Cobimetinib and vemurafenib for the treatment of melanoma

Characterization of two melanoma cell lines resistant to BRAF/MEK inhibitors (vemurafenib and cobimetinib)

BACKGROUND

BRAF (v-raf murine sarcoma viral oncogene homolog B1)/MEK (mitogen-activated protein kinase kinase) inhibitors are used for melanoma treatment. Unfortunately, patients treated with this combined therapy develop resistance to treatment quite quickly, but the mechanisms underlying this phenomenon are not yet fully understood. Here, we report and characterize two melanoma cell lines (WM9 and Hs294T) resistant to BRAF (vemurafenib) and MEK (cobimetinib) inhibitors.

METHODS

Cell viability was assessed via the XTT test. The level of selected proteins as well as activation of signaling pathways were evaluated using Western blotting. The expression of the chosen genes was assessed by RT-PCR. The distribution of cell cycle phases was analyzed by flow cytometry, and confocal microscopy was used to take photos of spheroids. The composition of cytokines secreted by cells was determined using a human cytokine array.

RESULTS

The resistant cells had increased survival and activation of ERK kinase in the presence of BRAF/MEK inhibitors. The IC50 values for these cells were over 1000 times higher than for controls. Resistant cells also exhibited elevated activation of AKT, p38, and JNK signaling pathways with increased expression of EGFR, ErbB2, MET, and PDGFRβ receptors as well as reduced expression of ErbB3 receptor. Furthermore, these cells demonstrated increased expression of genes encoding proteins involved in drug transport and metabolism. Resistant cells also exhibited features of epithelial-mesenchymal transition and cancer stem cells as well as reduced proliferation rate and elevated cytokine secretion.

CONCLUSIONS

In summary, this work describes BRAF/MEK-inhibitor-resistant melanoma cells, allowing for better understanding the underlying mechanisms of resistance. The results may thus contribute to the development of new, more effective therapeutic strategies.

Advances in Understanding and Management of Erdheim-Chester Disease

Erdheim Chester Disease (ECD) is a rare histiocytic disorder marked by infiltration of organs with CD68+ histiocytes. ECD stems from mutations of BRAF and MAP2K1 in hematopoietic stem and progenitor cells (HSPCs), which further differentiate into monocytes and histiocytes. Histopathology reveals lipid-containing histiocytes, which test positive for CD68 and CD133 in immunohistochemistry. Signs and symptoms vary and depend on the organ/s of manifestation. Definitive radiological results associated with ECD include hairy kidney, coated aorta, and cardiac pseudotumor. Treatment options primarily include anti-cytokine therapy and inhibitors of BRAF and MEK signaling.

Illuminating phenotypic drug responses of sarcoma cells to kinase inhibitors by phosphoproteomics

Kinase inhibitors (KIs) are important cancer drugs but often feature polypharmacology that is molecularly not understood. This disconnect is particularly apparent in cancer entities such as sarcomas for which the oncogenic drivers are often not clear. To investigate more systematically how the cellular proteotypes of sarcoma cells shape their response to molecularly targeted drugs, we profiled the proteomes and phosphoproteomes of 17 sarcoma cell lines and screened the same against 150 cancer drugs. The resulting 2550 phenotypic profiles revealed distinct drug responses and the cellular activity landscapes derived from deep (phospho)proteomes (9-10,000 proteins and 10-27,000 phosphorylation sites per cell line) enabled several lines of analysis. For instance, connecting the (phospho)proteomic data with drug responses revealed known and novel mechanisms of action (MoAs) of KIs and identified markers of drug sensitivity or resistance. All data is publicly accessible via an interactive web application that enables exploration of this rich molecular resource for a better understanding of active signalling pathways in sarcoma cells, identifying treatment response predictors and revealing novel MoA of clinical KIs.

UV-Induced Somatic Mutations Driving Clonal Evolution in Healthy Skin, Nevus, and Cutaneous Melanoma

Introduction: Due to its aggressiveness, cutaneous melanoma (CM) is responsible for most skin cancer-related deaths worldwide. The origin of CM is closely linked to the appearance of UV-induced somatic mutations in melanocytes present in normal skin or in CM precursor lesions (nevi or dysplastic nevi). In recent years, new NGS studies performed on CM tissue have increased the understanding of the genetic somatic changes underlying melanomagenesis and CM tumor progression. Methods: We reviewed the literature using all important scientific databases. All articles related to genomic mutations in CM as well as normal skin and nevi were included, in particular those related to somatic mutations produced by UV radiation. Conclusions: CM development and progression are strongly associated with exposure to UV radiation, although each melanoma subtype has different characteristic genetic alterations and evolutionary trajectories. While BRAF and NRAS mutations are common in the early stages of tumor development for most CM subtypes, changes in CDKN2A, TP53 and PTEN, together with TERT promoter mutations, are especially common in advanced stages. Additionally, large genome duplications, loss of heterozygosity, and copy number variations are hallmarks of metastatic disease. Finally, the mutations driving melanoma targeted-therapy drug resistance are also summarized. The complete sequential stages of clonal evolution leading to CM onset from normal skin or nevi are still unknown, so further studies are needed in this field to shed light on the molecular pathways involved in CM malignant transformation and in melanoma acquired drug resistance.

BRAF and MEK Inhibitors Affect Dendritic-Cell Maturation and T-Cell Stimulation

BRAF and MEK inhibitor (BRAFi/MEKi) combinations are currently the standard treatment for patients with BRAF^V600 mutant metastatic melanoma. Since the RAS/RAF/MEK/ERK-pathway is crucial for the function of different immune cells, we postulated an effect on their function and thus interference with anti-tumor immunity. Therefore, we examined the influence of BRAFi/MEKi, either as single agent or in combination, on the maturation of monocyte-derived dendritic cells (moDCs) and their interaction with T cells. DCs matured in the presence of vemurafenib or vemurafenib/cobimetinib altered their cytokine secretion and surface marker expression profile. Upon the antigen-specific stimulation of CD8⁺ and CD4⁺ T cells with these DCs or with T2.A1 cells in the presence of BRAFi/MEKi, we detected a lower expression of activation markers on and a lower cytokine secretion by these T cells. However, treatment with any of the inhibitors alone or in combination did not change the avidity of CD8⁺ T cells in peptide titration assays with T2.A1 cells. T-helper cell/DC interaction is a bi-directional process that normally results in DC activation. Vemurafenib and vemurafenib/cobimetinib completely abolished the helper T-cell-mediated upregulation of CD70, CD80, and CD86 but not CD25 on the DCs. The combination of dabrafenib/trametinib affected DC maturation and activation as well as T-cell activation less than combined vemurafenib/cobimetinib did. Hence, for a potential combination with immunotherapy, our data indicate the superiority of dabrafenib/trametinib treatment.

Mixture Cure Models in Oncology: A Tutorial and Practical Guidance

Novel cancer therapies are associated with survival patterns that differ from established therapies, which may include survival curves that plateau after a certain follow-up time point. A fraction of the patient population is then considered statistically cured and subject to the same mortality experience as the cancer-free general population. Mixture cure models have been developed to account for this characteristic. As compared to standard survival analysis, mixture cure models can often lead to profoundly different estimates of long-term survival, required for health economic evaluations. This tutorial is designed as a practical introduction to mixture cure models. Step-by-step instructions are provided for the entire implementation workflow, i.e., from gathering and combining data from different sources to fitting models using maximum likelihood estimation and model results interpretation. Two mixture cure models were developed to illustrate (1) an "uninformed" approach where the cure fraction is estimated from trial data and (2) an "informed" approach where the cure fraction is obtained from an external source (e.g., real-world data) used as an input to the model. These models were implemented in the statistical software R, with the freely available code on GitHub. The cure fraction can be estimated as an output from ("uninformed" approach) or used as an input to ("informed" approach) a mixture cure model. Mixture cure models suggest presumed estimates of long-term survival proportions, especially in instances where some fraction of patients is expected to be statistically cured. While this type of model may initially seem complex, it is straightforward to use and interpret. Mixture cure models have the potential to improve the accuracy of survival estimates for treatments associated with statistical cure, and the present tutorial outlines the interpretation and implementation of mixture cure models in R. This type of model will likely become more widely used in health economic analyses as novel cancer therapies enter the market.

Resistance to Molecularly Targeted Therapies in Melanoma

Malignant melanoma is the most aggressive type of skin cancer with invasive growth patterns. In 2021, 106,110 patients are projected to be diagnosed with melanoma, out of which 7180 are expected to die. Traditional methods like surgery, radiation therapy, and chemotherapy are not effective in the treatment of metastatic and advanced melanoma. Recent approaches to treat melanoma have focused on biomarkers that play significant roles in cell growth, proliferation, migration, and survival. Several FDA-approved molecular targeted therapies such as tyrosine kinase inhibitors (TKIs) have been developed against genetic biomarkers whose overexpression is implicated in tumorigenesis. The use of targeted therapies as an alternative or supplement to immunotherapy has revolutionized the management of metastatic melanoma. Although this treatment strategy is more efficacious and less toxic in comparison to traditional therapies, targeted therapies are less effective after prolonged treatment due to acquired resistance caused by mutations and activation of alternative mechanisms in melanoma tumors. Recent studies focus on understanding the mechanisms of acquired resistance to these current therapies. Further research is needed for the development of better approaches to improve prognosis in melanoma patients. In this article, various melanoma biomarkers including BRAF, MEK, RAS, c-KIT, VEGFR, c-MET and PI3K are described, and their potential mechanisms for drug resistance are discussed.

Adjuvant Therapy of High-Risk (Stages IIC-IV) Malignant Melanoma in the Post Interferon-Alpha Era: A Systematic Review and Meta-Analysis

Introduction

Multiple agents are approved in the adjuvant setting of completely resected high-risk (stages IIC–IV) malignant melanoma. Subgroups may benefit differently depending on the agent used. We performed a systematic review and meta-analysis to evaluate the efficiency and tolerability of available options in the post interferon era across following subgroups: patient age, stage, ulceration status, lymph node involvement, BRAF status.

Methods

The PubMed and Cochrane Library databases were searched without restriction in year of publication in June and September 2020. Data were extracted according to the PRISMA Guidelines from two authors independently and were pooled according to the random-effects model. The predefined primary outcome was recurrence-free survival (RFS). Post-data extraction it was noted that one trial (BRIM8) reported disease-free survival which was defined in the exact same way as RFS.

Results

Five prospective randomized placebo-controlled trials were included in the meta-analysis. The drug regimens included ipilimumab, pembrolizumab, nivolumab, nivolumab/ipilimumab, vemurafenib, and dabrafenib/trametinib. Adjuvant treatment was associated with a higher RFS than placebo (HR 0.57; 95% CI= 0.45–0.71). Nivolumab/ipilimumab in stage IV malignant melanoma was associated with the highest RFS benefit (HR 0.23; 97.5% CI= 0.12–0.45), followed by dabrafenib/trametinib in stage III BRAF-mutant melanoma (HR 0.49; 95% CI= 0.40–0.59). The presence of a BRAF mutation was associated with higher RFS rates (HR 0.30; 95% CI= 0.11–0.78) compared to the wildtype group (HR 0.60; 95% CI= 0.44–0.81). Patient age did not influence outcomes (≥65: HR 0.50; 95% CI= 0.36–0.70, <65: HR 0.58; 95% CI= 0.46–0.75). Immune checkpoint inhibitor monotherapy was associated with lower RFS in non-ulcerated melanoma. Patients with stage IIIA benefited equally from adjuvant treatment as those with stage IIIB/C. Nivolumab/ipilimumab and ipilimumab monotherapy were associated with higher toxicity.

Conclusion

Adjuvant therapy should not be withheld on account of advanced age or stage IIIA alone. The presence of a BRAF mutation is prognostically favorable in terms of RFS. BRAF/MEK inhibitors should be preferred in the adjuvant treatment of BRAF-mutant non-ulcerated melanoma.

Anti-tumor Drug Targets Analysis: Current Insight and Future Prospect

The incidence and mortality of malignant tumors are on the rise, which has become the second leading cause of death in the world. At present, anti-tumor drugs are one of the most common methods for treating cancer. In recent years, with the in-depth study of tumor biology and related disciplines, it has been gradually discovered that the essence of cell carcinogenesis is the infinite proliferation of cells caused by the disorder of cell signal transduction pathways, followed by a major shift in the concept of anti-tumor drugs research and development. The focus of research and development is shifting from traditional cytotoxic drugs to a new generation of anti-tumor drugs targeted at abnormal signaling system targets in tumor cells. In this review, we summarize the targets of anti-tumor drugs and analyse the molecular mechanisms of their effects, which lay a foundation for subsequent treatment, research and development.

Clinical outcomes of BRAF plus MEK inhibition in melanoma: A meta-analysis and systematic review

Background

Melanoma is a potentially fatal malignancy with poor prognosis. Several recent studies have demonstrated that combination therapy of BRAF and MEK inhibition achieved better curative effect and appeared less toxic effects. We conducted a meta‐analysis to evaluate the efficacy and safety between BRAF inhibition plus MEK inhibition combination therapy and BRAF inhibition monotherapy in melanoma patients.

Methods

We performed the search in PubMed, EMBASE, and the Cochrane Library from January 2010 to January 2019. Inclusion and exclusion of studies, assessment of quality, outcome measures, data extraction, and synthesis were independently accomplished by two reviewers. Revman 5.3 software was used for the meta‐analysis.

Results

Totally, seven randomized controlled trials involving 3146 patients met our inclusion criteria. Comparing the results of combination therapy and monotherapy, combination therapy significantly improved OS (RR = 1.13; 95% CI, 1.08, 1.19; P < 0.00001), ORR (RR = 1.36; 95% CI, 1.28, 1.45; P < 0.00001), PFS (RR = 0.57; 95% CI, 0.52, 0.63; P < 0.00001) and reduced deaths (RR = 0.78; 95% CI, 0.69, 0.88; P < 0.0001). Skin‐related adverse events such as hyperkeratosis, cutaneous squamous‐cell carcinoma were less compared with monotherapy. However, gastrointestinal events like nausea, diarrhea, and vomiting were at a higher frequency.

Conclusion

Doublet BRAF and MEK inhibition achieved better survival outcomes over single‐agent BRAF inhibition and occurred less skin‐related events, but gastrointestinal events were more in combination therapy.

Dual Kit/Aur Inhibitors as Chemosensitizing Agents for the Treatment of Melanoma: Design, Synthesis, Docking Studies and Functional Investigation

Melanoma is the most serious form of skin cancer but its medication is still far from being safe and thoroughly effective. The search of novel therapeutic approaches represents therefore a health emergency to push through eagerly. In this study, we describe a novel class of dual c-Kit/Aur inhibitors, characterized by a 1,2,4-triazole core and developed by a structure-based optimization of a previously developed hit, and report the evidence of their significance as drug candidates for the treatment of melanoma. Compound 6a, merging the best inhibitory profile against the target kinases, showed anti-proliferative efficacy against the human melanoma cell lines A2058, expressing the BRAF V600D mutation, and WM266-4, expressing BRAF V600E. Significantly, it displayed also a highly synergistic profile when tested in combination with vemurafenib, thus proving its efficacy not only per se but even in a combination therapy, which is nowadays acknowledged as the cornerstone approach of the forthcoming tumour management.

Skin adverse events in recently approved targeted therapies in solid malignancies

Targeted anticancer therapies are an important weapon in the fight against cancer. Targeted therapies interfere with specific molecules necessary for tumor growth and cancer progression. They are divided mainly to either monoclonal antibodies or small molecules inhibitors. Their primary objective is to target directly and precisely the cancer cells leading to a minimal side-effects profile. The dermatologic adverse reactions of these targeted therapies is different from those seen with classical cytotoxic chemotherapy. Rashes, xerosis, hand-foot-skin reaction and mucositis are the most frequent side effects. In this paper, we aim to present a comprehensive review of the dermatologic side effects of targeted therapies including, specific side effects related to recently, approved targeted therapies.

Safety Profiles and Pharmacovigilance Considerations for Recently Patented Anticancer Drugs: Cutaneous Melanoma

BACKGROUND

Malignant melanoma is a skin cancer responsible for 90% of cutaneous cancer- related deaths. In recent years, breakthroughs in treatment strategy have revolutionized the prognosis in both early and advanced melanoma patients. In particular, treatment with monoclonal antibodies targeting co-inhibitory checkpoints or specific molecular pathways leads to a new era of promising options, by prolonging the survival time of these patients. Moreover, unlike the chemotherapy that was used until some time ago, these new drugs have a good and more manageable toxicity profile. However, because of the recent introduction in clinical practice of the new agents, there is a learning curve among physicians regarding early recognition and management of the associated side effects.

OBJECTIVES

The analysis of the toxicity profiles of the different agents currently studied for the treatment of early and advanced melanoma, and the description of several relevant recent patents in this field, are the aims of this review.

METHODS

This is a systematically conducted review based on current clinical guidelines and on international Pharmacovigilance databases (AERS-Eudravigilance - WHO Vigibase).

RESULTS

Our systematic analysis outlines a comprehensive overview of the pharmacology, clinical application and the safety of recent anticancer drugs to treat melanoma, which can be an essential instrument for health professionals and researchers.

CONCLUSION

The new oncological therapies against melanoma are based on increasingly specific biological and immunological targets. For this reason, the potential toxicities that are expected from patients would be less relevant than the systemic "classical" chemotherapy. However, the new therapies are not free from the risk of causing adverse reactions, some of which must be managed promptly and appropriately; moreover, the multiplicity of the metabolic pathways exposes the new target therapies to relevant potential interactions. This review can help to understand how important it is not to underestimate potential adverse drug reactions related to new targeted therapies.

Acquired platelet antagonism: off-target antiplatelet effects of malignancy treatment with tyrosine kinase inhibitors

Platelets can contribute to tumor progression and metastasis. Cancer patients are at increased risk of thrombosis, and advanced stages of cancer are associated with thrombocytosis or increased platelet reactivity. Tyrosine kinase inhibitors (TKIs) are widely used as a targeted strategy for cancer treatment, with the aim of prolonging progression-free survival of the patients. Because of their broad kinase target spectrum, most TKIs inevitably have off-target effects. Platelets rely on tyrosine kinase activity for their activation. Frequently observed side effects are lowering of platelet count and inhibition of platelet functions, whether or not accompanied by an increased bleeding risk. In this review, we aim to give insights into: (i) 38 TKIs that are currently used for the treatment of different types of cancer, either on the market or in clinical trials; (ii) how distinct TKIs can inhibit activation mechanisms in platelets; and (iii) the clinical consequences of the antiplatelet effects of TKI treatment. For several TKIs, the knowledge on affinity for their targets does not align with the published effects on platelets and reported bleeding events. This review should raise awareness of the potential antiplatelet effects of several TKIs, which will be enhanced in the presence of antithrombotic drugs.

BRAF and MEK Inhibitors Influence the Function of Reprogrammed T Cells: Consequences for Adoptive T-Cell Therapy

BRAF and MEK inhibitors (BRAFi/MEKi), the standard treatment for patients with BRAF^V600 mutated melanoma, are currently explored in combination with various immunotherapies, notably checkpoint inhibitors and adoptive transfer of receptor-transfected T cells. Since two BRAFi/MEKi combinations with similar efficacy are approved, potential differences in their effects on immune cells would enable a rational choice for triple therapies. Therefore, we characterized the influence of the clinically approved BRAFi/MEKi combinations dabrafenib (Dabra) and trametinib (Tram) vs. vemurafenib (Vem) and cobimetinib (Cobi) on the activation and functionality of chimeric antigen receptor (CAR)-transfected T cells. We co-cultured CAR-transfected CD8⁺ T cells and target cells with clinically relevant concentrations of the inhibitors and determined the antigen-induced cytokine secretion. All BRAFi/MEKi reduced this release as single agents, with Dabra having the mildest inhibitory effect, and Dabra + Tram having a clearly milder inhibitory effect than Vem + Cobi. A similar picture was observed for the upregulation of the activation markers CD25 and CD69 on CAR-transfected T cells after antigen-specific stimulation. Most importantly, the cytolytic capacity of the CAR-T cells was significantly inhibited by Cobi and Vem + Cobi, whereas the other kinase inhibitors showed no effect. Therefore, the combination Dabra + Tram would be more suitable for combining with T-cell-based immunotherapy than Vem + Cobi.

Clinical Use of Precision Oncology Decision Support

PURPOSE

Precision oncology is hindered by the lack of decision support for determining the functional and therapeutic significance of genomic alterations in tumors and relevant clinically available options. To bridge this knowledge gap, we established a Precision Oncology Decision Support (PODS) team that provides annotations at the alteration-level and subsequently determined if clinical decision-making was influenced.

METHODS

Genomic alterations were annotated to determine actionability based on a variant's known or potential functional and/or therapeutic significance. The medical records of a subset of patients annotated in 2015 were manually reviewed to assess trial enrollment. A web-based survey was implemented to capture the reasons why genotype-matched therapies were not pursued.

RESULTS

PODS processed 1,669 requests for annotation of 4,084 alterations (2,254 unique) across 49 tumor types for 1,197 patients. 2,444 annotations for 669 patients included an actionable variant call: 32.5% actionable, 9.4% potentially, 29.7% unknown, 28.4% non-actionable. 66% of patients had at least one actionable/potentially actionable alteration. 20.6% (110/535) patients annotated enrolled on a genotype-matched trial. Trial enrolment was significantly higher for patients with actionable/potentially actionable alterations (92/333, 27.6%) than those with unknown (16/136, 11.8%) and non-actionable (2/66, 3%) alterations (p=0.00004). Actionable alterations in PTEN, PIK3CA, and ERBB2 most frequently led to enrollment on genotype-matched trials. Clinicians cited a variety of reasons why patients with actionable alterations did not enroll on trials.

CONCLUSION

Over half of alterations annotated were of unknown significance or non-actionable. Physicians were more likely to enroll a patient on a genotype-matched trial when an annotation supported actionability. Future studies are needed to demonstrate the impact of decision support on trial enrollment and oncologic outcomes.

Signaling pathways as therapeutic targets in biliary tract cancer

INTRODUCTION

The incidence of biliary tract cancer (BTC) is increasing, and the disease is frequently diagnosed during advanced stages, leading to poor overall survival. Limited treatment options are currently available and novel therapeutic approaches are needed. A number of completed clinical trials have evaluated the role of chemotherapy for BTC, demonstrating a marginal benefit. Thus, there is increased interest in applying targeted therapies for this disease. Areas covered: This review article summarizes the role of chemotherapeutic regimens for the treatment of BTC, and highlights key signal transduction pathways of interest for targeted inhibition. Of particular interest are the MEK or MAP2K (mitogen-activated protein kinase kinase), phosphatidylinositol-3 kinase (PI3K) and signal transducer and activator of transcription-3 (STAT3) pathways. We discuss the available data on several promising inhibitors of these pathways, both in the pre-clinical and clinical settings. Expert opinion: Future treatment strategies should address targeting of MEK, PI3K and STAT3 for BTC, with a focus on combined therapeutic approaches.

Approaches for Identifying Novel Targets in Precision Medicine: Lessons from DNA Repair

Genome stability is maintained by a number of elegant mechanisms, which sense and repair damaged DNA. Germline defects that compromise genomic integrity result in cancer predisposition, exemplified by rare syndromes caused by mutations in certain DNA repair genes. These individuals often exhibit other symptoms including progeria and neurodegeneration. Paradoxically, some of these deleterious genetic alterations provide novel therapeutic opportunities to target cancer cells; an excellent example of such an approach being the recent development of poly (ADP-ribose) polymerase inhibitors as the first 'synthetic lethal' medicine for patients with BRCA-mutant cancers. The therapeutic exploitation of synthetic lethal interactions has enabled a novel approach to personalised medicine based on continued molecular profiling of patient and tumour material. This profiling may also aid clinicians in the identification of specific drug resistance mechanisms following relapse, and enable appropriate modification of the therapeutic regimen. This chapter focuses on therapeutic strategies designed to target aspects of the DNA damage response, and examines emerging themes demonstrating mechanistic overlap between DNA repair and neurodegeneration.

Molecular Pathways: Maintaining MAPK Inhibitor Sensitivity by Targeting Nonmutational Tolerance

Targeting hyperactive MAPK signaling has proven to be an effective treatment for a variety of different cancers. Responses to the BRAF inhibitors vemurafenib and dabrafenib and the MEK inhibitors trametinib and cobimetinib are, however, transient, and complete remission is rarely observed; rather, outgrowth of resistant clones within progressed tumors appears inevitable. These resistant tumors display great heterogeneity, which poses a major challenge to any salvage therapy. Recent focus has, therefore, been on the early dynamics of inhibitor response during tumor regression. During this time, cells can persist in an adapted tolerant state, which results in a phase of nonmutational drug tolerance. In this article, we discuss how inhibition of the MAPK pathway leads to an adaptive rewiring that evolves from the relief of immediate negative feedback loops to short-term gene expression changes and adaptation of intracellular signaling. Tolerance can also be mediated by external signaling from the tumor microenvironment, which itself adapts upon treatment and the selection for cells with an innate drug-tolerant phenotype. In preclinical models, combination treatment with receptor tyrosine kinase (RTK) inhibitors (lapatinib and dasatinib), histone deacetylase (HDAC) inhibitors (vorinostat and entinostat), or drugs targeting cancer-specific mechanisms (nelfinavir in melanoma) can overcome this early tolerance. A better understanding of how nonmutational tolerance is created and supported may hold the key to better combinational strategies that maintain drug sensitivity. Clin Cancer Res; 22(24); 5966-70. ©2016 AACR.