Tumor heterogeneity and strategies to overcome kinase inhibitor resistance in cancer: lessons from melanoma

Targeted Therapies in Cancer: To Be or Not to Be, Selective

Development of targeted therapies in recent years revealed several nonchemotherapeutic options for patients. Chief among targeted therapies is small molecule kinase inhibitors targeting key oncogenic signaling proteins. Through competitive and noncompetitive inhibition of these kinases, and therefore the pathways they activate, cancers can be slowed or completely eradicated, leading to partial or complete remissions for many cancer types. Unfortunately, for many patients, resistance to targeted therapies, such as kinase inhibitors, ultimately develops and can necessitate multiple lines of treatment. Drug resistance can either be de novo or acquired after months or years of drug exposure. Since resistance can be due to several unique mechanisms, there is no one-size-fits-all solution to this problem. However, combinations that target complimentary pathways or potential escape mechanisms appear to be more effective than sequential therapy. Combinations of single kinase inhibitors or alternately multikinase inhibitor drugs could be used to achieve this goal. Understanding how to efficiently target cancer cells and overcome resistance to prior lines of therapy became imperative to the success of cancer treatment. Due to the complexity of cancer, effective treatment options in the future will likely require mixing and matching these approaches in different cancer types and different disease stages.

Nongenetic Mechanisms of Drug Resistance in Melanoma

Resistance to targeted and immune-based therapies limits cures in patients with metastatic melanoma. A growing number of reports have identified nongenetic primary resistance mechanisms including intrinsic microenvironment- and lineage plasticity–mediated processes serving critical functions in the persistence of disease throughout therapy. There is a temporally shifting spectrum of cellular identities fluidly occupied by therapy-persisting melanoma cells responsible for driving therapeutic resistance and metastasis. The key epigenetic, metabolic, and phenotypic reprogramming events requisite for the manifestation and maintenance of so-called persister melanoma populations remain poorly understood and underscore the need to comprehensively investigate actionable vulnerabilities. Here we attempt to integrate the field's observations on nongenetic mechanisms of drug resistance in melanoma. We postulate that the future design of therapeutic strategies specifically addressing therapy-persisting subpopulations of melanoma will improve the curative potential of therapy for patients with metastatic disease.

Validating and enabling phosphoglycerate dehydrogenase (PHGDH) as a target for fragment-based drug discovery in PHGDH-amplified breast cancer

3-Phosphoglycerate dehydrogenase (PHGDH) has recently been identified as an attractive target in cancer therapy as it links upregulated glycolytic flux to increased biomass production in cancer cells. PHGDH catalyses the first step in the serine synthesis pathway and thus diverts glycolytic flux into serine synthesis. We have used siRNA-mediated suppression of PHGDH expression to show that PHGDH is a potential therapeutic target in PHGDH-amplified breast cancer. Knockdown caused reduced proliferation in the PHGDH-amplified cell line MDA-MB-468, whereas breast cancer cells with low PHGDH expression or with elevated PHGDH expression in the absence of genomic amplification were not affected. As a first step towards design of a chemical probe for PHGDH, we report a fragment-based drug discovery approach for the identification of PHGDH inhibitors. We designed a truncated PHGDH construct that gave crystals which diffracted to high resolution, and could be used for fragment soaking. 15 fragments stabilising PHGDH were identified using a thermal shift assay and validated by X-ray crystallography and ITC competition experiments to exhibit 1.5-26.2 mM affinity for PHGDH. A structure-guided fragment growing approach was applied to the PHGDH binders from the initial screen, yielding greater understanding of the binding site and suggesting routes to achieve higher affinity NAD-competitive inhibitors.

Novel small molecule XPO1/CRM1 inhibitors induce nuclear accumulation of TP53, phosphorylated MAPK and apoptosis in human melanoma cells

XPO1/CRM1 is a key nuclear exporter protein that mediates translocation of numerous cellular regulatory proteins. We investigated whether XPO1 is a potential therapeutic target in melanoma using novel selective inhibitors of nuclear export (SINE). In vitro effects of SINE on cell growth and apoptosis were measured by MTS assay and flow cytometry [Annexin V/propidium iodide (PI)], respectively in human metastatic melanoma cell lines. Immunoblot analysis was used to measure nuclear localization of key cellular proteins. The in vivo activity of oral SINE was evaluated in NOD/SCID mice bearing A375 or CHL-1 human melanoma xenografts. SINE compounds induced cytostatic and pro-apoptotic effects in both BRAF wild type and mutant (V600E) cell lines at nanomolar concentrations. The cytostatic and pro-apoptotic effects of XPO1 inhibition were associated with nuclear accumulation of TP53, and CDKN1A induction in the A375 cell line with wild type TP53, while pMAPK accumulated in the nucleus regardless of TP53 status. The orally bioavailable KPT-276 and KPT-330 compounds significantly inhibited growth of A375 (p<0.0001) and CHL-1 (p = 0.0087) human melanoma cell lines in vivo at well tolerated doses. Inhibition of XPO1 using SINE represents a potential therapeutic approach for melanoma across cells with diverse molecular phenotypes by promoting growth inhibition and apoptosis.

[Cellular and molecular mechanisms of carcinogenic side effects and resistance to BRAF inhibitors in metastatic melanoma with BRAFV600 mutation: state of the knowledge]

Cutaneous melanoma is a malignant tumor with a high metastatic potential. If an early treatment is associated with a favorable outcome, the prognosis of metastatic melanoma remains poor. Advances in molecular characterization of cancers, notably the discovery of BRAF gene mutations in metastatic melanoma, allowed to the recent development of targeted therapies against mutated BRAF protein. Despite high tumor response rates observed in clinical trials, these new drugs are associated with frequent secondary tumor resistance occurrence and paradoxical carcinogenic side effects. The cellular and molecular mechanisms of these carcinogenic side effects and secondary resistance are not yet fully elucidated and are actually intensely studied. This review of the literature focus on the mechanisms of these carcinogenic side effects and on the tumor resistance associated with anti-BRAF targeted therapies.

Dabrafenib and its potential for the treatment of metastatic melanoma

The purpose of this study is to review the development of BRAF inhibitors, with emphasis on the trials conducted with dabrafenib (GSK2118436) and the evolving role of dabrafenib in treatment for melanoma patients. Fifty percent of cutaneous melanomas have mutations in BRAF, resulting in elevated activity of the mitogen-activated protein kinase signaling pathway. Dabrafenib inhibits the mutant BRAF (BRAF(mut)) protein in melanomas with BRAF(V600E) and BRAF(V600K) genotypes. BRAF(V600E) metastatic melanoma patients who receive dabrafenib treatment exhibit high clinical response rates and compared with dacarbazine chemotherapy, progression-free survival. Efficacy has also been demonstrated in BRAF(V600K) patients and in those with brain metastases. Dabrafenib has a generally mild and manageable toxicity profile. Cutaneous squamous cell carcinomas and pyrexia are the most significant adverse effects. Dabrafenib appears similar to vemurafenib with regard to efficacy but it is associated with less toxicity. It is expected that new combinations of targeted drugs, such as the combination of dabrafenib and trametinib (GSK1120212, a MEK inhibitor), will provide higher response rates and more durable clinical benefit than dabrafenib monotherapy.

Hotspot oncomutations: implications for personalized cancer treatment

Understanding the extent to which specific tumor mutations impact or mediate patient response to particular cancer therapies has become a rapidly increasing area of research. Recent research findings regarding four predominant mutational targets (KRAS, BRAF, EGFR and PIK3CA) show that these tumor mutations have predictive power for identifying which patients are likely to respond to particular therapies, and have prognostic significance irrespective of treatment. However, in this regard, the literature is frequently nuanced and sometimes contradictory. This lack of clarity may be due, at least in part, to the utilization of mutation detection methods with varying sensitivities across studies of different patient populations. Nevertheless, considerable evidence suggests minor tumor subpopulations may be contributing to inappropriate patient stratification, development of resistance to treatment, and the relapse that often follows treatment with molecularly targeted therapies. Consequently, mutant tumor subpopulations need to be considered in order to improve strategies for personalized cancer treatment.