The Intersection of Immune-Directed and Molecularly Targeted Therapy in Advanced Melanoma: Where We Have Been, Are, and Will Be

“In three years, four drugs have gained regulatory approval for the treatment of metastatic and unresectable melanoma, with at least seven other drugs having recently completed, currently in, or soon to be in phase III clinical testing. This amazing achievement has been made following a remarkable increase of knowledge in molecular biology and immunology that led to the identification of high-valued therapeutic targets and the clinical development of agents that effectively engage and inhibit these targets. The discovery of either effective molecularly targeted therapies or immunotherapies would have led to dramatic improvements to the standard-of-care treatment of melanoma. However, through parallel efforts that have showcased the efficacy of small-molecule BRAF and MAP–ERK kinase (MEK) inhibitors, as well as the immune checkpoint inhibitors, namely ipilimumab and the anti-PD1/PDL1 antibodies (lambrolizumab, nivolumab, MPDL3280), an opportunity exists to transform the treatment of melanoma specifically and cancer generally by exploring rational combinations of molecularly targeted therapies, immunotherapies, and molecular targeted therapies with immunotherapies. This overview presents the historical context to this therapeutic revolution, reviews the benefits and limitations of current therapies, and provides a look ahead at where the field is headed.”


CTLA-4 and PD-1/PD-L1 Blockade: New Immunotherapeutic Modalities with Durable Clinical Benefit in Melanoma Patients

“Immune checkpoint blockade with monoclonal antibodies directed at the inhibitory immune receptors CTLA-4, PD-1, and PD-L1 has emerged as a successful treatment approach for patients with advanced melanoma. Ipilimumab is the first agent associated with a documented improved overall survival benefit in this patient population. A striking attribute of CTLA-4 blockade is the durability of objective responses, leading to speculation of a possible cure for some patients. Many tumor responses achieved with PD-1 and PD-L1 inhibition were durable in the phase I trials and were seen in a higher proportion of patients with melanoma than typically observed with ipilimumab. Biomarker development to identify the subset of patients with melanoma who will achieve durable clinical benefit with checkpoint blockade is critical; tumor PD-L1 expression has been promising in early studies. The contrast between unprecedented response rates but limited durability of responses achieved with BRAF and MEK inhibition in BRAFV600-mutated melanoma and the impressive durability but relatively low rate of response achieved with immune checkpoint blockade is striking. Preclinical data on potential synergies between CTLA-4/PD-1/PD-L1 inhibition and MAPK-targeted therapy is emerging, and combined immune checkpoint blockade and MAPK inhibition are being explored in clinical trials. Other promising approaches to increase the number of patients with melanoma who benefit from durable responses with immune checkpoint blockade include concurrent or sequenced CTLA-4 and PD-1/PD-L1 inhibition and combination with other immunotherapeutic strategies.”


Bypass Mechanisms of Resistance to Receptor Tyrosine Kinase Inhibition in Lung Cancer

“Receptor tyrosine kinases (RTKs) are activated by somatic genetic alterations in a subset of cancers, and such cancers are often sensitive to specific inhibitors of the activated kinase. Two well-established examples of this paradigm include lung cancers with either EGFR mutations or ALK translocations. In these cancers, inhibition of the corresponding RTK leads to suppression of key downstream signaling pathways, such as the PI3K (phosphatidylinositol 3-kinase)/AKT and MEK (mitogen-activated protein kinase kinase)/ERK (extracellular signal–regulated kinase) pathways, resulting in cell growth arrest and death. Despite the initial clinical efficacy of ALK (anaplastic lymphoma kinase) and EGFR (epidermal growth factor receptor) inhibitors in these cancers, resistance invariably develops, typically within 1 to 2 years. Over the past several years, multiple molecular mechanisms of resistance have been identified, and some common themes have emerged. One is the development of resistance mutations in the drug target that prevent the drug from effectively inhibiting the respective RTK. A second is activation of alternative RTKs that maintain the signaling of key downstream pathways despite sustained inhibition of the original drug target. Indeed, several different RTKs have been implicated in promoting resistance to EGFR and ALK inhibitors in both laboratory studies and patient samples. In this mini-review, we summarize the concepts underlying RTK-mediated resistance, the specific examples known to date, and the challenges of applying this knowledge to develop improved therapeutic strategies to prevent or overcome resistance.”


mTORC1 Status Dictates Tumor Response to Targeted Therapeutics

“Genomics has revolutionized and personalized our approach to cancer therapy, with clinical trials now frequently involving patient stratification based on tumor genotype. Rational drug design specifically targeting the most common genetic events and aberrantly regulated pathways in human cancers makes this approach possible. However, our understanding of the wiring of oncogenic signaling networks and the key downstream effectors driving human cancers is incomplete, limiting our ability to predict clinical responses or identify mechanisms of resistance to targeted therapeutics. Recent studies in independent cancer lineages driven by distinct oncogenic signaling events point to a common downstream target, the mammalian (or mechanistic) target of rapamycin complex 1 (mTORC1), which dictates the cellular and clinical response to pathway-specific inhibitors. mTORC1 is a highly integrated signaling node that promotes anabolic cell growth and proliferation and lies downstream of multiple oncogenes and tumor suppressors, including those influencing the PI3K-Akt and RAS-RAF-MEK-ERK pathways. Studies are now suggesting that to effectively target the major oncogenic signaling pathway in a given tumor, mTORC1 must be inhibited, and that its sustained activation is a major mechanism of resistance to such targeted therapies.”


Bypass Mechanisms of Resistance to Receptor Tyrosine Kinase Inhibition in Lung Cancer

“Receptor tyrosine kinases (RTKs) are activated by somatic genetic alterations in a subset of cancers, and such cancers are often sensitive to specific inhibitors of the activated kinase. Two well-established examples of this paradigm include lung cancers with either EGFR mutations or ALK translocations. In these cancers, inhibition of the corresponding RTK leads to suppression of key downstream signaling pathways, such as the PI3K (phosphatidylinositol 3-kinase)/AKT and MEK (mitogen-activated protein kinase kinase)/ERK (extracellular signal–regulated kinase) pathways, resulting in cell growth arrest and death. Despite the initial clinical efficacy of ALK (anaplastic lymphoma kinase) and EGFR (epidermal growth factor receptor) inhibitors in these cancers, resistance invariably develops, typically within 1 to 2 years. Over the past several years, multiple molecular mechanisms of resistance have been identified, and some common themes have emerged. One is the development of resistance mutations in the drug target that prevent the drug from effectively inhibiting the respective RTK. A second is activation of alternative RTKs that maintain the signaling of key downstream pathways despite sustained inhibition of the original drug target. Indeed, several different RTKs have been implicated in promoting resistance to EGFR and ALK inhibitors in both laboratory studies and patient samples. In this mini-review, we summarize the concepts underlying RTK-mediated resistance, the specific examples known to date, and the challenges of applying this knowledge to develop improved therapeutic strategies to prevent or overcome resistance.”


Bypass Mechanisms of Resistance to Receptor Tyrosine Kinase Inhibition in Lung Cancer

“Receptor tyrosine kinases (RTKs) are activated by somatic genetic alterations in a subset of cancers, and such cancers are often sensitive to specific inhibitors of the activated kinase. Two well-established examples of this paradigm include lung cancers with either EGFR mutations or ALK translocations. In these cancers, inhibition of the corresponding RTK leads to suppression of key downstream signaling pathways, such as the PI3K (phosphatidylinositol 3-kinase)/AKT and MEK (mitogen-activated protein kinase kinase)/ERK (extracellular signal–regulated kinase) pathways, resulting in cell growth arrest and death. Despite the initial clinical efficacy of ALK (anaplastic lymphoma kinase) and EGFR (epidermal growth factor receptor) inhibitors in these cancers, resistance invariably develops, typically within 1 to 2 years. Over the past several years, multiple molecular mechanisms of resistance have been identified, and some common themes have emerged. One is the development of resistance mutations in the drug target that prevent the drug from effectively inhibiting the respective RTK. A second is activation of alternative RTKs that maintain the signaling of key downstream pathways despite sustained inhibition of the original drug target. Indeed, several different RTKs have been implicated in promoting resistance to EGFR and ALK inhibitors in both laboratory studies and patient samples. In this mini-review, we summarize the concepts underlying RTK-mediated resistance, the specific examples known to date, and the challenges of applying this knowledge to develop improved therapeutic strategies to prevent or overcome resistance.”


Mechanisms of Targeted Therapy Resistance Take a De-TOR

“The effectiveness of cancer therapeutics targeting signal transduction pathways is comprised of a diversity of mechanisms that drive de novo or acquired resistance. Two recent studies identify mTOR activation as a point of convergence of mechanisms that cause resistance to inhibitors of the Raf-MEK-ERK and PI3K signaling.”


Mechanism of MEK Inhibition Determines Efficacy in Mutant KRAS- Versus BRAF-Driven Cancers

“KRAS and BRAF activating mutations drive tumorigenesis through constitutive activation of the MAPK pathway. As these tumours represent an area of high unmet medical need, multiple allosteric MEK inhibitors, which inhibit MAPK signalling in both genotypes, are being tested in clinical trials. Impressive single-agent activity in BRAF-mutant melanoma has been observed; however, efficacy has been far less robust in KRAS-mutant disease1. Here we show that, owing to distinct mechanisms regulating MEK activation in KRAS- versus BRAF-driven tumours23, different mechanisms of inhibition are required for optimal antitumour activity in each genotype. Structural and functional analysis illustrates that MEK inhibitors with superior efficacy in KRAS-driven tumours (GDC-0623 and G-573, the former currently in phase I clinical trials) form a strong hydrogen-bond interaction with S212 in MEK that is critical for blocking MEK feedback phosphorylation by wild-type RAF.”


Mechanism of MEK Inhibition Determines Efficacy in Mutant KRAS- Versus BRAF-Driven Cancers

“KRAS and BRAF activating mutations drive tumorigenesis through constitutive activation of the MAPK pathway. As these tumours represent an area of high unmet medical need, multiple allosteric MEK inhibitors, which inhibit MAPK signalling in both genotypes, are being tested in clinical trials. Impressive single-agent activity in BRAF-mutant melanoma has been observed; however, efficacy has been far less robust in KRAS-mutant disease1. Here we show that, owing to distinct mechanisms regulating MEK activation in KRAS- versus BRAF-driven tumours23, different mechanisms of inhibition are required for optimal antitumour activity in each genotype. Structural and functional analysis illustrates that MEK inhibitors with superior efficacy in KRAS-driven tumours (GDC-0623 and G-573, the former currently in phase I clinical trials) form a strong hydrogen-bond interaction with S212 in MEK that is critical for blocking MEK feedback phosphorylation by wild-type RAF.”