Multiple Gastrointestinal Polyps in Patients Treated with BRAF Inhibitors

How I treat refractory/relapsed hairy cell leukemia with BRAF inhibitors

Hairy cell leukemia (HCL) responds very well to frontline chemotherapy with purine analogs (cladribine and pentostatine). However, approximately half of patients experience 1 or more relapses, which become progressively resistant to these myelotoxic and immunosuppressive agents. At progression, standard therapeutic options include a second course of purine analogs alone or in combination with rituximab and, upon second relapse, therapy with the anti-CD22 immunotoxin moxetumomab pasudotox. Furthermore, blockade of the mutant BRAF-V600E kinase (the pathogenetic hallmark of HCL) through orally available specific inhibitors (vemurafenib or dabrafenib) effaces the peculiar morphologic, phenotypic, and molecular identity of this disease and its typical antiapoptotic behavior and is emerging as an attractive chemotherapy-free strategy in various clinical scenarios. These include patients with, or at risk of, severe infections and, in a highly effective combination with rituximab, patients with relapsed or refractory HCL. Other treatments explored in clinical trials are BTK inhibition with ibrutinib and co-inhibition of BRAF (through dabrafenib or vemurafenib) and its downstream target MEK (through trametinib or cobimetinib). Here, we focus on our experience with BRAF inhibitors in clinical trials and as off-label use in routine practice by presenting 3 challenging clinical cases to illustrate their management in the context of all available treatment options.

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.

Targeting the Mutational Landscape of Bystander Cells: Drug-Promoted Blood Cancer From High-Prevalence Pre-neoplasias in Patients on BRAF Inhibitors

Drug-promoted cancers are increasingly recognized as a serious clinical problem in patients receiving BRAF inhibitory treatment. Here we report on a patient with BRAF mutant hairy cell leukemia and monoclonal B-cell lymphocytosis (MBL), who responded durably to BRAF/MEK inhibitors (BRAFi/MEKi) but experienced transformation of a RAS mutant MBL to chronic lymphocytic leukemia (CLL) with accelerated nodal progression. Hypothesizing that BRAFi triggered excessive MEK-ERK signaling in the MBL/CLL clone via the CRAF/RAS complex as previously described for BRAFi-induced cancers, BRAFi was discontinued inducing a rapid remission of the CLL on MEKi alone. Liquid biopsy monitoring showed a continuous increase of the MBL/CLL clone from the start of BRAFi/MEKi treatment followed by a rapid decline upon BRAFi withdrawal. Next-generation sequencing of a cohort of patients with MBL and monoclonal gammopathy of unclear significance (MGUS) revealed that almost one third of these cases harbored RAS mutations. In view of the population frequency of lymphatic pre-malignant conditions and the prevalence of RAS mutations in such cases, vigilant surveillance remains critical in patients treated with BRAF inhibitors.

Molecular targeted therapy of BRAF-mutant colorectal cancer

Over the past two decades, the molecular characterization of metastatic colorectal cancer (mCRC) has been revolutionized by the routine implementation of RAS and BRAF tests. As a result, it is now known that patients with mCRC harboring BRAF mutations experience a poor prognosis. Although it accounts for only 10% of mCRC, this group is heterogeneous; only the BRAF-V600E mutation, also observed in melanoma, is associated with a very poor prognosis. In terms of treatment, these patients do not benefit from therapeutics targeting the epidermal growth factor receptor (EGFR). In first-line chemotherapy, there are two main options; the first one is to use a triple chemotherapy combination of 5-fluorouracil, irinotecan, and oxaliplatin, with the addition of bevacizumab, because post hoc analysis of randomized trials have reported interesting results. The other option is to use double chemotherapy plus bevacizumab, since anti-EGFR seems to have modest activity in these patients. Only a small percentage of patients who experience failure of this first-line treatment receive second-line treatment. Monotherapy with BRAF inhibitors has failed in this setting, and different combinations have also been tested. Using the rationale that BRAF inhibitor monotherapy fails due to feedback activation of the EGFR pathway, BRAF inhibitors have been combined with anti-EGFR agents plus or minus MEK inhibitors; however, the results did not live up to the hopes raised by the concept. To date, the best results in second-line treatment have been obtained with a combination of vemurafenib, cetuximab, and irinotecan. Despite these advances, further improvements are needed.

ROS1 Rearrangement in a Case of Classic Biphasic Pulmonary Blastoma

Classic biphasic pulmonary blastoma (CBPB) is a rare and aggressive type of non-small cell lung carcinoma (NSCLC) presenting in adults in the fourth to fifth decade. The prognosis is poor and after surgical resection, therapeutic options are often limited. ROS1 is a proto-oncogene receptor tyrosine kinase that has been identified in some types of NSCLC. We report a case of a 36-year-old woman with CBPB, which was subsequently found to have a ROS1 rearrangement. This is the first reported case of a ROS1-rearranged CBPB. This finding has therapeutic implications as these tumors have the potential to be treated with receptor tyrosine kinase inhibitors.

Development of RET mutant cutaneous angiosarcoma during BRAF inhibitor therapy

Treatment with BRAF inhibitors may lead to paradoxical mitogen-activated protein kinase (MAPK) pathway activation and accelerated tumorigenesis in cells with preexisting oncogenic hits. This phenomenon manifests clinically in the development of squamous cell carcinomas (SCCs) and keratoacanthomas (KAs) in patients treated with BRAF inhibitors. Cases of extracutaneous malignancies associated with BRAF inhibitors have also been reported. We present a case of a patient who developed a cutaneous angiosarcoma 6 months after initiation of vemurafenib therapy. Next-generation sequencing (NGS) revealed a mutation in RET, which lies upstream of the MAPK pathway. This case highlights that treatment with BRAF inhibitors may promote the accelerated growth of secondary malignancies. Physician awareness of the spectrum of secondary malignancies associated with BRAF inhibitor treatment will support their early detection and treatment.

Molecular and Clinical Approach to Intra-abdominal Adverse Effects of Targeted Cancer Therapies

Targeted cancer therapies encompass an exponentially growing number of agents that involve a myriad of molecular pathways. To excel within this rapidly changing field of clinical oncology, radiologists must eschew traditional organ system-based approaches of cataloging adverse effects in favor of a conceptual framework that incorporates molecular mechanisms and associated clinical outcomes. Understanding molecular mechanisms that underlie imaging manifestations of adverse effects and known associations with treatment response allows radiologists to more effectively recognize adverse effects and differentiate them from tumor progression. Radiologists can therefore more effectively guide oncologists in the management of adverse effects and treatment decisions regarding continuation or cessation of drug therapy. Adverse effects from targeted cancer therapies can be classified into four categories: (a) category 1, on-target adverse effects associated with treatment response; (b) category 2, on-target adverse effects without associated treatment response; (c) category 3, off-target adverse effects; and (d) category 4, tumor necrosis-related adverse effects. This review focuses on adverse effects primarily within the abdomen and pelvis classified according to established or hypothesized molecular mechanisms and illustrated with images of classic examples and several potential emerging toxic effects. ^©RSNA, 2017.

PLX8394, a new generation BRAF inhibitor, selectively inhibits BRAF in colonic adenocarcinoma cells and prevents paradoxical MAPK pathway activation

BRAF inhibitors (BRAFi) are standard of care for the treatment of BRAF V600 mutation-driven metastatic melanoma, but can lead to paradoxical activation of the mitogen-activated protein kinase (MAPK) signalling pathway. This can result in the promotion of precancerous lesions and secondary neoplasms, mainly (but not exclusively) associated with pre-existing mutations in RAS genes. We previously reported a patient with synchronous BRAF-mutated metastatic melanoma and BRAF^wt/KRAS^G12D-metastatic colorectal cancer (CRC), whose CRC relapsed and progressed when treated with the BRAF inhibitor dabrafenib (GSK2118436). We used tissue from the resected CRC metastasis to derive a cell line, LM-COL-1, which directly and reliably mimicked the clinical scenario including paradoxical activation of the MAPK signalling pathway resulting in increased cell proliferation upon dabrafenib treatment. Novel BRAF inhibitors (PLX8394 and PLX7904), dubbed as “paradox breakers”, were developed to inhibit V600 mutated oncogenic BRAF without causing paradoxical MAPK pathway activation. In this study we used our LM-COL-1 model alongside multiple other CRC cell lines with varying mutational backgrounds to demonstrate and confirm that the paradox breaker PLX8394 retains on-target inhibition of mutated BRAF V600 without paradoxically promoting MAPK signalling.

Management of Treatment-Related Adverse Events with Agents Targeting the MAPK Pathway in Patients with Metastatic Melanoma

Tremendous progress has been made in the clinical landscape of advanced-stage BRAF V600-mutant melanoma treatment over the past 5 years. Targeted therapies that inhibit specific steps of the mitogen-activated protein kinase pathway have been shown to provide significant overall treatment benefit in patients with this difficult-to-treat disease. Combination therapy with BRAF and MEK inhibitors (dabrafenib plus trametinib or vemurafenib plus cobimetinib, respectively) has become standard of care. These agents are administered until disease progression or unacceptable toxicity occurs; thus, some patients may remain on maintenance therapy for an extended period of time, while toxicities may result in early discontinuation in other patients. Because the goal of treatment is to prolong survival with minimal impairment of quality of life, drug-related adverse events (AEs) require prompt management to ensure that patients derive the best possible benefit from therapy. Proper management depends on an understanding of which AEs are most likely BRAF or MEK inhibitor associated, thus providing a rationale for dose modification of the appropriate drug. Additionally, the unique safety profile of the chosen regimen may influence patient selection and monitoring. This review discusses the toxicity profiles of these agents, with a focus on the most commonly reported and serious AEs. Here, we offer practical guidance derived from our clinical experience for the optimal management of key drug-related AEs.

IMPLICATIONS FOR PRACTICE

Targeted therapy with BRAF plus MEK inhibitors has become the standard of care for patients with advanced-stage BRAF V600-mutant metastatic melanoma. To provide optimal therapeutic benefit to patients, clinicians need a keen understanding of the toxicity profiles of these drugs. Prompt identification and an understanding of which adverse events are most likely BRAF or MEK inhibitor associated provide a rationale for appropriate therapy adjustments. Practical recommendations derived from clinical experience are provided for management of key drug-related toxicities.

BRAF inhibitor treatment of melanoma causing colonic polyps: An alternative hypothesis

Colonic polyps may arise from BRAF inhibitor treatment of melanoma, possibly due to paradoxical activation of the mitogen-activated protein (MAP)-kinase pathway. In an alternative evidence based scenario, tubular colonic adenomas with APC gene mutations have also been identified in the context of BRAF inhibitor treatment, in the absence of mutations of MAPK genes. A minority of colorectal cancers develop by an alternative “serrated polyp pathway”. This article postulates a novel hypothesis, that the established phenotypic and molecular characteristics of serrated colonic polyps/CRC offer an intriguing insight into the pathobiology of BRAF inhibitor induced colonic polyps. Serrated polyps are characterized by a CpG island methylation phenotype, MLH1 silencing and cellular senescence. They also have BRAF mutations. The contention is that BRAF inhibitor induced polyps mimic the afore-described histology and molecular features of serrated polyps with the exception that instead of the presence of BRAF mutations they induce C-RAF homodimers and B-RAF: C-RAF heterodimers.