New frontiers against sorafenib resistance in renal cell carcinoma: From molecular mechanisms to predictive biomarkers

Renal cell carcinoma (RCC) is a highly vascularized tumor and prone to distant metastasis. Sorafenib is the first targeted multikinase inhibitor and first-line chemical drug approved for RCC therapy. In fact, only a small number of RCC patients benefit significantly from sorafenib treatment, while the growing prevalence of sorafenib resistance has become a major obstacle for drug therapy effectivity of sorafenib. The molecular mechanisms of sorafenib resistance in RCC are not completely understood by now. Herein, we comprehensively summarize the underlying mechanisms of sorafenib resistance and molecular biomarkers for predicting sorafenib responsiveness. Moreover, we outline strategies suitable for overcoming sorafenib resistance and prospect potential approaches for identifying biomarkers associated with sorafenib resistance in RCC, which contributes to guide individualized and precision drug therapy.

Immunomodulatory effects of BRAF and MEK inhibitors: Implications for Melanoma therapy

Targeted therapy with BRAF inhibitors (BRAFi) and MEK inhibitors (MEKi) provides rapid disease control with high response rates in patients with BRAF-mutant metastatic melanoma. However, the majority of patients develop resistance to therapy during the course of therapy. Immune checkpoint inhibitors show a slower onset of action with lower response rates, with responders showing sustained response. The combination of BRAFi/MEKi and immune checkpoint inhibitors combines the hope for a fast, reliable and lasting response to therapy. Preclinical data supports this hypothesis. With the help of the PubMed database, a comprehensive search and analysis of preclinical and clinical studies on the combination of BRAFi/MEKi with immune checkpoint inhibitors was performed and yielded the following results: 1) In vivo, BRAFi and MEKi have no negative effects on immune cells; BRAFi and MEKi generate 2) an immune stimulating tumor microenvironment, 3) an increased infiltration of immune cells into the tumors, 4) a better recognition of melanoma cells by immune effector cells, and 5) a better functionality of the immune effector cells. In addition, in vivo experiments 6) demonstrated a superiority of the combination treatment compared to the individual strategies in both BRAF-mutant and BRAF wild-type melanomas. In summary, available data show that both BRAFi and MEKi have beneficial effects on the antitumor immunity and the tumor microenvironment as a whole, which is mediated by different mechanisms. Currently, clinical studies are underway to investigate combinations of BRAFi and MEKi with immune checkpoint inhibitors. The results of these studies are eagerly awaited.

BRAF-mutated cells activate GCN2-mediated integrated stress response as a cytoprotective mechanism in response to vemurafenib

In BRAF-mutated melanoma cells, the BRAF inhibitor, vemurafenib, induces phosphorylation of eukaryotic initiation factor 2α (eIF2α) and subsequent induction of activating transcription factor 4 (ATF4), the central regulation node of the integrated stress response (ISR). While the ISR supports cellular adaptation to various stresses, the role of vemurafenib-triggered ISR has not been fully characterized. Here, we showed that in response to vemurafenib, BRAF-mutated melanoma and colorectal cancer cells rapidly induced the ISR as a cytoprotective mechanism through activation of general control nonderepressible 2 (GCN2), an eIF2α kinase sensing amino acid levels. The vemurafenib-triggered ISR, an event independent of downstream MEK inhibition, was specifically prevented by silencing GCN2, but not other eIF2α kinases, including protein kinase-like endoplasmic reticulum kinase, which transmits endoplasmic reticulum (ER) stress. Consistently, the ER stress gatekeeper, GRP78, was not induced by vemurafenib. Interestingly, ATF4 silencing by siRNA rendered BRAF-mutated melanoma cells sensitive to vemurafenib. Thus, the GCN2-mediated ISR can promote cellular adaptation to vemurafenib-induced stress, providing an insight into the development of drug resistance.