A brain-penetrant RAF dimer antagonist for the noncanonical BRAF oncoprotein of pediatric low-grade astrocytomas

Future perspective of targeted treatments in pediatric low-grade glioma (pLGG): the evolution of standard-of-care and challenges of a new era

While surgery, when possible, remains the mainstay of pediatric low-grade glioma (pLGG) management, adjuvant therapy has significantly evolved over time. Radiation therapy was commonly used in the late 1990s for tumors that could not be resected or recurred. This resulted in significant late morbidity in this population and mortality related to secondary malignancies and chronic health conditions. Chemotherapy became the mainstay of adjuvant therapy but children still experienced late morbidity secondary to exposure to multiple lines of treatment over time. Targeted therapies emerged after the identification of frequent genetic alterations in the mitogen activated protein kinase (MAPK) pathway including KIAA1549-BRAF fusions and BRAF-V600 mutations and the near universal upregulation of the MAPK pathway in these tumors. Both BRAF and MEK inhibitors have shown efficacy in the treatment of pLGG and have led to prolonged stability in some cases. Multiple phase III clinical trials are now comparing targeted therapy to standard-of-care chemotherapy regimens setting the stage for targeted therapy to replace chemotherapy as the first-line treatment in some cases. Targeted therapy, however, is not without its challenges. There are clear examples of resistance and mechanisms of resistance have not been fully elucidated. There is also no clear duration for these therapies and rebound growth is a well-known phenomenon especially in BRAF-V600 mutant tumors. Targeted therapies are also fairly recent developments and long-term toxicities and functional outcomes are still being monitored. Very young and adolescent/young adult LGGs also carry molecular features that may not be addressed by inhibition of the MAPK pathway. Adjuvant therapy for pLGG has evolved from radiation for all unresectable or residual tumors to molecularly driven targeted therapies with improved quality of life, late effects, and less off-target toxicities. While there is still much to learn in regard to newer targeted therapies for pLGG, the era of targeted therapies for pediatric LGG is upon us.

Molecular-targeted therapy for childhood low-grade glial and glioneuronal tumors

Since the discovery of the association between BRAF mutations and fusions in the development of childhood low-grade gliomas and the subsequent recognition that most childhood low-grade glial and glioneuronal tumors have aberrant signaling through the RAS/RAF/MAP kinase pathway, there has been a dramatic change in how these tumors are conceptualized. Many of the fusions and mutations present in these tumors are associated with molecular targets, which have agents in development or already in clinical use. Various agents, including MEK inhibitors, BRAF inhibitors, MTOR inhibitors and, in small subsets of patients NTRK inhibitors, have been used successfully to treat children with recurrent disease, after failure of conventional approaches such as surgery or chemotherapy. The relative benefits of chemotherapy as compared to molecular-targeted therapy for children with newly diagnosed gliomas and neuroglial tumors are under study. Already the combination of an MEK inhibitor and a BRAF inhibitor has been shown superior to conventional chemotherapy (carboplatin and vincristine) in newly diagnosed children with BRAF-V600E mutated low-grade gliomas and neuroglial tumors. However, the long-term effects of such molecular-targeted treatment are unknown. The potential use of molecular-targeted therapy in early treatment has made it mandatory that the molecular make-up of the majority of low-grade glial and glioneuronal tumors is known before initiation of therapy. The primary exception to this rule is in children with neurofibromatosis type 1 who, by definition, have NF1 loss; however, even in this population, gliomas arising in late childhood and adolescence or those not responding to conventional treatment may be candidates for biopsy, especially before entry on molecular-targeted therapy trials.

Tovorafenib: First Approval

Tovorafenib (OJEMDA™) is a once-weekly oral, selective, brain-penetrant, type II RAF kinase inhibitor being developed by Day One Biopharmaceuticals, Inc., under a license from Takeda Oncology, for the treatment of paediatric low-grade glioma (pLGG) and solid tumours. Most pLGGs harbour alterations in the MAPK pathway, such as a BRAF mutation or BRAF fusion, which result in aberrant intracellular signalling. Tovorafenib is an inhibitor of mutant BRAF V600E, wild-type BRAF and wild-type CRAF kinases and BRAF fusions. In April 2024, tovorafenib received its first approval in the USA for the treatment of patients aged ≥ 6 months with relapsed or refractory pLGGs harbouring a BRAF fusion or rearrangement, or BRAF V600 mutation. It received accelerated approval for this indication based on the response rate and duration of response achieved in this population in the ongoing, pivotal, phase 2 FIREFLY-1 study. Clinical development of tovorafenib is underway in numerous countries worldwide. This article summarizes the milestones in the development of tovorafenib leading to this first approval for relapsed or refractory pLGG with an activating BRAF alteration.

A critical review of RAF inhibitors in BRAF-mutated glioma treatment

BRAF gliomas have garnered significant attention in research due to the lack of effective treatments and their notable incidence, constituting 3% of all gliomas. This underlines the importance of investigating this area and the impact that targeted therapies could hold. This review discusses the development of targeted therapies for these tumors, examining the effectiveness of first-generation BRAF inhibitors such as Vemurafenib, Dabrafenib and Encorafenib, while addressing the challenges posed by paradoxical ERK activation. The advent of pan-RAF inhibitors, notably Tovorafenib, offers a promising advance, demonstrating enhanced efficacy and better penetration of the blood-brain barrier, without the issue of paradoxical activation. Nevertheless, continued research is essential to refine therapeutic strategies for BRAF-mutated gliomas, given the evolving nature of targeted therapy development.

Targeting RAF dimers in RAS mutant tumors: From biology to clinic

RAS mutations occur in approximately 30% of tumors worldwide and have a poor prognosis due to limited therapies. Covalent targeting of KRAS G12C has achieved significant success in recent years, but there is still a lack of efficient therapeutic approaches for tumors with non-G12C KRAS mutations. A highly promising approach is to target the MAPK pathway downstream of RAS, with a particular focus on RAF kinases. First-generation RAF inhibitors have been authorized to treat BRAF mutant tumors for over a decade. However, their use in RAS-mutated tumors is not recommended due to the paradoxical ERK activation mainly caused by RAF dimerization. To address the issue of RAF dimerization, type II RAF inhibitors have emerged as leading candidates. Recent clinical studies have shown the initial effectiveness of these agents against RAS mutant tumors. Promisingly, type II RAF inhibitors in combination with MEK or ERK inhibitors have demonstrated impressive efficacy in RAS mutant tumors. This review aims to clarify the importance of RAF dimerization in cellular signaling and resistance to treatment in tumors with RAS mutations, as well as recent progress in therapeutic approaches to address the problem of RAF dimerization in RAS mutant tumors.

BRAF - a tumour-agnostic drug target with lineage-specific dependencies

In June 2022, the FDA granted Accelerated Approval to the BRAF inhibitor dabrafenib in combination with the MEK inhibitor trametinib for the treatment of adult and paediatric patients (≥6 years of age) with unresectable or metastatic BRAFV600E-mutant solid tumours, except for BRAFV600E-mutant colorectal cancers. The histology-agnostic approval of dabrafenib plus trametinib marks the culmination of two decades of research into the landscape of BRAF mutations in human cancers, the biochemical mechanisms underlying BRAF-mediated tumorigenesis, and the clinical development of selective RAF and MEK inhibitors. Although the majority of patients with BRAFV600E-mutant tumours derive clinical benefit from BRAF inhibitor-based combinations, resistance to treatment develops in most. In this Review, we describe the biochemical basis for oncogenic BRAF-induced activation of MAPK signalling and pan-cancer and lineage-specific mechanisms of intrinsic, adaptive and acquired resistance to BRAF inhibitors. We also discuss novel RAF inhibitors and drug combinations designed to delay the emergence of treatment resistance and/or expand the population of patients with BRAF-mutant cancers who benefit from molecularly targeted therapies.

LOGGIC/FIREFLY-2: a phase 3, randomized trial of tovorafenib vs. chemotherapy in pediatric and young adult patients with newly diagnosed low-grade glioma harboring an activating RAF alteration

BACKGROUND

Pediatric low-grade glioma (pLGG) is essentially a single pathway disease, with most tumors driven by genomic alterations affecting the mitogen-activated protein kinase/ERK (MAPK) pathway, predominantly KIAA1549::BRAF fusions and BRAF V600E mutations. This makes pLGG an ideal candidate for MAPK pathway-targeted treatments. The type I BRAF inhibitor, dabrafenib, in combination with the MEK inhibitor, trametinib, has been approved by the United States Food and Drug Administration for the systemic treatment of BRAF V600E-mutated pLGG. However, this combination is not approved for the treatment of patients with tumors harboring BRAF fusions as type I RAF inhibitors are ineffective in this setting and may paradoxically enhance tumor growth. The type II RAF inhibitor, tovorafenib (formerly DAY101, TAK-580, MLN2480), has shown promising activity and good tolerability in patients with BRAF-altered pLGG in the phase 2 FIREFLY-1 study, with an objective response rate (ORR) per Response Assessment in Neuro-Oncology high-grade glioma (RANO-HGG) criteria of 67%. Tumor response was independent of histologic subtype, BRAF alteration type (fusion vs. mutation), number of prior lines of therapy, and prior MAPK-pathway inhibitor use.

METHODS

LOGGIC/FIREFLY-2 is a two-arm, randomized, open-label, multicenter, global, phase 3 trial to evaluate the efficacy, safety, and tolerability of tovorafenib monotherapy vs. current standard of care (SoC) chemotherapy in patients < 25 years of age with pLGG harboring an activating RAF alteration who require first-line systemic therapy. Patients are randomized 1:1 to either tovorafenib, administered once weekly at 420 mg/m2 (not to exceed 600 mg), or investigator's choice of prespecified SoC chemotherapy regimens. The primary objective is to compare ORR between the two treatment arms, as assessed by independent review per RANO-LGG criteria. Secondary objectives include comparisons of progression-free survival, duration of response, safety, neurologic function, and clinical benefit rate.

DISCUSSION

The promising tovorafenib activity data, CNS-penetration properties, strong scientific rationale combined with the manageable tolerability and safety profile seen in patients with pLGG led to the SIOPe-BTG-LGG working group to nominate tovorafenib for comparison with SoC chemotherapy in this first-line phase 3 trial. The efficacy, safety, and functional response data generated from the trial may define a new SoC treatment for newly diagnosed pLGG.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT05566795. Registered on October 4, 2022.

Pediatric low-grade glioma models: advances and ongoing challenges

Pediatric low-grade gliomas represent the most common childhood brain tumor class. While often curable, some tumors fail to respond and even successful treatments can have life-long side effects. Many clinical trials are underway for pediatric low-grade gliomas. However, these trials are expensive and challenging to organize due to the heterogeneity of patients and subtypes. Advances in sequencing technologies are helping to mitigate this by revealing the molecular landscapes of mutations in pediatric low-grade glioma. Functionalizing these mutations in the form of preclinical models is the next step in both understanding the disease mechanisms as well as for testing therapeutics. However, such models are often more difficult to generate due to their less proliferative nature, and the heterogeneity of tumor microenvironments, cell(s)-of-origin, and genetic alterations. In this review, we discuss the molecular and genetic alterations and the various preclinical models generated for the different types of pediatric low-grade gliomas. We examined the different preclinical models for pediatric low-grade gliomas, summarizing the scientific advances made to the field and therapeutic implications. We also discuss the advantages and limitations of the various models. This review highlights the importance of preclinical models for pediatric low-grade gliomas while noting the challenges and future directions of these models to improve therapeutic outcomes of pediatric low-grade gliomas.

The type II RAF inhibitor tovorafenib in relapsed/refractory pediatric low-grade glioma: the phase 2 FIREFLY-1 trial

BRAF genomic alterations are the most common oncogenic drivers in pediatric low-grade glioma (pLGG). Arm 1 (n = 77) of the ongoing phase 2 FIREFLY-1 (PNOC026) trial investigated the efficacy of the oral, selective, central nervous system-penetrant, type II RAF inhibitor tovorafenib (420 mg m-2 once weekly; 600 mg maximum) in patients with BRAF-altered, relapsed/refractory pLGG. Arm 2 (n = 60) is an extension cohort, which provided treatment access for patients with RAF-altered pLGG after arm 1 closure. Based on independent review, according to Response Assessment in Neuro-Oncology High-Grade Glioma (RANO-HGG) criteria, the overall response rate (ORR) of 67% met the arm 1 prespecified primary endpoint; median duration of response (DOR) was 16.6 months; and median time to response (TTR) was 3.0 months (secondary endpoints). Other select arm 1 secondary endpoints included ORR, DOR and TTR as assessed by Response Assessment in Pediatric Neuro-Oncology Low-Grade Glioma (RAPNO) criteria and safety (assessed in all treated patients and the primary endpoint for arm 2, n = 137). The ORR according to RAPNO criteria (including minor responses) was 51%; median DOR was 13.8 months; and median TTR was 5.3 months. The most common treatment-related adverse events (TRAEs) were hair color changes (76%), elevated creatine phosphokinase (56%) and anemia (49%). Grade ≥3 TRAEs occurred in 42% of patients. Nine (7%) patients had TRAEs leading to discontinuation of tovorafenib. These data indicate that tovorafenib could be an effective therapy for BRAF-altered, relapsed/refractory pLGG. ClinicalTrials.gov registration: NCT04775485 .

Targeting the RAS/RAF/MAPK pathway for cancer therapy: from mechanism to clinical studies

Metastatic dissemination of solid tumors, a leading cause of cancer-related mortality, underscores the urgent need for enhanced insights into the molecular and cellular mechanisms underlying metastasis, chemoresistance, and the mechanistic backgrounds of individuals whose cancers are prone to migration. The most prevalent signaling cascade governed by multi-kinase inhibitors is the mitogen-activated protein kinase (MAPK) pathway, encompassing the RAS-RAF-MAPK kinase (MEK)-extracellular signal-related kinase (ERK) pathway. RAF kinase is a primary mediator of the MAPK pathway, responsible for the sequential activation of downstream targets, such as MEK and the transcription factor ERK, which control numerous cellular and physiological processes, including organism development, cell cycle control, cell proliferation and differentiation, cell survival, and death. Defects in this signaling cascade are associated with diseases such as cancer. RAF inhibitors (RAFi) combined with MEK blockers represent an FDA-approved therapeutic strategy for numerous RAF-mutant cancers, including melanoma, non-small cell lung carcinoma, and thyroid cancer. However, the development of therapy resistance by cancer cells remains an important barrier. Autophagy, an intracellular lysosome-dependent catabolic recycling process, plays a critical role in the development of RAFi resistance in cancer. Thus, targeting RAF and autophagy could be novel treatment strategies for RAF-mutant cancers. In this review, we delve deeper into the mechanistic insights surrounding RAF kinase signaling in tumorigenesis and RAFi-resistance. Furthermore, we explore and discuss the ongoing development of next-generation RAF inhibitors with enhanced therapeutic profiles. Additionally, this review sheds light on the functional interplay between RAF-targeted therapies and autophagy in cancer.

Wild-type C-Raf gene dosage and dimerization drive prostate cancer metastasis

Mutated Ras and Raf kinases are well-known to promote cancer metastasis via flux through the Ras/Raf/MEK/ERK (mitogen-activated protein kinase [MAPK]) pathway. A role for non-mutated Raf in metastasis is also emerging, but the key mechanisms remain unclear. Elevated expression of any of the three wild-type Raf family members (C, A, or B) can drive metastasis. We utilized an in vivo model to show that wild-type C-Raf overexpression can promote metastasis of immortalized prostate cells in a gene dosage-dependent manner. Analysis of the transcriptomic and phosphoproteomic landscape indicated that C-Raf-driven metastasis is accompanied by upregulated MAPK signaling. Use of C-Raf mutants demonstrated that the dimerization domain, but not its kinase activity, is essential for metastasis. Endogenous Raf monomer knockouts revealed that C-Raf's ability to form dimers with endogenous Raf molecules is important for promoting metastasis. These data identify wild-type C-Raf heterodimer signaling as a potential target for treating metastatic disease.

Generation of patient-derived pediatric pilocytic astrocytoma in-vitro models using SV40 large T: evaluation of a modeling workflow

PURPOSE

Although pediatric low-grade gliomas (pLGG) are the most common pediatric brain tumors, patient-derived cell lines reflecting pLGG biology in culture are scarce. This also applies to the most common pLGG subtype pilocytic astrocytoma (PA). Conventional cell culture approaches adapted from higher-grade tumors fail in PA due to oncogene-induced senescence (OIS) driving tumor cells into arrest. Here, we describe a PA modeling workflow using the Simian Virus large T antigen (SV40-TAg) to circumvent OIS.

METHODS

18 pLGG tissue samples (17 (94%) histological and/or molecular diagnosis PA) were mechanically dissociated. Tumor cell positive-selection using A2B5 was perfomed in 8/18 (44%) cases. All primary cell suspensions were seeded in Neural Stem Cell Medium (NSM) and Astrocyte Basal Medium (ABM). Resulting short-term cultures were infected with SV40-TAg lentivirus. Detection of tumor specific alterations (BRAF-duplication and BRAF V600E-mutation) by digital droplet PCR (ddPCR) at defined time points allowed for determination of tumor cell fraction (TCF) and evaluation of the workflow. DNA-methylation profiling and gene-panel sequencing were used for molecular profiling of primary samples.

RESULTS

Primary cell suspensions had a mean TCF of 55% (+/- 23% (SD)). No sample in NSM (0/18) and ten samples in ABM (10/18) were successfully transduced. Three of these ten (30%) converted into long-term pLGG cell lines (TCF 100%), while TCF declined to 0% (outgrowth of microenvironmental cells) in 7/10 (70%) cultures. Young patient age was associated with successful model establishment.

CONCLUSION

A subset of primary PA cultures can be converted into long-term cell lines using SV40-TAg depending on sample intrinsic (patient age) and extrinsic workflow-related (e.g. type of medium, successful transduction) parameters. Careful monitoring of sample-intrinsic and extrinsic factors optimizes the process.

Phase 1 study of the pan-RAF inhibitor tovorafenib in patients with advanced solid tumors followed by dose expansion in patients with metastatic melanoma

PURPOSE

Genomic alterations of BRAF and NRAS are oncogenic drivers in malignant melanoma and other solid tumors. Tovorafenib is an investigational, oral, selective, CNS-penetrant, small molecule, type II pan‑RAF inhibitor. This first-in-human phase 1 study explored the safety and antitumor activity of tovorafenib.

METHODS

This two-part study in adult patients with relapsed or refractory advanced solid tumors included a dose escalation phase and a dose expansion phase including molecularly defined cohorts of patients with melanoma. Primary objectives were to evaluate the safety of tovorafenib administered once every other day (Q2D) or once weekly (QW), and to determine the maximum-tolerated and recommended phase 2 dose (RP2D) on these schedules. Secondary objectives included evaluation of antitumor activity and tovorafenib pharmacokinetics.

RESULTS

Tovorafenib was administered to 149 patients (Q2D n = 110, QW n = 39). The RP2D of tovorafenib was defined as 200 mg Q2D or 600 mg QW. In the dose expansion phase, 58 (73%) of 80 patients in Q2D cohorts and 9 (47%) of 19 in the QW cohort had grade ≥ 3 adverse events. The most common of these overall were anemia (14 patients, 14%) and maculo-papular rash (8 patients, 8%). Responses were seen in 10 (15%) of 68 evaluable patients in the Q2D expansion phase, including in 8 of 16 (50%) patients with BRAF mutation-positive melanoma naïve to RAF and MEK inhibitors. In the QW dose expansion phase, there were no responses in 17 evaluable patients with NRAS mutation-positive melanoma naïve to RAF and MEK inhibitors; 9 patients (53%) had a best response of stable disease. QW dose administration was associated with minimal accumulation of tovorafenib in systemic circulation in the dose range of 400-800 mg.

CONCLUSIONS

The safety profile of both schedules was acceptable, with QW dosing at the RP2D of 600 mg QW preferred for future clinical studies. Antitumor activity of tovorafenib in BRAF-mutated melanoma was promising and justifies continued clinical development across multiple settings.

CLINICALTRIALS

GOV IDENTIFIER

NCT01425008.

Targeting small GTPases: emerging grasps on previously untamable targets, pioneered by KRAS

Small GTPases including Ras, Rho, Rab, Arf, and Ran are omnipresent molecular switches in regulating key cellular functions. Their dysregulation is a therapeutic target for tumors, neurodegeneration, cardiomyopathies, and infection. However, small GTPases have been historically recognized as "undruggable". Targeting KRAS, one of the most frequently mutated oncogenes, has only come into reality in the last decade due to the development of breakthrough strategies such as fragment-based screening, covalent ligands, macromolecule inhibitors, and PROTACs. Two KRAS^G12C covalent inhibitors have obtained accelerated approval for treating KRAS^G12C mutant lung cancer, and allele-specific hotspot mutations on G12D/S/R have been demonstrated as viable targets. New methods of targeting KRAS are quickly evolving, including transcription, immunogenic neoepitopes, and combinatory targeting with immunotherapy. Nevertheless, the vast majority of small GTPases and hotspot mutations remain elusive, and clinical resistance to G12C inhibitors poses new challenges. In this article, we summarize diversified biological functions, shared structural properties, and complex regulatory mechanisms of small GTPases and their relationships with human diseases. Furthermore, we review the status of drug discovery for targeting small GTPases and the most recent strategic progress focused on targeting KRAS. The discovery of new regulatory mechanisms and development of targeting approaches will together promote drug discovery for small GTPases.

Structure and RAF family kinase isoform selectivity of type II RAF inhibitors tovorafenib and naporafenib

Upon activation by RAS, RAF family kinases initiate signaling through the MAP kinase cascade to control cell growth, proliferation, and differentiation. Among RAF isoforms (ARAF, BRAF, and CRAF), oncogenic mutations are by far most frequent in BRAF. The BRAFV600E mutation drives more than half of all malignant melanoma and is also found in many other cancers. Selective inhibitors of BRAFV600E (vemurafenib, dabrafenib, encorafenib) are used clinically for these indications, but they are not effective inhibitors in the context of oncogenic RAS, which drives dimerization and activation of RAF, nor for malignancies driven by aberrantly dimerized truncation/fusion variants of BRAF. By contrast, a number of "type II" RAF inhibitors have been developed as potent inhibitors of RAF dimers. Here, we compare potency of type II inhibitors tovorafenib (TAK-580) and naporafenib (LHX254) in biochemical assays against the three RAF isoforms and describe crystal structures of both compounds in complex with BRAF. We find that tovorafenib and naporafenib are most potent against CRAF but markedly less potent against ARAF. Crystal structures of both compounds with BRAFV600E or WT BRAF reveal the details of their molecular interactions, including the expected type II-binding mode, with full occupancy of both subunits of the BRAF dimer. Our findings have important clinical ramifications. Type II RAF inhibitors are generally regarded as pan-RAF inhibitors, but our studies of these two agents, together with recent work with type II inhibitors belvarafenib and naporafenib, indicate that relative sparing of ARAF may be a property of multiple drugs of this class.

Rare adult pilocytic astrocytoma of the septum pellucidum with novel RIN2::BRAF fusion

Pilocytic astrocytoma is mostly a pediatric tumor with the majority of patients under age 20. Although tumors can occur throughout neuraxis, most tumors are in the cerebellum and optic chiasm. Pilocytic astrocytoma in unusual locations is often associated with different genetic alterations than the classic KIAA1549::BRAF fusion. We report a rare adult pilocytic astrocytoma of the septum pellucidum that presented with progressive headache. A detailed genomic evaluation found a fusion between BRAF and a novel partner RIN2, a gene overexpressed in both low-grade glioma and glioblastoma. The RIN2::BRAF transcript encodes a chimeric protein containing a dimerization domain SH2 and an intact kinase domain, consistent with a prototypic oncogenic kinase rearrangement. In addition, we discuss the potential oncogenic mechanisms of BRAF signaling and its implication in targeted therapy with kinase inhibitors.

BRAF-mediated brain tumors in adults and children: A review and the Australian and New Zealand experience

The mitogen-activated protein kinase (MAPK) pathway signaling pathway is one of the most commonly mutated pathways in human cancers. In particular, BRAF alterations result in constitutive activation of the rapidly accelerating fibrosarcoma-extracellular signal-regulated kinase-MAPK significant pathway, leading to cellular proliferation, survival, and dedifferentiation. The role of BRAF mutations in oncogenesis and tumorigenesis has spurred the development of targeted agents, which have been successful in treating many adult cancers. Despite advances in other cancer types, the morbidity and survival outcomes of patients with glioma have remained relatively stagnant. Recently, there has been recognition that MAPK dysregulation is almost universally present in paediatric and adult gliomas. These findings, accompanying broad molecular characterization of gliomas, has aided prognostication and offered opportunities for clinical trials testing targeted agents. The use of targeted therapies in this disease represents a paradigm shift, although the biochemical complexities has resulted in unexpected challenges in the development of effective BRAF inhibitors. Despite these challenges, there are promising data to support the use of BRAF inhibitors alone and in combination with MEK inhibitors for patients with both low-grade and high-grade glioma across age groups. Safety and efficacy data demonstrate that many of the toxicities of these targeted agents are tolerable while offering objective responses. Newer clinical trials will examine the use of these therapies in the upfront setting. Appropriate duration of therapy and durability of response remains unclear in the glioma patient cohort. Longitudinal efficacy and toxicity data are needed. Furthermore, access to these medications remains challenging outside of clinical trials in Australia and New Zealand. Compassionate access is limited, and advocacy for mechanism of action-based drug approval is ongoing.

Integrated response analysis of pediatric low-grade gliomas during and after targeted therapy treatment

Background

Pediatric low-grade gliomas (pLGGs) are the most common central nervous system tumor in children, characterized by RAS/MAPK pathway driver alterations. Genomic advances have facilitated the use of molecular targeted therapies, however, their long-term impact on tumor behavior remains critically unanswered.

Methods

We performed an IRB-approved, retrospective chart and imaging review of pLGGs treated with off-label targeted therapy at Dana-Farber/Boston Children's from 2010 to 2020. Response analysis was performed for BRAFV600E and BRAF fusion/duplication-driven pLGG subsets.

Results

Fifty-five patients were identified (dabrafenib n = 15, everolimus n = 26, trametinib n = 11, and vemurafenib n = 3). Median duration of targeted therapy was 9.48 months (0.12-58.44). The 1-year, 3-year, and 5-year EFS from targeted therapy initiation were 62.1%, 38.2%, and 31.8%, respectively. Mean volumetric change for BRAFV600E mutated pLGG on BRAF inhibitors was -54.11%; median time to best volumetric response was 8.28 months with 9 of 12 (75%) objective RAPNO responses. Median time to largest volume post-treatment was 2.86 months (+13.49%); mean volume by the last follow-up was -14.02%. Mean volumetric change for BRAF fusion/duplication pLGG on trametinib was +7.34%; median time to best volumetric response was 6.71 months with 3 of 7 (43%) objective RAPNO responses. Median time to largest volume post-treatment was 2.38 months (+71.86%); mean volume by the last follow-up was +39.41%.

Conclusions

Our integrated analysis suggests variability in response by pLGG molecular subgroup and targeted therapy, as well as the transience of some tumor growth following targeted therapy cessation.

Resistance mechanisms in BRAFV600E paediatric high-grade glioma and current therapeutic approaches

Paediatric high-grade gliomas (pHGG) are aggressive central nervous system tumours with a poor prognosis. BRAFV600E mutant pHGGs can be treated with targeted BRAF inhibitors, which have shown both preclinical activity and potent clinical efficacy. Unfortunately, the development of drug resistance results in disease relapse or progression and is the primary cause of treatment failure. While there is a lot of data to explain mechanisms of resistance in other BRAFV600E tumours, comparatively little is known about the mechanisms of BRAF inhibitor resistance in BRAFV600E pHGG. Recent literature has identified aberrations in members of the RAS/RAF/ERK pathway, the PI3K/AKT/MTOR pathway and the cell cycle as major contributors to the resistance profile. A range of novel therapies have been suggested to overcome BRAF inhibitor drug resistance in BRAFV600E pHGG. This review will discuss the current literature available for BRAF inhibitor resistant BRAFV600E pHGGs and provide an overview of the currently available and proposed therapies.

Dual Inhibition of BRAF-MAPK and STAT3 Signaling Pathways in Resveratrol-Suppressed Anaplastic Thyroid Cancer Cells with BRAF Mutations

Anaplastic thyroid cancer is an extremely lethal malignancy without reliable treatment. BRAFV600E point mutation is common in ATCs, which leads to MAPK signaling activation and is regarded as a therapeutic target. Resveratrol inhibits ATC cell growth, while its impact on BRAF-MAPK signaling remains unknown. This study aims to address this issue by elucidating the statuses of BRAF-MAPK and STAT3 signaling activities in resveratrol-treated THJ-11T, THJ-16T, and THJ-21T ATC cells and Nthyori 3-1 thyroid epithelial cells. RT-PCR and Sanger sequencing revealed MKRN1-BRAF fusion mutation in THJ-16T, BRAF V600E point mutation in THJ-21T, and wild-type BRAF genes in THJ-11T and Nthyori 3-1 cells. Western blotting and immunocytochemical staining showed elevated pBRAF, pMEK, and pERK levels in THJ-16T and THJ-21T, but not in THJ-11T or Nthyori 3-1 cells. Calcein/PI, EdU, and TUNEL assays showed that compared with docetaxel and doxorubicin and MAPK-targeting dabrafenib and trametinib, resveratrol exerted more powerful inhibitory effects on mutant BRAF-harboring THJ-16T and THJ-21T cells, accompanied by reduced levels of MAPK pathway-associated proteins and pSTAT3. Trametinib- and dabrafenib-enhanced STAT3 activation was efficiently suppressed by resveratrol. In conclusion, resveratrol acts as dual BRAF-MAPK and STAT3 signaling inhibitor and a promising agent against ATCs with BRAF mutation.

KRAS as a Key Oncogene in the Clinical Precision Diagnosis and Treatment of Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDAC) is one of the most malignant tumors, with a 5-year survival rate of less than 10%. At present, the comprehensive treatment based on surgery, radiotherapy and chemotherapy has encountered a bottleneck, and targeted immunotherapy turns to be the direction of future development. About 90% of PDAC patients have KRAS mutations, and KRAS has been widely used in the diagnosis, treatment, and prognosis of PDAC in recent years. With the development of liquid biopsy and gene testing, KRAS is expected to become a new biomarker to assist the stratification and prognosis of PDAC patients. An increasing number of small molecule inhibitors acting on the KRAS pathway are being developed and put into the clinic, providing more options for PDAC patients.

Human induced pluripotent stem cell engineering establishes a humanized mouse platform for pediatric low-grade glioma modeling

A major obstacle to identifying improved treatments for pediatric low-grade brain tumors (gliomas) is the inability to reproducibly generate human xenografts. To surmount this barrier, we leveraged human induced pluripotent stem cell (hiPSC) engineering to generate low-grade gliomas (LGGs) harboring the two most common pediatric pilocytic astrocytoma-associated molecular alterations, NF1 loss and KIAA1549:BRAF fusion. Herein, we identified that hiPSC-derived neuroglial progenitor populations (neural progenitors, glial restricted progenitors and oligodendrocyte progenitors), but not terminally differentiated astrocytes, give rise to tumors retaining LGG histologic features for at least 6 months in vivo. Additionally, we demonstrated that hiPSC-LGG xenograft formation requires the absence of CD4 T cell-mediated induction of astrocytic Cxcl10 expression. Genetic Cxcl10 ablation is both necessary and sufficient for human LGG xenograft development, which additionally enables the successful long-term growth of patient-derived pediatric LGGs in vivo. Lastly, MEK inhibitor (PD0325901) treatment increased hiPSC-LGG cell apoptosis and reduced proliferation both in vitro and in vivo. Collectively, this study establishes a tractable experimental humanized platform to elucidate the pathogenesis of and potential therapeutic opportunities for childhood brain tumors.

Discovery of Raf Family Is a Milestone in Deciphering the Ras-Mediated Intracellular Signaling Pathway

The Ras-Raf-MEK-ERK signaling pathway, the first well-established MAPK pathway, plays essential roles in cell proliferation, survival, differentiation and development. It is activated in over 40% of human cancers owing to mutations of Ras, membrane receptor tyrosine kinases and other oncogenes. The Raf family consists of three isoforms, A-Raf, B-Raf and C-Raf. Since the first discovery of a truncated mutant of C-Raf as a transforming oncogene carried by a murine retrovirus, forty years of extensive studies have provided a wealth of information on the mechanisms underlying the activation, regulation and biological functions of the Raf family. However, the mechanisms by which activation of A-Raf and C-Raf is accomplished are still not completely understood. In contrast, B-Raf can be easily activated by binding of Ras-GTP, followed by cis-autophosphorylation of the activation loop, which accounts for the fact that this isoform is frequently mutated in many cancers, especially melanoma. The identification of oncogenic B-Raf mutations has led to accelerated drug development that targets Raf signaling in cancer. However, the effort has not proved as effective as anticipated, inasmuch as the mechanism of Raf activation involves multiple steps, factors and phosphorylation of different sites, as well as complex interactions between Raf isoforms. In this review, we will focus on the physiological complexity of the regulation of Raf kinases and their connection to the ERK phosphorylation cascade and then discuss the role of Raf in tumorigenesis and the clinical application of Raf inhibitors in the treatment of cancer.

Overcoming differential tumor penetration of BRAF inhibitors using computationally guided combination therapy

BRAF-targeted kinase inhibitors (KIs) are used to treat malignancies including BRAF-mutant non-small cell lung cancer, colorectal cancer, anaplastic thyroid cancer, and, most prominently, melanoma. However, KI selection criteria in patients remain unclear, as are pharmacokinetic/pharmacodynamic (PK/PD) mechanisms that may limit context-dependent efficacy and differentiate related drugs. To address this issue, we imaged mouse models of BRAF-mutant cancers, fluorescent KI tracers, and unlabeled drug to calibrate in silico spatial PK/PD models. Results indicated that drug lipophilicity, plasma clearance, faster target dissociation, and, in particular, high albumin binding could limit dabrafenib action in visceral metastases compared to other KIs. This correlated with retrospective clinical observations. Computational modeling identified a timed strategy for combining dabrafenib and encorafenib to better sustain BRAF inhibition, which showed enhanced efficacy in mice. This study thus offers principles of spatial drug action that may help guide drug development, KI selection, and combination.

Long-Term Follow-Up of Pediatric CNS Tumor Survivors—A Selection of Relevant Long-Term Issues

Introduction: Survivors of pediatric central nervous system (CNS) tumors are at high risk for late effects and long-term morbidity. The quality of survival became increasingly important, as advances in diagnostics, multimodal treatment strategies, and supportive care have led to significant increases in long-term survival. Aim: This review aims to provide a global overview of the potential late effects and long-term follow-up care of CNS tumor survivors, directed to trainees and practitioners with less targeted training in pediatric oncology. Late effects in CNS tumor survivors: A specific focus on CNS tumor survivors relies on cognitive and psychosocial late effects, as they may have an impact on education, professional career, independent living, and quality of life. Further important late effects in CNS tumor survivors include endocrine, metabolic, cardiovascular, and cerebrovascular diseases. Conclusions: Comprehensive long-term follow-up care is essential for pediatric CNS tumor survivors to improve their quality of survival and quality of life. An individualized approach, taking all potential late effects into account, and carried out by an interdisciplinary team, is recommended, and should continue into adulthood. Existing recommendations and guidelines on long-term follow-up care guide the multidisciplinary teams.

Recent Developments in Targeting RAS Downstream Effectors for RAS-Driven Cancer Therapy

Aberrant activity of oncogenic rat sarcoma virus (RAS) protein promotes tumor growth and progression. RAS-driven cancers comprise more than 30% of all human cancers and are refractory to frontline treatment strategies. Since direct targeting of RAS has proven challenging, efforts have been centered on the exploration of inhibitors for RAS downstream effector kinases. Two major RAS downstream signaling pathways, including the Raf/MEK/Erk cascade and the phosphatidylinositol-3-kinase (PI3K) pathway, have become compelling targets for RAS-driven cancer therapy. However, the main drawback in the blockade of a single RAS effector is the multiple levels of crosstalk and compensatory mechanisms between these two pathways that contribute to drug resistance against monotherapies. A growing body of evidence reveals that the sequential or synergistic inhibition of multiple RAS effectors is a more convenient route for the efficacy of cancer therapy. Herein, we revisit the recent developments and discuss the most promising modalities targeting canonical RAS downstream effectors for the treatment of RAS-driven cancers.

A structural perspective on targeting the RTK/Ras/MAP kinase pathway in cancer

Precision oncology is premised on identifying and drugging proteins and pathways that drive tumorigenesis or are required for survival of tumor cells. Across diverse cancer types, the signaling pathway emanating from receptor tyrosine kinases on the cell surface to RAS and the MAP kinase pathway is the most frequent target of oncogenic mutations, and key proteins in this signaling axis including EGFR, SHP2, RAS, BRAF, and MEK have long been a focus in cancer drug discovery. In this review, we provide an overview of historical and recent efforts to develop inhibitors targeting these nodes with an emphasis on the role that an understanding of protein structure and regulation has played in inhibitor discovery and characterization. Beyond its well-established role in structure-based drug design, structural biology has revealed mechanisms of allosteric regulation, distinct effects of activating oncogenic mutations, and other vulnerabilities that have opened new avenues in precision cancer drug discovery.

Buparlisib plus carboplatin or lomustine in patients with recurrent glioblastoma: a phase Ib/II, open-label, multicentre, randomised study

Background

Glioblastoma relapse is associated with activation of phosphatidylinositol 3-kinase (PI3K) signalling pathway. In preclinical studies, the pan-PI3K inhibitor buparlisib showed antitumour activity in glioma models.

Methods

This was a two-part, multicentre, phase Ib/II study in patients with recurrent glioblastoma pretreated with radiotherapy and temozolomide standard of care. Patients received buparlisib (80 mg or 100 mg once daily) plus carboplatin (area under the curve (AUC)=5 every 3 weeks), or buparlisib (60 mg once daily) plus lomustine (100 mg/m² every 6 weeks). The primary endpoint was to determine the maximum tolerable dose (MTD) and/or recommended phase II dose of buparlisib plus carboplatin or lomustine.

Results

Between 28 February 2014 and 7 July 2016, 35 patients were enrolled and treated with buparlisib plus carboplatin (n=17; buparlisib (80 mg) plus carboplatin, n=3; and buparlisib (100 mg) plus carboplatin, n=14), or buparlisib (60 mg) plus lomustine (n=18). The MTD of buparlisib was determined to be 100 mg per day in combination with carboplatin at an AUC of 5 every 3 weeks. The MTD of buparlisib in combination with lomustine could not be determined as it did not satisfy the MTD criteria per the Bayesian logistic regression model.

Conclusion

The overall safety profile of buparlisib remained unchanged, and no new or unexpected safety findings were reported in this study. Preliminary assessment for both combinations did not demonstrate sufficient antitumour activity compared with historical data on single-agent carboplatin or lomustine.

Trial registration number

NCT01934361.

ARAF mutations confer resistance to the RAF inhibitor belvarafenib in melanoma

Although RAF monomer inhibitors (type I.5, BRAF(V600)) are clinically approved for the treatment of BRAF^V600-mutant melanoma, they are ineffective in non-BRAF^V600 mutant cells^1-3. Belvarafenib is a potent and selective RAF dimer (type II) inhibitor that exhibits clinical activity in patients with BRAF^V600E- and NRAS-mutant melanomas. Here we report the first-in-human phase I study investigating the maximum tolerated dose, and assessing the safety and preliminary efficacy of belvarafenib in BRAF^V600E- and RAS-mutated advanced solid tumours (NCT02405065, NCT03118817). By generating belvarafenib-resistant NRAS-mutant melanoma cells and analysing circulating tumour DNA from patients treated with belvarafenib, we identified new recurrent mutations in ARAF within the kinase domain. ARAF mutants conferred resistance to belvarafenib in both a dimer- and a kinase activity-dependent manner. Belvarafenib induced ARAF mutant dimers, and dimers containing mutant ARAF were active in the presence of inhibitor. ARAF mutations may serve as a general resistance mechanism for RAF dimer inhibitors as the mutants exhibit reduced sensitivity to a panel of type II RAF inhibitors. The combination of RAF plus MEK inhibition may be used to delay ARAF-driven resistance and suggests a rational combination for clinical use. Together, our findings reveal specific and compensatory functions for the ARAF isoform and implicate ARAF mutations as a driver of resistance to RAF dimer inhibitors.

RAF-MEK-ERK pathway in cancer evolution and treatment

The RAF-MEK-ERK signaling cascade is a well-characterized MAPK pathway involved in cell proliferation and survival. The three-layered MAPK signaling cascade is initiated upon RTK and RAS activation. Three RAF isoforms ARAF, BRAF and CRAF, and their downstream MEK1/2 and ERK1/2 kinases constitute a coherently orchestrated signaling module that directs a range of physiological functions. Genetic alterations in this pathway are among the most prevalent in human cancers, which consist of numerous hot-spot mutations such as BRAF^V600E. Oncogenic mutations in this pathway often override otherwise tightly regulated checkpoints to open the door for uncontrolled cell growth and neoplasia. The crosstalk between the RAF-MEK-ERK axis and other signaling pathways further extends the proliferative potential of this pathway in human cancers. In this review, we summarize the molecular architecture and physiological functions of the RAF-MEK-ERK pathway with emphasis on its dysregulations in human cancers, as well as the efforts made to target the RAF-MEK-ERK module using small molecule inhibitors.

Inhibition of RAF dimers: it takes two to tango

The RAS-regulated RAF-MEK1/2-ERK1/2 pathway promotes cell proliferation and survival and RAS and BRAF proteins are commonly mutated in cancer. This has fuelled the development of small molecule kinase inhibitors including ATP-competitive RAF inhibitors. Type I and type I½ ATP-competitive RAF inhibitors are effective in BRAFV600E/K-mutant cancer cells. However, in RAS-mutant cells these compounds instead promote RAS-dependent dimerisation and paradoxical activation of wild-type RAF proteins. RAF dimerisation is mediated by two key regions within each RAF protein; the RKTR motif of the αC-helix and the NtA-region of the dimer partner. Dimer formation requires the adoption of a closed, active kinase conformation which can be induced by RAS-dependent activation of RAF or by the binding of type I and I½ RAF inhibitors. Binding of type I or I½ RAF inhibitors to one dimer partner reduces the binding affinity of the other, thereby leaving a single dimer partner uninhibited and able to activate MEK. To overcome this paradox two classes of drug are currently under development; type II pan-RAF inhibitors that induce RAF dimer formation but bind both dimer partners thus allowing effective inhibition of both wild-type RAF dimer partners and monomeric active class I mutant RAF, and the recently developed "paradox breakers" which interrupt BRAF dimerisation through disruption of the αC-helix. Here we review the regulation of RAF proteins, including RAF dimers, and the progress towards effective targeting of the wild-type RAF proteins.

Targetable BRAF and RAF1 Alterations in Advanced Pediatric Cancers

RAF family protein kinases signal through the MAPK pathway to orchestrate cellular proliferation, survival, and transformation. Identifying BRAF alterations in pediatric cancers is critically important as therapeutic agents targeting BRAF or MEK may be incorporated into the clinical management of these patients. In this study, we performed comprehensive genomic profiling on 3,633 pediatric cancer samples and identified a cohort of 221 (6.1%) cases with known or novel alterations in BRAF or RAF1 detected in extracranial solid tumors, brain tumors, or hematological malignancies. Eighty percent (176/221) of these tumors had a known-activating short variant (98, 55.7%), fusion (72, 40.9%), or insertion/deletion (6, 3.4%). Among BRAF altered cancers, the most common tumor types were brain tumors (74.4%), solid tumors (10.8%), hematological malignancies (9.1%), sarcomas (3.4%), and extracranial embryonal tumors (2.3%). RAF1 fusions containing intact RAF1 kinase domain (encoded by exons 10-17) were identified in seven tumors, including two novel fusions TMF1-RAF1 and SOX6-RAF1. Additionally, we highlight a subset of patients with brain tumor with positive clinical response to BRAF inhibitors, demonstrating the rationale for incorporating precision medicine into pediatric oncology. IMPLICATIONS FOR PRACTICE: Precision medicine has not yet gained a strong foothold in pediatric cancers. This study describes the landscape of BRAF and RAF1 genomic alterations across a diverse spectrum of pediatric cancers, primarily brain tumors, but also encompassing melanoma, sarcoma, several types of hematologic malignancy, and others. Given the availability of multiple U.S. Food and Drug Administration-approved BRAF inhibitors, identification of these alterations may assist with treatment decision making, as described here in three cases of pediatric cancer.

Advances in Targeted Therapies for Pediatric Brain Tumors

Purpose of Review

Over the last years, our understanding of the molecular biology of pediatric brain tumors has vastly improved. This has led to more narrowly defined subgroups of these tumors and has created new potential targets for molecularly driven therapies. This review presents an overview of the latest advances and challenges of implementing targeted therapies into the clinical management of pediatric brain tumors, with a focus on gliomas, craniopharyngiomas, and medulloblastomas.

Recent Findings

Pediatric low-grade gliomas (pLGG) show generally a low mutational burden with the mitogen-activated protein kinase (MAPK) signaling presenting a key driver for these tumors. Direct inhibition of this pathway through BRAF and/or MEK inhibitors has proven to be a clinically relevant strategy. More recently, MEK and IL-6 receptor inhibitors have started to be evaluated in the treatment for craniopharyngiomas. Aside these low-grade tumors, pediatric high-grade gliomas (pHGG) and medulloblastomas exhibit substantially greater molecular heterogeneity with various and sometimes unknown tumor driver alterations. The clinical benefit of different targeted therapy approaches to interfere with altered signaling pathways and restore epigenetic dysregulation is undergoing active clinical testing. For these multiple pathway-driven tumors, combination strategies will most likely be required to achieve clinical benefit.

Summary

The field of pediatric neuro-oncology made tremendous progress with regard to improved diagnosis setting the stage for precision medicine approaches over the last decades. The potential of targeted therapies has been clearly demonstrated for a subset of pediatric brain tumors. However, despite clear response rates, questions of sufficient blood-brain barrier penetration, optimal dosing, treatment duration as well as mechanisms of resistance and how these can be overcome with potential combination strategies need to be addressed in future investigations. Along this line, it is critical for future trials to define appropriate endpoints to assess therapy responses as well as short and long-term toxicities in the growing and developing child.

Anti-tumor activities of the new oral pan-RAF inhibitor, TAK-580, used as monotherapy or in combination with novel agents in multiple myeloma

Many RAS pathway inhibitors, including pan-RAF inhibitors, have shown significant anti-tumor activities in both solid and hematological tumors. The pan-RAF inhibitor, TAK-580, is a representative of the novel RAF inhibitors that act by disrupting RAF homo- or heterodimerization. In this study, we examined the anti-tumor effects of TAK-580 used as monotherapy or in combination with bortezomib, lenalidomide, or other novel agents in multiple myeloma (MM) cells in vitro. TAK-580 monotherapy potently targeted proteins in the RAS-RAF-MEK-ERK signaling pathway and induced potent cytotoxicity and apoptosis in MM cell lines and myeloma cells from patients with newly diagnosed and relapsed and/or refractory MM, compared with a representative RAF inhibitor, dabrafenib. Normal donor peripheral blood B lymphocytes and cord blood CD34-positive cells were not affected. Importantly, TAK-580 significantly inhibited phospho-FOXO3 and induced upregulation of Bim_L and Bim_S in a dose-dependent manner, finally leading to apoptosis in MM cells. Moreover, TAK-580 enhanced bortezomib-induced cytotoxicity and apoptosis in MM cells via the FOXO3-Bim axis and the terminal unfolded protein response. Importantly, TAK-580 also enhanced lenalidomide-induced cytotoxicity and apoptosis in MM cells. Taken together, our results provide the rationale for TAK-580 monotherapy and/or treatment in combination with novel agents to improve outcomes in patients with MM.

Prevalence of class I-III BRAF mutations among 114,662 cancer patients in a large genomic database

IMPACT STATEMENT

These data represent the largest aggregation of BRAF mutations within a single clinical database to our knowledge. The relative proportions of both BRAF V600 mutations and non-V600 mutations are informative in all cancers and by malignancy, and can serve as a definitive gold-standard for BRAF mutation cancer incidence by malignancy. The rate of BRAF mutation in human cancer in a real-world large database is lower than previously reported likely representing testing more broadly across tumor types. The relative percentages of Class II and Class III BRAF mutations are higher than previously reported, representing almost 35% of BRAF mutations in cancer. These findings provide support for the development of effective treatments for non-V600 BRAF mutations in cancer.

Genetic Heterogeneity of BRAF Fusion Kinases in Melanoma Affects Drug Responses

BRAF fusions are detected in numerous neoplasms, but their clinical management remains unresolved. We identified six melanoma lines harboring BRAF fusions representative of the clinical cases reported in the literature. Their unexpected heterogeneous responses to RAF and MEK inhibitors could be categorized upon specific features of the fusion kinases. Higher expression level correlated with resistance, and fusion partners containing a dimerization domain promoted paradoxical activation of the mitogen-activated protein kinase (MAPK) pathway and hyperproliferation in response to first- and second-generation RAF inhibitors. By contrast, next-generation αC-IN/DFG-OUT RAF inhibitors blunted paradoxical activation across all lines and had their therapeutic efficacy further increased in vitro and in vivo by combination with MEK inhibitors, opening perspectives in the clinical management of tumors harboring BRAF fusions.

The MAPK and AMPK signalings: interplay and implication in targeted cancer therapy

Cancer is characterized as a complex disease caused by coordinated alterations of multiple signaling pathways. The Ras/RAF/MEK/ERK (MAPK) signaling is one of the best-defined pathways in cancer biology, and its hyperactivation is responsible for over 40% human cancer cases. To drive carcinogenesis, this signaling promotes cellular overgrowth by turning on proliferative genes, and simultaneously enables cells to overcome metabolic stress by inhibiting AMPK signaling, a key singular node of cellular metabolism. Recent studies have shown that AMPK signaling can also reversibly regulate hyperactive MAPK signaling in cancer cells by phosphorylating its key components, RAF/KSR family kinases, which affects not only carcinogenesis but also the outcomes of targeted cancer therapies against the MAPK signaling. In this review, we will summarize the current proceedings of how MAPK-AMPK signalings interplay with each other in cancer biology, as well as its implications in clinic cancer treatment with MAPK inhibition and AMPK modulators, and discuss the exploitation of combinatory therapies targeting both MAPK and AMPK as a novel therapeutic intervention.

The non-linearity of RAF-MEK signaling in dendritic cells

Kinases form the major part of the druggable genome and their selective inhibition in human cancers has had reasonable clinical success. In contrast to tumorigenesis, the role of kinases in mediating immune responses is poorly understood. However, synergistic therapeutic regimens combining targeted therapy and immune therapy have been found to increase the median survival of tumor patients. In this context, we uncovered that RAF and MEK1/2 kinases, which are the integral parts of the classical MAPK cascade, have unique roles in driving DC differentiation and activation. RAF kinases are stabilized in their protein levels during DC differentiation and are obligatory for normal functioning of DCs. But, the targeting of MEK1/2 kinases with specific inhibitors did not phenocopy the effects observed with RAF inhibitors suggesting that RAF and MEK1/2 kinases may have specific and unique roles in driving immune responses, which deserves further studies to successfully administer these inhibitors in clinics.

Clinical Relevance of BRAF V600E Mutation Status in Brain Tumors with a Focus on a Novel Management Algorithm

The possible application of BRAF-targeted therapy in brain tumors is growing continuously. We have analyzed clinical strategies that address BRAF activation in primary brain tumors and verified current recommendations regarding screening for BRAF mutations. There is preliminary evidence for a range of positive responses in certain brain tumor types harboring the BRAF V600E mutation. National Comprehensive Cancer Network Guidelines for central nervous system cancers recommend screening for the BRAF V600E mutation in pilocytic astrocytoma, pleomorphic xanthoastrocytoma, and ganglioglioma. We suggest additional testing in glioblastomas WHO grade IV below the age of 30 years, especially those with epithelioid features, papillary craniopharyngiomas, and pediatric low-grade astrocytomas. BRAF-targeted therapy should be limited to the setting of a clinical trial. If the patient harboring a V600E mutation does not qualify for a trial, multimodality treatment is recommended. Dual inhibition of both RAF and MEK is expected to provide more potent and durable effects than anti-BRAF monotherapy. First-generation RAF inhibitors should be avoided. Gain-of-function mutations of EGFR and KIAA fusions may compromise BRAF-targeted therapy. BRAF alterations that result in MAPK pathway activation are common events in several types of brain tumors. BRAF V600E mutation emerges as a promising molecular target. The proposed algorithm was designed to help oncologists to provide the best therapeutic options for brain tumor patients.

RAS-targeted therapies: is the undruggable drugged?

RAS (KRAS, NRAS and HRAS) is the most frequently mutated gene family in cancers, and, consequently, investigators have sought an effective RAS inhibitor for more than three decades. Even 10 years ago, RAS inhibitors were so elusive that RAS was termed 'undruggable'. Now, with the success of allele-specific covalent inhibitors against the most frequently mutated version of RAS in non-small-cell lung cancer, KRAS^G12C, we have the opportunity to evaluate the best therapeutic strategies to treat RAS-driven cancers. Mutation-specific biochemical properties, as well as the tissue of origin, are likely to affect the effectiveness of such treatments. Currently, direct inhibition of mutant RAS through allele-specific inhibitors provides the best therapeutic approach. Therapies that target RAS-activating pathways or RAS effector pathways could be combined with these direct RAS inhibitors, immune checkpoint inhibitors or T cell-targeting approaches to treat RAS-mutant tumours. Here we review recent advances in therapies that target mutant RAS proteins and discuss the future challenges of these therapies, including combination strategies.

RAF kinase dimerization: implications for drug discovery and clinical outcomes

The RAF kinases activated by RAS GTPases regulate cell growth and division by signal transduction through the ERK cascade and mutations leading to constitutive activity are key drivers of human tumors, as are upstream activators including RAS and receptor tyrosine kinases. The development of first-generation RAF inhibitors, including vemurafenib (VEM) and dabrafenib led to initial excitement due to high response rates and profound regression of malignant melanomas carrying BRAF^V600E mutations. The excitement about these unprecedented response rates, however, was tempered by tumor unresponsiveness through both intrinsic and acquired drug-resistance mechanisms. In recent years much insight into the complexity of the RAS-RAF axis has been obtained and inactivation and signal transduction mechanisms indicate that RAF dimerization is a critical step in multiple cellular contexts and plays a key role in resistance. Both homo- and hetero-dimerization of BRAF and CRAF can modulate therapeutic response and disease progression in patients treated with ATP-competitive inhibitors and are therefore highly clinically significant. Ten years after the definition of the RAF dimer interface (DIF) by crystallography, this review focuses on the implications of RAF kinase dimerization in signal transduction and for drug development, both from a classical ATP-competitive standpoint and from the perspective of new therapeutic strategies including inhibiting dimer formation. A structural perspective of the DIF, how dimerization impacts inhibitor activation and the structure-based design of next-generation RAF kinase inhibitors with unique mechanisms of action is presented. We also discuss potential fields of application for DIF inhibitors, ranging from non-V600E oncoproteins and BRAF fusions to tumors driven by aberrant receptor tyrosine kinase or RAS signaling.

Targeting Aberrant RAS/RAF/MEK/ERK Signaling for Cancer Therapy

The RAS/RAF/MEK/ERK (MAPK) signaling cascade is essential for cell inter- and intra-cellular communication, which regulates fundamental cell functions such as growth, survival, and differentiation. The MAPK pathway also integrates signals from complex intracellular networks in performing cellular functions. Despite the initial discovery of the core elements of the MAPK pathways nearly four decades ago, additional findings continue to make a thorough understanding of the molecular mechanisms involved in the regulation of this pathway challenging. Considerable effort has been focused on the regulation of RAF, especially after the discovery of drug resistance and paradoxical activation upon inhibitor binding to the kinase. RAF activity is regulated by phosphorylation and conformation-dependent regulation, including auto-inhibition and dimerization. In this review, we summarize the recent major findings in the study of the RAS/RAF/MEK/ERK signaling cascade, particularly with respect to the impact on clinical cancer therapy.

Mitogenic and progenitor gene programmes in single pilocytic astrocytoma cells

Pilocytic astrocytoma (PA), the most common childhood brain tumor, is a low-grade glioma with a single driver BRAF rearrangement. Here, we perform scRNAseq in six PAs using methods that enabled detection of the rearrangement. When compared to higher-grade gliomas, a strikingly higher proportion of the PA cancer cells exhibit a differentiated, astrocyte-like phenotype. A smaller proportion of cells exhibit a progenitor-like phenotype with evidence of proliferation. These express a mitogen-activated protein kinase (MAPK) programme that was absent from higher-grade gliomas. Immune cells, especially microglia, comprise 40% of all cells in the PAs and account for differences in bulk expression profiles between tumor locations and subtypes. These data indicate that MAPK signaling is restricted to relatively undifferentiated cancer cells in PA, with implications for investigational therapies directed at this pathway.

Pilocytic astrocytoma is a low-grade pediatric glioma, characterized by a single BRAF rearrangement. Here, Reitman and colleagues use single-cell RNA sequencing to reveal molecular hallmarks of the disease that might be targeted therapeutically.

Oncogenic BRAF Alterations and Their Role in Brain Tumors

Alterations of the v-raf murine sarcoma viral oncogene homolog B (BRAF) have been extensively studied in several tumor entities and are known to drive cell growth in several tumor entities. Effective targeted therapies with mutation-specific small molecule inhibitors have been developed and established for metastasized malignant melanoma. The BRAF V600E mutation and KIAA1549-BRAF fusion are alterations found in several brain tumors and show a distinct prognostic impact in some entities. Besides the diagnostic significance for the classification of central nervous system tumors, these alterations present possible therapy targets that may be exploitable for oncological treatments, as it has been established for malignant melanomas. In this review the different central nervous system tumors harboring BRAF alterations are presented and the diagnostic significance, prognostic role, and therapeutic potential are discussed.

Brain Distribution and Active Efflux of Three panRAF Inhibitors: Considerations in the Treatment of Melanoma Brain Metastases

Targeted inhibition of RAF and MEK by molecularly targeted agents has been employed as a strategy to block aberrant mitogen-activated protein kinase (MAPK) signaling in melanoma. While the use of BRAF and MEK inhibitors, either as a single agent or in combination, improved efficacy in BRAF-mutant melanoma, initial responses are often followed by relapse due to acquired resistance. Moreover, some BRAF inhibitors are associated with paradoxical activation of the MAPK pathway, causing the development of secondary malignancies. The use of panRAF inhibitors, i.e., those that target all isoforms of RAF, may overcome paradoxical activation and resistance. The purpose of this study was to perform a quantitative assessment and evaluation of the influence of efflux mechanisms at the blood-brain barrier (BBB), in particular, Abcb1/P-glycoprotein (P-gp) and Abcg2/breast cancer resistance protein (Bcrp), on the brain distribution of three panRAF inhibitors: CCT196969 [1-(3-(tert-butyl)-1-phenyl-1H-pyrazol-5-yl)-3-(2-fluoro-4-((3-oxo-3,4-dihydropyrido[2,3-b]pyrazin-8-yl)oxy)phenyl)urea], LY3009120 1-(3,3-Dimethylbutyl)-3-(2-fluoro-4-methyl-5-(7-methyl-2-(methylamino)pyrido(2,3-d)pyrimidin-6-yl)phenyl)urea, and MLN2480 [4-pyrimidinecarboxamide, 6-amino-5-chloro-N-[(1R)-1-[5-[[[5-chloro-4-(trifluoromethyl)-2-pyridinyl]amino]carbonyl]-2-thiazolyl]ethyl]-]. In vitro studies using transfected Madin-Darby canine kidney II cells indicate that only LY3009120 and MLN2480 are substrates of Bcrp, and none of the three inhibitors are substrates of P-gp. The three panRAF inhibitors show high nonspecific binding in brain and plasma. In vivo studies in mice show that the brain distribution of CCT196969, LY3009120, and MLN2480 is limited, and is enhanced in transgenic mice lacking P-gp and Bcrp. While MLN2480 has a higher brain distribution, LY3009120 exhibits superior in vitro efficacy in patient-derived melanoma cell lines. The delivery of a drug to the site of action residing behind a functionally intact BBB, along with drug potency against the target, collectively play a critical role in determining in vivo efficacy outcomes.

Recent advances in intradural spinal tumors

Intradural spinal tumors are rare tumors of the central nervous system. Due to the eloquence of the spinal cord and its tracts, the compact architecture of the cord and nerves, and the infiltrative nature of some of these tumors, surgical resection is difficult to achieve without causing neurological deficits. Likewise, chemotherapy and radiotherapy are utilized more cautiously in the treatment of intradural spinal tumors than their cranial counterparts. Targeted therapies aimed at the genetic alterations and molecular biology tailored to these tumors would be helpful but are lacking.Here, we review the major types of intradural spinal tumors, with an emphasis on genetic alterations, molecular biology, and experimental therapies for these difficult to treat neoplasms.