Mutational activation of BRAF is the most prevalent genetic alteration in human melanoma, with ≥ 50% of tumours expressing the BRAF(V600E) oncoprotein1,2. Moreover, the marked tumour regression and improved survival of late-stage BRAF-mutated melanoma patients in response to treatment with vemurafenib demonstrates the essential role of oncogenic BRAF in melanoma maintenance3,4. However, as most patients relapse with lethal drug-resistant disease, understanding and preventing mechanism(s) of resistance is critical to providing improved therapy5. Here we investigate the cause and consequences of vemurafenib resistance using two independently derived primary human melanoma xeno-graft models in which drugresistanceisselected by continuous vemurafenib administration. In one of these models, resistant tumours show continued dependency on BRAF(V600E) → MEK → ERK signalling owing to elevated BRAF(V600E) expression. Most importantly, we demonstrate that vemurafenib-resistant melanomas become drug dependent for their continued proliferation, such that cessation of drug administration leads to regression of established drug-resistant tumours. We further demonstrate that a discontinuous dosing strategy, which exploits the fitness disadvantage displayed by drug-resistant cells in the absence of the drug, forestalls the onset of lethal drug-resistant disease. These data highlight the concept that drug-resistant cells may also display drug dependency, such that altered dosing may prevent the emergence of lethal drug resistance. Such observations may contribute to sustaining the durability of the vemurafenib response with the ultimate goal of curative therapy for the subset of melanoma patients with BRAF mutations.

Mutations at multiple sites in occur in cancer, suggesting that their mechanisms of activation might be different. We analyzed 17 tumor-associated MEK1 mutants and found that they drove ERK signaling autonomously or in a RAS/RAF-dependent manner. The latter are sensitive to feedback inhibition of RAF, which limits their functional output, and often cooccur with or mutations. They act as amplifiers of RAF signaling. In contrast, another class of mutants deletes a hitherto unrecognized negative regulatory segment of MEK1, is RAF- and phosphorylation-independent, is unaffected by feedback inhibition of upstream signaling, and drives high ERK output and transformation in the absence of RAF activity. Moreover, these RAF-independent mutants are insensitive to allosteric MEK inhibitors, which preferentially bind to the inactivated form of MEK1. All the mutants are sensitive to an ATP-competitive MEK inhibitor. Thus, our study comprises a novel therapeutic strategy for tumors driven by RAF-independent MEK1 mutants. Mutants with which MEK1 mutants coexist and their sensitivity to inhibitors are determined by allele-specific properties. This study shows the importance of functional characterization of mutant alleles in single oncogenes and identifies a new class of MEK1 mutants, insensitive to current MEK1 inhibitors but treatable with a new ATP-competitive inhibitor. .

Encorafenib, a selective BRAF inhibitor (BRAFi), has a pharmacologic profile that is distinct from that of other clinically active BRAFis. We evaluated encorafenib in a phase I study in patients with BRAFi treatment-naïve and pretreated -mutant melanoma. The pharmacologic activity of encorafenib was first characterized preclinically. Encorafenib monotherapy was then tested across a range of once-daily (50-700 mg) or twice-daily (75-150 mg) regimens in a phase I, open-label, dose-escalation and -expansion study in adult patients with histologically confirmed advanced/metastatic -mutant melanoma. Study objectives were to determine the maximum tolerated dose (MTD) and/or recommended phase II dose (RP2D), characterize the safety and tolerability and pharmacokinetic profile, and assess the preliminary antitumor activity of encorafenib. Preclinical data demonstrated that encorafenib inhibited BRAF V600E kinase activity with a prolonged off-rate and suppressed proliferation and tumor growth of V600E-mutant melanoma models. In the dose-escalation phase, 54 patients (29 BRAFi-pretreated and 25 BRAFi-naïve) were enrolled. Seven patients in the dose-determining set experienced dose-limiting toxicities. Encorafenib at a dose of 300 mg once daily was declared the RP2D. In the expansion phase, the most common all-cause adverse events were nausea (66%), myalgia (63%), and palmar-plantar erythrodysesthesia (54%). In BRAFi-naïve patients, the overall response rate (ORR) and median progression-free survival (mPFS) were 60% and 12.4 months [95% confidence interval (CI), 7.4-not reached (NR)]. In BRAFi-pretreated patients, the ORR and mPFS were 22% and 1.9 months (95% CI, 0.9-3.7). Once-daily dosing of single-agent encorafenib had a distinct tolerability profile and showed varying antitumor activity across BRAFi-pretreated and BRAFi-naïve patients with advanced/metastatic melanoma. .

Purpose An improved understanding of the molecular pathogenesis of brain metastases, one of the most common and devastating complications of advanced melanoma, may identify and prioritize rational therapeutic approaches for this disease. In particular, the identification of molecular differences between brain and extracranial metastases would support the need for the development of organ-specific therapeutic approaches. Experimental Design Hotspot mutations, copy number variations (CNV), global mRNA expression patterns, and quantitative analysis of protein expression and activation by reverse phase protein array (RPPA) analysis were evaluated in pairs of melanoma brain metastases and extracranial metastases from patients who had undergone surgical resection for both types of tumors. Results The status of 154 previously reported hotspot mutations, including driver mutations in BRAF and NRAS, were concordant in all evaluable patient-matched pairs of tumors. Overall patterns of CNV, mRNA expression, and protein expression were largely similar between the paired samples for individual patients. However, brain metastases demonstrated increased expression of several activation-specific protein markers in the PI3K/AKT pathway compared to the extracranial metastases. Conclusions These results add to the understanding of the molecular characteristics of melanoma brain metastases and support the rationale for additional testing of the PI3K/AKT pathway as a therapeutic target in these highly aggressive tumors.

Disregulated Wnt/β-catenin signaling has been linked to various human diseases, including cancers. Inhibitors of oncogenic Wnt signaling are likely to have a therapeutic effect in cancers. LRP5 and LRP6 are closely related membrane coreceptors for Wnt proteins. Using a phage-display library, we identified anti-LRP6 antibodies that either inhibit or enhance Wnt signaling. Two classes of LRP6 antagonistic antibodies were discovered: one class specifically inhibits Wnt proteins represented by Wnt1, whereas the second class specifically inhibits Wnt proteins represented by Wnt3a. Epitope-mapping experiments indicated that Wnt1 class-specific antibodies bind to the first propeller and Wnt3a class-specific antibodies bind to the third propeller of LRP6, suggesting that Wnt1-and Wnt3a-class proteins interact with distinct LRP6 propeller domains. This conclusion is further supported by the structural functional analysis of LRP5/6 and the finding that the Wnt antagonist Sclerostin interacts with the first propeller of LRP5/6 and preferentially inhibits the Wnt1-class proteins. We also show that Wnt1 or Wnt3a class-specific anti-LRP6 antibodies specifically block growth of MMTV-Wnt1 or MMTV-Wnt3 xenografts in vivo. Therapeutic application of these antibodies could be limited without knowing the type of Wnt proteins expressed in cancers. This is further complicated by our finding that bivalent LRP6 antibodies sensitize cells to the nonblocked class of Wnt proteins. The generation of a biparatopic LRP6 antibody blocks both Wnt1-and Wnt3a-mediated signaling without showing agonistic activity. Our studies provide insights into Wnt-induced LRP5/6 activation and show the potential utility of LRP6 antibodies in Wntdriven cancer.antibody therapeutics | cancer T he Wnt/β-catenin pathway regulates diverse biological processes during development and tissue homeostasis by modulating the protein stability of β-catenin (1-3). In the absence of extracellular Wnt proteins, cytoplasmic β-catenin is associated with the β-catenin destruction complex and degraded by ubiquitinmediated proteolysis. Wnt signals are transduced by two distinct receptors, the serpentine receptor Frizzled (Frz) and the singlespan transmembrane proteins LRP5 or LRP6. Wnt proteins promote the assembly of the Frz-LRP5/6 signaling complex and induce phosphorylation of LRP5 or LRP6. Phosphorylated LRP5 or LRP6 inactivates the β-catenin degradation complex, allowing stabilized β-catenin to enter the nucleus, bind to the TCF transcription factors, and act as a transcriptional coactivator.The extracellular domain of LRP5 or LRP6 contains four YWTD-type β-propeller domains each followed by an EGF-like domain and an LDLR domain. Each propeller contains six YWTD motifs that form a six-bladed β-propeller structure (4). Biochemical studies suggest that Wnt proteins physically interact with both Frz and LRP6 and induce the formation of an Frz-