body that synthesize melanin, a photoprotective pigment (
      • Lo J.A.
      • Fisher D.E.
      The melanoma revolution: from UV carcinogenesis to a new era in therapeutics.
      ). Melanoma can arise from pigment-producing cells in the eye, the gastrointestinal tract, genitalia, sinuses, and meninges but most commonly arises in the skin in the setting of UV injury. Melanoma is the fifth most common form of cancer in adults (men and women) and is the deadliest form of skin cancer (
      NCI. SEER-database
      Cancer Stat Facts: Melanoma of the Skin.
      ). The incidence of melanoma has been rising in the United States and worldwide (
      • Karimkhani C.
      • Green A.C.
      • Nijsten T.
      • Weinstock M.A.
      • Dellavalle R.P.
      • Naghavi M.
      • et al.
      The global burden of melanoma: results from the Global Burden of Disease Study 2015.
      ,
      • Schadendorf D.
      • van Akkooi A.C.J.
      • Berking C.
      • Griewank K.G.
      • Gutzmer R.
      • Hauschild A.
      • et al.
      Melanoma.
      ), with an estimated 96,480 adults (57,220 men and 39,260 women) diagnosed with melanoma in the United States in 2019, accounting for 5.5% of all new cancer cases and resulting in 7,230 deaths (1.2% of all cancer deaths).
      Melanoma is categorized by TNM staging to define patients with local disease (stage I–II), node-positive disease (stage III), and advanced or metastatic disease (stage IV). Current staging utilizes the AJCC 8th Edition (
      • Gershenwald J.E.
      • Scolyer R.A.
      • Hess K.R.
      • Sondak V.K.
      • Long G.V.
      • Ross M.I.
      • et al.
      Melanoma staging: evidence-based changes in the American Joint Committee on Cancer eighth edition cancer staging manual.
      ). Tumor thickness (Breslow depth), presence or absence of ulceration, mitotic rate, presence or absence of microsatellites and in-transit lesions, burden of lymph node disease, and presence or absence of distant metastasis are the key clinico-pathologic features for assigning a stage and/or assessing risk of recurrence. Most cutaneous melanomas are localized at the time of initial clinical presentation and are successfully treated with surgical excision with adequate margins (
      • Joyce D.
      • Skitzki J.J.
      Surgical management of primary cutaneous melanoma.
      ).
      Roughly half of cutaneous melanomas harbor oncogenic driver mutations in BRAF (
      Cancer Genome Atlas Network
      Genomic classification of cutaneous melanoma.
      ,
      • Davies H.
      • Bignell G.R.
      • Cox C.
      • Stephens P.
      • Edkins S.
      • Clegg S.
      • et al.
      Mutations of the BRAF gene in human cancer.
      ). RAS-RAF pathway alterations are frequently encountered in cutaneous melanoma, with 40–50% of melanomas harboring BRAF mutations, 20–30% with NRAS mutations, and 10–15% with mutations in NF1 (
      Cancer Genome Atlas Network
      Genomic classification of cutaneous melanoma.
      ) (Figure 1). BRAF V600E/K mutations are the most common (90%) abnormality in the BRAF gene. Non-V600E/K mutations have been observed, but the responsiveness to BRAF/MEK inhibition is less clear. At this time, the presence or absence of a BRAF V600E/K mutation is the primary actionable genomic data that influences eligibility for treatment. Other genomic alterations (e.g., mutations and/or amplifications in KIT) are observed in a proportion of patients with melanoma and can be used to guide treatment with KIT tyrosine kinase inhibitors (
      • Carvajal R.D.
      • Antonescu C.R.
      • Wolchok J.D.
      • Chapman P.B.
      • Roman R.A.
      • Teitcher J.
      • et al.
      KIT as a therapeutic target in metastatic melanoma.
      ). Other clinico-pathologic features associated with response to immunotherapy (e.g., PD-L1 expression and tumor mutational burden) are not sufficiently robust to drive clinical decision-making. Thus, TNM stage and presence or absence of a BRAF V600E/K mutation (for patients with stage III–IV) are the most crucial features used in determining eligibility for Food and Drug Administration–approved immunotherapy or targeted therapy options.
      Figure thumbnail gr1
      Figure 1Dysregulation of the MAPK signaling pathway in melanoma. Over 80% of melanomas possess genetic abnormalities in at least one key node in the MAPK signaling pathway. Oncogenic driver mutations in BRAF (V600E or V600K) are the most common genomic abnormalities observed in cutaneous melanoma, followed by mutations in NRAS, and the RAS GAP NF1. ERK, extracellular signal–regulated kinase; GAP, GTPase activating protein; MEK, MAPK kinase.
      Figure thumbnail gr2
      Figure 2Timeline of FDA-approved therapies for melanoma. FDA, Food and Drug Administration; D+T, dabrafenib + trametinib; Ipi, ipilimumab; nivo, nivolumab; T-VEC, talimogene laherparepvec.

       The melanoma oncologist’s toolkit in 2020

      The last ten years have witnessed a dramatic evolution in the treatment of patients with unresectable or metastatic melanoma, with development of immune checkpoint blockade strategies targeting the PD-1 and CTLA-4 coinhibitory receptors and MAPK molecular targeted therapy directed at oncogenic BRAF and MEK signaling pathways (Figure 2). Both approaches have proven effective in the treatment of advanced melanoma. Before 2010, the only primary Food and Drug Administration–approved treatments for advanced melanoma were high-dose IL-2 (
      • Atkins M.B.
      • Lotze M.T.
      • Dutcher J.P.
      • Fisher R.I.
      • Weiss G.
      • Margolin K.
      • et al.
      High-dose recombinant interleukin 2 therapy for patients with metastatic melanoma: analysis of 270 patients treated between 1985 and 1993.
      ) and dacarbazine (
      • Luke J.J.
      • Schwartz G.K.
      Chemotherapy in the management of advanced cutaneous malignant melanoma.
      ). In 2010, the results of the first phase I trial of the BRAF inhibitor (BRAFi) PLX4032 (vemurafenib [V]) was published (
      • Flaherty K.T.
      • Puzanov I.
      • Kim K.B.
      • Ribas A.
      • McArthur G.A.
      • Sosman J.A.
      • et al.
      Inhibition of mutated, activated BRAF in metastatic melanoma.
      ). That same year, the results of the first phase III trial of the anti–CTLA-4 mAb, ipilimumab, were published, demonstrating improved overall survival (
      • Hodi F.S.
      • O'Day S.J.
      • McDermott D.F.
      • Weber R.W.
      • Sosman J.A.
      • Haanen J.B.
      • et al.
      Improved survival with ipilimumab in patients with metastatic melanoma.
      ).
      Since 2010, nearly a dozen new treatments and treatment regimens for melanoma (Table 1) have been approved by the Food and Drug Administration, including four systemic immunotherapy treatments or combinations (ipilimumab, nivolumab, pembrolizumab, and combination ipilimumab-nivolumab), single-agent BRAFis (V and dabrafenib [D]), combination BRAFi/MEK inhibitor (MEKi) regimens (D + trametinib [T], V + cobimetinib [C], and encorafenib-binimetinib), and one intralesional immunotherapy involving a modified oncolytic herpes virus (talimogene laherparepvec [T-VEC]). These treatments gained their initial indications in advanced, unresectable melanoma, and several have since gained approval in the adjuvant setting. Here, we will consider the mechanism of action, efficacy, and toxicity of each class of treatments. For a more comprehensive review of the history and clinical development of these and other therapies in melanoma, we refer readers to other reviews (
      • Luke J.J.
      • Flaherty K.T.
      • Ribas A.
      • Long G.V.
      Targeted agents and immunotherapies: optimizing outcomes in melanoma.
      ,
      • Ribas A.
      • Wolchok J.D.
      Cancer immunotherapy using checkpoint blockade.
      ).
      Table 1Targeted and Immune Therapies for Melanoma
      AgentMechanismFDA-approved indications
      Targeted Therapies
      VemurafenibBRAF inhibitor- Unresectable/metastatic melanoma harboring BRAF V600E/K mutation
      CobimetinibMEK inhibitor- Unresectable/metastatic melanoma harboring BRAF V600E/K mutation
      Dabrafenib + trametinibBRAF inhibitor + MEK inhibitor- Unresectable/metastatic melanoma harboring BRAF V600E/K mutation

      - Adjuvant treatment of resected stage III BRAF V600E/K mutant melanoma
      Vemurafenib + cobimetinibBRAF inhibitor + MEK inhibitor- Unresectable/metastatic melanoma harboring BRAF V600E/K mutation
      Encorafenib + binimetinibBRAF inhibitor + MEK inhibitor- Unresectable/metastatic melanoma harboring BRAF V600E/K mutation
      Immunotherapies
      IpilimumabAnti–CTLA-4 monoclonal antibody- Unresectable/metastatic melanoma (regardless of BRAF status)

      - Adjuvant treatment of resected stage III melanoma (regardless of BRAF status)
      NivolumabAnti–PD-1 monoclonal antibody- Unresectable/metastatic melanoma (regardless of BRAF status)

      - Adjuvant treatment of resected stage III melanoma (regardless of BRAF status)
      PembrolizumabAnti–PD-1 monoclonal antibody- Unresectable/metastatic melanoma (regardless of BRAF status)

      - Adjuvant treatment of resected stage III melanoma (regardless of BRAF status)
      Ipilimumab-nivolumabAnti–CTLA-4 antibody + anti–PD-1 antibody- Unresectable/metastatic melanoma (regardless of BRAF status)
      T-VECModified, injectable oncolytic herpes virusLocal treatment of unresectable cutaneous, subcutaneous, and nodal lesions in patients with recurrent melanoma after surgery
      Abbreviations: FDA, Food and Drug Administration; MEK, MAPK kinase; T-VEC, talimogene laherpraepvec.

       Immunotherapy

      Immune checkpoint proteins (e.g., PD-1 and CTLA-4) are coinhibitory protein receptors expressed on the cell surface of lymphocytes, whose primary physiologic role is to maintain self-tolerance and limit inflammatory responses in normal tissues (
      • Keir M.E.
      • Butte M.J.
      • Freeman G.J.
      • Sharpe A.H.
      PD-1 and its ligands in tolerance and immunity.
      ,
      • Pardoll D.M.
      The blockade of immune checkpoints in cancer immunotherapy.
      ). The cognate ligands for PD-1 and CTLA-4 (e.g., PD-L1/PD-L2 and B7, respectively) are expressed on tumor cells or other immune cells and serve to restrain T-cell function (
      • Buchbinder E.I.
      • Desai A.
      CTLA-4 and PD-1 pathways: similarities, differences, and implications of their inhibition.
      ). In 2010, Hodi et al. published the results of the first phase III clinical trial demonstrating an improved overall survival in patients with metastatic melanoma treated with the anti–CTLA-4 antibody (
      • Hodi F.S.
      • O'Day S.J.
      • McDermott D.F.
      • Weber R.W.
      • Sosman J.A.
      • Haanen J.B.
      • et al.
      Improved survival with ipilimumab in patients with metastatic melanoma.
      ). Pooled analysis of patients treated with ipilimumab from 1,861 patients across 12 trials demonstrated three-year survival rates of roughly 20%, at which time the survival curve plateaued, supporting the durability of responses to CTLA-4 blockade (
      • Schadendorf D.
      • Hodi F.S.
      • Robert C.
      • Weber J.S.
      • Margolin K.
      • Hamid O.
      • et al.
      Pooled analysis of long-term survival data from Phase II and Phase III trials of ipilimumab in unresectable or metastatic melanoma.
      ). In 2014, the initial results of the first trials of mAbs targeting PD-1 were published, demonstrating clinical activity in melanoma, non–small cell lung cancer, and renal cell carcinoma (
      • Brahmer J.R.
      • Tykodi S.S.
      • Chow L.Q.
      • Hwu W.J.
      • Topalian S.L.
      • Hwu P.
      • et al.
      Safety and activity of anti-PD-L1 antibody in patients with advanced cancer.
      ,
      • Topalian S.L.
      • Hodi F.S.
      • Brahmer J.R.
      • Gettinger S.N.
      • Smith D.C.
      • McDermott D.F.
      • et al.
      Safety, activity, and immune correlates of anti-PD-1 antibody in cancer.
      ). Longer-term follow-up in patients with melanoma demonstrates overall response rates for first-line anti–PD-1 treatment are even higher (30–40% at 5 years) with ongoing durable responses in 70–80% of responding patients (
      • Hamid O.
      • Robert C.
      • Daud A.
      • Hodi F.S.
      • Hwu W.J.
      • Kefford R.
      • et al.
      Five-year survival outcomes for patients with advanced melanoma treated with pembrolizumab in KEYNOTE-001.
      ).
      Dual immune checkpoint blockade (ICB) with ipilimumab-nivolumab enhances response rates compared with single-agent ipilimumab or nivolumab in patients with metastatic melanoma (response rate, 58%), and both nivolumab containing arms demonstrated superior overall survival (OS) compared with single agent ipilimumab (
      • Larkin J.
      • Chiarion-Sileni V.
      • Gonzalez R.
      • Grob J.J.
      • Cowey C.L.
      • Lao C.D.
      • et al.
      Combined nivolumab and ipilimumab or monotherapy in untreated melanoma.
      ). However, >50% of patients also experienced significant (grade 3/4) toxicity from dual ICB resulting in treatment interruption or discontinuation. Nonoverlapping mechanisms of action may account for the differential clinical activity of these agents, as well as distinct toxicity profiles. Side effects from ICB therapy result from disruption of immunologic tolerance and manifest as immune-mediated reactions against healthy tissues, or so-called immune-related adverse events (irAEs) (
      • De Velasco G.
      • Je Y.
      • Bossé D.
      • Awad M.M.
      • Ott P.A.
      • Moreira R.B.
      • et al.
      Comprehensive meta-analysis of key immune-related adverse events from CTLA-4 and PD-1/PD-L1 inhibitors in cancer patients.
      ). The onset of irAEs is variable and unpredictable, but most grade 3/4 irAEs with ipilimumab-nivolumab occur during the 12-week induction phase (
      • Larkin J.
      • Chiarion-Sileni V.
      • Gonzalez R.
      • Grob J.J.
      • Rutkowski P.
      • Lao C.D.
      • et al.
      Five-year survival with combined nivolumab and ipilimumab in advanced melanoma.
      ). Importantly, progression-free survival and OS is similar for patients who had to discontinue treatment because of irAEs compared with the overall population (
      • Larkin J.
      • Chiarion-Sileni V.
      • Gonzalez R.
      • Grob J.J.
      • Rutkowski P.
      • Lao C.D.
      • et al.
      Five-year survival with combined nivolumab and ipilimumab in advanced melanoma.
      ). Five-year follow-up data confirms similar response and toxicity rates, with an impressive OS benefit: over half (52%) of patients in the ipilimumab-nivolumab arms still alive after 5 years and an impressive median treatment-free interval of 18.1 months, underscoring the durability of these responses (
      • Larkin J.
      • Chiarion-Sileni V.
      • Gonzalez R.
      • Grob J.J.
      • Rutkowski P.
      • Lao C.D.
      • et al.
      Five-year survival with combined nivolumab and ipilimumab in advanced melanoma.
      ). Despite these impressive data, it remains unclear which patients require dual ICB versus single-agent PD-1 blockade.
      To date, several putative biomarkers have shown associations with clinical response to PD-1 blockade, including pre-existing immune infiltrate (
      • Tumeh P.C.
      • Harview C.L.
      • Yearley J.H.
      • Shintaku I.P.
      • Taylor E.J.
      • Robert L.
      • et al.
      PD-1 blockade induces responses by inhibiting adaptive immune resistance.
      ), PD-L1 expression (
      • Daud A.I.
      • Wolchok J.D.
      • Robert C.
      • Hwu W.J.
      • Weber J.S.
      • Ribas A.
      • et al.
      Programmed death-ligand 1 expression and response to the anti-programmed death 1 antibody pembrolizumab in melanoma.
      ), and tumor mutational burden, although none of these are sufficiently robust that they drive clinical practice. Recently, integrative models incorporating clinical, genomic, and gene expression data have been developed that appear to be more robust than any of the individual features on their own (
      • Liu D.
      • Schilling B.
      • Liu D.
      • Sucker A.
      • Livingstone E.
      • Jerby-Amon L.
      • et al.
      Integrative molecular and clinical modeling of clinical outcomes to PD1 blockade in patients with metastatic melanoma.
      ). Given the lack of robust predictive biomarkers with a strong negative predictive value, the decision to treat with anti–PD-1 and/or anti–CTLA-4 mAbs may be driven by patient characteristics, including age, comorbid medical conditions (including autoimmune conditions), and burden of metastatic disease, although data supporting these practices are limited. One setting in which dual ICB appears superior to single-agent anti–PD-1 treatment is in patients with asymptomatic brain metastases. Given the very robust intracranial responses observed with dual PD-1 and CTLA-4 blockade in the phase II, single-arm Checkmate-204 trial (
      • Tawbi H.A.
      • Forsyth P.A.
      • Algazi A.
      • Hamid O.
      • Hodi F.S.
      • Moschos S.J.
      • et al.
      Combined nivolumab and ipilimumab in melanoma metastatic to the brain.
      ), dual ICB is emerging as the preferred regimen for patients with asymptomatic brain metastases, with an impressive 57% intracranial clinical benefit rate (compared with 56% extracranial benefit rate) and 26% complete response rate.
      Another form of immunotherapy active in cutaneous melanoma is the injectable agent T-VEC (Imlygic). T-VEC is a modified herpes virus engineered to replicate in tumor cells to produce the growth factor GM-CSF, which facilitates immune infiltration and antigen presentation to prime immune response (
      • Kaufman H.L.
      • Ruby C.E.
      • Hughes T.
      • Slingluff Jr., C.L.
      Current status of granulocyte-macrophage colony-stimulating factor in the immunotherapy of melanoma.
      ). Based on the results of the phase III OPTiM trial, intralesional injection with T-VEC was associated with improved response rate compared with GM-CSF alone (26.4% vs. 5.7%) (
      • Andtbacka R.H.
      • Kaufman H.L.
      • Collichio F.
      • Amatruda T.
      • Senzer N.
      • Chesney J.
      • et al.
      Talimogene laherparepvec improves durable response rate in patients with advanced melanoma.
      ,
      • Andtbacka R.H.I.
      • Collichio F.
      • Harrington K.J.
      • Middleton M.R.
      • Downey G.
      • Ӧhrling K.
      • et al.
      Final analyses of OPTiM: a randomized phase III trial of talimogene laherparepvec versus granulocyte-macrophage colony-stimulating factor in unresectable stage III–IV melanoma.
      ). Injectable lesions were cutaneous and accessible lymph nodes. Responses were observed in nearby and distant uninjected lesions, suggesting an immune priming effect. T-VEC has been combined with ICB (
      • Puzanov I.
      • Milhem M.M.
      • Minor D.
      • Hamid O.
      • Li A.
      • Chen L.
      • et al.
      Talimogene laherparepvec in combination with ipilimumab in previously untreated, unresectable stage IIIB-IV melanoma.
      ). A phase III trial evaluating pembrolizumab with and without T-VEC is underway (NCT02263508), and this strategy is also showing activity in soft tissue sarcoma (
      • Kelly C.M.
      • Antonescu C.R.
      • Bowler T.
      • Munhoz R.
      • Chi P.
      • Dickson M.A.
      • et al.
      Objective response rate among patients with locally advanced or metastatic sarcoma treated with talimogene laherparepvec in combination with pembrolizumab: a phase 2 clinical trial.
      ).

       Targeted therapy

      The era of molecular targeted therapy in cutaneous melanoma was ushered in following the discovery of BRAF mutations in several cancers including melanoma (
      • Davies H.
      • Bignell G.R.
      • Cox C.
      • Stephens P.
      • Edkins S.
      • Clegg S.
      • et al.
      Mutations of the BRAF gene in human cancer.
      ). This discovery led to the initial evaluation of BRAFis, with initial trials showing 50% response rates as a single agent in patients with metastatic melanoma (
      • Chapman P.B.
      • Hauschild A.
      • Robert C.
      • Haanen J.B.
      • Ascierto P.
      • Larkin J.
      • et al.
      Improved survival with vemurafenib in melanoma with BRAF V600E mutation.
      ,
      • Hauschild A.
      • Grob J.J.
      • Demidov L.V.
      • Jouary T.
      • Gutzmer R.
      • Millward M.
      • et al.
      Dabrafenib in BRAF-mutated metastatic melanoma: a multicentre, open-label, phase 3 randomised controlled trial.
      ). Given the clinical activity of single-agent MEK inhibition (
      • Flaherty K.T.
      • Robert C.
      • Hersey P.
      • Nathan P.
      • Garbe C.
      • Milhem M.
      • et al.
      Improved survival with MEK inhibition in BRAF-mutated melanoma.
      ) and appreciation of the importance of downstream MAPK pathway signaling, BRAFi/MEKi combinations were subsequently evaluated. D+T was the first BRAF-MEK combination approved for metastatic melanoma based on the two phase III clinical trials COMBI-v (
      • Robert C.
      • Karaszewska B.
      • Schachter J.
      • Rutkowski P.
      • Mackiewicz A.
      • Stroiakovski D.
      • et al.
      Improved overall survival in melanoma with combined dabrafenib and trametinib.
      ) and COMBI-d (
      • Long G.V.
      • Stroyakovskiy D.
      • Gogas H.
      • Levchenko E.
      • de Braud F.
      • Larkin J.
      • et al.
      Dabrafenib and trametinib versus dabrafenib and placebo for Val600 BRAF-mutant melanoma: a multicentre, double-blind, phase 3 randomised controlled trial.
      ), comparing D+T to single-agent V or D, respectively. D+T demonstrated response rates of 60–70% in the COMBI-v and COMBI-d trials, compared with 50% response rates with a single-agent BRAFi. Furthermore, the toxicity profile of combination D+T differed from D and V, with more pyrexia with D+T but a decreased incidence of keratoacanthoma and squamous cell carcinoma. The coBRIM trial evaluated combination V+C versus V and placebo, demonstrating an improved objective response rate with V+C (70%) versus V alone (50%) (
      • Larkin J.
      • Ascierto P.A.
      • Dréno B.
      • Atkinson V.
      • Liszkay G.
      • Maio M.
      • et al.
      Combined vemurafenib and cobimetinib in BRAF-mutated melanoma.
      ). The toxicity profile of V+C differs from D+T with more GI upset (diarrhea and nausea), fatigue, rash, liver enzyme abnormalities, and photosensitivity (from V) and less pyrexia compared with D+T. The randomized, phase III COLUMBUS trial evaluated a third BRAFi (encorafenib) and MEKi (binimetinib) versus V (
      • Dummer R.
      • Ascierto P.A.
      • Gogas H.J.
      • Arance A.
      • Mandala M.
      • Liszkay G.
      • et al.
      Encorafenib plus binimetinib versus vemurafenib or encorafenib in patients with BRAF-mutant melanoma (COLUMBUS): a multicentre, open-label, randomised phase 3 trial.
      ) demonstrating a median progression-free survival of 14.8 months versus 7.3 months.
      The available BRAFi/MEKi combinations are comparable in terms of efficacy, with response rates ranging from 60–70% and 18-month progression-free survival rates of 30–40% (
      • Dummer R.
      • Ascierto P.A.
      • Gogas H.J.
      • Arance A.
      • Mandala M.
      • Liszkay G.
      • et al.
      Encorafenib plus binimetinib versus vemurafenib or encorafenib in patients with BRAF-mutant melanoma (COLUMBUS): a multicentre, open-label, randomised phase 3 trial.
      ,
      • Larkin J.
      • Ascierto P.A.
      • Dréno B.
      • Atkinson V.
      • Liszkay G.
      • Maio M.
      • et al.
      Combined vemurafenib and cobimetinib in BRAF-mutated melanoma.
      ,
      • Long G.V.
      • Stroyakovskiy D.
      • Gogas H.
      • Levchenko E.
      • de Braud F.
      • Larkin J.
      • et al.
      Dabrafenib and trametinib versus dabrafenib and placebo for Val600 BRAF-mutant melanoma: a multicentre, double-blind, phase 3 randomised controlled trial.
      ,
      • Robert C.
      • Karaszewska B.
      • Schachter J.
      • Rutkowski P.
      • Mackiewicz A.
      • Stroiakovski D.
      • et al.
      Improved overall survival in melanoma with combined dabrafenib and trametinib.
      ) with distinct toxicity profiles. Direct head-to-head comparison of the available regimens is unlikely to be performed, but indirect side-by-side analysis of data from V+C, D+T, and encorafenib-binimetinib compared with V monotherapy (
      • Hamid O.
      • Cowey C.L.
      • Offner M.
      • Faries M.
      • Carvajal R.D.
      Efficacy, safety, and tolerability of approved combination BRAF and MEK inhibitor regimens for BRAF-mutant melanoma.
      ) revealed comparable progression-free survival and OS data. Median OS was 33.6 months for patients treated with encorafenib-binimetinib compared with 25.6 months with D+T and 22.3 months with V+C, but direct comparison is not possible across trials. The availability of three approved BRAFi/MEKi regimens provides multiple treatment options for patients with stage IV BRAF-V600E/K mutant melanoma.

       Combining immune therapy with targeted therapy

      Given the high response rates observed with targeted therapies and the durable responses observed with immunotherapies, the combination of these effective therapeutic strategies was a logical next step for patients with BRAF-mutant melanoma. Unfortunately, the initial experience combining ICB with targeted therapy proved more of a cautionary tale than a success story.
      • Ribas A.
      • Hodi F.S.
      • Callahan M.
      • Konto C.
      • Wolchok J.
      Hepatotoxicity with combination of vemurafenib and ipilimumab.
      published their experience with patients treated with ipilimumab and V with the observation of grade 3 hepatotoxicity. However, after the subsequent success of BRAF-MEK combinations and PD-1 blockade, combination approaches were revisited. Backed by preclinical evidence of improved efficacy (
      • Frederick D.T.
      • Piris A.
      • Cogdill A.P.
      • Cooper Z.A.
      • Lezcano C.
      • Ferrone C.R.
      • et al.
      BRAF inhibition is associated with enhanced melanoma antigen expression and a more favorable tumor microenvironment in patients with metastatic melanoma.
      ,
      • Hu-Lieskovan S.
      • Mok S.
      • Homet Moreno B.
      • Tsoi J.
      • Robert L.
      • Goedert L.
      • et al.
      Improved antitumor activity of immunotherapy with BRAF and MEK inhibitors in BRAF(V600E) melanoma.
      ), several anti–PD-(L)1/BRAFi/MEKi triplet therapy combinations have been evaluated in early-phase clinical trials with response rates greater than 70% and comparable rates of grade 3/4 toxicity (
      • Ascierto P.A.
      • Ferrucci P.F.
      • Fisher R.
      • Del Vecchio M.
      • Atkinson V.
      • Schmidt H.
      • et al.
      Dabrafenib, trametinib and pembrolizumab or placebo in BRAF-mutant melanoma.
      ,
      • Ribas A.
      • Lawrence D.
      • Atkinson V.
      • Agarwal S.
      • Miller Jr., W.H.
      • Carlino M.S.
      • et al.
      Combined BRAF and MEK inhibition with PD-1 blockade immunotherapy in BRAF-mutant melanoma.
      ,
      • Sullivan R.J.
      • Hamid O.
      • Gonzalez R.
      • Infante J.R.
      • Patel M.R.
      • Hodi F.S.
      • et al.
      Atezolizumab plus cobimetinib and vemurafenib in BRAF-mutated melanoma patients.
      ). Whether these combination approaches, with or without alterations in dose and schedule of BRAF/MEK targeted therapy, provide an OS benefit compared with either PD-1 blockade or combination BRAF/MEK inhibition in a prospective fashion remains to be seen.

       Frontline treatment of metastatic melanoma in 2020

      For most patients with metastatic melanoma, immunotherapy with PD-1 and/or CTLA-4 blockade is the preferred first-line regimen given the improved OS, response rate, and durability of response, which may allow patients to discontinue treatment. For patients unable to tolerate systemic immunotherapy, BRAF/MEK targeted therapy is active and associated with durable responses in one third of patients, especially in patients with normal lactate dehydrogenase and fewer than three different organ sites involved. Additionally, responses to BRAFi/MEKi therapy are usually brisk and may provide more rapid disease control for patients requiring urgent treatment. Whether efficacy and/or tolerability of BRAF/MEK targeted therapy as a second-line treatment is comparable with the front-line data of published phase III trials remains to be seen, but emerging retrospective data suggests that tolerability and efficacy may be diminished following PD-1 blockade (
      • Saab K.R.
      • Mooradian M.J.
      • Wang D.Y.
      • Chon J.
      • Xia C.Y.
      • Bialczak A.
      • et al.
      Tolerance and efficacy of BRAF plus MEK inhibition in patients with melanoma who previously have received programmed cell death protein 1-based therapy.
      ). Immunotherapy with PD-(L)1 blockade combined with BRAF/MEK targeted therapy has demonstrated improved response rates in early phase clinical trials, but incidence of toxicity is increased. Phase III trials are ongoing evaluating anti-PD-(L)1/BRAFi/MEKi triplets, and other trials evaluating alternate dosing schedules and lead-ins are underway. Strategies for patients who have progressed on front-line treatments include clinical trials, most of which are evaluating treatment combinations.

       Adjuvant systemic therapy for resected cutaneous melanoma

      For high-risk patients with resected melanoma, adjuvant systemic therapy is offered to reduce the risk of melanoma recurrence after surgery (
      • Eggermont A.M.M.
      • Robert C.
      • Ribas A.
      The new era of adjuvant therapies for melanoma.
      ). Patients with high-risk stage II melanoma (i.e., those that are >4 mm thick or >2 mm thick with ulceration; stage IIB–C) and patients with node-positive disease are at increased risk of recurrence and demonstrate worse melanoma-specific survival (
      • Gershenwald J.E.
      • Scolyer R.A.
      • Hess K.R.
      • Sondak V.K.
      • Long G.V.
      • Ross M.I.
      • et al.
      Melanoma staging: evidence-based changes in the American Joint Committee on Cancer eighth edition cancer staging manual.
      ). Before 2015, high-dose interferon alpha 2b was the only approved therapy for adjuvant treatment of resected high-risk stage II and stage III melanoma, with a consistent albeit modest benefit in recurrence-free survival (RFS) and to a lesser extent an OS benefit (
      • Ives N.J.
      • Suciu S.
      • Eggermont A.M.M.
      • Kirkwood J.
      • Lorigan P.
      • Markovic S.N.
      • et al.
      Adjuvant interferon-alpha for the treatment of high-risk melanoma: an individual patient data meta-analysis.
      ,
      • Kirkwood J.M.
      • Manola J.
      • Ibrahim J.
      • Sondak V.
      • Ernstoff M.S.
      • Rao U.
      • et al.
      A pooled analysis of eastern cooperative oncology group and intergroup trials of adjuvant high-dose interferon for melanoma.
      ,
      • Mocellin S.
      • Lens M.B.
      • Pasquali S.
      • Pilati P.
      • Chiarion Sileni V.
      Interferon alpha for the adjuvant treatment of cutaneous melanoma.
      ). In 2015, ipilimumab (anti–CTLA-4) was approved for the adjuvant treatment of all patients with stage III melanoma based on the results of EORTC 18071 (CA184-029), which demonstrated that high-dose ipilimumab (10 mg/kg) given every three weeks for a total of four doses improved RFS compared with placebo (
      • Eggermont A.M.
      • Chiarion-Sileni V.
      • Grob J.J.
      • Dummer R.
      • Wolchok J.D.
      • Schmidt H.
      • et al.
      Prolonged survival in Stage III melanoma with ipilimumab adjuvant therapy.
      ). The results of E1609 demonstrated that standard dose (3 mg/kg) ipilimumab improved RFS and OS compared with high-dose interferon alpha 2b (
      • Tarhini A.A.
      • Lee S.J.
      • Hodi F.S.
      • Rao U.N.M.
      • Cohen G.I.
      • Hamid O.
      • et al.
      Phase III Study of Adjuvant ipilimumab (3 or 10 mg/kg) versus High-Dose interferon alfa-2b for Resected High-Risk Melanoma: North American InterGroup E1609.
      ).
      In 2017, the Checkmate-238 study demonstrated improved RFS with nivolumab (anti–PD-1) compared with ipilimumab and with a more favorable toxicity profile (
      • Weber J.
      • Mandala M.
      • Del Vecchio M.
      • Gogas H.J.
      • Arance A.M.
      • Cowey C.L.
      • et al.
      Adjuvant Nivolumab versus ipilimumab in Resected Stage III or IV Melanoma.
      ). Comparable improvement in RFS was noted in the KEYNOTE-054 study, a randomized, placebo-controlled phase III trial of pembrolizumab (
      • Eggermont A.M.M.
      • Blank C.U.
      • Mandala M.
      • Long G.V.
      • Atkinson V.
      • Dalle S.
      • et al.
      Adjuvant Pembrolizumab versus Placebo in Resected Stage III Melanoma.
      ). Based on the results of these trials, both nivolumab and pembrolizumab were approved for adjuvant treatment of patients with resected stage III melanoma and patients with resected, oligometastatic stage IV melanoma. The efficacy of combined BRAFi/MEKi therapy has also been evaluated in the phase III COMBi-AD trial, which demonstrated improved RFS and OS compared with placebo for patients with BRAF V600E/K melanoma (
      • Long G.V.
      • Hauschild A.
      • Santinami M.
      • Atkinson V.
      • Mandalà M.
      • Chiarion-Sileni V.
      • et al.
      Adjuvant dabrafenib plus trametinib in Stage III BRAF-mutated melanoma.
      ). The results of this trial led to the approval of D and T for the adjuvant treatment of stage III melanoma. A recent exploratory biomarker analysis revealed increased tumor mutational burden and/or IFN-γ gene signature may identify patients more likely to respond to adjuvant D+T (
      • Dummer R.
      • Brase J.C.
      • Garrett J.
      • Campbell C.D.
      • Gasal E.
      • Squires M.
      • et al.
      Adjuvant dabrafenib plus trametinib versus placebo in patients with resected, BRAFV600-mutant, stage III melanoma (COMBI-AD): exploratory biomarker analyses from a randomised, phase 3 trial.
      ), although further validation is needed.
      The adjuvant options and surgical management of melanoma in 2020 look quite different from ten years ago (
      • Cohen J.V.
      • Buchbinder E.I.
      The evolution of adjuvant therapy for melanoma.
      ,
      • Eggermont A.M.M.
      • Robert C.
      • Ribas A.
      The new era of adjuvant therapies for melanoma.
      ). Completion lymph node dissection is no longer standard of care for patients with positive sentinel lymph node biopsy, and PD-1 mAb ICB treatment or BRAFi/MEKi therapy are the new frontline options for adjuvant therapy. Despite these advances, several unanswered questions remain. First, for patients with stage III BRAF V600E/K melanoma, it is unclear if PD-1 blockade or BRAFi/MEKi therapy is preferable. BRAFi/MEKi use is supported by a clear RFS and OS benefit, whereas PD-1 blockade with either nivolumab or pembrolizumab is supported only by RFS data currently. However, the 18-month RFS rates are comparable based on the available data (D+T is 67% compared with 66–71% for PD-1 blockade). Although adjuvant PD-1 blockade is generally more tolerable, with approximately 15% of patients stopping therapy because of adverse events compared with approximately 25% with D+T, the risk of severe or irreversible irAEs with PD-1 blockade in patients with modest risk of recurrence can limit enthusiasm for this approach. In all adjuvant trials to date (including COMBI-AD, KEYNOTE-054, and Checkmate-238), completion lymph node dissections were required before beginning adjuvant treatment. Completion lymph node dissection is no longer standard of care (
      • Faries M.B.
      • Thompson J.F.
      • Cochran A.J.
      • Andtbacka R.H.
      • Mozzillo N.
      • Zager J.S.
      • et al.
      Completion dissection or observation for sentinel-node metastasis in melanoma.
      ) and omitting completion lymph node dissection in favor or adjuvant therapy is reasonable and effective and spares patients significant risk of morbidity (
      • Eggermont A.M.M.
      • Robert C.
      • Ribas A.
      The new era of adjuvant therapies for melanoma.
      ). Risk stratification analysis of a cohort of 1,009 patients with 15-year follow-up further demonstrated low risk of recurrence (9%) for patients with a single melanoma deposit measuring <0.1 mm in a single sentinel lymph node compared with patients with deposits measuring 0.1–1.0 mm (16%) and >1.0 mm (25%) (
      • van der Ploeg A.P.
      • van Akkooi A.C.
      • Rutkowski P.
      • Nowecki Z.I.
      • Michej W.
      • Mitra A.
      • et al.
      Prognosis in patients with sentinel node-positive melanoma is accurately defined by the combined Rotterdam tumor load and Dewar topography criteria.
      ). A model incorporating primary tumor ulceration status and sentinel lymph node burden has been developed with a 1.0 mm threshold for distinguishing low- or intermediate- versus high-risk patients (
      • Verver D.
      • van Klaveren D.
      • van Akkooi A.C.J.
      • Rutkowski P.
      • Powell B.W.E.M.
      • Robert C.
      • et al.
      Risk stratification of sentinel node-positive melanoma patients defines surgical management and adjuvant therapy treatment considerations.
      ).
      For patients eligible for both adjuvant PD-1 blockade or BRAFi/MEKi therapy, deciding between these options is a challenge given the comparable improvement in RFS. In practice, factors influencing the choice between targeted therapy and ICB therapy incorporates stage and associated risk of recurrence, comorbid conditions, and side effect profile. For example, patients with stage IIIA melanoma (AJCC 8th Edition) can consider BRAFi/MEKi treatment if available or pursue surveillance but rarely opt for adjuvant PD-1 blockade. Given the improved activity of ICB therapy compared with BRAFi/MEKi targeted therapy in stage IV disease (discussed previously), some clinicians favor PD-1 blockade in this patient population although the current data are inadequate to guide formal recommendations. Management of patients who recur after adjuvant therapy includes surgical resection of locoregional disease, alternative active agents, and clinical trials.

       Future directions and unanswered questions

      Despite the dramatic improvement in clinical outcomes over the past decade owing to immunotherapy and targeted therapy in melanoma, not all patients respond to approved systemic therapies. Extensive preclinical, translational, and clinical research is ongoing to better understand the mechanisms of response and resistance to current therapies, develop rational next-generation treatments (and combinations), and develop more sophisticated models of melanoma that will support further preclinical and translational research.

       Biomarkers

      Although progress has been made in the identification of features associated with response and resistance to cancer immunotherapy, more work is clearly needed to establish reliable predictors of long-term response to ICB therapy. Development of a robust biomarker or gene signature to predict response and/or resistance will likely require sophisticated, integrated multiparameter analysis of genomic, transcriptomic, and clinical data, using matched patient samples obtained before and during ICB therapy in lesions that are responding and failing to respond to therapy.

       The role of the tumor

      With the advent of gene editing techniques and technologies, several genome-scale or sub–genome-scale CRISPR screens have been performed nominating novel genes and pathways that can render melanoma cells more sensitive to immune attack (
      • Manguso R.T.
      • Pope H.W.
      • Zimmer M.D.
      • Brown F.D.
      • Yates K.B.
      • Miller B.C.
      • et al.
      In vivo CRISPR screening identifies Ptpn2 as a cancer immunotherapy target.
      ,
      • Pan D.
      • Kobayashi A.
      • Jiang P.
      • Ferrari de Andrade L.
      • Tay R.E.
      • Luoma A.M.
      • et al.
      A major chromatin regulator determines resistance of tumor cells to T cell-mediated killing.
      ). To identify novel therapeutic targets in melanomas with impaired IFNγ signaling,
      • Vredevoogd D.W.
      • Kuilman T.
      • Ligtenberg M.A.
      • Boshuizen J.
      • Stecker K.E.
      • de Bruijn B.
      • et al.
      Augmenting immunotherapy impact by lowering tumor TNF cytotoxicity threshold.
      conducted a CRISPR screen using melanoma cell lines lacking the IFNγ receptor (IFNGR1). Such screens are valuable to sift through available targets and nominate druggable candidates for further exploration. It is important to note that most of these CRISPR screens to date evaluate the effect of gene deletion in tumor cells only, which cannot readily account for the effect of pharmacologic targeting of a given gene product in which tumor cells, immune cells, and stromal cells are all simultaneously engaged.
      Another key feature of tumors, including melanoma, is the adaptation to treatment leading to resistance. Several mechanisms of resistance to BRAF/MEK targeted therapy have been reported, largely involving bypass or alternative activation of the MAPK pathway (
      • Lim S.Y.
      • Menzies A.M.
      • Rizos H.
      Mechanisms and strategies to overcome resistance to molecularly targeted therapy for melanoma.
      ). Recently, alterations in cell state have been associated with differential drug sensitivity to BRAF/MEK inhibition, and concurrently drug screening efforts have been directed at the cells and cell states resistant to BRAF/MEK inhibition (
      • Eskiocak B.
      • McMillan E.A.
      • Mendiratta S.
      • Kollipara R.K.
      • Zhang H.
      • Humphries C.G.
      • et al.
      Biomarker accessible and chemically addressable mechanistic subtypes of BRAF melanoma.
      ,
      • Tsoi J.
      • Robert L.
      • Paraiso K.
      • Galvan C.
      • Sheu K.M.
      • Lay J.
      • et al.
      Multi-stage differentiation defines melanoma subtypes with differential vulnerability to drug-induced iron-dependent oxidative stress.
      ). Among these are epigenetic states characterized by low-MITF expression, usually associated with high expression of the epithelial-mesenchymal transition marker AXL. Resistance owing to altered melanoma cell states has been associated with minimal residual disease using patient-derived xenograft models (
      • Rambow F.
      • Rogiers A.
      • Marin-Bejar O.
      • Aibar S.
      • Femel J.
      • Dewaele M.
      • et al.
      Toward minimal residual disease-directed therapy in melanoma.
      ). The interplay between altered cell states acquired in the short-term following BRAF and/or MEK inhibition and later accumulation of key mutations is incompletely understood but may be related to alterations in multiple genes or pathways, associated with upregulation of error-prone polymerases and downregulation of DNA repair mechanisms (
      • Russo M.
      • Crisafulli G.
      • Sogari A.
      • Reilly N.M.
      • Arena S.
      • Lamba S.
      • et al.
      Adaptive mutability of colorectal cancers in response to targeted therapies.
      ).

       The role of the immune system

      Successful antitumor immune responses following ICB presumably requires reactivation and clonal proliferation of antigen-experienced T cells present in the tumor microenvironment (
      • Pardoll D.M.
      The blockade of immune checkpoints in cancer immunotherapy.
      ). Recently, single-cell characterization of tumor-infiltrating immune cells has permitted comprehensive evaluation of immune cell populations and immune cell states within a given immune cell population. CD8 T-cell states associated with response to PD-1 blockade include TCF7+ stem-like CD8 T cells with low expression of T-cell dysfunction and exhaustion markers (
      • Sade-Feldman M.
      • Yizhak K.
      • Bjorgaard S.L.
      • Ray J.P.
      • de Boer C.G.
      • Jenkins R.W.
      • et al.
      Defining T cell states associated with response to checkpoint immunotherapy in melanoma.
      ). Orthogonal data using a murine melanoma model supported these findings and further demonstrated that ICB does not reverse exhaustion of CD8 T cells (
      • Miller B.C.
      • Sen D.R.
      • Al Abosy R.
      • Bi K.
      • Virkud Y.V.
      • LaFleur M.W.
      • et al.
      Subsets of exhausted CD8+ T cells differentially mediate tumor control and respond to checkpoint blockade.
      ). Even when terminally exhausted, dysfunctional CD8 T cells participate directly in the antitumor immune response remains an area of active investigation, but given the altered epigenetic state of dysfunctional CD8 T cells, these changes may be irreversible (
      • Miller B.C.
      • Sen D.R.
      • Al Abosy R.
      • Bi K.
      • Virkud Y.V.
      • LaFleur M.W.
      • et al.
      Subsets of exhausted CD8+ T cells differentially mediate tumor control and respond to checkpoint blockade.
      ,
      • Sen D.R.
      • Kaminski J.
      • Barnitz R.A.
      • Kurachi M.
      • Gerdemann U.
      • Yates K.B.
      • et al.
      The epigenetic landscape of T cell exhaustion.
      ) and require recruitment of naïve immune cells to the tumor microenvironment. Several recent papers described the presence of tumor-associated tertiary lymphoid structures and associated B cells with improved clinical outcomes in patients with melanoma (
      • Cabrita R.
      • Lauss M.
      • Sanna A.
      • Donia M.
      • Skaarup Larsen M.
      • Mitra S.
      • et al.
      Tertiary lymphoid structures improve immunotherapy and survival in melanoma.
      ,
      • Helmink B.A.
      • Reddy S.M.
      • Gao J.
      • Zhang S.
      • Basar R.
      • Thakur R.
      • et al.
      B cells and tertiary lymphoid structures promote immunotherapy response.
      ) and soft-tissue sarcoma (
      • Petitprez F.
      • de Reyniès A.
      • Keung E.Z.
      • Chen T.W.
      • Sun C.M.
      • Calderaro J.
      • et al.
      B cells are associated with survival and immunotherapy response in sarcoma.
      ). Tumor-associated tertiary lymphoid structures (also known as ectopic lymph nodes) may be a site of enhanced antigen presentation where naïve lymphocytes can become antigen-experienced and primed for antitumor immune response following PD-1 blockade.
      Another key question focuses on the identity of the tumor-targeted antigens. There is broad consensus that UV-induced tumor-specific neoantigens may be recognized by CD8 T cells, but the overall mutational burden is an imperfect predictor of clinical response. It is also notable that autoimmune vitiligo—with some distinctive clinical features—is commonly observed in patients experiencing major tumor regressions with ICB. Although this could represent an epiphenomenon indicating overall immune activation by checkpoint inhibition, it is notable that vitiligo is virtually never seen when ICB is used to treat tumors other than melanoma. It thus remains to be seen whether melanocyte lineage antigens may also represent important functional immune targets in responding patients.

       Preclinical cancer models

      Development of more sophisticated preclinical tumor models, including evaluation of patient-derived human samples capable of preserving features of the tumor immune microenvironment, and improved understanding of other extratumoral features that influence immune responses may facilitate and accelerate preclinical and translational research efforts (
      • Friedman A.A.
      • Letai A.
      • Fisher D.E.
      • Flaherty K.T.
      Precision medicine for cancer with next-generation functional diagnostics.
      ,
      • Zitvogel L.
      • Pitt J.M.
      • Daillère R.
      • Smyth M.J.
      • Kroemer G.
      Mouse models in oncoimmunology.
      ). Patient-derived tumor models that preserve the immune contexture of the tumor microenvironment, including patient-derived organoids (
      • Neal J.T.
      • Li X.
      • Zhu J.
      • Giangarra V.
      • Grzeskowiak C.L.
      • Ju J.
      • et al.
      Organoid modeling of the tumor immune microenvironment.
      ), patient-derived organotypic tumor spheroids (
      • Jenkins R.W.
      • Aref A.R.
      • Lizotte P.H.
      • Ivanova E.
      • Stinson S.
      • Zhou C.W.
      • et al.
      Ex vivo profiling of PD-1 blockade using organotypic tumor spheroids.
      ), and models in which tumor material is combined with peripheral immune cells (
      • Dijkstra K.K.
      • Cattaneo C.M.
      • Weeber F.
      • Chalabi M.
      • van de Haar J.
      • Fanchi L.F.
      • et al.
      Generation of tumor-reactive T cells by co-culture of peripheral blood lymphocytes and tumor organoids.
      ) have been described in recent years. Such models and assays have shown promise in liquid malignancies (
      • Tyner J.W.
      • Tognon C.E.
      • Bottomly D.
      • Wilmot B.
      • Kurtz S.E.
      • Savage S.L.
      • et al.
      Functional genomic landscape of acute myeloid leukaemia.
      ) and more recently with solid tumors and conventional cytotoxic chemotherapy (
      • Ooft S.N.
      • Weeber F.
      • Dijkstra K.K.
      • McLean C.M.
      • Kaing S.
      • van Werkhoven E.
      • et al.
      Patient-derived organoids can predict response to chemotherapy in metastatic colorectal cancer patients.
      ,
      • Vlachogiannis G.
      • Hedayat S.
      • Vatsiou A.
      • Jamin Y.
      • Fernández-Mateos J.
      • Khan K.
      • et al.
      Patient-derived organoids model treatment response of metastatic gastrointestinal cancers.
      ). Novel function precision medicine approaches may be ideally suited to identify specific therapies, or therapeutic combinations, to optimize clinical activity and durability of clinical response for individual patients (
      • Smyth M.J.
      • Ngiow S.F.
      • Ribas A.
      • Teng M.W.
      Combination cancer immunotherapies tailored to the tumour microenvironment.
      ,
      • Spranger S.
      • Gajewski T.
      Rational combinations of immunotherapeutics that target discrete pathways.
      ).

       Next-generation therapies and combination approaches

      Clinical trials are already underway evaluating novel immune modulatory agents in combination with anti–PD-1/PD-L1 therapies in an effort to overcome innate resistance (
      • Jenkins R.W.
      • Barbie D.A.
      • Flaherty K.T.
      Mechanisms of resistance to immune checkpoint inhibitors.
      ,
      • O'Donnell J.S.
      • Long G.V.
      • Scolyer R.A.
      • Teng M.W.
      • Smyth M.J.
      Resistance to PD1/PDL1 checkpoint inhibition.
      ,
      • Sharma P.
      • Hu-Lieskovan S.
      • Wargo J.A.
      • Ribas A.
      Primary, adaptive, and acquired resistance to cancer immunotherapy.
      ). Despite increasing reports of rational combination strategies, these therapies remain one size fits all because of the lack of robust biomarkers to guide clinical decision-making. Recently, the results of two phase III trials comparing novel promising combination strategies were reported. The combination of the IDO inhibitor epacadostat with pembrolizumab showed no survival benefit compared with single-agent PD-1 blockade in the ECHO-301/KEYNOTE-252 phase III, placebo-controlled, randomized clinical trial (
      • Long G.V.
      • Dummer R.
      • Hamid O.
      • Gajewski T.F.
      • Caglevic C.
      • Dalle S.
      • et al.
      Epacadostat plus pembrolizumab versus placebo plus pembrolizumab in patients with unresectable or metastatic melanoma (ECHO-301/KEYNOTE-252): a phase 3, randomised, double-blind study.
      ). The results of the phase III IMPSIRE 170 trial comparing combination treatment with cobimetinib (MEKi) and atezolizumab (anti–PD-L1 mAb) versus pembrolizumab (anti–PD-1) are not yet published, but presentation at the European Society for Medical Oncology 2019 meeting indicated that the atezolizumab-cobimetinib combination failed to meet its primary endpoint in patients with BRAF wild-type melanoma (
      • Arance A.M.
      • Gogas H.
      • Dreno B.
      • Flaherty K.T.
      • Demidov L.
      • Stroyakovskiy D.
      • et al.
      Combination treatment with cobimetnib (C) and atezolizumab (A) vs pembrolizumab (P) in previously untreated patients (PTS) with BRAFV600 wild type (WT) advanced melanoma: primary analysis from the phase 3 IMSPIRE170 trial.
      ). With these recent high-profile negative trials and the expanding number of combination trials (
      • Tang J.
      • Shalabi A.
      • Hubbard-Lucey V.M.
      Comprehensive analysis of the clinical immuno-oncology landscape.
      ), there is renewed focus on the preclinical and early-phase clinical development of combination strategies.

      Conclusion

      Advances in molecular targeted therapy and immune checkpoint inhibition have led to unprecedented improvement in overall survival for patients with advanced melanoma. Single-agent PD-1 blockade and combination BRAFi/MEKi therapy have both demonstrated a long-term, 5-year OS benefit of 30–40%. Superior response rates have been demonstrated with combined PD-1/CTLA-4 blockade, with a numerically higher although not statistically significant OS benefit compared with single-agent PD-1 blockade. BRAF/MEK therapy and PD-1 blockade have supplanted high-dose interferon alpha 2b and ipilimumab as the preferred adjuvant treatment options for patients with stage III melanoma. Intense investigation is ongoing to identify effective treatment strategies for patients for whom ICB therapy and/or BRAF/MEK targeted therapy are ineffective. Given the durability of responses observed in patients successfully treated with ICB therapy, the vast majority of current melanoma clinical trials include an ICB backbone in an effort to match the high response rates seen with some targeted therapy responses with the durability of responses evidenced with cancer immunotherapy. Given the recent failures of several initially high-profile phase III combination immunotherapy trials coupled with the ever-increasing number of novel therapies and combination trials, there is an unmet need for novel approaches, tools, techniques, and methods for preclinical evaluation to better understand mechanisms of response and resistance to immune checkpoint inhibitors and next-generation antitumor immune modulatory drugs (
      • O'Donnell J.S.
      • Long G.V.
      • Scolyer R.A.
      • Teng M.W.
      • Smyth M.J.
      Resistance to PD1/PDL1 checkpoint inhibition.
      ,
      • Sharma P.
      • Hu-Lieskovan S.
      • Wargo J.A.
      • Ribas A.
      Primary, adaptive, and acquired resistance to cancer immunotherapy.
      ). Just as the advances of 2010 (BRAF targeted therapy and CTLA-4 blockade) represented new treatment paradigms rather than incremental improvement over the standard of care, progress over the next decade will likely require more than simply enhancing what is (e.g., anti–PD-1 therapy) and instead focusing on advancing toward what will be with an eye on overcoming resistance, novel biomarker strategies, and advances in precision functional medicine.

      ORCIDs

      Conflict of Interest

      RWJ is on the advisory board for XSphera Biosciences and has received research support from Monopteros Therapeutics. DEF is on the Board of Directors and a consultant for Soltego Inc. DEF has a financial interest in Soltego, Inc., a company developing SIK inhibitors for topical skin darkening treatments that might be used for a broad set of human applications. Dr. Fisher’s interests were reviewed and are managed by Massachusetts General Hospital and Partners HealthCare in accordance with their conflict of interest policies. RWJ has a financial interest in XSphera Biosciences Inc., a company focused on using ex vivo profiling technology to deliver functional, precision immune-oncology solutions for patients, providers, and drug development companies. RWJ’s interests were reviewed and are managed by Massachusetts General Hospital and Partners HealthCare in accordance with their conflict of interest policies.

      Acknowledgments

      Dr. Jenkins is supported by the National Cancer Institute ( 1K08CA226391 ), the Melanoma Research Alliance , the Karin Grunebaum Cancer Research Foundation , the V Foundation , and the Henri and Belinda Termeer Fund for Early Career Investigators in Systems Pharmacology. Dr. Fisher is supported by grants from NIH : 2P01 CA163222-06; 5R01 AR043369-23; 5R01CA222871-03; 5R01AR072304-03, and a grant from the Dr. Miriam and Sheldon G. Adelson Medical Research Foundation.

      Author Contributions

      Writing - Original Draft Preparation: RWJ; Writing - Review and Editing: DEF.

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